shear layer formation: Topics by Science.gov

Sample records for shear layer formation

Effects of Environment Forcing on Marine Boundary Layer Cloud-Drizzle Processes

NASA Astrophysics Data System (ADS)

Dong, X.

2017-12-01

Determining the factors affecting drizzle formation in marine boundary layer (MBL) clouds remains a challenge for both observation and modeling communities. To investigate the roles of vertical wind shear and buoyancy (static instability) in drizzle formation, ground-based observations from the Atmospheric Radiation Measurement (ARM) Program at the Azores are analyzed for two types of conditions. The type I clouds should last for at least five hours and more than 90% time must be non-drizzling, and then followed by at least two hours of drizzling periods while the type II clouds are characterized by mesoscale convection cellular (MCC) structures with drizzle occur every two to four hours. By analyzing the boundary layer wind profiles (direction and speed), it was found that either directional or speed shear is required to promote drizzle production in the type I clouds. Observations and a recent model study both suggest that vertical wind shear helps the production of turbulent kinetic energy (TKE), stimulates turbulence within cloud layer, and enhances drizzle formation near the cloud top. The type II clouds do not require strong wind shear to produce drizzle. The small values of lower-tropospheric stability (LTS) and negative Richardson number (Ri) in the type II cases suggest that boundary layer instability plays an important role in TKE production and cloud-drizzle processes. By analyzing the relationships between LTS and wind shear for all cases and all time periods, a stronger connection was found between LTS and wind directional shear than that between LTS and wind speed shear.
Effects of environment forcing on marine boundary layer cloud-drizzle processes: MBL Cloud-Drizzle Processes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu, Peng; Dong, Xiquan; Xi, Baike

Determining the factors affecting drizzle formation in marine boundary layer (MBL) clouds remains a challenge for both observation and modeling communities. To investigate the roles of vertical wind shear and buoyancy (static instability) in drizzle formation, ground-based observations from the Atmospheric Radiation Measurement (ARM) Program at the Azores are analyzed for two types of conditions. The type I clouds should last for at least five hours and more than 90% time must be non-drizzling, and then followed by at least two hours of drizzling periods while the type II clouds are characterized by mesoscale convection cellular (MCC) structures with drizzlemore » occur every two to four hours. By analyzing the boundary layer wind profiles (direction and speed), it was found that either directional or speed shear is required to promote drizzle production in the type I clouds. Observations and a recent model study both suggest that vertical wind shear helps the production of turbulent kinetic energy (TKE), stimulates turbulence within cloud layer, and enhances drizzle formation near the cloud top. The type II clouds do not require strong wind shear to produce drizzle. The small values of lower-tropospheric stability (LTS) and negative Richardson number ( Ri) in the type II cases suggest that boundary layer instability plays an important role in TKE production and cloud-drizzle processes. As a result, by analyzing the relationships between LTS and wind shear for all cases and all time periods, a stronger connection was found between LTS and wind directional shear than that between LTS and wind speed shear.« less
Effects of environment forcing on marine boundary layer cloud-drizzle processes: MBL Cloud-Drizzle Processes

DOE PAGES

Wu, Peng; Dong, Xiquan; Xi, Baike; ...

2017-04-20

Determining the factors affecting drizzle formation in marine boundary layer (MBL) clouds remains a challenge for both observation and modeling communities. To investigate the roles of vertical wind shear and buoyancy (static instability) in drizzle formation, ground-based observations from the Atmospheric Radiation Measurement (ARM) Program at the Azores are analyzed for two types of conditions. The type I clouds should last for at least five hours and more than 90% time must be non-drizzling, and then followed by at least two hours of drizzling periods while the type II clouds are characterized by mesoscale convection cellular (MCC) structures with drizzlemore » occur every two to four hours. By analyzing the boundary layer wind profiles (direction and speed), it was found that either directional or speed shear is required to promote drizzle production in the type I clouds. Observations and a recent model study both suggest that vertical wind shear helps the production of turbulent kinetic energy (TKE), stimulates turbulence within cloud layer, and enhances drizzle formation near the cloud top. The type II clouds do not require strong wind shear to produce drizzle. The small values of lower-tropospheric stability (LTS) and negative Richardson number ( Ri) in the type II cases suggest that boundary layer instability plays an important role in TKE production and cloud-drizzle processes. As a result, by analyzing the relationships between LTS and wind shear for all cases and all time periods, a stronger connection was found between LTS and wind directional shear than that between LTS and wind speed shear.« less
Magnetic Field Generation, Particle Energization and Radiation at Relativistic Shear Boundary Layers

NASA Astrophysics Data System (ADS)

Liang, Edison; Fu, Wen; Spisak, Jake; Boettcher, Markus

2015-11-01

Recent large scale Particle-in-Cell (PIC) simulations have demonstrated that in unmagnetized relativistic shear flows, strong transverse d.c. magnetic fields are generated and sustained by ion-dominated currents on the opposite sides of the shear interface. Instead of dissipating the shear flow free energy via turbulence formation and mixing as it is usually found in MHD simulations, the kinetic results show that the relativistic boundary layer stabilizes itself via the formation of a robust vacuum gap supported by a strong magnetic field, which effectively separates the opposing shear flows, as in a maglev train. Our new PIC simulations have extended the runs to many tens of light crossing times of the simulation box. Both the vacuum gap and supporting magnetic field remain intact. The electrons are energized to reach energy equipartition with the ions, with 10% of the total energy in electromagnetic fields. The dominant radiation mechanism is similar to that of a wiggler, due to oscillating electron orbits around the boundary layer.
Experimental Reacting Hydrogen Shear Layer Data at High Subsonic Mach Number

NASA Technical Reports Server (NTRS)

Chang, C. T.; Marek, C. J.; Wey, C.; Wey, C. C.

1996-01-01

The flow in a planar shear layer of hydrogen reacting with hot air was measured with a two-component laser Doppler velocimeter (LDV) system, a schlieren system, and OH fluorescence imaging. It was compared with a similar air-to-air case without combustion. The high-speed stream's flow speed was about 390 m/s, or Mach 0.71, and the flow speed ratio was 0.34. The results showed that a shear layer with reaction grows faster than one without; both cases are within the range of data scatter presented by the established data base. The coupling between the streamwise and the cross-stream turbulence components inside the shear layers was low, and reaction only increased it slightly. However, the shear layer shifted laterally into the lower speed fuel stream, and a more organized pattern of Reynolds stress was present in the reaction shear layer, likely as a result of the formation of a larger scale structure associated with shear layer corrugation from heat release. Dynamic pressure measurements suggest that coherent flow perturbations existed inside the shear layer and that this flow became more chaotic as the flow advected downstream. Velocity and thermal variable values are listed in this report for a computational fluid dynamics (CFD) benchmark.
Conditions for double layers in the earth's magnetosphere and perhaps in other astrophysical objects

NASA Technical Reports Server (NTRS)

Lyons, L. R.

1987-01-01

It is suggested that the features which govern the formation of the double layers are: (1) the divergence of the magnetospheric electric field, (2) the ionospheric conductivity, and (3) the current-voltage characteristics of auroral magnetic field lines. Also considered are conditions in other astrophysical objects that could lead to the formation of DLs in a manner analogous to what occurs in the earth's auroral zones. It is noted that two processes can drive divergent Pedersen currents within a collisional conducting layer: (1) sheared plasma flow applied anywhere along the magnetic field lines connected to the conducting layer and (2) a neutral flow with shear within the conducting layer.
Disruption of vertical motility by shear triggers formation of thin phytoplankton layers.

PubMed

Durham, William M; Kessler, John O; Stocker, Roman

2009-02-20

Thin layers of phytoplankton are important hotspots of ecological activity that are found in the coastal ocean, meters beneath the surface, and contain cell concentrations up to two orders of magnitude above ambient concentrations. Current interpretations of their formation favor abiotic processes, yet many phytoplankton species found in these layers are motile. We demonstrated that layers formed when the vertical migration of phytoplankton was disrupted by hydrodynamic shear. This mechanism, which we call gyrotactic trapping, can be responsible for the thin layers of phytoplankton commonly observed in the ocean. These results reveal that the coupling between active microorganism motility and ambient fluid motion can shape the macroscopic features of the marine ecological landscape.
Influence of Freestream and Forced Disturbances on the Shear Layers of a Square Prism

NASA Astrophysics Data System (ADS)

Lander, Daniel Chapman

Flow around the square prism, an archetypal bluff body, has applications in all areas of fluid mechanics: vibration, mixing, combustion and noise production to name a few. It also has distinct importance to wind loading on architectural and industrial structures such as tall buildings, bridges, and towers. The von-Karman (VK) vortex street is a major reason for its significance: a flow phenomenon which has received intense scrutiny from scientific and engineering communities for more than 100 years! However, the characteristics of the shear layers separating from the sharp edges, essential to the vortex shedding, have received comparatively little attention. This is surprising considering the Kelvin-Helmholtz (KH) instability of shear layers produce the first signatures of turbulence in the wake. Furthermore, the shear layers are conduits for the passage of vorticity between the boundary layer and the turbulent wake. Many details of their structure and role in the shedding process remain unexplored. This dissertation aims to address this deficiency. Specifically, this project considered the influence of three variables on the characteristics of the transition-to-turbulence in the square prism shear layers. These are: (1) Reynolds number; (2) freestream disturbances and (3) forced disturbances. In each case, the dynamics of the shear layer-wake interaction were considered. Particle image velocimetry and constant temperature anemometry measurements were used to document the shear layer during inception and evolution as it passes into the wake. With increasing Reynolds number, ReD = UinfinityD/nu, in the range 16,700-148,000, the transition-to-turbulence in the initially laminar shear layer moves toward separation. A coordinate system local to the time-averaged shear layer axis was used such that the tangent and normal velocities, turbulent stresses and gradient quantities could be obtained for the curved shear layer. Characteristic frequencies, lengths and transition points of the KH instability were documented and shown to exhibit features distinct from the plane mixing layer. The evolution of the integrated turbulent kinetic energy was documented and a linear region of growth was associated with the amplification of the KH instability. A scaling relationship of the Kelvin-Helmholtz to von-Karman frequencies was established for the square prism shear layer. ƒKH/ƒ VK was shown to be a power-law function of Re D, with differing characteristics to the much more studied circular cylinder. Increasing ReD up to ˜ 70,000 bolsters the Reynolds stresses in the shear layers as they enter the wake, shortening the wake formation length, LF. The shear layer diffusion length, LD was quantified and the Gerrard-Product, LF x LD, was introduced to account for constant St D in the presence of the reduced LF as function of ReD. A freestream disturbance condition with intensity □ u¯¯ 2¯ / U infinity = 0.065 and longitudinal integral length scale, Lxu = 0.33 was considered for the case of ReD = 50,000. Disturbances were introduced by means of small circular cylinder placed upstream of the stagnation streamline. The disturbance moved the time-averaged position of the shear layer towards the body but did not substantially alter the growth rate of its width. The "normal" transition-to-turbulence pathway, via laminar vortex formation and subsequent pairing of vortices in the initial stages of the shear layer was shown to be highly sensitive to external disturbances. The disturbance interrupted the typical transition pathway and was associated with a Bypass-transition mechanism, which subsequently increased the likelihood of intermittent shear layer reattachment on the downstream surface of the body. Triple decomposition was used to study the random and coherent components of the VK structures in the wake. Data indicated a narrowing and lengthening of the wake, which was accompanied by a rise in base pressure and a reduction in time-averaged drag. The unsteady coherent vorticity field revealed a streamwise elongation of the VK vortex structures, which complemented the time-averaged wake lengthening. It appears that the influence of freestream disturbances, in particular, by their stochastic nature, is to suppress the formation of the coherent structures in the shear layer. Forced disturbances imposed on the shear layers at the leading edges of the square prism were considered at ReD=16,700 for excitation frequencies ƒe = ƒ KH, ƒVK and 0. The response of the shear layer to forcing at steady and ƒVK frequencies had little impact on the time-averaged position or growth.
Gyrotactic trapping: A numerical study

NASA Astrophysics Data System (ADS)

Ghorai, S.

2016-04-01

Gyrotactic trapping is a mechanism proposed by Durham et al. ["Disruption of vertical motility by shear triggers formation of thin Phytoplankton layers," Science 323, 1067-1070 (2009)] to explain the formation of thin phytoplankton layer just below the ocean surface. This mechanism is examined numerically using a rational model based on the generalized Taylor dispersion theory. The crucial role of sedimentation speed in the thin layer formation is demonstrated. The effects of variation in different parameters on the thin layer formation are also investigated.
Molecular cloud formation in high-shear, magnetized colliding flows

NASA Astrophysics Data System (ADS)

Fogerty, E.; Frank, A.; Heitsch, F.; Carroll-Nellenback, J.; Haig, C.; Adams, M.

2016-08-01

The colliding flows (CF) model is a well-supported mechanism for generating molecular clouds. However, to-date most CF simulations have focused on the formation of clouds in the normal-shock layer between head-on colliding flows. We performed simulations of magnetized colliding flows that instead meet at an oblique-shock layer. Oblique shocks generate shear in the post-shock environment, and this shear creates inhospitable environments for star formation. As the degree of shear increases (I.e. the obliquity of the shock increases), we find that it takes longer for sink particles to form, they form in lower numbers, and they tend to be less massive. With regard to magnetic fields, we find that even a weak field stalls gravitational collapse within forming clouds. Additionally, an initially oblique collision interface tends to reorient over time in the presence of a magnetic field, so that it becomes normal to the oncoming flows. This was demonstrated by our most oblique shock interface, which became fully normal by the end of the simulation.
Protein Adsorption and Layer Formation at the Stainless Steel-Solution Interface Mediates Shear-Induced Particle Formation for an IgG1 Monoclonal Antibody.

PubMed

Kalonia, Cavan K; Heinrich, Frank; Curtis, Joseph E; Raman, Sid; Miller, Maria A; Hudson, Steven D

2018-03-05

Passage of specific protein solutions through certain pumps, tubing, and/or filling nozzles can result in the production of unwanted subvisible protein particles (SVPs). In this work, surface-mediated SVP formation was investigated. Specifically, the effects of different solid interface materials, interfacial shear rates, and protein concentrations on SVP formation were measured for the National Institute of Standards and Technology monoclonal antibody (NISTmAb), a reference IgG1 monoclonal antibody (mAb). A stainless steel rotary piston pump was used to identify formulation and process parameters that affect aggregation, and a flow cell (alumina or stainless steel interface) was used to further investigate the effect of different interface materials and/or interfacial shear rates. SVP particles produced were monitored using flow microscopy or flow cytometry. Neutron reflectometry and a quartz crystal microbalance with dissipation monitoring were used to characterize adsorption and properties of NISTmAb at the stainless steel interface. Pump/shear cell experiments showed that the NISTmAb concentration and interface material had a significant effect on SVP formation, while the effects of interfacial shear rate and passage number were less important. At the higher NISTmAb concentrations, the adsorbed protein became structurally altered at the stainless steel interface. The primary adsorbed layer remained largely undisturbed during flow, suggesting that SVP formation at high NISTmAb concentration was caused by the disruption of patches and/or secondary interactions.
Indentation-Induced Shear Band Formation in Thin-Film Multilayers

NASA Astrophysics Data System (ADS)

Bigelow, Shannon; Shen, Yu-Lin

2017-08-01

We report an exploratory investigation into the cause of shear band formation in multilayer thin-films subject to nanoindentation. The material system considered here is composed of alternating aluminum (Al) and silicon carbide (SiC) nanolayers, atop a silicon (Si) substrate. Finite element models are developed in an attempt to reproduce the shear banding phenomenon observed experimentally. By introducing strain softening into the material model for the hard SiC layers, shear bands can be seen to emerge from the indentation site in the finite element analysis. Broad implications, along with possible directions for future work, are discussed.
SUPERSONIC SHEAR INSTABILITIES IN ASTROPHYSICAL BOUNDARY LAYERS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Belyaev, Mikhail A.; Rafikov, Roman R., E-mail: rrr@astro.princeton.edu

Disk accretion onto weakly magnetized astrophysical objects often proceeds via a boundary layer (BL) that forms near the object's surface, in which the rotation speed of the accreted gas changes rapidly. Here, we study the initial stages of formation for such a BL around a white dwarf or a young star by examining the hydrodynamical shear instabilities that may initiate mixing and momentum transport between the two fluids of different densities moving supersonically with respect to each other. We find that an initially laminar BL is unstable to two different kinds of instabilities. One is an instability of a supersonicmore » vortex sheet (implying a discontinuous initial profile of the angular speed of the gas) in the presence of gravity, which we find to have a growth rate of order (but less than) the orbital frequency. The other is a sonic instability of a finite width, supersonic shear layer, which is similar to the Papaloizou-Pringle instability. It has a growth rate proportional to the shear inside the transition layer, which is of order the orbital frequency times the ratio of stellar radius to the BL thickness. For a BL that is thin compared to the radius of the star, the shear rate is much larger than the orbital frequency. Thus, we conclude that sonic instabilities play a dominant role in the initial stages of nonmagnetic BL formation and give rise to very fast mixing between disk gas and stellar fluid in the supersonic regime.« less
Seismic Performance of Composite Shear Walls Constructed Using Recycled Aggregate Concrete and Different Expandable Polystyrene Configurations.

PubMed

Liu, Wenchao; Cao, Wanlin; Zhang, Jianwei; Qiao, Qiyun; Ma, Heng

2016-03-02

The seismic performance of recycled aggregate concrete (RAC) composite shear walls with different expandable polystyrene (EPS) configurations was investigated. Six concrete shear walls were designed and tested under cyclic loading to evaluate the effect of fine RAC in designing earthquake-resistant structures. Three of the six specimens were used to construct mid-rise walls with a shear-span ratio of 1.5, and the other three specimens were used to construct low-rise walls with a shear-span ratio of 0.8. The mid-rise and low-rise shear walls consisted of an ordinary recycled concrete shear wall, a composite wall with fine aggregate concrete (FAC) protective layer (EPS modules as the external insulation layer), and a composite wall with sandwiched EPS modules as the insulation layer. Several parameters obtained from the experimental results were compared and analyzed, including the load-bearing capacity, stiffness, ductility, energy dissipation, and failure characteristics of the specimens. The calculation formula of load-bearing capacity was obtained by considering the effect of FAC on composite shear walls as the protective layer. The damage process of the specimen was simulated using the ABAQUS Software, and the results agreed quite well with those obtained from the experiments. The results show that the seismic resistance behavior of the EPS module composite for shear walls performed better than ordinary recycled concrete for shear walls. Shear walls with sandwiched EPS modules had a better seismic performance than those with EPS modules lying outside. Although the FAC protective layer slightly improved the seismic performance of the structure, it undoubtedly slowed down the speed of crack formation and the stiffness degradation of the walls.
Seismic Performance of Composite Shear Walls Constructed Using Recycled Aggregate Concrete and Different Expandable Polystyrene Configurations

PubMed Central

Liu, Wenchao; Cao, Wanlin; Zhang, Jianwei; Qiao, Qiyun; Ma, Heng

2016-01-01

The seismic performance of recycled aggregate concrete (RAC) composite shear walls with different expandable polystyrene (EPS) configurations was investigated. Six concrete shear walls were designed and tested under cyclic loading to evaluate the effect of fine RAC in designing earthquake-resistant structures. Three of the six specimens were used to construct mid-rise walls with a shear-span ratio of 1.5, and the other three specimens were used to construct low-rise walls with a shear-span ratio of 0.8. The mid-rise and low-rise shear walls consisted of an ordinary recycled concrete shear wall, a composite wall with fine aggregate concrete (FAC) protective layer (EPS modules as the external insulation layer), and a composite wall with sandwiched EPS modules as the insulation layer. Several parameters obtained from the experimental results were compared and analyzed, including the load-bearing capacity, stiffness, ductility, energy dissipation, and failure characteristics of the specimens. The calculation formula of load-bearing capacity was obtained by considering the effect of FAC on composite shear walls as the protective layer. The damage process of the specimen was simulated using the ABAQUS Software, and the results agreed quite well with those obtained from the experiments. The results show that the seismic resistance behavior of the EPS module composite for shear walls performed better than ordinary recycled concrete for shear walls. Shear walls with sandwiched EPS modules had a better seismic performance than those with EPS modules lying outside. Although the FAC protective layer slightly improved the seismic performance of the structure, it undoubtedly slowed down the speed of crack formation and the stiffness degradation of the walls. PMID:28773274
A Theory of Density Layering in Stratified Turbulence using Statistical State Dynamics

NASA Astrophysics Data System (ADS)

Fitzgerald, J.; Farrell, B.

2016-12-01

Stably stratified turbulent fluids commonly develop density structures that are layered in the vertical direction (e.g., Manucharyan et al., 2015). Within layers, density is approximately constant and stratification is weak. Between layers, density varies rapidly and stratification is strong. A common explanation for the existence of layers invokes the negative diffusion mechanism of Phillips (1972) & Posmentier (1977). The physical principle underlying this mechanism is that the flux-gradient relationship connecting the turbulent fluxes of buoyancy to the background stratification must have the special property of weakening fluxes with strengthening gradient. Under these conditions, the evolution of the stratification is governed by a negative diffusion problem which gives rise to spontaneous layer formation. In previous work on stratified layering, this flux-gradient property is often assumed (e.g, Posmentier, 1977) or drawn from phenomenological models of turbulence (e.g., Balmforth et al., 1998).In this work we develop the theoretical underpinnings of layer formation by applying stochastic turbulence modeling and statistical state dynamics (SSD) to predict the flux-gradient relation and analyze layer formation directly from the equations of motion. We show that for stochastically-forced homogeneous 2D Boussinesq turbulence, the flux-gradient relation can be obtained analytically and indicates that the fluxes always strengthen with stratification. The Phillips mechanism thus does not operate in this maximally simplified scenario. However, when the problem is augmented to include a large scale background shear, we show that the flux-gradient relationship is modified so that the fluxes weaken with stratification. Sheared and stratified 2D Boussinesq turbulence thus spontaneously forms density layers through the Phillips mechanism. Using SSD (Farrell & Ioannou 2003), we obtain a closed, deterministic dynamics for the stratification and the statistical turbulent state. We show that density layers form as a linear instability of the sheared turbulence, associated with a supercritical bifurcation. We further show that SSD predicts the nonlinear equilibration and maintenance of the layers, and captures the phenomena of layer growth and mergers (Radko, 2007).
Structure-rheology relationship in a sheared lamellar fluid.

PubMed

Jaju, S J; Kumaran, V

2016-03-01

The structure-rheology relationship in the shear alignment of a lamellar fluid is studied using a mesoscale model which provides access to the lamellar configurations and the rheology. Based on the equations and free energy functional, the complete set of dimensionless groups that characterize the system are the Reynolds number (ργL(2)/μ), the Schmidt number (μ/ρD), the Ericksen number (μγ/B), the interface sharpness parameter r, the ratio of the viscosities of the hydrophilic and hydrophobic parts μ(r), and the ratio of the system size and layer spacing (L/λ). Here, ρ and μ are the fluid density and average viscosity, γ is the applied strain rate, D is the coefficient of diffusion, B is the compression modulus, μ(r) is the maximum difference in the viscosity of the hydrophilic and hydrophobic parts divided by the average viscosity, and L is the system size in the cross-stream direction. The lattice Boltzmann method is used to solve the concentration and momentum equations for a two dimensional system of moderate size (L/λ=32) and for a low Reynolds number, and the other parameters are systematically varied to examine the qualitative features of the structure and viscosity evolution in different regimes. At low Schmidt numbers where mass diffusion is faster than momentum diffusion, there is fast local formation of randomly aligned domains with "grain boundaries," which are rotated by the shear flow to align along the extensional axis as time increases. This configuration offers a high resistance to flow, and the layers do not align in the flow direction even after 1000 strain units, resulting in a viscosity higher than that for an aligned lamellar phase. At high Schmidt numbers where momentum diffusion is fast, the shear flow disrupts layers before they are fully formed by diffusion, and alignment takes place by the breakage and reformation of layers by shear, resulting in defects (edge dislocations) embedded in a background of nearly aligned layers. At high Ericksen number where the viscous forces are large compared to the restoring forces due to layer compression and bending, shear tends to homogenize the concentration field, and the viscosity decreases significantly. At very high Ericksen number, shear even disrupts the layering of the lamellar phase. At low Ericksen number, shear results in the formation of well aligned layers with edge dislocations. However, these edge dislocations take a long time to anneal; the relatively small misalignment due to the defects results in a large increase in viscosity due to high layer stiffness and due to shear localization, because the layers between defects get pinned and move as a plug with no shear. An increase in the viscosity contrast between the hydrophilic and hydrophobic parts does not alter the structural characteristics during alignment. However, there is a significant increase in the viscosity, due to pinning of the layers between defects, which results in a plug flow between defects and a localization of the shear to a part of the domain.
Formation and propagation of Love waves in a surface layer with a P-wave source. Technical report

DOE Office of Scientific and Technical Information (OSTI.GOV)

Florence, A.L.; Miller, S.A.

The objective of this research is to investigate experimentally, and support with theoretical calculations, the formation and propagation of Love waves from a P-wave source due to scattering at material heterogeneities. The P-wave source is a spherical piezoelectric crystal cast in a surface layer of rock simulant overlaying a higher impedance granite substrate. Excitation of the piezoelectric crystal with a known voltage applies a spherical compressional pulse of known amplitude to the surrounding medium. Lateral heterogeneities cast in the surface layer convert incident P-wave energy into shear waves. The horizontally polarized shear waves (SH waves) trapped in the surface layermore » wave guide are the Love waves we will measure at the surface.« less
Shear-induced laning transition in a confined colloidal film

NASA Astrophysics Data System (ADS)

Gerloff, Sascha; Vezirov, Tarlan A.; Klapp, Sabine H. L.

2017-06-01

Using Brownian dynamics simulations, we investigate a dense system of charged colloids exposed to shear flow in a confined (slit-pore) geometry. The equilibrium system at zero flow consists of three well-pronounced layers with a squarelike crystalline in-plane structure. We demonstrate that, for sufficiently large shear rates, the middle layer separates into two sublayers where the particles organize into moving lanes with opposite velocities. The formation of this "microlaned" state results in a destruction of the applied shear profile; it also has a strong impact on the structure of the system, and on its rheology as measured by the elements of the stress tensor. At higher shear rates, we observe a disordered state and finally a recrystallization reminiscent of the behavior of bilayer films. We also discuss the system size dependence and the robustness of the microlaned state against variations of the slit-pore width. In fact, for a pore width allowing for four layers, we observe a similar shear-induced state in which the system splits into two domains with opposite velocities.
Modeling Periodic Adiabatic Shear Bands Evolution in a 304L Stainless Steel Thick-Walled Cylinder

NASA Astrophysics Data System (ADS)

Liu, Mingtao; Hu, Haibo; Fan, Cheng; Tang, Tiegang

2015-06-01

The self-organization of multiple shear bands in a 304L stainless steel thick-walled cylinder (TWC) was numerically studied. The microstructures of material lead to the non-uniform distribution of local yield stress, which plays a key role in the formation of spontaneous shear localization. We introduced a probability factor satisfied Gauss distribution into the macroscopic constitutive relationship to describe the non-uniformity of local yield stress. Using the probability factor, the initiation and propagation of multiple shear bands in TWC were numerically replicated in our 2D FEM simulation. Experimental results in the literature indicate that the machined surface at the internal boundary of a 304L stainless steel cylinder provides a work-hardened layer (about 20 μm) which has significantly different microstructures from base material. The work-hardened layer leads to the phenomenon that most shear bands are in clockwise or counterclockwise direction. In our simulation, periodic oriented perturbations were applied to describe the grain orientation in the work-hardened layer, and the spiral pattern of shear bands was successfully replicated.

Experimental studies of combustion in a two dimensional free shear layer

NASA Technical Reports Server (NTRS)

Pitz, R. W.; Daily, J. W.

1979-01-01

The effect of combustion on the turbulent free shear layer formed at a rearward facing step has been studied. Schlieren movies confirm the importance of large scale vortices in determining entrainment and mixing behavior. The movies, long exposure schlieren photographs, and laser anemometry velocity profiles are used to observe the spreading rate of the layer and to study the vortex formation process. It is concluded that to first order, the primary effect of combustion is felt through the change in density ratio across the layer and acceleration of the flow due to volumetric expansion of the fluid in a confined duct.
Morphological Simulation of Phase Separation Coupled Oscillation Shear and Varying Temperature Fields

NASA Astrophysics Data System (ADS)

Wang, Heping; Li, Xiaoguang; Lin, Kejun; Geng, Xingguo

2018-05-01

This paper explores the effect of the shear frequency and Prandtl number ( Pr) on the procedure and pattern formation of phase separation in symmetric and asymmetric systems. For the symmetric system, the periodic shear significantly prolongs the spinodal decomposition stage and enlarges the separated domain in domain growth stage. By adjusting the Pr and shear frequency, the number and orientation of separated steady layer structures can be controlled during domain stretch stage. The numerical results indicate that the increase in Pr and decrease in the shear frequency can significantly increase in the layer number of the lamellar structure, which relates to the decrease in domain size. Furthermore, the lamellar orientation parallel to the shear direction is altered into that perpendicular to the shear direction by further increasing the shear frequency, and also similar results for larger systems. For asymmetric system, the quantitative analysis shows that the decrease in the shear frequency enlarges the size of separated minority phases. These numerical results provide guidance for setting the optimum condition for the phase separation under periodic shear and slow cooling.
Velocity shear, turbulent saturation, and steep plasma gradients in the scrape-off layer of inner-wall limited tokamaks

DOE PAGES

Halpern, Federico D.; Ricci, Paolo

2016-12-19

The narrow power decay-length (λ q), recently found in the scrape-off layer (SOL) of inner wall limited (IWL) discharges in tokamaks, is studied using 3D, flux-driven, global two fluid turbulence simulations. The formation of the steep plasma profiles is found to arise due to radially sheared E×B poloidal flows. A complex interaction between sheared flows and parallel plasma currents outflowing into the sheath regulates the turbulent saturation, determining the transport levels. We quantify the effects of sheared flows, obtaining theoretical estimates in agreement with our non-linear simulations. As a result, analytical calculations suggest that the IWL λ q is roughlymore » equal to the turbulent correlation length.« less
Geologic map of the Kings Mountain and Grover quadrangles, Cleveland and Gaston Counties, North Carolina, and Cherokee and York Counties, South Carolina

USGS Publications Warehouse

Horton, J. Wright

2006-01-01

This geologic map of the Kings Mountain and Grover 7.5-minute quadrangles, N.C.-S.C., straddles a regional geological boundary between the Inner Piedmont and Carolina terranes. The Kings Mountain sequence (informal name) on the western flank of the Carolina terrane in this area includes the Neoproterozoic Battleground and Blacksburg Formations. The Battleground Formation has a lower part consisting of metavolcanic rocks and interlayered schist, and an upper part consisting of quartz-sericite phyllite and schist interlayered with quartz-pebble metaconglomerate, aluminous quartzite, micaceous quartzite, manganiferous rock, and metavolcanic rocks. The Blacksburg Formation consists of phyllitic metasiltstone interlayered with thinner units of marble, laminated micaceous quartzite, hornblende gneiss, and amphibolite. Layered metamorphic rocks of the Inner Piedmont terrane include muscovite-biotite gneiss, muscovite schist, and amphibolite. The Kings Mountain sequence has been intruded by metatonalite and metatrondhjemite (Neoproterozoic), metadiorite and metagabbro (Paleozoic), and High Shoals Granite (Pennsylvanian). Layered metamorphic rocks of the Inner Piedmont in this area have been intruded by Toluca Granite (Ordovician?), Cherryville Granite and associated pegmatite (Mississippian), and spodumene pegmatite (Mississippian). Diabase dikes (early Jurassic) are locally present throughout the area. Ductile fault zones of regional scale include the Kings Mountain and Kings Creek shear zones. In this area, the Kings Mountain shear zone forms the boundary between the Inner Piedmont and Carolina terranes, and the Kings Creek shear zone separates the Battleground Formation from the Blacksburg Formation. Structural styles change across the Kings Mountain shear zone from steeply-dipping layers, foliations, and folds on the southeast to gently- and moderately-dipping layers, foliations, and recumbent folds on the northwest. Mineral assemblages in the Kings Mountain sequence show a westward decrease from upper amphibolite facies (sillimanite zone) near the High Shoals Granite on the east side of the map to greenschist (epidote-amphibolite) facies in the south-central part of the area near the Kings Mountain shear zone. Amphibolite-facies mineral assemblages in the Inner Piedmont terrane increase in grade from the kyanite zone near the Kings Mountain shear zone to the sillimanite zone in the northwest part of the map. Surficial deposits include alluvium in the stream valleys and colluvium along ridges and steep slopes. These quadrangles are unusual in their richness and variety of mineral deposits, which include spodumene (lithium), cassiterite (tin), mica, feldspar, silica, clay, marble, kyanite and sillimanite, barite, manganese, sand and gravel, gold, pyrite, and iron. (Abstract from pamphlet.)
Geologic Map of the Kings Mountain and Grover Quadrangles, Cleveland and Gaston Counties, North Carolina, and Cherokee and York Counties, South Carolina

USGS Publications Warehouse

Horton, J. Wright

2008-01-01

This geologic map of the Kings Mountain and Grover 7.5-min quadrangles, N.C.-S.C., straddles a regional geological boundary between the Inner Piedmont and Carolina terranes. The Kings Mountain sequence (informal name) on the western flank of the Carolina terrane in this area includes the Neoproterozoic Battleground and Blacksburg Formations. The Battleground Formation has a lower part consisting of metavolcanic rocks and interlayered schist and an upper part consisting of quartz-sericite phyllite and schist interlayered with quartz-pebble metaconglomerate, aluminous quartzite, micaceous quartzite, manganiferous rock, and metavolcanic rocks. The Blacks-burg Formation consists of phyllitic metasiltstone interlayered with thinner units of marble, laminated micaceous quartzite, hornblende gneiss, and amphibolite. Layered metamorphic rocks of the Inner Piedmont terrane include muscovite-biotite gneiss, muscovite schist, and amphibolite. The Kings Mountain sequence has been intruded by metatonalite and metatrondhjemite (Neoproterozoic), metagabbro and metadiorite (Paleozoic?), and the High Shoals Granite (Pennsylvanian). Layered metamorphic rocks of the Inner Piedmont in this area have been intruded by the Toluca Granite (Ordovician?), the Cherryville Granite and associated pegmatite (Mississippian), and spodumene pegmatite (Mississippian). Diabase dikes (early Jurassic) are locally present throughout the area. Ductile fault zones of regional scale include the Kings Mountain and Kings Creek shear zones. In this area, the Kings Mountain shear zone forms the boundary between the Inner Piedmont and Carolina terranes, and the Kings Creek shear zone separates the Battleground Formation from the Blacksburg Formation. Structural styles change across the Kings Mountain shear zone from steeply dipping layers, foliations, and folds on the southeast to gently and moderately dipping layers, foliations, and recumbent folds on the northwest. Mineral assemblages in the Kings Mountain sequence show a westward decrease from upper amphibolite facies (sillimanite zone) near the High Shoals Granite in the eastern side of the map area to upper greenschist (epidote-amphibolite) facies in the south-central part of the area near the Kings Mountain shear zone. Amphibolite-facies mineral assemblages in the Inner Piedmont terrane increase in grade from the kyanite zone near the Kings Mountain shear zone to the sillimanite zone in the northwestern part of the map area. Surficial deposits include alluvium in the stream valleys and colluvium along ridges and steep slopes. These quadrangles are unusual in the richness and variety of the mineral deposits that they contain, which include spodumene (lithium), cassiterite (tin), mica, feldspar, silica, clay, marble, kyanite and sillimanite, barite, manganese, sand and gravel, gold, pyrite, and iron.
A numerical study of multiple adiabatic shear bands evolution in a 304LSS thick-walled cylinder

NASA Astrophysics Data System (ADS)

Liu, Mingtao; Hu, Haibo; Fan, Cheng; Tang, Tiegang

2017-01-01

The self-organization of multiple shear bands in a 304L stainless steel(304LSS) thick-walled cylinder (TWC) was numerically studied. The microstructures of material lead to the non-uniform distribution of the local yield stress, which play a key role in the formation of spontaneous shear localization. We introduced a probability factor satisfied the Gaussian distribution into the macroscopic constitutive relationship to describe the non-uniformity of local yield stress. Using the probability factor, the initiation and propagation of multiple shear bands in TWC were numerically replicated in our 2D FEM simulation. Experimental results in the literature indicated that the machined surface at the internal boundary of a 304L stainless steel cylinder provides a work-hardened layer (about 20˜30μm) which has significantly different microstructures from the base material. The work-hardened layer leads to the phenomenon that most shear bands propagate along a given direction, clockwise or counterclockwise. In our simulation, periodical single direction spiral perturbations were applied to describe the grain orientation in the work-hardened layer, and the single direction spiral pattern of shear bands was successfully replicated.
Dust to planetesimals - Settling and coagulation in the solar nebula

NASA Technical Reports Server (NTRS)

Weidenschilling, S. J.

1980-01-01

The behavior of solid particles in a low-mass solar nebula during settling to the central plane and the formation of planetesimals is discussed. The gravitational instability in a dust layer and collisional accretion are examined as possible mechanisms of planetesimal formation. The shear between the gas and a dust layer is considered along with the differences in the planetesimal formation mechanisms between the inner and outer nebula. A numerical model for computing simultaneous coagulation and settling is described.
Study of Diffusion Barrier for Solder/ n-Type Bi2Te3 and Bonding Strength for p- and n-Type Thermoelectric Modules

NASA Astrophysics Data System (ADS)

Lin, Wen-Chih; Li, Ying-Sih; Wu, Albert T.

2018-01-01

This paper investigates the interfacial reaction between Sn and Sn3Ag0.5Cu (SAC305) solder on n-type Bi2Te3 thermoelectric material. An electroless Ni-P layer successfully suppressed the formation of porous SnTe intermetallic compound at the interface. The formation of the layers between Bi2Te3 and Ni-P indicates that Te is the dominant diffusing species. Shear tests were conducted on both Sn and SAC305 solder on n- and p-type Bi2Te3 with and without a Ni-P barrier layer. Without a Ni-P layer, porous SnTe would result in a more brittle fracture. A comparison of joint strength for n- and p-type thermoelectric modules is evaluated by the shear test. Adding a diffusion barrier increases the mechanical strength by 19.4% in n-type and 74.0% in p-type thermoelectric modules.
Experimental Analysis of Hydraulic Fracture Growth and Acoustic Emission Response in a Layered Formation

NASA Astrophysics Data System (ADS)

Ning, Li; Shicheng, Zhang; Yushi, Zou; Xinfang, Ma; Shan, Wu; Yinuo, Zhang

2018-04-01

Microseismic/acoustic emission (AE) monitoring is an essential technology for understanding hydraulic fracture (HF) geometry and stimulated reservoir volume (SRV) during hydraulic fracturing in unconventional reservoirs. To investigate HF growth mechanisms and features of induced microseismic/AE events in a layered formation, laboratory fracturing experiments were performed on shale specimens (30 cm × 30 cm × 30 cm) with multiple bedding planes (BPs) under triaxial stresses. AE monitoring was used to reveal the spatial distribution and hypocenter mechanisms of AE events induced by rock failure. Computerized tomography scanning was used to observe the internal fracture geometry. Experimental results showed that the various HF geometries could be obviously distinguished based on injection pressure curves and AE responses. Fracture complexity was notably increased when vertically growing HFs connected with and opened more BPs. The formation of a complex fracture network was generally indicated by frequent fluctuations in injection pressure curves, intense AE activity, and three-dimensionally distributed AE events. Investigations of the hypocenter mechanisms revealed that shear failure/event dominated in shale specimens. Shear and tensile events were induced in hydraulically connected regions, and shear events also occurred around BPs that were not hydraulically connected. This led to an overestimation of HF height and SRV in layered formations based on the AE location results. The results also showed that variable injection rate and using plugging agent were conducive in promoting HF to penetrate through the weak and high-permeability BPs, thereby increasing the fracture height.
Different slip systems controlling crystallographic preferred orientation and intracrystalline deformation of amphibole in mylonites from the Neyriz mantle diapir, Iran

NASA Astrophysics Data System (ADS)

Elyaszadeh, Ramin; Prior, David J.; Sarkarinejad, Khalil; Mansouri, Hadiseh

2018-02-01

A deformed layered gabbro and a mylonitic gabbro sample from the marginal shear zone of the Neyriz mantle diapir in Iran were analyzed using electron backscatter diffraction (EBSD). Both samples have the common amphibole crystallographic preferred orientation (CPO) in which (100) lies perpendicular to foliation and <001> parallel to lineation. Amphibole grains in the layered gabbro sample have little internal deformation, whereas in the mylonitic gabbro sample the amphibole grains are strongly distorted and contain low angle grain boundaries. There is a subtle change in CPO as a function of grain size in the mylonitic gabbro. Coarse grains (porphyroclasts) have a (100) <001> CPO oriented with the main foliation reference frame whilst fine grains have a (100) <001> CPO oriented with the C‧ shear bands. Detailed analysis of porphyroclast distortions and subgrain boundary trace analysis suggests that hard slip systems, most particularly (110) <1-10> control intracrystalline deformation. Schmid factor analysis suggest that these slip systems are not involved in foliation formation but are linked kinematically to C‧ shear bands. It is unlikely that the slip systems that control intracrystalline deformation are important in CPO formation. We interpret that subgrain rotation recrystallization lead to grain size reduction and the elongate recrystallized grains were rotated towards the C‧ shear bands by grain boundary sliding. This rigid body rotation, possibly in combination with easy slip on (100) <001> are considered the main cause of CPO formation. Amphibole zonation patterns in the layered gabbro sample suggest that oriented growth of amphibole may have contributed to CPO.
Cake properties in ultrafiltration of TiO2 fine particles combined with HA: in situ measurement of cake thickness by fluid dynamic gauging and CFD calculation of imposed shear stress for cake controlling.

PubMed

Du, Xing; Qu, Fangshu; Liang, Heng; Li, Kai; Chang, Haiqing; Li, Guibai

2016-05-01

In this study, the cake buildup of TiO2 fine particles in the presence of humid acid (HA) and cake layer controlling during ultrafiltration (UF) were investigated. Specifically, we measured the cake thickness using fluid dynamic gauging (FDG) method under various solution conditions, including TiO2 concentration (0.1-0.5 g/L), HA concentration (0-5 mg/L, total organic carbon (TOC)), and pH values (e.g., 4, 6 and 10), and calculated the shear stress distribution induced by stirring using computational fluid dynamics (CFD) to analyze the cake layer controlling conditions, including the operation flux (50-200 L m(-2) h(-1)) and TiO2 concentration (0.1-0.5 g/L). It was found that lower TiO2/HA concentration ratio could lead to exceedingly severe membrane fouling because of the formation of a relatively denser cake layer by filling the voids of cake layer with HA, and pH was essential for cake layer formation owing to the net repulsion between particles. Additionally, it was observed that shear stress was rewarding for mitigating cake growth under lower operation flux as a result of sufficient back-transport forces, and exhibited an excellent performance on cake layer controlling in lower TiO2 concentrations due to slight interaction forces on the vicinity of membrane.
What can asymmetry tell us? Investigation of asymmetric versus symmetric pinch and swell structures in nature and simulation

NASA Astrophysics Data System (ADS)

Gardner, Robyn; Piazolo, Sandra; Daczko, Nathan

2015-04-01

Pinch and swell structures occur from microscopic to landscape scales where a more competent layer in a weaker matrix is deformed by pure shear, often in rifting environments. The Anita Shear Zone (ASZ) in Fiordland, New Zealand has an example of landscape scale (1 km width) asymmetric pinch and swell structures developed in ultramafic rocks. Field work suggests that the asymmetry is a result of variations in the surrounding 'matrix' flow properties as the ultramafic band is surrounded to the east by an orthogneiss (Milford Orthogneiss) and to the west by a paragneiss (Thurso Paragneiss). In addition, there is a narrow and a much wider shear zone between the ultramafics and the orthogneiss and paragneiss, respectively. Detailed EBSD analysis of samples from a traverse across the pinch and swell structure indicate the ultramafics in the shear zone on the orthogneiss side have larger grain size than the ultramafics in the shear zone on the paragneiss side. Ultramafic samples from the highly strained paragneiss and orthogneiss shear zones show dislocation creep behaviour, and, on the paragneiss side, also significant deformation by grain boundary sliding. To test if asymmetry of pinch and swell structures can be used to derive the rheological properties of not only the pinch and swell lithologies, but also of the matrix, numerical simulations were performed. Numerical modelling of pure shear (extension) was undertaken with (I) initially three layers and then (II) five layers by adding soft high strain zones on both sides of the rheological hard layer. The matrix was given first symmetric, then asymmetric viscosity. Matrix viscosity was found to impact the formation of pinch and swell structures with the weaker layer causing increased tortuosity of the competent layer edge due to increased local differential stress. Results highlight that local, rheologically soft layers and the relative viscosity of matrix both impact significantly the shape and symmetry of developing pinch and swell structures.
Contribution of peculiar shear motions to large-scale structure

NASA Technical Reports Server (NTRS)

Mueler, Hans-Reinhard; Treumann, Rudolf A.

1994-01-01

Self-gravitating shear flow instability simulations in a cold dark matter-dominated expanding Einstein-de Sitter universe have been performed. When the shear flow speed exceeds a certain threshold, self-gravitating Kelvin-Helmoholtz instability occurs, forming density voids and excesses along the shear flow layer which serve as seeds for large-scale structure formation. A possible mechanism for generating shear peculiar motions are velocity fluctuations induced by the density perturbations of the postinflation era. In this scenario, short scales grow earlier than large scales. A model of this kind may contribute to the cellular structure of the luminous mass distribution in the universe.
High-efficiency exfoliation of large-area mono-layer graphene oxide with controlled dimension.

PubMed

Park, Won Kyu; Yoon, Yeojoon; Song, Young Hyun; Choi, Su Yeon; Kim, Seungdu; Do, Youngjin; Lee, Junghyun; Park, Hyesung; Yoon, Dae Ho; Yang, Woo Seok

2017-11-27

In this work, we introduce a novel and facile method of exfoliating large-area, single-layer graphene oxide using a shearing stress. The shearing stress reactor consists of two concentric cylinders, where the inner cylinder rotates at controlled speed while the outer cylinder is kept stationary. We found that the formation of Taylor vortex flow with shearing stress can effectively exfoliate the graphite oxide, resulting in large-area single- or few-layer graphene oxide (GO) platelets with high yields (>90%) within 60 min of reaction time. Moreover, the lateral size of exfoliated GO sheets was readily tunable by simply controlling the rotational speed of the reactor and reaction time. Our approach for high-efficiency exfoliation of GO with controlled dimension may find its utility in numerous industrial applications including energy storage, conducting composite, electronic device, and supporting frameworks of catalyst.
New-type steel plate with ultra high crack-arrestability

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ishikawa, T.; Nomiyama, Y.; Hagiwara, Y.

1995-12-31

A new-type steel plate has been developed by controlling the microstructure of the surface layers. The surface layer consists of ultra fine grain ferrite microstructure, which provides excellent fracture toughness even at cryogenic temperature. When an unstable brittle crack propagates in the developed steel plate, shear-lips can be easily formed due to the surface layers with ultra fine grain microstructure. Since unstable running crack behavior is strongly affected by side-ligaments (shear-lips), which are associated with extensive plastic deformation, enhanced formation of the shear-lips can improve crack arrestability. This paper describes the developed steel plates of HT500MPa tensile strength class formore » shipbuilding use. Fracture mechanics investigations using large-scale fracture testings (including ultrawide duplex ESSO tests) clarified that the developed steel plates have ultra high crack-arrestability. It was also confirmed that the plates possess sufficient properties, including weldability and workability, for ship building use.« less
The emergence of asymmetric normal fault systems under symmetric boundary conditions

NASA Astrophysics Data System (ADS)

Schöpfer, Martin P. J.; Childs, Conrad; Manzocchi, Tom; Walsh, John J.; Nicol, Andrew; Grasemann, Bernhard

2017-11-01

Many normal fault systems and, on a smaller scale, fracture boudinage often exhibit asymmetry with one fault dip direction dominating. It is a common belief that the formation of domino and shear band boudinage with a monoclinic symmetry requires a component of layer parallel shearing. Moreover, domains of parallel faults are frequently used to infer the presence of a décollement. Using Distinct Element Method (DEM) modelling we show, that asymmetric fault systems can emerge under symmetric boundary conditions. A statistical analysis of DEM models suggests that the fault dip directions and system polarities can be explained using a random process if the strength contrast between the brittle layer and the surrounding material is high. The models indicate that domino and shear band boudinage are unreliable shear-sense indicators. Moreover, the presence of a décollement should not be inferred on the basis of a domain of parallel faults alone.
Structures Formed in Experimentally Sheared Artificial Fault Gouge: Precise Statistical Measurements

NASA Astrophysics Data System (ADS)

Dilov, T.; Yoshida, S.; Kato, A.; Nakatani, M.; Mochizuki, H.; Otsuki, K.

2004-12-01

The physical parameters governing earthquakes change with the ongoing formation and evolution of structures, formed in the course of a single or multiple earthquakes, within a particular fault zone or in a broad volume containing interacting tectonic faults. Our precise knowledge of these complex phenomena is still elusive. Especially, works considering geometrical evolution of shear structures under controlled conditions are rare. In order to gain some insights we accomplished a set of 12 laboratory experiments using a servo-controlled direct-shear apparatus, under room temperature and without controlling the air humidity. Two fault gouge layers (industrially produced quartz powder, average particle size of 5 μ m, and pre-shear thickness of 1.5, 2.0 and 3.0 mm,) were sandwiched between three granite blocks. The middle block was slid in order to create frictional structures within the simulated gouge. The total imposed shear strain varies between 0.14 and 11.80. The post-shear gouge layer thickness ranges from 0.99-2.11 mm. Each experiment was run under a constant normal stress (varying from 10-44 MPa through the experiments) and at a constant shear velocity (0.07, 0.7 and 7 μ m/s, through the experiments). Later, in cross-sections of solidified by epoxy glue gouge (parallel to the shear direction, normal to the gouge walls,) we quantified the numerous R-shears, according to their density distribution, fracture thickness (measured perpendicularly to the fracture walls), fracture angle and morphology, and fracture length. In gouge views parallel to the sliding blocks, we measured fracture length and along-strike R-shear morphology. Although the latter data are with lower quality, both observational sets provide precise statistical fracture data as well snapshots of evolving 3D structures. We observe shear localization with decreasing gouge layer thickness and with increasing normal stress. The average density of major fractures increases from 2.83 to 3.67 [fracture/cm] for decrease of the post-shear gouge layer thickness. This is at the expense of a considerable decrease of visible more diffusive minor fractures. On the other hand, the fractures formed at lower normal stress are more irregular and show average fracture density of 4.48 [fracture/cm]. The latter decreases down to 3.64 at higher normal stress, as the fracture morphology becomes more regular. The fracture density increases abruptly from zero, after a small total shear strain (0.15-0.50), and later the change is slower or none with the increase of the total shear strain; the fractures are already localized and they accommodate most of the brittle deformation. Also we observe weak polarity in fracture development in accordance to the sliding sense, especially in the subset of fractures starting from the gouge wall and dying out within the gouge layer. More such fractures are developed along the leading part of the sliding blocks. Our results throw new light over the formation and development of fault-related structures and their dependency on the earthquake-governing physical parameters.
The role of zonal flows and predator–prey oscillations in triggering the formation of edge and core transport barriers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schmitz, Lothar; Zeng, Lei; Rhodes, Terry L.

2014-04-24

Here, we present direct evidence of low frequency, radially sheared, turbulence-driven flows (zonal flows (ZFs)) triggering edge transport barrier formation preceding the L- to H-mode transition via periodic turbulence suppression in limit-cycle oscillations (LCOs), consistent with predator–prey dynamics. The final transition to edge-localized mode-free H-mode occurs after the equilibrium E × B flow shear increases due to ion pressure profile evolution. ZFs are also observed to initiate formation of an electron internal transport barrier (ITB) at the q = 2 rational surface via local suppression of electron-scale turbulence. Multi-channel Doppler backscattering (DBS) has revealed the radial structure of the ZF-induced shear layer and the E × B shearing rate, ω E×B, in both barrier types. During edge barrier formation, the shearing rate lags the turbulence envelope during the LCO by 90°, transitioning to anti-correlation (180°) when the equilibrium shear dominates the turbulence-driven flow shear due to the increasing edge pressure gradient. The time-dependent flow shear and the turbulence envelope are anti-correlated (180° out of phase) in the electron ITB. LCOs with time-reversed evolution dynamics (transitioning from an equilibrium-flow dominated to a ZF-dominated state) have also been observed during the H–L back-transition and are potentially of interest for controlled ramp-down of the plasma stored energy and pressure (normalized to the poloidal magnetic field)more » $$\\beta_{\\theta} =2\\mu_{0} n{( {T_{{\\rm e}} +T_{{\\rm i}}})}/{B_{\\theta}^{2}}$$ in ITER.« less
The role of zonal flows and predator-prey oscillations in triggering the formation of edge and core transport barriers

NASA Astrophysics Data System (ADS)

Schmitz, L.; Zeng, L.; Rhodes, T. L.; Hillesheim, J. C.; Peebles, W. A.; Groebner, R. J.; Burrell, K. H.; McKee, G. R.; Yan, Z.; Tynan, G. R.; Diamond, P. H.; Boedo, J. A.; Doyle, E. J.; Grierson, B. A.; Chrystal, C.; Austin, M. E.; Solomon, W. M.; Wang, G.

2014-07-01

We present direct evidence of low frequency, radially sheared, turbulence-driven flows (zonal flows (ZFs)) triggering edge transport barrier formation preceding the L- to H-mode transition via periodic turbulence suppression in limit-cycle oscillations (LCOs), consistent with predator-prey dynamics. The final transition to edge-localized mode-free H-mode occurs after the equilibrium E × B flow shear increases due to ion pressure profile evolution. ZFs are also observed to initiate formation of an electron internal transport barrier (ITB) at the q = 2 rational surface via local suppression of electron-scale turbulence. Multi-channel Doppler backscattering (DBS) has revealed the radial structure of the ZF-induced shear layer and the E × B shearing rate, ωE×B, in both barrier types. During edge barrier formation, the shearing rate lags the turbulence envelope during the LCO by 90°, transitioning to anti-correlation (180°) when the equilibrium shear dominates the turbulence-driven flow shear due to the increasing edge pressure gradient. The time-dependent flow shear and the turbulence envelope are anti-correlated (180° out of phase) in the electron ITB. LCOs with time-reversed evolution dynamics (transitioning from an equilibrium-flow dominated to a ZF-dominated state) have also been observed during the H-L back-transition and are potentially of interest for controlled ramp-down of the plasma stored energy and pressure (normalized to the poloidal magnetic field) \\beta_{\\theta} =2\\mu_{0} n{( {T_{e} +T_{i}})}/{B_{\\theta}^{2}} in ITER.
Interfacial crystalline structures in injection over-molded polypropylene and bond strength.

PubMed

Yan, Bowen; Wu, Hong; Jiang, Genjie; Guo, Shaoyun; Huang, Jian

2010-11-01

This paper describes interfacial crystalline structures found in injection overmolded polypropylene components and the relationship of these structures to bond strength between the components. The combined effects of the development of hierarchical gradient structures and the particular thermomechanical environment near the interface on the interfacial crystalline structures were investigated in detail by PLM, SEM, DSC, WAXD, and infrared dichroism spectroscopy. The experimental results showed that during molding there was competitive formation of interfacial crystalline structures consisted of "shish-kebab" layer (SKL) and a transcrystalline layers (TCL). Variation in shear stress (controlled by injection pressure and injection speed) plays an important role in the formation of the SKL. The formation of TCL is influenced by the thermal environment, namely melt temperature and mold temperature. Increasing within certain limits, interfacial temperature and the thermal gradient near the interface promotes β-iPP growth. The relationship between interfacial crystalline structures and interfacial bond strength was established by lap shear measurement. The interfacial bond strength is improved by enhancing the formation of TCL, but reduced if SKL predominates.

The effect of a shear boundary layer on the stability of a capillary jet

NASA Astrophysics Data System (ADS)

Ganan-Calvo, Alfonso; Montanero, Jose M.; Herrada, Miguel A.

2014-11-01

The generic stabilization effect of a shear boundary layer over the free surface of a capillary jet is here studied from analytical (asymptotic), numerical and experimental approaches. In first place, we show the consistency of the proposed asymptotic analysis by a linear stability (numerical) analysis of the Navier-Stokes equations for a finite boundary layer thickness. We show how the convective-to-absolute instability transition departs drastically from the flat velocity profile case as the axial coordinate becomes closer to the origin of the boundary layer development. For large enough axial distances from that origin, Rayleigh's dispersion relation is recovered. A collection of experimental observations is analyzed from the perspective provided by these results. We propose a systematic framework to the dynamics of capillary jets issued from a nozzle, either by direct injection into a quiescent atmosphere or in a co-flow (e.g. gas flow-focused jets), which exhibit peculiarities now definitely attributable in first order to the formation of shear boundary layers. Partial support from the Ministry of Economy and Competitiveness, Junta de Extremadura, and Junta de Andalucia (Spain) through Grant Nos. DPI2010-21103, GR10047, P08-TEP-04128, and TEP-7465, respectively, is gratefully acknowledged.
Dynamo action and magnetic buoyancy in convection simulations with vertical shear

NASA Astrophysics Data System (ADS)

Guerrero, G.; Käpylä, P.

2011-10-01

A hypothesis for sunspot formation is the buoyant emergence of magnetic flux tubes created by the strong radial shear at the tachocline. In this scenario, the magnetic field has to exceed a threshold value before it becomes buoyant and emerges through the whole convection zone. In this work we present the results of direct numerical simulations of compressible turbulent convection that include a vertical shear layer. Like the solar tachocline, the shear is located at the interface between convective and stable layers. We follow the evolution of a random seed magnetic field with the aim of study under what conditions it is possible to excite the dynamo instability and whether the dynamo generated magnetic field becomes buoyantly unstable and emerges to the surface as expected in the flux-tube context. We find that shear and convection are able to amplify the initial magnetic field and form large-scale elongated magnetic structures. The magnetic field strength depends on several parameters such as the shear amplitude, the thickness and location of the shear layer, and the magnetic Reynolds number (Rm). Models with deeper and thicker shear layers allow longer storage and are more favorable for generating a mean magnetic field. Models with higher Rm grow faster but saturate at slightly lower levels. Whenever the toroidal magnetic field reaches amplitudes greater a threshold value which is close to the equipartition value, it becomes buoyant and rises into the convection zone where it expands and forms mushroom shape structures. Some events of emergence, i.e., those with the largest amplitudes of the amplified field, are able to reach the very uppermost layers of the domain. These episodes are able to modify the convective pattern forming either broader convection cells or convective eddies elongated in the direction of the field. However, in none of these events the field preserves its initial structure. The back-reaction of the magnetic field on the fluid is also observed in lower values of the turbulent velocity and in perturbations of approximately three per cent on the shear profile.
Water-tunnel experiments on an oscillating airfoil at RE equals 21,000

NASA Technical Reports Server (NTRS)

Mcalister, K. W.; Carr, L. W.

1978-01-01

Flow visualization experiments were performed in a water tunnel on a modified NACA 0012 airfoil undergoing large amplitude harmonic oscillations in pitch. Hydrogen bubbles were used to: (1) create a conveniently striated and well preserved set of inviscid flow markers; and (2) to expose the succession of events occurring within the viscous domain during the onset of dynamic stall. Unsteady effects were shown to have an important influence on the progression of flow reversal along the airfoil surface prior to stall. A region of reversed flow underlying a free shear layer was found to momentarily exist over the entire upper surface without any appreciable disturbance of the viscous-inviscid boundary. A flow protuberance was observed to develop near the leading edge, while minor vortices evolve from an expanding instability of the free shear layer over the rear portion of the airfoil. The complete breakdown of this shear layer culminates in the successive formation of two dominant vortices.
Mixed layers of sodium caseinate + dextran sulfate: influence of order of addition to oil-water interface.

PubMed

Jourdain, Laureline S; Schmitt, Christophe; Leser, Martin E; Murray, Brent S; Dickinson, Eric

2009-09-01

We report on the interfacial properties of electrostatic complexes of protein (sodium caseinate) with a highly sulfated polysaccharide (dextran sulfate). Two routes were investigated for preparation of adsorbed layers at the n-tetradecane-water interface at pH = 6. Bilayers were made by the layer-by-layer deposition technique whereby polysaccharide was added to a previously established protein-stabilized interface. Mixed layers were made by the conventional one-step method in which soluble protein-polysaccharide complexes were adsorbed directly at the interface. Protein + polysaccharide systems gave a slower decay of interfacial tension and stronger dilatational viscoelastic properties than the protein alone, but there was no significant difference in dilatational properties between mixed layers and bilayers. Conversely, shear rheology experiments exhibited significant differences between the two kinds of interfacial layers, with the mixed system giving much stronger interfacial films than the bilayer system, i.e., shear viscosities and moduli at least an order of magnitude higher. The film shear viscoelasticity was further enhanced by acidification of the biopolymer mixture to pH = 2 prior to interface formation. Taken together, these measurements provide insight into the origin of previously reported differences in stability properties of oil-in-water emulsions made by the bilayer and mixed layer approaches. Addition of a proteolytic enzyme (trypsin) to both types of interfaces led to a significant increase in the elastic modulus of the film, suggesting that the enzyme was adsorbed at the interface via complexation with dextran sulfate. Overall, this study has confirmed the potential of shear rheology as a highly sensitive probe of associative electrostatic interactions and interfacial structure in mixed biopolymer layers.
Formative Processes of a Sliding Zone in Pelitic Schist - Implications of Microscopic Analyses on High-quality Drilled Cores

NASA Astrophysics Data System (ADS)

Yamasaki, S.; Chigira, M.

2009-04-01

Pelitic schist has been known to be easily deformed by gravitational force to form characteristic topographic and geologic features, but little is known about how they develop. This is mainly due to the fact that deformed politic schist is so fragile that it could not be obtained from subsurface without disturbance. We analyzed high-quality undisturbed cores obtained by using a sophisticated drilling technique from two typical pelitic schist landslide sites in Japan. We made analyses on physical, chemical, mineralogical properties and observations from mesoscopic to microscopic rock textures of these cores and found that a special layering of rock-forming minerals determines the locations of shearing by gravity and that there is specific water-rock interaction processes in pelitic schist. Pelitic schist consists of thinly alternating beds of black layers and quartz-rich layers, and a black layer has numerous microscopic layers containing abundant pyrite and graphite grains (pyrite-graphite layers). Many of the black layers were observed to have microfractures connected to open cracks, suggesting that relatively thick, continuous black layers are easily sheared to form an incipient sliding layer. Thus unevenly distributed pyrite-graphite layers likely to determine the potential location of microscopic slip in a rock mass. Shear displacement along black layers occurs unevenly, depending upon the microscopic heterogeneity in mineral composition as well as undulating shape of the layers. Open micro-cracks nearly perpendicular to the schistosity were commonly observed in quartz-rich layers in contact with black layers, suggesting that the shearing occurred with heterogeneous displacements along the black layer and that it occurred under the low confining pressure. This is in the incipient stage of a fracture zone. When shearing occurs along two thick neighboring black layers, the rock in between would be fractured, rotated and pulverized. In some cases, quartz-rich layers were fractured in a brittle manner and their fragments were rearranged to form micro-folds. Rocks are thus pulverized with multiple shear surfaces. Incipient fracture zones and their surroundings have many voids because they are made under low confining pressures near the ground surface, so oxidizing surface water easily percolates through them. Oxidizing water reacts with pyrite which is contained in pelitic schist, producing sulfuric acid through. The rocks therefore become deteriorated by the water-rock interaction and would be easily deformed. Such a combination of the physical processes of deformation and fracturing and the chemical process of weathering develop a sliding zone.
Microstructures and rheology of a calcite-shale thrust fault

NASA Astrophysics Data System (ADS)

Wells, Rachel K.; Newman, Julie; Wojtal, Steven

2014-08-01

A thin (˜2 cm) layer of extensively sheared fault rock decorates the ˜15 km displacement Copper Creek thrust at an exposure near Knoxville, TN (USA). In these ultrafine-grained (<0.3 μm) fault rocks, interpenetrating calcite grains form an interconnected network around shale clasts. One cm below the fault rock layer, sedimentary laminations in non-penetratively deformed footwall shale are cut by calcite veins, small faults, and stylolites. A 350 μm thick calcite vein separates the fault rocks and footwall shale. The vein is composed of layers of (1) coarse calcite grains (>5 μm) that exhibit a lattice preferred orientation (LPO) with pores at twin-twin and twin-grain boundary intersections, and (2) ultrafine-grained (0.3 μm) calcite that exhibits interpenetrating grain boundaries, four-grain junctions and lacks a LPO. Coarse calcite layers crosscut ultrafine-grained layers indicating intermittent vein formation during shearing. Calcite in the fault rock layer is derived from vein calcite and grain-size reduction of calcite took place by plasticity-induced fracture. The ultrafine-grained calcite deformed primarily by diffusion-accommodated grain boundary sliding and formed an interconnected network around shale clasts within the shear zone. The interconnected network of ultrafine-grained calcite indicates that calcite, not shale, was the weak phase in this fault zone.
Spontaneous formation of electric current sheets and the origin of solar flares

NASA Technical Reports Server (NTRS)

Low, B. C.; Wolfson, R.

1988-01-01

It is demonstrated that the continuous boundary motion of a sheared magnetic field in a tenuous plasma with an infinite electrical conductivity can induce the formation of multiple electric current sheets in the interior plasma. In response to specific footpoint displacements, the quadrupolar magnetic field considered is shown to require the formation of multiple electric current sheets as it achieves a force-free state. Some of the current sheets are found to be of finite length, running along separatrix lines of force which separate lobes of magnetic flux. It is suggested that current sheets in the form of infinitely thin magnetic shear layers may be unstable to resistive tearing, a process which may have application to solar flares.
Mapping the Dynamics of Shear Stress—Induced Structural Changes in Endothelial Cells

PubMed Central

Mott, Rosalind E.; Helmke, Brian P.

2009-01-01

Hemodynamic shear stress regulates endothelial cell biochemical processes that govern cytoskeletal contractility, focal adhesion dynamics, and extracellular matrix assembly. Since shear stress causes rapid strain focusing at discrete locations in the cytoskeleton, we hypothesized that shear stress coordinately alters structural dynamics in the cytoskeleton, focal adhesion sites, and extracellular matrix on a time scale of minutes. Using multi-wavelength 4-D fluorescence microscopy, we measured the displacement of rhodamine-fibronectin and of GFP-labeled actin, vimentin, paxillin, and/or vinculin in aortic endothelial cells before and after onset of steady unidirectional shear stress. In the cytoskeleton, the onset of shear stress increased actin polymerization into lamellipodia, altered the angle of lateral displacement of actin stress fibers and vimentin filaments, and decreased centripetal remodeling of actin stress fibers in both subconfluent and confluent cell layers. Shear stress induced the formation of new focal complexes and reduced the centripetal remodeling of focal adhesions in regions of new actin polymerization. The structural dynamics of focal adhesions and the fibronectin matrix varied with cell density. In subconfluent cell layers, shear stress onset decreased the displacement of focal adhesions and fibronectin fibrils. In confluent monolayers, the direction of fibronectin and focal adhesion displacement shifted significantly towards the downstream direction within one minute after onset of shear stress. These spatially coordinated rapid changes in the structural dynamics of cytoskeleton, focal adhesions, and extracellular matrix are consistent with focusing of mechanical stress and/or strain near major sites of shear stress-mediated mechanotransduction. PMID:17855768
Combined action of transverse oscillations and uniform cross-flow on vortex formation and pattern of a circular cylinder

NASA Astrophysics Data System (ADS)

Lam, K. M.; Liu, P.; Hu, J. C.

2010-07-01

This paper attempts to study the roles of lateral cylinder oscillations and a uniform cross-flow in the vortex formation and wake modes of an oscillating circular cylinder. A circular cylinder is given lateral oscillations of varying amplitudes (between 0.28 and 1.42 cylinder-diameters) in a slow uniform flow stream (Reynolds number=284) to produce the 2S, 2P and P+S wake modes. Detailed flow information is obtained with time-resolved particle-image velocimetry and the phase-locked averaging techniques. In the 2S and 2P mode, the flow speeds relative to the cylinder movement are less than the uniform flow velocity and it is found that initial formation of a vortex is caused by shear-layer separation of the uniform flow on the cylinder. Subsequent development of the shear-layer vortices is affected by the lateral cylinder movement. At small cylinder oscillation amplitudes, vortices are shed in synchronization with the cylinder movement, resulting in the 2S mode. The 2P mode occurs at larger cylinder oscillation amplitudes at which each shear-layer vortex is found to undergo intense stretching and eventual bifurcation into two separate vortices. The P+S mode occurs when the cylinder moving speeds are, for most of the time, higher than the speed of the uniform flow. These situations are found at fast and large-amplitude cylinder oscillations in which the flow relative to the cylinder movement takes over the uniform flow in governing the initial vortex formation. The formation stages of vortices from the cylinder are found to bear close resemblance to those of a vortex street pattern of a cylinder oscillating in an otherwise quiescent fluid at Keulegan-Carpenter numbers around 16. Vortices in the inclined vortex street pattern so formed are then convected downstream by the uniform flow as the vortex pairs in the 2P mode.
The Silurian Hoedongri Formation in the Taebaeksan Basin of Korea Revisited: its Significance in the Tectonic Reconstruction of East Asia

NASA Astrophysics Data System (ADS)

Chae, S.; Hong, J.; Jung, S.; Ree, J.

2011-12-01

The Silurian Hoedongri Formation of the Taebaeksan Basin of South Korea has been used as a key unit to the correlation of tectonic provinces of East Asia since the South China craton (or Yangtz block) contains Silurian-Devonian sequences as well as Cambrian-Ordovician ones in the Paleozoic basins while the North China craton (or Sino-Korea block) is devoid of Silurian-Devonian sequences. In the Biryongdong area near the type locality of the Hoedongri Formation, it has been reported that the gray limestone of the Hoedongri Formation unconformably overlies brownish gray limestone of the Ordovician Haengmae Formation. However, our detailed examination on the Biryongdong section reveals that both of the brownish gray and gray limestones are mylonitic marbles with the boundary between the two units being a healed fault breccia zone (~ 12 m thick). The main difference of the two units is that repeated cycles of plastic deformation and fracturing occurred in the underlying brownish gray marble ('Haengmae') while the gray marble ('Hoedongri') deformed mainly by intracrystalline plasticity. The mylonitic foliation strikes NW with a low to moderate dip angle (20-60°) to NE. The ridge-in-groove type lineation on foliation surface trends NNW. The shape-preferred foliation of elongated calcite grains are oblique to the mylonitic foliation defined by layers with a grain-size variation, indicating a top-to-the-SSE shear sense. The mylonitic marble consists of elongated remnant grains (80-120 μm) with deformation twins and dynamically recrystallized matrix grains (10-40 μm). Grain boundaries and twin boundaries are lobate or wavy, indicating dynamic boundary migration. Some layers of the gray mylonitic marble are composed entirely of larger (80-120 μm) elongated calcite grains. In the brownish gray mylonitic marble unit, layers of brittle fracturing overprinting mylonitic foliation occur. In some of these layers, fragments (several cm - tens of cm) of the mylonitic marble are angular to subangular with coarse calcite fillings between the fragments. In other layers, mylonitic marble fragments are elongated with matrix foliation wrapping around them. These features suggest repeated cycles of plastic deformation and fracturing. Tectonic significance of this shear zone (at least 90 m thick) is not clear at present, and the regional extent and absolute age constraint of the shear zone should be clarified.
Interaction of Particles and Turbulence in the Solar Nebula

NASA Technical Reports Server (NTRS)

Dacles-Mariani, Jennifer S.; Dobrovolskis, A. R.; Cuzzi, J. N.; DeVincenzi, Donald L. (Technical Monitor)

1996-01-01

The most widely accepted theories for the formation of the Solar system claim that small solid particles continue to settle into a thin layer at the midplane of the Solar nebula until it becomes gravitationally unstable and collapses directly into km-sized planetesimals. This scenario has been challenged on at least two grounds: (1) due to turbulence, the particles may not settle into a thin layer, and (2) a thin layer may not be unstable. The Solar nebula contains at least three sources of turbulence: radial shear, vertical shear, and thermal convection. The first of these is small and probably negligible, while the last is poorly understood. However, the second contribution is likely to be substantial. The particle-rich layer rotates at nearly the Keplerian speed, but the surrounding gaseous nebula rotates slower because it is partly supported by pressure. The resulting shear generates a turbulent boundary layer which stirs the particles away from the midplane, and forestalls gravitational instability. Our previous work used a 'zero-equation' (Prandtl) model to predict the intensity of shear-generated turbulence, and enabled us to demonstrate numerically that settling of particles to the midplane is self-limiting. However, we neglected the possibility that mass loading by particles might damp the turbulence. To explore this, we have developed a more sophisticated 'one-equation' model which incorporates local generation, transport, and dissipation of turbulence, as well as explicit damping of turbulence by particles. We also include a background level of global turbulence to represent other sources. Our results indicate that damping flattens the distribution of particles somewhat, but that background turbulence thickens the particle layer.
Early Cretaceous Ductile Deformation of Marbles from the Western Hills of Beijing, North China Craton

NASA Astrophysics Data System (ADS)

Feng, H.; Liu, J.

2017-12-01

During the Early Cretaceous tectonic lithosphere extension, the pre-mesozoic rocks from the Western Hills in the central part of the North China Craton suffered from weak metamorphism but intense shear deformation. The prominent features of the deformation structures are the coexisting layer-parallel shear zones and intrafolia folds, and the along-strike thickness variations of the marble layers from the highly sheared Mesoproterozoic Jing'eryu Formation. Platy marbles are well-developed in the thinner layers, while intrafolia folds are often observed in the thicker layers. Most folds are tight recumbent folds and their axial planes are parallel to the foliations and layerings of the marbles. The folds are A-type folds with hinges being always paralleling to the stretching lineations consistently oriented at 130°-310° directions throughout the entire area. SPO and microstructural analyses of the sheared marbles suggest that the thicker layers suffered from deformations homogeneously, while strain localization can be distinguished in the thinner layers. Calcite twin morphology and CPO analysis indicate that the deformation of marbles from both thinner and thicker layers happened at temperatures of 300 to 500°C. The above analysis suggests that marbles in the thicker layers experienced a progressive sequence of thermodynamic events: 1) regional metamorphism, 2) early ductile deformation dominated by relatively higher temperature conditions, during which all the mineral particles elongated and oriented limitedly and the calcite grains are deformed mainly by mechanical twinning, and 3) late superimposition of relatively lower temperature deformation and recrystallization, which superposed the early deformation, and made the calcites finely granulated, elongated and oriented by dynamical recrystallization along with other grains. Marbles from the thinner layers, however, experienced a similar, but different sequence of thermo-dynamic events, i.e. regional metamorphism, early ductile deformation and weak superimposition by subsequent deformation, which caused the development of the strain localization. It is also shown that the intensity of progressive superimposition deformation contributed to the thinning and thickening of the marble layers.
Flow structure and aerodynamic performance of a hovering bristled wing in low Re

NASA Astrophysics Data System (ADS)

Lee, Seunghun; Lahooti, Mohsen; Kim, Daegyoum

2017-11-01

Previous studies on a bristled wing have mainly focused on simple kinematics of the wing such as translation or rotation. The aerodynamic performance of a bristled wing in a quasi-steady phase is known to be comparable to that of a smooth wing without a gap because shear layers in the gaps of the bristled wing are sufficiently developed to block the gaps. However, we point out that, in the starting transient phase where the shear layers are not fully developed, the force generation of a bristled wing is not as efficient as that of a quasi-steady state. The performance in the transient phase is important to understand the aerodynamics of a bristled wing in an unsteady motion. In the hovering motion, due to repeated stroke reversals, the formation and development of shear layers inside the gaps is repeated in each stroke. In this study, a bristled wing in hovering is numerically investigated in the low Reynolds number of O(10). We especially focus on the development of shear layers during a stroke reversal and its effect on the overall propulsive performance. Although the aerodynamic force generation is slightly reduced due to the gap vortices, the asymmetric behavior of vortices in a gap between bristles during a stroke reversal makes the bristled wing show higher lift to drag ratio than a smooth wing.
Near-inertial kinetic energy budget of the mixed layer and shear evolution in the transition layer in the Arabian Sea during the monsoons

NASA Astrophysics Data System (ADS)

Majumder, Sudip; Tandon, Amit; Rudnick, Daniel L.; Thomas Farrar, J.

2015-09-01

We present the horizontal kinetic energy (KE) balance of near-inertial currents in the mixed layer and explain shear evolution in the transition layer using observations from a mooring at 15.26° N in the Arabian Sea during the southwest monsoon. The highly sheared and stratified transition layer at the mixed-layer base varies between 5 m and 35 m and correlates negatively with the wind stress. Results from the mixed layer near-inertial KE (NIKE) balance suggest that wind energy at times can energize the transition layer and at other times is fully utilized within the mixed layer. A simple two layer model is utilized to study the shear evolution in the transition layer and shown to match well with observations. The shear production in this model arises from alignment of wind stress and shear. Although the winds are unidirectional during the monsoon, the shear in the transition layer is predominantly near-inertial. The near-inertial shear bursts in the observations show the same phasing and magnitude at near-inertial frequencies as the wind-shear alignment term.
Strain localization in the lower crust: brittle precursors versus lithological heterogeneities (Musgrave Ranges, Central Australia)

NASA Astrophysics Data System (ADS)

Hawemann, Friedrich; Mancktelow, Neil; Wex, Sebastian; Pennacchioni, Giorgio; Camacho, Alfredo

2016-04-01

The Davenport shear zone in Central Australia is a strike-slip ductile shear zone developed during the Petermann Orogeny (~ 550 Ma). The conditions of shearing are estimated to be amphibolite-eclogite facies (650 °C, 1.2 GPa). The up to seven kilometre thick mylonite zone encloses several large low strain domains with excellent exposure, thus allowing a thorough study of the initiation of shear zones. Quartzo-feldspathic gneisses and granitoids inherit a suite of lithological heterogeneities such as quartz-rich pegmatites, mafic layers and dykes. When in a favourable orientation to the shortening direction, these rheologically different pre-existing layers might be expected to localize deformation. However, with the singular exception of long, continuous and fine-grained dolerite dykes, this is not observed. Quartz-rich pegmatites are mostly unsheared, even if in a favourable orientation, and sometimes boudinaged or folded. There are instead many shear zones only a few mm to cm in width, extending up to tens of metres, which are in fact oriented at a very high angle to the shortening direction. Parallel to these, a network of little to moderately overprinted brittle fractures are observed, commonly marked by pseudotachylyte (pst) and sometimes new biotite. Shear reactivation of these precursor fractures is generally limited to the length of the initial fracture and typically re-uses and shears the pst. The recrystallized mineral assemblage in the sheared pst consists of Cpx+Grt+Fsp±Ky and is the same to that in the adjacent sheared gneiss, with the same PT estimates (650 °C, 1.2 GPa). In some cases, multiple generations of cross-cutting and sheared pst demonstrate alternating fracture and flow during progressive shear zone development and a clear tendency for subsequent pst formation to also localize in the existing shear zone. The latest pst may be both unsheared and unrecrystallized (no grt) and is probably related to a late stage, still localized within the same shear zone. The observation that pst is preferentially sheared indicates that it is weaker than the host rock, although their bulk compositions are about the same, suggesting that the governing factors for localization are the finer grain size and the elongate, nearly planar geometry of the original pst generation zone. The same may be true of the sheared dolerite dykes, which are long, narrow and generally finer grained than the surrounding gneiss or granite. Although quartz-rich pegmatites are not preferred sites of localization, quartzo-feldspathic mylonites are fully recrystallized with a relatively coarse grain size (typically > 50 microns) typical of rather low long-term flow stress. We therefore propose that localization in the lower crust only occurs on long planar layers with a finer grain size that can promote weakening by grain-size sensitive creep. Coarser-grained lithological layers and boundaries are not exploited during the initiation of a shear zone and, in particular, quartz-rich layers are not preferentially sheared.
Interaction of viscous and inviscid instability modes in separation-bubble transition

NASA Astrophysics Data System (ADS)

Brinkerhoff, Joshua R.; Yaras, Metin I.

