Dynamics of Aerosol Particles in Stationary, Isotropic Turbulence
NASA Technical Reports Server (NTRS)
Collins, Lance R.; Meng, Hui
2004-01-01
A detailed study of the dynamics of sub-Kolmogorov-size aerosol particles in stationary isotropic turbulence has been performed. The study combined direct numerical simulations (DNS; directed by Prof. Collins) and high-resolution experimental measurements (directed by Prof. Meng) under conditions of nearly perfect geometric and parametric overlap. The goal was to measure the accumulation of particles in low-vorticity regions of the flow that arises from the effect commonly referred to as preferential concentration. The grant technically was initiated on June 13, 2000; however, funding was not available until July 11, 2000. The grant was originally awarded to Penn State University (numerical simulations) and SUNY-Buffalo (experiments); however, Prof. Collins effort was moved to Cornell University on January 2002 when he joined that university. He completed the study there. A list of the specific tasks that were completed under this study is presented.
Streamlines in stationary homogeneous isotropic turbulence and fractal-generated turbulence
NASA Astrophysics Data System (ADS)
Boschung, J.; Peters, N.; Laizet, S.; Vassilicos, J. C.
2016-04-01
We compare streamline statistics in stationary homogeneous isotropic turbulence and in turbulence generated by a fractal square grid. We examine streamline segments characterised by the velocity difference {{Δ }}u and the distance l between extremum points. We find close agreement between the stationary homogeneous isotropic turbulence and the decay region of the fractal-generated turbulence as well as the production region of the fractal flow for small segments. The statistics of larger segments are very similar for the isotropic turbulence and the decay region, but differ for the production region. Specifically, we examine the first, second and third conditional mean < {[{{Δ }}u]}n| l> . Noticeably, non-vanishing < {[{{Δ }}u]}n| l> for n=1,3 are due to an asymmetry of positive and negative segments, i.e. those for which {{Δ }}u\\gt 0 and {{Δ }}u\\lt 0, respectively. This asymmetry is not only kinematic, but is also due to dissipative effects and therefore < {[{{Δ }}u]}n| l> contains cascade information.
Linearly Forced Isotropic Turbulence
NASA Technical Reports Server (NTRS)
Lundgren, T. S.
2003-01-01
Stationary isotropic turbulence is often studied numerically by adding a forcing term to the Navier-Stokes equation. This is usually done for the purpose of achieving higher Reynolds number and longer statistics than is possible for isotropic decaying turbulence. It is generally accepted that forcing the Navier-Stokes equation at low wave number does not influence the small scale statistics of the flow provided that there is wide separation between the largest and smallest scales. It will be shown, however, that the spectral width of the forcing has a noticeable effect on inertial range statistics. A case will be made here for using a broader form of forcing in order to compare computed isotropic stationary turbulence with (decaying) grid turbulence. It is shown that using a forcing function which is directly proportional to the velocity has physical meaning and gives results which are closer to both homogeneous and non-homogeneous turbulence. Section 1 presents a four part series of motivations for linear forcing. Section 2 puts linear forcing to a numerical test with a pseudospectral computation.
DNS of Shock / Isotropic Turbulence Interaction
NASA Astrophysics Data System (ADS)
Grube, Nathan; Taylor, Ellen; Martín, Pino
2010-11-01
We discuss DNS of Shock / Isotropic Turbulence Interactions (SITI). We vary the incoming turbulence Mach number up to 0.8 and the convective Mach number up to 5 in order to determine their effects on the interaction. These cases are challenging due to the presence of shocklets in the incoming turbulence as well as significant motion of the main shock. Shock-capturing must be used at all points while still maintaining low enough numerical dissipation to preserve the turbulent fluctuations. We use the linearly- and nonlinearly-optimized Weighted Essentially Non-Oscillatory (WENO) method[1,2]. Particular attention is paid to the inflow boundary condition, where we find the use of snapshots of "frozen" turbulence from decaying isotropic box simulations to be unsatisfactory. We instead use time-varying inflow data generated by a separate forced isotropic turbulence simulation with a specified convection speed. This allows us to access flow conditions where the assumptions of Taylor's Hypothesis are not met. 1.) Mart'in, M.P., Taylor, E.M., Wu, M., and Weirs, V.G., JCP 220(1) 270-89, 2006. 2.) Taylor, E.M., Wu, M., and Mart'in, M.P., JCP 223(1) 384-97, 2007.
Modification of homogeneous and isotropic turbulence by solid particles
NASA Astrophysics Data System (ADS)
Hwang, Wontae
2005-12-01
Particle-laden flows are prevalent in natural and industrial environments. Dilute loadings of small, heavy particles have been observed to attenuate the turbulence levels of the carrier-phase flow, up to 80% in some cases. We attempt to increase the physical understanding of this complex phenomenon by studying the interaction of solid particles with the most fundamental type of turbulence, which is homogeneous and isotropic with no mean flow. A flow facility was developed that could create air turbulence in a nearly-spherical chamber by means of synthetic jet actuators mounted on the corners. Loudspeakers were used as the actuators. Stationary turbulence and natural decaying turbulence were investigated using two-dimensional particle image velocimetry for the base flow qualification. Results indicated that the turbulence was fairly homogeneous throughout the measurement domain and very isotropic, with small mean flow. The particle-laden flow experiments were conducted in two different environments, the lab and in micro-gravity, to examine the effects of particle wakes and flow structure distortion caused by settling particles. The laboratory experiments showed that glass particles with diameters on the order of the turbulence Kolmogorov length scale attenuated the fluid turbulent kinetic energy (TKE) and dissipation rate with increasing particle mass loadings. The main source of fluid TKE production in the chamber was the speakers, but the loss of potential energy of the settling particles also resulted in a significant amount of production of extra TKE. The sink of TKE in the chamber was due to the ordinary fluid viscous dissipation and extra dissipation caused by particles. This extra dissipation could be divided into "unresolved" dissipation caused by local velocity disturbances in the vicinity of the small particles and dissipation caused by large-scale flow distortions from particle wakes and particle clusters. The micro-gravity experiments in NASA's KC-135
Asymptotic behavior of curvature of surface elements in isotropic turbulence
NASA Technical Reports Server (NTRS)
Girimaji, S. S.
1991-01-01
The asymptotic behavior of the curvature of material elements in turbulence is investigated using Lagrangian velocity-gradient time series obtained from direct numerical simulations of isotropic turbulence. Several material-element ensembles of different initial curvatures and shapes are studied. It is found that, at long times, the (first five) moments of the logarithm of characteristic curvature and shape factor asymptote to values that are independent of the initial curvature or shape. This evidence strongly suggests that the asymptotic pdf's of the curvature and shape of material elements are stationary and independent of initial conditions. Irrespective of initial curvature or shape, the asymptotic shape of a material surface is cylindrical with a high probability.
Some Basic Laws of Isotropic Turbulent Flow
NASA Technical Reports Server (NTRS)
Loitsianskii, L. G.
1945-01-01
An Investigation is made of the diffusion of artificially produced turbulence behind screens or other turbulence producers. The method is based on the author's concept of disturbance moment as a certain theoretically well-founded measure of turbulent disturbances.
NASA Technical Reports Server (NTRS)
Eaton, John; Hwang, Wontae; Cabral, Patrick
2002-01-01
the addition of gravity as a variable parameter may help us to better understand the physics of turbulence attenuation. The experiments are conducted in a turbulence chamber capable of producing stationary or decaying isotropic turbulence with nearly zero mean flow and Taylor microscale Reynolds numbers up to nearly 500. The chamber is a 410 mm cubic box with the corners cut off to make it approximately spherical. Synthetic jet turbulence generators are mounted in each of the eight corners of the box. Each generator consists of a loudspeaker forcing a plenum and producing a pulsed jet through a 20 mm diameter orifice. These synthetic jets are directed into ejector tubes pointing towards the chamber center. The ejector tubes increase the jet mass flow and decrease the velocity. The jets then pass through a turbulence grid. Each of the eight loudspeakers is forced with a random phase and frequency. The resulting turbulence is highly Isotropic and matches typical behavior of grid turbulence. Measurements of both phases are acquired using particle image velocimetry (PIV). The gas is seeded with approximately 1 micron diameter seeding particles while the solid phase is typically 150 micron diameter spherical glass particles. A double-pulsed YAG laser and a Kodak ES-1.0 10-bit PIV camera provide the PIV images. Custom software is used to separate the images into individual images containing either gas-phase tracers or large particles. Modern high-resolution PIV algorithms are then used to calculate the velocity field. A large set of image pairs are acquired for each case, then the results are averaged both spatially and over the ensemble of acquired images. The entire apparatus is mounted in two racks which are carried aboard NASA's KC-135 Flying Microgravity Laboratory. The rack containing the turbulence chamber, the laser head, and the camera floats freely in the airplane cabin (constrained by competent NASA personnel) to minimize g-jitter.
Sudden relaminarisation and lifetimes in forced isotropic turbulence
NASA Astrophysics Data System (ADS)
Linkmann, Moritz; Morozov, Alexander
2015-11-01
We demonstrate an unexpected connection between isotropic turbulence and wall-bounded shear flows. We perform direct numerical simulations of isotropic turbulence forced at large scales at moderate Reynolds numbers and observe sudden transitions from chaotic dynamics to a spatially simple flow, analogous to the laminar state in wall bounded shear flows. We find that the survival probabilities of turbulence are exponential and the typical lifetimes increase super-exponentially with the Reynolds number, similar to results on relaminarisation of localised turbulence in pipe and plane Couette flow. Results from simulations subjecting the observed large-scale flow to random perturbations of variable amplitude demonstrate that it is a linearly stable simple exact solution that can be destabilised by a finite-amplitude perturbation, like the Hagen-Poiseuille profile in pipe flow. Our results suggest that both isotropic turbulence and wall-bounded shear flows qualitatively share the same phase-space dynamics.
The energy decay in self-preserving isotropic turbulence revisited
NASA Technical Reports Server (NTRS)
Speziale, Charles G.; Bernard, Peter S.
1992-01-01
The assumption of self-preservation allows for an analytical determination of the energy decay in isotropic turbulence. Here, the self-preserving isotropic decay problem is analyzed, yielding a more complete picture of self-serving isotropic turbulence. It is proven rigorously that complete self-serving isotropic turbulence admits two general types of asymptotic solutions: one where the turbulent kinetic energy K approximately t (exp -1) and one where K approximately t (sup alpha) with an exponent alpha greater than 1 that is determined explicitly by the initial conditions. By a fixed point analysis and numerical integration of the exact one-point equations, it is demonstrated that the K approximately t (exp -1) and where K approximately t (sup -alpha) with an exponent alpha greater than 1 that is determined explicitly by the initial conditions. By a fixed point analysis and numerical integration of the exact one-point equations, it is demonstrated that the K approximately t (exp -1) power law decay is the asymptotically consistent high Reynolds number solution; the K approximately 1 (sup -alpha) decay law is only achieved in the limit as t yields infinity and the turbulence Reynolds number vanishes. Arguments are provided which indicate that a K approximately t (exp -1) power law decay is the asymptotic state toward which a complete self-preserving isotropic turbulence is driven at high Reynolds numbers in order to resolve the imbalance between vortex stretching and viscous diffusion.
The energy decay in self-preserving isotropic turbulence revisited
NASA Technical Reports Server (NTRS)
Speziale, Charles G.; Bernard, Peter S.
1991-01-01
The assumption of self-preservation allows for an analytical determination of the energy decay in isotropic turbulence. Here, the self-preserving isotropic decay problem is analyzed, yielding a more complete picture of self-serving isotropic turbulence. It is proven rigorously that complete self-serving isotropic turbulence admits two general types of asymptotic solutions: one where the turbulent kinetic energy K approximately t (exp -1) and one where K approximately t (sup alpha) with an exponent alpha greater than 1 that is determined explicitly by the initial conditions. By a fixed point analysis and numerical integration of the exact one-point equations, it is demonstrated that the K approximately t (exp -1) and where K approximately t (sup -alpha) with an exponent alpha greater than 1 that is determined explicitly by the initial conditions. By a fixed point analysis and numerical integration of the exact one point equations, it is demonstrated that the K approximately t (exp -1) power law decay is the asymptotically consistent high Reynolds number solution; the K approximately 1 (sup - alpha) decay law is only achieved in the limit as t yields infinity and the turbulence Reynolds number vanishes. Arguments are provided which indicate that a K approximately t (exp -1) power law decay is the asymptotic state towards which a complete self-preseving isotropic turbulence is driven at high Reynolds numbers in order to resolve the imbalance between vortex stretching and viscous diffusion.
Laboratory Study of Homogeneous and Isotropic Turbulence at High Reynolds Number
NASA Astrophysics Data System (ADS)
Pecenak, Zachary; Dou, Zhongwang; Yang, Fan; Cao, Lujie; Liang, Zach; Meng, Hui
2013-11-01
To study particle dynamics modified by isotropic turbulence at high Reynolds numbers and provide experimental data for DNS validation, we have developed a soccer-ball-shaped truncated icosahedron turbulence chamber with 20 adjoining hexagon surfaces, 12 pentagon surfaces and twenty symettrically displaced fans, which form an enclosed chamber of 1m diameter. We use Particle Image Velocimetry (PIV) technique to characterize the base turbulent flow, using different PIV set ups to capture various characteristic scales of turbulence. Results show that the stationary isotropic turbulence field is a spherical domain with diameter of 40 mm with quasi-zero mean velocities. The maximum rms velocity is ~1.5 m/s, corresponding to a Taylor microscale Re of 450. We extract from the PIV velocity field the whole set of turbulent flow parameters including: turbulent kinetic energy, turbulent intensity, kinetic energy dissipation rate, large eddy length and time scales, the Kolmogorov length, time and velocity scales, Taylor microscale and Re, which are critical to the study of inter-particle statistics modified by turbulence. This research is funded by an NSF grant CBET-0967407.
Statistics of pressure and pressure gradient in homogeneous isotropic turbulence
NASA Technical Reports Server (NTRS)
Gotoh, T.; Rogallo, R. S.
1994-01-01
The statistics of pressure and pressure gradient in stationary isotropic turbulence are measured within direct numerical simulations at low to moderate Reynolds numbers. It is found that the one-point pdf of the pressure is highly skewed and that the pdf of the pressure gradient is of stretched exponential form. The power spectrum of the pressure P(k) is found to be larger than the corresponding spectrum P(sub G)(k) computed from a Gaussian velocity field having the same energy spectrum as that of the DNS field. The ratio P(k)/P(sub G)(k), a measure of the pressure-field intermittence, grows with wavenumber and Reynolds number as -R(sub lambda)(exp 1/2)log(k/k(sub d)) for k less than k(sub d)/2 where k(sub d) is the Kolmogorov wavenumber. The Lagrangian correlations of pressure gradient and velocity are compared and the Lagrangian time scale of the pressure gradient is observed to be much shorter than that of the velocity.
Large-deviation statistics of vorticity stretching in isotropic turbulence
NASA Astrophysics Data System (ADS)
Johnson, Perry L.; Meneveau, Charles
2016-03-01
A key feature of three-dimensional fluid turbulence is the stretching and realignment of vorticity by the action of the strain rate. It is shown in this paper, using the cumulant-generating function, that the cumulative vorticity stretching along a Lagrangian path in isotropic turbulence obeys a large deviation principle. As a result, the relevant statistics can be described by the vorticity stretching Cramér function. This function is computed from a direct numerical simulation data set at a Taylor-scale Reynolds number of Reλ=433 and compared to those of the finite-time Lyapunov exponents (FTLE) for material deformation. As expected, the mean cumulative vorticity stretching is slightly less than that of the most-stretched material line (largest FTLE), due to the vorticity's preferential alignment with the second-largest eigenvalue of strain rate and the material line's preferential alignment with the largest eigenvalue. However, the vorticity stretching tends to be significantly larger than the second-largest FTLE, and the Cramér functions reveal that the statistics of vorticity stretching fluctuations are more similar to those of the largest FTLE. In an attempt to relate the vorticity stretching statistics to the vorticity magnitude probability density function in statistically stationary conditions, a model Kramers-Moyal equation is constructed using the statistics encoded in the Cramér function. The model predicts a stretched-exponential tail for the vorticity magnitude probability density function, with good agreement for the exponent but significant difference (35%) in the prefactor.
Diffusion of Heat from a Line Source in Isotropic Turbulence
NASA Technical Reports Server (NTRS)
Uberoi, Mahinder S; Corrsin, Stanley
1953-01-01
An experimental and analytical study has been made of some features of the turbulent heat diffusion behind a line heated wire stretched perpendicular to a flowing isotropic turbulence. The mean temperature distributions have been measured with systematic variations in wind speed, size of turbulence-producing grid, and downstream location of heat source. The nature of the temperature fluctuation field has been studied. A comparison of Lagrangian and Eulerian analyses for diffusion in a nondecaying turbulence yields an expression for turbulent-heat-transfer coefficient in terms of turbulence velocity and a Lagrangian "scale." the ratio of Eulerian to Lagrangian microscale has been determined theoretically by generalization of a result of Heisenberg and with arbitrary constants taken from independent sources, shows rough agreement with experimental results. A convenient form has been deduced for the criterion of interchangeability of instantaneous space and time derivatives in a flowing turbulence.
Studies of Shock Wave Interactions with Homogeneous and Isotropic Turbulence
NASA Technical Reports Server (NTRS)
Briassulis, G.; Agui, J.; Watkins, C. B.; Andreopoulos, Y.
1998-01-01
A nearly homogeneous nearly isotropic compressible turbulent flow interacting with a normal shock wave has been studied experimentally in a large shock tube facility. Spatial resolution of the order of 8 Kolmogorov viscous length scales was achieved in the measurements of turbulence. A variety of turbulence generating grids provide a wide range of turbulence scales. Integral length scales were found to substantially decrease through the interaction with the shock wave in all investigated cases with flow Mach numbers ranging from 0.3 to 0.7 and shock Mach numbers from 1.2 to 1.6. The outcome of the interaction depends strongly on the state of compressibility of the incoming turbulence. The length scales in the lateral direction are amplified at small Mach numbers and attenuated at large Mach numbers. Even at large Mach numbers amplification of lateral length scales has been observed in the case of fine grids. In addition to the interaction with the shock the present work has documented substantial compressibility effects in the incoming homogeneous and isotropic turbulent flow. The decay of Mach number fluctuations was found to follow a power law similar to that describing the decay of incompressible isotropic turbulence. It was found that the decay coefficient and the decay exponent decrease with increasing Mach number while the virtual origin increases with increasing Mach number. A mechanism possibly responsible for these effects appears to be the inherently low growth rate of compressible shear layers emanating from the cylindrical rods of the grid.
Charge pariticle transport in the non-isotropic turbulences
NASA Astrophysics Data System (ADS)
Sun, P.; Jokipii, J. R.
2015-12-01
The scattering and diffusion of energetic charged particles is not only important for understanding phenomena such as diffusive shock acceleration but it also is a natural probe of the statistical characteristics of magnetohydrodynamic (MHD) turbulence. Although Parker's transport equation (Parker 1965) allows us to describe the propagation of charged particles, the transport coefficients needed in the equation must be determined. Using Quasi-Linear Theory (QLT, e.g. Jokipii (1966)), one finds that coefficients can be related to the correlation function or power spectrum of homogeneous magnetic turbulence. However, different turbulence models will generally have a different influence on particle's scattering and diffusion. Among those models developed in MHD Turbulence, such as isotropic, Slab plus 2D (Tu & Marsch 1993; Gray et al 1996; Bieber et al 1996), etc. Here, using test-particle orbit simulations to calculate the transport coefficients, we study particle transport in synthesized asymmetric turbulence using the form first proposed by Goldreich & Sridhar (1995). We developed and introduce a systematic method to synthesize scale-dependent non-isotropic magnetic turbulences. We also developed and introduce a method to synthesize the 3d turbulent magnetic field from the observed solar wind time series dataset. We present the comparison of their effects on charge particle transport with previous theories and models.
Experimental study of premixed flames in intense isotropic turbulence
Bedat, B.; Cheng, R.K.
1994-04-01
A methodology for investigating premixed turbulent flames propagating in intense isotropic turbulence has been developed. The burner uses a turbulence generator developed by Videto and Santavicca and the flame is stabilized by weak-swirl generated by air injectors. This set-up produces stable premixed turbulent flames under a wide range of mixture conditions and turbulence intensities. The experiments are designed to investigate systematically the changes in flame structures for conditions which can be classified as wrinkled laminar flames, corrugated flames and flames with distributed reaction zones. Laser Doppler anemometry and Rayleigh scattering techniques are used to determine the turbulence and scalar statistics. In the intense turbulence, the flames are found to produce very little changes in the mean and rams velocities. Their flame speed increase linearly with turbulence intensity as for wrinkled laminar flames. The Rayleigh scattering pdfs for flames within the distributed reaction zone regime are distinctly bimodal. The probabilities of the reacting states (i.e. contributions from within the reaction zone) is not higher than those of wrinkled laminar flame. These results show that there is no drastic changes in flame structures at Karlovitz number close to unity. This suggest that the Klimov-Williams criterion under-predicts the resilience of wrinkled flamelets to intense turbulence.
Computation of the sound generated by isotropic turbulence
NASA Technical Reports Server (NTRS)
Sarkar, S.; Hussaini, M. Y.
1993-01-01
The acoustic radiation from isotropic turbulence is computed numerically. A hybrid direct numerical simulation approach which combines direct numerical simulation (DNS) of the turbulent flow with the Lighthill acoustic analogy is utilized. It is demonstrated that the hybrid DNS method is a feasible approach to the computation of sound generated by turbulent flows. The acoustic efficiency in the simulation of isotropic turbulence appears to be substantially less than that in subsonic jet experiments. The dominant frequency of the computed acoustic pressure is found to be somewhat larger than the dominant frequency of the energy-containing scales of motion. The acoustic power in the simulations is proportional to epsilon (M(sub t))(exp 5) where epsilon is the turbulent dissipation rate and M(sub t) is the turbulent Mach number. This is in agreement with the analytical result of Proudman (1952), but the constant of proportionality is smaller than the analytical result. Two different methods of computing the acoustic power from the DNS data bases yielded consistent results.
Computation of large-scale statistics in decaying isotropic turbulence
NASA Technical Reports Server (NTRS)
Chasnov, Jeffrey R.
1993-01-01
We have performed large-eddy simulations of decaying isotropic turbulence to test the prediction of self-similar decay of the energy spectrum and to compute the decay exponents of the kinetic energy. In general, good agreement between the simulation results and the assumption of self-similarity were obtained. However, the statistics of the simulations were insufficient to compute the value of gamma which corrects the decay exponent when the spectrum follows a k(exp 4) wave number behavior near k = 0. To obtain good statistics, it was found necessary to average over a large ensemble of turbulent flows.
A non-isotropic multiple-scale turbulence model
NASA Technical Reports Server (NTRS)
Chen, C. P.
1990-01-01
A newly developed non-isotropic multiple scale turbulence model (MS/ASM) is described for complex flow calculations. This model focuses on the direct modeling of Reynolds stresses and utilizes split-spectrum concepts for modeling multiple scale effects in turbulence. Validation studies on free shear flows, rotating flows and recirculating flows show that the current model perform significantly better than the single scale k-epsilon model. The present model is relatively inexpensive in terms of CPU time which makes it suitable for broad engineering flow applications.
Model of non-stationary, inhomogeneous turbulence
NASA Astrophysics Data System (ADS)
Bragg, Andrew D.; Kurien, Susan; Clark, Timothy T.
2016-07-01
We compare results from a spectral model for non-stationary, inhomogeneous turbulence (Besnard et al. in Theor Comp Fluid Dyn 8:1-35, 1996) with direct numerical simulation (DNS) data of a shear-free mixing layer (SFML) (Tordella et al. in Phys Rev E 77:016309, 2008). The SFML is used as a test case in which the efficacy of the model closure for the physical-space transport of the fluid velocity field can be tested in a flow with inhomogeneity, without the additional complexity of mean-flow coupling. The model is able to capture certain features of the SFML quite well for intermediate to long times, including the evolution of the mixing-layer width and turbulent kinetic energy. At short-times, and for more sensitive statistics such as the generation of the velocity field anisotropy, the model is less accurate. We propose two possible causes for the discrepancies. The first is the local approximation to the pressure-transport and the second is the a priori spherical averaging used to reduce the dimensionality of the solution space of the model, from wavevector to wavenumber space. DNS data are then used to gauge the relative importance of both possible deficiencies in the model.
Bulk viscosity effect on freely decaying compressible homogeneous isotropic turbulence
NASA Astrophysics Data System (ADS)
Pan, Shaowu; Johnsen, Eric
2015-11-01
Despite growing interests in compressible turbulence, the effect of bulk viscosity has been long ignored. For certain gases, the bulk viscosity may be 1000 times greater than the shear viscosity and thus modify energy transfer and dissipation mechanisms. In this study, we use direct numerical simulations to investigate the role of bulk viscosity on decaying isotropic compressible turbulence. Our results show that bulk viscosity exhibits a negligible decrease on enstrophy, but moderate and significant increases on the turbulent kinetic energy and Taylor-scale Reynolds number, respectively. A Helmholtz decomposition of the velocity field indicates that the bulk viscosity has a negligible effect on the solenoidal part, but exhibits a cross-scale effect on the dilatational component.
Hindered Energy Cascade in Highly Helical Isotropic Turbulence
NASA Astrophysics Data System (ADS)
Stepanov, Rodion; Golbraikh, Ephim; Frick, Peter; Shestakov, Alexander
2015-12-01
The conventional approach to the turbulent energy cascade, based on Richardson-Kolmogorov phenomenology, ignores the topology of emerging vortices, which is related to the helicity of the turbulent flow. It is generally believed that helicity can play a significant role in turbulent systems, e.g., supporting the generation of large-scale magnetic fields, but its impact on the energy cascade to small scales has never been observed. We suggest, for the first time, a generalized phenomenology for isotropic turbulence with an arbitrary spectral distribution of the helicity. We discuss various scenarios of direct turbulent cascades with new helicity effect, which can be interpreted as a hindering of the spectral energy transfer. Therefore, the energy is accumulated and redistributed so that the efficiency of nonlinear interactions will be sufficient to provide a constant energy flux. We confirm our phenomenology by high Reynolds number numerical simulations based on a shell model of helical turbulence. The energy in our model is injected at a certain large scale only, whereas the source of helicity is distributed over all scales. In particular, we found that the helical bottleneck effect can appear in the inertial interval of the energy spectrum.
Hindered Energy Cascade in Highly Helical Isotropic Turbulence.
Stepanov, Rodion; Golbraikh, Ephim; Frick, Peter; Shestakov, Alexander
2015-12-01
The conventional approach to the turbulent energy cascade, based on Richardson-Kolmogorov phenomenology, ignores the topology of emerging vortices, which is related to the helicity of the turbulent flow. It is generally believed that helicity can play a significant role in turbulent systems, e.g., supporting the generation of large-scale magnetic fields, but its impact on the energy cascade to small scales has never been observed. We suggest, for the first time, a generalized phenomenology for isotropic turbulence with an arbitrary spectral distribution of the helicity. We discuss various scenarios of direct turbulent cascades with new helicity effect, which can be interpreted as a hindering of the spectral energy transfer. Therefore, the energy is accumulated and redistributed so that the efficiency of nonlinear interactions will be sufficient to provide a constant energy flux. We confirm our phenomenology by high Reynolds number numerical simulations based on a shell model of helical turbulence. The energy in our model is injected at a certain large scale only, whereas the source of helicity is distributed over all scales. In particular, we found that the helical bottleneck effect can appear in the inertial interval of the energy spectrum. PMID:26684120
Interaction of a converging spherical shock wave with isotropic turbulence
NASA Astrophysics Data System (ADS)
Bhagatwala, Ankit; Lele, Sanjiva K.
2012-08-01
Simulations of converging spherical shock waves propagating through a region of compressible isotropic turbulence are carried out. Both converging and reflected phases of the shock are studied. Effect of the reflected phase of the shock is found to be quite different from the expanding shock in the Taylor blast wave-turbulence interaction problem. Vorticity and turbulent kinetic energy are amplified due to passage of the shock. Similar to the latter problem, the vorticity-dilatation term is primarily responsible for the observed behavior. This is confirmed through Eulerian and Lagrangian statistics. Transverse vorticity amplification is compared with linear planar shock-turbulence theory. The smallest eddies, represented by the Kolmogorov scale, decrease in size after passing through the converging shock and this is shown to be related to a decrease in kinematic viscosity and increase in dissipation behind the converging shock. Distortion of the shock due to turbulence is also investigated and quantified. Turbulence also affects maximum compression achieved at the point of shock reflection, when the shock radius is at a minimum. This decrease in compression is quantified by comparing with pure shock simulations.
Suppression of self-organized structure coarsening in homogenous isotropic turbulence
NASA Astrophysics Data System (ADS)
Takagi, Youhei
2014-11-01
Self-organized structure by spinodal decomposition is often seen in quenched binary mixture. Complex network structure is formed through coarsening process of self-organized structure when the phase separation due to spinodal decomposition proceeds. The phase separation governed by the Cahn-Hilliard equation have been well investigated for stationary fluid in previous studies, however, the turbulent effect on the formation of structures was not fully discussed. In this study, we carried out a numerical simulation for homogenous isotropic turbulence with phase separation, the relation between turbulent vortex formation and self-organized structure coarsening. The governing equations are incompressible Navier-Stokes equation considering phase separation force and Cahn-Hilliard equation with the chemical potential based on the Landau-Ginzburg free energy. From the identification and visualization of turbulent structures, it was found that the local entrainment of small eddy structure suppressed the coarsening process of self-organized structure. The energy used in phase separation was related to the initial process of vortex sheet-tube transition in turbulent flow, and the energy cascade from large turbulent structure to small eddy was different from that without phase separation.
The signature of initial production mechanisms in isotropic turbulence decay
NASA Astrophysics Data System (ADS)
Meldi, M.
2016-03-01
In the present work the quantification of the time-lasting effects of production mechanisms in homogeneous isotropic turbulence decay is addressed. The analysis is developed through the use of theoretical tools as well as numerical calculations based on the eddy damped quasinormal Markovian (EDQNM) model. In both cases a modified Lin equation is used, which accounts for production mechanisms as proposed by Meldi, Lejemble, and Sagaut ["On the emergence of non-classical decay regimes in multiscale/fractal generated isotropic turbulence," J. Fluid Mech. 756, 816-843 (2014)]. The approaches used show that an exponential decay law can be observed if the intensity of the forcing is strong enough to drive the turbulence dynamics, before a power-law decay is eventually attained. The EDQNM numerical results indicate that the exponential regime can persist for long evolution times, longer than the observation time in grid turbulence experiments. A rigorous investigation of the self-similar behavior of the pressure spectrum has been performed by a comprehensive comparison of EDQNM data with direct numerical simulation (DNS)/experiments in the literature. While DNS and free decay EDQNM simulations suggest the need of a very high Reλ threshold in order to observe a clear -7/3 slope of the pressure inertial range, experimental data and forced EDQNM calculations indicate a significantly lower value. This observation suggests that the time-lasting effects of production mechanisms, which cannot be excluded in experiments, play a role in the lack of general agreement with classical numerical approaches. These results reinforce the urge to evolve the numerical simulation state of the art towards the prediction of realistic physical states.
Non-isotropic turbulence effects on spray combustion
NASA Technical Reports Server (NTRS)
Kim, Y. M.; Shang, H. M.; Chen, C. P.
1991-01-01
A numerical model for the prediction of local properties of statistically stationary spray-combusting flows is evaluated by comparison with experimental data. To appraise the relative performance of turbulence models, computations were carried out by the k-epsilon model and the algebraic stress model. The present numerical results show the qualitative agreement with experimental data. In terms of overall local flow properties, the algebraic stress model improves a degree of conformity to the experimental data due to its ability to introduce the nonisotropic turbulence effects. Two swirl numbers are considered to investigate the influence of swirl on the droplet evaporation and trajectories, and the effects of droplet/turbulence interactions in flow properties. It is found that the large swirl produces a higher evaporationn rate, and more intensive turbulent mixing and burning. The discrepancies observed in the results are attributed mainly to uncertainties in the initial spray size and velocity distributions, the droplet/wall impingement interaction, the combustion model with the fast chemistry and the turbulence models dealing with the strong streamline curvature and complex interactions between the dispersed droplets and the continuous gas-phase flows.
Long-time behavior of material-surface curvature in isotropic turbulence
NASA Technical Reports Server (NTRS)
Girimaji, S. S.
1992-01-01
The behavior at large times of the curvature of material elements in turbulence is investigated using Lagrangian velocity-gradient time series obtained from direct numerical simulations of isotropic turbulence. The main objectives are: to study the asymptotic behavior of the pdf curvature as a function of initial curvature and shape; and to establish whether the curvature of an initially plane material element goes to a stationary probability distribution. The evidence available in the literature about the asymptotic curvature-pdf of initially flat surfaces is ambiguous, and the conjecture is that it is quasi-stationary. In this work several material-element ensembles of different initial curvatures and shapes are studied. It is found that, at long times the moments of the logarithm of curvature are independent of the initial pdf of curvature. This, it is argued, supports the view that the curvature attains a stationary distribution at long times. It is also shown that, irrespective of initial shape or curvature, the shape of any material element at long times is cylindrical with a high probability.
Large Deviation Statistics of Vorticity Stretching in Isotropic Turbulence
NASA Astrophysics Data System (ADS)
Johnson, Perry; Meneveau, Charles
2015-11-01
A key feature of 3D fluid turbulence is the stretching/re-alignment of vorticity by the action of the strain-rate. It is shown using the cumulant-generating function that cumulative vorticity stretching along a Lagrangian path in isotropic turbulence behaves statistically like a sum of i.i.d. variables. The Cramer function for vorticity stretching is computed from the JHTDB isotropic DNS (Reλ = 430) and compared to those of the finite-time Lyapunov exponents (FTLE) for material deformation. As expected the mean cumulative vorticity stretching is slightly less than that of the most-stretched material line (largest FTLE), due to the vorticity's preferential alignment with the second-largest eigenvalue of strain-rate and the material line's preferential alignment with the largest eigenvalue. However, the vorticity stretching tends to be significantly larger than the second-largest FTLE, and the Cramer functions reveal that the statistics of vorticity stretching fluctuations are more similar to those of largest FTLE. A model Fokker-Planck equation is constructed by approximating the viscous destruction of vorticity with a deterministic non-linear relaxation law matching conditional statistics, while the fluctuations in vorticity stretching are modelled by stochastic noise matching the statistics encoded in the Cramer function. The model predicts a stretched-exponential tail for the vorticity magnitude PDF, with good agreement for the exponent but significant error (30-40%) in the pre-factor. Supported by NSF Graduate Fellowship (DGE-1232825) and NSF Grant CMMI-0941530.
Subfilter scalar-flux vector orientation in homogeneous isotropic turbulence.
Verma, Siddhartha; Blanquart, G
2014-06-01
The geometric orientation of the subfilter-scale scalar-flux vector is examined in homogeneous isotropic turbulence. Vector orientation is determined using the eigenframe of the resolved strain-rate tensor. The Schmidt number is kept sufficiently large so as to leave the velocity field, and hence the strain-rate tensor, unaltered by filtering in the viscous-convective subrange. Strong preferential alignment is observed for the case of Gaussian and box filters, whereas the sharp-spectral filter leads to close to a random orientation. The orientation angle obtained with the Gaussian and box filters is largely independent of the filter width and the Schmidt number. It is shown that the alignment direction observed numerically using these two filters is predicted very well by the tensor-diffusivity model. Moreover, preferred alignment of the scalar gradient vector in the eigenframe is shown to mitigate any probable issues of negative diffusivity in the tensor-diffusivity model. Consequentially, the model might not suffer from solution instability when used for large eddy simulations of scalar transport in homogeneous isotropic turbulence. Further a priori tests indicate poor alignment of the Smagorinsky and stretched vortex model predictions with the exact subfilter flux. Finally, strong filter dependence of subfilter scalar-flux orientation suggests that explicit filtering may be preferable to implicit filtering in large eddy simulations. PMID:25019887
Taylor length-scale size particles in Isotropic Turbulence
NASA Astrophysics Data System (ADS)
Lucci, Francesco
The present study investigates the two-way coupling effects of finite-size solid spherical particles on decaying isotropic turbulence using an immersed boundary method. The conventional point particle assumption is valid only in the case of particles with a diameter, dp, much smaller than the Kolmogorov length scale, eta. In a simulation with particles of diameter dp > eta the flow around each particle needs to be resolved. In this study, we use a method similar to that of Uhlmann(2005) [55] that adapts the Immersed Boundary(IB) Method developed by Peskin [38] to simulate the flow around suspended spherical solid particles. The main idea of the method is to distribute a number of Lagrangian points uniformly over the surface of the particle. A force is applied at each Lagrangian point to represent the momentum exchange between the particle and the surrounding fluid. An analytic three-point delta function is used to distribute the force to the Eulerian grid points saddling the spherical surface to satisfy the no-slip condition at each Lagrangian point. Decaying turbulence is simulated in a periodic box with a uniform mesh of up to (512)3 grid points and an initial microscale Reynolds number of up to Relambda0 = 110. We compare the single phase flow (SPF) with particle-laden flows with particles of different diameters. The density of the particle varies from 2.56 to 10 times that of the fluid. The effects of the particles on the temporal development of turbulence kinetic energy E(t), its dissipation rate epsilon( t), its two-way coupling rate of change Ψp( t) and frequency spectra E(o) are discussed. In this study, in contrast to particles with dp < eta [15], particles with dp > eta always increase the dissipation rate of turbulence kinetic energy, epsilon( t). In addition, Ψp(t) is always positive, whereas it can be positive or negative for particles with dp < eta. The balance between these two effects caused E(t) to be smaller than that of the single-phase flow
Coherent Vortex Simulations of 3D isotropic turbulence
NASA Astrophysics Data System (ADS)
Goldstein, Daniel E.; Vasilyev, Oleg V.; Kevlahan, Nicholas K.-R.
2006-11-01
This is the first of three talks on the wavelet filter based dynamically adaptive eddy capturing computational methodology that unifies variable fidelity simulation approaches such as wavelet-based DNS, Coherent Vortex Simulation (CVS), and Stochastic Coherent Adaptive Large Eddy Simulation. The commonality of these approaches is their ability to identify and ``track" on an adaptive mesh energetic coherent vortical structures. In CVS the velocity field is decomposed into two orthogonal parts: a coherent, inhomogeneous, non-Gaussian component and an incoherent, homogeneous, Gaussian component. This separation of coherent and incoherent components is achieved by wavelet thresholding which can be viewed as a non-linear filter that depends on each flow realization. The essence of the CVS approach is to solve for the coherent non-Gaussian component of a turbulent flow field. It has been shown previously that second generation bi-orthogonal wavelet threshold filtering is able to decompose a turbulent velocity field such that the total resulting SGS dissipation is approximately zero. This physically allows a CVS simulation to recover low order statistics with no SGS model. In this work CVS simulations of decaying incompressible 3D isotropic turbulence are compared to DNS results. -6pt
Local topology of energy transport in isotropic turbulence
NASA Astrophysics Data System (ADS)
Boschung, Jonas; Meneveau, Charles
2012-11-01
Similar to the velocity vector field, whose tangent (stream) lines represent how fluid volume (or mass in constant density flows) is transported in the flow, it is of interest to consider the vector field corresponding to the transport of mechanical energy (Meyers & Meneveau, 2012). The transport includes advection and viscous diffusion. In order to characterize the local topology of this vector field in turbulence, we examine statistical properties of its gradient field. This energy transport field is not divergence-free, due to dissipation and unsteady changes of kinetic energy. Therefore, the first invariant (the trace) of its gradient tensor is not zero, as in compressible flow. The three invariants PE, QE and RE of the energy transport gradient tensor are analyzed using concepts developed earlier for analysis of compressible flows. Data from DNS of isotropic turbulence is used, from the JHU database (Li et al. 2008, JoT), as well as other sources. Contracting node-like topology occurs very frequently, consistent with the dissipative nature of fluid turbulence. Further topological properties are established based on conditional PDFs of the invariants, and flow visualizations are used to develop insights into the local structure of the energy transport vector field. This work is supported by project CMMI-0941530. The authors also thank Prof. J. Meyers, Prof. N. Peters and Mr. P. Schaefer for interesting discussions on this topic.
Energy transfer and constrained simulations in isotropic turbulence
NASA Technical Reports Server (NTRS)
Jimenez, Javier
1993-01-01
The defining characteristic of turbulent flows is their ability to dissipate energy, even in the limit of zero viscosity. The Euler equations, if constrained in such a way that the velocity derivatives remain bounded, conserve energy. But when they arise as the limit of the Navier-Stokes (NS) equations, when the Reynolds number goes to infinity, there is persuasive empirical evidence that the gradients become singular as just the right function of Re for the dissipation to remain non-zero and to approach a well defined limit. It is generally believed that this limiting value of the dissipation is a property of the Euler equations themselves, independent of the particular dissipative mechanism involved, and that it can be normalized with the large scale properties of the turbulent flow (e.g. the kinetic energy per unit volume u'(exp 2)/2, and the integral scale L) without reference to the Reynolds number or to other dissipative quantities. This is usually taken to imply that the low wave number end of the energy spectrum, far from the dissipative range, is also independent of the particular mechanism chosen to dispose of the energy transfer. In the following sections, we present some numerical experiments on the effect of substituting different dissipation models into the truncated Euler equations. We will see that the effect is mainly felt in the 'near dissipation' range of the energy spectrum, but that this range can be quite wide in some cases, contaminating a substantial range of wave numbers. In the process, we will develop a 'practical' approximation to the subgrid energy transfer in isotropic turbulence, and we will gain insight into the structure of the nonlinear interactions among turbulent scales of comparable size, and into the nature of energy backscatter. Some considerations on future research directions are offered at the end.
Fully developed isotropic turbulence: Symmetries and exact identities
NASA Astrophysics Data System (ADS)
Canet, Léonie; Delamotte, Bertrand; Wschebor, Nicolás
2015-05-01
We consider the regime of fully developed isotropic and homogeneous turbulence of the Navier-Stokes equation with a stochastic forcing. We present two gauge symmetries of the corresponding Navier-Stokes field theory and derive the associated general Ward identities. Furthermore, by introducing a local source bilinear in the velocity field, we show that these symmetries entail an infinite set of exact and local relations between correlation functions. They include in particular the Kármán-Howarth relation and another exact relation for a pressure-velocity correlation function recently derived in G. Falkovich, I. Fouxon, and Y. Oz [J. Fluid Mech. 644, 465 (2010)], 10.1017/S0022112009993429 that we further generalize.
Clustering of vertically constrained passive particles in homogeneous isotropic turbulence
NASA Astrophysics Data System (ADS)
De Pietro, Massimo; van Hinsberg, Michel A. T.; Biferale, Luca; Clercx, Herman J. H.; Perlekar, Prasad; Toschi, Federico
2015-05-01
We analyze the dynamics of small particles vertically confined, by means of a linear restoring force, to move within a horizontal fluid slab in a three-dimensional (3D) homogeneous isotropic turbulent velocity field. The model that we introduce and study is possibly the simplest description for the dynamics of small aquatic organisms that, due to swimming, active regulation of their buoyancy, or any other mechanism, maintain themselves in a shallow horizontal layer below the free surface of oceans or lakes. By varying the strength of the restoring force, we are able to control the thickness of the fluid slab in which the particles can move. This allows us to analyze the statistical features of the system over a wide range of conditions going from a fully 3D incompressible flow (corresponding to the case of no confinement) to the extremely confined case corresponding to a two-dimensional slice. The background 3D turbulent velocity field is evolved by means of fully resolved direct numerical simulations. Whenever some level of vertical confinement is present, the particle trajectories deviate from that of fluid tracers and the particles experience an effectively compressible velocity field. Here, we have quantified the compressibility, the preferential concentration of the particles, and the correlation dimension by changing the strength of the restoring force. The main result is that there exists a particular value of the force constant, corresponding to a mean slab depth approximately equal to a few times the Kolmogorov length scale η , that maximizes the clustering of the particles.
DNS of fully resolved spherical particles dispersed in isotropic turbulence
NASA Astrophysics Data System (ADS)
Lucci, Francesco; Ferrante, Antonino; Elghobashi, Said
2008-11-01
Our DNS study concerns the interactions between decaying isotropic turbulence and solid spherical particles with diameter, d, larger than the Kolmogorov length scale, η. We employ an Immersed Boundary method similar to that of Uhlmann (JCP, 2005) to resolve the flow around 6400 spherical particles with a volume fraction of φv=0.1. The monosize particles have a diameter, d = 16 ηo. Our simulations, with 256^3 mesh points and Reλ0= 75, cover a range of 38 <=τp/τKo<=149, for the ratio of the particle response time to the initial Kolmogorov time scale. A Lagrangian approach is used to compute the frequency spectrum of the turbulence kinetic energy (TKE) of the fluid phase. The effects of varying τp/τKo on the spectrum and the decay rate of TKE are discussed. The effects of the formation of the particle boundary layer on the viscous dissipation rate of TKE are also discussed.
NASA Astrophysics Data System (ADS)
Goto, Susumu; Kida, Shigeo
1999-07-01
The Lagrangian direct-interaction approximation developed previously by the present authors [S. Kida and S. Goto, J. Fluid Mech. 345, 307 (1997)] is applied to a passive scalar field in isotropic turbulence. We examine the behavior of solutions to the resultant closure equations for the correlation function of the scalar field for arbitrary values of the Schmidt number, and show systematically that the solutions are completely consistent with the phenomenological theories on the scalar spectral function by Obukhov (1949), Corrsin (1951), Batchelor et al. (1959), and Batchelor (1959). The universal forms of the function in the statistically stationary state are obtained by solving the closure equations numerically in the whole wave number range for each case of moderate, extremely large, and small values of the Schmidt number.
Preferential concentration of heavy particles in compressible isotropic turbulence
NASA Astrophysics Data System (ADS)
Zhang, Qingqing; Liu, Han; Ma, Zongqiang; Xiao, Zuoli
2016-05-01
Numerical simulations of particle-laden compressible isotropic turbulence with Taylor Reynolds number Reλ ˜ 100 are conducted by using a high-order turbulence solver, which is based on high-order compact finite difference method in the whole flow domain and localized artificial diffusivities for discontinuities. For simplicity, only one-way coupling (i.e., the influence of fluid on particles) between the carrier flow and particles is considered. The focus is on the study of the preferential concentration of heavy particles in dissipative scale of turbulence and the underlying mechanisms. Firstly, the effect of Stokes number (St) on the particle distribution in flow of Mach 1.01 (referred to as high-Mach-number case in this study) is investigated as a necessary supplementation for the previous studies in incompressible and weakly compressible flows. It turns out that heavy particles with Stokes number close to unity exhibit the strongest preferential concentration, which is in agreement with the observation in incompressible flow. All types of heavy particles have a tendency to accumulate in high-density regions of the background flow. While all kinds of particles dominantly collect in low-vorticity regions, intermediate and large particles (St = 1 and St = 5) are also found to collect in high-vorticity regions behind the randomly formed shocklets. Secondly, the impact of turbulent Mach number (Mt) (or the compressibility) of the carrier flow on the spatial distribution of the particles with St = 1 is discussed using the simulated compressible flows with Mt being 0.22, 0.68, and 1.01, respectively. In low-Mach-number flow, particles tend to concentrate in regions of low vorticity due to the centrifuge effect of vortices and particle concentration decreases monotonically with the increasing vorticity magnitude. As Mach number increases, the degree of particle clustering is slightly weakened in low-vorticity regions but is enhanced in high-vorticity regions, which
Symmetries and the approach to statistical equilibrium in isotropic turbulence
NASA Astrophysics Data System (ADS)
Clark, Timothy T.; Zemach, Charles
1998-11-01
The relaxation in time of an arbitrary isotropic turbulent state to a state of statistical equilibrium is identified as a transition to a state which is invariant under a symmetry group. We deduce the allowed self-similar forms and time-decay laws for equilibrium states by applying Lie-group methods (a) to a family of scaling symmetries, for the limit of high Reynolds number, as well as (b) to a unique scaling symmetry, for nonzero viscosity or nonzero hyperviscosity. This explains why a diverse collection of turbulence models, going back half a century, arrived at the same time-decay laws, either through derivations embedded in the mechanics of a particular model, or through numerical computation. Because the models treat the same dynamical variables having the same physical dimensions, they are subject to the same scaling invariances and hence to the same time-decay laws, independent of the eccentricities of their different formulations. We show in turn, by physical argument, by an explicitly solvable analytical model, and by numerical computation in more sophisticated models, that the physical mechanism which drives (this is distinct from the mathematical circumstance which allows) the relaxation to equilibrium is the cascade of turbulence energy toward higher wave numbers, with the rate of cascade approaching zero in the low wave-number limit and approaching infinity in the high wave-number limit. Only the low-wave-number properties of the initial state can influence the equilibrium state. This supplies the physical basis, beyond simple dimensional analysis, for quantitative estimates of relaxation times. These relaxation times are estimated to be as large as hundreds or more times the initial dominant-eddy cycle times, and are determined by the large-eddy cycle times. This mode of analysis, applied to a viscous turbulent system in a wind tunnel with typical initial laboratory parameters, shows that the time necessary to reach the final stage of decay is
Carroll, Jonathan J.; Frank, Adam; Blackman, Eric G.
2010-10-10
Feedback from protostellar outflows can influence the nature of turbulence in star-forming regions even if they are not the primary source of velocity dispersion for all scales of molecular clouds. For the rate and power expected in star-forming regions, we previously (Carroll et al.) demonstrated that outflows could drive supersonic turbulence at levels consistent with the scaling relations from Matzner although with a steeper velocity power spectrum than expected for an isotropically driven supersonic turbulent cascade. Here, we perform higher resolution simulations and combine simulations of outflow driven turbulence with those of isotropically forced turbulence. We find that the presence of outflows within an ambient isotropically driven turbulent environment produces a knee in the velocity power spectrum at the outflow scale and a steeper slope at sub-outflow scales than for a purely isotropically forced case. We also find that the presence of outflows flattens the density spectrum at large scales effectively reducing the formation of large-scale turbulent density structures. These effects are qualitatively independent of resolution. We have also carried out Principal Component Analysis (PCA) for synthetic data from our simulations. We find that PCA as a tool for identifying the driving scale of turbulence has a misleading bias toward low amplitude large-scale velocity structures even when they are not necessarily the dominant energy containing scales. This bias is absent for isotropically forced turbulence but manifests strongly for collimated outflow driven turbulence.
Pressure and higher-order spectra for homogeneous isotropic turbulence
NASA Technical Reports Server (NTRS)
Pullin, D. I.; Rogallo, R. S.
1994-01-01
The spectra of the pressure, and other higher-order quantities including the dissipation, the enstrophy, and the square of the longitudinal velocity derivative are computed using data obtained from direct numerical simulation of homogeneous isotropic turbulence at Taylor-Reynolds numbers R(sub lambda) in the range 38 - 170. For the pressure spectra we find reasonable collapse in the dissipation range (of the velocity spectrum) when scaled in Kolmogorov variables and some evidence, which is not conclusive, for the existence of a k(exp -7/3) inertial range where k = absolute value of K, is the modulus of the wavenumber. The power spectra of the dissipation, the enstrophy, and the square of the longitudinal velocity derivative separate in the dissipation range, but appear to converge together in the short inertial range of the simulations. A least-squares curve-fit in the dissipation range for one value of R(sub lambda) = 96 gives a form for the spectrum of the dissipation as k(exp 0)exp(-Ck eta), for k(eta) greater than 0.2, where eta is the Kolmogorov length and C is approximately equal to 2.5.
Energy transfer and dissipation in forced isotropic turbulence.
McComb, W D; Berera, A; Yoffe, S R; Linkmann, M F
2015-04-01
A model for the Reynolds-number dependence of the dimensionless dissipation rate C(ɛ) was derived from the dimensionless Kármán-Howarth equation, resulting in C(ɛ)=C(ɛ,∞)+C/R(L)+O(1/R(L)(2)), where R(L) is the integral scale Reynolds number. The coefficients C and C(ɛ,∞) arise from asymptotic expansions of the dimensionless second- and third-order structure functions. This theoretical work was supplemented by direct numerical simulations (DNSs) of forced isotropic turbulence for integral scale Reynolds numbers up to R(L)=5875 (R(λ)=435), which were used to establish that the decay of dimensionless dissipation with increasing Reynolds number took the form of a power law R(L)(n) with exponent value n=-1.000±0.009 and that this decay of C(ɛ) was actually due to the increase in the Taylor surrogate U(3)/L. The model equation was fitted to data from the DNS, which resulted in the value C=18.9±1.3 and in an asymptotic value for C(ɛ) in the infinite Reynolds-number limit of C(ɛ,∞)=0.468±0.006. PMID:25974586
Interacting scales and energy transfer in isotropic turbulence
NASA Technical Reports Server (NTRS)
Zhou, YE
1993-01-01
The dependence of the energy transfer process on the disparity of the interacting scales is investigated in the inertial and far-dissipation ranges of isotropic turbulence. The strategy for generating the simulated flow fields and the choice of a disparity parameter to characterize the scaling of the interactions is discussed. The inertial range is found to be dominated by relatively local interactions, in agreement with the Kolmogorov assumption. The far-dissipation is found to be dominated by relatively non-local interactions, supporting the classical notion that the far-dissipation range is slaved to the Kolmogorov scales. The measured energy transfer is compared with the classical models of Heisenberg, Obukhov, and the more detailed analysis of Tennekes and Lumley. The energy transfer statistics measured in the numerically simulated flows are found to be nearly self-similar for wave numbers in the inertial range. Using the self-similar form measured within the limited scale range of the simulation, an 'ideal' energy transfer function and the corresponding energy flux rate for an inertial range of infinite extent are constructed. From this flux rate, the Kolmogorov constant is calculated to be 1.5, in excellent agreement with experiments.
Inverse energy cascade in three-dimensional isotropic turbulence.
Biferale, Luca; Musacchio, Stefano; Toschi, Federico
2012-04-20
We study the statistical properties of homogeneous and isotropic three-dimensional (3D) turbulent flows. By introducing a novel way to make numerical investigations of Navier-Stokes equations, we show that all 3D flows in nature possess a subset of nonlinear evolution leading to a reverse energy transfer: from small to large scales. Up to now, such an inverse cascade was only observed in flows under strong rotation and in quasi-two-dimensional geometries under strong confinement. We show here that energy flux is always reversed when mirror symmetry is broken, leading to a distribution of helicity in the system with a well-defined sign at all wave numbers. Our findings broaden the range of flows where the inverse energy cascade may be detected and rationalize the role played by helicity in the energy transfer process, showing that both 2D and 3D properties naturally coexist in all flows in nature. The unconventional numerical methodology here proposed, based on a Galerkin decimation of helical Fourier modes, paves the road for future studies on the influence of helicity on small-scale intermittency and the nature of the nonlinear interaction in magnetohydrodynamics. PMID:22680722
Testing a similarity theory for isotropic turbulence on DNS data.
NASA Astrophysics Data System (ADS)
Melander, Mogens; Fabijonas, Bruce
2006-11-01
Using direct numerical simulations, we consider the issue of self-similarity in 3D incompressible isotropic turbulence. The starting point for our investigation is a similarity theory we have developed on the basis of high Reynolds number shell model calculations. Like Kolmogorov's 1941 theory, our theory calls for similarity across all scales in the inertial range. Unlike K41, our theory does not fail on account of intermittency, but is developed to blossom in that environment. To observe self-similarity, it is essential that the correct variables are used, otherwise one sees only intermittency. The correct variables are reasonably easy to spot for the shell model, but they are more difficult to identify for the full Navier-Stokes equations. Moreover, one has to overcome the fact that the DNS has lower Reynolds numbers than in the shell model simulations so that the inertial range is shorter. Using the technique ESS, we clear this obstacle with only a minor modification to the theory. The DNS data then collapse on the theoretical pdf at all scales.
Energy transfer and dissipation in forced isotropic turbulence
NASA Astrophysics Data System (ADS)
McComb, W. D.; Berera, A.; Yoffe, S. R.; Linkmann, M. F.
2015-04-01
A model for the Reynolds-number dependence of the dimensionless dissipation rate Cɛ was derived from the dimensionless Kármán-Howarth equation, resulting in Cɛ=Cɛ ,∞+C /RL+O (1 /RL2) , where RL is the integral scale Reynolds number. The coefficients C and Cɛ ,∞ arise from asymptotic expansions of the dimensionless second- and third-order structure functions. This theoretical work was supplemented by direct numerical simulations (DNSs) of forced isotropic turbulence for integral scale Reynolds numbers up to RL=5875 (Rλ=435 ), which were used to establish that the decay of dimensionless dissipation with increasing Reynolds number took the form of a power law RLn with exponent value n =-1.000 ±0.009 and that this decay of Cɛ was actually due to the increase in the Taylor surrogate U3/L . The model equation was fitted to data from the DNS, which resulted in the value C =18.9 ±1.3 and in an asymptotic value for Cɛ in the infinite Reynolds-number limit of Cɛ ,∞=0.468 ±0.006 .
The structure of intense vorticity in homogeneous isotropic turbulence
NASA Technical Reports Server (NTRS)
Jimenez, J.; Wray, A. A.; Saffman, P. G.; Rogallo, R. S.
1992-01-01
The structure of the intense vorticity regions is studied in numerically simulated homogeneous, isotropic, equilibrium turbulent flow fields at four different Reynolds numbers in the range Re(sub lambda) = 36-171. In accordance with previous investigators, this vorticity is found to be organized in coherent, cylindrical or ribbon-like, vortices ('worms'). A statistical study suggests that they are just especially intense features of the background, O(omega'), vorticity. Their radii scale with the Kolmogorov microscale and their lengths with the integral scale of the flow. An interesting observation is that the Reynolds number based on the circulation of the intense vortices, gamma/nu, increases monotonically with Re(sub lambda), raising the question of the stability of the structures in the limit of Re(sub lambda) approaching infinity. One and two-dimensional statistics of vorticity and strain are presented; they are non-gaussian, and the behavior of their tails depends strongly on the Reynolds number. There is no evidence of convergence to a limiting distribution in our range of Re(sub lambda), even though the energy spectra and the energy dissipation rate show good asymptotic properties in the higher Reynolds number cases. Evidence is presented to show that worms are natural features of the flow and that they do not depend on the particular forcing scheme.
Effect of freestream isotropic turbulence on heat transfer from a sphere
NASA Astrophysics Data System (ADS)
Bagchi, Prosenjit; Kottam, Kirit
2008-07-01
We consider direct numerical simulation (DNS) based on pseudospectral methods to study the heat transfer around a stationary sphere held at a constant temperature and subject to an ambient turbulent velocity and temperature condition. The sphere Reynolds number is in the range of 63-400, and the sphere diameter (d) varies from one to eight times the Kolmogorov scale (η). The ambient turbulent field is isotropic, and the Taylor microscale Reynolds number Rλ varies from 38 to 240. Results from two sets of DNS are presented. In the first set, the ambient velocity field is turbulent, but the ambient temperature is held constant. In the second set of simulations, both the ambient velocity and the temperature fields are turbulent. These two sets of simulations allow us to isolate the role of freestream velocity fluctuations and temperature fluctuations in modifying the mean and time-dependent heat transfer from the sphere. The mean Nusselt number is observed to be independent of Rλ. It is shown that the freestream turbulence does not have any significant effect on the mean Nusselt number, and the available correlations for a steady and uniform ambient can predict the mean Nusselt number under the turbulent ambient condition. The instantaneous Nusselt number, however, can differ significantly from the mean, and can be negative in case of large temperature fluctuation in the far field. The instantaneous Nusselt number obtained from the DNS is analyzed and compared with the analytical expression for the unsteady thermal response of a sphere. It is shown that the thermal added-mass effect is small for d /η≈1 but introduces spurious oscillation at higher d. The thermal history effect is shown to be insignificant for all d /η. Properties of the thermal wake in the presence of the turbulent velocity and temperature fields are studied. The mean thermal wake is observed to be shorter in streamwise direction and wider in crossflow direction in a turbulent ambient than that
A Stochastic Model for the Relative Motion of High Stokes Number Particles in Isotropic Turbulence
NASA Astrophysics Data System (ADS)
Dhariwal, Rohit; Rani, Sarma; Koch, Donald
2014-11-01
In the current study, a novel analytical closure for the diffusion current in the PDF equation is presented that is applicable to high-inertia particle pairs with Stokes numbers Str >> 1 . Here Str is a Stokes number based on the time-scale τr of eddies whose size scales with pair separation r. Using this closure, Langevin equations were solved to evolve particle-pair relative velocities and separations in stationary isotropic turbulence. The Langevin equation approach enables the simulation of the full PDF of pair relative motion, instead of only the first few moments of the PDF as is the case in a moments-based approach. Accordingly, PDFs Ω (U | r) and Ω (Ur | r) are computed for various separations r, where the former is the PDF of relative velocity U and the latter is the PDF of the radial component of relative velocity Ur, both conditioned upon the separation r. Consistent with the DNS study of Sundaram & Collins, the Langevin simulations capture the transition of Ω (U | r) from being Gaussian at integral-scale separations to an exponential PDF at Kolmogorov-scale separations. The radial distribution functions (RDFs) computed from these simulations also show reasonable quantitative agreement with those from the DNS of Fevrier et al.
NASA Astrophysics Data System (ADS)
Bassenne, Maxime; Urzay, Javier; Park, George I.; Moin, Parviz
2016-03-01
This study investigates control-based forcing methods for incompressible homogeneous-isotropic turbulence forced linearly in physical space which result in constant turbulent kinetic energy, constant turbulent dissipation (also constant enstrophy), or a combination of the two based on a least-squares error minimization. The methods consist of proportional controllers embedded in the forcing coefficients. During the transient, the controllers adjust the forcing coefficients such that the controlled quantity achieves very early a minimal relative error with respect to its target stationary value. Comparisons of these forcing methods are made with the non-controlled approaches of Rosales and Meneveau ["Linear forcing in numerical simulations of isotropic turbulence: Physical space implementations and convergence properties," Phys. Fluids 17, 095106 (2005)] and Carroll and Blanquart ["A proposed modification to Lundgren's physical space velocity forcing method for isotropic turbulence," Phys. Fluids 25, 105114 (2013)], using direct numerical simulations (DNS) and large-eddy simulations (LES). The results indicate that the proposed constant-energetics forcing methods shorten the transient period from a user-defined artificial flow field to Navier-Stokes turbulence while maintaining steadier statistics. Additionally, the proposed method of constant kinetic-energy forcing behaves more robustly in coarse LES when initial conditions are employed that favor the occurrence of subgrid-scale backscatter, whereas the other approaches fail to provide physical turbulent flow fields. For illustration, the proposed forcing methods are applied to dilute particle-laden homogeneous-isotropic turbulent flows; the results serve to highlight the influences of the forcing strategies on the disperse-phase statistics.
Local energy transfer and nonlocal interactions in homogeneous, isotropic turbulence
NASA Technical Reports Server (NTRS)
Domaradzki, J. Andrzej; Rogallo, Robert S.
1990-01-01
Detailed computations were made of energy transfer among the scales of motion in incompressible turbulent fields at low Reynolds numbers, generated by direct numerical simulations. It was observed that, although the transfer resulted from triad interactions that were nonlocal in k space, the energy always transferred locally. The energy transfer calculated from the eddy-damped quasi-normal Markovian (EDQNM) theory of turbulence at low Reynolds numbers is in excellent agreement with the results of the numerical simulations. At high Reynolds numbers, the EDQNM theory predicts the same transfer mechanism in the inertial range that is observed at low Reynolds numbers.
Sweeping and straining effects in sound generation by high Reynolds number isotropic turbulence
NASA Technical Reports Server (NTRS)
Zhou, YE; Rubinstein, Robert
1995-01-01
The sound radiated by isotropic turbulence is computed using inertial range scaling expressions for the relevant two time and two point correlations. The result depends on whether the decay of Eulerian time correlations is dominated by large scale sweeping or by local straining: the straining hypothesis leads to an expression for total acoustic power, whereas the sweeping hypothesis leads to a more recent result.
Settling of almost neutrally buoyant particles in homogeneous isotropic turbulence
NASA Astrophysics Data System (ADS)
van Hinsberg, Michel; Clercx, Herman; Toschi, Federico
2015-11-01
Settling of particles in a turbulent flow occurs in various industrial and natural phenomena, examples are clouds and waste water treatment. It is well known that turbulence can enhance the settling velocity of particles. Many studies have been done, numerically and experimentally to investigate this behavior for the case of ``heavy'' particles, with particle to fluid density ratios above 100. Here we investigate the case of almost neutrally buoyant particles, i.e. density ratios between 1 and 100. In the case of light particles the Maxey-Riley equations cannot be simplified to only the Stokes drag and gravity force as pressure gradient, added mass and Basset history force are important as well. We investigate the influence of these forces on the settling velocity of particles and show that the extra forces can both increase or decrease the settling velocity, depending on the combination of the Stokes number and gravity applied.
Energy transfer in isotropic turbulence at low Reynolds numbers
NASA Technical Reports Server (NTRS)
Domaradzki, J. A.; Rogallo, R. S.
1988-01-01
Detailed measurements were made of energy transfer among the scales of motion in incompressible turbulent fields at low Reynolds numbers generated by direct numerical simulation. It was observed that although the transfer resulted from triad interactions that were non-local in k space, the energy always transferred locally. The results are consistent with the notion of non-uniform advection of small weak eddies by larger and stronger ones, similar to transfer processes in the far dissipation range at high Reynolds numbers.