2011-12-01

This paper describes numerical simulations that are used to examine the interaction of viscous and inviscid instability modes in laminar-to-turbulent transition in a separation bubble. The results of a direct numerical simulation are presented in which separation of a laminar boundary-layer occurs in the presence of an adverse streamwise pressure gradient. The simulation is performed at low freestream-turbulence levels and at a flow Reynolds number and pressure distribution approximating those typically encountered on the suction side of low-pressure turbine blades in a gas-turbine engine. The simulation results reveal the development of a viscous instability upstream of the point of separation which produces streamwise-oriented vortices in the attached laminar boundary layer. These vortices remain embedded in the flow downstream of separation and are carried into the separated shear layer, where they are amplified by the local adverse pressure-gradient and contribute to the formation of coherent hairpin-like vortices. A strong interaction is observed between these vortices and the inviscid instability that typically dominates the shear layer in the separated zone. The interaction is noted to determine the spanwise extent of the vortical flow structures that periodically shed from the downstream end of the separated shear layer. The structure of the shed vortical flow structures is examined and compared with the coherent structures typically observed within turbulent boundary layers.
Numerical simulation of viscoelastic layer rearrangement in polymer melts using OpenFOAM®

DOE Office of Scientific and Technical Information (OSTI.GOV)

Köpplmayr, Thomas, E-mail: tkoepplmayr@gmail.com; Mayrhofer, Elias

In addition to their shear-thinning behavior, polymer melts are characterized by first and second normal stress differences, which cause secondary motions. Polymer coextrusion processes involve viscoelastic two-phase flows that influence layer formation. Using polymer melts with different pigmentation makes visible the layers deformed by second normal stress differences. We used a new solver for the OpenFOAM CFD toolbox which handles viscoelastic two-phase flows. A derivative of the volume-of-fluid (VoF) methodology was employed to describe the interface. Different types of polymer melt, such as polyethylene (PE), polypropylene (PP) and polyethylene terephthalate (PET) were investigated. In a coextrusion process, the less viscousmore » phase usually tends to encapsulate the more viscous one. However, the different viscoelastic properties of the melts also influence interface deformation. The materials were characterized by small-amplitude oscillatory-shear rheometry, and a multimode Giesekus model was used to fit shear viscosity, storage and loss modulus. Our simulations also took interfacial tension into account. Experimental observations and corresponding numerical simulations were found to be in good accordance.« less
Effect of vortical structures on velocity and turbulent fields in the near region of an impinging turbulent jet

NASA Astrophysics Data System (ADS)

Yadav, Harekrishna; Agrawal, Amit

2018-03-01

This experimental study pertains to the formation of a secondary peak in heat transfer distribution for an axisymmetric turbulent impinging submerged jet. The analysis of instantaneous fields is undertaken at various Reynolds numbers based upon the bulk velocity and nozzle diameter (Re = 1300-10 000) and surface spacings (L/D = 0.25-6). Our analysis shows that flow separation and reattachment correspond to decrease/increase in local pressure and are caused by primary vortices; these are further linked to the location of maxima in streamwise and cross-stream velocities. It is further observed that the locations of maxima and minima in velocities are linked to fluctuations in rms velocities and thickening/thinning of the boundary layer. The vortices transported along the surface either coalesce among themselves or combine with other eddies to form a primary vortex. The primary vortex while getting convected downstream makes multiple interactions with the inner shear layer and causes waviness in instantaneous flow fields. In their later stage, the primary vortex moves away from the wall and accelerates, while the flow decelerates in the inner shear layer. The accelerated fluid in the outer shear layer pulls the downstream fluid from the inner shear layer and leads to the formation of a secondary vortex. After a certain distance downstream, the secondary vortex rolling between the primary vortex and the wall eventually breaks down, while the flow reattaches to the wall. The behavior of time average and instantaneous velocity fields suggests that unsteadiness in the heat transfer is linked to the location of maximum streamwise velocity, location of flow attachment, location of rms velocity, and thickness of the boundary layer. The instantaneous velocity fields show that for a given surface spacing, the chances for the appearance of the secondary vortex reduce with an increase in Reynolds number because of the reduction in space available for the secondary vortex to develop. It is further deduced that the strength of the secondary vortex is primarily dependent upon the strength of the primary vortex. However, the velocity field estimated using the linear stochastic estimation technique shows a tendency for the formation of the secondary vortex at higher Reynolds number, suggesting that most measurements do not resolve them well. Our analysis explains the reason for the appearance of the secondary peak in heat transfer distribution and helps resolve the contradictions in the literature regarding this phenomenon.
PIV measurements in the near wakes of hollow cylinders with holes

NASA Astrophysics Data System (ADS)

Firat, Erhan; Ozkan, Gokturk M.; Akilli, Huseyin

2017-05-01

The wake flows behind fixed, hollow, rigid circular cylinders with two rows of holes connecting the front and rear stagnation lines were investigated using particle image velocimetry (PIV) for various combinations of three hole diameters, d = 0.1 D, 0.15 D, and 0.20 D, six hole-to-hole distances, l = 2 d, 3 d, 4 d, 5 d, 6 d, and 7 d, and ten angles of incidence ( α), from 0° to 45° in steps of 5°, at a Reynolds number of Re = 6,900. Time-averaged velocity distributions, instantaneous and time-averaged vorticity patterns, time-averaged streamline topology, and hot spots of turbulent kinetic energy occurred through the interaction of shear layers from the models were presented to show how the wake flow was modified by the presence of the self-issuing jets with various momentums emanating from the downstream holes. In general, as hole diameter which is directly related to jet momentum increased, the values of time-averaged wake characteristics (length of time-averaged recirculation region, vortex formation length, length of shear layers, and gap between the shear layers) increased. Irrespective to d and l tested, the values of the vortex formation length of the models are greater than that of the cylinder without hole (reference model). That is, vortex formation process was shifted downstream by aid of jets. It was found that time-averaged wake characteristics were very sensitive to α. As α increased, the variation of these characteristics can be modeled by exponential decay functions. The effect of l on the three-dimensional vortex shedding patterns in the near wake of the models was also discussed.
CFD modelling of nocturnal low-level jet effects on wind energy related variables

NASA Astrophysics Data System (ADS)

Sogachev, Andrey; Mann, Jakob; Dellwik, Ebba; Ejsing Jørgensen, Hans

2010-05-01

The development of a wind speed maximum in the nocturnal boundary layer, referred to as a low-level jet (LLJ), is a common feature of the vertical structure of the atmospheric boundary layer (ABL). Characterizing and understanding LLJ streams is growing in importance as wind turbines are being built larger and taller to take advantage of higher wind speeds at increased heights. We used a computational fluid dynamics (CFD) model to explore LLJs effect on wind speed, wind directional and speed shear inside the surface layer 40 - 130 m, where their physical measurements are not trivial and still rare today. We used the one-dimensional version of the ABL model SCADIS (Sogachev et al. 2002: Tellus 54:784-819). The unique feature of the model, based on a two-equation closure approach, is the treatment of buoyancy effects in a universal way, which overcomes the uncertainties with model coefficients for non-shear source/sink terms (Sogachev, 2009: Boundary Layer Meteor. 130:423-435). From a variety of mechanisms suggested for formation of LLJs, such as inertial oscillations, baroclinicity over sloping terrain, and land-sea breeze effects, the one-dimensional ABL model is capable of simulating only the first one. However, that mechanism, which is caused by the diurnal oscillation of eddy viscosity, is often responsible for jet formation. Sensitivity tests carried out showed that SCADIS captures the most prominent features of the LLJ, including its vertical structure as well as its diurnal phase and amplitude. We simulated ABL pattern under conditions typical for LLJ formation (a fair day on July 1, a flat low-roughness underlying surface) at 30 and 50o latitudes. Diurnal variability of wind speed and turbulence intensity at four levels of 40, 70, 100 and 130 m above ground and of wind and directional shear between those levels were analysed. Despite of small differences in LLJ structure the properties of LLJ important for wind energy production are still common for two latitudes. Along with the wind speed increase in night time the turbulence intensity decreases and, as it was confirmed by many experiments, are insignificant in comparison with midday values (both factors are favourable for wind production). However, wind and directional shear across the entire layer occupied by hypothetical wind turbine rotors (between 40 - 130 m) provide different wind conditions above and below the turbine hub. For example, the shear exponent was higher than 0.65 during most part of night (below 0.08 at midday) and direction shear was sometimes higher than 0.3 degree per meter (about 0 at midday). Most extreme values of both parameters occurred at dawn when turbulence starts to develop. This creates large amounts of stress on the turbines, causing difficulties in their operation and fatigue issues over time. The model will have to be coupled to an aeroelastic model to be able to predict quantatively the consequences for power production and dynamic loads on wind turbines.

Excited waves in shear layers

NASA Technical Reports Server (NTRS)

Bechert, D. W.

1982-01-01

The generation of instability waves in free shear layers is investigated. The model assumes an infinitesimally thin shear layer shed from a semi-infinite plate which is exposed to sound excitation. The acoustical shear layer excitation by a source further away from the plate edge in the downstream direction is very weak while upstream from the plate edge the excitation is relatively efficient. A special solution is given for the source at the plate edge. The theory is then extended to two streams on both sides of the shear layer having different velocities and densities. Furthermore, the excitation of a shear layer in a channel is calculated. A reference quantity is found for the magnitude of the excited instability waves. For a comparison with measurements, numerical computations of the velocity field outside the shear layer were carried out.
Experimental study on the signs of particulate structures formation in annular geometry of rapid granular shear flows

NASA Astrophysics Data System (ADS)

Ritvanen, J.; Jalali, P.

2009-06-01

Rapid granular shear flow is a classical example in granular materials which exhibits both fluid-like and solid-like behaviors. Another interesting feature of rapid granular shear flows is the formation of ordered structures upon shearing. Certain amount of granular material, with uniform size distribution, is required to be loaded in the container in order to shear it under stable conditions. This work concerns the experimental study of rapid granular shear flows in annular Couette geometry. The flow is induced by continuous rotation of the plate over the top of the granular bed in an annulus. The compressive pressure, driving torque, instantaneous bed height from three symmetric locations and rotational speed of the shearing plate are measured. The annulus has a capacity of up to 15 kg of spherical steel balls of 3 mm in diameter. Rapid shear flow experiments are performed in one compressive force and rotation rate. The sensitivity of fluctuations is then investigated by different means through monodisperse packing. In this work, we present the results of the experiments showing how the flow properties depend on the amount of loaded granular material which is varied by small amounts between different experiments. The flow can exist in stable (fixed behavior) and unstable (time-dependent behavior) regimes as a function of the loaded material. We present the characteristics of flow to detect the formation of any additional structured layer in the annulus. As a result, an evolution graph for the bed height has been obtained as material is gradually added. This graph shows how the bed height grows when material increases. Using these results, the structure inside the medium can be estimated at extreme stable and unstable conditions.
Layering, melting, and recrystallization of a close-packed micellar crystal under steady and large-amplitude oscillatory shear flows

DOE Office of Scientific and Technical Information (OSTI.GOV)

López-Barrón, Carlos R., E-mail: carlos.r.lopez-barron@exxonmobil.com; Wagner, Norman J.; Porcar, Lionel

2015-05-15

The rheology and three-dimensional microstructure of a concentrated viscoelastic solution of the triblock copolymer poly(ethylene oxide){sub 106}-poly(propylene oxide){sub 68}-poly(ethylene oxide){sub 106} (Pluronic F127) in the protic ionic liquid ethylammonium nitrate are measured by small angle neutron scattering (SANS) under flow in three orthogonal planes. This solution's shear-thinning viscosity is due to the formation of two-dimensional hexagonal close-packed (HCP) sliding layer structure. Shear-melting of the crystalline structure is observed without disruption of the self-assembled micelles, resulting in a change in flow properties. Spatially resolved measurements in the 1–2 plane reveal that both shear-melting and sliding are not uniform across the Couettemore » gap. Melting and recrystallization of the HCP layers occur cyclically during a single large amplitude oscillatory shear (LAOS) cycle, in agreement with the “stick-slip” flow mechanism proposed by Hamley et al. [Phys. Rev. E 58, 7620–7628 (1998)]. Analysis of 3D “structural” Lissajous curves show that the cyclic melting and sliding are direct functions of the strain rate amplitude and show perfect correlation with the cyclic stress response during LAOS. Both viscosity and structural order obey the Delaware–Rutgers rule. Combining rheology with in situ spatiotemporally resolved SANS is demonstrated to elucidate the structural origins of the nonlinear rheology of complex fluids.« less
Layered Fault Rocks Below the West Salton Detachment Fault (WSDF), CA Record Multiple Seismogenic? Slip Events and Transfer of Material to a Fault Core

NASA Astrophysics Data System (ADS)

Axen, G. J.; Luther, A. L.; Selverstone, J.; Mozley, P.

2011-12-01

Unique layered cataclasites (LCs) occur locally along footwall splays, S of the ~N-dipping, top-E WSDF. They are well exposed in a NW-plunging antiform that folds the LCs and their upper and lower bounding faults. Layers range from very fine-grained granular shear zones 1-2 mm thick and cm's to m's long, to medium- to coarse-grained isotropic granular cataclasite with floating clasts up to 4-5 cm diameter in layers up to ~30 cm thick and 3 to >10 m long. The top, N-flank contact is ~5 m structurally below the main WSDF. Maximum thickness of the LCs is ~5 m on the S flank of the antiform, where the upper 10-50 cm of LCs are composed of relatively planar layers that are subparallel to the upper fault, which locally displays ultracataclasite. Deeper layers are folded into open to isoclinal folds and are faulted. Most shear-sense indicators show N-side-to-E or -SE slip, and include: (1) aligned biotite flakes and mm-scale shear bands that locally define a weak foliation dipping ~ESE, (2) sharp to granular shears, many of which merge up or down into fine-grained layers and, in the base of the overlying granodiorite, (3) primary reidel shears and (4) folded pegmatite dikes. Biotite is unaltered and feldspars are weakly to strongly altered to clays and zeolites. Zeolites also grew in pores between clasts. XRF analyses suggest minimal chemical alteration. The upper fault is sharp and relatively planar, carries granular to foliated cataclasitic granodiorite that grades up over ~2-4 m into punky, microcracked but plutonic-textured rock with much of the feldspar alteration seen in LC clasts. Some upper-plate reidels bend into parallelism with the top fault and bound newly formed LC layers. The basal fault truncates contorted layers and lacks evidence of layers being added there. We infer that the deeper, contorted layers are older and that the LC package grew upward by transfer of cataclasized slices from the overlying granodiorite while folding was ongoing. Particle-size distributions reflect constrained comminution and shear localization (slopes of ~3-3.5 on log-log plots of grain size vs. no. of grains > grain size). The LCs require episodic slip events that probably record dozens of seismic cycles. Foliation likely records post- or interseismic creep. Geometric complexities among the WSDF footwall splays presumably caused episodic dilation that allowed accumulation and folding of the LCs. Mechanical processes dominated over chemical processes. A key question is why the LCs apparently were stronger than the overlying granodiorite, leading to formation of new LC layers rather than significant reworking of older layers.
The Granular Blasius Problem: High inertial number granular flows

NASA Astrophysics Data System (ADS)

Tsang, Jonathan; Dalziel, Stuart; Vriend, Nathalie

2017-11-01

The classical Blasius problem considers the formation of a boundary layer through the change at x = 0 from a free-slip to a no-slip boundary beneath an otherwise steady uniform flow. Discrete particle model (DPM) simulations of granular gravity currents show that a similar phenomenon exists for a steady flow over a uniformly sloped surface that is smooth upstream (allowing slip) but rough downstream (imposing a no-slip condition). The boundary layer is a region of high shear rate and therefore high inertial number I; its dynamics are governed by the asymptotic behaviour of the granular rheology as I -> ∞ . The μ(I) rheology asserts that dμ / dI = O(1 /I2) as I -> ∞ , but current experimental evidence is insufficient to confirm this. We show that `generalised μ(I) rheologies', with different behaviours as I -> ∞ , all permit the formation of a boundary layer. We give approximate solutions for the velocity profile under each rheology. The change in boundary condition considered here mimics more complex topography in which shear stress increases in the streamwise direction (e.g. a curved slope). Such a system would be of interest in avalanche modelling. EPSRC studentship (Tsang) and Royal Society Dorothy Hodgkin Fellowship (Vriend).
Dynamo action and magnetic buoyancy in convection simulations with vertical shear

NASA Astrophysics Data System (ADS)

Guerrero, G.; Käpylä, P. J.

2011-09-01

Context. A hypothesis for sunspot formation is the buoyant emergence of magnetic flux tubes created by the strong radial shear at the tachocline. In this scenario, the magnetic field has to exceed a threshold value before it becomes buoyant and emerges through the whole convection zone. Aims: We follow the evolution of a random seed magnetic field with the aim of study under what conditions it is possible to excite the dynamo instability and whether the dynamo generated magnetic field becomes buoyantly unstable and emerges to the surface as expected in the flux-tube context. Methods: We perform numerical simulations of compressible turbulent convection that include a vertical shear layer. Like the solar tachocline, the shear is located at the interface between convective and stable layers. Results: We find that shear and convection are able to amplify the initial magnetic field and form large-scale elongated magnetic structures. The magnetic field strength depends on several parameters such as the shear amplitude, the thickness and location of the shear layer, and the magnetic Reynolds number (Rm). Models with deeper and thicker tachoclines allow longer storage and are more favorable for generating a mean magnetic field. Models with higher Rm grow faster but saturate at slightly lower levels. Whenever the toroidal magnetic field reaches amplitudes greater a threshold value which is close to the equipartition value, it becomes buoyant and rises into the convection zone where it expands and forms mushroom shape structures. Some events of emergence, i.e. those with the largest amplitudes of the initial field, are able to reach the very uppermost layers of the domain. These episodes are able to modify the convective pattern forming either broader convection cells or convective eddies elongated in the direction of the field. However, in none of these events the field preserves its initial structure. The back-reaction of the magnetic field on the fluid is also observed in lower values of the turbulent velocity and in perturbations of approximately three per cent on the shear profile. Conclusions: The results indicate that buoyancy is a common phenomena when the magnetic field is amplified through dynamo action in a narrow layer. It is, however, very hard for the field to rise up to the surface without losing its initial coherence.
Shear layer excitation, experiment versus theory

NASA Technical Reports Server (NTRS)

Bechert, D. W.; Stahl, B.

1984-01-01

The acoustical excitation of shear layers is investigated. Acoustical excitation causes the so-called orderly structures in shear layers and jets. Also, the deviations in the spreading rate between different shear layer experiments are due to the same excitation mechanism. Measurements in the linear interaction region close to the edge from which the shear layer is shed are examined. Two sets of experiments (Houston 1981 and Berlin 1983/84) are discussed. The measurements were carried out with shear layers in air using hot wire anemometers and microphones. The agreement between these measurements and the theory is good. Even details of the fluctuating flow field correspond to theoretical predictions, such as the local occurrence of negative phase speeds.
Optimization of MBR hydrodynamics for cake layer fouling control through CFD simulation and RSM design.

PubMed

Yang, Min; Yu, Dawei; Liu, Mengmeng; Zheng, Libing; Zheng, Xiang; Wei, Yuansong; Wang, Fang; Fan, Yaobo

2017-03-01

Membrane fouling is an important issue for membrane bioreactor (MBR) operation. This paper aims at the investigation and the controlling of reversible membrane fouling due to cake layer formation and foulants deposition by optimizing MBR hydrodynamics through the combination of computational fluid dynamics (CFD) and design of experiment (DOE). The model was validated by comparing simulations with measurements of liquid velocity and dissolved oxygen (DO) concentration in a lab-scale submerged MBR. The results demonstrated that the sludge concentration is the most influencing for responses including shear stress, particle deposition propensity (PDP), sludge viscosity and strain rate. A medium sludge concentration of 8820mgL -1 is optimal for the reduction of reversible fouling in this submerged MBR. The bubble diameter is more decisive than air flowrate for membrane shear stress due to its role in sludge viscosity. The optimal bubble diameter was at around 4.8mm for both of shear stress and PDP. Copyright © 2016 Elsevier Ltd. All rights reserved.
Vertical wind shear characteristics that promote supercell-to-MCS transitions

NASA Astrophysics Data System (ADS)

Peters, J. M.

2017-12-01

What causes supercells to transition into MCSs in some situations, but not others? To explore this question, I first examined observed environmental characteristics of supercell events when MCSs formed, and compared them to the analogous environmental characteristics of supercell events when MCSs did not form. During events when MCS growth occurred, 0-1 km (low-level) vertical wind shear was stronger and 0-10 km (deep-layer) vertical wind shear was weaker than the wind shear during events when MCS growth did not occur. Next, I used idealized simulations of supercell thunderstorms to understand the connections between low-level and deep-layer shear and MCS growth. Compared to simulations with strong deep-layer shear, the simulations with weak deep-layer shear had rain in the storm's forward-flank downdraft (FFD) that fell closer to the updraft, fell through storm-moistened air and evaporated less, and produced a more intense FFD. Compared to simulations with weak low-level shear, the simulations with stronger low-level shear showed enhanced northward low-level hydrometeor transport into the FFD. Environments with strong low-level shear and weak deep-layer shear therefore conspired to produce a storm with a more intense FFD cold pool, when compared to environments with weak low-level shear and/or strong deep-layer shear. This strong FFD periodically disrupted the supercells' mesocyclones, and favorably interacted with westerly wind shear to produce widespread linear convection initiation, which drove MCS growth. These results suggest that increasing low-level wind shear after dark - while commonly assumed to enhance tornado potential - may in fact drive MCS growth and reduce tornado potential, unless it is combined with sufficiently strong deep layer shear.
Effect of processing parameters on the formation of C{sub f}/LAS composites/Ag−Cu−Ti/TC4 brazed joint

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Duo; Niu, Hongwei

C{sub f}/LAS composites were successfully jointed to TC4 alloy with Ag−Cu−Ti filler by vacuum brazing. The interfacial microstructure of TC4/C{sub f}/LAS composites joints was characterized by employing scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-diffraction (XRD) and transmission electron microscopy (TEM). The determination of the thin interfacial reaction layer (TiSi{sub 2} + TiC layer) was realized by TEM. The effect of holding time on the interfacial microstructure and shear strength were investigated. With the increasing holding time, the thickness of diffusion layer, Ti{sub 3}Cu{sub 4} layer, and TiSi{sub 2} + TiC layer increased obviously, on the contrary, that ofmore » Ti−Cu intermetallic compound layers decreased gradually. Besides, blocky Ti{sub 3}Cu{sub 4} phase was coarsened when the joint was brazed at 890 °C for 20 min, which deteriorated the mechanical properties of the joint dramatically. The interfacial evolution of TC4/C{sub f}/LAS composites joint and the formation of TiSi{sub 2}, TiC, Ti{sub 3}Cu{sub 4}, TiCu and Ti{sub 2}Cu phases were expounded. The maximum shear strength of 26.4 MPa was obtained when brazed at 890 °C for 10 min. - Highlights: •The thin interface reaction layer was determined to be TiSi{sub 2} + TiC layer by TEM. •Holding time had influence on the interfacial microstructure and joint properties. •Microstructural evolution mechanism and reactions of brazed joints were expounded.« less
Probing nonlinear rheology layer-by-layer in interfacial hydration water.

PubMed

Kim, Bongsu; Kwon, Soyoung; Lee, Manhee; Kim, Q Hwan; An, Sangmin; Jhe, Wonho

2015-12-22

Viscoelastic fluids exhibit rheological nonlinearity at a high shear rate. Although typical nonlinear effects, shear thinning and shear thickening, have been usually understood by variation of intrinsic quantities such as viscosity, one still requires a better understanding of the microscopic origins, currently under debate, especially on the shear-thickening mechanism. We present accurate measurements of shear stress in the bound hydration water layer using noncontact dynamic force microscopy. We find shear thickening occurs above ∼ 10(6) s(-1) shear rate beyond 0.3-nm layer thickness, which is attributed to the nonviscous, elasticity-associated fluidic instability via fluctuation correlation. Such a nonlinear fluidic transition is observed due to the long relaxation time (∼ 10(-6) s) of water available in the nanoconfined hydration layer, which indicates the onset of elastic turbulence at nanoscale, elucidating the interplay between relaxation and shear motion, which also indicates the onset of elastic turbulence at nanoscale above a universal shear velocity of ∼ 1 mm/s. This extensive layer-by-layer control paves the way for fundamental studies of nonlinear nanorheology and nanoscale hydrodynamics, as well as provides novel insights on viscoelastic dynamics of interfacial water.
Advances in active control and optimization in turbulence

NASA Astrophysics Data System (ADS)

Freeman, Aaron Paul

The main objective of this research is to explore the effectiveness of pulsed plasma actuators for turbulence control. In particular, a pulsed plasma actuator is used in this research to implement active control, in the form of a localized body force, over turbulent separated shear layers. Applications of tins research include controlling the formation and distribution of large scale turbulent structures and optimizing turbulence-aberrated laser propagation. This research is primarily experimental, with the motivation for the work derived from theoretical analysis of a turbulent shear layer. The experimental work is considered within two primary flow regimes, compressible and incompressible. For both cases, a turbulent shear layer is generated and then forced with plasma which is introduced periodically at frequencies ranging between 1.0 kHz and 25.0 kHz. The Reynolds numbers, based on visual thickness, of the compressible and incompressible flows investigated in this research are 6.0 106 and 8.0 104 respectively. Experimental results for the compressible case, based on Shack-Hartmann profiling of turbulence-aberrated laser wavefronts, for laser propagation through forced and unforced shear flows show reductions in the laser aberrations of up to 27.5% with a pulsing frequency of 5.0 kHz as well as increases of up to 16.9% with a pulsing frequency of 1.0 kHz. Other pulsing frequencies within the specified range were experimental analyzed and found to exhibit little or no significant change in the laser aberrations compared to the unforced case. The direct results from the Shack-Hartmann wavefront sensor are used to calculate the power spectra of the recorded Optical Path Difference profiles to verify the correlation between large aero-optical aberrations and propagation through large turbulent structures. Shadowgraph imaging of the compressible flow field was conducted to visually demonstrate the same. The experimental procedure for the incompressible shear layer involves imaging the flow field using fog-Mie scattering. The analysis for the resulting incompressible shear layer images include investigations of the distribution of large scale structures and the associated effects that periodic forcing has on the shear layer relating to mixing enhancement and scalar geometry. The effects of periodic forcing on mixing will be determined based on the scalar probability density function and the scalar power spectrum. In addition, the geometry of the scalar interfaces will be examined in terms of the generalized fractal dimension to determine the effects that periodic forcing has on the scale dependency of self-similarity within the flow field. Results from the experiments for the incompressible shear layer show that mixing can be increased by up to 8.4% as determined based on increases within the intermediate scalar probability density function and decreased by as much as 30.8% at forcing frequencies of 25.0 kHz and 1.0 kHz respectively. Additionally, this research shows that the extent of the range of scales of geometrical self-similarity of iso-concentration interfaces extracted from the flow images can be increased by up to 75.0% or reduced by as much as 75.0% depending on the forcing frequency applied. These results show that aero-optical interactions in a compressible shear layer as well as both mixing and the interfacial geometry in incompressible shear layers can be substantially modified by the periodic forcing.
Bumps and Ridges: Trabeculation Effects in Embryonic Heart Development

NASA Astrophysics Data System (ADS)

Battista, Nicholas; Lane, Andrea; Miller, Laura

2014-11-01

Trabeculae form in developing zebrafish hearts for Re on the order of 0.1; effects of trabeculae in this flow is not well understood. Dynamic processes, such as vortex formation, are important in the generation of shear at the endothelial surface layer and strains at the epithelial layer, which aid in proper morphology and functionality. In this study, CFD is used to quantify the effects of Re and idealized trabeculae height on the resulting flows.
The Flowfield Characteristics of a Mach 2 Diamond Jet

NASA Technical Reports Server (NTRS)

Washington, Donnell; Alvi, Farrukh S.; Krothapalli, Anjanevulu

1997-01-01

The potential for using a novel diamond-shaped nozzle which may allow for superior mixing characteristics of supersonic jets without significant thrust losses is explored. The results of flow visualization and pressure measurements indicate the presence of distinct structures in the shear layers, not normally observed in shear layers of axisymmetric and rectangular jets. As characteristics of these features suggests that they are a manifestation of significant streamwise vorticity in the shear layers. Despite the distinct nature of the flowfield structure of the present shear layer, the global growth rates of this shear layer were found to be very similar to its two-dimensional and axisymmetric counterparts. These and other observations suggest that the presence of streamwise vorticity may not play a significant role in the global development of a compressible shear layer.
Evolution of fracture and fault-controlled fluid pathways in carbonates of the Albanides fold-thrust belt

USGS Publications Warehouse

Graham, Wall B.R.; Girbacea, R.; Mesonjesi, A.; Aydin, A.

2006-01-01

The process of fracture and fault formation in carbonates of the Albanides fold-thrust belt has been systematically documented using hierarchical development of structural elements from hand sample, outcrop, and geologic-map scales. The function of fractures and faults in fluid migration was elucidated using calcite cement and bitumen in these structures as a paleoflow indicator. Two prefolding pressure-solution and vein assemblages were identified: an overburden assemblage and a remote tectonic stress assemblage. Sheared layer-parallel pressure-solution surfaces of the overburden assemblage define mechanical layers. Shearing of mechanical layers associated with folding resulted in the formation of a series of folding assemblage fractures at different orientations, depending on the slip direction of individual mechanical layers. Prefolding- and folding-related fracture assemblages together formed fragmentation zones in mechanical layers and are the sites of incipient fault localization. Further deformation along these sites was accommodated by rotation and translation of fragmented rock, which formed breccia and facilitated fault offset across multiple mechanical layers. Strike-slip faults formed by this process are organized in two sets in an apparent conjugate pattern. Calcite cement and bitumen that accumulated along fractures and faults are evidence of localized fluid flow along fault zones. By systematic identification of fractures and faults, their evolution, and their fluid and bitumen contents, along with subsurface core and well-log data, we identify northeast-southwest-trending strike-slip faults and the associated structures as dominant fluid pathways in the Albanides fold-thrust belt. Copyright ?? 2006. The American Association of Petroleum Geologists. All rights reserved.
Large-Amplitude Long-Wave Instability of a Supersonic Shear Layer

NASA Technical Reports Server (NTRS)

Messiter, A. F.

1995-01-01

For sufficiently high Mach numbers, small disturbances on a supersonic vortex sheet are known to grow in amplitude because of slow nonlinear wave steepening. Under the same external conditions, linear theory predicts slow growth of long-wave disturbances to a thin supersonic shear layer. An asymptotic formulation is given here which adds nonzero shear-layer thickness to the weakly nonlinear formulation for a vortex sheet. Spatial evolution is considered, for a spatially periodic disturbance having amplitude of the same order, in Reynolds number, as the shear-layer thickness. A quasi-equilibrium inviscid nonlinear critical layer is found, with effects of diffusion and slow growth appearing through nonsecularity condition. Other limiting cases are also considered, in an attempt to determine a relationship between the vortex-sheet limit and the long-wave limit for a thin shear layer; there appear to be three special limits, corresponding to disturbances of different amplitudes at different locations along the shear layer.
Impact of the Diurnal Cycle of the Atmospheric Boundary Layer on Wind-Turbine Wakes: A Numerical Modelling Study

NASA Astrophysics Data System (ADS)

Englberger, Antonia; Dörnbrack, Andreas

2018-03-01

The wake characteristics of a wind turbine for different regimes occurring throughout the diurnal cycle are investigated systematically by means of large-eddy simulation. Idealized diurnal cycle simulations of the atmospheric boundary layer are performed with the geophysical flow solver EULAG over both homogeneous and heterogeneous terrain. Under homogeneous conditions, the diurnal cycle significantly affects the low-level wind shear and atmospheric turbulence. A strong vertical wind shear and veering with height occur in the nocturnal stable boundary layer and in the morning boundary layer, whereas atmospheric turbulence is much larger in the convective boundary layer and in the evening boundary layer. The increased shear under heterogeneous conditions changes these wind characteristics, counteracting the formation of the night-time Ekman spiral. The convective, stable, evening, and morning regimes of the atmospheric boundary layer over a homogeneous surface as well as the convective and stable regimes over a heterogeneous surface are used to study the flow in a wind-turbine wake. Synchronized turbulent inflow data from the idealized atmospheric boundary-layer simulations with periodic horizontal boundary conditions are applied to the wind-turbine simulations with open streamwise boundary conditions. The resulting wake is strongly influenced by the stability of the atmosphere. In both cases, the flow in the wake recovers more rapidly under convective conditions during the day than under stable conditions at night. The simulated wakes produced for the night-time situation completely differ between heterogeneous and homogeneous surface conditions. The wake characteristics of the transitional periods are influenced by the flow regime prior to the transition. Furthermore, there are different wake deflections over the height of the rotor, which reflect the incoming wind direction.
Reynolds number influence on the formation of vortical structures on a pitching flat plate.

PubMed

Widmann, Alexander; Tropea, Cameron

2017-02-06

The impact of chord-based Reynolds number on the formation of leading-edge vortices (LEVs) on unsteady pitching flat plates is investigated. The influence of secondary flow structures on the shear layer feeding the LEV and the subsequent topological change at the leading edge as the result of viscous processes are demonstrated. Time-resolved velocity fields are measured using particle image velocimetry simultaneously in two fields of view to correlate local and global flow phenomena in order to identify unsteady boundary-layer separation and the subsequent flow structures. Finally, the Reynolds number is identified as a parameter that is responsible for the transition in mechanisms leading to LEV detachment from an aerofoil, as it determines the viscous response of the boundary layer in the vortex-wall interaction.
Reynolds number influence on the formation of vortical structures on a pitching flat plate

PubMed Central

Tropea, Cameron

2017-01-01

The impact of chord-based Reynolds number on the formation of leading-edge vortices (LEVs) on unsteady pitching flat plates is investigated. The influence of secondary flow structures on the shear layer feeding the LEV and the subsequent topological change at the leading edge as the result of viscous processes are demonstrated. Time-resolved velocity fields are measured using particle image velocimetry simultaneously in two fields of view to correlate local and global flow phenomena in order to identify unsteady boundary-layer separation and the subsequent flow structures. Finally, the Reynolds number is identified as a parameter that is responsible for the transition in mechanisms leading to LEV detachment from an aerofoil, as it determines the viscous response of the boundary layer in the vortex–wall interaction. PMID:28163871
A model for thin layer formation by delayed particle settling at sharp density gradients

NASA Astrophysics Data System (ADS)

Prairie, Jennifer C.; White, Brian L.

2017-02-01

Thin layers - regions where plankton or particles accumulate vertically on scales of a few meters or less - are common in coastal waters, and have important implications for both trophic dynamics and carbon cycling. These features can form by a variety of biological and physical mechanisms, including localized growth, shear-thinning, and directed swimming. An additional mechanism may result in the formation of thin layers of marine aggregates, which have been shown to decrease their settling velocity when passing through sharp density gradients, a behavior termed delayed settling. Here, we apply a simple vertical advection-diffusion model to predict the properties of aggregate thin layers formed by this process. We assume a constant vertical flux of particles from the surface, which is parameterized by observations from laboratory experiments with marine aggregates. The formation, maintenance, and shape of the layers are described in relation to non-dimensional numbers that depend on environmental conditions and particle settling properties. In particular, model results demonstrate layer intensity and sharpness both increase with higher Péclet number (Pe), that is, under conditions with weaker mixing relative to layer formation. Similarly, more intense and sharper layers are found when the delayed settling behavior of aggregates is characterized by a lower velocity minimum. The model also predicts layers that are vertically asymmetric and highly "peaky" when compared with a Gaussian distribution, features often seen in thin layers in natural environments. Lastly, by comparing model predictions with observations of thin layers in the field, we are able to gain some insight into the applicability of delayed settling as a thin layer formation mechanism in different environmental conditions.

Fault zone processes in mechanically layered mudrock and chalk

NASA Astrophysics Data System (ADS)

Ferrill, David A.; Evans, Mark A.; McGinnis, Ronald N.; Morris, Alan P.; Smart, Kevin J.; Wigginton, Sarah S.; Gulliver, Kirk D. H.; Lehrmann, Daniel; de Zoeten, Erich; Sickmann, Zach

2017-04-01

A 1.5 km long natural cliff outcrop of nearly horizontal Eagle Ford Formation in south Texas exposes northwest and southeast dipping normal faults with displacements of 0.01-7 m cutting mudrock, chalk, limestone, and volcanic ash. These faults provide analogs for both natural and hydraulically-induced deformation in the productive Eagle Ford Formation - a major unconventional oil and gas reservoir in south Texas, U.S.A. - and other mechanically layered hydrocarbon reservoirs. Fault dips are steep to vertical through chalk and limestone beds, and moderate through mudrock and clay-rich ash, resulting in refracted fault profiles. Steeply dipping fault segments contain rhombohedral calcite veins that cross the fault zone obliquely, parallel to shear segments in mudrock. The vertical dimensions of the calcite veins correspond to the thickness of offset competent beds with which they are contiguous, and the slip parallel dimension is proportional to fault displacement. Failure surface characteristics, including mixed tensile and shear segments, indicate hybrid failure in chalk and limestone, whereas shear failure predominates in mudrock and ash beds - these changes in failure mode contribute to variation in fault dip. Slip on the shear segments caused dilation of the steeper hybrid segments. Tabular sheets of calcite grew by repeated fault slip, dilation, and cementation. Fluid inclusion and stable isotope geochemistry analyses of fault zone cements indicate episodic reactivation at 1.4-4.2 km depths. The results of these analyses document a dramatic bed-scale lithologic control on fault zone architecture that is directly relevant to the development of porosity and permeability anisotropy along faults.
Including fluid shear viscosity in a structural acoustic finite element model using a scalar fluid representation

PubMed Central

Cheng, Lei; Li, Yizeng; Grosh, Karl

2013-01-01

An approximate boundary condition is developed in this paper to model fluid shear viscosity at boundaries of coupled fluid-structure system. The effect of shear viscosity is approximated by a correction term to the inviscid boundary condition, written in terms of second order in-plane derivatives of pressure. Both thin and thick viscous boundary layer approximations are formulated; the latter subsumes the former. These approximations are used to develop a variational formation, upon which a viscous finite element method (FEM) model is based, requiring only minor modifications to the boundary integral contributions of an existing inviscid FEM model. Since this FEM formulation has only one degree of freedom for pressure, it holds a great computational advantage over the conventional viscous FEM formulation which requires discretization of the full set of linearized Navier-Stokes equations. The results from thick viscous boundary layer approximation are found to be in good agreement with the prediction from a Navier-Stokes model. When applicable, thin viscous boundary layer approximation also gives accurate results with computational simplicity compared to the thick boundary layer formulation. Direct comparison of simulation results using the boundary layer approximations and a full, linearized Navier-Stokes model are made and used to evaluate the accuracy of the approximate technique. Guidelines are given for the parameter ranges over which the accurate application of the thick and thin boundary approximations can be used for a fluid-structure interaction problem. PMID:23729844
Including fluid shear viscosity in a structural acoustic finite element model using a scalar fluid representation.

PubMed

Cheng, Lei; Li, Yizeng; Grosh, Karl

2013-08-15

An approximate boundary condition is developed in this paper to model fluid shear viscosity at boundaries of coupled fluid-structure system. The effect of shear viscosity is approximated by a correction term to the inviscid boundary condition, written in terms of second order in-plane derivatives of pressure. Both thin and thick viscous boundary layer approximations are formulated; the latter subsumes the former. These approximations are used to develop a variational formation, upon which a viscous finite element method (FEM) model is based, requiring only minor modifications to the boundary integral contributions of an existing inviscid FEM model. Since this FEM formulation has only one degree of freedom for pressure, it holds a great computational advantage over the conventional viscous FEM formulation which requires discretization of the full set of linearized Navier-Stokes equations. The results from thick viscous boundary layer approximation are found to be in good agreement with the prediction from a Navier-Stokes model. When applicable, thin viscous boundary layer approximation also gives accurate results with computational simplicity compared to the thick boundary layer formulation. Direct comparison of simulation results using the boundary layer approximations and a full, linearized Navier-Stokes model are made and used to evaluate the accuracy of the approximate technique. Guidelines are given for the parameter ranges over which the accurate application of the thick and thin boundary approximations can be used for a fluid-structure interaction problem.
Microstructural Evolution of Ni-Sn Transient Liquid Phase Sintering Bond during High-Temperature Aging

NASA Astrophysics Data System (ADS)

Feng, Hongliang; Huang, Jihua; Peng, Xianwen; Lv, Zhiwei; Wang, Yue; Yang, Jian; Chen, Shuhai; Zhao, Xingke

2018-05-01

For high-temperature-resistant packaging of new generation power chip, a chip packaging simulation structure of Ni/Ni-Sn/Ni was bonded by a transient liquid-phase sintering process. High-temperature aging experiments were carried out to investigate joint heat stability. The microstructural evolution and mechanism during aging, and mechanical properties after aging were analyzed. The results show that the 30Ni-70Sn bonding layer as-bonded at 340°C for 240 min is mainly composed of Ni3Sn4 and residual Ni particles. When aged at 350°C, because of the difficulty of nucleation for Ni3Sn and quite slow growth of Ni3Sn2, the bonding layer is stable and the strength of that doesn't change obviously with aging time. When aging temperature increased to 500°C, however, the residual Ni particles were gradually dissolved and the bonding layer formed a stable structure with dominated Ni3Sn2 after 36 h. Meanwhile, due to the volume shrinkage (4.43%) from Ni3Sn2 formation, a number of voids were formed. The shear strength shows an increase, resulting from Ni3Sn2 formation, but then it decreases slightly caused by voids. After aging at 500°C for 100 h, shear strength is still maintained at 29.6 MPa. In addition, the mechanism of void formation was analyzed and microstructural evolution model was also established.
Investigation to determine the vulnerability of reclaimed land to building collapse using near surface geophysical method

NASA Astrophysics Data System (ADS)

Adewoyin, O. O.; Joshua, E. O.; Akinyemi, M. L.; Omeje, M.; Joel, E. S.

2017-05-01

Adequate knowledge of the geology and the structures of the subsurface would assist engineers in the best way to carry out constructions to avoid building collapse. In this study, near surface seismic refraction method was used to determine the geotechnical parameters of the subsurface, the results obtained were correlated with the result of borehole data drilled in the study area. The results of seismic refraction method delineated mostly two distinct layers with the first layer having the lower geotechnical parameters. It was observed that in the first layer, the Young’s modulus ranged from 0.168 to 0.458 GPa, shear modulus ranged between 0.068 and 0.185 GPa, the bulk modulus ranged between 0.106 and 0.287 GPa while the bearing capacity ranged from 0.083 to 0.139 MPa. On the other hand, in the second layer, the Young’s modulus ranged between 3.717 and 7.018 GPa, shear modulus ranged from 1.500 to 2.830 GPa while the bulk modulus ranged from 2.383 to 4.449 GPa. Significantly, the formation of the second layer appeared to be more competent than the first layer, therefore engineering construction in this geological setting is recommended to be founded on the second layer at depth ranging between 7 and 16 m.
Strain localisation in mechanically layered rocks beneath detachment zones: insights from numerical modelling

NASA Astrophysics Data System (ADS)

Le Pourhiet, L.; Huet, B.; Labrousse, L.; Yao, K.; Agard, P.; Jolivet, L.

2013-04-01

We have designed a series of fully dynamic numerical simulations aimed at assessing how the orientation of mechanical layering in rocks controls the orientation of shear bands and the depth of penetration of strain in the footwall of detachment zones. Two parametric studies are presented. In the first one, the influence of stratification orientation on the occurrence and mode of strain localisation is tested by varying initial dip of inherited layering in the footwall with regard to the orientation of simple shear applied at the rigid boundary simulating a rigid hanging wall, all scaling and rheological parameter kept constant. It appears that when Mohr-Coulomb plasticity is being used, shear bands are found to localise only when the layering is being stretched. This corresponds to early deformational stages for inital layering dipping in the same direction as the shear is applied, and to later stages for intial layering dipping towards the opposite direction of shear. In all the cases, localisation of the strain after only γ=1 requires plastic yielding to be activated in the strong layer. The second parametric study shows that results are length-scale independent and that orientation of shear bands is not sensitive to the viscosity contrast or the strain rate. However, decreasing or increasing strain rate is shown to reduce the capacity of the shear zone to localise strain. In the later case, the strain pattern resembles a mylonitic band but the rheology is shown to be effectively linear. Based on the results, a conceptual model for strain localisation under detachment faults is presented. In the early stages, strain localisation occurs at slow rates by viscous shear instabilities but as the layered media is exhumed, the temperature drops and the strong layers start yielding plastically, forming shear bands and localising strain at the top of the shear zone. Once strain localisation has occured, the deformation in the shear band becomes extremely penetrative but the strength cannot drop since the shear zone has a finite thickness.
The Sensitivity of Large-Eddy Simulation to Local and Nonlocal Drag Coefficients at the Lower Boundary

NASA Technical Reports Server (NTRS)

Schowalter, D. G.; DeCroix, D. S.; Lin, Y. L.; Arya, S. P.; Kaplan, M. L.

1996-01-01

It was found that the homogeneity of the surface drag coefficient plays an important role in the large scale structure of turbulence in large-eddy simulation of the convective atmospheric boundary layer. Particularly when a ground surface temperature was specified, large horizontal anisotropies occurred when the drag coefficient depended upon local velocities and heat fluxes. This was due to the formation of streamwise roll structures in the boundary layer. In reality, these structures have been found to form when shear is approximately balanced by buoyancy. The present cases, however, were highly convective. The formation was caused by particularly low values of the drag coefficient at the entrance to thermal plume structures.
Mechanism of the formation of hollow spherical granules using a high shear granulator.

PubMed

Asada, Takumi; Nishikawa, Mitsunori; Ochiai, Yasushi; Noguchi, Shuji; Kimura, Shin-Ichiro; Iwao, Yasunori; Itai, Shigeru

2018-05-30

Recently, we have developed a novel granulation technology to manufacture hollow spherical granules (HSGs) for controlled-release formulations; however, the mechanism of the granulation is still unclear. The aim of this study is to determine the mechanism of the formation of the HSGs using a high shear granulator. Samples of granulated material were collected at various times during granulation and were investigated using scanning electron microscope and X-ray computed tomography. It was observed that the granulation proceeded by drug layering to the polymer, followed by formation of a hollow in the granule. In addition, it was also found that generation of a crack in the adhered drug layer and air flow into the granules might be involved in forming the hollow in the structure. Observation of the granulation of formulations with different types of drugs and polymers indicated that negative pressure in the granules occurred and the granules caved in when the hollow was formed. The hollow-forming speed and the shell density of the hollow granules depended on the particular drug and polymer. Taken together, the granulation mechanism of HSGs was determined and this information will be valuable for HSGs technology development. Copyright © 2018 Elsevier B.V. All rights reserved.
Size effects on the martensitic phase transformation of NiTi nanograins

NASA Astrophysics Data System (ADS)

Waitz, T.; Antretter, T.; Fischer, F. D.; Simha, N. K.; Karnthaler, H. P.

2007-02-01

The analysis of nanocrystalline NiTi by transmission electron microscopy (TEM) shows that the martensitic transformation proceeds by the formation of atomic-scale twins. Grains of a size less than about 50 nm do not transform to martensite even upon large undercooling. A systematic investigation of these phenomena was carried out elucidating the influence of the grain size on the energy barrier of the transformation. Based on the experiment, nanograins were modeled as spherical inclusions containing (0 0 1) compound twinned martensite. Decomposition of the transformation strains of the inclusions into a shear eigenstrain and a normal eigenstrain facilitates the analytical calculation of shear and normal strain energies in dependence of grain size, twin layer width and elastic properties. Stresses were computed analytically for special cases, otherwise numerically. The shear stresses that alternate from twin layer to twin layer are concentrated at the grain boundaries causing a contribution to the strain energy scaling with the surface area of the inclusion, whereas the strain energy induced by the normal components of the transformation strain and the temperature dependent chemical free energy scale with the volume of the inclusion. In the nanograins these different energy contributions were calculated which allow to predict a critical grain size below which the martensitic transformation becomes unlikely. Finally, the experimental result of the atomic-scale twinning can be explained by analytical calculations that account for the transformation-opposing contributions of the shear strain and the twin boundary energy of the twin-banded morphology of martensitic nanograins.
Blunt body near wake flow field at Mach 6

NASA Technical Reports Server (NTRS)

Horvath, Thomas J.; McGinley, Catherine B.; Hannemann, Klaus

1996-01-01

Tests were conducted in a Mach 6 flow to examine the reattachment process of an axisymmetric free shear layer associated with the near wake of a 70 deg. half angle, spherically blunted cone with a cylindrical after body. Model angle of incidence was fixed at 0 deg. and free-stream Reynolds numbers based on body diameter ranged from 0.5 x 10(exp 6) to 4 x 10(exp 6). The sensitivity of wake shear layer transition on reattachment heating was investigated. The present perfect gas study was designed to compliment results obtained previously in facilities capable of producing real gas effects. The instrumented blunted cone model was designed primarily for testing in high enthalpy hypervelocity shock tunnels in both this country and abroad but was amenable for testing in conventional hypersonic blowdown wind tunnels as well. Surface heating rates were inferred from temperature - time histories from coaxial surface thermocouples on the model forebody and thin film resistance gages along the model base and cylindrical after body. General flow feature (bow shock, wake shear layer, and recompression shock) locations were visually identified by schlieren photography. Mean shear layer position and growth were determined from intrusive pitot pressure surveys. In addition, wake surveys with a constant temperature hot-wire anemometer were utilized to qualitatively characterize the state of the shear layer prior to reattachment. Experimental results were compared to laminar perfect gas predictions provided by a 3-D Navier Stokes code (NSHYP). Shear layer impingement on the instrumented cylindrical after body resulted in a localized heating maximum that was 21 to 29 percent of the forebody stagnation point heating. Peak heating resulting from the reattaching shear layer was found to be a factor of 2 higher than laminar predictions, which suggested a transitional shear layer. Schlieren flow visualization and fluctuating voltage time histories and spectra from the hot wire surveys across the shear layer substantiate this observation. The sensitivity of surface heating to forebody roughness was characterized for a reattaching shear layer. For example, at R(sub infinity), d = 4 x 10(exp 6), when the shear layer was transitional, the magnitude of peak heating from shear layer impingement was reduced by approximately 24 percent when transition grit was applied to the forebody. The spatial location of the local peak, however, remained unchanged.
Design, fabrication, and testing of an ultrasonic de-icing system for helicopter rotor blades

NASA Astrophysics Data System (ADS)

Palacios, Jose Luis

A low-power, non-thermal ultrasonic de-icing system is introduced as a possible substitute for current electro-thermal systems. The system generates delaminating ultrasonic transverse shear stresses at the interface of accreted ice. A PZT-4 disk driven at 28.5 KHz (radial resonance of the disk) instantaneously de-bonds 2 mm thick freezer ice layers. The ice layers are accreted to a 0.7 mm thick, 30.4 cm x 30.4 cm steel plate at an environment temperature of -20°C. A power input of 50 Watts is applied to the actuator (50 V, 19.6 KV/m), which translates to a de-icing power of 0.07 W/cm2. A finite element model of the actuator bonded to the isotropic plate is used to guide the design of the system, and predicts the transverse shear stresses at the ice interface. Wind tunnel icing tests were conducted to demonstrate the potential use of the proposed system under impact icing conditions. Both glaze ice and rime ice were generated on steel and composite plates by changing the cloud conditions of the wind tunnel. Continuous ultrasonic vibration prevented impact ice formation around the actuator location at an input power not exceeding 0.18 W/cm 2 (1.2 W/in2). As ice thickness reached a critical thickness of approximately 1.2 mm, shedding occurred on those locations where ultrasonic transverse shear stresses exceeded the shear adhesion strength of the ice. Finite element transverse shear stress predictions correlate with observed experimental impact ice de-bonding behavior. To increase the traveling distance of propagating ultrasonic waves, ultrasonic shear horizontal wave modes are studied. Wave modes providing large modal interface transverse shear stress concentration coefficients (ISCC) between the host structure (0.7 mm thick steel plate) and accreted ice (2.5 mm thick ice layer) are identified and investigated for a potential increase in the wave propagation distance. Ultrasonic actuators able to trigger these optimum wave modes are designed and fabricated. Despite exciting wave modes with high ISCC values, instantaneous ice de-bonding is not observed at input powers under 100 Watts. The two triggered ultrasonic wave modes of the structure occur at high excitation frequencies, 202 KHz and 500 KHz respectively. At these frequencies, the ultrasonic actuators do not provide large enough transverse shear stresses to exceed the shear adhesion strength of the ice layer. Neither the actuator exciting the SH1 mode (202 KHz), nor the actuator triggering the SH2 mode (500 KHz) instantaneously de-bonds ice layers with an input power under 100 Watts.
Shear strength of metal-sapphire contacts

NASA Technical Reports Server (NTRS)

Pepper, S. V.

1976-01-01

The shear strength of polycrystalline Ag, Cu, Ni, and Fe contacts on clean (0001) sapphire has been studied in ultrahigh vacuum. Both clean metal surfaces and surfaces exposed to O2, Cl2, and C2H4 were used. The results indicate that there are two sources of strength of Al2O3-metal contacts: an intrinsic one that depends on the particular clean metal in contact with Al2O3 and an additional one due to intermediate films. The shear strength of the clean metal contacts correlated directly with the free energy of oxide formation for the lowest metal oxide, in accord with the hypothesis that a chemical bond is formed between metal cations and oxygen anions in the sapphire surface. Contacts formed by metals exposed to chlorine exhibited uniformly low shear strength indicative of van der Waals bonding between chlorinated metal surfaces and sapphire. Contacts formed by metals exposed to oxygen exhibited enhanced shear strength, in accord with the hypothesis that an intermediate oxide layer increases interfacial strength.
Year-Long Vertical Velocity Statistics Derived from Doppler Lidar Data for the Continental Convective Boundary Layer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Berg, Larry K.; Newsom, Rob K.; Turner, David D.

One year of Coherent Doppler Lidar (CDL) data collected at the U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) site in Oklahoma is analyzed to provide profiles of vertical velocity variance, skewness, and kurtosis for cases of cloud-free convective boundary layers. The variance was scaled by the Deardorff convective velocity scale, which was successful when the boundary layer depth was stationary but failed in situations when the layer was changing rapidly. In this study the data are sorted according to time of day, season, wind direction, surface shear stress, degree of instability, and wind shear across the boundary-layer top. Themore » normalized variance was found to have its peak value near a normalized height of 0.25. The magnitude of the variance changes with season, shear stress, and degree of instability, but was not impacted by wind shear across the boundary-layer top. The skewness was largest in the top half of the boundary layer (with the exception of wintertime conditions). The skewness was found to be a function of the season, shear stress, wind shear across the boundary-layer top, with larger amounts of shear leading to smaller values. Like skewness, the vertical profile of kurtosis followed a consistent pattern, with peak values near the boundary-layer top (also with the exception of wintertime data). The altitude of the peak values of kurtosis was found to be lower when there was a large amount of wind shear at the boundary-layer top.« less
Measurements in the annular shear layer of high subsonic and under-expanded round jets

NASA Astrophysics Data System (ADS)

Feng, Tong; McGuirk, James J.

2016-01-01

An experimental study has been undertaken to document compressibility effects in the annular shear layers of axisymmetric jets. Comparison is made of the measured flow development with the well-documented influence of compressibility in planar mixing layers. High Reynolds number (~106) and high Mach number jets issuing from a convergent nozzle at nozzle pressure ratios (NPRs) from 1.28 to 3.0 were measured using laser Doppler anemometry instrumentation. Detailed radial profile data are reported, particularly within the potential core region, for mean velocity, turbulence rms, and turbulence shear stress. For supercritical NPRs the presence of the pressure waves in the inviscid shock cell region as the jet expanded back to ambient pressure was found to exert a noticeable effect on shear layer location, causing this to shift radially outwards at high supercritical NPR conditions. After a boundary layer to free shear layer transition zone, the turbulence development displayed a short region of similarity before adjustment to near-field merged jet behaviour. Peak turbulence rms reduction due to compressibility was similar to that observed in planar layers with radial rms suppression much stronger than axial. Comparison of the compressibility-modified annular shear layer growth rate with planar shear layer data on the basis of the convective Mach number ( M C) showed notable differences; in the annular shear layer, compressibility effects began at lower M C and displayed a stronger reduction in growth. For high Mach number aerospace propulsion applications involving round jets, the current measurements represent a new data set for the calibration/validation of compressibility-affected turbulence models.
Study of the Relation between the Resonance Behavior of Thickness Shear Mode (TSM) Sensors and the Mechanical Characteristics of Biofilms.

PubMed

Castro, Pedro; Elvira, Luis; Maestre, Juan Ramón; Montero de Espinosa, Francisco

2017-06-15

This work analyzes some key aspects of the behavior of sensors based on piezoelectric Thickness Shear Mode (TSM) resonators to study and monitor microbial biofilms. The operation of these sensors is based on the analysis of their resonance properties (both resonance frequency and dissipation factor) that vary in contact with the analyzed sample. This work shows that different variations during the microorganism growth can be detected by the sensors and highlights which of these changes are indicative of biofilm formation. TSM sensors have been used to monitor in real time the development of Staphylococcus epidermidis and Escherichia coli biofilms, formed on the gold electrode of the quartz crystal resonators, without any coating. Strains with different ability to produce biofilm have been tested. It was shown that, once a first homogeneous adhesion of bacteria was produced on the substrate, the biofilm can be considered as a semi-infinite layer and the quartz sensor reflects only the viscoelastic properties of the region immediately adjacent to the resonator, not being sensitive to upper layers of the biofilm. The experiments allow the microrheological evaluation of the complex shear modulus ( G * = G ' + jG ″) of the biofilm at 5 MHz and at 15 MHz, showing that the characteristic parameter that indicates the adhesion of a biofilm for the case of S. epidermidis and E. coli , is an increase in the resonance frequency shift of the quartz crystal sensor, which is connected with an increase of the real shear modulus, related to the elasticity or stiffness of the layer. In addition both the real and the imaginary shear modulus are frequency dependent at these high frequencies in biofilms.
Finite element simulation of thickness changes in laminate during thermoforming

NASA Astrophysics Data System (ADS)

White, K. D.; Sherwood, J. A.

2017-10-01

This paper discusses a numerical investigation of thickness changes of Dyneema HB80, a cross-ply thermoplastic lamina, during a helmet thermoforming process. The main mode of deformation during the preform phase of manufacture is in-plane shearing of the fabric. A laminate undergoes varying degrees of shear to conform to the geometric variations over the surface of the preform shape. Decreases in areal coverage that occur with increases in the local shear angle will lead to a resulting increase in local thickness. During the consolidation phase, multiple preform layers are compressed in a set of matched tools, and the compounding of the thickness variations can adversely affect the uniformity of pressure distribution between matched die tooling. Pressure variations over the surface of the part can lead to incomplete consolidation of the ply stack, as well as weakened, resin-rich areas. Because wrinkling of the composite reinforcement, incomplete consolidation and resin-rich areas can result in a compromised structural performance, it is important that the manufacturing process be well understood so it can be designed to mitigate formation of such defects. In the current work, the material properties derived from shear, bending and tensile tests are implemented in a finite element model of the cross-ply lamina. The finite element model uses a hybrid discrete mesoscopic approach, and deep-draw forming of the material is simulated to investigate its formability to a hemispherical geometry. Thickening of the lamina resulting from shear deformation is investigated and incorporated into models single-layer preform simulations. The simulation results are used to inform the design of multiple-layer preforms to mitigate the development of thin regions and out-of-plane waves to ensure complete, uniform consolidation.
Evolution of fault zones in carbonates with mechanical stratigraphy - Insights from scale models using layered cohesive powder

NASA Astrophysics Data System (ADS)

van Gent, Heijn W.; Holland, Marc; Urai, Janos L.; Loosveld, Ramon

2010-09-01

We present analogue models of the formation of dilatant normal faults and fractures in carbonate fault zones, using cohesive hemihydrate powder (CaSO 4·½H 2O). The evolution of these dilatant fault zones involves a range of processes such as fragmentation, gravity-driven breccia transport and the formation of dilatant jogs. To allow scaling to natural prototypes, extensive material characterisation was done. This showed that tensile strength and cohesion depend on the state of compaction, whereas the friction angle remains approximately constant. In our models, tensile strength of the hemihydrate increases with depth from 9 to 50 Pa, while cohesion increases from 40 to 250 Pa. We studied homogeneous and layered material sequences, using sand as a relatively weak layer and hemihydrate/graphite mixtures as a slightly stronger layer. Deformation was analyzed by time-lapse photography and Particle Image Velocimetry (PIV) to calculate the evolution of the displacement field. With PIV the initial, predominantly elastic deformation and progressive localization of deformation are observed in detail. We observed near-vertical opening-mode fractures near the surface. With increasing depth, dilational shear faults were dominant, with releasing jogs forming at fault-dip variations. A transition to non-dilatant shear faults was observed near the bottom of the model. In models with mechanical stratigraphy, fault zones are more complex. The inferred stress states and strengths in different parts of the model agree with the observed transitions in the mode of deformation.
Anomalous shear band characteristics and extra-deep shock-affected zone in Zr-based bulk metallic glass treated with nanosecond laser peening.

PubMed

Wei, Yanpeng; Xu, Guangyue; Zhang, Kun; Yang, Zhe; Guo, Yacong; Huang, Chenguang; Wei, Bingchen

2017-03-07

The effects of nanosecond laser peening on Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 metallic glass were investigated in this study. The peening treatment produced an extra-deep shock-affected zone compared to crystal metal. As opposed to the conventional shear bands, numerous arc shear bands appeared and aggregated in the vertical direction of the laser beam, forming basic units for accommodating plastic deformation. The arc shear bands exhibited short and discrete features near the surface of the material, then grew longer and fewer at deeper peened layer depths, which was closely related to the laser shock wave attenuation. An energy dissipation model was established based on Hugoniot Elastic Limit and shear band characteristics to represent the formation of an extra-deep shock-affected zone. The results presented here suggest that the bulk modification of metallic glass with a considerable affected depth is feasible. Further, they reveal that nanosecond laser peening is promising as an effective approach to tuning shear bands for improved MGs ductility.
Observations of an Intermediate Layer During the Coqui II Campaign

NASA Technical Reports Server (NTRS)

Bishop, R. L.; Earle, G. D.; Herrero, F. A.; Bateman, T. T.

2000-01-01

NASA sounding rocket 21.114, launched March 7, 1998, during the Coqui II campaign, provided neutral wind and plasma density measurements of a weak intermediate layer. The layer was centered near 140 km and had an approximate peak plasma density of 2200 cc. The measured winds were typically less than 40 m/s, in agreement with wind shear formation theory and coincident density observations. The data obtained during the flight allow us to explore the plasma density structure and wind field morphology of the intermediate layer. Coupled with simultaneous data from Arecibo Observatory, the upleg and downleg density profiles provide three spatially separated measurements that enable the first detailed investigation of the horizontal extent and variation of an intermediate layer.
Competitions between Rayleigh-Taylor instability and Kelvin-Helmholtz instability with continuous density and velocity profiles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ye, W. H.; He, X. T.; CAPT, Peking University, Beijing 100871

2011-02-15

In this research, competitions between Rayleigh-Taylor instability (RTI) and Kelvin-Helmholtz instability (KHI) in two-dimensional incompressible fluids within a linear growth regime are investigated analytically. Normalized linear growth rate formulas for both the RTI, suitable for arbitrary density ratio with continuous density profile, and the KHI, suitable for arbitrary density ratio with continuous density and velocity profiles, are obtained. The linear growth rates of pure RTI ({gamma}{sub RT}), pure KHI ({gamma}{sub KH}), and combined RTI and KHI ({gamma}{sub total}) are investigated, respectively. In the pure RTI, it is found that the effect of the finite thickness of the density transition layermore » (L{sub {rho}}) reduces the linear growth of the RTI (stabilizes the RTI). In the pure KHI, it is found that conversely, the effect of the finite thickness of the density transition layer increases the linear growth of the KHI (destabilizes the KHI). It is found that the effect of the finite thickness of the density transition layer decreases the ''effective'' or ''local'' Atwood number (A) for both the RTI and the KHI. However, based on the properties of {gamma}{sub RT}{proportional_to}{radical}(A) and {gamma}{sub KH}{proportional_to}{radical}(1-A{sup 2}), the effect of the finite thickness of the density transition layer therefore has a completely opposite role on the RTI and the KHI noted above. In addition, it is found that the effect of the finite thickness of the velocity shear layer (L{sub u}) stabilizes the KHI, and for the most cases, the combined effects of the finite thickness of the density transition layer and the velocity shear layer (L{sub {rho}=}L{sub u}) also stabilize the KHI. Regarding the combined RTI and KHI, it is found that there is a competition between the RTI and the KHI because of the completely opposite effect of the finite thickness of the density transition layer on these two kinds of instability. It is found that the competitions between the RTI and the KHI depend, respectively, on the Froude number, the density ratio of the light fluid to the heavy one, and the finite thicknesses of the density transition layer and the velocity shear layer. Furthermore, for the fixed Froude number, the linear growth rate ratio of the RTI to the KHI decreases with both the density ratio and the finite thickness of the density transition layer, but increases with the finite thickness of the velocity shear layer and the combined finite thicknesses of the density transition layer and the velocity shear layer (L{sub {rho}=}L{sub u}). In summary, our analytical results show that the effect of the finite thickness of the density transition layer stabilizes the RTI and the overall combined effects of the finite thickness of the density transition layer and the velocity shear layer (L{sub {rho}=}L{sub u}) also stabilize the KHI. Thus, it should be included in applications where the transition layer effect plays an important role, such as the formation of large-scale structures (jets) in high energy density physics and astrophysics and turbulent mixing.« less

Origin of Shear Stability and Compressive Ductility Enhancement of Metallic Glasses by Metal Coating

PubMed Central

Sun, B. A.; Chen, S. H.; Lu, Y. M.; Zhu, Z. G.; Zhao, Y. L.; Yang, Y.; Chan, K. C.; Liu, C. T.

2016-01-01

Metallic glasses (MGs) are notorious for the poor macroscopic ductility and to overcome the weakness various intrinsic and extrinsic strategies have been proposed in past decades. Among them, the metal coating is regarded as a flexible and facile approach, yet the physical origin is poorly understood due to the complex nature of shear banding process. Here, we studied the origin of ductile enhancement in the Cu-coating both experimentally and theoretically. By examining serrated shear events and their stability of MGs, we revealed that the thin coating layer plays a key role in stopping the final catastrophic failure of MGs by slowing down shear band dynamics and thus retarding its attainment to a critical instable state. The mechanical analysis on interplay between the coating layer and shear banding process showed the enhanced shear stability mainly comes from the lateral tension of coating layer induced by the surface shear step and the bonding between the coating layer and MGs rather than the layer thickness is found to play a key role in contributing to the shear stability. PMID:27271435
A critical review of the experimental data for developed free turbulent shear layers

NASA Technical Reports Server (NTRS)

Birch, S. F.; Eggers, J. M.

1973-01-01

Experimental shear layer data are reviewed and the results are compared to numerical predictions for three test cases. It was concluded from the study that many, if not most, of the apparent inconsistencies which exist in the interpretation of the experimental data for free shear layers result from confusing data taken in developed turbulent flows with those taken in transitional or developing flows. Other conclusions drawn from the study include the following: (1) The effects of Mach number are more uncertain primarily because of limited data and the absence of any turbulence measurements for supersonic shear layers. (2) The data available for heterogeneous shear layers are not sufficient to clearly establish the effect of density ratio on mixing rate.
Strengthening Effect of Incremental Shear Deformation on Ti Alloy Clad Plate with a Ni-Based Alloy Laser-Clad Layer

NASA Astrophysics Data System (ADS)

Zhao, W.; Zha, G. C.; Kong, F. X.; Wu, M. L.; Feng, X.; Gao, S. Y.

2017-05-01

A Ti-6Al-4V alloy clad plate with a Tribaloy 700 alloy laser-clad layer is subjected to incremental shear deformation, and we evaluate the structural evolution and mechanical properties of the specimens. Results indicate the significance of the incremental shear deformation on the strengthening effect. The wear resistance and Vickers hardness of the laser-clad layer are enhanced due to increased dislocation density. The incremental shear deformation can increase the bonding strength of the laser-clad layer and the corresponding substrate and can break the columnar crystals in the laser-clad layer near the interface. These phenomena suggest that shear deformation eliminates the defects on the interface of the laser-clad layer and the substrate. Substrate hardness is evidently improved, and the strengthening effect is caused by the increased dislocation density and shear deformation. This deformation can then transform the α- and β-phases in the substrate into a high-intensity ω-phase.
Stability diagram for dense suspensions of model colloidal Al2O3 particles in shear flow.

PubMed

Hecht, Martin; Harting, Jens; Herrmann, Hans J

2007-05-01

In Al2O3 suspensions, depending on the experimental conditions, very different microstructures can be found, comprising fluidlike suspensions, a repulsive structure, and a clustered microstructure. For technical processing in ceramics, the knowledge of the microstructure is of importance, since it essentially determines the stability of a workpiece to be produced. To enlighten this topic, we investigate these suspensions under shear by means of simulations. We observe cluster formation on two different length scales: the distance of nearest neighbors and on the length scale of the system size. We find that the clustering behavior does not depend on the length scale of observation. If interparticle interactions are not attractive the particles form layers in the shear flow. The results are summarized in a stability diagram.
The effects of forcing on a single stream shear layer and its parent boundary layer

NASA Technical Reports Server (NTRS)

Haw, Richard C.; Foss, John F.

1990-01-01

Forcing and its effect on fluid flows has become an accepted tool in the study and control of flow systems. It has been used both as a diagnostic tool, to explore the development and interaction of coherent structures, and as a method of controlling the behavior of the flow. A number of forcing methods have been used in order to provide a perturbation to the flow; among these are the use of an oscillating trailing edge, acoustically driven slots, external acoustic forcing, and mechanical piston methods. The effect of a planar mechanical piston forcing on a single stream shear layer is presented; it can be noted that this is one of the lesser studied free shear layers. The single stream shear layer can be characterized by its primary flow velocity scale and the thickness of the separating boundary layer. The velocity scale is constant over the length of the flow field; theta (x) can be used as a width scale to characterize the unforced shear layer. In the case of the forced shear layer the velocity field is a function of phase time and definition of a width measure becomes somewhat problematic.
Shear-layer structures in near-wall turbulence

NASA Technical Reports Server (NTRS)

Johansson, A. V.; Alfredsson, P. H.; Kim, J.

1987-01-01

The structure of internal shear layer observed in the near-wall region of turbulent flows is investigated by analyzing flow fields obtained from numerical simulations of channel and boundary-layer flows. It is found that the shear layer is an important contributor to the turbulence production. The conditionally averaged production at the center of the structure was almost twice as large as the long-time mean value. The shear-layer structure is also found to retain its coherence over streamwise distances on the order of a thousand viscous length units, and propagates with a constant velocity of about 10.6 u sub rho throughout the near wall region.
Forced free-shear layer measurements

NASA Technical Reports Server (NTRS)

Leboeuf, Richard L.

1994-01-01

Detailed three-dimensional three-component phase averaged measurements of the spanwise and streamwise vorticity formation and evolution in acoustically forced plane free-shear flows have been obtained. For the first time, phase-averaged measurements of all three velocity components have been obtained in both a mixing layer and a wake on three-dimensional grids, yielding the spanwise and streamwise vorticity distributions without invoking Taylor's hypothesis. Initially, two-frequency forcing was used to phase-lock the roll-up and first pairing of the spanwise vortical structures in a plane mixing layer. The objective of this study was to measure the near-field vortical structure morphology in a mixing layer with 'natural' laminar initial boundary layers. For the second experiment the second and third subharmonics of the fundamental roll-up frequency were added to the previous two-frequency forcing in order to phase-lock the roll-up and first three pairings of the spanwise rollers in the mixing layer. The objective of this study was to determine the details of spanwise scale changes observed in previous time-averaged measurements and flow visualization of unforced mixing layers. For the final experiment, single-frequency forcing was used to phase-lock the Karman vortex street in a plane wake developing from nominally two-dimensional laminar initial boundary layers. The objective of this study was to compare measurements of the three-dimensional structure in a wake developing from 'natural' initial boundary layers to existing models of wake vortical structure.
Origin of leucite-rich and sanidine-rich flow layers in the Leucite Hills Volcanic Field, Wyoming

NASA Astrophysics Data System (ADS)

Gunter, W. D.; Hoinkes, Georg; Ogden, Palmer; Pajari, G. E.

1990-09-01

Two types of orendite (sanidine-phlogopite lamproite) and wyomingite (leucite-phlogopite lamproite) intraflow layering are present in the ultrapotassic Leucite Hills Volcanic Field, Wyoming. In large-scale layering, wyomingites are confined to the base of the flow, while in centimeter-scale layering, orendite and wyomingite alternate throughout the flow. The mineralogy of the orendites and wyomingites are the same; only the relative amount of each mineral vary substantially. The chemical compositions of adjacent layers of wyomingite and orendite are almost identical except for water. The centimeter-scale flow layering probably represents fossil streamlines of the lava and therefore defines the path of circulation of the viscous melt. Toward the front of the flow, the layers are commonly folded. Structures present which are indicative that the flows may have possessed a yield strength are limb shears, boudinage, and slumping. Phlogopite phenocrysts are poorly aligned in the orendite layers, while they are often in subparallel alignment in the wyomingite layers; and they are used as a measure of shearing intensity during emplacement of the flow. Vesicle volumes are concentrated in the orendite layers. In the large-scale layering, a discontinuous base rubble zone of autobreccia is overlain by a thin platy zone followed by a massive zone which composes more than the upper 75% of the flow. Consequently, we feel that the origin of the layering may be related to shearing. Two extremes in the geometry of shearing are proposed: closely spaced, thin, densely sheared layers separated by discrete intervals throughout a lava flow as in the centimeter-scale layering and classical plug flow where all the shearing is confined to the base as in the large-scale layering. A mechanism is proposed which causes thixotropic behavior and localizes shearing: the driving force is the breakdown of molecular water to form T-OH bonds which establishes a chemical potential gradient for water in the melt. The higher activity of water in the nonsheared regions allows sandine to crystallize, whereas the lower activity of water in the areas of active shearing causes leucite to crystallize.
Near-body vorticity dynamics of a square cylinder subjected to an inline pulsatile free stream flow

NASA Astrophysics Data System (ADS)

Krishnan, Hrisheekesh; Agrawal, Amit; Sharma, Atul; Sheridan, John

2016-09-01

In the present work, the effect of an inflow sinusoidal excitation that is superimposed over the mean flow on the vortex-shedding characteristics of a square cylinder is studied. The frequency of pulsation is varied around the natural vortex-shedding frequency, and the amplitude of pulsation is varied moderately in comparison to the cylinder diameter, at a fixed Reynolds number (=100). A flow regime map is prepared and compared with the experimental results, which are available for a circular cylinder that is subjected to inline excitation. We correlate the spectra to the corresponding flow regime. Visualization of the vorticity contours reveals that the significant interaction of the base-region vorticities with the main shear layer vorticities is important in the mechanism of formation of the several vortex-shedding modes. The strength and sign of base region vorticity with respect to the shear layers has a fundamental role to play in the mechanism of formation. It is hypothesized that the similarity in vortex-shedding modes across different excitation types, bluff body geometry, and for different parameters is due to the similarity in the underlying vorticity dynamics.
Near-field Development of a Turbulent Mixing Layer Periodically Forced by a Bimorph PVDF Film Actuator

NASA Astrophysics Data System (ADS)

Naka, Yoshitsugu; Tsuboi, Ken-Ichiro; Kametani, Yukinori; Fukagata, Koji; Obi, Shinnosuke

We have performed experiments in a turbulent mixing layer with periodic forcing introduced by a Piezo Film Actuator (PFA). Three different lengths of PFAs have been used, and the effects of various combinations of forcing amplitudes and frequencies are investigated. The forcing at the first and second sub-harmonic frequencies against the natural frequency enhances the development of the thickness of the mixing layer: the mixing layer spreads due to the forcing. On the other hand, the forcing near the natural frequency suppresses the development: the mean velocity gradient becomes steeper than the no control case. The vector pattern of the periodic velocity components indicated the formation of the vortical structure. By forcing at the natural and its first sub-harmonic frequencies, two counter-rotating vortices are clearly observed in one period of forcing. By forcing at second sub-harmonic frequency, the vortical structure is found only in the downstream region. The distribution of the periodic Reynolds shear stress significantly varies with the forcing frequency and it takes a positive value when forcing occurs near the natural frequency. However, the total value of the Reynolds shear stress remains negative due to the contribution of the turbulent components.
Compressibility effects in the shear layer over a rectangular cavity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Beresh, Steven J.; Wagner, Justin L.; Casper, Katya M.

2016-10-26

we studied the influence of compressibility on the shear layer over a rectangular cavity of variable width in a free stream Mach number range of 0.6–2.5 using particle image velocimetry data in the streamwise centre plane. As the Mach number increases, the vertical component of the turbulence intensity diminishes modestly in the widest cavity, but the two narrower cavities show a more substantial drop in all three components as well as the turbulent shear stress. Furthermore, this contrasts with canonical free shear layers, which show significant reductions in only the vertical component and the turbulent shear stress due to compressibility.more » The vorticity thickness of the cavity shear layer grows rapidly as it initially develops, then transitions to a slower growth rate once its instability saturates. When normalized by their estimated incompressible values, the growth rates prior to saturation display the classic compressibility effect of suppression as the convective Mach number rises, in excellent agreement with comparable free shear layer data. The specific trend of the reduction in growth rate due to compressibility is modified by the cavity width.« less
Vertical suspended sediment fluxes observed from a formation of underwater gliders

NASA Astrophysics Data System (ADS)

Merckelbach, Lucas; Riethmueller, Rolf

2014-05-01

In order to understand and predict the pathways and deposition of fine sediments in coastal regions valid parameterisations of the fluxes across interfaces (sea bed - water column or a pycnocline) are paramount. Traditionally, these parameterisations are based on the concept of a critical shear stress, but more recently a probabilistic approach has been proposed, in which the resuspension of sediment is assumed to have a certain likelihood, depending on the external forcing. Both approaches find their justification, to some extent, from the results of laboratory experiments, however, in-situ data, essential for model validation, are scarce. In this study we develop a field method to estimate the (fine) sediment fluxes between the seabed and the water column, and across the pycnocline. The method is applied to a stratified shallow sea (the North Sea in Summer). In order to assess the results, these fluxes are interpreted in terms of bottom shear stress and current shear between upper and lower layer, respectively. The method was tested in an experiment with two underwater gliders in Summer 2013 in the German Bight. Both gliders were equipped with optical backscatter sensors, the measurements of which serve as a proxy for suspended sediment concentration. The profiling character of the gliders allows to calculate the rate of change of the layer-averaged sediment concentration, as observed by the platform. The local, Lagrangian rate of change of sediment concentration is the balance between the fluxes across the layer's interfaces. Due to a horizontal speed of the glider of about 0.5 m/s, horizontal gradients in sediment concentration cause the observed and the local rate of change of sediment concentration to be significantly different. The novelty of this experiment was that the two gliders were flown in a rigid formation, where one glider trailed the other at a more or less constant distance of 5 km, controlled by an algorithm. This allowed the local rate of change to be quantified - and therefore the net fluxes across the interfaces - by accounting for the effects of horizontal gradients. The validity of this method was assessed by comparing the settling and resuspension/entrainment fluxes with physical drivers: current shear near the pycnocline and bottom shear, with wind effects and tidal motion as proxies, respectively.
An experimental study of combustion: The turbulent structure of a reacting shear layer formed at a rearward-facing step. Ph.D. Thesis. Final Report

NASA Technical Reports Server (NTRS)

Pitz, R. W.

1981-01-01

A premixed propane-air flame is stabilized in a turbulent free shear layer formed at a rearward-facing step. The mean and rms averages of the turbulent velocity flow field were determined by LDV for both reacting and non-reacting flows. The reaching flow was visualized by high speed schlieren photography. Large scale structures dominate the reacting shear layer. The growth of the large scale structures is tied to the propagation of the flame. The linear growth rate of the reacting shear layer defined by the mean velocity profiles is unchanged by combustion but the virtual origin is shifted downstream. The reacting shear layer based on the mean velocity profiles is shifted toward the recirculation zone and the reattachments lengths are shortened by 30%.
Prediction of turbulent shear layers in turbomachines

NASA Technical Reports Server (NTRS)

Bradshaw, P.

1974-01-01

The characteristics of turbulent shear layers in turbomachines are compared with the turbulent boundary layers on airfoils. Seven different aspects are examined. The limits of boundary layer theory are investigated. Boundary layer prediction methods are applied to analysis of the flow in turbomachines.
Dynamics of the Sediment Plume Over the Yangtze Bank in the Yellow and East China Seas

NASA Astrophysics Data System (ADS)

Luo, Zhifa; Zhu, Jianrong; Wu, Hui; Li, Xiangyu

2017-12-01

A distinct sediment plume exists over the Yangtze Bank in the Yellow and East China Seas (YECS) in winter, but it disappears in summer. Based on satellite color images, there are two controversial viewpoints about the formation mechanism for the sediment plume. One viewpoint is that the sediment plume forms because of cross-shelf sediment advection of highly turbid water along the Jiangsu coast. The other viewpoint is that the formation is caused by local bottom sediment resuspension and diffused to the surface layer through vertical turbulent mixing. The dynamic mechanism of the sediment plume formation has been unclear until now. This issue was explored by using a numerical sediment model in the present paper. Observed wave, current, and sediment data from 29 December 2016 to 16 January 2017 were collected near the Jiangsu coast and used to validate the model. The results indicated that the model can reproduce the hydrodynamic and sediment processes. Numerical experiments showed that the bottom sediment could be suspended by the bottom shear stress and diffuse to the surface layer by vertical mixing in winter; however, the upward diffusion is restricted by the strong stratification in summer. The sediment plume is generated locally due to bottom sediment resuspension primarily via tide-induced bottom shear stress rather than by cross-shelf sediment advection over the Yangtze Bank.
Turbulence measurement in a reacting and non-reacting shear layer at a high subsonic Mach number

NASA Technical Reports Server (NTRS)

Chang, C. T.; Marek, C. J.; Wey, C.; Jones, R. A.; Smith, M. J.