Grid-generated isotropic homogeneous turbulence at high Reynolds numbers
NASA Technical Reports Server (NTRS)
Rosen, G.
1981-01-01
Consideration is given to an empirical formula for the longitudinal correlation function for grid-generated incompressible fluid turbulence at Reynolds numbers above 12,800. The formula, which relates the longitudinal correlation function to the inverse cube of a dimensionless geometrical ratio, is shown to minimize the global correlation integrals into which the two-point velocity correlation tensor has been substituted subject to a global constraint on the Sobolev concomitent of the longitudinal correlation function. Furthermore, the energy spectrum function associated with the empirical formula is shown to satisfy a tertiary Helmholtz-type linear condition throughout the initial period of decay.
Mixing of a passive scalar in isotropic and sheared homogeneous turbulence
NASA Technical Reports Server (NTRS)
Shirani, E.; Ferziger, J. H.; Reynolds, W. C.
1981-01-01
In order to calculate the velocity and scalar fields, the three dimensional, time-dependent equations of motion and the diffusion equation were solved numerically. The following cases were treated: isotropic, homogeneous turbulence with decay of a passive scalar; and homogeneous turbulent shear flow with a passive scalar whose mean varies linearly in the spanwise direction. The solutions were obtained at relatively low Reynolds numbers so that all of the turbulent scales could be resolved without modeling. Turbulent statistics such as integral length scales, Taylor microscales, Kolmogorov length scale, one- and two-point correlations of velocity-velocity and velocity-scalar, turbulent Prandtl/Schmidt number, r.m.s. values of velocities, the scalar quantity and pressure, skewness, decay rates, and decay exponents were calculated. The results are compared with the available expermental results, and good agreement is obtained.
Decay Power Law in, High Intensity, Isotropic Turbulent Flow
NASA Astrophysics Data System (ADS)
Koster, Timothy; Puga, Alejandro; Larue, John
2014-11-01
In the study reported here, isotropy is determined using the measure proposed by George (1992), where isotropy corresponds to those downstream positions where the product of the Taylor Reynolds number and the skewness of the velocity derivative is a constant. Straight forward approach can be used which is based on the observation of Batchelor (1953), that the square of the Talor micorscale is linearly related to downstream distance relative to the virtual origin. The fact that the decay of downstream velocity variance is described by a power law is shown to imply power law behavior for various other parameters such as the dissipation, the integral length scale, the Taylor microscale, the Kolmogorov microscale and the Taylor Reynolds number and that there is an algebraic relationship between the various power law exponents. Results are presented for mean velocities of 6 and 8 m/s for the downstream decay of the parameters listed in the preceding. The corresponding values of the Taylor Reynolds number at the start of the isotropic region are 290 and 400, and the variance decay exponent and virtual origin are found to be respectively -1.707 and -1.298 and -27.95 and -5.757. The exponents in the decay law for the other parameters are found to be within +/- 3% of the expected values. University of California Irvine Research Funds.
Theoretical and computational studies of isotropic homogeneous turbulence
NASA Astrophysics Data System (ADS)
Huang, Mei-Jiau
Numerical simulations are presented for viscous incompressible homogeneous turbulent flows with periodic boundary conditions. Our numerical method is based on the spectral Fourier method. Rogallo's code is modified and extended to trace fluid particles and simulate the evolution of material line elements. The first part of the thesis is about modifying and applying the code to simulate a passive vector field convected and stretched by the so called ABC flows in the presence of viscosity. The correlation of the geometry of the physical structures of the passive vector with the external straining is investigated. It is observed that most amplifications either occur in the neighborhoods of local unstable manifolds of the stagnation points of the ABC flows, if they exist, especially those with only one positive eigenvalue, or they are confined within the chaotic regions of the ABC flows if there is no stagnation point. The second part of the thesis is an investigation of the power-law energy decay of turbulence. Two decay exponents, 1.24 and 1.54, are measured from simulations. A new similarity form for the double and triple velocity autocorrelation functions using the Taylor microscale as the scaling, consistent with the Karman-Howarth equation and a power-law energy decay, is proposed and compared with numerical results. The proposed similarity form seems applicable at small to intermediate Reynolds number. For flows with very large Reynolds number, an expansion form of energy spectrum is proposed instead. The third part of the thesis is a presentation of the Lagrangian data collected by tracking fluid particles in decaying turbulent flows. The mean growth rates of the magnitudes of material line elements, that of the vorticity due to nonlinear forces, and the mean principal rates of strain tensors are found to be proportional to the square root of the mean enstrophy. The proportional coefficients remain constant during the decay. The mean angles between material
Theoretical and Computational Studies of Isotropic Homogeneous Turbulence
NASA Astrophysics Data System (ADS)
Huang, Mei-Jiau
Numerical simulations are presented for viscous incompressible homogeneous turbulent flows with periodic boundary conditions. Our numerical method is based on the spectral Fourier method. Rogallo's code is modified and extended to trace fluid particles and simulate the evolution of material line elements. The first part of the thesis is about modifying and applying the code to simulate a passive vector field convected and stretched by the so-called ABC flows in the presence of viscosity. The correlation of the geometry of the physical structures of the passive vector with the external straining is investigated. It is observed that most amplifications either occur in the neighborhoods of local unstable manifolds of the stagnation points of the ABC flows, if they exist, especially those with only one positive eigenvalue, or they are confined within the chaotic regions of the ABC flows if there is no stagnation point. The second part of the thesis is an investigation of the power-law energy decay of turbulence. Two decay exponents, 1.24 and 1.54, are measured from simulations. A new similarity form for the double and triple velocity autocorrelation functions using the Taylor microscale as the scaling, consistent with the Karman-Howarth equation and a power-law energy decay, is proposed and compared with numerical results. The proposed similarity form seems applicable at small to intermediate Reynolds number. For flows with very large Reynolds number, an expansion form of energy spectrum is proposed instead. The third part of the thesis is a presentation of the Lagrangian data collected by tracking fluid particles in decaying turbulent flows. The mean growth rates of the magnitudes of material line elements, that of the vorticity due to nonlinear forces, and the mean principal rates of strain tensors are found to be proportional to the square root of the mean enstrophy. The proportional coefficients remain constant during the decay. The mean angles between material
The radiated noise from isotropic turbulence and heated jets
NASA Technical Reports Server (NTRS)
Lilley, G. M.
1995-01-01
Our understanding of aerodynamic noise has its foundations in the work of Sir James Lighthill (1952), which was the first major advance in acoustics since the pioneering work of Lord Rayleigh in the last century. The combination of Lighthill's theory of aerodynamic noise as applied to turbulent flows and the experimental growing database from the early 1950's was quickly exploited by various jet propulsion engine designers in reducing the noise of jet engines at takeoff and landing to levels marginally acceptable to communities living in the neighborhoods of airports. The success in this noise containment led to the rapid growth of fast economical subsonic civil transport aircraft worldwide throughout the 1960's and has continued to the present day. One important factor in this success story has been the improvements in the engine cycle that have led to both reductions in specific fuel consumption and noise. The second is the introduction of Noise Certification, which specifies the maximum noise levels at takeoff and landing that all aircraft must meet before they can be entered on the Civil Aircraft Register. The growing interest in the development of a new supersonic civil transport to replace 'Concorde' in the early years of the next century has led to a resurgence of interest in the more challenging problem of predicting the noise of hot supersonic jets and developing means of aircraft noise reduction at takeoff and landing to meet the standards now accepted for subsonic Noise Certification. The prediction of aircraft noise to the accuracy required to meet Noise Certification requirements has necessitated reliance upon experimental measurements and empirically derived laws based on the available experimental data bases. These laws have their foundation in the results from Lighthill's theory, but in the case of jet noise, where the noise is generated in the turbulent mixing region with the external ambient fluid, the complexity of the turbulent motion has
Short-time evolution of Lagrangian velocity gradient correlations in isotropic turbulence
NASA Astrophysics Data System (ADS)
Fang, L.; Bos, W. J. T.; Jin, G. D.
2015-12-01
We show by direct numerical simulation (DNS) that the Lagrangian cross correlation of velocity gradients in homogeneous isotropic turbulence increases at short times, whereas its auto-correlation decreases. Kinematic considerations allow to show that two invariants of the turbulent velocity field determine the short-time velocity gradient correlations. In order to get a more intuitive understanding of the dynamics for longer times, heuristic models are proposed involving the combined action of local shear and rotation. These models quantitatively reproduce the effects and disentangle the different physical mechanisms leading to the observations in the DNS.
How isotropic are turbulent flows generated by using periodic conditions in a cube?
NASA Astrophysics Data System (ADS)
Qin, Z. C.; Fang, L.; Fang, J.
2016-03-01
In numerical simulations, "isotropic" turbulent flows are always generated by using periodic conditions. We show that these periodic conditions mathematically lead to large-scale anisotropy which can be about 10% of the mean values, and thus prevent existing post-processing results from being accurate. A decomposition method by employing spherical harmonics is then proposed to distinguish this scale-dependent anisotropy effect from others and to minimize the related post-processing error.
New forcing scheme to sustain particle-laden homogeneous and isotropic turbulence
NASA Astrophysics Data System (ADS)
Mallouppas, G.; George, W. K.; van Wachem, B. G. M.
2013-08-01
This paper evaluates the effect of forcing to sustain turbulence on the transfer function of the fluid with particles suspended in a homogeneous and isotropic flow. As mentioned by Lucci et al. ["Modulation of isotropic turbulence by particles of Taylor length-scale size," J. Fluid Mech. 650, 5-55 (2010), 10.1017/S0022112009994022], there are three limitations of forcing particle-laden homogeneous and isotropic turbulence: (a) large fluctuations on the temporal evolution of the kinetic energy are created when forcing is active at low wavenumbers, (b) the redistribution of energy is affected when forcing is performed over all wavenumbers, and (c) the nonlinear transfer function of the fluid due to the triadic interactions is affected when forcing is active over a wavenumber range. These limitations make the interpretation of the effects of particles on the energy spectrum of the fluid difficult. A new forcing scheme in physical space has been designed which avoids these limitations in wavenumber space, so the spectral effects of particles can be evaluated. The performance of this forcing scheme is tested using Direct Numerical Simulations. It is shown that the nonlinear transfer function of the fluid with the current forcing scheme is only affected at the wavenumbers it is acting, consistent with the theory. Even so, the spatial coherence and phase spectra between the two-way coupling and the fluid computed from the simulations show that the new forcing scheme is only moderately correlated even for the forced wavenumbers, with correlation coefficient typically about 10%.
Attenuation of Gas Turbulence by a Nearly Stationary Dispersion of Fine Particles
NASA Technical Reports Server (NTRS)
Paris, A. D.; Eaton, J. K.; Hwang, W.
1999-01-01
particles fall through the measurement volume. Measurements will be acquired using a high resolution image velocimetry (PIV) system being developed specifically for work in particle-laden flows. The measurements will include the decay of the turbulence kinetic energy under various particle loadings. The spatial spectra of the turbulence will also be measured. In a second set of experiments, the interaction of a single eddy with a collection of nearly stationary particles will be examined. The eddy will be a vortex ring emitted by a jet pulse through an orifice. The distortion of the vortex under the influence of the particles will be examined to gain a better understanding of how fine particles can cause such large reductions in turbulence levels. This experiment could not be conducted in terrestrial gravity because the high particle velocities would overwhelm the relatively low speed motion of the vortex ring. This experimental program is just getting underway. The initial challenge is to build a closed facility containing reasonably homogeneous and isotropic turbulence with zero mean velocity. Our approach is to use a set of synthetic jets mounted on the periphery of a transparent plexiglass box to create the turbulence. A synthetic jet is a plenum chamber with an orifice open to the volume of interest. The volume of the chamber fluctuates periodically so alternately a jet is ejected from the volume or flow is drawn back in as a sink. The asymmetry of this situation results in a net transport of momentum and kinetic energy into the volume of interest. The present apparatus includes eight synthetic jets each powered independently by a six inch loudspeaker. The synthetic jets discharge through ejector tubes to increase the scale of the turbulence. Construction of the apparatus is now complete and preliminary flow visualization studies have been conducted. The PIV system is also under development. A compact dual-pulse YAG laser has been acquired as the light source and
NASA Technical Reports Server (NTRS)
Ling, S. C.; Saad, A.
1977-01-01
The energetic isotropic turbulence generated by a waterfall of low head was found to be developed in part through the unstable two-phase flow of entrained air bubbles. The resulting turbulent field had a turbulent Reynolds number in excess of 20,000 and maintained a self-similar structure throughout the decay period studied. The present study may provide some insight into the structure of turbulence produced by breaking waves over the ocean.
NASA Astrophysics Data System (ADS)
Canet, Léonie; Delamotte, Bertrand; Wschebor, Nicolás
2016-06-01
We investigate the regime of fully developed homogeneous and isotropic turbulence of the Navier-Stokes (NS) equation in the presence of a stochastic forcing, using the nonperturbative (functional) renormalization group (NPRG). Within a simple approximation based on symmetries, we obtain the fixed-point solution of the NPRG flow equations that corresponds to fully developed turbulence both in d =2 and 3 dimensions. Deviations to the dimensional scalings (Kolmogorov in d =3 or Kraichnan-Batchelor in d =2 ) are found for the two-point functions. To further analyze these deviations, we derive exact flow equations in the large wave-number limit, and show that the fixed point does not entail the usual scale invariance, thereby identifying the mechanism for the emergence of intermittency within the NPRG framework. The purpose of this work is to provide a detailed basis for NPRG studies of NS turbulence; the determination of the ensuing intermittency exponents is left for future work.
The role of wall confinement on the decay rate of an initially isotropic turbulent field
NASA Astrophysics Data System (ADS)
Dowling, David R.; Movahed, Pooya; Johnsen, Eric
2014-11-01
The problem of freely decaying isotropic turbulence has been the subject of intensive research during the past few decades due to its importance for modeling purposes. While isotropy and periodic boundary conditions assumptions simplify the analysis, large-scale anisotropy (e.g., caused by rotation, shear, acceleration or walls) is in practice present in most turbulent flows and affects flow dynamics across different scales, as well as the kinetic energy decay. We investigate the role of wall confinement and viscous dissipation on the decay rate of an initially isotropic field for confining volumes of different aspect ratios. We first generate an isotropic velocity field in a cube with periodic boundary conditions. Next, using this field, we change the boundary conditions to no-slip walls on all sides. These walls restrict the initial field to a confined geometry and also provide an additional viscous dissipation mechanism. The problem is considered for confining volumes of different aspect ratios by adjusting the initial field. The change in confining volume introduces an additional length scale to the problem. Direct numerical simulation of the proposed set-up is used to verify the scaling arguments for the decay rate of kinetic energy. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation Grant Number ACI-1053575.
Water Vapor Turbulence Profiles in Stationary Continental Convective Mixed Layers
Turner, D. D.; Wulfmeyer, Volker; Berg, Larry K.; Schween, Jan
2014-10-08
The U.S. Department of Energy Atmospheric Radiation Measurement (ARM) program’s Raman lidar at the ARM Southern Great Plains (SGP) site in north-central Oklahoma has collected water vapor mixing ratio (q) profile data more than 90% of the time since October 2004. Three hundred (300) cases were identified where the convective boundary layer was quasi-stationary and well-mixed for a 2-hour period, and q mean, variance, third order moment, and skewness profiles were derived from the 10-s, 75-m resolution data. These cases span the entire calendar year, and demonstrate that the q variance profiles at the mixed layer (ML) top changes seasonally, but is more related to the gradient of q across the interfacial layer. The q variance at the top of the ML shows only weak correlations (r < 0.3) with sensible heat flux, Deardorff convective velocity scale, and turbulence kinetic energy measured at the surface. The median q skewness profile is most negative at 0.85 zi, zero at approximately zi, and positive above zi, where zi is the depth of the convective ML. The spread in the q skewness profiles is smallest between 0.95 zi and zi. The q skewness at altitudes between 0.6 zi and 1.2 zi is correlated with the magnitude of the q variance at zi, with increasingly negative values of skewness observed lower down in the ML as the variance at zi increases, suggesting that in cases with larger variance at zi there is deeper penetration of the warm, dry free tropospheric air into the ML.
On the dynamics of small-scale vorticity in isotropic turbulence
NASA Technical Reports Server (NTRS)
Jimenez, Javier; Wray, A. A.
1994-01-01
It was previously shown that the strong vorticity in isotropic turbulence is organized into tubular vortices ('worms') whose properties were characterized through the use of full numerical simulations at several Reynolds numbers. At the time most of the observations were kinematic, and several scaling laws were discovered for which there was no theoretical explanation. In the meantime, further analysis of the same fields yielded new information on the generation of the vortices, and it was realized that even if they had to be formed by stretching, they were at any given moment actually compressed at many points of their axes. This apparent contradiction was partially explained by postulating axial inertial waves induced by the nonuniformity of the vortex cores, which helped to 'spread' the axial strain and allowed the vortices to remain compact even if not uniformly stretched. The existence of such solutions was recently proved numerically. The present report discusses a set of new numerical simulations of isotropic turbulence, and a reanalysis of the old ones, in an effort to prove or disprove the presence of these waves in actual turbulent flows and to understand the dynamics, as opposed to the kinematics, of the vortices.
The isotropic nature of the background turbulence spectra in the solar wind
NASA Astrophysics Data System (ADS)
Wang, X.; Tu, C. Y.; He, J.; Marsch, E.; Wang, L.
2014-12-01
At the high-frequency end of the inertial range, the solar wind turbulence power spectrum was recently found to be anisotropic with respect to the direction of local magnetic field, as an evidence for the presence of a "critical balance" style turbulence cascade. However, we find that the spectral anisotropy seems to result from intermittent structures. The following two independent studies corroborate this statement by showing that the power spectra of the background turbulence, in which there are no intermittent structures, have an isotropic nature. In Study 1, we remove the wavelet coefficients of the local intermittency with large partial variance increment (PVI), and find the spectral indices of the magnetic field are 1.63±0.02, independent of the angle θRB between the direction of the local background magnetic field and the radial direction. In Study 2, we make a statistical study on the magnetic field spectral indices obtained by using Fast Fourier Transform on 40 time series, in which no intermittent structures appear. We find that for the time series with 0o<θRB <6o, the probability distribution of the observed spectral indices peaks at -1.7, while the -2 index predicted by the "critical balance" theory rarely appears. For the time series with 84 o <θRB <90 o, the probability distribution of the indices peaks at -1.5. Considering the uncertainty of the statistics, these results show that the background-turbulence spectra are nearly isotropic with respect to θRB, which may be consistent with some explanations based on hydrodynamic turbulence theory.
NASA Astrophysics Data System (ADS)
Xanthos, Savvas; Gong, Minwei; Andreopoulos, Yiannis
2010-01-01
Further analysis of the experimental data of the velocity gradient tensor first published by Xanthos et al. [J. Fluid Mech. 584, 301 (2007)] has been carried out and new results are reported here to provide additional insights on the effects of expansion waves interacting with isotropic turbulence. The flow field was generated by the reflection of an incoming shock wave at the open end of a large scale shock tube facility which interacted with the induced flow behind the incident shock wave which passed through a turbulence generating grid. In the present configuration the interaction is free from streamline curvature effects, which cause additional effects on turbulence. The strength of the applied expansive straining was 240 s-1. Rectangular pattern grids of different mesh sizes were used to generate isotropic and homogeneous turbulence with turbulent Reynolds number Reλ based on Taylor's microscale between 450 and 488. Lateral vorticity fluctuations and fluctuations of enstrophy and all stretching vector components are drastically reduced during the interaction. Residual attenuation in the postinteraction flow field was found only in the lateral vorticity fluctuations and in the longitudinal stretching term S11Ω1. Helicity and the helicity angle were computed from the data and the orientation angle of the vorticity vector in reference to the velocity vector was determined. Large fluctuations of the helicity angle were observed which extend from 0° to 180° with most probable values close to 30° and 130° and a mean value of 85°. Rotational dissipation rate was found to be high at these angles. The time-dependent signals of enstrophy, vortex stretching/tilting vector, and dissipation rate were found to exhibit a rather strong intermittent behavior which is characterized by high amplitude bursts followed by low level activities. It was found that the observed strong dissipative events are mostly associated with strong activities in the longitudinal stretching
Fang, L; Zhang, Y J; Fang, J; Zhu, Y
2016-08-01
We show by direct numerical simulations (DNSs) that in different types of isotropic turbulence, the fourth-order statistical invariants have approximately a linear relation, which can be represented by a straight line in the phase plane, passing two extreme states: the Gaussian state and the restricted Euler state. Also, each DNS case corresponds to an equilibrium region that is roughly Reynolds-dependent. In addition, both the time reversal and the compressibility effect lead to nonequilibrium transition processes in this phase plane. This observation adds a new restriction on the mean-field theory. PMID:27627399
The upper atmosphere of Uranus - A critical test of isotropic turbulence models
NASA Technical Reports Server (NTRS)
French, R. G.; Elliot, J. L.; Sicardy, B.; Nicholson, P.; Matthews, K.
1982-01-01
Observations of the August 15, 1980, Uranus occultation of KM 12, obtained from Cerro Tololo InterAmerican Observatory, European Southern Observatory, and Cerro Las Campanas Observatory, are used to compare the atmospheric structure at points separated by approximately 140 km along the planetary limb. The results reveal striking, but by no means perfect correlation of the light curves, ruling out isotropic turbulence as the cause of the light curve spikes. The atmosphere is strongly layered, and any acceptable turbulence model must accommodate the axial ratios of greater than about 60 which are observed. The mean temperature of the atmosphere is 150 plus or minus 15 K for the region near number density 10 to the 14th per cu cm. Derived temperature variations of vertical scale approximately 130 km and amplitude plus or minus 5 K are in agreement for all stations, and correlated spikes correspond to low-amplitude temperature variations with a vertical scale of several kilometers.
Modulation of isotropic turbulence by deformable droplets of Taylor lengthscale size
NASA Astrophysics Data System (ADS)
Dodd, Michael; Ferrante, Antonino
2014-11-01
We investigate the effects of finite-size deformable droplets on decaying isotropic turbulence via direct numerical simulation (DNS). DNS is performed using the two-fluid pressure-correction method by Dodd and Ferrante [J. Comput. Phys. 273, 416 (2014)] coupled with the volume of fluid method by Baraldi et al. [Comput. & Fluids 96, 322 (2014)]. We fully-resolve the flow around and inside 3130 droplets of Taylor lengthscale size, resulting in a droplet volume fraction of 0.05. The initial Taylor lengthscale Reynolds number is Reλ0 = 75 , and the computational mesh has 10243 grid points. We analyze the effects on turbulence modulation of varying the droplet- to carrier-fluid viscosity ratio (1 <=μd /μc <= 100) and the droplet Weber number based on the r.m.s velocity of turbulence (0 . 1 <=Werms <= 5). We discuss how varying these parameters affects the turbulence kinetic energy budget, and explain the physical mechanisms for such modulation. This work was supported by the National Science Foundation CAREER Award, Grant Number OCI-1054591.
On the decay of homogeneous nearly isotropic turbulence behind active fractal grids
NASA Astrophysics Data System (ADS)
Thormann, Adrien; Meneveau, Charles
2012-11-01
The study of decaying isotropic turbulent flow is an important point of reference for turbulence theories and numerical simulations. For the past several decades, most experimental results appear to favor power-law decay with exponents between -1.2 and -1.4, approximately. More recently, fractal-generated turbulence (Hurst & Vassilicos, PoF 2007, and subsequent papers) using multi-scale passive grids suggest possible faster decay, and non-trivial behavior especially near the grid, where the mean velocity is spatially evolving. In order to generate spatially homogeneous flow using multi-scale injection of kinetic energy at high Reynolds numbers, we use a new type of active-grid consisting of winglets with various fractal shapes. We test space-filling fractal shaped winglets as well as Sierpisky-carpet and Apollonian packing type fractal shapes. Data are acquired using X-wire thermal anemometry. Tests of homogeneity of mean flow and turbulence intensity will be presented as well as decay of kinetic energy and spectral characteristics of the flow. This research is supported by NSF-CBET-1033942. The assistance of Ms. Imbi Salasoo and Mr. Nathan Greene in designing and building the fractal winglets is much appreciated. The authors also thank Mr. Vince Rolin for his assistance with the active grid.
Model for drop coalescence in a locally isotropic turbulent flow field.
Narsimhan, Ganesan
2004-04-01
The proposed model views drop coalescence in a turbulent flow field as a two-step process consisting of formation of a doublet due to drop collisions followed by coalescence of the individual droplets in a doublet due to the drainage of the intervening film of continuous phase under the action of colloidal (van der Waals and electrostatic) and random turbulent forces. The turbulent flow field was assumed to be locally isotropic. A first-passage-time analysis was employed for the random process of intervening continuous-phase film thickness between the two drops of a doublet in order to evaluate the first two moments of coalescence-time distribution of the doublet. The average drop coalescence time of the doublet was dependent on the barrier for coalescence due to the net repulsive force (net effect of colloidal repulsive and turbulent attractive forces). The predicted average drop coalescence time was found to be smaller for larger turbulent energy dissipation rates, smaller surface potentials, larger drop sizes, larger ionic strengths, and larger drop size ratios of unequal-sized drop pairs. The predicted average drop coalescence time was found to decrease whenever the ratio of average turbulent force to repulsive force barrier became larger. The calculated coalescence-time distribution was broader, with a higher standard deviation, at lower energy dissipation rates, higher surface potentials, smaller drop sizes, and smaller size ratios of unequal drop pairs. The model predictions of average coalescence-rate constants for tetradecane-in-water emulsions stabilized by sodium dodecyl sulfate (SDS) in a high-pressure homogenizer agreed fairly well with the inferred experimental values as reported by Narsimhan and Goel (J. Colloid Interface Sci. 238 (2001) 420-432) at different homogenizer pressures and SDS concentrations. PMID:14985038
Mass dependency of turbulent parameters in stationary glow discharge plasmas
Titus, J. B.; Alexander, A. B.; Wiggins, D. L.; Johnson, J. A. III
2013-05-15
A direct current glow discharge tube is used to determine how mass changes the effects of certain turbulence characteristics in a weakly ionized gas. Helium, neon, argon, and krypton plasmas were created, and an axial magnetic field, varied from 0.0 to 550.0 Gauss, was used to enhance mass dependent properties of turbulence. From the power spectra of light emission variations associated with velocity fluctuations, determination of mass dependency on turbulent characteristic unstable modes, energy associated with turbulence, and the rate at which energy is transferred from scale to scale are measured. The magnetic field strength is found to be too weak to overcome particle diffusion to the walls to affect the turbulence in all four types of plasmas, though mass dependency is still detected. Though the total energy and the rate at which the energy moves between scales are mass invariant, the amplitude of the instability modes that characterize each plasma are dependent on mass.
Magnetic Field Line Random Walk in Isotropic Turbulence with Varying Mean Field
NASA Astrophysics Data System (ADS)
Sonsrettee, W.; Subedi, P.; Ruffolo, D.; Matthaeus, W. H.; Snodin, A. P.; Wongpan, P.; Chuychai, P.; Rowlands, G.; Vyas, S.
2016-08-01
In astrophysical plasmas, the magnetic field line random walk (FLRW) plays an important role in guiding particle transport. The FLRW behavior is scaled by the Kubo number R=(b/{B}0)({{\\ell }}\\parallel /{{\\ell }}\\perp ) for rms magnetic fluctuation b, large-scale mean field {{\\boldsymbol{B}}}0, and coherence scales parallel ({{\\ell }}\\parallel ) and perpendicular ({{\\ell }}\\perp ) to {{\\boldsymbol{B}}}0. Here we use a nonperturbative analytic framework based on Corrsin’s hypothesis, together with direct computer simulations, to examine the R-scaling of the FLRW for varying B 0 with finite b and isotropic fluctuations with {{\\ell }}\\parallel /{{\\ell }}\\perp =1, instead of the well-studied route of varying {{\\ell }}\\parallel /{{\\ell }}\\perp for b \\ll {B}0. The FLRW for isotropic magnetic fluctuations is also of astrophysical interest regarding transport processes in the interstellar medium. With a mean field, fluctuations may have variance anisotropy, so we consider limiting cases of isotropic variance and transverse variance (with b z = 0). We obtain analytic theories, and closed-form solutions for extreme cases. Padé approximants are provided to interpolate all versions of theory and simulations to any B 0. We demonstrate that, for isotropic turbulence, Corrsin-based theories generally work well, and with increasing R there is a transition from quasilinear to Bohm diffusion. This holds even with b z = 0, when different routes to R\\to ∞ are mathematically equivalent; in contrast with previous studies, we find that a Corrsin-based theory with random ballistic decorrelation works well even up to R = 400, where the effects of trapping are barely perceptible in simulation results.
NASA Astrophysics Data System (ADS)
Farge, Marie; Roussel, Olivier; Schneider, Kai
2004-11-01
We compare the extraction of coherent vortices in 3D homogeneous isotropic turbulence computed by DNS using either orthogonal or biorthogonal wavelets. The method is based on a wavelet decomposition of the vorticity field and a subsequent thresholding of the wavelet coefficients (PRL, 87(5), 2001, Phys. Fluids 15(10), 2003). The coherent vorticity is reconstructed from few strong wavelet coefficients while the incoherent vorticity is reconstructed from the remaining weak coefficients. In the orthogonal case the choice of the threshold is motivated from statistical denoising theory and has no adjustable parameters. Using 3% of the coefficients we show that both decompositions extract the coherent vortices out of the turbulent flow. They contain 99.6% of the energy and retain 74% and 68% of the enstrophy in the orthogonal and biorthogonal case, respectively. Concerning the incoherent background flow, it is structureless and decorrelated for the orthogonal decomposition, with a Gaussian velocity PDF. In contrast, the biorthogonal decomposition yields a background flow which exhibits quasi-2D sheet-like structures with an exponetial velocity PDF instead. In conclusion, modeling the incoherent background flow might be more difficult using biorthogonal wavelets for the CVS (Coherent Vortex Simulation, cf. Flow, Turbulence and Combustion 66(4), 2001).
Creating Only Isotropic Homogeneous Turbulence in Liquid Helium near Absolute Zero
NASA Astrophysics Data System (ADS)
Ihas, G. G.; Thompson, K. J.; Labbe, G.; McClintock, P. V. E.
2012-02-01
Flow through a grid is a standard method to produce isotropic, homogeneous turbulence for laboratory study. This technique has been used to generate quantum turbulence (QT) above 1 K in superfluid heliumootnotetextS. R. Stalp, L. Skrbek, and R. J. Donnelly, Phys. Rev. Lett. 82, 4831 (1999). where QT seems to mimic classical turbulence. Efforts have been made recentlyootnotetextG. G. Ihas, G. Labbe, S-c. Liu, and K. J. Thompson, J. Low Temp. Phys. 150, 384 (2008). to make similar measurements near absolute zero, where there is an almost total absence of normal fluid and hence classical viscosity. This presents the difficulty that most motive force devices produce heat which overwhelms the phenomena being investigated. The process of designing and implimenting a ``dissipation-free'' motor for pulling a grid through superfluid helium at millikelvin temperatures has resulted in the development of new techniques which have broad application in low temperature research. Some of these, such as Meissner-affect magnetic drives, capacitive and inductive position sensors, and magnetic centering devices will be described. Heating results for devices which can move in a controlled fashion from very low speed up to 10 cm/s will be presented. Acknowledgement: We thank W.F. Vinen for many useful discussions.
NASA Astrophysics Data System (ADS)
Hammond, Adam; Dou, Zhongwang; Tripathi, Anjan; Liang, Zach; Meng, Hui
2015-11-01
Study of droplet collision and cloud formation should consider the effects of both turbulence and electrostatic charge on particle dynamics. We present the first experimental observation of radial relative velocity (RV) of charged particles in homogeneous and isotropic turbulence (HIT). Charges on particles were generated through triboelectric effect between the inner wall of the chamber and the particles. To measure charge distribution, a particle-laden head-on impinging flow mimicking our HIT chamber conditions was built and holographic particle tracking was applied to quantify particle charges by measuring their displacements in an electric field. Particles were observed to have opposite charges. Next, in our HIT chamber, we measured particle RV by a novel 4-frame particle tracking velocimetry technique with and without charges on particles, wherein charges were neutralized by coating the interior of the HIT chamber with conductive carbon paint. We compared RV under the same turbulence conditions between charged particles and neutral particles and observed that when particles were oppositely charged, their mean inward RV increased at small separation distances. This result is consistent with recent theory and simulations (Lu and Shaw, Physics of Fluids, 2015). This work was supported by the National Science Foundation through a Collaborative Research Grant CBET-0967407.
NASA Astrophysics Data System (ADS)
Sekimoto, Atsushi; Dong, Siwei; Jiménez, Javier
2016-03-01
Statistically stationary and homogeneous shear turbulence (SS-HST) is investigated by means of a new direct numerical simulation code, spectral in the two horizontal directions and compact-finite-differences in the direction of the shear. No remeshing is used to impose the shear-periodic boundary condition. The influence of the geometry of the computational box is explored. Since HST has no characteristic outer length scale and tends to fill the computational domain, long-term simulations of HST are "minimal" in the sense of containing on average only a few large-scale structures. It is found that the main limit is the spanwise box width, Lz, which sets the length and velocity scales of the turbulence, and that the two other box dimensions should be sufficiently large (Lx ≳ 2Lz, Ly ≳ Lz) to prevent other directions to be constrained as well. It is also found that very long boxes, Lx ≳ 2Ly, couple with the passing period of the shear-periodic boundary condition, and develop strong unphysical linearized bursts. Within those limits, the flow shows interesting similarities and differences with other shear flows, and in particular with the logarithmic layer of wall-bounded turbulence. They are explored in some detail. They include a self-sustaining process for large-scale streaks and quasi-periodic bursting. The bursting time scale is approximately universal, ˜20S-1, and the availability of two different bursting systems allows the growth of the bursts to be related with some confidence to the shearing of initially isotropic turbulence. It is concluded that SS-HST, conducted within the proper computational parameters, is a very promising system to study shear turbulence in general.
NASA Astrophysics Data System (ADS)
Borgas, Michael S.; Yeung, P. K.
2004-03-01
A new model for Lagrangian particle-pair separation in turbulent flows is developed and compared with data from direct numerical simulations (DNS) of isotropic turbulence. The model formulation emphasizes (i) non-Gaussian behaviour in Eulerian and Lagrangian statistics, (ii) the occurrence of large separation velocities, (iii) the role of straining and streaming flow structure as recognized in kinematic simulations of turbulence, and (iv) the role of conditionally averaged accelerations in stochastic modelling of turbulent relative dispersion. Previous stochastic models of relative dispersion have produced unrealistic behaviour, particularly in the dissipation subrange where viscous effects are important, which have led to questions on the adequacy of stochastic modelling. However, this failure can now be recognized as inadequate detail in formulation, which is explained and rectified in this paper. The model is quasi-one-dimensional in nature, and is focused on the statistics of particle-pair separation distance and its rate of change, referred to as the separation speed. Detailed comparisons are presented at several Reynolds numbers using the DNS database reported in a companion paper (Part 1). Up to fourth-order moments for these quantities are examined, as well as the separation-distance probability density function, which is discussed in the context of recent claims of Richardson scaling in the literature. The model is able to account for the spatial representation of straining regions as well as incompressibility of the flow, and is shown to reproduce strong non-Gaussianity and intermittency in the Lagrangian separation statistics observed in DNS. Comparisons with recent physical experiments are also remarkably consistent. This work demonstrates that stochastic models when properly formulated are effective and efficient representations of the dispersion process and this general class of models therefore possess great utility for calculations of both one
Parametric Study of Decay of Homogeneous Isotropic Turbulence Using Large Eddy Simulation
NASA Technical Reports Server (NTRS)
Swanson, R. C.; Rumsey, Christopher L.; Rubinstein, Robert; Balakumar, Ponnampalam; Zang, Thomas A.
2012-01-01
Numerical simulations of decaying homogeneous isotropic turbulence are performed with both low-order and high-order spatial discretization schemes. The turbulent Mach and Reynolds numbers for the simulations are 0.2 and 250, respectively. For the low-order schemes we use either second-order central or third-order upwind biased differencing. For higher order approximations we apply weighted essentially non-oscillatory (WENO) schemes, both with linear and nonlinear weights. There are two objectives in this preliminary effort to investigate possible schemes for large eddy simulation (LES). One is to explore the capability of a widely used low-order computational fluid dynamics (CFD) code to perform LES computations. The other is to determine the effect of higher order accuracy (fifth, seventh, and ninth order) achieved with high-order upwind biased WENO-based schemes. Turbulence statistics, such as kinetic energy, dissipation, and skewness, along with the energy spectra from simulations of the decaying turbulence problem are used to assess and compare the various numerical schemes. In addition, results from the best performing schemes are compared with those from a spectral scheme. The effects of grid density, ranging from 32 cubed to 192 cubed, on the computations are also examined. The fifth-order WENO-based scheme is found to be too dissipative, especially on the coarser grids. However, with the seventh-order and ninth-order WENO-based schemes we observe a significant improvement in accuracy relative to the lower order LES schemes, as revealed by the computed peak in the energy dissipation and by the energy spectrum.
Forward and backward in time dispersion of fluid and inertial particles in isotropic turbulence
NASA Astrophysics Data System (ADS)
Bragg, Andrew D.; Ireland, Peter J.; Collins, Lance R.
2016-01-01
In this paper, we investigate both theoretically and numerically the Forward-In-Time (FIT) and Backward-In-Time (BIT) dispersion of fluid and inertial particle-pairs in isotropic turbulence. Fluid particles are known to separate faster BIT than FIT in three-dimensional turbulence, and we find that inertial particles do the same. However, we find that the irreversibility in the inertial particle dispersion is in general much stronger than that for fluid particles. For example, the ratio of the BIT to FIT mean-square separation can be up to an order of magnitude larger for the inertial particles than for the fluid particles. We also find that for both the inertial and fluid particles, the irreversibility becomes stronger as the scale of their separation decreases. Regarding the physical mechanism for the irreversibility, we argue that whereas the irreversibility of fluid particle-pair dispersion can be understood in terms of a directional bias arising from the energy transfer process in turbulence, inertial particles experience an additional source of irreversibility arising from the non-local contribution to their velocity dynamics, a contribution that vanishes in the limit St → 0, where St is the particle Stokes number. For each given initial (final, in the BIT case) separation, r0, there is an optimum value of St for which the dispersion irreversibility is strongest, as such particles are optimally affected by both sources of irreversibility. We derive analytical expressions for the BIT, mean-square separation of inertial particles and compare the predictions with numerical data obtained from a Reλ ≈ 582 (where Reλ is the Taylor Reynolds number) Direct Numerical Simulation (DNS) of particle-laden isotropic turbulent flow. The small-time theory, which in the dissipation range is valid for times ≤max[Stτη, τη] (where τη is the Kolmogorov time scale), is in excellent agreement with the DNS. The theory for long-times is in good agreement with the DNS
He, G.; Doolen, G.D.; Chen, S.
1999-12-01
The longitudinal structure function (LSF) and the transverse structure function (TSF) in isotropic turbulence are calculated using a vortex model. The vortex model is composed of the Rankine and Burgers vortices which have the exponential distributions in the vortex Reynolds number and vortex radii. This model exhibits a power law in the inertial range and satisfies the minimal condition of isotropy that the second-order exponent of the LSF in the inertial range is equal to that of the TSF. Also observed are differences between longitudinal and transverse structure functions caused by intermittency. These differences are related to their scaling differences which have been previously observed in experiments and numerical simulations. {copyright} {ital 1999 American Institute of Physics.}
Passive scalar convective-diffusive subrange for low Prandtl numbers in isotropic turbulence
NASA Astrophysics Data System (ADS)
Briard, A.; Gomez, T.
2015-01-01
In this Rapid Communication, we study the behavior of a strongly diffusive passive scalar field T submitted to a freely decaying, homogeneous and isotropic turbulence with eddy-damped quasinormal Markovian simulations. We present a new subrange located between the k-17 /3 inertial-diffusive subrange and the Kolmogorov wave number kη. This subrange is generated by small-scale convection linked to kη that balances diffusion effects. Thus, we build a typical length scale kCD -1 based on convection and diffusion and give an expression for the shape of the passive scalar spectrum in this subrange ET˜√{Pr}k-11 /3 using physical arguments. This result unifies two different theories coming from Batchelor [G. K. Batchelor, J. Fluid. Mech. 5, 113 (1959), 10.1017/S002211205900009X] and Chasnov [J. Chasnov et al., Phys. Fluids A 1, 1698 (1989), 10.1063/1.857535] and explains results previously obtained experimentally.
Isotropic boundary adapted wavelets for coherent vorticity extraction in turbulent channel flows
NASA Astrophysics Data System (ADS)
Farge, Marie; Sakurai, Teluo; Yoshimatsu, Katsunori; Schneider, Kai; Morishita, Koji; Ishihara, Takashi
2015-11-01
We present a construction of isotropic boundary adapted wavelets, which are orthogonal and yield a multi-resolution analysis. We analyze DNS data of turbulent channel flow computed at a friction-velocity based Reynolds number of 395 and investigate the role of coherent vorticity. Thresholding of the wavelet coefficients allows to split the flow into two parts, coherent and incoherent vorticity. The statistics of the former, i.e., energy and enstrophy spectra, are close to the ones of the total flow, and moreover the nonlinear energy budgets are well preserved. The remaining incoherent part, represented by the large majority of the weak wavelet coefficients, corresponds to a structureless, i.e., noise-like, background flow and exhibits an almost equi-distribution of energy.
NASA Astrophysics Data System (ADS)
Gotoh, Toshiyuki
2012-11-01
Spectrum of passive scalar variance at very high Schmidt number up to 1000 in isotropic steady turbulence has been studied by using very high resolution DNS. Gaussian random force and scalar source which are isotropic and white in time are applied at low wavenumber band. Since the Schmidt number is very large, the system was integrated for 72 large eddy turn over time for the system to forgot the initial state. It is found that the scalar spectrum attains the asymptotic k-1 spectrum in the viscous-convective range and the constant CB is found to be 5.7 which is larger than 4.9 obtained by DNS under the uniform mean scalar gradient. Reasons for the difference are inferred as the Reynolds number effect, anisotropy, difference in the scalar injection, duration of time average, and the universality of the constant is discussed. The constant CB is also compared with the prediction by the Lagrangian statistical theory for the passive scalar. The scalar spectrum in the far diffusive range is found to be exponential, which is consistent with the Kraichnan's spectrum. However, the Kraichnan spectrum was derived under the assumption that the velocity field is white in time, therefore theoretical explanation of the agreement needs to be explored. Grant-in-Aid for Scientific Research No. 21360082, Ministry of Education, Culture, Sports, Science and Technology of Japan.
Large fluctuations of the nonlinearities in isotropic turbulence. Anisotropic filtering analysis
NASA Astrophysics Data System (ADS)
Tordella, D.; Di Savino, S.; Sitzia, L.
2014-09-01
Using a Navier-Stokes isotropic turbulent field numerically simulated in a box with a discretization of 10243 (Biferale et al., 2005), we show that the probability of having a stretching-tilting larger than a few times the local enstrophy is low. By using an anisotropic kind of filter in the Fourier space, where wavenumbers that have at least one component below a threshold or inside a range are removed, we analyze these survival statistics when the large, the small inertial or the small inertial and dissipation scales are filtered out. By considering a flow obtained by randomizing the phases of the Fourier modes, and applying our filtering techniques, we identified clearly the properties attributable to turbulence. It can be observed that, in the unfiltered isotropic Navier-Stokes field, the probability of the ratio (|ωṡ∇U|/|) being higher than a given threshold is higher than in the fields where the large scales were filtered out. At the same time, it is lower than in the fields where the small inertial and dissipation range of scales is filtered out. This is basically due to the suppression of compact structures in the ranges that have been filtered in different ways. The partial removal of the background of filaments and sheets does not have a first order effect on these statistics. These results are discussed in the light of a hypothesized relation between vortical filaments, sheets and blobs in physical space and in Fourier space. The study in fact can be viewed as a kind of test for this idea and tries to highlight its limits. We conclude that a qualitative relation in physical space and in Fourier space can be supposed to exist for blobs only. That is for the near isotropic structures which are sufficiently described by a single spatial scale and do not suffer from the disambiguation problem as filaments and sheets do. Information is also given on the filtering effect on statistics concerning the inclination of the strain rate tensor eigenvectors with
NASA Astrophysics Data System (ADS)
Collins, Lance; Bragg, Andrew; Ireland, Peter
2014-11-01
In this talk, we consider the physical mechanism for the clustering of inertial particles in the inertial range of turbulence. By comparisons with DNS data we demonstrate that the mechanism in the theory of Zaichik et al. (Phys. Fluids 19, 113308, 2007) quantitatively describes the clustering of particles in the inertial range. We then analyze the theory for isotropic turbulence in the limit Reλ --> ∞ . For arbitrary St (Stokes number), there exists a separation in the inertial range beyond which Str << 1 , where Str is the Stokes number based on the eddy turnover timescale at separation r. The inertial-range clustering in this limit can be understood to be due to the preferential sampling of the coarse-grained velocity gradient tensor at that scale. At smaller separations, there may be transitions to Str ~ 1 , where a path history symmetry breaking effect dominates the clustering mechanism, and in some cases Str >> 1 , which implies ballistic behavior and a flat RDF. The scaling for each of these regimes is derived and compared to DNS, where applicable. Finally, we compare the results with the ``sweep-stick'' mechanism by Coleman and Vassilicos (Phys. Fluids 21, 113301, 2009) and discuss the similarities and differences between the two theories.
NASA Astrophysics Data System (ADS)
Dou, Zhongwang; Pecenak, Zachary; Liang, Zach; Cao, Lujie; Ireland, Peter; Collins, Lance; Meng, Hui
2015-11-01
Particle-pair radial relative velocity (RV) in turbulence plays a critical role in droplet collision and cloud formation. Both simulations and experiments are performed to better understand RV of inertial particles in homogeneous and isotropic turbulence (HIT). However, past experimental measurement of particle RV statistics exhibited large deviations from DNS results (de Jong et al., 2010). In the current study, we identified intrinsic limitations in our previous study and devised a 4-frame particle tracking velocimetry technique to measure particle RV. In a second-generation, enclosed, fan-driven HIT chamber, both tracer and inertial particles were studied at R_ λ of 366. The experimentally measured RV statistics were compared with DNS with excellent agreement. Additionally, for both kinds of particles, the mean inward RV vs. particle separation distance r also matched very well with DNS, but at near-zero r, experimental values were slightly higher. To investigate the cause of this discrepancy, we compared DNS of both mono- and tri-dispersed particles. We found that the tri-dispersed particles exhibited higher mean inward RV at small r than any mono-dispersed particles. This suggests that the increase of mean inward RV in the experiment could be due to the Stokes number (St) distribution present in the particles, while DNS employed single St values. This work was supported by the National Science Foundation through a Collaborative Research Grant CBET-0967407.
The rotation and translation of non-spherical particles in homogeneous isotropic turbulence
NASA Astrophysics Data System (ADS)
Byron, Margaret
The motion of particles suspended in environmental turbulence is relevant to many scientific fields, from sediment transport to biological interactions to underwater robotics. At very small scales and simple shapes, we are able to completely mathematically describe the motion of inertial particles; however, the motion of large aspherical particles is significantly more complex, and current computational models are inadequate for large or highly-resolved domains. Therefore, we seek to experimentally investigate the coupling between freely suspended particles and ambient turbulence. A better understanding of this coupling will inform not only engineering and physics, but the interactions between small aquatic organisms and their environments. In the following pages, we explore the roles of shape and buoyancy on the motion of passive particles in turbulence, and allow these particles to serve as models for meso-scale aquatic organisms. We fabricate cylindrical and spheroidal particles and suspend them in homogeneous, isotropic turbulence that is generated via randomly-actuated jet arrays. The particles are fabricated with agarose hydrogel, which is refractive-index-matched to the surrounding fluid (water). Both the fluid and the particle are seeded with passive tracers, allowing us to perform Particle Image Velocimetry (PIV) simultaneously on the particle and fluid phase. To investigate the effects of shape, particles are fabricated at varying aspect ratios; to investigate the effects of buoyancy, particles are fabricated at varying specific gravities. Each particle type is freely suspended at a volume fraction of F=0.1%, for which four-way coupling interactions are negligible. The suspended particles are imaged together with the surrounding fluid and analyzed using stereoscopic PIV, which yields three velocity components in a two-dimensional measurement plane. Using image thresholding, the results are separated into simultaneous fluid-phase and solid-phase velocity
NASA Astrophysics Data System (ADS)
Onishi, Ryo; Vassilicos, J. C.
2014-11-01
This study investigates the collision statistics of inertial particles in inverse-cascading 2D homogeneous isotropic turbulence by means of a direct numerical simulation (DNS). A collision kernel model for particles with small Stokes number (St) in 2D flows is proposed based on the model of Saffman & Turner (1956) (ST56 model). The DNS results agree with this 2D version of the ST56 model for St < 0.1. It is then confirmed that our DNS results satisfy the 2D version of the spherical formulation of the collision kernel. The fact that the flatness factor stays around 3 in our 2D flow confirms that the present 2D turbulent flow is nearly intermittency-free. Collision statistics for St = 0.1, 0.4 and 0.6, i.e. for St <1, are obtained from the present 2D DNS and compared with those obtained from the three-dimensional (3D) DNS of Onishi et al. (2013). We have observed that the 3D radial distribution function at contact (g(R), the so-called clustering effect) decreases for St = 0.4 and 0.6 with increasing Reynolds number, while the 2D g(R) does not show a significant dependence on Reynolds number. This observation supports the view that the Reynolds-number dependence of g(R) observed in three dimensions is due to internal intermittency of the 3D turbulence. We have further investigated the local St, which is a function of the local flow strain rates, and proposed a plausible mechanism that can explain the Reynolds-number dependence of g(R).
Large-deviation joint statistics of the finite-time Lyapunov spectrum in isotropic turbulence
NASA Astrophysics Data System (ADS)
Johnson, Perry L.; Meneveau, Charles
2015-08-01
One of the hallmarks of turbulent flows is the chaotic behavior of fluid particle paths with exponentially growing separation among them while their distance does not exceed the viscous range. The maximal (positive) Lyapunov exponent represents the average strength of the exponential growth rate, while fluctuations in the rate of growth are characterized by the finite-time Lyapunov exponents (FTLEs). In the last decade or so, the notion of Lagrangian coherent structures (which are often computed using FTLEs) has gained attention as a tool for visualizing coherent trajectory patterns in a flow and distinguishing regions of the flow with different mixing properties. A quantitative statistical characterization of FTLEs can be accomplished using the statistical theory of large deviations, based on the so-called Cramér function. To obtain the Cramér function from data, we use both the method based on measuring moments and measuring histograms and introduce a finite-size correction to the histogram-based method. We generalize the existing univariate formalism to the joint distributions of the two FTLEs needed to fully specify the Lyapunov spectrum in 3D flows. The joint Cramér function of turbulence is measured from two direct numerical simulation datasets of isotropic turbulence. Results are compared with joint statistics of FTLEs computed using only the symmetric part of the velocity gradient tensor, as well as with joint statistics of instantaneous strain-rate eigenvalues. When using only the strain contribution of the velocity gradient, the maximal FTLE nearly doubles in magnitude, highlighting the role of rotation in de-correlating the fluid deformations along particle paths. We also extend the large-deviation theory to study the statistics of the ratio of FTLEs. The most likely ratio of the FTLEs λ1 : λ2 : λ3 is shown to be about 4:1:-5, compared to about 8:3:-11 when using only the strain-rate tensor for calculating fluid volume deformations. The results
Large-deviation joint statistics of the finite-time Lyapunov spectrum in isotropic turbulence
Johnson, Perry L. Meneveau, Charles
2015-08-15
One of the hallmarks of turbulent flows is the chaotic behavior of fluid particle paths with exponentially growing separation among them while their distance does not exceed the viscous range. The maximal (positive) Lyapunov exponent represents the average strength of the exponential growth rate, while fluctuations in the rate of growth are characterized by the finite-time Lyapunov exponents (FTLEs). In the last decade or so, the notion of Lagrangian coherent structures (which are often computed using FTLEs) has gained attention as a tool for visualizing coherent trajectory patterns in a flow and distinguishing regions of the flow with different mixing properties. A quantitative statistical characterization of FTLEs can be accomplished using the statistical theory of large deviations, based on the so-called Cramér function. To obtain the Cramér function from data, we use both the method based on measuring moments and measuring histograms and introduce a finite-size correction to the histogram-based method. We generalize the existing univariate formalism to the joint distributions of the two FTLEs needed to fully specify the Lyapunov spectrum in 3D flows. The joint Cramér function of turbulence is measured from two direct numerical simulation datasets of isotropic turbulence. Results are compared with joint statistics of FTLEs computed using only the symmetric part of the velocity gradient tensor, as well as with joint statistics of instantaneous strain-rate eigenvalues. When using only the strain contribution of the velocity gradient, the maximal FTLE nearly doubles in magnitude, highlighting the role of rotation in de-correlating the fluid deformations along particle paths. We also extend the large-deviation theory to study the statistics of the ratio of FTLEs. The most likely ratio of the FTLEs λ{sub 1} : λ{sub 2} : λ{sub 3} is shown to be about 4:1:−5, compared to about 8:3:−11 when using only the strain-rate tensor for calculating fluid volume
The length distribution of streamline segments in homogeneous isotropic decaying turbulence
NASA Astrophysics Data System (ADS)
Schaefer, P.; Gampert, M.; Peters, N.
2012-04-01
by Schaefer et al. ["Fast and slow changes of the length of gradient trajectories in homogenous shear turbulence," in Advances in Turbulence XII, edited by B. Eckhardt (Springer-Verlag, Berlin, 2009), pp. 565-572] we will refer to the morphological part of the evolution of streamline segments as slow changes while the topological part of the evolution is referred to as fast changes. This separation yields a transport equation for the probability density function (pdf) P(l) of the arclength l of streamline segments in which the slow changes translate into a convection and a diffusion term when terms up to second order are included and the fast changes yield integral terms. The overall temporal evolution (morphological and topological) of the arclength l of streamline segments is analyzed and associated with the motion of the above isosurface. This motion is diffusion controlled for small segments, while large segments are mainly subject to strain and pressure fluctuations. The convection velocity corresponds to the first order jump moment, while the diffusion term includes the second order jump moment. It is concluded, both theoretically and from direct numerical simulations (DNS) data of homogeneous isotropic decaying turbulence at two different Reynolds numbers, that the normalized first order jump moment is quasi-universal, while the second order one is proportional to the inverse of the square root of the Taylor based Reynolds number Re_{λ }^{-1/2}. Its inclusion thus represents a small correction in the limit of large Reynolds numbers. Numerical solutions of the pdf equation yield a good agreement with the pdf obtained from the DNS data. The interplay of viscous drift acting on small segments and linear strain acting on large segments yield, as it has already been concluded for dissipation elements, that the mean length of streamline segments should scale with Taylor microscale.
MAGNETIC FIELD LINE RANDOM WALK IN ISOTROPIC TURBULENCE WITH ZERO MEAN FIELD
Sonsrettee, W.; Ruffolo, D.; Snodin, A. P.; Wongpan, P.; Subedi, P.; Matthaeus, W. H.; Chuychai, P. E-mail: david.ruf@mahidol.ac.th E-mail: pat.wongpan@postgrad.otago.ac.nz E-mail: prasub@udel.edu
2015-01-01
In astrophysical plasmas, magnetic field lines often guide the motions of thermal and non-thermal particles. The field line random walk (FLRW) is typically considered to depend on the Kubo number R = (b/B {sub 0})(ℓ{sub ∥}/ℓ ) for rms magnetic fluctuation b, large-scale mean field B {sub 0}, and parallel and perpendicular coherence scales ℓ{sub ∥} and ℓ , respectively. Here we examine the FLRW when R → ∞ by taking B {sub 0} → 0 for finite b{sub z} (fluctuation component along B {sub 0}), which differs from the well-studied route with b{sub z} = 0 or b{sub z} << B {sub 0} as the turbulence becomes quasi-two-dimensional (quasi-2D). Fluctuations with B {sub 0} = 0 are typically isotropic, which serves as a reasonable model of interstellar turbulence. We use a non-perturbative analytic framework based on Corrsin's hypothesis to determine closed-form solutions for the asymptotic field line diffusion coefficient for three versions of the theory, which are directly related to the k {sup –1} or k {sup –2} moment of the power spectrum. We test these theories by performing computer simulations of the FLRW, obtaining the ratio of diffusion coefficients for two different parameterizations of a field line. Comparing this with theoretical ratios, the random ballistic decorrelation version of the theory agrees well with the simulations. All results exhibit an analog to Bohm diffusion. In the quasi-2D limit, previous works have shown that Corrsin-based theories deviate substantially from simulation results, but here we find that as B {sub 0} → 0, they remain in reasonable agreement. We conclude that their applicability is limited not by large R, but rather by quasi-two-dimensionality.
Magnetic Field Line Random Walk in Isotropic Turbulence with Zero Mean Field
NASA Astrophysics Data System (ADS)
Sonsrettee, W.; Subedi, P.; Ruffolo, D.; Matthaeus, W. H.; Snodin, A. P.; Wongpan, P.; Chuychai, P.
2015-01-01
In astrophysical plasmas, magnetic field lines often guide the motions of thermal and non-thermal particles. The field line random walk (FLRW) is typically considered to depend on the Kubo number R = (b/B 0)(l∥/l) for rms magnetic fluctuation b, large-scale mean field B 0, and parallel and perpendicular coherence scales l∥ and l, respectively. Here we examine the FLRW when R → ∞ by taking B 0 → 0 for finite bz (fluctuation component along B 0), which differs from the well-studied route with bz = 0 or bz Lt B 0 as the turbulence becomes quasi-two-dimensional (quasi-2D). Fluctuations with B 0 = 0 are typically isotropic, which serves as a reasonable model of interstellar turbulence. We use a non-perturbative analytic framework based on Corrsin's hypothesis to determine closed-form solutions for the asymptotic field line diffusion coefficient for three versions of the theory, which are directly related to the k -1 or k -2 moment of the power spectrum. We test these theories by performing computer simulations of the FLRW, obtaining the ratio of diffusion coefficients for two different parameterizations of a field line. Comparing this with theoretical ratios, the random ballistic decorrelation version of the theory agrees well with the simulations. All results exhibit an analog to Bohm diffusion. In the quasi-2D limit, previous works have shown that Corrsin-based theories deviate substantially from simulation results, but here we find that as B 0 → 0, they remain in reasonable agreement. We conclude that their applicability is limited not by large R, but rather by quasi-two-dimensionality.
Forces on stationary particles in near-bed turbulent flows
Schmeeckle, M.W.; Nelson, J.M.; Shreve, R.L.
2007-01-01
In natural flows, bed sediment particles are entrained and moved by the fluctuating forces, such as lift and drag, exerted by the overlying flow on the particles. To develop a better understanding of these forces and the relation of the forces to the local flow, the downstream and vertical components of force on near-bed fixed particles and of fluid velocity above or in front of them were measured synchronously at turbulence-resolving frequencies (200 or 500 Hz) in a laboratory flume. Measurements were made for a spherical test particle fixed at various heights above a smooth bed, above a smooth bed downstream of a downstream-facing step, and in a gravel bed of similarly sized particles as well as for a cubical test particle and 7 natural particles above a smooth bed. Horizontal force was well correlated with downstream velocity and not correlated with vertical velocity or vertical momentum flux. The standard drag formula worked well to predict the horizontal force, but the required value of the drag coefficient was significantly higher than generally used to model bed load motion. For the spheres, cubes, and natural particles, average drag coefficients were found to be 0.76, 1.36, and 0.91, respectively. For comparison, the drag coefficient for a sphere settling in still water at similar particle Reynolds numbers is only about 0.4. The variability of the horizontal force relative to its mean was strongly increased by the presence of the step and the gravel bed. Peak deviations were about 30% of the mean force for the sphere over the smooth bed, about twice the mean with the step, and 4 times it for the sphere protruding roughly half its diameter above the gravel bed. Vertical force correlated poorly with downstream velocity, vertical velocity, and vertical momentum flux whether measured over or ahead of the test particle. Typical formulas for shear-induced lift based on Bernoulli's principle poorly predict the vertical forces on near-bed particles. The
Power and Nonpower Laws of Passive Scalar Moments Convected by Isotropic Turbulence
NASA Astrophysics Data System (ADS)
Gotoh, Toshiyuki; Watanabe, Takeshi
2015-09-01
The scaling behavior of the moments of two passive scalars that are excited by two different methods and simultaneously convected by the same isotropic steady turbulence at Rλ=805 and Sc=0.72 is studied by using direct numerical simulation with N =40963 grid points. The passive scalar θ is excited by a random source that is Gaussian and white in time, and the passive scalar q is excited by the mean uniform scalar gradient. In the inertial convective range, the n th-order moments of the scalar increment δ θ (r ) do not obey a simple power law, but have the local scaling exponents ξnθ+βnlog (r /r*) with βn>0 . In contrast, the local scaling exponents of q have well-developed plateaus and saturate with increasing order. The power law of passive scalar moments is not trivial. The universality of passive scalars is found not in the moments, but in the normalized moments.
Power and nonpower laws of passive scalar moments convected by isotropic turbulence.
Gotoh, Toshiyuki; Watanabe, Takeshi
2015-09-11
The scaling behavior of the moments of two passive scalars that are excited by two different methods and simultaneously convected by the same isotropic steady turbulence at R_{λ}=805 and Sc=0.72 is studied by using direct numerical simulation with N=4096^{3} grid points. The passive scalar θ is excited by a random source that is Gaussian and white in time, and the passive scalar q is excited by the mean uniform scalar gradient. In the inertial convective range, the nth-order moments of the scalar increment δθ(r) do not obey a simple power law, but have the local scaling exponents ξ_{n}^{θ}+β_{n}log(r/r_{*}) with β_{n}>0. In contrast, the local scaling exponents of q have well-developed plateaus and saturate with increasing order. The power law of passive scalar moments is not trivial. The universality of passive scalars is found not in the moments, but in the normalized moments. PMID:26406833
Miura, H.; Araki, K.
2014-07-15
Hall effects on local structures in homogeneous, isotropic, and incompressible magnetohydrodynamic turbulence are studied numerically. The transition of vortices from sheet-like to tubular structures induced by the Hall term is found, while the kinetic energy spectrum does not distinguish the two types of structures. It is shown by the use of the sharp low-pass filter that the transition occurs not only in the scales smaller than the ion skin depth but also in a larger scale. The transition is related with the forward energy transfer in the spectral space. Analyses by the use of the sharp low-pass filter show that the nonlinear energy transfer associated with the Hall term is dominated by the forward transfer and relatively local in the wave number space. A projection of the simulation data to a Smagorinsky-type sub-grid-scale model shows that the high wave number component of the Hall term may possibly be replaced by the model effectively.
Clustering of vertically constrained passive particles in homogeneous and isotropic turbulence
NASA Astrophysics Data System (ADS)
van Hinsberg, Michel; de Pietro, Massimo; Biferale, Luca; Clercx, Herman; Toschi, Federico
2014-11-01
We analyze the dynamics of small particles confined within a horizontal fluid slab in a three-dimensional (3D) homogenous isotropic turbulent velocity field. Particles can freely move horizontally as fluid tracers but are vertically confined around a given horizontal plane via a simple linear restoring force. The present model may be considered as the simplest description for the dynamics of small aquatic organisms that, due to swimming, active regulation of their buoyancy or other mechanisms, are capable to maintain themselves in a shallow horizontal layer somewhere below the free surface of oceans or lakes. In the model varying the strength of the restoring force can control the thickness of the fluid slab in which the particles can move. Whenever some confinement is present, particle trajectories deviate from fluid tracers and experience an effectively compressible velocity field. We report a quantification of this effective compressibility as well as a quantification of preferential concentration of tracer particles in terms of the correlation dimension. We found that there exists a particular value of the force constant, corresponding to a mean slab depth approximately equal to a few times the Kolmogorov length scale, that maximizes the clustering of the particles. This work is part of the research programmes 11PR2841 and FP112 of the Foundation for Fundamental Research on Matter (FOM), which is part of the Netherlands Organisation for Scientific Research (NWO). The work was partially funded by ERC Grant No 339032.
A priori study of subgrid-scale flux of a passive scalar in isotropic homogeneous turbulence
Chumakov, Sergei
2008-01-01
We perform a direct numerical simulation (DNS) of forced homogeneous isotropic turbulence with a passive scalar that is forced by mean gradient. The DNS data are used to study the properties of subgrid-scale flux of a passive scalar in the framework of large eddy simulation (LES), such as alignment trends between the flux, resolved, and subgrid-scale flow structures. It is shown that the direction of the flux is strongly coupled with the subgrid-scale stress axes rather than the resolved flow quantities such as strain, vorticity, or scalar gradient. We derive an approximate transport equation for the subgrid-scale flux of a scalar and look at the relative importance of the terms in the transport equation. A particular form of LES tensor-viscosity model for the scalar flux is investigated, which includes the subgrid-scale stress. Effect of different models for the subgrid-scale stress on the model for the subgrid-scale flux is studied.
NASA Astrophysics Data System (ADS)
Miura, H.; Araki, K.
2014-07-01
Hall effects on local structures in homogeneous, isotropic, and incompressible magnetohydrodynamic turbulence are studied numerically. The transition of vortices from sheet-like to tubular structures induced by the Hall term is found, while the kinetic energy spectrum does not distinguish the two types of structures. It is shown by the use of the sharp low-pass filter that the transition occurs not only in the scales smaller than the ion skin depth but also in a larger scale. The transition is related with the forward energy transfer in the spectral space. Analyses by the use of the sharp low-pass filter show that the nonlinear energy transfer associated with the Hall term is dominated by the forward transfer and relatively local in the wave number space. A projection of the simulation data to a Smagorinsky-type sub-grid-scale model shows that the high wave number component of the Hall term may possibly be replaced by the model effectively.
Fichtl, G.H.
1983-09-01
When designing a wind energy converison system (WECS), it may be necessary to take into account the distribution of wind across the disc of rotation. The specific engineering applications include structural strength, fatigue, and control. This wind distribution consists of two parts, namely that associated with the mean wind profile and that associated with the turbulence velocity fluctuation field. The work reported herein is aimed at the latter, namely the distribution of turbulence velocity fluctuations across the WECS disk of rotation. A theory is developed for the two-time covariance matrix for turbulence velocity vector components for wind energy conversion system (WECS) design. The theory is developed for homogeneous and iotropic turbulance with the assumption that Taylor's hypothesis is valid. The Eulerian turbulence velocity vector field is expanded about the hub of the WECS. Formulae are developed for the turbulence velocity vector component covariance matrix following the WECS blade elements. It is shown that upon specification of the turbulence energy spectrum function and the WECS rotation rate, the two-point, two-time covariance matrix of the turbulent flow relative to the WECS bladed elements is determined. This covariance matrix is represented as the sum of nonstationary and stationary contributions. Generalized power spectral methods are used to obtain two-point, double frequency power spectral density functions for the turbulent flow following the blade elements. The Dryden turbulence model is used to demonstrate the theory. A discussion of linear system response analysis is provided to show how the double frequency turbulence spectra might be used to calculate response spectra of a WECS to turbulent flow. Finally the spectrum of the component of turbulence normal to the WECS disc of rotation, following the blade elements, is compared with experimental results.
NASA Astrophysics Data System (ADS)
Meneveau, Charles; Yang, Yunke; Perlman, Eric; Wan, Minpin; Burns, Randal; Szalay, Alex; Chen, Shiyi; Eyink, Gregory
2008-11-01
A public database system archiving a direct numerical simulation (DNS) data set of isotropic, forced turbulence is used for studying basic turbulence dynamics. The data set consists of the DNS output on 1024-cubed spatial points and 1024 time-samples spanning about one large-scale turn-over timescale. This complete space-time history of turbulence is accessible to users remotely through an interface that is based on the Web-services model (see http://turbulence.pha.jhu.edu). Users may write and execute analysis programs on their host computers, while the programs make subroutine-like calls that request desired parts of the data over the network. The architecture of the database is briefly explained, as are some of the new functions such as Lagrangian particle tracking and spatial box-filtering. These tools are used to evaluate and compare subgrid stresses and models.
NASA Astrophysics Data System (ADS)
Dou, Zhongwang; Pecenak, Zachary K.; Cao, Lujie; Woodward, Scott H.; Liang, Zach; Meng, Hui
2016-03-01
Enclosed flow apparatuses with negligible mean flow are emerging as alternatives to wind tunnels for laboratory studies of homogeneous and isotropic turbulence (HIT) with or without aerosol particles, especially in experimental validation of Direct Numerical Simulation (DNS). It is desired that these flow apparatuses generate HIT at high Taylor-microscale Reynolds numbers ({{R}λ} ) and enable accurate measurement of turbulence parameters including kinetic energy dissipation rate and thereby {{R}λ} . We have designed an enclosed, fan-driven, highly symmetric truncated-icosahedron ‘soccer ball’ airflow apparatus that enables particle imaging velocimetry (PIV) and other whole-field flow measurement techniques. To minimize gravity effect on inertial particles and improve isotropy, we chose fans instead of synthetic jets as flow actuators. We developed explicit relations between {{R}λ} and physical as well as operational parameters of enclosed HIT chambers. To experimentally characterize turbulence in this near-zero-mean flow chamber, we devised a new two-scale PIV approach utilizing two independent PIV systems to obtain both high resolution and large field of view. Velocity measurement results show that turbulence in the apparatus achieved high homogeneity and isotropy in a large central region (48 mm diameter) of the chamber. From PIV-measured velocity fields, we obtained turbulence dissipation rates and thereby {{R}λ} by using the second-order velocity structure function. A maximum {{R}λ} of 384 was achieved. Furthermore, experiments confirmed that the root mean square (RMS) velocity increases linearly with fan speed, and {{R}λ} increases with the square root of fan speed. Characterizing turbulence in such apparatus paves the way for further investigation of particle dynamics in particle-laden homogeneous and isotropic turbulence.
Analysis of subgrid models using direct and large-eddy simulations of isotropic turbulence
NASA Astrophysics Data System (ADS)
Menon, S.; Yeung, P. K.
1994-12-01
Direct and large eddy simulations of forced and decaying isotropic turbulence have been performed using a pseudospectral and a finite-difference code. Subgrid models that include a one-equation subgrid kinetic energy model with and without a stochastic backscatter forcing term and a new scale similarity model have been analyzed in both Fourier space and physical space. The Fourier space analysis showed that the energy transfer across the cutoff wavenumber k(sub c) is dominated by local interaction. The correlation between the exact and the modeled (by a spectral eddy viscosity) nonlinear terms and the subgrid energy transfer in physical space was found to be quite low. In physical space, a similar correlation analysis was carried out using top hat filtering. Results show that the subgrid stress and the energy flux predicted by the subgrid models correlates very well with the exact data. The scale similarity model showed very high correlation for reasonable grid resolution. However, with decrease in grid resolution, the scale similarity model became more uncorrelated, when compared to the kinetic energy subgrid model. The subgrid models were then used for large-eddy simulations for a range of Reynolds number. It was determined that the dissipation was modeled poorly and that the correlation with the exact results was quite low for all the models. In general, for coarse grid resolution, the scale similarity model consistently showed very low correlation while the kinetic energy model showed a relatively higher correlation. These results suggest that to use the scale similarity model relatively fine grid resolution may be required, whereas, the kinetic energy model could be used even in coarse grid.
NASA Astrophysics Data System (ADS)
Aliseda, Alberto; Bourgoin, Mickael; Eswirp Collaboration
2014-11-01
We present preliminary results from a recent grid turbulence experiment conducted at the ONERA wind tunnel in Modane, France. The ESWIRP Collaboration was conceived to probe the smallest scales of a canonical turbulent flow with very high Reynolds numbers. To achieve this, the largest scales of the turbulence need to be extremely big so that, even with the large separation of scales, the smallest scales would be well above the spatial and temporal resolution of the instruments. The ONERA wind tunnel in Modane (8 m -diameter test section) was chosen as a limit of the biggest large scales achievable in a laboratory setting. A giant inflatable grid (M = 0.8 m) was conceived to induce slowly-decaying homogeneous isotropic turbulence in a large region of the test section, with minimal structural risk. An international team or researchers collected hot wire anemometry, ultrasound anemometry, resonant cantilever anemometry, fast pitot tube anemometry, cold wire thermometry and high-speed particle tracking data of this canonical turbulent flow. While analysis of this large database, which will become publicly available over the next 2 years, has only started, the Taylor-scale Reynolds number is estimated to be between 400 and 800, with Kolmogorov scales as large as a few mm . The ESWIRP Collaboration is formed by an international team of scientists to investigate experimentally the smallest scales of turbulence. It was funded by the European Union to take advantage of the largest wind tunnel in Europe for fundamental research.
NASA Astrophysics Data System (ADS)
Higdon, J. C.
1986-10-01
A model of anisotropic plasma fluids is developed to examine the origin of the spectra of random, electron-density variations inferred by Armstrong, Cordes, and Rickett from measurements of pulsar radio signals. These electron variations are interpreted as density components of an anisotropic stationary mode of nonlinear magnetogasdynamics-tangetial pressure balances. It is demonstrated that turbulent flows, generated by the disruption of H I clouds in O star H II regions, reproduce well the mean electron spectrum inferred by Armstrong et al., if the relative rms density variation, >(n-n0)2<1/2/n0 is 0.125, where n0 is the mean density.
Gomez, T; Sagaut, P; Schilling, O; Zhou, Y
2006-07-05
A spectral subggrid-scale eddy viscosity and magnetic resisitivity model based on the eddy-damped quasi-normal Markovian (EDQNM) spectral kinetic and magnetic energy transfer presented in [12] is used in large-eddy simulation (LES) of large kinetic and magnetic Reynold number magneto-hydrodynamic (MHD) turbulence. The proposed model is assessed via a posteri tests on three-dimensional, incompressible, isotropic, non-helical, freely-decaying MHD turbulence at asymptotically large Reynolds numbers. Using LES with an initial condition characterized by an Alfv{acute e}n ratio of kinetic to magnetic energy {tau}{sub A} equal to unity, it is shown that at the kinetic energy spectrum E{sub K}(k) and magnetic energy spectrum E{sub M}(k) exhibit Kolmogorov -5/3 inertial subrange scalings in the LES, consistent with the EDQNM model.
NASA Astrophysics Data System (ADS)
Dong, Siwei; Sekimoto, Atsushi; Jiménez, Javier
2013-11-01
The rough independence of the logarithmic layer (LL) of wall-bounded turbulence from the details of the buffer and outer layers, suggests that the interaction of the turbulent fluctuations with the mean shear may be mimicked by statistically-stationary homogeneous shear turbulence (SS-HST) in a finite box. We study SS-HST in boxes for which the statistics best agree with those of the LL. Both flows share similar Corrsin shear parameters, and Reynolds-stress and vorticity anisotropies. Two-point correlation functions show that u and w are constrained by the simulation box and are respectively shorter and narrower for SS-HST than for the LL, but v and the vorticity are roughly of the same size in both flows when Reλ is similar. The transient bursting of v in both flows is quite similar to the linear Orr mechanism, with time scales that are of the same order in both flows. In both cases, a streamwise velocity streak forms and breaks down quasi periodically, and the break down is accompanied by an enhanced flux of momentum, in the form of large-scale ejections and sweeps. Funded by the ERC Multiflow program and CSC.
On the weakly anisotropic nature of the time-stationary turbulence in the solar wind
NASA Astrophysics Data System (ADS)
Wang, Xin; Tu, Chuanyi; He, Jiansen; Marsch, Eckart; Wang, Linghua
2016-03-01
At the frequency range from 0.01 Hz to 0.1 Hz, the power spectrum of the fluctuations in the solar wind turbulence was recently observed to be anisotropic with respect to the direction of local mean magnetic field (B0). These observations are considered as evidence for a "critical balance" style cascade. However, we find that the anisotropy of the spectral index seems to be very weak, if we use continuous time series which are time-stationary and have nearly constant local B0. We apply the fast fourier transform (FFT) on these time series selected from the eight-year magnetic field (B) and flow velocity (V) data observed by the WIND spacecraft in the high-speed solar wind. Our results show that the FFT spectral indices of the time series with B0 nearly parallel and perpendicular to the Sun-to-Earth radial direction are not significantly different. This work provides new clues on the nature of the anisotropy of the solar wind turbulence and thus will improve our understanding of the turbulent energy cascade.
NASA Astrophysics Data System (ADS)
Laskowski, Gregory M.; Durbin, Paul A.
2007-01-01
Serpentine passages are found in a number of engineering applications including turbine blade cooling passages. The design of effective cooling passages for high-temperature turbine blades depends in part on the ability to predict heat transfer, thus requiring an accurate representation of the turbulent flow field. These passages are subjected to strong curvature and rotational effects, and the resulting turbulent flow field is fairly complex. An understanding of the flow physics for flows with strong curvature and rotation is required in order to improve the design of turbine blade cooling passages. Experimental measurements of certain turbulence quantities for such configurations can be challenging to obtain, especially near solid surfaces, making the serpentine passage an ideal candidate for a direct numerical simulation (DNS). A DNS study has been conducted to investigate the coupled effect of strong curvature and rotation by simulating turbulent flow through a fully developed, smooth wall, round-ended, isothermal serpentine channel subjected to orthogonal mode rotation. The geometry investigated has an average radius of curvature Rc/δ=2.0 in the curved section and dimensions 12πδ×2δ×3πδ in the streamwise, transverse, and spanwise directions. The computational domain consists of periodic inflow/outflow boundaries, two solid wall boundaries, and periodic boundaries in the spanwise direction. The simulations were conducted for Reynolds number, Reb=5600, and rotation numbers, Rob ,z=0 and 0.32. Differences observed between the stationary and rotating cases are discussed in terms of the mean velocity, secondary flow, and Reynolds stresses.
NASA Astrophysics Data System (ADS)
Joly, A.; Moulin, F.; Violeau, D.; Astruc, D.
2012-10-01
The prediction of solid bodies transport (such as algae, debris, sediment grains, or corrosion deposits) is a necessary requirement in many industrial or environmental processes. The physical processes involved cover a wide range of processes, from tidal flow to turbulent eddies and particle drag. A stochastic model was therefore developed to link the different scales of the physical processes where it was assumed that the particles are dilute enough that they do not affect the flow or the motion of other particles while being large enough that each particle does not follow exactly the fluid motions (i.e., macro-particles). The stochastic model is built in such a way that it uses Reynolds-averaged fluid properties to predict trajectories of individual particles. This model was then tested using experimental measurements obtained for isotropic particles released in semi-homogeneous turbulence. The turbulent flow was generated using a pair of oscillating grids and was characterized using particle image velocimetry measurements. The trajectories of the particles were measured using a pair of high resolution cameras. The comparison between the experimental data and different numerical models gives satisfactory results.
NASA Astrophysics Data System (ADS)
Donateo, Antonio; Cava, Daniela; Contini, Daniele
2013-04-01
In atmospheric turbulent flows, variables describing the motion undergo random and stochastic fluctuations. In turbulence studies the hypotheses of stationarity and ergodicity of time series is required in order to obtain estimates of ensemble averages from the temporal averages obtained from single runs. In atmosphere, however, equivalence between the two averages is just approximated because of non stationarity often inherent to atmospheric time series. Typically non-stationary conditions are driven by weather or internal boundary layer changing, for example for the presence of gravity waves or simply for the slow diurnal evolution of the boundary layer. The individuation of non-stationary cases is important for measurements of turbulent fluxes using the eddy covariance method generally applied to 30 minutes averages. Moreover it is necessary to have an analytical/parametric stationarity test, which can be used in real time determination of turbulent fluxes, for example in Fluxnet network. Nowadays different stationarity tests are proposed in literature and they are substantially used by scientific community (Foken & Wichura, 1996; Mahrt, 1998; Affre et al., 2000). In this work several time series have been analysed with the three different stationarity tests and a comparison of their performances has been developed. The stationarity tests have been applied to different scalars (temperature, ultrafine particles number concentration, carbon dioxide and water vapour concentration). All the time series come from measurements in different sites and are collected over different canopies: iced surface (in Antarctica), urban or suburban surface (Italy) and vegetal canopy over forests (both in Italy and USA). In total 6 different sites have been analysed and the performances of the stationarity tests do not seem to be site dependent. The correlation of their performances as a function of local micro-meteorological conditions have been analysed. All the three tests show
NASA Technical Reports Server (NTRS)
Bennett, Floyd V.; Yntema, Robert T.
1959-01-01
Several approximate procedures for calculating the bending-moment response of flexible airplanes to continuous isotropic turbulence are presented and evaluated. The modal methods (the mode-displacement and force-summation methods) and a matrix method (segmented-wing method) are considered. These approximate procedures are applied to a simplified airplane for which an exact solution to the equation of motion can be obtained. The simplified airplane consists of a uniform beam with a concentrated fuselage mass at the center. Airplane motions are limited to vertical rigid-body translation and symmetrical wing bending deflections. Output power spectra of wing bending moments based on the exact transfer-function solutions are used as a basis for the evaluation of the approximate methods. It is shown that the force-summation and the matrix methods give satisfactory accuracy and that the mode-displacement method gives unsatisfactory accuracy.
NASA Astrophysics Data System (ADS)
Tóth-Katona, Tibor; Gleeson, James T.
2004-01-01
Fluctuations of the injected electric power during electroconvection (EHC) of liquid crystals are reported in both the conductive and the dielectric regime of convection. The amplitude and the frequency of the fluctuations, as well as the probability density functions have been compared in these two regimes and substantial differences have been found both in defect turbulence of EHC and at the DSM1→DSM2 transition.
NASA Astrophysics Data System (ADS)
Coleman, S. W.; Vassilicos, J. C.
2009-11-01
Our work focuses on the sweep-stick mechanism of particle clustering in turbulent flows introduced by Chen et al. [L. Chen, S. Goto, and J. C. Vassilicos, "Turbulent clustering of stagnation points and inertial particles," J. Fluid Mech. 553, 143 (2006)] for two-dimensional (2D) inverse cascading homogeneous, isotropic turbulence (HIT), whereby heavy particles cluster in a way that mimics the clustering of zero-acceleration points. We extend this phenomenology to three-dimensional (3D) HIT, where it was previously reported that zero-acceleration points were extremely rare. Having obtained a unified mechanism we quantify the Stokes number dependency of the probability of the heavy particles to be at zero-acceleration points and show that in the inertial range of Stokes numbers, the sweep-stick mechanism is dominant over the conventionally proposed mechanism of heavy particles being centrifuged from high vorticity regions to high strain regions. Finally, having a clustering coincidence between particles and zero-acceleration points, both in 2D and 3D HIT, motivates us to demonstrate the sweep and stick parts of the mechanism in both dimensions. The sweeping of regions of low acceleration regions by the local fluid velocity in both flows is demonstrated by introducing a velocity of the acceleration field. Finally, the stick part is demonstrated by showing that heavy particles statistically move with the same velocity as zero-acceleration points, while moving away from any nonzero-acceleration region, irrespective of their Stokes number. These results explain the clustering of inertial particles given the clustering of zero-acceleration points.
Flow structure interaction between a flexible cantilever beam and isotropic turbulence
NASA Astrophysics Data System (ADS)
Vogel, Andrew; Morvan, Thomas; Goushcha, Oleg; Andreopoulos, Yiannis; Elvin, Niell
2015-11-01
In the present experimental work we consider the degree of distortion of isotropy and homogeneity of grid turbulence caused by the presence of a thin flexible cantilever beam immersed in the flow aligned in the longitudinal direction. Beams of various rigidities and lengths were used in the experiments. Piezoelectric patches were attached to the beams which provided an output voltage proportional to the strain and therefore proportional to the beam's deflection. The experiments were carried out in a large scale wind tunnel and hot-wires were used to measure turbulence intensity in the vicinity of the beams for various values of the ratio of aerodynamic loading to beam's rigidity. It was found that the flow field distortion depends on the rigidity of the beam. For very rigid beams this distortion is of the order of the boundary layer thickness developing over the beam while for very flexible beams the distorted region is of the order of the beam's tip deflection. Analysis of the time-dependent signals indicated some correlation between the frequency of beam's vibration and flow structures detected. Supported by NSF Grant: CBET #1033117.
NASA Astrophysics Data System (ADS)
Donzis, Diego; Yakhot, Victor; Sreenivasan, K. R.
2015-11-01
Most approaches to understand turbulence have sought universal behavior believed to manifest at high Reynolds numbers (Rλ). However, recent theory and simulations suggest that universal characteristics, such as the non-trivial anomalous scaling exponents of moments of velocity gradients, emerge even at very low Rλ at which no inertial range exists. Furthermore, with decreasing Reynolds numbers, a transition occurs from fully intermittent turbulence to (approximately) Gaussian behavior at an apparently universal critical Rλ. A potential implication of these observations is that significant information concerning the inertial range (e.g. scaling exponents) is already manifest in the dissipation range at very low Rλ. Thus, high Rλ properties can be studied with well-resolved low-Rλ simulations instead of marginally resolved high-Reynolds flows. The focus of this talk is to explore signatures of universality at high-Reynolds numbers in the dissipation range of highly resolved DNS (kmax η ~ O (20)) for Rλ up to 90, and decaying simulations close to the critical Rλ. In addition to statistics of velocity gradients and dissipation we explore evidence of Beltramization as suggested in past theoretical work.
NASA Astrophysics Data System (ADS)
Ireland, Peter J.; Bragg, Andrew D.; Collins, Lance R.
2016-06-01
In Part I of this study, we analyzed the motion of inertial particles in isotropic turbulence in the absence of gravity using direct numerical simulation (DNS). Here, in Part II, we introduce gravity and study its effect over a wide range of flow Reynolds numbers, Froude numbers, and particle Stokes numbers. We see that gravity causes particles to sample the flow more uniformly and reduces the time particles can spend interacting with the underlying turbulence. We also find that gravity tends to increase inertial particle accelerations, and we introduce a model to explain that effect. We then analyze the particle relative velocities and radial distribution functions (RDFs), which are generally seen to be independent of Reynolds number for low and moderate Kolmogorov-scale Stokes numbers $St$. We see that gravity causes particle relative velocities to decrease, and that the relative velocities have higher scaling exponents with gravity. We observe that gravity has a non-trivial effect on clustering, acting to decrease clustering at low $St$ and to increase clustering at high $St$. By considering the effect of gravity on the clustering mechanisms described in the theory of Zaichik & Alipchenkov (New J. Phys., 11:103018, 2009), we provide an explanation for this non-trivial effect of gravity. We also show that when the effects of gravity are accounted for in the theory of Zaichik & Alipchenkov, the results compare favorably with DNS. The relative velocities and RDFs exhibit considerable anisotropy at small separations, and this anisotropy is quantified using spherical harmonic functions. We use the relative velocities and the RDFs to compute the particle collision kernels, and find that the collision kernel remains as it was for the case without gravity, namely nearly independent of Reynolds number for low and moderate $St$.
Rotational motion of elongated particles in isotropic turbulent flow: statistical perspective
NASA Astrophysics Data System (ADS)
Zhao, Lihao; Andersson, Helge; Variano, Evan
2014-11-01
We consider the rotational motion of non-spherical particles in turbulent flow, comparing the statistics of particles' angular velocity to the corresponding quantities computed in the fluid phase. We use numerical (DNS) and laboratory measurements for particles that are both larger and smaller than the Kolmogorov lengthscale. The particles are spheroids or rods, with aspect ratios between 1 and 10. We will discuss the subtleties of defining a meaningful Stokes number for these particles, focusing on the effect of asphericity and the fact that our interest is in rotation and not translation. Comparing the probability density function of angular velocity between fluid and particle phase indicates that the angular velocity of particles has a narrower distribution than that of the fluid phase, and that. particles do respond to extreme events in the fluid phase. The first four moments of the PDFs are analyzed, and these show that the ``filtering'' effect is very similar between DNS and lab experiments, despite differences in particle sizes and mass. We propose a nondimensional curve for predicting the magnitude of the filtering effect, and discuss the implications of this curve for the definition of Stokes number, as discussed earlier. This work has been supported by grants from the Peder Sather Center for Advanced Study at UC Berkeley and from the Research Council of Norway (Contract No. 213917/F20).
NASA Astrophysics Data System (ADS)
Jin, Guodong; He, Guo-Wei
2015-11-01
Clustering and intermittency in radial relative velocity (RRV) of heavy particles of same size settling in turbulent flows can be remarkably changed due to gravity. Clustering is monotonically reduced at Stokes number less than 1 under gravity due to the disability of the centrifugal mechanism, however it is non-monotonically enhanced at Stokes number greater than 1 due to the multiplicative amplification in the case that the proposed effective Kubo number is less than 1. Although gravity causes monotonical reduction in the rms of RRV of particles at a given Stokes number with decreasing Froude number, the variation tendency in the tails of standardized PDF of RRV versus Froude number is obviously different: the tails become narrower at a small Stokes number, while they become broader at a large Stokes number. The mechanism of this variation stems from the compromise between the following two competing factors. The mitigation of correlation of particle positions and the regions of high strain rate which are more intermittent reduces the intermittency in RRV at small Stokes numbers, while the significant reduction in the backward-in-time relative separations will make particle pairs see small-scale structures, leading to a higher intermittency in RRV at large Stokes numbers. NSAF of China (grant number U1230126); NSFC (grant numbers 11072247 and 11232011).
NASA Astrophysics Data System (ADS)
Vazquez, M. S.; Rodriguez, W. V.
2005-04-01
From large eddy simulations of heated square ducts, we study the effect of non-stationary structures over the flow thermal behavior. Indeed the net decrease of the turbulent intensity over the heated wall, the velocity temperature correlation intensity is enhanced. This correlation which represents the turbulent heat transfer is fed by turbulent structures of high intensity, but with a low frequency. These turbulent structures create high amplitude temperature fluctuations. A conditional statistical study is performed for isolating these turbulent structures and knowing its local and global effects over the flow.
NASA Astrophysics Data System (ADS)
Ireland, Peter J.; Bragg, Andrew D.; Collins, Lance R.
2016-06-01
In this study, we analyze the statistics of both individual inertial particles and inertial particle pairs in direct numerical simulations of homogeneous isotropic turbulence in the absence of gravity. The effect of the Taylor microscale Reynolds number $R_\\lambda$ on the particle statistics is examined over the largest range to date (from $R_\\lambda = 88-597$). We first explore the effect of preferential sampling on the single-particle statistics, and use our understanding of preferential sampling to provide a physical explanation for many of the trends in the particle velocity gradients, kinetic energies, and accelerations at low $St$. As $St$ increases, inertial filtering effects become more important, causing the particle kinetic energies and accelerations to decrease. We then consider particle-pair statistics, and focus our attention on the relative velocities and radial distribution functions (RDFs) of the particles. The relative velocity statistics indicate that preferential-sampling effects are important for $St \\lesssim 0.1$ and that path-history/non-local effects become increasingly important for $St \\gtrsim 0.2$. The lower-order relative velocity statistics are only weakly sensitive to changes in Reynolds number at low $St$. We find that the RDFs peak near $St$ of order unity, that they exhibit power-law scaling for low and intermediate $St$, and that they are largely independent of Reynolds number for low and intermediate $St$. We also observe that at large $St$, changes in the RDF are related to changes the scaling exponents of the relative velocity variances. The particle collision kernel is found to be largely insensitive to the flow Reynolds number, suggesting that relatively low-Reynolds-number simulations may be able to capture much of the relevant physics of droplet collisions and growth in the adiabatic cores of atmospheric clouds.
A mathematical model of turbulence in flows with uniform stationary velocity gradients
NASA Technical Reports Server (NTRS)
Zak, M. A.
1982-01-01
Certain cases of turbulence as a postinstability state of a fluid in motion modeled by the introduction of multivalued velocity fields are examined. The turbulence is regarded as occurring in the form of random pulsations which grow until the external energy input in the average flow is balanced by the dissipated energy of pulsations by means of turbulent friction. Closed form analytic solutions are shown to be possible when the considered velocity fields, the pulsation velocity and the fluid velocity, are decoupled.
NASA Astrophysics Data System (ADS)
Longo, Sandro
2011-01-01
This paper is a companion paper to a study devoted to the analysis of experimental instantaneous fluid levels and three-component fluid velocity measurements in a stationary flow field generated by a Crump weir in a laboratory flume, using an ultrasonic distance sensor and a three-probe arrangement of an ultrasonic Doppler velocity profiler (UVP) (Longo in Exp Fluids, doi: 10.1007/s00348-010-0881-5, 2010). Whereas Longo (Exp Fluids, doi: 10.1007/s00348-010-0881-5, 2010) deals with a general overview of the problem, the description of the experiments and the analysis of the free surface statistics and relevant scales, the present manuscript is devoted to a detailed analysis of the turbulence and the correlation with the free surface. The data are elaborated by obtaining the macroturbulence Reynolds tensor, using conditional averages based on free surface-fluctuation statistics. We also compute the two-point correlations of fluid velocity, the micro-scale and the integral scale, the correlation between free surface and the turbulence beneath. A free surface-boundary layer was detected having a thickness proportional to the root mean square of the free surface-height series and with a velocity scale that correlates well with the free surface-elevation time gradient. Most of the relevant state variables, such as the mean velocity and Reynolds stress components, collapse to a single curve if scaled appropriately. There are many indicators that a specific regime occurs that has an optimal tuning between the free surface and turbulence. In this regime, the length scales are considered as an indicator.
Gaussian Multiplicative Chaos for Symmetric Isotropic Matrices
NASA Astrophysics Data System (ADS)
Chevillard, Laurent; Rhodes, Rémi; Vargas, Vincent
2013-02-01
Motivated by isotropic fully developed turbulence, we define a theory of symmetric matrix valued isotropic Gaussian multiplicative chaos. Our construction extends the scalar theory developed by J.P. Kahane in 1985.
Rotor noise due to atmospheric turbulence ingestion. I - Fluid mechanics
NASA Technical Reports Server (NTRS)
Simonich, J. C.; Amiet, R. K.; Schlinker, R. H.; Greitzer, E. M.
1986-01-01
In the present analytical procedure for the prediction of helicopter rotor noise generation due to the ingestion of atmospheric turbulence, different models for turbulence fluid mechanics and the ingestion process are combined. The mean flow and turbulence statistics associated with the atmospheric boundary layer are modeled with attention to the effects of atmospheric stability length, windspeed, and altitude. The turbulence field can be modeled as isotropic, locally stationary, and homogeneous. For large mean flow contraction ratios, accurate predictions of turbulence vorticity components at the rotor face requires the incorporation of the differential drift of fluid particles on adjacent streamlines.
NASA Technical Reports Server (NTRS)
Kerr, R. A.
1983-01-01
In a three dimensional simulation higher order derivative correlations, including skewness and flatness factors, are calculated for velocity and passive scalar fields and are compared with structures in the flow. The equations are forced to maintain steady state turbulence and collect statistics. It is found that the scalar derivative flatness increases much faster with Reynolds number than the velocity derivative flatness, and the velocity and mixed derivative skewness do not increase with Reynolds number. Separate exponents are found for the various fourth order velocity derivative correlations, with the vorticity flatness exponent the largest. Three dimensional graphics show strong alignment between the vorticity, rate of strain, and scalar-gradient fields. The vorticity is concentrated in tubes with the scalar gradient and the largest principal rate of strain aligned perpendicular to the tubes. Velocity spectra, in Kolmogorov variables, collapse to a single curve and a short minus 5/3 spectral regime is observed.
NASA Astrophysics Data System (ADS)
Pujolle-Robic, C.; Olmsted, P. D.; Noirez, L.
2002-08-01
This letter describes the non-linear rheology of the isotropic phase of a thermotropic side chain liquid-crystal polymer (SCLCP), from which we infer a flow-induced iso- tropic-to-nematic (IN) phase transition above a critical shear stress and construct non-equilib- rium phase diagrams. In contrast to the well-studied wormlike-micellar solutions and predictions for simple liquid-crystalline systems, the critical stress does not vanish as the equilibrium transition temperature is approached from the above. We postulate that this is due to: i) the coupling between mesogens and the polymer backbone, whose equilibrium oblate nematic backbone conformation contrasts with the prolate non-equilibrium conformation; and ii) the peculiar topological constraints in SCLCP melts, which have been previously postulated as leading to long-lived clusters.
NASA Astrophysics Data System (ADS)
Dhariwal, Rohit; Rani, Sarma; Koch, Donald
2015-11-01
In an earlier work, Rani, Dhariwal, and Koch (JFM, Vol. 756, 2014) developed an analytical closure for the diffusion current in the PDF transport equation describing the relative motion of high-Stokes-number particle pairs in isotropic turbulence. In this study, an improved closure was developed for the diffusion coefficient, such that the motion of the particle-pair center of mass is taken into account. Using the earlier and the new analytical closures, Langevin simulations of pair relative motion were performed for four particle Stokes numbers, Stη = 10 , 20 , 40 , 80 and at two Taylor micro-scale Reynolds numbers Reλ = 76 , 131 . Detailed comparisons of the analytical model predictions with those of DNS were undertaken. It is seen that the pair relative motion statistics obtained from the improved theory show excellent agreement with the DNS statistics. The radial distribution functions (RDFs), and relative velocity PDFs obtained from the improved-closure-based Langevin simulations are found to be in very good agreement with those from DNS. It was found that the RDFs and relative velocity RMS increased with Reλ for all Stη . The collision kernel also increased strongly with Reλ , since it depended on the RDF and the radial relative velocities.
NASA Astrophysics Data System (ADS)
Nguyen van yen, Romain; Farge, Marie; Schneider, Kai
2012-02-01
Classical statistical theories of turbulence have shown their limitations, in that they cannot predict much more than the energy spectrum in an idealized setting of statistical homogeneity and stationarity. We explore the applicability of a conditional statistical modeling approach: can we sort out what part of the information should be kept, and what part should be modeled statistically, or, in other words, “dissipated”? Our mathematical framework is the initial value problem for the two-dimensional (2D) Euler equations, which we approximate numerically by solving the 2D Navier-Stokes equations in the vanishing viscosity limit. In order to obtain a good approximation of the inviscid dynamics, we use a spectral method and a resolution going up to 8192 2. We introduce a macroscopic concept of dissipation, relying on a split of the flow between coherent and incoherent contributions: the coherent flow is constructed from the large wavelet coefficients of the vorticity field, and the incoherent flow from the small ones. In previous work, a unique threshold was applied to all wavelet coefficients, while here we also consider the effect of a scale by scale thresholding algorithm, called scale-wise coherent vorticity extraction. We study the statistical properties of the coherent and incoherent vorticity fields, and the transfers of enstrophy between them, and then use these results to propose, within a maximum entropy framework, a simple model for the incoherent vorticity. In the framework of this model, we show that the flow velocity can be predicted accurately in the L2 norm for about 10 eddy turnover times.
Power fluctuations, large deviations and turbulence
Bandi, Mahesh M; Chumakov, Sergei; Connaughton, Colm P
2008-01-01
We study local power fluctuations in numerical simulations of stationary, homogenous, isotropic turbulence in two and three dimensions with Gaussian forcing. Due to the near-Gaussianity of the one-point velocity distribution, the probability distribution function (pdf) of the local power is well modeled by the pdf of the product of two joint normally distributed variables. In appropriate units, this distribution is calculated exactly and shown to satisfy a Fluctuation Relation (FR) with a coefficient which depends on {epsilon}.
Active control for turbulent premixed flame simulations
Bell, John B.; Day, Marcus S.; Grcar, Joseph F.; Lijewski, Michael J.
2004-03-26
Many turbulent premixed flames of practical interest are statistically stationary. They occur in combustors that have anchoring mechanisms to prevent blow-off and flashback. The stabilization devices often introduce a level of geometric complexity that is prohibitive for detailed computational studies of turbulent flame dynamics. As a result, typical detailed simulations are performed in simplified model configurations such as decaying isotropic turbulence or inflowing turbulence. In these configurations, the turbulence seen by the flame either decays or, in the latter case, increases as the flame accelerates toward the turbulent inflow. This limits the duration of the eddy evolutions experienced by the flame at a given level of turbulent intensity, so that statistically valid observations cannot be made. In this paper, we apply a feedback control to computationally stabilize an otherwise unstable turbulent premixed flame in two dimensions. For the simulations, we specify turbulent in flow conditions and dynamically adjust the integrated fueling rate to control the mean location of the flame in the domain. We outline the numerical procedure, and illustrate the behavior of the control algorithm. We use the simulations to study the propagation and the local chemical variability of turbulent flame chemistry.
Structure and scales in turbulence modeling
NASA Astrophysics Data System (ADS)
Reynolds, W. C.; Langer, C. A.; Kassinos, S. C.
2002-07-01
The enstrophy of the large-scale energy-containing turbulence is proposed as the second turbulence scale for use, in conjunction with the turbulence energy, in two-scale one-point engineering turbulence models. Its transport equation is developed in general and modeled for homogeneous turbulence in terms of the two scales and our new one-point structure tensors. The model produces the correct behavior of the scales for both two- and three-dimensional turbulence. Constants in the high Reynolds number model are evaluated only by reference to asymptotic analysis for decaying turbulence in stationary and rotating frames, and this model is then shown to provide an excellent prediction of homogeneous turbulent shear flow when used with the structure tensors for that flow. The low Reynolds number constant in the model is evaluated using the asymptotic decay rate for isotropic turbulence at zero Reynolds number, and numerical simulations of decay for intermediate Reynolds numbers are used to establish one remaining constant, the value of which does not affect high Reynolds number predictions.
NASA Astrophysics Data System (ADS)
van Hout, Rene; Eisma, Jerke; Overmars, Edwin; Elsinga, Gerrit; Westerweel, Jerry
2015-11-01
Time resolved tomographic PIV measurements (acquisition rate 250Hz) were performed in a turbulent boundary layer (TBL) on the side wall of an open channel, water flow facility (cross section 60x60cm, Wx H) , 3.5m downstream of the inlet at a bulk flow velocity of Ub = 0.17m/s (Reb =Ub H / ν = 102x103, δ0 . 99 = 5 . 0 cm, Reθ = 891). The measurement volume was a horizontal slab (6x1.5x6cm3, lx wx h) extending from the side wall, 30cm above the bottom. The Tomo-PIV setup comprised four high-speed ImagerPro HS cameras (2016x2016pixels), a high-speed laser (Nd:YLF, Darwin Duo 80M, Quantronix), optics/prisms and data acquisition/processing software (LaVision, DaVis8.2). A sphere with diameter, D = 6mm (D+ = 51, ``+'' denotes inner wall scaling), was positioned at y = 37.5 and 5.4mm (y+ = 319 and 46) from the wall (measured from the sphere's center). The latter position covers most of the buffer layer while the former is well in the outer layer. Sphere Reynolds numbers based on D and the average streamwise velocity at the sphere's center were 984 (y+ = 319) and 684 (y+ = 46). Results show the interaction between the coherent turbulence structures in the TBL and those generated in the sphere's wake. Total and partial destruction of the log-law layer is observed when the sphere is positioned in the buffer and outer layer, respectively.
NASA Astrophysics Data System (ADS)
Frisch, Uriel
1996-01-01
Written five centuries after the first studies of Leonardo da Vinci and half a century after A.N. Kolmogorov's first attempt to predict the properties of flow, this textbook presents a modern account of turbulence, one of the greatest challenges in physics. "Fully developed turbulence" is ubiquitous in both cosmic and natural environments, in engineering applications and in everyday life. Elementary presentations of dynamical systems ideas, probabilistic methods (including the theory of large deviations) and fractal geometry make this a self-contained textbook. This is the first book on turbulence to use modern ideas from chaos and symmetry breaking. The book will appeal to first-year graduate students in mathematics, physics, astrophysics, geosciences and engineering, as well as professional scientists and engineers.
Relaminarization under stationary vortices
NASA Astrophysics Data System (ADS)
Breidenthal, Robert
2005-11-01
Flow visualization reveals that a turbulent boundary layer is relaminarized when stationary streamwise vortices are introduced. Following a suggestion of Balle, the vortices are stabilized by large streamwise ``Karman'' grooves in a wavy wall. In a water tunnel, upstream vortex generators place a large streamwise vortex in the middle of each groove, at the stationary point where Prandtl's vortex force vanishes. According to a theory by Cotel, the wall fluxes of a turbulent boundary layer should decline to laminar values under such ``persistent'' vortices. The observed relaminarization is consistent with this theory and with previous measurements of heat transfer by Touel and Balle. However, the structure of the transverse flow resembles the cats-eye pattern of a temporal shear layer rather than the anticipated von Karman wake. The cats-eye pattern corresponds to the forced shear layers of Oster-Wygnanski and Roberts, who found that the Reynolds stresses and mixing rate also decline to laminar values.
Diffusion of Sound Waves in a Turbulent Atmosphere
NASA Technical Reports Server (NTRS)
Lyon, Richard H.
1960-01-01
The directional and frequency diffusion of a plane monochromatic 2 sound wave in statistically homogeneous, isotropic, and stationary turbulence is analyzed theoretically. The treatment is based on the diffusion equation for the energy density of sound waves, using the scattering cross section derived by Kraichnan for the type of turbulence assumed here. A form for the frequency-wave number spectrum of the turbulence is adopted which contains the pertinent parameters of the flow and is adapted to ease of calculation. A new approach to the evaluation of the characteristic period of the flow is suggested. This spectrum is then related to the scattering cross section. Finally, a diffusion equation is derived as a small-angle scattering approximation to the rigorous transport equation. The rate of spread of the incident wave in frequency and direction is calculated, as well as the power spectrum and autocorrelation for the wave.
Spectral multigrid methods for the solution of homogeneous turbulence problems
NASA Technical Reports Server (NTRS)
Erlebacher, G.; Zang, T. A.; Hussaini, M. Y.
1987-01-01
New three-dimensional spectral multigrid algorithms are analyzed and implemented to solve the variable coefficient Helmholtz equation. Periodicity is assumed in all three directions which leads to a Fourier collocation representation. Convergence rates are theoretically predicted and confirmed through numerical tests. Residual averaging results in a spectral radius of 0.2 for the variable coefficient Poisson equation. In general, non-stationary Richardson must be used for the Helmholtz equation. The algorithms developed are applied to the large-eddy simulation of incompressible isotropic turbulence.
Measuring the cascade rate in anisotropic turbulence through 3rd order structure functions.
NASA Astrophysics Data System (ADS)
Verdini, Andrea; Landi, Simone; Hellinger, Petr
2014-05-01
We employ the Von-Karman-Howart-Yaglom-Politano-Poquet (KHYPP)law, to compute the cascade rate by means of 3rd order structure functions in homogeneous, forced, DNS at high resolution. We consider first the isotropic case (no guide field) and verify that the cascade rate is consistent with the dissipation rate. Then we consider an anisotropic case (with guide field) for which the isotropic KHYPP law does not apply. We compute the parallel and perpendicular cascade rates and find that the latter basically accounts for the total dissipation rate, as expected for anisotropic turbulence. Also, the cascade rate derived from the isotropic law is found to be a good approximation for the total cascade rate. Recent works have shown that the hypothesis of stationary turbulence must be probably relaxed in the solar wind. We present preliminary results on the measure of the cascade rate in the expanding solar wind, obtained with DNS of MHD turbulence in the expanding box model. Such model incorporates the basic physic of expansion thus inducing anisotropies driven by both the magnetic field and expansion, along with an energy decrease due to the conservation of linear invariants (angular momentum and magnetic flux). The correction due to non-stationary conditions is found to be important and to become negligible only at small scales, thus suggesting that solar wind measurements over- estimate the actual cascade rate.
A Comparative Study of Stationary and Non-stationary Wind Models Using Field Measurements
NASA Astrophysics Data System (ADS)
Chen, Jun; Hui, Michael C. H.; Xu, Y. L.
2007-01-01
We present a comparative study of the conventional stationary wind speed model and a newly proposed non-stationary wind speed model using field measurements. The concept of, and the differences between, the two wind models are briefly reviewed. Wind data recorded by a field measurement system for wind turbulence parameters (FMS-WTP) of 1-year duration are analyzed using the two wind models. Comparisons were made between the wind characteristics obtained from the two models, including hourly mean wind speed, turbulence intensity, the wind spectrum, integral length scale, root coherence function and probability density function. The effects of wind types (monsoon or typhoon), statistical properties (stationary or non-stationary), and surface roughness (open-sea fetch or overland fetch) on wind characteristics are discussed. The comparative study demonstrates that the non-stationary wind model appears to be more appropriate than the conventional stationary wind speed model for characterizing turbulent winds of one-hour duration over complex terrain.
Structure of wind-shear turbulence
NASA Technical Reports Server (NTRS)
Trevino, G.; Laituri, T. R.
1988-01-01
The statistical characteristics of wind-shear turbulence are modelled. Isotropic turbulence serves as the basis of comparison for the anisotropic turbulence which exists in wind shear. The question of how turbulence scales in a wind shear is addressed from the perspective of power spectral density.
Structure of wind-shear turbulence
NASA Technical Reports Server (NTRS)
Trevino, G.; Laituri, T. R.
1989-01-01
The statistical characteristics of wind shear turbulence are modelled. Isotropic turbulence serves as the basis of comparison for the anisotropic turbulence which exists in wind shear. The question of turbulence scales in wind shear is addressed from the perspective of power spectral density.
The effects of anisotropic free-stream turbulence on turbulent boundary layer behavior
NASA Technical Reports Server (NTRS)
Liang-Wei, F.; Hoffman, J. A.
1985-01-01
The effects of near-isotropic and highly anisotropic free-stream turbulence on mean flow properties of the turbulence structure of turbulent boundary layers in a near zero pressure gradient flow has been experimentally evaluated. Turbulence levels vary from 0.5% to 8.0% and the momentum thickness Reynolds number varies from 800 to 1100. The results indicate that the effects of free-stream turbulence on the classical boundary layer properties for near-isotropic turbulence which have been published by other investigators are similar to the case of highly anisotropic turbulence fields, while the effects of free-stream turbulence on the properties of the turbulent structure within the boundary layer for the case of near-isotropic turbulence are quite different compared to the highly anisotropic case.
Structure of the isotropic transport operators in three independent space variables
NASA Technical Reports Server (NTRS)
Abu-Shumays, I. K.; Bareiss, E. H.
1969-01-01
Based on the idea of separation of variables, a spectral theory for the three-dimensional, stationary, isotropic transport operator in a vector space of complex-valued Borel functions results in continuous sets of regular and generalized eigenfunctions.
Electron magnetohydrodynamics: dynamics and turbulence.
Lyutikov, Maxim
2013-11-01
We consider dynamics and turbulent interaction of whistler modes within the framework of inertialess electron magnetohydrodynamics (EMHD). We argue that there is no energy principle in EMHD: any stationary closed configuration is neutrally stable. On the other hand, the relaxation principle, the long term evolution of a weakly dissipative system towards Taylor-Beltrami state, remains valid in EMHD. We consider the turbulent cascade of whistler modes. We show that (i) harmonic whistlers are exact nonlinear solutions; (ii) collinear whistlers do not interact (including counterpropagating); (iii) waves with the same value of the wave vector k(1)=k(2) do not interact; (iv) whistler modes have a dispersion that allows a three-wave decay, including into a zero frequency mode; (v) the three-wave interaction effectively couples modes with highly different wave numbers and propagation angles. In addition, linear interaction of a whistler with a single zero mode can lead to spatially divergent structures via parametric instability. All these properties are drastically different from MHD, so that the qualitative properties of the Alfvén turbulence can not be transferred to the EMHD turbulence. We derive the Hamiltonian formulation of EMHD, and using Bogoliubov transformation reduce it to the canonical form; we calculate the matrix elements for the three-wave interaction of whistlers. We solve numerically the kinetic equation and show that, generally, the EMHD cascade develops within a broad range of angles, while transiently it may show anisotropic, nearly two-dimensional structures. Development of a cascade depends on the forcing (nonuniversal) and often fails to reach a steady state. Analytical estimates predict the spectrum of magnetic fluctuations for the quasi-isotropic cascade [proportionality]k(-2). The cascade remains weak (not critically balanced). The cascade is UV local, while the infrared locality is weakly (logarithmically) violated. PMID:24329368
Turbulence in homogeneous shear flows
NASA Astrophysics Data System (ADS)
Pumir, Alain
1996-11-01
Homogeneous shear flows with an imposed mean velocity U=Syx̂ are studied in a period box of size Lx×Ly×Lz, in the statistically stationary turbulent state. In contrast with unbounded shear flows, the finite size of the system constrains the large-scale dynamics. The Reynolds number, defined by Re≡SL2y/ν varies in the range 2600⩽Re⩽11300. The total kinetic energy and enstrophy in the volume of numerical integration have large peaks, resulting in fluctuations of kinetic energy of order 30%-50%. The mechanism leading to these fluctuations is very reminiscent of the ``streaks'' responsible for the violent bursts observed in turbulent boundary layers. The large scale anisotropy of the flow, characterized by the two-point correlation tensor
Electron magnetohydrodynamics: Dynamics and turbulence
NASA Astrophysics Data System (ADS)
Lyutikov, Maxim
2013-11-01
We consider dynamics and turbulent interaction of whistler modes within the framework of inertialess electron magnetohydrodynamics (EMHD). We argue that there is no energy principle in EMHD: any stationary closed configuration is neutrally stable. On the other hand, the relaxation principle, the long term evolution of a weakly dissipative system towards Taylor-Beltrami state, remains valid in EMHD. We consider the turbulent cascade of whistler modes. We show that (i) harmonic whistlers are exact nonlinear solutions; (ii) collinear whistlers do not interact (including counterpropagating); (iii) waves with the same value of the wave vector k1=k2 do not interact; (iv) whistler modes have a dispersion that allows a three-wave decay, including into a zero frequency mode; (v) the three-wave interaction effectively couples modes with highly different wave numbers and propagation angles. In addition, linear interaction of a whistler with a single zero mode can lead to spatially divergent structures via parametric instability. All these properties are drastically different from MHD, so that the qualitative properties of the Alfvén turbulence can not be transferred to the EMHD turbulence. We derive the Hamiltonian formulation of EMHD, and using Bogoliubov transformation reduce it to the canonical form; we calculate the matrix elements for the three-wave interaction of whistlers. We solve numerically the kinetic equation and show that, generally, the EMHD cascade develops within a broad range of angles, while transiently it may show anisotropic, nearly two-dimensional structures. Development of a cascade depends on the forcing (nonuniversal) and often fails to reach a steady state. Analytical estimates predict the spectrum of magnetic fluctuations for the quasi-isotropic cascade ∝k-2. The cascade remains weak (not critically balanced). The cascade is UV local, while the infrared locality is weakly (logarithmically) violated.
Stationary and non-stationary nonlinear optical spectroscopy on surface polaritons
NASA Technical Reports Server (NTRS)
Ponath, H. E.
1984-01-01
A phenomenological theory is given for non-stationary electromagnetic surface waves propagating along the boundary plane between two homogeneous isotropic media. The description of nonlinear optical effects using shortened wave equations is demonstrated for spontaneous and simulated Raman scattering processes on surface polaritons.
Isotropic sequence order learning.
Porr, Bernd; Wörgötter, Florentin
2003-04-01
In this article, we present an isotropic unsupervised algorithm for temporal sequence learning. No special reward signal is used such that all inputs are completely isotropic. All input signals are bandpass filtered before converging onto a linear output neuron. All synaptic weights change according to the correlation of bandpass-filtered inputs with the derivative of the output. We investigate the algorithm in an open- and a closed-loop condition, the latter being defined by embedding the learning system into a behavioral feedback loop. In the open-loop condition, we find that the linear structure of the algorithm allows analytically calculating the shape of the weight change, which is strictly heterosynaptic and follows the shape of the weight change curves found in spike-time-dependent plasticity. Furthermore, we show that synaptic weights stabilize automatically when no more temporal differences exist between the inputs without additional normalizing measures. In the second part of this study, the algorithm is is placed in an environment that leads to closed sensor-motor loop. To this end, a robot is programmed with a prewired retraction reflex reaction in response to collisions. Through isotropic sequence order (ISO) learning, the robot achieves collision avoidance by learning the correlation between his early range-finder signals and the later occurring collision signal. Synaptic weights stabilize at the end of learning as theoretically predicted. Finally, we discuss the relation of ISO learning with other drive reinforcement models and with the commonly used temporal difference learning algorithm. This study is followed up by a mathematical analysis of the closed-loop situation in the companion article in this issue, "ISO Learning Approximates a Solution to the Inverse-Controller Problem in an Unsupervised Behavioral Paradigm" (pp. 865-884). PMID:12689389
Rodlike localized structure in isotropic pattern-forming systems
NASA Astrophysics Data System (ADS)
Bordeu, Ignacio; Clerc, Marcel G.
2015-10-01
Stationary two-dimensional localized structures have been observed in a wide variety of dissipative systems. The existence, stability properties, dynamical evolution, and bifurcation diagram of an azimuthal symmetry breaking, rodlike localized structure in the isotropic prototype model of pattern formation, the Swift-Hohenberg model, is studied. These rodlike structures persist under the presence of nongradient perturbations. Interaction properties of the rodlike structures are studied. This allows us to envisage the possibility of different crystal-like configurations.
NON-PREMIXED TURBULENT JET FLAMES
The paper, part of a general investigation of mixing and chemical reaction in turbulent jets, concerns the length of non-premixed turbulent jet flames in a stationary environment. Experimental results for the turbulent flame length of chemically reacting jets in water show both i...
Suppression of turbulent resistivity in turbulent Couette flow
NASA Astrophysics Data System (ADS)
Si, Jiahe; Colgate, Stirling A.; Sonnenfeld, Richard G.; Nornberg, Mark D.; Li, Hui; Colgate, Arthur S.; Westpfahl, David J.; Romero, Van D.; Martinic, Joe
2015-07-01
Turbulent transport in rapidly rotating shear flow very efficiently transports angular momentum, a critical feature of instabilities responsible both for the dynamics of accretion disks and the turbulent power dissipation in a centrifuge. Turbulent mixing can efficiently transport other quantities like heat and even magnetic flux by enhanced diffusion. This enhancement is particularly evident in homogeneous, isotropic turbulent flows of liquid metals. In the New Mexico dynamo experiment, the effective resistivity is measured using both differential rotation and pulsed magnetic field decay to demonstrate that at very high Reynolds number rotating shear flow can be described entirely by mean flow induction with very little contribution from correlated velocity fluctuations.
Suppression of turbulent resistivity in turbulent Couette flow
Si, Jiahe Sonnenfeld, Richard G.; Colgate, Arthur S.; Westpfahl, David J.; Romero, Van D.; Martinic, Joe; Colgate, Stirling A.; Li, Hui; Nornberg, Mark D.
2015-07-15
Turbulent transport in rapidly rotating shear flow very efficiently transports angular momentum, a critical feature of instabilities responsible both for the dynamics of accretion disks and the turbulent power dissipation in a centrifuge. Turbulent mixing can efficiently transport other quantities like heat and even magnetic flux by enhanced diffusion. This enhancement is particularly evident in homogeneous, isotropic turbulent flows of liquid metals. In the New Mexico dynamo experiment, the effective resistivity is measured using both differential rotation and pulsed magnetic field decay to demonstrate that at very high Reynolds number rotating shear flow can be described entirely by mean flow induction with very little contribution from correlated velocity fluctuations.
NASA Astrophysics Data System (ADS)
Kühnen, Jakob; Hof, Björn
2015-11-01
We show that a simple modification of the velocity profile in a pipe can lead to a complete collapse of turbulence and the flow fully relaminarises. The annihilation of turbulence is achieved by a steady manipulation of the streamwise velocity component alone, greatly reducing control efforts. Several different control techniques are presented: one with a local modification of the flow profile by means of a stationary obstacle, one employing a nozzle injecting fluid through a small gap at the pipe wall and one with a moving wall, where a part of the pipe is shifted in the streamwise direction. All control techniques act on the flow such that the streamwise velocity profile becomes more flat and turbulence gradually grows faint and disappears. In a smooth straight pipe the flow remains laminar downstream of the control. Hence a reduction in skin friction by a factor of 8 and more can be accomplished. Stereoscopic PIV-measurements and movies of the development of the flow during relaminarisation are presented.
Sedimentation of finite-size particles in quiescent and turbulent environments
NASA Astrophysics Data System (ADS)
Brandt, Luca; Fornari, Walter; Picano, Francesco
2015-11-01
Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows. We present Direct Numerical Simulations of sedimentation in quiescent and turbulent environments using an Immersed Boundary Method to study the behavior of finite-size particles in homogeneous isotropic turbulence. The particle radius is approximately 6 Komlogorov lengthscales, the volume fraction 0.5% and 1% and the density ratio 1.02. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. The reduction with respect to a single particle in quiescent fluid is about 12% in dilute conditions. The probability density function of the particle velocity is almost Gaussian in a turbulent flow, whereas it displays large positive tails in quiescent fluid. These tails are associated to the intermittent fast sedimentation of particle pairs in drafting-kissing-tumbling motions. Using the concept of mean relative velocity we estimate the mean drag coefficient from empirical formulas and show that non stationary effects, related to vortex shedding, explain the increased reduction in mean settling velocity in a turbulent environment. This work was supported by the European Research Council Grant No. ERC-2013- CoG-616186, TRITOS.
Isotropic Monte Carlo Grain Growth
Energy Science and Technology Software Center (ESTSC)
2013-04-25
IMCGG performs Monte Carlo simulations of normal grain growth in metals on a hexagonal grid in two dimensions with periodic boundary conditions. This may be performed with either an isotropic or a misorientation - and incliantion-dependent grain boundary energy.
Transversely isotropic poroelasticity arising from thin isotropic layers
Berryman, J.G.
1996-11-01
Percolation phenomena play central roles in the field of poroelasticity, where two distinct sets of percolating continua intertwine. A connected solid frame forms the basis of the elastic behavior of a poroelastic medium in the presence of confining forces, while connected pores permit a percolating fluid (if present) to influence the mechanical response of the system from within. The present paper discusses isotropic and anisotropic poroelastic media and establishes general formulas for the behavior of transversely isotropic poroelasticity arising from laminations of isotropic components. The Backus averaging method is shown to provide elementary means of constructing general formulas. The results for confined fluids are then compared with the more general Gassmann formulas that must be satisfied by any anisotropic poroelastic medium and found to be in complete agreement.
Effect of turbulence on the drag and lift of a particle
NASA Astrophysics Data System (ADS)
Bagchi, P.; Balachandar, S.
2003-11-01
A direct numerical simulation (DNS) is used to study the effect of a freestream isotropic turbulent flow on the drag and lift forces on a spherical particle. The particle diameter is about 1.5-10 times the Kolmogorov scale, the particle Reynolds number is about 60-600, and the freestream turbulence intensity is about 10%-25%. The isotropic turbulent field considered here is stationary, i.e., frozen in time. It is shown that the freestream turbulence does not have a substantial and systematic effect on the time-averaged mean drag. The standard drag correlation based on the instantaneous or mean relative velocity results in a reasonably accurate prediction of the mean drag obtained from the DNS. However, the accuracy of prediction of the instantaneous drag decreases with increasing particle size. For the smaller particles, the low frequency oscillations in the DNS drag are well captured by the standard drag, but for the larger particles significant differences exist even for the low frequency components. Inclusion of the added-mass and history forces, computed based on the fluid velocity at the center of the particle, does not improve the prediction. Different estimates of the fluid velocity seen by the particle are examined. It is shown that the mean drag is insensitive to the fluid velocity measured at the particle center, or obtained by averaging over a fluid volume of the order of the particle size. The fluctuations diminish as the size of the averaging volume increases. The effect of increasing freestream turbulence intensity for the same particle size is studied. Fluctuations in the drag and lift forces are shown to scale with the mean drag and freestream intensity. The standard drag without the added-mass and history forces provides the best approximation to the DNS result.
Charged Particle Diffusion in Isotropic Random Static Magnetic Fields
NASA Astrophysics Data System (ADS)
Subedi, P.; Sonsrettee, W.; Matthaeus, W. H.; Ruffolo, D. J.; Wan, M.; Montgomery, D.
2013-12-01
Study of the transport and diffusion of charged particles in a turbulent magnetic field remains a subject of considerable interest. Research has most frequently concentrated on determining the diffusion coefficient in the presence of a mean magnetic field. Here we consider Diffusion of charged particles in fully three dimensional statistically isotropic magnetic field turbulence with no mean field which is pertinent to many astrophysical situations. We classify different regions of particle energy depending upon the ratio of Larmor radius of the charged particle to the characteristic outer length scale of turbulence. We propose three different theoretical models to calculate the diffusion coefficient each applicable to a distinct range of particle energies. The theoretical results are compared with those from computer simulations, showing very good agreement.
Turbulent solutions of equations of fluid motion
NASA Technical Reports Server (NTRS)
Deissler, R. G.
1985-01-01
Some turbulent solutions of the unaveraged Navier-Stokes equations (equations of fluid motion) are reviewed. Those equations are solved numerically in order to study the nonlinear physics of incompressible turbulent flow. The three components of the mean-square velocity fluctuations are initially equal for the conditions chosen. The resulting solutions show characteristics of turbulence, such as the linear and nonlinear excitation of small-scale fluctuations. For the stronger fluctuations the initially nonrandom flow develops into an apparently random turbulence. The cases considered include turbulence that is statistically homogeneous or inhomogeneous and isotropic or anisotropic. A statistically steady-state turbulence is obtained by using a spatially periodic body force. Various turbulence processes, including the transfer of energy between eddy sizes and between directional components and the production, dissipation, and spatial diffusion of turbulence, are considered. It is concluded that the physical processes occurring in turbulence can be profitably studied numerically.
Interaction of a free flame front with a turbulence field
NASA Technical Reports Server (NTRS)
Tucker, Maurice
1956-01-01
Small-perturbation spectral-analysis techniques are used to obtain the root-mean-square flame-generated turbulence velocities and the attenuating pressure fluctuations stemming from interaction of a constant-pressure flame front with a field of isotropic turbulence in the absence of turbulence decay processes.
Bumblebee Flight in Heavy Turbulence
NASA Astrophysics Data System (ADS)
Engels, T.; Kolomenskiy, D.; Schneider, K.; Lehmann, F.-O.; Sesterhenn, J.
2016-01-01
High-resolution numerical simulations of a tethered model bumblebee in forward flight are performed superimposing homogeneous isotropic turbulent fluctuations to the uniform inflow. Despite tremendous variation in turbulence intensity, between 17% and 99% with respect to the mean flow, we do not find significant changes in cycle-averaged aerodynamic forces, moments, or flight power when averaged over realizations, compared to laminar inflow conditions. The variance of aerodynamic measures, however, significantly increases with increasing turbulence intensity, which may explain flight instabilities observed in freely flying bees.
NASA Astrophysics Data System (ADS)
Schlickeiser, R.
2011-05-01
A new transport theory of cosmic rays in magnetized space plasmas with axisymmetric incompressible magnetic turbulence is developed extending the quasilinear approximation to the particle orbit. Arbitrary gyrophase deviations from the unperturbed spiral orbits in the uniform magnetic field are allowed. For quasi-stationary and spatially homogeneous magnetic turbulence, we derive the small Larmor radius approximation gyrophase-averaged cosmic ray Fokker-Planck coefficients. The generalized Fokker-Planck coefficients correctly reduce to their known quasilinear values in the corresponding limit. New forms of the quasilinear Fokker-Planck coefficients in axisymmetric turbulence are derived which no longer involve infinite sums of products of Bessel functions, which facilitate their numerical computation for specified turbulence field correlation tensors. The Fokker-Planck coefficients for arbitrary phase orbits of the cosmic ray particles provide strict upper limits for the perpendicular and pitch-angle Fokker-Planck coefficients, which in turn yield strict upper and lower limits for the perpendicular and parallel spatial diffusion coefficients, respectively, describing the spatial diffusion of the isotropic part of the cosmic ray phase space density. For the associated mean free paths, we find for this general case that the product of the minimum parallel mean free path with the sum of the maximum perpendicular mean free paths equals R 2 L , where RL denotes the cosmic ray gyroradius.
NASA Astrophysics Data System (ADS)
Rost, J. C.; Porkolab, M.; Dorris, J. R.; Marinoni, A.; Burrell, K. H.
2012-10-01
The Phase Contrast Imaging (PCI) diagnostic on DIII-D provides a line-integrated measurement of density fluctuations covering wavenumbers 2 to 30 cm-1. An outer gap scan during QH-mode with stationary plasma parameters allowed the PCI to sample a large range in kr/kθ. A narrow peak in turbulence amplitude is seen near the LCFS. The ExB Doppler shift allows the location to be determined precisely, showing two distinct regions of turbulence at 0.5 and 0.2 cm inside the LCFS with kr>0 and kr<0 respectively, consistent with the expected effects of shear in the Er well. PCI measurements at 200 kHz show that kθ=0.8 cm-1 with poloidal correlation length Lθ=6 cm. Using a simple non-isotropic turbulence model, we find that kr=3 cm-1 and Lr=0.5 cm, with n/n˜25% in the pedestal for this high-kr turbulence. These fluctuations, which are outside the parameter range accessible to most turbulence diagnostics, are large enough in amplitude to play a role in setting the pedestal structure. These PCI observations are qualitatively similar to those made in ELM-free H-mode and between ELMs suggesting that similar large kr turbulence may be important.
Particle Settling in Low Energy Turbulence
NASA Astrophysics Data System (ADS)
Allen, Rachel; MacVean, Lissa; Tse, Ian; Mazzaro, Laura; Stacey, Mark; Variano, Evan
2014-11-01
Particle settling velocities can be altered by turbulence. In turbulence, dense particles may get trapped in convergent flow regions, and falling particles may be swept towards the downward side of turbulent eddies, resulting in enhanced settling velocities. The degree of velocity enhancement may depend on the Stokes number, the Rouse number, and the turbulent Reynolds number. In a homogeneous, isotropic turbulence tank, we tested the effects of particle size and type, suspended sediment concentration, and level of turbulence on the settling velocities of particles typically found in muddy estuaries. Two Acoustic Doppler Velocimeters (ADVs), separated vertically, measured turbulent velocities and suspended sediment concentrations, which yield condition dependent settling velocities, via ∂/á C ñ ∂ t = -∂/∂ z (ws á C ñ + á w ' C ' ñ) . These results are pertinent to fine sediment transport in estuaries, where high concentrations of suspended material are transported and impacted by low energy turbulence.
Cascade modeling of single and two-phase turbulence
NASA Astrophysics Data System (ADS)
Bolotnov, Igor A.