1993-01-01

The results of two component velocity and turbulence measurements are presented which were obtained on a planar reacting shear layer burning hydrogen. Quantitative LDV and temperature measurements are presented with and without chemical reaction within the shear layer at a velocity ratio of 0.34 and a high speed Mach number of 0.7. The comparison showed that the reacting shear layer grew faster than that without reaction. Using a reduced width coordinate, the reacting and non-reacting profiles were very similar. The peak turbulence for both cases was 20 percent.
Coupling with ocean mixed layer leads to intraseasonal variability in tropical deep convection: Evidence from cloud-resolving simulations

NASA Astrophysics Data System (ADS)

Anber, Usama; Wang, Shuguang; Sobel, Adam

2017-03-01

The effect of coupling a slab ocean mixed layer to atmospheric convection is examined in cloud-resolving model (CRM) simulations in vertically sheared and unsheared environments without Coriolis force, with the large-scale circulation parameterized using the Weak Temperature Gradient (WTG) approximation. Surface fluxes of heat and moisture as well as radiative fluxes are fully interactive, and the vertical profile of domain-averaged horizontal wind is strongly relaxed toward specified profiles with vertical shear that varies from one simulation to the next. Vertical wind shear is found to play a critical role in the simulated behavior. There exists a threshold value of the shear strength above which the coupled system develops regular oscillations between deep convection and dry nonprecipitating states, similar to those found earlier in a much more idealized model which did not consider wind shear. The threshold value of the vertical shear found here varies with the depth of the ocean mixed layer. The time scale of the spontaneously generated oscillations also varies with mixed layer depth, from 10 days with a 1 m deep mixed layer to 50 days with a 10 m deep mixed layer. The results suggest the importance of the interplay between convection organized by vertical wind shear, radiative feedbacks, large-scale dynamics, and ocean mixed layer heat storage in real intraseasonal oscillations.
Characterization of Unsteady Flow Structures Near Leading-Edge Slat. Part 1; PIV Measurements

NASA Technical Reports Server (NTRS)

Jenkins, Luther N.; Khorrami, Mehdi R.; Choudhari, Meelan

2004-01-01

A comprehensive computational and experimental study has been performed at the NASA Langley Research Center as part of the Quiet Aircraft Technology (QAT) Program to investigate the unsteady flow near a leading-edge slat of a two-dimensional, high-lift system. This paper focuses on the experimental effort conducted in the NASA Langley Basic Aerodynamics Research Tunnel (BART) where Particle Image Velocimetry (PIV) data was acquired in the slat cove and at the slat trailing edge of a three-element, high-lift model at 4, 6, and 8 degrees angle of attack and a freestream Mach Number of 0.17. Instantaneous velocities obtained from PIV images are used to obtain mean and fluctuating components of velocity and vorticity. The data show the recirculation in the cove, reattachment of the shear layer on the slat lower surface, and discrete vortical structures within the shear layer emanating from the slat cusp and slat trailing edge. Detailed measurements are used to examine the shear layer formation at the slat cusp, vortex shedding at the slat trailing edge, and convection of vortical structures through the slat gap. Selected results are discussed and compared with unsteady, Reynolds-Averaged Navier-Stokes (URANS) computations for the same configuration in a companion paper by Khorrami, Choudhari, and Jenkins (2004). The experimental dataset provides essential flow-field information for the validation of near-field inputs to noise prediction tools.
Fiber optic plantar pressure/shear sensor

NASA Astrophysics Data System (ADS)

Soetanto, William; Nguyen, Ngoc T.; Wang, Wei-Chih

2011-04-01

A full-scale foot pressure/shear sensor that has been developed to help diagnose the cause of ulcer formation in diabetic patients is presented. The design involves a tactile sensor array using intersecting optical fibers embedded in soft elastomer. The basic configuration incorporates a mesh that is comprised of two sets of parallel optical fiber plane; the planes are configured so the parallel rows of fiber of the top and bottom planes are perpendicular to each other. Threedimensional information is determined by measuring the loss of light from each of the waveguide to map the overall pressure distribution and the shifting of the layers relative to each other. In this paper we will present the latest development on the fiber optic plantar pressure/shear sensor which can measure normal force up from 19.09 kPa to 1000 kPa.
Fundamental Processes of Atomization in Fluid-Fluid Flows

NASA Technical Reports Server (NTRS)

McCready, M. J.; Chang, H.-C.; Leighton, D. T.

2001-01-01

This report outlines the major results of the grant "Fundamental Processes of Atomization in Fluid-Fluid Flows." These include: 1) the demonstration that atomization in liquid/liquid shear flow is driven by a viscous shear instability that triggers the formation of a long thin sheet; 2) discovery of a new mode of interfacial instability for oscillatory two-layer systems whereby a mode that originates within the less viscous liquid phase causes interfacial deformation as the oscillation proceeds; 3) the demonstration that rivulet formation from gravity front occurs because the local front shape specified by gravity and surface tension changes from a nose to a wedge geometry, thus triggering a large increase in viscous resistance; and 4) extension of the studies on nonlinear wave evolution on falling films and in stratified flow, particularly the evolution towards large-amplitude solitary waves that tend to generate drops.

Micro-mechanisms of Surface Defects Induced on Aluminum Alloys during Plastic Deformation at Elevated Temperatures

NASA Astrophysics Data System (ADS)

Gali, Olufisayo A.

Near-surface deformed layers developed on aluminum alloys significantly influence the corrosion and tribological behavior as well as reduce the surface quality of the rolled aluminum. The evolution of the near-surface microstructures induced on magnesium containing aluminum alloys during thermomechanical processing has been investigated with the aim generating an understanding of the influence of individual forming parameters on its evolution and examine the microstructure of the roll coating induced on the mating steel roll through material transfer during rolling. The micro-mechanisms related to the various features of near-surface microstructure developed during tribological conditions of the simulated hot rolling process were identified. Thermomechanical processing experiments were performed with the aid of hot rolling (operating temperature: 550 to 460 °C, 4, 10 and 20 rolling pass schedules) and hot forming (operating temperature: 350 to 545 °C, strain rate: 4 x 10-2 s-1) tribo-simulators. The surface, near-surface features and material transfer induced during the elevated temperature plastic deformation were examined and characterized employing optical interferometry, SEM/EDS, FIB and TEM. Near-surface features characterized on the rolled aluminum alloys included; cracks, fractured intermetallic particles, aluminum nano-particles, oxide decorated grain boundaries, rolled-in oxides, shingles and blisters. These features were related to various individual rolling parameters which included, the work roll roughness, which induced the formation of shingles, rolling marks and were responsible for the redistribution of surface oxide and the enhancements of the depth of the near-surface damage. The enhanced stresses and strains experienced during rolling were related to the formation and propagation of cracks, the nanocrystalline structure of the near-surface layers and aluminum nano-particles. The mechanism of the evolution of the near-surface microstructure were determined to include grain boundary sliding which induced the cracks at the surface and subsurface of the alloy, magnesium diffusion to free surfaces, crack propagation from shear stresses and the shear strains inducing the nanocrystalline grain structure, the formation of shingles by the shear deformation of micro-wedges induced by the work roll grooves, and the deformation of this oxide covered micro-wedges inducing the rolled-in oxides. Magnesium diffusion to free surfaces was identified as inducing crack healing due to the formation of MgO within cracks and was responsible for the oxide decorated grain boundaries. An examination of the roll coating revealed a complex layered microstructure that was induced through tribo-chemical and mechanical entrapment mechanisms. The microstructure of the roll coating suggested that the work roll material and the rolled aluminum alloy were essential in determining its composition and structure. Subsequent hot forming processes revealed the rich oxide-layer of the near-surface microstructure was beneficial for reducing the coefficient of friction during tribological contact with the steel die. Damage to the microstructure include cracks induced from grain boundary sliding of near-surface grains and the formation of oxide fibres within cracks of the near-surface deformed layers.
New concepts for Reynolds stress transport equation modeling of inhomogeneous flows

NASA Technical Reports Server (NTRS)

Perot, J. Blair; Moin, Parviz

1993-01-01

The ability to model turbulence near solid walls and other types of boundaries is important in predicting complex engineering flows. Most turbulence modeling has concentrated either on flows which are nearly homogeneous or isotropic, or on turbulent boundary layers. Boundary layer models usually rely very heavily on the presence of mean shear and the production of turbulence due to that mean shear. Most other turbulence models are based on the assumption of quasi-homogeneity. However, there are many situations of engineering interest which do not involve large shear rates and which are not quasi-homogeneous or isotropic. Shear-free turbulent boundary layers are the prototypical example of such flows, with practical situations being separation and reattachment, bluff body flow, high free-stream turbulence, and free surface flows. Although these situations are not as common as the variants of the flat plate turbulent boundary layer, they tend to be critical factors in complex engineering situations. The models developed are intended to extend classical quasi-homogeneous models into regions of large inhomogeneity. These models do not rely on the presence of mean shear or production, but are still applicable when those additional effects are included. Although the focus is on shear-free boundary layers as tests for these models, results for standard shearing boundary layers are also shown.
Structural Characterization of the Foliated-Layered Gabbro Transition in Wadi Tayin of the Samail Ophiolite, Oman; Oman Drilling Project Holes GT1A and GT2A

NASA Astrophysics Data System (ADS)

Deans, J. R.; Crispini, L.; Cheadle, M. J.; Harris, M.; Kelemen, P. B.; Teagle, D. A. H.; Matter, J. M.; Takazawa, E.; Coggon, J. A.

2017-12-01

Oman Drilling Project Holes GT1A and GT2A were drilled into the Wadi Tayin massif, Samail ophiolite and both recovered ca. 400 m of continuous core through a section of the layered gabbros and the foliated-layered gabbro transition. Hole GT1A is cut by a discrete fault system including localized thin ultracataclastic fault zones. Hole GT2A is cut by a wider zone of brittle deformation and incipient brecciation. Here we report the structural history of the gabbros reflecting formation at the ridge to later obduction. Magmatic and high temperature history- 1) Both cores exhibit a pervasive, commonly well-defined magmatic foliation delineated by plagioclase, olivine and in places clinopyroxene. Minor magmatic deformation is present. 2) The dip of the magmatic foliation varies cyclically, gradually changing dip by 30o from gentle to moderate over a 50 m wavelength. 3) Layering is present throughout both cores, is defined by changes in mode and grain size ranging in thickness from 2 cm to 3 m and is commonly sub-parallel to the foliation. 4) There are no high temperature crystal-plastic shear zones in the core. Key observations include: no simple, systematic shallowing of dip with depth across the foliated-layered gabbro transition and layering is continuous across this transition. Cyclic variation of magmatic foliation dip most likely reflects the process of plate separation at the ridge axis. Near-axis faulting- i) On or near-axis structures consist of epidote-amphibole bearing hydraulic breccias and some zones of intense cataclasis with intensely deformed epidote and seams of clay and chlorite accompanied by syntectonic alteration of the wall rock. Early veins are filled with amphibole, chlorite, epidote, and anhydrite. ii) The deformation ranges from brittle-ductile, causing local deflection of the magmatic foliation, to brittle offset of the foliation and core and mantle structures in anhydrite veins. iii) The prevalent sense of shear is normal and slickenfibers indicate oblique offset. Obduction related faulting- i) Low temperature brittle faults and veins with laumontite, clay, and gypsum crosscut all structures. ii) Faults show a reverse sense of shear and crosscut, possibly reactivate, normal faults. Our observations suggest formation of reverse faults and late veins during obduction of the ophiolite.
Decay of the supersonic turbulent wakes from micro-ramps

NASA Astrophysics Data System (ADS)

Sun, Z.; Schrijer, F. F. J.; Scarano, F.; van Oudheusden, B. W.

2014-02-01

The wakes resulting from micro-ramps immersed in a supersonic turbulent boundary layer at Ma = 2.0 are investigated by means of particle image velocimetry. Two micro-ramps are investigated with height of 60% and 80% of the undisturbed boundary layer, respectively. The measurement domain is placed at the symmetry plane of the ramp and encompasses the range from 10 to 32 ramp heights downstream of the ramp. The decay of the flow field properties is evaluated in terms of time-averaged and root-mean-square (RMS) statistics. In the time-averaged flow field, the recovery from the imparted momentum deficit and the decay of upwash motion are analyzed. The RMS fluctuations of the velocity components exhibit strong anisotropy at the most upstream location and develop into a more isotropic regime downstream. The self-similarity properties of velocity components and fluctuation components along wall-normal direction are followed. The investigation of the unsteady large scale motion is carried out by means of snapshot analysis and by a statistical approach based on the spatial auto-correlation function. The Kelvin-Helmholtz (K-H) instability at the upper shear layer is observed to develop further with the onset of vortex pairing. The average distance between vortices is statistically estimated using the spatial auto-correlation. A marked transition with the wavelength increase is observed across the pairing regime. The K-H instability, initially observed only at the upper shear layer also begins to appear in the lower shear layer as soon as the wake is elevated sufficiently off the wall. The auto-correlation statistics confirm the coherence of counter-rotating vortices from the upper and lower sides, indicating the formation of vortex rings downstream of the pairing region.
Crustal flow at the margin of high plateaux: A lithospheric-scale experimental approach

NASA Astrophysics Data System (ADS)

Bajolet, Flora; Chardon, Dominique; Gapais, Denis; Martinod, Joseph; Kermarrec, Jean-Jacques

2010-05-01

A serie of analogue models was performed in order to explore the mechanisms of exhumation of high grade rocks at the margin of high plateaux. Experiments are scaled for gravity and simulate convergence between a hot, weak and thin lithosphere lacking a resistant mantle layer (high plateau, HP) and a cold and thick cratonic lithosphere (CL). The HP consists in a three-layer crust made of a low-viscosity silicone simulating partially molten lower crust (PMLC), overlaid by a medium-viscosity silicone simulating the middle crust, and a thin sand layer modelling the brittle upper crust. The CL is made of three layers, from bottom to top: a high-viscosity silicone (resistant mantle layer), a medium-viscosity silicone (lower crust) and a sand layer (upper crust). The model lithospheres float on a low-viscosity and dense solution of sodium polytungstate, simulating the asthenosphere. A set of laterally constrained experiments was run by changing the velocity of convergence, and the strength / thickness of the layers, to explore various degrees of coupling amongst lithospheric layers and between the two lithospheres. Several sets of experiments with comparable parameters were performed and stopped at different amounts of shortening, then frozen and cut for observation on serial cross-sections. For all experiments, the same kinematic scenario occurs. First, shortening affects preferentially the HP. Shortening proceeds by homogeneous thickening of the entire ductile crust and the formation of pop-downs of upper brittle crust after preferential development of HP-verging thrust faults. The crust rapidly acquired a double thickness under the HP, whereas the inner parts of the CL became moderately thickened as a continental subduction of CL mantle initiates under the HP. The part of the PMLC in contact with the CL starts to form a CL-verging antiform evolving into a wedge-shaped channel being injected into the lower crust of the CL. The channel is exhumed by slip along the reverse shear zone acting as the ramp accommodating subduction of the CL mantle below the HP. Injection of PMLC induces far field horizontal displacements of lower crust of the CL towards the foreland. The main foreland-verging thrusts affecting the CL form at that time. After a certain amount of injection and amplification, the roof of the antiform is horizontally sheared backward (i.e., toward the HP) along a flat shear zone whose upper wall coincides with the brittle-ductile transition. This shear zone emerges as the latest back thrust developed in the model, which bounds the outermost pop-down formed in the HP. These results suggest the amplification of a domal antiform resulting in injection of a non-cylindrical channel of PMLC from under HP into the crust of the CL, producing large finite exhumation of the PMLC even in the absence of erosion at the margin of HP. Erosion would favour greater exhumation ending with the formation of a dome of PMLC at the surface, accompanied by back tilting (and consecutive reorganization) of the flat shear zone accommodating return flow of mid/upper crust toward the HP above the channel. Analogy with the Himalayan-Tibet orogen suggests the South Tibetan detachment system may result from such a late reorganization in the exhumation of the Higher Himalaya Crystalline. The experiments provide constraints on the initiation stages of crustal flow at the margin of HP and may allow refining the channel flow model.
Influence of type-I fimbriae and fluid shear stress on bacterial behavior and multicellular architecture of early Escherichia coli biofilms at single-cell resolution.

PubMed

Wang, Liyun; Keatch, Robert; Zhao, Qi; Wright, John A; Bryant, Clare E; Redmann, Anna L; Terentjev, Eugene M

2018-01-12

Biofilm formation on abiotic surfaces in food and medical industry can cause severe contamination and infection, yet how biological and physical factors determine cellular architecture of early biofilms and bacterial behavior of the constituent cells remains largely unknown. In this study we examine the specific role of type-I fimbriae in nascent stages of biofilm formation and the response of micro-colonies to environmental flow shear at single-cell resolution. The results show that type-I fimbriae are not required for reversible adhesion from plankton, but critical for irreversible adhesion of Escherichia coli ( E.coli ) MG1655 forming biofilms on polyethylene terephthalate (PET) surfaces. Besides establishing a firm cell-surface contact, the irreversible adhesion seems necessary to initiate the proliferation of E.coli on the surface. After application of shear stress, bacterial retention is dominated by the 3D architecture of colonies independent of the population and the multi-layered structure could protect the embedded cells from being insulted by fluid shear, while cell membrane permeability mainly depends on the biofilm population and the duration time of the shear stress. Importance Bacterial biofilms could lead to severe contamination problems in medical devices and food processing equipment. However, biofilms are usually studied at a rough macroscopic level, thus little is known about how individual bacterial behavior within biofilms and multicellular architecture are influenced by bacterial appendages (e.g. pili/fimbriae) and environmental factors during early biofilm formation. We apply Confocal Laser Scanning Microscopy (CLSM) to visualize E.coli micro-colonies at single-cell resolution. Our findings suggest that type-I fimbriae are vital to the initiation of bacterial proliferation on surfaces and that the responses of biofilm architecture and cell membrane permeability of constituent bacteria to fluid shear stress are different, which are respectively regulated by the 3D morphology and the population of micro-colonies. Copyright © 2018 American Society for Microbiology.
Experimental study of combustion in a turbulent free shear layer formed at a rearward facing step

NASA Technical Reports Server (NTRS)

Pitz, R. W.; Daily, J. W.

1981-01-01

A premixed propane-air flame is stabilized in a turbulent free shear layer formed at a rearward facing step. The mean and rms averages of the turbulent velocity flow field are determined by LDV for both reacting (equivalence ratio 0.57) and nonreacting flows (Reynolds number 15,000-37,000 based on step height). The effect of combustion is to shift the layer toward the recirculation zone and reduce the flame spread. For reacting flow, the growth rate is unchanged except very near the step. The probability density function of the velocity is bimodial near the origin of the reacting layer and single-peaked but often skewed elsewhere. Large-scale structures dominate the reacting shear layer. Measurements of their passing frequency from LDV are consistent with high-speed Schlieren movies of the reacting layer and indicate that the coalescence rate of the eddies in the shear layer is reduced by combustion.
Optical based tactile shear and normal load sensor

DOEpatents

Salisbury, Curt Michael

2015-06-09

Various technologies described herein pertain to a tactile sensor that senses normal load and/or shear load. The tactile sensor includes a first layer and an optically transparent layer bonded together. At least a portion of the first layer is made of optically reflective material. The optically transparent layer is made of resilient material (e.g., clear silicone rubber). The tactile sensor includes light emitter/light detector pair(s), which respectively detect either normal load or shear load. Light emitter(s) emit light that traverses through the optically transparent layer and reflects off optically reflective material of the first layer, and light detector(s) detect and measure intensity of reflected light. When a normal load is applied, the optically transparent layer compresses, causing a change in reflected light intensity. When shear load is applied, a boundary between optically reflective material and optically absorptive material is laterally displaced, causing a change in reflected light intensity.
Modern CFD applications for the design of a reacting shear layer facility

NASA Technical Reports Server (NTRS)

Yu, S. T.; Chang, C. T.; Marek, C. J.

1991-01-01

The RPLUS2D code, capable of calculating high speed reacting flows, was adopted to design a compressible shear layer facility. In order to create reacting shear layers at high convective Mach numbers, hot air streams at supersonic speeds, rendered by converging-diverging nozzles, must be provided. A finite rate chemistry model is used to simulate the nozzle flows. Results are compared with one-dimensional solutions at chemical equilibrium. Additionally, a two equation turbulence model with compressibility effects was successfully incorporated with the RPLUS code. The model was applied to simulate a supersonic shear layer. Preliminary results show favorable comparisons with the experimental data.
Experimental Study of Transitional Flow Behavior in a Simulated Low Pressure Turbine

NASA Technical Reports Server (NTRS)

Sohn, Ki Hyeon; DeWitt, Kenneth J.

1998-01-01

A detailed investigation of the flow physics occurring on the suction side of a simulated Low Pressure Turbine (LPT) blade was performed. A contoured upper wall was designed to simulate the pressure distribution of an actual LPT airfoil onto a flat lower plate. The experiments were carried out for the Reynolds numbers of 35,000, 70,000, 100,000 and 250,000 with four levels of freestream turbulence ranging from 1% to 4%. For the three lower Reynolds numbers, the boundary layer on the flat plate was separated and formed a bubble. The size of laminar separation bubble was measured to be inversely proportional to the freestream turbulence levels and Reynolds numbers. However, no separation was observed for the Re = 250,000 case. The transition on a separated flow was found to proceed through the formation of turbulent spots in the free shear layer as evidenced in the intermittency profiles for Re = 35,000, 70,000 and 100,000. Spectral data show no evidence of Kelvin-Helmholtz or Tollmien-Schlichting instability waves in the free shear layer over a separation bubble (bypass transition). However, the flow visualization revealed the large vortex structures just outside of the bubble and their development to turbulent flow for Re = 50,000, which is similar to that in the free shear layer (separated-flow transition). Therefore, it is fair to say that the bypass and separated-flow transition modes coexist in the transitional flows over the separation bubble for certain conditions. Transition onset and end locations and length determined from intermittency profiles decrease as Reynolds number and freestream turbulence levels increase.
Experimental Study of Transitional Flow Behavior in a Simulated Low Pressure Turbine

NASA Technical Reports Server (NTRS)

Sohn, Ki Hyeon; DeWitt, Kenneth J.

2007-01-01

A detailed investigation of the flow physics occurring on the suction side of a simulated Low Pressure Turbine (LPT) blade was performed. A contoured upper wall was designed to simulate the pressure distribution of an actual LPT airfoil onto a flat lower plate. The experiments were carried out for the Reynolds numbers of 35,000, 70,000, 100,000, and 250,000 with four levels of freestream turbulence ranging from 1 to 4 percent. For the three lower Reynolds numbers, the boundary layer on the flat plate was separated and formed a bubble. The size of laminar separation bubble was measured to be inversely proportional to the freestream turbulence levels and Reynolds numbers. However, no separation was observed for the Re = 250,000 case. The transition on a separated flow was found to proceed through the formation of turbulent spots in the free shear layer as evidenced in the intermittency profiles for Re = 35,000, 70,000, and 100,000. Spectral data show no evidence of Kelvin-Helmholtz of Tollmien-Schlichting instability waves in the free shear layer over a separation bubble (bypass transition). However, the flow visualization revealed the large vortex structures just outside of the bubble and their development to turbulent flow for Re = 50,000, which is similar to that in the free shear layer (separated-flow transition). Therefore, it is fair to say that the bypass and separated-flow transition modes coexist in the transition flows over the separation bubble of certain conditions. Transition onset and end locations and length determined from intermittency profiles decreased as Reynolds number and freestream turbulence levels increase.
Late-Time Mixing Sensitivity to Initial Broadband Surface Roughness in High-Energy-Density Shear Layers

DOE PAGES

Flippo, K. A.; Doss, F. W.; Kline, J. L.; ...

2016-11-23

While using a large volume high-energy-density fluid shear experiment ( 8.5 cm 3 ) at the National Ignition Facility, we have demonstrated for the first time the ability to significantly alter the evolution of a supersonic sheared mixing layer by controlling the initial conditions of that layer. Furthermore, by altering the initial surface roughness of the tracer foil, we demonstrate the ability to transition the shear mixing layer from a highly ordered system of coherent structures to a randomly ordered system with a faster growing mix layer, indicative of strong mixing in the layer at a temperature of severalmore » tens of electron volts and at near solid density. Moreover, simulations using a turbulent-mix model show good agreement with the experimental results and poor agreement without turbulent mix.« less
Ross Sea Till Properties: Implications for Ice Sheet Bed Interaction

NASA Astrophysics Data System (ADS)

Halberstadt, A. R.; Anderson, J. B.; Simkins, L.; Prothro, L. O.; Bart, P. J.

2015-12-01

Since the discovery of a pervasive shearing till layer underlying Ice Stream B, the scientific community has categorized subglacial diamictons as either deformation till or lodgement till primarily based on shear strength. Deformation till is associated with streaming ice, formed through subglacial deformation of unconsolidated sediments. Lodgement till is believed to be deposited by the plastering of sediment entrained at the base of slow-flowing ice onto a rigid bed. Unfortunately, there has been a paucity of quantitative data on the spatial distribution of shear strength across the continental shelf. Cores collected from the Ross Sea on cruises NBP1502 and NBP9902 provide a rich dataset that can be used to interpret till shear strength variability. Till strengths are analyzed within the context of: (1) geologic substrate; (2) water content and other geotechnical properties; (3) ice sheet retreat history; and (4) geomorphic framework. Tills display a continuum of shear strengths rather than a bimodal distribution, suggesting that shear strength cannot be used to distinguish between lodgement and deformation till. Where the substrate below the LGM unconformity is comprised of older lithified deposits, till shear strengths are both highly variable within the till unit, as well as highly variable between cores. Conversely, where ice streams flowed across unconsolidated Plio-Pleistocene deposits, shear strengths are low and less variable within the unit and between cores. This suggests greater homogenization of cannibalized tills, and possibly a deeper pervasive shear layer. Coarser-grained tills are observed on banks and bank slopes, with finer tills in troughs. Highly variable and more poorly sorted tills are found in close proximity to sediment-based subglacial meltwater channels, attesting to a change in ice-bed interaction as subglacial water increases. Pellets (rounded sedimentary clasts of till matrix) are observed in Ross Sea cores, suggesting a history of deformation responsible for pellet formation. Till strength was measured in a variety of environments, including mega-scale lineations and grounding zone wedges; ongoing work focuses on evaluating till shear strengths within a geomorphic context. These analyses are used to re-evaluate till genesis, transport, and characterization.
The mean and turbulent flow structure of a weak hydraulic jump

NASA Astrophysics Data System (ADS)

Misra, S. K.; Kirby, J. T.; Brocchini, M.; Veron, F.; Thomas, M.; Kambhamettu, C.

2008-03-01

The turbulent air-water interface and flow structure of a weak, turbulent hydraulic jump are analyzed in detail using particle image velocimetry measurements. The study is motivated by the need to understand the detailed dynamics of turbulence generated in steady spilling breakers and the relative importance of the reverse-flow and breaker shear layer regions with attention to their topology, mean flow, and turbulence structure. The intermittency factor derived from turbulent fluctuations of the air-water interface in the breaker region is found to fit theoretical distributions of turbulent interfaces well. A conditional averaging technique is used to calculate ensemble-averaged properties of the flow. The computed mean velocity field accurately satisfies mass conservation. A thin, curved shear layer oriented parallel to the surface is responsible for most of the turbulence production with the turbulence intensity decaying rapidly away from the toe of the breaker (location of largest surface curvature) with both increasing depth and downstream distance. The reverse-flow region, localized about the ensemble-averaged free surface, is characterized by a weak downslope mean flow and entrainment of water from below. The Reynolds shear stress is negative in the breaker shear layer, which shows that momentum diffuses upward into the shear layer from the flow underneath, and it is positive just below the mean surface indicating a downward flux of momentum from the reverse-flow region into the shear layer. The turbulence structure of the breaker shear layer resembles that of a mixing layer originating from the toe of the breaker, and the streamwise variations of the length scale and growth rate are found to be in good agreement with observed values in typical mixing layers. All evidence suggests that breaking is driven by a surface-parallel adverse pressure gradient and a streamwise flow deceleration at the toe of the breaker. Both effects force the shear layer to thicken rapidly, thereby inducing a sharp free surface curvature change at the toe.
Sheared boundary layers in turbulent Rayleigh-Benard convection

NASA Astrophysics Data System (ADS)

Solomon, T. H.; Gollub, J. P.

1990-05-01

Thermal boundary layers in turbulent Rayleigh-Benard convection are studied experimentally using a novel system in which the convecting fluid is sheared from below with a flowing layer of mercury. Oscillatory shear substantially alters the spatial structure and frequency of the eruptions, with minimal effect on the heat flux (less than 5 percent). The temperature probability distribution function (PDF) just above the lower boundary layer changes from Gaussian to exponential without significant changes in the interior PDF. Implications for theories of 'hard' turbulence are discussed.
Gasdynamic simulations of the solar wind interaction with Venus - Boundary layer formation

NASA Astrophysics Data System (ADS)

McGary, J. E.

1993-05-01

A 2D gasdynamic simulation of the mass-loaded solar wind flow around the dayside of Venus is presented. For average ionopause conditions near 300 km, the simulations show that mass loading from the pickup of oxygen ions produces a boundary layer of finite thickness along the ionopause. Within this layer and toward the ionopause, the temperature decreases and the total mass density increases significantly. Furthermore, there is a shear in the bulk flow velocity across the boundary layer, such that the tangential flow decreases in speed as the ionopause is approached and remains low along the ionopause which is consistent with Pioneer Venus observations. Numerical simulations are carried out for various mass addition rates and demonstrate that the boundary layer develops when oxygen ion production exceeds approximately 2 x 10 exp 5/cu m per s.
Acoustics-turbulence interaction

NASA Technical Reports Server (NTRS)

Hussain, A. K. M. F.; Zaman, K. B. M. O.

1977-01-01

An investigation of the instability frequency was undertaken. Measurements revealed that the hot wire probe induces and sustains stable upstream oscillation of the free shear layer. The characteristics of the free shear layer tone are found to be different from the slit jet wedge edgetone phenomenon. The shear tone induced by a plane wedge in a plane free shear layer was then examined in order to further document the phenomenon. The eigenvalues and eigenfunctions of the tone fundamental show agreement with the spatial stability theory. A comprehensive summary of the results is also included.
Formation of Microbial Streamers by Flow-Induced Shear and Their Hydrodynamic Effects

NASA Astrophysics Data System (ADS)

Gong, J.; Olsen, K. A.; Nguyen, T.; Tice, M. M.; 2012; 2013, G. C.

2014-12-01

Microbial streamers are productive elements of surface-attached microbial communities that paradoxically seem to roughen mats under rapid, high shear flows, potentially exposing the mat to greater risk of erosion. They are common features found in modern hot-spring outflow channels, yet their formation mechanisms and effects on mat erosion are poorly understood. We test a hypothesis that streamers are produced by shear-induced viscoelastic deformation, and that streamers grow to heal detached turbulent boundary layers. Laboratory flume experiments were conducted using Particle Image/Tracking Velocimetry (PIV/PTV) to gain quantitative insights into the behavior of flows around small projections constructed from 3D-printed plastics or hydrated EPS gels, as well as artificial streamers. The combined use of fabricated hard and viscoelastic shapes, tracer particles, sheet lasers and high speed cameras allowed visualization of flows and quantitative measurements. Results show that primary and secondary flows (backflow behind projections) combine to produce deformations that drive the elongation of the top and ultimately initiate streamer formation. With insufficient secondary flows, streamers are not able to rise up from the basal mat. This implies that a combination of sufficient topographic relief and flow strength is required for streamers to form. In addition, flow measurements indicate that the presence of artificial streamers made the surface hydraulically smoother, and in effect reducing bed shear at the base. These results suggest a novel set of feedbacks that could reduce net mat erosion in energetic flows, and could help guide the evaluation of biosignatures in sedimentary rocks deposited in the presence of microbial mats.
LES/RANS Simulation of a Supersonic Reacting Wall Jet

NASA Technical Reports Server (NTRS)

Edwards, Jack R.; Boles, John A.; Baurle, Robert A.

2010-01-01

This work presents results from large-eddy / Reynolds-averaged Navier-Stokes (LES/RANS) simulations of the well-known Burrows-Kurkov supersonic reacting wall-jet experiment. Generally good agreement with experimental mole fraction, stagnation temperature, and Pitot pressure profiles is obtained for non-reactive mixing of the hydrogen jet with a non-vitiated air stream. A lifted flame, stabilized between 10 and 22 cm downstream of the hydrogen jet, is formed for hydrogen injected into a vitiated air stream. Flame stabilization occurs closer to the hydrogen injection location when a three-dimensional combustor geometry (with boundary layer development resolved on all walls) is considered. Volumetric expansion of the reactive shear layer is accompanied by the formation of large eddies which interact strongly with the reaction zone. Time averaged predictions of the reaction zone structure show an under-prediction of the peak water concentration and stagnation temperature, relative to experimental data and to results from a Reynolds-averaged Navier-Stokes calculation. If the experimental data can be considered as being accurate, this result indicates that the present LES/RANS method does not correctly capture the cascade of turbulence scales that should be resolvable on the present mesh. Instead, energy is concentrated in the very largest scales, which provide an over-mixing effect that excessively cools and strains the flame. Predictions improve with the use of a low-dissipation version of the baseline piecewise parabolic advection scheme, which captures the formation of smaller-scale structures superimposed on larger structures of the order of the shear-layer width.
SplitRacer - a new Semi-Automatic Tool to Quantify And Interpret Teleseismic Shear-Wave Splitting

NASA Astrophysics Data System (ADS)

Reiss, M. C.; Rumpker, G.

2017-12-01

We have developed a semi-automatic, MATLAB-based GUI to combine standard seismological tasks such as the analysis and interpretation of teleseismic shear-wave splitting. Shear-wave splitting analysis is widely used to infer seismic anisotropy, which can be interpreted in terms of lattice-preferred orientation of mantle minerals, shape-preferred orientation caused by fluid-filled cracks or alternating layers. Seismic anisotropy provides a unique link between directly observable surface structures and the more elusive dynamic processes in the mantle below. Thus, resolving the seismic anisotropy of the lithosphere/asthenosphere is of particular importance for geodynamic modeling and interpretations. The increasing number of seismic stations from temporary experiments and permanent installations creates a new basis for comprehensive studies of seismic anisotropy world-wide. However, the increasingly large data sets pose new challenges for the rapid and reliably analysis of teleseismic waveforms and for the interpretation of the measurements. Well-established routines and programs are available but are often impractical for analyzing large data sets from hundreds of stations. Additionally, shear wave splitting results are seldom evaluated using the same well-defined quality criteria which may complicate comparison with results from different studies. SplitRacer has been designed to overcome these challenges by incorporation of the following processing steps: i) downloading of waveform data from multiple stations in mseed-format using FDSNWS tools; ii) automated initial screening and categorizing of XKS-waveforms using a pre-set SNR-threshold; iii) particle-motion analysis of selected phases at longer periods to detect and correct for sensor misalignment; iv) splitting analysis of selected phases based on transverse-energy minimization for multiple, randomly-selected, relevant time windows; v) one and two-layer joint-splitting analysis for all phases at one station by simultaneously minimizing their transverse energy - this includes the analysis of null measurements. vi) comparison of results with theoretical splitting parameters determined for one, two, or continuously-varying anisotropic layer(s). Examples for the application of SplitRacer will be presented.

Nanomanufacturing of silicon surface with a single atomic layer precision via mechanochemical reactions.

PubMed

Chen, Lei; Wen, Jialin; Zhang, Peng; Yu, Bingjun; Chen, Cheng; Ma, Tianbao; Lu, Xinchun; Kim, Seong H; Qian, Linmao

2018-04-18

Topographic nanomanufacturing with a depth precision down to atomic dimension is of importance for advancement of nanoelectronics with new functionalities. Here we demonstrate a mask-less and chemical-free nanolithography process for regio-specific removal of atomic layers on a single crystalline silicon surface via shear-induced mechanochemical reactions. Since chemical reactions involve only the topmost atomic layer exposed at the interface, the removal of a single atomic layer is possible and the crystalline lattice beneath the processed area remains intact without subsurface structural damages. Molecular dynamics simulations depict the atom-by-atom removal process, where the first atomic layer is removed preferentially through the formation and dissociation of interfacial bridge bonds. Based on the parametric thresholds needed for single atomic layer removal, the critical energy barrier for water-assisted mechanochemical dissociation of Si-Si bonds was determined. The mechanochemical nanolithography method demonstrated here could be extended to nanofabrication of other crystalline materials.
Experimental study of the free surface velocity field in an asymmetrical confluence

NASA Astrophysics Data System (ADS)

Creelle, Stephan; Mignot, Emmanuel; Schindfessel, Laurent; De Mulder, Tom

2017-04-01

The hydrodynamic behavior of open channel confluences is highly complex because of the combination of different processes that interact with each other. To gain further insights in how the velocity uniformization between the upstream channels and the downstream channel is proceeding, experiments are performed in a large scale 90 degree angled concrete confluence flume with a chamfered rectangular cross-section and a width of 0.98m. The dimensions and lay-out of the flume are representative for a prototype scale confluence in e.g. drainage and irrigation systems. In this type of engineered channels with sharp corners the separation zone is very large and thus the velocity difference between the most contracted section and the separation zone is pronounced. With the help of surface particle tracking velocimetry the velocity field is recorded from upstream of the confluence to a significant distance downstream of the confluence. The resulting data allow to analyze the evolution of the incoming flows (with a developed velocity profile) that interact with the stagnation zone and each other, causing a shear layer between the two bulk flows. Close observation of the velocity field near the stagnation zone shows that there are actually two shear layers in the vicinity of the upstream corner. Furthermore, the data reveals that the shear layer observed more downstream between the two incoming flows is actually one of the two shear layers next to the stagnation zone that continues, while the other shear layer ceases to exist. The extensive measurement domain also allows to study the shear layer between the contracted section and the separation zone. The shear layers of the stagnation zone between the incoming flows and the one between the contracted flow and separation zone are localized and parameters such as the maximum gradient, velocity difference and width of the shear layer are calculated. Analysis of these data shows that the shear layer between the incoming flows disappears quite quickly, because of the severe flow contraction that aids the flow uniformization. This is also accelerated because of a flow redistribution process that starts already upstream of the confluence, resulting in a lower than expected velocity difference over the shear layer between the bulk of the incoming flows. In contrast, the shear layer between the contracted section and the separation zone proves to be of a significantly higher order of magnitude, with large turbulent structures appearing that get transported far downstream. In conclusion, the resulting understanding of this analysis of velocity fields with a larger field of view shows that when analyzing confluence hydrodynamics, one should pay ample attention to analyze data far enough up and downstream to assess all the relevant processes.
Propagation of sound waves through a linear shear layer: A closed form solution

NASA Technical Reports Server (NTRS)

Scott, J. N.

1978-01-01

Closed form solutions are presented for sound propagation from a line source in or near a shear layer. The analysis was exact for all frequencies and was developed assuming a linear velocity profile in the shear layer. This assumption allowed the solution to be expressed in terms of parabolic cyclinder functions. The solution is presented for a line monopole source first embedded in the uniform flow and then in the shear layer. Solutions are also discussed for certain types of dipole and quadrupole sources. Asymptotic expansions of the exact solutions for small and large values of Strouhal number gave expressions which correspond to solutions previously obtained for these limiting cases.
Streamline curvature in supersonic shear layers

NASA Technical Reports Server (NTRS)

Kibens, V.

1992-01-01

Results of an experimental investigation in which a curved shear layer was generated between supersonic flow from a rectangular converging/diverging nozzle and the freestream in a series of open channels with varying radii of curvature are reported. The shear layers exhibit unsteady large-scale activity at supersonic pressure ratios, indicating increased mixing efficiency. This effect contrasts with supersonic flow in a straight channel, for which no large-scale vortical structure development occurs. Curvature must exceed a minimum level before it begins to affect the dynamics of the supersonic shear layer appreciably. The curved channel flows are compared with reference flows consisting of a free jet, a straight channel, and wall jets without sidewalls on a flat and a curved plate.
Time-Accurate Simulations and Acoustic Analysis of Slat Free-Shear Layer

NASA Technical Reports Server (NTRS)

Khorrami, Mehdi R.; Singer, Bart A.; Berkman, Mert E.

2001-01-01

A detailed computational aeroacoustic analysis of a high-lift flow field is performed. Time-accurate Reynolds Averaged Navier-Stokes (RANS) computations simulate the free shear layer that originates from the slat cusp. Both unforced and forced cases are studied. Preliminary results show that the shear layer is a good amplifier of disturbances in the low to mid-frequency range. The Ffowcs-Williams and Hawkings equation is solved to determine the acoustic field using the unsteady flow data from the RANS calculations. The noise radiated from the excited shear layer has a spectral shape qualitatively similar to that obtained from measurements in a corresponding experimental study of the high-lift system.
Compression Fracture of CFRP Laminates Containing Stress Intensifications.

PubMed

Leopold, Christian; Schütt, Martin; Liebig, Wilfried V; Philipkowski, Timo; Kürten, Jonas; Schulte, Karl; Fiedler, Bodo

2017-09-05

For brittle fracture behaviour of carbon fibre reinforced plastics (CFRP) under compression, several approaches exist, which describe different mechanisms during failure, especially at stress intensifications. The failure process is not only initiated by the buckling fibres, but a shear driven fibre compressive failure beneficiaries or initiates the formation of fibres into a kink-band. Starting from this kink-band further damage can be detected, which leads to the final failure. The subject of this work is an experimental investigation on the influence of ply thickness and stacking sequence in quasi-isotropic CFRP laminates containing stress intensifications under compression loading. Different effects that influence the compression failure and the role the stacking sequence has on damage development and the resulting compressive strength are identified and discussed. The influence of stress intensifications is investigated in detail at a hole in open hole compression (OHC) tests. A proposed interrupted test approach allows identifying the mechanisms of damage initiation and propagation from the free edge of the hole by causing a distinct damage state and examine it at a precise instant of time during fracture process. Compression after impact (CAI) tests are executed in order to compare the OHC results to a different type of stress intensifications. Unnotched compression tests are carried out for comparison as a reference. With this approach, a more detailed description of the failure mechanisms during the sudden compression failure of CFRP is achieved. By microscopic examination of single plies from various specimens, the different effects that influence the compression failure are identified. First damage of fibres occurs always in 0°-ply. Fibre shear failure leads to local microbuckling and the formation and growth of a kink-band as final failure mechanisms. The formation of a kink-band and finally steady state kinking is shifted to higher compressive strains with decreasing ply thickness. Final failure mode in laminates with stress intensification depends on ply thickness. In thick or inner plies, damage initiates as shear failure and fibre buckling into the drilled hole. The kink-band orientation angle is changing with increasing strain. In outer or thin plies shear failure of single fibres is observed as first damage and the kink-band orientation angle is constant until final failure. Decreasing ply thickness increases the unnotched compressive strength. When stress intensifications are present, the position of the 0°-layer is critical for stability under compression and is thus more important than the ply thickness. Central 0°-layers show best results for OHC and CAI strength due to higher bending stiffness and better supporting effect of the adjacent layers.
Compression Fracture of CFRP Laminates Containing Stress Intensifications

PubMed Central

Schütt, Martin; Philipkowski, Timo; Kürten, Jonas; Schulte, Karl

2017-01-01

For brittle fracture behaviour of carbon fibre reinforced plastics (CFRP) under compression, several approaches exist, which describe different mechanisms during failure, especially at stress intensifications. The failure process is not only initiated by the buckling fibres, but a shear driven fibre compressive failure beneficiaries or initiates the formation of fibres into a kink-band. Starting from this kink-band further damage can be detected, which leads to the final failure. The subject of this work is an experimental investigation on the influence of ply thickness and stacking sequence in quasi-isotropic CFRP laminates containing stress intensifications under compression loading. Different effects that influence the compression failure and the role the stacking sequence has on damage development and the resulting compressive strength are identified and discussed. The influence of stress intensifications is investigated in detail at a hole in open hole compression (OHC) tests. A proposed interrupted test approach allows identifying the mechanisms of damage initiation and propagation from the free edge of the hole by causing a distinct damage state and examine it at a precise instant of time during fracture process. Compression after impact (CAI) tests are executed in order to compare the OHC results to a different type of stress intensifications. Unnotched compression tests are carried out for comparison as a reference. With this approach, a more detailed description of the failure mechanisms during the sudden compression failure of CFRP is achieved. By microscopic examination of single plies from various specimens, the different effects that influence the compression failure are identified. First damage of fibres occurs always in 0°-ply. Fibre shear failure leads to local microbuckling and the formation and growth of a kink-band as final failure mechanisms. The formation of a kink-band and finally steady state kinking is shifted to higher compressive strains with decreasing ply thickness. Final failure mode in laminates with stress intensification depends on ply thickness. In thick or inner plies, damage initiates as shear failure and fibre buckling into the drilled hole. The kink-band orientation angle is changing with increasing strain. In outer or thin plies shear failure of single fibres is observed as first damage and the kink-band orientation angle is constant until final failure. Decreasing ply thickness increases the unnotched compressive strength. When stress intensifications are present, the position of the 0°-layer is critical for stability under compression and is thus more important than the ply thickness. Central 0°-layers show best results for OHC and CAI strength due to higher bending stiffness and better supporting effect of the adjacent layers. PMID:28872623
Shear flow of one-component polarizable fluid in a strong electric field

NASA Astrophysics Data System (ADS)

Sun, J. M.; Tao, R.

1996-04-01

A shear flow of one-component polarizable fluid in a strong electric field has a structural transition at a critical shear stress. When the shear stress is increased from zero up to the critical shear stress, the flow (in the x direction) has a flowing-chain (FC) structure, consisting of tilted or broken chains along the field (z direction). At the critical shear stress, the FC structure gives way to a flowing-hexagonal-layered (FHL) structure, consisting of several two-dimensional layers which are parallel to the x-z plane. Within one layer, particles form strings in the flow direction. Strings are constantly sliding over particles in strings right beneath. The effective viscosity drops dramatically at the structural change. As the shear stress reduces, the FHL structure persists even under a stress-free state if the thermal fluctuation is very weak. This structure change in the charging and discharging process produces a large hysteresis.
Bending stiffness and interlayer shear modulus of few-layer graphene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, Xiaoming; Yi, Chenglin; Ke, Changhong, E-mail: cke@binghamton.edu

2015-03-09

Interlayer shear deformation occurs in the bending of multilayer graphene with unconstrained ends, thus influencing its bending rigidity. Here, we investigate the bending stiffness and interlayer shear modulus of few-layer graphene through examining its self-folding conformation on a flat substrate using atomic force microscopy in conjunction with nonlinear mechanics modeling. The results reveal that the bending stiffness of 2–6 layers graphene follows a square-power relationship with its thickness. The interlayer shear modulus is found to be in the range of 0.36–0.49 GPa. The research findings show that the weak interlayer shear interaction has a substantial stiffening effect for multilayer graphene.
Evolution of inviscid Kelvin-Helmholtz instability from a piecewise linear shear layer

NASA Astrophysics Data System (ADS)

Guha, Anirban; Rahmani, Mona; Lawrence, Gregory

2012-11-01

Here we study the evolution of 2D, inviscid Kelvin-Helmholtz instability (KH) ensuing from a piecewise linear shear layer. Although KH pertaining to smooth shear layers (eg. Hyperbolic tangent profile) has been thorough investigated in the past, very little is known about KH resulting from sharp shear layers. Pozrikidis and Higdon (1985) have shown that piecewise shear layer evolves into elliptical vortex patches. This non-linear state is dramatically different from the well known spiral-billow structure of KH. In fact, there is a little acknowledgement that elliptical vortex patches can represent non-linear KH. In this work, we show how such patches evolve through the interaction of vorticity waves. Our work is based on two types of computational methods (i) Contour Dynamics: a boundary-element method which tracks the evolution of the contour of a vortex patch using Lagrangian marker points, and (ii) Direct Numerical Simulation (DNS): an Eulerian pseudo-spectral method heavily used in studying hydrodynamic instability and turbulence.
Electron Debye scale Kelvin-Helmholtz instability: Electrostatic particle-in-cell simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Sang-Yun; Lee, Ensang, E-mail: eslee@khu.ac.kr; Kim, Khan-Hyuk

2015-12-15

In this paper, we investigated the electron Debye scale Kelvin-Helmholtz (KH) instability using two-dimensional electrostatic particle-in-cell simulations. We introduced a velocity shear layer with a thickness comparable to the electron Debye length and examined the generation of the KH instability. The KH instability occurs in a similar manner as observed in the KH instabilities in fluid or ion scales producing surface waves and rolled-up vortices. The strength and growth rate of the electron Debye scale KH instability is affected by the structure of the velocity shear layer. The strength depends on the magnitude of the velocity and the growth ratemore » on the velocity gradient of the shear layer. However, the development of the electron Debye scale KH instability is mainly determined by the electric field generated by charge separation. Significant mixing of electrons occurs across the shear layer, and a fraction of electrons can penetrate deeply into the opposite side fairly far from the vortices across the shear layer.« less
Microstructure and rheology of particle stabilized emulsions: Effects of particle shape and inter-particle interactions.

PubMed

Katepalli, Hari; John, Vijay T; Tripathi, Anubhav; Bose, Arijit

2017-01-01

Using fumed and spherical silica particles of similar hydrodynamic size, we investigated the effects of particle shape and inter-particle interactions on the formation, stability and rheology of bromohexadecane-in-water Pickering emulsions. The interparticle interactions were varied from repulsive to attractive by modifying the salt concentration in the aqueous phase. Optical microscope images revealed smaller droplet sizes for the fumed silica stabilized emulsions. All the emulsions remained stable for several weeks. Cryo-SEM images of the emulsion droplets showed a hexagonally packed single layer of particles at oil-water interfaces in emulsions stabilized with silica spheres, irrespective of the nature of the inter-particle interactions. Thus, entropic, excluded volume interactions dominate the fate of spherical particles at oil-water interfaces. On the other hand, closely packed layers of particles were observed at oil-water interfaces for the fumed silica stabilized emulsions for both attractive and repulsive interparticle interactions. At the high salt concentrations, attractive inter-particles interactions led to aggregation of fumed silica particles, and multiple layers of these particles were then observed on the droplet surfaces. A network of fumed silica particles was also observed between the emulsion droplets, suggesting that enthalpic interactions are responsible for the determining particle configurations at oil-water interfaces as well as in the aqueous phase. Steady shear viscosity measurements over a range of shear stresses, as well as oscillatory shear measurements at 1Hz confirm the presence of a network in fumed silica suspensions and emulsions, and the lack of such a network when spherical particles are used. The fractal structure of fumed silica leads to several contact points and particle interlocking in the water as well as on the bromohexadecane-water interfaces, with corresponding effects on the structure and rheology of the emulsions. The attenuation of droplet motion due to the formation of a particle network can be exploited for stabilizing emulsions and for modulating their rheology. Copyright © 2016 Elsevier Inc. All rights reserved.
Chocolate tablet aspects of cytherean Meshkenet Tessera

NASA Technical Reports Server (NTRS)

Raitala, J.

1993-01-01

Meshkenet Tessera structures were mapped from Magellan data and several resemblances to chocolate tablet boudinage were found. The complex fault sets display polyphase tectonic sequences of a few main deformation phases. Shear and tension have contributed to the areal deformation. Main faults cut the 1600-km long Meshkenet Tessera highland into bar-like blocks which have ridge and groove pattern oriented along or at high angles to the faults. The first approach to the surface block deformation is an assumption of initial parallel shear faulting followed by a chocolate tablet boudinage. Major faults which cut Meshkenet Tessera into rectangular blocks have been active repetitively while two progressive or superposed boudinage set formations have taken place at high angles during the relaxational or flattening type deformation of the area. Chocolate tablet boudinage is caused by a layer-parallel two-dimensional extension resulting in fracturing of the competent layer. Such structures, defined by two sets of boudin neck lines at right angles to each other, have been described by a number of authors. They develop in a flattening type of bulk deformation or during superposed deformation where the rock is elongated in two dimensions parallel to the surface. This is an attempt to describe and understand the formation and development of structures of Meshkenet Tessera which has complicated fault structures.
Experimental Study of Shock Generated Compressible Vortex Ring

NASA Astrophysics Data System (ADS)

Das, Debopam; Arakeri, Jaywant H.; Krothapalli, Anjaneyulu

2000-11-01

Formation of a compressible vortex ring and generation of sound associated with it is studied experimentally. Impulse of a shock wave is used to generate a vortex ring from the open end of a shock-tube. Vortex ring formation process has been studied in details using particle image Velocimetry (PIV). As the shock wave exits the tube it diffracts and expands. A circular vortex sheet forms at the edge and rolls up into a vortex ring. Far field microphone measurement shows that the acoustic pressure consists of a spike due to shock wave followed by a low frequency pressure wave of decaying nature, superimposed with high frequency pressure wave. Acoustic waves consist of waves due to expansion, waves formed in the tube during diaphragm breakage and waves associated with the vortex ring and shear-layer vortices. Unsteady evolution of the vortex ring and shear-layer vortices in the jet behind the ring is studied by measuring the velocity field using PIV. Corresponding vorticity field, circulation around the vortex core and growth rate of the vortex core is calculated from the measured velocity field. The velocity field in a compressible vortex ring differs from that of an incompressible ring due to the contribution from both shock and vortex ring.
Lower thermospheric-enhanced sodium layers observed at low latitude and possible formation: Case studies

NASA Astrophysics Data System (ADS)

Xue, X. H.; Dou, X. K.; Lei, J.; Chen, J. S.; Ding, Z. H.; Li, T.; Gao, Q.; Tang, W. W.; Cheng, X. W.; Wei, K.

2013-05-01

We report two lower thermospheric-enhanced sodium layer (TeSL) cases observed at a low-latitude station, Lijiang, China (26.7°N, 100.0°E), on 10 March and 10 April 2012, respectively. The TeSLs in the two cases were located at altitudes near 122 and 112 km, respectively. In addition, strong sporadic sodium layers (SSLs) near 100 km accompanied the TeSL observed on 10 March 2012. Both the TeSLs and SSLs exhibited tidal-induced downward motion. The adjacent ground-based and space-borne ionospheric radio observations showed strong Es layers before the appearance of the TeSLs, suggesting an " Es- TeSLs (SSLs)" chain formed through the tidal wind shear mechanism. Assuming that the vertical tidal wavelengths remain unchanged, it is found that in different regions caused by the tidal wind shear, different TeSLs evolution processes are expected: (1) in a tidal-convergence region, a TeSL/SSL with a downward propagation phase is enhanced due to a rapid decrease in the Na+lifetime at the lower altitude; (2) in an ion convergence-divergence interface region, a TeSL/SSL will still follow the tidal downward phase progression, but sodium density does not exhibit evident enhancement; and (3) when a TeSL/SSL enters into a tidal wind-divergence zone, the layer density tends to decrease.
Stress and strain evolution of folding rocks

NASA Astrophysics Data System (ADS)

Llorens, Maria-Gema; Griera, Albert; Bons, Paul; Gomez-Rivas, Enrique; Weikusat, Ilka

2015-04-01

One of the main objectives of structural geology is to unravel rock deformation histories. Fold shapes can be used to estimate the orientation and amount of strain associated with folding. However, much more information on rheology and kinematics can potentially be extracted from fold geometries (Llorens et al., 2013a). We can study the development of folds, quantify the relationships between the different parameters that determine their geometries and estimate their mechanical evolution. This approach allows us to better understand and predict not only rock but also ice deformation. One of the main parameters in fold development is the viscosity contrast between the folding layer and the matrix in which it is embedded (m), since it determines the initial fold wavelength and the amplification rate of the developing folds. Moreover, non-linear viscous rheology influences fold geometry too (Llorens et al., 2013b). We present a series of 2-dimensional simulations of folding of viscous single layers in pure and simple shear. We vary different parameters in order to compare and determine their influence on the resulting fold patterns and the associated mechanical response of the material. To perform these simulations we use the software platform ELLE (www.elle.ws) with the non-linear viscous finite element code BASIL. The results show that layers thicken at the beginning of deformation in all simulations, and visible folds start earlier or later depending on the viscosity contrast. When folds start to nucleate the layer maximum shear strain decreases, moving away from the theoretical trend for homogeneous strain (no folding). This allows the accurate determination of the onset of folding. Maximum deviatoric stresses are higher in power-law than in linear-viscosity materials, and it is initially double in pure shear than in simple shear conditions. Therefore, folding a competent layer requires less work in simple than in pure shear. The maximum deviatoric stress difference between pure and simple shear is less pronounced in power-law materials. It also depends on the original orientation of the layer relative to the shear plane, being the shortening rate initially relatively low when the layer makes a low angle with the shear plane. The mechanical behaviour is similar in pure and simple shear when the layer is oriented at a relative high angle (45°). M-G Llorens, PD Bons, A Griera and E Gomez-Rivas (2013a) When do folds unfold during progressive shear?. Geology, 41, 563-566. M-G Llorens, PD Bons, A Griera, E Gomez-Rivas and LA Evans (2013b) Single layer folding in simple shear. Journal of Structural Geology, 50, 209-220.
Active control of jet flowfields

NASA Astrophysics Data System (ADS)

Kibens, Valdis; Wlezien, Richard W.

1987-06-01

Passive and active control of jet shear layer development were investigated as mechanisms for modifying the global characteristics of jet flowfields. Slanted and stepped indeterminate origin (I.O.) nozzles were used as passive, geometry-based control devices which modified the flow origins. Active control techniques were also investigated, in which periodic acoustic excitation signals were injected into the I.O. nozzle shear layers. Flow visualization techniques based on a pulsed copper-vapor laser were used in a phase-conditioned image acquisition mode to assemble optically averaged sets of images acquired at known times throughout the repetition cycle of the basic flow oscillation period. Hot wire data were used to verify the effect of the control techniques on the mean and fluctuating flow properties. The flow visualization images were digitally enhanced and processed to show locations of prominent vorticity concentrations. Three-dimensional vortex interaction patterns were assembled in a format suitable for movie mode on a graphic display workstation, showing the evolution of three-dimensional vortex system in time.
Unfolding single- and multilayers

NASA Astrophysics Data System (ADS)

Llorens, Maria-Gema; Bons, Paul D.; Griera, Albert; Gomez-Rivas, Enrique

2014-05-01

When planar structures (e.g. sedimentary layers, veins, dykes, cleavages, etc.) are subjected to deformation, they have about equal chances to be shortened or stretched. The most common shortening and stretching structures are folds and boudinage, respectively. However, boudinage requires additional deformation mechanisms apart from viscous flow, like formation of fractures or strain localization. When folded layers are subjected to extension, they could potentially unfold back to straight layers. Although probably not uncommon, this would be difficult to recognize. Open questions are whether folded layers can unfold, what determines their mechanical behaviour and how we can recognize them in the field. In order to approach these questions, we present a series of numerical experiments that simulate stretching of previously folded single- and multi-layers in simple shear, using the two dimensional numerical modelling platform ELLE, including the finite element module BASIL that calculates viscous deformation. We investigate the parameters that affect a fold train once it rotates into the extensional field. The results show that the unfolding process strongly depends on the viscosity contrast between the layer and matrix (Llorens et al., 2013). Layers do not completely unfold when they experience softening before or during the stretching process or when other neighbouring competent layers prevent them from unfolding. The foliation refraction patterns are the main indicators of unfolded folds. Additionally, intrafolial folds and cusp-like folds adjacent to straight layers, as well as variations in fold amplitudes and limb lengths of irregular folds can also be used as indicators of stretching of a layer after shortening and folding. References: Llorens, M-.G., Bons, P.D., Griera, A. and Gomez-Rivas, E. 2013. When do folds unfold during progressive shear?. Geology, 41, 563-566.
Features of sound propagation through and stability of a finite shear layer

NASA Technical Reports Server (NTRS)

Koutsoyannis, S. P.

1976-01-01

The plane wave propagation, the stability and the rectangular duct mode problems of a compressible inviscid linearly sheared parallel, but otherwise homogeneous flow, are shown to be governed by Whittaker's equation. The exact solutions for the perturbation quantities are essentially Whittaker M-functions. A number of known results are obtained as limiting cases of exact solutions. For the compressible finite thickness shear layer it is shown that no resonances and no critical angles exist for all Mach numbers, frequencies and shear layer velocity profile slopes except in the singular case of the vortex sheet.
Global distribution of neutral wind shear associated with sporadic E layers derived from GAIA

NASA Astrophysics Data System (ADS)

Shinagawa, H.; Miyoshi, Y.; Jin, H.; Fujiwara, H.

2017-04-01

There have been a number of papers reporting that the statistical occurrence rate of the sporadic E (Es) layer depends not only on the local time and season but also on the geographical location, implying that geographical and seasonal dependence in vertical neutral wind shear is one of the factors responsible for the geographical and seasonal dependence in Es layer occurrences rate. To study the role of neutral wind shear in the global distribution of the Es layer occurrence rate, we employ a self-consistent atmosphere-ionosphere coupled model called GAIA (Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy), which incorporates meteorological reanalysis data in the lower atmosphere. The average distribution of neutral wind shear in the lower thermosphere is derived for the June-August and December-February periods, and the global distribution of vertical ion convergence is obtained to estimate the Es layer occurrence rate. It is found that the local and seasonal dependence of neutral wind shear is an important factor in determining the dependence of the Es layer occurrence rate on geographical distribution and seasonal variation. However, there are uncertainties in the simulated vertical neutral wind shears, which have larger scales than the observed wind shear scales. Furthermore, other processes such as localization of magnetic field distribution, background metallic ion distribution, ionospheric electric fields, and chemical processes of metallic ions are also likely to make an important contribution to geographical distribution and seasonal variation of the Es occurrence rate.

Influence of Stress State, Stress Orientation, and Rock Properties on the Development of Deformation-Band 'Ladder' Arrays in Porous Sandstone

NASA Astrophysics Data System (ADS)

Schultz, R. A.; Soliva, R.; Fossen, H.

2013-12-01

Deformation bands in porous rocks tend to develop into spatially organized arrays that display a variety of lengths and thicknesses, and their geometries and arrangements are of interest with respect to fluid flow in reservoirs. Field examples of deformation band arrays in layered clastic sequences suggest that the development of classic deformation band arrays, such as ladders and conjugate sets, and the secondary formation of through-going faults appear to be related to the physical properties of the host rock, the orientation of stratigraphic layers relative to the far-field stress state, and the evolution of the local stress state within the developing array. We have identified several field examples that demonstrate changes in band properties, such as type and orientation, as a function of one or more of these three main factors. Normal-sense deformation-band arrays such as those near the San Rafael Swell (Utah) develop three-dimensional ladder-style arrays at a high angle to the maximum compression direction; these cataclastic shear bands form at acute angles to the maximum compression not very different from that of the optimum frictional sliding plane, thus facilitating the eventual nucleation of a through-going fault. At Orange quarry (France), geometrically conjugate sets of reverse-sense compactional shear bands form with angles to the maximum compression direction that inhibit fault nucleation within them; the bands in this case also form at steep enough angles to bedding that stratigraphic heterogeneities within the deforming formation were apparently not important. Two exposures of thrust-sense ladders at Buckskin Gulch (Utah) demonstrate the importance of host-rock properties, bedding-plane involvement, and local stress perturbations on band-array growth. In one ladder, thrust-sense shear deformation bands nucleated along suitably oriented bedding planes, creating overprinting sets of compaction bands that can be attributed to layer properties and local stress changes near the shear-band tips. Two other ladder exposures preserve compaction bands having nearly perpendicular orientations relative the bounding shear bands that define contractional stepovers that also nucleated on bedding planes. These cases suggest that local stress changes within a deformation-band stepover may lead to either rotation of bands or changes in band type relative to bands formed outside the stepover. The development of the common geometries of deformation band arrays, such as ladders, and the deformation paths to faulting thus depend on a combination of stress state, stress orientation, and rock properties.
Acoustic wave propagation in a temporal evolving shear-layer for low-Mach number perturbations

NASA Astrophysics Data System (ADS)

Hau, Jan-Niklas; Müller, Björn

2018-01-01

We study wave packets with the small perturbation/gradient Mach number interacting with a smooth shear-layer in the linear regime of small amplitude perturbations. In particular, we investigate the temporal evolution of wave packets in shear-layers with locally curved regions of variable size using non-modal linear analysis and direct numerical simulations of the two-dimensional gas-dynamical equations. Depending on the wavenumber of the initially imposed wave packet, three different types of behavior are observed: (i) The wave packet passes through the shear-layer and constantly transfers energy back to the mean flow. (ii) It is turned around (or reflected) within the sheared region and extracts energy from the base flow. (iii) It is split into two oppositely propagating packages when reaching the upper boundary of the linearly sheared region. The conducted direct numerical simulations confirm that non-modal linear stability analysis is able to predict the wave packet dynamics, even in the presence of non-linearly sheared regions. In the light of existing studies in this area, we conclude that the sheared regions are responsible for the highly directed propagation of linearly generated acoustic waves when there is a dominating source, as it is the case for jet flows.
Complex strain fields

NASA Astrophysics Data System (ADS)

Bradshaw, P.

Computational techniques for accounting for extra strain rates, abnormal distributions of delta-U/delta-y, fluctuating strain rates, and the effects of body forces in modeling shear flows are discussed. Consideration is given to simple shears where the extra strain rate does not affect turbulence, thin shear layers, moderately thin shear layers, and strongly distorted flows. Attention is given to formulations based on the exact transport equations for Reynolds stress as derived from the time-averaged Navier-Stokes equations. Extra strain rates arise from curvature, lateral divergence, and bulk compression, with Coriolis forces accounting for the first, intensification of the spanwise vorticity for the second, and compression or dilation of the shear layer producing the third. The curvature forces, e.g., buoyancy and Coriolis forces, are responsible for hurricanes and tornadoes.
Modelling hazardous surface hoar layers in the mountain snowpack over space and time

NASA Astrophysics Data System (ADS)

Horton, Simon Earl

Surface hoar layers are a common failure layer in hazardous snow slab avalanches. Surface hoar crystals (frost) initially form on the surface of the snow, and once buried can remain a persistent weak layer for weeks or months. Avalanche forecasters have difficulty tracking the spatial distribution and mechanical properties of these layers in mountainous terrain. This thesis presents numerical models and remote sensing methods to track the distribution and properties of surface hoar layers over space and time. The formation of surface hoar was modelled with meteorological data by calculating the downward flux of water vapour from the atmospheric boundary layer. The timing of surface hoar formation and the modelled crystal size was verified at snow study sites throughout western Canada. The major surface hoar layers over several winters were predicted with fair success. Surface hoar formation was modelled over various spatial scales using meteorological data from weather forecast models. The largest surface hoar crystals formed in regions and elevation bands with clear skies, warm and humid air, cold snow surfaces, and light winds. Field surveys measured similar regional-scale patterns in surface hoar distribution. Surface hoar formation patterns on different slope aspects were observed, but were not modelled reliably. Mechanical field tests on buried surface hoar layers found layers increased in shear strength over time, but had persistent high propensity for fracture propagation. Layers with large crystals and layers overlying hard melt-freeze crusts showed greater signs of instability. Buried surface hoar layers were simulated with the snow cover model SNOWPACK and verified with avalanche observations, finding most hazardous surface hoar layers were identified with a structural stability index. Finally, the optical properties of surface hoar crystals were measured in the field with spectral instruments. Large plate-shaped crystals were less reflective at shortwave infrared wavelengths than other common surface snow grains. The methods presented in this thesis were developed into operational products that model hazardous surface hoar layers in western Canada. Further research and refinements could improve avalanche forecasts in regions prone to hazardous surface hoar layers.
Flow Phenomena in the Very Near Wake of a Flat Plate with a Circular Trailing Edge

NASA Technical Reports Server (NTRS)

Rai, Man Mohan

2014-01-01

The very near wake of a flat plate with a circular trailing edge, exhibiting pronounced shedding of wake vortices, is investigated with data from a direct numerical simulation. The separating boundary layers are turbulent and statistically identical thus resulting in a wake that is symmetric in the mean. The focus here is on the instability of the detached shear layers, the evolution of rib-vortex induced localized regions of reverse flow that detach from the main body of reverse flow in the trailing edge region and convect downstream, and phaseaveraged velocity statistics in the very near wake. The detached shear layers are found to exhibit unstable behavior intermittently, including the development of shear layer vortices as in earlier cylinder flow investigations with laminar separating boundary layers. Only a small fraction of the separated turbulent boundary layers undergo this instability, and form the initial shed vortices. Pressure spectra within the shear layers show a broadband peak at a multiple of shedding frequency. Phase-averaged intensity and shear stress distributions of the randomly fluctuating component of velocity are compared with those obtained in the near wake. The distributions of the production terms in the transport equations for the turbulent stresses are also provided.
Comparison with Analytical Solution: Generation and Radiation of Acoustic Waves from a 2-D Shear Layer

NASA Technical Reports Server (NTRS)

Dahl, Milo D.

2000-01-01

An acoustic source inside of a 2-D jet excites an instability wave in the shear layer resulting in sound radiating away from the shear layer. Solve the linearized Euler equations to predict the sound radiation outside of the jet. The jet static pressure is assumed to be constant. The jet flow is parallel and symmetric about the x-axis. Use a symmetry boundary condition along the x-axis.
An experimental study of transmission, reflection and scattering of sound in a free jet flight simulation facility and comparison with theory

NASA Technical Reports Server (NTRS)

Ahuja, K. K.; Tanna, H. K.; Tester, B. J.

1981-01-01

When a free jet (or open jet) is used as a wind tunnel to simulate the effects of flight on model noise sources, it is necessary to calibrate out the effects of the free jet shear layer on the transmitted sound, since the shear layer is absent in the real flight case. In this paper, a theoretical calibration procedure for this purpose is first summarized; following this, the results of an experimental program, designed to test the validity of the various components of the calibration procedure, are described. The experiments are conducted by using a point sound source located at various axial positions within the free jet potential core. By using broadband excitation and cross-correlation methods, the angle changes associated with ray paths across the shear layer are first established. Measurements are then made simultaneously inside and outside the free jet along the proper ray paths to determine the amplitude changes across the shear layer. It is shown that both the angle and amplitude changes can be predicted accurately by theory. It is also found that internal reflection at the shear layer is significant only for large ray angles in the forward quadrant where total internal reflection occurs. Finally, the effects of sound absorption and scattering by the shear layer turbulence are also examined experimentally.
Sensor for Direct Measurement of the Boundary Shear Stress in Fluid Flow

NASA Technical Reports Server (NTRS)

Bao, Xiaoqi; Badescu, Mircea; Bar-Cohen, Yoseph; Lih, Shyh-Shiuh; Sherrit, Stewart; Chang, Zensheu; Chen, Beck; Widholm, Scott; Ostlund, Patrick

2011-01-01

The formation of scour patterns at bridge piers is driven by the forces at the boundary of the water flow. In most experimental scour studies, indirect processes have been applied to estimate the shear and normal stress using measured velocity profiles. The estimations are based on theoretical models and associated assumptions. However, the turbulence flow fields and boundary layer in the pier-scour region are very complex. In addition, available turbulence models cannot account accurately for the bed roughness effect. Direct measurement of the boundary shear and normal stress and their fluctuations are attractive alternatives. However, this approach is a challenging one especially for high spatial resolution and high fidelity measurements. The authors designed and fabricated a prototype miniature shear stress sensor including an EDM machined floating plate and a high-resolution laser optical encoder. Tests were performed both in air as well as operation in water with controlled flow. The sensor sensitivity, stability and signal-to-noise level were measured and evaluated. The detailed test results and a discussion of future work will be presented in this paper.
High order accurate solutions of viscous problems

NASA Technical Reports Server (NTRS)

Hayder, M. Ehtesham; Turkel, Eli

1993-01-01

We consider a fourth order extension to MacCormack's scheme. The original extension was fourth order only for the inviscid terms but was second order for the viscous terms. We show how to modify the viscous terms so that the scheme is uniformly fourth order in the spatial derivatives. Applications are given to some boundary layer flows. In addition, for applications to shear flows the effect of the outflow boundary conditions are very important. We compare the accuracy of several of these different boundary conditions for both boundary layer and shear flows. Stretching at the outflow usually increases the oscillations in the numerical solution but the addition of a filtered sponge layer (with or without stretching) reduces such oscillations. The oscillations are generated by insufficient resolution of the shear layer. When the shear layer is sufficiently resolved then oscillations are not generated and there is less of a need for a nonreflecting boundary condition.
A hybrid molecular dynamics study on the non-Newtonian rheological behaviors of shear thickening fluid.

PubMed

Chen, Kaihui; Wang, Yu; Xuan, Shouhu; Gong, Xinglong

2017-07-01

To investigate the microstructural evolution dependency on the apparent viscosity in shear-thickening fluids (STFs), a hybrid mesoscale model combined with stochastic rotation dynamics (SRD) and molecular dynamics (MD) is used. Muller-Plathe reverse perturbation method is adopted to analyze the viscosities of STFs in a two-dimensional model. The characteristic of microstructural evolution of the colloidal suspensions under different shear rate is studied. The effect of diameter of colloidal particles and the phase volume fraction on the shear thickening behavior is investigated. Under low shear rate, the two-atom structure is formed, because of the strong particle attractions in adjacent layers. At higher shear rate, the synergetic pair structure extends to layered structure along flow direction because of the increasing hydrodynamics action. As the shear rate rises continuously, the layered structure rotates and collides with other particles, then turned to be individual particles under extension or curve string structure under compression. Finally, at the highest shear rate, the strings curve more severely and get into two-dimensional cluster. The apparent viscosity of the system changes from shear-thinning behavior to the shear-thickening behavior. This work presents valuable information for further understanding the shear thickening mechanism. Copyright © 2017 Elsevier Inc. All rights reserved.
The effect of non-Newtonian viscosity on the stability of the Blasius boundary layer

NASA Astrophysics Data System (ADS)

Griffiths, P. T.; Gallagher, M. T.; Stephen, S. O.

2016-07-01

We consider, for the first time, the stability of the non-Newtonian boundary layer flow over a flat plate. Shear-thinning and shear-thickening flows are modelled using a Carreau constitutive viscosity relationship. The boundary layer equations are solved in a self-similar fashion. A linear asymptotic stability analysis, that concerns the lower-branch structure of the neutral curve, is presented in the limit of large Reynolds number. It is shown that the lower-branch mode is destabilised and stabilised for shear-thinning and shear-thickening fluids, respectively. Favourable agreement is obtained between these asymptotic predictions and numerical results obtained from an equivalent Orr-Sommerfeld type analysis. Our results indicate that an increase in shear-thinning has the effect of significantly reducing the value of the critical Reynolds number, this suggests that the onset of instability will be significantly advanced in this case. This postulation, that shear-thinning destabilises the boundary layer flow, is further supported by our calculations regarding the development of the streamwise eigenfunctions and the relative magnitude of the temporal growth rates.
Boundary layers at the interface of two different shear flows

NASA Astrophysics Data System (ADS)

Weidman, Patrick D.; Wang, C. Y.

2018-05-01

We present solutions for the boundary layer between two uniform shear flows flowing in the same direction. In the upper layer, the flow has shear strength a, fluid density ρ1, and kinematic viscosity ν1, while the lower layer has shear strength b, fluid density ρ2, and kinematic viscosity ν2. Similarity transformations reduce the boundary-layer equations to a pair of ordinary differential equations governed by three dimensionless parameters: the shear strength ratio γ = b/a, the density ratio ρ = ρ2/ρ1, and the viscosity ratio ν = ν2/ν1. Further analysis shows that an affine transformation reduces this multi-parameter problem to a single ordinary differential equation which may be efficiently integrated as an initial-value problem. Solutions of the original boundary-value problem are shown to agree with the initial-value integrations, but additional dual and quadruple solutions are found using this method. We argue on physical grounds and through bifurcation analysis that these additional solutions are not tenable. The present problem is applicable to the trailing edge flow over a thin airfoil with camber.
Visualization of boundary-layer development on turbomachine blades with liquid crystals

NASA Technical Reports Server (NTRS)

Vanzante, Dale E.; Okiishi, Theodore H.

1991-01-01

This report documents a study of the use of liquid crystals to visualize boundary layer development on a turbomachine blade. A turbine blade model in a linear cascade of blades was used for the tests involved. Details of the boundary layer development on the suction surface of the turbine blade model were known from previous research. Temperature sensitive and shear sensitive liquid crystals were tried as visual agents. The temperature sensitive crystals were very effective in their ability to display the location of boundary layer flow separation and reattachment. Visualization of natural transition from laminar to turbulent boundary layer flow with the temperature sensitive crystals was possible but subtle. The visualization of separated flow reattachment with the shear sensitive crystals was easily accomplished when the crystals were allowed to make a transition from the focal-conic to a Grandjean texture. Visualization of flow reattachment based on the selective reflection properties of shear sensitive crystals was achieved only marginally because of the larger surface shear stress and shear stress gradient levels required for more dramatic color differences.
Experimental assessment of theory for refraction of sound by a shear layer

NASA Technical Reports Server (NTRS)

Schlinker, R. H.; Amiet, R. K.

1978-01-01

The refraction angle and amplitude changes associated with sound transmission through a circular, open-jet shear layer were studied in a 0.91 m diameter open jet acoustic research tunnel. Free stream Mach number was varied from 0.1 to 0.4. Good agreement between refraction angle correction theory and experiment was obtained over the test Mach number, frequency and angle measurement range for all on-axis acoustic source locations. For off-axis source positions, good agreement was obtained at a source-to-shear layer separation distance greater than the jet radius. Measureable differences between theory and experiment occurred at a source-to-shear layer separation distance less than one jet radius. A shear layer turbulence scattering experiment was conducted at 90 deg to the open jet axis for the same free stream Mach numbers and axial source locations used in the refraction study. Significant discrete tone spectrum broadening and tone amplitude changes were observed at open jet Mach numbers above 0.2 and at acoustic source frequencies greater than 5 kHz. More severe turbulence scattering was observed for downstream source locations.
Understanding the desensitizing mechanism of olefin in explosives: shear slide of mixed HMX-olefin systems.

PubMed

Zhang, Chaoyang; Cao, Xia; Xiang, Bin

2012-04-01

We simulated the shear slide behavior of typical mixed HMX-olefin systems and the effect of thickness of olefin layers (4-22 Å) on the behavior at a molecular level by considering two cases: bulk shear and interfacial shear. The results show that: (1) the addition of olefin into HMX can reduce greatly the shear sliding barriers relative to the pure HMX in the two cases, suggesting that the desensitizing mechanism of olefin is controlled dominantly by its good lubricating property; (2) the change of interaction energy in both systoles of shear slide is strongly dominated by van der Waals interaction; and (3) the thickness of olefin layers in the mixed explosives can influence its desensitizing efficiency. That is, the excessive thinness of olefin layers in the mixed explosive systems, for example, several angstroms, can lead to very high sliding barriers.
Features of sound propagation through and stability of a finite shear layer

NASA Technical Reports Server (NTRS)

Koutsoyannis, S. P.

1977-01-01

The plane wave propagation, the stability, and the rectangular duct mode problems of a compressible, inviscid, linearly sheared, parallel, homogeneous flow are shown to be governed by Whittaker's equation. The exact solutions for the perturbation quantities are essentially the Whittaker M-functions where the nondimensional quantities have precise physical meanings. A number of known results are obtained as limiting cases of the exact solutions. For the compressible finite thickness shear layer it is shown that no resonances and no critical angles exist for all Mach numbers, frequencies, and shear layer velocity profile slopes except in the singular case of the vortex sheet.
Effect of smear layer thickness and pH of self-adhesive resin cements on the shear bond strength to dentin.

PubMed

Ebrahimi Chaharom, Mohammad Esmaeel; Ajami, Amir Ahmad; Bahari, Mahmoud; Rezazadeh, Haleh

2017-01-01

There are concerns in relation to the bonding efficacy of self-adhesive resin cements to dentin covered with the smear layer. This study aims to evaluate the effect of smear layer thickness and different pH values of self-adhesive resin cements on the shear bond strength to dentin. The dentin on the buccal and lingual surfaces of 48 sound human premolars were abraded with 60- and 600-grit silicon carbide papers to achieve thick and thin smear layers, respectively. The samples were divided into three groups (n = 16) based on the cement pH: Rely-X Unicem (RXU) (pH < 2); Clearfil SA Luting (CSL) (pH = 3); and Speed CEM (SPC) (pH = 4.5). In each group, composite resin blocks were bonded to the buccal and lingual surfaces. After 24 h, the shear bond strength values were measured in MPa, and the failure modes were evaluated under a stereomicroscope. Data were analyzed with two-way ANOVA and post hoc least significant difference tests (P < 0.05). Cement pH had a significant effect on the shear bond strength (P = 0.02); however, the smear layer thickness had no significant effect on the shear bond strength (P > 0.05). The cumulative effect of these variables was not significant, either (P = 0.11). The shear bond strengths of SPC and CSL self-adhesive resin cements were similar and significantly lower than that of RXU. The smear layer thickness was not a determining factor for the shear bond strength value of self-adhesive resin cements.
Fan-structure wave as a source of earthquake instability

NASA Astrophysics Data System (ADS)

Tarasov, Boris

2015-04-01

Today frictional shear resistance along pre-existing faults is considered to be the lower limit on rock shear strength at confined compression corresponding to the seismogenic layer. This determines the lithospheric strength and the primary earthquake mechanism associated with frictional stick-slip instability on pre-existing faults. This paper introduces a recently identified shear rupture mechanism providing a paradoxical feature of hard rocks - the possibility of shear rupture propagation through the highly confined intact rock mass at shear stress levels significantly less than frictional strength. In the new mechanism the rock failure, associated with consecutive creation of small slabs (known as 'domino-blocks') from the intact rock in the rupture tip, is driven by a fan-shaped domino structure representing the rupture head. The fan-head combines such unique features as: extremely low shear resistance (below the frictional strength), self-sustaining stress intensification in the rupture tip (providing easy formation of new domino-blocks), and self-unbalancing conditions in the fan-head (making the failure process inevitably spontaneous and violent). An important feature of the fan-mechanism is the fact that for the initial formation of the fan-structure an enhanced local shear stress is required, however, after completion of the fan-structure it can propagate as a dynamic wave through intact rock mass at shear stresses below the frictional strength. Paradoxically low shear strength of pristine rocks provided by the fan-mechanism determines the lower limit of the lithospheric strength and favours the generation of new faults in pristine rocks in preference to frictional stick-slip instability along pre-existing faults. The new approach reveals an alternative role of pre-existing faults in earthquake activity: they represent local stress concentrates in pristine rock adjoining the fault where special conditions for the fan-mechanism nucleation are created, while further dynamic propagation of the new fault (earthquake) occurs at low field stresses even below the frictional strength. However, the proximity of the pre-existing discontinuities to the area of instability caused by the fan mechanism creates the illusion of stick-slip instability on the pre-existing faults, thus concealing the real situation.
Effect of particle-particle shearing on the bioleaching of sulfide minerals.

PubMed

Chong, N; Karamanev, D G; Margaritis, A

2002-11-05

The biological leaching of sulfide minerals, used for the production of gold, copper, zinc, cobalt, and other metals, is very often carried out in slurry bioreactors, where the shearing between sulfide particles is intensive. In order to be able to improve the efficiency of the bioleaching, it is of significant importance to know the effect of particle shearing on the rate of leaching. The recently proposed concept of ore immobilization allowed us to study the effect of particle shearing on the rate of sulfide (pyrite) leaching by Thiobacillus ferrooxidans. Using this concept, we designed two very similar bioreactors, the main difference between which was the presence and absence of particle-particle shearing. It was shown that when the oxygen mass transfer was not the rate-limiting step, the rate of bioleaching in the frictionless bioreactor was 2.5 times higher than that in a bioreactor with particle friction (shearing). The concentration of free suspended cells in the frictionless bioreactor was by orders of magnitude lower than that in the frictional bioreactor, which showed that particle friction strongly reduces the microbial attachment to sulfide surface, which, in turn, reduces the rate of bioleaching. Surprisingly, it was found that formation of a layer of insoluble iron salts on the surface of sulfide particles is much slower under shearless conditions than in the presence of particle-particle shearing. This was explained by the effect of particle friction on liquid-solid mass transfer rate. The results of this study show that reduction of the particle friction during bioleaching of sulfide minerals can bring important advantages not only by increasing significantly the bioleaching rate, but also by increasing the rate of gas-liquid oxygen mass transfer, reducing the formation of iron precipitates and reducing the energy consumption. One of the efficient methods for reduction of particle friction is ore immobilization in a porous matrix. Copyright 2002 Wiley Periodicals, Inc. Biotechnol Bioeng 80: 349-357, 2002.
Sediment transport and fluid mud layer formation in the macro-tidal Chikugo river estuary during a fortnightly tidal cycle

NASA Astrophysics Data System (ADS)

Azhikodan, Gubash; Yokoyama, Katsuhide

2018-03-01

The erosion and deposition dynamics of fine sediment in a highly turbid estuarine channel were successfully surveyed during the period from August 29 to September 12, 2009 using an echo sounder in combination with a high-resolution acoustic Doppler current profiler. Field measurements were conducted focusing on the tide driven dynamics of suspended sediment concentration (SSC), and fluid mud at the upstream of the macrotidal Chikugo river estuary during semidiurnal and fortnightly tidal cycles. Morphological evolution was observed especially during the spring tide over a period of two weeks. The elevation of the channel bed was stable during neap tide, but it underwent fluctuations when the spring tide occurred owing to the increase in the velocity and shear stress. Two days of time lag were observed between the maximum SSC and peak tidal flow, which resulted in the asymmetry between neap-to-spring and spring-to-neap transitions. During the spring tide, a hysteresis loop was observed between shear stress and SSC, and its direction was different during flood and ebb tides. Although both fine sediments and flocs were dominant during flood tides, only fine sediments were noticed during ebb tides. Hence, the net elevation change in the bed was positive, and sedimentation took place during the semilunar tidal cycle. Finally, a bed of consolidated mud was deposited on the initial bed, and the height of the channel bed increased by 0.9 m during the two-week period. The observed hysteretic effect between shear stress and SSC during the spring tides, and the asymmetrical neap-spring-neap tidal cycle influenced the near-bed sediment dynamics of the channel, and led to the formation of a fluid mud layer at the bottom of the river.

Leaping shampoo glides on a 500-nm-thick lubricating air layer

NASA Astrophysics Data System (ADS)

Li, Erqiang; Lee, Sanghyun; Marston, Jeremy; Bonito, Andrea; Thoroddsen, Sigurdur

2013-11-01

When a stream of shampoo is fed onto a pool in one's hand, a jet can leap sideways or rebound from the liquid surface in an intriguing phenomenon known as the Kaye effect. Earlier studies have debated whether non-Newtonian effects are the underlying cause of this phenomenon, making the jet glide on top of a shear-thinning liquid layer, or whether an entrained air layer is responsible. Herein we show unambiguously that the jet slides on a lubricating air layer [Lee et al., Phys. Rev. E 87, 061001 (2013)]. We identify this layer by looking through the pool liquid and observing its rupture into fine micro-bubbles. The resulting micro-bubble sizes suggest that the thickness of this air layer is around 500 nm. This thickness estimate is also supported by the tangential deceleration of the jet during the rebounding, with the shear stress within the thin air layer sufficient for the observed deceleration. Particle tracking within the jet shows uniform velocity, with no pronounced shear, which would be required for shear-thinning effects. The role of the surfactant may primarily be to stabilize the air film.
Impedance method for measuring shear elasticity of liquids

NASA Astrophysics Data System (ADS)

Badmaev, B. B.; Dembelova, T. S.; Damdinov, B. B.; Gulgenov, Ch. Zh.

2017-11-01

Experimental results of studying low-frequency (74 kHz) shear elasticity of polymer liquids by the impedance method (analogous to the Mason method) are presented. A free-volume thick liquid layer is placed on the horizontal surface of a piezoelectric quartz crystal with dimensions 34.7 × 12 × 5.5 cm. The latter performs tangential vibrations at resonance frequency. The liquid layer experiences shear strain, and shear waves should propagate in it. From the theory of the method, it follows that, with an increase in the layer thickness, both real and imaginary resonance frequency shifts should exhibit damped oscillations and tend to limiting values. For the liquids under study, the imaginary frequency shift far exceeds the real one, which testifies to the presence of bulk shear elasticity.
Application of a Reynolds stress turbulence model to the compressible shear layer

NASA Technical Reports Server (NTRS)

Sarkar, S.; Balakrishnan, L.

1990-01-01

Theoretically based turbulence models have had success in predicting many features of incompressible, free shear layers. However, attempts to extend these models to the high-speed, compressible shear layer have been less effective. In the present work, the compressible shear layer was studied with a second-order turbulence closure, which initially used only variable density extensions of incompressible models for the Reynolds stress transport equation and the dissipation rate transport equation. The quasi-incompressible closure was unsuccessful; the predicted effect of the convective Mach number on the shear layer growth rate was significantly smaller than that observed in experiments. Having thus confirmed that compressibility effects have to be explicitly considered, a new model for the compressible dissipation was introduced into the closure. This model is based on a low Mach number, asymptotic analysis of the Navier-Stokes equations, and on direct numerical simulation of compressible, isotropic turbulence. The use of the new model for the compressible dissipation led to good agreement of the computed growth rates with the experimental data. Both the computations and the experiments indicate a dramatic reduction in the growth rate when the convective Mach number is increased. Experimental data on the normalized maximum turbulence intensities and shear stress also show a reduction with increasing Mach number.
Pseudotachylyte formation vs. mylonitization - repeated cycles of seismic fracture and aseismic creep in the middle crust (Woodroffe Thrust, Central Australia)

NASA Astrophysics Data System (ADS)

Wex, Sebastian; Mancktelow, Neil; Hawemann, Friedrich; Camacho, Alfredo; Pennacchioni, Giorgio

2014-05-01

The Musgrave Ranges in Central Australia provide excellent exposure of the shallowly south-dipping Woodroffe Thrust, which placed ~1200 Ma granulites onto amphibolite facies gneisses. This ~400 km long E-W structure developed under mid-crustal conditions during the intracratonic Petermann Orogeny around 550 Ma. From field observations and measurements, the shortening direction is constrained to be N-S and the movement sense top-to-north. Ductile deformation during this process almost entirely localized in the footwall rocks, developing a zone of mylonites, ultramylonites and sheared pseudotachylytes, several hundred metres wide, with pseudotachylyte abundance rapidly decreasing further into the footwall. In contrast, the hanging wall behaved in a predominantly brittle manner, producing significant volumes of pseudotachylyte breccia and isolated veins, but was otherwise mostly unaffected and only weakly foliated. The difference in rheological behaviour is reflected in the pseudotachylyte fabric, which is dominantly sheared in the footwall and largely unsheared in the hanging wall. Low-strain domains in the footwall show that localized shearing initiated along pseudotachylyte veins and that shear zones and mylonitic foliations were in turn exploited by subsequent pseudotachylyte veins. Neither phyllonitization nor synkinematic growth of new muscovite is observed. In contrast to models with a simple brittle-to-viscous transition, these observations show that a continuous cycle of brittle fracturing and shearing is active in dry mid-crustal environments. The products of multiple earthquakes and ductile overprint, repeatedly exploiting the same structural discontinuity, are composite layers of sheared pseudotachylyte. In the Woodroffe Thrust, these layers are numerous and frequently observed parallel to the foliation in the footwall mylonites. The thickest of these sheared pseudotachylyte horizons (~15 m thick) mark the immediate contact to the hanging wall and almost entirely consist of pseudotachylyte matrix. Particularly in the footwall, but locally also in the hanging wall, shear strain can additionally be concentrated along the margins of dolerite dykes, whose mineral assemblages will be studied to determine the metamorphic conditions that were active during development of the Woodroffe Thrust.
Oxygen inhibition layer of composite resins: effects of layer thickness and surface layer treatment on the interlayer bond strength.

PubMed

Bijelic-Donova, Jasmina; Garoushi, Sufyan; Lassila, Lippo V J; Vallittu, Pekka K

2015-02-01

An oxygen inhibition layer develops on surfaces exposed to air during polymerization of particulate filling composite. This study assessed the thickness of the oxygen inhibition layer of short-fiber-reinforced composite in comparison with conventional particulate filling composites. The effect of an oxygen inhibition layer on the shear bond strength of incrementally placed particulate filling composite layers was also evaluated. Four different restorative composites were selected: everX Posterior (a short-fiber-reinforced composite), Z250, SupremeXT, and Silorane. All composites were evaluated regarding the thickness of the oxygen inhibition layer and for shear bond strength. An equal amount of each composite was polymerized in air between two glass plates and the thickness of the oxygen inhibition layer was measured using a stereomicroscope. Cylindrical-shaped specimens were prepared for measurement of shear bond strength by placing incrementally two layers of the same composite material. Before applying the second composite layer, the first increment's bonding site was treated as follows: grinding with 1,000-grit silicon-carbide (SiC) abrasive paper, or treatment with ethanol or with water-spray. The inhibition depth was lowest (11.6 μm) for water-sprayed Silorane and greatest (22.9 μm) for the water-sprayed short-fiber-reinforced composite. The shear bond strength ranged from 5.8 MPa (ground Silorane) to 36.4 MPa (water-sprayed SupremeXT). The presence of an oxygen inhibition layer enhanced the interlayer shear bond strength of all investigated materials, but its absence resulted in cohesive and mixed failures only with the short-fiber-reinforced composite. Thus, more durable adhesion with short-fiber-reinforced composite is expected. © 2014 Eur J Oral Sci.
Structure of high and low shear-stress events in a turbulent boundary layer

NASA Astrophysics Data System (ADS)

Gomit, G.; de Kat, R.; Ganapathisubramani, B.

2018-01-01

Simultaneous particle image velocimetry (PIV) and wall-shear-stress sensor measurements were performed to study structures associated with shear-stress events in a flat plate turbulent boundary layer at a Reynolds number Reτ≈4000 . The PIV field of view covers 8 δ (where δ is the boundary layer thickness) along the streamwise direction and captures the entire boundary layer in the wall-normal direction. Simultaneously, wall-shear-stress measurements that capture the large-scale fluctuations were taken using a spanwise array of hot-film skin-friction sensors (spanning 2 δ ). Based on this combination of measurements, the organization of the conditional wall-normal and streamwise velocity fluctuations (u and v ) and of the Reynolds shear stress (-u v ) can be extracted. Conditional averages of the velocity field are computed by dividing the histogram of the large-scale wall-shear-stress fluctuations into four quartiles, each containing 25% of the occurrences. The conditional events corresponding to the extreme quartiles of the histogram (positive and negative) predominantly contribute to a change of velocity profile associated with the large structures and in the modulation of the small scales. A detailed examination of the Reynolds shear-stress contribution related to each of the four quartiles shows that the flow above a low wall-shear-stress event carries a larger amount of Reynolds shear stress than the other quartiles. The contribution of the small and large scales to this observation is discussed based on a scale decomposition of the velocity field.
Fluid Effects on Shear for Seismic Waves in Finely Layered Porous Media

DOE Office of Scientific and Technical Information (OSTI.GOV)

Berryman, J G

Although there are five effective shear moduli for any layered VTI medium, one and only one effective shear modulus of the layered system (namely the uniaxial shear) contains all the dependence of pore fluids on the elastic or poroelastic constants that can be observed in vertically polarized shear waves. Pore fluids can increase the magnitude the shear energy stored in this modulus by an amount that ranges from the smallest to the largest effective shear moduli of the VTI system. But, since there are five shear moduli in play, the overall increase in shear energy due to fluids is reducedmore » by a factor of about 5 in general. We can therefore give definite bounds on the maximum increase of overall shear modulus, being about 20% of the allowed range as liquid is fully substituted for gas. An attendant increase of density (depending on porosity and fluid density) by approximately 5 to 10% decreases the shear wave speed and, thereby, partially offsets the effect of this shear modulus increase. The final result is an increase of shear wave speed on the order of 5 to 10%. This increase is shown to be possible under most favorable circumstances - i.e. when the shear modulus fluctuations are large (resulting in strong anisotropy) and the medium behaves in an undrained fashion due to fluid trapping. At frequencies higher than seismic (such as sonic and ultrasonic waves for well-logging or laboratory experiments), resulting short response times also produce the requisite undrained behavior and, therefore, fluids also affect shear waves at high frequencies by increasing rigidity.« less
Study on shear strengthening of RC continuous T-beams using different layers of CFRP strips

DOE Office of Scientific and Technical Information (OSTI.GOV)

Alferjani, M. B. S.; Samad, A. A. Abdul; Mohamad, Noridah

2015-05-15

Carbon fiber reinforced polymer (CFRP) laminates are externally bonded to reinforced concrete (RC) members to provide additional strength such as flexural, shear, etc. However, this paper presents the results of an experimental investigation for enhancing the shear capacity of reinforced concrete (RC) continuous T- beams using different layers of CFRP wrapping schemes. A total of three concrete beams were tested and various sheet configurations and layouts were studied to determine their effects on ultimate shear strength and shear capacity of the beams. One beam was kept as control beams, while other beams were strengthened with externally bonded CFRP strips withmore » three side bonding and one or two layers of CFRP strips. From the test results, it was found that all schemes were found to be effective in enhancing the shear strength of RC beams. It was observed that the strength increases with the number of sheet layers provided the most effective strengthening for RC continuous T- beam. Beam strengthened using this scheme showed 23.21% increase in shear capacity as compared to the control beam. Two prediction models available in literature were used for computing the contribution of CFRP strips and compared with the experimental results.« less
Microstructures and Mechanical Properties of Friction Stir Spot Welded Aluminum Alloy AA2014

NASA Astrophysics Data System (ADS)

Babu, S.; Sankar, V. S.; Janaki Ram, G. D.; Venkitakrishnan, P. V.; Madhusudhan Reddy, G.; Prasad Rao, K.

2013-01-01

Friction stir spot welding (FSSW) is a relatively recent development, which can provide a superior alternative to resistance spot welding and riveting for fabrication of aluminum sheet metal structures. In the current work, FSSW experiments were conducted in 3-mm thick sheets of aluminum alloy 2014 in T4 and T6 conditions, with and without Alclad layers. The effects of tool geometry and welding process parameters on joint formation were investigated. A good correlation between process parameters, bond width, hook height, joint strength, and fracture mode was observed. The presence of Alclad layers and the base metal temper condition were found to have no major effect on joint formation and joint strength. Friction stir spot welds produced under optimum conditions were found to be superior to riveted joints in lap-shear and cross-tension tests. The prospects of FSSW in aluminum sheet metal fabrication are discussed.
Interfacial Reaction and IMC Growth of an Ultrasonically Soldered Cu/SAC305/Cu Structure during Isothermal Aging

PubMed Central

Long, Weifeng; Hu, Xiaowu; Fu, Yanshu

2018-01-01

In order to accelerate the growth of interfacial intermetallic compound (IMC) layers in a soldering structure, Cu/SAC305/Cu was first ultrasonically spot soldered and then subjected to isothermal aging. Relatively short vibration times, i.e., 400 ms and 800 ms, were used for the ultrasonic soldering. The isothermal aging was conducted at 150 °C for 0, 120, 240, and 360 h. The evolution of microstructure, the IMC layer growth mechanism during aging, and the shear strength of the joints after aging were systemically investigated. Results showed the following. (i) Formation of intermetallic compounds was accelerated by ultrasonic cavitation and streaming effects, the thickness of the interfacial Cu6Sn5 layer increased with aging time, and a thin Cu3Sn layer was identified after aging for 360 h. (ii) The growth of the interfacial IMC layer of the ultrasonically soldered Cu/SAC305/Cu joints followed a linear function of the square root of the aging time, revealing a diffusion-controlled mechanism. (iii) The tensile shear strength of the joint decreased to a small extent with increasing aging time, owing to the combined effects of IMC grain coarsening and the increase of the interfacial IMC. (iv) Finally, although the fracture surfaces and failure locations of the joint soldered with 400 ms and 800 ms vibration times show similar characteristics, they are influenced by the aging time. PMID:29316625
Effects of unidirectional flow shear stresses on the formation, fractal microstructure and rigidity of incipient whole blood clots and fibrin gels.

PubMed

Badiei, N; Sowedan, A M; Curtis, D J; Brown, M R; Lawrence, M J; Campbell, A I; Sabra, A; Evans, P A; Weisel, J W; Chernysh, I N; Nagaswami, C; Williams, P R; Hawkins, K

2015-01-01

Incipient clot formation in whole blood and fibrin gels was studied by the rheometric techniques of controlled stress parallel superposition (CSPS) and small amplitude oscillatory shear (SAOS). The effects of unidirectional shear stress on incipient clot microstructure, formation kinetics and elasticity are reported in terms of the fractal dimension (df) of the fibrin network, the gel network formation time (TGP) and the shear elastic modulus, respectively. The results of this first haemorheological application of CSPS reveal the marked sensitivity of incipient clot microstructure to physiologically relevant levels of shear stress, these being an order of magnitude lower than have previously been studied by SAOS. CSPS tests revealed that exposure of forming clots to increasing levels of shear stress produces a corresponding elevation in df, consistent with the formation of tighter, more compact clot microstructures under unidirectional flow. A corresponding increase in shear elasticity was recorded. The scaling relationship established between shear elasticity and df for fibrin clots and whole blood confirms the fibrin network as the dominant microstructural component of the incipient clot in terms of its response to imposed stress. Supplementary studies of fibrin clot formation by rheometry and microscopy revealed the substantial additional network mass required to increase df and provide evidence to support the hypothesis that microstructural changes in blood clotted under unidirectional shear may be attributed to flow enhanced thrombin generation and activation. CSPS also identified a threshold value of unidirectional shear stress above which no incipient clot formation could be detected. CSPS was shown to be a valuable haemorheological tool for the study of the effects of physiological and pathological levels of shear on clot properties.
Identification of the Viscous Superlayer on the Low-Speed Side of a Single-Stream Shear Layer

NASA Astrophysics Data System (ADS)

Foss, John; Peabody, Jason

2010-11-01

Image pairs (elevation/plan views) have been acquired of a smoke streakline originating in the irrotational region on the low-speed side of a high Re single-stream shear layer of Morris and Foss (2003). The viscous superlayer (VSL) is identified as the terminus of the streak; 1800 such images provide VSL position statistics. Hot-wire data acquired concurrently at the shear layer edge and interior are used to investigate the relationship between these velocity magnitudes and the large-scale motions. Distinctive features (plumes) along the streakline are tracked between images to provide discrete irrotational region velocity magnitudes and material trajectories. A non-diffusive marker, introduced in the separating (high speed) boundary layer and imaged at x/θo=352, has revealed an unexpected bias in the streak-defined VSL locations. The interpretation of this bias clarifies the induced flow patterns in the entrainment region. The observations are consistent with a conception of the large-scale shear layer motions as "billows" of vortical fluid separated by re-entrant "wedges" of irrotational fluid, per Phillips (1972). Morris, S.C. and Foss, J.F. (2003). "Turbulent Boundary Layer to Single Stream Shear Layer: The Transition Region." Journal of Fluid Mechanics. Vol. 494, pp. 187-221. Phillips, O. M. (1972). "The Entrainment Interface." Journal of Fluid Mechanics. Vol. 51, pp. 97-118.
An experimental study of transmission, reflection and scattering of sound in a free-jet flight simulation facility and comparison with theory

NASA Technical Reports Server (NTRS)

Ahuja, K. K.; Tester, B. J.; Tanna, H. K.; Searle, N.

1977-01-01

Acoustic time delays across a free-jet shear layer are measured and compared with predictions based on (1) ray paths refracted abruptly across a cylindrical vortex sheet and (2) ray paths traced through a more realistic diverging flow model. The close agreement between measurement and theory confirms that Snell's law provides an accurate prediction of wavefront refraction or angle changes across a diverging shear layer. Microphones are placed on calculated ray paths to determine the coherent transmission and internal reflection characteristics of the shear layer and also the scattering of sound by the shear-layer turbulence. The transmission data essentially verify the proposed, theoretical calibration factor which forms part of a computational procedure that is being developed to convert model jet data from a free-jet facility to inflight conditions.
Experimental validation of non-uniformity effect of the radial electric field on the edge transport barrier formation in JT-60U H-mode plasmas

PubMed Central

Kamiya, K.; Itoh, K.; Itoh, S.-I.

2016-01-01

The turbulent structure formation, where strongly-inhomogeneous turbulence and global electromagnetic fields are self-organized, is a fundamental mechanism that governs the evolution of high-temperature plasmas in the universe and laboratory (e.g., the generation of edge transport barrier (ETB) of the H-mode in the toroidal plasmas). The roles of inhomogeneities of radial electric field (Er) are known inevitable. In this mechanism, whether the first derivative of Er (shear) or the second derivative of Er (curvature) works most is decisive in determining the class of nontrivial solutions (which describe the barrier structure). Here we report the experimental identification of the essential role of the Er-curvature on the ETB formation, for the first time, based on the high-spatiotemporal resolution spectroscopic measurement. We found the decisive importance of Er-curvature on ETB formation during ELM-free phase, but there is only a low correlation with the Er-shear value at the peak of normalized ion temperature gradient. Furthermore, in the ELMing phase, the effect of curvature is also quantified in terms of the relationship between pedestal width and thickness of the layer of inhomogeneous Er. This is the fundamental basis to understand the structure of transport barriers in fusion plasmas. PMID:27480931
Volcanism and Fluvio-Glacial Processes on the Interior Layered Deposits of Valles Marineris, Mars?

NASA Astrophysics Data System (ADS)

Chapman, M. G.

2005-12-01

The Interior Layered Deposits (ILDs) in Valles Marineris have been suggested to be possible sub-ice volcanoes. Recent images also show evidence of possible fluvio-glacial processes on the ILDs and hence volcano/ice/water interaction. For example, Mars Express Mission anaglyph from Orbit 334 of central Ophir and Candor Chasmata, THEMIS image V10551002, and MOC images E1700142 and E190020 show 2 ILD mounds in central Candor Chasma that have been sheared off at approximately equal elevations by some material that has been subsequently removed. Level shearing of ILD rock materials and subsequent removal of the abrasive material, suggest ice erosion and glacial processes because glacial ice is mobile enough to grind the rock and can melt away. Another adjacent ILD mound in Central Candor shows an abrupt flank termination and damming of material, rather than flank scour. The dammed material appears to be layers piled up in a ridge at the ILD base. This relation is observed on the HRSC anaglyph and MOC images E0101343 and E201146. Another ILD in Melas Chasma, seen on MOC image M0804981, shows lobes of flank material that terminate along a lineation; possibly suggesting lobe confinement against subsequently removed material. This morphology can also be observed on the flank of the Gangis Chasma ILD in MOC image M0705587. A possible terrestrial volcanic analog for this ILD flank morphology is the Helgafell hyaloclasitic ridge (tindar) in Iceland (Chapman et al., 2004), the eastern flank of which has a linear termination interpreted as largely unmodified and caused by hyalotuff material banked against a former ice wall that has since melted away (Schopka et al., 2003). Glacial shearing of some ILDs and confined banking of other ILDs suggest that these mounds formed at different times, as the sheared ILD likely predated ice and the confined ILD may have formed concurrently with ice. Alternatively, the banking may have been due to lack of shear forces (static ice) and confined post-depositional avalanche deposits. However, exposed in the banked cliff faces are near horizontal bedding planes that can be traced upslope into angled flank layers; a relation that may suggest ice concurrent with volcanic ILD formation (Chapman and Smellie, in press). In addition to glacial processes, many Mars ILDs show fluvial gullies cut into mostly low lying flank deposits. Gullies are eroded into all sides of the ILDs including their north-facing slopes, so solar heating likely did not generate the gullies. Although formal work on the subject is lacking, ongoing terrestrial observation by the author (on an edifice north of Helgafell and in Gjalp eruption films) indicate fluvial erosion of subglacial volcanoes on Earth may be concurrent with their formation, occurring after edifices rise above their surrounding ice-confined meltwater lake. Remnant ice on the top of the edifices can melt to generate streams that erode the growing volcanic flanks.
Microstructural Evolution during Mid-Crustal Shear Zone Thickening and Thinning, Mount Irene Detachment Zone, Fiordland, New Zealand

NASA Astrophysics Data System (ADS)

Negrini, M.; Smith, S. A. F.; Scott, J.; Rooney, J. S.; Demurtas, M.

2016-12-01

Recent work has shown that ductile shear zones experience cyclic variations in stress and strain rate due to, for example, elastic loading from earthquake slip on brittle faults or the presence of rigid particles and asperities within the shear zone. Such non-steady state flow conditions can promote microstructural changes including a decrease in grain sizes followed by a switch in the main deformation mechanisms. Understanding the microstructural changes that occur during non steady-state deformation is therefore critical in evaluating shear zone rheology. The Mount Irene shear zone formed during Cretaceous extension in the middle crust and was active at temperatures of 600°C and pressures of 6 kbar. The shear zone localized in a basal calcite marble layer typically 3-5 m thick containing hundreds of thin (mm-cm) calc-silicate bands that are now parallel to the shear zone boundaries. The lower boundary of the shear zone preserves meter-scale undulations that cause the shear zone to be squeezed in to regions that are <1.5 m thick. The calc-silicate bands act as "flow markers" and allow individual shear zone layers to be traced continuously through thick and thin regions, implying that the mylonites experienced cyclic variations in stress and strain rate. Calc-mylonite samples collected from the same layer close to the base of the shear zone reveal that layer thinning was accompanied by progressive microstructural changes including intense twinning, stretching and flattening of large calcite porphyroclasts as well as the development of interconnected networks of recrystallized calcite aggregates. EBSD analysis shows that the recrystallized aggregates contain polygonal calcite grains with microstructures (e.g. grain quadruple junctions) similar to those reported for neighbor-switching processes associated with grain boundary sliding and superplasticity. Ongoing and future work will utilize samples from across the full thickness of the shear zone to determine key microstructural changes and deformation mechanisms that accommodated shear zone thinning and thickening during non-steady state deformation.
Mechanics of Unidirectional Fiber-Reinforced Composites: Buckling Modes and Failure Under Compression Along Fibers

NASA Astrophysics Data System (ADS)

Paimushin, V. N.; Kholmogorov, S. A.; Gazizullin, R. K.

2018-01-01

One-dimensional linearized problems on the possible buckling modes of an internal or peripheral layer of unidirectional multilayer composites with rectilinear fibers under compression in the fiber direction are considered. The investigations are carried out using the known Kirchhoff-Love and Timoshenko models for the layers. The binder, modeled as an elastic foundation, is described by the equations of elasticity theory, which are simplified in accordance to the model of a transversely soft layer and are integrated along the transverse coordinate considering the kinematic coupling relations for a layer and foundation layers. Exact analytical solutions of the problems formulated are found, which are used to calculate a composite made of an HSE 180 REM prepreg based on a unidirectional carbon fiber tape. The possible buckling modes of its internal and peripheral layers are identified. Calculation results are compared with experimental data obtained earlier. It is concluded that, for the composite studied, the flexural buckling of layers in the uniform axial compression of specimens along fibers is impossible — the failure mechanism is delamination with buckling of a fiber bundle according to the pure shear mode. It is realized (due to the low average transverse shear modulus) at the value of the ultimate compression stress equal to the average shear modulus. It is shown that such a shear buckling mode can be identified only on the basis of equations constructed using the Timoshenko shear model to describe the deformation process of layers.
Microstructure, texture and colour development during crust formation on whole muscle chicken fillets.

PubMed

Barbut, S

2013-01-01

1. The development of crust during a 22-min period was evaluated in an oven, and in previously cooked-in-bag products (no crust) placed in an oven for 10 min. The oven-roasted products started to develop a thin (2-4 μm) crust layer after 4 min. At that point, the colour of the fillets turned white but no browning was observed. As roasting time increased, crust thickness and shear force increased, the product turned brown and eventually black at certain spots. 2. Light microscopy revealed the shrinking of muscle fibres close to the surface, as they also lost water. At a certain point, tears between the different layers started to appear. The inner muscle fibres also progressively shrank and the spaces between them increased. Microscopy of cook-in-bag products revealed no crust formation during heating. Upon moving to the oven, crust started to form but was much faster compared with the other products. 3. Cook-in-the-bag samples showed a higher rate of cook loss during the first 12 min (to internal 70°C) compared with oven heating. This could have been due to the fast heating rate in water and/or no crust formation. 4. White colour was fully formed on water-cooked fillets within 2 min (L* = 83), while it was gradually forming on oven-roasted samples (max L* of 79 after 12 min). 5. Shear force measurements showed an increase in both treatments up to 18 min, with a decrease thereafter (when dry crust started to crack).
Structural development of the Red Hill portion of the Feather River ultramafic complex, Pulmas County, California

DOE Office of Scientific and Technical Information (OSTI.GOV)

Weisenberg, C.W.

1979-01-01

The Feather River Ultramafic Complex is a partially serpentinized body of metamorphosed alpine peridotite and gabbro that lies along the northern part of the Melones fault zone, a NNW trending belt in the Northern Sierra Nevada. The complex was studied in the area of Red Hill, near the canyon of the North Fork, Feather River. The complex is separated from the Calaveras Terrain and Arlington Formation country rocks by steep faults; the Melones Fault on the east and the Rich Bar Fault on the west. Units recognized within the complex include Rich Bar metamorphic rocks, peridotite, metaperidotite, tremolite-olivine schist, hornblendemore » schist, and layered metagabbro. The Rich Bar metamorphic rocks are tectonic slices of amphibolite grade hornblende schist, mica schist, and quartzite found along the Rich Bar Fault. The complex shows evidence of 4 major events. E-1 (Pennsylvania-Permian) was formation of the peridotite-gabbro complex. E-2 (Permo-Triassic) consisted of pervasive shearing parallel to the Rich Bar Fault associated with initial emplacement within the Sierra Nevada. E-3 is believed to be compression and metamorphism (serpentinization) associated with the Nevadan orogeny. E-4 was associated with intrusion of nearby plutons. The regional association of the complex with late paleozoic arc volcanics of the Taylorsville area suggest formation near or under an island arc. Metamorphism during emplacement indicates association with the arc at that time. Left-lateral shear during emplacement along the Rich Bar Fault indicates NW directed thrusting when the layering in metagabbro is rotated to horizontal.« less
Lattice Boltzmann Study of Bubbles on a Patterned Superhydrophobic Surface under Shear Flow

NASA Astrophysics Data System (ADS)

Chen, Wei; Wang, Kai; Hou, Guoxiang; Leng, Wenjun

2018-01-01

This paper studies shear flow over a 2D patterned superhydrophobic surface using lattice Boltzmann method (LBM). Single component Shan-Chen multiphase model and Carnahan-Starling EOS are adopted to handle the liquid-gas flow on superhydrophobic surface with entrapped micro-bubbles. The shape of bubble interface and its influence on slip length under different shear rates are investigated. With increasing shear rate, the bubble interface deforms. Then the contact lines are depinned from the slot edges and move downstream. When the shear rate is high enough, a continuous gas layer forms. If the protrusion angle is small, the gas layer forms and collapse periodically, and accordingly the slip length changes periodically. While if the protrusion angle is large, the gas layer is steady and separates the solid wall from liquid, resulting in a very large slip length.

Turbulent shear stresses in compressible boundary layers

NASA Technical Reports Server (NTRS)

Laderman, A. J.; Demetriades, A.

1979-01-01

Hot-wire anemometer measurements of turbulent shear stresses in a Mach 3 compressible boundary layer were performed in order to investigate the effects of heat transfer on turbulence. Measurements were obtained by an x-probe in a flat plate, zero pressure gradient, two dimensional boundary layer in a wind tunnel with wall to freestream temperature ratios of 0.94 and 0.71. The measured shear stress distributions are found to be in good agreement with previous results, supporting the contention that the shear stress distribution is essentially independent of Mach number and heat transfer for Mach numbers from incompressible to hypersonic and wall to freestream temperature ratios of 0.4 to 1.0. It is also found that corrections for frequency response limitations of the electronic equipment are necessary to determine the correct shear stress distribution, particularly at the walls.
Aerodynamics of a translating comb-like plate inspired by a fairyfly wing

NASA Astrophysics Data System (ADS)

Lee, Seung Hun; Kim, Daegyoum

2017-08-01

Unlike the smooth wings of common insects or birds, micro-scale insects such as the fairyfly have a distinctive wing geometry, comprising a frame with several bristles. Motivated by this peculiar wing geometry, we experimentally investigated the flow structure of a translating comb-like wing for a wide range of gap size, angle of attack, and Reynolds number, Re = O(10) - O(103), and the correlation of these parameters with aerodynamic performance. The flow structures of a smooth plate without a gap and a comb-like plate are significantly different at high Reynolds number, while little difference was observed at the low Reynolds number of O(10). At low Reynolds number, shear layers that were generated at the edges of the tooth of the comb-like plate strongly diffuse and eventually block a gap. This gap blockage increases the effective surface area of the plate and alters the formation of leading-edge and trailing-edge vortices. As a result, the comb-like plate generates larger aerodynamic force per unit area than the smooth plate. In addition to a quasi-steady phase after the comb-like plate travels several chords, we also studied a starting phase of the shear layer development when the comb-like plate begins to translate from rest. While a plate with small gap size can generate aerodynamic force at the starting phase as effectively as at the quasi-steady phase, the aerodynamic force drops noticeably for a plate with a large gap because the diffusion of the developing shear layers is not enough to block the gap.
N-BODY SIMULATION OF PLANETESIMAL FORMATION THROUGH GRAVITATIONAL INSTABILITY AND COAGULATION. II. ACCRETION MODEL

DOE Office of Scientific and Technical Information (OSTI.GOV)

Michikoshi, Shugo; Kokubo, Eiichiro; Inutsuka, Shu-ichiro, E-mail: michikoshi@cfca.j, E-mail: kokubo@th.nao.ac.j, E-mail: inutsuka@tap.scphys.kyoto-u.ac.j

2009-10-01

The gravitational instability of a dust layer is one of the scenarios for planetesimal formation. If the density of a dust layer becomes sufficiently high as a result of the sedimentation of dust grains toward the midplane of a protoplanetary disk, the layer becomes gravitationally unstable and spontaneously fragments into planetesimals. Using a shearing box method, we performed local N-body simulations of gravitational instability of a dust layer and subsequent coagulation without gas and investigated the basic formation process of planetesimals. In this paper, we adopted the accretion model as a collision model. A gravitationally bound pair of particles ismore » replaced by a single particle with the total mass of the pair. This accretion model enables us to perform long-term and large-scale calculations. We confirmed that the formation process of planetesimals is the same as that in the previous paper with the rubble pile models. The formation process is divided into three stages: the formation of nonaxisymmetric structures; the creation of planetesimal seeds; and their collisional growth. We investigated the dependence of the planetesimal mass on the simulation domain size. We found that the mean mass of planetesimals formed in simulations is proportional to L {sup 3/2} {sub y}, where L{sub y} is the size of the computational domain in the direction of rotation. However, the mean mass of planetesimals is independent of L{sub x} , where L{sub x} is the size of the computational domain in the radial direction if L{sub x} is sufficiently large. We presented the estimation formula of the planetesimal mass taking into account the simulation domain size.« less
The Effect of Saturation on Shear Wave Anisotropy in a Transversely Isotropic Medium

NASA Astrophysics Data System (ADS)

Li, W.; Pyrak-Nolte, L. J.

2010-12-01

Seismic monitoring of fluid distributions in the subsurface requires an understanding of the effect of fluid saturation on the anisotropic properties of layered media. Austin Chalk is a carbonate rock composed mainly of calcite (99.9%) with fine bedding caused by a weakly-directed fabric. In this paper, we assess the shear-wave anisotropy of Austin Chalk and the effect of saturation on interpreting anisotropy based on shear wave velocity, attenuation and spectral content as a function of saturation. In the laboratory, we performed full shear-waveform measurements on several dry cubic samples of Austin Chalk with dimensions 50mm x 50mm x 50mm. Two shear-wave contact transducers (central Frequency 1 MHz) were use to send and receive signals. Data was collected for three orthogonal orientations of the sample and as a function of shear wave polarization relative to the layers in the sample. For the waves propagated parallel to the layers, both fast and slow shear waves were observed with velocities of 3444 m/s and 3193 m/s, respectively. It was noted that the minimum and maximum shear wave velocities did not occur when the shear wave polarization were perpendicular or parallel to the layering in the sample but occurred at an orientation of ~25 degrees from the normal to the layers. The sample was then vacuum saturated with water for approximately ~15 hours. The same measurements were performed on the saturated sample as those on the dry sample. Both shear wave velocities observed decreased upon water-saturation with corresponding velocities of 3155 m/s and 2939 m/s, respectively. In the dry condition the difference between the fast and slow shear wave velocities was 250 m/s. This difference decreased to 215 m/s after fluid saturation. In both the dry and saturated condition, the shear wave velocity for waves propagated perpendicularly to the layers was independent of polarization and had the same magnitude as that of the slow shear wave. A wavelet analysis was performed to determine changes in the spectral content of the signals upon saturation as well velocity dispersion. We found that (1) low frequency components exhibit a larger difference in time delay between the fast and slow shear waves for the water-saturated condition than for the dry condition; (2) that high frequency components have relatively small differences in time delay between the dry and saturated conditions; and (3) the dominant frequency shifted to lower frequencies for the fast shear wave upon saturation while no change in dominant frequency was observed for the slow shear wave upon saturation. Thus, fluid saturation affects shear velocity as well as the spectral content of the signal. Acknowledgments: The authors wish to acknowledge support of this work by the Geosciences Research Program, Office of Basic Energy Sciences US Department of Energy (DE-FG02-09ER16022), by Exxon Mobil Upstream Research Company and the GeoMathematical Imaging Group at Purdue University.
Extremely high wall-shear stress events in a turbulent boundary layer

NASA Astrophysics Data System (ADS)

Pan, Chong; Kwon, Yongseok

2018-04-01

The present work studies the fluctuating characteristics of the streamwise wall-shear stress in a DNS of a turbulent boundary layer at Re τ =1500 from a structural view. The two-dimensional field of the fluctuating friction velocity u‧ τ (x,z) is decomposed into the large- and small-scale components via a recently proposed scale separation algorithm, Quasi-bivariate Variational Mode Decomposition (QB-VMD). Both components are found to be dominated by streak-like structures, which can be regarded as the wall signature of the inner-layer streaks and the outer-layer LSMs, respectively. Extreme positive/negative wall-shear stress fluctuation events are detected in the large-scale component. The former’s occurrence frequency is nearly one order of magnitude higher than the latter; therefore, they contribute a significant portion of the long tail of the wall-shear stress distribution. Both two-point correlations and conditional averages show that these extreme positive wall-shear stress events are embedded in the large-scale positive u‧ τ streaks. They seem to be formed by near-wall ‘splatting’ process, which are related to strong finger-like sweeping (Q4) events originated from the outer-layer positive LSMs.
Refraction and scattering of sound by a shear layer

NASA Technical Reports Server (NTRS)

Schlinker, R. H.; Amiet, R. K.

1980-01-01

The angle and amplitude changes for acoustic waves refracted by a circular open jet shear layer were determined. The generalized refraction theory was assessed experimentally for on axis and off axis acoustic source locations as source frequency varied from 1 kHz to 10 kHz and free stream Mach number varied from 0.1 to 0.4. Angle and amplitude changes across the shear layer show good agreement with theory. Experiments confirm that the refraction theory is independent of shear layer thickness, acoustic source frequency, and source type. A generalized theory is, thus, available for correcting far field noise data acquired in open jet test facilities. The effect of discrete tone scattering by the open jet turbulent shear layer was also studied. Scattering effects were investigated over the same Mach number range as frequency varied from 5 kHz to 15 kHz. Attenuation of discrete tone amplitude and tone broadening were measured as a function of acoustic source position and radiation angle. Scattering was found to be stronger at angles close to the open jet axis than at 90 deg, and becomes stronger as the acoustic source position shifts downstream. A scattering analysis provided an estimate of the onset of discrete tone scattering.
Effects of the shear layer growth rate on the supersonic jet noise

NASA Astrophysics Data System (ADS)

Ozawa, Yuta; Nonomura, Taku; Oyama, Akira; Mamori, Hiroya; Fukushima, Naoya; Yamamoto, Makoto

2017-11-01

Strong acoustic waves emitted from rocket plume might damage to rocket payloads because their payloads consist of fragile structure. Therefore, understanding and prediction of acoustic wave generation are of importance not only in science, but also in engineering. The present study makes experiments of a supersonic jet flow at the Mach number of 2.0 and investigates a relationship between growth rate of a shear layer and noise generation of the supersonic jet. We conducted particle image velocimetry (PIV) and acoustic measurements for three different shaped nozzles. These nozzles were employed to control the condition of a shear layer of the supersonic jet flow. We applied single-pixel ensemble correlation method (Westerweel et al., 2004) for the PIV images to obtain high-resolution averaged velocity profiles. This correlation method enabled us to obtain detailed data of the shear layer. For all cases, acoustic measurements clearly shows the noise source position at the end of a potential core of the jet. In the case where laminar to turbulent transition occurred in the shear layer, the sound pressure level increased by 4 dB at the maximum. This research is partially supported by Presto, JST (JPMJPR1678) and KAKENHI (25709009 and 17H03473).
Depinning and heterogeneous dynamics of colloidal crystal layers under shear flow

NASA Astrophysics Data System (ADS)

Gerloff, Sascha; Klapp, Sabine H. L.

2016-12-01

Using Brownian dynamics (BD) simulations and an analytical approach we investigate the shear-induced, nonequilibrium dynamics of dense colloidal suspensions confined to a narrow slit-pore. Focusing on situations where the colloids arrange in well-defined layers with solidlike in-plane structure, the confined films display complex, nonlinear behavior such as collective depinning and local transport via density excitations. These phenomena are reminiscent of colloidal monolayers driven over a periodic substrate potential. In order to deepen this connection, we present an effective model that maps the dynamics of the shear-driven colloidal layers to the motion of a single particle driven over an effective substrate potential. This model allows us to estimate the critical shear rate of the depinning transition based on the equilibrium configuration, revealing the impact of important parameters, such as the slit-pore width and the interaction strength. We then turn to heterogeneous systems where a layer of small colloids is sheared with respect to bottom layers of large particles. For these incommensurate systems we find that the particle transport is dominated by density excitations resembling the so-called "kink" solutions of the Frenkel-Kontorova (FK) model. In contrast to the FK model, however, the corresponding "antikinks" do not move.
Simulation of Vortex Structure in Supersonic Free Shear Layer Using Pse Method

NASA Astrophysics Data System (ADS)

Guo, Xin; Wang, Qiang

The method of parabolized stability equations (PSE) are applied in the analysis of nonlinear stability and the simulation of flow structure in supersonic free shear layer. High accuracy numerical techniques including self-similar basic flow, high order differential method, appropriate transformation and decomposition of nonlinear terms are adopted and developed to solve the PSE effectively for free shear layer. The spatial evolving unstable waves which dominate the flow structure are investigated through nonlinear coupling spatial marching methods. The nonlinear interactions between harmonic waves are further analyzed and instantaneous flow field are obtained by adding the harmonic waves into basic flow. Relevant data agree well with that of DNS. The results demonstrate that T-S wave does not keeping growing exponential as the linear evolution, the energy transfer to high order harmonic modes and finally all harmonic modes get saturation due to the nonlinear interaction; Mean flow distortion is produced by the nonlinear interaction between the harmonic and its conjugate harmonic, makes great change to the average flow and increases the thickness of shear layer; PSE methods can well capture the large scale nonlinear flow structure in the supersonic free shear layer such as vortex roll-up, vortex pairing and nonlinear saturation.
Generation and Radiation of Acoustic Waves from a 2D Shear Layer

NASA Technical Reports Server (NTRS)

Dahl, Milo D.

2000-01-01

A thin free shear layer containing an inflection point in the mean velocity profile is inherently unstable. Disturbances in the flow field can excite the unstable behavior of a shear layer, if the appropriate combination of frequencies and shear layer thicknesses exists, causing instability waves to grow. For other combinations of frequencies and thicknesses, these instability waves remain neutral in amplitude or decay in the downstream direction. A growing instability wave radiates noise when its phase velocity becomes supersonic relative to the ambient speed of sound. This occurs primarily when the mean jet flow velocity is supersonic. Thus, the small disturbances in the flow, which themselves may generate noise, have generated an additional noise source. It is the purpose of this problem to test the ability of CAA to compute this additional source of noise. The problem is idealized such that the exciting disturbance is a fixed known acoustic source pulsating at a single frequency. The source is placed inside of a 2D jet with parallel flow; hence, the shear layer thickness is constant. With the source amplitude small enough, the problem is governed by the following set of linear equations given in dimensional form.
A pitfall in shallow shear-wave refraction surveying

USGS Publications Warehouse

Xia, J.; Miller, R.D.; Park, C.B.; Wightman, E.; Nigbor, R.

2002-01-01

The shallow shear-wave refraction method works successfully in an area with a series of horizontal layers. However, complex near-surface geology may not fit into the assumption of a series of horizontal layers. That a plane SH-wave undergoes wave-type conversion along an interface in an area of nonhorizontal layers is theoretically inevitable. One real example shows that the shallow shear-wave refraction method provides velocities of a converted wave rather than an SH- wave. Moreover, it is impossible to identify the converted wave by refraction data itself. As most geophysical engineering firms have limited resources, an additional P-wave refraction survey is necessary to verify if velocities calculated from a shear-wave refraction survey are velocities of converted waves. The alternative at this time may be the surface wave method, which can provide reliable S-wave velocities, even in an area of velocity inversion (a higher velocity layer underlain by a lower velocity layer). ?? 2002 Elsevier Science B.V. All rights reserved.
Localization instability and the origin of regularly- spaced faults in planetary lithospheres

NASA Astrophysics Data System (ADS)

Montesi, Laurent Gilbert Joseph

2002-10-01

Brittle deformation is not distributed uniformly in planetary lithospheres but is instead localized on faults and ductile shear zones. In some regions such as the Central Indian Basin or martian ridged plains, localized shear zones display a characteristic spacing. This pattern can constrain the mechanical structure of the lithosphere if a model that includes the development of localized shear zones and their interaction with the non- localizing levels of the lithosphere is available. I construct such a model by modifying the buckling analysis of a mechanically-stratified lithosphere idealization, by allowing for rheologies that have a tendency to localize. The stability of a rheological system against localization is indicated by its effective stress exponent, ne. That quantity must be negative for the material to have a tendency to localize. I show that a material deforming brittly or by frictional sliding has ne < 0. Localization by shear heating or grain size feedback in the ductile field requires significant deviations from non-localized deformation conditions. The buckling analysis idealizes the lithosphere as a series of horizontal layers of different mechanical properties. When this model is subjected to horizontal extension or compression, infinitesimal perturbation of its interfaces grow at a rate that depends on their wavelength. Two superposed instabilities develop if ne < 0 in a layer overlying a non-localizing substratum. One is the classical buckling/necking instability. The other gives rise to regularly-spaced localized shear zones, with a spacing proportional to the thickness of the localizing layer, and dependent on n e. I call that second instability the localization instability. Using the localization instability, the depth to which fault penetrate in the Indian Ocean and in martian ridged plains can be constrained from the ridge spacing. The result are consistent with earthquake data in the Indian Ocean and radiogenic heat production on Mars. It is therefore possible that the localization instability exerts a certain control on the formation of fault patterns in planetary lithospheres. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253- 1690.)
Simulations of free shear layers using a compressible k-epsilon model

NASA Technical Reports Server (NTRS)

Yu, S. T.; Chang, C. T.; Marek, C. J.

1991-01-01

A two-dimensional, compressible Navier-Stokes equations with a k-epsilon turbulence model are solved numerically to simulate the flows of compressible free shear layers. The appropriate form of k and epsilon equations for compressible flows are discussed. Sarkar's modeling is adopted to simulate the compressibility effects in the k and epsilon equations. The numerical results show that the spreading rate of the shear layers decreases with increasing convective Mach number. In addition, favorable comparison was found between the calculated results and Goebel and Dutton's experimental data.
Simulations of free shear layers using a compressible kappa-epsilon model

NASA Technical Reports Server (NTRS)

Yu, S. T.; Chang, C. T.; Marek, C. J.

1991-01-01

A two-dimensional, compressible Navier-Stokes equation with a k-epsilon turbulence model is solved numerically to simulate the flow of a compressible free shear layer. The appropriate form of k and epsilon equations for compressible flow is discussed. Sarkar's modeling is adopted to simulate the compressibility effects in the k and epsilon equations. The numerical results show that the spreading rate of the shear layers decreases with increasing convective Mach number. In addition, favorable comparison was found between the calculated results and experimental data.
Sensor for Boundary Shear Stress in Fluid Flow

NASA Technical Reports Server (NTRS)

Bao, Xiaoqi; Badescu, Mircea; Sherrit, Stewart; Bar-Cohen, Yoseph; Lih, Shyh-Shiuh; Chang, Zensheu; Trease, Brian P.; Kerenyi, Kornel; Widholm, Scott E.; Ostlund, Patrick N.

2012-01-01

The formation of scour patterns at bridge piers is driven by the forces at the boundary of the water flow. In most experimental scour studies, indirect processes have been applied to estimate the shear stress using measured velocity profiles. The estimations are based on theoretical models and associated assumptions. However, the turbulence flow fields and boundary layer in the pier-scour region are very complex and lead to low-fidelity results. In addition, available turbulence models cannot account accurately for the bed roughness effect. Direct measurement of the boundary shear stress, normal stress, and their fluctuations are attractive alternatives. However, most direct-measurement shear sensors are bulky in size or not compatible to fluid flow. A sensor has been developed that consists of a floating plate with folded beam support and an optical grid on the back, combined with a high-resolution optical position probe. The folded beam support makes the floating plate more flexible in the sensing direction within a small footprint, while maintaining high stiffness in the other directions. The floating plate converts the shear force to displacement, and the optical probe detects the plate s position with nanometer resolution by sensing the pattern of the diffraction field of the grid through a glass window. This configuration makes the sensor compatible with liquid flow applications.
Growth Behavior of Intermetallic Compounds at SnAgCu/Ni and Cu Interfaces

NASA Astrophysics Data System (ADS)

Qi, Lihua; Huang, Jihua; Zhang, Hua; Zhao, Xingke; Wang, Haitao; Cheng, Donghai

2010-02-01

The growth behavior of reaction-formed intermetallic compounds (IMCs) at Sn3.5Ag0.5Cu/Ni and Cu interfaces under thermal-shear cycling conditions was investigated. The results show that the morphology of (Cu x Ni1- x )6Sn5 and Cu6Sn5 IMCs formed both at Sn3.5Ag0.5Cu/Ni and Cu interfaces gradually changed from scallop-like to chunk-like, and different IMC thicknesses developed with increasing thermal-shear cycling time. Furthermore, Cu6Sn5 IMC growth rate at the Sn3.5Ag0.5Cu/Cu interface was higher than that of (Cu x Ni1- x )6Sn5 IMC under thermal-shear cycling. Compared to isothermal aging, thermal-shear cycling led to only one Cu6Sn5 layer at the interface between SnAgCu solder and Cu substrate after 720 cycles. Moreover, Ag3Sn IMC was dispersed uniformly in the solder after reflow. The planar Ag3Sn formed near the interface changed remarkably and merged together to large platelets with increasing cycles. The mechanism of formation of Cu6Sn5, (Cu x Ni1- x )6Sn5 and Ag3Sn IMCs during thermal-shear cycling process was investigated.
In-plane and Interlaminar Shear Strength of a Unidirectional Hi-nicalon Fiber-reinforced Celsian Matrix Composite

NASA Technical Reports Server (NTRS)

Uenal, O.; Bansal, N. P.

2000-01-01

In-plane and interlaminar shear strength of a unidirectional SiC fiber-reinforced (BaSr)Al2Si2O8 celsian composite were measured by the double-notch shear test method between room temperature and 1200 C. The interlaminar shear strength was lower than the in-plane shear strength at all temperatures. Stress analysis, using finite element modeling, indicated that shear stress concentration was not responsible for the observed difference in strength. Instead, the difference in layer architecture and thus, the favorable alignment of fiber-rich layers with the shear plane in the interlaminar specimens appears to be the reason for the low strength of this composite. A rapid decrease in strength was observed with temperature due to softening of the glassy phase in the material.
The Structure of Vertical Wind Shear in Tropical Cyclone Environments: Implications for Forecasting and Predictability

NASA Astrophysics Data System (ADS)

Finocchio, Peter M.

The vertical wind shear measured between 200 and 850 hPa is commonly used to diagnose environmental interactions with a tropical cyclone (TC) and to forecast the storm's intensity and structural evolution. More often than not, stronger vertical shear within this deep layer prohibits the intensification of TCs and leads to predictable asymmetries in precipitation. But such bulk measures of vertical wind shear can occasionally mislead the forecaster. In the first part of this dissertation, we use a series of idealized numerical simulations to examine how a TC responds to changing the structure of unidirectional vertical wind shear while fixing the 200-850-hPa shear magnitude. These simulations demonstrate a significant intensity response, in which shear concentrated in shallow layers of the lower troposphere prevents vortex intensification. We attribute the arrested development of TCs in lower-level shear to the intrusion of mid-level environmental air over the surface vortex early in the simulations. Convection developing on the downshear side of the storm interacts with the intruding air so as to enhance the downward flux of low-entropy air into the boundary layer. We also construct a two-dimensional intensity response surface from a set of simulations that sparsely sample the joint shear height-depth parameter space. This surface reveals regions of the two-parameter space for which TC intensity is particularly sensitive. We interpret these parameter ranges as those which lead to reduced intensity predictability. Despite the robust response to changing the shape of a sheared wind profile in idealized simulations, we do not encounter such sensitivity within a large set of reanalyzed TCs in the Northern Hemisphere. Instead, there is remarkable consistency in the structure of reanalyzed wind profiles around TCs. This is evident in the distributions of two new parameters describing the height and depth of vertical wind shear, which highlight a clear preference for shallow layers of upper-level shear. Many of the wind profiles tested in the idealized simulations have shear height or depth values on the tails of these distributions, suggesting that the environmental wind profiles around real TCs do not exhibit enough structural variability to have the clear statistical relationship to intensity change that we expected. In the final part of this dissertation, we use the reanalyzed TC environments to initialize ensembles of idealized simulations. Using a new modeling technique that allows for time-varying environments, these simulations examine the predictability implications of exposing a TC to different structures and magnitudes of vertical wind shear during its life cycle. We find that TCs in more deeply distributed vertical wind shear environments have a more uncertain intensity evolution than TCs exposed to shallower layers of upper-level shear. This higher uncertainty arises from a more marginal boundary layer environment that the deeply distributed shear establishes, which enhances the TC sensitivity to the magnitude of deep-layer shear. Simulated radar reflectivity also appears to evolve in a more uncertain fashion in environments with deeply distributed vertical shear. However, structural predictability timescales, computed as the time it takes for errors in the amplitude or phase of azimuthal asymmetries of reflectivity to saturate, are similar for wind profiles with shallow upper-level shear and deeply distributed shear. Both ensembles demonstrate predictability timescales of two to three days for the lowest azimuthal wavenumbers of amplitude and phase. As the magnitude of vertical wind shear increases to universally destructive levels, structural and intensity errors begin to decrease. Shallow upper-level shear primes the TC for a more pronounced recovery in the predictability of the wavenumber-one precipitation structure in stronger shear. The recovered low-wavenumber predictability of TC precipitation structure and the collapse in intensity spread in strong shear suggests that vertical wind shear is most effective at reducing TC predictability when its magnitude is near the threshold between favorable and unfavorable values and when it is deeply distributed through the troposphere. By isolating the effect of the environmental flow, the simulations and analyses in this dissertation offer a unique understanding of how vertical wind shear affects TCs. In particular, the results have important implications for designing and implementing future environmental observing strategies that will be critical for improving forecasts of these destructive storms.
Effects of shear on the magnetic footprint and stochastic layer in double-null divertor tokamak

NASA Astrophysics Data System (ADS)

Farhat, Hamidullah; Punjabi, Alkesh; Ali, Halima

2006-10-01

We have developed a new area-preserving map, called the Adjustable Shear Map, to calculate effects of shear on the magnetic footprint and stochastic layer in double-null divertor tokamak. The map is given by equationsxn+1=xn-kyn[(1-yn^2 )(1+syn)+sxn+1^2 ),yn+1=yn+kxn+1[1+s(xn+1^2 +yn^2 )]. k is the map parameter and s is the shear parameter. O-point of the map is (0, 0), and the X-points are (0, 1), and (0, -1). For s=0, k=0.6, the last good surface is y=0.9918 with q ˜3. Here we will report on the effects of shear on the stochastic layer and magnetic footprint as the shear parameter is varied from 0 to -1. Here we will report the preliminary results on the effect of shear on the magnetic foot print and the stochastic layer where the shear parameter s has values between -1 and 0. using method of maps [1-4]. This work is done under the DOE grant number DE-FG02-01ER54624. 1. A. Punjabi, A. Boozer, and A. Verma, Phys. Rev. lett., 69, 3322 (1992). 2. H. Ali, A. Punjabi, and A. Boozer, Phys. Plasmas 11, 4527 (2004). 3. A. Punjabi, H. Ali, and A. Boozer, Phys. Plasmas 10, 3992 (2003). 4. A. Punjabi, H. Ali, and A. Boozer, Phys. Plasmas 4, 337 (1997).
Cold-Flow Study of Low Frequency Pressure Instability in Hybrid Rocket Motors

NASA Technical Reports Server (NTRS)

Jenkins, Rhonald M.

1997-01-01

Past experience with hybrid rockets has shown that certain motor operating conditions are conducive to the formation of low frequency pressure oscillations, or flow instabilities, within the motor. Both past and present work in the hybrid propulsion community acknowledges deficiencies in the understanding of such behavior, though it seems probable that the answer lies in an interaction between the flow dynamics and the combustion heat release. Knowledge of the fundamental flow dynamics is essential to the basic understanding of the overall stability problem. A first step in this direction was a study conducted at NASA Marshall Space Flight Center (MSFC), centered around a laboratory-scale two dimensional water flow model of a hybrid rocket motor. Principal objectives included: (1) visualization of flow and measurement of flow velocity distributions: (2) assessment of the importance of shear layer instabilities in driving motor pressure oscillations; (3) determination of the interactions between flow induced shear layers with the mainstream flow, the secondary (wall) throughflow, and solid boundaries; (4) investigation of the interactions between wall flow oscillations and the mainstream flow pressure distribution.

On Instability of Geostrophic Current with Linear Vertical Shear at Length Scales of Interleaving

NASA Astrophysics Data System (ADS)

Kuzmina, N. P.; Skorokhodov, S. L.; Zhurbas, N. V.; Lyzhkov, D. A.

2018-01-01

The instability of long-wave disturbances of a geostrophic current with linear velocity shear is studied with allowance for the diffusion of buoyancy. A detailed derivation of the model problem in dimensionless variables is presented, which is used for analyzing the dynamics of disturbances in a vertically bounded layer and for describing the formation of large-scale intrusions in the Arctic basin. The problem is solved numerically based on a high-precision method developed for solving fourth-order differential equations. It is established that there is an eigenvalue in the spectrum of eigenvalues that corresponds to unstable (growing with time) disturbances, which are characterized by a phase velocity exceeding the maximum velocity of the geostrophic flow. A discussion is presented to explain some features of the instability.
Investigation of the influence of textiles and surface treatments on blistering using a novel simulant.

PubMed

Guerra, C; Schwartz, C J

2012-02-01

Friction blisters occur when shear loading causes the separation of dermal layers. Consequences range from minor pain to life-threatening infection. Past research in blister formation has focused on in vivo experiments, which complicate a mechanics-based study of the phenomenon. A Synthetic Skin Simulant Platform (3SP) approach was developed to investigate the effect of textile fabrics (t-shirt knit and denim cottons) and surface treatments (dry and wet lubricants) on blister formation. 3SP samples consist of bonded elastomeric layers that are surrogates for various dermal layers. These layers display frictional and mechanical properties similar to their anatomical analogues. Blistering was assessed by the measurement of deboned area between layers. Denim caused greater blistering than did the t-shirt knit cotton, and both lubricants significantly reduced blister area and surface damage. A triglyceride-based lubricant had a more pronounced effect on blister reduction than corn starch. The triglyceride lubricant used with t-shirt knit cotton resulted in no blisters being formed. The performance of the 3SP approach follows previously reported frictional behavior of skin in vivo. The results of textile and surface treatment performance suggest that future 3SP iterations can be focused on specific anatomical sites based on application type. © 2011 John Wiley & Sons A/S.
Some aspects of the problem of secondary eyewall formation in idealized three-dimensional nonlinear simulations

NASA Astrophysics Data System (ADS)

Menelaou, K.; Yau, M. K.; Martinez, Y.

2014-09-01

Some aspects of the problem of secondary eyewall formation (SEF) are investigated with the aid of an idealized model. A series of experiments are conducted, starting with a strong annular vortex embedded in a quiescent background flow and forced by the sustained heating associated with a spiral rainband (control experiment). Following this, two experiments are configured to assess the impact of vertical wind shear (VWS) in the SEF process. The importance of the boundary layer force imbalance is finally investigated in a number of simulations in which surface and boundary layer physics are included. From the control experiment, it is found that in the absence of background environmental flow, the sustained latent heating associated with a spiral rainband can form a secondary eyewall even in the absence of a frictional boundary layer. The presence of VWS acts negatively in the SEF process by disrupting the organization of the potential vorticity induced by the rainband. When boundary layer physics is included, some similarities with previous studies are seen, but there is no SEF. These results suggest that the boundary layer most likely contributes to, rather than initiate, a secondary eyewall. This article was corrected on 10 OCT 2014. See the end of the full text for details.
A Computational Study of Shear Layer Receptivity

NASA Astrophysics Data System (ADS)

Barone, Matthew; Lele, Sanjiva

2002-11-01

The receptivity of two-dimensional, compressible shear layers to local and external excitation sources is examined using a computational approach. The family of base flows considered consists of a laminar supersonic stream separated from nearly quiescent fluid by a thin, rigid splitter plate with a rounded trailing edge. The linearized Euler and linearized Navier-Stokes equations are solved numerically in the frequency domain. The flow solver is based on a high order finite difference scheme, coupled with an overset mesh technique developed for computational aeroacoustics applications. Solutions are obtained for acoustic plane wave forcing near the most unstable shear layer frequency, and are compared to the existing low frequency theory. An adjoint formulation to the present problem is developed, and adjoint equation calculations are performed using the same numerical methods as for the regular equation sets. Solutions to the adjoint equations are used to shed light on the mechanisms which control the receptivity of finite-width compressible shear layers.
Turbulent boundary layer on a convex, curved surface

NASA Technical Reports Server (NTRS)

Gillis, J. C.; Johnston, J. P.; Kays, W. M.; Moffat, R. J.

1980-01-01

The effects of strong convex curvature on boundary layer turbulence were investigated. The data gathered on the behavior of Reynolds stress suggested the formulation of a simple turbulence model. Three sets of data were taken on two separate facilities. Both rigs had flow from a flat surface, over a convex surface with 90 deg of turning, and then onto a flat recovery surface. The geometry was adjusted so that, for both rigs, the pressure gradient along the test surface was zero - thus avoiding any effects of streamwise acceleration on the wall layers. Results show that after a sudden introduction of curvature, the shear stress in the outer part of the boundary layer is sharply diminished and is even slightly negative near the edge. The wall shear also drops off quickly downstream. In contrast, when the surface suddenly becomes flat again, the wall shear and shear stress profiles recover very slowly towards flat wall conditions.
Structure of a reattaching supersonic shear flow

NASA Technical Reports Server (NTRS)

Samimy, M.; Abu-Hijleh, B. A. K.

1988-01-01

A Mach 1.83 fully developed turbulent boundary layer with boundary layer thickness, free stream velocity, and Reynolds number of 7.5 mm, 476 m/s, and 6.2 x 10 to the 7th/m, respectively, was separated at a 25.4-mm backward step and formed a shear layer. Fast-response pressure transducers, schlieren photography, and LDV were used to study the structure of this reattaching shear flow. The preliminary results show that large-scale relatively organized structures with limited spanwise extent form in the free shear layer. Some of these structures appear to survive the recompression and reattachment processes, while others break down into smaller scales and the flow becomes increasingly three-dimensional. The survived large-scale structures lose their organization through recompression/reattachment, but regain it after reattachment. The structures after reattachment form a 40-45-degree angle relative to the free stream and deteriorate gradually as they move downstream.
An experimental investigation of a turbulent shear flow with separation, reverse flow, and reattachment

NASA Astrophysics Data System (ADS)

Ruderich, R.; Fernholz, H. H.

1986-02-01

Attention is given to the turbulent and disturbed flow over a bluff plate having a long splitter plate in its plane-of-symmetry, so that the flow separates at the sharp bevelled edge of the bluff plate, forms a free shear layer above the reverse flow region, and reattaches on the splitter plate over a narrow region that is curved in spanwise direction. Hot wire and pulsed wire anemometry were used to measure mean velocity, Reynolds shear stress and Reynolds normal stress distributions, and spectra and integral length-scales were measured to investigate the state and structure of the flow. Mean and fluctuating qualities showed a self-similar behavior in a short region upstream of the reattachment, as well as 'profile-similarity' in the separated shear layer and along the splitter plate downstream from reattachment. No flapping or reattaching shear layer was observed.
Stable plume rise in a shear layer.

PubMed

Overcamp, Thomas J

2007-03-01

Solutions are given for plume rise assuming a power-law wind speed profile in a stably stratified layer for point and finite sources with initial vertical momentum and buoyancy. For a constant wind speed, these solutions simplify to the conventional plume rise equations in a stable atmosphere. In a shear layer, the point of maximum rise occurs further downwind and is slightly lower compared with the plume rise with a constant wind speed equal to the wind speed at the top of the stack. If the predictions with shear are compared with predictions for an equivalent average wind speed over the depth of the plume, the plume rise with shear is higher than plume rise with an equivalent average wind speed.
Taylor instability in the shock layer on a Jovian atmosphere entry probe.

NASA Technical Reports Server (NTRS)

Compton, D. L.

1972-01-01

Investigation of the Taylor instability relative to the dynamical instability whose presence in the shock layer on a spacecraft entering the Jovian atmosphere is to be expected because of the difference in velocity across the shear layer. Presented calculations show that the Taylor instability at the interface between shock-heated freestream gas and ablation products is inconsequential in comparison to the shear layer instability.
Viscoelastic Properties of Extracellular Polymeric Substances Can Strongly Affect Their Washing Efficiency from Reverse Osmosis Membranes.

PubMed

Ferrando Chavez, Diana Lila; Nejidat, Ali; Herzberg, Moshe

2016-09-06

The role of the viscoelastic properties of biofouling layers in their removal from the membrane was studied. Model fouling layers of extracellular polymeric substances (EPS) originated from microbial biofilms of Pseudomonas aeruginosa PAO1 differentially expressing the Psl polysaccharide were used for controlled washing experiments of fouled RO membranes. In parallel, adsorption experiments and viscoelastic modeling of the EPS layers were conducted in a quartz crystal microbalance with dissipation (QCM-D). During the washing stage, as shear rate was elevated, significant differences in permeate flux recovery between the three different EPS layers were observed. According to the amount of organic carbon remained on the membrane after washing, the magnitude of Psl production provides elevated resistance of the EPS layer to shear stress. The highest flux recovery during the washing stage was observed for the EPS with no Psl. Psl was shown to elevate the layer's shear modulus and shear viscosity but had no effect on the EPS adhesion to the polyamide surface. We conclude that EPS retain on the membrane as a result of the layer viscoelastic properties. These results highlight an important relation between washing efficiency of fouling layers from membranes and their viscoelastic properties, in addition to their adhesion properties.
Bounds on strain in large Tertiary shear zones of SE Asia from boudinage restoration

NASA Astrophysics Data System (ADS)

Lacassin, R.; Leloup, P. H.; Tapponnier, P.

1993-06-01

We have used surface-balanced restoration of stretched, boudinaged layers to estimate minimum amounts of finite strain in the mylonitic gneisses of the Oligo-Miocene Red River-Ailao Shan shear zone (Yunnan, China) and of the Wang Chao shear zone (Thailand). The layer-parallel extension values thus obtained range between 250 and 870%. We discuss how to use such extension values to place bounds on amounts of finite shear strain in these large crustal shear zones. Assuming simple shear, these values imply minimum total and late shear strains of, respectively, 33 ± 6 and 7 ± 3 at several sites along the Red River-Ailao Shan shear zone. For the Wang Chao shear zone a minimum shear strain of 7 ± 4 is deduced. Assuming homogeneous shear would imply that minimum strike-slip displacements along these two left-lateral shear zones, which have been interpreted to result from the India-Asia collision, have been of the order of 330 ± 60 km (Red River-Ailao Shan) and 35 ± 20 km (Wang Chao).
Near-wall similarity in a pressure-driven three-dimensional turbulent boundary layer

NASA Technical Reports Server (NTRS)

Pierce, F. J.; Mcallister, J. E.

1980-01-01

Mean velocity, measured wall pressure and wall shear stress fields were made in a three dimensional pressure-driven turbulent boundary layer created by a cylinder with trailing edge placed normal to a flat plate floor. The direct force wall shear stress measurements were made with floating element direct force sensing shear meter that responded to both the magnitude and direction of the local wall shear stress. The ability of 10 near wall similarity models to describe the near wall velocity field for the measured flow under a wide range of skewing conditions and a variety of pressure gradient and wall shear vector orientations was used.
Predictions and Experimental Microstructural Characterization of High Strain Rate Failure Modes in Layered Aluminum Composites

NASA Astrophysics Data System (ADS)

Khanikar, Prasenjit

Different aluminum alloys can be combined, as composites, for tailored dynamic applications. Most investigations pertaining to metallic alloy layered composites, however, have been based on quasi-static approaches. The dynamic failure of layered metallic composites, therefore, needs to be characterized in terms of strength, toughness, and fracture response. A dislocation-density based crystalline plasticity formulation, finite-element techniques, rational crystallographic orientation relations and a new fracture methodology were used to predict the failure modes associated with the high strain rate behavior of aluminum layered composites. Two alloy layers, a high strength alloy, aluminum 2195, and an aluminum alloy 2139, with high toughness, were modeled with representative microstructures that included precipitates, dispersed particles, and different grain boundary (GB) distributions. The new fracture methodology, based on an overlap method and phantom nodes, is used with a fracture criteria specialized for fracture on different cleavage planes. One of the objectives of this investigation, therefore, was to determine the optimal arrangements of the 2139 and 2195 aluminum alloys for a metallic layered composite that would combine strength, toughness and fracture resistance for high strain-rate applications. Different layer arrangements were investigated for high strain-rate applications, and the optimal arrangement was with the high toughness 2139 layer on the bottom, which provided extensive shear strain localization, and the high strength 2195 layer on the top for high strength resistance. The layer thickness of the bottom high toughness layer also affected the bending behavior of the roll-boned interface and the potential delamination of the layers. Shear strain localization, dynamic cracking and delamination were the mutually competing failure mechanisms for the layered metallic composite, and control of these failure modes can be optimized for high strain-rate applications. The second major objective of this investigation was the use of recently developed dynamic fracture formulations to model and analyze the crack nucleation and propagation of aluminum layered composites subjected to high strain rate loading conditions and how microstructural effects, such as precipitates, dispersed particles, and GB orientations affect failure evolution. This dynamic fracture approach is used to investigate crack nucleation and crack growth as a function of the different microstructural characteristics of each alloy in layered composites with and without pre-existing cracks. The zigzag nature of the crack paths were mainly due to the microstructural features, such as precipitates and dispersed particles distributions and orientations ahead of the crack front, and it underscored the capabilities of the fracture methodology. The evolution of dislocation density and the formation of localized shear slip contributed to the blunting of the propagating crack. Extensive geometrical and thermal softening due to the localized plastic slip also affected crack path orientations and directions. These softening mechanisms resulted in the switching of cleavage planes, which affected crack path orientations. Interface delamination can also have an important role in the failure and toughening of the layered composites. Different scenarios of delamination were investigated, such as planar crack growth and crack penetration into the layers. The presence of brittle surface oxide platelets in the interface region also significantly influenced the interface delamination process. Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Optical Microscopy (OM) characterization provided further physical insights and validation of the predictive capabilities. The inherent microstructural features of each alloy play a significant role in the dynamic fracture, shear strain localization, and interface delamination of the layered metallic composite. These microstructural features, such as precipitates, dispersed particles, and GB orientations and distributions can be optimized for desired behavior of metallic composites.
Interaction between a normal shock wave and a turbulent boundary layer at high transonic speeds. Part 2: Wall shear stress

NASA Technical Reports Server (NTRS)

Liou, M. S.; Adamson, T. C., Jr.

1979-01-01

An analysis is presented of the flow in the two inner layers, the Reynolds stress sublayer and the wall layer. Included is the calculation of the shear stress at the wall in the interaction region. The limit processes considered are those used for an inviscid flow.
A simple model for the prediction of the discrete stiffness states of a homogeneous electrostatically tunable multi-layer beam

NASA Astrophysics Data System (ADS)

Bergamini, A.; Christen, R.; Motavalli, M.

2007-04-01

The adaptive modification of the mechanical properties of structures has been described as a key to a number of new or enhanced technologies, ranging from prosthetics to aerospace applications. Previous work reported the electrostatic tuning of the bending stiffness of simple sandwich structures by modifying the shear stress transfer parameters at the interface between faces and the compliant core of the sandwich. For this purpose, the choice of a sandwich structure presented considerable experimental advantages, such as the ability to obtain a large increase in stiffness by activating just two interfaces between the faces and the core of the beam. The hypothesis the development of structures with tunable bending stiffness is based on, is that by applying a normal stress at the interface between two layers of a multi-layer structure it is possible to transfer shear stresses from one layer to the other by means of adhesion or friction forces. The normal stresses needed to generate adhesion or friction can be generated by an electrostatic field across a dielectric layer interposed between the layers of a structure. The shear stress in the cross section of the structure (e.g. a beam) subjected to bending forces is transferred in full, if sufficiently large normal stresses and an adequate friction coefficient at the interface are given. Considering beams with a homogeneous cross-section, in which all layers are made of the same material and have the same width, eliminates the need to consider parameters such as the shear modulus of the material and the shear stiffness of the core, thus making the modelling work easier and the results more readily understood. The goal of the present work is to describe a numerical model of a homogeneous multi-layer beam. The model is validated against analytical solutions for the extreme cases of interaction at the interface (no friction and a high level of friction allowing for full shear stress transfer). The obtained model is used to better understand the processes taking place at the interfaces between layers, demonstrate the existence of discrete stiffness states and to find guidance for the selection of suitable dielectric layers for the generation of the electrostatic normal stresses needed for the shear stress transfer at the interface.
Shear-induced surface alignment of polymer dispersed liquid crystal microdroplets on the boundary layer

NASA Technical Reports Server (NTRS)

Parmar, D. S.; Singh, J. J.

1993-01-01

Polymer dispersed liquid crystal thin films have been deposited on a glass substrate, utilizing the processes of polymerization and solvent evaporation induced phase separation. Liquid crystal microdroplets trapped on the upper surface of the thin film respond to the shear stress due to air or gas flow on the surface layer. Response to an applied step shear stress input on the surface layer has been measured by measuring the time response of the transmitted light intensity. Initial results on the measurements of the light transmission as a function of the air flow differential pressure indicate that these systems offer features suitable for boundary layer and gas flow sensors.
Analytic Study of Three-Dimensional Rupture Propagation in Strike-Slip Faulting with Analogue Models

NASA Astrophysics Data System (ADS)

Chan, Pei-Chen; Chu, Sheng-Shin; Lin, Ming-Lang

2014-05-01

Strike-slip faults are high angle (or nearly vertical) fractures where the blocks have moved along strike way (nearly horizontal). Overburden soil profiles across main faults of Strike-slip faults have revealed the palm and tulip structure characteristics. McCalpin (2005) has trace rupture propagation on overburden soil surface. In this study, we used different offset of slip sandbox model profiles to study the evolution of three-dimensional rupture propagation by strike -slip faulting. In strike-slip faults model, type of rupture propagation and width of shear zone (W) are primary affecting by depth of overburden layer (H), distances of fault slip (Sy). There are few research to trace of three-dimensional rupture behavior and propagation. Therefore, in this simplified sandbox model, investigate rupture propagation and shear zone with profiles across main faults when formation are affecting by depth of overburden layer and distances of fault slip. The investigators at the model included width of shear zone, length of rupture (L), angle of rupture (θ) and space of rupture. The surface results was follow the literature that the evolution sequence of failure envelope was R-faults, P-faults and Y-faults which are parallel to the basement fault. Comparison surface and profiles structure which were curved faces and cross each other to define 3-D rupture and width of shear zone. We found that an increase in fault slip could result in a greater width of shear zone, and proposed a W/H versus Sy/H relationship. Deformation of shear zone showed a similar trend as in the literature that the increase of fault slip resulted in the increase of W, however, the increasing trend became opposite after a peak (when Sy/H was 1) value of W was reached (small than 1.5). The results showed that the W width is limited at a constant value in 3-D models by strike-slip faulting. In conclusion, this study helps evaluate the extensions of the shear zone influenced regions for strike-slip faults.
Compensation of shear waves in photoacoustic tomography with layered acoustic media.

PubMed

Schoonover, Robert W; Anastasio, Mark A

2011-10-01

An image reconstruction formula is presented for photoacoustic computed tomography that accounts for conversion between longitudinal and shear waves in a planar-layered acoustic medium. We assume the optical absorber that produces the photoacoustic wave field is embedded in a single fluid layer and any elastic solid layers present are separated by one or more fluid layers. The measurement aperture is assumed to be planar. Computer simulation studies are conducted to demonstrate and investigate the proposed reconstruction formula.
Mantle flow tectonics - The influence of a ductile lower crust and implications for the formation of topographic uplands on Venus

NASA Technical Reports Server (NTRS)

Bindschadler, Duane L.; Parmentier, E. Marc

1990-01-01

The crust and mantle of Venus can be represented by a model of a layered structure stratified in both density and viscosity. This structure consists of a brittle-elastic upper crustal layer; a ductile weaker crustal layer; a strong upper mantle layer, about 10 percent denser than the crust; and a weaker substrate, representing the portion of the mantle in which convective flow occurs which is a primary source of large-scale topographic and tectonic features. This paper examines the interactions between these four layers and the mantle flow driven by thermal or compositional variations. Solutions are found for a flow driven by a buoyancy-force distribution within the mantle and by relief at the surface and crust-mantle boundary. It is shown that changes in crustal thickness are driven by vertical normal stresses due to mantle flow and by shear coupling of horizontal mantle flow into the crust.
Experimental characterization of broadband electrostatic noise due to plasma compression

NASA Astrophysics Data System (ADS)

Dubois, Ami M.; Thomas, Edward, Jr.; Amatucci, William E.; Ganguli, Gurudas

2015-11-01

For a wide variety of laboratory and space plasma environments, theory states that plasmas are unstable to transverse shear flows over a very broad frequency range, where the shear scale length (LE) compared to the ion gyro-radius (ρi) determines the character of the shear-driven instability that may prevail. During active periods in the Earth's magnetosphere, such sheared flows are intensified and broadband electrostatic noise (BEN) is often observed by satellites traversing natural boundary layers. An interpenetrating magnetized plasma configuration is used to create a transverse velocity shear profile similar to that found at natural space plasma boundary layers. The continuous variation and the associated transition of the instability regimes driven by the shear flow mechanism are demonstrated in a single laboratory experiment. For the first time, broadband wave emission, which is correlated to increasing/decreasing stress (i.e., ρi/LE) on a plasma boundary layer, is found under controlled and repeatable conditions. This result provides evidence that the compression/relaxation of a plasma boundary layer leads to a BEN signature and holds out the promise for understanding the cause and effect of the in situ observation of BEN by satellites. This project was supported with funding from the U.S. Department of Energy, the Defense Threat Reduction Agency, and NRL Base Funds.

Large scale structures in a turbulent boundary layer and their imprint on wall shear stress

NASA Astrophysics Data System (ADS)

Pabon, Rommel; Barnard, Casey; Ukeiley, Lawrence; Sheplak, Mark

2015-11-01

Experiments were performed on a turbulent boundary layer developing on a flat plate model under zero pressure gradient flow. A MEMS differential capacitive shear stress sensor with a 1 mm × 1 mm floating element was used to capture the fluctuating wall shear stress simultaneously with streamwise velocity measurements from a hot-wire anemometer traversed in the wall normal direction. Near the wall, the peak in the cross correlation corresponds to an organized motion inclined 45° from the wall. In the outer region, the peak diminishes in value, but is still significant at a distance greater than half the boundary layer thickness, and corresponds to a structure inclined 14° from the wall. High coherence between the two signals was found for the low-frequency content, reinforcing the belief that large scale structures have a vital impact on wall shear stress. Thus, estimation of the wall shear stress from the low-frequency velocity signal will be performed, and is expected to be statistically significant in the outer boundary layer. Additionally, conditionally averaged mean velocity profiles will be presented to assess the effects of high and low shear stress. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1315138.
On the turbulent friction layer for rising pressure

NASA Technical Reports Server (NTRS)

Wieghardt, K; Tillmann, W

1951-01-01

Among the information presented are included displacement, momentum, and kinetic energy thicknesses, shearing stress distributions across boundary layer, and surface friction coefficients. The Gruschwitz method and its modifications are examined and tested. An energy theorem for the turbulent boundary layer is introduced and discussed but does not lead to a method for the prediction of the behavior of the turbulent boundary layer because relations for the shearing stress and the surface friction are lacking.
Effect of inhomogeneity due to temperature on the propagation of shear waves in an anisotropic layer

NASA Astrophysics Data System (ADS)

Prasad, Bishwanath; Pal, Prakash Chandra; Kundu, Santimoy; Prasad, Narayan

2017-07-01

The present paper is concerned with the propagation of shear waves in an anisotropic inhomogeneous layer whose elastic constants are functions of temperature. The dependence of material properties on temperature gives rise to inhomogeneity of the layer which is one of the trivial characteristics of the constituent layers of earth which may cause due to the presence of various types of elements and compounds beneath the earth. The layer is lying over a rigid foundation and there is no loading on the upper boundary. The dispersion equation of shear waves has been obtained in closed form. Numerical computations are performed and graphs are plotted to show the effect of inhomogeneity and anisotropy factors on the dimensionless phase velocity. It is found that the phase velocity is considerably influenced by the inhomogeneity and anisotropy of the layer.
Modeling shear-induced particle ordering and deformation in a dense soft particle suspension

NASA Astrophysics Data System (ADS)

Liao, Chih-Tang; Wu, Yi-Fan; Chien, Wei; Huang, Jung-Ren; Chen, Yeng-Long

2017-11-01

We apply the lattice Boltzmann method and the bead-spring network model of deformable particles (DPs) to study shear-induced particle ordering and deformation and the corresponding rheological behavior for dense DP suspensions confined in a narrow gap under steady external shear. The particle configuration is characterized with small-angle scattering intensity, the real-space 2D local order parameter, and the particle shape factors including deformation, stretching and tilt angles. We investigate how particle ordering and deformation vary with the particle volume fraction ϕ (=0.45-0.65) and the external shear rate characterized with the capillary number Ca (=0.003-0.191). The degree of particle deformation increases mildly with ϕ but significantly with Ca. Under moderate shear rate (Ca = 0.105), the inter-particle structure evolves from string-like ordering to layered hexagonal close packing (HCP) as ϕ increases. A long wavelength particle slithering motion emerges for sufficiently large ϕ. For ϕ = 0.61, the structure maintains layered HCP for Ca = 0.031-0.143 but gradually becomes disordered for larger and smaller Ca. The correlation in particle zigzag movements depends sensitively on ϕ and particle ordering. Layer-by-layer analysis reveals how the non-slippery hard walls affect particle ordering and deformation. The shear-induced reconfiguration of DPs observed in the simulation agrees qualitatively with experimental results of sheared uniform emulsions. The apparent suspension viscosity increases with ϕ but exhibits much weaker dependence compared to hard-sphere suspensions, indicating that particle deformation and unjamming under shear can significantly reduce the viscous stress. Furthermore, the suspension shear-thins, corresponding to increased inter-DP ordering and particle deformation with Ca. This work provides useful insights into the microstructure-rheology relationship of concentrated deformable particle suspensions.
Modeling shear-induced particle ordering and deformation in a dense soft particle suspension.