The analysis of turbulent two-phase flows requires closure models in order to perform reliable computational multiphase fluid dynamics (CFMD) analyses. A turbulence cascade model, which tracks the evolution of the turbulent kinetic energy between the various eddy sizes, has been developed for the analysis of the single and bubbly two-phase turbulence. Various flows are considered including the decay of isotropic grid-induced turbulence, uniform shear flow and turbulent channel flow. The model has been developed using a "building block" approach by moving from modeling of simpler turbulent flows (i.e., homogeneous, isotropic decay) to more involved turbulent flows (i.e., non-homogeneous channel flow). The spectral cascade-transport model's performance has been assessed against a number of experimental and direct numerical simulation (DNS) results.
Scaling of turbulence and turbulent mixing using Terascale numerical simulations
NASA Astrophysics Data System (ADS)
Donzis, Diego A.
Fundamental aspects of turbulence and turbulent mixing are investigated using direct numerical simulations (DNS) of stationary isotropic turbulence, with Taylor-scale Reynolds numbers (Rlambda) ranging from 8 to 650 and Schmidt numbers (Sc) from 1/8 to 1024. The primary emphasis is on important scaling issues that arise in the study of intermittency, mixing and turbulence under solid-body rotation. Simulations up to 20483 in size have been performed using large resource allocations on Terascale computers at leading supercomputing centers. Substantial efforts in algorithmic development have also been undertaken and resulted in a new code based on a two-dimensional domain decomposition which allows the use of very large number of processors. Benchmark tests indicate very good parallel performance for resolutions up to 40963 on up to 32768 processors, which is highly promising for future simulations at higher resolutions and processor counts eventually to approach Petascale levels. Investigation of intermittency through the statistics of dissipation and enstrophy in a series of simulations at the same Reynolds number but different resolution indicate that accurate results in high-order moments require a higher degree of fine-scale resolution than commonly practiced. However, statistics up to fourth order are satisfactory if the grid spacing is not larger than Komogorov scale, without the requirement of a clear analytic range for corresponding structure functions as suggested by recent theories. Results from highly resolved simulations provide support for a modified resolution criterion derived in this work for structure functions of different orders and as a function of Rlambda. At the highest Reynolds number in our simulations (400 and 650) dissipation and enstrophy exhibit extreme fluctuations of O(1000) the mean which have not been studied in the literature before. The far tails of the probability density functions of dissipation and enstrophy appear to coincide
New Exact Relations for Helicities in Hall Magnetohydrodynamic Turbulence
NASA Astrophysics Data System (ADS)
Banerjee, Supratik; Galtier, Sebastien
2016-04-01
Hall magnetohydrodynamics is a mono-fluid plasma model appropriate for probing Final{some of the} physical processes (other than pure kinetic effects) at length scales smaller than the scales of standard MHD. In sub-ionic space plasma turbulence (e.g. the solar wind) this fluid model has been proved to be useful. Three-dimensional incompressible Hall magnetohydrodynamics (MHD) possesses three inviscid invariants which are the total energy, the magnetic helicity and the generalized helicity. In this presentation, we would like to discuss new exact relations for helicities (magnetic helicities and generalized helicities) which are derived for homogeneous stationary (not necessarily isotropic) Hall MHD turbulence (and also for its inertialess electron MHD limit) in the asymptotic limit of large Reynolds numbers. The universal laws are written only in terms of mixed second-order structure functions, i.e. the scalar product of two different increments and are written simply as ηM = di < δ ( {b} × {j}) \\cdot δ {b} >, with ηM the average magnetic helicity flux rate, {b} the magnetic field, {j} the current and ± ηG = < δ ( {v} × {Ω} ) \\cdot δ {Ω} > , with ηM the average generalized helicity flux rate, {v} the fluid velocity and {Ω} = {b} + dI {ω} being the generalized helicity where ω is simply the fluid vorticity ( = nabla × {v}). It provides, therefore, a direct measurement of the dissipation rates for the corresponding helicities even in case of an anisotropic plasma turbulence. This study shows that the generalized helicity cascade is strongly linked to the left polarized fluctuations while the magnetic helicity cascade is linked to the right polarized fluctuations. The newly derived relations also show that like energy, a non-zero helicity flux can only be associated to a departure of Beltrami flow state. {Reference} S. Banerjee & S. Galtier, {Chiral Exact Relations for Helicities in Hall Magnetohydrodynamic Turbulence} (submitted).
Estimating three-demensional energy transfer in isotropic turbulence
NASA Technical Reports Server (NTRS)
Li, K. S.; Helland, K. N.; Rosenblatt, M.
1980-01-01
To obtain an estimate of the spectral transfer function that indicates the rate of decay of energy, an x-wire probe was set at a fixed position, and two single wire probes were set at a number of locations in the same plane perpendicular to the mean flow in the wind tunnel. The locations of the single wire probes are determined by pseudo-random numbers (Monte Carlo). Second order spectra and cross spectra are estimated. The assumption of isotropy relative to second order spectra is examined. Third order spectra are also estimated corresponding to the positions specified. A Monte Carlo Fourier transformation of the downstream bispectra corresponding to integration across the plane perpendicular to the flow is carried out assuming isotropy. Further integration is carried out over spherical energy shells.
Early isotropization of the Glasma
NASA Astrophysics Data System (ADS)
Epelbaum, T.
2014-11-01
A first principle treatment of the early stages of a heavy ion collision within the Color Glass Condensate framework seems to indicate an early hydrodynamization of the initially out of equilibrium Quark Gluon Plasma. Contrasting with the leading order CGC result, the improved calculation presented here provides evidence for a fast isotropization of the pressure tensor, as well as an anomalously small shear viscosity over entropy ratio.
Eddy fluxes in baroclinic turbulence
NASA Astrophysics Data System (ADS)
Thompson, Andrew F.
The eddy heat flux generated by the statistically equilibrated baroclinic instability of a uniform, horizontal temperature gradient is studied using a two-mode quasigeostrophic model. An overview of the dependence of the eddy diffusivity of heat Dtau on the planetary potential vorticity gradient beta, the bottom friction kappa, the deformation radius lambda, the vertical shear of the large-scale flow 2U and the domain size L is provided at 70 numerical simulations with beta = 0 (f-plane) and 110 simulations with beta ≠ 0 (beta-plane). Strong, axisymmetric, well-separated baroclinic vortices dominate the equilibrated barotropic vorticity and temperature fields of f-plane turbulence. The heat flux arises from a systematic northward (southward) migration of anti-cyclonic (cyclonic) eddies with warm (cold) fluid trapped in the cores. Zonal jets form spontaneously on the beta-plane, and stationary, isotropic, jet-scale eddies align within the strong eastward-flowing regions of the jets. In both studies, the vortices and jets give rise to a strong anti-correlation between the barotropic vorticity zeta and the temperature field tau. The baroclinic mode is also an important contributor to dissipation by bottom friction and energizes the barotropic mode at scales larger than lambda. This in part explains why previous parameterizations for the eddy heat flux based on Kolmogorovian cascade theories are found to be unreliable. In a separate study, temperature and salinity profiles obtained with expendable conductivity, temperature and depth (XCTD) probes within Drake Passage, Southern Ocean are used to analyze the turbulent diapycnal eddy diffusivity kappa rho to a depth of 1000 meters. The Polar Front separates two dynamically different regions with strong, surface-intensified mixing north of the Front. South of the Polar Front mixing is weaker and peaks at a depth of approximately 500 m, near the local temperature maximum. Peak values of kapparho are found to exceed 10-3 m
Calculation of Turbulent Expansion Processes
NASA Technical Reports Server (NTRS)
Tollmien, Walter
1945-01-01
On the basis of certain formulas recently established by L. Prandtl for the turbulent interchange of momentum in stationary flows, various cases of "free turbulence" - that is, of flows without boundary walls - are treated in the present report. Prandtl puts the apparent shearing stress introduced by the turbulent momentum interchange. This present report deals first with the mixing of an air stream of uniform velocity with the adjacent still air, than with the expansion or diffusion of an air jet in the surrounding air space.
Anisotropic turbulence in the solar wind
NASA Technical Reports Server (NTRS)
Matthaeus, W. H.; Bieber, J. W.; Zank, G. P.
1995-01-01
Solar wind turbulence has been viewed traditionally as composed of parallel propagating ('slab' fluctuations) or otherwise as isotropic turbulence. A variety of recent investigations, reviewed here, indicate that the spectrum may contain a significant admixture of two dimensional fluctuations, having variations mainly perpendicular to the local magnetic field. These indications come from simulations, from the theory of nearly incompressible MHD, from cosmic ray transport studies and from transport theory for solar wind turbulence, as well as from interpretations of direct observations. Thus, solar wind turbulence may be more like bundles of spaghetti than like parallel phase fronts.
Stirring turbulence with turbulence
NASA Astrophysics Data System (ADS)
Cekli, Hakki Ergun; Joosten, René; van de Water, Willem
2015-12-01
We stir wind-tunnel turbulence with an active grid that consists of rods with attached vanes. The time-varying angle of these rods is controlled by random numbers. We study the response of turbulence on the statistical properties of these random numbers. The random numbers are generated by the Gledzer-Ohkitani-Yamada shell model, which is a simple dynamical model of turbulence that produces a velocity field displaying inertial-range scaling behavior. The range of scales can be adjusted by selection of shells. We find that the largest energy input and the smallest anisotropy are reached when the time scale of the random numbers matches that of the largest eddies of the wind-tunnel turbulence. A large mismatch of these times creates a highly intermittent random flow with interesting but quite anomalous statistics.
Stationary nonlinear Airy beams
Lotti, A.; Faccio, D.; Couairon, A.; Papazoglou, D. G.; Panagiotopoulos, P.; Tzortzakis, S.; Abdollahpour, D.
2011-08-15
We demonstrate the existence of an additional class of stationary accelerating Airy wave forms that exist in the presence of third-order (Kerr) nonlinearity and nonlinear losses. Numerical simulations and experiments, in agreement with the analytical model, highlight how these stationary solutions sustain the nonlinear evolution of Airy beams. The generic nature of the Airy solution allows extension of these results to other settings, and a variety of applications are suggested.
A Transversely Isotropic Thermoelastic Theory
NASA Technical Reports Server (NTRS)
Arnold, S. M.
1989-01-01
A continuum theory is presented for representing the thermoelastic behavior of composites that can be idealized as transversely isotropic. This theory is consistent with anisotropic viscoplastic theories being developed presently at NASA Lewis Research Center. A multiaxial statement of the theory is presented, as well as plane stress and plane strain reductions. Experimental determination of the required material parameters and their theoretical constraints are discussed. Simple homogeneously stressed elements are examined to illustrate the effect of fiber orientation on the resulting strain distribution. Finally, the multiaxial stress-strain relations are expressed in matrix form to simplify and accelerate implementation of the theory into structural analysis codes.
Induced piezoelectricity in isotropic biomaterial.
Zimmerman, R L
1976-12-01
Isotropic material can be made to exhibit piezoelectric effects by the application of a constant electric field. For insulators, the piezoelectric strain constant is proportional to the applied electric field and for semiconductors, an additional out-of-phase component of piezoelectricity is proportional to the electric current density in the sample. The two induced coefficients are proportional to the strain-dependent dielectric constant (depsilon/dS + epsilon) and resistivity (drho/dS - rho), respectively. The latter is more important at frequencies such that rhoepsilonomega less than 1, often the case in biopolymers. Signals from induced piezoelectricity in nature may be larger than those from true piezoelectricity. PMID:990389
Stationary Vortices in Karman Grooves. I.Vortex Growth Rate
NASA Astrophysics Data System (ADS)
Balle, Gregory J.; Kier, Thiemo M.; Breidenthal, Robert E.
1999-11-01
The effect of a stationary vortex on wall fluxes in turbulent flow is predicted by the vortex persistence theory of turbulence. As a first step to test the theory, the feasibility of holding a vortex sufficiently stationary whilst embedded in a turbulent boundary layer is investi gated. Exploiting the stationarity of von Karman vortices in a wake, the dividing stream line is replaced by a wavy wall. Vortex generators are accurately positioned in the valleys (the "Karman grooves") so that the resulting streamwise vortices correspond to those in the vortex street. Complex potential theory predicts stationary points for such vortices, while there are no such points near a flat wall. Flow visualization experiments explore the basic properties of these vortices. In comparison to vortices near a flat plate, the growth rate of the stationary vortex in a Karman groove is reduced dramatically, i.e. by about an order of magnitude. This is consistent with the idea that the stationarity, persistence and growth rate of a turbulent vortex are intimately linked. Passive control of near-wall stream wise vortices is demonstrated, a foundation for the next step, measurement of a wall flux.
Damage spreading in 2-dimensional isotropic and anisotropic Bak-Sneppen models
NASA Astrophysics Data System (ADS)
Bakar, B.; Tirnakli, U.
2008-03-01
We implement the damage spreading technique on 2-dimensional isotropic and anisotropic Bak-Sneppen models. Our extensive numerical simulations show that there exists a power-law sensitivity to the initial conditions at the statistically stationary state (self-organized critical state). Corresponding growth exponent α for the Hamming distance and the dynamical exponent z are calculated. These values allow us to observe a clear data collapse of the finite size scaling for both versions of the Bak-Sneppen model. Moreover, it is shown that the growth exponent of the distance in the isotropic and anisotropic Bak-Sneppen models is strongly affected by the choice of the transient time.
Isotropic Negative Thermal Expansion Metamaterials.
Wu, Lingling; Li, Bo; Zhou, Ji
2016-07-13
Negative thermal expansion materials are important and desirable in science and engineering applications. However, natural materials with isotropic negative thermal expansion are rare and usually unsatisfied in performance. Here, we propose a novel method to achieve two- and three-dimensional negative thermal expansion metamaterials via antichiral structures. The two-dimensional metamaterial is constructed with unit cells that combine bimaterial strips and antichiral structures, while the three-dimensional metamaterial is fabricated by a multimaterial 3D printing process. Both experimental and simulation results display isotropic negative thermal expansion property of the samples. The effective coefficient of negative thermal expansion of the proposed models is demonstrated to be dependent on the difference between the thermal expansion coefficient of the component materials, as well as on the circular node radius and the ligament length in the antichiral structures. The measured value of the linear negative thermal expansion coefficient of the three-dimensional sample is among the largest achieved in experiments to date. Our findings provide an easy and practical approach to obtaining materials with tunable negative thermal expansion on any scale. PMID:27333052
Particle dispersion models and drag coefficients for particles in turbulent flows
NASA Technical Reports Server (NTRS)
Crowe, C. T.; Chung, J. N.; Troutt, T. R.
1988-01-01
Some of the concepts underlying particle dispersion due to turbulence are reviewed. The traditional approaches to particle dispersion in homogeneous, stationary turbulent fields are addressed, and recent work on particle dispersion in large scale turbulent structures is reviewed. The state of knowledge of particle drag coefficients in turbulent gas-particle flows is also reviewed.
Simonsson, Ivar; Mostad, Petter
2016-07-01
Probability calculations for relationship inference based on DNA tests are often performed with computer packages such as Familias. When mutations are assumed to be a possibility, one may notice a curious and problematic effect of including untested parents: results tend to change slightly. In this paper, we trace this effect back to fundamental model-formulating issues which can only be resolved by using stationary mutation models. We present several methods for obtaining such stationary mutation matrices from original mutation matrices, and evaluate essential properties of these methods. Our conclusion is that typically, stationary mutation models can be obtained, but for many types of markers, it may be impossible to combine specific biologically reasonable requirements for a mutation matrix with the requirement of stationarity. PMID:27231805
Nucleation and transients at the onset of vortex turbulence
NASA Technical Reports Server (NTRS)
Huber, Greg; Alstrom, Preben; Bohr, Tomas
1992-01-01
We present analytical and numerical results that explain the transient turbulent dynamics observed in the complex Ginzburg-Landau equation. Just below the transition to turbulence, we observe that metastable turbulent states break down by the nucleation and growth of single-vortex droplets, leading to a 'frozen' state with a low (but finite) density of stationary vortices. We derive the relation between nucleation time and radius, and determine their dependence on the distance to the turbulence transition line.
Spherical cloaking with homogeneous isotropic multilayered structures.
Qiu, Cheng-Wei; Hu, Li; Xu, Xiaofei; Feng, Yijun
2009-04-01
We propose a practical realization of electromagnetic spherical cloaking by layered structure of homogeneous isotropic materials. By mimicking the classic anisotropic cloak by many alternating thin layers of isotropic dielectrics, the permittivity and permeability in each isotropic layer can be properly determined by effective medium theory in order to achieve invisibility. The model greatly facilitates modeling by Mie theory and realization by multilayer coating of dielectrics. Eigenmode analysis is also presented to provide insights of the discretization in multilayers. PMID:19518392
ANISOTROPY LENGTHENS THE DECAY TIME OF TURBULENCE IN MOLECULAR CLOUDS
Hansen, Charles E.; McKee, Christopher F.; Klein, Richard I.
2011-09-01
The decay of isothermal turbulence with velocity anisotropy is investigated using computational simulations and synthetic observations. We decompose the turbulence into isotropic and anisotropic components with total velocity dispersions {sigma}{sub iso} and {sigma}{sub ani}, respectively. We find that the decay rate of the turbulence depends on the crossing time of the isotropic component only. A cloud of size L with significant anisotropy in its turbulence has a dissipation time, t{sub diss} = L/(2{sigma}{sub iso}). This translates into turbulent energy decay rates on the cloud scale that can be much lower for anisotropic turbulence than for isotropic turbulence. To help future observations determine whether observed molecular clouds have the level of anisotropy required to maintain the observed level of turbulence over their lifetimes, we performed a principal component analysis on our simulated clouds. Even with projection effects washing out the anisotropic signal, there is a measurable difference in the axis-constrained principal component analysis performed in directions parallel and perpendicular to the direction of maximum velocity dispersion. When this relative difference, {psi}, is 0.1, there is enough anisotropy for the dissipation time to triple the expected isotropic value. We provide a fit for converting {psi} into an estimate for the dissipation time, t{sub diss}.
Helicopter rotor noise due to ingestion of atmospheric turbulence
NASA Astrophysics Data System (ADS)
Simonich, J. C.; Amiet, R. K.; Schlinker, R. H.; Greitzer, E. M.
1986-05-01
A theoretical study was conducted to develop an analytical prediction method for helicopter main rotor noise due to the ingestion of atmospheric turbulence. This study incorporates an atmospheric turbulence model, a rotor mean flow contraction model and a rapid distortion turbulence model which together determine the statistics of the non-isotropic turbulence at the rotor plane. Inputs to the combined mean inflow and turbulence models are controlled by atmospheric wind characteristics and helicopter operating conditions. A generalized acoustic source model was used to predict the far field noise generated by the non-isotropic flow incident on the rotor. Absolute levels for acoustic spectra and directivity patterns were calculated for full scale helicopters, without the use of empirical or adjustable constants. Comparisons between isotropic and non-isotropic turbulence at the rotor face demonstrated pronounced differences in acoustic spectra. Turning and contraction of the flow for hover and low speed vertical ascent cases result in a 3 dB increase in the acoustic spectrum energy and a 10 dB increase in tone levels. Compared to trailing edge noise, turbulence ingestion noise is the dominant noise mechanism below approximately 30 rotor harmonics, while above 100 harmonics, trailing edge noise levels exceed turbulence ingestion noise by 25 dB.
Helicopter rotor noise due to ingestion of atmospheric turbulence
NASA Technical Reports Server (NTRS)
Simonich, J. C.; Amiet, R. K.; Schlinker, R. H.; Greitzer, E. M.
1986-01-01
A theoretical study was conducted to develop an analytical prediction method for helicopter main rotor noise due to the ingestion of atmospheric turbulence. This study incorporates an atmospheric turbulence model, a rotor mean flow contraction model and a rapid distortion turbulence model which together determine the statistics of the non-isotropic turbulence at the rotor plane. Inputs to the combined mean inflow and turbulence models are controlled by atmospheric wind characteristics and helicopter operating conditions. A generalized acoustic source model was used to predict the far field noise generated by the non-isotropic flow incident on the rotor. Absolute levels for acoustic spectra and directivity patterns were calculated for full scale helicopters, without the use of empirical or adjustable constants. Comparisons between isotropic and non-isotropic turbulence at the rotor face demonstrated pronounced differences in acoustic spectra. Turning and contraction of the flow for hover and low speed vertical ascent cases result in a 3 dB increase in the acoustic spectrum energy and a 10 dB increase in tone levels. Compared to trailing edge noise, turbulence ingestion noise is the dominant noise mechanism below approximately 30 rotor harmonics, while above 100 harmonics, trailing edge noise levels exceed turbulence ingestion noise by 25 dB.
Macroscopic simulation of isotropic permanent magnets
NASA Astrophysics Data System (ADS)
Bruckner, Florian; Abert, Claas; Vogler, Christoph; Heinrichs, Frank; Satz, Armin; Ausserlechner, Udo; Binder, Gernot; Koeck, Helmut; Suess, Dieter
2016-03-01
Accurate simulations of isotropic permanent magnets require to take the magnetization process into account and consider the anisotropic, nonlinear, and hysteretic material behaviour near the saturation configuration. An efficient method for the solution of the magnetostatic Maxwell equations including the description of isotropic permanent magnets is presented. The algorithm can easily be implemented on top of existing finite element methods and does not require a full characterization of the hysteresis of the magnetic material. Strayfield measurements of an isotropic permanent magnet and simulation results are in good agreement and highlight the importance of a proper description of the isotropic material.
Kinematics of velocity and vorticity correlations in turbulent flow
NASA Technical Reports Server (NTRS)
Bernard, P. S.
1983-01-01
The kinematic problem of calculating second-order velocity moments from given values of the vorticity covariance is examined. Integral representation formulas for second-order velocity moments in terms of the two-point vorticity correlation tensor are derived. The special relationships existing between velocity moments in isotropic turbulence are expressed in terms of the integral formulas yielding several kinematic constraints on the two-point vorticity correlation tensor in isotropic turbulence. Numerical evaluation of these constraints suggests that a Gaussian curve may be the only form of the longitudinal velocity correlation coefficient which is consistent with the requirement of isotropy. It is shown that if this is the case, then a family of exact solutions to the decay of isotropic turbulence may be obtained which contains Batchelor's final period solution as a special case. In addition, the computed results suggest a method of approximating the integral representation formulas in general turbulent shear flows.
Surface Stress with Non-stationary Weak Winds and Stable Stratification
NASA Astrophysics Data System (ADS)
Mahrt, L.; Thomas, Christoph K.
2016-04-01
The behaviour of turbulent transport in the weak-wind, stably-stratified, boundary layer over land is examined in terms of the non-stationarity of the wind field using measurements from three field programs. These field programs include towers ranging from 12 to 20 m in height and an extensive horizontal network of sonic anemometers. The relationship of the friction velocity to the stratification and non-stationary submeso motions is investigated from several points of view and nominally quantified. The relationship of the turbulence to the stratification is less systematic than expected partly due to enhancement of the turbulence by submeso motions. Cause and effect relationships are difficult to isolate because the non-stationary momentum flux significantly modifies the profile of the non-stationary mean flow. The link between the turbulence and accelerations at the surface is examined in terms of the changing vertical structure of the wind profile and sudden increases in the downward transport of momentum.
Constitutive modeling for isotropic materials
NASA Technical Reports Server (NTRS)
Lindholm, Ulric S.; Chan, Kwai S.
1986-01-01
The objective of the program is to evaluate and develop existing constitutive models for use in finite-element structural analysis of turbine engine hot section components. The class of constitutive equation studied is considered unified in that all inelastic deformation including plasticity, creep, and stress relaxation are treated in a single term rather than a classical separation of plasticity (time independent) and creep (time dependent) behavior. The unified theories employed also do not utilize the classical yield surface or plastic potential concept. The models are constructed from an appropriate flow law, a scalar kinetic relation between strain rate, temperature and stress, and evolutionary equations for internal variables describing strain or work hardening, both isotropic and directional (kinematic). This and other studies have shown that the unified approach is particularly suited for determining the cyclic behavior of superalloy type blade and vane materials and is entirely compatible with three-dimensional inelastic finite-element formulations. The behavior was examined of a second nickel-base alloy, MAR-M247, and compared it with the Bodner-Partom model, further examined procedures for determining the material-specific constants in the models, and exercised the MARC code for a turbine blade under simulated flight spectrum loading. Results are summarized.
Constitutive modeling for isotropic materials
NASA Technical Reports Server (NTRS)
Chan, K. S.; Lindholm, U. S.; Bodner, S. R.
1988-01-01
The third and fourth years of a 4-year research program, part of the NASA HOST Program, are described. The program goals were: (1) to develop and validate unified constitutive models for isotropic materials, and (2) to demonstrate their usefulness for structural analysis of hot section components of gas turbine engines. The unified models selected for development and evaluation were those of Bodner-Partom and of Walker. The unified approach for elastic-viscoplastic constitutive equations is a viable method for representing and predicting material response characteristics in the range where strain rate and temperature dependent inelastic deformations are experienced. This conclusion is reached by extensive comparison of model calculations against the experimental results of a test program of two high temperature Ni-base alloys, B1900+Hf and Mar-M247, over a wide temperature range for a variety of deformation and thermal histories including uniaxial, multiaxial, and thermomechanical loading paths. The applicability of the Bodner-Partom and the Walker models for structural applications has been demonstrated by implementing these models into the MARC finite element code and by performing a number of analyses including thermomechanical histories on components of hot sections of gas turbine engines and benchmark notch tensile specimens. The results of the 4-year program have been published in four annual reports. The results of the base program are summarized in this report. The tasks covered include: (1) development of material test procedures, (2) thermal history effects, and (3) verification of the constitutive model for an alternative material.
HYDROCARBON POLLUTANTS FROM STATIONARY SOURCES
The report gives results of a study of hydrocarbon pollutants from stationary sources. Early in the study, readily available information was assembled on stationary sources of hydrocarbon emissions and effluents. Information was also obtained on process descriptions, operating pa...
Characterizing inertial and convective optical turbulence by detrended fluctuation analysis
NASA Astrophysics Data System (ADS)
Funes, Gustavo; Figueroa, Eduardo; Gulich, Damián.; Zunino, Luciano; Pérez, Darío. G.
2013-10-01
Atmospheric turbulence is usually simulated at the laboratory by generating convective free flows with hot surfaces, or heaters. It is tacitly assumed that propagation experiments in this environment are comparable to those usually found outdoors. Nevertheless, it is unclear under which conditions the analogy between convective and isotropic turbulence is valid; that is, obeying Kolmogorov isotropic models. For instance, near-ground-level turbulence often is driven by shear ratchets deviating from established inertial models. In this case, a value for the structure constant can be obtained but it would be unable to distinguish between both classes of turbulence. We have performed a conceptually simple experiment of laser beam propagation through two types of artificial turbulence: isotropic turbulence generated by a turbulator [Proc. SPIE 8535, 853508 (2012)], and convective turbulence by controlling the temperature of electric heaters. In both cases, a thin laser beam propagates across the turbulent path, and its wandering is registered by a position sensor detector. The strength of the optical turbulence, in terms of the structure constant, is obtained from the wandering variance. It is expressed as a function of the temperature difference between cold and hot sources in each setup. We compare the time series behaviour for each turbulence with increasing turbulence strength by estimating the Hurst exponent, H, through detrended fluctuation analysis (DFA). Refractive index fluctuations are inherently fractal; this characteristic is reflected in their spectra power-law dependence—in the inertial range. This fractal behaviour is inherited by time series of optical quantities, such as the wandering, by the occurrence of long-range correlations. By analyzing the wandering time series with this technique, we are able to correlate the turbulence strength to the value of the Hurt exponent. Ultimately, we characterize both types of turbulence.
Broken symmetry in ideal magnetohydrodynamic turbulence
NASA Technical Reports Server (NTRS)
Shebalin, John V.
1993-01-01
A numerical study of the long-time evolution of a number of cases of inviscid, isotropic, incompressible, three-dimensional fluid, and magneto-fluid turbulence has been completed. The results confirm that ideal magnetohydrodynamic turbulence is non-ergodic if there is no external magnetic field present. This is due essentially to a canonical symmetry being broken in an arbitrary dynamical representation. The broken symmetry manifests itself as a coherent structure, i.e., a non-zero time-averaged part of the turbulent magnetic field. The coherent structure is observed, in one case, to contain about eighteen percent of the total energy.
Local structures of homogeneous Hall MHD turbulence
NASA Astrophysics Data System (ADS)
Miura, H.; Araki, K.
2011-12-01
Local structures of decaying homogeneous and isotropic Hall MHD turbulence are studied by means of direct numerical simulations. Regions of strong vorticity and strong current density in Hall MHD turbulence are compared to those of single-fluid MHD turbulence. An analysis by the use of a low-pass filter reveals that the introduction of the Hall term can modify not only small-scale structures of the current density but also structures of the vorticity field, especially at the scales smaller than the ion skin depth.
Outflow Driven Turbulence in Star Forming Clouds
NASA Astrophysics Data System (ADS)
Frank, Adam
Setting young stellar object jets and outflows in their broadest context requires an understanding of outflows as “feedback” in the development of molecular cloud turbulence and the determination of star formation efficiencies. In this contribution I review our group’s recent studies exploring relationships between protostellar outflows and turbulence in molecular clouds. We first present studies of turbulence and fossil cavities driven by YSO outflows using numerical simulations which track the evolution of single transient jets driven into a turbulent medium. Our simulations show both the effect of turbulence on outflow structures and, conversely, the effect of outflows on the ambient turbulence. These studies demonstrate that individual transient outflows have the capacity to re-energize decaying turbulence. Next we present simulations of multiple interacting jets. We show that turbulence can readily be sustained by these interactions and show that it is possible to broadly characterize an effective driving scale of the outflows. Comparing the velocity spectrum obtained in our studies to that of an isotropically forced control we show that in outflow driven turbulence a power law of the form E(k) ∝ k - β is indeed achieved. However we find a steeper spectrum β ˜ 3 is obtained in outflow driven turbulence models than in isotropically forced simulations β ˜ 2. 0. Taken together both studies provide broad support for the conclusion that fossil cavities driven by decaying jets can provide a source of turbulence and feedback which mediate star formation processes in molecular clouds. Whether this does obtain in real clouds remains a point which must be demonstrated
Turbulent energy flux generated by shock/homogeneous-turbulence interaction
NASA Astrophysics Data System (ADS)
Sinha, Krishnendu; Quadros, Russell; Larsson, Johan
2015-11-01
High-speed turbulent flows with shock waves are characterized by high localized surface heat transfer rates. Computational predictions are often inaccurate due to the limitations in modeling of the unclosed turbulent energy flux in the highly non-equilibrium regions of shock interaction. In this paper, we investigate the turbulent energy flux generated when homogeneous isotropic turbulence passes through a nominally normal shock wave. We use linear interaction analysis where the incoming turbulence is idealized as being composed of a collection of two-dimensional planar vorticity waves, and the shock wave is taken to be a discontinuity. The nature of the post-shock turbulent energy flux is predicted to be strongly dependent on the incidence angle of the incoming waves. The energy flux correlation is also decomposed into its vortical, entropy and acoustic contributions to understand its rapid non-monotonic variation behind the shock. Three-dimensional statistics, calculated by integrating two-dimensional results over a prescribed upstream energy spectrum, are compared with available direct numerical simulation data. A detailed budget of the governing equation is also considered in order to gain insight into the underlying physics.
Tunable dynamics of microtubule-based active isotropic gels
Henkin, Gil; DeCamp, Stephen J.; Chen, Daniel T. N.; Sanchez, Tim; Dogic, Zvonimir
2014-01-01
We investigate the dynamics of an active gel of bundled microtubules (MTs) that is driven by clusters of kinesin molecular motors. Upon the addition of ATP, the coordinated action of thousands of molecular motors drives the gel to a highly dynamical turbulent-like state that persists for hours and is only limited by the stability of constituent proteins and the availability of the chemical fuel. We characterize how enhanced transport and emergent macroscopic flows of active gels depend on relevant molecular parameters, including ATP, kinesin motor and depletant concentrations, MT volume fraction, as well as the stoichiometry of the constituent motor clusters. Our results show that the dynamical and structural properties of MT-based active gels are highly tunable. They also indicate existence of an optimal concentration of molecular motors that maximize far-from-equilibrium activity of active isotropic MT gels. PMID:25332391
Two-dimensional convective turbulence
Gruzinov, A.V.; Kukharkin, N.; Sudan, R.N.
1996-02-01
We show that 2D {bold E{times}B} ionospheric turbulence of the electron density in the equatorial electrojet is isomorphic to the viscous convection of an ordinary fluid in a porous medium due to temperature gradients. Numerical simulations reveal the strong anisotropy in the turbulence, which consists of rising hot bubbles and falling cool bubbles. These bubbles break up into fingers leading to the formation of stable shear flows. After reaching a quasisteady state, the omnidirectional energy spectrum approaches a {ital k}{sup {minus}2} behavior, rather than {ital k}{sup {minus}5/3} as expected from isotropic turbulence. Physical mechanisms that lead to anisotropy are analyzed. {copyright} {ital 1996 The American Physical Society.}
Large eddy simulation of longitudinal stationary vortices
NASA Astrophysics Data System (ADS)
Sreedhar, Madhu; Ragab, Saad
1994-07-01
The response of longitudinal stationary vortices when subjected to random perturbations is investigated using temporal large-eddy simulation. Simulations are obtained for high Reynolds numbers and at a low subsonic Mach number. The subgrid-scale stress tensor is modeled using the dynamic eddy-viscosity model. The generation of large-scale structures due to centrifugal instability and their subsequent breakdown to turbulence is studied. The following events are observed. Initially, ring-shaped structures appear around the vortex core. These structures are counter-rotating vortices similar to the donut-shaped structures observed in a Taylor-Couette flow between rotating cylinders. These structures subsequently interact with the vortex core resulting in a rapid decay of the vortex. The turbulent kinetic energy increases rapidly until saturation, and then a period of slow decay prevails. During the period of maximum turbulent kinetic energy, the normalized mean circulation profile exhibits a logarithmic region, in agreement with the universal inner profile of Hoffman and Joubert [J. Fluid Mech. 16, 395 (1963)].
Dynamics of quantum turbulence of different spectra
Walmsley, Paul; Zmeev, Dmitry; Pakpour, Fatemeh; Golov, Andrei
2014-01-01
Turbulence in a superfluid in the zero-temperature limit consists of a dynamic tangle of quantized vortex filaments. Different types of turbulence are possible depending on the level of correlations in the orientation of vortex lines. We provide an overview of turbulence in superfluid 4He with a particular focus on recent experiments probing the decay of turbulence in the zero-temperature regime below 0.5 K. We describe extensive measurements of the vortex line density during the free decay of different types of turbulence: ultraquantum and quasiclassical turbulence in both stationary and rotating containers. The observed decays and the effective dissipation as a function of temperature are compared with theoretical models and numerical simulations. PMID:24704876
Effects of free-stream turbulence on turbulent boundary layers with mild adverse pressure gradients
NASA Technical Reports Server (NTRS)
Hoffmann, J. A.; Kassir, S. M.
1988-01-01
The influence of near isotropic free-stream turbulence on the shape factors and skin friction coefficients of turbulent boundary layers is presented for the cases of zero and mild adverse pressure gradients. With free-stream turbulence, improved fluid mixing occurs in boundary layers with adverse pressure gradients relative to the zero pressure gradient condition, with the same free-stream turbulence intensity and length scale. Stronger boundary layers with lower shape factors occur as a result of a lower ratio of the integral scale of turbulence to the boundary layer thickness, and to vortex stretching of the turbulent eddies in the free-stream, both of which act to improve the transmission of momentum from the free-stream to the boundary layers.
NASA Technical Reports Server (NTRS)
Hoffmann, Jon A.
1988-01-01
The influence of near isotropic free-stream turbulence on the shape factors and skin friction coefficients of turbulent bounday layers is presented for the cases of zero and mild adverse pressure gradients. With free-stream turbulence, improved fluid mixing occurs in boundary layers with adverse pressure gradients relative to the zero pressure gradient condition, with the same free-stream turbulence intensity and length scale. Stronger boundary layers with lower shape factors occur as a result of a lower ratio of the integral scale of turbulence to the boundary layer thickness, and to vortex stretching of the turbulent eddies in the free stream, both of which act to improve the transmission of momentum from the free stream to the boundary layers.
NASA Technical Reports Server (NTRS)
Hoffmann, J. A.; Kassir, S. M.; Larwood, S. M.
1989-01-01
The influence of near isotropic free-stream turbulence on the shape factors and skin friction coefficients of turbulent boundary layers is presented for the cases of zero and mild adverse pressure gradients. With free-stream turbulence, improved fluid mixing occurs in boundary layers with adverse pressure gradients relative to the zero pressure gradient condition, with the same free-stream turbulence intensity and length scale. Stronger boundary layers with lower shape factors occur as a result of a lower ratio of the integral scale of turbulence to the boundary layer thickness, and to vortex stretching of the turbulent eddies in the free-stream, both of which act to improve the transmission of momentum from the free-stream to the boundary layers.
Hierarchical structures in a turbulent pipe flow
NASA Astrophysics Data System (ADS)
Zou, Zhengping; Zhu, Yuanjie; Zhou, Mingde; She, Zhen-Su
2003-12-01
A hierarchical structure (HS) analysis ( β-test and γ-test) is applied to a fully developed turbulent pipe flow. Velocity signals are measured at two cross sections in the pipe and at a series of radial locations from the pipe wall. Particular attention is paid to the variation of turbulent statistics at wall units 10< y+<3000. It is shown that at all locations the velocity fluctuations satisfy the She-Leveque hierarchical symmetry (Phys. Rev. Lett. 72 (1994) 336). The measured HS parameters, β and γ, are interpreted in terms of the variation of fluid structures. Intense anisotropic fluid structures generated near the wall appear to be more singular than the most intermittent structures in isotropic turbulence and appear to be more outstanding compared to the background fluctuations; this yields a more intermittent velocity signal with smaller γ and β. As turbulence migrates into the logarithmic region, small-scale motions are generated by an energy cascade and large-scale organized structures emerge which are also less singular than the most intermittent structures of isotropic turbulence. At the center, turbulence is nearly isotropic, and β and γ are close to the 1994 She-Leveque predictions. A transition is observed from the logarithmic region to the center in which γ drops and the large-scale organized structures break down. We speculate that it is due to the growing eddy viscosity effects of widely spread turbulent fluctuations in a similar way as in the breakdown of the Taylor vortices in a turbulent Couette-Taylor flow at high Reynolds numbers.
Spatial and velocity statistics of inertial particles in turbulent flows
NASA Astrophysics Data System (ADS)
Bec, J.; Biferale, L.; Cencini, M.; Lanotte, A. S.; Toschi, F.
2011-12-01
Spatial and velocity statistics of heavy point-like particles in incompressible, homogeneous, and isotropic three-dimensional turbulence is studied by means of direct numerical simulations at two values of the Taylor-scale Reynolds number Reλ ~ 200 and Reλ ~ 400, corresponding to resolutions of 5123 and 20483 grid points, respectively. Particles Stokes number values range from St ≈ 0.2 to 70. Stationary small-scale particle distribution is shown to display a singular -multifractal- measure, characterized by a set of generalized fractal dimensions with a strong sensitivity on the Stokes number and a possible, small Reynolds number dependency. Velocity increments between two inertial particles depend on the relative weight between smooth events - where particle velocity is approximately the same of the fluid velocity-, and caustic contributions - when two close particles have very different velocities. The latter events lead to a non-differentiable small-scale behaviour for the relative velocity. The relative weight of these two contributions changes at varying the importance of inertia. We show that moments of the velocity difference display a quasi bi-fractal-behavior and that the scaling properties of velocity increments for not too small Stokes number are in good agreement with a recent theoretical prediction made by K. Gustavsson and B. Mehlig arXiv: 1012.1789v1 [physics.flu-dyn], connecting the saturation of velocity scaling exponents with the fractal dimension of particle clustering.
Simulation and Modeling of Homogeneous, Compressed Turbulence.
NASA Astrophysics Data System (ADS)
Wu, Chung-Teh
Low Reynolds number homogeneous turbulence undergoing low Mach number isotropic and one-dimensional compression has been simulated by numerically solving the Navier-Stokes equations. The numerical simulations were carried out on a CYBER 205 computer using a 64 x 64 x 64 mesh. A spectral method was used for spatial differencing and the second -order Runge-Kutta method for time advancement. A variety of statistical information was extracted from the computed flow fields. These include three-dimensional energy and dissipation spectra, two-point velocity correlations, one -dimensional energy spectra, turbulent kinetic energy and its dissipation rate, integral length scales, Taylor microscales, and Kolmogorov length scale. It was found that the ratio of the turbulence time scale to the mean-flow time scale is an important parameter in these flows. When this ratio is large, the flow is immediately affected by the mean strain in a manner similar to that predicted by rapid distortion theory. When this ratio is small, the flow retains the character of decaying isotropic turbulence initially; only after the strain has been applied for a long period does the flow accumulate a significant reflection of the effect of mean strain. In these flows, the Kolmogorov length scale decreases rapidly with increasing total strain, due to the density increase that accompanies compression. Results from the simulated flow fields were used to test one-point-closure, two-equation turbulence models. The two-equation models perform well only when the compression rate is small compared to the eddy turn-over rate. A new one-point-closure, three-equation turbulence model which accounts for the effect of compression is proposed. The new model accurately calculates four types of flows (isotropic decay, isotropic compression, one-dimensional compression, and axisymmetric expansion flows) for a wide range of strain rates.
NASA Technical Reports Server (NTRS)
Goldstein, M. E.; Rosenbaum, B. M.
1972-01-01
A model, based on Lighthill's theory, for predicting aerodynamic noise from a turbulent shear flow is developed. This model is a generalization of the one developed by Ribner. Unlike Ribner's model, it does not require that the turbulent correlations factor into space and time-dependent parts. It replaces his assumption of isotropic. turbulence by the more realistic one of axisymmetric turbulence. The implications of the model for jet noise are discussed.
NASA Astrophysics Data System (ADS)
Podesta, J. J.
It is known that Kolmogorov's four-fifths law for statistically homogeneous and isotropic turbulence can be generalized to anisotropic turbulence. This fundamental result for homogeneous anisotropic turbulence says that in the inertial range the divergence of the vector third-order moment |v(r) is constant and is equal to -4, where is the dissipation rate of the turbulence. This law can be extended to incompressible magnetohydrodyamic (MHD) turbulence where statistical isotropy is often not valid due, for example, to the presence of a large-scale magnetic field. Laws for anisotropic incompressible MHD turbulence were first derived by Politano and Pouquet. In this paper, the laws for vector third-order moments in homogeneous non-isotropic incompressible MHD turbulence are derived by a technique due to Frisch that clarifies the relationship between the energy flux in Fourier space and the vector third-order moments in physical space. This derivation is different from the original derivation of Politano and Pouquet which is based on the Kn-Howarth equation, and provides some new physical insights. Separate laws are derived for the cascades of energy, cross-helicity and magnetic-helicity, the three ideal invariants of incompressible MHD for flows in three dimensions. These laws are of fundamental importance in the theory of MHD turbulence where non-isotropic turbulence is much more prevalent than isotropic turbulence.
Experimental assessment of helicopter rotor turbulence ingestion noise in hover
NASA Technical Reports Server (NTRS)
Simonich, J. C.; Schlinker, R. H.; Amiet, R. K.
1989-01-01
An experiment was conducted to assess the accuracy of a theory for non-isotropic turbulence ingestion. In order to generate non-isotropic turbulence in a controlled environment, a scale model rotor in a closed chamber was used so that the turbulence generated by the rotor was reingested by the recirculating flow. Simultaneous measurements of turbulence inflow properties and far field acoustics were acquired. Measurements confirmed that the inflow turbulence was highly non-isotropic. The measured aerodynamic properties were used as inputs for the noise prediction procedure. The general agreement between the non-isotropic noise prediction procedure and the experiment was good, although the procedure generally overpredicts the quasi-tonal low to mid range frequencies and underpredicts the higher broadband signals. The predicted sound power level as a function of polar angle was in close agreement with measurements, except near the rotor plane, which is not modeled by the present analysis. It is determined that the most sensitive parameter influencing the predicted noise was the turbulence intensity.
Backward two-particle dispersion in turbulence: asymptotic behaviors at high Reynolds number
NASA Astrophysics Data System (ADS)
Yeung, Pui-Kuen; Buaria, D.; Sawford, B. L.
2015-11-01
Backward relative dispersion of fluid elements and diffusing substances or property markers is central to a Lagrangian view of turbulent mixing, but data are not readily available. Recently we have devised a numerical approach based on massively parallel processing of the trajectories of many billions of particle pairs, and have used it to obtain results in simulations of stationary isotropic turbulence up to 4096 in size and Taylor-scale Reynolds number up to 1000, with a wide range of initial separations. Backward dispersion is faster than forward, especially at intermediate times after the ballistic range and before long-time diffusive behavior is reached. Richardson scaling is demonstrated for the mean-squared separation, with forward and backward Richardson constants estimated to be 0.55 and 1.5 respectively, which are close to or comparable to other estimates. However, because of persistent dissipation sub-range effects no corresponding scaling was observed for higher order moments. An effort is made to analyze theoretically several key characteristics such as asymmetry in time and exponential growth of third and fourth moments at early times. Related results for marked entities that diffuse relative to the fluid will also be briefly addressed. Supported by NSF Grant CBET-1039037 (Fluid Dynamics Program).
Ceramic stationary gas turbine
Roode, M. van
1995-10-01
The performance of current industrial gas turbines is limited by the temperature and strength capabilities of the metallic structural materials in the engine hot section. Because of their superior high-temperature strength and durability, ceramics can be used as structural materials for hot section components (blades, nozzles, combustor liners) in innovative designs at increased turbine firing temperatures. The benefits include the ability to increase the turbine inlet temperature (TIT) to about 1200{degrees}C ({approx}2200{degrees}F) or more with uncooled ceramics. It has been projected that fully optimized stationary gas turbines would have a {approx}20 percent gain in thermal efficiency and {approx}40 percent gain in output power in simple cycle compared to all metal-engines with air-cooled components. Annual fuel savings in cogeneration in the U.S. would be on the order of 0.2 Quad by 2010. Emissions reductions to under 10 ppmv NO{sub x} are also forecast. This paper describes the progress on a three-phase, 6-year program sponsored by the U.S. Department of Energy, Office of Industrial Technologies, to achieve significant performance improvements and emissions reductions in stationary gas turbines by replacing metallic hot section components with ceramic parts. Progress is being reported for the period September 1, 1994, through September 30, 1995.
Ceramic stationary gas turbine
Roode, M. van
1995-12-31
The performance of current industrial gas turbines is limited by the temperature and strength capabilities of the metallic structural materials in the engine hot section. Because of their superior high-temperature strength and durability, ceramics can be used as structural materials for hot section components (blades, nozzles, combustor liners) in innovative designs at increased turbine firing temperatures. The benefits include the ability to increase the turbine inlet temperature (TIT) to about 1200{degrees}C ({approx}2200{degrees}F) or more with uncooled ceramics. It has been projected that fully optimized stationary gas turbines would have a {approx}20 percent gain in thermal efficiency and {approx}40 percent gain in output power in simple cycle compared to all metal-engines with air-cooled components. Annual fuel savings in cogeneration in the U.S. would be on the order of 0.2 Quad by 2010. Emissions reductions to under 10 ppmv NO{sub x} are also forecast. This paper describes the progress on a three-phase, 6-year program sponsored by the U.S. Department of Energy, Office of Industrial Technologies, to achieve significant performance improvements and emissions reductions in stationary gas turbines by replacing metallic hot section components with ceramic parts. Progress is being reported for the period September 1, 1994, through September 30, 1995.
Gyrokinetic modelling of stationary electron and impurity profiles in tokamaks
Skyman, A. Tegnered, D. Nordman, H. Strand, P.
2014-09-15
Particle transport due to Ion Temperature Gradient (ITG)/Trapped Electron Mode (TEM) turbulence is investigated using the gyrokinetic code GENE. Both a reduced quasilinear treatment and nonlinear simulations are performed for typical tokamak parameters corresponding to ITG dominated turbulence. The gyrokinetic results are compared and contrasted with results from a computationally efficient fluid model. A selfconsistent treatment is used, where the stationary local profiles are calculated corresponding to zero particle flux simultaneously for electrons and trace impurities. The scaling of the stationary profiles with magnetic shear, safety factor, electron-to-ion temperature ratio, collisionality, toroidal sheared rotation, plasma β, triangularity, and elongation is investigated. In addition, the effect of different main ion mass on the zero flux condition is discussed. The electron density gradient can significantly affect the stationary impurity profile scaling. It is therefore expected that a selfconsistent treatment will yield results more comparable to experimental results for parameter scans where the stationary background density profile is sensitive. This is shown to be the case in scans over magnetic shear, collisionality, elongation, and temperature ratio, for which the simultaneous zero flux electron and impurity profiles are calculated. A slight asymmetry between hydrogen, deuterium, and tritium with respect to profile peaking is obtained, in particular, for scans in collisionality and temperature ratio.
The spatio-temporal spectrum of turbulent flows.
Clark di Leoni, P; Cobelli, P J; Mininni, P D
2015-12-01
Identification and extraction of vortical structures and of waves in a disorganised flow is a mayor challenge in the study of turbulence. We present a study of the spatio-temporal behavior of turbulent flows in the presence of different restitutive forces. We show how to compute and analyse the spatio-temporal spectrum from data stemming from numerical simulations and from laboratory experiments. Four cases are considered: homogeneous and isotropic turbulence, rotating turbulence, stratified turbulence, and water wave turbulence. For homogeneous and isotropic turbulence, the spectrum allows identification of sweeping by the large-scale flow. For rotating and for stratified turbulence, the spectrum allows identification of the waves, precise quantification of the energy in the waves and in the turbulent eddies, and identification of physical mechanisms such as Doppler shift and wave absorption in critical layers. Finally, in water wave turbulence the spectrum shows a transition from gravity-capillary waves to bound waves as the amplitude of the forcing is increased. PMID:26701711
Global scale-invariant dissipation in collisionless plasma turbulence.
Kiyani, K H; Chapman, S C; Khotyaintsev, Yu V; Dunlop, M W; Sahraoui, F
2009-08-14
A higher-order multiscale analysis of the dissipation range of collisionless plasma turbulence is presented using in situ high-frequency magnetic field measurements from the Cluster spacecraft in a stationary interval of fast ambient solar wind. The observations, spanning five decades in temporal scales, show a crossover from multifractal intermittent turbulence in the inertial range to non-Gaussian monoscaling in the dissipation range. This presents a strong observational constraint on theories of dissipation mechanisms in turbulent collisionless plasmas. PMID:19792654
Turbulence generation in homogeneous dilute particle- laden flows
NASA Astrophysics Data System (ADS)
Chen, Jeng-Horng
Homogeneous turbulence generated by the motion of particles in dispersed multiphase flows was studied both theoretically and experimentally, motivated by applications to sprays, particle-laden jets, bubble plumes and rainstorms, among others. The experiments involved uniform fluxes of monodisperse spherical particles falling through a slow upflow of air. Particle fluxes and phase velocities were measured by sampling and phase-discriminating laser Doppler velocimetry (LDV), respectively. Measured particle velocities included mean and fluctuating streamwise and cross-stream velocities and probability density functions (PDF's). Measured continuous-phase velocities included mean and fluctuating streamwise and cross-stream velocities, PDF's and the higher moments of velocity fluctuations such as skewness and kurtosis, energy spectra of velocity fluctuations and integral length scales based on streamwise velocity fluctuations. Continuous-phase velocity measurements included conditional averages for particle wake disturbances and the turbulent inter-wake region surrounding these disturbances as well as overall flow properties. Present and earlier results in the literature provided particle Reynolds numbers of 38-990, particle volume fractions less than 0.01% and turbulence intensities (normalized by mean particle relative velocities) of 0.1-10.0%. Theory included characterization of particle wake disturbances as laminar-like turbulent wakes observed for intermediate particle Reynolds numbers in turbulent environments, characterization of the turbulent inter-wake region by analogy to grid-generated isotropic turbulence, and estimation of overall flow properties by conditional averaging of the properties of the wake disturbances and the turbulent inter-wake region. Present measurements showed that particle wake disturbances during turbulence generation were properly characterized by the properties of laminar-like turbulent wakes. The turbulent inter-wake region was
Stationary engineering handbook
Petrocelly, K.L.
1989-01-01
Years ago, the only qualifications you needed to become to become an operating engineer were the ability to shovel large chunks of coal through small furnace doors and the fortitude to sweat profusely for hours without fainting. As a consequence of technological evolution, the engineer's coal shovels have been replaced with computers and now perspiration is more the result of job stress than exposure to high temperatures. The domain of the operator has been extended far beyond the smoke-filled caverns that once encased him, out into the physical plant, and his responsibilities have been expanded accordingly. Unlike his less sophisticated predecessor, today's technician must be well versed in all aspects of the operation. The field of power plant operations has become a full-fledged profession and its principals are called Stationary Engineers. This book addresses the areas of responsibility and the education and skills needed for successful operation of building services equipment.
Shock-turbulence interaction and the generation of noise
NASA Technical Reports Server (NTRS)
Ribner, H S
1954-01-01
Interaction of convected field of turbulence with shock wave is analyzed to yield modified turbulence, entropy spottiness, and noise generated downstream of the shock. Analysis is generalization of single-spectrum-wave treatment of NACA-TN-2864. Formulas for spectra and correlations are obtained. Numerical calculations yield curves of rms velocity components, temperature, pressure, and noise in db against Mach number for m = 1 to infinity; both isotropic and strongly axisymmetric (lateral/longitudinal = 36/1) initial turbulence are treated. In either case, turbulence of 0.1 percent longitudinal component generates about 120 dbs of noise.
Consistent Initial Conditions for the DNS of Compressible Turbulence
NASA Technical Reports Server (NTRS)
Ristorcelli, J. R.; Blaisdell, G. A.
1996-01-01
Relationships between diverse thermodynamic quantities appropriate to weakly compressible turbulence are derived. It is shown that for turbulence of a finite turbulent Mach number there is a finite element of compressibility. A methodology for generating initial conditions for the fluctuating pressure, density and dilatational velocity is given which is consistent with finite Mach number effects. Use of these initial conditions gives rise to a smooth development of the flow, in contrast to cases in which these fields are specified arbitrarily or set to zero. Comparisons of the effect of different types of initial conditions are made using direct numerical simulation of decaying isotropic turbulence.
Premixed Turbulent Flame Propagation in Microgravity
NASA Technical Reports Server (NTRS)
Menon, Suresh
1999-01-01
A combined numerical-experimental study has been carried out to investigate the structure and propagation characteristics of turbulent premixed flames with and without the influence of buoyancy. Experimentally, the premixed flame characteristics are studied in the wrinkled regime using a Couette flow facility and an isotropic flow facility in order to resolve the scale of flame wrinkling. Both facilities were chosen for their ability to achieve sustained turbulence at low Reynolds number. This implies that conventional diagnostics can be employed to resolve the smallest scales of wrinkling. The Couette facility was also built keeping in mind the constraints imposed by the drop tower requirements. Results showed that the flow in this Couette flow facility achieves full-developed turbulence at low Re and all turbulence statistics are in good agreement with past measurements on large-scale facilities. Premixed flame propagation studies were then carried out both using the isotropic box and the Couette facility. Flame imaging showed that fine scales of wrinkling occurs during flame propagation. Both cases in Ig showed significant buoyancy effect. To demonstrate that micro-g can remove this buoyancy effect, a small drop tower was built and drop experiments were conducted using the isotropic box. Results using the Couette facility confirmed the ability to carry out these unique reacting flow experiments at least in 1g. Drop experiments at NASA GRC were planned but were not completed due to termination of this project.
Experiments on the interaction between hydrodynamic turbulence and surface waves
NASA Astrophysics Data System (ADS)
Jamin, Timothee; Berhanu, Michael; Falcon, Eric
2014-11-01
Different regimes of interaction between hydrodynamic turbulence and a free surface are investigated in a meter scale basin. A homogeneous and isotropic turbulence is generated by an 8×8 array of jets pointing upwards at the bottom of the tank. The 64 jets are driven individually to reach a random spatiotemporal forcing pattern and produce an intense turbulence. Using fluid velocity measurements, we characterize the turbulence obtained with this setup, then we investigate free-surface deformations induced by hydrodynamic turbulence. In a second stage an electromechanical shaker will generate gravity-capillary waves at the free surface. We aim to study reduction or amplification of surface waves and then measure energy exchange between hydrodynamic turbulence and wave turbulence. This work was supported by the DGA-CNRS Ph.D program and ANR Turbulon 12-BS04-0005.
Coherence in Turbulence: New Perspective
NASA Astrophysics Data System (ADS)
Levich, Eugene
2009-07-01
It is claimed that turbulence in fluids is inherently coherent phenomenon. The coherence shows up clearly as strongly correlated helicity fluctuations of opposite sign. The helicity fluctuations have cellular structure forming clusters that are actually observed as vorticity bands and coherent structures in laboratory turbulence, direct numerical simulations and most obviously in atmospheric turbulence. The clusters are named BCC - Beltrami Cellular Clusters - because of the observed nearly total alignment of the velocity and vorticity fields in each particular cell, and hence nearly maximal possible helicity in each cell; although when averaged over all the cells the residual mean helicity in general is small and does not play active dynamical role. The Beltrami like fluctuations are short-lived and stabilize only in small and generally contiguous sub-domains that are tending to a (multi)fractal in the asymptotic limit of large Reynolds numbers, Re → ∞. For the model of homogeneous isotropic turbulence the theory predicts the leading fractal dimension of BCC to be: DF = 2.5. This particular BCC is responsible for generating the Kolmogorov -5/3 power law energy spectrum. The most obvious role that BCC play dynamically is that the nonlinear interactions in them are relatively reduced, due to strong spatial alignment between the velocity field v(r, t) and the vorticity field ω(r, t) = curlv(r, t), while the physical quantities typically best characterizing turbulence intermittency, such as entrophy, vorticity stretching and generation, and energy dissipation are maximized in and near them. The theory quantitatively relates the reduction of nonlinear inter-actions to the BCC fractal dimension DF and subsequent turbulence intermittency. It is further asserted that BCC is a fundamental feature of all turbulent flows, e.g., wall bounded turbulent flows, atmospheric and oceanic flows, and their leading fractal dimension remains invariant and universal in these flows
Studying Turbulence Using Numerical Simulation Databases, 2. Proceedings of the 1988 Summer Program
NASA Technical Reports Server (NTRS)
1988-01-01
The focus of the program was on the use of direct numerical simulations of turbulent flow for study of turbulence physics and modeling. A special interest was placed on turbulent mixing layers. The required data for these investigations were generated from four newly developed codes for simulation of time and spatially developing incompressible and compressible mixing layers. Also of interest were the structure of wall bounded turbulent and transitional flows, evaluation of diagnostic techniques for detection of organized motions, energy transfer in isotropic turbulence, optical propagation through turbulent media, and detailed analysis of the interaction of vortical structures.
PDF turbulence modeling and DNS
NASA Technical Reports Server (NTRS)
Hsu, A. T.
1992-01-01
The problem of time discontinuity (or jump condition) in the coalescence/dispersion (C/D) mixing model is addressed in probability density function (pdf). A C/D mixing model continuous in time is introduced. With the continuous mixing model, the process of chemical reaction can be fully coupled with mixing. In the case of homogeneous turbulence decay, the new model predicts a pdf very close to a Gaussian distribution, with finite higher moments also close to that of a Gaussian distribution. Results from the continuous mixing model are compared with both experimental data and numerical results from conventional C/D models. The effect of Coriolis forces on compressible homogeneous turbulence is studied using direct numerical simulation (DNS). The numerical method used in this study is an eight order compact difference scheme. Contrary to the conclusions reached by previous DNS studies on incompressible isotropic turbulence, the present results show that the Coriolis force increases the dissipation rate of turbulent kinetic energy, and that anisotropy develops as the Coriolis force increases. The Taylor-Proudman theory does apply since the derivatives in the direction of the rotation axis vanishes rapidly. A closer analysis reveals that the dissipation rate of the incompressible component of the turbulent kinetic energy indeed decreases with a higher rotation rate, consistent with incompressible flow simulations (Bardina), while the dissipation rate of the compressible part increases; the net gain is positive. Inertial waves are observed in the simulation results.
Isotropic Growth of Graphene toward Smoothing Stitching.
Zeng, Mengqi; Tan, Lifang; Wang, Lingxiang; Mendes, Rafael G; Qin, Zhihui; Huang, Yaxin; Zhang, Tao; Fang, Liwen; Zhang, Yanfeng; Yue, Shuanglin; Rümmeli, Mark H; Peng, Lianmao; Liu, Zhongfan; Chen, Shengli; Fu, Lei
2016-07-26
The quality of graphene grown via chemical vapor deposition still has very great disparity with its theoretical property due to the inevitable formation of grain boundaries. The design of single-crystal substrate with an anisotropic twofold symmetry for the unidirectional alignment of graphene seeds would be a promising way for eliminating the grain boundaries at the wafer scale. However, such a delicate process will be easily terminated by the obstruction of defects or impurities. Here we investigated the isotropic growth behavior of graphene single crystals via melting the growth substrate to obtain an amorphous isotropic surface, which will not offer any specific grain orientation induction or preponderant growth rate toward a certain direction in the graphene growth process. The as-obtained graphene grains are isotropically round with mixed edges that exhibit high activity. The orientation of adjacent grains can be easily self-adjusted to smoothly match each other over a liquid catalyst with facile atom delocalization due to the low rotation steric hindrance of the isotropic grains, thus achieving the smoothing stitching of the adjacent graphene. Therefore, the adverse effects of grain boundaries will be eliminated and the excellent transport performance of graphene will be more guaranteed. What is more, such an isotropic growth mode can be extended to other types of layered nanomaterials such as hexagonal boron nitride and transition metal chalcogenides for obtaining large-size intrinsic film with low defect. PMID:27403842
Fluctuations of energy flux in wave turbulence.
Falcon, Eric; Aumaître, Sébastien; Falcón, Claudio; Laroche, Claude; Fauve, Stéphan
2008-02-15
We report that the power driving gravity and capillary wave turbulence in a statistically stationary regime displays fluctuations much stronger than its mean value. We show that its probability density function (PDF) has a most probable value close to zero and involves two asymmetric roughly exponential tails. We understand the qualitative features of the PDF using a simple Langevin-type model. PMID:18352479
NASA Technical Reports Server (NTRS)
Liu, Xiao-Feng; Thomas, Flint O.; Nelson, Robert C.
2001-01-01
Turbulence kinetic energy (TKE) is a very important quantity for turbulence modeling and the budget of this quantity in its transport equation can provide insight into the flow physics. Turbulence kinetic energy budget measurements were conducted for a symmetric turbulent wake flow subjected to constant zero, favorable and adverse pressure gradients in year-three of research effort. The purpose of this study is to clarify the flow physics issues underlying the demonstrated influence of pressure gradient on wake development and provide experimental support for turbulence modeling. To ensure the reliability of these notoriously difficult measurements, the experimental procedure was carefully designed on the basis of an uncertainty analysis. Four different approaches, based on an isotropic turbulence assumption, a locally axisymmetric homogeneous turbulence assumption, a semi-isotropy assumption and a forced balance of the TKE equation, were applied for the estimate of the dissipation term. The pressure transport term is obtained from a forced balance of the turbulence kinetic energy equation. This report will present the results of the turbulence kinetic energy budget measurement and discuss their implication on the development of strained turbulent wakes.
On the application of Rice's exceedance statistics to atmospheric turbulence.
NASA Technical Reports Server (NTRS)
Chen, W. Y.
1972-01-01
Discrepancies produced by the application of Rice's exceedance statistics to atmospheric turbulence are examined. First- and second-order densities from several data sources have been measured for this purpose. Particular care was paid to each selection of turbulence that provides stationary mean and variance over the entire segment. Results show that even for a stationary segment of turbulence, the process is still highly non-Gaussian, in spite of a Gaussian appearance for its first-order distribution. Data also indicate strongly non-Gaussian second-order distributions. It is therefore concluded that even stationary atmospheric turbulence with a normal first-order distribution cannot be considered a Gaussian process, and consequently the application of Rice's exceedance statistics should be approached with caution.
Quantifying and scaling airplane performance in turbulence
NASA Astrophysics Data System (ADS)
Richardson, Johnhenri R.
This dissertation studies the effects of turbulent wind on airplane airspeed and normal load factor, determining how these effects scale with airplane size and developing envelopes to account for them. The results have applications in design and control of aircraft, especially small scale aircraft, for robustness with respect to turbulence. Using linearized airplane dynamics and the Dryden gust model, this dissertation presents analytical and numerical scaling laws for airplane performance in gusts, safety margins that guarantee, with specified probability, that steady flight can be maintained when stochastic wind gusts act upon an airplane, and envelopes to visualize these safety margins. Presented here for the first time are scaling laws for the phugoid natural frequency, phugoid damping ratio, airspeed variance in turbulence, and flight path angle variance in turbulence. The results show that small aircraft are more susceptible to high frequency gusts, that the phugoid damping ratio does not depend directly on airplane size, that the airspeed and flight path angle variances can be parameterized by the ratio of the phugoid natural frequency to a characteristic turbulence frequency, and that the coefficient of variation of the airspeed decreases with increasing airplane size. Accompanying numerical examples validate the results using eleven different airplanes models, focusing on NASA's hypothetical Boeing 757 analog the Generic Transport Model and its operational 5.5% scale model, the NASA T2. Also presented here for the first time are stationary flight, where the flight state is a stationary random process, and the stationary flight envelope, an adjusted steady flight envelope to visualize safety margins for stationary flight. The dissertation shows that driving the linearized airplane equations of motion with stationary, stochastic gusts results in stationary flight. It also shows how feedback control can enlarge the stationary flight envelope by alleviating
Horton, W.; Hu, G.