PubMed

Liao, Chih-Tang; Wu, Yi-Fan; Chien, Wei; Huang, Jung-Ren; Chen, Yeng-Long

2017-11-01

We apply the lattice Boltzmann method and the bead-spring network model of deformable particles (DPs) to study shear-induced particle ordering and deformation and the corresponding rheological behavior for dense DP suspensions confined in a narrow gap under steady external shear. The particle configuration is characterized with small-angle scattering intensity, the real-space 2D local order parameter, and the particle shape factors including deformation, stretching and tilt angles. We investigate how particle ordering and deformation vary with the particle volume fraction ϕ (=0.45-0.65) and the external shear rate characterized with the capillary number Ca (=0.003-0.191). The degree of particle deformation increases mildly with ϕ but significantly with Ca. Under moderate shear rate (Ca = 0.105), the inter-particle structure evolves from string-like ordering to layered hexagonal close packing (HCP) as ϕ increases. A long wavelength particle slithering motion emerges for sufficiently large ϕ. For ϕ = 0.61, the structure maintains layered HCP for Ca = 0.031-0.143 but gradually becomes disordered for larger and smaller Ca. The correlation in particle zigzag movements depends sensitively on ϕ and particle ordering. Layer-by-layer analysis reveals how the non-slippery hard walls affect particle ordering and deformation. The shear-induced reconfiguration of DPs observed in the simulation agrees qualitatively with experimental results of sheared uniform emulsions. The apparent suspension viscosity increases with ϕ but exhibits much weaker dependence compared to hard-sphere suspensions, indicating that particle deformation and unjamming under shear can significantly reduce the viscous stress. Furthermore, the suspension shear-thins, corresponding to increased inter-DP ordering and particle deformation with Ca. This work provides useful insights into the microstructure-rheology relationship of concentrated deformable particle suspensions.
CDUCT-LaRC Status - Shear Layer Refraction and Noise Radiation

NASA Technical Reports Server (NTRS)

Nark, Douglas M.; Farassat, F.

2006-01-01

A proposed boundary condition accounting for shear layer effects within the Ffowcs Williams-Hawkings radiation module of the CDUCT-LaRC code is investigated. The development and numerical justification of the boundary condition formulation are reviewed. An initial assessment of the effectiveness of the shear layer correction is conducted through comparison with experimental data. Preliminary results indicate that the correction provides physically meaningful modifications of the baseline predicted directivity patterns. Trends of peak directivity steepening and shifting that appeared in predicted patterns were found to follow similar structures in measured data, particularly at higher radiation angles.
On the eigenfrequencies of elastic shear waves propagating in an inhomogeneous layer

NASA Astrophysics Data System (ADS)

Khachatryan, V. M.

2018-04-01

In this work, we consider the problem of eigenfrequencies of elastic shear waves propagating in a layer whose Young’s modulus and density are functions of the longitudinal coordinate. Taking into account the material inhomogeneity makes the problem of the eigenfrequencies of the waves propagating in the layer more complicated. In this paper, the problem of pure shear is considered. To solve the problem, we use an integral formula which allows us to represent the general solution of the original equation with variable coefficients in terms of the general solution of the accompanying equation with constant coefficients.
Unsteady behavior of a reattaching shear layer

NASA Technical Reports Server (NTRS)

Driver, D. M.; Seegmiller, H. L.; Marvin, J.

1983-01-01

A detailed investigation of the unsteadiness in a reattaching, turbulent shear layer is reported. Laser-Doppler velocimeter measurements were conditionally sampled on the basis of instantaneous flow direction near reattachment. Conditions of abnormally short reattachment and abnormally long reattachment were considered. Ensemble-averaging of measurements made during these conditions was used to obtain mean velocities and Rreynolds stresses. In the mean flow, conditional streamlines show a global change in flow pattern which correlates with wall-flow direction. This motion can loosely be described as a 'flapping' of the shear layer. Tuft probes show that the flow direction reversals occur quite randomly and are shortlived. Streses shown also vary with the change in flow pattern. Yet, the global'flapping' motion does not appear to contribute significantly to the stress in the flow. A second type of unsteady motion was identified. Spectral analysis of both wall static pressure and streamwise velocity shows that most of the energy in the flow resides in frequencies that are significantly lower than that of the turbulence. The dominant frequency is at a Strouhal number equal to 0.2, which is the characteristic frequency of roll-up and pairing of vortical structure seen in free shear layers. It is conjectured that the 'flapping' is a disorder of the roll-up and pairing process occurring in the shear layer.
Hydrogen jet combustion in a scramjet combustor with the rearwall-expansion cavity

NASA Astrophysics Data System (ADS)

Zhang, Yan-Xiang; Wang, Zhen-Guo; Sun, Ming-Bo; Yang, Yi-Xin; Wang, Hong-Bo

2018-03-01

This study is carried out to experimentally investigate the combustion characteristics of the hydrogen jet flame stabilized by the rearwall-expansion cavity in a model scramjet combustor. The flame distributions are characterized by the OH* spontaneous emission images, and the dynamic features of the flames are studied through the high speed framing of the flame luminosity. The combustion modes are further analyzed based on the visual flame structure and wall pressure distributions. Under the present conditions, the combustion based on the rearwall-expansion cavity appears in two distinguished modes - the typical cavity shear-layer stabilized combustion mode and the lifted-shear-layer stabilized combustion mode. In contrast with the shear-layer stabilized mode, the latter holds stronger flame. The transition from shear-layer stabilized combustion mode to lifted-shear-layer stabilized mode usually occurs when the equivalence ratio is high enough. While the increases of the offset ratio and upstream injection distance both lead to weaker jet-cavity interactions, cause longer ignition delay, and thus delay the mode transition. The results reveal that the rearwall-expansion cavity with an appropriate offset ratio should be helpful in delaying mode transition and preventing thermal choke, and meanwhile just brings minor negative impact on the combustion stability and efficiency.
Simultaneous wall-shear-stress and wide-field PIV measurements in a turbulent boundary layer

NASA Astrophysics Data System (ADS)

Gomit, Guillaume; Fourrie, Gregoire; de Kat, Roeland; Ganapathisubramani, Bharathram

2015-11-01

Simultaneous particle image velocimetry (PIV) and hot-film shear stress sensor measurements were performed to study the large-scale structures associated with shear stress events in a flat plate turbulent boundary layer at a high Reynolds number (Reτ ~ 4000). The PIV measurement was performed in a streamwise-wall normal plane using an array of six high resolution cameras (4 ×16MP and 2 ×29MP). The resulting field of view covers 8 δ (where δ is the boundary layer thickness) in the streamwise direction and captures the entire boundary layer in the wall-normal direction. The spatial resolution of the measurement is approximately is approximately 70 wall units (1.8 mm) and sampled each 35 wall units (0.9 mm). In association with the PIV setup, a spanwise array of 10 skin-friction sensors (spanning one δ) was used to capture the footprint of the large-scale structures. This combination of measurements allowed the analysis of the three-dimensional conditional structures in the boundary layer. Particularly, from conditional averages, the 3D organisation of the wall normal and streamwise velocity components (u and v) and the Reynolds shear stress (-u'v') related to a low and high shear stress events can be extracted. European Research Council Grant No-277472-WBT.
The Magnetohydrodynamic Kelvin-Helmholtz Instability. III. The Role of Sheared Magnetic Field in Planar Flows

NASA Astrophysics Data System (ADS)

Jeong, Hyunju; Ryu, Dongsu; Jones, T. W.; Frank, Adam

2000-01-01

We have carried out simulations of the nonlinear evolution of the magnetohydrodynamic (MHD) Kelvin-Helmholtz (KH) instability for compressible fluids in 2.5 dimensions, extending our previous work by Frank et al. and Jones et al. In the present work we have simulated flows in the x-y plane in which a ``sheared'' magnetic field of uniform strength smoothly rotates across a thin velocity shear layer from the z-direction to the x-direction, aligned with the flow field. The sonic Mach number of the velocity transition is unity. Such flows containing a uniform field in the x-direction are linearly stable if the magnetic field strength is great enough that the Alfvénic Mach number MA=U0/cA<2. That limit does not apply directly to sheared magnetic fields, however, since the z-field component has almost no influence on the linear stability. Thus, if the magnetic shear layer is contained within the velocity shear layer, the KH instability may still grow, even when the field strength is quite large. So, here we consider a wide range of sheared field strengths covering Alfvénic Mach numbers, MA=142.9 to 2. We focus on dynamical evolution of fluid features, kinetic energy dissipation, and mixing of the fluid between the two layers, considering their dependence on magnetic field strength for this geometry. There are a number of differences from our earlier simulations with uniform magnetic fields in the x-y plane. For the latter, simpler case we found a clear sequence of behaviors with increasing field strength ranging from nearly hydrodynamic flows in which the instability evolves to an almost steady cat's eye vortex with enhanced dissipation, to flows in which the magnetic field disrupts the cat's eye once it forms, to, finally, flows that evolve very little before field-line stretching stabilizes the velocity shear layer. The introduction of magnetic shear can allow a cat's eye-like vortex to form, even when the field is stronger than the nominal linear instability limit given above. For strong fields that vortex is asymmetric with respect to the preliminary shear layer, however, so the subsequent dissipation is enhanced over the uniform field cases of comparable field strength. In fact, so long as the magnetic field achieves some level of dynamical importance during an eddy turnover time, the asymmetries introduced through the magnetic shear will increase flow complexity and, with that, dissipation and mixing. The degree of the fluid mixing between the two layers is strongly influenced by the magnetic field strength. Mixing of the fluid is most effective when the vortex is disrupted by magnetic tension during transient reconnection, through local chaotic behavior that follows.
Direct simulations of chemically reacting turbulent mixing layers, part 2

NASA Technical Reports Server (NTRS)

Metcalfe, Ralph W.; Mcmurtry, Patrick A.; Jou, Wen-Huei; Riley, James J.; Givi, Peyman

1988-01-01

The results of direct numerical simulations of chemically reacting turbulent mixing layers are presented. This is an extension of earlier work to a more detailed study of previous three dimensional simulations of cold reacting flows plus the development, validation, and use of codes to simulate chemically reacting shear layers with heat release. Additional analysis of earlier simulations showed good agreement with self similarity theory and laboratory data. Simulations with a two dimensional code including the effects of heat release showed that the rate of chemical product formation, the thickness of the mixing layer, and the amount of mass entrained into the layer all decrease with increasing rates of heat release. Subsequent three dimensional simulations showed similar behavior, in agreement with laboratory observations. Baroclinic torques and thermal expansion in the mixing layer were found to produce changes in the flame vortex structure that act to diffuse the pairing vortices, resulting in a net reduction in vorticity. Previously unexplained anomalies observed in the mean velocity profiles of reacting jets and mixing layers were shown to result from vorticity generation by baroclinic torques.
Shear flow of dense granular materials near smooth walls. I. Shear localization and constitutive laws in the boundary region.

PubMed

Shojaaee, Zahra; Roux, Jean-Noël; Chevoir, François; Wolf, Dietrich E

2012-07-01

We report on a numerical study of the shear flow of a simple two-dimensional model of a granular material under controlled normal stress between two parallel smooth frictional walls moving with opposite velocities ± V. Discrete simulations, which are carried out with the contact dynamics method in dense assemblies of disks, reveal that, unlike rough walls made of strands of particles, smooth ones can lead to shear strain localization in the boundary layer. Specifically, we observe, for decreasing V, first a fluidlike regime (A), in which the whole granular layer is sheared, with a homogeneous strain rate except near the walls, then (B) a symmetric velocity profile with a solid block in the middle and strain localized near the walls, and finally (C) a state with broken symmetry in which the shear rate is confined to one boundary layer, while the bulk of the material moves together with the opposite wall. Both transitions are independent of system size and occur for specific values of V. Transient times are discussed. We show that the first transition, between regimes A and B, can be deduced from constitutive laws identified for the bulk material and the boundary layer, while the second one could be associated with an instability in the behavior of the boundary layer. The boundary zone constitutive law, however, is observed to depend on the state of the bulk material nearby.
Loading direction-dependent shear behavior at different temperatures of single-layer chiral graphene sheets

NASA Astrophysics Data System (ADS)

Zhao, Yang; Dong, Shuhong; Yu, Peishi; Zhao, Junhua

2018-06-01

The loading direction-dependent shear behavior of single-layer chiral graphene sheets at different temperatures is studied by molecular dynamics (MD) simulations. Our results show that the shear properties (such as shear stress-strain curves, buckling strains, and failure strains) of chiral graphene sheets strongly depend on the loading direction due to the structural asymmetry. The maximum values of both the critical buckling shear strain and the failure strain under positive shear deformation can be around 1.4 times higher than those under negative shear deformation. For a given chiral graphene sheet, both its failure strain and failure stress decrease with increasing temperature. In particular, the amplitude to wavelength ratio of wrinkles for different chiral graphene sheets under shear deformation using present MD simulations agrees well with that from the existing theory. These findings provide physical insights into the origins of the loading direction-dependent shear behavior of chiral graphene sheets and their potential applications in nanodevices.
Shear-lag analysis about an internally-dropped ply

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vizzini, A.J.

1995-12-31

The region around a terminated ply is modeled as several elastic layers separated by shear regions. A shear-lag analysis is then performed allowing for the thickness of the elastic and shear layers to vary. Boundary conditions, away for the ply drop, are based on the deflections determined by a finite element model. The interlaminar stresses are compared against those generated by the finite element model for tapered laminates under pure extension, pure bending, and extension-bending coupling. The shear-lag analysis predicts the interlaminar shear at and near the ply drop for pure extension and in cases involving bending if the deflectionsmore » due to bending are removed. The interlaminar shear stress and force equilibrium are used to determine the interlaminar normal stress. The trends in the interlaminar normal stress shown by the finite element model are partially captured by the shear-lag analysis. This simple analysis indicates that the mechanism for load transfer about a ply drop is primarily due to shear transfer through the resin rich areas.« less
The effects of forcing on a single stream shear layer and its parent boundary layer

NASA Technical Reports Server (NTRS)

Haw, R. C.; Foss, J. F.

1989-01-01

The detailed response of a large single-stream shear layer to a sinusoidal forcing at x = 0 is quantitatively defined. Phase-averaged data are used to characterize the increased disturbance convection velocity and a width measure of the disturbance field. These findings are consistent with and complement those of Fiedler and Mensing (1985).
Characterization of the Shear Layer in a Mach 3 Shock/Turbulent Boundary Layer Interaction

NASA Astrophysics Data System (ADS)

Helm, Clara; Priebe, Stephan; Li, Justine; Dupont, Pierre; Martin, Pino

2013-11-01

The unsteady motion of fully separated shock and turbulent boundary layers interactions (STBLIs) is characterized by an energized low-frequency motion that is two orders of magnitude lower than that of the incoming turbulence. In addition, the spectra shows significant energy content at frequency that is between the characteristic low frequency and the higher frequency motions of the incoming turbulence. The intermediate frequency content is hypothesized to be associated with the existence of Kelvin-Helmholtz type structures, which form in the shear layer downstream of the separation shock and are shed near the reattachment point downstream of the interaction. The current research is concerned with investigating the origins of the intermediate frequencies, and how they may be related to or possibly influence the low-frequency unsteadiness. Specifically, LES data of a Mach 3 STBLI over a 24o ramp are used to estimate convection velocities within the shear layer downstream of the shock. In addition, Brown and Thomas type correlations are used to estimate time and length scales of the eddies in the shear layer. This work is supported by the Air Force Office of Scientific Research under grant AF/9550-10-1-0164.
The effects of spatial inhomogeneities on flow through the endothelial surface layer.

PubMed

Leiderman, Karin M; Miller, Laura A; Fogelson, Aaron L

2008-05-21

Flow through the endothelial surface layer (the glycocalyx and adsorbed plasma proteins) plays an important but poorly understood role in cell signaling through a process known as mechanotransduction. Characterizing the flow rates and shear stresses throughout this layer is critical for understanding how flow-induced ionic currents, deformations of transmembrane proteins, and the convection of extracellular molecules signal biochemical events within the cell, including cytoskeletal rearrangements, gene activation, and the release of vasodilators. Previous mathematical models of flow through the endothelial surface layer are based upon the assumptions that the layer is of constant hydraulic permeability and constant height. These models also assume that the layer is continuous across the endothelium and that the layer extends into only a small portion of the vessel lumen. Results of these models predict that fluid shear stress is dissipated through the surface layer and is thus negligible near endothelial cell membranes. In this paper, such assumptions are removed, and the resultant flow rates and shear stresses through the layer are described. The endothelial surface layer is modeled as clumps of a Brinkman medium immersed in a Newtonian fluid. The width and spacing of each clump, hydraulic permeability, and fraction of the vessel lumen occupied by the layer are varied. The two-dimensional Navier-Stokes equations with an additional Brinkman resistance term are solved using a projection method. Several fluid shear stress transitions in which the stress at the membrane shifts from low to high values are described. These transitions could be significant to cell signaling since the endothelial surface layer is likely dynamic in its composition, density, and height.
On various refined theories in the bending analysis of angle-ply laminates

NASA Astrophysics Data System (ADS)

Savithri, S.; Varadan, T. K.

1992-05-01

The accuracies of six shear-deformation theories are compared by analyzing the bending of angle-ply laminates and studying the results in the light of exact solutions. The shear-deformation theories used are those by: Ren (1986), Savithri and Varadan (1990), Bhaskar and Varadan (1991), Murakami (1986), and Pandya and Kant (1988), and combinations of these. The analytical methods are similar in that the number of unknown variables in the displacement field is independent of the number of layers in the laminate. The model by Ren is based on a parabolic distribution of transverse shear stresses in each laminate layer. This model is shown to give good predictions of deflections and stresses in two-layer antisymmetric and three-layer symmetric angle-ply laminates.
High temperature pseudotachylytes and ductile shear zones in dry rocks from the continental lower crust (Lofoten, Norway)

NASA Astrophysics Data System (ADS)

Menegon, Luca; Pennacchioni, Giorgio; Harris, Katherine; Wood, Elliot

2014-05-01

Understanding the mechanisms of initiation and growth of shear zones under lower crustal conditions is of fundamental importance when assessing lithosphere rheology and strength. In this study we investigate brittle-ductile shear zones developed under lower crustal conditions in anorthosites from Nusfjord, Lofoten (northern Norway). Steep ductile shear zones trend E-W to ESE-WSW and have a stretching lineation plunging steeply to the SSW or SSE. The shear sense is normal (south block down to the south) as indicated by SC and SC' fabrics and sigmoidal foliations. The shear zone show a mylonitic to ultramylonitic fabric, sharp boundaries to the host anorthosites, and abundant anastomosing dark fine-grained layers along the main foliation. The fine-grained layers localized much of the strain. Relatively lower strain domains within or adjacent to shear zones indicate that the fine dark bands of mylonites represent transposed pseudotachylyte which still locally preserve the pristine structures such as chilled margins, breccia textures with angular clasts of the host rock and injection veins; intersecting veins of pseudotachylyte record multiple stages of seismic slip. The orientation of injection veins and marker offset along the most preserved pseudotachylyte fault veins indicate approximately a sinistral strike slip kinematic during faulting event responsible for the friction-induced melting. These observations indicate that ductile shear zones exploited pre-existing brittle fault zones including a network of pseudotachylytes, and that the fine-grained "ultramylonites" derive from former fine-grained pseudotachylytes. The pseudotachylyte microstructure is dominated by plagioclase microlites dispersed in a groundmass of fine-grained clinopyroxene. Clinopyroxene recrystallizes in the damage zone flanking the pseudotachylytes, indicating high metamorphic grade during pseudotachylyte formation. Small idioblastic or cauliflower garnet are scattered through the matrix and overgrow the plagioclase porphyroclasts; in some cases small garnets nucleated along thin microfractures discordant to the pseudotachylyte vein or along the pseudotachylyte boundary. In the host rock garnet form thin continuous coronitic rims surrounding biotite and opaque and discontinuous one around pyroxene. The mineral assemblage of ultramlylonites is also consistent with high grade metamorphic conditions (recrystallized plagioclase and clinopyroxene, biotite and amphibole). Nucleation of ductile shear zones is dictated by the availability of pseudotachylyte veins; remarkably, lithological boundaries have not been exploited by ductile shear zones. Brittle deformation and extreme grain size reduction are likely to be necessary conditions in order to promote ductile strain localization in dry rocks in the lower crust.

New Heating Mechanism of Asteroids in Protoplanetary Disks

NASA Astrophysics Data System (ADS)

Menzel, Raymond L.; Roberge, W. G.

2013-10-01

Heating of asteroids in the early solar system has been mainly attributed to two mechanisms: the decay of short-lived radionuclides and the unipolar induction mechanism originally proposed in a classic series of papers by Sonett and collaborators. As originally conceived, unipolar induction heating is the result of the dissipation of current inside the body driven by a “motional electric field”, which appears in the asteroid’s reference frame when it is immersed in a fully-ionized, magnetized T-Tauri solar wind. However we point out a subtle conceptual error in the way that the electric field is calculated. Strictly speaking, the motional electric field used by Sonett et al. is the electric field in the free-streaming plasma far from the asteroid. For realistic assumptions about the plasma density in protoplanetary disks, the interaction between the plasma and asteroid cause the formation of a shear layer, in which the motional electric field decreases and even vanishes at the asteroid surface. We reexamine and improve the induction heating mechanism by: (1) correcting this conceptual error by using non-ideal multifluid MHD to self consistently calculate the velocity, magnetic, and electric fields in and around the shear layer; and (2) considering more realistic environments and scenarios that are consistent with current theories about protoplanetary disks. We present solutions for two highly idealized flows, which demonstrate that the electric field inside the asteroid is actually produced by magnetic field gradients in the shear layer, and can either vanish or be comparable to the fields predicted by Sonett et al. depending on the flow geometry. We term this new mechanism “electrodynamic heating”, calculate its possible upper limits, and compare them to heating generated by the decay of short-lived radionuclides.
Heat release effects in a turbulent, reacting shear layer

NASA Astrophysics Data System (ADS)

Hermanson, James Carl

The effects of heat release were studied in a planar, gaseous reacting mixing layer formed between free streams containing hydrogen and fluorine in inert diluents. Sufficiently high concentrations of reactants were employed to produce adiabatic flame temperature rises of up to 940 K (1240 K absolute). The Reynolds number at the measuring station, based on velocity difference, 1% temperature thickness and cold kinematic viscosity was approximately 6x10^4. The temperature field was measured with cold wire resistance thermometers and thermocouples. Flow visualization was accomplished by schlieren spark and motion picture photography. Mean velocity information was extracted from mean pitot probe dynamic pressure measurements.Though the displacement thickness of the layer, for zero streamwise pressure gradient, increased with increasing heat release, the actual growth rate of the layer did not increase, but instead decreased slightly. The overall entrainment into the layer was seen to be substantially reduced as a consequence of heat release. Calculations showed that the decrease in layer growth rate can be accounted for by a corresponding reduction in turbulent shear stress.The mean temperature rise profiles, normalized by the adiabatic flame temperature rise, were not greatly changed in shape by heat release. A small decrease in normalized mean temperature rise with heat release was observed. Large scale coherent structures were observed to persist at all levels of heat release in this investigation. The mean structure spacing decreased with increasing temperature. This decrease exceeded the rate of layer growth rate reduction, and suggests that the mechanisms of vortex amalgamation were, to some extent, inhibited by heat release.Imposition of a favorable pressure gradient resulted in additional thinning of the layer, and caused a slight increase in the mixing and amount of chemical product formation. The change in layer growth rate can be shown to be related to a change in free stream velocity ratio induced by pressure gradient.
The footprints of Saharan air layer and lightning on the formation of tropical depressions over the eastern Atlantic Ocean

NASA Astrophysics Data System (ADS)

Centeno Delgado, Diana C.; Chiao, Sen

2015-02-01

The roles of the Saharan Air Layer (SAL) and lightning during genesis of Tropical Depression (TD) 8 (2006) and TD 12 (2010) were investigated in relation to the interaction of the dust outbreaks with each system and their surrounding environment. This study applied data collected from the 2006 NASA African Monsoon Multidisciplinary Analysis and 2010 Genesis and Rapid Intensification Processes projects. Satellite observations from METEOSAT and Moderate Resolution Imaging Spectroradiometer (MODIS)—Aerosol Optical Depth (AOD) were also employed for the study of the dust content. Lightning activity data from the Met Office Arrival Time Difference (ATD) system were used as another parameter to correlate moist convective overturning and a sign of cyclone formation. The AOD and lightning analysis for TD 8 demonstrated the time-lag connection through their positive contribution to TC-genesis. TD 12 developed without strong dust outbreak, but with lower wind shear (2 m s-1) and an organized Mesoscale Convective System (MCS). Overall, the results from the combination of various data analyses in this study support the fact that both systems developed under either strong or weak dust conditions. From these two cases, the location (i.e., the target area) of strong versus weak dust outbreaks, in association with lightning, were essential interactions that impacted TC-genesis. While our dust footprints hypothesis applied under strong dust conditions (i.e., TD 8), other factors (e.g., vertical wind shear, pre-existing vortex and trough location, thermodynamics) need to be evaluated as well. The results from this study suggest that the SAL is not a determining factor that affects the formation of tropical cyclones (i.e., TD 8 and TD 12).
Coherent motion in excited free shear flows

NASA Technical Reports Server (NTRS)

Wygnanski, Israel J.; Petersen, Robert A.

1987-01-01

The application of the inviscid instability approach to externally excited turbulent free shear flows at high Reynolds numbers is explored. Attention is given to the cases of a small-deficit plane turbulent wake, a plane turbulent jet, an axisymmetric jet, the nonlinear evolution of instabilities in free shear flows, the concept of the 'preferred mode', vortex pairing in turbulent mixing layers, and experimental results for the control of free turbulent shear layers. The special features often attributed to pairing or to the preferred mode are found to be difficult to comprehend; the concept of feedback requires further substantiation in the case of incompressible flow.
Effects of free-stream turbulence intensity on transition in a laminar separation bubble formed over an airfoil

NASA Astrophysics Data System (ADS)

Istvan, Mark S.; Yarusevych, Serhiy

2018-03-01

The laminar-to-turbulent transition process in a laminar separation bubble formed over a NACA 0018 airfoil is investigated experimentally. All experiments are performed for an angle of attack of 4°, chord Reynolds numbers of 80,000 and 125,000, and free-stream turbulence intensities between 0.06 and 1.99%. The results show that increasing the level of free-stream turbulence intensity leads to a decrease in separation bubble length, attributed to a downstream shift in mean separation and an upstream shift in mean reattachment, the later ascribed to an upstream shift in mean transition. Maximum spatial amplification rates of disturbances in the separated shear layer decrease with increasing free-stream turbulence intensity, implying that the larger initial amplitudes of disturbances are solely responsible for the upstream shift in mean transition and as a result mean reattachment. At the baseline level of turbulence intensity, coherent structures forming in the aft portion of the bubble are characterized by strong spanwise coherence at formation, and undergo spanwise deformations leading to localized breakup in the vicinity of mean reattachment. As the level of free-stream turbulence intensity is increased, the spanwise coherence of the shear layer rollers is reduced, and spanwise undulations in the vortex filaments start to take place at the mean location of roll-up. At the highest level of turbulence intensity investigated, streamwise streaks originating in the boundary layer upstream of the separation bubble are observed within the bubble. These streaks signify an onset of bypass transition upstream of the separation bubble, which gives rise to a highly three-dimensional shear layer roll-up. A quantitative analysis of the associated changes in salient characteristics of the coherent structures is presented, connecting the effect of elevated free-stream turbulence intensity on the time-averaged and dynamic characteristics of the separation bubble.
Rotor Vortex Filaments: Living on the Slipstream's Edge

NASA Technical Reports Server (NTRS)

Young, Larry A.

1997-01-01

The purpose of this paper is to gain a better understanding of rotor wake evolution in hover and axial flow by deriving an analytical solution for the time dependent behavior of vortex filament circulation and core size. This solution is applicable only for vortex filaments in the rotor far-wake. A primarily inviscid vortex/shear layer interaction (where the slipstream boundary is modeled as a shear layer) has been identified in this analytical treatment. This vortex/shear layer interaction results in decreasing, vortex filament circulation and core size with time. The inviscid vortex/shear layer interaction is shown, in a first-order treatment, to be of greater magnitude than viscous diffusion effects. The rate of contraction, and ultimate collapse, of the vortex filament core is found to be directly proportional to the rotor inflow velocity. This new insight into vortex filament decay promises to help reconcile several disparate observations made in the literature and will, hopefully, promote new advances in theoretical modeling of rotor wakes.
Quantitative kinematic analysis within the Khlong Marui shear zone, southern Thailand

NASA Astrophysics Data System (ADS)

Kanjanapayont, Pitsanupong; Grasemann, Bernhard; Edwards, Michael A.; Fritz, Harald

2012-02-01

The NNE trending Khlong Marui shear zone has a strong geomorphic signal with marked fault-strike parallel topographic ridges. The lithologies within the strike-slip zone mainly consist of vertical layers of mylonitic meta-sedimentary rocks associated with orthogneisses, mylonitic granites, and pegmatitic veins. The pegmatitic veins concordantly intrude the mylonitic foliation but were sheared at the rims indicating syn-kinematic emplacement. Microstructures and mineral assemblages suggest that the rocks in the area have been metamorphosed at amphibolite facies and low to medium greenschist facies by the first deformation. The Khlong Marui shear zone was deformed under dextral simple shear flow with a small finite strain. The ductile-to-brittle deformation involves a period of exhumation of lenses of higher grade rocks together with low grade fault rocks probably associated with positive flower structures. The final stage brittle deformation is reflected by normal faulting and formation of proto-cataclasites to cataclasites of the original mylonitic meta-sedimentary host rock. Although clear age-constraints are still missing, we use regional relationships to speculate that earlier dextral strike-slip displacement of the Khlong Marui shear zone was related to the West Burma and Shan-Thai collision and subduction along the Sunda Trench in the Late Cretaceous, while the major exhumation period of the ductile lens was tectonically influenced by the early India-Asia collision. The changing stress field has responded by switching from dextral strike-slip to normal faulting in the Khlong Marui shear zone, and is associated with "escape tectonics" arising from the overall India-Asia collision.
Numerical Simulation of Vortex Ring Formation in the Presence of Background Flow: Implications for Squid Propulsion

NASA Astrophysics Data System (ADS)

Jiang, Houshuo; Grosenbaugh, Mark A.

2002-11-01

Numerical simulations are used to study the laminar vortex ring formation in the presence of background flow. The numerical setup includes a round-headed axisymmetric body with a sharp-wedged opening at the posterior end where a column of fluid is pushed out by a piston inside the body. The piston motion is explicitly included into the simulations by using a deforming mesh. The numerical method is verified by simulating the standard vortex ring formation process in quiescent fluid for a wide range of piston stroke to cylinder diameter ratios (Lm/D). The results from these simulations confirm the existence of a universal formation time scale (formation number) found by others from experimental and numerical studies. For the case of vortex ring formation by the piston/cylinder arrangement in a constant background flow (i.e. the background flow is in the direction of the piston motion), the results show that a smaller fraction of the ejected circulation is delivered into the leading vortex ring, thereby decreasing the formation number. The mechanism behind this reduction is believed to be related to the modification of the shear layer profile between the jet flow and the background flow by the external boundary layer on the outer surface of the cylinder. In effect, the vorticity in the jet is cancelled by the opposite signed vorticity in the external boundary layer. Simulations using different end geometries confirm the general nature of the phenomenon. The thrust generated from the jet and the drag forces acting on the body are calculated with and without background flow for different piston programs. The implications of these results for squid propulsion are discussed.
Effect of Interfacial Reaction on the Mechanical Performance of Steel to Aluminum Dissimilar Ultrasonic Spot Welds

NASA Astrophysics Data System (ADS)

Xu, Lei; Wang, Li; Chen, Ying-Chun; Robson, Joe D.; Prangnell, Philip B.

2016-01-01

The early stages of formation of intermetallic compounds (IMC) have been investigated in dissimilar aluminum to steel welds, manufactured by high power (2.5 kW) ultrasonic spot welding (USW). To better understand the influence of alloy composition, welds were produced between a low-carbon steel (DC04) and two different aluminum alloys (6111 and 7055). The joint strengths were measured in lap shear tests and the formation and growth behavior of IMCs at the weld interface were characterized by electron microscopy, for welding times from 0.2 to 2.4 seconds. With the material combinations studied, the η (Fe2Al5) intermetallic phase was found to form first, very rapidly in the initial stage of welding, with a discontinuous island morphology. Continuous layers of η and then θ (FeAl3) phase were subsequently seen to develop on extending the welding time to greater than 0.7 second. The IMC layer formed in the DC04-AA7055 combination grew thicker than for the DC04-AA6111 welds, despite both weld sets having near identical thermal histories. Zinc was also found to be dissolved in the IMC phases when welding with the AA7055 alloy. After post-weld aging of the aluminum alloy, fracture in the lap shear tests always occurred along the joint interface; however, the DC04-AA6111 welds had higher fracture energy than the DC04-AA7055 combination.
A Multi-scale Refined Zigzag Theory for Multilayered Composite and Sandwich Plates with Improved Transverse Shear Stresses

NASA Technical Reports Server (NTRS)

Iurlaro, Luigi; Gherlone, Marco; Di Sciuva, Marco; Tessler, Alexander

2013-01-01

The Refined Zigzag Theory (RZT) enables accurate predictions of the in-plane displacements, strains, and stresses. The transverse shear stresses obtained from constitutive equations are layer-wise constant. Although these transverse shear stresses are generally accurate in the average, layer-wise sense, they are nevertheless discontinuous at layer interfaces, and thus they violate the requisite interlaminar continuity of transverse stresses. Recently, Tessler applied Reissner's mixed variational theorem and RZT kinematic assumptions to derive an accurate and efficient shear-deformation theory for homogeneous, laminated composite, and sandwich beams, called RZT(m), where "m" stands for "mixed". Herein, the RZT(m) for beams is extended to plate analysis, where two alternative assumptions for the transverse shear stresses field are examined: the first follows Tessler's formulation, whereas the second is based on Murakami's polynomial approach. Results for elasto-static simply supported and cantilever plates demonstrate that Tessler's formulation results in a powerful and efficient structural theory that is well-suited for the analysis of multilayered composite and sandwich panels.
Predicting equilibrium states with Reynolds stress closures in channel flow and homogeneous shear flow

NASA Technical Reports Server (NTRS)

Abid, R.; Speziale, C. G.

1993-01-01

Turbulent channel flow and homogeneous shear flow have served as basic building block flows for the testing and calibration of Reynolds stress models. A direct theoretical connection is made between homogeneous shear flow in equilibrium and the log-layer of fully-developed turbulent channel flow. It is shown that if a second-order closure model is calibrated to yield good equilibrium values for homogeneous shear flow it will also yield good results for the log-layer of channel flow provided that the Rotta coefficient is not too far removed from one. Most of the commonly used second-order closure models introduce an ad hoc wall reflection term in order to mask deficient predictions for the log-layer of channel flow that arise either from an inaccurate calibration of homogeneous shear flow or from the use of a Rotta coefficient that is too large. Illustrative model calculations are presented to demonstrate this point which has important implications for turbulence modeling.
Predicting equilibrium states with Reynolds stress closures in channel flow and homogeneous shear flow

NASA Technical Reports Server (NTRS)

Abid, R.; Speziale, C. G.

1992-01-01

Turbulent channel flow and homogeneous shear flow have served as basic building block flows for the testing and calibration of Reynolds stress models. A direct theoretical connection is made between homogeneous shear flow in equilibrium and the log-layer of fully-developed turbulent channel flow. It is shown that if a second-order closure model is calibrated to yield good equilibrium values for homogeneous shear flow it will also yield good results for the log-layer of channel flow provided that the Rotta coefficient is not too far removed from one. Most of the commonly used second-order closure models introduce an ad hoc wall reflection term in order to mask deficient predictions for the log-layer of channel flow that arise either from an inaccurate calibration of homogeneous shear flow or from the use of a Rotta coefficient that is too large. Illustrative model calculations are presented to demonstrate this point which has important implications for turbulence modeling.
The Effects of Shear Strain, Fabric, and Porosity Evolution on Elastic and Mechanical Properties of Clay-Rich Fault Gouge

NASA Astrophysics Data System (ADS)

Kenigsberg, A.; Saffer, D. M.; Riviere, J.; Marone, C.

2017-12-01

Ultrasonic/seismic waves are widely used for probing fault zone elastic and mechanical properties (gouge composition, frictional strength, density) and elastic properties (Vp, Vs, bulk and shear moduli), as it can provide insight into key processes and fault properties during shearing. These include fabric and force chain formation, porosity evolution, and fault zone stiffness, which are in turn factors in fault slip, damage, and healing. We report on a suite of direct shear experiments on synthetic fault gouge composed of 50% smectite /50% quartz at a normal stress of 25 MPa, in which we use ultrasonic wave transmission to continuously monitor compressional and shear wave velocities (Vp, Vs) up to shear strains of 25, while simultaneously measuring friction and monitoring the evolution of density and porosity. We find that wavespeeds vary with shear strain, due to fabric development and the evolution of density and porosity. The coefficient of friction peaks at μ .47 at a shear strain of .5 - 1, decreases to a steady state value of μ .43 by shear strains of 4.5- 6 and then remains rather constant to shear strains of 6 - 25, consistent with previous work. Density increases rapidly from 1.78 g/cm3 to 1.83 g/cm3 at shear strains from 0-2 (porosity decreases from 33% to 25% over that range), and then more gradually increases to a density of 2.08 g/cm3 (porosity of 21%) at a shear strain of 25. Vp increases from 2400 m/s to 2900 m/s during the onset of shear until a shear strain of 3, and then decreases to 2400-2500 by shear strain of 7-9. At shear strains above 9, Vp slowly increases as the layer becomes denser and less porous. We interpret the co-evolving changes in friction, porosity, and elastic moduli/wavespeed to reflect fabric development and alignment of clay particles as a function of shearing. More specifically, the decrease in Vp at a shear strain of 3 reflects the clay particles gradually aligning. Once the particles are aligned, the gradual increase of Vp at shear strains of 7-9 reflects near complete alignment and increased compaction and density. This interpretation is supported by SEM imaging and analysis of a suite of experiments stopped at different shear strains.
Non-linear coherent mode interactions and the control of shear layers

NASA Technical Reports Server (NTRS)

Nikitopoulos, D. E.; Liu, J. T. C.

1990-01-01

A nonlinear integral formulation, based on local linear stability considerations, is used to study the collective interactions between discrete wave-modes associated with large-scale structures and the mean flow in a developing shear layer. Aspects of shear layer control are examined in light of the sensitivity of these interactions to the initial frequency parameter, modal energy contents and modal phases. Manipulation of the large-scale structure is argued to be an effective means of controlling the flow, including the small-scale turbulence dominated region far downstream. Cases of fundamental, 1st and 2nd subharmonic forcing are discussed in conjunction with relevant experiments.
Optical Properties of Compressible Inhomogeneous Shear Layers Relevant to High Power Lasers.

DTIC Science & Technology

1987-09-30

trend of laser development towards shorter wavelenghts , the fluid optics challenge is increased con- siderahly. In general, the conditioning of the gas...tion pattern of laser beams passing through the layer. We hoped to under- ", stand and to predict compressible shear layer growth rate and optical per...layer growth rates for jet Mach numbers of 0.1, 0.3 and n.6 were measured using a Mach-Zehnder interferometer. Interferograms using a He-Ne laser source
Modeling interface shear behavior of granular materials using micro-polar continuum approach

NASA Astrophysics Data System (ADS)

Ebrahimian, Babak; Noorzad, Ali; Alsaleh, Mustafa I.

2018-01-01

Recently, the authors have focused on the shear behavior of interface between granular soil body and very rough surface of moving bounding structure. For this purpose, they have used finite element method and a micro-polar elasto-plastic continuum model. They have shown that the boundary conditions assumed along the interface have strong influences on the soil behavior. While in the previous studies, only very rough bounding interfaces have been taken into account, the present investigation focuses on the rough, medium rough and relatively smooth interfaces. In this regard, plane monotonic shearing of an infinite extended narrow granular soil layer is simulated under constant vertical pressure and free dilatancy. The soil layer is located between two parallel rigid boundaries of different surface roughness values. Particular attention is paid to the effect of surface roughness of top and bottom boundaries on the shear behavior of granular soil layer. It is shown that the interaction between roughness of bounding structure surface and the rotation resistance of bounding grains can be modeled in a reasonable manner through considered Cosserat boundary conditions. The influence of surface roughness is investigated on the soil shear strength mobilized along the interface as well as on the location and evolution of shear localization formed within the layer. The obtained numerical results have been qualitatively compared with experimental observations as well as DEM simulations, and acceptable agreement is shown.
Study of the Transition from MRI to Magnetic Turbulence via Parasitic Instability by a High-order MHD Simulation Code

NASA Astrophysics Data System (ADS)

Hirai, Kenichiro; Katoh, Yuto; Terada, Naoki; Kawai, Soshi

2018-02-01

Magnetic turbulence in accretion disks under ideal magnetohydrodynamic (MHD) conditions is expected to be driven by the magneto-rotational instability (MRI) followed by secondary parasitic instabilities. We develop a three-dimensional ideal MHD code that can accurately resolve turbulent structures, and carry out simulations with a net vertical magnetic field in a local shearing box disk model to investigate the role of parasitic instabilities in the formation process of magnetic turbulence. Our simulations reveal that a highly anisotropic Kelvin–Helmholtz (K–H) mode parasitic instability evolves just before the first peak in turbulent stress and then breaks large-scale shear flows created by MRI. The wavenumber of the enhanced parasitic instability is larger than the theoretical estimate, because the shear flow layers sometimes become thinner than those assumed in the linear analysis. We also find that interaction between antiparallel vortices caused by the K–H mode parasitic instability induces small-scale waves that break the shear flows. On the other hand, at repeated peaks in the nonlinear phase, anisotropic wavenumber spectra are observed only in the small wavenumber region and isotropic waves dominate at large wavenumbers unlike for the first peak. Restructured channel flows due to MRI at the peaks in nonlinear phase seem to be collapsed by the advection of small-scale shear structures into the restructured flow and resultant mixing.
Interfacial Shear Strength and Adhesive Behavior of Silk Ionomer Surfaces.

PubMed

Kim, Sunghan; Geryak, Ren D; Zhang, Shuaidi; Ma, Ruilong; Calabrese, Rossella; Kaplan, David L; Tsukruk, Vladimir V

2017-09-11

The interfacial shear strength between different layers in multilayered structures of layer-by-layer (LbL) microcapsules is a crucial mechanical property to ensure their robustness. In this work, we investigated the interfacial shear strength of modified silk fibroin ionomers utilized in LbL shells, an ionic-cationic pair with complementary ionic pairing, (SF)-poly-l-glutamic acid (Glu) and SF-poly-l-lysine (Lys), and a complementary pair with partially screened Coulombic interactions due to the presence of poly(ethylene glycol) (PEG) segments and SF-Glu/SF-Lys[PEG] pair. Shearing and adhesive behavior between these silk ionomer surfaces in the swollen state were probed at different spatial scales and pressure ranges by using functionalized atomic force microscopy (AFM) tips as well as functionalized colloidal probes. The results show that both approaches were consistent in analyzing the interfacial shear strength of LbL silk ionomers at different spatial scales from a nanoscale to a fraction of a micron. Surprisingly, the interfacial shear strength between SF-Glu and SF-Lys[PEG] pair with partially screened ionic pairing was greater than the interfacial shear strength of the SF-Glu and SF-Lys pair with a high density of complementary ionic groups. The difference in interfacial shear strength and adhesive strength is suggested to be predominantly facilitated by the interlayer hydrogen bonding of complementary amino acids and overlap of highly swollen PEG segments.
Shear horizontal guided wave modes to infer the shear stiffness of adhesive bond layers.

PubMed

Le Crom, Bénédicte; Castaings, Michel

2010-04-01

This paper presents a non-destructive, ultrasonic technique to evaluate the quality of bonds between substrates. Shear-horizontally polarized (SH) wave modes are investigated to infer the shear stiffness of bonds, which is necessarily linked to the shear resistance that is a critical parameter for bonded structures. Numerical simulations are run for selecting the most appropriate SH wave modes, i.e., with higher sensitivity to the bond than to other components, and experiments are made for generating-detecting pre-selected SH wave modes and for measuring their phase velocities. An inverse problem is finally solved, consisting of the evaluation of the shear stiffness modulus of a bond layer at different curing times between a metallic plate and a composite patch, such assembly being investigated in the context of repair of aeronautical structures.
Separate and combined effects of static stability and shear variation on the baroclinic instability of a two-layer current

NASA Technical Reports Server (NTRS)

Hyun, J. M.

1981-01-01

Quasi-geostrophic disturbance instability characteristics are studied in light of a linearized, two-layer Eady model in which both the static stability and the zonal current shear are uniform but different in each layer. It is shown that the qualitative character of the instability is determined by the sign of the basic-state potential vorticity gradient at the layer interface, and that there is a qualitative similarity between the effects of Richardson number variations due to changes in static stability and those due to changes in shear. The two-layer model is also used to construct an analog of the Williams (1974) continuous model of generalized Eady waves, the basic state in that case having zero potential vorticity gradient in the interior. The model results are in good agreement with the earlier Williams findings.

Shear Band Formation in Plastic-Bonded Explosives (PBX)

NASA Astrophysics Data System (ADS)

Dey, Thomas N.; Johnson, James N.

1997-07-01

Adiabatic shear bands can be a source of ignition and lead to detonation. At low to moderate deformation rates, 10--1000 s-1, two other mechanisms can also give rise to shear bands. These mechanisms are: softening caused by micro-cracking and (2) a constitutive response with a non-associated flow rule as is observed in granular material such as soil. Brittle behavior at small strains and the granular nature of HMX suggest that PBX-9501 constitutive behavior may be similar to sand. A constitutive model for each of these mechanims is studied in a series of calculations. A viscoelastic constitutive model for PBX-9501 softens via a statistical crack model, based on the work of Dienes (1986). A sand model is used to provide a non-associated flow rule. Both models generate shear band formation at 1--2% strain at nominal strain rates at and below 1000 s-1. Shear band formation is suppressed at higher strain rates. The sand model gives qualitative agreement for location and orientation of shear bands observed in a punch experiment. Both mechanisms may accelerate the formation of adiabatic shear bands.
Spatio-temporal characteristics of large scale motions in a turbulent boundary layer from direct wall shear stress measurement

NASA Astrophysics Data System (ADS)

Pabon, Rommel; Barnard, Casey; Ukeiley, Lawrence; Sheplak, Mark

2016-11-01

Particle image velocimetry (PIV) and fluctuating wall shear stress experiments were performed on a flat plate turbulent boundary layer (TBL) under zero pressure gradient conditions. The fluctuating wall shear stress was measured using a microelectromechanical 1mm × 1mm floating element capacitive shear stress sensor (CSSS) developed at the University of Florida. The experiments elucidated the imprint of the organized motions in a TBL on the wall shear stress through its direct measurement. Spatial autocorrelation of the streamwise velocity from the PIV snapshots revealed large scale motions that scale on the order of boundary layer thickness. However, the captured inclination angle was lower than that determined using the classic method by means of wall shear stress and hot-wire anemometry (HWA) temporal cross-correlations and a frozen field hypothesis using a convection velocity. The current study suggests the large size of these motions begins to degrade the applicability of the frozen field hypothesis for the time resolved HWA experiments. The simultaneous PIV and CSSS measurements are also used for spatial reconstruction of the velocity field during conditionally sampled intense wall shear stress events. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1315138.
Effect of Oxygen Inhibition Layer of Universal Adhesives on Enamel Bond Fatigue Durability and Interfacial Characteristics With Different Etching Modes.

PubMed

Ouchi, H; Tsujimoto, A; Nojiri, K; Hirai, K; Takamizawa, T; Barkmeier, W W; Latta, M A; Miyazaki, M

The purpose of this study was to evaluate the effect of the oxygen inhibition layer of universal adhesive on enamel bond fatigue durability and interfacial characteristics with different etching modes. The three universal adhesives used were Scotchbond Universal Adhesive (3M ESPE, St Paul, MN, USA), Adhese Universal (Ivoclar Vivadent, Schaan, Lichtenstein), and G-Premio Bond (GC, Tokyo, Japan). The initial shear bond strength and shear fatigue strength to enamel was determined in the presence and absence of the oxygen inhibition layer, with and without phosphoric acid pre-etching. The water contact angle was also measured in all groups using the sessile drop method. The enamel bonding specimens with an oxygen inhibition layer showed significantly higher (p<0.05) initial shear bond strengths and shear fatigue strengths than those without, regardless of the adhesive type and etching mode. Moreover, the water contact angles on the specimens with an oxygen inhibition layer were significantly lower (p<0.05) than on those without, regardless of etching mode. The results of this study suggest that the oxygen inhibition layer of universal adhesives significantly increases the enamel bond fatigue durability and greatly changes interfacial characteristics, suggesting that the bond fatigue durability and interfacial characteristics of these adhesives strongly rely on its presence.
Shear band formation in plastic bonded explosive (PBX)

NASA Astrophysics Data System (ADS)

Dey, T. N.; Johnson, J. N.

1998-07-01

Adiabatic shear bands can be a source of ignition and lead to detonation. At low to moderate deformation rates, 10-1000 s-1, two other mechanisms can also give rise to shear bands. These mechanisms are: 1) softening caused by micro-cracking and 2) a constitutive response with a non-associated flow rule as is observed in granular material such as soil. Brittle behavior at small strains and the granular nature of HMX suggest that PBX-9501 constitutive behavior may be similar to sand. A constitutive model for the first of these mechanisms is studied in a series of calculations. This viscoelastic constitutive model for PBX-9501 softens via a statistical crack model. A sand model is used to provide a non-associated flow rule and detailed results will be reported elsewhere. Both models generate shear band formation at 1-2% strain at nominal strain rates at and below 1000 s-1. Shear band formation is suppressed at higher strain rates. Both mechanisms may accelerate the formation of adiabatic shear bands.
Progressive softening of brittle-ductile transition due to interplay between chemical and deformation processes

NASA Astrophysics Data System (ADS)

Jeřábek, Petr; Bukovská, Zita; Morales, Luiz F. G.

2017-04-01

The micro-scale shear zones (shear bands) in granitoids from the South Armorican Shear Zone reflect localization of deformation and progressive weakening in the conditions of brittle-ductile transition. We studied microstructures in the shear bands with the aim to establish their P-T conditions and to derive stress and strain rates for specific deformation mechanisms. The evolving microstructure within shear bands documents switches in deformation mechanisms related to positive feedbacks between deformation and chemical processes and imposes mechanical constraints on the evolution of the brittle-ductile transition in the continental transform fault domains. The metamorphic mineral assemblage present in the shear bands indicate their formation at 300-350 ˚ C and 100-400 MPa. Focusing on the early development of shear bands, we identified three stages of shear band evolution. The early stage I associated with initiation of shear bands occurs via formation of microcracks with possible yielding differential stress of up to 250 MPa (Diamond and Tarantola, 2015). Stage II is associated with subgrain rotation recrystallization and dislocation creep in quartz and coeval dissolution-precipitation creep of microcline. Recrystallized quartz grains in shear bands show continual increase in size, and decrease in stress and strain rates from 94 MPa to 17-26 MPa (Stipp and Tullis, 2003) and 3.8*10-12 s-1- 1.8*10-14 s-1 (Patterson and Luan, 1990) associated with deformation partitioning into weaker microcline layer and shear band widening. The quartz mechanical data allowed us to set some constrains for coeval dissolution-precipitation of microcline which at our estimated P-T conditions suggests creep at 17-26 MPa differential stress and 3.8*10-13 s-1 strain rate. Stage III is characterized by localized slip along interconnected white mica bands accommodated by dislocation creep at strain rate 3.8*10-12 s-1 and stress 9.36 MPa (Mares and Kronenberg, 1993). The studied example documents a competition between shear zone widening and narrowing mechanisms, i.e. distributed and localized deformation, depending on the specific mineral phase and deformation mechanism active in each moment of the shear zone evolution. In addition, our mechanical data point to dynamic evolution of the studied brittle-ductile transition characterized by major weakening to strengths ˜10 MPa. Such non-steady-state evolution may be common in crustal shear zones especially when phase transformations are involved. References: Diamond, L. W., and A. Tarantola (2015), Interpretation of fluid inclusions in quartz deformed by weak ductile shearing: Reconstruction of differential stress magnitudes and pre-deformation fluid properties, Earth Planet. Sci. Lett., 417, 107-119. Mares, V. M., and A. K. Kronenberg (1993), Experimental deformation of muscovite, J. Struct. Geol., 15(9), 1061-1075. Paterson, M. S., and F. C. Luan (1990), Quartzite rheology under geological conditions, Geol. Soc. London, Spec. Publ., 54(1), 299-307. Stipp, M., and J. Tullis (2003), The recrystallized grain size piezometer for quartz, Geophys. Res. Lett., 30(21), 1-5.
Influence of thickness and permeability of endothelial surface layer on transmission of shear stress in capillaries

NASA Astrophysics Data System (ADS)

Zhang, SongPeng; Zhang, XiangJun; Tian, Yu; Meng, YongGang; Lipowsky, Herbert

2015-07-01

The molecular coating on the surface of microvascular endothelium has been identified as a barrier to transvascular exchange of solutes. With a thickness of hundreds of nanometers, this endothelial surface layer (ESL) has been treated as a porous domain within which fluid shear stresses are dissipated and transmitted to the solid matrix to initiate mechanotransduction events. The present study aims to examine the effects of the ESL thickness and permeability on the transmission of shear stress throughout the ESL. Our results indicate that fluid shear stresses rapidly decrease to insignificant levels within a thin transition layer near the outer boundary of the ESL with a thickness on the order of ten nanometers. The thickness of the transition zone between free fluid and the porous layer was found to be proportional to the square root of the Darcy permeability. As the permeability is reduced ten-fold, the interfacial fluid and solid matrix shear stress gradients increase exponentially two-fold. While the interfacial fluid shear stress is positively related to the ESL thickness, the transmitted matrix stress is reduced by about 50% as the ESL thickness is decreased from 500 to 100 nm, which may occur under pathological conditions. Thus, thickness and permeability of the ESL are two main factors that determine flow features and the apportionment of shear stresses between the fluid and solid phases of the ESL. These results may shed light on the mechanisms of force transmission through the ESL and the pathological events caused by alterations in thickness and permeability of the ESL.
Observations of the Morning Development of the Urban Boundary Layer Over London, UK, Taken During the ACTUAL Project

NASA Astrophysics Data System (ADS)

Halios, Christos H.; Barlow, Janet F.

2018-03-01

The study of the boundary layer can be most difficult when it is in transition and forced by a complex surface, such as an urban area. Here, a novel combination of ground-based remote sensing and in situ instrumentation in central London, UK, is deployed, aiming to capture the full evolution of the urban boundary layer (UBL) from night-time until the fully-developed convective phase. In contrast with the night-time stable boundary layer observed over rural areas, the night-time UBL is weakly convective. Therefore, a new approach for the detection of the morning-transition and rapid-growth phases is introduced, based on the sharp, quasi-linear increase of the mixing height. The urban morning-transition phase varied in duration between 0.5 and 4 h and the growth rate of the mixing layer during the rapid-growth phase had a strong positive relationship with the convective velocity scale, and a weaker, negative relationship with wind speed. Wind shear was found to be higher during the night-time and morning-transition phases than the rapid-growth phase and the shear production of turbulent kinetic energy near the mixing-layer top was around six times larger than surface shear production in summer, and around 1.5 times larger in winter. In summer under low winds, low-level jets dominated the UBL, and shear production was greater than buoyant production during the night-time and the morning-transition phase near the mixing-layer top. Within the rapid-growth phase, buoyant production dominated at the surface, but shear production dominated in the upper half of the UBL. These results imply that regional flows such as low-level jets play an important role alongside surface forcing in determining UBL structure and growth.
The dynamics and control of fluctuating pressure loads in the reattachment region of a supersonic free shear layer

NASA Technical Reports Server (NTRS)

Smits, A. J.

1990-01-01

The primary aim is to investigate the mechanisms which cause the unsteady wall-pressure fluctuations in shock wave turbulent shear layer interactions. The secondary aim is to find means to reduce the magnitude of the fluctuating pressure loads by controlling the unsteady shock motion. The particular flow proposed for study is the unsteady shock wave interaction formed in the reattachment zone of a separated supersonic flow. Similar flows are encountered in many practical situations, and they are associated with high levels of fluctuating wall pressure. Wall pressure fluctuations were measured in the reattachment region of the supersonic free shear layer. The free shear layer was formed by the separation of a Mach 2.9 turbulent boundary layer from a backward facing step. Reattachment occurred on a 20 deg ramp. By adjusting the position of the ramp, the base pressure was set equal to the freestream pressure, and the free shear layer formed in the absence of a separation shock. An array of flush-mounted, miniature, high-frequency pressure transducers was used to make multichannel measurements of the fluctuating wall pressure in the vicinity of the reattachment region. Contrary to previous observations of this flow, the reattachment region was found to be highly unsteady, and the pressure fluctuations were found to be significant. The overall behavior of the wall pressure loading is similar in scale and magnitude to the unsteadiness of the wall pressure field in compression ramp flows at the same Mach number. Rayleigh scattering was used to visualize the instantaneous shock structure in the streamwise and spanwise direction. Spanwise wrinkles on the order of half the boundary layer thickness were observed.
Impact of grain size evolution on necking and pinch-and-swell formation in calcite layers

NASA Astrophysics Data System (ADS)

Schmalholz, Stefan Markus; Duretz, Thibault

2017-04-01

The formation of necking zones and the associated formation of pinch-and-swell structure is one form of strain localization in extending, competent layers. Natural pinch-and-swell structure in centimetre-thick calcite layers typically shows a reduction of grain size from swell towards pinch. However, the impact of grain size evolution on necking and pinch-and-swell formation is incompletely understood. We perform zero-dimensional (0D) and 2D thermo-mechanical numerical simulations to quantify the impact of grain size evolution on necking for extension rates between 10-12s^-1and10^-14 s-1 and temperatures around 350°C. For a combination of diffusion and dislocation creep we calculate grain size evolution according to the paleowattmeter (grain size is proportional to mechanical work rate) or the paleopiezometer (grain size is proportional to stress). Numerical results fit two observations: (i) grain size reduction from swells towards pinches, and (ii) dislocation creep dominated deformation in swells and significant contribution of diffusion creep in pinches. Modelled grain size in pinches (10 to 60 μm) and swells (70 to 800 μm) is close to observed grain size in pinches (15 to 27 μm) and in swells (250 to 1500 μm). Grain size evolution has only a minor impact on necking suggesting that grain size evolution is a consequence, and not the cause of necking. Viscous shear heating and grain size evolution had a negligible thermal impact in the simulations.
Experimental data and model for the turbulent boundary layer on a convex, curved surface

NASA Technical Reports Server (NTRS)

Gillis, J. C.; Johnson, J. P.; Moffat, R. J.; Kays, W. M.

1981-01-01

Experiments were performed to determine how boundary layer turbulence is affected by strong convex curvature. The data gathered on the behavior of the Reynolds stress suggested the formulation of a simple turbulence model. Data were taken on two separate facilities. Both rigs had flow from a flat surface, over a convex surface with 90 deg of turning and then onto a flat recovery surface. The geometry was adjusted so that, for both rigs, the pressure gradient along the test surface was zero. Two experiments were performed at delta/R approximately 0.10, and one at weaker curvature with delta/R approximately 0.05. Results show that after a sudden introduction of curvature the shear stress in the outer part of the boundary layer is sharply diminished and is even slightly negative near the edge. The wall shear also drops off quickly downstream. When the surface suddenly becomes flat again, the wall shear and shear stress profiles recover very slowly towards flat wall conditions. A simple turbulence model, which was based on the theory that the Prandtl mixing length in the outer layer should scale on the velocity gradient layer, was shown to account for the slow recovery.
Hypersonic merged layer blunt body flows with wakes

NASA Technical Reports Server (NTRS)

Jain, Amolak C.; Dahm, Werner K.

1991-01-01

An attempt is made here to understand the basic physics of the flowfield with wake on a blunt body of revolution under hypersonic rarefied conditions. A merged layer model of flow is envisioned. Full steady-state Navier-Stokes equations in spherical polar coordinate system are computed from the surface with slip and temperature jump conditions to the free stream by the Accelerated Successive Replacement method of numerical integration. Analysis is developed for bodies of arbitrary shape, but actual computations have been carried out for a sphere and sphere-cone body. Particular attention is paid to set the limit of the onset of separation, wake closure, shear-layer impingement, formation and dissipation of the shocks in the flowfield. Validity of the results is established by comparing the present results for sphere with the corresponding results of the SOFIA code in the common region of their validity and with the experimental data.
Stably Stratified Atmospheric Boundary Layers

NASA Astrophysics Data System (ADS)

Mahrt, L.

2014-01-01

Atmospheric boundary layers with weak stratification are relatively well described by similarity theory and numerical models for stationary horizontally homogeneous conditions. With common strong stratification, similarity theory becomes unreliable. The turbulence structure and interactions with the mean flow and small-scale nonturbulent motions assume a variety of scenarios. The turbulence is intermittent and may no longer fully satisfy the usual conditions for the definition of turbulence. Nonturbulent motions include wave-like motions and solitary modes, two-dimensional vortical modes, microfronts, intermittent drainage flows, and a host of more complex structures. The main source of turbulence may not be at the surface, but rather may result from shear above the surface inversion. The turbulence is typically not in equilibrium with the nonturbulent motions, sometimes preventing the formation of an inertial subrange. New observational and analysis techniques are expected to advance our understanding of the very stable boundary layer.
Method of making gas diffusion layers for electrochemical cells

DOEpatents

Frisk, Joseph William; Boand, Wayne Meredith; Larson, James Michael

2002-01-01

A method is provided for making a gas diffusion layer for an electrochemical cell comprising the steps of: a) combining carbon particles and one or more surfactants in a typically aqueous vehicle to make a preliminary composition, typically by high shear mixing; b) adding one or more highly fluorinated polymers to said preliminary composition by low shear mixing to make a coating composition; and c) applying the coating composition to an electrically conductive porous substrate, typically by a low shear coating method.
An alternative assessment of second-order closure models in turbulent shear flows

NASA Technical Reports Server (NTRS)

Speziale, Charles G.; Gatski, Thomas B.

1994-01-01

The performance of three recently proposed second-order closure models is tested in benchmark turbulent shear flows. Both homogeneous shear flow and the log-layer of an equilibrium turbulent boundary layer are considered for this purpose. An objective analysis of the results leads to an assessment of these models that stands in contrast to that recently published by other authors. A variety of pitfalls in the formulation and testing of second-order closure models are uncovered by this analysis.
Dynamic Stall Control Using Plasma Actuators

NASA Astrophysics Data System (ADS)

Webb, Nathan; Singhal, Achal; Castaneda, David; Samimy, Mo

2017-11-01

Dynamic stall occurs in many applications, including sharp maneuvers of fixed wing aircraft, wind turbines, and rotorcraft and produces large unsteady aerodynamic loads that can lead to flutter and mechanical failure. This work uses flow control to reduce the unsteady loads by excitation of instabilities in the shear layer over the separated region using nanosecond pulse driven dielectric barrier discharge (NS-DBD) plasma actuators. These actuators have been shown to effectively delay or mitigate static stall. A wide range of flow parameters were explored in the current work: Reynolds number (Re = 167,000 to 500,000), reduced frequency (k = 0.025 to 0.075), and excitation Strouhal number (Ste = 0 to 10). Based on the results, three major conclusions were drawn: (a) Low Strouhal number excitation (Ste <0.5) results in oscillatory aerodynamic loads in the stalled stage of dynamic stall; (b) All excitation resulted in earlier flow reattachment; and (c) Excitation at progressively higher Ste weakened and eventually eliminated the dynamic stall vortex (DSV), thereby dramatically reducing the unsteady loading. The decrease in the strength of the DSV is achieved by the formation of shear layer coherent structures that bleed the leading-edge vorticity prior to the ejection of the DSV.
A general integral form of the boundary-layer equation for incompressible flow with an application to the calculation of the separation point of turbulent boundary layers

NASA Technical Reports Server (NTRS)

Tetervin, Neal; Lin, Chia Chiao

1951-01-01

A general integral form of the boundary-layer equation, valid for either laminar or turbulent incompressible boundary-layer flow, is derived. By using the experimental finding that all velocity profiles of the turbulent boundary layer form essentially a single-parameter family, the general equation is changed to an equation for the space rate of change of the velocity-profile shape parameter. The lack of precise knowledge concerning the surface shear and the distribution of the shearing stress across turbulent boundary layers prevented the attainment of a reliable method for calculating the behavior of turbulent boundary layers.
Design study of the geometry of the blanking tool to predict the burr formation of Zircaloy-4 sheet

NASA Astrophysics Data System (ADS)

Ha, Jisun; Lee, Hyungyil; Kim, Dongchul; Kim, Naksoo

2013-12-01

In this work, we investigated factors that influence burr formation for zircaloy-4 sheet used for spacer grids of nuclear fuel roads. Factors we considered are geometric factors of punch. We changed clearance and velocity in order to consider the failure parameters, and we changed shearing angle and corner radius of L-shaped punch in order to consider geometric factors of punch. First, we carried out blanking test with failure parameter of GTN model using L-shaped punch. The tendency of failure parameters and geometric factors that affect burr formation by analyzing sheared edges is investigated. Consequently, geometric factor's influencing on the burr formation is also high as failure parameters. Then, the sheared edges and burr formation with failure parameters and geometric factors is investigated using FE analysis model. As a result of analyzing sheared edges with the variables, we checked geometric factors more affect burr formation than failure parameters. To check the reliability of the FE model, the blanking force and the sheared edges obtained from experiments are compared with the computations considering heat transfer.
An integral turbulent kinetic energy analysis of free shear flows

NASA Technical Reports Server (NTRS)

Peters, C. E.; Phares, W. J.

1973-01-01

Mixing of coaxial streams is analyzed by application of integral techniques. An integrated turbulent kinetic energy (TKE) equation is solved simultaneously with the integral equations for the mean flow. Normalized TKE profile shapes are obtained from incompressible jet and shear layer experiments and are assumed to be applicable to all free turbulent flows. The shear stress at the midpoint of the mixing zone is assumed to be directly proportional to the local TKE, and dissipation is treated with a generalization of the model developed for isotropic turbulence. Although the analysis was developed for ducted flows, constant-pressure flows were approximated with the duct much larger than the jet. The axisymmetric flows under consideration were predicted with reasonable accuracy. Fairly good results were also obtained for the fully developed two-dimensional shear layers, which were computed as thin layers at the boundary of a large circular jet.
Meteor radar wind over Chung-Li (24.9°N, 121°E), Taiwan, for the period 10-25 November 2012 which includes Leonid meteor shower: Comparison with empirical model and satellite measurements

NASA Astrophysics Data System (ADS)

Su, C. L.; Chen, H. C.; Chu, Y. H.; Chung, M. Z.; Kuong, R. M.; Lin, T. H.; Tzeng, K. J.; Wang, C. Y.; Wu, K. H.; Yang, K. F.

2014-08-01

The neutral winds in the mesosphere and lower thermosphere (MLT) region are measured by a newly installed meteor trail detection system (or meteor radar) at Chung-Li, Taiwan, for the period 10-25 November 2012, which includes the Leonid meteor shower period. In this study, we use the 3 m field-aligned plasma irregularities in the sporadic E (Es) region in combination with the International Geomagnetic Reference Field model to calibrate the system phase biases such that the true positions of the meteor trails can be correctly determined with interferometry technique. The horizontal wind velocities estimated from the radial velocities of the meteor trails and their locations by using a least squares method show that the diurnal tide dominates the variation of the MLT neutral wind with time over Chung-Li, which is in good agreement with the horizontal wind model (HWM07) prediction. However, harmonic analysis reveals that the amplitudes of the mean wind, diurnal, and semidiurnal tides of the radar-measured winds in height range 82-100 km are systematically larger than those of the model-predicted winds by up to a factor of 3. A comparison shows that the overall pattern of the height-local time distribution of the composite radar-measured meteor wind is, in general, consistent with that of the TIMED Doppler Interferometer-observed wind, which is dominated by a diurnal oscillation with downward phase progression at a rate of about 1.3 km/h. The occurrences of the Es layers retrieved from fluctuations of the amplitude and excess phase of the GPS signal received by the FORMOSAT-3/COSMIC satellites during the GPS radio occultation (RO) process are compared with the shear zones of the radar-measured meteor wind and HWM07 wind. The result shows that almost all of the RO-retrieved Es layers occur within the wind shear zones that favor the Es layer formation based on the wind shear theory, suggesting that the primary physical process responsible for the Es layer events retrieved from the scintillations of the GPS RO signal is very likely the plasma convergence effect of the neutral wind shear.
Structural state diagram of concentrated suspensions of jammed soft particles in oscillatory shear flow

NASA Astrophysics Data System (ADS)

Khabaz, Fardin; Cloitre, Michel; Bonnecaze, Roger T.

2018-03-01

In a recent study [Khabaz et al., Phys. Rev. Fluids 2, 093301 (2017), 10.1103/PhysRevFluids.2.093301], we showed that jammed soft particle glasses (SPGs) crystallize and order in steady shear flow. Here we investigate the rheology and microstructures of these suspensions in oscillatory shear flow using particle-dynamics simulations. The microstructures in both types of flows are similar, but their evolutions are very different. In both cases the monodisperse and polydisperse suspensions form crystalline and layered structures, respectively, at high shear rates. The crystals obtained in the oscillatory shear flow show fewer defects compared to those in the steady shear. SPGs remain glassy for maximum oscillatory strains less than about the yield strain of the material. For maximum strains greater than the yield strain, microstructural and rheological transitions occur for SPGs. Polydisperse SPGs rearrange into a layered structure parallel to the flow-vorticity plane for sufficiently high maximum shear rates and maximum strains about 10 times greater than the yield strain. Monodisperse suspensions form a face-centered cubic (FCC) structure when the maximum shear rate is low and hexagonal close-packed (HCP) structure when the maximum shear rate is high. In steady shear, the transition from a glassy state to a layered one for polydisperse suspensions included a significant induction strain before the transformation. In oscillatory shear, the transformation begins to occur immediately and with different microstructural changes. A state diagram for suspensions in large amplitude oscillatory shear flow is found to be in close but not exact agreement with the state diagram for steady shear flow. For more modest amplitudes of around one to five times the yield strain, there is a transition from a glassy structure to FCC and HCP crystals, at low and high frequencies, respectively, for monodisperse suspensions. At moderate frequencies, the transition is from glassy to HCP via an intermediate FCC phase.

Graduate engineering research participation in aeronautics

NASA Technical Reports Server (NTRS)

Roberts, A. S., Jr.

1984-01-01

Graduate student engineering research in aeronautics at Old Dominion University is surveyed. Student participation was facilitated through a NASA sponsored university program which enabled the students to complete degrees. Research summaries are provided and plans for the termination of the grant program are outlined. Project topics include: Failure modes for mechanically fastened joints in composite materials; The dynamic stability of an earth orbiting satellite deploying hinged appendages; The analysis of the Losipescu shear test for composite materials; and the effect of boundary layer structure on wing tip vortex formation and decay.
Further Examination of the Thermodynamic Modification of the Inflow Layer of Tropical Cyclones by Vertical Wind Shear

DTIC Science & Technology

2013-01-11

Q. J. Roy. Meteor. Soc., in review, 2012. Srivastava , R . C.: A model of intense downdrafts driven by the melt- ing and evaporation of precipitation, J...formation and intensity of downdrafts ( Srivastava , 1987). 5A brief discussion of the potential consequences of the envi- ronmental wind profile can be found...the time series is shown. (b) As in (a) but for RMN68 (da k r d), 10RMN68 (light red), ICE68 (dark grey), and 10ICE68 (light grey). Note that the tilt
Influence of the Saharan Air Layer on Atlantic tropical cyclone formation during the period 1-12 September 2003

NASA Astrophysics Data System (ADS)

Pan, Weiyu; Wu, Liguang; Shie, Chung-Lin

2011-01-01

Atmospheric Infrared Sounder (AIRS) data show that the Saharan air layer (SAL) is a dry, warm, and well-mixed layer between 950 and 500 hPa over the tropical Atlantic, extending westward from the African coast to the Caribbean Sea. The formations of both Hurricane Isabel and Tropical Depression 14 (TD14) were accompanied with outbreaks of SAL air during the period 1-12 September 2003, although TD14 failed to develop into a named tropical cyclone. The influence of the SAL on their formations is investigated by examining data from satellite observations and numerical simulations, in which AIRS data are incorporated into the MM5 model through the nudging technique. Analyses of the AIRS and simulation data suggest that the SAL may have played two roles in the formation of tropical cyclones during the period 1-12 September 2003. First, the outbreaks of SAL air on 3 and 8 September enhanced the transverse-vertical circulation with the rising motion along the southern edge of the SAL and the sinking motion inside the SAL, triggering the development of two tropical disturbances associated with Hurricane Isabel and TD14. Second, in addition to the reduced environmental humidity and enhanced static stability in the lower troposphere, the SAL dry air intruded into the inner region of these tropical disturbances as their cyclonic flows became strong. This effect may have slowed down the formation of Isabel and inhibited TD14 becoming a named tropical cyclone, while the enhanced vertical shear contributed little to tropical cyclone formation during this period. The 48-h trajectory calculations confirm that the parcels from the SAL can be transported into the inner region of an incipient tropical cyclone.
Sediment resuspension characteristics in Baltimore Harbor, Maryland

USGS Publications Warehouse

Maa, J.P.-Y.; Sanford, L.; Halka, J.P.

1998-01-01

Critical bed shear stress for sediment resuspension and sediment erosion rate were measured in-situ at sites from inner to outer Baltimore Harbor using the VIMS Sea Carousel. Clay mineral contents and biological conditions were almost the same at the four study sites. The experimental results indicated that the erosion rate increased from the outer harbor toward the inner harbor with a maximum difference of about 10 times at an excess bed shear stress of 0.1 Pa. The measured critical bed shear stress strongly depended on the existence of a fluff layer. It was approximately 0.05 Pa if a fluff layer existed, and increases to about 0.1 Pa in the absence of a fluff layer.
Turbulent shear layers in confining channels

NASA Astrophysics Data System (ADS)

Benham, Graham P.; Castrejon-Pita, Alfonso A.; Hewitt, Ian J.; Please, Colin P.; Style, Rob W.; Bird, Paul A. D.

2018-06-01

We present a simple model for the development of shear layers between parallel flows in confining channels. Such flows are important across a wide range of topics from diffusers, nozzles and ducts to urban air flow and geophysical fluid dynamics. The model approximates the flow in the shear layer as a linear profile separating uniform-velocity streams. Both the channel geometry and wall drag affect the development of the flow. The model shows good agreement with both particle image velocimetry experiments and computational turbulence modelling. The simplicity and low computational cost of the model allows it to be used for benchmark predictions and design purposes, which we demonstrate by investigating optimal pressure recovery in diffusers with non-uniform inflow.
Control of a three-dimensional turbulent shear layer by means of oblique vortices

NASA Astrophysics Data System (ADS)

Jürgens, Werner; Kaltenbach, Hans-Jakob

2018-04-01

The effect of local forcing on the separated, three-dimensional shear layer downstream of a backward-facing step is investigated by means of large-eddy simulation for a Reynolds number based on the step height of 10,700. The step edge is either oriented normal to the approaching turbulent boundary layer or swept at an angle of 40°. Oblique vortices with different orientation and spacing are generated by wavelike suction and blowing of fluid through an edge parallel slot. The vortices exhibit a complex three-dimensional structure, but they can be characterized by a wavevector in a horizontal section plane. In order to determine the step-normal component of the wavevector, a method is developed based on phase averages. The dependence of the wavevector on the forcing parameters can be described in terms of a dispersion relation, the structure of which indicates that the disturbances are mainly convected through the fluid. The introduced vortices reduce the size of the recirculation region by up to 38%. In both the planar and the swept case, the most efficient of the studied forcings consists of vortices which propagate in a direction that deviates by more than 50° from the step normal. These vortices exhibit a spacing in the order of 2.5 step heights. The upstream shift of the reattachment line can be explained by increased mixing and momentum transport inside the shear layer which is reflected in high levels of the Reynolds shear stress -ρ \\overline{u'v'}. The position of the maximum of the coherent shear stress is found to depend linearly on the wavelength, similar to two-dimensional free shear layers.
A three-dimensional spectral algorithm for simulations of transition and turbulence

NASA Technical Reports Server (NTRS)

Zang, T. A.; Hussaini, M. Y.

1985-01-01

A spectral algorithm for simulating three dimensional, incompressible, parallel shear flows is described. It applies to the channel, to the parallel boundary layer, and to other shear flows with one wall bounded and two periodic directions. Representative applications to the channel and to the heated boundary layer are presented.
Transverse jet shear layer instabilities and their control

NASA Astrophysics Data System (ADS)

Karagozian, Ann

2013-11-01

The jet in crossflow, or transverse jet, is a canonical flowfield that has relevance to engineering systems ranging from dilution jets and film cooling for gas turbine engines to thrust vector control and fuel injection in high speed aerospace vehicles to environmental control of effluent from chimney and smokestack plumes. Over the years, our UCLA Energy and Propulsion Research Lab's studies on this flowfield have focused on the dynamics of the vorticity associated with equidensity and variable density jets in crossflow, including the stability characteristics of the jet's upstream shear layer. A range of different experimental diagnostics have been used to study the jet's upstream shear layer, whereby a transition from convectively unstable behavior at high jet-to-crossflow momentum flux ratios to absolutely unstable flow at low momentum flux and/or density ratios is identified. These differences in shear layer stability characteristics have a profound effect on how one employs external excitation to control jet penetration, spread, and mixing, depending on the flow regime and specific engineering application. These control strategies, and challenges for future research directions, will be identified in this presentation.
Turbulence, combustion, pollutant, and stability characterization of a premixed, step combustor

NASA Technical Reports Server (NTRS)

Ganji, A. T.; Sawyer, R. F.

1980-01-01

A two dimensional combustion tunnel was constructed to study a lean premixed turbulent propane/air flame stablized behind a rearward facing step. Studied were: (1) the existence and importance of large coherent structures in turbulent reacting and nonreacting free shear layers behind the steps; (2) the effect of inlet temperature and reference velocity on combustion efficiency; (3) CO, NO2 and NO sub x production in the flame; and (4) the blowout and upstream propagation of the flame. In the ranges studied, the large coherent structures dominated both the reacting and the nonreacting free shear layers behind the step. The growth of the vortices and the propagation of the flamer were intimately linked. Vortex pairing was observed to be one of the mechanisms for introduction of fresh reactants into the shear layer and growth of the shear layer. Probe composition measurements of the flame showed that, in the recirculation zone, the reaction was above 99 percent complete, CO and unburnt hydrocarbons were above the equilibrium level NO sub x concentration was far below the equilibrium level and NO2 comprised a negligible fraction of NO sub x.
Effect of a delta tab on fine scale mixing in a turbulent two-stream shear layer

NASA Technical Reports Server (NTRS)

Foss, J. K.; Zaman, K. B. M. Q.

1996-01-01

The fine scale mixing produced by a delta tab in a shear layer has been studied experimentally. The tab was placed at the trailing edge of a splitter plate which produced a turbulent two-stream mixing layer. The tab apex tilted downstream and into the high speed stream. Hot-wire measurements in the 3-D space behind the tab detailed the three velocity components as well as the small scale population distributions. These small scale eddies, which represent the peak in the dissipation spectrum, were identified and counted using the Peak-Valley-Counting technique. It was found that the small scale populations were greater in the shear region behind the tab, with the greatest increase occurring where the shear layer underwent a sharp turn. This location was near, but not coincident, with the core of the streamwise vortex, and away from the region exhibiting maximum turbulence intensity. Moreover, the tab increased the most probably frequency and strain rate of the small scales. It made the small scales smaller and more energetic.
Time-Accurate Simulations and Acoustic Analysis of Slat Free-Shear-Layer. Part 2

NASA Technical Reports Server (NTRS)

Khorrami, Mehdi R.; Singer, Bart A.; Lockard, David P.

2002-01-01

Unsteady computational simulations of a multi-element, high-lift configuration are performed. Emphasis is placed on accurate spatiotemporal resolution of the free shear layer in the slat-cove region. The excessive dissipative effects of the turbulence model, so prevalent in previous simulations, are circumvented by switching off the turbulence-production term in the slat cove region. The justifications and physical arguments for taking such a step are explained in detail. The removal of this excess damping allows the shear layer to amplify large-scale structures, to achieve a proper non-linear saturation state, and to permit vortex merging. The large-scale disturbances are self-excited, and unlike our prior fully turbulent simulations, no external forcing of the shear layer is required. To obtain the farfield acoustics, the Ffowcs Williams and Hawkings equation is evaluated numerically using the simulated time-accurate flow data. The present comparison between the computed and measured farfield acoustic spectra shows much better agreement for the amplitude and frequency content than past calculations. The effect of the angle-of-attack on the slat's flow features radiated acoustic field are also simulated presented.
Baseline climatology of extremely high vertical wind shears' values over Europe based on ERA-Interim reanalysis

NASA Astrophysics Data System (ADS)

Palarz, Angelika; Celiński-Mysław, Daniel

2017-04-01

The dominant role in the development of deep convection is played by kinematic and thermodynamic conditions, as well as atmospheric circulation, land cover and local relief. Severe thunderstorms are considerably more likely to form in environments with large values of convective available potential energy (CAPE) and significant magnitude of vertical wind shears (VWSs). According to the most recent research, the tropospheric wind shears have an important influence on intensity, longevity and organisation of the primary convective systems - bow echoes, squall lines and supercell thunderstorms. This study, in turn, examines the role of wind structure in controlling the spatial and temporal variability of VWSs over Europe. Considering the importance of the kinematic conditions for the convective systems formation, research is limited exclusively to 0-1 km, 0-3 km and 0-6 km wind shears. In order to compute the VWS' values, the data derived from ERA-Interim reanalysis for the period 1981-2015 was applied. It consisted of U and V wind components with 12-hourly sampling and horizontal resolution of 0.75×0.75°. The VWS' values were calculated as wind difference between two levels - this entails that the hodograph's shape was not considered (e.g. Clark 2013, Pucik et. al 2015). We have analysed both VWS' mean values (MN) and frequency of VWSs exceeding assumed thresholds (FQ). Taking into account previous studies (e.g. Rasmussen & Blanchard 1998, Schneider et al. 2006, Schaumann & Przybylinski 2012), the thresholds for extremely high values of vertical wind shears were set at 10 m/s for 0-1 km shear, 15 m/s for 0-3 km shear and 18 m/s for 0-6 km shear. Both MN and FQ values were characterised by strong temporal variability, as well as significant spatial differentiation over the research area. A clear diurnal cycle was identified in the case of 0-1 km shear, while seasonal variability was typical for 0-3 km and 0-6 km shears. Regardless of the season, 0-1 km shear reached higher MN and FQ values at 00 UTC than at 12 UTC. Moreover, its spatial distribution showed distinct differences linked to the underlying surface type. Surface energy budget seems to be an important factor contributing to the diurnal and spatial variability of VWSs - it generates the formation of local air circulation leading to modification of the wind direction and speed in the boundary layer. For 0-3 km and 0-6 km shears, a noticeable spatial differentiation between land and sea areas was not recognised. The significantly higher MN and FQ values over the land were found exclusively in the case of 0-3 km shear during the winter, particularly over the Mediterranean region. In the middle troposphere, the VWS' fluctuations (0-3 and 0-6 km shears) are primarily determined by the seasonal changes in atmospheric circulation patterns over the research area.
The Statitical and Case studies of the Thermospheric Enhanced Sodium Layers (TeSLs)

NASA Astrophysics Data System (ADS)

Xue, X.; Li, G.; She, C. Y.; Dou, X.

2015-12-01

We report the thermospheric enhanced sodium layers (TeSLs) observed at low and middle latitude region. Based on the statistical results of the TeSLs observed at Hainan, China (20.0N), a low latitude region, during the year 2011 - 2012, we found a good correlation between the TeSLs and the ionospheric counterparts in E region. For nine of the total 10 TeSLs, which were observed by a sodium lidar, the adjacent ionospheric observations from the COSMIC radio occultation and ionosondes exhibited abrupt perturbations in the RO SNR profiles and spread Es in the ionograms, respectively, indicating the existence of large-scale complex Es. Further, all the TeSLs, which had the co-observations by a VHF radar located nearby, were accompanied by the E region field-aligned irregularity (FAI) echoes. And seven FAIs (7/10) showed evident upwelling structure covering altitudes of 100 -- 140 km, well correlated with the development of the TeSLs. The occurrence of the large-scale complex Es possibly implies the direct altitude modulation of the horizontal Es layers by the atmospheric waves or the strong eastward polarization electric fields, which contribute the formation the FAI structures. In the course of the altitude modulation of the Es layers, sufficient ions (including sodium ions) and electrons could be accumulated in the upper altitude during the upward motion of the FAI plasma and benefits the formation of TeSLs through the chemical reaction. Two TeSL cases observed at Fort Collins, CO (20.0N), a middle latitude region, during day of year (DOY) 177 - 179 and DOY 191 during the year 2003. The enhanced sodium density in the lower thermospheric region provided the simultaneous observations for the horizontal wind and temperature. The TeSLs observed at Fort Collins had the similar feathers, i.e., they appeared at approximately 110 km and propagated with a downward speed of 1.5 - 2.5 km/hr, meanwhile, the higher temperature with approximately 50K increase and strong horizontal wind shear were accompanied with the TeSLs. Further investigation indicated that the TeSLs might be caused mainly by the ions vertical convergence due to horizontal wind-shear induced by the semi-diurnal tides, and the temperature enhancement at the altitude of 110 km had little contributions to the formation of TeSLs.
Two Dimensional (2D) P-Aramid Dry Multi-Layered Woven Fabrics Deformational Behaviour for Technical Applications

NASA Astrophysics Data System (ADS)

Abtew, M. A.; Loghin, C.; Cristian, I.; Boussu, F.; Bruniaux, P.; Chen, Y.; Wang, L.

2018-06-01

In today’s scenario for the various technical applications, from composites to body armour, the material mouldability along with its mechanical property become very important. In the present study, two dimensional (2D) woven fabrics made of para-aramid high performance fibres in multi-layer dry structure were used for investigating different forming characteristics. The different layers were arranged with 0°/90° orientation for deep drawing formability test to analyse the effect of number of layers and blank-holder pressure (BHP) during the test. Specific preforming device with low speed forming process and predefined hemispherical shape of punch has been applied. Using fine photographic analysis, some important 2D multi-layer fabrics forming characteristics i.e., material drawing-in, surface shear angle etc. from the imposed deformation have been observed, measured and analysed for better understanding and co MPa rison. The result revealed that the mouldability behaviour of the multi-layered dry textile fabric preforms is directional, and closely dependent on blank-holding pressure and number of layers. This indicates both parameters should be carefully considered while material deformation to avoid the formation of wrinkling and maintain other mechanical properties on final application.
Numerical simulation of stability and stability control of high speed compressible rotating couette flow

NASA Technical Reports Server (NTRS)

Biringen, Sedat; Hatay, Ferhat F.

1993-01-01

The nonlinear temporal evolution of disturbances in compressible flow between infinitely long, concentric cylinders is investigated through direct numerical simulations of the full, three-dimensional Navier-Stokes and energy equations. Counter-rotating cylinders separated by wide gaps are considered with supersonic velocities of the inner cylinder. Initially, the primary disturbance grows exponentially in accordance with linear stability theory. As the disturbances evolve, higher harmonics and subharmonics are generated in a cascading order eventually reaching a saturation state. Subsequent highly nonlinear stages of the evolution are governed by the interaction of the disturbance modes, particularly the axial subharmonics. Nonlinear evolution of the disturbance field is characterized by the formation of high-shear layers extending from the inner cylinder towards the center of the gap in the form of jets similar to the ejection events in transitional and turbulent wall-bounded shear flows.
Application of Formation MicroScanner Log in Structural Analysis of an Ultra-slow Spreading Environment, ODP Hole 1105A

NASA Astrophysics Data System (ADS)

Zarian, P.; Casey, J. F.; Miller, J.

2002-12-01

One of the unconventional research efforts that have been directed to reveal the structure of the lower oceanic crust is the application of Formation MicroScanner log in an ultra-slow spreading environment. Hole 1105A was cored during ODP Leg 179 to a depth of 158m on the Atlantis Platform in the Southwest Indian Ridge with a relative high recovery of 82.8% of gabbroic rocks. Open-hole logs, including FMS borehole images were acquired after the coring operation. Detailed modal, grain size and microstructural analyses of 147 thin sections reveal the relationships between deformation intensity, modal composition and rheology. The microstructural analyses showed that the majority of the oxide-rich gabbros exhibit high deformation intensity textures, which are present only in a few of the oxide-free gabbros. The oxide-rich gabbros represent ductile deformation zones and control the location of major deformation zones within these rocks. Oxide-rich layers can be clearly identified on electrical images and enabled the identification of different deformation features.The calibration of borehole images with core data provides us with more detailed information about the geometry of these oxide-rich layers within the magma chamber such as the dip and true pseudo-stratigraphic thickness of these layers. Also within the intervals of no core recovery, electrical images provide valuable information of the borehole wall. Brittle deformation features such as natural mineralized fractures can be documented throughout the borehole. Statistical results show that oxide-rich layers are relatively steep with an average dip of about 50 degrees and have a dominant E-W strike which is parallel to the ridge. These layers may represent the insitu crystallization of cummulate layers on a magma chamber wall elongated parallel to the ridge, or they may represent shear zones associated with near surface high angle normal faults that cut the main low angle detachment surface which caused the unroofing of the lower oceanic crust at the inner corner of the Ridge-Transform Intersection. These ductile shear zones appear to strike in a ridge parallel orientation, may have acted as syntectonic permeable pathways for fractionated melts infiltrated during the unroofing. Core-log integration also demonstrates the capabilities of electrical borehole images for structural analyses within a hard rock environment.
Anisotropy of Earth's D'' layer and stacking faults in the MgSiO3 post-perovskite phase.

PubMed

Oganov, Artem R; Martonák, Roman; Laio, Alessandro; Raiteri, Paolo; Parrinello, Michele

2005-12-22

The post-perovskite phase of (Mg,Fe)SiO3 is believed to be the main mineral phase of the Earth's lowermost mantle (the D'' layer). Its properties explain numerous geophysical observations associated with this layer-for example, the D'' discontinuity, its topography and seismic anisotropy within the layer. Here we use a novel simulation technique, first-principles metadynamics, to identify a family of low-energy polytypic stacking-fault structures intermediate between the perovskite and post-perovskite phases. Metadynamics trajectories identify plane sliding involving the formation of stacking faults as the most favourable pathway for the phase transition, and as a likely mechanism for plastic deformation of perovskite and post-perovskite. In particular, the predicted slip planes are {010} for perovskite (consistent with experiment) and {110} for post-perovskite (in contrast to the previously expected {010} slip planes). Dominant slip planes define the lattice preferred orientation and elastic anisotropy of the texture. The {110} slip planes in post-perovskite require a much smaller degree of lattice preferred orientation to explain geophysical observations of shear-wave anisotropy in the D'' layer.
Case study of simultaneous observations of sporadic sodium layer, E-region field-aligned irregularities and sporadic E layer at low latitude of China

NASA Astrophysics Data System (ADS)

Xie, H. Y.; Ning, B. Q.; Zhao, X. K.; Hu, L. H.

2017-03-01

Using the Na lidar at Haikou (20.0°N, 110.3°E), the VHF coherent radar and the digital ionosonde both at Sanya (18.4°N, 109.6°E), cases of simultaneous observations of sporadic sodium layer (SSL), E-region field-aligned irregularities (FAI) and sporadic E layer (Es) in the mesosphere and lower thermosphere (MLT) region at low latitude of China are studied. It is found that SSL occurs simultaneously or follows the enhancement of Es and FAI. The Es, FAI and SSL descend slowly with time which is mostly controlled by the diurnal tide (DT). Besides, the interaction of gravity wave (GW) with tides can cause oscillations in FAI and SSL. Our observations support the neutralization of ions for SSL formation: when the metallic ions layer descents to the altitudes where models predict, the sodium ions convert rapidly to atomic Na that may form an SSL event. Moreover, the SSL peak density will increase (decrease) in the convergence (divergence) vertical shear region of zonal wind.
Wind Shear Effects on the Structure and Dynamics of the Daytime Atmospheric Boundary Layer

NASA Astrophysics Data System (ADS)

Haghshenas, Armin; Mellado, Juan Pedro

2017-04-01

The daytime atmospheric boundary layer (ABL), in which the positive buoyancy flux at the surface creates convective instability and generates turbulence, has been a subject of extensive research during the last century. However, fewer studies have considered wind shear in detail and most of them are single-case studies. So most of the available theories and parameterizations have not been sufficiently tested over a wide range of atmospheric conditions. Moreover, since previous numerical studies were mostly carried out by large eddy simulation, a complete understanding of the physics of the problem is still missing due to the lack of information about the small-scale dynamics. Specifically, despite the consensus in the community that wind shear enhances the entrainment process, the amount of enhancement is still matter of contention. In order to investigate the effects of wind shear on the structure and dynamics of the ABL in detail, direct numerical simulations are used in this study. Shear is prescribed by a height-constant velocity in the troposphere and the simulation runs until a fully turbulent, quasi-equilibrium regime is observed. Despite the simplification of neglecting the Coriolis force, our configuration reproduces the main features observed in the previous studies, which had taken the Coriolis force into account. As a novelty compared to previous single-case studies, we introduce a dimensionless parameter that allows us to study systematically any combination of surface buoyancy flux, buoyancy stratification, and wind shear; We refer to this dimensionless number as shear number. Seven simulations with shear numbers ranging from 0 (no wind) to 20 (moderate wind) are conducted; this range of shear numbers corresponds to wind strength from 0 to 15 m/s in the free troposphere for typical midday atmospheric conditions. In general, we find that shear effects are negligibly small when the shear number is below 10, and for larger values the effects remain constrained inside the entrainment zone and surface layer. This critical shear number is justified by scrutinizing the turbulence regimes (convective and mechanical) within the entrainment zone in the sense that, for this shear number, the turbulence transport of turbulence kinetic energy inside the entrainment zone equals the shear-production rate. Following this analysis a critical flux Richardson number of 0.6 inside the entrainment zone is found. In particular, we observe the following: First, the mean buoyancy and total buoyancy flux inside the mixed layer remain invariant under a change of shear number and they follow the free-convection scaling laws. Second, the height of minimum buoyancy flux increases due to shear effects, but just moderately (less than 5%). Nevertheless, this increment represents a growth of entrainment zone's thickness by 50% for shear numbers of the order of 20. Third, we observe that for shear numbers larger than 10, the entrainment flux ratio grows by up to 50% in an early state of ABL development. We provide explicit parameterizations of all these shear effects.
Calculation of free turbulent mixing by interaction approach.

NASA Technical Reports Server (NTRS)

Morel, T.; Torda, T. P.

1973-01-01

The applicability of Bradshaw's interaction hypothesis to two-dimensional free shear flows was investigated. According to it, flows with velocity extrema may be considered to consist of several interacting layers. The hypothesis leads to a new expression for the shear stress which removes the usual restriction that shear stress vanishes at the velocity extremum. The approach is based on kinetic energy and the length scale equations. The compressible flow equations are simplified by restriction to low Mach numbers, and the range of their applicability is discussed. The empirical functions of the turbulence model are found here to be correlated with the spreading rate of the shear layer. The analysis demonstrates that the interaction hypothesis is a workable concept.

Exact coherent structures in an asymptotically reduced description of parallel shear flows

NASA Astrophysics Data System (ADS)

Beaume, Cédric; Knobloch, Edgar; Chini, Gregory P.; Julien, Keith

2015-02-01

A reduced description of shear flows motivated by the Reynolds number scaling of lower-branch exact coherent states in plane Couette flow (Wang J, Gibson J and Waleffe F 2007 Phys. Rev. Lett. 98 204501) is constructed. Exact time-independent nonlinear solutions of the reduced equations corresponding to both lower and upper branch states are found for a sinusoidal, body-forced shear flow. The lower branch solution is characterized by fluctuations that vary slowly along the critical layer while the upper branch solutions display a bimodal structure and are more strongly focused on the critical layer. The reduced equations provide a rational framework for investigations of subcritical spatiotemporal patterns in parallel shear flows.
Thrombus Formation at High Shear Rates.

PubMed

Casa, Lauren D C; Ku, David N

2017-06-21

The final common pathway in myocardial infarction and ischemic stroke is occlusion of blood flow from a thrombus forming under high shear rates in arteries. A high-shear thrombus forms rapidly and is distinct from the slow formation of coagulation that occurs in stagnant blood. Thrombosis at high shear rates depends primarily on the long protein von Willebrand factor (vWF) and platelets, with hemodynamics playing an important role in each stage of thrombus formation, including vWF binding, platelet adhesion, platelet activation, and rapid thrombus growth. The prediction of high-shear thrombosis is a major area of biofluid mechanics in which point-of-care testing and computational modeling are promising future directions for clinically relevant research. Further research in this area will enable identification of patients at high risk for arterial thrombosis, improve prevention and treatment based on shear-dependent biological mechanisms, and improve blood-contacting device design to reduce thrombosis risk.
Predictions of the shear response of (Mg,Fe)SiO3 post-perovskite

NASA Astrophysics Data System (ADS)

Metsue, A.; Tsuchiya, T.

2011-12-01

Observation of seismic data put in forth evidence of a spatial anisotropy in the seismic wave velocities in the D'' layer, the lowermost part of the mantle. (Mg,Fe)SiO3 post-perovskite (PPv) is thought to be the most abundant phase in this part of the mantle, and this mineral exhibits a strong elastic anisotropy and may contribute significantly to the seismic anisotropy in the D'' layer. However, the seismic anisotropy cannot be expressed at the rock scale if the orientations of the grains are distributed randomly. Consequently, the formation of lattice preferred orientations with an anisotropic mechanism of plasticity, such as dislocation creep, can cause the seismic anisotropy in the D'' layer. Some experiments have been done on the plasticity of pure and Fe-bearing MgSiO3 post-perovskite and lead to textures of deformation dominated by the (100) and (110) slip planes (Merkel et al., 2007) and by the (001) slip plane (Miyagi et al., 2010). On the other hand, theoretical calculations on the dislocations mobility on pure MgSiO3 (Carrez et al., 2007; Metsue et al., 2009) suggested a texture dominated by the (010) slip plane. A first step to understanding the mechanisms of plasticity and, therefore, the shear wave splitting occurring in the deep Earth is to test the response of the PPv phase to a plastic shear in a geophysical relevant composition. In this study, we present new results from first-principles calculations on the shear response of pure and ferrous iron-bearing MgSiO3 PPv. The originality of this work is the use of internally consistent LSDA+U formalism to accurately describe the local interactions between the d-states of iron. About 8% of iron is incorporated in the high spin state as a Mg substitution defect, since several studies suggest that iron is in the high spin in the D'' layer pressure range (Stackhouse et al., 2006; Metsue and Tsuchiya, 2011). We also performed the calculations for incorporated iron in the low spin state if an eventual spin transition of Fe occurs. The response of the PPv to a plastic shear is investigated at 120 GPa through the calculations of the Generalized Stacking Faults (GSF) energy in pure and iron-bearing systems for ten potential {hkl} slip systems, since these latter are not well constrained for the PPv phase. The GSF energies are obtained by shearing homogeneously half of an infinite crystal over the other half for every slip plane and give the value of the ideal shear stress (ISS), which can be defined as the theoretical elastic limit of the crystal. The [100](001) slip system in pure and iron-bearing phases exhibits the lowest ISS and may play an important role in the plastic deformation of the PPv phase. The activation of this slip system is compatible with the observed shear wave splitting VSH>VSV. We show that incorporation of iron decreases the GSF energy and the ISS of all slip systems. We discuss the plastic anisotropy of pure and iron-bearing phases from the values of the ISS and the orientation of applied tensile stress. Our results suggest that the incorporation of ferrous iron in the PPv phase has a limited effect on its plastic anisotropy.
Formation of Cu-type shear bands and their influence on deformation and texture of rolled f.c.c. {l_brace}112{r_brace}<111> single crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wagner, P.; Engler, O.; Luecke, K.

1995-10-01

Microstructural and textural evolution during rolling were investigated in (112)[11{bar 1}] single crystals of Al, Cu, and homogeneous supersaturated Al1.8wt%Cu. After a rolling degree of 30% the initial C-orientation (112)[11{bar 1}] of all three materials has rotated towards the so called D-orientation (4411)[1111{bar 8}]. While in the non-shear banding Al the D-orientation remains stable up to high rolling degrees, in the shear banding materials Cu and Al-Cu it rotates back to the initial C-orientation simultaneously with the formation of shear bands. This orientation change is explained by a rigid body rotation due to the special geometry of a deformation withmore » unidirectional shear bands. With the onset of shear band formation also strong orientation scatterings about the transverse direction appear in the pole figures. These scatterings are located inside the shear bands as well as their vicinity. They are due to the strong shear deformation and the resulting reaction stresses occurring in the shear bands and in their vicinity, respectively.« less
A fundamental study on the structural integrity of magnesium alloys joined by friction stir welding

NASA Astrophysics Data System (ADS)

Rao, Harish Mangebettu

The goal of this research is to study the factors that influence the physical and mechanical properties of lap-shear joints produced using friction stir welding. This study focuses on understanding the effect of tool geometry and weld process parameters including the tool rotation rate, tool plunge depth and dwell time on the mechanical performance of similar magnesium alloy and dissimilar magnesium to aluminum alloy weld joints. A variety of experimental activities were conducted including tensile and fatigue testing, fracture surface and failure analysis, microstructure characterization, hardness measurements and chemical composition analysis. An investigation on the effect of weld process conditions in friction stir spot welding of magnesium to magnesium produced in a manner that had a large effective sheet thickness and smaller interfacial hook height exhibited superior weld strength. Furthermore, in fatigue testing of friction stir spot welded of magnesium to magnesium alloy, lap-shear welds produced using a triangular tool pin profile exhibited better fatigue life properties compared to lap-shear welds produced using a cylindrical tool pin profile. In friction stir spot welding of dissimilar magnesium to aluminum, formation of intermetallic compounds in the stir zone of the weld had a dominant effect on the weld strength. Lap-shear dissimilar welds with good material mixture and discontinues intermetallic compounds in the stir zone exhibited superior weld strength compared to lap-shear dissimilar welds with continuous formation of intermetallic compounds in the stir zone. The weld structural geometry like the interfacial hook, hook orientation and bond width also played a major role in influencing the weld strength of the dissimilar lap-shear friction stir spot welds. A wide scatter in fatigue test results was observed in friction stir linear welds of aluminum to magnesium alloys. Different modes of failure were observed under fatigue loading including crack propagation into the top sheet, into the bottom sheet, and interfacial separation. Investigation of the tested welds revealed that the voids in the weld nugget reduced the weld strength, resulting in lower fatigue life. A thin layer of IMCs formed along the faying surface which accelerated the fatigue failure.
Issue of Changes in Adhesion of Bitumen Sheet to Primary Layer over the Course of Time in Multilayer Waterproofing during Shear Testing

NASA Astrophysics Data System (ADS)

Plachý, Jan; Vysoká, Jana; Vejmelka, Radek; Horský, Jan; Vacek, Vítězslav

2017-10-01

This paper is based on research dealing with defects that appear on concrete bridge decks with an insulating layer from asphalt strips on the interface between the asphalt strip and its basis. The durability and lifespan of the bearing structure of concrete bridge is determined by insulating layer that constitutes, together with the primary layer and a protective layer, the insulation system of the concrete bridge deck. Paints based on low viscosity epoxy resigns are one of the possibilities of primary layer implementation. These paints may be performed as anchoring-impregnation paints that usually represent single layer paint on the bridge deck surface. Sealing layer is another variant. Sealing layer is a multilayer consisting of anchoring- impregnation paint and sealing paint. The primary layers mainly provide vapour closing of the concrete surface, and partly, through roughening the surface, contribute to adhesion of bitumen (asphalt) insulation (waterproofing) layer. Application of the primary layer has been spreading in the Czech Republic since the 1990s. Now, after approximately 30 years of use defects in these epoxy based sealing layers at the interface between primary layer and waterproofing layer of reinforced bitumen sheets (RBS) are being solved in the Czech Republic. After performance of the first test focusing on breaking-strength, it was found that the strength between the asphalt and the primary belt layer in some types of low-viscosity resin-epoxy decreases and after a certain period of time again increases, depending on the time. Tensile strength test is carried out on a sample of asphalt strip, which is fused onto the substrate with a primer coat. It was therefore proceeded to test the shear adhesion. Testing of the shear adhesion is conducted on the entire concrete deck waterproofing system. It was supposed that the decrease of adhesion at this test become evident in higher extent. Adhesion tests in shear were performed on the primary layer consisting of an anchoring impregnation coating and sealing layer.
Experimental study of a free turbulent shear flow at Mach 19 with electron-beam and conventional probes. [flow measurement

NASA Technical Reports Server (NTRS)

Harvey, W. P.; Hunter, W. D., Jr.

1975-01-01

An experimental study of the initial development region of a hypersonic turbulent free mixing layer was made. Data were obtained at three stations downstream of a M = 19 nozzle over a Reynolds range of 1.3 million to 3.3 million per meter and at a total temperature of about 1670 K. In general, good agreement was obtained between electron-beam and conventional probe measurements of local mean flow parameters. Measurements of fluctuating density indicated that peak root-mean-square (rms) levels are higher in the turbulent free mixing layer than in boundary layers for Mach numbers less than 9. The intensity of rms density fluctuations in the free stream is similar in magnitude to pressure fluctuations in high Mach number flows. Spectrum analyses of the measured fluctuating density through the shear layer indicate significant fluctuation energy at the lower frequencies (0.2 to 5 kHZ) which correspond to large-scale disturbances in the high-velocity region of the shear layer.
Ultrasonic shear wave couplant

DOEpatents

Kupperman, David S.; Lanham, Ronald N.

1985-01-01

Ultrasonically testing of an article at high temperatures is accomplished by the use of a compact layer of a dry ceramic powder as a couplant in a method which involves providing an ultrasonic transducer as a probe capable of transmitting shear waves, coupling the probe to the article through a thin compact layer of a dry ceramic powder, propagating a shear wave from the probe through the ceramic powder and into the article to develop echo signals, and analyzing the echo signals to determine at least one physical characteristic of the article.
Ultrasonic shear wave couplant

DOEpatents

Kupperman, D.S.; Lanham, R.N.

1984-04-11

Ultrasonically testing of an article at high temperatures is accomplished by the use of a compact layer of a dry ceramic powder as a couplant in a method which involves providing an ultrasonic transducer as a probe capable of transmitting shear waves, coupling the probe to the article through a thin compact layer of a dry ceramic powder, propagating a shear wave from the probe through the ceramic powder and into the article to develop echo signals, and analyzing the echo signals to determine at least one physical characteristic of the article.
Ceramic-metal composites prepared via tape casting and melt infiltration methods

NASA Astrophysics Data System (ADS)

Kim, Hyun Jun

Melt infiltration of preforms prepared by tape casting and lamination has been accomplished using a short-time infiltration process that significantly suppresses reaction product formation. For layered materials produced via infiltration of laminated ceramic tapes, of particular interest is the effect that a large change in microstructure has on infiltration, phase formation, and mechanical properties. Hardness of the fine scale composite layers is approximately three times higher than coarse scale layers, due to greater strength of the fine B4C network. Fractography showed that crack propagation occurred by brittle fracture of the carbide and ductile extension of the metal. Despite large differences in hardness, the fracture mode of the fine and coarse scale microstructures appears identical. Fluid flow modeling for tape casting was conducted with a Newtonian slurry under a parallel blade, and the effect of beveling the blade based on a one dimensional flow model is shown. The discussion on slurry deformation after the blade exit suggests that the mode of slurry deformation depends on the relative importance of the pressure gradient and wall shear and that the existence of zero shear plane might have a negative effect on particle alignment in the tape. The analysis of the flow under a beveled blade predicts that this configuration is more advantageous than the parallel blade for productivity and parallel blade is better for producing uniform particle alignment and thinner tape. Also, the one dimensional flow model for the beveled blade is shown to be a valid approximation of the fluid behavior below a blade angle of 45 degrees. The flow visualization study on tape casting was conducted with a transparent apparatus and model slurry. Most investigators have concluded that the shear stress between the doctor blade and moving carrier causes the particle alignment, but, according to the result of visualization experiment, some degree of particle alignment is already established in the reservoir. The fluid flow concept of tape casting is incorporated with a metal infiltration technique to prepare the ceramic-metal composites with tailored porosity and pore orientation. Boron carbide-aluminum system was used to prepare the composites, and its stiffness constants were investigated. The aligned metal ligaments rarely affect the stiffness constant anisotropy which appears to be caused by tape casting operation.
Sinuous flow in metals

PubMed Central

Yeung, Ho; Viswanathan, Koushik; Compton, Walter Dale; Chandrasekar, Srinivasan

2015-01-01

Annealed metals are surprisingly difficult to cut, involving high forces and an unusually thick “chip.” This anomaly has long been explained, based on ex situ observations, using a model of smooth plastic flow with uniform shear to describe material removal by chip formation. Here we show that this phenomenon is actually the result of a fundamentally different collective deformation mode—sinuous flow. Using in situ imaging, we find that chip formation occurs via large-amplitude folding, triggered by surface undulations of a characteristic size. The resulting fold patterns resemble those observed in geophysics and complex fluids. Our observations establish sinuous flow as another mesoscopic deformation mode, alongside mechanisms such as kinking and shear banding. Additionally, by suppressing the triggering surface undulations, sinuous flow can be eliminated, resulting in a drastic reduction of cutting forces. We demonstrate this suppression quite simply by the application of common marking ink on the free surface of the workpiece material before the cutting. Alternatively, prehardening a thin surface layer of the workpiece material shows similar results. Besides obvious implications to industrial machining and surface generation processes, our results also help unify a number of disparate observations in the cutting of metals, including the so-called Rehbinder effect. PMID:26216980
Sinuous flow in metals.

PubMed

Yeung, Ho; Viswanathan, Koushik; Compton, Walter Dale; Chandrasekar, Srinivasan

2015-08-11

Annealed metals are surprisingly difficult to cut, involving high forces and an unusually thick "chip." This anomaly has long been explained, based on ex situ observations, using a model of smooth plastic flow with uniform shear to describe material removal by chip formation. Here we show that this phenomenon is actually the result of a fundamentally different collective deformation mode--sinuous flow. Using in situ imaging, we find that chip formation occurs via large-amplitude folding, triggered by surface undulations of a characteristic size. The resulting fold patterns resemble those observed in geophysics and complex fluids. Our observations establish sinuous flow as another mesoscopic deformation mode, alongside mechanisms such as kinking and shear banding. Additionally, by suppressing the triggering surface undulations, sinuous flow can be eliminated, resulting in a drastic reduction of cutting forces. We demonstrate this suppression quite simply by the application of common marking ink on the free surface of the workpiece material before the cutting. Alternatively, prehardening a thin surface layer of the workpiece material shows similar results. Besides obvious implications to industrial machining and surface generation processes, our results also help unify a number of disparate observations in the cutting of metals, including the so-called Rehbinder effect.
Interaction between a normal shock wave and a turbulent boundary layer at high transonic speeds. II - Wall shear stress

NASA Technical Reports Server (NTRS)

Liou, M. S.; Adamson, T. C., Jr.

1980-01-01

Asymptotic methods are used to calculate the shear stress at the wall for the interaction between a normal shock wave and a turbulent boundary layer on a flat plate. A mixing length model is used for the eddy viscosity. The shock wave is taken to be strong enough that the sonic line is deep in the boundary layer and the upstream influence is thus very small. It is shown that unlike the result found for laminar flow an asymptotic criterion for separation is not found; however, conditions for incipient separation are computed numerically using the derived solution for the shear stress at the wall. Results are compared with available experimental measurements.
Direct measurements of wall shear stress by buried wire gages in a shock-wave boundary-layer interaction region

NASA Technical Reports Server (NTRS)

Murthy, V. S.; Rose, W. C.

1977-01-01

Detailed measurements of wall shear stress (skin friction) were made with specially developed buried wire gages in the interaction regions of a Mach 2.9 turbulent boundary layer with externally generated shocks. Separation and reattachment points inferred by these measurements support the findings of earlier experiments which used a surface oil flow technique and pitot profile measurements. The measurements further indicate that the boundary layer tends to attain significantly higher skin-friction values downstream of the interaction region as compared to upstream. Comparisons between measured wall shear stress and published results of some theoretical calculation schemes show that the general, but not detailed, behavior is predicted well by such schemes.
LES-ODT Simulations of Turbulent Reacting Shear Layers

NASA Astrophysics Data System (ADS)

Hoffie, Andreas; Echekki, Tarek

2012-11-01

Large-eddy simulations (LES) combined with the one-dimensional turbulence (ODT) simulations of a spatially developing turbulent reacting shear layer with heat release and high Reynolds numbers were conducted and compared to results from direct numerical simulations (DNS) of the same configuration. The LES-ODT approach is based on LES solutions for momentum on a coarse grid and solutions for momentum and reactive scalars on a fine ODT grid, which is embedded in the LES computational domain. The shear layer is simulated with a single-step, second-order reaction with an Arrhenius reaction rate. The transport equations are solved using a low Mach number approximation. The LES-ODT simulations yield reasonably accurate predictions of turbulence and passive/reactive scalars' statistics compared to DNS results.
Comparison of the initial development of shear layers in two-dimensional and axisymmetric ejector configurations

NASA Technical Reports Server (NTRS)

Dufflocq, M.; Benjamin, M. A.; Roan, V. P.

1993-01-01

A two-phase experimental investigation designed to study the development of shear layers in axisymmetric and two-dimensional single-nozzle ejectors has been completed. In this study, combinations of similar and dissimilar gases were used as the supersonic primary and subsonic secondary. Test cases included combinations of air/air, argon/air and helium/air as the supersonic primary and subsonic secondary, respectively. Similar flow conditions were studied for each ejector configuration. Mixing of the gases occurred in a constant-area tube, where the inlet pressure was maintained at 34.5 kPa. The cases studied resulted in convective Mach numbers that range between 0.06 and 1.9. The data gathered shows differences between the initial shear-layer development for the two ejector geometries, and also between the different test cases studied for each ejector configuration. The measured growth rates for the axisymmetric ejector are more than twice those measured for the two-dimensional ejector. However, in both cases the results show that compressibility has a reducing effect on the growth rate. Further, in the region immediately after the inlet to the mixing tube, compressibility seems to affect the ejector shear layers in a manner similar to that of two-stream two-dimensional mixing layers.
Lear jet boundary layer/shear layer laser propagation experiments

NASA Technical Reports Server (NTRS)

Gilbert, K.

1980-01-01

Optical degradations of aircraft turbulent boundary layers with shear layers generated by aerodynamic fences are analyzed. A collimated 2.5 cm diameter helium-neon laser (0.63 microns) traversed the approximate 5 cm thick natural aircraft boundary layer in double pass via a reflective airfoil. In addition, several flights examined shear layer-induced optical degradation. Flight altitudes ranged from 1.5 to 12 km, while Mach numbers were varied from 0.3 to 0.8. Average line spread function (LSF) and Modulation Transfer Function (MTF) data were obtained by averaging a large number of tilt-removed curves. Fourier transforming the resulting average MTF yields an LSF, thus affording a direct comparison of the two optical measurements. Agreement was good for the aerodynamic fence arrangement, but only fair in the case of a turbulent boundary layer. Values of phase variance inferred from the LSF instrument for a single pass through the random flow and corrected for a large aperture ranged from 0.08 to 0.11 waves (lambda = .63 microns) for the boundary layer. Corresponding values for the fence vary from 0.08 to 0.16 waves. Extrapolation of these values to 10.6 microns suggests negligible degradation for a CO2 laser transmitted through a 5 cm thick, subsonic turbulent boundary layer.
Mean flow field and surface heating produced by unequal shock interactions at hypersonic speeds

NASA Technical Reports Server (NTRS)

Birch, S. F.; Rudy, D. H.

1975-01-01

Mean velocity profiles were measured in a free shear layer produced by the interaction of two unequal strength shock waves at hypersonic free-stream Mach numbers. Measurements were made over a unit Reynolds number range of 3,770,000 per meter to 17,400,000 per meter based on the flow on the high velocity side of the shear layer. The variation in measured spreading parameters with Mach number for the fully developed flows is consistent with the trend of the available zero velocity ratio data when the Mach numbers for the data given in this study are taken to be characteristic Mach numbers based on the velocity difference across the mixing layer. Surface measurements in the shear-layer attachment region of the blunt-body model indicate peak local heating and static pressure consistent with other published data. Transition Reynolds numbers were found to be significantly lower than those found in previous data.
Bias of shear wave elasticity measurements in thin layer samples and a simple correction strategy.

PubMed

Mo, Jianqiang; Xu, Hao; Qiang, Bo; Giambini, Hugo; Kinnick, Randall; An, Kai-Nan; Chen, Shigao; Luo, Zongping

2016-01-01

Shear wave elastography (SWE) is an emerging technique for measuring biological tissue stiffness. However, the application of SWE in thin layer tissues is limited by bias due to the influence of geometry on measured shear wave speed. In this study, we investigated the bias of Young's modulus measured by SWE in thin layer gelatin-agar phantoms, and compared the result with finite element method and Lamb wave model simulation. The result indicated that the Young's modulus measured by SWE decreased continuously when the sample thickness decreased, and this effect was more significant for smaller thickness. We proposed a new empirical formula which can conveniently correct the bias without the need of using complicated mathematical modeling. In summary, we confirmed the nonlinear relation between thickness and Young's modulus measured by SWE in thin layer samples, and offered a simple and practical correction strategy which is convenient for clinicians to use.
Reynolds number invariance of the structure inclination angle in wall turbulence.

PubMed

Marusic, Ivan; Heuer, Weston D C

2007-09-14

Cross correlations of the fluctuating wall-shear stress and the streamwise velocity in the logarithmic region of turbulent boundary layers are reported over 3 orders of magnitude change in Reynolds number. These results are obtained using hot-film and hot-wire anemometry in a wind tunnel facility, and sonic anemometers and a purpose-built wall-shear stress sensor in the near-neutral atmospheric surface layer on the salt flats of Utah's western desert. The direct measurement of fluctuating wall-shear stress in the atmospheric surface layer has not been available before. Structure inclination angles are inferred from the cross correlation results and are found to be invariant over the large range of Reynolds number. The findings justify the prior use of low Reynolds number experiments for obtaining structure angles for near-wall models in the large-eddy simulation of atmospheric surface layer flows.

Recent insights into instability and transition to turbulence in open-flow systems

NASA Technical Reports Server (NTRS)

Morkovin, Mark V.

1988-01-01

Roads to turbulence in open-flow shear layers are interpreted as sequences of often competing instabilities. These correspond to primary and higher order restructurings of vorticity distributions which culminate in convected spatial disorder (with some spatial coherence on the scale of the shear layer) traditionally called turbulence. Attempts are made to interpret these phenomena in terms of concepts of convective and global instabilities on one hand, and of chaos and strange attractors on the other. The first is fruitful, and together with a review of mechanisms of receptivity provides a unifying approach to understanding and estimating transition to turbulence. In contrast, current evidence indicates that concepts of chaos are unlikely to help in predicting transition in open-flow systems. Furthermore, a distinction should apparently be made between temporal chaos and the convected spatial disorder of turbulence past Reynolds numbers where boundary layers and separated shear layers are formed.
LIF measurements of scalar mixing in turbulent shear layers

NASA Technical Reports Server (NTRS)

Karasso, Paris S.; Mungal, M. G.

1993-01-01

The structure of shear layer flows at high Reynolds numbers remains a very interesting problem. Straight mixing layers have been studied and yielded information on the probability density function (pdf) of a passive scalar across the layer. Konrad and Koochesfahani & Dimotakis measured the pdf of the mixture fraction for mixing layers of moderate Reynolds numbers, each about 25,000 (Re based on velocity difference and visual thickness). Their measurements showed a 'non-marching' pdf (central hump which is invariant from edge to edge across the layer), a result which is linked to the visualizations of the spanwise Kelvin-Helmholtz (K-H) instability mode, which is the primary instability for plane shear layer flows. A secondary instability mode, the Taylor-Gortler (T-G) instability, which is associated with streamwise vortical structures, has also been observed in shear layers. Image reconstruction by Jimenez et al. and volume renderings by Karasso & Mungal at low Re numbers have demonstrated that the K-H and the T-G instability modes occur simultaneously in a non-mutually destructive way, evidence that supports the quasi two-dimensional aspect of these flows and the non-marching character of the pdf at low Reynolds numbers. At higher Re numbers though, the interaction of these two instability modes is still unclear and may affect the mixing process. In this study, we perform measurements of the concentration pdf of plane mixing layers for different operating conditions. At a speed ratio of r = U(sub 1)/U(sub 2) = 4:1, we examine three Reynolds number cases: Re = 14,000, Re = 31,000, and Re = 62,000. Some other Re number cases' results, not presented in detail, are invoked to explain the behavior of the pdf of the concentration field. A case of r = 2.6:1 at Re = 20,000 is also considered. The planar laser-induced fluorescence technique is used to yield quantitative measurements. The different Re are obtained by changing the velocity magnitudes of the two streams. The question of resolution of these measurements is addressed. In order to investigate the effects of the initial conditions on the development and the structure of the mixing layer, the boundary layer on the high-speed side of the splitter plate is tripped. The average concentration and the average mixed fluid concentration are also calculated to further understand the changes in the shear layer for the different cases examined.
The effects of buoyancy on shear-induced melt bands in a compacting porous medium

NASA Astrophysics Data System (ADS)

Butler, S. L.

2009-03-01

It has recently been shown [Holtzman, B., Groebner, N., Zimmerman, M., Ginsberg, S., Kohlstedt, D., 2003. Stress-driven melt segregation in partially molten rocks. Geochem. Geophys. Geosyst. 4, Art. No. 8607; Holtzman, B.K., Kohlstedt, D.L., 2007. Stress-driven melt segregation and strain partitioning in partially molten rocks: effects of stress and strain. J. Petrol. 48, 2379-2406] that when partially molten rock is subjected to simple shear, bands of high and low porosity are formed at a particular angle to the direction of instantaneous maximum extension. These have been modeled numerically and it has been speculated that high porosity bands may form an interconnected network with a bulk, effective permeability that is enhanced in a direction parallel to the bands. As a result, the bands may act to focus mantle melt towards the axis of mid-ocean ridges [Katz, R.F., Spiegelman, M., Holtzman, B., 2006. The dynamics of melt and shear localization in partially molten aggregates. Nature 442, 676-679]. In this contribution, we examine the combined effects of buoyancy and matrix shear on a deforming porous layer. The linear theory of Spiegelman [Spiegelman, M., 1993. Flow in deformable porous media. Part 1. Simple analysis. J. Fluid Mech. 247, 17-38; Spiegelman, M., 2003. Linear analysis of melt band formation by simple shear. Geochem. Geophys. Geosyst. 4, doi:10.1029/2002GC000499, Article 8615] and Katz et al. [Katz, R.F., Spiegelman, M., Holtzman, B., 2006. The dynamics of melt and shear localization in partially molten aggregates. Nature 442, 676-679] is generalized to include both the effects of buoyancy and matrix shear on a deformable porous layer with strain-rate dependent rheology. The predictions of linear theory are compared with the early time evolution of our 2D numerical model and they are found to be in excellent agreement. For conditions similar to the upper mantle, buoyancy forces can be similar to or much greater than matrix shear-induced forces. The results of the numerical model indicate that bands form when buoyancy forces are large and that these can significantly alter the direction of the flow of liquid away from vertical. The bands form at angles similar to the angle of maximum instantaneous growth rate. Consequently, for strongly strain-rate dependent rheology, there may be two sets of bands formed that are symmetric about the direction of maximum compressive stress in the background mantle flow. This second set of bands would reduce the efficiency with which melt bands would focus melts towards the ridge axis.
Dynamic Negative Compressibility of Few-Layer Graphene, h-BN, and MoS2

NASA Astrophysics Data System (ADS)

Neves, Bernardo; Barboza, Ana Paula; Chacham, Helio; Oliveira, Camilla; Fernandes, Thales; Martins Ferreira, Erlon; Archanjo, Braulio; Batista, Ronaldo; Oliveira, Alan

2013-03-01

We report a novel mechanical response of few-layer graphene, h-BN, and MoS2 to the simultaneous compression and shear by an atomic force microscope (AFM) tip. The response is characterized by the vertical expansion of these two-dimensional (2D) layered materials upon compression. Such effect is proportional to the applied load, leading to vertical strain values (opposite to the applied force) of up to 150%. The effect is null in the absence of shear, increases with tip velocity, and is anisotropic. It also has similar magnitudes in these solid lubricant materials (few-layer graphene, h-BN, and MoS2), but it is absent in single-layer graphene and in few-layer mica and Bi2Se3. We propose a physical mechanism for the effect where the combined compressive and shear stresses from the tip induce dynamical wrinkling on the upper material layers, leading to the observed flake thickening. The new effect (and, therefore, the proposed wrinkling) is reversible in the three materials where it is observed.[2] Financial support from CNPq, Fapemig, Rede Nacional de Pesquisa em Nanotubos de Carbono and INCT-Nano-Carbono
Shear-induced crystallization of a dense rapid granular flow: hydrodynamics beyond the melting point.

PubMed

Khain, Evgeniy; Meerson, Baruch

2006-06-01

We investigate shear-induced crystallization in a very dense flow of monodisperse inelastic hard spheres. We consider a steady plane Couette flow under constant pressure and neglect gravity. We assume that the granular density is greater than the melting point of the equilibrium phase diagram of elastic hard spheres. We employ a Navier-Stokes hydrodynamics with constitutive relations all of which (except the shear viscosity) diverge at the crystal-packing density, while the shear viscosity diverges at a smaller density. The phase diagram of the steady flow is described by three parameters: an effective Mach number, a scaled energy loss parameter, and an integer number m: the number of half-oscillations in a mechanical analogy that appears in this problem. In a steady shear flow the viscous heating is balanced by energy dissipation via inelastic collisions. This balance can have different forms, producing either a uniform shear flow or a variety of more complicated, nonlinear density, velocity, and temperature profiles. In particular, the model predicts a variety of multilayer two-phase steady shear flows with sharp interphase boundaries. Such a flow may include a few zero-shear (solidlike) layers, each of which moving as a whole, separated by fluidlike regions. As we are dealing with a hard sphere model, the granulate is fluidized within the "solid" layers: the granular temperature is nonzero there, and there is energy flow through the boundaries of the solid layers. A linear stability analysis of the uniform steady shear flow is performed, and a plausible bifurcation diagram of the system, for a fixed m, is suggested. The problem of selection of m remains open.
Radio Emission from Three-dimensional Relativistic Hydrodynamic Jets: Observational Evidence of Jet Stratification

NASA Astrophysics Data System (ADS)

Aloy, Miguel-Angel; Gómez, José-Luis; Ibáñez, José-María; Martí, José-María; Müller, Ewald

2000-01-01

We present the first radio emission simulations from high-resolution three-dimensional relativistic hydrodynamic jets; these simulations allow us to study the observational implications of the interaction between the jet and the external medium. This interaction gives rise to a stratification of the jet in which a fast spine is surrounded by a slow high-energy shear layer. The stratification (in particular, the large specific internal energy and slow flow in the shear layer) largely determines the emission from the jet. If the magnetic field in the shear layer becomes helical (e.g., resulting from an initial toroidal field and an aligned field component generated by shear), the emission shows a cross section asymmetry, in which either the top or the bottom of the jet dominates the emission. This, as well as limb or spine brightening, is a function of the viewing angle and flow velocity, and the top/bottom jet emission predominance can be reversed if the jet changes direction with respect to the observer or if it presents a change in velocity. The asymmetry is more prominent in the polarized flux because of field cancellation (or amplification) along the line of sight. Recent observations of jet cross section emission asymmetries in the blazar 1055+018 can be explained by assuming the existence of a shear layer with a helical magnetic field.
Examining the Displacement of Energy during Formation of Shear Bands

NASA Astrophysics Data System (ADS)

Hernandez, M.; Hilley, G. E.

2011-12-01

M.X. Hernandez, G. Hilley Department of Geological and Environmental Sciences, Stanford University, Stanford, CA This study has originated from an experimental (sandbox) setting that we have previously used to document the link between the kinematics and dynamics of deforming sand in the verge of frictional failure. Our initial experimental setting included a load control system that allowed us to track the changes in load, that when applied to the sand, deform and generate individual shear bands or localized faults. Over the course of earlier experiments, three cameras located at different positions outside the sandbox monitored the movement throughout the run. This current stage of analysis includes using computer programs such as QuickTime to create image sequences of the shear band formation, and Microsoft Excel to visually graph and plot each data sequence. This allows us to investigate the correlation between changes in work measured within our experiments, the construction of topography, slip along shear bands, and the creation of new shear bands. We observed that the measured load generally increased during the experiment to maintain a constant displacement rate as the sand wedge thickened and modeled topography increased. Superposed on this trend were periodic drops in load that appeared temporally coincident with the formation of shear bands in the sand. Using the time series of the loads applied during the experiment, changes in the position of the backstop over time, and the loads measured before, during, and after the time of each shear band formation, we are examining the fraction of the apples work that is absorbed by friction and shear band formation, and what fraction of the apples work is expended in increasing the potential energy of the thickening sand wedge. Our results indicate that before the formation of a continuous shear band, the rate of work done on the sand by the experimental apparatus decreases. This may suggest that once formed, work done against friction in the sand is significantly less than that expended in forming shear bands and thickening the sand wedge.
Wall shear stress in intracranial aneurysms and adjacent arteries☆

PubMed Central

Wang, Fuyu; Xu, Bainan; Sun, Zhenghui; Wu, Chen; Zhang, Xiaojun

2013-01-01

Hemodynamic parameters play an important role in aneurysm formation and growth. However, it is difficult to directly observe a rapidly growing de novo aneurysm in a patient. To investigate possible associations between hemodynamic parameters and the formation and growth of intracranial aneurysms, the present study constructed a computational model of a case with an internal carotid artery aneurysm and an anterior communicating artery aneurysm, based on the CT angiography findings of a patient. To simulate the formation of the anterior communicating artery aneurysm and the growth of the internal carotid artery aneurysm, we then constructed a model that virtually removed the anterior communicating artery aneurysm, and a further two models that also progressively decreased the size of the internal carotid artery aneurysm. Computational simulations of the fluid dynamics of the four models were performed under pulsatile flow conditions, and wall shear stress was compared among the different models. In the three aneurysm growth models, increasing size of the aneurysm was associated with an increased area of low wall shear stress, a significant decrease in wall shear stress at the dome of the aneurysm, and a significant change in the wall shear stress of the parent artery. The wall shear stress of the anterior communicating artery remained low, and was significantly lower than the wall shear stress at the bifurcation of the internal carotid artery or the bifurcation of the middle cerebral artery. After formation of the anterior communicating artery aneurysm, the wall shear stress at the dome of the internal carotid artery aneurysm increased significantly, and the wall shear stress in the upstream arteries also changed significantly. These findings indicate that low wall shear stress may be associated with the initiation and growth of aneurysms, and that aneurysm formation and growth may influence hemodynamic parameters in the local and adjacent arteries. PMID:25206394
Importance of intrinsic properties of dense caseinate dispersions for structure formation.

PubMed

Manski, Julita M; van Riemsdijk, Lieke E; van der Goot, Atze J; Boom, Remko M

2007-11-01

Rheological measurements of dense calcium caseinate and sodium caseinate dispersions (> or =15%) provided insight into the factors determining shear-induced structure formation in caseinates. Calcium caseinate at a sufficiently high concentration (30%) was shown to form highly anisotropic structures during shearing and concurrent enzymatic cross-linking. In contrast, sodium caseinate formed isotropic structures using similar processing conditions. The main difference between the two types of caseinates is the counterion present, and as a consequence, the size of structural elements and their interactions. The rheological behavior of calcium caseinate and sodium caseinate reflected these differences, yielding non-monotonic and shear thinning flow behavior for calcium caseinate whereas sodium caseinate behaved only slightly shear thinning. It appears that the intrinsic properties of the dense caseinate dispersions, which are reflected in their rheological behavior, affect the structure formation that was found after applying shear. Therefore, rheological measurements are useful to obtain an indication of the structure formation potential of caseinate dispersions.
Characterization of ignition transient processes in kerosene-fueled model scramjet engine by dual-pulse laser-induced plasma

NASA Astrophysics Data System (ADS)

Li, Xipeng; Liu, Weidong; Pan, Yu; Yang, Leichao; An, Bin; Zhu, Jiajian

2018-03-01

Dual-pulse laser-induced plasma ignition of kerosene in cavity at model scramjet engine is studied. The simulated flight condition is Ma 6 at 30 km, and the isolator entrance has a Mach number of 2.92, a total pressure of 2.6 MPa and a stagnation temperature of 1650 K. Two independent laser pulses at 532 nm with a pulse width of 10 ns, a diameter of 12 mm and a maximum energy of 300 mJ are focused into cavity for ignition. The flame structure and propagation during transient ignition processes are captured by simultaneous CH* and OH* chemiluminescence imaging. The entire ignition process of kerosene can be divided into five stages, which are referred as turbulent dissipation stage, quasi-stable state, combustion enhancement stage, reverting stage and combustion stabilization stage. A local closed loop of propagations of the burning mixtures from the shear layer into the recirculation zone of cavity is revealed, which the large-scale eddy in the shear layer plays a key role. The enhancement of mass exchange between shear layer and the recirculation zone of cavity could promote the flame propagation process and enhance the ignition capability as well as extend the ignition limits. A cavity shear-layer stabilized combustion of kerosene is established in the supersonic flow roughly 3.3 ms after the laser pulse. Chemical reactions mainly occur in the shear layer and the near-wall zone downstream of the cavity. The distribution of OH* is thicker than CH* at stable combustion condition.
Cavity-actuated supersonic mixing and combustion control

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yu, K.H.; Schadow, K.C.

1994-11-01

Compressible shear layers in supersonic jets are quite stable and spread very slowly compared with incompressible shear layers. In this paper, a novel use of a cavity-actuated forcing technique is demonstrated for increasing the spreading rate of compressible shear layers. Periodic modulations were applied to Mach 2.0 reacting and nonreacting jets using the cavities that were attached at the exit of a circular supersonic nozzle. The effect of cavity-actuated forcing was studied as a function of the cavity geometry, in particular, the length and the depth of the cavity. When the cavities were tuned to certain frequencies, large-scale highly coherentmore » structures were produced in the shear layers substantially increasing the growth rate. The cavity excitation was successfully applied to both cold and hot supersonic jets. When applied to cold Mach 2.0 air jets. the cavity-actuated forcing increased the spreading rate of the initial shear layers with the convective Mach number (M[sub C]) of 0.85 by a factor of three. For high-temperature Mach 2.0 jets with M[sub C] of 1.4, a 50% increase in the spreading rate was observed with the forcing. Finally, the cavity-actuated forcing was applied to reacting supersonic jets with ethylene-oxygen afterburning. For this case, the forcing caused a 20%--30% reduction in the afterburning flame length and modified the afterburning intensity significantly. The direction of the modification depended on the characteristics of the afterburning flames. The intensity was reduced with forcing for unstable flames with weak afterburning while it was increased for stable flames with strong afterburning.« less
Linearised dynamics and non-modal instability analysis of an impinging under-expanded supersonic jet

NASA Astrophysics Data System (ADS)

Karami, Shahram; Stegeman, Paul C.; Theofilis, Vassilis; Schmid, Peter J.; Soria, Julio

2018-04-01

Non-modal instability analysis of the shear layer near the nozzle of a supersonic under-expanded impinging jet is studied. The shear layer instability is considered to be one of the main components of the feedback loop in supersonic jets. The feedback loop is observed in instantaneous visualisations of the density field where it is noted that acoustic waves scattered by the nozzle lip internalise as shear layer instabilities. A modal analysis describes the asymptotic limit of the instability disturbances and fails to capture short-time responses. Therefore, a non-modal analysis which allows the quantitative description of the short-time amplification or decay of a disturbance is performed by means of a local far-field pressure pulse. An impulse response analysis is performed which allows a wide range of frequencies to be excited. The temporal and spatial growths of the disturbances in the shear layer near the nozzle are studied by decomposing the response using dynamic mode decomposition and Hilbert transform analysis. The short-time response shows that disturbances with non-dimensionalised temporal frequencies in the range of 1 to 4 have positive growth rates in the shear layer. The Hilbert transform analysis shows that high non-dimensionalised temporal frequencies (>4) are dampened immediately, whereas low non-dimensionalised temporal frequencies (<1) are neutral. Both dynamic mode decomposition and Hilbert transform analysis show that spatial frequencies between 1 and 3 have positive spatial growth rates. Finally, the envelope of the streamwise velocity disturbances reveals the presence of a convective instability.
Instabilities in a staircase stratified shear flow

NASA Astrophysics Data System (ADS)

Ponetti, G.; Balmforth, N. J.; Eaves, T. S.

2018-01-01

We study stratified shear flow instability where the density profile takes the form of a staircase of interfaces separating uniform layers. Internal gravity waves riding on density interfaces can resonantly interact due to a background shear flow, resulting in the Taylor-Caulfield instability. The many steps of the density profile permit a multitude of interactions between different interfaces, and a rich variety of Taylor-Caulfield instabilities. We analyse the linear instability of a staircase with piecewise-constant density profile embedded in a background linear shear flow, locating all the unstable modes and identifying the strongest. The interaction between nearest-neighbour interfaces leads to the most unstable modes. The nonlinear dynamics of the instabilities are explored in the long-wavelength, weakly stratified limit (the defect approximation). Unstable modes on adjacent interfaces saturate by rolling up the intervening layer into a distinctive billow. These nonlinear structures coexist when stacked vertically and are bordered by the sharp density gradients that are the remnants of the steps of the original staircase. Horizontal averages remain layer-like.
Statistical assessment of optical phase fluctuations through turbulent mixing layers

NASA Astrophysics Data System (ADS)

Gardner, Patrick J.; Roggemann, Michael C.; Welsh, Byron M.; Bowersox, Rodney D.

1995-09-01

A lateral shearing interferometer is used to measure the slope of perturbed wavefronts after propagating through turbulent shear flows. This provides a two-dimensional flow visualization technique which is nonintrusive. The slope measurements are used to reconstruct the phase of the turbulence-corrupted wave front. Experiments were performed on a plane shear mixing layer of helium and nitrogen gas at fixed velocities, for five locations in the flow development. The two gases, having a density ratio of approximately seven, provide an effective means of simulating compressible shear layers. Statistical autocorrelation functions and structure functions are computed on the reconstructed phase maps. The autocorrelation function results indicate that the turbulence-induced phase fluctuations are not wide-sense stationary. The structure functions exhibit statistical homogeneity, indicating the phase fluctuation are stationary in first increments. However, the turbulence-corrupted phase is not isotropic. A five-thirds power law is shown to fit one-dimensional, orthogonal slices of the structure function, with scaling coefficients related to the location in the flow.
Shear bond strengths of an indirect composite layering material to a tribochemically silica-coated zirconia framework material.

PubMed

Iwasaki, Taro; Komine, Futoshi; Fushiki, Ryosuke; Kubochi, Kei; Shinohara, Mitsuyo; Matsumura, Hideo

2016-01-01

This study evaluated shear bond strengths of a layering indirect composite material to a zirconia framework material treated with tribochemical silica coating. Zirconia disks were divided into two groups: ZR-PRE (airborne-particle abrasion) and ZR-PLU (tribochemical silica coating). Indirect composite was bonded to zirconia treated with one of the following primers: Clearfil Ceramic Primer (CCP), Clearfil Mega Bond Primer with Clearfil Porcelain Bond Activator (MGP+Act), ESPE-Sil (SIL), Estenia Opaque Primer, MR. Bond, Super-Bond PZ Primer Liquid A with Liquid B (PZA+PZB), and Super-Bond PZ Primer Liquid B (PZB), or no treatment. Shear bond testing was performed at 0 and 20,000 thermocycles. Post-thermocycling shear bond strengths of ZR-PLU were higher than those of ZR-PRE in CCP, MGP+Act, SIL, PZA+PZB, and PZB groups. Application of silane yielded better durable bond strengths of a layering indirect composite material to a tribochemically silica-coated zirconia framework material.
Kelvin-Helmholtz waves in extratropical cyclones passing over mountain ranges: KH Waves in Extratropical Cyclones over Mountain Ranges

DOE Office of Scientific and Technical Information (OSTI.GOV)

Medina, Socorro; Houze, Robert A.

2016-02-19

Kelvin–Helmholtz billows with horizontal scales of 3–4 km have been observed in midlatitude cyclones moving over the Italian Alps and the Oregon Cascades when the atmosphere was mostly statically stable with high amounts of shear and Ri < 0.25. In one case, data from a mobile radar located within a windward facing valley documented a layer in which the shear between down-valley flow below 1.2 km and strong upslope cross-barrier flow above was large. Several episodes of Kelvin–Helmholtz waves were observed within the shear layer. The occurrence of the waves appears to be related to the strength of the shear:more » when the shear attained large values, an episode of billows occurred, followed by a sharp decrease in the shear. The occurrence of large values of shear and Kelvin–Helmholtz billows over two different mountain ranges suggests that they may be important features occurring when extratropical cyclones with statically stable flow pass over mountain ranges.« less
Midlatitude Sporadic E Layers: Physical mechanisms and observational characteristics

NASA Astrophysics Data System (ADS)

Haldoupis, Christos

The sporadic E and its abbreviation Es refer to rather thin layers of metallic ions which form in the dynamo region of the Earth’s ionosphere, mostly between 100 and 130 km, where ion motion is controlled by collisions with the neutrals, making them move with the local winds, while electrons are strongly magnetized. The physics of sporadic E relies on the Windshear theory, in which vertical shears in the horizontal wind can form layers of long-lived metallic ions through the combined action of ion-neutral collisional coupling and geomagnetic Lorentz forcing. Once formed, the layers tend to gradually descend with time downwards below 100 km where they eventually disappear because of 3-body recombination that becomes increasingly effective in depleting them of their plasma. This is a comprehensive presentation on sporadic E, a topic which has been researched extensively over many decades. First, it reviews the basics of windshear theory and its plasma convergence mechanisms and then updates our physical understanding through a synthesis of key observational characteristics and findings. The emphasis is placed on the wind shear control of the diurnal and sub-diurnal variability and altitude descent of sporadic E layers and the key role behind this of atmospheric tides, which also drive the formation of upper E region descending intermediate layers (IDL). The evidence suggests that IDLs constitute an integral part within a broader sporadic E layer system. The presentation summarizes observations that establish a role also for the planetary waves which, through the modulation of tides, affect sporadic E layer occurrence and longer-term variability. In addition, findings are presented that provide a better physical insight in relation with the seasonal dependence and the global occurrence of sporadic E layers. The observational facts imply that sporadic E is controlled macroscopically mostly by tidal and planetary wind atmospheric dynamics, the Earth’s magnetic field and the mean meteoric deposition. Finally, traces of sporadic E in altitude and time may help evaluate and validate neutral wind model predictions in the lower thermosphere.
On investigating wall shear stress in two-dimensional plane turbulent wall jets

NASA Astrophysics Data System (ADS)

Mehdi, Faraz; Johansson, Gunnar; White, Christopher; Naughton, Jonathan

2012-11-01

Mehdi & White [Exp Fluids 50:43-51(2011)] presented a full momentum integral based method for determining wall shear stress in zero pressure gradient turbulent boundary layers. They utilized the boundary conditions at the wall and at the outer edge of the boundary layer. A more generalized expression is presented here that uses just one boundary condition at the wall. The method is mathematically exact and has an advantage of having no explicit streamwise gradient terms. It is successfully applied to two different experimental plane turbulent wall jet datasets for which independent estimates of wall shear stress were known. Complications owing to experimental inaccuracies in determining wall shear stress from the proposed method are also discussed.
Structure and dynamics of shear bands in amorphous–crystalline nanolaminates

DOE PAGES

Guo, Wei; Gan, Bin; Molina-Aldareguia, Jon M.; ...

2015-08-03

In this paper, the velocities of shear bands in amorphous CuZr/crystalline Cu nanolaminates were quantified as a function of strain rate and crystalline volume fraction. A rate-dependent transition in flow response was found in a 100 nm CuZr/10 nm Cu nanolaminates. When increasing the Cu layer thickness from 10 nm to 100 nm, the instantaneous velocity of the shear band in these nanolaminates decreases from 11.2 μm/s to <~500 nm/s. Finally, atom probe tomography and transmission election microcopy observation revealed that in post-deformed pillars both grain rotation in the crystalline portion and non-diffusive crystallization in the amorphous layer affect themore » viscosity of shear bands.« less
Climatological characteristics of high altitude wind shear and lapse rate layers

NASA Technical Reports Server (NTRS)

Ehernberger, L. J.; Guttman, N. B.

1981-01-01

Indications of the climatological distribution of wind shear and temperature lapse and inversion rates as observed by rawinsonde measurements over the western United States are recorded. Frequencies of the strongest shear, lapse rates, and inversion layer strengths were observed for a 1 year period of record and were tabulated for the lower troposphere, the upper troposphere, and five altitude intervals in the lower stratosphere. Selected bivariate frequencies were also tabulated. Strong wind shears, lapse rates, and inversion are observed less frequently as altitude increases from 175 millibars to 20 millibars. On a seasonal basis the frequencies were higher in winter than in summer except for minor influences due to increased tropopause altitude in summer and the stratospheric wind reversal in the spring and fall.

Torque controlled rotary-shear experiments reveal pseudotachilites formation-dynamics and precursor events

NASA Astrophysics Data System (ADS)

Tisato, Nicola; Cordonnier, Benoit; De Siena, Luca; Lavier, Luc; Di Toro, Giulio

2017-04-01

Except few cases, rotary shear tests, which are designed to study dynamic friction and strengthening/weakening mechanisms in seismogenic faults, are performed by imposing, to the specimens, a slipping velocity that is pre-defined. This approach has been adopted from engineering that typically, tests man-made objects that, when functioning, spin or slide at a pre-defined velocity under a pre-defined load. On the other hand, natural earthquakes are the effect of a rupture that nucleates, propagates and arrests in the subsurface. These three phases, and the consequent emerging fault slipping velocity, are controlled by the accumulated and released energy around the seismogenic fault before, during and after the earthquake. Thus, imposing the slipping velocity in laboratory experiments might not represent the best option to uncover many aspects of earthquake nucleation and fault slipping dynamics. Here we present some experiments performed with an innovative rotary shear apparatus that uses a clock-spring that when winded provides to the rotating sample a linearly increasing torque. Thus, the nucleation of simulated events occur spontaneously when the shear stress on the slipping surface overcomes the static friction times the normal load that is controlled by a deadweight. In addition, this method allows studying precursory seismic events resembling natural slow-slip earthquakes. We report some preliminary results for a transparent polymer that has melting point 340 K and allows observing the slipping surface (i.e., the contact between the two samples). By coupling: i) the rotary shear apparatus, ii) a video camera recording at 60 fps and a iii) laser pointer we observed the formation and evolution of a melt film that forms in the slipping surface after a phase of "dry" stick-slip. After each seismic event the melt layer solidify forming a pseudotachilite that partially welds the slipping surfaces. We also present the mechanical data that show rupture strengthening in concomitance with the formation of the pseudotachilite. Eventually, the original and "welded" slipping surfaces stopped "fracturing" and the failure occurred between the sample and the sample-holder giving insights about fault healing-reactivation cycle. On the light of these data we will discuss the methodology and the results, including some precursory slow-slip events, draw some conclusions and provide outlook for future studies.
Low-damage direct patterning of silicon oxide mask by mechanical processing

PubMed Central

2014-01-01

To realize the nanofabrication of silicon surfaces using atomic force microscopy (AFM), we investigated the etching of mechanically processed oxide masks using potassium hydroxide (KOH) solution. The dependence of the KOH solution etching rate on the load and scanning density of the mechanical pre-processing was evaluated. Particular load ranges were found to increase the etching rate, and the silicon etching rate also increased with removal of the natural oxide layer by diamond tip sliding. In contrast, the local oxide pattern formed (due to mechanochemical reaction of the silicon) by tip sliding at higher load was found to have higher etching resistance than that of unprocessed areas. The profile changes caused by the etching of the mechanically pre-processed areas with the KOH solution were also investigated. First, protuberances were processed by diamond tip sliding at lower and higher stresses than that of the shearing strength. Mechanical processing at low load and scanning density to remove the natural oxide layer was then performed. The KOH solution selectively etched the low load and scanning density processed area first and then etched the unprocessed silicon area. In contrast, the protuberances pre-processed at higher load were hardly etched. The etching resistance of plastic deformed layers was decreased, and their etching rate was increased because of surface damage induced by the pre-processing. These results show that etching depth can be controlled by controlling the etching time through natural oxide layer removal and mechanochemical oxide layer formation. These oxide layer removal and formation processes can be exploited to realize low-damage mask patterns. PMID:24948891
Mechanism and microstructures in Ga2O3 pseudomartensitic solid phase transition.

PubMed

Zhu, Sheng-Cai; Guan, Shu-Hui; Liu, Zhi-Pan

2016-07-21

Solid-to-solid phase transition, although widely exploited in making new materials, challenges persistently our current theory for predicting its complex kinetics and rich microstructures in transition. The Ga2O3α-β phase transformation represents such a common but complex reaction with marked change in cation coordination and crystal density, which was known to yield either amorphous or crystalline products under different synthetic conditions. Here we, via recently developed stochastic surface walking (SSW) method, resolve for the first time the atomistic mechanism of Ga2O3α-β phase transformation, the pathway of which turns out to be the first reaction pathway ever determined for a new type of diffusionless solid phase transition, namely, pseudomartensitic phase transition. We demonstrate that the sensitivity of product crystallinity is caused by its multi-step, multi-type reaction pathway, which bypasses seven intermediate phases and involves all types of elementary solid phase transition steps, i.e. the shearing of O layers (martensitic type), the local diffusion of Ga atoms (reconstructive type) and the significant lattice dilation (dilation type). While the migration of Ga atoms across the close-packed O layers is the rate-determining step and yields "amorphous-like" high energy intermediates, the shearing of O layers contributes to the formation of coherent biphase junctions and the presence of a crystallographic orientation relation, (001)α//(201[combining macron])β + [120]α//[13[combining macron]2]β. Our experiment using high-resolution transmission electron microscopy further confirms the theoretical predictions on the atomic structure of biphase junction and the formation of (201[combining macron])β twin, and also discovers the late occurrence of lattice expansion in the nascent β phase that grows out from the parent α phase. By distinguishing pseudomartensitic transition from other types of mechanisms, we propose general rules to predict the product crystallinity of solid phase transition. The new knowledge on the kinetics of pseudomartensitic transition complements the theory of diffusionless solid phase transition.
Wingtip Vortices and Free Shear Layer Interaction in the Vicinity of Maximum Lift to Drag Ratio Lift Condition

NASA Astrophysics Data System (ADS)

Memon, Muhammad Omar

Cost-effective air-travel is something everyone wishes for when it comes to booking flights. The continued and projected increase in commercial air travel advocates for energy efficient airplanes, reduced carbon footprint, and a strong need to accommodate more airplanes into airports. All of these needs are directly affected by the magnitudes of drag these aircraft experience and the nature of their wingtip vortex. A large portion of the aerodynamic drag results from the airflow rolling from the higher pressure side of the wing to the lower pressure side, causing the wingtip vortices. The generation of this particular drag is inevitable however, a more fundamental understanding of the phenomenon could result in applications whose benefits extend much beyond the relatively minuscule benefits of commonly-used winglets. Maximizing airport efficiency calls for shorter intervals between takeoffs and landings. Wingtip vortices can be hazardous for following aircraft that may fly directly through the high-velocity swirls causing upsets at vulnerably low speeds and altitudes. The vortex system in the near wake is typically more complex since strong vortices tend to continue developing throughout the near wake region. Several chord lengths distance downstream of a wing, the so-called fully rolled up wing wake evolves into a combination of a discrete wingtip vortex pair and a free shear layer. Lift induced drag is generated as a byproduct of downwash induced by the wingtip vortices. The parasite drag results from a combination of form/pressure drag and the upper and lower surface boundary layers. These parasite effects amalgamate to create the free shear layer in the wake. While the wingtip vortices embody a large portion of the total drag at lifting angles, flow properties in the free shear layer also reveal their contribution to the aerodynamic efficiency of the aircraft. Since aircraft rarely cruise at maximum aerodynamic efficiency, a better understanding of the balance between the lift induced drag (wingtip vortices) and parasite drag (free shear layer) can have a significant impact. Particle Image Velocimetry (PIV) experiments were performed at a) a water tunnel at ILR Aachen, Germany, and b) at the University of Dayton Low Speed Wind Tunnel in the near wake of an AR 6 wing with a Clark-Y airfoil to investigate the characteristics of the wingtip vortex and free shear layer at angles of attack in the vicinity of maximum aerodynamic efficiency for the wing. The data was taken 1.5 and 3 chord lengths downstream of the wing at varying free-stream velocities. A unique exergy-based technique was introduced to quantify distinct changes in the wingtip vortex axial core flow. The existence of wingtip vortex axial core flow transformation from wake-like (velocity less-than the freestream) to jet-like (velocity greater-than the freestream) behavior in the vicinity of the maximum (L/D) angles was observed. The exergy-based technique was able to identify the change in the out of plane profile and corresponding changes in the L/D performance. The resulting velocity components in and around the free shear layer in the wing wake showed counter flow in the cross-flow plane presumably corresponding to behavior associated with the flow over the upper and lower surfaces of the wing. Even though the velocity magnitudes in the free shear layer in cross-flow plane are a small fraction of the freestream velocity ( 10%), significant directional flow was observed. An indication of the possibility of the transfer of momentum (from inboard to outboard of the wing) was identified through spanwise flow corresponding to the upper and lower surfaces through the free shear layer in the wake. A transition from minimal cross flow in the free shear layer to a well-established shear flow in the spanwise direction occurs in the vicinity of maximum lift-to-drag ratio (max L/D) angle of attack. A distinctive balance between the lift induced drag and parasite drag was identified. Improved understanding of this relationship could be extended not only to improve aircraft performance through the reduction of lift induced drag, but also to air vehicle performance in off-design cruise conditions.
Passive scalars chaotic dynamics induced by two vortices in a two-layer geophysical flow with shear and rotation

NASA Astrophysics Data System (ADS)

Ryzhov, Eugene

2015-11-01

Vortex motion in shear flows is of great interest from the point of view of nonlinear science, and also as an applied problem to predict the evolution of vortices in nature. Considering applications to the ocean and atmosphere, it is well-known that these media are significantly stratified. The simplest way to take stratification into account is to deal with a two-layer flow. In this case, vortices perturb the interface, and consequently, the perturbed interface transits the vortex influences from one layer to another. Our aim is to investigate the dynamics of two point vortices in an unbounded domain where a shear and rotation are imposed as the leading order influence from some generalized perturbation. The two vortices are arranged within the bottom layer, but an emphasis is on the upper-layer fluid particle motion. Point vortices induce singular velocity fields in the layer they belong to, however, in the other layers of a multi-layer flow, they induce regular velocity fields. The main feature is that singular velocity fields prohibit irregular dynamics in the vicinity of the singular points, but regular velocity fields, provided optimal conditions, permit irregular dynamics to extend almost in every point of the corresponding phase space.
Ultrasonic soldering of Cu alloy using Ni-foam/Sn composite interlayer.

PubMed

Xiao, Yong; Wang, Qiwei; Wang, Ling; Zeng, Xian; Li, Mingyu; Wang, Ziqi; Zhang, Xingyi; Zhu, Xiaomeng

2018-07-01

In this study, Cu alloy joints were fabricated with a Ni-foam reinforced Sn-based composite solder with the assistance of ultrasonic vibration. Effects of ultrasonic soldering time on the microstructure and mechanical properties of Cu/Ni-Sn/Cu joints were investigated. Results showed that exceptional metallurgic bonding could be acquired with the assistance of ultrasonic vibration using a self-developed Ni-foam/Sn composite solder. For joint soldered for 5 s, a (Cu,Ni) 6 Sn 5 intermetallic compound (IMC) layer was formed on the Cu substrate surface, Ni skeletons distributed randomly in the soldering seam and a serrated (Ni,Cu) 3 Sn 4 IMC layer was formed on the Ni skeleton surface. Increasing the soldering time to 20 s, the (Ni,Cu) 3 Sn 4 IMC layer grew significantly and exhibited a loose porous structure on the Ni skeleton surface. Further increase the soldering time to 30 s, Ni skeletons were largely dissolved in the Sn base solder, and micro-sized (Ni,Cu) 3 Sn 4 particles were formed and dispersed homogeneously in the soldering seam. The formation of (Ni,Cu) 3 Sn 4 particles was mainly ascribed to acoustic cavitations induced erosion and grain refining effects. The joint soldered for 30 s exhibited the highest shear strength of 64.9 ± 3.3 MPa, and the shearing failure mainly occurred at the soldering seam/Cu substrate interface. Copyright © 2018 Elsevier B.V. All rights reserved.
High strength, surface porous polyether-ether-ketone for load-bearing orthopaedic implants

PubMed Central

Evans, Nathan T.; Torstrick, F. Brennan; Lee, Christopher S.D.; Dupont, Kenneth M.; Safranski, David L.; Chang, W. Allen; Macedo, Annie E.; Lin, Angela; Boothby, Jennifer M.; Whittingslow, Daniel C.; Carson, Robert A.; Guldberg, Robert E.; Gall, Ken

2015-01-01

Despite its widespread clinical use in load-bearing orthopaedic implants, polyether-ether-ketone (PEEK) is often associated with poor osseointegration. In this study, a surface porous PEEK material (PEEK-SP) was created using a melt extrusion technique. The porous layer thickness was 399.6±63.3 µm and possessed a mean pore size of 279.9±31.6 µm, strut spacing of 186.8±55.5 µm, porosity of 67.3±3.1%, and interconnectivity of 99.9±0.1%. Monotonic tensile tests showed that PEEK-SP preserved 73.9% of the strength (71.06±2.17 MPa) and 73.4% of the elastic modulus (2.45±0.31 GPa) of as-received, injection molded PEEK. PEEK-SP further demonstrated a fatigue strength of 60.0 MPa at one million cycles, preserving 73.4% of the fatigue resistance of injection molded PEEK. Interfacial shear testing showed the pore layer shear strength to be 23.96±2.26 MPa. An osseointegration model in the rat revealed substantial bone formation within the pore layer at 6 and 12 weeks via µCT and histological evaluation. Ingrown bone was more closely apposed to the pore wall and fibrous tissue growth was reduced in PEEK-SP when compared to non-porous PEEK controls. These results indicate that PEEK-SP could provide improved osseointegration while maintaining the structural integrity necessary for load-bearing orthopaedic applications. PMID:25463499
Large-scale ordering of nanoparticles using viscoelastic shear processing.

PubMed

Zhao, Qibin; Finlayson, Chris E; Snoswell, David R E; Haines, Andrew; Schäfer, Christian; Spahn, Peter; Hellmann, Goetz P; Petukhov, Andrei V; Herrmann, Lars; Burdet, Pierre; Midgley, Paul A; Butler, Simon; Mackley, Malcolm; Guo, Qixin; Baumberg, Jeremy J

2016-06-03

Despite the availability of elaborate varieties of nanoparticles, their assembly into regular superstructures and photonic materials remains challenging. Here we show how flexible films of stacked polymer nanoparticles can be directly assembled in a roll-to-roll process using a bending-induced oscillatory shear technique. For sub-micron spherical nanoparticles, this gives elastomeric photonic crystals termed polymer opals showing extremely strong tunable structural colour. With oscillatory strain amplitudes of 300%, crystallization initiates at the wall and develops quickly across the bulk within only five oscillations. The resulting structure of random hexagonal close-packed layers is improved by shearing bidirectionally, alternating between two in-plane directions. Our theoretical framework indicates how the reduction in shear viscosity with increasing order of each layer accounts for these results, even when diffusion is totally absent. This general principle of shear ordering in viscoelastic media opens the way to manufacturable photonic materials, and forms a generic tool for ordering nanoparticles.
Simulating stick-slip failure in a sheared granular layer using a physics-based constitutive model

DOE PAGES

Lieou, Charles K. C.; Daub, Eric G.; Guyer, Robert A.; ...

2017-01-14

In this paper, we model laboratory earthquakes in a biaxial shear apparatus using the Shear-Transformation-Zone (STZ) theory of dense granular flow. The theory is based on the observation that slip events in a granular layer are attributed to grain rearrangement at soft spots called STZs, which can be characterized according to principles of statistical physics. We model lab data on granular shear using STZ theory and document direct connections between the STZ approach and rate-and-state friction. We discuss the stability transition from stable shear to stick-slip failure and show that stick slip is predicted by STZ when the applied shearmore » load exceeds a threshold value that is modulated by elastic stiffness and frictional rheology. Finally, we also show that STZ theory mimics fault zone dilation during the stick phase, consistent with lab observations.« less
Large-scale ordering of nanoparticles using viscoelastic shear processing

PubMed Central

Zhao, Qibin; Finlayson, Chris E.; Snoswell, David R. E.; Haines, Andrew; Schäfer, Christian; Spahn, Peter; Hellmann, Goetz P.; Petukhov, Andrei V.; Herrmann, Lars; Burdet, Pierre; Midgley, Paul A.; Butler, Simon; Mackley, Malcolm; Guo, Qixin; Baumberg, Jeremy J.

2016-01-01

Despite the availability of elaborate varieties of nanoparticles, their assembly into regular superstructures and photonic materials remains challenging. Here we show how flexible films of stacked polymer nanoparticles can be directly assembled in a roll-to-roll process using a bending-induced oscillatory shear technique. For sub-micron spherical nanoparticles, this gives elastomeric photonic crystals termed polymer opals showing extremely strong tunable structural colour. With oscillatory strain amplitudes of 300%, crystallization initiates at the wall and develops quickly across the bulk within only five oscillations. The resulting structure of random hexagonal close-packed layers is improved by shearing bidirectionally, alternating between two in-plane directions. Our theoretical framework indicates how the reduction in shear viscosity with increasing order of each layer accounts for these results, even when diffusion is totally absent. This general principle of shear ordering in viscoelastic media opens the way to manufacturable photonic materials, and forms a generic tool for ordering nanoparticles. PMID:27255808
Generation and Radiation of Acoustic Waves from a 2-D Shear Layer using the CE/SE Method

NASA Technical Reports Server (NTRS)

Loh, Ching Y.; Wang, Xiao Y.; Chang, Sin-Chung; Jorgenson, Philip C. E.