1998-07-01
The origin of plasma turbulence from currents and spatial gradients in plasmas is described and shown to lead to the dominant transport mechanism in many plasma regimes. A wide variety of turbulent transport mechanism exists in plasmas. In this survey the authors summarize some of the universally observed plasma transport rates.
Static spherically symmetric wormholes with isotropic pressure
NASA Astrophysics Data System (ADS)
Cataldo, Mauricio; Liempi, Luis; Rodríguez, Pablo
2016-06-01
In this paper we study static spherically symmetric wormhole solutions sustained by matter sources with isotropic pressure. We show that such spherical wormholes do not exist in the framework of zero-tidal-force wormholes. On the other hand, it is shown that for the often used power-law shape function there are no spherically symmetric traversable wormholes sustained by sources with a linear equation of state p = ωρ for the isotropic pressure, independently of the form of the redshift function ϕ (r). We consider a solution obtained by Tolman at 1939 for describing static spheres of isotropic fluids, and show that it also may describe wormhole spacetimes with a power-law redshift function, which leads to a polynomial shape function, generalizing a power-law shape function, and inducing a solid angle deficit.
One-Dimensional Simulation of Isotropic Radiation
NASA Technical Reports Server (NTRS)
Anspaugh, B. E.; Downing, R. G.
1987-01-01
Solar cells tested for effects of radiation in unidirectional beam. Cam groove imparts cosecant-function velocity to solar cells on rotating target plate. Cells on stationary plate above rotating one absorb steady perpendicular radiation from test source.
Stationary Engineering Laboratory Manual--2.
ERIC Educational Resources Information Center
Steingress, Frederick M.; Frost, Harold J.
The Stationary Engineering Laboratory Manual 2 was designed for vocational/technical high school students who have received instruction in the basics of stationary engineering. It was developed for students who will be operating a live plant and who will be responsible for supplying steam for heating, cooking, and baking. Each lesson in the manual…
Stationary Engineering. Science Manual--2.
ERIC Educational Resources Information Center
Frost, Harold J.; Steingress, Frederick M.
This second-year student manual contains 140 brief related science lessons applying science and math to trade activities in the field of stationary engineering. The lessons are organized into 16 units: (1) Introduction to Stationary Engineering, (2) Engineering Fundamentals, (3) Steam Boilers, (4) Boiler Fittings, (5) Boilerroom System, (6)…
Negative Poisson's ratio materials via isotropic interactions.
Rechtsman, Mikael C; Stillinger, Frank H; Torquato, Salvatore
2008-08-22
We show that under tension a classical many-body system with only isotropic pair interactions in a crystalline state can, counterintuitively, have a negative Poisson's ratio, or auxetic behavior. We derive the conditions under which the triangular lattice in two dimensions and lattices with cubic symmetry in three dimensions exhibit a negative Poisson's ratio. In the former case, the simple Lennard-Jones potential can give rise to auxetic behavior. In the latter case, a negative Poisson's ratio can be exhibited even when the material is constrained to be elastically isotropic. PMID:18764632
Negative Poisson's Ratio Materials via Isotropic Interactions
NASA Astrophysics Data System (ADS)
Rechtsman, Mikael C.; Stillinger, Frank H.; Torquato, Salvatore
2008-08-01
We show that under tension a classical many-body system with only isotropic pair interactions in a crystalline state can, counterintuitively, have a negative Poisson’s ratio, or auxetic behavior. We derive the conditions under which the triangular lattice in two dimensions and lattices with cubic symmetry in three dimensions exhibit a negative Poisson’s ratio. In the former case, the simple Lennard-Jones potential can give rise to auxetic behavior. In the latter case, a negative Poisson’s ratio can be exhibited even when the material is constrained to be elastically isotropic.
Turbulent flows near flat plates
NASA Astrophysics Data System (ADS)
Kambe, R.; Imamura, T.; Doi, M.
1980-08-01
The method to study the effect of the plate moving in the homogeneous or isotropic turbulence is presented. The crucial point of this method is to solve the Orr-Sommerfeld like equation, which is satisfied by the kernel of the Wiener-Hermite expansion of the velocity field, under the inhomogeneous boundary condition. In the special case of constant mean flow, our method gives the same result as that of Hunt and Graham and succeeds in explaining the experimental results of Thomas and Hancock. The method is also applied to the case of nonuniform mean flow, where the shear effect comes up.
A Quadratic Closure for Compressible Turbulence
Futterman, J A
2008-09-16
We have investigated a one-point closure model for compressible turbulence based on third- and higher order cumulant discard for systems undergoing rapid deformation, such as might occur downstream of a shock or other discontinuity. In so doing, we find the lowest order contributions of turbulence to the mean flow, which lead to criteria for Adaptive Mesh Refinement. Rapid distortion theory (RDT) as originally applied by Herring closes the turbulence hierarchy of moment equations by discarding third order and higher cumulants. This is similar to the fourth-order cumulant discard hypothesis of Millionshchikov, except that the Millionshchikov hypothesis was taken to apply to incompressible homogeneous isotropic turbulence generally, whereas RDT is applied only to fluids undergoing a distortion that is 'rapid' in the sense that the interaction of the mean flow with the turbulence overwhelms the interaction of the turbulence with itself. It is also similar to Gaussian closure, in which both second and fourth-order cumulants are retained. Motivated by RDT, we develop a quadratic one-point closure for rapidly distorting compressible turbulence, without regard to homogeneity or isotropy, and make contact with two equation turbulence models, especially the K-{var_epsilon} and K-L models, and with linear instability growth. In the end, we arrive at criteria for Adaptive Mesh Refinement in Finite Volume simulations.
Small-scale universality in fluid turbulence
Schumacher, Jörg; Scheel, Janet D.; Krasnov, Dmitry; Donzis, Diego A.; Yakhot, Victor; Sreenivasan, Katepalli R.
2014-01-01
Turbulent flows in nature and technology possess a range of scales. The largest scales carry the memory of the physical system in which a flow is embedded. One challenge is to unravel the universal statistical properties that all turbulent flows share despite their different large-scale driving mechanisms or their particular flow geometries. In the present work, we study three turbulent flows of systematically increasing complexity. These are homogeneous and isotropic turbulence in a periodic box, turbulent shear flow between two parallel walls, and thermal convection in a closed cylindrical container. They are computed by highly resolved direct numerical simulations of the governing dynamical equations. We use these simulation data to establish two fundamental results: (i) at Reynolds numbers Re ∼ 102 the fluctuations of the velocity derivatives pass through a transition from nearly Gaussian (or slightly sub-Gaussian) to intermittent behavior that is characteristic of fully developed high Reynolds number turbulence, and (ii) beyond the transition point, the statistics of the rate of energy dissipation in all three flows obey the same Reynolds number power laws derived for homogeneous turbulence. These results allow us to claim universality of small scales even at low Reynolds numbers. Our results shed new light on the notion of when the turbulence is fully developed at the small scales without relying on the existence of an extended inertial range. PMID:25024175
Numerical experiments in homogeneous turbulence
NASA Technical Reports Server (NTRS)
Rogallo, R. S.
1981-01-01
The direct simulation methods developed by Orszag and Patternson (1972) for isotropic turbulence were extended to homogeneous turbulence in an incompressible fluid subjected to uniform deformation or rotation. The results of simulations for irrotational strain (plane and axisymmetric), shear, rotation, and relaxation toward isotropy following axisymmetric strain are compared with linear theory and experimental data. Emphasis is placed on the shear flow because of its importance and because of the availability of accurate and detailed experimental data. The computed results are used to assess the accuracy of two popular models used in the closure of the Reynolds-stress equations. Data from a variety of the computed fields and the details of the numerical methods used in the simulation are also presented.
Extreme events in computational turbulence
Yeung, P. K.; Zhai, X. M.; Sreenivasan, Katepalli R.
2015-01-01
We have performed direct numerical simulations of homogeneous and isotropic turbulence in a periodic box with 8,1923 grid points. These are the largest simulations performed, to date, aimed at improving our understanding of turbulence small-scale structure. We present some basic statistical results and focus on “extreme” events (whose magnitudes are several tens of thousands the mean value). The structure of these extreme events is quite different from that of moderately large events (of the order of 10 times the mean value). In particular, intense vorticity occurs primarily in the form of tubes for moderately large events whereas it is much more “chunky” for extreme events (though probably overlaid on the traditional vortex tubes). We track the temporal evolution of extreme events and find that they are generally short-lived. Extreme magnitudes of energy dissipation rate and enstrophy occur simultaneously in space and remain nearly colocated during their evolution. PMID:26424452
Biomimetic Isotropic Nanostructures for Structural Coloration
Forster, Jason D.; Noh, Heeso; Liew, Seng Fatt; Saranathan, Vinodkumar; Schreck, Carl F.; Yang, Lin; Park, Jin-Gyu; Prum, Richard O.; Mochrie, Simon G.J.; O'Hern, Corey S.; Cao, Hui; Dufresne, Eric R.
2010-08-09
The self-assembly of films that mimic color-producing nanostructures in bird feathers is described. These structures are isotropic and have a characteristic length-scale comparable to the wavelength of visible light. Structural colors are produced when wavelength-independent scattering is suppressed by limiting the optical path length through geometry or absorption.
Hierarchical structures in a turbulent pipe flow
NASA Astrophysics Data System (ADS)
She, Zhen-Su; Zou, Zhengping; Zhu, Yuanjie; Zhou, Mingde
2003-11-01
Statistical structures of a series of longitudinal velocity fluctuation signals at different distances (10
Rotating turbulence under "precession-like" perturbation.
Iyer, Kartik P; Mazzitelli, Irene; Bonaccorso, Fabio; Pouquet, Annick; Biferale, Luca
2015-12-01
The effects of changing the orientation of the rotation axis on homogeneous turbulence is considered. We perform direct numerical simulations on a periodic box of 1024(3) grid points, where the orientation of the rotation axis is changed (a) at a fixed time instant (b) regularly at time intervals commensurate with the rotation time scale. The former is characterized by a dominant inverse energy cascade whereas in the latter, the inverse cascade is stymied due to the recurrent changes in the rotation axis resulting in a strong forward energy transfer and large-scale structures that resemble those of isotropic turbulence. PMID:26637337
Transversely isotropic elasticity imaging of cancellous bone.
Shore, Spencer W; Barbone, Paul E; Oberai, Assad A; Morgan, Elise F
2011-06-01
To measure spatial variations in mechanical properties of biological materials, prior studies have typically performed mechanical tests on excised specimens of tissue. Less invasive measurements, however, are preferable in many applications, such as patient-specific modeling, disease diagnosis, and tracking of age- or damage-related degradation of mechanical properties. Elasticity imaging (elastography) is a nondestructive imaging method in which the distribution of elastic properties throughout a specimen can be reconstructed from measured strain or displacement fields. To date, most work in elasticity imaging has concerned incompressible, isotropic materials. This study presents an extension of elasticity imaging to three-dimensional, compressible, transversely isotropic materials. The formulation and solution of an inverse problem for an anisotropic tissue subjected to a combination of quasi-static loads is described, and an optimization and regularization strategy that indirectly obtains the solution to the inverse problem is presented. Several applications of transversely isotropic elasticity imaging to cancellous bone from the human vertebra are then considered. The feasibility of using isotropic elasticity imaging to obtain meaningful reconstructions of the distribution of material properties for vertebral cancellous bone from experiment is established. However, using simulation, it is shown that an isotropic reconstruction is not appropriate for anisotropic materials. It is further shown that the transversely isotropic method identifies a solution that predicts the measured displacements, reveals regions of low stiffness, and recovers all five elastic parameters with approximately 10% error. The recovery of a given elastic parameter is found to require the presence of its corresponding strain (e.g., a deformation that generates ɛ₁₂ is necessary to reconstruct C₁₂₁₂), and the application of regularization is shown to improve accuracy. Finally
Poissonian steady states: From stationary densities to stationary intensities
NASA Astrophysics Data System (ADS)
Eliazar, Iddo
2012-10-01
Markov dynamics are the most elemental and omnipresent form of stochastic dynamics in the sciences, with applications ranging from physics to chemistry, from biology to evolution, and from economics to finance. Markov dynamics can be either stationary or nonstationary. Stationary Markov dynamics represent statistical steady states and are quantified by stationary densities. In this paper, we generalize the notion of steady state to the case of general Markov dynamics. Considering an ensemble of independent motions governed by common Markov dynamics, we establish that the entire ensemble attains Poissonian steady states which are quantified by stationary Poissonian intensities and which hold valid also in the case of nonstationary Markov dynamics. The methodology is applied to a host of Markov dynamics, including Brownian motion, birth-death processes, random walks, geometric random walks, renewal processes, growth-collapse dynamics, decay-surge dynamics, Ito diffusions, and Langevin dynamics.
Turbulence modeling and experiments
NASA Technical Reports Server (NTRS)
Shabbir, Aamir
1992-01-01
The best way of verifying turbulence is to do a direct comparison between the various terms and their models. The success of this approach depends upon the availability of the data for the exact correlations (both experimental and DNS). The other approach involves numerically solving the differential equations and then comparing the results with the data. The results of such a computation will depend upon the accuracy of all the modeled terms and constants. Because of this it is sometimes difficult to find the cause of a poor performance by a model. However, such a calculation is still meaningful in other ways as it shows how a complete Reynolds stress model performs. Thirteen homogeneous flows are numerically computed using the second order closure models. We concentrate only on those models which use a linear (or quasi-linear) model for the rapid term. This, therefore, includes the Launder, Reece and Rodi (LRR) model; the isotropization of production (IP) model; and the Speziale, Sarkar, and Gatski (SSG) model. Which of the three models performs better is examined along with what are their weaknesses, if any. The other work reported deal with the experimental balances of the second moment equations for a buoyant plume. Despite the tremendous amount of activity toward the second order closure modeling of turbulence, very little experimental information is available about the budgets of the second moment equations. Part of the problem stems from our inability to measure the pressure correlations. However, if everything else appearing in these equations is known from the experiment, pressure correlations can be obtained as the closing terms. This is the closest we can come to in obtaining these terms from experiment, and despite the measurement errors which might be present in such balances, the resulting information will be extremely useful for the turbulence modelers. The purpose of this part of the work was to provide such balances of the Reynolds stress and heat
Quenching and Anisotropy of Hydromagnetic Turbulent Transport
NASA Astrophysics Data System (ADS)
Karak, Bidya Binay; Rheinhardt, Matthias; Brandenburg, Axel; Käpylä, Petri J.; Käpylä, Maarit J.
2014-11-01
Hydromagnetic turbulence affects the evolution of large-scale magnetic fields through mean-field effects like turbulent diffusion and the α effect. For stronger fields, these effects are usually suppressed or quenched, and additional anisotropies are introduced. Using different variants of the test-field method, we determine the quenching of the turbulent transport coefficients for the forced Roberts flow, isotropically forced non-helical turbulence, and rotating thermal convection. We see significant quenching only when the mean magnetic field is larger than the equipartition value of the turbulence. Expressing the magnetic field in terms of the equipartition value of the quenched flows, we obtain for the quenching exponents of the turbulent magnetic diffusivity about 1.3, 1.1, and 1.3 for Roberts flow, forced turbulence, and convection, respectively. However, when the magnetic field is expressed in terms of the equipartition value of the unquenched flows, these quenching exponents become about 4, 1.5, and 2.3, respectively. For the α effect, the exponent is about 1.3 for the Roberts flow and 2 for convection in the first case, but 4 and 3, respectively, in the second. In convection, the quenching of turbulent pumping follows the same power law as turbulent diffusion, while for the coefficient describing the {\\boldsymbolΩ} × \\boldsymbol {{J}} effect nearly the same quenching exponent is obtained as for α. For forced turbulence, turbulent diffusion proportional to the second derivative along the mean magnetic field is quenched much less, especially for larger values of the magnetic Reynolds number. However, we find that in corresponding axisymmetric mean-field dynamos with dominant toroidal field the quenched diffusion coefficients are the same for the poloidal and toroidal field constituents.
Quenching and anisotropy of hydromagnetic turbulent transport
Karak, Bidya Binay; Brandenburg, Axel; Rheinhardt, Matthias; Käpylä, Petri J.; Käpylä, Maarit J.
2014-11-01
Hydromagnetic turbulence affects the evolution of large-scale magnetic fields through mean-field effects like turbulent diffusion and the α effect. For stronger fields, these effects are usually suppressed or quenched, and additional anisotropies are introduced. Using different variants of the test-field method, we determine the quenching of the turbulent transport coefficients for the forced Roberts flow, isotropically forced non-helical turbulence, and rotating thermal convection. We see significant quenching only when the mean magnetic field is larger than the equipartition value of the turbulence. Expressing the magnetic field in terms of the equipartition value of the quenched flows, we obtain for the quenching exponents of the turbulent magnetic diffusivity about 1.3, 1.1, and 1.3 for Roberts flow, forced turbulence, and convection, respectively. However, when the magnetic field is expressed in terms of the equipartition value of the unquenched flows, these quenching exponents become about 4, 1.5, and 2.3, respectively. For the α effect, the exponent is about 1.3 for the Roberts flow and 2 for convection in the first case, but 4 and 3, respectively, in the second. In convection, the quenching of turbulent pumping follows the same power law as turbulent diffusion, while for the coefficient describing the Ω×J effect nearly the same quenching exponent is obtained as for α. For forced turbulence, turbulent diffusion proportional to the second derivative along the mean magnetic field is quenched much less, especially for larger values of the magnetic Reynolds number. However, we find that in corresponding axisymmetric mean-field dynamos with dominant toroidal field the quenched diffusion coefficients are the same for the poloidal and toroidal field constituents.
NASA Astrophysics Data System (ADS)
Furukawa, Junichi; Noguchi, Yoshiki; Hirano, Toshisuke; Williams, Forman A.
2002-07-01
The density change across premixed flames propagating in turbulent flows modifies the turbulence. The nature of that modification depends on the regime of turbulent combustion, the burner design, the orientation of the turbulent flame and the position within the flame. The present study addresses statistically stationary turbulent combustion in the flame-sheet regime, in which the laminar-flame thickness is less than the Kolmogorov scale, for flames stabilized on a vertically oriented cylindrical burner having fully developed upward turbulent pipe flow upstream from the exit. Under these conditions, rapidly moving wrinkled laminar flamelets form the axisymmetric turbulent flame brush that is attached to the burner exit. Predictions have been made of changes in turbulence properties across laminar flamelets in such situations, but very few measurements have been performed to test the predictions. The present work measures individual velocity changes and changes in turbulence across flamelets at different positions in the turbulent flame brush for three different equivalence ratios, for comparison with theory.
ERIC Educational Resources Information Center
Hanratty, Thomas J.
1980-01-01
This paper gives an account of research on the structure of turbulence close to a solid boundary. Included is a method to study the flow close to the wall of a pipe without interferring with it. (Author/JN)
NASA Astrophysics Data System (ADS)
Nazarenko, Sergey
2015-07-01
Wave turbulence is the statistical mechanics of random waves with a broadband spectrum interacting via non-linearity. To understand its difference from non-random well-tuned coherent waves, one could compare the sound of thunder to a piece of classical music. Wave turbulence is surprisingly common and important in a great variety of physical settings, starting with the most familiar ocean waves to waves at quantum scales or to much longer waves in astrophysics. We will provide a basic overview of the wave turbulence ideas, approaches and main results emphasising the physics of the phenomena and using qualitative descriptions avoiding, whenever possible, involved mathematical derivations. In particular, dimensional analysis will be used for obtaining the key scaling solutions in wave turbulence - Kolmogorov-Zakharov (KZ) spectra.
Higher order stationary subspace analysis
NASA Astrophysics Data System (ADS)
Panknin, Danny; von Bünau, Paul; Kawanabe, Motoaki; Meinecke, Frank C.; Müller, Klaus-Robert
2016-03-01
Non-stationarity in data is an ubiquitous problem in signal processing. The recent stationary subspace analysis procedure (SSA) has enabled to decompose such data into a stationary subspace and a non-stationary part respectively. Algorithmically only weak non- stationarities could be tackled by SSA. The present paper takes the conceptual step generalizing from the use of first and second moments as in SSA to higher order moments, thus defining the proposed higher order stationary subspace analysis procedure (HOSSA). The paper derives the novel procedure and shows simulations. An obvious trade-off between the necessity of estimating higher moments and the accuracy and robustness with which they can be estimated is observed. In an ideal setting of plenty of data where higher moment information is dominating our novel approach can win against standard SSA. However, with limited data, even though higher moments actually dominate the underlying data, still SSA may arrive on par.
Simulation and modeling of homogeneous, compressed turbulence
NASA Technical Reports Server (NTRS)
Wu, C. T.; Ferziger, J. H.; Chapman, D. R.
1985-01-01
Low Reynolds number homogeneous turbulence undergoing low Mach number isotropic and one-dimensional compression was simulated by numerically solving the Navier-Stokes equations. The numerical simulations were performed on a CYBER 205 computer using a 64 x 64 x 64 mesh. A spectral method was used for spatial differencing and the second-order Runge-Kutta method for time advancement. A variety of statistical information was extracted from the computed flow fields. These include three-dimensional energy and dissipation spectra, two-point velocity correlations, one-dimensional energy spectra, turbulent kinetic energy and its dissipation rate, integral length scales, Taylor microscales, and Kolmogorov length scale. Results from the simulated flow fields were used to test one-point closure, two-equation models. A new one-point-closure, three-equation turbulence model which accounts for the effect of compression is proposed. The new model accurately calculates four types of flows (isotropic decay, isotropic compression, one-dimensional compression, and axisymmetric expansion flows) for a wide range of strain rates.
Simulation and modeling of homogeneous, compressed turbulence
NASA Astrophysics Data System (ADS)
Wu, C. T.; Ferziger, J. H.; Chapman, D. R.
1985-05-01
Low Reynolds number homogeneous turbulence undergoing low Mach number isotropic and one-dimensional compression was simulated by numerically solving the Navier-Stokes equations. The numerical simulations were performed on a CYBER 205 computer using a 64 x 64 x 64 mesh. A spectral method was used for spatial differencing and the second-order Runge-Kutta method for time advancement. A variety of statistical information was extracted from the computed flow fields. These include three-dimensional energy and dissipation spectra, two-point velocity correlations, one-dimensional energy spectra, turbulent kinetic energy and its dissipation rate, integral length scales, Taylor microscales, and Kolmogorov length scale. Results from the simulated flow fields were used to test one-point closure, two-equation models. A new one-point-closure, three-equation turbulence model which accounts for the effect of compression is proposed. The new model accurately calculates four types of flows (isotropic decay, isotropic compression, one-dimensional compression, and axisymmetric expansion flows) for a wide range of strain rates.
Stationary Fuel Cell Evaluation (Presentation)
Kurtz, J.; Wipke, K.; Sprik, S.; Ramsden, T.; Ainscough, C.
2012-05-01
This powerpoint presentation discusses its objectives: real world operation data from the field and state-of-the-art lab; collection; analysis for independent technology validation; collaboration with industry and end users operating stationary fuel cell systems and reporting on technology status, progress and technical challenges. The approach and accomplishments are: A quarterly data analysis and publication of first technical stationary fuel cell composite data products (data through June 2012).
Spatiotemporal intermittency in the torsional Couette flow between a rotating and a stationary disk
NASA Astrophysics Data System (ADS)
Cros, A.; Le Gal, P.
2002-11-01
This work is devoted to the experimental study of the transition to turbulence of a flow confined in a narrow gap between a rotating and a stationary disk. When the fluid layer thickness is of the same order of magnitude as the boundary layer depths, the azimuthal velocity axial gradient is nearly constant and this rotating disk flow tends to be a torsional Couette flow. As in the plane Couette flow or the Taylor-Couette flow, transition to turbulence occurs via the appearance of turbulent domains inside a laminar background. In the rotating disk case, the nucleation of turbulent spirals, previously called "solitary waves" in the rotating disk flow literature, is connected to the birth of structural defects in a periodic underlying roll pattern. As the rotation rate is increased, the lifetime of these turbulent structures increases until a threshold is reached where they then form permanent turbulent spirals arranged nearly periodically all around a circumference. However, since the number of these turbulent spirals decreases with the rotational frequency, the transition to a fully turbulent regime is not achieved. Thus the turbulent fraction of the pattern saturates to a value lower than 0.5. After a geometrical description of the structures, we present a statistical analysis of sizes and lifetimes of the turbulent and laminar domains in order to compare this transition to already observed spatiotemporal intermittent behavior.
Effect of Contraction on Turbulence and Temperature Fluctuations Generated by a Warm Grid
NASA Technical Reports Server (NTRS)
Mills, Robert R., Jr.; Corrsin, Stanley
1959-01-01
Hot-wire anemometer measurements were made of several statistical properties of approximately homogeneous and isotropic fields of turbulence and temperature fluctuations generated by a warm grid in a uniform airstream sent through a 4-to-1 contraction. These measurements were made both in the contraction and in the axisymmetric domain farther downstream. In addition to confirming the well-known turbulence anisotropy induced by strain, the data show effects on the skewnesses of both longitudinal velocity fluctuation (which has zero skewness in isotropic turbulence) and its derivative. The concomitant anisotropy in the temperature field accelerates the decay of temperature fluctuations.
Rapid Distortion Theory in astrophysical turbulence
NASA Astrophysics Data System (ADS)
Safonov, Sergey; Petrosyan, Arakel
2016-04-01
In this report, we study statistical properties of astrophysical turbulent plasma flows using Rapid Distortion Theory (RDT). The core assumption is that the turbulence responds to the external distortion so fast, that inertial and viscous forces result in a negligible change in velocity distribution. Thus it is assumed that the response to the external effect takes place in the time interval much smaller than turbulence decay time. This allows to linearize equations and to derive equations for second moments of turbulence. We apply RDT to incompressible turbulent MHD flows distorted with external magnetic field and linear velocity shear in cases of rotating and non-rotating plasma. It is shown that even with a strong nonlinearity many properties of turbulence can be qualitatively studied using a linear theory. A closed system of linear equations for velocity and magnetic field fluctuations is derived. Development of initially isotropic turbulence and transition to anisotropy are studied. Equations for fluid, current and cross helicity are derived. Differences in cases of rotating and non-rotating flows are discussed. Changes introduced by considering Hall effect are discussed.
Anisotropy in turbulent flows and in turbulent transport
NASA Astrophysics Data System (ADS)
Biferale, Luca; Procaccia, Itamar
2005-07-01
The problem of anisotropy and its effects on the statistical theory of high Reynolds number (Re) turbulence (and turbulent transport) is intimately related and intermingled with the problem of the universality of the (anomalous) scaling exponents of structure functions. Both problems had seen tremendous progress in the last 5 years. In this review we present a detailed description of the new tools that allow effective data analysis and systematic theoretical studies such as to separate isotropic from anisotropic aspects of turbulent statistical fluctuations. Employing the invariance of the equations of fluid mechanics to all rotations, we show how to decompose the (tensorial) statistical objects in terms of the irreducible representation of the SO(d) symmetry group (with d being the dimension, d=2 or 3). This device allows a discussion of the scaling properties of the statistical objects in well-defined sectors of the symmetry group, each of which is determined by the “angular momenta” sector numbers (j,m). For the case of turbulent advection of passive scalar or vector fields, this decomposition allows rigorous statements to be made: (i) the scaling exponents are universal, (ii) the isotropic scaling exponents are always leading, (iii) the anisotropic scaling exponents form a discrete spectrum which is strictly increasing as a function of j. This emerging picture offers a complete understanding of the decay of anisotropy upon going to smaller and smaller scales. Next, we explain how to apply the SO(3) decomposition to the statistical Navier-Stokes theory. We show how to extract information about the scaling behavior in the isotropic sector. Doing so furnishes a systematic way to assess the universality of the scaling exponents in this sector, clarifying the anisotropic origin of the many measurements that claimed the opposite. A systematic analysis of direct numerical simulations (DNS) of the Navier-Stokes equations and of experiments provides a strong support
Pattern-fluid interpretation of chemical turbulence
NASA Astrophysics Data System (ADS)
Scholz, Christian; Schröder-Turk, Gerd E.; Mecke, Klaus
2015-04-01
The spontaneous formation of heterogeneous patterns is a hallmark of many nonlinear systems, from biological tissue to evolutionary population dynamics. The standard model for pattern formation in general, and for Turing patterns in chemical reaction-diffusion systems in particular, are deterministic nonlinear partial differential equations where an unstable homogeneous solution gives way to a stable heterogeneous pattern. However, these models fail to fully explain the experimental observation of turbulent patterns with spatio-temporal disorder in chemical systems. Here we introduce a pattern-fluid model as a general concept where turbulence is interpreted as a weakly interacting ensemble obtained by random superposition of stationary solutions to the underlying reaction-diffusion system. The transition from turbulent to stationary patterns is then interpreted as a condensation phenomenon, where the nonlinearity forces one single mode to dominate the ensemble. This model leads to better reproduction of the experimental concentration profiles for the "stationary phases" and reproduces the turbulent chemical patterns observed by Q. Ouyang and H. L. Swinney [Chaos 1, 411 (1991), 10.1063/1.165851].
Pattern-fluid interpretation of chemical turbulence.
Scholz, Christian; Schröder-Turk, Gerd E; Mecke, Klaus
2015-04-01
The spontaneous formation of heterogeneous patterns is a hallmark of many nonlinear systems, from biological tissue to evolutionary population dynamics. The standard model for pattern formation in general, and for Turing patterns in chemical reaction-diffusion systems in particular, are deterministic nonlinear partial differential equations where an unstable homogeneous solution gives way to a stable heterogeneous pattern. However, these models fail to fully explain the experimental observation of turbulent patterns with spatio-temporal disorder in chemical systems. Here we introduce a pattern-fluid model as a general concept where turbulence is interpreted as a weakly interacting ensemble obtained by random superposition of stationary solutions to the underlying reaction-diffusion system. The transition from turbulent to stationary patterns is then interpreted as a condensation phenomenon, where the nonlinearity forces one single mode to dominate the ensemble. This model leads to better reproduction of the experimental concentration profiles for the "stationary phases" and reproduces the turbulent chemical patterns observed by Q. Ouyang and H. L. Swinney [Chaos 1, 411 (1991)]. PMID:25974562
NASA Technical Reports Server (NTRS)
Biedron, Robert T.; Vatsa, Veer N.; Atkins, Harold L.
2005-01-01
We apply an unsteady Reynolds-averaged Navier-Stokes (URANS) solver for unstructured grids to unsteady flows on moving and stationary grids. Example problems considered are relevant to active flow control and stability and control. Computational results are presented using the Spalart-Allmaras turbulence model and are compared to experimental data. The effect of grid and time-step refinement are examined.
A study of local anisotropy in globally isotropic incompressible MHD
NASA Astrophysics Data System (ADS)
Milano, L. J.; Dmitruk, P.; Matthaeus, W. H.; Montgomery, D.
2000-10-01
It is a well known fact that in presence of a DC applied field, MHD turbulence develops spectral anisotropy from an isotropic initial condition [1]. Typically, the reduced spectrum is steeper in the direction of the magnetic field than it is in any transverse direction. Theoretical insight into the origin of this effect has been derived from simulations in which there is a uniform DC magnetic field, but suggestions of a similar anisotropy is seen in various laboratory devices and also in the solar wind [2,3]. One might expect that a DC field is not essential, and it is the local mean field that is responsible. Here we investigate the occurence of local anisotropy in 3 dimensional MHD, i.e. we search for a local version of the spectral anisotropy effect. We perform 3D MHD pseudo-spectral incompressible relaxation simulations, and compute structure functions accumulated according to whether the separation is parallel to, or transverse to, the local magnetic field. Preliminary results show that correlations decay slower in the locally averaged magnetic field direction. [1] J. Shebalin, W. Matthaeus and D. Montgomery, J. Plasma Phys. 29, 525 (1983) [2] W.H. Matthaeus, M.L. Goldsteon and D.A. Roberts, J. Geophys. Res. 95, 20 673 (1990) [3] J. Armstrong, W. Coles, M. Kojima and B. Rickett, Ap. J. 358, 685 (1990)
Applications of direct numerical simulation of turbulence in second order closures
NASA Technical Reports Server (NTRS)
Shih, Tsan-Hsing; Lumley, John L.
1995-01-01
This paper discusses two methods of developing models for the rapid pressure-strain correlation term in the Reynolds stress transport equation using direct numerical simulation (DNS) data. One is a perturbation about isotropic turbulence, the other is a perturbation about two-component turbulence -- an extremely anisotropic turbulence. A model based on the latter method is proposed and is found to be very promising when compared with DNS data and other models.
Statistical Properties of the T-Exponential of Isotropically Distributed Random Matrices
NASA Astrophysics Data System (ADS)
Il'yn, A. S.; Sirota, V. A.; Zybin, K. P.
2016-05-01
A functional method for calculating averages of the time-ordered exponential of a continuous isotropic random N× N matrix process is presented. The process is not assumed to be Gaussian. In particular, the Lyapunov exponents and higher correlation functions of the T-exponent are derived from the statistical properties of the process. The approach may be of use in a wide range of physical problems. For example, in theory of turbulence the account of non-gaussian statistics is very important since the non-Gaussian behavior is responsible for the time asymmetry of the energy flow.
Anisotropy in MHD turbulence due to a mean magnetic field
NASA Technical Reports Server (NTRS)
Shebalin, J. V.; Matthaeus, W. H.; Montgomery, D.
1982-01-01
The development of anisotropy in an initially isotropic spectrum is studied numerically for two-dimensional magnetohydrodynamic turbulence. The anisotropy develops due to the combined effects of an externally imposed dc magnetic field and viscous and resistive dissipation at high wave numbers. The effect is most pronounced at high mechanical and magnetic Reynolds numbers. The anisotropy is greater at the higher wave numbers.
On curve and surface stretching in turbulent flow
NASA Technical Reports Server (NTRS)
Etemadi, Nassrollah
1989-01-01
Cocke (1969) proved that in incompressible, isotropic turbulence the average material line (material surface) elements increase in comparison with their initial values. Good estimates of how much they increase in terms of the eigenvalues of the Green deformation tensor were rigorously obtained.
NASA Technical Reports Server (NTRS)
Tchen, C. M.
1986-01-01
Theoretical and numerical works in atmospheric turbulence have used the Navier-Stokes fluid equations exclusively for describing large-scale motions. Controversy over the existence of an average temperature gradient for the very large eddies in the atmosphere suggested that a new theoretical basis for describing large-scale turbulence was necessary. A new soliton formalism as a fluid analogue that generalizes the Schrodinger equation and the Zakharov equations has been developed. This formalism, processing all the nonlinearities including those from modulation provided by the density fluctuations and from convection due to the emission of finite sound waves by velocity fluctuations, treats large-scale turbulence as coalescing and colliding solitons. The new soliton system describes large-scale instabilities more explicitly than the Navier-Stokes system because it has a nonlinearity of the gradient type, while the Navier-Stokes has a nonlinearity of the non-gradient type. The forced Schrodinger equation for strong fluctuations describes the micro-hydrodynamical state of soliton turbulence and is valid for large-scale turbulence in fluids and plasmas where internal waves can interact with velocity fluctuations.
Zonal flow generation and its feedback on turbulence production in drift wave turbulence
NASA Astrophysics Data System (ADS)
Pushkarev, Andrey V.; Bos, Wouter J. T.; Nazarenko, Sergey V.
2013-04-01
Plasma turbulence described by the Hasegawa-Wakatani equations is simulated numerically for different models and values of the adiabaticity parameter C. It is found that for low values of C turbulence remains isotropic, zonal flows are not generated and there is no suppression of the meridional drift waves and particle transport. For high values of C, turbulence evolves towards highly anisotropic states with a dominant contribution of the zonal sector to the kinetic energy. This anisotropic flow leads to a decrease of turbulence production in the meridional sector and limits the particle transport across the mean isopycnal surfaces. This behavior allows to consider the Hasegawa-Wakatani equations a minimal PDE model, which contains the drift-wave/zonal-flow feedback loop mechanism.
Zonal flow generation and its feedback on turbulence production in drift wave turbulence
Pushkarev, Andrey V.; Bos, Wouter J. T.; Nazarenko, Sergey V.
2013-04-15
Plasma turbulence described by the Hasegawa-Wakatani equations is simulated numerically for different models and values of the adiabaticity parameter C. It is found that for low values of C turbulence remains isotropic, zonal flows are not generated and there is no suppression of the meridional drift waves and particle transport. For high values of C, turbulence evolves towards highly anisotropic states with a dominant contribution of the zonal sector to the kinetic energy. This anisotropic flow leads to a decrease of turbulence production in the meridional sector and limits the particle transport across the mean isopycnal surfaces. This behavior allows to consider the Hasegawa-Wakatani equations a minimal PDE model, which contains the drift-wave/zonal-flow feedback loop mechanism.
ODTLES : a model for 3D turbulent flow based on one-dimensional turbulence modeling concepts.
McDermott, Randy; Kerstein, Alan R.; Schmidt, Rodney Cannon
2005-01-01
This report describes an approach for extending the one-dimensional turbulence (ODT) model of Kerstein [6] to treat turbulent flow in three-dimensional (3D) domains. This model, here called ODTLES, can also be viewed as a new LES model. In ODTLES, 3D aspects of the flow are captured by embedding three, mutually orthogonal, one-dimensional ODT domain arrays within a coarser 3D mesh. The ODTLES model is obtained by developing a consistent approach for dynamically coupling the different ODT line sets to each other and to the large scale processes that are resolved on the 3D mesh. The model is implemented computationally and its performance is tested and evaluated by performing simulations of decaying isotropic turbulence, a standard turbulent flow benchmarking problem.
Stationary density profiles in the Alcator C-mod tokamak
Kesner, J.; Ernst, D.; Hughes, J.; Mumgaard, R.; Shiraiwa, S.; Whyte, D.; Scott, S.
2012-12-15
In the absence of an internal particle source, plasma turbulence will impose an intrinsic relationship between an inwards pinch and an outwards diffusion resulting in a stationary density profile. The Alcator C-mod tokamak utilizes RF heating and current drive so that fueling only occurs in the vicinity of the separatrix. Discharges that transition from L-mode to I-mode are seen to maintain a self-similar stationary density profile as measured by Thomson scattering. For discharges with negative magnetic shear, an observed rise of the safety factor in the vicinity of the magnetic axis appears to be accompanied by a decrease of electron density, qualitatively consistent with the theoretical expectations.
On the subgrid-scale modeling of compressible turbulence
NASA Technical Reports Server (NTRS)
Speziale, C. G.; Erlebacher, G.; Zang, T. A.; Hussaini, M. Y.
1987-01-01
A subgrid-scale model recently derived for use in the large-eddy simulation of compressible turbulent flows is examined from a fundamental theoretical and computational standpoint. It is demonstrated that this model, which is applicable only to compressible turbulent flows in the limit of small density fluctuations, correlates somewhat poorly with the results of direct numerical simulations of compressible isotropic turbulence at low Mach numbers. An alternative model, based on Favre-filtered fields, is suggested which appears to reduce these limitations.
The subgrid-scale modeling of compressible turbulence
NASA Technical Reports Server (NTRS)
Speziale, C. G.; Hussaini, M. Y.; Erlebacher, G.; Zang, T. A.
1988-01-01
A subgrid-scale model recently derived for use in the large-eddy simulation of compressible turbulent flows is examined from a fundamental theoretical and computational standpoint. It is demonstrated that this model, which is applicable only to compressible turbulent flows in the limit of small density fluctuations, correlates somewhat poorly with the results of direct numerical simulations of compressible isotropic turbulence at low Mach numbers. An alternative model, based on Favre-filtered fields, is suggested which appears to reduce these limitations.
Large-eddy simulation of a turbulent mixing layer
NASA Technical Reports Server (NTRS)
Mansour, N. N.; Ferziger, J. H.; Reynolds, W. C.
1978-01-01
The three dimensional, time dependent (incompressible) vorticity equations were used to simulate numerically the decay of isotropic box turbulence and time developing mixing layers. The vorticity equations were spatially filtered to define the large scale turbulence field, and the subgrid scale turbulence was modeled. A general method was developed to show numerical conservation of momentum, vorticity, and energy. The terms that arise from filtering the equations were treated (for both periodic boundary conditions and no stress boundary conditions) in a fast and accurate way by using fast Fourier transforms. Use of vorticity as the principal variable is shown to produce results equivalent to those obtained by use of the primitive variable equations.
PDF methods for combustion in high-speed turbulent flows
NASA Technical Reports Server (NTRS)
Pope, Stephen B.
1995-01-01
This report describes the research performed during the second year of this three-year project. The ultimate objective of the project is extend the applicability of probability density function (pdf) methods from incompressible to compressible turbulent reactive flows. As described in subsequent sections, progress has been made on: (1) formulation and modelling of pdf equations for compressible turbulence, in both homogeneous and inhomogeneous inert flows; and (2) implementation of the compressible model in various flow configurations, namely decaying isotropic turbulence, homogeneous shear flow and plane mixing layer.
Adams, Allan; Chesler, Paul M; Liu, Hong
2014-04-18
We construct turbulent black holes in asymptotically AdS4 spacetime by numerically solving Einstein's equations. Using the AdS/CFT correspondence we find that both the dual holographic fluid and bulk geometry display signatures of an inverse cascade with the bulk geometry being well approximated by the fluid-gravity gradient expansion. We argue that statistically steady-state black holes dual to d dimensional turbulent flows have horizons whose area growth has a fractal-like structure with fractal dimension D=d+4/3. PMID:24785028
Stationary surgical smoke evacuation systems.
2001-03-01
Two types of systems are available for evacuating the surgical smoke created by electrosurgery and laser surgery: portable and stationary surgical smoke evacuation systems. While portable systems dominate the market today, stationary systems are an alternative worth considering--even though they are still in their infancy, with fewer than 90 systems installed to date. Stationary systems represent a major commitment on the part of the healthcare facility. Several system components must be installed as part of the physical plant (for instance, within the walls), making the system a permanent fixture in the surgical suite. Installation of these systems is often carried out during building construction or major renovation--although the systems can be cost-effective even if no renovations are planned. For this Evaluation, we tested three stationary systems. All three are adequate to capture surgical smoke and evacuate it from the operating room. These systems are easy to use, are quietter than their portable counterparts, and require minimal user maintenance. They represent an excellent option for most hospitals actively evacuating surgical smoke. In this article, we discuss the factors to consider when selecting from among these systems. We also offer guidance on choosing between stationary systems and portable ones. PMID:11321758
Turbulence in Compressible Flows
NASA Technical Reports Server (NTRS)
1997-01-01
Lecture notes for the AGARD Fluid Dynamics Panel (FDP) Special Course on 'Turbulence in Compressible Flows' have been assembled in this report. The following topics were covered: Compressible Turbulent Boundary Layers, Compressible Turbulent Free Shear Layers, Turbulent Combustion, DNS/LES and RANS Simulations of Compressible Turbulent Flows, and Case Studies of Applications of Turbulence Models in Aerospace.
Turbulent Flow Between Rotating Cylinders
NASA Technical Reports Server (NTRS)
Shih-I, Pai
1943-01-01
The turbulent air flow between rotating cylinders was investigated. The distributions of mean speed and of turbulence were measured in the gap between a rotating inner and a stationary outer cylinder. The measurements led to the conclusion that the turbulent flow in the gap cannot be considered two dimensional, but that a particular type of secondary motion takes place. It is shown that the experimentally found velocity distribution can be fully understood under the assumption that this secondary motion consists of three-dimensional ring-shape vortices. The vortices occur only in pairs, and their number and size depend on the speed of the rotating cylinder; the number was found to decrease with increasing speed. The secondary motion has an essential part in the transmission of the moment of momentum. In regions where the secondary motion is negligible, the momentum transfer follows the laws known for homologous turbulence. Ring-shape vortices are known to occur in the laminar flow between rotating cylinders, but it was hitherto unknown that they exist even at speeds that are several hundred times the critical limit.
A one-equation turbulence model for recirculating flows
NASA Astrophysics Data System (ADS)
Zhang, Yang; Bai, JunQiang; Xu, JingLei; Li, Yi
2016-06-01
A one-equation turbulence model which relies on the turbulent kinetic energy transport equation has been developed to predict the flow properties of the recirculating flows. The turbulent eddy-viscosity coefficient is computed from a recalibrated Bradshaw's assumption that the constant a 1 = 0.31 is recalibrated to a function based on a set of direct numerical simulation (DNS) data. The values of dissipation of turbulent kinetic energy consist of the near-wall part and isotropic part, and the isotropic part involves the von Karman length scale as the turbulent length scale. The performance of the new model is evaluated by the results from DNS for fully developed turbulence channel flow with a wide range of Reynolds numbers. However, the computed result of the recirculating flow at the separated bubble of NACA4412 demonstrates that an increase is needed on the turbulent dissipation, and this leads to an advanced tuning on the self-adjusted function. The improved model predicts better results in both the non-equilibrium and equilibrium flows, e.g. channel flows, backward-facing step flow and hump in a channel.
Some Results Relevant to Statistical Closures for Compressible Turbulence
NASA Technical Reports Server (NTRS)
Ristorcelli, J. R.
1998-01-01
For weakly compressible turbulent fluctuations there exists a small parameter, the square of the fluctuating Mach number, that allows an investigation using a perturbative treatment. The consequences of such a perturbative analysis in three different subject areas are described: 1) initial conditions in direct numerical simulations, 2) an explanation for the oscillations seen in the compressible pressure in the direct numerical simulations of homogeneous shear, and 3) for turbulence closures accounting for the compressibility of velocity fluctuations. Initial conditions consistent with small turbulent Mach number asymptotics are constructed. The importance of consistent initial conditions in the direct numerical simulation of compressible turbulence is dramatically illustrated: spurious oscillations associated with inconsistent initial conditions are avoided, and the fluctuating dilatational field is some two orders of magnitude smaller for a compressible isotropic turbulence. For the isotropic decay it is shown that the choice of initial conditions can change the scaling law for the compressible dissipation. A two-time expansion of the Navier-Stokes equations is used to distinguish compressible acoustic and compressible advective modes. A simple conceptual model for weakly compressible turbulence - a forced linear oscillator is described. It is shown that the evolution equations for the compressible portions of turbulence can be understood as a forced wave equation with refraction. Acoustic modes of the flow can be amplified by refraction and are able to manifest themselves in large fluctuations of the compressible pressure.
NASA Technical Reports Server (NTRS)
Bardina, Jorge E.
1995-01-01
The objective of this work is to develop, verify, and incorporate the baseline two-equation turbulence models which account for the effects of compressibility into the three-dimensional Reynolds averaged Navier-Stokes (RANS) code and to provide documented descriptions of the models and their numerical procedures so that they can be implemented into 3-D CFD codes for engineering applications.
Talbot, L.; Cheng, R.K.
1993-12-01
Turbulent combustion is the dominant process in heat and power generating systems. Its most significant aspect is to enhance the burning rate and volumetric power density. Turbulent mixing, however, also influences the chemical rates and has a direct effect on the formation of pollutants, flame ignition and extinction. Therefore, research and development of modern combustion systems for power generation, waste incineration and material synthesis must rely on a fundamental understanding of the physical effect of turbulence on combustion to develop theoretical models that can be used as design tools. The overall objective of this program is to investigate, primarily experimentally, the interaction and coupling between turbulence and combustion. These processes are complex and are characterized by scalar and velocity fluctuations with time and length scales spanning several orders of magnitude. They are also influenced by the so-called {open_quotes}field{close_quotes} effects associated with the characteristics of the flow and burner geometries. The authors` approach is to gain a fundamental understanding by investigating idealized laboratory flames. Laboratory flames are amenable to detailed interrogation by laser diagnostics and their flow geometries are chosen to simplify numerical modeling and simulations and to facilitate comparison between experiments and theory.
Scaling laws in magnetized plasma turbulence
Boldyrev, Stanislav
2015-06-28
Interactions of plasma motion with magnetic fields occur in nature and in the laboratory in an impressively broad range of scales, from megaparsecs in astrophysical systems to centimeters in fusion devices. The fact that such an enormous array of phenomena can be effectively studied lies in the existence of fundamental scaling laws in plasma turbulence, which allow one to scale the results of analytic and numerical modeling to the sized of galaxies, velocities of supernovae explosions, or magnetic fields in fusion devices. Magnetohydrodynamics (MHD) provides the simplest framework for describing magnetic plasma turbulence. Recently, a number of new features of MHD turbulence have been discovered and an impressive array of thought-provoking phenomenological theories have been put forward. However, these theories have conflicting predictions, and the currently available numerical simulations are not able to resolve the contradictions. MHD turbulence exhibits a variety of regimes unusual in regular hydrodynamic turbulence. Depending on the strength of the guide magnetic field it can be dominated by weakly interacting Alfv\\'en waves or strongly interacting wave packets. At small scales such turbulence is locally anisotropic and imbalanced (cross-helical). In a stark contrast with hydrodynamic turbulence, which tends to ``forget'' global constrains and become uniform and isotropic at small scales, MHD turbulence becomes progressively more anisotropic and unbalanced at small scales. Magnetic field plays a fundamental role in turbulent dynamics. Even when such a field is not imposed by external sources, it is self-consistently generated by the magnetic dynamo action. This project aims at a comprehensive study of universal regimes of magnetic plasma turbulence, combining the modern analytic approaches with the state of the art numerical simulations. The proposed study focuses on the three topics: weak MHD turbulence, which is relevant for laboratory devices, the solar
Mixing and chemical reaction in sheared and nonsheared homogeneous turbulence
NASA Technical Reports Server (NTRS)
Leonard, Andy D.; Hill, James C.
1992-01-01
Direct numerical simulations were made to examine the local structure of the reaction zone for a moderately fast reaction between unmixed species in decaying, homogeneous turbulence and in a homogeneous turbulent shear flow. Pseudospectral techniques were used in domains of 64 exp 3 and higher wavenumbers. A finite-rate, single step reaction between non-premixed reactants was considered, and in one case temperature-dependent Arrhenius kinetics was assumed. Locally intense reaction rates that tend to persist throughout the simulations occur in locations where the reactant concentration gradients are large and are amplified by the local rate of strain. The reaction zones are more organized in the case of a uniform mean shear than in isotropic turbulence, and regions of intense reaction rate appear to be associated with vortex structures such as horseshoe vortices and fingers seen in mixing layers. Concentration gradients tend to align with the direction of the most compressive principal strain rate, more so in the isotropic case.
Effects of axisymmetric contractions on turbulence of various scales
NASA Technical Reports Server (NTRS)
Tan-Atichat, J.; Nagib, H. M.; Drubka, R. E.
1980-01-01
Digitally acquired and processed results from an experimental investigation of grid generated turbulence of various scales through and downstream of nine matched cubic contour contractions ranging in area ratio from 2 to 36, and in length to inlet diameter ratio from 0.25 to 1.50 are reported. An additional contraction with a fifth order contour was also utilized for studying the shape effect. Thirteen homogeneous and nearly isotropic test flow conditions with a range of turbulence intensities, length scales and Reynolds numbers were generated and used to examine the sensitivity of the contractions to upstream turbulence. The extent to which the turbulence is altered by the contraction depends on the incoming turbulence scales, the total strain experienced by the fluid, as well as the contraction ratio and the strain rate. Varying the turbulence integral scale influences the transverse turbulence components more than the streamwise component. In general, the larger the turbulence scale, the lesser the reduction in the turbulence intensity of the transverse components. Best agreement with rapid distortion theory was obtained for large scale turbulence, where viscous decay over the contraction length was negligible, or when a first order correction for viscous decay was applied to the results.
Isotropic homogeneous universe with viscous fluid
Santos, N.O.; Dias, R.S.; Banerjee, A.
1985-04-01
Exact solutions are obtained for the isotropic homogeneous cosmological model with viscous fluid. The fluid has only bulk viscosity and the viscosity coefficient is taken to be a power function of the mass density. The equation of state assumed obeys a linear relation between mass density and pressure. The models satisfying Hawking's energy conditions are discussed. Murphy's model is only a special case of this general set of solutions and it is shown that Murphy's conclusion that the introduciton of bulk viscosity can avoid the occurrence of space-time singularity at finite past is not, in general, valid.
Lagrangian statistics of light particles in turbulence
NASA Astrophysics Data System (ADS)
Mercado, Julián Martínez; Prakash, Vivek N.; Tagawa, Yoshiyuki; Sun, Chao; Lohse, Detlef; (International CollaborationTurbulence Research)
2012-05-01
We study the Lagrangian velocity and acceleration statistics of light particles (micro-bubbles in water) in homogeneous isotropic turbulence. Micro-bubbles with a diameter db = 340 μm and Stokes number from 0.02 to 0.09 are dispersed in a turbulent water tunnel operated at Taylor-Reynolds numbers (Reλ) ranging from 160 to 265. We reconstruct the bubble trajectories by employing three-dimensional particle tracking velocimetry. It is found that the probability density functions (PDFs) of the micro-bubble acceleration show a highly non-Gaussian behavior with flatness values in the range 23 to 30. The acceleration flatness values show an increasing trend with Reλ, consistent with previous experiments [G. Voth, A. La Porta, A. M. Crawford, J. Alexander, and E. Bodenschatz, "Measurement of particle accelerations in fully developed turbulence," J. Fluid Mech. 469, 121 (2002)], 10.1017/S0022112002001842 and numerics [T. Ishihara, Y. Kaneda, M. Yokokawa, K. Itakura, and A. Uno, "Small-scale statistics in highresolution direct numerical simulation of turbulence: Reynolds number dependence of one-point velocity gradient statistics," J. Fluid Mech. 592, 335 (2007)], 10.1017/S0022112007008531. These acceleration PDFs show a higher intermittency compared to tracers [S. Ayyalasomayajula, Z. Warhaft, and L. R. Collins, "Modeling inertial particle acceleration statistics in isotropic turbulence," Phys. Fluids. 20, 095104 (2008)], 10.1063/1.2976174 and heavy particles [S. Ayyalasomayajula, A. Gylfason, L. R. Collins, E. Bodenschatz, and Z. Warhaft, "Lagrangian measurements of inertial particle accelerations in grid generated wind tunnel turbulence," Phys. Rev. Lett. 97, 144507 (2006)], 10.1103/PhysRevLett.97.144507 in wind tunnel experiments. In addition, the micro-bubble acceleration autocorrelation function decorrelates slower with increasing Reλ. We also compare our results with experiments in von Kármán flows and point-particle direct numerical simulations with periodic
The dynamics of variable-density turbulence
Sandoval, D.L.
1995-11-01
The dynamics of variable-density turbulent fluids are studied by direct numerical simulation. The flow is incompressible so that acoustic waves are decoupled from the problem, and implying that density is not a thermodynamic variable. Changes in density occur due to molecular mixing. The velocity field, is in general, divergent. A pseudo-spectral numerical technique is used to solve the equations of motion. Three-dimensional simulations are performed using a grid size of 128{sup 3} grid points. Two types of problems are studied: (1) the decay of isotropic, variable-density turbulence, and (2) buoyancy-generated turbulence in a fluid with large density fluctuations. In the case of isotropic, variable-density turbulence, the overall statistical decay behavior, for the cases studied, is relatively unaffected by the presence of density variations when the initial density and velocity fields are statistically independent. The results for this case are in quantitative agreement with previous numerical and laboratory results. In this case, the initial density field has a bimodal probability density function (pdf) which evolves in time towards a Gaussian distribution. The pdf of the density field is symmetric about its mean value throughout its evolution. If the initial velocity and density fields are statistically dependent, however, the decay process is significantly affected by the density fluctuations. For the case of buoyancy-generated turbulence, variable-density departures from the Boussinesq approximation are studied. The results of the buoyancy-generated turbulence are compared with variable-density model predictions. Both a one-point (engineering) model and a two-point (spectral) model are tested against the numerical data. Some deficiencies in these variable-density models are discussed and modifications are suggested.
Structure in turbulent thermal convection
NASA Astrophysics Data System (ADS)
Balachandar, S.
1992-12-01
Small-scale features of vorticity, strain rate, and temperature gradients are considered in a Rayleigh-Bénard convection. The results reported are from a direct numerical simulation of turbulent convection performed in a rectangular box of aspect ratio 2√2 at a Rayleigh number of 6.5×106 and a Prandtl number of 0.72. In agreement with earlier results [Ashurst et al., Phys. Fluids 30, 2343 (1987) and Ruetsch and Maxey, Phys. Fluids A 3, 1587 (1991)], the intermediate strain rate is on an average positive, but the ratio of alpha, beta, and gamma strain rates are measured to be 5.3:1.0:-6.3. This result differs from the earlier result of 3:1:-4 obtained in homogeneous isotropic and shear turbulences. Buoyancy-induced vorticity production makes significant contribution to the overall enstrophy balance, especially close to the boundaries. Vorticity production by buoyancy is exclusively in the horizontal direction and is balanced by preferred production by stretching and tilting in the vertical direction, due to the preferred alignment of extensional alpha strain rate with the vertical direction. Such directional alignment of vorticity, strain rate, and scalar gradient is explained on the basis of preferred spatial orientation of coherent structures in thermal turbulence.
Second-order closure models for supersonic turbulent flows
NASA Technical Reports Server (NTRS)
Speziale, Charles G.; Sarkar, Sutanu
1991-01-01
Recent work on the development of a second-order closure model for high-speed compressible flows is reviewed. This turbulent closure is based on the solution of modeled transport equations for the Favre-averaged Reynolds stress tensor and the solenoidal part of the turbulent dissipation rate. A new model for the compressible dissipation is used along with traditional gradient transport models for the Reynolds heat flux and mass flux terms. Consistent with simple asymptotic analyses, the deviatoric part of the remaining higher-order correlations in the Reynolds stress transport equations are modeled by a variable density extension of the newest incompressible models. The resulting second-order closure model is tested in a variety of compressible turbulent flows which include the decay of isotropic turbulence, homogeneous shear flow, the supersonic mixing layer, and the supersonic flat-plate turbulent boundary layer. Comparisons between the model predictions and the results of physical and numerical experiments are quite encouraging.
Analysis of two-equation turbulence models for recirculating flows
NASA Technical Reports Server (NTRS)
Thangam, S.
1991-01-01
The two-equation kappa-epsilon model is used to analyze turbulent separated flow past a backward-facing step. It is shown that if the model constraints are modified to be consistent with the accepted energy decay rate for isotropic turbulence, the dominant features of the flow field, namely the size of the separation bubble and the streamwise component of the mean velocity, can be accurately predicted. In addition, except in the vicinity of the step, very good predictions for the turbulent shear stress, the wall pressure, and the wall shear stress are obtained. The model is also shown to provide good predictions for the turbulence intensity in the region downstream of the reattachment point. Estimated long time growth rates for the turbulent kinetic energy and dissipation rate of homogeneous shear flow are utilized to develop an optimal set of constants for the two equation kappa-epsilon model. The physical implications of the model performance are also discussed.
Are Eddy Covariance series stationary?
Technology Transfer Automated Retrieval System (TEKTRAN)
Spectral analysis via a discrete Fourier transform is used often to examine eddy covariance series for cycles (eddies) of interest. Generally the analysis is performed on hourly or half-hourly data sets collected at 10 or 20 Hz. Each original series is often assumed to be stationary. Also automated ...
Stationary measure in the multiverse
Linde, Andrei; Vanchurin, Vitaly; Winitzki, Sergei E-mail: vitaly@cosmos2.phy.tufts.edu
2009-01-15
We study the recently proposed ''stationary measure'' in the context of the string landscape scenario. We show that it suffers neither from the ''Boltzmann brain'' problem nor from the ''youngness'' paradox that makes some other measures predict a high CMB temperature at present. We also demonstrate a good performance of this measure in predicting the results of local experiments, such as proton decay.
Cui, Linyan
2015-06-01
Analytic expressions for the temporal power spectra of irradiance fluctuations and angle of arrival (AOA) fluctuations are derived for optical waves propagating through weak anisotropic non-Kolmogorov atmospheric turbulence. In the derivation, the anisotropic non-Kolmogorov spectrum is adopted, which adopts the assumption of circular symmetry in the orthogonal plane throughout the path and the same degree of anisotropy along the propagation direction for all the turbulence cells. The final expressions consider simultaneously the anisotropic factor and general spectral power law values. When the anisotropic factor equals one (corresponding to the isotropic turbulence), the derived temporal power spectral models have good consistency with the known results for the isotropic turbulence. Numerical calculations show that the increased anisotropic factor alleviates the atmospheric turbulence's influence on the final expressions. PMID:26367055
Isotropization of nematic liquid crystals by TMDSC
Chen, Wei; Dadmun, M.; Zhang, Ge; Boller, A.; Wunderlich, B. |
1997-12-01
Temperature-modulated differential scanning calorimetry (TMDSC) and traditional DSC are used to study the transition between the nematic liquid crystalline state and the isotropic liquid for two small molecules [4,4{prime}-azoxyanisole and N,N`-bis(4-n-octyloxybenzal)-1,4-phenylenediamine] and one macromolecule (4,4{prime}-dihydroxy-{alpha}-methylstilbene copolymerized with a 1:1 molar mixture of 1,7-dibromoheptane and 1,9-dibromononane). The DSC measurements with 4,4{prime}-azoxyanisole were used for temperature calibration with varying heating and cooling rates. Quasi-isothermal TMDSC with small temperature amplitude and standard TMDSC with underlying heating and cooling rates were utilized to analyze the breadth of the transitions. It could be verified that the isotropization transition of a nematic liquid crystal is, indeed, reversible for all three molecules. The nature of the transition changes, however, from relatively sharp, for small, rigid molecules, to about three kelvins wide for the small molecule with flexible ends, to as broad as 20 K for the macromolecule. It was also demonstrated that quantitative heats of fusion of sharp transitions can be extracted from TMDSC, but only from the time-domain heat-flow signal.
Depletion induced isotropic-isotropic phase separation in suspensions of rod-like colloids.
Jungblut, S; Tuinier, R; Binder, K; Schilling, T
2007-12-28
When non-adsorbing polymers are added to an isotropic suspension of rod-like colloids, the colloids effectively attract each other via depletion forces. We performed Monte Carlo simulations to study the phase diagram of such rod-polymer mixture. The colloidal rods were modeled as hard spherocylinders; the polymers were described as spheres of the same diameter as the rods. The polymers may overlap with no energy cost, while the overlap of polymers and rods is forbidden. Large amounts of depletant cause phase separation of the mixture. We estimated the phase boundaries of isotropic-isotropic coexistence both in the bulk and in confinement. To determine the phase boundaries we applied the grand canonical ensemble using successive umbrella sampling [J. Chem. Phys. 120, 10925 (2004)], and we performed a finite size scaling analysis to estimate the location of the critical point. The results are compared with predictions of the free volume theory developed by Lekkerkerker and Stroobants [Nuovo Cimento D 16, 949 (1994)]. We also give estimates for the interfacial tension between the coexisting isotropic phases and analyze its power-law behavior on the approach of the critical point. PMID:18163708
Multidimensional stationary probability distribution for interacting active particles.
Maggi, Claudio; Marconi, Umberto Marini Bettolo; Gnan, Nicoletta; Di Leonardo, Roberto
2015-01-01
We derive the stationary probability distribution for a non-equilibrium system composed by an arbitrary number of degrees of freedom that are subject to Gaussian colored noise and a conservative potential. This is based on a multidimensional version of the Unified Colored Noise Approximation. By comparing theory with numerical simulations we demonstrate that the theoretical probability density quantitatively describes the accumulation of active particles around repulsive obstacles. In particular, for two particles with repulsive interactions, the probability of close contact decreases when one of the two particle is pinned. Moreover, in the case of isotropic confining potentials, the radial density profile shows a non trivial scaling with radius. Finally we show that the theory well approximates the "pressure" generated by the active particles allowing to derive an equation of state for a system of non-interacting colored noise-driven particles. PMID:26021260
NASA Technical Reports Server (NTRS)
Rubesin, Morris W.
1987-01-01
Recent developments at several levels of statistical turbulence modeling applicable to aerodynamics are briefly surveyed. Emphasis is on examples of model improvements for transonic, two-dimensional flows. Experience with the development of these improved models is cited to suggest methods of accelerating the modeling process necessary to keep abreast of the rapid movement of computational fluid dynamics into the computation of complex three-dimensional flows.
Diffusive Cosmic-ray Acceleration at Relativistic Shock Waves with Magnetostatic Turbulence
NASA Astrophysics Data System (ADS)
Schlickeiser, R.
2015-08-01
The analytical theory of diffusive cosmic-ray acceleration at parallel stationary shock waves with magnetostatic turbulence is generalized to arbitrary shock speeds {V}{{s}}={β }1c, including, in particular, relativistic speeds. This is achieved by applying the diffusion approximation to the relevant Fokker-Planck particle transport equation formulated in the mixed comoving coordinate system. In this coordinate system, the particle's momentum coordinates p and μ ={p}\\parallel /p are taken in the rest frame of the streaming plasma, whereas the time and space coordinates are taken in the observer's system. For magnetostatic slab turbulence, the diffusion-convection transport equation for the isotropic (in the rest frame of the streaming plasma) part of the particle's phase space density is derived. For a step-wise shock velocity profile, the steady-state diffusion-convection transport equation is solved. For a symmetric pitch-angle scattering Fokker-Planck coefficient, {D}μ μ (-μ )={D}μ μ (μ ), the steady-state solution is independent of the microphysical scattering details. For nonrelativistic mono-momentum particle injection at the shock, the differential number density of accelerated particles is a Lorentzian-type distribution function, which at large momenta approaches a power-law distribution function N(p≥slant {p}c)\\propto {p}-ξ with the spectral index ξ ({β }1)=1+[3/({{{Γ }}}1\\sqrt{{r}2-{β }12}-1)(1+3{β }12)]. For nonrelativistic ({β }1\\ll 1) shock speeds, this spectral index agrees with the known result ξ ({β }1\\ll 1)≃ (r+2)/(r-1), whereas for ultrarelativistic ({{{Γ }}}1\\gg 1) shock speeds the spectral index value is close to unity.