2000-01-01

In the present work, the generation and radiation of acoustic waves from a 2-D shear layer problem is considered. An acoustic source inside of a 2-D jet excites an instability wave in the shear layer, resulting in sound Mach radiation. The numerical solution is obtained by solving the Euler equations using the space time conservation element and solution element (CE/SE) method. Linearization is achieved through choosing a small acoustic source amplitude. The Euler equations are nondimensionalized as instructed in the problem statement. All other conditions are the same except that the Crocco's relation has a slightly different form. In the following, after a brief sketch of the CE/SE method, the numerical results for this problem are presented.
Simulating the formation of Hurricane Isabel (2003) with AIRS data

NASA Astrophysics Data System (ADS)

Wu, Liguang; Braun, Scott A.; Qu, John J.; Hao, Xianjun

2006-02-01

Using the AIRS retrieved temperature and humidity profiles, the Saharan Air Layer (SAL) influence on the formation of Hurricane Isabel (2003) is simulated numerically with the MM5 model. The warmth and dryness of the SAL (the thermodynamic effect) is assimilated by use of the nudging technique, which enables the model thermodynamic state to be relaxed to the profiles of the AIRS retrieved data for the regions without cloud contamination. By incorporating the AIRS data, MM5 better simulates the large-scale flow patterns and the timing and location of the formation of Hurricane Isabel and its subsequent track. By comparing with an experiment without nudging of the AIRS data, it is shown that the SAL may have delayed the formation of Hurricane Isabel and inhibited the development of another tropical disturbance to the east. This case study confirms the argument by Dunion and Velden (2004) that the SAL can suppress Atlantic tropical cyclone activity by increasing the vertical wind shear, reducing the mean relative humidity, and stabilizing the environment at lower levels.
An experimental investigation of flow-induced oscillations of the Bruel and Kjaer in-flow microphone

NASA Technical Reports Server (NTRS)

Fields, Richard S., Jr.

1995-01-01

One source contributing to wind tunnel background noise is microphone self-noise. An experiment was conducted to investigate the flow-induced acoustic oscillations of Bruel & Kjaer (B&K) in-flow microphones. The results strongly suggest the B&K microphone cavity behaves more like an open cavity. Their cavity acoustic oscillations are likely caused by strong interactions between the cavity shear layer and the cavity trailing edge. But the results also suggest that cavity shear layer oscillations could be coupled with cavity acoustic resonance to generate tones. Detailed flow velocity measurements over the cavity screen have shown inflection points in the mean velocity profiles and high disturbance and spectral intensities in the vicinity of the cavity trailing edge. These results are the evidence for strong interactions between cavity shear layer oscillations and the cavity trailing edge. They also suggest that beside acoustic signals, the microphone inside the cavity has likely recorded hydrodynamic pressure oscillations, too. The results also suggest that the forebody shape does not have a direct effect on cavity oscillations. For the FITE (Flow Induced Tone Eliminator) microphone, it is probably the forebody length and the resulting boundary layer turbulence that have made it work. Turbulence might have thickened the boundary layer at the separation point, weakened the shear layer vortices, or lifted them to miss impinging on the cavity trailing edge. In addition, the study shows that the cavity screen can modulate the oscillation frequency but not the cavity acoustic oscillation mechanisms.
Chronic shear induces caveolae formation and alters ERK and Akt responses in endothelial cells

NASA Technical Reports Server (NTRS)

Boyd, Nolan L.; Park, Heonyong; Yi, Hong; Boo, Yong Chool; Sorescu, George P.; Sykes, Michelle; Jo, Hanjoong

2003-01-01

Caveolae are plasmalemmal domains enriched with cholesterol, caveolins, and signaling molecules. Endothelial cells in vivo are continuously exposed to shear conditions, and their caveolae density and location may be different from that of static cultured cells. Here, we show that chronic shear exposure regulates formation and localization of caveolae and caveolin-1 in bovine aortic endothelial cells (BAEC). Chronic exposure (1 or 3 days) of BAEC to laminar shear increased the total number of caveolae by 45-48% above static control. This increase was due to a rise in the luminal caveolae density without changing abluminal caveolae numbers or increasing caveolin-1 mRNA and protein levels. Whereas some caveolin-1 was found in the plasma membrane in static-cultured cells, it was predominantly localized in the Golgi. In contrast, chronic shear-exposed cells showed intense caveolin-1 staining in the luminal plasma membrane with minimum Golgi association. The preferential luminal localization of caveolae may play an important role in endothelial mechanosensing. Indeed, we found that chronic shear exposure (preconditioning) altered activation patterns of two well-known shear-sensitive signaling molecules (ERK and Akt) in response to a step increase in shear stress. ERK activation was blunted in shear preconditioned cells, whereas the Akt response was accelerated. These results suggest that chronic shear stimulates caveolae formation by translocating caveolin-1 from the Golgi to the luminal plasma membrane and alters cell signaling responses.
Dissolved oxygen as a key parameter to aerobic granule formation.

PubMed

Sturm, B S McSwain; Irvine, R L

2008-01-01

Much research has asserted that high shear forces are necessary for the formation of aerobic granular sludge in Sequencing Batch Reactors (SBRs). In order to distinguish the role of shear and dissolved oxygen on granule formation, two separate experiments were conducted with three bench-scale SBRs. In the first experiment, an SBR was operated with five sequentially decreasing superficial upflow gas velocities ranging from 1.2 to 0.4 cm s(-1). When less than 1 cm s(-1) shear was applied to the reactor, aerobic granules disintegrated into flocs, with corresponding increases in SVI and effluent suspended solids. However, the dissolved oxygen also decreased from 8 mg L(-1) to 5 mg L(-1), affecting the Feast/Famine regime in the SBR and the substrate removal kinetics. A second experiment operated two SBRs with an identical shear force of 1.2 cm s(-1), but two dissolved oxygen concentrations. Even when supplied a high shear force, aerobic granules could not form at a dissolved oxygen less than 5 mg L(-1), with a Static Fill. These results indicate that the substrate removal kinetics and dissolved oxygen are more significant to granule formation than shear force. Copyright IWA Publishing 2008.
Nutrient interleaving below the mixed layer of the Kuroshio Extension Front

NASA Astrophysics Data System (ADS)

Nagai, Takeyoshi; Clayton, Sophie

2017-08-01

Nitrate interleaving structures were observed below the mixed layer during a cruise to the Kuroshio Extension in October 2009. In this paper, we investigate the formation mechanisms for these vertical nitrate anomalies, which may be an important source of nitrate to the oligotrphoc surface waters south of the Kuroshio Extension Front. We found that nitrate concentrations below the main stream of the Kuroshio Extension were elevated compared to the ambient water of the same density ( σ 𝜃 = 23.5-25). This appears to be analogous to the "nutrient stream" below the mixed layer, associated with the Gulf Stream. Strong turbulence was observed above the vertical nitrate anomaly, and we found that this can drive a large vertical turbulent nitrate flux >O (1 mmol N m-2 day-1). A realistic, high-resolution (2 km) numerical simulation reproduces the observed Kuroshio nutrient stream and nitrate interleaving structures, with similar lateral and vertical scales. The model results suggest that the nitrate interleaving structures are first generated at the western side of the meander crest on the south side of the Kuroshio Extension, where the southern tip of the mixed layer front is under frontogenesis. Lagrangian analyses reveal that the vertical shear of geostrophic and subinertial ageostrophic flow below the mixed layer tilts the existing along-isopycnal nitrate gradient of the Kuroshio nutrient stream to form nitrate interleaving structures. This study suggests that the multi-scale combination of (i) the lateral stirring of the Kuroshio nutrient stream by developed mixed layer fronts during fall to winter, (ii) the associated tilting of along-isopycnal nitrate gradient of the nutrient stream by subinertial shear, which forms vertical interleaving structures, and (iii) the strong turbulent diffusion above them, may provide a route to supply nutrients to oligotrophic surface waters on the south side of the Kuroshio Extension.
Long-time observation of meteor induced layers with ionosonde

NASA Astrophysics Data System (ADS)

Yusupov, Kamil; Akchurin, Adel

2016-07-01

It is considered that the main theory explaining appearance of sporadic E is the theory of wind shear, which means (includes) the presence and movement of nodes converging tidal wind through the height region of the most frequent occurrence Es (120-140km) [Mathew et. all, 1998]. However, the appearance of intense layers, following its name, are sporadic, and such variability cannot to explain by the influence of tidal waves only. Another indication inconsistency theory of wind shear is the appearance of so-called transient Es layers [Maruiama, 2003]. The distinctive feature of this trace is the high critical frequency (> 5 MHz), a constant height, weak amplitude, all trace semitransparent and short lifetime [Maruiama et. all, 2003 and 2008 and references there]. Because of duration, such layer is opposite to the traditional persistent Es layer, which we do not consider in this paper. Various researchers have used different terms for such spontaneous Es, it is meteor echo, meteor induced Es, spontaneously formed sporadic Es patches resulting of the Fresnel scattering from a region of enhanced plasma density along the meteor trail, transitory Es and transient Es. Since the term transient Es is unstable, to avoid confusion, we will stick to this term. Since meteor echo is not fully satisfy this term by some parameter, we will describe the properties of transient Es based on the ionogram properties and not from physics of its origin. We used data from our ionosonde with one-minute ionogram repetition rate for 2010-2014 years. For processing performed a method are using to select beatings and the ionosphere reflectivity of the layers by means A-, H-and AΣ-map [Akchurin, 2011; Yusupov, 2014]. This maps allow to collect transient Es appearance over a long-time. Such statistics comparison with meteor showers activity showed good agreement. It shows the presence of the transient Es formation mechanism, which coupling with meteors.
3D modeling of unconstrained HPT process: role of strain gradient on high deformed microstructure formation

NASA Astrophysics Data System (ADS)

Ben Kaabar, A.; Aoufi, A.; Descartes, S.; Desrayaud, C.

2017-05-01

During tribological contact’s life, different deformation paths lead to the formation of high deformed microstructure, in the near-surface layers of the bodies. The mechanical conditions (high pressure, shear) occurring under contact, are reproduced through unconstrained High Pressure Torsion configuration. A 3D finite element model of this HPT test is developed to study the local deformation history leading to high deformed microstructure with nominal pressure and friction coefficient. For the present numerical study the friction coefficient at the interface sample/anvils is kept constant at 0.3; the material used is high purity iron. The strain distribution in the sample bulk, as well as the main components of the strain gradients according to the spatial coordinates are investigated, with rotation angle of the anvil.
Analysing hydro-mechanical behaviour of reinforced slopes through centrifuge modelling

NASA Astrophysics Data System (ADS)

Veenhof, Rick; Wu, Wei

2017-04-01

Every year, slope instability is causing casualties and damage to properties and the environment. The behaviour of slopes during and after these kind of events is complex and depends on meteorological conditions, slope geometry, hydro-mechanical soil properties, boundary conditions and the initial state of the soils. This study describes the effects of adding reinforcement, consisting of randomly distributed polyolefin monofilament fibres or Ryegrass (Lolium), on the behaviour of medium-fine sand in loose and medium dense conditions. Direct shear tests were performed on sand specimens with different void ratios, water content and fibre or root density, respectively. To simulate the stress state of real scale field situations, centrifuge model tests were conducted on sand specimens with different slope angles, thickness of the reinforced layer, fibre density, void ratio and water content. An increase in peak shear strength is observed in all reinforced cases. Centrifuge tests show that for slopes that are reinforced the period until failure is extended. The location of shear band formation and patch displacement behaviour indicate that the design of slope reinforcement has a significant effect on the failure behaviour. Future research will focus on the effect of plant water uptake on soil cohesion.
An adaptive extended finite element method for the analysis of agglomeration of colloidal particles in a flowing fluid

DOE Office of Scientific and Technical Information (OSTI.GOV)

Choi, Young Joon; Jorshari, Razzi Movassaghi; Djilali, Ned

2015-03-10

Direct numerical simulations of the flow-nanoparticle interaction in a colloidal suspension are presented using an extended finite element method (XFEM) in which the dynamics of the nanoparticles is solved in a fully-coupled manner with the flow. The method is capable of accurately describing solid-fluid interfaces without the need of boundary-fitted meshes to investigate the dynamics of particles in complex flows. In order to accurately compute the high interparticle shear stresses and pressures while minimizing computing costs, an adaptive meshing technique is incorporated with the fluid-structure interaction algorithm. The particle-particle interaction at the microscopic level is modeled using the Lennard-Jones (LJ)more » potential and the corresponding potential parameters are determined by a scaling procedure. The study is relevant to the preparation of inks used in the fabrication of catalyst layers for fuel cells. In this paper, we are particularly interested in investigating agglomeration of the nanoparticles under external shear flow in a sliding bi-periodic Lees-Edwards frame. The results indicate that the external shear has a crucial impact on the structure formation of colloidal particles in a suspension.« less

Understanding roles of E × B flow and magnetic shear on the formation of internal and edge transport barriers using two-field bifurcation concept

NASA Astrophysics Data System (ADS)

Chatthong, B.; Onjun, T.

2016-01-01

A set of heat and particle transport equations with the inclusion of E × B flow and magnetic shear is used to understand the formation and behaviors of edge transport barriers (ETBs) and internal transport barriers (ITBs) in tokamak plasmas based on two-field bifurcation concept. A simple model that can describe the E × B flow shear and magnetic shear effect in tokamak plasma is used for anomalous transport suppression with the effect of bootstrap current included. Consequently, conditions and formations of ETB and ITB can be visualized and studied. It can be seen that the ETB formation depends sensitively on the E × B flow shear suppression with small dependence on the magnetic shear suppression. However, the ITB formation depends sensitively on the magnetic shear suppression with a small dependence on the E × B flow shear suppression. Once the H-mode is achieved, the s-curve bifurcation diagram is modified due to an increase of bootstrap current at the plasma edge, resulting in reductions of both L-H and H-L transition thresholds with stronger hysteresis effects. It is also found that both ITB and ETB widths appear to be governed by heat or particle sources and the location of the current peaking. In addition, at a marginal flux just below the L-H threshold, a small perturbation in terms of heat or density fluctuation can result in a transition, which can remain after the perturbation is removed due to the hysteresis effect.
The formation of planetary disks and winds: an ultraviolet view

NASA Astrophysics Data System (ADS)

Gómez de Castro, Ana I.

2009-04-01

Planetary systems are angular momentum reservoirs generated during star formation. This accretion process produces very powerful engines able to drive the optical jets and the molecular outflows. A fraction of the engine energy is released into heating thus the temperature of the engine ranges from the 3000 K of the inner disk material to the 10 MK in the areas where magnetic reconnection occurs. There are important unsolved problems concerning the nature of the engine, its evolution and the impact of the engine in the chemical evolution of the inner disk. Of special relevance is the understanding of the shear layer between the stellar photosphere and the disk; this layer controls a significant fraction of the magnetic field building up and the subsequent dissipative processes ougth to be studied in the UV. This contribution focus on describing the connections between 1 Myr old suns and the Sun and the requirements for new UV instrumentation to address their evolution during this period. Two types of observations are shown to be needed: monitoring programmes and high resolution imaging down to, at least, milliarsecond scales.
Evolution and transition mechanisms of internal swirling flows with tangential entry

NASA Astrophysics Data System (ADS)

Wang, Yanxing; Wang, Xingjian; Yang, Vigor

2018-01-01

The characteristics and transition mechanisms of different states of swirling flow in a cylindrical chamber have been numerically investigated using the Galerkin finite element method. The effects of the Reynolds number and swirl level were examined, and a unified theory connecting different flow states was established. The development of each flow state is considered as a result of the interaction and competition between basic mechanisms: (1) the centrifugal effect, which drives an axisymmetric central recirculation zone (CRZ); (2) flow instabilities, which develop at the free shear layer and the central solid-body rotating flow; (3) the bouncing and restoring effects of the injected flow, which facilitate the convergence of flow on the centerline and the formation of bubble-type vortex breakdown; and (4) the damping effect of the end-induced flow, which suppresses the development of the instability waves. The results show that the CRZ, together with the free shear layer on its surface, composes the basic structure of swirling flow. The development of instability waves produces a number of discrete vortex cores enclosing the CRZ. The azimuthal wave number is primarily determined by the injection angle. Generally, the wave number is smaller at a higher injection angle, due to the reduction of the perimeter of the free shear layer. At the same time, the increase in the Reynolds number facilitates the growth of the wave number. The end-induced flow tends to reduce the wave number near the head end and causes a change in wave number from the head end to the downstream region. Spiral-type vortex breakdown can be considered as a limiting case at a high injection angle, with a wave number equal to 0 near the head end and equal to 1 downstream. At lower Reynolds numbers, the bouncing and restoring effect of the injected flow generates bubble-type vortex breakdown.
Frictional `non-aging' of fault mirror surfaces?: Insight from friction experiments on Carrara marble

NASA Astrophysics Data System (ADS)

Park, Y.; Ree, J. H.; Hirose, T.

2016-12-01

Mirror-like fault surfaces (or fault mirror: FM) have recently been suggested as a precursor of unstable slip (thus indicative of seismic slip). Frictional aging of fault surfaces (increase in static friction during interseismic period) is a common phenomenon of fault surfaces, resulting from increase in contact area or in bond strength between asperities with time. Despite the importance of FM in earthquake faulting, the frictional-aging behavior of FM has never been studied. To understand the frictional-aging behavior of FM, slide-hold-slide friction experiments were done on carbonate FM and powdered gouge of former carbonate FM (PG hereafter) using low-to-high-velocity-rotary-shear apparatus, at a slip rate of 1 μm s-1 a normal stress of 1.5 MPa, room temperature and room humidity condition. The sheared PG specimens showed a logarithmic positive relationship between static friction and holding time, consistent with Dieterich-type healing behavior. In contrast, the sheared FM specimens showed little effect of holding time on static friction. The slip surface of FM specimens consists of densely-packed and sintered nano-particles while that of PG specimens is composed of loose nano-particles. It has been known that yield strength of a material increases dramatically with size-decreasing grains being nano-particles. Since FM is a layer of densely-packed and sintered nanoparticles, enhanced strength of FM may inhibit growth of real contact area of fault surfaces during hold time. Furthermore, sintered particles composing FM have less pore space than loose gouge layer, and thus there would be a less chance of strengthening by pore space reduction, inter-particle meniscus formation or water adsorption onto the particles surface in the FM layer. Our preliminary result suggests that carbonate FM's may impede the recovery of fault strength during interseismic period, resulting in less possibility of earthquake nucleation. Reduced frictional healing may be a common phenomenon of FM's in other materials too once they are composed of sintered nano-particles.
Shear Strains, Strain Rates and Temperature Changes in Adiabatic Shear Bands

DTIC Science & Technology

1980-05-01

X14A. It has been found that when bainitic and martensitic steels are sheared adiabatically, a layer of material within ths shear zone is altezed and...Sooiety for Metals, Metals Park, Ohio, 1978, pp. 148-0. 21 TABLE II SOLID-STATE TRANSFORMATIONS IN BAINITIC STEEL TRANSFORMATION TRANSFORMATION...shear, thermoplastic, plasticity, plastic deformation, armor, steel IL AnSRACT ( -=nba asoa.tm a naeoesM iN faity by bleak n bet/2972 Experiments
The formation of Laurentia: Evidence from shear wave splitting

NASA Astrophysics Data System (ADS)

Liddell, Mitch V.; Bastow, Ian; Darbyshire, Fiona; Gilligan, Amy; Pugh, Stephen

2017-12-01

The northern Hudson Bay region in Canada comprises several Archean cratonic nuclei, assembled by a number of Paleoproterozoic orogenies including the Trans-Hudson Orogen (THO) and the Rinkian-Nagssugtoqidian Orogen. Recent debate has focused on the extent to which these orogens have modern analogues such as the Himalayan-Karakoram-Tibet Orogen. Further, the structure of the lithospheric mantle beneath the Hudson Strait and southern Baffin Island is potentially indicative of Paleoproterozoic underthrusting of the Superior plate beneath the Churchill collage. Also in question is whether the Laurentian cratonic root is stratified, with a fast, depleted, Archean core underlain by a slower, younger, thermally-accreted layer. Plate-scale process that create structures such as these are expected to manifest as measurable fossil seismic anisotropic fabrics. We investigate these problems via shear wave splitting, and present the most comprehensive study to date of mantle seismic anisotropy in northern Laurentia. Strong evidence is presented for multiple layers of anisotropy beneath Archean zones, consistent with the episodic development model of stratified cratonic keels. We also show that southern Baffin Island is underlain by dipping anisotropic fabric, where underthrusting of the Superior plate beneath the Churchill has previously been interpreted. This provides direct evidence of subduction-related deformation at 1.8 Ga, implying that the THO developed with modern plate-tectonic style interactions.
Modeling of flow-induced shear stress applied on 3D cellular scaffolds: Implications for vascular tissue engineering.

PubMed

Lesman, Ayelet; Blinder, Yaron; Levenberg, Shulamit

2010-02-15

Novel tissue-culture bioreactors employ flow-induced shear stress as a means of mechanical stimulation of cells. We developed a computational fluid dynamics model of the complex three-dimensional (3D) microstructure of a porous scaffold incubated in a direct perfusion bioreactor. Our model was designed to predict high shear-stress values within the physiological range of those naturally sensed by vascular cells (1-10 dyne/cm(2)), and will thereby provide suitable conditions for vascular tissue-engineering experiments. The model also accounts for cellular growth, which was designed as an added cell layer grown on all scaffold walls. Five model variants were designed, with geometric differences corresponding to cell-layer thicknesses of 0, 50, 75, 100, and 125 microm. Four inlet velocities (0.5, 1, 1.5, and 2 cm/s) were applied to each model. Wall shear-stress distribution and overall pressure drop calculations were then used to characterize the relation between flow rate, shear stress, cell-layer thickness, and pressure drop. The simulations showed that cellular growth within 3D scaffolds exposes cells to elevated shear stress, with considerably increasing average values in correlation to cell growth and inflow velocity. Our results provide in-depth analysis of the microdynamic environment of cells cultured within 3D environments, and thus provide advanced control over tissue development in vitro. 2009 Wiley Periodicals, Inc.
Structure and strength at the bonding interface of a titanium-segmented polyurethane composite through 3-(trimethoxysilyl) propyl methacrylate for artificial organs.

PubMed

Sakamoto, Harumi; Doi, Hisashi; Kobayashi, Equo; Yoneyama, Takayuki; Suzuki, Yoshiaki; Hanawa, Takao

2007-07-01

The objective of this study was to investigate the structure and strength at the bonding interface of a titanium (Ti)-segmented polyurethane (SPU) composite through (3-trimethoxysilyl) propyl methacrylate (gamma-MPS) for artificial organs. The effects of the thickness of the gamma-MPS layer on the shear bonding strength between Ti and SPU were investigated. Ti disks were immersed in various concentrations of gamma-MPS solutions for several immersion times. The depth profiles of elements and the thickness of the gamma-MPS layer were determined by glow discharge optical emission spectroscopy and ellipsometry, respectively. The bonding stress at the Ti/gamma-MPS/SPU interface was evaluated with a shear bonding test. Furthermore, the fractured surface of a Ti-SPU composite was observed by optical microscopy and characterized using X-ray photoelectron spectroscopy. Consequently, the thickness of the gamma-MPS layer was controlled by the concentration of the gamma-MPS solution and immersion time. The shear bonding stress at the interface increased with the increase of the thickness of the gamma-MPS layer. Therefore, the control of the thickness of the gamma-MPS layer is significant to increase the shear bonding stress at the Ti/gamma-MPS/SPU interface. These results are significant to create composites for artificial organs consisting of other metals and polymers. Copyright 2007 Wiley Periodicals, Inc.
Analytic and Computational Perspectives of Multi-Scale Theory for Homogeneous, Laminated Composite, and Sandwich Beams and Plates

NASA Technical Reports Server (NTRS)

Tessler, Alexander; Gherlone, Marco; Versino, Daniele; DiSciuva, Marco

2012-01-01

This paper reviews the theoretical foundation and computational mechanics aspects of the recently developed shear-deformation theory, called the Refined Zigzag Theory (RZT). The theory is based on a multi-scale formalism in which an equivalent single-layer plate theory is refined with a robust set of zigzag local layer displacements that are free of the usual deficiencies found in common plate theories with zigzag kinematics. In the RZT, first-order shear-deformation plate theory is used as the equivalent single-layer plate theory, which represents the overall response characteristics. Local piecewise-linear zigzag displacements are used to provide corrections to these overall response characteristics that are associated with the plate heterogeneity and the relative stiffnesses of the layers. The theory does not rely on shear correction factors and is equally accurate for homogeneous, laminated composite, and sandwich beams and plates. Regardless of the number of material layers, the theory maintains only seven kinematic unknowns that describe the membrane, bending, and transverse shear plate-deformation modes. Derived from the virtual work principle, RZT is well-suited for developing computationally efficient, C(sup 0)-continuous finite elements; formulations of several RZT-based elements are highlighted. The theory and its finite element approximations thus provide a unified and reliable computational platform for the analysis and design of high-performance load-bearing aerospace structures.
Study on the Failure and Energy Absorption Mechanism of Multilayer Explosively Welded Plates Impacted by Spherical Fragments

NASA Astrophysics Data System (ADS)

Zhou, N.; Wang, J. X.; Tang, S. Z.; Tao, Q. C.; Wang, M. X.

2018-01-01

A stereomicroscope, microscopic metallograph, scanning electron microscope, and the ANSYS/LS-DYNA 3D finite-element code were employed to investigate the failure and energy absorption mechanism of two-layer steel/aluminum and three-layer steel/aluminum/steel and aluminum/steel/aluminum explosively welded composite plates impacted by spherical fragments. The effects of layer number, target order, and the combination state of interfaces on the failure and energy absorption mechanism are analyzed based on experimental and numerical results. Results showed that the effect of the combination state of interfaces on the failure mode was pronounced the most compared with other factors. The failure mechanism of the front and middle plates were shearing and plugging, and that of rear plate was ductile deformation when the tied interface failed by tension (or by shearing and plugging when the interface combination remained connected). A narrow adiabatic shear band was formed in the locally yielding plate damaged by shearing and plugging during the penetration process. The amount of energy needed to completely perforate the three-layer composite target was greater than that for a two-layer composite target with the same areal density and total thickness. The protective performance of the steel/aluminum/steel target was better than that of the aluminum/steel/aluminum target with the same areal density.
Analytic and Computational Perspectives of Multi-Scale Theory for Homogeneous, Laminated Composite, and Sandwich Beams and Plates

NASA Technical Reports Server (NTRS)

Tessler, Alexander; Gherlone, Marco; Versino, Daniele; Di Sciuva, Marco

2012-01-01

This paper reviews the theoretical foundation and computational mechanics aspects of the recently developed shear-deformation theory, called the Refined Zigzag Theory (RZT). The theory is based on a multi-scale formalism in which an equivalent single-layer plate theory is refined with a robust set of zigzag local layer displacements that are free of the usual deficiencies found in common plate theories with zigzag kinematics. In the RZT, first-order shear-deformation plate theory is used as the equivalent single-layer plate theory, which represents the overall response characteristics. Local piecewise-linear zigzag displacements are used to provide corrections to these overall response characteristics that are associated with the plate heterogeneity and the relative stiffnesses of the layers. The theory does not rely on shear correction factors and is equally accurate for homogeneous, laminated composite, and sandwich beams and plates. Regardless of the number of material layers, the theory maintains only seven kinematic unknowns that describe the membrane, bending, and transverse shear plate-deformation modes. Derived from the virtual work principle, RZT is well-suited for developing computationally efficient, C0-continuous finite elements; formulations of several RZT-based elements are highlighted. The theory and its finite elements provide a unified and reliable computational platform for the analysis and design of high-performance load-bearing aerospace structures.
Improved two-equation k-omega turbulence models for aerodynamic flows

NASA Technical Reports Server (NTRS)

Menter, Florian R.

1992-01-01

Two new versions of the k-omega two-equation turbulence model will be presented. The new Baseline (BSL) model is designed to give results similar to those of the original k-omega model of Wilcox, but without its strong dependency on arbitrary freestream values. The BSL model is identical to the Wilcox model in the inner 50 percent of the boundary-layer but changes gradually to the high Reynolds number Jones-Launder k-epsilon model (in a k-omega formulation) towards the boundary-layer edge. The new model is also virtually identical to the Jones-Lauder model for free shear layers. The second version of the model is called Shear-Stress Transport (SST) model. It is based on the BSL model, but has the additional ability to account for the transport of the principal shear stress in adverse pressure gradient boundary-layers. The model is based on Bradshaw's assumption that the principal shear stress is proportional to the turbulent kinetic energy, which is introduced into the definition of the eddy-viscosity. Both models are tested for a large number of different flowfields. The results of the BSL model are similar to those of the original k-omega model, but without the undesirable freestream dependency. The predictions of the SST model are also independent of the freestream values and show excellent agreement with experimental data for adverse pressure gradient boundary-layer flows.
Diamond anvil cell for spectroscopic investigation of materials at high temperature, high pressure and shear

DOEpatents

Westerfield, Curtis L.; Morris, John S.; Agnew, Stephen F.

1997-01-01

Diamond anvil cell for spectroscopic investigation of materials at high temperature, high pressure and shear. A cell is described which, in combination with Fourier transform IR spectroscopy, permits the spectroscopic investigation of boundary layers under conditions of high temperature, high pressure and shear.
Effect of tree roots on a shear zone: modeling reinforced shear stress.

Treesearch

Kazutoki Abe; Robert R. Ziemer

1991-01-01

Tree roots provide important soil reinforcement that impoves the stability of hillslopes. After trees are cut and roots begin to decay, the frequency of slope failures can increase. To more fully understand the mechanics of how tree roots reinforce soil, fine sandy soil containing pine roots was placed in a large shear box in horizontal layers and sheared across a...
A Classification of Subaqueous Density Flows Based on Transformations From Proximal to Distal Regions

NASA Astrophysics Data System (ADS)

Hermidas, Navid; Eggenhuisen, Joris; Luthi, Stefan; Silva Jacinto, Ricardo; Toth, Ferenc; Pohl, Florian

2017-04-01

Transformations of a subaqueous density flow from proximal to distal regions are investigated. A classification of these transformations based on the state of the free shear and boundary layers and existence of a plug layer during transition from a debris flow to a turbidity current is presented. A connection between the emplaced deposit by the flow and the relevant flow type is drawn through the results obtained from a series of laboratory flume experiments. These were performed using 9%, 15%, and 21% sediment mixture concentrations composed of sand, silt, clay, and tap water, on varying bed slopes of 6°, 8°, and 9.5°, and with discharge rates of 10[m3/h] and 15[m3/h]. Stress-controlled rheometry experiments were performed on the mixtures to obtain apparent viscosity data. A classification was developed based on the imposed flow conditions, where a cohesive flow may fall within one of five distinct flow types: 1) a cohesive plug flow (PF) with a laminar free shear and boundary layers, 2) a top transitional plug flow (TTPF) containing a turbulent free shear layer, a plug layer, and a laminar boundary layer, 3) a complete transitional plug flow (CTPF) consisting of a turbulent free shear and boundary layers and a plug, 4) a transitional turbidity current (TTC) with a turbulent free shear layer and a laminar boundary layer, and, 5) a completely turbulent turbidity current (TC). During the experiments, flow type PF resulted in en masse deposition of a thick uniform ungraded muddy sand mixture, which was emplaced once the yield stress overcame the gravitational forces within the tail region of the flow. Flow type TTPF resulted in deposition of a thin ungraded basal clean sand layer during the run. This layer was covered by a muddy sand deposit from the tail. Flow type TTC did not deposit any sediment during the run. A uniform muddy sand mixture was emplaced by the tail of the flow. Flow type TC resulted in deposition of poorly sorted massive bottom sand layer. This layer was overlain by either a muddy sand mixture or a sand and silt planar lamination. Flow type CTPF was not observed during the experiments. Furthermore, it was observed that flows which are in transition from a TTC to a TTPF result in a thin bottom clean sand layer covered by a banded transitional interval. This was overlain by a muddy sand layer and a very thin clean sand layer, resulting from traction by dilute turbulent wake. In all cases a mud cap was emplaced on top of the deposit after the runs were terminated.
An experimental study of scalar mixing in curved shear layers

NASA Technical Reports Server (NTRS)

Karasso, P. S.; Mungal, M. G.

1990-01-01

This report describes the work being undertaken to study the scalar mixing in curved shear layers. First, the motivation for this work and its objectives are described. Second, a description of the experimental rig that has been built is given. Third, some preliminary results (flow visualizations) are discussed, and finally, future steps that will be taken to complete the study are outlined.
Inventory of File sref.t03z.pgrb212_SPC.prob_3hrly.gri

Science.gov Websites

-GWD analysis Zonal Flux of Gravity Wave Stress [prob] prob =1 002 entire atmosphere (considered as a as a single layer) VUCSH analysis Vertical U-Component Shear [prob] prob =2 004 entire atmosphere (considered as a single layer) VUCSH analysis Vertical U-Component Shear [prob] prob =3 005 surface APCP 0-3
Inventory of File sref.t03z.pgrb216_SPC.prob_3hrly.gri

Science.gov Websites

-GWD analysis Zonal Flux of Gravity Wave Stress [prob] prob =1 002 entire atmosphere (considered as a as a single layer) VUCSH analysis Vertical U-Component Shear [prob] prob =2 004 entire atmosphere (considered as a single layer) VUCSH analysis Vertical U-Component Shear [prob] prob =3 005 surface APCP 0-3
Inventory of File sref.t03z.pgrb243_SPC.prob_3hrly.gri

Science.gov Websites

-GWD analysis Zonal Flux of Gravity Wave Stress [prob] prob =1 002 entire atmosphere (considered as a as a single layer) VUCSH analysis Vertical U-Component Shear [prob] prob =2 004 entire atmosphere (considered as a single layer) VUCSH analysis Vertical U-Component Shear [prob] prob =3 005 surface APCP 0-3
A new finite element code for the study of strain-localization under strike-slip faults

NASA Astrophysics Data System (ADS)

Rodríguez-González, J.; Montesi, L.

2016-12-01

Shear localization under strike-slip faults in ductile conditions remains a matter of debate. The rheology of rocks in the ductile regime is fundamentally strain-rate hardening, which complicates the understanding of the formation of narrow shear zones. Localized shear zones are present in a variety of scales, including kilometric structures at plate boundaries. To compensate for strain-rate hardening, shear zones must be weaker than their surroundings thanks to some weakening mechanism that works at multiple length scales. Mechanisms as shear heating or grain size reduction have been invoked to explain localization of deformation, but none of these mechanisms can work in scales that range from 1 to 1000 km. Layered fabric development has been suggested as a candidate to develop localized shear zones at multiple scales. To test this hypothesis, we have developed a new software that uses the Finite Element Method library deal.II written in C++. We solve the elasticity equations for elastic and Maxwell visco-elastic mediums. A key component required to study strain localization is adaptive mesh refinement. The code automatically identifies those regions in which the deformation is being localized and will increase the resolution. We benchmark the code and test its accuracy using analytical solutions of strike-slip deformation with different boundary conditions. We simulate the instantaneous deformation caused by two kinds of dislocations: a free fault subject to a far field traction and fault with an imposed displacement. We also simulate the visco-elastic relaxation following a strike-slip dislocation. We show that deal.II is a flexible library, suitable for different problems, which will prove useful to study the mechanisms that can lead to strain localization.

Absolute/convective secondary instabilities and the role of confinement in free shear layers

NASA Astrophysics Data System (ADS)

Arratia, Cristóbal; Mowlavi, Saviz; Gallaire, François

2018-05-01

We study the linear spatiotemporal stability of an infinite row of equal point vortices under symmetric confinement between parallel walls. These rows of vortices serve to model the secondary instability leading to the merging of consecutive (Kelvin-Helmholtz) vortices in free shear layers, allowing us to study how confinement limits the growth of shear layers through vortex pairings. Using a geometric construction akin to a Legendre transform on the dispersion relation, we compute the growth rate of the instability in different reference frames as a function of the frame velocity with respect to the vortices. This approach is verified and complemented with numerical computations of the linear impulse response, fully characterizing the absolute/convective nature of the instability. Similar to results by Healey on the primary instability of parallel tanh profiles [J. Fluid Mech. 623, 241 (2009), 10.1017/S0022112008005284], we observe a range of confinement in which absolute instability is promoted. For a parallel shear layer with prescribed confinement and mixing length, the threshold for absolute/convective instability of the secondary pairing instability depends on the separation distance between consecutive vortices, which is physically determined by the wavelength selected by the previous (primary or pairing) instability. In the presence of counterflow and moderate to weak confinement, small (large) wavelength of the vortex row leads to absolute (convective) instability. While absolute secondary instabilities in spatially developing flows have been previously related to an abrupt transition to a complex behavior, this secondary pairing instability regenerates the flow with an increased wavelength, eventually leading to a convectively unstable row of vortices. We argue that since the primary instability remains active for large wavelengths, a spatially developing shear layer can directly saturate on the wavelength of such a convectively unstable row, by-passing the smaller wavelengths of absolute secondary instability. This provides a wavelength selection mechanism, according to which the distance between consecutive vortices should be sufficiently large in comparison with the channel width in order for the row of vortices to persist. We argue that the proposed wavelength selection criteria can serve as a guideline for experimentally obtaining plane shear layers with counterflow, which has remained an experimental challenge.
DNS and LES of a Shear-Free Mixing Layer

NASA Technical Reports Server (NTRS)

Knaepen, B.; Debliquy, O.; Carati, D.

2003-01-01

The purpose of this work is twofold. First, given the computational resources available today, it is possible to reach, using DNS, higher Reynolds numbers than in Briggs et al.. In the present study, the microscale Reynolds numbers reached in the low- and high-energy homogeneous regions are, respectively, 32 and 69. The results reported earlier can thus be complemented and their robustness in the presence of increased turbulence studied. The second aim of this work is to perform a detailed and documented LES of the shear-free mixing layer. In that respect, the creation of a DNS database at higher Reynolds number is necessary in order to make meaningful LES assessments. From the point of view of LES, the shear-free mixing-layer is interesting since it allows one to test how traditional LES models perform in the presence of an inhomogeneity without having to deal with difficult numerical issues. Indeed, as argued in Briggs et al., it is possible to use a spectral code to study the shear-free mixing layer and one can thus focus on the accuracy of the modelling while avoiding contamination of the results by commutation errors etc. This paper is organized as follows. First we detail the initialization procedure used in the simulation. Since the flow is not statistically stationary, this initialization procedure has a fairly strong influence on the evolution. Although we will focus here on the shear-free mixing layer, the method proposed in the present work can easily be used for other flows with one inhomogeneous direction. The next section of the article is devoted to the description of the DNS. All the relevant parameters are listed and comparison with the Veeravalli & Warhaft experiment is performed. The section on the LES of the shear-free mixing layer follows. A detailed comparison between the filtered DNS data and the LES predictions is presented. It is shown that simple eddy viscosity models perform very well for the present test case, most probably because the flow seems to be almost isotropic in the small-scale range that is not resolved by the LES.
A Study of the Unstable Modes in High Mach Number Gaseous Jets and Shear Layers

NASA Astrophysics Data System (ADS)

Bassett, Gene Marcel

1993-01-01

Instabilities affecting the propagation of supersonic gaseous jets have been studied using high resolution computer simulations with the Piecewise-Parabolic-Method (PPM). These results are discussed in relation to jets from galactic nuclei. These studies involve a detailed treatment of a single section of a very long jet, approximating the dynamics by using periodic boundary conditions. Shear layer simulations have explored the effects of shear layers on the growth of nonlinear instabilities. Convergence of the numerical approximations has been tested by comparing jet simulations with different grid resolutions. The effects of initial conditions and geometry on the dominant disruptive instabilities have also been explored. Simulations of shear layers with a variety of thicknesses, Mach numbers and densities perturbed by incident sound waves imply that the time for the excited kink modes to grow large in amplitude and disrupt the shear layer is taug = (546 +/- 24) (M/4)^{1.7 } (Apert/0.02) ^{-0.4} delta/c, where M is the jet Mach number, delta is the half-width of the shear layer, and A_ {pert} is the perturbation amplitude. For simulations of periodic jets, the initial velocity perturbations set up zig-zag shock patterns inside the jet. In each case a single zig-zag shock pattern (an odd mode) or a double zig-zag shock pattern (an even mode) grows to dominate the flow. The dominant kink instability responsible for these shock patterns moves approximately at the linear resonance velocity, nu_ {mode} = cextnu_ {relative}/(cjet + c_ {ext}). For high resolution simulations (those with 150 or more computational zones across the jet width), the even mode dominates if the even penetration is higher in amplitude initially than the odd perturbation. For low resolution simulations, the odd mode dominates even for a stronger even mode perturbation. In high resolution simulations the jet boundary rolls up and large amounts of external gas are entrained into the jet. In low resolution simulations this entrainment process is impeded by numerical viscosity. The three-dimensional jet simulations behave similarly to two-dimensional jet runs with the same grid resolutions.
Similarity scaling of turbulence in small temperate lake: implication for gas flux: implication for gas flux

NASA Astrophysics Data System (ADS)

Tedford, E. W.; MacIntyre, S.; Miller, S. D.; Czikowsky, M. J.

2013-12-01

The actively mixing layer, or surface layer, is the region of the upper mixed layer of lakes, oceans and the atmosphere directly influenced by wind, heating and cooling. Turbulence within the surface mixing layer has a direct impact on important ecological processes. The Monin-Obukhov length scale (LMO) is a critical length scale used in predicting and understanding turbulence in the actively mixed layer. On the water side of the air-water interface, LMO is defined as: LMO=-u*^3/(0.4 JB0) where u*, the shear velocity, is defined as (τ/rho)^0.5 where τ is the shear stress and rho is the density of water and JBO is the buoyancy flux at the surface. Above the depth equal to the absolute value of the Monin-Obukhov length scale (zMO), wind shear is assumed to dominate the production of turbulent kinetic energy (TKE). Below zMO, the turbulence is assumed to be suppressed when JB0 is stabilizing (warming surface waters) and enhanced when the buoyancy flux is destabilizing (cooling surface waters). Our observed dissipations were well represented using the canonical similarity scaling equations. The Monin-Obukhov length scale was generally effective in separating the surface-mixing layer into two regions: an upper region, dominated by wind shear; and a lower region, dominated by buoyancy flux. During both heating and cooling and above a depth equal to |LMO|, turbulence was dominated by wind shear and dissipation followed law of the wall scaling although was slightly augmented by buoyancy flux during both heating and cooling. Below a depth equal to |LMO| during cooling, dissipation was nearly uniform with depth. Although distinguishing between an upper region of the actively mixing layer dominated by wind stress and a lower portion dominated by buoyancy flux is typically accurate the most accurate estimates of dissipation include the effects of both wind stress and buoyancy flux throughout the actively mixed layer. We demonstrate and discuss the impact of neglecting the non-dominant forcing (buoyancy flux above zMO and wind stress below zMO) above and below zMO.
Understanding the Accretion Engine in Pre-main Sequence Stars

NASA Astrophysics Data System (ADS)

Gómez de Castro, Ana I.

2009-05-01

Planetary systems are angular momentum reservoirs generated during star formation as a result of the joint action of gravity and angular momentum conservation. The accretion process drives to the generation of powerful engines able to drive the optical jets and the molecular outflows. A fraction of the engine energy is released into heating the circumstellar plasma to temperatures between 3000 K to 10 MK depending on the plasma location and density. There are very important unsolved problems concerning the nature of the engine, its evolution and its impact in the chemical evolution of the disk. Of special relevance is the understanding of the shear layer between the stellar photosphere and the disk; this layer controls a significant fraction of the magnetic field building up and the subsequent dissipative processes ought to be studied in the UV.
Two-dimensional nonlinear finite element analysis of well damage due to reservoir compaction, well-to-well interactions, and localization on weak layers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hilbert, L.B. Jr.; Fredrich, J.T.; Bruno, M.S.

1996-05-01

In this paper the authors present the results of a coupled nonlinear finite element geomechanics model for reservoir compaction and well-to-well interactions for the high-porosity, low strength diatomite reservoirs of the Belridge field near Bakersfield, California. They show that well damage and failures can occur under the action of two distinct mechanisms: shear deformations induced by pore compaction, and subsidence, and shear deformations due to well-to-well interactions during production or water injection. They show such casting damage or failure can be localized to weak layers that slide or slip under shear due to subsidence. The magnitude of shear displacements andmore » surface subsidence agree with field observations.« less
Isothermal Oxidation of Magnetite to Hematite in Air and Cyclic Reduction/Oxidation Under Carbon Looping Combustion Conditions

NASA Astrophysics Data System (ADS)

Simmonds, Tegan; Hayes, Peter C.

2017-12-01

In the carbon looping combustion process the oxygen carrier is regenerated through oxidation in air; this process has been simulated by the oxidation of dense synthetic magnetite for selected temperatures and times. The oxidation of magnetite in air is shown to occur through the formation of dense hematite layers on the particle surface. This dense hematite forms through lath type shear transformations or solid-state diffusion through the product layer. Cyclic reduction in CO-CO2/oxidation in air of hematite single crystals has been carried out under controlled laboratory conditions at 1173 K (900 °C). It has been shown that the initial reduction step is critical to determining the product microstructure, which consists of gas pore dendrites in the magnetite matrix with blocky hematite formed on the pore surfaces. The progressive growth of the magnetite layer with the application of subsequent cycles appears to continue until no original hematite remains, after which physical disintegration of the particles takes place.
The stabilizing effect of compressibility in turbulent shear flow

NASA Technical Reports Server (NTRS)

Sarkar, S.

1994-01-01

Direct numerical simulation of turbulent homogeneous shear flow is performed in order to clarify compressibility effects on the turbulence growth in the flow. The two Mach numbers relevant to homogeneous shear flow are the turbulent Mach number M(t) and the gradient Mach number M(g). Two series of simulations are performed where the initial values of M(g) and M(t) are increased separately. The growth rate of turbulent kinetic energy is observed to decrease in both series of simulations. This 'stabilizing' effect of compressibility on the turbulent energy growth rate is observed to be substantially larger in the DNS series where the initial value of M(g) is changed. A systematic companion of the different DNS cues shows that the compressibility effect of reduced turbulent energy growth rate is primarily due to the reduced level of turbulence production and not due to explicit dilatational effects. The reduced turbulence production is not a mean density effect since the mean density remains constant in compressible homogeneous shear flow. The stabilizing effect of compressibility on the turbulence growth is observed to increase with the gradient Mach number M(g) in the homogeneous shear flow DNS. Estimates of M(g) for the mixing and the boundary layer are obtained. These estimates show that the parameter M(g) becomes much larger in the high-speed mixing layer relative to the high-speed boundary layer even though the mean flow Mach numbers are the same in the two flows. Therefore, the inhibition of turbulent energy production and consequent 'stabilizing' effect of compressibility on the turbulence (over and above that due to the mean density variation) is expected to be larger in the mixing layer relative to the boundary layer in agreement with experimental observations.
Aspects of turbulent-shear-layer dynamics and mixing

NASA Astrophysics Data System (ADS)

Slessor, Michael David

Experiments have been conducted in the GALCIT Supersonic Shear Layer Facility to investigate some aspects of high-Reynolds-number, turbulent, shear-layer flows in both incompressible- and compressible-flow regimes. Experiments designed to address several issues were performed; effects of inflow boundary conditions, freestream conditions (supersonic/subsonic flow), and compressibility, on both large-scale dynamics and small-scale mixing, are described. Chemically-reacting and non-reacting flows were investigated, the former relying on the (Hsb2 + NO)/Fsb2 chemical system, in the fast-kinetic regime, to infer the structure and amount of molecular-scale mixing through use of "flip" experiments. A variety of experimental techniques, including a color-schlieren visualization system developed as part of this work, were used to study the flows. Both inflow conditions and compressibility are found to have significant effects on the flow. In particular, inflow conditions are "remembered" for long distances downstream, a sensitivity similar to that observed in low-dimensionality, non-linear (chaotic) systems. The global flowfields (freestreams coupled by the shear layer) of transonic flows exhibit a sensitivity to imposed boundary conditions, a.e., local area ratios. A previously-proposed mode-selection rule for turbulent-structure convection speeds, based on the presence of a lab-frame subsonic freestream, was experimentally demonstrated to be incorrect. Compressibility, when decoupled from ail other parameters, e.g., Reynolds number, velocity and density ratios, etc., reduces large-scale entrainment and turbulent growth, but slightly enhances small-scale mixing, with an associated change in the structure of the molecularly-mixed fluid. This reduction in shear-layer growth rate is examined and a new parameter that interprets compressibility as an energy-exchange mechanism is proposed. The parameter reconciles and collapses experimentally-observed growth rates.
LED Die-Bonded on the Ag/Cu Substrate by a Sn-BiZn-Sn Bonding System

NASA Astrophysics Data System (ADS)

Tang, Y. K.; Hsu, Y. C.; Lin, E. J.; Hu, Y. J.; Liu, C. Y.

2016-12-01

In this study, light emitting diode (LED) chips were die-bonded on a Ag/Cu substrate by a Sn-BixZn-Sn bonding system. A high die-bonding strength is successfully achieved by using a Sn-BixZn-Sn ternary system. At the bonding interface, there is observed a Bi-segregation phenomenon. This Bi-segregation phenomenon solves the problems of the brittle layer-type Bi at the joint interface. Our shear test results show that the bonding interface with Bi-segregation enhances the shear strength of the LED die-bonding joints. The Bi-0.3Zn and Bi-0.5Zn die-bonding cases have the best shear strength among all die-bonding systems. In addition, we investigate the atomic depth profile of the deposited Bi-xZn layer by evaporating Bi-xZn E-gun alloy sources. The initial Zn content of the deposited Bi-Zn alloy layers are much higher than the average Zn content in the deposited Bi-Zn layers.
Initial development of the two-dimensional ejector shear layer - Experimental results

NASA Technical Reports Server (NTRS)

Benjamin, M. A.; Dufflocq, M.; Roan, V. P.

1993-01-01

An experimental investigation designed to study the development of shear layers in a two-dimensional single-nozzle ejector has been completed. In this study, combinations of air/air, argon/air, helium/air, and air/helium were used as the supersonic primary and subsonic secondary, respectively. Mixing of the gases occurred in a constant-area tube 39.1 mm high by 25.4 mm wide, where the inlet static pressure was maintained at 35 kPa. The cases studied resulted in convective Mach numbers between 0.058 and 1.64, density ratios between 0.102 and 3.49, and velocity ratios between 0.065 and 0.811. The resulting data shows the differences in the shear-layer development for the various combinations of independent variables utilized in the investigation. The normalized growth-rates in the near-field were found to be similar to two-dimensional mixing layers. These results have enhanced the ability to analyze and design ejector systems as well as providing a better understanding of the physics.
Magnetoelastic shear wave propagation in pre-stressed anisotropic media under gravity

NASA Astrophysics Data System (ADS)

Kumari, Nirmala; Chattopadhyay, Amares; Singh, Abhishek K.; Sahu, Sanjeev A.

2017-03-01

The present study investigates the propagation of shear wave (horizontally polarized) in two initially stressed heterogeneous anisotropic (magnetoelastic transversely isotropic) layers in the crust overlying a transversely isotropic gravitating semi-infinite medium. Heterogeneities in both the anisotropic layers are caused due to exponential variation (case-I) and linear variation (case-II) in the elastic constants with respect to the space variable pointing positively downwards. The dispersion relations have been established in closed form using Whittaker's asymptotic expansion and were found to be in the well-agreement to the classical Love wave equations. The substantial effects of magnetoelastic coupling parameters, heterogeneity parameters, horizontal compressive initial stresses, Biot's gravity parameter, and wave number on the phase velocity of shear waves have been computed and depicted by means of a graph. As a special case, dispersion equations have been deduced when the two layers and half-space are isotropic and homogeneous. The comparative study for both cases of heterogeneity of the layers has been performed and also depicted by means of graphical illustrations.
Instability of a shear layer between multicomponent fluids at supercritical pressure

NASA Astrophysics Data System (ADS)

Fu, Qing-fei; Zhang, Yun-xiao; Mo, Chao-jie; Yang, Li-jun

2018-04-01

The temporal instability of a thin shear layer lying between streams of two components of fluids has been studied. The effects of density profile of the layer on the instability behavior were mainly considered. The detailed density profile was obtained through Linear Gradient Theory. The eigenvalue problem was calculated, and the temporal instability curves were obtained for the thermodynamic parameters, e.g. pressure and temperature. The results show that, increase of pressure leads to the increase of the maximum growth rate. However, increasing pressure has opposite effects on the disturbances with small and large wave length. The increase of temperature causes the decrease of disturbance growth rate. The instability behavior of the shear layers was determined mainly by the interval between the inflections of the velocity and density profiles, and the maximum density gradient. The total effects, determined by coupling density stratification, and interval between the inflections of the velocity and density profiles, were quite distinct for different ranges of temperature and pressure.
Influence of shear forces on the aggregation and sedimentation behavior of cerium dioxide (CeO2) nanoparticles under different hydrochemical conditions

NASA Astrophysics Data System (ADS)

Lv, Bowen; Wang, Chao; Hou, Jun; Wang, Peifang; Miao, Lingzhan; Li, Yi; Ao, Yanhui; Yang, Yangyang; You, Guoxiang; Xu, Yi

2016-07-01

This study contributed to a better understanding of the behavior of nanoparticles (NPs) in dynamic water. First, the aggregation behavior of CeO2 NPs at different pH values in various salt solutions was examined to determine the appropriate hydrochemical conditions for hydrodynamics study. Second, the aggregation behavior of CeO2 NPs under different shear forces was investigated at pH 4 and ionic strength 0 in various salt solutions to find out whether shear forces could influence the stability of the nanoparticles and if yes, how. Also, five-stage sedimentation tests were conducted to understand the influence of shear stress on the vertical distribution of CeO2 NPs in natural waters. The aggregation test showed that the shear force could increase the collision efficiency between NPs during aggregation and cause a relatively large mass of NPs to remain in suspension. Consequently, the nanoparticles had a greater possibility of continued aggregation. The sedimentation test under static conditions indicated that a large mass of NPs (>1000 nm) sink to the bottom layer, leaving only small aggregates dispersed in the upper or middle layer of the solution. However, later sedimentation studies under stirring conditions demonstrated that shear forces can disrupt this stratification phenomenon. These results suggest that shear forces can influence the spatial distribution of NPs in natural waters, which might lead to different toxicities of CeO2 NPs to aquatic organisms distributed in the different water layers. This study contributes to a better understanding of nanomaterial toxicology and provides a way for further research.
A Theoretical Model of Drumlin Formation Based on Observations at Múlajökull, Iceland

NASA Astrophysics Data System (ADS)

Iverson, N. R.; McCracken, R. G.; Zoet, L. K.; Benediktsson, Í. Ö.; Schomacker, A.; Johnson, M. D.; Woodard, J.

2017-12-01

The drumlin field at the surge-type glacier, Múlajökull, provides an unusual opportunity to build a model of drumlin formation based on field observations in a modern drumlin-forming environment. These observations indicate that surges deposit till layers that drape the glacier forefield, conform to drumlin surfaces, and are deposited in shear. Observations also indicate that erosion helps create drumlin relief, effective stresses in subglacial till are highest between drumlins, and during quiescent flow, crevasses on the glacier surface overlie drumlins while subglacial channels occupy intervening swales. In the model, we consider gentle undulations on the bed bounded by subglacial channels at low water pressure. During quiescent flow, slip of temperate ice across these undulations and basal water flow toward bounding channels create an effective stress distribution that maximizes till entrainment in ice on the heads and flanks of drumlins. Crevasses amplify this effect but are not necessary for it. During surges, effective stresses are uniformly low, and the bed shears pervasively. Vigorous basal melting during surges releases debris from ice and deposits it on the bed, with deposition augmented by transport in the deforming bed. As surge cycles progress, drumlins migrate downglacier and grow at increasing rates, due to positive feedbacks that depend on drumlin height. Drumlin growth can be accompanied by either net aggradation or erosion of the bed, and drumlin heights and stratigraphy generally correspond with observations. This model highlights that drumlin growth can reflect instabilities other than those of bed shear instability models, which require heuristic till transport assumptions.
Switching from pure- into simple-shear mode during uplift of the Altiplano plateau (Central Andes)

NASA Astrophysics Data System (ADS)

Babeyko, A. Yu.; Sobolev, S. V.

2003-04-01

The Altiplano plateau of the Central Andes is the second greatest plateau in the world after Tibet with an average elevation of about 4 km formed as a result of ocean-continent collision between subducting Nasca plate on the west and Brazilian shield on the east. According to the well known Isacks (1988) scenario, the Cenozoic evolution of the plateau started ca. 30 Ma in response to the retreat of the flat-subducted Nasca plate. Astenospheric material, which replaced the retreated plate, thermally thinned and softened the overlying lithosphere. The Altiplano crust, being pushed by the Brazilian shield from the east, was first shortened in a pure-shear mode and reached 60-70 km in thickness. At ca. 8-10 Ma deformation changed to a simple-shear mode: it was ceased in the upper crust of the plateau and migrated eastwards, into the Subandean, while the plateau itself continued to grow due to ongoing shortening in the lower crust. We employ numerical 2D thermomechanical modelling to test the above scenario and to evaluate the key parameters, which account for the transition from pure- to simple- shear style of the lithosphere-scale deformation under pure-shear boundary condition. As a numerical tool we use explicit finite difference/finite element lagrangian code with markers tracking material properties. The model contains rheologically different layers representing sediments, felsic and mafic crust, lithospheric mantle, and astenosphere. Rheological laws are Mohr-Coloumb elasto-plastic with softening and Maxwell visco-elastic with nonlinear power-law creep. Initial and boundary conditions simulate thermal activation of the Altiplano lithosphere by upwelling astenosphere as well as its westward pushing by the cold Brazilian shield with constant velocity. We found that model shortening always occurs in a pure-shear mode unless the uppermost crust of the Brazilian shield becomes during the deformation considerably weaker than the Altiplano upper crust (drop of friction coefficient down to 0.05-0.1). This weakening may be attributed to more pronounced plastic softening in thick layer of the Paleozoic sediments covering the shield. Another nessesary condition is formation of a prominent (2-3 km) topographic step between the plateau and foreland before the beginning of the second phase. This topographic step is explained by initial localization of the pure-shear-type deformation under the Altiplano, where the crust is hotter and more felsic than the crust of the Brazilian shield.
A test of the double-shearing model of flow for granular materials

USGS Publications Warehouse

Savage, J.C.; Lockner, D.A.

1997-01-01

The double-shearing model of flow attributes plastic deformation in granular materials to cooperative slip on conjugate Coulomb shears (surfaces upon which the Coulomb yield condition is satisfied). The strict formulation of the double-shearing model then requires that the slip lines in the material coincide with the Coulomb shears. Three different experiments that approximate simple shear deformation in granular media appear to be inconsistent with this strict formulation. For example, the orientation of the principal stress axes in a layer of sand driven in steady, simple shear was measured subject to the assumption that the Coulomb failure criterion was satisfied on some surfaces (orientation unspecified) within the sand layer. The orientation of the inferred principal compressive axis was then compared with the orientations predicted by the double-shearing model. The strict formulation of the model [Spencer, 1982] predicts that the principal stress axes should rotate in a sense opposite to that inferred from the experiments. A less restrictive formulation of the double-shearing model by de Josselin de Jong [1971] does not completely specify the solution but does prescribe limits on the possible orientations of the principal stress axes. The orientations of the principal compression axis inferred from the experiments are probably within those limits. An elastoplastic formulation of the double-shearing model [de Josselin de Jong, 1988] is reasonably consistent with the experiments, although quantitative agreement was not attained. Thus we conclude that the double-shearing model may be a viable law to describe deformation of granular materials, but the macroscopic slip surfaces will not in general coincide with the Coulomb shears.
Passive imaging of hydrofractures in the South Belridge diatomite

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ilderton, D.C.; Patzek, T.W.; Rector, J.W.

1996-03-01

The authors present the results of a seismic analysis of two hydrofractures spanning the entire diatomite column (1,110--1,910 ft or 338--582 m) in Shell`s Phase 2 steam drive pilot in South Belridge, California. These hydrofractures were induced at two depths (1,110--1,460 and 1,560--1,910 ft) and imaged passively using the seismic energy released during fracturing. The arrivals of shear waves from the cracking rock (microseismic events) were recorded at a 1 ms sampling rate by 56 geophones in three remote observation wells, resulting in 10 GB of raw data. These arrival times were then inverted for the event locations, from whichmore » the hydrofracture geometry was inferred. A five-dimensional conjugate-gradient algorithm with a depth-dependent, but otherwise constant shear wave velocity model (CVM) was developed for the inversions. To validate CVM, they created a layered shear wave velocity model of the formation and used it to calculate synthetic arrival times from known locations chosen at various depths along the estimated fracture plane. These arrival times were then inverted with CVM and the calculated locations compared with the known ones, quantifying the systematic error associated with the assumption of constant shear wave velocity. They also performed Monte Carlo sensitivity analyses on the synthetic arrival times to account for all other random errors that exist in field data. After determining the limitations of the inversion algorithm, they hand-picked the shear wave arrival times for both hydrofractures and inverted them with CVM.« less
Geomechanical response of permafrost-associated hydrate deposits to depressurization-induced gas production

USGS Publications Warehouse

Rutqvist, J.; Moridis, G.J.; Grover, T.; Collett, T.

2009-01-01

In this simulation study, we analyzed the geomechanical response during depressurization production from two known hydrate-bearing permafrost deposits: the Mallik (Northwest Territories, Canada) deposit and Mount Elbert (Alaska, USA) deposit. Gas was produced from these deposits at constant pressure using horizontal wells placed at the top of a hydrate layer (HL), located at a depth of about 900??m at the Mallik site and 600??m at the Mount Elbert site. The simulation results show that general thermodynamic and geomechanical responses are similar for the two sites, but with substantially higher production and more intensive geomechanical responses at the deeper Mallik deposit. The depressurization-induced dissociation begins at the well bore and then spreads laterally, mainly along the top of the HL. The depressurization results in an increased shear stress within the body of the receding hydrate and causes a vertical compaction of the reservoir. However, its effects are partially mitigated by the relatively stiff permafrost overburden, and compaction of the HL is limited to less than 0.4%. The increased shear stress may lead to shear failure in the hydrate-free zone bounded by the HL overburden and the downward-receding upper dissociation interface. This zone undergoes complete hydrate dissociation, and the cohesive strength of the sediment is low. We determined that the likelihood of shear failure depends on the initial stress state as well as on the geomechanical properties of the reservoir. The Poisson's ratio of the hydrate-bearing formation is a particularly important parameter that determines whether the evolution of the reservoir stresses will increase or decrease the likelihood of shear failure.
Development of Naphthalene PLIF for Visualizing Ablation Products From a Space Capsule Heat Shield

NASA Technical Reports Server (NTRS)

Combs, C. S.; Clemens, N. T.; Danehy, P. M.

2014-01-01

The Orion Multi-Purpose Crew Vehicle (MPCV) will use an ablative heat shield. To better design this heat shield and others that will undergo planetary entry, an improved understanding of the ablation process would be beneficial. Here, a technique developed at The University of Texas at Austin that uses planar laser-induced fluorescence (PLIF) of a low-temperature sublimating ablator (naphthalene) to enable visualization of the ablation products in a hypersonic flow is applied. Although high-temperature ablation is difficult and expensive to recreate in a laboratory environment, low-temperature sublimation creates a limited physics problem that can be used to explore ablation-product transport in a hypersonic flow-field. In the current work, a subscale capsule reentry vehicle model with a solid naphthalene heat shield has been tested in a Mach 5 wind tunnel. The PLIF technique provides images of the spatial distribution of sublimated naphthalene in the heat-shield boundary layer, separated shear layer, and backshell recirculation region. Visualizations of the capsule shear layer using both naphthalene PLIF and Schlieren imaging compared favorably. PLIF images have shown high concentrations of naphthalene in the capsule separated flow region, intermittent turbulent structures on the heat shield surface, and interesting details of the capsule shear layer structure. It was shown that, in general, the capsule shear layer appears to be more unsteady at lower angels of attack. The PLIF images demonstrated that during a wind tunnel run, as the model heated up, the rate of naphthalene ablation increased, since the PLIF signal increased steadily over the course of a run. Additionally, the shear layer became increasingly unsteady over the course of a wind tunnel run, likely because of increased surface roughness but also possibly because of the increased blowing. Regions with a relatively low concentration of naphthalene were also identified in the capsule backshell recirculation region and are most likely the result of cross-flow-induced vortices on the capsule afterbody.

Ductile shear zones beneath strike-slip faults: Implications for the thermomechanics of the San Andreas fault zone

USGS Publications Warehouse

Thatcher, W.; England, P.C.

1998-01-01

We have carried out two-dimensional (2-D) numerical experiments on the bulk flow of a layer of fluid that is driven in a strike-slip sense by constant velocities applied at its boundaries. The fluid has the (linearized) conventional rheology assumed to apply to lower crust/upper mantle rocks. The temperature dependence of the effective viscosity of the fluid and the shear heating that accompanies deformation have been incorporated into the calculations, as has thermal conduction in an overlying crustal layer. Two end-member boundary conditions have been considered, corresponding to a strong upper crust driving a weaker ductile substrate and a strong ductile layer driving a passive, weak crust. In many cases of practical interest, shear heating is concentrated close to the axial plane of the shear zone for either boundary condition. For these cases, the resulting steady state temperature field is well approximated by a cylindrical heat source embedded in a conductive half-space at a depth corresponding to the top of the fluid layer. This approximation, along with the application of a theoretical result for one-dimensional shear zones, permits us to obtain simple analytical approximations to the thermal effects of 2-D ductile shear zones for a range of assumed rheologies and crustal geotherms, making complex numerical calculations unnecessary. Results are compared with observable effects on heat flux near the San Andreas fault using constraints on the slip distribution across the entire fault system. Ductile shearing in the lower crust or upper mantle can explain the observed increase in surface heat flux southeast of the Mendocino triple junction and match the amplitude of the regional heat flux anomaly in the California Coast Ranges. Because ductile dissipation depends only weakly on slip rate, faults moving only a few millimeters per year can be important heat sources, and the superposition of effects of localized ductile shearing on both currently active and now inactive strands of the San Andreas system can explain the breadth of the heat flux anomaly across central California.
Shear-layer correction after Amiet under consideration of additional temperature gradient. Working diagrams for correction of signals

NASA Technical Reports Server (NTRS)

Dobrzynski, W.

1984-01-01

Amiet's correction scheme for sound wave transmission through shear-layers is extended to incorporate the additional effects of different temperatures in the flow-field in the surrounding medium at rest. Within a parameter-regime typical for acoustic measurements in wind tunnels amplitude- and angle-correction is calculated and plotted systematically to provide a data base for the test engineer.
Simultaneous Multiple-Location Separation Control

NASA Technical Reports Server (NTRS)

Greenblatt, David (Inventor)

2009-01-01

A method of controlling a shear layer for a fluid dynamic body introduces first periodic disturbances into the fluid medium at a first flow separation location. Simultaneously, second periodic disturbances are introduced into the fluid medium at a second flow separation location. A phase difference between the first and second periodic disturbances is adjusted to control flow separation of the shear layer as the fluid medium moves over the fluid dynamic body.
Diamond anvil cell for spectroscopic investigation of materials at high temperature, high pressure and shear

DOEpatents

Westerfield, C.L.; Morris, J.S.; Agnew, S.F.

1997-01-14

Diamond anvil cell is described for spectroscopic investigation of materials at high temperature, high pressure and shear. A cell is described which, in combination with Fourier transform IR spectroscopy, permits the spectroscopic investigation of boundary layers under conditions of high temperature, high pressure and shear. 4 figs.
Turbulent kinetic energy equation and free mixing

NASA Technical Reports Server (NTRS)

Morel, T.; Torda, T. P.; Bradshaw, P.

1973-01-01

Calculation of free shear flows was carried out to investigate the usefulness of several concepts which were previously successfully applied to wall flows. The method belongs to the class of differential approaches. The turbulence is taken into account by the introduction of one additional partial differential equation, the transport equation for the turbulent shear stress. The structure of turbulence is modeled after Bradshaw et al. This model was used successfully in boundary layers and its applicability to other flows is demonstrated. The work reported differs substantially from that of an earlier attempt to use this approach for calculation of free flows. The most important difference is that the region around the center line is treated by invoking the interaction hypothesis (concerning the structure of turbulence in the regions separated by the velocity extrema). The compressibility effects on shear layer spreading at low and moderate Mach numbers were investigated. In the absence of detailed experiments in free flows, the evidence from boundary layers that at low Mach numbers the structure of turbulence is unaffected by the compressibility was relied on. The present model was tested over a range of self-preserving and developing flows including pressure gradients using identical empirical input. The dependence of the structure of turbulence on the spreading rate of the shear layer was established.
Impact of finite rate chemistry on the hydrodynamic stability of shear flows in turbulent lean premixed combustion

NASA Astrophysics Data System (ADS)

Dagan, Yuval; Ghoniem, Ahmed

2017-11-01

Recent experimental observations show that the dynamic response of a reactive flow is strongly impacted by the fuel chemistry. In order to gain insight into some of the underlying mechanisms we formulate a new linear stability model that incorporates the impact of finite rate chemistry on the hydrodynamic stability of shear flows. Contrary to previous studies which typically assume that the velocity field is independent of the kinetic rates, the velocity field in our study is coupled with the temperature field. Using this formulation, we reproduce previous results, e.g., most unstable global modes, obtained for non-reacting shear flow. Moreover, we show that these modes are significantly altered in frequency and gain by the presence of a reaction region within the shear layer. This qualitatively agrees with results of our recent experimental and numerical studies, which show that the flame surface location relative to the shear layer influences the stability characteristics in combustion tunnels. This study suggests a physical explanation for the observed impact of finite rate chemistry on shear flow stability.
Diurnal forcing of planetary atmospheres

NASA Technical Reports Server (NTRS)

Houben, Howard C.

1991-01-01

A free convection parameterization has been introduced into the Mars Planetary Boundary Layer Model (MPBL). Previously, the model would fail to generate turbulence under conditions of zero wind shear, even when statically unstable. This in turn resulted in erroneous results at the equator, for example, when the lack of Coriolis forcing allowed zero wind conditions. The underlying cause of these failures was the level 2 second-order turbulence closure scheme which derived diffusivities as algebraic functions of the Richardson number (the ratio of static stability to wind shear). In the previous formulation, the diffusivities were scaled by the wind shear--a convenient parameter since it is non-negative. This was the drawback that all diffusivities are zero under conditions of zero shear (viz., the free convection case). The new scheme tests for the condition of zero shear in conjunction with static instability and recalculates the diffusivities using a static stability scaling. The results for a simulation of the equatorial boundary layer at autumnal equinox are presented. (Note that after some wind shear is generated, the model reverts to the traditional diffusivity calculation.)
Analysis of passive damping in thick composite structures

NASA Technical Reports Server (NTRS)

Saravanos, D. A.

1993-01-01

Computational mechanics for the prediction of damping and other dynamic characteristics in composite structures of general thicknesses and laminations are presented. Discrete layer damping mechanics that account for the representation of interlaminar shear effects in the material are summarized. Finite element based structural mechanics for the analysis of damping are described, and a specialty finite element is developed. Applications illustrate the quality of the discrete layer damping mechanics in predicting the damped dynamic characteristics of composite structures with thicker sections and/or laminate configurations that induce interlaminar shear. The results also illustrate and quantify the significance of interlaminar shear damping in such composite structures.
Predictions of High Strain Rate Failure Modes in Layered Aluminum Composites

NASA Astrophysics Data System (ADS)

Khanikar, Prasenjit; Zikry, M. A.

2014-01-01

A dislocation density-based crystalline plasticity formulation, specialized finite-element techniques, and rational crystallographic orientation relations were used to predict and characterize the failure modes associated with the high strain rate behavior of aluminum layered composites. Two alloy layers, a high strength alloy, aluminum 2195, and an aluminum alloy 2139, with high toughness, were modeled with representative microstructures that included precipitates, dispersed particles, and different grain boundary distributions. Different layer arrangements were investigated for high strain rate applications and the optimal arrangement was with the high toughness 2139 layer on the bottom, which provided extensive shear strain localization, and the high strength 2195 layer on the top for high strength resistance The layer thickness of the bottom high toughness layer also affected the bending behavior of the roll-bonded interface and the potential delamination of the layers. Shear strain localization, dynamic cracking, and delamination are the mutually competing failure mechanisms for the layered metallic composite, and control of these failure modes can be used to optimize behavior for high strain rate applications.
Turbulence and mixing from optimal perturbations to a stratified shear layer

NASA Astrophysics Data System (ADS)

Kaminski, Alexis; Caulfield, C. P.; Taylor, John

2014-11-01

The stability and mixing of stratified shear layers is a canonical problem in fluid dynamics with relevance to flows in the ocean and atmosphere. The Miles-Howard theorem states that a necessary condition for normal-mode instability in parallel, inviscid, steady stratified shear flows is that the gradient Richardson number, Rig is less than 1/4 somewhere in the flow. However, substantial transient growth of non-normal modes may be possible at finite times even when Rig > 1 / 4 everywhere in the flow. We have calculated the ``optimal perturbations'' associated with maximum perturbation energy gain for a stably-stratified shear layer. These optimal perturbations are then used to initialize direct numerical simulations. For small but finite perturbation amplitudes, the optimal perturbations grow at the predicted linear rate initially, but then experience sufficient transient growth to become nonlinear and susceptible to secondary instabilities, which then break down into turbulence. Remarkably, this occurs even in flows for which Rig > 1 / 4 everywhere. We will describe the nonlinear evolution of the optimal perturbations and characterize the resulting turbulence and mixing.
Diagnostics of boundary layer transition by shear stress sensitive liquid crystals

NASA Astrophysics Data System (ADS)

Shapoval, E. S.

2016-10-01

Previous research indicates that the problem of boundary layer transition visualization on metal models in wind tunnels (WT) which is a fundamental question in experimental aerodynamics is not solved yet. In TsAGI together with Khristianovich Institute of Theoretical and Applied Mechanics (ITAM) a method of shear stress sensitive liquid crystals (LC) which allows flow visualization was proposed. This method allows testing several flow conditions in one wind tunnel run and does not need covering the investigated model with any special heat-insulating coating which spoils the model geometry. This coating is easily applied on the model surface by spray or even by brush. Its' thickness is about 40 micrometers and it does not spoil the surface quality. At first the coating obtains some definite color. Under shear stress the LC coating changes color and this change is proportional to shear stress. The whole process can be visually observed and during the tests it is recorded by camera. The findings of the research showed that it is possible to visualize boundary layer transition, flow separation, shock waves and the flow image on the whole. It is possible to predict that the proposed method of shear stress sensitive liquid crystals is a promise for future research.
Instability of a Lamellar Phase under Shear Flow: Formation of Multilamellar Vesicles

NASA Astrophysics Data System (ADS)

Courbin, L.; Delville, J. P.; Rouch, J.; Panizza, P.

2002-09-01

The formation of closed-compact multilamellar vesicles (referred to in the literature as the ``onion texture'') obtained upon shearing lamellar phases is studied using small-angle light scattering and cross-polarized microscopy. By varying the shear rate γ ˙, the gap cell D, and the smectic distance d, we show that: (i)the formation of this structure occurs homogeneously in the cell at a well-defined wave vector qi, via a strain-controlled process, and (ii)the value of qi varies as (dγ ˙/D)1/3. These results strongly suggest that formation of multilamellar vesicles may be monitored by an undulation (buckling) instability of the membranes, as expected from theory.
Coupling of bias-induced crystallographic shear planes with charged domain walls in ferroelectric oxide thin films

DOE PAGES

Han, Myung-Geun; Garlow, Joseph A.; Bugnet, Matthieu; ...

2016-09-02

Polar discontinuity at interfaces plays deterministic roles in charge transport, magnetism, and even superconductivity of functional oxides. To date, most polar discontinuity problems have been explored in hetero-interfaces between two dissimilar materials. Here, we show that charged domain walls (CDWs) in epitaxial thin films of ferroelectric PbZr 0.2Ti 0.8O 3 are strongly coupled to polar interfaces through the formation of ½<101>{h0l} type crystallographic shear planes (CSPs). Using atomic resolution imaging and spectroscopy we illustrate that the CSPs consist of both conservative and nonconservative segments when coupled to the CDWs, where necessary compensating charges for stabilizing the CDWs are associated withmore » vacancies at the CSPs. Lasly, the CDW/CSP coupling yields an atomically narrow domain walls, consisting of a single atomic layer of oxygen. This study shows that the CDW/CSP coupling is a fascinating venue to develop emergent material properties.« less
Generation of large-scale intrusions at baroclinic fronts: an analytical consideration with a reference to the Arctic Ocean

NASA Astrophysics Data System (ADS)

Kuzmina, Natalia

2016-12-01

Analytical solutions are found for the problem of instability of a weak geostrophic flow with linear velocity shear accounting for vertical diffusion of buoyancy. The analysis is based on the potential-vorticity equation in a long-wave approximation when the horizontal scale of disturbances is considered much larger than the local baroclinic Rossby radius. It is hypothesized that the solutions found can be applied to describe stable and unstable disturbances of the planetary scale with respect, in particular, to the Arctic Ocean, where weak baroclinic fronts with typical temporal variability periods on the order of several years or more have been observed and the β effect is negligible. Stable (decaying with time) solutions describe disturbances that, in contrast to the Rossby waves, can propagate to both the west and east, depending on the sign of the linear shear of geostrophic velocity. The unstable (growing with time) solutions are applied to explain the formation of large-scale intrusions at baroclinic fronts under the stable-stable thermohaline stratification observed in the upper layer of the Polar Deep Water in the Eurasian Basin. The suggested mechanism of formation of intrusions can be considered a possible alternative to the mechanism of interleaving at the baroclinic fronts due to the differential mixing.
DOE Office of Scientific and Technical Information (OSTI.GOV)

Fetfatsidis, K. A.; Sherwood, J. A.

NCFs (Non-Crimp Fabrics) are commonly used in the design of wind turbine blades and other complex systems due to their ability to conform to complex shapes without the wrinkling that is typically experienced with woven fabrics or prepreg tapes. In the current research, a form of vacuum assisted resin transfer molding known as SCRIMP registered is used to manufacture wind turbine blades. Often, during the compacting of the fabric layers by the vacuum pressure, several plies may bunch together out-of-plane and form wave defects. When the resin is infused, the areas beneath the waves become resin rich and can compromisemore » the structural integrity of the blade. A reliable simulation tool is valuable to help predict where waves and other defects may appear as a result of the manufacturing process. Forming simulations often focus on the in-plane shearing and tensile behavior of fabrics and do not necessarily consider the bending stiffness of the fabrics, which is important to predict the formation of wrinkles and/or waves. This study incorporates experimentally determined in-plane shearing, tensile, and bending stiffness information of NCFs into a finite element model (ABAQUS/Explicit) of a 9-meter wind turbine blade to investigate the mechanical behaviors that can lead to the formation of waves as a result of the manufacturing process.« less
The high-energy-density counterpropagating shear experiment and turbulent self-heating

DOE PAGES

Doss, F. W.; Fincke, J. R.; Loomis, E. N.; ...

2013-12-06

The counterpropagating shear experiment has previously demonstrated the ability to create regions of shockdriven shear, balanced symmetrically in pressure and experiencing minimal net drift. This allows for the creation of a high-Mach-number high-energy-density shear environment. New data from the counterpropagating shear campaign is presented, and both hydrocode modeling and theoretical analysis in the context of a Reynolds-averaged-Navier-Stokes model suggest turbulent dissipation of energy from the supersonic flow bounding the layer is a significant driver in its expansion. A theoretical minimum shear flow Mach number threshold is suggested for substantial thermal-turbulence coupling.
Holt film wall shear instrumentation for boundary layer transition research

NASA Technical Reports Server (NTRS)

Schneider, Steven P.

1994-01-01

Measurements of the performance of hot-film wall-shear sensors were performed to aid development of improved sensors. The effect of film size and substrate properties on the sensor performance was quantified through parametric studies carried out both electronically and in a shock tube. The results show that sensor frequency response increases with decreasing sensor size, while at the same time sensitivity decreases. Substrate effects were also studied, through parametric variation of thermal conductivity and heat capacity. Early studies used complex dual-layer substrates, while later studies were designed for both single-layer and dual-layer substrates. Sensor failures and funding limitations have precluded completion of the substrate thermal-property tests.
Shear properties evaluation of a truss core of sandwich beams

NASA Astrophysics Data System (ADS)

Wesolowski, M.; Ludewicz, J.; Domski, J.; Zakrzewski, M.

2017-10-01

The open-cell cores of sandwich structures are locally bonded to the face layers by means of adhesive resin. The sandwich structures composed of different parent materials such as carbon fibre composites (laminated face layers) and metallic core (aluminium truss core) brings the need to closely analyse their adhesive connections which strength is dominated by the shear stress. The presented work considers sandwich beams subjected to the static tests in the 3-point bending with the purpose of estimation of shear properties of the truss core. The main concern is dedicated to the out-of plane shear modulus and ultimate shear stress of the aluminium truss core. The loading of the beam is provided by a static machine. For the all beams the force - deflection history is extracted by means of non-contact optical deflection measurement using PONTOS system. The mode of failure is identified for each beam with the corresponding applied force. A flexural rigidity of the sandwich beams is also discussed based on force - displacement plots.
On multiple solutions of non-Newtonian Carreau fluid flow over an inclined shrinking sheet

NASA Astrophysics Data System (ADS)

Khan, Masood; Sardar, Humara; Gulzar, M. Mudassar; Alshomrani, Ali Saleh

2018-03-01

This paper presents the multiple solutions of a non-Newtonian Carreau fluid flow over a nonlinear inclined shrinking surface in presence of infinite shear rate viscosity. The governing boundary layer equations are derived for the Carreau fluid with infinite shear rate viscosity. The suitable transformations are employed to alter the leading partial differential equations to a set of ordinary differential equations. The consequential non-linear ODEs are solved numerically by an active numerical approach namely Runge-Kutta Fehlberg fourth-fifth order method accompanied by shooting technique. Multiple solutions are presented graphically and results are shown for various physical parameters. It is important to state that the velocity and momentum boundary layer thickness reduce with increasing viscosity ratio parameter in shear thickening fluid while opposite trend is observed for shear thinning fluid. Another important observation is that the wall shear stress is significantly decreased by the viscosity ratio parameter β∗ for the first solution and opposite trend is observed for the second solution.
Novel Composites for Wing and Fuselage Applications: Speedy Nonlinear Analysis of Postbuckled Panels in Shear (SNAPPS)

NASA Technical Reports Server (NTRS)

Sharp, Dave; Sobel, Larry

1997-01-01

A simple and rapid analysis method, consisting of a number of modular, 'strength-of-materials-type' models, is presented for predicting the nonlinear response and stiffener separation of postbuckled, flat, composite, shear panels. The analysis determines the maximum principal tensile stress in the skin surface layer under to toe. Failure is said to occur when this stress reaches the mean transverse tensile strength of the layer. The analysis methodology consists of a number of closed-form equations that can easily be used in a 'hand analysis. For expediency, they have been programmed into a preliminary design code called SNAPPS (Speedy Nonlinear Analysis of Postbuckled Panels in Shear), which rapidly predicts postbuckling response of the panel for each value of the applied shear load. SNAPPS response and failure predictions were found to agree well with test results for three panels with widely different geometries, laminates and stiffnesses. Design guidelines are given for increasing the load-carrying capacity of stiffened, composite shear panels.