Direct numerical simulation of turbulent reacting flows
Chen, J.H.
1993-12-01
The development of turbulent combustion models that reflect some of the most important characteristics of turbulent reacting flows requires knowledge about the behavior of key quantities in well defined combustion regimes. In turbulent flames, the coupling between the turbulence and the chemistry is so strong in certain regimes that is is very difficult to isolate the role played by one individual phenomenon. Direct numerical simulation (DNS) is an extremely useful tool to study in detail the turbulence-chemistry interactions in certain well defined regimes. Globally, non-premixed flames are controlled by two limiting cases: the fast chemistry limit, where the turbulent fluctuations. In between these two limits, finite-rate chemical effects are important and the turbulence interacts strongly with the chemical processes. This regime is important because industrial burners operate in regimes in which, locally the flame undergoes extinction, or is at least in some nonequilibrium condition. Furthermore, these nonequilibrium conditions strongly influence the production of pollutants. To quantify the finite-rate chemistry effect, direct numerical simulations are performed to study the interaction between an initially laminar non-premixed flame and a three-dimensional field of homogeneous isotropic decaying turbulence. Emphasis is placed on the dynamics of extinction and on transient effects on the fine scale mixing process. Differential molecular diffusion among species is also examined with this approach, both for nonreacting and reacting situations. To address the problem of large-scale mixing and to examine the effects of mean shear, efforts are underway to perform large eddy simulations of round three-dimensional jets.
Evolution of the shock front and turbulence structures in the shock/turbulence interaction
NASA Technical Reports Server (NTRS)
Kevlahan, N.; Mahesh, K.; Lee, S.
1992-01-01
The interaction of a weak shock front with isotropic turbulence has been investigated using Direct Numerical Simulation (DNS). Two problems were considered: the ability of the field equation (the equation for a propagating surface) to model the shock; and a quantitative study of the evolution of turbulence structure using the database generated by Lee et al. Field equation model predictions for front shape have been compared with DNS results; good agreement is found for shock wave interaction with 2D turbulence and for a single steady vorticity wave. In the interaction of 3D isotropic turbulence with a normal shock, strong alignment of vorticity with the intermediate eigenvector of the rate of strain tensor (S(sup *)(sub ij) = S(sub ij) - (1/3)(delta(sub ij))(S(sub kk))) is seen to develop upstream of the shock and to be further amplified on passage through the shock. Vorticity tends to align at 90 deg to the largest eigenvector, but there is no preferred alignment with the smallest eigenvector. Upstream of the shock, the alignments continue to develop even after the velocity derivative skewness saturates. There is a significant tendency, which increases with time throughout the computational domain, for velocity to align with vorticity. The alignment between velocity and vorticity is strongest in eddy regions and weakest in convergence regions.
Numerical studies of microscopic oil droplets under intense turbulence
NASA Astrophysics Data System (ADS)
Snyder, Murray; Knio, Omar
2008-11-01
The rise of small oil droplets in water experiencing isotropic turbulence conditions is analyzed computationally under four different turbulence intensities. The computational method combines DNS of the turbulent flow with Lagrangian tracking of the slightly buoyant droplets using a dynamical equation with buoyancy, virtual mass, pressure, drag, lift and history forces. In our recent work, Snyder et al. (2008), we showed that the puzzling behavior observed by Friedman and Katz (2002), where the rise velocity of droplets smaller than 800 μm in diameter is enhanced by turbulence whereas the rise of larger droplets is retarded, could be explained by significant drop then enhancement of the droplet drag coefficient, and corresponding drop in the virtual mass coefficient. In this study we use the same technique to explain the recent experimental results of Gopalan and Katz (2008), who also showed both suppression and enhancement of droplet rise velocities. Using drag and virtual mass coefficients which vary with Reynolds number, our computations approximate the experimental behavior observed by Gopalan and Katz in isotropic turbulence with 79, 100 and 151 μm Kolmogorov length scales. Combined with close agreement with the Friedman and Katz results, with an 88 μm Kolmogorov scale, our results provide further evidence that both the quasi-steady drag and virtual mass coefficients may be heavily modified under intense turbulence.
Direct simulation of compressible turbulence in a shear flow
NASA Technical Reports Server (NTRS)
Sarkar, S.; Erlebacher, G.; Hussaini, M. Y.
1991-01-01
Compressibility effects on the turbulence in homogeneous shear flow are investigated. The growth of the turbulent kinetic energy was found to decrease with increasing Mach number: a phenomenon which is similar to the reduction of turbulent velocity intensities observed in experiments on supersonic free shear layers. An examination of the turbulent energy budget shows that both the compressible dissipation and the pressure-dilatation contribute to the decrease in the growth of kinetic energy. The pressure-dilatation is predominantly negative in homogeneous shear flow, in contrast to its predominantly positive behavior in isotropic turbulence. The different signs of the pressure-dilatation are explained by theoretical consideration of the equations for the pressure variance and density variance. Previously, the following results were obtained for isotropic turbulence: (1) the normalized compressible dissipation is of O(M(sub t)(exp 2)); and (2) there is approximate equipartition between the kinetic and potential energies associated with the fluctuating compressible mode. Both of these results were substantiated in the case of homogeneous shear. The dilatation field is significantly more skewed and intermittent than the vorticity field. Strong compressions seem to be more likely than strong expansions.
Measuring turbulent fluid dispersion using laser induced phosphorescence
NASA Astrophysics Data System (ADS)
van der Voort, Dennis; Dam, Nico; van de Water, Willem; Kunnen, Rudie; Clercx, Herman; van Heijst, Gertjan
2015-11-01
Fluid dispersion due to turbulence is an important subject in both natural and engineering processes, from cloud formation to turbulent mixing and liquid spray combustion. The combination of small scales and often high velocities results in few experimental techniques that can follow the course of events. We introduce a novel technique, which measures the dispersion of ``tagged'' fluid particles by means of laser-induced phosphorescence, using a solution containing a europium-based molecular complex with a relatively long phosphorescence half-life. This technique is used to measure transport processes in both the dispersion of droplets in homogeneous isotropic turbulence and the dispersion of fluid of near-nozzle spray breakup processes. By tagging a small amount of droplets/fluid via laser excitation, the tagged droplets can be tracked in a Lagrangian way. The absolute dispersion of the droplets can be measured in a variety of turbulent flows. Using this technique it is shows that droplets around St =τp /τη ~ 1 (Stokes number) disperse faster than true fluid tracers in homogeneous isotropic turbulence, as well as differences between longitudinal and radial dispersion in turbulent sprays. This work is part of the research programme of the Foundation for Fundamental Research on Matter (FOM), which is part of the Dutch Organisation for Scientific Research (NWO).
Recalculation of the Isotropic H Functions
NASA Astrophysics Data System (ADS)
Hiroi, Takahiro
1994-06-01
The isotropic H functions defined in radiative transfer theory by Chandrasekhar (Radiative Transfer, Dover, New York, 1960) have been numerically recalculated for a larger number of single scattering albedo (w) values. The obtained H functions should be accurate to five decimal places at least. The H function values become highly nonlinear as w approaches 1, where calculations were performed with many more points than in Chandrasekhar (1960). A simple linear interpolation of Chandrasekhar's H function table could result in as much as 8% error in the H function values and 16% in the H2 values that appear in multiple scattering terms such as in Hapke (J. Geophys. Res. 86, 3039-3054, 1981). On the other hand, Hapke's approximations (J. Geophys. Res. 86, 3039-3054, 1981; Theory of Reflectance and Emittance Spectroscopy, Cambridge Univ. Press, New York, 1993) give H values within 4.1% and 0.8% error, respectively.
Isotropic MD simulations of dynamic brittle fracture
Espanol, P.; Rubio, M.A.; Zuniga, I.
1996-12-01
The authors present results obtained by molecular dynamics simulations on the propagation of fast cracks in triangular 2D lattices. Their aim is to simulate Mode 1 fracture of brittle isotropic materials. They propose a force law that respects the isotropy of the material. The code yields the correct imposed sound c{sub {parallel}}, shear c{sub {perpendicular}} and surface V{sub R} wave speeds. Different notch lengths are systematically studied. They observed that initially the cracks are linear and always branch at a particular critical velocity c* {approx} 0.8V{sub R} and that this occurs when the crack tip reaches the position of a front emitted from the initial crack tip and propagating at a speed c = 0.68V{sub R}.
Isotropic expansion of an inhomogeneous universe
NASA Astrophysics Data System (ADS)
Geng, Wei-Jian; Lü, H.
2014-10-01
We propose a cosmological model that describes isotropic expansion of an inhomogeneous universe. The energy-momentum tensor that creates the spatial inhomogeneity may not affect the uniform expansion scaling factor a(t) in the Friedmann-Lemaître-Robertson-Walker-like metrics. Such an energy-momentum tensor may not be exotic; in fact any linear or nonlinear σ model has this feature. We show that the classical spatial inhomogeneity can be embedded in both inflation models and the traditional cosmological expansion by perfect fluid. The spatial inhomogeneity resembles the primordial quantum perturbation that was frozen in the comoving frame. We obtain some exact inhomogeneous solutions with spherical or axial symmetries. We also show that some of our cosmological models can be viewed as the dynamical black hole formation.
New bounds on isotropic Lorentz violation
Chris Carone; Marc Sher; Marc Vanderhaeghen
2006-09-19
Violations of Lorentz invariance that appear via operators of dimension four or less are completely parameterized in the Standard Model Extension (SME). In the pure photonic sector of the SME, there are nineteen dimensionless, Lorentz-violating parameters. Eighteen of these have experimental upper bounds ranging between 10{sup -11} and 10{sup -32}; the remaining parameter, ktr, is isotropic and has a much weaker bound of order 10{sup -4}. In this Brief Report, we point out that ktr gives a significant contribution to the anomalous magnetic moment of the electron and find a new upper bound of order 10{sup -8}. With reasonable assumptions, we further show that this bound may be improved to 10{sup -14} by considering the renormalization of other Lorentz-violating parameters that are more tightly constrained. Using similar renormalization arguments, we also estimate bounds on Lorentz violating parameters in the pure gluonic sector of QCD.
Isotropic and anisotropic surface wave cloaking techniques
NASA Astrophysics Data System (ADS)
McManus, T. M.; La Spada, L.; Hao, Y.
2016-04-01
In this paper we compare two different approaches for surface waves cloaking. The first technique is a unique application of Fermat’s principle and requires isotropic material properties, but owing to its derivation is limited in its applicability. The second technique utilises a geometrical optics approximation for dealing with rays bound to a two dimensional surface and requires anisotropic material properties, though it can be used to cloak any smooth surface. We analytically derive the surface wave scattering behaviour for both cloak techniques when applied to a rotationally symmetric surface deformation. Furthermore, we simulate both using a commercially available full-wave electromagnetic solver and demonstrate a good level of agreement with their analytically derived solutions. Our analytical solutions and simulations provide a complete and concise overview of two different surface wave cloaking techniques.
Quantization ambiguities in isotropic quantum geometry
NASA Astrophysics Data System (ADS)
Bojowald, Martin
2002-10-01
Some typical quantization ambiguities of quantum geometry are studied within isotropic models. Since this allows explicit computations of operators and their spectra, one can investigate the effects of ambiguities in a quantitative manner. It is shown that these ambiguities do not affect the fate of the classical singularity, demonstrating that the absence of a singularity in loop quantum cosmology is a robust implication of the general quantization scheme. The calculations also allow conclusions about modified operators in the full theory. In particular, using holonomies in a non-fundamental representation of SU(2) to quantize connection components turns out to lead to significant corrections to classical behaviour at macroscopic volume for large values of the spin of the chosen representation.
Elastic constants of layers in isotropic laminates.
Heyliger, Paul R; Ledbetter, Hassel; Kim, Sudook; Reimanis, Ivar
2003-11-01
The individual laminae elastic constants in multilayer laminates composed of dissimilar isotropic layers were determined using ultrasonic-resonance spectroscopy and the linear theory of elasticity. Ultrasonic resonance allows one to measure the free-vibration response spectrum of a traction-free solid under periodic vibration. These frequencies depend on pointwise density, laminate dimensions, layer thickness, and layer elastic constants. Given a material with known mass but unknown constitution, this method allows one to extract the elastic constants and density of the constituent layers. This is accomplished by measuring the frequencies and then minimizing the differences between these and those calculated using the theory of elasticity for layered media to select the constants that best replicate the frequency-response spectrum. This approach is applied to a three-layer, unsymmetric laminate of WpCu, and very good agreement is found with the elastic constants of the two constituent materials. PMID:14649998
Statistical theories of Langmuir turbulence. II - Subsonic to sonic transition
NASA Technical Reports Server (NTRS)
Dubois, D. F.; Rose, H. A.; Nicholson, D. R.
1985-01-01
The subsonic limit of the quadratic direct interaction approximation (DIA) applied to the Zakharov equations is compared with the cubic DIA applied to the nonlinear Schroedinger equation, which is the subsonic limit of the Zakharov equations. Comparisons with Monte Carlo simulations of a truncated system show that the first theory more accurately describes the regime of stationary turbulence, while the second theory more accurately describes the subsonic evolution of the modulational instability. The weak turbulence limits of the two theories describe the sonic and subsonic regimes, respectively. The addition of vertex corrections to the DIA leads to a hybrid weak turbulence theory that smoothly interpolates between the sonic and subsonic regimes.
Cui, Linyan; Xue, Bindang; Zhou, Fugen
2016-04-01
In this study, the modified anisotropic turbulence refractive-index fluctuations spectral model is derived based on the extended Rytov approximation theory for the theoretical investigations of optical plane and spherical waves propagating through moderate-to-strong anisotropic non-Kolmogorov turbulence. The anisotropic factor which parameterizes the asymmetry of turbulence cells or eddies in the horizontal and vertical directions is introduced. The general spectral power law in the range of 3-4 is also considered compared with the conventional classic value of 11/3 for Kolmogorov turbulence. Based on the modified anisotropic turbulence refractive-index fluctuations spectrum, the analytic expressions of the irradiance scintillation index are also derived for optical plane and spherical waves propagating through moderate-to-strong anisotropic non-Kolmogorov turbulence. They are applicable in a wide range of turbulence strengths and can reduce correctly to the previously published results in the special cases of weak anisotropic turbulence and moderate-to-strong isotropic turbulence. Calculations are performed to analyze the derived models. PMID:27140754
Subgrid or Reynolds stress-modeling for three-dimensional turbulence computations
NASA Technical Reports Server (NTRS)
Rubesin, M. W.
1975-01-01
A review is given of recent advances in two distinct computational methods for evaluating turbulence fields, namely, statistical Reynolds stress modeling and turbulence simulation, where large eddies are followed in time. It is shown that evaluation of the mean Reynolds stresses, rather than use of a scalar eddy viscosity, permits an explanation of streamline curvature effects found in several experiments. Turbulence simulation, with a new volume averaging technique and third-order accurate finite-difference computing is shown to predict the decay of isotropic turbulence in incompressible flow with rather modest computer storage requirements, even at Reynolds numbers of aerodynamic interest.
MHD Turbulence and Magnetic Dynamos
NASA Technical Reports Server (NTRS)
Shebalin, John V
2014-01-01
Incompressible magnetohydrodynamic (MHD) turbulence and magnetic dynamos, which occur in magnetofluids with large fluid and magnetic Reynolds numbers, will be discussed. When Reynolds numbers are large and energy decays slowly, the distribution of energy with respect to length scale becomes quasi-stationary and MHD turbulence can be described statistically. In the limit of infinite Reynolds numbers, viscosity and resistivity become zero and if these values are used in the MHD equations ab initio, a model system called ideal MHD turbulence results. This model system is typically confined in simple geometries with some form of homogeneous boundary conditions, allowing for velocity and magnetic field to be represented by orthogonal function expansions. One advantage to this is that the coefficients of the expansions form a set of nonlinearly interacting variables whose behavior can be described by equilibrium statistical mechanics, i.e., by a canonical ensemble theory based on the global invariants (energy, cross helicity and magnetic helicity) of ideal MHD turbulence. Another advantage is that truncated expansions provide a finite dynamical system whose time evolution can be numerically simulated to test the predictions of the associated statistical mechanics. If ensemble predictions are the same as time averages, then the system is said to be ergodic; if not, the system is nonergodic. Although it had been implicitly assumed in the early days of ideal MHD statistical theory development that these finite dynamical systems were ergodic, numerical simulations provided sufficient evidence that they were, in fact, nonergodic. Specifically, while canonical ensemble theory predicted that expansion coefficients would be (i) zero-mean random variables with (ii) energy that decreased with length scale, it was found that although (ii) was correct, (i) was not and the expected ergodicity was broken. The exact cause of this broken ergodicity was explained, after much
Statistical Mechanics of Turbulent Dynamos
NASA Technical Reports Server (NTRS)
Shebalin, John V.
2014-01-01
Incompressible magnetohydrodynamic (MHD) turbulence and magnetic dynamos, which occur in magnetofluids with large fluid and magnetic Reynolds numbers, will be discussed. When Reynolds numbers are large and energy decays slowly, the distribution of energy with respect to length scale becomes quasi-stationary and MHD turbulence can be described statistically. In the limit of infinite Reynolds numbers, viscosity and resistivity become zero and if these values are used in the MHD equations ab initio, a model system called ideal MHD turbulence results. This model system is typically confined in simple geometries with some form of homogeneous boundary conditions, allowing for velocity and magnetic field to be represented by orthogonal function expansions. One advantage to this is that the coefficients of the expansions form a set of nonlinearly interacting variables whose behavior can be described by equilibrium statistical mechanics, i.e., by a canonical ensemble theory based on the global invariants (energy, cross helicity and magnetic helicity) of ideal MHD turbulence. Another advantage is that truncated expansions provide a finite dynamical system whose time evolution can be numerically simulated to test the predictions of the associated statistical mechanics. If ensemble predictions are the same as time averages, then the system is said to be ergodic; if not, the system is nonergodic. Although it had been implicitly assumed in the early days of ideal MHD statistical theory development that these finite dynamical systems were ergodic, numerical simulations provided sufficient evidence that they were, in fact, nonergodic. Specifically, while canonical ensemble theory predicted that expansion coefficients would be (i) zero-mean random variables with (ii) energy that decreased with length scale, it was found that although (ii) was correct, (i) was not and the expected ergodicity was broken. The exact cause of this broken ergodicity was explained, after much
Magnetohydrodynamic Turbulence
NASA Astrophysics Data System (ADS)
Montgomery, David C.
2004-01-01
Magnetohydrodynamic (MHD) turbulence theory is modeled on neutral fluid (Navier-Stokes) turbulence theory, but with some important differences. There have been essentially no repeatable laboratory MHD experiments wherein the boundary conditions could be controlled or varied and a full set of diagnostics implemented. The equations of MHD are convincingly derivable only in the limit of small ratio of collision mean-free-paths to macroscopic length scales, an inequality that often goes the other way for magnetofluids of interest. Finally, accurate information on the MHD transport coefficients-and thus, the Reynolds-like numbers that order magnetofluid behavior-is largely lacking; indeed, the algebraic expressions used for such ingredients as the viscous stress tensor are often little more than wishful borrowing from fluid mechanics. The one accurate thing that has been done extensively and well is to solve the (strongly nonlinear) MHD equations numerically, usually in the presence of rectangular periodic boundary conditions, and then hope for the best when drawing inferences from the computations for those astrophysical and geophysical MHD systems for which some indisputably turbulent detailed data are available, such as the solar wind or solar prominences. This has led to what is perhaps the first field of physics for which computer simulations are regarded as more central to validating conclusions than is any kind of measurement. Things have evolved in this way due to a mixture of the inevitable and the bureaucratic, but that is the way it is, and those of us who want to work on the subject have to live with it. It is the only game in town, and theories that have promised more-often on the basis of some alleged ``instability''-have turned out to be illusory.
Noise of a model helicopter rotor due to ingestion of turbulence
NASA Technical Reports Server (NTRS)
Paterson, R. W.; Amiet, R. K.
1979-01-01
A theoretical and experimental investigation of the noise of a model helicoper rotor due to ingestion of turbulence was conducted. Experiments were performed with a 0.76 m dia, articulated model rotor for a range of inflow turbulence and rotor operating conditions. Inflow turbulence levels varied from approximately 2 to 19 percent and tip Mach number was varied from 0.3 to 0.52. Test conditions included ingestion of a atmospheric turbulence in outdoor hover as well as ingestion of grid generated isotropic turbulence in the wind tunnel airstream. In wind tunnel testing, both forward flight and vertical ascent (climb) were simulated. Far field noise spectra and directivity were measured in addition to incident turbulence intensities, length scales, and spectra. Results indicate that ingestion of atmospheric turbulence is the dominant helicopter rotor hover noise mechanism at the moderate to high frequencies which determine perceived noise level.
Towed-Grid Studies of Quantum Turbulence
NASA Astrophysics Data System (ADS)
Liu, Shu-Chen; Labbe, Greg; Ihas, Gary G.
2006-11-01
We produce Homogeneous Isotropic Quantum Turbulence (HIQT) in liquid helium at 20 mK to compare with classical experiments and theories. Specifically, in the absence of viscosity, through what path does the turbulence decay? To produce HIQT we must quickly accelerate a grid to about 1 m/s in a channel of superfluid helium, tow it for 1 cm at a nearly constant speed, and then stop it equally quickly. To avoid joule and eddy current heating of the liquid helium, a magnetically shielded superconducting linear motor has been built, guided by simulations, along with the current pulses control program written in LabView with an embedded C compiler. The simulations, design process, and the experimental data demonstrating the functioning motor will be presented. We measure the HIQT energy decay mechanism using a calorimetric technique. Recent theory suggests the decay occurs through a Kelvin-wave cascade on the vortex lines which couples the initially large turbulent eddies to the short wavelength phonon spectrum of the liquid, yielding a characteristic rate of temperature rise. Doped germanium thermometers less than 300 μm diameter immersed in the turbulent helium allow fast calorimetric measurements to be made. The decay of turbulence is detected by the rate of temperature rise in the isolated cell after the grid is pulled.
High efficiency stationary hydrogen storage
Hynek, S.; Fuller, W.; Truslow, S.
1995-09-01
Stationary storage of hydrogen permits one to make hydrogen now and use it later. With stationary hydrogen storage, one can use excess electrical generation capacity to power an electrolyzer, and store the resultant hydrogen for later use or transshipment. One can also use stationary hydrogen as a buffer at fueling stations to accommodate non-steady fueling demand, thus permitting the hydrogen supply system (e.g., methane reformer or electrolyzer) to be sized to meet the average, rather than the peak, demand. We at ADL designed, built, and tested a stationary hydrogen storage device that thermally couples a high-temperature metal hydride to a phase change material (PCM). The PCM captures and stores the heat of the hydriding reaction as its own heat of fusion (that is, it melts), and subsequently returns that heat of fusion (by freezing) to facilitate the dehydriding reaction. A key component of this stationary hydrogen storage device is the metal hydride itself. We used nickel-coated magnesium powder (NCMP) - magnesium particles coated with a thin layer of nickel by means of chemical vapor deposition (CVD). Magnesium hydride can store a higher weight fraction of hydrogen than any other practical metal hydride, and it is less expensive than any other metal hydride. We designed and constructed an experimental NCM/PCM reactor out of 310 stainless steel in the form of a shell-and-tube heat exchanger, with the tube side packed with NCMP and the shell side filled with a eutectic mixture of NaCL, KCl, and MgCl{sub 2}. Our experimental results indicate that with proper attention to limiting thermal losses, our overall efficiency will exceed 90% (DOE goal: >75%) and our overall system cost will be only 33% (DOE goal: <50%) of the value of the delivered hydrogen. It appears that NCMP can be used to purify hydrogen streams and store hydrogen at the same time. These prospects make the NCMP/PCM reactor an attractive component in a reformer-based hydrogen fueling station.
Explosive turbulent magnetic reconnection.
Higashimori, K; Yokoi, N; Hoshino, M
2013-06-21
We report simulation results for turbulent magnetic reconnection obtained using a newly developed Reynolds-averaged magnetohydrodynamics model. We find that the initial Harris current sheet develops in three ways, depending on the strength of turbulence: laminar reconnection, turbulent reconnection, and turbulent diffusion. The turbulent reconnection explosively converts the magnetic field energy into both kinetic and thermal energy of plasmas, and generates open fast reconnection jets. This fast turbulent reconnection is achieved by the localization of turbulent diffusion. Additionally, localized structure forms through the interaction of the mean field and turbulence. PMID:23829741
An investigation of turbulent transport in the extreme lower atmosphere
NASA Technical Reports Server (NTRS)
Koper, C. A., Jr.; Sadeh, W. Z.
1975-01-01
A model in which the Lagrangian autocorrelation is expressed by a domain integral over a set of usual Eulerian autocorrelations acquired concurrently at all points within a turbulence box is proposed along with a method for ascertaining the statistical stationarity of turbulent velocity by creating an equivalent ensemble to investigate the flow in the extreme lower atmosphere. Simultaneous measurements of turbulent velocity on a turbulence line along the wake axis were carried out utilizing a longitudinal array of five hot-wire anemometers remotely operated. The stationarity test revealed that the turbulent velocity is approximated as a realization of a weakly self-stationary random process. Based on the Lagrangian autocorrelation it is found that: (1) large diffusion time predominated; (2) ratios of Lagrangian to Eulerian time and spatial scales were smaller than unity; and, (3) short and long diffusion time scales and diffusion spatial scales were constrained within their Eulerian counterparts.
Investigating source processes of isotropic events
NASA Astrophysics Data System (ADS)
Chiang, Andrea
explosion. In contrast, recovering the announced explosive yield using seismic moment estimates from moment tensor inversion remains challenging but we can begin to put error bounds on our moment estimates using the NSS technique. The estimation of seismic source parameters is dependent upon having a well-calibrated velocity model to compute the Green's functions for the inverse problem. Ideally, seismic velocity models are calibrated through broadband waveform modeling, however in regions of low seismicity velocity models derived from body or surface wave tomography may be employed. Whether a velocity model is 1D or 3D, or based on broadband seismic waveform modeling or the various tomographic techniques, the uncertainty in the velocity model can be the greatest source of error in moment tensor inversion. These errors have not been fully investigated for the nuclear discrimination problem. To study the effects of unmodeled structures on the moment tensor inversion, we set up a synthetic experiment where we produce synthetic seismograms for a 3D model (Moschetti et al., 2010) and invert these data using Green's functions computed with a 1D velocity mode (Song et al., 1996) to evaluate the recoverability of input solutions, paying particular attention to biases in the isotropic component. The synthetic experiment results indicate that the 1D model assumption is valid for moment tensor inversions at periods as short as 10 seconds for the 1D western U.S. model (Song et al., 1996). The correct earthquake mechanisms and source depth are recovered with statistically insignificant isotropic components as determined by the F-test. Shallow explosions are biased by the theoretical ISO-CLVD tradeoff but the tectonic release component remains low, and the tradeoff can be eliminated with constraints from P wave first motion. Path-calibration to the 1D model can reduce non-double-couple components in earthquakes, non-isotropic components in explosions and composite sources and improve
Rotational surfaces in isotropic spaces satisfying weingarten conditions
NASA Astrophysics Data System (ADS)
Öğrenmiş, Alper Osman
2016-07-01
In this paper, we study the rotational surfaces in the isotropic 3-space 𝕀3 satisfying Weingarten conditions in terms of the relative curvature K (analogue of the Gaussian curvature) and the isotropic mean curvature H. In particular, we classify such surfaces of linear Weingarten type in 𝕀3.
Philosophies and fallacies in turbulence modeling
NASA Astrophysics Data System (ADS)
Spalart, Philippe R.
2015-04-01
We present a set of positions, likely to be controversial, on turbulence modeling for the Reynolds-Averaged Navier Stokes (RANS) equations. The paper has three themes. First is what we call the "fundamental paradox" of turbulence modeling, between the local character of the Partial Differential Equations strongly favored by CFD methods and the nonlocal physical nature of turbulence. Second, we oppose two philosophies. The "Systematic" philosophy attempts to model the exact transport equations for the Reynolds stresses or possibly higher moments term by term, gradually relegating the Closure Problem to higher moments and invoking the "Principle of Receding Influence" (although rarely formulating it). In contrast, the "Openly Empirical" philosophy produces models which satisfy strict constraints such as Galilean invariance, but lack an explicit connection with terms in the exact turbulence equations. The prime example is the eddy-viscosity assumption. Third, we explain a series of what we perceive as fallacies, many of them widely held and by senior observers, in turbulence knowledge, leading to turbulence models. We divide them into "hard" fallacies for which a short mathematical argument demonstrates that a particular statement is wrong or meaningless, and "soft" fallacies for which approximate physical arguments can be opposed, but we contend that a clear debate is overdue and wishful thinking has been involved. Some fallacies appear to be "intermediate." An example in the hard class is the supposed isotropy of the diagonal Reynolds stresses. Examples in the soft class are the need to match the decay rate of isotropic turbulence, and the value of realizability in a model. Our hope is to help the direct effort in this field away from simplistic and hopeless lines of work, and to foster debates.
Methods of separation of variables in turbulence theory
NASA Technical Reports Server (NTRS)
Tsuge, S.
1978-01-01
Two schemes of closing turbulent moment equations are proposed both of which make double correlation equations separated into single-point equations. The first is based on neglected triple correlation, leading to an equation differing from small perturbed gasdynamic equations where the separation constant appears as the frequency. Grid-produced turbulence is described in this light as time-independent, cylindrically-isotropic turbulence. Application to wall turbulence guided by a new asymptotic method for the Orr-Sommerfeld equation reveals a neutrally stable mode of essentially three dimensional nature. The second closure scheme is based on an assumption of identity of the separated variables through which triple and quadruple correlations are formed. The resulting equation adds, to its equivalent of the first scheme, an integral of nonlinear convolution in the frequency describing a role due to triple correlation of direct energy-cascading.
Anisotropic magnetohydrodynamic turbulence in a strong external magnetic field
NASA Technical Reports Server (NTRS)
Montgomery, D.; Turner, L.
1981-01-01
A strong external dc magnetic field introduces a basic anisotropy in incompressible MHD turbulence. The modifications that this is likely to produce in the properties of the turbulence are investigated for high Reynolds numbers. It is found that the turbulent spectrum splits into two parts: (1) an essentially two-dimensional spectrum with both the velocity field and the magnetic fluctuations perpendicular to the dc magnetic field, and (2) a generally weaker and more nearly isotropic spectrum of Alfven waves. These results are discussed in relation to measurements from the Culham-Harwell Zeta pinch device and the UCLA Macrorotor tokamak, as well as in relation to measurements of MHD turbulence in the solar wind.
Anisotropic magnetohydrodynamic turbulence in a strong external magnetic field
NASA Technical Reports Server (NTRS)
Montgomery, D.; Turner, L.
1981-01-01
A strong external dc magnetic field introduces a basic anisotropy into incompressible magnetohydrodynamic turbulence. The modifications that this is likely to produce in the properties of the turbulence are explored for the high Reynolds number case. The conclusion is reached that the turbulent spectrum splits into two parts: an essentially two dimensional spectrum with both the velocity field and magnetic fluctuations perpendicular to the dc magnetic field, and a generally weaker and more nearly isotropic spectrum of Alfven waves. A minimal characterization of the spectral density tensors is given. Similarities to measurements from the Culham-Harwell Zeta pinch device and the UCLA Macrotor Tokamak are remarked upon, as are certain implications for the Belcher and Davis measurements of magnetohydrodynamic turbulence in the solar wind.
Numerical simulation of turbulence in the presence of shear
NASA Technical Reports Server (NTRS)
Shaanan, S.; Ferziger, J. H.; Reynolds, W. C.
1975-01-01
The numerical calculations are presented of the large eddy structure of turbulent flows, by use of the averaged Navier-Stokes equations, where averages are taken over spatial regions small compared to the size of the computational grid. The subgrid components of motion are modeled by a local eddy-viscosity model. A new finite-difference scheme is proposed to represent the nonlinear average advective term which has fourth-order accuracy. This scheme exhibits several advantages over existing schemes with regard to the following: (1) the scheme is compact as it extends only one point away in each direction from the point to which it is applied; (2) it gives better resolution for high wave-number waves in the solution of Poisson equation, and (3) it reduces programming complexity and computation time. Examples worked out in detail are the decay of isotropic turbulence, homogeneous turbulent shear flow, and homogeneous turbulent shear flow with system rotation.
Single particle measurements of material line stretching in turbulence: Experiments
NASA Astrophysics Data System (ADS)
Kramel, Stefan; Tympel, Saskia; Toschi, Federico; Voth, Greg
2015-11-01
We find that particles in the shape of chiral dipoles display a preferential rotation direction in three dimensional isotropic turbulence. The particles consist of two helical ends with opposite chirality that are connected by a straight rod. They are fabricated using 3D printing and have an aspect ratio of 10 and a length in the inertial range of our flow between oscillating grids. Due to their high aspect ratio, they move like material lines. Because material lines align with the extentional eigenvectors of the velocity gradient tensor they experience a mean stretching in turbulence. The stretching of a chiral dipole produces a rotation about the dipole axis and so chiral dipoles experience a non-zero mean spinning rate in turbulence. These results provide a first direct experimental measurement of the rate of material line stretching in turbulence.
Accumulation of motile elongated micro-organisms in turbulence
NASA Astrophysics Data System (ADS)
Zhan, Caijuan; Sardina, Gaetano; Lushi, Enkeleida; Brandt, Luca
2014-01-01
We study the effect of turbulence on marine life by performing numerical simulations of motile microorganisms, modelled as prolate spheroids, in isotropic homogeneous turbulence. We show that the clustering and patchiness observed in laminar flows, linear shear and vortex flows, are significantly reduced in a three-dimensional turbulent flow mainly because of the complex topology; elongated micro-orgamisms show some level of clustering in the case of swimmers without any preferential alignment whereas spherical swimmers remain uniformly distributed. Micro-organisms with one preferential swimming direction (e.g. gyrotaxis) still show significant clustering if spherical in shape, whereas prolate swimmers remain more uniformly distributed. Due to their large sensitivity to the local shear, these elongated swimmers react slower to the action of vorticity and gravity and therefore do not have time to accumulate in a turbulent flow. These results show how purely hydrodynamic effects can alter the ecology of microorganisms that can vary their shape and their preferential orientation.
Isotropic microscale mechanical properties of coral skeletons
Pasquini, Luca; Molinari, Alan; Fantazzini, Paola; Dauphen, Yannicke; Cuif, Jean-Pierre; Levy, Oren; Dubinsky, Zvy; Caroselli, Erik; Prada, Fiorella; Goffredo, Stefano; Di Giosia, Matteo; Reggi, Michela; Falini, Giuseppe
2015-01-01
Scleractinian corals are a major source of biogenic calcium carbonate, yet the relationship between their skeletal microstructure and mechanical properties has been scarcely studied. In this work, the skeletons of two coral species: solitary Balanophyllia europaea and colonial Stylophora pistillata, were investigated by nanoindentation. The hardness HIT and Young's modulus EIT were determined from the analysis of several load–depth data on two perpendicular sections of the skeletons: longitudinal (parallel to the main growth axis) and transverse. Within the experimental and statistical uncertainty, the average values of the mechanical parameters are independent on the section's orientation. The hydration state of the skeletons did not affect the mechanical properties. The measured values, EIT in the 76–77 GPa range, and HIT in the 4.9–5.1 GPa range, are close to the ones expected for polycrystalline pure aragonite. Notably, a small difference in HIT is observed between the species. Different from corals, single-crystal aragonite and the nacreous layer of the seashell Atrina rigida exhibit clearly orientation-dependent mechanical properties. The homogeneous and isotropic mechanical behaviour of the coral skeletons at the microscale is correlated with the microstructure, observed by electron microscopy and atomic force microscopy, and with the X-ray diffraction patterns of the longitudinal and transverse sections. PMID:25977958
Crossover from isotropic to directed percolation
NASA Astrophysics Data System (ADS)
Zhou, Zongzheng; Yang, Ji; Ziff, Robert M.; Deng, Youjin
2012-08-01
We generalize the directed percolation (DP) model by relaxing the strict directionality of DP such that propagation can occur in either direction but with anisotropic probabilities. We denote the probabilities as p↓=ppd and p↑=p(1-pd), with p representing the average occupation probability and pd controlling the anisotropy. The Leath-Alexandrowicz method is used to grow a cluster from an active seed site. We call this model with two main growth directions biased directed percolation (BDP). Standard isotropic percolation (IP) and DP are the two limiting cases of the BDP model, corresponding to pd=1/2 and pd=0,1 respectively. In this work, besides IP and DP, we also consider the 1/2
Crossover from isotropic to directed percolation.
Zhou, Zongzheng; Yang, Ji; Ziff, Robert M; Deng, Youjin
2012-08-01
We generalize the directed percolation (DP) model by relaxing the strict directionality of DP such that propagation can occur in either direction but with anisotropic probabilities. We denote the probabilities as p(↓) = pp(d) and p(↑) = p(1-p(d)), with p representing the average occupation probability and p(d) controlling the anisotropy. The Leath-Alexandrowicz method is used to grow a cluster from an active seed site. We call this model with two main growth directions biased directed percolation (BDP). Standard isotropic percolation (IP) and DP are the two limiting cases of the BDP model, corresponding to p(d) =1/2 and p(d) = 0,1 respectively. In this work, besides IP and DP, we also consider the 1/2 < p(d) <1 region. Extensive Monte Carlo simulations are carried out on the square and the simple-cubic lattices, and the numerical data are analyzed by finite-size scaling. We locate the percolation thresholds of the BDP model for p(d) = 0.6 and 0.8, and determine various critical exponents. These exponents are found to be consistent with those for standard DP. We also determine the renormalization exponent associated with the asymmetric perturbation due to p(d)-1/2 ≠ 0 near IP, and confirm that such an asymmetric scaling field is relevant at IP. PMID:23005718
Constitutive modeling for isotropic materials (HOST)
NASA Technical Reports Server (NTRS)
Lindholm, Ulric S.; Chan, Kwai S.; Bodner, S. R.; Weber, R. M.; Walker, K. P.; Cassenti, B. N.
1984-01-01
The results of the first year of work on a program to validate unified constitutive models for isotropic materials utilized in high temperature regions of gas turbine engines and to demonstrate their usefulness in computing stress-strain-time-temperature histories in complex three-dimensional structural components. The unified theories combine all inelastic strain-rate components in a single term avoiding, for example, treating plasticity and creep as separate response phenomena. An extensive review of existing unified theories is given and numerical methods for integrating these stiff time-temperature-dependent constitutive equations are discussed. Two particular models, those developed by Bodner and Partom and by Walker, were selected for more detailed development and evaluation against experimental tensile, creep and cyclic strain tests on specimens of a cast nickel base alloy, B19000+Hf. Initial results comparing computed and test results for tensile and cyclic straining for temperature from ambient to 982 C and strain rates from 10(exp-7) 10(exp-3) s(exp-1) are given. Some preliminary date correlations are presented also for highly non-proportional biaxial loading which demonstrate an increase in biaxial cyclic hardening rate over uniaxial or proportional loading conditions. Initial work has begun on the implementation of both constitutive models in the MARC finite element computer code.
Constitutive modeling for isotropic materials (HOST)
NASA Technical Reports Server (NTRS)
Lindholm, U. S.; Chan, K. S.; Bodner, S. R.; Weber, R. M.; Walker, K. P.; Cassenti, B. N.
1985-01-01
This report presents the results of the second year of work on a problem which is part of the NASA HOST Program. Its goals are: (1) to develop and validate unified constitutive models for isotropic materials, and (2) to demonstrate their usefulness for structural analyses of hot section components of gas turbine engines. The unified models selected for development and evaluation are that of Bodner-Partom and Walker. For model evaluation purposes, a large constitutive data base is generated for a B1900 + Hf alloy by performing uniaxial tensile, creep, cyclic, stress relation, and thermomechanical fatigue (TMF) tests as well as biaxial (tension/torsion) tests under proportional and nonproportional loading over a wide range of strain rates and temperatures. Systematic approaches for evaluating material constants from a small subset of the data base are developed. Correlations of the uniaxial and biaxial tests data with the theories of Bodner-Partom and Walker are performed to establish the accuracy, range of applicability, and integability of the models. Both models are implemented in the MARC finite element computer code and used for TMF analyses. Benchmark notch round experiments are conducted and the results compared with finite-element analyses using the MARC code and the Walker model.
Limits to Poisson's ratio in isotropic materials
NASA Astrophysics Data System (ADS)
Mott, P. H.; Roland, C. M.
2009-10-01
A long-standing question is why Poisson’s ratio ν nearly always exceeds 0.2 for isotropic materials, whereas classical elasticity predicts ν to be between -1 to (1)/(2) . We show that the roots of quadratic relations from classical elasticity divide ν into three possible ranges: -1<ν≤0 , 0≤ν≤(1)/(5) , and (1)/(5)≤ν<(1)/(2) . Since elastic properties are unique there can be only one valid set of roots, which must be (1)/(5)≤ν<(1)/(2) for consistency with the behavior of real materials. Materials with Poisson’s ratio outside of this range are rare, and tend to be either very hard (e.g., diamond, beryllium etc.) or porous (e.g., auxetic foams); such substances have more complex behavior than can be described by classical elasticity. Thus, classical elasticity is inapplicable whenever ν<(1)/(5) , and the use of the equations from classical elasticity for such materials is inappropriate.
Isotropic thaw subsidence in undisturbed permafrost landscapes
NASA Astrophysics Data System (ADS)
Shiklomanov, Nikolay I.; Streletskiy, Dmitry A.; Little, Jonathon D.; Nelson, Frederick E.
2013-12-01
in undisturbed terrain within some regions of the Arctic reveal limited correlation between increasing air temperature and the thickness of the seasonally thawed layer above ice-rich permafrost. Here we describe landscape-scale, thaw-induced subsidence lacking the topographic contrasts associated with thermokarst terrain. A high-resolution, 11 year record of temperature and vertical movement at the ground surface from contrasting physiographic regions of northern Alaska, obtained with differential global positioning systems technology, indicates that thaw of an ice-rich layer at the top of permafrost has produced decimeter-scale subsidence extending over the entire landscapes. Without specialized observation techniques the subsidence is not apparent to observers at the surface. This "isotropic thaw subsidence" explains the apparent stability of active layer thickness records from some landscapes of northern Alaska, despite warming near-surface air temperatures. Integrated over extensive regions, it may be responsible for thawing large volumes of carbon-rich substrate and could have negative impacts on infrastructure.
Statistical turbulence theory and turbulence phenomenology
NASA Technical Reports Server (NTRS)
Herring, J. R.
1973-01-01
The application of deductive turbulence theory for validity determination of turbulence phenomenology at the level of second-order, single-point moments is considered. Particular emphasis is placed on the phenomenological formula relating the dissipation to the turbulence energy and the Rotta-type formula for the return to isotropy. Methods which deal directly with most or all the scales of motion explicitly are reviewed briefly. The statistical theory of turbulence is presented as an expansion about randomness. Two concepts are involved: (1) a modeling of the turbulence as nearly multipoint Gaussian, and (2) a simultaneous introduction of a generalized eddy viscosity operator.
Simulations of transition and turbulence on the Navier-Stokes computer
NASA Technical Reports Server (NTRS)
Krist, S. E.; Zang, T. A.
1987-01-01
The Navier-Stokes Computer (NSC) consists of multiple local memory parallel processors interconnected in a hypercube network. Efficient implementation of algorithms on the NSC thus requires the effective utilization of both the coarse and fine grain paralelism inherent in the architectural design. The basic approach to implementing an algorithm on the NSC is presented herein. The particular finite-difference algorithm considered was developed for performing transition and turbulence simulations by direct solution of the time-dependent incompressible Navier-Stokes equations. The suitability of this algorithm for performing simulations of the isotropic turbulence problem is verified from computations performed on a Cray 2. Projected timing results for the algorithm on the NSC itself are presented for both the isotropic turbulence and laminar turbulent transition problems.
Stochastic modeling of turbulent reacting flows
NASA Technical Reports Server (NTRS)
Fox, R. O.; Hill, J. C.; Gao, F.; Moser, R. D.; Rogers, M. M.
1992-01-01
Direct numerical simulations of a single-step irreversible chemical reaction with non-premixed reactants in forced isotropic turbulence at R(sub lambda) = 63, Da = 4.0, and Sc = 0.7 were made using 128 Fourier modes to obtain joint probability density functions (pdfs) and other statistical information to parameterize and test a Fokker-Planck turbulent mixing model. Preliminary results indicate that the modeled gradient stretching term for an inert scalar is independent of the initial conditions of the scalar field. The conditional pdf of scalar gradient magnitudes is found to be a function of the scalar until the reaction is largely completed. Alignment of concentration gradients with local strain rate and other features of the flow were also investigated.
Energy spectra in elasto-inertial turbulence
NASA Astrophysics Data System (ADS)
Valente, P. C.; da Silva, C. B.; Pinho, F. T.
2016-07-01
Direct numerical simulations of statistically steady homogeneous isotropic turbulence in viscoelastic fluids described by the FENE-P model are presented. Emphasis is given to large polymer relaxation times compared to the eddy turnover time, which is a regime recently termed elasto-inertial turbulence. In this regime the polymers are ineffective in dissipating kinetic energy but they play a lead role in transferring kinetic energy to the small solvent scales which turns out to be concomitant with the depletion of the usual non-linear energy cascade. However, we show that the non-linear interactions are still highly active, but they lead to no net downscale energy transfer because the forward and reversed energy cascades are nearly balanced. Finally, we show that the tendency for a steeper elasto-inertial power-law spectra is reversed for large polymer relaxation times and the spectra tend towards the usual k-5/3 functional form.
A Jet-Stirred Apparatus for Turbulent Combustion Experiments
NASA Astrophysics Data System (ADS)
Davani, Abbasali; Ronney, Paul
2015-11-01
A novel jet-stirred combustion chamber is designed to study turbulent premixed flames. In the new approach, multiple impinging turbulent jets are used to stir the mixture. It is well known that pair of counterflowing turbulent jets produces nearly a constant intensity (u') along the jet axes. In this study, different numbers of impinging jets in various configurations are used to produce isotropic turbulence intensity. FLUENT simulations have been conducted to assess the viability of the proposed chamber. In order to be able to compare different configurations, three different non dimensional indices are introduces. Mean flow index; Homogeneity index, and Isotropicity index. Using these indices one can compare various chambers including conventional Fan-stirred Reactors. Results show that a concentric inlet/outlet chamber (CAIO) with 8 inlets and 8 outlets with inlet velocity of 20 m/s and initial intensity of 15% produces near zero mean flow and 2.5 m/s turbulence intensity which is much more higher than reported values for Fan-stirred chamber. This research was sponsored by National Science Foundation.
The analysis and modeling of dilatational terms in compressible turbulence
NASA Technical Reports Server (NTRS)
Sarkar, S.; Erlebacher, G.; Hussaini, M. Y.; Kreiss, H. O.
1989-01-01
It is shown that the dilatational terms that need to be modeled in compressible turbulence include not only the pressure-dilatation term but also another term - the compressible dissipation. The nature of these dilatational terms in homogeneous turbulence is explored by asymptotic analysis of the compressible Navier-Stokes equations. A non-dimensional parameter which characterizes some compressible effects in moderate Mach number, homogeneous turbulence is identified. Direct numerical simulations (DNS) of isotropic, compressible turbulence are performed, and their results are found to be in agreement with the theoretical analysis. A model for the compressible dissipation is proposed; the model is based on the asymptotic analysis and the direct numerical simulations. This model is calibrated with reference to the DNS results regarding the influence of compressibility on the decay rate of isotropic turbulence. An application of the proposed model to the compressible mixing layer has shown that the model is able to predict the dramatically reduced growth rate of the compressible mixing layer.
MEASUREMENTS AND COMPUTATIONS OF FUEL DROPLET TRANSPORT IN TURBULENT FLOWS
Joseph Katz and Omar Knio
2007-01-10
The objective of this project is to study the dynamics of fuel droplets in turbulent water flows. The results are essential for development of models capable of predicting the dispersion of slightly light/heavy droplets in isotropic turbulence. Since we presently do not have any experimental data on turbulent diffusion of droplets, existing mixing models have no physical foundations. Such fundamental knowledge is essential for understanding/modeling the environmental problems associated with water-fuel mixing, and/or industrial processes involving mixing of immiscible fluids. The project has had experimental and numerical components: 1. The experimental part of the project has had two components. The first involves measurements of the lift and drag forces acting on a droplet being entrained by a vortex. The experiments and data analysis associated with this phase are still in progress, and the facility, constructed specifically for this project is described in Section 3. In the second and main part, measurements of fuel droplet dispersion rates have been performed in a special facility with controlled isotropic turbulence. As discussed in detail in Section 2, quantifying and modeling the of droplet dispersion rate requires measurements of their three dimensional trajectories in turbulent flows. To obtain the required data, we have introduced a new technique - high-speed, digital Holographic Particle Image Velocimetry (HPIV). The technique, experimental setup and results are presented in Section 2. Further information is available in Gopalan et al. (2005, 2006). 2. The objectives of the numerical part are: (1) to develop a computational code that combines DNS of isotropic turbulence with Lagrangian tracking of particles based on integration of a dynamical equation of motion that accounts for pressure, added mass, lift and drag forces, (2) to perform extensive computations of both buoyant (bubbles) and slightly buoyant (droplets) particles in turbulence conditions
Determining the alpha dynamo parameter in incompressible homogeneous magnetohydrodynamic turbulence
NASA Technical Reports Server (NTRS)
Matthaeus, W. H.; Goldstein, M. L.; Lantz, S. R.
1983-01-01
Alpha, an important parameter in dynamo theory, is proportional to either the kinetic, current, magnetic, or velocity helicity of the fluctuating magnetic field and fluctuating velocity field. The particular helicity to which alpha is proportional depends on the assumptions used in deriving the first order smoothed equations that describe the alpha effect. In two cases, when alpha is proportional to either the magnetic helicity or velocity helicity, alpha is determined experimentally from two point measurements of the fluctuating fields in incompressible, homogeneous turbulence having arbitrary symmetry. For the other two possibilities, alpha is determined if the turbulence is isotropic.
Grid superfluid turbulence and intermittency at very low temperature
NASA Astrophysics Data System (ADS)
Krstulovic, Giorgio
2016-06-01
Low-temperature grid-generated turbulence is investigated by using numerical simulations of the Gross-Pitaevskii equation. The statistics of regularized velocity increments are studied. Increments of the incompressible velocity are found to be skewed for turbulent states. Results are later confronted with the (quasi) homogeneous and isotropic Taylor-Green flow, revealing the universality of the statistics. For this flow, the statistics are found to be intermittent and a Kolmogorov constant close to the one of classical fluid is found for the second-order structure function.
Turbulent Sediment Suspension and Induced Ripple Dynamics Absent Mean Shear
NASA Astrophysics Data System (ADS)
Johnson, B. A.; Cowen, E.
2014-12-01
The uprush and backwash phases in the swash zone, the region of the beach that is alternately covered and uncovered by wave run-up, are fundamentally different events. Backwash is dominated by a growing boundary layer where the turbulence is set by the bed shear stress. In this phase traditional boundary layer turbulence models and Shields-type critical stress pickup functions work well. However, the uprush phase, while often viewed in the context of traditional boundary layer turbulence models, has little in common with the backwash phase. During uprush, the entire water column is turbulent, as it rapidly advects well-stirred highly turbulent flow generated offshore from breaking waves or collapsing bores. Turbulence levels in the uprush are several times higher than turbulent boundary layer theory would predict and hence the use of a boundary layer model to predict turbulence levels during uprush grossly under predicts the turbulence and subsequent sediment suspension in the swash zone. To study the importance of this advected turbulence to sediment suspension we conduct experiments in a water tank designed to generate horizontally homogeneous isotropic turbulence absent mean shear using randomly actuated synthetic jet arrays suspended above both a solid glass plate and a narrowly graded sediment bed. Using jet arrays with different jet spacings allows the generation of high Reynolds number turbulence with variable integral length scales, which we hypothesize control the characteristic length scales in the induced ripple field. Particle image velocimetry and acoustic Doppler velocimetry measurements are used to characterize the near-bed flow and this unique turbulent boundary layer. Metrics include the mean flow and turbulence intensities and stresses, temporal and spatial spectra, dissipation of turbulent kinetic energy, and integral length scales of the turbulence. We leverage our unique dataset to compare the flows over impermeable fixed and permeable mobile
The dynamics of variable-density turbulence
Sandoval, D.L.
1995-11-01
The dynamics of variable-density turbulent fluids are studied by direct numerical simulation. The flow is incompressible so that acoustic waves are decoupled from the problem, and implying that density is not a thermodynamic variable. Changes in density occur due to molecular mixing. The velocity field is, in general, divergent. A pseudo-spectral numerical technique is used to solve the equations of motion. Three-dimensional simulations are performed using a grid size of 128{sup 3} grid points. Two types of problems are studied: (1) the decay of isotropic, variable-density turbulence, and (2) buoyancy-generated turbulence in a fluid with large density fluctuations (such that the Boussinesq approximation is not valid). In the case of isotropic, variable-density turbulence, the overall statistical decay behavior, for the cases studied, is relatively unaffected by the presence of density variations when the initial density and velocity fields are statistically independent. The results for this case are in quantitative agreement with previous numerical and laboratory results. In this case, the initial density field has a bimodal probability density function (pdf) which evolves in time towards a Gaussian distribution. The pdf of the density field is symmetric about its mean value throughout its evolution. If the initial velocity and density fields are statistically dependent, however, the decay process is significantly affected by the density fluctuations. For this case, the pdf of the density becomes asymmetric about its mean value during the early stages of its evolution. It is argued that these asymmetries in the pdf of the density field are due to different entrainment rates, into the mixing region, that favor the high speed fluid.
Application of a new K-tau model to near wall turbulent flows
NASA Technical Reports Server (NTRS)
Thangam, S.; Abid, R.; Speziale, Charles G.
1991-01-01
A recently developed K-tau model for near wall turbulent flows is applied to two severe test cases. The turbulent flows considered include the incompressible flat plate boundary layer with the adverse pressure gradients and incompressible flow past a backward facing step. Calculations are performed for this two-equation model using an anisotropic as well as isotropic eddy-viscosity. The model predictions are shown to compare quite favorably with experimental data.
Energy Transfer in Rotating Turbulence
NASA Technical Reports Server (NTRS)
Cambon, Claude; Mansour, Nagi N.; Godeferd, Fabien S.; Rai, Man Mohan (Technical Monitor)
1995-01-01
The influence or rotation on the spectral energy transfer of homogeneous turbulence is investigated in this paper. Given the fact that linear dynamics, e.g. the inertial waves regime tackled in an RDT (Rapid Distortion Theory) fashion, cannot Affect st homogeneous isotropic turbulent flow, the study of nonlinear dynamics is of prime importance in the case of rotating flows. Previous theoretical (including both weakly nonlinear and EDQNM theories), experimental and DNS (Direct Numerical Simulation) results are gathered here and compared in order to give a self-consistent picture of the nonlinear effects of rotation on tile turbulence. The inhibition of the energy cascade, which is linked to a reduction of the dissipation rate, is shown to be related to a damping due to rotation of the energy transfer. A model for this effect is quantified by a model equation for the derivative-skewness factor, which only involves a micro-Rossby number Ro(sup omega) = omega'/(2(OMEGA))-ratio of rms vorticity and background vorticity as the relevant rotation parameter, in accordance with DNS and EDQNM results fit addition, anisotropy is shown also to develop through nonlinear interactions modified by rotation, in an intermediate range of Rossby numbers (Ro(omega) = (omega)' and Ro(omega)w greater than 1), which is characterized by a marco-Rossby number Ro(sup L) less than 1 and Ro(omega) greater than 1 which is characterized by a macro-Rossby number based on an integral lengthscale L and the micro-Rossby number previously defined. This anisotropy is mainly an angular drain of spectral energy which tends to concentrate energy in tile wave-plane normal to the rotation axis, which is exactly both the slow and the two-dimensional manifold. In Addition, a polarization of the energy distribution in this slow 2D manifold enhances horizontal (normal to the rotation axis) velocity components, and underlies the anisotropic structure of the integral lengthscales. Finally is demonstrated the
VERTICAL STRUCTURE OF STATIONARY ACCRETION DISKS WITH A LARGE-SCALE MAGNETIC FIELD
Bisnovatyi-Kogan, G. S.; Lovelace, R. V. E. E-mail: RVL1@cornell.edu
2012-05-10
In earlier works we pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because the hydrodynamic and/or magnetorotational instabilities are suppressed high in the disk where the magnetic and radiation pressures are larger than the plasma thermal pressure. Here, we calculate the vertical profiles of the stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity and the fact that the turbulence vanishes at the surface of the disk. Also, here we require that the radial accretion speed be zero at the disk's surface and we assume that the ratio of the turbulent viscosity to the turbulent magnetic diffusivity is of order unity. Thus, at the disk's surface there are three boundary conditions. As a result, for a fixed dimensionless viscosity {alpha}-value, we find that there is a definite relation between the ratio R of the accretion power going into magnetic disk winds to the viscous power dissipation and the midplane plasma-{beta}, which is the ratio of the plasma to magnetic pressure in the disk. For a specific disk model with R of order unity we find that the critical value required for a stationary solution is {beta}{sub c} Almost-Equal-To 2.4r/({alpha}h), where h is the disk's half thickness. For weaker magnetic fields, {beta} > {beta}{sub c}, we argue that the poloidal field will advect outward while for {beta} < {beta}{sub c} it will advect inward. Alternatively, if the disk wind is negligible (R<<1), there are stationary solutions with {beta} >> {beta}{sub c}.
Coupling Turbulence in Hybrid LES-RANS Techniques
NASA Technical Reports Server (NTRS)
Woodruff, Stephen L.
2011-01-01
A formulation is proposed for hybrid LES-RANS computations that permits accurate computations during resolution changes, so that resolution may be changed at will in order to employ only as much resolution in each subdomain as is required by the physics. The two components of this formulation, establishing the accuracy of a hybrid model at constant resolutions throughout the RANS-to-LES range and maintaining that accuracy when resolution is varied, are demonstrated for decaying, homogeneous, isotropic turbulence.
Reverse Energy Cascade in Turbulent Weakly Ionized Plasmas
NASA Technical Reports Server (NTRS)
Williams, Kyron; Appartaim, R.; Belay, K.; Johnson, J. A., III
1998-01-01
For systems far from equilibrium, the neglect of a role for viscous effects in turbulence may be generally inappropriate when the relaxation time for the molecular process approaches the local flow time (Orou et al. (1996)). Furthermore, for stationary collisional plasmas, the conventional Reynolds number is irrelevant under circumstances where the standard features of turbulence in ordinary gases are observed in the plasma (Johnson et al. (1987)). The current theoretical understanding of these turbulent phenomenon is particularly inadequate for turbulence associated with ionizing shock waves; generally speaking, thermodynamic, acoustic and pressure fluctuations are all seen as amplified across the shock wave followed by a dramatic decay (relaminarization) usually attributed to a lack of importance of viscosity in the turbulent regions. This decay would be accelerated when the flow speed is also reduced due to the importance usually given to the conventional Reynolds number (which is directly proportional to velocity) as a quality of turbulence index. However, evidence supporting this consensus is lacking. By contrast, recent evidence of vanishing triple correlations form De Silva et al. (1996) provides strong support for early theoretical speculation of inherently molecular effects in macroscopic turbulence in Tsuge (1974). This specifically suggests that the role of compressive effects ordinarily associated with the shock wave could be significantly muted by the existence of a strongly turbulent local environment. There is also more recent theoretical speculation (Frisch et al. (1984)) of an inherently and previously unsuspected non-dissipative nature to turbulence, with energy conservation being nurtured by reverse energy cascades in the turbulent fluctuation spectra. Furthermore, the role which might be played by fluctuations on quantum mechanical phenomena and variations in molecular parameters is completely unknown, especially of the sort which might be found
Adaptive waveguide bends with homogeneous, nonmagnetic, and isotropic materials.
Han, Tiancheng; Qiu, Cheng-Wei; Tang, Xiaohong
2011-01-15
We propose a method for adaptive waveguide bends using homogeneous, nonmagnetic, and isotropic materials, which simplifies the parameters of the bends to the utmost extent. The proposed bend has an adaptive and compact shape because of all the flat boundaries. The nonmagnetic property is realized by selecting OB'/OC = 0.5. Only two nonmagnetic isotropic dielectrics are needed throughout, and the transmission is not sensitive to nonmagnetic isotropic dielectrics. Results validate and illustrate these functionalities, which make the bend much easier to fabricate and apply, owing to its simple parameters, compact shape, and versatility in connecting different waveguides. PMID:21263493
A phenomenological treatment of rotating turbulence
NASA Technical Reports Server (NTRS)
Zhou, YE
1995-01-01
The strong similarity between the magnetohydrodynamic (MHD) turbulence and initially isotropic turbulence subject to rotation is noted. We then apply the MHD phenomenologies of Kraichnan and Matthaeus & Zhou to rotating turbulence. When the turbulence is subject to a strong rotation, the energy spectrum is found to scale as E(k) = C(sub Omega)(Omega(sub epsilon))(sup 1/2)k(sup -2), where Omega is the rotation rate, k is the wavenumber, and epsilon is the dissipation rate. This spectral form is consistent with a recent letter by Zeman. However, here the constant C(sub Omega) is found to be related to the Kolmogorov constant and is estimated in the range 1.22 - 1.87 for the typical values of the latter constant. A 'rule' that relates spectral transfer times to the eddy turnover time and the time scale for decay of the triple correlations is deduced. A hypothesis for the triple correlation decay rate leads to the spectral law which varies between the '-5/3' (without rotation) and '-2' laws (with strong rotation). For intermediate rotation rates, the spectrum varies according to the value of a dimensionless parameter that measures the strength of the rotation wavenumber k(sub Omega) = (Omega(sup 3)/epsiolon)(sup 1/2) relative to the wavenumber k. An eddy viscosity is derived with an explicit dependence on the rotation rate.
A Monte Carlo simulation technique for low-altitude, wind-shear turbulence
NASA Technical Reports Server (NTRS)
Bowles, Roland L.; Laituri, Tony R.; Trevino, George
1990-01-01
A case is made for including anisotropy in a Monte Carlo flight simulation scheme of low-altitude wind-shear turbulence by means of power spectral density. This study attempts to eliminate all flight simulation-induced deficiencies in the basic turbulence model. A full-scale low-altitude wind-shear turbulence simulation scheme is proposed with particular emphasis on low cost and practicality for near-ground flight. The power spectral density statistic is used to highlight the need for realistic estimates of energy transfer associated with low-altitude wind-shear turbulence. The simulation of a particular anisotropic turbulence model is shown to be a relatively simple extension from that of traditional isotropic (Dryden) turbulence.
On the dynamics of magnetorotational turbulent stresses
NASA Astrophysics Data System (ADS)
Ogilvie, G. I.
2003-04-01
The turbulent stresses that lead to angular momentum transport in accretion discs have often been treated as resulting from an isotropic effective viscosity, related to the pressure through the alpha parametrization of Shakura and Sunyaev. This simple approach may be adequate for the simplest aspects of accretion disc theory, and was necessitated historically by an incomplete understanding of the origin of the turbulence. More recently, Balbus and Hawley have shown that the magnetorotational instability provides a robust mechanism of generating turbulent Reynolds and Maxwell stresses in sufficiently ionized discs. The alpha viscosity model fails to describe numerous aspects of this process. The present paper introduces a new analytical model that aims to represent more faithfully the dynamics of magnetorotational turbulent stresses and bridge the gap between analytical studies and numerical simulations. Covariant evolutionary equations for the mean Reynolds and Maxwell tensors are presented, which correctly include the linear interaction with the mean flow. Non-linear and dissipative effects, in the absence of an imposed magnetic flux and in the limit of large Reynolds number and magnetic Reynolds number, are modelled through five non-linear terms that represent known physical processes and are strongly constrained by symmetry properties and dimensional considerations. The resulting model explains the development of statistically steady, anisotropic turbulent stresses in the shearing sheet, a local representation of a differentially rotating disc, in agreement with numerical simulations. It also predicts that purely hydrodynamic turbulence is not sustained in a flow that adequately satisfies Rayleigh's stability criterion. The model is usually formally hyperbolic and therefore `causal', and guarantees the realizability of the stress tensors. It should be particularly useful in understanding the dynamics of warped, eccentric and tidally distorted discs, non
Transport of magnetic turbulence in supernova remnants
NASA Astrophysics Data System (ADS)
Brose, R.; Telezhinsky, I.; Pohl, M.
2016-08-01
Context. Supernova remnants are known as sources of Galactic cosmic rays for their nonthermal emission of radio waves, X-rays, and gamma rays. However, the observed soft broken power-law spectra are hard to reproduce within standard acceleration theory based on the assumption of Bohm diffusion and steady-state calculations. Aims: We point out that a time-dependent treatment of the acceleration process together with a self-consistent treatment of the scattering turbulence amplification is necessary. Methods: We numerically solve the coupled system of transport equations for cosmic rays and isotropic Alfvénic turbulence. The equations are coupled through the growth rate of turbulence determined by the cosmic-ray gradient and the spatial diffusion coefficient of cosmic rays determined by the energy density of the turbulence. The system is solved on a comoving expanding grid extending upstream for dozens of shock radii, allowing for the self-consistent study of cosmic-ray diffusion in the vicinity of their acceleration site. The transport equation for cosmic rays is solved in a test-particle approach. Results: We demonstrate that the system is typically not in a steady state. In fact, even after several thousand years of evolution, no equilibrium situation is reached. The resulting time-dependent particle spectra strongly differ from those derived assuming a steady state and Bohm diffusion. Our results indicate that proper accounting for the evolution of the scattering turbulence and hence the particle diffusion coefficient is crucial for the formation of the observed soft spectra. In any case, the need to continuously develop magnetic turbulence upstream of the shock introduces nonlinearity in addition to that imposed by cosmic-ray feedback.
NASA Technical Reports Server (NTRS)
Zhang, W.; Diamond, P. H.; Vishniac, E. T.
1994-01-01
Recently, the magnetic shearing instability (MSI) has been proposed as a dynamical mechanism for angular momentum transport in accretion disks (Balbus & Hawley 1991; Hawley & Balbus 1991). In this paper, the nonlinear dynamics of MSI modes in the presence of a vertical magnetic field B(sub 0) is discussed. In particular, the saturation levels of the fluctuating fields, the angular momentum flux, and the energy dissipation mechanism, are examined in detail. It is shown that MSI induces strong magnetohydrodynamic (MHD) turbulence in a range of wavenumbers 1/H is less than K is less than or equal to Omega/V(sub A)(sub 0)), where H is the thickness, Omega is the rotation frequency of the disk, and V(sub A)(sub 0) is the Alfven velocity. Despite the fact that the linear growth rate of MSI is maximal at small-scale (i.e., k is approximately Omega/V(sub A(sub 0)), angular momentum transport due to MSI turbulence is dominated by the magnetic Reynolds stress driven by large-scale modes (k is approximately 1/H). It is shown that the amplitude of low k(sub r) MSI eddies is limited primarily by subscale shear flow instability. Thus, dominant MSI cells are quasi-isotropic. In a stationary state, the effective Shakura-Sunyaev 'alpha' value is predicted to be of order V(sub A)(sub 0)0/C(sub s). In addition, the veritcal magnetic-field-induced MSI cells convert vertical magnetic field B(sub 0) into azimuthal magnetic field B(sub theta) in the disk. The generation of azimuthal magnetic field in turn introduces new physical processes, such as dynamo activity and azimuthal MSI turbulence. We conclude that it is not possible to decouple vertical MSI saturation from azimuthal MSI evolution. Low-frequency MSI cells are shown to co-exist with high-frequency radial buoyancy or internal waves. We show that modulational interaction between waves on these two frequency ranges is usually weak in the case when mean magnetic field is vertical. Thus, MSI and internal wave dynamics must be
Constitutive modeling for isotropic materials (HOST)
NASA Technical Reports Server (NTRS)
Chan, Kwai S.; Lindholm, Ulric S.; Bodner, S. R.; Hill, Jeff T.; Weber, R. M.; Meyer, T. G.
1986-01-01
The results of the third year of work on a program which is part of the NASA Hot Section Technology program (HOST) are presented. The goals of this program are: (1) the development of unified constitutive models for rate dependent isotropic materials; and (2) the demonstration of the use of unified models in structural analyses of hot section components of gas turbine engines. The unified models selected for development and evaluation are those of Bodner-Partom and of Walker. A test procedure was developed for assisting the generation of a data base for the Bodner-Partom model using a relatively small number of specimens. This test procedure involved performing a tensile test at a temperature of interest that involves a succession of strain-rate changes. The results for B1900+Hf indicate that material constants related to hardening and thermal recovery can be obtained on the basis of such a procedure. Strain aging, thermal recovery, and unexpected material variations, however, preluded an accurate determination of the strain-rate sensitivity parameter is this exercise. The effects of casting grain size on the constitutive behavior of B1900+Hf were studied and no particular grain size effect was observed. A systematic procedure was also developed for determining the material constants in the Bodner-Partom model. Both the new test procedure and the method for determining material constants were applied to the alternate material, Mar-M247 . Test data including tensile, creep, cyclic and nonproportional biaxial (tension/torsion) loading were collected. Good correlations were obtained between the Bodner-Partom model and experiments. A literature survey was conducted to assess the effects of thermal history on the constitutive behavior of metals. Thermal history effects are expected to be present at temperature regimes where strain aging and change of microstructure are important. Possible modifications to the Bodner-Partom model to account for these effects are outlined
Inverse Energy Cascades in Rotating Turbulence
NASA Astrophysics Data System (ADS)
Rosenberg, D. L.; Marino, R.; Mininni, P.; Pouquet, A.
2013-12-01
We present the results of direct numerical simulations (DNS) of rapidly rotating turbulent flows on grids of 20483 grid points that are forced at intermediate scales. Injection of energy at such scales at small Rossby numbers (~0.04) leads to a direct cascade toward small scales and an inverse cascade toward large scales. These results essentially validate those obtained using large eddy simulation (LES) (Sen et al., PRE 86:036319 (2012)): for a (helical) forcing that injects energy largely in 2D modes, the large scale energy spectrum scales as kperp-5/3, consistent with Kolmogorov-Kraichnan-Batchelor-Leith phenomenology; for a nonhelical isotropic forcing, the large scale energy spectrum scales as kperp-3. The (helical) anisotropic forcing DNS solution, like that of the LES models, shows a k-1 isotropic energy spectrum, which Sen et al. attribute to a large scale shear. The higher resolution of the DNS runs allows us to carry out probability distribution and conditional analyses that show that this interpretation may, in fact, be consistent with wall-bounded turbulent shear flow.
DNS and LIA analysis of the shock turbulence interaction
NASA Astrophysics Data System (ADS)
Livescu, Daniel; Ryu, Jaiyoung
2014-11-01
The interaction between isotropic turbulence and a normal shock wave is studied using Direct Numerical Simulations (DNS), with all flow scales (including the shock width) accurately solved, and the Linear Interaction Analysis (LIA). The turbulence quantities from DNS converge to the LIA solutions as the turbulent Mach number, Mt, becomes small, even at low upstream Reynolds numbers. This reconciles a long time open question about the role of LIA and establishes it as a reliable prediction tool for turbulence-shock interaction problems when there is a significant separation between the shock width and turbulence scales and Mt is low, which is encountered in many practical applications. The final LIA formulas are extended to investigate detailed turbulence physics. The extended LIA relations are used to show consistency with the DNS results and study the interaction at high Ms, where the resolution requirements make DNS studies unfeasible. The results show that the shock wave significantly changes the topology of the turbulent structures, with a symmetrization of the third invariant of the velocity gradient tensor and (Ms mediated) of the PDF of the longitudinal velocity derivatives, and an Ms dependent increase in the correlation between strain and rotation.
Inlet free-stream turbulence effects on diffuser performance
NASA Technical Reports Server (NTRS)
Hoffman, J. A.; Gonzales, G.
1983-01-01
The performance of a subsonic two dimensional diffuser was experimentally evaluated as a function of inlet free-stream turbulence parameters. Anisotropic inlet free-stream turbulence with the eddy axis perpendicular to the flow and parallel to the diverging walls of the diffuser appears to be more effective at transmitting energy to the diverging walls of the diffuser, thereby improving diffuser performance, as compared to isotropic turbulence or anisotropic turbulence with the eddy axis perpendicular to the diverging walls of the diffuser. The pressure recovery of the diffuser was found to be strongly dependent upon the inlet free-stream total turbulence intensity, was independent of eddy size for large eddy dimensions, and was dependent upon eddy size for small eddy dimensions. The improvement in the diffuser's static pressure recovery coefficient at a total included divergence angle of 20 deg, compared to the low inlet turbulence case, was found to be as much as 21 times larger than the pressure loss across the turbulence generators.
Spectral analysis of structure functions and their scaling exponents in forced isotropic turbulence
NASA Astrophysics Data System (ADS)
Linkmann, Moritz; McComb, W. David; Yoffe, Samuel; Berera, Arjun
2014-11-01
The pseudospectral method, in conjunction with a new technique for obtaining scaling exponents ζn from the structure functions Sn (r) , is presented as an alternative to the extended self-similarity (ESS) method and the use of generalized structure functions. We propose plotting the ratio | Sn (r) /S3 (r) | against the separation r in accordance with a standard technique for analysing experimental data. This method differs from the ESS technique, which plots the generalized structure functions Gn (r) against G3 (r) , where G3 (r) ~ r . Using our method for the particular case of S2 (r) we obtain the new result that the exponent ζ2 decreases as the Taylor-Reynolds number increases, with ζ2 --> 0 . 679 +/- 0 . 013 as Rλ --> ∞ . This supports the idea of finite-viscosity corrections to the K41 prediction for S2, and is the opposite of the result obtained by ESS. The pseudospectral method permits the forcing to be taken into account exactly through the calculation of the energy input in real space from the work spectrum of the stirring forces. The combination of the viscous and the forcing corrections as calculated by the pseudospectral method is shown to account for the deviation of S3 from Kolmogorov's ``four-fifths''-law at all scales. This work has made use of the resources provided by the UK supercomputing service HECToR, made available through the Edinburgh Compute and Data Facility (ECDF). A. B. is supported by STFC, S. R. Y. and M. F. L. are funded by EPSRC.
Spectral analysis of structure functions and their scaling exponents in forced isotropic turbulence.
McComb, W D; Yoffe, S R; Linkmann, M F; Berera, A
2014-11-01
The pseudospectral method, in conjunction with a technique for obtaining scaling exponents ζ_{n} from the structure functions S_{n}(r), is presented as an alternative to the extended self-similarity (ESS) method and the use of generalized structure functions. We propose plotting the ratio |S_{n}(r)/S_{3}(r)| against the separation r in accordance with a standard technique for analyzing experimental data. This method differs from the ESS technique, which plots S_{n}(r) against S_{3}(r), with the assumption S_{3}(r)∼r. Using our method for the particular case of S_{2}(r) we obtain the result that the exponent ζ_{2} decreases as the Taylor-Reynolds number increases, with ζ_{2}→0.679±0.013 as R_{λ}→∞. This supports the idea of finite-viscosity corrections to the K41 prediction for S_{2}, and is the opposite of the result obtained by ESS. The pseudospectral method also permits the forcing to be taken into account exactly through the calculation of the energy input in real space from the work spectrum of the stirring forces. PMID:25493884
Infrared properties of the energy spectrum in freely decaying isotropic turbulence.
McComb, W D
2016-01-01
The low wave number expansion of the energy spectrum takes the well known form E(k,t)=E_{2}(t)k^{2}+E_{4}(t)k^{4}+⋯, where the coefficients are weighted integrals against the correlation function C(r,t). We show that expressing E(k,t) in terms of the longitudinal correlation function f(r,t) immediately yields E_{2}(t)=0 by cancellation. We verify that the same result is obtained using the correlation function C(r,t), provided only that f(r,t) falls off faster than r^{-3} at large values of r. As power-law forms are widely studied for the purpose of establishing bounds, we consider the family of model correlations f(r,t)=α_{n}(t)r^{-n}, for positive integer n, at large values of the separation r. We find that for the special case n=3, the relationship connecting f(r,t) and C(r,t) becomes indeterminate, and (exceptionally) E_{2}≠0, but that this solution is unphysical in that the viscous term in the Kármán-Howarth equation vanishes. Lastly, we show that E_{4}(t) is independent of time, without needing to assume the exponential decrease of correlation functions at large distances. PMID:26871151
Spectral approach to finite Reynolds number effects on Kolmogorov's 4/5 law in isotropic turbulence
NASA Astrophysics Data System (ADS)
Tchoufag, J.; Sagaut, P.; Cambon, C.
2012-01-01
The Kolmogorov's 4/5 law is often considered as the sole exact relationship of inertial range statistics. Its asymptotic character, however, has been evidenced, investigating the finite Reynolds number (FRN) effect for the third-order structure function S3(r) (e.g., for longitudinal velocity increments with r separation length) using variants of the Kármán-Howarth equation in physical space. Similar semi-empirical fits were proposed for the maximum of the normalized structure function, C3 = -maxrS3(r)/(ɛr), expressing C3 - 4/5 as a power law of the Taylor-based Reynolds number. One of the most complete studies in this domain is by Antonia and Burratini [J. Fluid Mech. 550, 175 (2006)]. Considering that these studies are based on a model for the unsteady second-order structure function S2(r,t), with no explicit model for the third-order structure function itself, we propose to revisit the FRN effect by a spectral approach, in the line of Qian [Phys. Rev. E 55, 337 (1997), Phys. Rev. E 60, 3409 (1999)]. The spectral transfer term T(k,t), from which S3(r,t) is derived by an exact quadrature, is directly calculated by solving the Lin equation for the energy spectrum E(k,t), closed by a standard triadic (or three-point) theory, here Eddy Damped Quasi Normal Markovian. We show that the best spectral approach to the FRN effect is found by separately investigating the negative (largest scales) and positive (smaller scales) bumps of the transfer term, and not only by looking at the maximum of the spectral flux or maxk ∫k∞T(p ,t)dp→ɛ. In the forced case, previous results are well reproduced, with Reynolds numbers as high as Reλ = 5 000 to nearly recover the 4/5 value. In the free decay case, the general trend is recovered as well, with an even higher value of Reλ = 50 000, but the EDQNM plots are systematically below those in Antonia and Burattini [J. Fluid Mech. 550, 175 (2006)]. This is explained by the sensitivity to initial data for E(k) in solving the Lin equation at moderate Reynolds numbers. Accordingly, an ad hoc initialization yields results consistent with the experimental spectrum measurements of Comte-Bellot and Corrsin [J. Fluid Mech. 48(2), 273 (1971)], from which S3(r) are recalculated. Present results show that the dispersion observed in existing data at low Reynolds number may be due to sensitivity to initial spectrum shape, a feature of the flow which is not under control in most of laboratory experiments.
Pattern formation and mass transfer under stationary solutal Marangoni instability.
Schwarzenberger, Karin; Köllner, Thomas; Linde, Hartmut; Boeck, Thomas; Odenbach, Stefan; Eckert, Kerstin
2014-04-01
According to the seminal theory by Sternling and Scriven, solutal Marangoni convection during mass transfer of surface-active solutes may occur as either oscillatory or stationary instability. With strong support of Manuel G. Velarde, a combined initiative of experimental works, in particular to mention those of Linde, Wierschem and coworkers, and theory has enabled a classification of dominant wave types of the oscillatory mode and their interactions. In this way a rather comprehensive understanding of the nonlinear evolution of the oscillatory instability could be achieved. A comparably advanced state-of-the-art with respect to the stationary counterpart seemed to be out of reach a short time ago. Recent developments on both the numerical and experimental side, in combination with assessing an extensive number of older experiments, now allow one to draw a more unified picture. By reviewing these works, we show that three main building blocks exist during the nonlinear evolution: roll cells, relaxation oscillations and relaxation oscillations waves. What is frequently called interfacial turbulence results from the interaction between these partly coexisting basic patterns which may additionally occur in different hierarchy levels. The second focus of this review lies on the practical importance of such convection patterns concerning their influence on mass transfer characteristics. Particular attention is paid here to the interaction between Marangoni and buoyancy effects which frequently complicates the pattern formation even more. To shed more light on these dependencies, new simulations regarding the limiting case of stabilizing density stratification and vanishing buoyancy are incorporated. PMID:24456800
Ohira, Yutaka
2013-04-10
We consider particle acceleration by large-scale incompressible turbulence with a length scale larger than the particle mean free path. We derive an ensemble-averaged transport equation of energetic charged particles from an extended transport equation that contains the shear acceleration. The ensemble-averaged transport equation describes particle acceleration by incompressible turbulence (turbulent shear acceleration). We find that for Kolmogorov turbulence, the turbulent shear acceleration becomes important on small scales. Moreover, using Monte Carlo simulations, we confirm that the ensemble-averaged transport equation describes the turbulent shear acceleration.
Geophysical and astrophysical turbulence
NASA Astrophysics Data System (ADS)
Moffatt, H. K.
Spiral structures in two-dimensional turbulence are studied and a theory (Moffatt, 1985, 1986) which regards fully three-dimensional turbulence as an agglomeration of 'random vortex sheets and coherent helical structures' is reviewed. Consideration is given to the process by which current-sheet discontinuities may appear during magnetic relaxation. Within the framework of dynamo theory, the determination of the generation coefficient and the turbulent diffusivity in mean-field electrodynamics for turbulence with helicity in the limit of a large magnetic Reynolds number is discussed. Certain features of 'chromospheric turbulence' (i.e., turbulence in the solar atmosphere outside the photosphere) are also examined.
Extracting stationary segments from non-stationary synthetic and cardiac signals
NASA Astrophysics Data System (ADS)
Rodríguez, María. G.; Ledezma, Carlos A.; Perpiñán, Gilberto; Wong, Sara; Altuve, Miguel
2015-01-01
Physiological signals are commonly the result of complex interactions between systems and organs, these interactions lead to signals that exhibit a non-stationary behaviour. For cardiac signals, non-stationary heart rate variability (HRV) may produce misinterpretations. A previous work proposed to divide a non-stationary signal into stationary segments by looking for changes in the signal's properties related to changes in the mean of the signal. In this paper, we extract stationary segments from non-stationary synthetic and cardiac signals. For synthetic signals with different signal-to-noise ratio levels, we detect the beginning and end of the stationary segments and the result is compared to the known values of the occurrence of these events. For cardiac signals, RR interval (cardiac cycle length) time series, obtained from electrocardiographic records during stress tests for two populations (diabetic patients with cardiovascular autonomic neuropathy and control subjects), were divided into stationary segments. Results on synthetic signals reveal that the non-stationary sequence is divided into more stationary segments than needed. Additionally, due to HRV reduction and exercise intolerance reported on diabetic cardiovascular autonomic neuropathy patients, non-stationary RR interval sequences from these subjects can be divided into longer stationary segments compared to the control group.
Space plasma physics: I - Stationary processes
NASA Technical Reports Server (NTRS)
Hasegawa, Akira; Sato, Tetsuya
1989-01-01
The physics of stationary processes in space plasmas is examined theoretically in an introduction intended for graduate students. The approach involves the extensive use of numerical simulations. Chapters are devoted to fundamental principles, small-amplitude waves, and the stationary solar plasma system; typical measurement data and simulation results are presented graphically.
Stationary rotating strings as relativistic particle mechanics
Ogawa, Kouji; Ishihara, Hideki; Saito, Shinya; Kozaki, Hiroshi; Nakano, Hiroyuki
2008-07-15
Stationary rotating strings can be viewed as geodesic motions in appropriate metrics in two-dimensional space. We obtain all solutions describing stationary rotating strings in flat spacetime as an application. These rotating strings have infinite length with various wiggly shapes. Averaged value of the string energy, the angular momentum, and the linear momentum along the string are discussed.
Stationary Engineering Laboratory--2. Teacher's Guide.
ERIC Educational Resources Information Center
Steingress, Frederick M.; Frost, Harold J.
The Stationary Engineering Laboratory Manual 2 Teacher's Guide was designed as an aid to the instructors of vocational-technical high school students who have received instruction in the basics of stationary engineering. The course of study was developed for students who will be operating a live plant and who will be responsible for supplying…
On the entrainment dynamics of inergodic, non-stationary flows
NASA Astrophysics Data System (ADS)
Rosi, Giuseppe; Rival, David
2014-11-01
Entrainment is typically studied through the conditional averaging along the turbulent non-turbulent interface (TNTI) of ergodic flows. However, this method is unsuitable for inergodic, non-stationary flows, as the TNTI is non-similar at different points in space and time. To understand how a TNTI's mean time dependence effects entrainment, the current study investigates the transport of irrotational fluid into a vortex forming behind an accelerating plate. The plate accelerates to a final velocity within a full-, half- and quarter-chord tow. Phase-averaged, planar, particle tracking velocimetry data is acquired and the forward finite-time Lyapunov exponent and vorticity fields are used to identify the TNTI. The TNTI is then represented by a contour, which is used to approximate the entrainment rate and investigate the transport mechanisms across the TNTI. Early results show that increasing acceleration suppresses vortex growth and entrainment. We hypothesize that shear-layer structure is integral to entrainment by altering the feeding rate of rotational fluid and the TNTI's convexity. The hypothesis is tested by altering plate-edge geometry and by varying the final chord-based Reynolds number from 5000 to 20 000. Natural Sciences and Engineering Council of Canada.
Extraction of stationary components in biosignal discrimination.
Martinez-Vargas, J D; Cardenas-Pena, D; Castellanos-Dominguez, G
2012-01-01
Biosignal recordings are widely used in the medical environment to support the evaluation and the diagnosis of pathologies. Nevertheless, the main difficulty lies in the non-stationary behavior of the biosignals, difficulting the obtention of patterns characterizing the changes in physiological or pathological states. Thus, the obtention of the stationary and non-stationary components of a biosignal is still an open issue. This work proposes a methodology to detect time-homogeneities based on time-frequency analysis with aim to extract the non-stationary behavior of the biosignal. Results show an increase in the stationarity and in the distance between classes of the reconstructions from the enhanced time-frequency representations. The stationary components extracted with the proposed approach can be used to solve biosignal classification problems. PMID:23365817
Landau damping in a turbulent setting
Plunk, G. G.
2013-03-15
To address the problem of Landau damping in kinetic turbulence, we consider the forcing of the linearized Vlasov equation by a stationary random source. It is found that the time-asymptotic density response is dominated by resonant particle interactions that are synchronized with the source. The energy consumption of this response is calculated, implying an effective damping rate, which is the main result of this paper. Evaluating several cases, it is found that the effective damping rate can differ from the Landau damping rate in magnitude and also, remarkably, in sign. A limit is demonstrated in which the density and current become phase-locked, which causes the effective damping to be negligible; this result offers a fresh perspective from which to reconsider recent observations of kinetic turbulence satisfying critical balance.
NASA Astrophysics Data System (ADS)
Liu, Yangwei; Lu, Lipeng; Fang, Le; Gao, Feng
2011-06-01
The correlation between the velocity helicity and the energy backscatter is proved in a DNS case of 256 3-grid homogeneous isotropic decaying turbulence. The helicity is then proposed to be employed to improve turbulence models and SGS models. Then Spalart-Allmaras turbulence model (SA) is modified with the helicity to take account of the energy backscatter, which is significant in the region of corner separation in compressors. By comparing the numerical results with experiments, it can be concluded that the modification for SA model with helicity can appropriately represent the energy backscatter, and greatly improves the predictive accuracy for simulating the corner separation flow in compressors.
Fluidic harvesters in free stream turbulence undergoing flow-induced vibrations or flutter
NASA Astrophysics Data System (ADS)
Gomez, Joan; Azadeh Ranjbar, Vahid; Goushcha, Oleg; Andreopoulos, Yiannis; Elvin, Niell
2015-11-01
In the present experimental work we investigated the performance of fluidic harvesters consisting of cylindrical body mounted of the tip of a flexible beam in the presence of nearly homogeneous and isotropic turbulence. Circular, semi-circular and square shapes have been tested. It was found that turbulence interferes with resonance conditions between the flow and the structure in the case of vortex induced vibrations and has absolutely no effect in flutter dominated case. As a result, turbulence increases the power output of non-linear harvesters subjected to vortex induces vibration and it has no effect in harvester under flutter conditions. Supported by NSF Grant: CBET #1033117.
Stationary Plasma Thruster Plume Emissions
NASA Technical Reports Server (NTRS)
Manzella, David H.
1994-01-01
The emission spectrum from a xenon plasma produced by a Stationary Plasma Thruster provided by the Ballistic Missile Defense Organization (BMDO) was measured. Approximately 270 individual Xe I, Xe II, and XE III transitions were identified. A total of 250 mW of radiated optical emission was estimated from measurements taken at the thruster exit plane. There was no evidence of erosion products in the emission signature. Ingestion and ionization of background gas at elevated background pressure was detected. The distribution of excited states could be described by temperatures ranging from fractions of 1 eV to 4 eV with a high degree of uncertainty due to the nonequilibrium nature of this plasma. The plasma was over 95 percent ionized at the thruster exit plane. Between 10 and 20 percent of the ions were doubly charged. Two modes of operation were identified. The intensity of plasma emission increased by a factor of two during operation in an oscillatory mode. The transfer between the two modes of operation was likely related to unidentified phenomena occurring on a time scale of minutes.
Stationary Plasma Thruster Plume Characteristics
NASA Technical Reports Server (NTRS)
Myers, Roger M.; Manzella, David H.
1994-01-01
Stationary Plasma Thrusters (SPT's) are being investigated for application to a variety of near-term missions. This paper presents the results of a preliminary study of the thruster plume characteristics which are needed to assess spacecraft integration requirements. Langmuir probes, planar probes, Faraday cups, and a retarding potential analyzer were used to measure plume properties. For the design operating voltage of 300 V the centerline electron density was found to decrease from approximately 1.8 x 10 exp 17 cubic meters at a distance of 0.3 m to 1.8 X 10 exp 14 cubic meters at a distance of 4 m from the thruster. The electron temperature over the same region was between 1.7 and 3.5 eV. Ion current density measurements showed that the plume was sharply peaked, dropping by a factor of 2.6 within 22 degrees of centerline. The ion energy 4 m from the thruster and 15 degrees off-centerline was approximately 270 V. The thruster cathode flow rate and facility pressure were found to strongly affect the plume properties. In addition to the plume measurements, the data from the various probe types were used to assess the impact of probe design criteria
Instability of stationary liquid sheets.
Ardekani, A M; Joseph, D D
2009-03-31
The rupture of a 3D stationary free liquid film under the competing effects of surface tension and van der Waals forces is studied as a linearized stability problem in a purely irrotational analysis utilizing the dissipation method. The results of the foregoing analysis are compared with a 2D long-wave approximation that has given rise to an extensive literature on the rupture problem. The irrotational and long-wave approximations are here compared with the exact 2D solution. The exact solution and the two approximate theories give the same results for infinitely long waves. The problem considered depends on two dimensionless parameters, the Hamaker number and the Ohnesorge number. The Hamaker number is a dimensionless number defined as a measure of the ratio of van der Waals forces to surface tension. The exact solution and the two approximate solutions differ by < 1% when the Hamaker number is small for all values of the Ohnesorge number. When the Ohhnesorge number is close to one, as in the case of water films separated by distance 100 A, the long-wave approximation overestimates and the potential flow approximation underestimates the exact solution by similar small amounts. The high accuracy of the dissipation method shows that the effects of vorticity are small for small to moderate Hamaker numbers. PMID:19279213
Distinguishing ichthyogenic turbulence from geophysical turbulence
NASA Astrophysics Data System (ADS)
Pujiana, Kandaga; Moum, James N.; Smyth, William D.; Warner, Sally J.
2015-05-01
Measurements of currents and turbulence beneath a geostationary ship in the equatorial Indian Ocean during a period of weak surface forcing revealed unexpectedly strong turbulence beneath the surface mixed layer. Coincident with the turbulence was a marked reduction of the current speeds registered by shipboard Doppler current profilers, and an increase in their variability. At a mooring 1 km away, measurements of turbulence and currents showed no such anomalies. Correlation with the shipboard echo sounder measurements indicate that these nighttime anomalies were associated with fish aggregations beneath the ship. The fish created turbulence by swimming against the strong zonal current in order to remain beneath the ship, and their presence affected the Doppler speed measurements. The principal characteristics of the resultant ichthyogenic turbulence are (i) low wave number roll-off of shear spectra in the inertial subrange relative to geophysical turbulence, (ii) Thorpe overturning scales that are small compared with the Ozmidov scale, and (iii) low mixing efficiency. These factors extend previous findings by Gregg and Horne (2009) to a very different biophysical regime and support the general conclusion that the biological contribution to mixing the ocean via turbulence is negligible.
On the Space-Time Structure of Sheared Turbulence
NASA Astrophysics Data System (ADS)
de Maré, Martin; Mann, Jakob
2016-04-01
We develop a model that predicts all two-point correlations in high Reynolds number turbulent flow, in both space and time. This is accomplished by combining the design philosophies behind two existing models, the Mann spectral velocity tensor, in which isotropic turbulence is distorted according to rapid distortion theory, and Kristensen's longitudinal coherence model, in which eddies are simultaneously advected by larger eddies as well as decaying. The model is compared with data from both observations and large-eddy simulations and is found to predict spatial correlations comparable to the Mann spectral tensor and temporal coherence better than any known model. Within the developed framework, Lagrangian two-point correlations in space and time are also predicted, and the predictions are compared with measurements of isotropic turbulence. The required input to the models, which are formulated as spectral velocity tensors, can be estimated from measured spectra or be derived from the rate of dissipation of turbulent kinetic energy, the friction velocity and the mean shear of the flow. The developed models can, for example, be used in wind-turbine engineering, in applications such as lidar-assisted feed forward control and wind-turbine wake modelling.
Characterization of Turbulent Flows for Turbulence Modeling
NASA Astrophysics Data System (ADS)
Reynolds, W. C.; Haire, S. L.
1998-11-01
A diagram for the characterization of turbulent flows using the invariants of the mean velocity gradient tensor is introduced. All mean flows, from irrotationally strained flows to shearing flows, to purely rotational flows, can be identified on this diagram. Different flow fields which occupy the same region on the diagram are said to be comprised of the same topological features. The current state of turbulence modeling can be identified on the diagram based on the type of mean flow fields which can be accurately computed. Regions on the diagram can be shown for which current capabilities in turbulence modeling fail to accurately resolve the turbulent structures. Relevant mean field topology is identified for future work in turbulence modeling. Using this analysis, we suggest a number of flows to be computed by DNS or LES and used as testing cases for new models.
Is 2-D turbulence relevant in the atmosphere?
NASA Astrophysics Data System (ADS)
Lovejoy, Shaun; Schertzer, Daniel
2010-05-01
Starting with (Taylor, 1935), the paradigm of isotropic (and scaling!) turbulence was developed initially for laboratory applications, but following (Kolmogorov, 1941), three dimensional isotropic turbulence was progressively applied to the atmosphere. Since the atmosphere is strongly stratified, a single wide scale range model which is both isotropic and scaling is not possible so that theorists had to immediately choose between the two symmetries: isotropy or scale invariance. Following the development of models of two dimensional isotropic turbulence ((Fjortoft, 1953), but especially (Kraichnan, 1967) and (Charney, 1971)), the mainstream choice was to first make the convenient assumption of isotropy and to drop wide range scale invariance. Starting at the end of the 1970's this "isotropy primary" (IP) paradigm has lead to a series of increasingly complex isotropic 2D/isotropic 3D models of atmospheric dynamics which continue to dominate the theoretical landscape. Justifications for IP approaches have focused almost exclusively on the horizontal statistics of the horizontal wind in both numerical models and analyses and from aircraft campaigns, especially the highly cited GASP (Nastrom and Gage, 1983), (Gage and Nastrom, 1986; Nastrom and Gage, 1985) and MOZAIC (Cho and Lindborg, 2001) experiments. Since understanding the anisotropy clearly requires comparisons between horizontal and vertical statistics/structures this focus has been unfortunate. Over the same thirty year period that 2D/3D isotropic models were being elaborated, evidence slowly accumulated in favour of the opposite theoretical choice: to drop the isotropy assumption but to retain wide range scaling. The models in the alternative paradigm are scaling but strongly anisotropic with vertical sections of structures becoming increasingly stratified at larger and larger scales albeit in a power law manner; we collectively refer to these as "SP" for "scaling primary" approaches. Early authors explicitly
Statistical theory of turbulent incompressible multimaterial flow
Kashiwa, B.
1987-10-01
Interpenetrating motion of incompressible materials is considered. ''Turbulence'' is defined as any deviation from the mean motion. Accordingly a nominally stationary fluid will exhibit turbulent fluctuations due to a single, slowly moving sphere. Mean conservation equations for interpenetrating materials in arbitrary proportions are derived using an ensemble averaging procedure, beginning with the exact equations of motion. The result is a set of conservation equations for the mean mass, momentum and fluctuational kinetic energy of each material. The equation system is at first unclosed due to integral terms involving unknown one-point and two-point probability distribution functions. In the mean momentum equation, the unclosed terms are clearly identified as representing two physical processes. One is transport of momentum by multimaterial Reynolds stresses, and the other is momentum exchange due to pressure fluctuations and viscous stress at material interfaces. Closure is approached by combining careful examination of multipoint statistical correlations with the traditional physical technique of kappa-epsilon modeling for single-material turbulence. This involves representing the multimaterial Reynolds stress for each material as a turbulent viscosity times the rate of strain based on the mean velocity of that material. The multimaterial turbulent viscosity is related to the fluctuational kinetic energy kappa, and the rate of fluctuational energy dissipation epsilon, for each material. Hence a set of kappa and epsilon equations must be solved, together with mean mass and momentum conservation equations, for each material. Both kappa and the turbulent viscosities enter into the momentum exchange force. The theory is applied to (a) calculation of the drag force on a sphere fixed in a uniform flow, (b) calculation of the settling rate in a suspension and (c) calculation of velocity profiles in the pneumatic transport of solid particles in a pipe.
Non-stationary random ground vibration due to loads moving along a railway track
NASA Astrophysics Data System (ADS)
Lu, Feng; Gao, Qiang; Lin, J. H.; Williams, F. W.
2006-11-01
The pseudo-excitation method (PEM) and the precise integration algorithm are combined to compute the non-stationary random ground vibration caused by loads moving along a railway track at constant speed. The rails are modeled as a single infinite Euler beam connected to sleepers and hence to ballast. This ballast rests on the ground, which is assumed to consist of layered transversely isotropic soil. The equations of motion of the system are established in a Cartesian coordinate system which moves with the loads. The non-stationary power spectral density and the time-dependent standard deviation can be derived conveniently by means of PEM, while the precise integration algorithm for two-point boundary value problems is applied to the solution of the equations of motion in the frequency/wavenumber domain. By virtue of the transverse isotropic property of the layered soils, the threefold iteration process in the frequency/wavenumber domain is reduced into a twofold iteration process. Hence the computational efficiency is improved considerably.
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.
Symmetry Breaking Drift of Particles Settling in Homogeneous Shear Turbulence
NASA Astrophysics Data System (ADS)
van Hinsberg, M. A. T.; Clercx, H. J. H.; Toschi, Federico
2016-08-01
We investigate the influence of shear on the gravitational settling of heavy inertial particles in homogeneous shear turbulence (HST). In addition to the well-known enhanced settling velocity, observed for heavy inertial particles in homogeneous isotropic turbulence (HIT), a horizontal drift velocity is also observed in the shearing direction due to the presence of a nonzero mean vorticity (introducing symmetry breaking due to the mean shear). This drift velocity is due to the combination of shear, gravity, and turbulence, and all three of these elements are needed for this effect to occur. We extend the mechanism responsible for the enhanced settling velocity in HIT to the case of HST. Two separate regimes are observed, characterized by positive or negative drift velocity, depending on the particle settling velocity.
Fluid Dynamics Prize Otto Laporte Lecture:Turbulence and Aeroacoustics
NASA Astrophysics Data System (ADS)
Comte-Bellot, Genevieve
2014-11-01
Some significant advances obtained over the years for two closely related fields, Turbulence and Aeroacoustics, are presented. Particular focus is placed on experimental results and on physical mechanisms. For example, for a 2D channel flow, skewness factors of velocity fluctuations are discussed. The study of isotropic turbulence generated by grids in the «Velvet wind tunnel» of Stanley Corrsin, constitutes a masterpiece. Of particular note are the Eulerian memory times, analysed for all wavenumbers. Concerning hot-wire anemometry, the potential of the new constant voltage technique is presented. Some results obtained with Particule Image Velocimetry are also reported. Two flow control examples are illustrated: lift generation for a circular cylinder, and noise reduction for a high speed jet. Finally, the propagation of acoustic waves through turbulence is considered. Experimental data are here completed by numerical simulations showing the possible occurrence of caustics.
Symmetry Breaking Drift of Particles Settling in Homogeneous Shear Turbulence.
van Hinsberg, M A T; Clercx, H J H; Toschi, Federico
2016-08-01
We investigate the influence of shear on the gravitational settling of heavy inertial particles in homogeneous shear turbulence (HST). In addition to the well-known enhanced settling velocity, observed for heavy inertial particles in homogeneous isotropic turbulence (HIT), a horizontal drift velocity is also observed in the shearing direction due to the presence of a nonzero mean vorticity (introducing symmetry breaking due to the mean shear). This drift velocity is due to the combination of shear, gravity, and turbulence, and all three of these elements are needed for this effect to occur. We extend the mechanism responsible for the enhanced settling velocity in HIT to the case of HST. Two separate regimes are observed, characterized by positive or negative drift velocity, depending on the particle settling velocity. PMID:27541467
COSMIC RAY TRANSPORT THROUGH GYRORESONANCE INSTABILITY IN COMPRESSIBLE TURBULENCE
Yan Huirong; Lazarian, A. E-mail: alazarian@wisc.edu
2011-04-10
We study the nonlinear growth of kinetic gyroresonance instability of cosmic rays (CRs) induced by large-scale compressible turbulence. This feedback of CRs on turbulence was shown to induce an important scattering mechanism in addition to direct interaction with the compressible turbulence. The linear growth is bound to saturate due to the wave-particle interactions. By balancing the increase of CR anisotropy via the large-scale compression and its decrease via the wave-particle scattering, we find the steady-state solutions. The nonlinear suppression due to the wave-particle scattering limits the energy range of CRs that can excite the instabilities and be scattered by the induced slab waves. The direct interaction with large-scale compressible modes still appears to be the dominant mechanism for isotropization of high-energy CRs (>100 GeV).
Time Correlations and the Frequency Spectrum of Sound Radiated by Turbulent Flows
NASA Technical Reports Server (NTRS)
Rubinstein, Robert; Zhou, Ye
1997-01-01
Theories of turbulent time correlations are applied to compute frequency spectra of sound radiated by isotropic turbulence and by turbulent shear flows. The hypothesis that Eulerian time correlations are dominated by the sweeping action of the most energetic scales implies that the frequency spectrum of the sound radiated by isotropic turbulence scales as omega(exp 4) for low frequencies and as omega(exp -3/4) for high frequencies. The sweeping hypothesis is applied to an approximate theory of jet noise. The high frequency noise again scales as omega(exp -3/4), but the low frequency spectrum scales as omega(exp 2). In comparison, a classical theory of jet noise based on dimensional analysis gives omega(exp -2) and omega(exp 2) scaling for these frequency ranges. It is shown that the omega(exp -2) scaling is obtained by simplifying the description of turbulent time correlations. An approximate theory of the effect of shear on turbulent time correlations is developed and applied to the frequency spectrum of sound radiated by shear turbulence. The predicted steepening of the shear dominated spectrum appears to be consistent with jet noise measurements.
Genetics Home Reference: autosomal dominant congenital stationary night blindness
... stationary night blindness autosomal dominant congenital stationary night blindness Enable Javascript to view the expand/collapse boxes. ... Close All Description Autosomal dominant congenital stationary night blindness is a disorder of the retina , which is ...
Evaluation of imaging geometry for stationary chest tomosynthesis
NASA Astrophysics Data System (ADS)
Shan, Jing; Tucker, Andrew W.; Lee, Yueh Z.; Heath, Michael D.; Wang, Xiaohui; Foos, David; Lu, Jianping; Zhou, Otto
2014-03-01
We have recently demonstrated the feasibility of stationary digital chest tomosynthesis (s-DCT) using a dis- tributed carbon nanotube x-ray source array. The technology has the potential to increase the imaging resolution and speed by eliminating source motion. In addition, the flexibility in the spatial configuration of the individual sources allows new tomosynthesis imaging geometries beyond the linear scanning mode used in the conventional systems. In this paper, we report the preliminary results on the effects of the tomosynthesis imaging geometry on the image quality. The study was performed using a bench-top s-DCT system consisting of a CNT x-ray source array and a flat-panel detector. System MTF and ASF are used as quantitative measurement of the in-plane and in-depth resolution. In this study geometries with the x-ray sources arranged in linear, square, rectangular and circular configurations were investigated using comparable imaging doses. Anthropomorphic chest phantom images were acquired and reconstructed for image quality assessment. It is found that wider angular coverage results in better in-depth resolution, while the angular span has little impact on the in-plane resolution in the linear geometry. 2D source array imaging geometry leads to a more isotropic in-plane resolution, and better in-depth resolution compared to 1D linear imaging geometry with comparable angular coverage.
Characteristics of the laminar-turbulent edge in transitional boundary layers
NASA Astrophysics Data System (ADS)
Lee, Jin; Zaki, Tamer
2015-11-01
Characteristics of the boundary separating the laminar and turbulent regions in a transitional boundary layer are examined using a time series of three-dimensional flow fields extracted from direct numerical simulations (DNS). In order to accurately mimic boundary-layer experiments perturbed by grid turbulence, the current simulation includes the leading edge of the flat plate and the incoming homogeneous isotropic turbulence. The Reynolds number based on the momentum thickness reaches up to 1400, and high-resolution three-dimensional flow fields of the DNS data will be publicly accessible via the Johns Hopkins Turbulence Database (JHTDB). The laminar-turbulence discrimination algorithm isolates the turbulence spots within the transition zone and the bounding surface of the fully-turbulent flow. Attention is placed on the cross-stream surface between the transition zone and fully-turbulent boundary layer. The shape of this interface is dictated by a balance between downstream advection, destabilization of upstream flow and merging of turbulence spots. Conditionally sampled statistics are examined across the interface, and are also compared to the downstream equilibrium turbulent boundary layer.
A Two-length Scale Turbulence Model for Single-phase Multi-fluid Mixing
Schwarzkopf, J. D.; Livescu, D.; Baltzer, J. R.; Gore, R. A.; Ristorcelli, J. R.
2015-09-08
A two-length scale, second moment turbulence model (Reynolds averaged Navier-Stokes, RANS) is proposed to capture a wide variety of single-phase flows, spanning from incompressible flows with single fluids and mixtures of different density fluids (variable density flows) to flows over shock waves. The two-length scale model was developed to address an inconsistency present in the single-length scale models, e.g. the inability to match both variable density homogeneous Rayleigh-Taylor turbulence and Rayleigh-Taylor induced turbulence, as well as the inability to match both homogeneous shear and free shear flows. The two-length scale model focuses on separating the decay and transport length scales, as the two physical processes are generally different in inhomogeneous turbulence. This allows reasonable comparisons with statistics and spreading rates over such a wide range of turbulent flows using a common set of model coefficients. The specific canonical flows considered for calibrating the model include homogeneous shear, single-phase incompressible shear driven turbulence, variable density homogeneous Rayleigh-Taylor turbulence, Rayleigh-Taylor induced turbulence, and shocked isotropic turbulence. The second moment model shows to compare reasonably well with direct numerical simulations (DNS), experiments, and theory in most cases. The model was then applied to variable density shear layer and shock tube data and shows to be in reasonable agreement with DNS and experiments. Additionally, the importance of using DNS to calibrate and assess RANS type turbulence models is highlighted.
Scaling of turbulent flame speed for expanding flames with Markstein diffusion considerations
NASA Astrophysics Data System (ADS)
Chaudhuri, Swetaprovo; Wu, Fujia; Law, Chung K.
2013-09-01
In this paper we clarify the role of Markstein diffusivity, which is the product of the planar laminar flame speed and the Markstein length, on the turbulent flame speed and its scaling, based on experimental measurements on constant-pressure expanding turbulent flames. Turbulent flame propagation data are presented for premixed flames of mixtures of hydrogen, methane, ethylene, n-butane, and dimethyl ether with air, in near-isotropic turbulence in a dual-chamber, fan-stirred vessel. For each individual fuel-air mixture presented in this work and the recently published iso-octane data from Leeds, normalized turbulent flame speed data of individual fuel-air mixtures approximately follow a ReT,f0.5 scaling, for which the average radius is the length scale and thermal diffusivity is the transport property of the turbulence Reynolds number. At a given ReT,f, it is experimentally observed that the normalized turbulent flame speed decreases with increasing Markstein number, which could be explained by considering Markstein diffusivity as the leading dissipation mechanism for the large wave number flame surface fluctuations. Consequently, by replacing thermal diffusivity with the Markstein diffusivity in the turbulence Reynolds number definition above, it is found that normalized turbulent flame speeds could be scaled by ReT,M0.5 irrespective of the fuel, equivalence ratio, pressure, and turbulence intensity for positive Markstein number flames.
Scaling of turbulent flame speed for expanding flames with Markstein diffusion considerations.
Chaudhuri, Swetaprovo; Wu, Fujia; Law, Chung K
2013-09-01
In this paper we clarify the role of Markstein diffusivity, which is the product of the planar laminar flame speed and the Markstein length, on the turbulent flame speed and its scaling, based on experimental measurements on constant-pressure expanding turbulent flames. Turbulent flame propagation data are presented for premixed flames of mixtures of hydrogen, methane, ethylene, n-butane, and dimethyl ether with air, in near-isotropic turbulence in a dual-chamber, fan-stirred vessel. For each individual fuel-air mixture presented in this work and the recently published iso-octane data from Leeds, normalized turbulent flame speed data of individual fuel-air mixtures approximately follow a Re_{T,f}^{0.5} scaling, for which the average radius is the length scale and thermal diffusivity is the transport property of the turbulence Reynolds number. At a given Re_{T,f}^{}, it is experimentally observed that the normalized turbulent flame speed decreases with increasing Markstein number, which could be explained by considering Markstein diffusivity as the leading dissipation mechanism for the large wave number flame surface fluctuations. Consequently, by replacing thermal diffusivity with the Markstein diffusivity in the turbulence Reynolds number definition above, it is found that normalized turbulent flame speeds could be scaled by Re_{T,M}^{0.5} irrespective of the fuel, equivalence ratio, pressure, and turbulence intensity for positive Markstein number flames. PMID:24125342
Introduction to quantum turbulence.
Barenghi, Carlo F; Skrbek, Ladislav; Sreenivasan, Katepalli R
2014-03-25
The term quantum turbulence denotes the turbulent motion of quantum fluids, systems such as superfluid helium and atomic Bose-Einstein condensates, which are characterized by quantized vorticity, superfluidity, and, at finite temperatures, two-fluid behavior. This article introduces their basic properties, describes types and regimes of turbulence that have been observed, and highlights similarities and differences between quantum turbulence and classical turbulence in ordinary fluids. Our aim is also to link together the articles of this special issue and to provide a perspective of the future development of a subject that contains aspects of fluid mechanics, atomic physics, condensed matter, and low-temperature physics. PMID:24704870
Modeling Compressed Turbulence
Israel, Daniel M.
2012-07-13
From ICE to ICF, the effect of mean compression or expansion is important for predicting the state of the turbulence. When developing combustion models, we would like to know the mix state of the reacting species. This involves density and concentration fluctuations. To date, research has focused on the effect of compression on the turbulent kinetic energy. The current work provides constraints to help development and calibration for models of species mixing effects in compressed turbulence. The Cambon, et al., re-scaling has been extended to buoyancy driven turbulence, including the fluctuating density, concentration, and temperature equations. The new scalings give us helpful constraints for developing and validating RANS turbulence models.
Introduction to quantum turbulence
Barenghi, Carlo F.; Skrbek, Ladislav; Sreenivasan, Katepalli R.
2014-01-01
The term quantum turbulence denotes the turbulent motion of quantum fluids, systems such as superfluid helium and atomic Bose–Einstein condensates, which are characterized by quantized vorticity, superfluidity, and, at finite temperatures, two-fluid behavior. This article introduces their basic properties, describes types and regimes of turbulence that have been observed, and highlights similarities and differences between quantum turbulence and classical turbulence in ordinary fluids. Our aim is also to link together the articles of this special issue and to provide a perspective of the future development of a subject that contains aspects of fluid mechanics, atomic physics, condensed matter, and low-temperature physics. PMID:24704870
NASA Astrophysics Data System (ADS)
Brunk, Brett Kenneth
1997-11-01
Pollutant and particle transport in estuaries is affected by a multitude of physical, chemical and biological processes. In this research the importance of equilibrium sorption and turbulent coagulation were studied. Sorption in estuaries was modeled using phenanthrene, bacterial extracellular polymer and kaolinite clay as surrogates for a hydrophobic organic pollutant, dissolved organic matter and inorganic suspended sediment, respectively. Experiments over a range of estuarine salinities showed that ionic strength had the largest effect on the extent of sorption, while the effect of extracellular polymer coatings on the mineral surfaces was insignificant. Further calculations using typical estuarine suspended sediment concentrations indicated that equilibrium sorption could not fully account for the solid/solution phase distribution of hydrophobic organic compounds in the estuarine water column. For particles that are small compared to the length scales of turbulence, the rate of coagulation is related to the dynamics of the smallest turbulent eddies since they have the highest shear rate. Experimental and theoretical effort focused on determining the coagulation rate of spherical particles in isotropic turbulence. A pair diffusion approximation valid for rapidly fluctuating flows was used to calculate the rate of coagulation in a randomly varying isotropic linear flow field. Dynamic simulations of particle coagulation in Gaussian turbulence were computed over a range of representative values of particle-particle interactions (i.e, hydrodynamic interactions and van der Waals attraction) and total strain (i.e., the product of the strain rate and its time scale). The computed coagulation rates for isotropic turbulence differed from analytical approximations valid at large and small total strain. As expected, particle interactions were found to be significant. Experimental measurements of coagulation in grid-generated turbulence were obtained by measuring the loss
Fluctuation-induced dielectric permittivity in the isotropic phase of cholesteric liquid crystals
NASA Astrophysics Data System (ADS)
Mukherjee, Prabir K.; Das, Asok K.
2016-03-01
The temperature and pressure dependence of the static dielectric permittivity in the isotropic phase of the isotropic to cholesteric phase transition is calculated using Landau-de Gennes’s fluctuation theory, allowing spatial variation of the orientational order parameter. A comparison is made with experimental data available in the isotropic phase of the isotropic to cholesteric phase transition.
Separation of stationary and non-stationary sources with a generalized eigenvalue problem.
Hara, Satoshi; Kawahara, Yoshinobu; Washio, Takashi; von Bünau, Paul; Tokunaga, Terumasa; Yumoto, Kiyohumi
2012-09-01
Non-stationary effects are ubiquitous in real world data. In many settings, the observed signals are a mixture of underlying stationary and non-stationary sources that cannot be measured directly. For example, in EEG analysis, electrodes on the scalp record the activity from several sources located inside the brain, which one could only measure invasively. Discerning stationary and non-stationary contributions is an important step towards uncovering the mechanisms of the data generating system. To that end, in Stationary Subspace Analysis (SSA), the observed signal is modeled as a linear superposition of stationary and non-stationary sources, where the aim is to separate the two groups in the mixture. In this paper, we propose the first SSA algorithm that has a closed form solution. The novel method, Analytic SSA (ASSA), is more than 100 times faster than the state-of-the-art, numerically stable, and guaranteed to be optimal when the covariance between stationary and non-stationary sources is time-constant. In numerical simulations on wide range of settings, we show that our method yields superior results, even for signals with time-varying group-wise covariance. In an application to geophysical data analysis, ASSA extracts meaningful components that shed new light on the Pi 2 pulsations of the geomagnetic field. PMID:22551683
NASA Astrophysics Data System (ADS)
Lee, Kurnchul; Venugopal, Vishnu; Girimaji, Sharath S.
2016-08-01
Return-to-isotropy and kinetic-potential energy equipartition are two fundamental pressure-moderated energy redistributive processes in anisotropic compressible turbulence. Pressure-strain correlation tensor redistributes energy among various Reynolds stress components and pressure-dilatation is responsible for energy reallocation between dilatational kinetic and potential energies. The competition and interplay between these pressure-based processes are investigated in this study. Direct numerical simulations (DNS) of low turbulent Mach number dilatational turbulence are performed employing the hybrid thermal Lattice Boltzman method (HTLBM). It is found that a tendency towards equipartition precedes proclivity for isotropization. An evolution towards equipartition has a collateral but critical effect on return-to-isotropy. The preferential transfer of energy from strong (rather than weak) Reynolds stress components to potential energy accelerates the isotropization of dilatational fluctuations. Understanding of these pressure-based redistributive processes is critical for developing insight into the character of compressible turbulence.
Power spectral density analysis of wind-shear turbulence for related flight simulations. M.S. Thesis
NASA Technical Reports Server (NTRS)
Laituri, Tony R.
1988-01-01
Meteorological phenomena known as microbursts can produce abrupt changes in wind direction and/or speed over a very short distance in the atmosphere. These changes in flow characteristics have been labelled wind shear. Because of its adverse effects on aerodynamic lift, wind shear poses its most immediate threat to flight operations at low altitudes. The number of recent commercial aircraft accidents attributed to wind shear has necessitated a better understanding of how energy is transferred to an aircraft from wind-shear turbulence. Isotropic turbulence here serves as the basis of comparison for the anisotropic turbulence which exists in the low-altitude wind shear. The related question of how isotropic turbulence scales in a wind shear is addressed from the perspective of power spectral density (psd). The role of the psd in related Monte Carlo simulations is also considered.
Theory and modeling of atmospheric turbulence, part 1
NASA Technical Reports Server (NTRS)
1984-01-01
The cascade transfer which is the only function to describe the mode coupling as the result of the nonlinear hydrodynamic state of turbulence is discussed. A kinetic theory combined with a scaling procedure was developed. The transfer function governs the non-linear mode coupling in strong turbulence. The master equation is consistent with the hydrodynamical system that describes the microdynamic state of turbulence and has the advantages to be homogeneous and have fewer nonlinear terms. The modes are scaled into groups to decipher the governing transport processes and statistical characteristics. An equation of vorticity transport describes the microdynamic state of two dimensional, isotropic and homogeneous, geostrophic turbulence. The equation of evolution of the macrovorticity is derived from group scaling in the form of the Fokker-Planck equation with memory. The microdynamic state of turbulence is transformed into the Liouville equation to derive the kinetic equation of the singlet distribution in turbulence. The collision integral contains a memory, which is analyzed with pair collision and the multiple collision. Two other kinetic equations are developed in parallel for the propagator and the transition probability for the interaction among the groups.
Turbulent Amplification and Structure of the Intracluster Magnetic Field
NASA Astrophysics Data System (ADS)
Beresnyak, Andrey; Miniati, Francesco
2016-02-01
We compare DNS calculations of homogeneous isotropic turbulence with the statistical properties of intracluster turbulence from the Matryoshka Run and find remarkable similarities between their inertial ranges. This allowed us to use the time-dependent statistical properties of intracluster turbulence to evaluate dynamo action in the intracluster medium, based on earlier results from a numerically resolved nonlinear magneto-hydrodynamic turbulent dynamo. We argue that this approach is necessary (a) to properly normalize dynamo action to the available intracluster turbulent energy and (b) to overcome the limitations of low Re affecting current numerical models of the intracluster medium. We find that while the properties of intracluster magnetic field are largely insensitive to the value and origin of the seed field, the resulting values for the Alfvén speed and the outer scale of the magnetic field are consistent with current observational estimates, basically confirming the idea that the magnetic field in today’s galaxy clusters is a record of its past turbulent activity.
Mean flow and anisotropic cascades in decaying 2D turbulence
NASA Astrophysics Data System (ADS)
Liu, Chien-Chia; Cerbus, Rory; Gioia, Gustavo; Chakraborty, Pinaki
2015-11-01
Many large-scale atmospheric and oceanic flows are decaying 2D turbulent flows embedded in a non-uniform mean flow. Despite its importance for large-scale weather systems, the affect of non-uniform mean flows on decaying 2D turbulence remains unknown. In the absence of mean flow it is well known that decaying 2D turbulent flows exhibit the enstrophy cascade. More generally, for any 2D turbulent flow, all computational, experimental and field data amassed to date indicate that the spectrum of longitudinal and transverse velocity fluctuations correspond to the same cascade, signifying isotropy of cascades. Here we report experiments on decaying 2D turbulence in soap films with a non-uniform mean flow. We find that the flow transitions from the usual isotropic enstrophy cascade to a series of unusual and, to our knowledge, never before observed or predicted, anisotropic cascades where the longitudinal and transverse spectra are mutually independent. We discuss implications of our results for decaying geophysical turbulence.
Buckingham, A.C.; Siekhaus, W.J.
1982-09-27
The unsteady, non-similar, chemically reactive, turbulent boundary layer equations are modified for gas plus dispersed solid particle mixtures, for gas phase turbulent combustion reactions and for heterogeneous gas-solid surface erosive reactions. The exterior (ballistic core) edge boundary conditions for the solutions are modified to include dispersed particle influences on core propellant combustion-generated turbulence levels, combustion reactants and products, and reaction-induced, non-isentropic mixture states. The wall surface (in this study it is always steel) is considered either bare or coated with a fixed particle coating which is conceptually non-reactive, insulative, and non-ablative. Two families of solutions are compared. These correspond to: (1) consideration of gas-borne, free-slip, almost spontaneously mobile (motile) solid particle additives which influence the turbulent heat transfer at the uncoated steel surface and, in contrast, (2) consideration of particle-free, gas phase turbulent heat transfer to the insulated surface coated by stationary particles. Significant differences in erosive heat transfer are found in comparing the two families of solutions over a substantial range of interior ballistic flow conditions. The most effective influences on reducing erosive heat transfer appear to favor mobile, gas-borne particle additives.
Coherent structures in ion temperature gradient turbulence-zonal flow
Singh, Rameswar; Singh, R.; Kaw, P.; Gürcan, Ö. D.; Diamond, P. H.
2014-10-15
Nonlinear stationary structure formation in the coupled ion temperature gradient (ITG)-zonal flow system is investigated. The ITG turbulence is described by a wave-kinetic equation for the action density of the ITG mode, and the longer scale zonal mode is described by a dynamic equation for the m = n = 0 component of the potential. Two populations of trapped and untrapped drift wave trajectories are shown to exist in a moving frame of reference. This novel effect leads to the formation of nonlinear stationary structures. It is shown that the ITG turbulence can self-consistently sustain coherent, radially propagating modulation envelope structures such as solitons, shocks, and nonlinear wave trains.
A k-{\\varepsilon} turbulence closure model of an isothermal dry granular dense matter
NASA Astrophysics Data System (ADS)
Fang, Chung
2016-07-01
The turbulent flow characteristics of an isothermal dry granular dense matter with incompressible grains are investigated by the proposed first-order k-{\\varepsilon} turbulence closure model. Reynolds-filter process is applied to obtain the balance equations of the mean fields with two kinematic equations describing the time evolutions of the turbulent kinetic energy and dissipation. The first and second laws of thermodynamics are used to derive the equilibrium closure relations satisfying turbulence realizability conditions, with the dynamic responses postulated by a quasi-linear theory. The established closure model is applied to analyses of a gravity-driven stationary flow down an inclined moving plane. While the mean velocity decreases monotonically from its value on the moving plane toward the free surface, the mean porosity increases exponentially; the turbulent kinetic energy and dissipation evolve, respectively, from their minimum and maximum values on the plane toward their maximum and minimum values on the free surface. The evaluated mean velocity and porosity correspond to the experimental outcomes, while the turbulent dissipation distribution demonstrates a similarity to that of Newtonian fluids in turbulent shear flows. When compared to the zero-order model, the turbulent eddy evolution tends to enhance the transfer of the turbulent kinetic energy and plane shearing across the flow layer, resulting in more intensive turbulent fluctuation in the upper part of the flow. Solid boundary as energy source and sink of the turbulent kinetic energy becomes more apparent in the established first-order model.
A k-{\\varepsilon} turbulence closure model of an isothermal dry granular dense matter
NASA Astrophysics Data System (ADS)
Fang, Chung
2015-07-01
The turbulent flow characteristics of an isothermal dry granular dense matter with incompressible grains are investigated by the proposed first-order k-{\\varepsilon} turbulence closure model. Reynolds-filter process is applied to obtain the balance equations of the mean fields with two kinematic equations describing the time evolutions of the turbulent kinetic energy and dissipation. The first and second laws of thermodynamics are used to derive the equilibrium closure relations satisfying turbulence realizability conditions, with the dynamic responses postulated by a quasi-linear theory. The established closure model is applied to analyses of a gravity-driven stationary flow down an inclined moving plane. While the mean velocity decreases monotonically from its value on the moving plane toward the free surface, the mean porosity increases exponentially; the turbulent kinetic energy and dissipation evolve, respectively, from their minimum and maximum values on the plane toward their maximum and minimum values on the free surface. The evaluated mean velocity and porosity correspond to the experimental outcomes, while the turbulent dissipation distribution demonstrates a similarity to that of Newtonian fluids in turbulent shear flows. When compared to the zero-order model, the turbulent eddy evolution tends to enhance the transfer of the turbulent kinetic energy and plane shearing across the flow layer, resulting in more intensive turbulent fluctuation in the upper part of the flow. Solid boundary as energy source and sink of the turbulent kinetic energy becomes more apparent in the established first-order model.
Large-scale flows and coherent structure phenomena in flute turbulence
Sandberg, I.; Andrushchenko, Zh.N.; Pavlenko, V.P.
2005-04-15
The properties of zonal and streamer flows in the flute mode turbulence are investigated. The stability criteria and the frequency of these flows are determined in terms of the spectra of turbulent fluctuations. Furthermore, it is shown that zonal flows can undergo a further nonlinear evolution leading to the formation of long-lived coherent structures which consist of self-bound wave packets supporting stationary shear layers, and thus can be characterized as regions with a reduced level of anomalous transport.
Acceleration Statistics in Rotating and Sheared Turbulence
NASA Astrophysics Data System (ADS)
Jacobitz, Frank; Schneider, Kai; Bos, Wouter; Farge, Marie
2012-11-01
Acceleration statistics are of fundamental interest in turbulence ranging from theoretical questions to modeling of dispersion processes. Direct numerical simulations of sheared and rotating homogeneous turbulence are performed with different ratios of Coriolis parameter to shear rate. The statistics of Lagrangian and Eulerian acceleration are studied with a particular focus on the influence of the rotation ratio and also on the scale dependence of the statistics. The probability density functions (pdfs) of both Lagrangian and Eulerian acceleration show a strong and similar influence on the rotation ratio. The flatness further quantifies its influence and yields values close to three for strong rotation. For moderate and vanishing rotation, the flatness of the Eulerian acceleration is larger than that of the Lagrangian acceleration, contrary to previous results for isotropic turbulence. A wavelet-based scale-dependent analysis shows that the flatness of both Eulerian and Lagrangian acceleration increases as scale decreases. For strong rotation, the Eulerian acceleration is more intermittent than the Lagrangian acceleration, while the opposite result is obtained for moderate rotation.
Cui, Linyan; Xue, Bindang
2015-09-01
Theoretical and experimental investigations have shown that the atmospheric turbulence exhibits both anisotropic and non-Kolmogorov properties. Very recent analyses of angle of arrival (AOA) fluctuations of an optical wave in anisotropic non-Kolmogorov turbulence have adopted the assumption that the propagation path was in the z-direction with circular symmetry of turbulence cells maintained in the orthogonal xy-plane throughout the path, and one single anisotropy factor was adopted in the orthogonal xy-plane to parameterize the asymmetry of turbulence cells or eddies in both horizontal and vertical directions. In this work, the circular symmetry assumption of turbulence cells or eddies in the orthogonal xy-plane is no longer required, and two anisotropy parameters are introduced in the orthogonal xy-plane to investigate the AOA fluctuations. In addition, deviations from the classic 11/3 spectral power law behavior for Kolmogorov turbulence are allowed by assuming spectral power law value variations between 3 and 4. With the Rytov approximation theory, new theoretical models for the variance of AOA fluctuations are developed for optical plane and spherical waves propagating through weak anisotropic non-Kolmogorov atmospheric turbulence. When the two anisotropic parameters are equal to each other, they reduce correctly to the recently published results (the circular symmetry assumption of turbulence cells or eddies in the orthogonal xy-plane was adopted). Furthermore, when these two anisotropic parameters equal one, they reduce correctly to the previously published analytic expressions for the cases of optical wave propagation through weak isotropic non-Kolmogorov turbulence. PMID:26367438
NASA Astrophysics Data System (ADS)
Lee, Gyung Su.
This thesis is devoted to two studies of low-frequency turbulence in toroidally confined plasma. Low-frequency turbulence is believed to play an important role in anomalous transport in toroidal confinement devices. The first study pertains the the development of an analytic theory of ion-temperature-gradient-driven turbulence in tokamaks. Energy-conserving, renormalized spectrum equations are derived and solved in order to obtain the spectra of stationary ion-temperature-gradient-driven turbulence. Corrections to mixing-length estimates are calculated explicitly. The resulting anomalous ion thermal diffusivity is derived and is found to be consistent with experimentally-deduced ion thermal diffusivities. The associated electron thermal diffusivity, particle and heat-pinch velocities are also calculated. The effects of impurity gradients on saturated ion-temperature-gradient-driven turbulence are discussed and a related explanation of density profile steepening during Z-mode operation is proposed. The second study is devoted to the role of multiple helicity nonlinear interactions of tearing modes and dynamics of magnetic relaxation in a high-temperature current-carrying plasma. To extend the resistive MHD theory of magnetic fluctuations and dynamo activity observed in the reversed field pinch, the fluid equations for high-temperature regime are derived and basic nonlinear interaction mechanism and the effects of diamagnetic corrections to the MHD turbulence theory are studied for the case of fully developed, densely packed turbulence. Modifications to the MHD dynamo theory and anomalous thermal transport and confinement scaling predictions are examined.