Influences of the Saharan Air Layer on the Formation and Intensification of Hurricane Isabel (2003): Analysis of AIRS data and Numerical Simulation

NASA Astrophysics Data System (ADS)

Wu, L.; Braun, S. A.

2006-12-01

Over the past two decades, little advance has been made in prediction of tropical cyclone intensity while substantial improvements have been made in forecasting hurricane tracks. One reason is that we don't well understand the physical processes that govern tropical cyclone intensity. Recent studies have suggested that the Saharan Air Layer (SAL) may be yet another piece of the puzzle in advancing our understanding of tropical cyclone intensity change in the Atlantic basin. The SAL is an elevated mixed layer, forming as air moves across the vast Sahara Desert, in particular during boreal summer months. The SAL contains warm, dry air as well as a substantial amount of mineral dust, which can affect radiative heating and modify cloud processes. Using the retrieved temperature and humidity profiles from the AIRS suite on the NASA Aqua satellite, the SAL and its influences on the formation and intensification of Hurricane Isabel (2003) are analyzed and simulated with MM5. When the warmth and dryness of the SAL (the thermodynamic effect) is considered by relaxing the model thermodynamic state to the AIRS profiles, MM5 can well simulate the large-scale flow patterns and the activity of Hurricane Isabel in terms of the timing and location of formation and the subsequent track. Compared with the experiment without nudging the AIRS data, it is suggested that the simulated SAL effect may delay the formation and intensification of Hurricane Isabel. This case study generally confirms the argument by Dunion and Velden (2004) that the SAL can suppress Atlantic tropical cyclone activity by increasing the vertical wind shear, reducing the mean relative humidity, and stabilizing the environment at lower levels.
Improved Bond Strength of Cyanoacrylate Adhesives Through Nanostructured Chromium Adhesion Layers

NASA Astrophysics Data System (ADS)

Gobble, Kyle; Stark, Amelia; Stagon, Stephen P.

2016-09-01

The performance of many consumer products suffers due to weak and inconsistent bonds formed to low surface energy polymer materials, such as polyolefin-based high-density polyethylene (HDPE), with adhesives, such as cyanoacrylate. In this letter, we present an industrially relevant means of increasing bond shear strength and consistency through vacuum metallization of chromium thin films and nanorods, using HDPE as a prototype material and cyanoacrylate as a prototype adhesive. For the as received HDPE surfaces, unmodified bond shear strength is shown to be only 0.20 MPa with a standard deviation of 14 %. When Cr metallization layers are added onto the HDPE at thicknesses of 50 nm or less, nanorod-structured coatings outperform continuous films and have a maximum bond shear strength of 0.96 MPa with a standard deviation of 7 %. When the metallization layer is greater than 50 nm thick, continuous films demonstrate greater performance than nanorod coatings and have a maximum shear strength of 1.03 MPa with a standard deviation of 6 %. Further, when the combination of surface roughening with P400 grit sandpaper and metallization is used, 100-nm-thick nanorod coatings show a tenfold increase in shear strength over the baseline, reaching a maximum of 2.03 MPa with a standard deviation of only 3 %. The substantial increase in shear strength through metallization, and the combination of roughening with metallization, may have wide-reaching implications in consumer products which utilize low surface energy plastics.
Impact of vertical wind shear on roll structure in idealized hurricane boundary layers

NASA Astrophysics Data System (ADS)

Wang, Shouping; Jiang, Qingfang

2017-03-01

Quasi-two-dimensional roll vortices are frequently observed in hurricane boundary layers. It is believed that this highly coherent structure, likely caused by the inflection-point instability, plays an important role in organizing turbulent transport. Large-eddy simulations are conducted to investigate the impact of wind shear characteristics, such as the shear strength and inflection-point level, on the roll structure in terms of its spectral characteristics and turbulence organization. A mean wind nudging approach is used in the simulations to maintain the specified mean wind shear without directly affecting turbulent motions. Enhancing the radial wind shear expands the roll horizontal scale and strengthens the roll's kinetic energy. Increasing the inflection-point level tends to produce a narrow and sharp peak in the power spectrum at the wavelength consistent with the roll spacing indicated by the instantaneous turbulent fields. The spectral tangential momentum flux, in particular, reaches a strong peak value at the roll wavelength. In contrast, the spectral radial momentum flux obtains its maximum at the wavelength that is usually shorter than the roll's, suggesting that the roll radial momentum transport is less efficient than the tangential because of the quasi-two-dimensionality of the roll structure. The most robust rolls are produced in a simulation with the highest inflection-point level and relatively strong radial wind shear. Based on the spectral analysis, the roll-scale contribution to the turbulent momentum flux can reach 40 % in the middle of the boundary layer.
Natural constraints on the rheology of the lower continental crust (Musgrave Ranges, Central Australia)

NASA Astrophysics Data System (ADS)

Hawemann, Friedrich; Mancktelow, Neil; Wex, Sebastian; Camacho, Alfredo; Pennacchioni, Giorgio

2015-04-01

Current models and extrapolated laboratory data generally predict viscous flow in the lower continental crust and any localized brittle deformation at these depths has been proposed to reflect downward propagation of the frictional-viscous transition zone during short-term seismic events and related high strain rates. Better natural constraints on this proposed rheological behaviour can be obtained directly from currently exposed lower crust that has not been strongly overprinted during its exhumation. One of the largest and best preserved lower crustal sections is located in the Musgrave Ranges, Central Australia. The Petermann Orogeny (550 Ma) in this area is characterized by the development of localized shear zones on a wide range of scales, overprinting water-deficient granulites of Musgravian age (1.2 Ga) as well as younger granites and gabbros. Shearing is rarely localized on lithological inhomogeneities, but rather on precursor fractures and on commonly associated pseudotachylytes. The only exception is that older dolerite dykes are often exploited, possibly because they are planar layers of markedly smaller grain size. Sheared pseudotachylyte often appears caramel-coloured in the field and has a fine grained assemblage of Grt+Cpx+Fsp. Multiple generations of pseudotachylyte formed broadly coeval with shearing are indicated by clasts of sheared pseudotachylyte within pseudotachylyte veins that then themselves subsequently sheared. The ductile shear zones formed under sub-eclogitic conditions of ca. 650°C and 1.2 GPa, generally typical of the lower continental crust. However, the P-T conditions during pseudotachylyte formation cannot be readily determined using classical geothermobarometry, because of the fine grain sizes and possible disequilibrium. The software "Xmaptools" (by Pierre Lanari) allows the quantification of X-ray maps produced by EDS or WDS. It provides both very precise definition of local mineral compositions for exchange geothermobarometry on a statistical basis, and an estimate of the bulk pseudotachylyte composition for small areas, avoiding clasts and heterogeneous composition of the former melt. The combination with thermodynamic modelling using PerpleX is used to test the results from geothermobarometry. The estimated conditions are similar to the ductile shear zones and support evidence for synchronous action of brittle faulting and viscous shearing in the lower crust.
Three-dimensional shear wave velocity structure in the Atlantic upper mantle

NASA Astrophysics Data System (ADS)

James, Esther Kezia Candace

Oceanic lithosphere constitutes the upper boundary layer of the Earth's convecting mantle. Its structure and evolution provide a vital window on the dynamics of the mantle and important clues to how the motions of Earth's surface plates are coupled to convection in the mantle below. The three-dimensional shear-velocity structure of the upper mantle beneath the Atlantic Ocean is investigated to gain insight into processes that drive formation of oceanic lithosphere. Travel times are measured for approximately 10,000 fundamental-mode Rayleigh waves, in the period range 30-130 seconds, traversing the Atlantic basin. Paths with >30% of their length through continental upper mantle are excluded to maximize sensitivity to the oceanic upper mantle. The lateral distribution of Rayleigh wave phase velocity in the Atlantic upper mantle is explored with two approaches. One, phase velocity is allowed to vary only as a function of seafloor age. Two, a general two-dimensional parameterization is utilized in order to capture perturbations to age-dependent structure. Phase velocity shows a strong dependence on seafloor age, and removing age-dependent velocity from the 2-D maps highlights areas of anomalously low velocity, almost all of which are proximal to locations of hotspot volcanism. Depth-dependent variations in vertically-polarized shear velocity (Vsv) are determined with two sets of 3-D models: a layered model that requires constant VSV in each depth layer, and a splined model that allows VSV to vary continuously with depth. At shallow depths (˜75 km) the seismic structure shows the expected dependence on seafloor age. At greater depths (˜200 km) high-velocity lithosphere is found only beneath the oldest seafloor; velocity variations beneath younger seafloor may result from temperature or compositional variations within the asthenosphere. The age-dependent phase velocities are used to constrain temperature in the mantle and show that, in contrast to previous results for the Pacific, phase velocities for the Atlantic are not consistent with a half-space cooling model but are best explained by a plate-cooling model with thickness of 75 km and mantle temperature of 1400°C. Comparison with data such as basalt chemistry and seafloor elevation helps to separate thermal and compositional effects on shear velocity.
Creep cavitation bands control porosity and fluid flow in lower crustal shear zones

DOE Office of Scientific and Technical Information (OSTI.GOV)

Menegon, Luca; Fusseis, Florian; Stunitz, Holger

2015-03-01

Shear zones channelize fluid flow in Earth’s crust. However, little is known about deep crustal fluid migration and how fluids are channelized and distributed in a deforming lower crustal shear zone. This study investigates the deformation mechanisms, fluid-rock interaction, and development of porosity in a monzonite ultramylonite from Lofoten, northern Norway. The rock was deformed and transformed into an ultramylonite under lower crustal conditions (temperature = 700–730 °C, pressure = 0.65–0.8 GPa). The ultramylonite consists of feldspathic layers and domains of amphibole + quartz + calcite, which result from hydration reactions of magmatic clinopyroxene. The average grain size in bothmore » domains is <25 mm. Microstructural observations and electron backscatter diffraction analysis are consistent with diffusion creep as the dominant deformation mechanism in both domains. Festoons of isolated quartz grains define C'-type bands in feldspathic layers. These quartz grains do not show a crystallographic preferred orientation. The alignment of quartz grains is parallel to the preferred elongation of pores in the ultramylonites, as evidenced from synchrotron X-ray microtomography. Such C'-type bands are interpreted as creep cavitation bands resulting from diffusion creep deformation associated with grain boundary sliding. Mass-balance calculation indicates a 2% volume increase during the protolith-ultramylonite transformation, which is consistent with synkinematic formation of creep cavities producing dilatancy. Thus, this study presents evidence that creep cavitation bands may control deep crustal porosity and fluid flow. Nucleation of new phases in creep cavitation bands inhibits grain growth and enhances the activity of grain size–sensitive creep, thereby stabilizing strain localization in the polymineralic ultramylonites.« less
Understanding Transition to Turbulence in Shear Layers.

DTIC Science & Technology

1983-05-01

Imperfect bifurcations ; circular Couette flow 24 0.02.06 Interpretations of experiments on Poiseuille flow 26 ( a ) Absence of turbulence far Re > Re 27 (b...instability of the shear layer. This is a very effective com- bination, which fuels most flow-conditioned acoustic resonances, includ- ing organ pipes and...variable density as encountered in aeronautics are *confined to Appendix A , Sections A .07, A .08, A .10- A .15, and Chapter 10. Effects of stratification as
Adiabatic shear mechanisms for the hard cutting process

NASA Astrophysics Data System (ADS)

Yue, Caixu; Wang, Bo; Liu, Xianli; Feng, Huize; Cai, Chunbin

2015-05-01

The most important consequence of adiabatic shear phenomenon is formation of sawtooth chip. Lots of scholars focused on the formation mechanism of sawtooth, and the research often depended on experimental approach. For the present, the mechanism of sawtooth chip formation still remains some ambiguous aspects. This study develops a combined numerical and experimental approach to get deeper understanding of sawtooth chip formation mechanism for Polycrystalline Cubic Boron Nitride (PCBN) tools orthogonal cutting hard steel GCr15. By adopting the Johnson-Cook material constitutive equations, the FEM simulation model established in this research effectively overcomes serious element distortions and cell singularity in high strain domain caused by large material deformation, and the adiabatic shear phenomenon is simulated successfully. Both the formation mechanism and process of sawtooth are simulated. Also, the change features regarding the cutting force as well as its effects on temperature are studied. More specifically, the contact of sawtooth formation frequency with cutting force fluctuation frequency is established. The cutting force and effect of cutting temperature on mechanism of adiabatic shear are investigated. Furthermore, the effects of the cutting condition on sawtooth chip formation are researched. The researching results show that cutting feed has the most important effect on sawtooth chip formation compared with cutting depth and speed. This research contributes a better understanding of mechanism, feature of chip formation in hard turning process, and supplies theoretical basis for the optimization of hard cutting process parameters.
Linear and weakly nonlinear aspects of free shear layer instability, roll-up, subharmonic interaction and wall influence

NASA Technical Reports Server (NTRS)

Cain, A. B.; Thompson, M. W.

1986-01-01

The growth of the momentum thickness and the modal disturbance energies are examined to study the nature and onset of nonlinearity in a temporally growing free shear layer. A shooting technique is used to find solutions to the linearized eigenvalue problem, and pseudospectral weakly nonlinear simulations of this flow are obtained for comparison. The roll-up of a fundamental disturbance follows linear theory predictions even with a 20 percent disturbance amplitude. A weak nonlinear interaction of the disturbance creates a finite-amplitude mean shear stress which dominates the growth of the layer momentum thickness, and the disturbance growth rate changes until the fundamental disturbance dominates. The fundamental then becomes an energy source for the harmonic, resulting in an increase in the growth rate of the subharmonic over the linear prediction even when the fundamental has no energy to give. Also considered are phase relations and the wall influence.
Laminated beams: deflection and stress as a function of epoxy shear modulus

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bialek, J.

1976-01-01

The large toroidal field coil deflections observed during the PLT power test are due to the poor shear behavior of the insulation material used between layers of copper. Standard techniques for analyzing such laminated structures do not account for this effect. This paper presents an analysis of laminated beams that corrects this deficiency. The analysis explicitly models the mechanical behavior of each layer in a laminated beam and hence avoids the pitfalls involved in any averaging technique. In particular, the shear modulus of the epoxy in a laminated beam (consisting of alternate layers of metal and epoxy) may span themore » entire range of values from zero to classical. Solution of the governing differential equations defines the stress, strain, and deflection for any point within a laminated beam. The paper summarizes these governing equations and also includes a parametric study of a simple laminated beam.« less
Magnetic fabrics of drumlins of the Green Bay Lobe: Implications for the bed-deformation model of drumlin formation

NASA Astrophysics Data System (ADS)

Vreeland, Nicholas Paul

According to some theories, subglacial deformation of sediment is the process of sediment transport most responsible for drumlin formation. If so, strain indicators in the sediment should yield deformation patterns that are systematically related to drumlin morphology. Clast fabrics have been used most commonly to make inferences about strain patterns in drumlins but with a wide range of sometimes divergent interpretations. These divergent interpretations reflect, in part, a lack of experimental control on the relationship between the state of strain and resulting fabrics. Herein, fabrics determined from the anisotropy of magnetic susceptibility (AMS) of till within selected drumlins of the Green Bay Lobe are used to study the role of bed deformation in drumlin formation. AMS fabrics are a proxy for fabrics formed by non-equant, silt-sized, magnetite grains. Unlike past fabric studies of drumlins, laboratory deformation experiments conducted with this till provide a quantitative foundation for inferring strain magnitude, shearing direction, and shear-plane orientations from fabrics determined from principal directions of magnetic susceptibility (k1, k2, and k3). Intact till samples were collected from transects in seven drumlins in Dane, Dodge, Jefferson, Waupaca, and Waushara counties of south-central Wisconsin, by both exploiting five existing outcrops and collecting 42 89 mm-diameter cores and sub-sampling them. Overall, ˜2800 samples were collected for AMS analysis, and 112 AMS fabrics were computed. Much of the till sampled (84% of fabrics) has k1 fabric strengths weaker than the lower 95% confidence limit for till (S1< 0.82) sheared to moderate strains (˜10), suggesting the till has been deformed but to strains too small to indicate that bed deformation was the principal till transport mechanism. Three of five drumlins studied have k1 fabric orientations that are not symmetrically disposed about the local flow direction indicated by drumlins. Rather, these fabrics are oriented 7-25° to the southeast of the drumlin orientations, consistent with reinterpreted microfabric data collected from nearby drumlins (Evenson, 1971). Furthermore, in all drumlins, orientations of shear planes inferred from principal susceptibilities deviate markedly from the local surface slopes of drumlins, with a 23.8° average difference between the poles to inferred shear planes and to local slopes. We infer that the drumlin fabric was set by basal till deformation during glacier flow to the southeast prior to drumlin formation and that drumlinization did not significantly reset the fabric. Thus, these drumlins are inferred to have been formed by differential erosion of a pre-existing till layer by processes unrelated to bed deformation.
Direct in situ observation of metallic glass deformation by real-time nano-scale indentation

NASA Astrophysics Data System (ADS)

Gu, Lin; Xu, Limei; Zhang, Qingsheng; Pan, Deng; Chen, Na; Louzguine-Luzgin, Dmitri V.; Yao, Ke-Fu; Wang, Weihua; Ikuhara, Yuichi

2015-03-01

A common understanding of plastic deformation of metallic glasses (MGs) at room temperature is that such deformation occurs via the formation of runaway shear bands that usually lead to catastrophic failure of MGs. Here we demonstrate that inhomogeneous plastic flow at nanoscale can evolve in a well-controlled manner without further developing of shear bands. It is suggested that the sample undergoes an elasto-plastic transition in terms of quasi steady-state localized shearing. During this transition, embryonic shear localization (ESL) propagates with a very slow velocity of order of ~1 nm/s without the formation of a hot matured shear band. This finding further advances our understanding of the microscopic deformation process associated with the elasto-plastic transition and may shed light on the theoretical development of shear deformation in MGs.
Seismic imaging of the lithosphere beneath Hudson Bay: Episodic growth of the Laurentian mantle keel

NASA Astrophysics Data System (ADS)

Darbyshire, Fiona A.; Eaton, David W.; Bastow, Ian D.

2013-07-01

The Hudson Bay basin in northern Canada conceals one of the major collisional zones of the Canadian Shield, the Trans-Hudson Orogen (THO), which marks the Paleoproterozoic collision between the Archean Superior and Western Churchill cratons at ˜1.9-1.8Ga. Improved knowledge of upper mantle structure beneath the region is essential to establish the nature of the THO, specifically whether Himalayan-style plate tectonics operated in Paleoproterozoic times. Detailed seismological constraints on lithospheric architecture are also required to advance our understanding of the mechanism and timing of keel formation. We use surface wave tomography to illuminate new details of the lithospheric architecture of the Hudson Bay region, resolving both seismic wavespeed and azimuthal anisotropy. Phase velocity maps are calculated from fundamental-mode Rayleigh wave dispersion curves, then used to construct a 3D model exploring upper mantle structure to depths of ˜300km. Fast shear wavespeeds suggest a lithospheric thickness varying from ˜180km to almost 280 km beneath the Hudson Bay region. The new study confirms previous inferences that there is no correlation between crustal ages and lithospheric thickness. Patterns of shear wavespeed and azimuthal anisotropy indicate a layered lithosphere. In the uppermost mantle, both the highest velocities and the anisotropic fast directions wrap around the Bay. This structure is likely related to the formation processes of the Paleozoic intracratonic basin. At greater depth (˜70-150km) we resolve two high-wavespeed cores separated by a relatively narrow near-vertical lower-velocity curtain. This internal architecture is suggested to result from the terminal phase of a modern-style plate-tectonic collision between the Archean Superior and Churchill cratons during the Trans-Hudson orogeny, entrapping juvenile Proterozoic material. The lower lithosphere (≥160km depth) has a relatively homogeneous wavespeed structure across the region, with distinct patterns of anisotropy closely resembling the subsurface geometry of the THO. We interpret this basal layer as juvenile or reworked material accreted to the base of the existing cratonic lithosphere during or soon after the Trans-Hudson orogeny. The formation of the Laurentian keel thus likely occurred in multiple phases, with a basal layer developing in post-Archean times, during the THO.
High-strength, surface-porous polyether-ether-ketone for load-bearing orthopedic implants.

PubMed

Evans, Nathan T; Torstrick, F Brennan; Lee, Christopher S D; Dupont, Kenneth M; Safranski, David L; Chang, W Allen; Macedo, Annie E; Lin, Angela S P; Boothby, Jennifer M; Whittingslow, Daniel C; Carson, Robert A; Guldberg, Robert E; Gall, Ken

2015-02-01

Despite its widespread clinical use in load-bearing orthopedic implants, polyether-ether-ketone (PEEK) is often associated with poor osseointegration. In this study, a surface-porous PEEK material (PEEK-SP) was created using a melt extrusion technique. The porous layer was 399.6±63.3 μm thick and possessed a mean pore size of 279.9±31.6 μm, strut spacing of 186.8±55.5 μm, porosity of 67.3±3.1% and interconnectivity of 99.9±0.1%. Monotonic tensile tests showed that PEEK-SP preserved 73.9% of the strength (71.06±2.17 MPa) and 73.4% of the elastic modulus (2.45±0.31 GPa) of as-received, injection-molded PEEK. PEEK-SP further demonstrated a fatigue strength of 60.0 MPa at one million cycles, preserving 73.4% of the fatigue resistance of injection-molded PEEK. Interfacial shear testing showed the pore layer shear strength to be 23.96±2.26 MPa. An osseointegration model in the rat revealed substantial bone formation within the pore layer at 6 and 12 weeks via microcomputed tomography and histological evaluation. Ingrown bone was more closely apposed to the pore wall and fibrous tissue growth was reduced in PEEK-SP when compared to non-porous PEEK controls. These results indicate that PEEK-SP could provide improved osseointegration while maintaining the structural integrity necessary for load-bearing orthopedic applications. Copyright © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Structural basis for unique hierarchical cylindrites induced by ultrahigh shear gradient in single natural fiber reinforced poly(lactic acid) green composites.

PubMed

Xu, Huan; Xie, Lan; Jiang, Xin; Hakkarainen, Minna; Chen, Jing-Bin; Zhong, Gan-Ji; Li, Zhong-Ming

2014-05-12

A local shear flow field was feasibly generated by pulling the ramie fiber in single fiber reinforced poly(lactic acid) (PLA) composites. This was featured by an ultrahigh shear gradient with a maximum shear rate up to 1500 s(-1), a level comparable to that frequently occurring during the practical polymer processing. To distinguish shear-induced self-nucleation and ramie fiber-induced heterogeneous nucleation, the shear history was classified by pulling the fiber for 5 s (pulled sample) and pulling out the fiber during 10 s (pulled-out sample), while the static fiber-induced crystallization was carried out as the counterpart. As a result of the ultrahigh shear gradient, the combination of primary shear-induced nucleation in the central region and secondary nucleation in the outer layer assembled the unique hierarchical superstructures. By comparing the architectural configurations of interphases formed in the static, pulled, and pulled-out samples, it was shown that the hierarchical cylindrites underwent the process of self-nucleation driven by the applied shear flow, very different from the formation of fiber-induced transcrystallinity (TC) triggered by the heterogeneous nucleating sites at the static fiber surface. The twisting of transcrystallized lamellae may take place due to the spatial hindrance induced by the incredibly dense nuclei under the intense shearing flow, as observed in the synchrotron X-ray diffraction patterns. The influence of chain characteristics on the crystalline morphology was further explored by adding a small amount of poly(ethylene glycol) (PEG) to enhance the molecular mobility of PLA. It was of interest to find that the existence of PEG not only facilitated the growth rates of TC and cylindrites but also improved the preferential orientation of PLA chains and thus expanded the ordered regions. We unearthed lamellar units that were composed of rich fibrillar extended chain crystals (diameter of 50-80 nm). These results are of importance to shed light on tailoring crystalline morphology for natural fibers reinforced green composite materials. Of immense practical significance, too, is the crystalline evolution that has been tracked in the simple model penetrated with an ultrahigh shear gradient, which researchers have so far been unable to replicate during the practical melt processing, such as extrusion and injection molding.
Evidence for Seismic and Aseismic Slip along a Foreland Thrust Fault, Southern Appalachians

NASA Astrophysics Data System (ADS)

Newman, J.; Wells, R. K.; Holyoke, C. W.; Wojtal, S. F.

2013-12-01

Studies of deformation along ancient thrust faults form the basis for much of our fundamental understanding of fault and shear zone processes. These classic studies interpreted meso- and microstructures as formed during aseismic creep. Recent experimental studies, and studies of naturally deformed rocks in seismically active regions, reveal similar microstructures to those observed locally in a carbonate foreland thrust from the southern Appalachians, suggesting that this thrust fault preserves evidence of both seismic and aseismic deformation. The Copper Creek thrust, TN, accommodated 15-20 km displacement, at depths of 4-6 km, as estimated from balanced cross-sections. At the Diggs Gap exposure of the Copper Creek thrust, an approximately 2 cm thick, vein-like shear zone separates shale layers in the hanging wall and footwall. The shear zone is composed of anastomosing layers of ultrafine-grained calcite and/or shale as well as aggregate clasts of ultrafine-grained calcite or shale. The boundary between the shear zone and the hanging wall is sharp, with slickensides along the boundary, parallel to the shear zone movement direction. A 350 μm-thick layer of ultrafine-grained calcite separates the shear zone and the footwall. Fault parallel and perpendicular calcite veins are common in the footwall and increase in density towards the shear zone. Microstructures within the vein-like shear zone that are similar to those observed in experimental studies of unstable slip include: ultrafine-grained calcite (~0.34 μm), nano-aggregate clasts (100-300 nm), injection structures, and vein-wrapped and matrix-wrapped clasts. Not all structures within the shear zone and ultrafine-grained calcite layer suggest seismic slip. Within the footwall veins and calcite aggregate clasts within the shear zone, pores at twin-twin intersections suggest plasticity-induced fracturing as the main mechanism for grain size reduction. Interpenetrating grain boundaries in ultrafine-grained calcite and a lack of a lattice preferred orientation suggest ultrafine-grained calcite deformed by diffusion creep accommodated grain boundary sliding. These structures suggest a strain-rate between 10-15 - 10-11 s-1, using calcite flow laws at temperatures 150-250 °C. Microstructures suggest both seismic and aseismic slip along this ancient fault zone. During periods of aseismic slip, deformation is accommodated by plasticity-induced fracturing and diffusion creep. Calcite veins suggest an increase in pore-fluid pressure, contributing to fluidized and unstable flow, but also providing the calcite that deformed by diffusion creep during aseismic creep.
An Experimental Study of a Separated/Reattached Flow Behind a Backward-Facing Step. Re(sub h) = 37,000

NASA Technical Reports Server (NTRS)

Jovic, Srba

1996-01-01

An experimental study was carried out to investigate turbulent structure of a two-dimensional incompressible separating/reattaching boundary layer behind a backward-facing step. Hot-wire measurement technique was used to measure three Reynolds stresses and higher-order mean products of velocity fluctuations. The Reynolds number, Re(sub h), based on the step height, h, and the reference velocity, U(sub 0), was 37,000. The upstream oncoming flow was fully developed turbulent boundary layer with the Re(sub theta) = 3600. All turbulent properties, such as Reynolds stresses, increase dramatically downstream of the step within an internally developing mixing layer. Distributions of dimensionless mean velocity, turbulent quantities and antisymmetric distribution of triple velocity products in the separated free shear layer suggest that the shear layer above the recirculating region strongly resembles free-shear mixing layer structure. In the reattachment region close to the wall, turbulent diffusion term balances the rate of dissipation since advection and production terms appear to be negligibly small. Further downstream, production and dissipation begin to dominate other transport processes near the wall indicating the growth of an internal turbulent boundary layer. In the outer region, however, the flow still has a memory of the upstream disturbance even at the last measuring station of 51 step-heights. The data show that the structure of the inner layer recovers at a much faster rate than the outer layer structure. The inner layer structure resembles the near-wall structure of a plane zero pressure-gradient turbulent boundary layer (plane TBL) by 25h to 30h, while the outer layer structure takes presumably over 100h.
Beyond the Electrostatic Ionosphere: Dynamic Coupling of the Magnetosphere and Ionosphere

NASA Astrophysics Data System (ADS)

Lysak, R. L.; Song, Y.

2017-12-01

Many models of magnetospheric dynamics treat the ionosphere as a height-integrated slab in which the electric fields are electrostatic. However, in dynamic situations, the coupling between magnetosphere and ionosphere is achieved by the propagation of shear Alfvén waves. Hall effects lead to a coupling of shear Alfvén and fast mode waves, resulting in an inductive electric field and a compressional component of the magnetic field. It is in fact this compressional magnetic field that is largely responsible for the magnetic fields seen on the ground. A fully inductive ionosphere model is required to describe this situation. The shear Alfvén waves are affected by the strong gradient in the Alfvén speed above the ionosphere, setting up the ionospheric Alfvén resonator with wave periods in the 1-10 second range. These waves develop a parallel electric field on small scales that can produce a broadband acceleration of auroral electrons, which form the Alfvénic aurora. Since these electrons are relatively low in energy (hundreds of eV to a few keV), they produce auroral emissions as well as ionization at higher altitudes. Therefore, they can produce localized columns of ionization that lead to structuring in the auroral currents due to phase mixing or feedback interactions. This implies that the height-integrated description of the ionosphere is not appropriate in these situations. These considerations suggest that the Alfvénic aurora may, at least in some cases, act as a precursor to the development of a quasi-static auroral arc. The acceleration of electrons and ions produces a density cavity at higher altitudes that favors the formation of parallel electric fields. Furthermore, the precipitating electrons will produce secondary and backscattered electrons that provide a necessary population for the formation of double layers. These interactions strongly suggest that the simple electrostatic boundary condition often assumed is inadequate to describe auroral arc formation.
Shear-induced partial translational ordering of a colloidal solid

NASA Astrophysics Data System (ADS)

Ackerson, B. J.; Clark, N. A.

1984-08-01

Highly charged submicrometer plastic spheres suspended in water at low ionic strength will order spontaneously into bcc crystals or polycrystals. A simple linear shear orients and disorders these crystals by forcing (110) planes to stack normal to the shear gradient and to slide relative to each other with a <111> direction parallel to the solvent flow. In this paper we analyze in detail the disordering and flow processes occurring beyond the intrinsic elastic limit of the bcc crystal. We are led to a model in which the flow of a colloidal crystal is interpreted as a fundamentally different process from that found in atomic crystals. In the colloidal crystal the coupling of particle motion to the background fluid forces a homogeneous flow, where every layer is in motion relative to its neighboring layers. In contrast, the plastic flow in an atomic solid is defect mediated flow. At the lowest applied stress, the local bcc order in the colloidal crystal exhibits shear strains both parallel and perpendicular to the direction of the applied stress. The magnitude of these deformations is estimated using the configurational energy for bcc and distorted bcc crystals, assuming a screened Coulomb pair interaction between colloidal particles. As the applied stress is increased, the intrinsic elastic limit of the crystal is exceeded and the crystal begins to flow with adjacent layers executing an oscillatory path governed by the balance of viscous and screened Coulomb forces. The path takes the structure from the bcc1 and bcc2 twins observed at zero shear to a distorted two-dimensional hcp structure at moderate shear rates, with a loss of interlayer registration as the shear is increased. This theoretical model is consistent with other experimental observations, as well.
Kelvin-Helmholtz instability in a single-component atomic superfluid

NASA Astrophysics Data System (ADS)

Baggaley, A. W.; Parker, N. G.

2018-05-01

We demonstrate an experimentally feasible method for generating the classical Kelvin-Helmholtz instability in a single-component atomic Bose-Einstein condensate. By progressively reducing a potential barrier between two counterflowing channels, we seed a line of quantized vortices, which precede to form progressively larger clusters, mimicking the classical roll-up behavior of the Kelvin-Helmholtz instability. This cluster formation leads to an effective superfluid shear layer, formed through the collective motion of many quantized vortices. From this we demonstrate a straightforward method to measure the effective viscosity of a turbulent quantum fluid in a system with a moderate number of vortices, within the range of current experimental capabilities.

Shear-induced morphology transition and microphase separation in a lamellar phase doped with clay particles.

PubMed

Nettesheim, Florian; Grillo, Isabelle; Lindner, Peter; Richtering, Walter

2004-05-11

We report on the influence of shear on a nonionic lamellar phase of tetraethyleneglycol monododecyl ether (C12E4) in D2O containing clay particles (Laponite RD). The system was studied by means of small-angle light scattering (SALS) and small-angle neutron scattering (SANS) under shear. The SANS experiments were conducted using a H2O/D2O mixture of the respective scattering length density to selectively match the clay scattering. The rheological properties show the familiar shear thickening regime associated with the formation of multilamellar vesicles (MLVs) and a shear thinning regime at higher stresses. The variation of viscosity is less pronounced as commonly observed. In the shear thinning regime, depolarized SALS reveals an unexpectedly strong variation of the MLV size. SANS experiments using the samples with lamellar contrast reveal a change in interlamellar spacing of up to 30% at stresses that lead to MLV formation. This change is much more pronounced than the change observed, when shear suppresses thermal bilayer undulations. Microphase separation occurs, and as a consequence, the lamellar spacing decreases drastically. The coincidence of the change in lamellar spacing and the onset of MLV formation is a strong indication for a morphology-driven microphase separation.
High attenuation in MgSiO3 post-perovskite due to [100] dislocation glide under D'' conditions: an atomic scale study

NASA Astrophysics Data System (ADS)

Cordier, P.; Goryaeva, A.; Carrez, P.

2016-12-01

Dislocation motion in crystalline materials represents one of the most efficient mechanisms to produce plastic shear, the key mechanism for CPO development. Previous atomistic simulations show that MgSiO3 ppv is characterized by remarkably low lattice friction opposed to the glide of straight [100] screw dislocations in (010), while glide in (001) requires one order of magnitude larger stress values [1]. At finite temperature, dislocation glide occurs through nucleation and propagation of kink-pairs, i.e. dislocation does not move as a straight line, but partly bows out over the Peierls potential. We propose a theoretical study of a kink-pair formation mechanism for [100] screw dislocations in MgSiO3 ppv employing the line tension (LT) model [2] in conjunction with ab-initio atomic-scale modeling. The dislocation line tension, which plays a key role in dislocation dynamics, is computed at atomic scale as the energy increase resulting from individual atomic displacements due to the nucleation of a bow out. The estimated kink-pair formation enthalpy gives an access to evolution of critical resolved shear stress (CRSS) with temperature. Our results clearly demonstrate that at the lower mantle conditions, lattice friction in ppv vanishes for temperatures above ca. 600 K, i.e. ppv deforms in the athermal regime in contrast to the high-lattice friction bridgmanite [3]. Moreover, in the Earth's mantle, high-pressure Mg-ppv can be expected to be as ductile as MgO. Our simulations demonstrate that ppv contributes to a weak layer at the base of the mantle which is likely to promote alignment of (010) planes. In addition to that, we show that the high mobility of [100] dislocations results in a decrease of the apparent shear modulus (up to 15%) which contributes to a decrease of the shear wave velocity of about 7% and suggest that ppv induces energy dissipation and strong seismic attenuation in the D" layer. References[1] Goryaeva A, Carrez Ph & Cordier P (2015) Modeling defects and plasticity in MgSiO3 post-perovskite: Part 2 - screw and edge [100] dislocations. Phys. Chem. Miner. 45:793-803 [2] Seeger A (1984) in "Dislocations", CNRS, Paris, p. 141. [3] Kraych A, Carrez Ph & Cordier P (2016) On dislocation glide in MgSiO3 bridgmanite at high pressure and high-temperature. Earth Planet. Sci. Lett. submitted.
Reduction of friction stress of ethylene glycol by attached hydrogen ions

PubMed Central

Li, Jinjin; Zhang, Chenhui; Deng, Mingming; Luo, Jianbin

2014-01-01

In the present work, it is shown that the friction stress of ethylene glycol can decrease by an order of magnitude to achieve superlubricity if there are hydrogen ions attached on the friction surfaces. An ultra-low friction coefficient (μ = 0.004) of ethylene glycol between Si3N4 and SiO2 can be obtained with the effect of hydrogen ions. Experimental result indicates that the hydrogen ions adsorbed on the friction surfaces forming a hydration layer and the ethylene glycol in the contact region forming an elastohydrodynamic film are the two indispensable factors for the reduction of friction stress. The mechanism of superlubricity is attributed to the extremely low shear strength of formation of elastohydrodynamic film on the hydration layer. This finding may introduce a new approach to reduce friction coefficient of liquid by attaching hydrogen ions on friction surfaces. PMID:25428584
Transformation of multiwall carbon nanotubes to onions with layers cross-linked by sp3 bonds under high pressure and shear deformation

NASA Astrophysics Data System (ADS)

Pankov, A. M.; Bredikhina, A. S.; Kulnitskiy, B. A.; Perezhogin, I. A.; Skryleva, E. A.; Parkhomenko, Yu. N.; Popov, M. Yu.; Blank, V. D.

2017-08-01

A pressure-induced phase transition of multiwall carbon nanotubes (MWNT) to a new structure at room temperature is studied using a shear diamond anvil cell, X-ray photoelectron spectra (XPS), transmission electron microscope (TEM) and Raman procedures. We observe a cardinal pressure-induced change in the nanoparticles shape from multi-shell tubes to multi-shell spheres. MWNT transforms to onions with layers cross-linked by sp3 bonds under the 45-65 GPa compressive stress combined with shear deformation at room temperature. TEM and XPS results show that about 40% of the carbon atoms in the new phase are sp3-bounded.
A novel boundary layer sensor utilizing domain switching in ferroelectric liquid crystals

NASA Technical Reports Server (NTRS)

Parmar, D. S.

1991-01-01

This paper describes the design and the principles of operation of a novel sensor for the optical detection of a shear stress field induced by air or gas flow on a rigid surface. The detection relies on the effects of shear-induced optical switching in ferroelectric liquid crystals. It is shown that the method overcomes many of the limitations of similar measuring techniques including those using cholesteric liquid crystals. The present method offers a preferred alternative for flow visualization and skin friction measurements in wind-tunnel experiments on laminar boundary layer transition investigations. A theoretical model for the optical response to shear stress is presented together with a schematic diagram of the experimental setup.
The measurement of shear stress and total heat flux in a nonadiabatic turbulent hypersonic boundary layer

NASA Technical Reports Server (NTRS)

Mikulla, V.; Horstman, C. C.

1975-01-01

Turbulent shear stress and direct turbulent total heat-flux measurements have been made across a nonadiabatic, zero pressure gradient, hypersonic boundary layer by using specially designed hot-wire probes free of strain-gauging and wire oscillation. Heat-flux measurements were in reasonably good agreement with values obtained by integrating the energy equation using measured profiles of velocity and temperature. The shear-stress values deduced from the measurements, by assuming zero correlation of velocity and pressure fluctuations, were lower than the values obtained by integrating the momentum equation. Statistical properties of the cross-correlations are similar to corresponding incompressible measurements at approximately the same momentum-thickness Reynolds number.
Wall shear stress estimates in coronary artery constrictions

NASA Technical Reports Server (NTRS)

Back, L. H.; Crawford, D. W.

1992-01-01

Wall shear stress estimates from laminar boundary layer theory were found to agree fairly well with the magnitude of shear stress levels along coronary artery constrictions obtained from solutions of the Navier Stokes equations for both steady and pulsatile flow. The relatively simple method can be used for in vivo estimates of wall shear stress in constrictions by using a vessel shape function determined from a coronary angiogram, along with a knowledge of the flow rate.
Longitudinally polarized shear wave optical coherence elastography (Conference Presentation)

NASA Astrophysics Data System (ADS)

Miao, Yusi; Zhu, Jiang; Qi, Li; Qu, Yueqiao; He, Youmin; Gao, Yiwei; Chen, Zhongping

2017-02-01

Shear wave measurement enables quantitative assessment of tissue viscoelasticity. In previous studies, a transverse shear wave was measured using optical coherence elastography (OCE), which gives poor resolution along the force direction because the shear wave propagates perpendicular to the applied force. In this study, for the first time to our knowledge, we introduce an OCE method to detect a longitudinally polarized shear wave that propagates along the force direction. The direction of vibration induced by a piezo transducer (PZT) is parallel to the direction of wave propagation, which is perpendicular to the OCT beam. A Doppler variance method is used to visualize the transverse displacement. Both homogeneous phantoms and a side-by-side two-layer phantom were measured. The elastic moduli from mechanical tests closely matched to the values measured by the OCE system. Furthermore, we developed 3D computational models using finite element analysis to confirm the shear wave propagation in the longitudinal direction. The simulation shows that a longitudinally polarized shear wave is present as a plane wave in the near field of planar source due to diffraction effects. This imaging technique provides a novel method for the assessment of elastic properties along the force direction, which can be especially useful to image a layered tissue.
Use of shear horizontal waves to distinguish adhesive thickness variation from reduction in bonding strength.

PubMed

Predoi, Mihai Valentin; Ech Cherif El Kettani, Mounsif; Leduc, Damien; Pareige, Pascal; Coné, Khadidiatou

2015-08-01

The capability of shear horizontal (SH) guided waves, to evaluate geometrical imperfections in a bonding layer, is investigated. SH waves are used in a three-layer structure in which the adhesive layer has variable thickness. It is proven that the SH waves are adapting to the local thickness of the adhesive layer (adiabatic waves). This is particularly useful in case of small thickness variations, which is of technical interest. The influence of thickness and stiffness of the adhesive layer on the wavenumbers are investigated. The selected SH2 mode is proven to be very sensitive to the adhesive layer thickness variation in the given frequency range and considerably less sensitive to the adhesive stiffness variation. This property is due to its specific displacement field and is important in practical applications, such as inspection techniques based on SH waves, in order to avoid false alarms.
Turbulence modeling of free shear layers for high-performance aircraft

NASA Technical Reports Server (NTRS)

Sondak, Douglas L.

1993-01-01

The High Performance Aircraft (HPA) Grand Challenge of the High Performance Computing and Communications (HPCC) program involves the computation of the flow over a high performance aircraft. A variety of free shear layers, including mixing layers over cavities, impinging jets, blown flaps, and exhaust plumes, may be encountered in such flowfields. Since these free shear layers are usually turbulent, appropriate turbulence models must be utilized in computations in order to accurately simulate these flow features. The HPCC program is relying heavily on parallel computers. A Navier-Stokes solver (POVERFLOW) utilizing the Baldwin-Lomax algebraic turbulence model was developed and tested on a 128-node Intel iPSC/860. Algebraic turbulence models run very fast, and give good results for many flowfields. For complex flowfields such as those mentioned above, however, they are often inadequate. It was therefore deemed that a two-equation turbulence model will be required for the HPA computations. The k-epsilon two-equation turbulence model was implemented on the Intel iPSC/860. Both the Chien low-Reynolds-number model and a generalized wall-function formulation were included.
The dependence of the strength and thickness of field-aligned currents on solar wind and ionospheric parameters

PubMed Central

Johnson, Jay R.; Wing, Simon

2017-01-01

Sheared plasma flows at the low-latitude boundary layer (LLBL) correlate well with early afternoon auroral arcs and upward field-aligned currents. We present a simple analytic model that relates solar wind and ionospheric parameters to the strength and thickness of field-aligned currents (Λ) in a region of sheared velocity, such as the LLBL. We compare the predictions of the model with DMSP observations and find remarkably good scaling of the upward region 1 currents with solar wind and ionospheric parameters in region located at the boundary layer or open field lines at 1100–1700 magnetic local time. We demonstrate that Λ~nsw−0.5 and Λ ~ L when Λ/L < 5 where L is the auroral electrostatic scale length. The sheared boundary layer thickness (Δm) is inferred to be around 3000 km, which appears to have weak dependence on Vsw. J‖ has dependencies on Δm, Σp, nsw, and Vsw. The analytic model provides a simple way to organize data and to infer boundary layer structures from ionospheric data. PMID:29057194
How do closed-compact multi-lamellar droplets form under shear flow? A possible mechanism

NASA Astrophysics Data System (ADS)

Courbin, L.; Pons, R.; Rouch, J.; Panizza, P.

2003-01-01

The formation of closed-compact multi-lamellar droplets obtained upon shearing both a lamellar phase (Lα) and a two-phase separated lamellar-sponge (Lα-L3) mixture is investigated as a function of the shear rate dot gamma, using small-angle light scattering (SALS) and cross-polarized optical microscopy. In both systems the formation of droplets occurs homogeneously in the cell at a well-defined wave vector qe propto dot gamma1/3 via a strain-controlled process. These results suggest that the formation of droplets may be monitored in both systems by a buckling instability of the lamellae as predicted from a recent theory.
A novel method for visualising and quantifying through-plane skin layer deformations.

PubMed

Gerhardt, L-C; Schmidt, J; Sanz-Herrera, J A; Baaijens, F P T; Ansari, T; Peters, G W M; Oomens, C W J

2012-10-01

Skin is a multilayer composite and exhibits highly non-linear, viscoelastic, anisotropic material properties. In many consumer product and medical applications (e.g. during shaving, needle insertion, patient re-positioning), large tissue displacements and deformations are involved; consequently large local strains in the skin tissue can occur. Here, we present a novel imaging-based method to study skin deformations and the mechanics of interacting skin layers of full-thickness skin. Shear experiments and real-time video recording were combined with digital image correlation and strain field analysis to visualise and quantify skin layer deformations during dynamic mechanical testing. A global shear strain of 10% was applied to airbrush-patterned porcine skin (thickness: 1.2-1.6mm) using a rotational rheometer. The recordings were analysed with ARAMIS image correlation software, and local skin displacement, strain and stiffness profiles through the skin layers determined. The results of this pilot study revealed inhomogeneous skin deformation, characterised by a gradual transition from a low (2.0-5.0%; epidermis) to high (10-22%; dermis) shear strain regime. Shear moduli ranged from 20 to 130kPa. The herein presented method will be used for more extended studies on viable human skin, and is considered a valuable foundation for further development of constitutive models which can be used in advanced finite element analyses of skin. Copyright © 2012 Elsevier Ltd. All rights reserved.
Measurement of Phased Array Point Spread Functions for Use with Beamforming

NASA Technical Reports Server (NTRS)

Bahr, Chris; Zawodny, Nikolas S.; Bertolucci, Brandon; Woolwine, Kyle; Liu, Fei; Li, Juan; Sheplak, Mark; Cattafesta, Louis

2011-01-01

Microphone arrays can be used to localize and estimate the strengths of acoustic sources present in a region of interest. However, the array measurement of a region, or beam map, is not an accurate representation of the acoustic field in that region. The true acoustic field is convolved with the array s sampling response, or point spread function (PSF). Many techniques exist to remove the PSF's effect on the beam map via deconvolution. Currently these methods use a theoretical estimate of the array point spread function and perhaps account for installation offsets via determination of the microphone locations. This methodology fails to account for any reflections or scattering in the measurement setup and still requires both microphone magnitude and phase calibration, as well as a separate shear layer correction in an open-jet facility. The research presented seeks to investigate direct measurement of the array's PSF using a non-intrusive acoustic point source generated by a pulsed laser system. Experimental PSFs of the array are computed for different conditions to evaluate features such as shift-invariance, shear layers and model presence. Results show that experimental measurements trend with theory with regard to source offset. The source shows expected behavior due to shear layer refraction when observed in a flow, and application of a measured PSF to NACA 0012 aeroacoustic trailing-edge noise data shows a promising alternative to a classic shear layer correction method.
Fluid Shearing for Accelerated Chemical Reactions - Fluid Mechanics in the VFD

NASA Astrophysics Data System (ADS)

Leivadarou, Evgenia; Dalziel, Stuart; G. K. Batchelor Laboratory, Department of Applied Mathematics; Theoretical Physics Team

2016-11-01

The Vortex Fluidic Device (VFD) is a rapidly rotating tube that can operate under continuous flow with a jet feeding liquid reactants to the tube's hemispherical base. It is a new 'green' approach to the organic synthesis with many industrial applications in cosmetics, protein folding and pharmaceutical production. The rate of reaction in the VFD is enhanced when the collision rate is increased. The aim of the project is to explain the fluid mechanics and optimize the performance of the device. One contribution to the increased yield is believed to be the high levels of shear stress. We attempt to enhance the shear stress by achieving high velocity gradients in the boundary layers. Another factor is the uncontrolled vibrations due to imperfections in the bearings and therefore it is important to assess their influence in the initial spreading. The surface area of the film should be maximized with respect to the rotation rate, geometry and orientation of the tube, flow rate, wettability and contact line dynamics. Experiments are presented for a flat disk and a curved bowl, establishing the optimum height of release, rotation rate and tube orientation. Vibrations were imposed to investigate the changes in the film formation. We discuss the implications of our results in the VFD.
FLUID EVOLUTION AND MINERAL REACTIONS DURING SHEAR ZONE FORMATION AT NUSFJORD, LOFOTEN, NORWAY (Invited)

NASA Astrophysics Data System (ADS)

Kullerud, K.

2009-12-01

At Nusfjord in Lofoten, Norway, three 0.3 - 3 m thick shear zones occur in a gabbro-anorthosite. During deformation, the shear zones were infiltrated by a hydrous fluid enriched in Cl. In the central parts of the shear zones, fluid-rock interaction resulted in complete break-down of the primary mafic silicates. Complete hydration of these minerals to Cl-free amphibole and biotite suggests that the hydrous fluid was present in excess during deformation in these parts of the shear zones. Along the margins of the shear zones, however, the igneous mafic silicates (Cpx, Bt, Opx) were only partly overgrown by hydrous minerals. Here, Cl-enriched minerals (Amph, Bt, Scp, Ap) can be observed. Amphibole shows compositions covering the range 0.1 - 4.0 wt % Cl within single thin sections. Mineral textures and extreme compositional variations of the Cl-bearing minerals indicate large chemical gradients of the fluid phase. Relics of primary mafic silicates and compositionally zoned reaction coronas around primary mafic silicates suggest that the free fluid was totally consumed before the alteration of the primary phases were completed. The extreme variations in the Cl-content of amphibole are inferred to monitor a gradual desiccation of the Cl-bearing grain-boundary fluid during fluid-mineral reactions accordingly: 1) The first amphibole that formed during the reactions principally extracted water from the fluid, resulting in a slight increase in the Cl content of the fluid. 2) Continued amphibole-forming reactions resulted in gradual consumption of the free fluid phase, principally by extracting water from the fluid, resulting in an increase in its Cl-content. Higher Cl-content of the fluid resulted in higher Cl-content of the equilibrium amphibole. 3) The most Cl-enriched amphibole (4 wt % Cl) formed in equilibrium with the last volumes of the grain-boundary fluid, which had evolved to a highly saline solution. Mineral reactions within a 1-2 thick zone of the host rock along the contact to the shear zones indicate a more complicated involvement of fluids during shear zone formation than described above. Apparently, fluids have been transported laterally from the outer parts of the shear zones into the gabbro-anorthosite along thin recrystallized zones of plagioclase. The fluid that infiltrated the undeformed host rock of the shear zones resulted in formation of Cl-free amphibole and garnet between the primary mafic minerals and plagioclase. A working hypothesis is that narrow fractures formed within the host rock, outside the sheared rock during shear zone formation. During shear zone formation, the central parts of the shear zones were completely hydrated by an externally derived Cl-bearing hydrous fluid. Some of the fluid migrated to the marginal parts of the shear zones and evolved to a highly saline solution. However, during desiccation of the fluid along the marginal parts of the shear zones, some of the fluid escaped along narrow fractures into the host rock of the shear zones. The Cl-free amphibole that formed from this fluid suggests that the narrow pathways of the fluid provided a path for water transport, but acted as a filter for the much larger ions of Cl.
Mechanisms of lamellar collagen formation in connective tissues.

PubMed

Ghazanfari, Samaneh; Khademhosseini, Ali; Smit, Theodoor H

2016-08-01

The objective of tissue engineering is to regenerate functional tissues. Engineering functional tissues requires an understanding of the mechanisms that guide the formation and evolution of structure in the extracellular matrix (ECM). In particular, the three-dimensional (3D) collagen fiber arrangement is important as it is the key structural determinant that provides mechanical integrity and biological function. In this review, we survey the current knowledge on collagen organization mechanisms that can be applied to create well-structured functional lamellar tissues and in particular intervertebral disc and cornea. Thus far, the mechanisms behind the formation of cross-aligned collagen fibers in the lamellar structures is not fully understood. We start with cell-induced collagen alignment and strain-stabilization behavior mechanisms which can explain a single anisotropically aligned collagen fiber layer. These mechanisms may explain why there is anisotropy in a single layer in the first place. However, they cannot explain why a consecutive collagen layer is laid down with an alternating alignment. Therefore, we explored another mechanism, called liquid crystal phasing. While dense concentrations of collagen show such behavior, there is little evidence that the conditions for liquid crystal phasing are actually met in vivo. Instead, lysyl aldehyde-derived collagen cross-links have been found essential for correct lamellar matrix deposition. Furthermore, we suggest that supra-cellular (tissue-level) shear stress may be instrumental in the alignment of collagen fibers. Understanding the potential mechanisms behind the lamellar collagen structure in connective tissues will lead to further improvement of the regeneration strategies of functional complex lamellar tissues. Copyright © 2016 Elsevier Ltd. All rights reserved.
Rough-to-smooth transition of an equilibrium neutral constant stress layer. [atmospheric flow over rough terrain

NASA Technical Reports Server (NTRS)

Logan, E., Jr.; Fichtl, G. H.

1975-01-01

A model is proposed for low-level atmospheric flows over terrains of changing roughness length, such as those found at the windward end of landing strips adjoining rough terrain. The proposed model is used to develop a prediction technique for calculating transition wind and shear-stress profiles in the region following surface roughness discontinuity. The model for the transition region comprises two layers: a logarithmic layer and a buffer layer. The flow is assumed to be steady, two-dimensional, and incompressible, with neutral hydrostatic stability. A diagram is presented for a typical wind profile in the transition region, obtained from the logarithmic and velocity defect profiles using shear stress calculated by relevant equations.
Numerical investigations of two-degree-of-freedom vortex-induced vibration in shear flow

NASA Astrophysics Data System (ADS)

Zhang, Hui; Liu, Mengke; Han, Yang; Li, Jian; Gui, Mingyue; Chen, Zhihua

2017-06-01

Exponential-polar coordinates attached to a moving cylinder are used to deduce the stream function-vorticity equations for two-degree-of-freedom vortex-induced vibration, the initial and boundary conditions, and the distribution of the hydrodynamic force, which consists of the vortex-induced force, inertial force, and viscous damping force. The fluid-structure interactions occurring from the motionless cylinder to the steady vibration are investigated numerically, and the variations of the flow field, pressure, lift/drag, and cylinder displacement are discussed. Both the dominant vortex and the cylinder shift, whose effects are opposite, affect the shear layer along the transverse direction and the secondary vortex along the streamwise direction. However, the effect of the cylinder shift is larger than that of the dominant vortices. Therefore, the former dominates the total effects of the flow field. Moreover, the symmetry of the flow field is broken with the increasing shear rate. With the effect of the background vortex, the upper vortices are strengthened, and the lower vortices are weakened; thus, the shear layer and the secondary vortices induced by the upper shedding vortices are strengthened, while the shear layer and the secondary vortices induced by the lower shedding vortices are weakened. Therefore, the amplitudes of the displacement and drag/lift dominated by the upper vortex are larger than those of the displacement and drag/lift dominated by the lower vortex.
Relativistic Shear Flow between Electron-Ion and Electron-Positron Plasmas and Astrophysical Applications

NASA Astrophysics Data System (ADS)

Liang, Edison; Fu, Wen; Böttcher, Markus

2017-10-01

We present particle-in-cell simulation results of relativistic shear boundary layers between electron-ion and electron-positron plasmas and discuss their potential applications to astrophysics. Specifically, we find that in the case of a fast electron-positron spine surrounded by a slow-moving or stationary electron-ion sheath, lepton acceleration proceeds in a highly anisotropic manner due to electromagnetic fields created at the shear interface. While the highest-energy leptons still produce a beaming pattern (as seen in the quasi-stationary frame of the sheath) of order 1/Γ, where Γ is the bulk Lorentz factor of the spine, for lower-energy particles, the beaming is much less pronounced. This is in stark contrast to the case of pure electron-ion shear layers, in which anisotropic particle acceleration leads to significantly narrower beaming patterns than 1/Γ for the highest-energy particles. In either case, shear-layer acceleration is expected to produce strongly angle-dependent lepton (hence, emanating radiation) spectra, with a significantly harder spectrum in the forward direction than viewed from larger off-axis angles, much beyond the regular Doppler boosting effect from a co-moving isotropic lepton distribution. This may solve the problem of the need for high (and apparently arbitrarily chosen) minimum Lorentz factors of radiating electrons, often plaguing current blazar and GRB jet modeling efforts.

Effect of different thickness h-BN coatings on interface shear strength of quartz fiber reinforced Sisbnd Osbnd Csbnd N composite

NASA Astrophysics Data System (ADS)

Wang, Shubin; Zheng, Yu

2014-02-01

Hexagonal boron nitride (h-BN) coatings with different thickness were prepared on quartz fibers to improve mechanical properties of quartz fiber reinforced Sisbnd Osbnd Csbnd N composite. Scanning electron microscopy (SEM), push-out test and single edge notched beam (SENB) in three point bending test were employed to study morphology, interface shear strength and fracture toughness of the composite. The results showed that h-BN coatings changed the crack growth direction and weaken the interface shear strength efficiently. When the h-BN coating was 308.2 nm, the interface shear strength was about 5.2 MPa, which was about one-quarter of that of the sample without h-BN coatings. After the heating process for obtaining composite, the h-BN nanometer-sized grains would grow up to micron-sized hexagonal grains. Different thickness h-BN coatings had different structure. When the coatings were relatively thin, the hexagonal grains were single layer structure, and when the coatings were thicker, the hexagonal grains were multiple layer structure. This multiple layer interface phase would consume more power of cracks, thus interface shear strength of the composite decreased steadily with the increasing of h-BN coatings thickness. When the coating thickness was 238.8 nm, KIC reaches the peak value 3.8 MPa m1/2, which was more than two times of that of composites without h-BN coatings.
Relativistic Shear Flow between Electron–Ion and Electron–Positron Plasmas and Astrophysical Applications

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liang, Edison; Fu, Wen; Böttcher, Markus

We present particle-in-cell simulation results of relativistic shear boundary layers between electron–ion and electron–positron plasmas and discuss their potential applications to astrophysics. Specifically, we find that in the case of a fast electron–positron spine surrounded by a slow-moving or stationary electron–ion sheath, lepton acceleration proceeds in a highly anisotropic manner due to electromagnetic fields created at the shear interface. While the highest-energy leptons still produce a beaming pattern (as seen in the quasi-stationary frame of the sheath) of order 1/Γ, where Γ is the bulk Lorentz factor of the spine, for lower-energy particles, the beaming is much less pronounced. Thismore » is in stark contrast to the case of pure electron–ion shear layers, in which anisotropic particle acceleration leads to significantly narrower beaming patterns than 1/Γ for the highest-energy particles. In either case, shear-layer acceleration is expected to produce strongly angle-dependent lepton (hence, emanating radiation) spectra, with a significantly harder spectrum in the forward direction than viewed from larger off-axis angles, much beyond the regular Doppler boosting effect from a co-moving isotropic lepton distribution. This may solve the problem of the need for high (and apparently arbitrarily chosen) minimum Lorentz factors of radiating electrons, often plaguing current blazar and GRB jet modeling efforts.« less
Bmp2 Deletion Causes an Amelogenesis Imperfecta Phenotype Via Regulating Enamel Gene Expression

PubMed Central

GUO, FENG; FENG, JUNSHENG; WANG, FENG; LI, WENTONG; GAO, QINGPING; CHEN, ZHUO; SHOFF, LISA; DONLY, KEVIN J.; GLUHAK-HEINRICH, JELICA; CHUN, YONG HEE PATRICIA; HARRIS, STEPHEN E.; MACDOUGALL, MARY; CHEN, SHUO

2015-01-01

Although Bmp2 is essential for tooth formation, the role of Bmp2 during enamel formation remains unknown in vivo. In this study, the role of Bmp2 in regulation of enamel formation was investigated by the Bmp2 conditional knock out (Bmp2 cKO) mice. Teeth of Bmp2 cKO mice displayed severe and profound phenotypes with asymmetric and misshaped incisors as well as abrasion of incisors and molars. Scanning electron microscopy analysis showed that the enamel layer was hypoplastic and enamel lacked a typical prismatic pattern. Teeth from null mice were much more brittle as tested by shear and compressive moduli. Expression of enamel matrix protein genes, amelogenin, enamelin, and enamel-processing proteases, Mmp-20 and Klk4 was reduced in the Bmp2 cKO teeth as reflected in a reduced enamel formation. Exogenous Bmp2 up-regulated those gene expressions in mouse enamel organ epithelial cells. This result for the first time indicates Bmp2 signaling is essential for proper enamel development and mineralization in vivo. PMID:25545831
Bmp2 deletion causes an amelogenesis imperfecta phenotype via regulating enamel gene expression.

PubMed

Guo, Feng; Feng, Junsheng; Wang, Feng; Li, Wentong; Gao, Qingping; Chen, Zhuo; Shoff, Lisa; Donly, Kevin J; Gluhak-Heinrich, Jelica; Chun, Yong Hee Patricia; Harris, Stephen E; MacDougall, Mary; Chen, Shuo

2015-08-01

Although Bmp2 is essential for tooth formation, the role of Bmp2 during enamel formation remains unknown in vivo. In this study, the role of Bmp2 in regulation of enamel formation was investigated by the Bmp2 conditional knock out (Bmp2 cKO) mice. Teeth of Bmp2 cKO mice displayed severe and profound phenotypes with asymmetric and misshaped incisors as well as abrasion of incisors and molars. Scanning electron microscopy analysis showed that the enamel layer was hypoplastic and enamel lacked a typical prismatic pattern. Teeth from null mice were much more brittle as tested by shear and compressive moduli. Expression of enamel matrix protein genes, amelogenin, enamelin, and enamel-processing proteases, Mmp-20 and Klk4 was reduced in the Bmp2 cKO teeth as reflected in a reduced enamel formation. Exogenous Bmp2 up-regulated those gene expressions in mouse enamel organ epithelial cells. This result for the first time indicates Bmp2 signaling is essential for proper enamel development and mineralization in vivo. © 2015 Wiley Periodicals, Inc.
Interfacial microstructure and shear strength of reactive air brazed oxygen transport membrane ceramic-metal alloy joints

NASA Astrophysics Data System (ADS)

FR, Wahid Muhamad; Yoon, Dang-Hyok; Raju, Kati; Kim, Seyoung; Song, Kwang-sup; Yu, Ji Haeng

2018-01-01

To fabricate a multi-layered structure for maximizing oxygen production, oxygen transport membrane (OTM) ceramics need to be joined or sealed hermetically metal supports for interfacing with the peripheral components of the system. Therefore, in this study, Ag-10 wt% CuO was evaluated as an effective filler material for the reactive air brazing of dense Ce0.9Gd0.1O2-δ-La0.7Sr0.3MnO3±δ (GDC-LSM) OTM ceramics. Thermal decomposition in air and wetting behavior of the braze filler was performed. Reactive air brazing was performed at 1050 °C for 30 min in air to join GDC-LSM with four different commercially available high temperature-resistant metal alloys, such as Crofer 22 APU, Inconel 600, Fecralloy, and AISI 310S. The microstructure and elemental distribution of the ceramic-ceramic and ceramic-metal interfaces were examined from polished cross-sections. The mechanical shear strength at room temperature for the as-brazed and isothermally aged (800 °C for 24 h) joints of all the samples was compared. The results showed that the strength of the ceramic-ceramic joints was decreased marginally by aging; however, in the case of metal-ceramic joints, different decreases in strengths were observed according to the metal alloy used, which was explained based on the formation of different oxide layers at the interfaces.
Microgravity Effects on Transendothelial Transport

NASA Technical Reports Server (NTRS)

Tarbell, John M.

1996-01-01

The Endothelial Cell (EC) layer which lines blood vessels from the aorta to the capillaries provides the principal barrier to transport of water and solutes between blood and underlying tissue. Endothelial cells are continuously exposed to the mechanical shearing force (shear stress) and normal force (pressure) imposed by flowing blood on their surface, and they are adapted to this mechanical environment. When the cardiovascular system is exposed to microgravity, the mechanical environmental of endothelial cells is perturbed drastically and the transport properties of EC layers are altered in response. We have shown recently that step changes in shear stress have an acute effect on transport properties of EC layers in a cell culture model, and several recent studies in different vessels of live animals have confirmed the shear-dependent transport properties of the endothelium. We hypothesize that alterations in mechanical forces induced by microgravity and their resultant influence on transendothelial transport of water and solutes are, in large measure, responsible for the characteristic cephalad fluid shift observed in humans experiencing microgravity. To study the effects of altered mechanical forces on transendothelial transport and to test pharmacologic agents as counter measures to microgravity induced fluid shifts we have proposed ground-based studies using well defined cell culture models.
Pyroclast/snow interactions and thermally driven slurry formation. Part 1: Theory for monodisperse grain beds

USGS Publications Warehouse

Walder, J.S.

2000-01-01

Lahars are often produced as pyroclastic flows move over snow. This phenomenon involves a complicated interplay of mechanical and thermal processes that need to be separated to get at the fundamental physics. The thermal physics of pyroclast/snow interactions form the focus of this paper. A theoretical model is developed of heat- and mass transfer at the interface between a layer of uniformly sized pyroclasts and an underlying bed of snow, for the case in which there is no relative shear motion between pyroclasts and snow. A microscale view of the interface is required to properly specify boundary conditions. The physical model leads to the prediction that the upward flux of water vapor - which depends upon emplacement temperature, pyroclast grain size, pyroclast-layer thickness, and snow permeability - is sometimes sufficient to fluidize the pyroclasts. Uniform fluidization is usually unstable to bubble formation, which leads to vigorous convection of the pyroclasts themselves. Thus, predicted threshold conditions for fluidization are tantamount to predicted thresholds for particle convection. Such predictions are quantitatively in good agreement with results of experiments described in part 2 of this paper. Because particle convection commonly causes scour of the snow bed and transformation of the pyroclast layer to a slurry, there exists a 'thermal scour' process for generating lahars from pyroclastic flows moving over snow regardless of the possible role of mechanical scour.
Depth-Dependent Transverse Shear Properties of the Human Corneal Stroma

PubMed Central

Petsche, Steven J.; Chernyak, Dimitri; Martiz, Jaime; Levenston, Marc E.

2012-01-01

Purpose. To measure the transverse shear modulus of the human corneal stroma and its profile through the depth by mechanical testing, and to assess the validity of the hypothesis that the shear modulus will be greater in the anterior third due to increased interweaving of lamellae. Methods. Torsional rheometry was used to measure the transverse shear properties of 6 mm diameter buttons of matched human cadaver cornea pairs. One cornea from each pair was cut into thirds through the thickness with a femtosecond laser and each stromal third was tested individually. The remaining intact corneas were tested to measure full stroma shear modulus. The shear modulus from a 1% shear strain oscillatory test was measured at various levels of axial compression for all samples. Results. After controlling for axial compression, the transverse shear moduli of isolated anterior layers were significantly higher than central and posterior layers. Mean modulus values at 0% axial strain were 7.71 ± 6.34 kPa in the anterior, 1.99 ± 0.45 kPa in the center, 1.31 ± 1.01 kPa in the posterior, and 9.48 ± 2.92 kPa for full thickness samples. A mean equilibrium compressive modulus of 38.7 ± 8.6 kPa at 0% axial strain was calculated from axial compression measured during the shear tests. Conclusions. Transverse shear moduli are two to three orders of magnitude lower than tensile moduli reported in the literature. The profile of shear moduli through the depth displayed a significant increase from posterior to anterior. This gradient supports the hypothesis and corresponds to the gradient of interwoven lamellae seen in imaging of stromal cross-sections. PMID:22205608
Wind shear training applications for 91/135

NASA Technical Reports Server (NTRS)

Arbon, ED

1991-01-01

The requirement for wind shear training of all pilots has been demonstrated too often by the accident statistics of past years. Documents were developed to train airline crews on specific aircraft and to teach recognition of the meteorological conditions that are conducive to wind shear and microburst formation. A Wind Shear Training Aid program is discussed.
Behaviour of Mechanically Laminated CLT Members

NASA Astrophysics Data System (ADS)

Kuklík, P.; Velebil, L.

2015-11-01

Cross laminated timber (CLT) is one of the structural building systems based on the lamination of multiple layers, where each layer is oriented perpendicularly to each other. Recent requirements are placed to develop an alternative process based on the mechanical lamination of the layers, which is of particular interest to our research group at the University Centre for Energy Efficient Buildings. The goal is to develop and verify the behaviour of mechanically laminated CLT wall panels exposed to shear stresses in the plane. The shear resistance of mechanically jointed CLT is ensured by connecting the layers by screws. The paper deals with the experimental analysis focused on the determination of the torsional stiffness and the slip modulus of crossing areas for different numbers of orthogonally connected layers. The results of the experiments were compared with the current analytical model.
Stratospheric mountain wave attenuation in positive and negative ambient wind shear

NASA Astrophysics Data System (ADS)

Kruse, C. G.; Smith, R. B.

2016-12-01

Recently, much has been learned about the vertical propagation and attenuation of mountain waves launched by the Southern Alps of New Zealand (NZ) from the Deep Propagating Gravity Wave Experiment (DEEPWAVE) field campaign. Over NZ, approximately half of mountain wave events are strongly attenuated in a lower-stratospheric "valve layer," defined as a layer of reduced wind with no critical levels. Within a valve layer, negative wind shear causes mountain waves steepen and attenuate, with the amount of transmitted momentum flux controlled by the minimum wind speed within the layer. The other half of wave events are deep (propagating to 35+ km), usually with positive wind shear. Within these deep events, increasing amplitude with decreasing density causes mountain waves to attenuate gradually (after spatial/temporal averaging). Global reanalyses indicate that this valve layer is a climatological feature in the wintertime mid-latitudes above the subtropical jet, while deep events and gradual attenuation occur over higher latitudes below the polar stratospheric jet. The local physics of mountain wave attenuation in positive and negative ambient wind shear are investigated using realistic winter-long (JJA) 6-km resolution Weather Research and Forecasting (WRF) model simulations over the Andes. Attention is given to the spatiotemporal variability of wave attenuation and the various factors driving this variability (e.g. variability in wave generation, ambient conditions at attenuation level, inherent wave-induced instabilities). Mesoscale potential vorticity generation is used as an indicator of wave attenuation. Additionally, regionally integrated wave momentum flux and gravity wave drag (GWD) within WRF are quantified and compared with parameterized quantities in the MERRA1 and 2 reanalyses.
The Scaling of Broadband Shock-Associated Noise with Increasing Temperature

NASA Technical Reports Server (NTRS)

Miller, Steven A.

2012-01-01

A physical explanation for the saturation of broadband shock-associated noise (BBSAN) intensity with increasing jet stagnation temperature has eluded investigators. An explanation is proposed for this phenomenon with the use of an acoustic analogy. For this purpose the acoustic analogy of Morris and Miller is examined. To isolate the relevant physics, the scaling of BBSAN at the peak intensity level at the sideline ( = 90 degrees) observer location is examined. Scaling terms are isolated from the acoustic analogy and the result is compared using a convergent nozzle with the experiments of Bridges and Brown and using a convergent-divergent nozzle with the experiments of Kuo, McLaughlin, and Morris at four nozzle pressure ratios in increments of total temperature ratios from one to four. The equivalent source within the framework of the acoustic analogy for BBSAN is based on local field quantities at shock wave shear layer interactions. The equivalent source combined with accurate calculations of the propagation of sound through the jet shear layer, using an adjoint vector Green s function solver of the linearized Euler equations, allows for predictions that retain the scaling with respect to stagnation pressure and allows for the accurate saturation of BBSAN with increasing stagnation temperature. This is a minor change to the source model relative to the previously developed models. The full development of the scaling term is shown. The sources and vector Green s function solver are informed by steady Reynolds-Averaged Navier-Stokes solutions. These solutions are examined as a function of stagnation temperature at the first shock wave shear layer interaction. It is discovered that saturation of BBSAN with increasing jet stagnation temperature occurs due to a balance between the amplification of the sound propagation through the shear layer and the source term scaling.A physical explanation for the saturation of broadband shock-associated noise (BBSAN) intensity with increasing jet stagnation temperature has eluded investigators. An explanation is proposed for this phenomenon with the use of an acoustic analogy. For this purpose the acoustic analogy of Morris and Miller is examined. To isolate the relevant physics, the scaling of BBSAN at the peak intensity level at the sideline psi = 90 degrees) observer location is examined. Scaling terms are isolated from the acoustic analogy and the result is compared using a convergent nozzle with the experiments of Bridges and Brown and using a convergent-divergent nozzle with the experiments of Kuo, McLaughlin, and Morris at four nozzle pressure ratios in increments of total temperature ratios from one to four. The equivalent source within the framework of the acoustic analogy for BBSAN is based on local field quantities at shock wave shear layer interactions. The equivalent source combined with accurate calculations of the propagation of sound through the jet shear layer, using an adjoint vector Green s function solver of the linearized Euler equations, allows for predictions that retain the scaling with respect to stagnation pressure and allows for the accurate saturation of BBSAN with increasing stagnation temperature. This is a minor change to the source model relative to the previously developed models. The full development of the scaling term is shown. The sources and vector Green s function solver are informed by steady Reynolds-Averaged Navier-Stokes solutions. These solutions are examined as a function of stagnation temperature at the first shock wave shear layer interaction. It is discovered that saturation of BBSAN with increasing jet stagnation temperature occurs due to a balance between the amplification of the sound propagation through the shear layer and the source term scaling.
Why do naked singularities form in gravitational collapse? II

DOE Office of Scientific and Technical Information (OSTI.GOV)

Joshi, Pankaj S.; Goswami, Rituparno; Dadhich, Naresh

We examine physical features that could lead to formation of a naked singularity rather than black hole, as end state of spherical collapse. Generalizing earlier results on dust collapse to general type I matter fields, it is shown that collapse always creates black hole if shear vanishes or density is homogeneous. It follows that nonzero shear is a necessary condition for singularity to be visible to external observers, when trapped surface formation is delayed by shearing forces or inhomogeneity within the collapsing cloud.
Influence of laser power on microstructure and mechanical properties of laser welded-brazed Mg to Ni coated Ti alloys

NASA Astrophysics Data System (ADS)

Tan, Caiwang; Lu, Qingshuang; Chen, Bo; Song, Xiaoguo; Li, Liqun; Feng, Jicai; Wang, Yang

2017-03-01

AZ31B Magnesium (Mg) and Ti-6Al-4V titanium (Ti) alloys with Ni coating were joined by laser welding-brazing process using AZ92 Mg based filler. The influence of laser power on microstructure and mechanical properties were investigated. Ni coating was found to significantly promote good wetting-spreading ability of molten filler on the Ti sheet. Acceptable joints without obvious defects were obtained within a relatively wide processing window. In the process metallurgical bonding was achieved by the formation of Ti3Al phase at direct irradiation zone and Al-Ni phase followed by a layer of Mg-Al-Ni ternary compound adjacent to the fusion zone at the intermediate zone. The thickness of reaction layers increased slowly with the increasing laser power. The tensile-shear test indicated that joints produced at the laser power of 1300 W reached 2387 N fracture load, representing 88.5% joint efficiency with respect to the Mg base metal. The corresponding failure occurred in the fusion zone of the Mg base metal, while joints fractured at the interface at lower/higher laser power due to the crack or excessive intermetallic compound (IMC) formation along the interface.
Control Al/Mg intermetallic compound formation during ultrasonic-assisted soldering Mg to Al.

PubMed

Xu, Zhiwu; Li, Zhengwei; Li, Jiaqi; Ma, Zhipeng; Yan, Jiuchun

2018-09-01

To prevent the formation of Al/Mg intermetallic compounds (IMCs) of Al 3 Mg 2 and Al 12 Mg 17 , dissimilar Al/Mg were ultrasonic-assisted soldered using Sn-based filler metals. A new IMC of Mg 2 Sn formed in the soldered joints during this process and it was prone to crack at large thickness. The thickness of Mg 2 Sn was reduced to 22 μm at 285 °C when using Sn-3Cu as the filler metal. Cracks were still observed inside the blocky Mg 2 Sn. The thickness of Mg 2 Sn was significantly reduced when using Sn-9Zn as the filler metal. A 17 μm Mg 2 Sn layer without crack was obtained at a temperature of 200 °C, ultrasonic power of Mode I, and ultrasonic time of 2 s. The shear strengths of the joints using Sn-9Zn was much higher than those using Sn-3Cu because of the thinner Mg 2 Sn layer in the former joints. Sn whiskers were prevented by using Sn-9Zn. A cavitation model during ultrasonic assisted soldering was proposed. Copyright © 2018 Elsevier B.V. All rights reserved.
Instability-induced ordering, universal unfolding and the role of gravity in granular Couette flow

NASA Astrophysics Data System (ADS)

Alam, Meheboob; Arakeri, V. H.; Nott, P. R.; Goddard, J. D.; Herrmann, H. J.

2005-01-01

Linear stability theory and bifurcation analysis are used to investigate the role of gravity in shear-band formation in granular Couette flow, considering a kinetic-theory rheological model. We show that the only possible state, at low shear rates, corresponds to a "plug" near the bottom wall, in which the particles are densely packed and the shear rate is close to zero, and a uniformly sheared dilute region above it. The origin of such plugged states is shown to be tied to the spontaneous symmetry-breaking instabilities of the gravity-free uniform shear flow, leading to the formation of ordered bands of alternating dilute and dense regions in the transverse direction, via an infinite hierarchy of pitchfork bifurcations. Gravity plays the role of an "imperfection", thus destroying the "perfect" bifurcation structure of uniform shear. The present bifurcation problem admits universal unfolding of pitchfork bifurcations which subsequently leads to the formation of a sequence of a countably infinite number of "isolas", with the solution structures being a modulated version of their gravity-free counterpart. While the solution with a plug near the bottom wall looks remarkably similar to the shear-banding phenomenon in dense slow granular Couette flows, a "floating" plug near the top wall is also a solution of these equations at high shear rates. A two-dimensional linear stability analysis suggests that these floating plugged states are unstable to long-wave travelling disturbances.The unique solution having a bottom plug can also be unstable to long waves, but remains stable at sufficiently low shear rates. The implications and realizability of the present results are discussed in the light of shear-cell experiments under "microgravity" conditions.
Evaluation of Full Reynolds Stress Turbulence Models in FUN3D

NASA Technical Reports Server (NTRS)

Dudek, Julianne C.; Carlson, Jan-Renee

2017-01-01

Full seven-equation Reynolds stress turbulence models are a relatively new and promising tool for todays aerospace technology challenges. This paper uses two stress-omega full Reynolds stress models to evaluate challenging flows including shock-wave boundary layer interactions, separation and mixing layers. The Wilcox and the SSGLRR full second-moment Reynolds stress models are evaluated for four problems: a transonic two-dimensional diffuser, a supersonic axisymmetric compression corner, a compressible planar shear layer, and a subsonic axisymmetric jet. Simulation results are compared with experimental data and results using the more commonly used Spalart-Allmaras (SA) one-equation and the Menter Shear Stress Transport (SST) two-equation models.
Characteristics of turbulence in boundary layer with zero pressure gradient

NASA Technical Reports Server (NTRS)

Klebanoff, P S

1955-01-01

The results of an experimental investigation of a turbulent boundary layer with zero pressure gradient are presented. Measurements with the hot-wire anemometer were made of turbulent energy and turbulent shear stress, probability density and flattening factor of u-fluctuation (fluctuation in x-direction), spectra of turbulent energy and shear stress, and turbulent dissipation. The importance of the region near the wall and the inadequacy of the concept of local isotropy are demonstrated. Attention is given to the energy balance and the intermittent character of the outer region of the boundary layer. Also several interesting features of the spectral distribution of the turbulent motions are discussed.
Analysis of the leading edge effects on the boundary layer transition

NASA Technical Reports Server (NTRS)

Chow, Pao-Liu

1990-01-01

A general theory of boundary layer control by surface heating is presented. Some analytical results for a simplified model, i.e., the optimal control of temperature fluctuations in a shear flow are described. The results may provide a clue to the effectiveness of the active feedback control of a boundary layer flow by wall heating. In a practical situation, the feedback control may not be feasible from the instrumentational point of view. In this case the vibrational control introduced in systems science can provide a useful alternative. This principle is briefly explained and applied to the control of an unstable wavepacket in a parallel shear flow.
Relation between boundary slip mechanisms and waterlike fluid behavior.

PubMed

Ternes, Patricia; Salcedo, Evy; Barbosa, Marcia C

2018-03-01

The slip of a fluid layer in contact with a solid confining surface is investigated for different temperatures and densities using molecular dynamic simulations. We show that for an anomalous waterlike fluid the slip goes as follows: for low levels of shear, defect slip appears and is related to the particle exchange between the fluid layers; at high levels of shear, global slip occurs and is related to the homogeneous distribution of the fluid in the confining surfaces. The oscillations in the transition velocity from defect to global slip are shown to be associated with changes in the layering distribution in the anomalous fluid.

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