Isotropic and anisotropic bouncing cosmologies in Palatini gravity
Barragan, Carlos; Olmo, Gonzalo J.
2010-10-15
We study isotropic and anisotropic (Bianchi I) cosmologies in Palatini f(R) and f(R,R{sub {mu}{nu}R}{sup {mu}{nu}}) theories of gravity with a perfect fluid and consider the existence of nonsingular bouncing solutions in the early universe. We find that all f(R) models with isotropic bouncing solutions develop shear singularities in the anisotropic case. On the contrary, the simple quadratic model R+aR{sup 2}/R{sub P}+R{sub {mu}{nu}R}{sup {mu}{nu}/}R{sub P} exhibits regular bouncing solutions in both isotropic and anisotropic cases for a wide range of equations of state, including dust (for a<0) and radiation (for arbitrary a). It thus represents a purely gravitational solution to the big bang singularity and anisotropy problems of general relativity without the need for exotic (w>1) sources of matter/energy or extra degrees of freedom.
Subtraction threshold for an isotropic fluorescence emission difference microscope
NASA Astrophysics Data System (ADS)
Wang, Nan; Kobayashi, Takayoshi
2015-12-01
Isotropic fluorescence emission difference microscopy proposed recently provides a simple method to enhance the spatial resolution in three-dimensions (3D) for fluorescence imaging. However, the subtraction threshold to achieve the condition for appropriately resolving the sample in 3D have not been studied. Then the subtraction factors used in this type of microscopes are still experientially chosen. Based on vector diffraction theory and a 3D numerical model developed here, the subtraction threshold is numerically investigated for the isotropic fluorescence subtraction microscopy. The subtraction factors and peak intensities at the threshold are obtained and comparied both in lateral and axial planes for achieving most appropriate subtraction and inspecting the isotropic characteristic. The effects of radius ratios of implemented 0-π annular phase plate for generating three dimensional donut spot on the subtracted resolution, peak intensity and negative sidebands are also discussed.
Stress waves in transversely isotropic media: The homogeneous problem
NASA Technical Reports Server (NTRS)
Marques, E. R. C.; Williams, J. H., Jr.
1986-01-01
The homogeneous problem of stress wave propagation in unbounded transversely isotropic media is analyzed. By adopting plane wave solutions, the conditions for the existence of the solution are established in terms of phase velocities and directions of particle displacements. Dispersion relations and group velocities are derived from the phase velocity expressions. The deviation angles (e.g., angles between the normals to the adopted plane waves and the actual directions of their propagation) are numerically determined for a specific fiber-glass epoxy composite. A graphical method is introduced for the construction of the wave surfaces using magnitudes of phase velocities and deviation angles. The results for the case of isotropic media are shown to be contained in the solutions for the transversely isotropic media.
INSTRUMENTAL SENSING OF STATIONARY SOURCE EMISSIONS
Remote sensing methods offer a number of advantages over contact measurement methods in the area of enforcement and surveillance of emissions from stationary sources. Several techniques have been developed that can measure the gas concentration, effluent velocity, and particulate...
Dampers for Stationary Labyrinth Seals
NASA Technical Reports Server (NTRS)
El-Aini, Yehia; Mitchell, William; Roberts, Lawrence; Montgomery, Stuart; Davis, Gary
2011-01-01
Vibration dampers have been invented that are incorporated as components within the stationary labyrinth seal assembly. These dampers are intended to supplement other vibration-suppressing features of labyrinth seals in order to reduce the incidence of high-cycle-fatigue failures, which have been known to occur in the severe vibratory environments of jet engines and turbopumps in which labyrinth seals are typically used. A vibration damper of this type includes several leaf springs and/or a number of metallic particles (shot) all held in an annular seal cavity by a retaining ring. The leaf springs are made of a spring steel alloy chosen, in conjunction with design parameters, to maintain sufficient preload to ensure effectiveness of damping at desired operating temperatures. The cavity is vented via a small radial gap between the retaining ring and seal housing. The damping mechanism is complex. In the case of leaf springs, the mechanism is mainly friction in the slippage between the seal housing and individual dampers. In the case of a damper that contains shot, the damping mechanism includes contributions from friction between individual particles, friction between particles and cavity walls, and dissipation of kinetic energy of impact. The basic concept of particle/shot vibration dampers has been published previously; what is new here is the use of such dampers to suppress traveling-wave vibrations in labyrinth seals. Damping effectiveness depends on many parameters, including, but not limited to, coefficient of friction, mode shape, and frequency and amplitude of vibrational modes. In tests, preloads of the order of 6 to 15 lb (2.72 to 6.8 kilograms) per spring damper were demonstrated to provide adequate damping levels. Effectiveness of shot damping of vibrations having amplitudes from 20 to 200 times normal terrestrial gravitational acceleration (196 to 1,960 meters per square second) and frequencies up to 12 kHz was demonstrated for shot sizes from 0.032 to
Self-Organized Stationary States of Tokamaks.
Jardin, S C; Ferraro, N; Krebs, I
2015-11-20
We demonstrate that in a 3D resistive magnetohydrodynamic simulation, for some parameters it is possible to form a stationary state in a tokamak where a saturated interchange mode in the center of the discharge drives a near helical flow pattern that acts to nonlinearly sustain the configuration by adjusting the central loop voltage through a dynamo action. This could explain the physical mechanism for maintaining stationary nonsawtoothing "hybrid" discharges, often referred to as "flux pumping." PMID:26636854
Stationary phase deposition based on onium salts
Wheeler, David R.; Lewis, Patrick R.; Dirk, Shawn M.; Trudell, Daniel E.
2008-01-01
Onium salt chemistry can be used to deposit very uniform thickness stationary phases on the wall of a gas chromatography column. In particular, the stationary phase can be bonded to non-silicon based columns, especially microfabricated metal columns. Non-silicon microfabricated columns may be manufactured and processed at a fraction of the cost of silicon-based columns. In addition, the method can be used to phase-coat conventional capillary columns or silicon-based microfabricated columns.
Self-Organized Stationary States of Tokamaks
Jardin, S. C.; Ferraro, N.; Krebs, I.
2015-11-01
We demonstrate that in a 3D resistive magnetohydrodynamic simulation, for some parameters it is possible to form a stationary state in a tokamak where a saturated interchange mode in the center of the discharge drives a near helical flow pattern that acts to nonlinearly sustain the configuration by adjusting the central loop voltage through a dynamo action. This could explain the physical mechanism for maintaining stationary nonsawtoothing "hybrid" discharges, often referred to as "flux pumping."
Multi-Spacecraft Turbulence Analysis Methods
NASA Astrophysics Data System (ADS)
Horbury, Tim S.; Osman, Kareem T.
Turbulence is ubiquitous in space plasmas, from the solar wind to supernova remnants, and on scales from the electron gyroradius to interstellar separations. Turbulence is responsible for transporting energy across space and between scales and plays a key role in plasma heating, particle acceleration and thermalisation downstream of shocks. Just as with other plasma processes such as shocks or reconnection, turbulence results in complex, structured and time-varying behaviour which is hard to measure with a single spacecraft. However, turbulence is a particularly hard phenomenon to study because it is usually broadband in nature: it covers many scales simultaneously. One must therefore use techniques to extract information on multiple scales in order to quantify plasma turbulence and its effects. The Cluster orbit takes the spacecraft through turbulent regions with a range of characteristics: the solar wind, magnetosheath, cusp and magnetosphere. In each, the nature of the turbulence (strongly driven or fully evolved; dominated by kinetic effects or largely on fluid scales), as well as characteristics of the medium (thermalised or not; high or low plasma sub- or super-Alfvenic) mean that particular techniques are better suited to the analysis of Cluster data in different locations. In this chapter, we consider a range of methods and how they are best applied to these different regions. Perhaps the most studied turbulent space plasma environment is the solar wind, see Bruno and Carbone [2005]; Goldstein et al. [2005] for recent reviews. This is the case for a number of reasons: it is scientifically important for cosmic ray and solar energetic particle scattering and propagation, for example. However, perhaps the most significant motivations for studying solar wind turbulence are pragmatic: large volumes of high quality measurements are available; the stability of the solar wind on the scales of hours makes it possible to identify statistically stationary intervals to
Preliminary Results of Heat Transfer from a Stationary and Rotating Ellipsoidal Spinner
NASA Technical Reports Server (NTRS)
vonGlahn, U.
1953-01-01
Convective heat-transfer coefficients in dry air were obtained for an ellipsoidal spinner of 30-inch maximum diameter for both stationary and rotating operation over a range of conditions including airspeeds up to 275 miles per hour, rotational speeds up to 1200 rpm, and angles of attack of zero and 40 The results are presented in terms of Nusselt numbers, Reynolds numbers, and convective heat-transfer coefficients. The studies included both uniform heating densities over the spinner and uniform surface temperatures.. In general, the results showed that rotation will increase the convective heat transfer from a spinner, especially in the turbulent-flow regions. Rotation of the spinner at 1200 rpm and at a free-stream velocity of 275 miles per hour increased the Nusselt number parameter in the turbulent-flow region by 32 percent over that obtained with a stationary spinner; whereas in the nose region, where the flow was laminar, an increase of only 18 percent was observed. Transition from laminar to turbulent flow occurred over a large range of Reynolds numbers primarily because of surface roughness of the spinner. Operation at an angle of attack of 40 had only small effects on the local convective heat transfer for the model studied.
Unexpected collapses during isotropic consolidation of model granular materials
NASA Astrophysics Data System (ADS)
Doanh, Thiep; Le Bot, Alain; Abdelmoula, Nouha; Gribaa, Lassad; Hans, Stéphane; Boutin, Claude
2016-02-01
This paper reports the unexpected instantaneous instabilities of idealized granular materials under simple isotropic drained compression. Specimens of monosized glass beads submitted to isotropic compression exhibit a series of local collapses under undetermined external stress with partial liquefaction, experience sudden volumetric compaction and axial contraction of various amplitude. Short-lived excess pore water pressure vibrates like an oscillating underdamped system in the first dynamic transient phase and rapidly disperses in the subsequent longer dissipation phase. However, very dense samples maintain a collapse-free behaviour below a threshold void ratio e30col at 30 kPa of stress. The potential mechanisms that could explain these spontaneous collapses are discussed.
Concurrence-based entanglement measures for isotropic states
Rungta, Pranaw; Caves, Carlton M.
2003-01-01
We discuss properties of entanglement measures called I-concurrence and tangle. For a bipartite pure state, I-concurrence and tangle are simply related to the purity of the marginal density operators. The I-concurrence (tangle) of a bipartite mixed state is the minimum average I-concurrence (tangle) of ensemble decompositions of pure states of the joint density operator. Terhal and Vollbrecht [Phys. Rev. Lett. 85, 2625 (2000)] have given an explicit formula for the entanglement of formation of isotropic states in arbitrary dimensions. We use their formalism to derive comparable expressions for the I-concurrence and tangle of isotropic states.
An analytical model for permeability of isotropic porous media
NASA Astrophysics Data System (ADS)
Yang, Xiaohu; Lu, Tian Jian; Kim, Tongbeum
2014-06-01
We demonstrate that permeability of isotropic porous media e.g., open-cell foams can be analytically presented as a function of two morphological parameters: porosity and pore size. Adopting a cubic unit cell model, an existing tortuosity model from the branching algorithm method is incorporated into a generalized permeability model. The present model shows that dimensionless permeability significantly increases as the porosity of isotropic porous media and unifies the previously reported data in a wide range of porosity (ɛ=0.55-0.98) and pore size (Dp=0.254 mm-5.08 mm).
Equilibrium Shapes for Isotropic Elastic Tubes in the Planar Case
NASA Astrophysics Data System (ADS)
Xu, Qing-Hua; Zhou, Xiao-Hua; Liu, Yuan-Sheng; Wu, Ke-Jian; Wen, Jun
2013-05-01
When making an isotropic elastic shell into a curving tube, the crimp energy and bending energy determine the equilibrium shapes of the tube. In this study, we established a model to explore the elastic behavior of a tube made of an elastic shell. Two typical shapes: torus shape and periodic shape are discussed by studying the equilibrium shape equations in the planar case. Our study reveals that the crimp energy for an isotropic elastic tube is innegligible and will induce abundant shapes. It also reveals that varicose vein is more likely to occur when the blood vessels become thicker, which is in accordance with the clinic experiments.
A note on antenna models in a warm isotropic plasma
NASA Technical Reports Server (NTRS)
Singh, N.
1980-01-01
The electron-transparent and electron-reflecting models of antennas in a warm isotropic plasma are reexamined. It is shown that a purely electrical treatment of both the models without an explicit use of the boundary condition on electron velocity yields the same results as those previously obtained through an electromechanical treatment. The essential difference between the two models is that for the electron-reflecting model, fields are nonzero only in the exterior region, while for the electron-transparent model, they are nonzero both in the exterior and interior regions of the antenna. This distinction helps in clarifying some misconceptions about these models of antennas in warm isotropic plasma.
Stationary phase in the yeast Saccharomyces cerevisiae.
Werner-Washburne, M; Braun, E; Johnston, G C; Singer, R A
1993-01-01
Growth and proliferation of microorganisms such as the yeast Saccharomyces cerevisiae are controlled in part by the availability of nutrients. When proliferating yeast cells exhaust available nutrients, they enter a stationary phase characterized by cell cycle arrest and specific physiological, biochemical, and morphological changes. These changes include thickening of the cell wall, accumulation of reserve carbohydrates, and acquisition of thermotolerance. Recent characterization of mutant cells that are conditionally defective only for the resumption of proliferation from stationary phase provides evidence that stationary phase is a unique developmental state. Strains with mutations affecting entry into and survival during stationary phase have also been isolated, and the mutations have been shown to affect at least seven different cellular processes: (i) signal transduction, (ii) protein synthesis, (iii) protein N-terminal acetylation, (iv) protein turnover, (v) protein secretion, (vi) membrane biosynthesis, and (vii) cell polarity. The exact nature of the relationship between these processes and survival during stationary phase remains to be elucidated. We propose that cell cycle arrest coordinated with the ability to remain viable in the absence of additional nutrients provides a good operational definition of starvation-induced stationary phase. PMID:8393130
Stationary Liquid Fuel Fast Reactor
Yang, Won Sik; Grandy, Andrew; Boroski, Andrew; Krajtl, Lubomir; Johnson, Terry
2015-09-30
For effective burning of hazardous transuranic (TRU) elements of used nuclear fuel, a transformational advanced reactor concept named SLFFR (Stationary Liquid Fuel Fast Reactor) was proposed based on stationary molten metallic fuel. The fuel enters the reactor vessel in a solid form, and then it is heated to molten temperature in a small melting heater. The fuel is contained within a closed, thick container with penetrating coolant channels, and thus it is not mixed with coolant nor flow through the primary heat transfer circuit. The makeup fuel is semi- continuously added to the system, and thus a very small excess reactivity is required. Gaseous fission products are also removed continuously, and a fraction of the fuel is periodically drawn off from the fuel container to a processing facility where non-gaseous mixed fission products and other impurities are removed and then the cleaned fuel is recycled into the fuel container. A reference core design and a preliminary plant system design of a 1000 MWt TRU- burning SLFFR concept were developed using TRU-Ce-Co fuel, Ta-10W fuel container, and sodium coolant. Conservative design approaches were adopted to stay within the current material performance database. Detailed neutronics and thermal-fluidic analyses were performed to develop a reference core design. Region-dependent 33-group cross sections were generated based on the ENDF/B-VII.0 data using the MC2-3 code. Core and fuel cycle analyses were performed in theta-r-z geometries using the DIF3D and REBUS-3 codes. Reactivity coefficients and kinetics parameters were calculated using the VARI3D perturbation theory code. Thermo-fluidic analyses were performed using the ANSYS FLUENT computational fluid dynamics (CFD) code. Figure 0.1 shows a schematic radial layout of the reference 1000 MWt SLFFR core, and Table 0.1 summarizes the main design parameters of SLFFR-1000 loop plant. The fuel container is a 2.5 cm thick cylinder with an inner radius of 87.5 cm. The fuel
Turbulent and directed plasma motions in solar flares
NASA Technical Reports Server (NTRS)
Fludra, A.; Bentley, R. D.; Lemen, J. R.; Jakimiec, J.; Sylwester, J.
1989-01-01
An improved method for fitting asymmetric soft X-ray line profiles from solar flares is presented. A two-component model is used where one component represents the total emission from directed upflow plasma and the other the emission from the plasma at rest. Unlike previous methods, the width of the moving component is independent from that of the stationary component. Time variations of flare plasma characteristics (i.e., temperature, emission measure of moving and stationary plasma, upflow and turbulent velocities) are derived from the Ca XIX and Fe XXV spectra recorded by the Bent Crystal Spectrometer on the Solar Maximum Mission. The fitting technique provides a statistical estimation for the uncertainties in the fitting parameters. The relationship between the directed and turbulent motions has been studied, and a correlation of the random and directed motions has been found in some flares with intensive plasma upflows. Mean temperatures of the upflowing and stationary plasmas are compared for the first time from ratios of calcium to iron X-ray line intensities. Finally, evidence for turbulent motions and the possibility of plasma upflow late into the decay phase is presented and discussed.
Turbulent Convection: Old and New Models
NASA Astrophysics Data System (ADS)
Canuto, V. M.
1996-08-01
This paper contains (1) a physical argument to show that the one-eddy MLT model underestimates the convective flux Fc in the high-efficiency regime, while it overestimates Fc in the low-efficiency regime, and (2) a new derivation of the Fc(MLT) using a turbulence model in the one-eddy approximation. (3) We forsake the one-eddy approximation and adopt the Kolmogorov spectrum to represent the turbulent energy spectrum. The resulting Fc > Fc(MLT) in the high-efficiency regime, and Fc
Experiments in Advective and Turbulent Hyporheic Pumping
NASA Astrophysics Data System (ADS)
Mccluskey, A. H.; Grant, S.; Stewardson, M. J.
2014-12-01
Hyporheic exchange (HE) is the mixing of stream and subsurface waters beneath the sediment-water interface (SWI). At the patch and reach scales, HE is dominated by periodic upwelling and downwelling zones, induced by pressure variation and processes within the turbulent boundary layer (TBL). This can be caused by (1) the geometry of the stream, imposing a stationary wave at the SWI or (2) by a travelling wave associated with the propagation of turbulent pressure waves generated from the TBL. Case (1) has generally been the favoured model of hyporheic exchange and has been referred to as hyporheic 'pumping' by Elliott and Brooks, and subsequently others. Case (2) can be termed turbulent pumping, and has been proposed as a mechanism to model the combined effects of turbulent dispersion alongside steady-state advection. While this has been represented numerically and analytically, conjecture remains about the physical representation of these combined processes. We present initial results from experiments undertaken to classify the spatial and temporal characteristics of pressure variation at and beneath the SWI, with a periodic sinusoidal geometry of wavelength 0.28m and height 0.02m. As an initial characterisation, the advective flow profile has been examined using time-lapse photography of dyes released across the span of a periodic downwelling zone. These tracer tests confirmed delineation of isolated upwelling and downwelling cells as noted by previous authors in modelling studies. However, their distribution deviates from the typical pumping pattern with increased discharge and stream gradient. Empirical orthogonal function (EOF) analysis of high frequency (250Hz) pressure measurements, sampled at an array along the centroid of the flume underneath one wavelength gave further insight into the spatial distribution of turbulent signatures arising from roughness-generated turbulence. A turbulent frequency of 6-10Hz dominates, however the penetration depth appears to
NASA Astrophysics Data System (ADS)
Gray, Morgan; Petit, Cyril; Rodionov, Sergey; Bertino, Laurent; Bocquet, Marc; Fusco, Thierry
2013-12-01
We propose a new algorithm for an AO control law which allows to reduce the computation burden in the case of an Extremely Large Telescope and to deal with a non stationary behavior of the atmospheric turbulence. This approach uses Ensemble Transform Kalman Filter (ETKF) and localizations by domains decomposition: the assimilation is split into local domains on the pupil of the telescope and each of the update data assimilation for each domain is performed independently. This kind of assimilation enables parallel computation of much less data during the update stage. This is a Kalman Filter adaptation for large scale systems with a non stationary turbulence when the explicit storage and manipulation of extremely large covariance matrices are impossible. This distributed parallel environment implementation is highlighted and studied in the context of an ELT application. First simulation results are proposed to assess our theoretical analysis and to demonstrate the potentiality of this new approach for an AO control law on ELTs.
NASA Technical Reports Server (NTRS)
Goldstein, M. L.; Klimas, A. J.; Sandri, G.
1975-01-01
The Fokker-Planck coefficient for pitch-angle scattering, appropriate for cosmic rays in homogeneous stationary magnetic turbulence is computed without making any specific assumptions concerning the statistical symmetries of the random field. The Fokker-Planck coefficient obtained can be used to compute the parallel diffusion coefficient for high-energy cosmic rays propagating in the presence of strong turbulence, or for low-energy cosmic rays in the presence of weak turbulence. Because of the generality of magnetic turbulence allowed for in the analysis, special interplanetary magnetic field features, such as discontinuities or particular wave modes, can be included rigorously.
Secondary Instability of Stationary Crossflow Vortices in Mach 6 Boundary Layer Over a Circular Cone
NASA Technical Reports Server (NTRS)
Li, Fei; Choudhari, Meelan M.; Paredes-Gonzalez, Pedro; Duan, Lian
2015-01-01
Hypersonic boundary layer flows over a circular cone at moderate incidence can support strong crossflow instability. Due to more efficient excitation of stationary crossflow vortices by surface roughness, such boundary layer flows may transition to turbulence via rapid amplification of the high-frequency secondary instabilities of finite amplitude stationary crossflow vortices. The amplification characteristics of these secondary instabilities are investigated for crossflow vortices generated by an azimuthally periodic array of roughness elements over a 7-degree half-angle circular cone in a Mach 6 free stream. Depending on the local amplitude of the stationary crossflow mode, the most unstable secondary disturbances either originate from the second (i.e., Mack) mode instabilities of the unperturbed boundary layer or correspond to genuine secondary instabilities that reduce to stable disturbances at sufficiently small amplitudes of the stationary crossflow vortex. The predicted frequencies of dominant secondary disturbances are similar to those measured during wind tunnel experiments at Purdue University and the Technical University of Braunschweig, Germany.
Casanova, S.; Schlickeiser, R.
2012-02-01
Recently, a new transport theory of cosmic rays in magnetized space plasmas extending the quasilinear approximation to the particle orbit has been developed for the case of an axisymmetric incompressible magnetic turbulence. Here, we generalize the approach to the important physical case of a compressible plasma. As previously obtained in the case of an incompressible plasma, we allow arbitrary gyrophase deviations from the unperturbed spiral orbits in the uniform magnetic field. For the case of quasi-stationary and spatially homogeneous magnetic turbulence we derive, in the small Larmor radius approximation, gyrophase-averaged cosmic-ray Fokker-Planck coefficients. Upper limits for the perpendicular and pitch-angle Fokker-Planck coefficients and for the perpendicular and parallel spatial diffusion coefficients are presented.
Turbulence generation by waves
Kaftori, D.; Nan, X.S.; Banerjee, S.
1995-12-31
The interaction between two-dimensional mechanically generated waves, and a turbulent stream was investigated experimentally in a horizontal channel, using a 3-D LDA synchronized with a surface position measuring device and a micro-bubble tracers flow visualization with high speed video. Results show that although the wave induced orbital motion reached all the way to the wall, the characteristics of the turbulence wall structures and the turbulence intensity close to the wall were not altered. Nor was the streaky nature of the wall layer. On the other hand, the mean velocity profile became more uniform and the mean friction velocity was increased. Close to the free surface, the turbulence intensity was substantially increased as well. Even in predominantly laminar flows, the introduction of 2-D waves causes three dimensional turbulence. The turbulence enhancement is found to be proportional to the wave strength.
NASA Astrophysics Data System (ADS)
Adhikari, L.; Zank, G. P.; Hunana, P.; Bruno, R.; Telloni, D.; Marino, R.
2016-03-01
Turbulence in the solar wind is ubiquitous. To understand the transport of low-frequency turbulence in the sub- and super-Alfvénic solar wind flow, Zank et al. 2012 developed an extensive turbulence transport model that describes the evolution of the energy in forward and backward propagating modes, the total turbulent energy, the cross-helicity, the residual energy, the correlation lengths corresponding to forward and backward propagating modes, and the correlation length of the residual energy. Adhikari et. al. 2015 presented the first detailed solution of Zank et al., and found good agreement between the Zank et al. model and observations. Here, we solve the 1D steady-state turbulence transport equations with and without sources of turbulence, and show that all the identified sources are required to reproduce the theoretical results to be consistent with the observations.
Kerstein, A.R.
1996-12-31
One-Dimensional Turbulence is a new turbulence modeling strategy involving an unsteady simulation implemented in one spatial dimension. In one dimension, fine scale viscous and molecular-diffusive processes can be resolved affordably in simulations at high turbulence intensity. The mechanistic distinction between advective and molecular processes is thereby preserved, in contrast to turbulence models presently employed. A stochastic process consisting of mapping {open_quote}events{close_quote} applied to a one-dimensional velocity profile represents turbulent advection. The local event rate for given eddy size is proportional to the velocity difference across the eddy. These properties cause an imposed shear to induce an eddy cascade analogous in many respects to the eddy cascade in turbulent flow. Many scaling and fluctuation properties of self-preserving flows, and of passive scalars introduced into these flows, are reproduced.
Effects of prestresses on mechanical properties of isotropic graphite materials
NASA Astrophysics Data System (ADS)
Oku, T.; Kurumada, A.; Imamura, Y.; Kawamata, K.; Shiraishi, M.
1998-10-01
Graphite materials which are used for plasma facing components and other components are subjected to stresses due to the high heat flux from the fusion plasma. Some mechanical properties of graphite materials can change due to the prestresses. The property changes should be considered for the design of the plasma facing components. The purpose of this study is to examine the effects of prestresses on the mechanical properties of isotropic graphite materials. Compressive prestresses were applied to two kinds of isotropic fine-grained graphites (IG-430 and IG-11) at 298 K (both), 1873 K (IG-11), 2273 K (IG-11) and 2283 K (IG-430). As a result, the decrease in Young's modulus for IG-430 due to high-temperature prestressing was 56% which was much larger than the 6.4% that was due to prestressing at 298 K. The results for IG-11 were the same as those for IG-430 graphite. This finding was considered to be due primarily to a difference in degree of the preferred orientation of crystallites in the graphite on the basis of the Bacon anisotropy factor (BAF) obtained from X-ray diffraction measurement of the prestressed specimens. Furthermore, high-temperature compressive prestressing produced an increase in the strength of the isotropic graphite, although room temperature prestressing produced no such effect. The results obtained here suggest that the isotropic graphite which is subjected to high-temperature compressive stresses can become anisotropic in service.
On the accuracy and fitting of transversely isotropic material models.
Feng, Yuan; Okamoto, Ruth J; Genin, Guy M; Bayly, Philip V
2016-08-01
Fiber reinforced structures are central to the form and function of biological tissues. Hyperelastic, transversely isotropic material models are used widely in the modeling and simulation of such tissues. Many of the most widely used models involve strain energy functions that include one or both pseudo-invariants (I4 or I5) to incorporate energy stored in the fibers. In a previous study we showed that both of these invariants must be included in the strain energy function if the material model is to reduce correctly to the well-known framework of transversely isotropic linear elasticity in the limit of small deformations. Even with such a model, fitting of parameters is a challenge. Here, by evaluating the relative roles of I4 and I5 in the responses to simple loadings, we identify loading scenarios in which previous models accounting for only one of these invariants can be expected to provide accurate estimation of material response, and identify mechanical tests that have special utility for fitting of transversely isotropic constitutive models. Results provide guidance for fitting of transversely isotropic constitutive models and for interpretation of the predictions of these models. PMID:27136091
A Simple Mechanical Model for the Isotropic Harmonic Oscillator
ERIC Educational Resources Information Center
Nita, Gelu M.
2010-01-01
A constrained elastic pendulum is proposed as a simple mechanical model for the isotropic harmonic oscillator. The conceptual and mathematical simplicity of this model recommends it as an effective pedagogical tool in teaching basic physics concepts at advanced high school and introductory undergraduate course levels. (Contains 2 figures.)
Optical torque on small bi-isotropic particles.
Nieto-Vesperinas, Manuel
2015-07-01
We establish the equations for the time-averaged optical torque on dipolar bi-isotropic particles. Due to the interference of the scattered fields, it has a term additional to the one that is commonly employed in theory and experiments. Its consequences for conservation of energy, angular momentum, and effects like negative torques are discussed. PMID:26125357
Semiclassical States Associated with Isotropic Submanifolds of Phase Space
NASA Astrophysics Data System (ADS)
Guillemin, V.; Uribe, A.; Wang, Z.
2016-05-01
We define classes of quantum states associated with isotropic submanifolds of cotangent bundles. The classes are stable under the action of semiclassical pseudo-differential operators and covariant under the action of semiclassical Fourier integral operators. We develop a symbol calculus for them; the symbols are symplectic spinors. We outline various applications.
Orientation statistics and settling velocity of ellipsoids in decaying turbulence
NASA Astrophysics Data System (ADS)
Siewert, C.; Kunnen, R. P. J.; Meinke, M.; Schröder, W.
2014-06-01
Motivated by applications in technology as well as in other disciplines where the motion of particles in a turbulent flow field is important, the orientation and settling velocity of ellipsoidal particles in a spatially decaying isotropic turbulent flow are numerically investigated. With respect to cloud microphysics ellipsoidal particles of various shapes are interpreted as archetypes of regular ice crystals, i.e., plates and columns approximated by oblate and prolate ellipsoids. The motion of 19 million small and heavy ellipsoidal particles is tracked by a Lagrangian point-particle model based on Stokes flow conditions. Five types of ellipsoids of revolution such as prolates, spheres, and oblates are considered. The orientation and settling velocity statistics are gathered at six turbulence intensities characterized by the turbulent kinetic energy dissipation rate ranging from 30 to 250 cm2s- 3. It is shown that the preferential orientation of ellipsoids is disturbed by the turbulent fluctuations of the fluid forces and moments. As the turbulence intensity increases the orientation probability distribution becomes more and more uniform. That is, the settling velocity of the ellipsoids is influenced by the turbulence level since the drag force is dependent on the orientation. The effect is more pronounced, the longer the prolate or the flatter the oblate is. The theoretical settling velocity based on the orientation probability of the non-spherical particles is smaller than that found in the simulation. The results show the existence of the preferential sweeping phenomenon also for non-spherical particles. These two effects of turbulence on the motion of ellipsoids change the settling velocity and as such the swept volume, that is expected to result in modified collision probabilities of ellipsoid-shaped particles.
Pulsating instability and self-acceleration of fast turbulent flames
NASA Astrophysics Data System (ADS)
Poludnenko, Alexei Y.
2015-01-01
A series of three-dimensional numerical simulations is used to study the intrinsic stability of high-speed turbulent flames. Calculations model the interaction of a fully resolved premixed flame with a highly subsonic, statistically steady, homogeneous, isotropic turbulence. The computational domain is unconfined to prevent the onset of thermoacoustic instabilities. We consider a wide range of turbulent intensities and system sizes, corresponding to the Damköhler numbers Da = 0.1 - 6.0. These calculations show that turbulent flames in the regimes considered are intrinsically unstable. In particular, we find three effects. (1) Turbulent flame speed, ST, develops pulsations with the observed peak-to-peak amplitude ST max / ST min > 10 and a characteristic time scale close to a large-scale eddy turnover time. Such variability is caused by the interplay between turbulence, which continuously creates the flame surface, and highly intermittent flame collisions, which consume the flame surface. (2) Unstable burning results in the periodic pressure build-up and the formation of pressure waves or shocks, when ST approaches or exceeds the speed of a Chapman-Jouguet deflagration. (3) Coupling of pressure gradients formed during pulsations with density gradients across the flame leads to the anisotropic amplification of turbulence inside the flame volume and flame acceleration. Such process, which is driven by the baroclinic term in the vorticity transport equation, is a reacting-flow analog of the mechanism underlying the Richtmyer-Meshkov instability. With the increase in turbulent intensity, the limit-cycle instability discussed here transitions to the regime described in our previous work, in which the growth of ST becomes unbounded and produces a detonation.
Tangling clustering instability for small particles in temperature stratified turbulence
NASA Astrophysics Data System (ADS)
Elperin, T.; Kleeorin, N.; Liberman, M.; Rogachevskii, I.
2013-08-01
We study tangling clustering instability of inertial particles in a temperature stratified turbulence with small finite correlation time. It is shown that the tangling mechanism in the temperature stratified turbulence strongly increases the degree of compressibility of particle velocity field. This results in the strong decrease of the threshold for the excitation of the tangling clustering instability even for small particles. The tangling clustering instability in the temperature stratified turbulence is essentially different from the inertial clustering instability that occurs in non-stratified isotropic and homogeneous turbulence. While the inertial clustering instability is caused by the centrifugal effect of the turbulent eddies, the mechanism of the tangling clustering instability is related to the temperature fluctuations generated by the tangling of the mean temperature gradient by the velocity fluctuations. Temperature fluctuations produce pressure fluctuations and cause particle accumulations in regions with increased instantaneous pressure. It is shown that the growth rate of the tangling clustering instability is by sqrtRe (ell _0 / L_T)^2 / (3 Ma)^4 times larger than that of the inertial clustering instability, where Re is the Reynolds number, Ma is the Mach number, ℓ0 is the integral turbulence scale, and LT is the characteristic scale of the mean temperature variations. It is found that depending on the parameters of the turbulence and the mean temperature gradient there is a preferential particle size at which the particle clustering due to the tangling clustering instability is more effective. The particle number density inside the cluster after the saturation of this instability can be by several orders of magnitude larger than the mean particle number density. It is also demonstrated that the evaporation of droplets drastically changes the tangling clustering instability, e.g., it increases the instability threshold in the droplet radius. The
Non-stationary dynamics in the bouncing ball: A wavelet perspective
Behera, Abhinna K. Panigrahi, Prasanta K.; Sekar Iyengar, A. N.
2014-12-01
The non-stationary dynamics of a bouncing ball, comprising both periodic as well as chaotic behavior, is studied through wavelet transform. The multi-scale characterization of the time series displays clear signatures of self-similarity, complex scaling behavior, and periodicity. Self-similar behavior is quantified by the generalized Hurst exponent, obtained through both wavelet based multi-fractal detrended fluctuation analysis and Fourier methods. The scale dependent variable window size of the wavelets aptly captures both the transients and non-stationary periodic behavior, including the phase synchronization of different modes. The optimal time-frequency localization of the continuous Morlet wavelet is found to delineate the scales corresponding to neutral turbulence, viscous dissipation regions, and different time varying periodic modulations.
Non-stationary dynamics in the bouncing ball: a wavelet perspective.
Behera, Abhinna K; Iyengar, A N Sekar; Panigrahi, Prasanta K
2014-12-01
The non-stationary dynamics of a bouncing ball, comprising both periodic as well as chaotic behavior, is studied through wavelet transform. The multi-scale characterization of the time series displays clear signatures of self-similarity, complex scaling behavior, and periodicity. Self-similar behavior is quantified by the generalized Hurst exponent, obtained through both wavelet based multi-fractal detrended fluctuation analysis and Fourier methods. The scale dependent variable window size of the wavelets aptly captures both the transients and non-stationary periodic behavior, including the phase synchronization of different modes. The optimal time-frequency localization of the continuous Morlet wavelet is found to delineate the scales corresponding to neutral turbulence, viscous dissipation regions, and different time varying periodic modulations. PMID:25554027
Neef, M; Kruse, K
2014-11-01
We study the dynamics of an active polar fluid in the interstitial space between two fixed coaxial cylinders. For sufficiently large expansive or contractive active stresses, the fluid presents roll instabilities of axially symmetric states leading to the spontaneous formation of vortices in the flow field. These vortices are either stationary or travel around the inner cylinder. Increasing the activity further, our numerical solutions indicate the existence of active turbulence that coexists with regular vortex solutions. PMID:25493812
Effects of small scale energy injection on large scales in turbulent reaction flows
NASA Astrophysics Data System (ADS)
Xuan, Yuan
2014-11-01
Turbulence causes the generation of eddies of various length scales. In turbulent non-reacting flows, most of the kinetic energy is contained in large scale turbulent structures and dissipated at small scales. This energy cascade process from large scales to small scales provides the foundation of a lot of turbulence models, especially for Large Eddy Simulations. However, in turbulent reacting flows, chemical energy is converted locally to heat and therefore deploys energy at the smallest scales. As such, effects of small scale energy injection due to combustion on large scale turbulent motion may become important. These effects are investigated in the case of auto-ignition under homogeneous isotropic turbulence. Impact of small scale heat release is examined by comparing various turbulent statistics (e.g. energy spectrum, two-point correlation functions, and structure functions) in the reacting case to the non-reacting case. Emphasis is placed on the identification of the most relevant turbulent quantities in reflecting such small-large scale interactions.
Investigation of turbulence modulation in particle-laden flows using the lattice Boltzmann method.
NASA Astrophysics Data System (ADS)
Peng, Cheng; Geneva, Nicholas; Min, Haoda; Wang, Lian-Ping
2015-11-01
Turbulent modulation by finite-size solid particles has been studied experimentally and numerically in the past several decades. Previous studies have revealed that resolving the interfaces between particle surfaces and fluid is crucial to properly include finite-size effects on local fluid turbulence. Finite-size particles also produce pseudo-turbulence that may not decay locally, leading to a stronger nonlinear dependence of the level of turbulence modulation on the particle volume fraction. In this study we apply the lattice Boltzmann method (LBM) to perform interface-resolved simulations of turbulent particle-laden flow, focusing on local turbulence dynamics at the scale of particle size. We will discuss the accuracy of this mesoscopic approach when compared to other macroscopic methods. We consider both fully developed homogeneous isotropic (HI) turbulent flows and turbulent channel flows laden with finite-size particles. The particle volume fraction is around 10% and the particle-to-fluid density ratio is of the order of one. Conditional statistics as a function of distance from the moving particle surfaces are studied in detail, and are used to help interpret global turbulence modulation by particles. Grid convergence of these conditional statistics will be discussed.
NASA Astrophysics Data System (ADS)
Bihlo, Alexander; Dos Santos Cardoso-Bihlo, Elsa Maria; Nave, Jean-Christophe; Popovych, Roman
2012-11-01
Various subgrid-scale closure models break the invariance of the Euler or Navier-Stokes equations and thus violate the geometric structure of these equations. A method is shown which allows one to systematically derive invariant turbulence models starting from non-invariant turbulence models and thus to correct artificial symmetry-breaking. The method is illustrated by finding invariant hyperdiffusion schemes to be applied in the two-dimensional turbulence problem.
A new approach to the study of the turbulence layer
NASA Technical Reports Server (NTRS)
Levenson, Ruth A.
1993-01-01
Turbulent thermal convection is of considerable importance in fluid dynamical transport phenomena occurring, for example, in the planetary boundary layer of the Earth, the interiors of stars, and accretion disks. In particular, during a significant portion of the evolutionary phase of many stars having convectively unstable cores or outer envelopes, a substantial fraction of energy is transported from the central layers to the outer layers by thermal convection. Moreover, as much of the interior of a star is in highly turbulent motion, a complete theory of stellar structure and evolution requires the explicit consideration of turbulence in order to have expressions for the turbulent quantities arising in the stellar structure equations, and particularly, the turbulent fluxes that appear in the total flux conservation equation, such as the convective flux, kinetic energy flux, etc. A reliable quantification of these fluxes continues to present a challenge in astrophysical fluid dynamics, primarily because astrophysical turbulence is almost always fully-developed and nearly inviscid, and therefore governed by strong nonlinear interactions that distribute the energy among a very wide spectrum of eddies with scales ranging from the characteristic dimension of the flow to those sufficiently small to be affected by viscous dissipation. Furthermore, astrophysical flows are invariably compressible, anisotropic, and inhomogeneous, which requires the consideration of the dynamics of longitudinal modes and their interaction with the transverse modes, as well as complicated boundary conditions. In order to reach a compromise between analytical and numerical tractability and the basic physics of turbulent convection, we have constructed a model of stationary turbulent convection that yields various turbulence statistics, including the convective flux, that are required in stellar evolution models.
NASA Astrophysics Data System (ADS)
Soboleva, O. N.; Kurochkina, E. P.
2016-01-01
The effective coefficients in the problem of the acoustic wave propagation have been calculated for a multiscale 3D isotropic medium using a subgrid modeling approach. The density and the elastic stiffness have been represented mathematically by the Kolmogorov multiplicative cascades, which, to date, appear to be the only mechanisms for generating a stationary multifractal fields with a log-stable probability distribution. The fields with the stable distribution are described with the help of linear combination random values ?, ? and weight coefficients ?, ?, which satisfy certain conditions in the nodes of spatial grid ?. The parameters of the stable distribution of the random values ?, ? are equal: ?, ?, ?, ?. The wavelength is assumed to be large as compared with the scale of heterogeneities of the medium. We consider the regime in which the waves propagate over a distance of the typical wave length in source. The theoretical results obtained in this paper are compared with the results of a direct 3D numerical simulation.
Nicolleau, F; Sung, K-S; Vassilicos, J C
2008-10-01
In this paper different arguments are put forward to explain why two-particle diffusion is depleted in the direction of stratification of a stably stratified turbulence. Kinematic simulations (KSs) which reproduce that depletion are used to shed light on the responsible mechanisms. The local horizontal divergence is studied and comparisons are made with two-dimensional kinematic simulation. The probability density function of the horizontal divergence of the velocity field is not a Dirac distribution in the presence of stratification but a Gaussian and this Gaussian does not depend on the Froude number. The number of stagnation points in the KS of three-dimensional strongly stratified turbulence is found virtually identical to what it is in KS of three-dimensional isotropic turbulence. However, the root mean square horizontal and vertical stagnation point velocities of the stratified turbulence are both larger than their counterparts in isotropic turbulence that latter getting progressively smaller as the Reynolds number increases. Therefore, the strong stratification destroys the persistence of the stagnation points. The main reason for the depletion, however, seems to have to be sought in the effect of stratification on the strain rate tensor. The stratification does lead to a depletion of the average square strain rate tensor, as well as of all average square strain rate eigenvalues. We conclude that it is these effects of stratification on the strain rate tensor that explain the depletion of the horizontal turbulent pair diffusion. PMID:18999525
Effects of Gravity on Sheared Turbulence Laden with Bubbles or Droplets
NASA Technical Reports Server (NTRS)
Elghobashi, Said; Lasheras, Juan
1999-01-01
The objective of this numerical/experimental study is to improve the understanding of the effects of gravity on the two-way interaction between dispersed particles (bubbles or liquid droplets) and the carrier turbulent flow. The first phase of the project considers isotropic turbulence. Turbulent homogeneous shear flows laden with droplets/bubbles will be studied in the next phase. The experiments reported here are concerned with the dispersion of liquid droplets by homogeneous turbulence under various gravitational conditions and the effect of these droplets on the evolution of the turbulence of the carrier fluid (air). Direct numerical simulations (DNS) of bubble - laden isotropic decaying turbulence are performed using the two-fluid approach (TF) instead of the Eulerian-Lagrangian approach (EL). The motivation for using the TF formulation is that EL requires considerable computational resources especially for the case of two-way coupling where the instantaneous trajectories of a large number of individual bubbles need to be computed. The TF formulation is developed by spatially averaging the instantaneous equations of the carrier flow and bubble phase over a scale of the order of the Kolmogorov length scale which, in our case, is much larger than the bubble diameter. On that scale, the bubbles are treated as a continuum (without molecular diffusivity) characterized by the bubble phase velocity field and concentration (volume fraction). The bubble concentration, C, is assumed small enough to neglect the bubble-bubble interactions.
Convergence of third-order velocity structure functions in axisymmetric turbulence
NASA Astrophysics Data System (ADS)
Godeferd, Fabien; Delache, Alexandre
2011-11-01
Kolmogorov theory (1941) for isotropic turbulence establishes asymptotic scaling laws for the statistics of n-th order structure functions at high Reynolds number, in terms of dissipation ɛ and separation distance r for the velocity increment δu . A famed relationship is the -4/5 law. When the turbulent flow is anisotropic, due to external distortions (background rotation,...) to inhomogeneities or initial conditions (jets, ``isotropic'' grid turbulence), such laws may fail. We examine the applicability of the K41 predictions for third-order moments of velocity structure functions, and evaluate low Reynolds number effects and anisotropic effects on the departure with the -4/5 law. We consider rotating or stably stratified turbulence, whose statistics are obtained by Direct Numerical Simulations or by a two-point statistical model allowing to reach high Reynolds numbers. We link anisotropic spectral statistics for energy transfer with <(δu) 3 > and derive physical space statistics from spectral data of the statistical model. Although K41 scalings may arguably not apply to anisotropic turbulence, some justifications for anisotropic turbulence statistics can be provided (Taylor et al. PRE 2003) by specific data processing in DNS.
NASA Astrophysics Data System (ADS)
Jejjala, Vishnu; Minic, Djordje; Ng, Y. Jack; Tze, Chia-Hsiung
We propose a string theory of turbulence that explains the Kolmogorov scaling in 3+1 dimensions and the Kraichnan and Kolmogorov scalings in 2+1 dimensions. This string theory of turbulence should be understood in light of the AdS/CFT dictionary. Our argument is crucially based on the use of Migdal's loop variables and the self-consistent solutions of Migdal's loop equations for turbulence. In particular, there is an area law for turbulence in 2+1 dimensions related to the Kraichnan scaling.
Tactical missile turbulence problems
NASA Technical Reports Server (NTRS)
Dickson, Richard E.
1987-01-01
Of particular interest is atmospheric turbulence in the atmospheric boundary layer, since this affects both the launch and terminal phase of flight, and the total flight for direct fire systems. Brief discussions are presented on rocket artillery boost wind problems, mean wind correction, turbulent boost wind correction, the Dynamically Aimed Free Flight Rocket (DAFFR) wind filter, the DAFFR test, and rocket wake turbulence problems. It is concluded that many of the turbulence problems of rockets and missiles are common to those of aircraft, such as structural loading and control system design. However, these problems have not been solved at this time.
Commutative law for products of infinitely large isotropic random matrices.
Burda, Zdzislaw; Livan, Giacomo; Swiech, Artur
2013-08-01
Ensembles of isotropic random matrices are defined by the invariance of the probability measure under the left (and right) multiplication by an arbitrary unitary matrix. We show that the multiplication of large isotropic random matrices is spectrally commutative and self-averaging in the limit of infinite matrix size N→∞. The notion of spectral commutativity means that the eigenvalue density of a product ABC... of such matrices is independent of the order of matrix multiplication, for example, the matrix ABCD has the same eigenvalue density as ADCB. In turn, the notion of self-averaging means that the product of n independent but identically distributed random matrices, which we symbolically denote by AAA..., has the same eigenvalue density as the corresponding power A(n) of a single matrix drawn from the underlying matrix ensemble. For example, the eigenvalue density of ABCCABC is the same as that of A(2)B(2)C(3). We also discuss the singular behavior of the eigenvalue and singular value densities of isotropic matrices and their products for small eigenvalues λ→0. We show that the singularities at the origin of the eigenvalue density and of the singular value density are in one-to-one correspondence in the limit N→∞: The eigenvalue density of an isotropic random matrix has a power-law singularity at the origin ~|λ|(-s) with a power sε(0,2) when and only when the density of its singular values has a power-law singularity ~λ(-σ) with a power σ=s/(4-s). These results are obtained analytically in the limit N→∞. We supplement these results with numerical simulations for large but finite N and discuss finite-size effects for the most common ensembles of isotropic random matrices. PMID:24032775
NASA Technical Reports Server (NTRS)
Wang, Chi R.
1988-01-01
Boundary layer flow and turbulence transport analyses to study the influence of the free-stream turbulence on the surface heat transfer rate and the skin friction around the stagnation point of a circular cylinder in a turbulent flow are presented. The analyses are formulated with the turbulent boundary layer equations, the Reynolds stress transport equations and the k - epsilon two-equation turbulence modeling. The analyses are used to calculate the time-averaged turbulence double correlations, the mean flow properties, the surface heat transfer rate and the skin friction with an isotropic turbulence in the freestream. The analytical results are described and compared with the existing experimental measurements. Depending on the free-stream turbulence properties, the turbulence kinetic energy can increase or decrease as the flow moves toward the surface. However, the turbulence kinetic energy induces large Reynolds normal stresses at the boundary layer edge. The Reynolds normal stresses change the boundary layer profiles of the time-averaged double correlations of the velocity and temperature fluctuations, the surface heat transfer rate and the skin friction. The free-stream turbulence dissipation rate can affect the stagnation-point heat transfer rate but the influence of the free-stream temperature fluctuation on the heat transfer rate is insignificant.
Spatially isotropic four-dimensional imaging with dual-view plane illumination microscopy
Wu, Yicong; Wawrzusin, Peter; Senseney, Justin; Fischer, Robert S; Christensen, Ryan; Santella, Anthony; York, Andrew G; Winter, Peter W; Waterman, Clare M; Bao, Zhirong; Colón-Ramos, Daniel A; McAuliffe, Matthew; Shroff, Hari
2014-01-01
Optimal four-dimensional imaging requires high spatial resolution in all dimensions, high speed and minimal photobleaching and damage. We developed a dual-view, plane illumination microscope with improved spatiotemporal resolution by switching illumination and detection between two perpendicular objectives in an alternating duty cycle. Computationally fusing the resulting volumetric views provides an isotropic resolution of 330 nm. As the sample is stationary and only two views are required, we achieve an imaging speed of 200 images/s (i.e., 0.5 s for a 50-plane volume). Unlike spinning-disk confocal or Bessel beam methods, which illuminate the sample outside the focal plane, we maintain high spatiotemporal resolution over hundreds of volumes with negligible photobleaching. To illustrate the ability of our method to study biological systems that require high-speed volumetric visualization and/or low photobleaching, we describe microtubule tracking in live cells, nuclear imaging over 14 h during nematode embryogenesis and imaging of neural wiring during Caenorhabditis elegans brain development over 5 h. PMID:24108093
Inhomogeneous turbulence in magnetic reconnection
NASA Astrophysics Data System (ADS)
Yokoi, Nobumitsu
2016-07-01
Turbulence is expected to play an essential role in enhancing magnetic reconnection. Turbulence associated with magnetic reconnection is highly inhomogeneous: it is generated by inhomogeneities of the field configuration such as the velocity shear, temperature gradient, density stratification, magnetic shear, etc. This self-generated turbulence affects the reconnection through the turbulent transport. In this reconnection--turbulence interaction, localization of turbulent transport due to dynamic balance between several turbulence effects plays an essential role. For investigating inhomogeneous turbulence in a strongly nonlinear regime, closure or turbulence modeling approaches provide a powerful tool. A turbulence modeling approach for the magnetic reconnection is introduced. In the model, the mean-field equations with turbulence effects incorporated are solved simultaneously with the equations of turbulent statistical quantities that represent spatiotemporal properties of turbulence under the effect of large-scale field inhomogeneities. Numerical simulations of this Reynolds-averaged turbulence model showed that self-generated turbulence enhances magnetic reconnection. It was pointed out that reconnection states may be divided into three category depending on the turbulence level: (i) laminar reconnection; (ii) turbulent reconnection, and (iii) turbulent diffusion. Recent developments in this direction are also briefly introduced, which includes the magnetic Prandtl number dependence, spectral evolution, and guide-field effects. Also relationship of this fully nonlinear turbulence approach with other important approaches such as plasmoid instability reconnection will be discussed.
Extended inertial range phenomenology of magnetohydrodynamic turbulence
NASA Technical Reports Server (NTRS)
Matthaeus, William H.; Zhou, YE
1989-01-01
A phenomenological treatment of the inertial range of isotropic statistically steady magnetohydrodynamic turbulence is presented, extending the theory of Kraichnan (1965). The role of Alfven wave propagation is treated on equal footing with nonlinear convection, leading to a simple generalization of the relations between the times characteristic of wave propagation, convection, energy transfer, and decay of triple correlations. The theory leads to a closed-form steady inertial range spectral law that reduces to the Kraichnan and Kolmogorov laws in appropriate limits. The Kraichnan constant is found to be related in a simple way to the Kolmogorov constant; for typical values of the latter constant, the former has values in the range 1.22-1.87. Estimates of the time scale associated with spectral transfer of energy also emerge from the new approach, generalizing previously presented 'golden rules' for relating the spectral transfer time scale to the Alfven and eddy-turnover time scales.
Toward the large-eddy simulation of compressible turbulent flows
NASA Technical Reports Server (NTRS)
Erlebacher, G.; Hussaini, M. Y.; Speziale, C. G.; Zang, T. A.
1990-01-01
New subgrid-scale models for the large-eddy simulation of compressible turbulent flows are developed and tested based on the Favre-filtered equations of motion for an ideal gas. A compressible generalization of the linear combination of the Smagorinsky model and scale-similarity model, in terms of Favre-filtered fields, is obtained for the subgrid-scale stress tensor. An analogous thermal linear combination model is also developed for the subgrid-scale heat flux vector. The two dimensionless constants associated with these subgrid-scale models are obtained by correlating with the results of direct numerical simulations of compressible isotropic turbulence performed on a 96(exp 3) grid using Fourier collocation methods. Extensive comparisons between the direct and modeled subgrid-scale fields are provided in order to validate the models. A large-eddy simulation of the decay of compressible isotropic turbulence (conducted on a coarse 32(exp 3) grid) is shown to yield results that are in excellent agreement with the fine grid direct simulation. Future applications of these compressible subgrid-scale models to the large-eddy simulation of more complex supersonic flows are discussed briefly.
Broken Ergodicity in Two-Dimensional Homogeneous Magnetohydrodynamic Turbulence
NASA Technical Reports Server (NTRS)
Shebalin, John V.
2010-01-01
Two-dimensional (2-D) homogeneous magnetohydrodynamic (MHD) turbulence has many of the same qualitative features as three-dimensional (3-D) homogeneous MHD turbulence.The se features include several ideal invariants, along with the phenomenon of broken ergodicity. Broken ergodicity appears when certain modes act like random variables with mean values that are large compared to their standard deviations, indicating a coherent structure or dynamo.Recently, the origin of broken ergodicity in 3-D MHD turbulence that is manifest in the lowest wavenumbers was explained. Here, a detailed description of the origins of broken ergodicity in 2-D MHD turbulence is presented. It will be seen that broken ergodicity in ideal 2-D MHD turbulence can be manifest in the lowest wavenumbers of a finite numerical model for certain initial conditions or in the highest wavenumbers for another set of initial conditions.T he origins of broken ergodicity in ideal 2-D homogeneous MHD turbulence are found through an eigen analysis of the covariance matrices of the modal probability density functions.It will also be shown that when the lowest wavenumber magnetic field becomes quasi-stationary, the higher wavenumber modes can propagate as Alfven waves on these almost static large-scale magnetic structures
Fully developed turbulence in slugs of pipe flows
NASA Astrophysics Data System (ADS)
Cerbus, Rory; Liu, Chien-Chia; Sakakibara, Jun; Gioia, Gustavo; Chakraborty, Pinaki
2015-11-01
Despite over a century of research, transition to turbulence in pipe flows remains a mystery. In theory the flow remains laminar for arbitrarily large Reynolds number, Re. In practice, however, the flow transitions to turbulence at a finite Re whose value depends on the disturbance, natural or artificial, in the experimental setup. The flow remains in the transition state for a range of Re ~ 0 (1000) ; for larger Re the flow becomes fully developed. The transition state for Re > 3000 consists of axially segregated regions of laminar and turbulent patches. These turbulent patches, known as slugs, grow as they move downstream. Their lengths span anywhere between a few pipe diameters to the whole length of the pipe. Here we report Stereo Particle Image Velocimetry measurements in the cross-section of the slugs. Notwithstanding the continuous growth of the slugs, we find that the mean velocity and stress profiles in the slugs are indistinguishable from that of statistically-stationary fully-developed turbulent flows. Our results are independent of the length of the slugs. We contrast our results with the well-known work of Wygnanski & Champagne (1973), whose measurements, we argue, are insufficient to draw a clear conclusion regarding fully developed turbulence in slugs.
Comparison of hot wire/laser velocimeter turbulence intensity measurements
NASA Technical Reports Server (NTRS)
Meyers, J. F.; Wilkinson, S. P.
1982-01-01
The question of whether a random measure of particle velocities yields a good statistical estimate of the stationary condition of the turbulence flow field was investigated by comparing hot-wire and laser velocimeter turbulence intensity measurements. Great care was taken to insure that the instrument precision of both the laser velocimeter and hot wire was maximized. In this attempt to reduce the measurement uncertainties in the hot wire, direct digitization of the analog output signal was performed with point-by-point conversion to velocity through a spline fit calibration curve and the turbulence intensity function was calculated statistically. Frequent calibrations of the hot wire were performed using the laser velocimeter as the velocity standard to account for the presence of the small seed particles in the air flow and signal drift in the hot wire.
RECONNECTION OUTFLOW GENERATED TURBULENCE IN THE SOLAR WIND
Vörös, Z.; Sasunov, Y. L.; Zaqarashvili, T. V.; Khodachenko, M.; Semenov, V. S.; Bruno, R.
2014-12-10
Petschek-type time-dependent reconnection (TDR) and quasi-stationary reconnection (QSR) models are considered to understand reconnection outflow structures and the generation of local turbulence in the solar wind. Comparing TDR/QSR model predictions of the outflow structures with actual measurements shows that both models can explain the data equally well. It is demonstrated that the outflows can often generate more or less spatially extended turbulent boundary layers. The structure of a unique extended reconnection outflow is investigated in detail. The analysis of spectral scalings and spectral break locations shows that reconnection can change the local field and plasma conditions which may support different local turbulent dissipation mechanisms at their characteristic wavenumbers.
Effects of wall curvature on turbulence statistics
NASA Technical Reports Server (NTRS)
Moser, R. D.; Moin, P.
1985-01-01
A three-dimensional, time-dependent, direct numerical simulation of low-Reynolds number turbulent flow in a mildly curved channel was performed, and the results examined to determine the mechanism by which curvature affects wall-bounded turbulent shear flows. A spectral numerical method with about one-million modes was employed, and no explicit subgrid scale model was used. The effects of curvature on this flow were determined by comparing the concave and convex sides of the channel. The observed effects are consistent with experimental observations for mild curvature. The most significant difference in the turbulence statistics between the concave and convex sides is in the Reynolds shear stress. This is accompanied by significant differences in the terms of the Reynolds shear stress balance equations. In addition, it was found that stationary Taylor-Goertler vortices were present and that they had a significant effect on the flow by contributing to the mean Reynolds shear stress, and by enhancing the difference between the wall shear stresses.
Decaying Turbulence in the Generalised Burgers Equation
NASA Astrophysics Data System (ADS)
Boritchev, Alexandre
2014-10-01
We consider the generalised Burgers equation where f is strongly convex and ν is small and positive. We obtain sharp estimates for Sobolev norms of u (upper and lower bounds differ only by a multiplicative constant). Then, we obtain sharp estimates for the dissipation length scale and the small-scale quantities which characterise the decaying Burgers turbulence, i.e., the structure functions and the energy spectrum. The proof uses a quantitative version of an argument by Aurell et al. (J Fluid Mech 238:467-486, 1992). Note that we are dealing with decaying, as opposed to stationary turbulence. Thus, our estimates are not uniform in time. However, they hold on a time interval [ T 1, T 2], where T 1 and T 2 depend only on f and the initial condition, and do not depend on the viscosity. These results allow us to obtain a rigorous theory of the one-dimensional Burgers turbulence in the spirit of Kolmogorov's 1941 theory. In particular, we obtain two results which hold in the inertial range. On one hand, we explain the bifractal behaviour of the moments of increments, or structure functions. On the other hand, we obtain an energy spectrum of the form k -2. These results remain valid in the inviscid limit.
The theoretical model of atmospheric turbulence spectrum in surface layer
NASA Astrophysics Data System (ADS)
Liu, Shida; Liu, Shikuo; Xin, Guojun; Liang, Fuming
1994-12-01
It is shown that the slope of energy spectrum obtained from the velocity solution of Kdv—Burgers equation lies between —5/3 and—2 in the dilogarithmic coordinates paper. The spectrum is very close to one of Kolmogorov's isotropic turbulence and Frisch's intermittent turbulence in inertial region. In this paper, the Kdv-Burgers equation to describe atmospheric boundary layer turbulence is obtained. In the equation, the 1 / R e corresponds to dissipative coefficient v, R /2 t to dispersive coefficient β, then ( v/2 β)2 corresponds to 1 / R 2 e • Ri. We prove that the wave number corresponding to maximum energy spectrum S(k) decreases with the decrease of stability (i.e., the increase of ( v / 2 β)2 in eddy—containing region. And the spectrim amplitude decreases with the increase of ( v / 2 β)2 (i.e., the decrease of stability). These results are consistent with actual turbulence spectrum of atmospheric surface layer from turbulence data.
Nonhelical inverse transfer of a decaying turbulent magnetic field.
Brandenburg, Axel; Kahniashvili, Tina; Tevzadze, Alexander G
2015-02-20
In the presence of magnetic helicity, inverse transfer from small to large scales is well known in magnetohydrodynamic (MHD) turbulence and has applications in astrophysics, cosmology, and fusion plasmas. Using high resolution direct numerical simulations of magnetically dominated self-similarly decaying MHD turbulence, we report a similar inverse transfer even in the absence of magnetic helicity. We compute for the first time spectral energy transfer rates to show that this inverse transfer is about half as strong as with helicity, but in both cases the magnetic gain at large scales results from velocity at similar scales interacting with smaller-scale magnetic fields. This suggests that both inverse transfers are a consequence of universal mechanisms for magnetically dominated turbulence. Possible explanations include inverse cascading of the mean squared vector potential associated with local near two dimensionality and the shallower k^{2} subinertial range spectrum of kinetic energy forcing the magnetic field with a k^{4} subinertial range to attain larger-scale coherence. The inertial range shows a clear k^{-2} spectrum and is the first example of fully isotropic magnetically dominated MHD turbulence exhibiting weak turbulence scaling. PMID:25763960
Two phenomenological constants explain similarity laws in stably stratified turbulence.
Katul, Gabriel G; Porporato, Amilcare; Shah, Stimit; Bou-Zeid, Elie
2014-02-01
In stably stratified turbulent flows, the mixing efficiency associated with eddy diffusivity for heat, or equivalently the turbulent Prandtl number (Pr(t)), is fraught with complex dynamics originating from the scalewise interplay between shear generation of turbulence and its dissipation by density gradients. A large corpus of data and numerical simulations agree on a near-universal relation between Pr(t) and the Richardson number (R(i)), which encodes the relative importance of buoyancy dissipation to mechanical production of turbulent kinetic energy. The Pr(t)-R(i) relation is shown to be derivable solely from the cospectral budgets for momentum and heat fluxes if a Rotta-like return to isotropy closure for the pressure-strain effects and Kolmogorov's theory for turbulent cascade are invoked. The ratio of the Kolmogorov to the Kolmogorov-Obukhov-Corrsin phenomenological constants, and a constant associated with isotropization of the production whose value (= 3/5) has been predicted from Rapid Distortion Theory, explain all the macroscopic nonlinearities. PMID:25353571
Modeling of Turbulent Flow in Electromagnetically Levitated Metal Droplets
NASA Technical Reports Server (NTRS)
Berry, S.; Hyers, R. W.; Abedian, B.; Racz, L. M.; Rose, M. Franklin (Technical Monitor)
2001-01-01
This article details an effort to improve the understanding and prediction of turbulent flow inside a droplet of molten metal levitated in an electromagnetic field. It is shown that the flow field in a test case, a nickel droplet levitated under microgravity conditions, is in the transitional regime between laminar and turbulent flow. Past research efforts have used laminar, enhanced viscosity, and k-epsilon turbulence models to describe the flow. The method highlighted in our study is the renormalization group (RNG) algorithm. We show that an accurate description of the turbulent eddy viscosity is critical in order to obtain realistic velocity fields, and that the turbulent eddy viscosity cannot be uniform in levitated droplets. The RNG method does not impose isotropic length or time scales on the flow field, thus allowing such nonuniform features to be captured. A number of other materials processing applications exhibit similarly complex flow characteristics, such as highly recirculating, transitional, and free surface flows, for which this modeling approach may prove useful.
Acceleration PDFs of particles in rotating turbulent convection
NASA Astrophysics Data System (ADS)
Clercx, Herman; Perlekar, Prasad; Lavezzo, Valentina; Toschi, Federico
2012-11-01
Particle dispersion in buoyancy-driven rotating turbulent flows has direct relevance for many industrial and environmental applications. We have used a Lattice Boltzmann Method coupled with Lagrangian particle tracking algorithm to investigate the behaviour of passive and inertial particles released in turbulent rotating Rayleigh-Bénard (RB) convection. The flow domain is horizontally periodic and vertically confined. Both the gravity and the rotation vector are oriented in the vertical direction. Here we present the results of the acceleration PDFs of particles in both non-rotating and strongly rotating RB convection. It is found that the bulk acceleration PDF in non-rotating RB turbulence is like in homogeneous isotropic turbulence whereas rotation introduces anisotropy similar to acceleration PDFs obtained from experiments in (isothermal) forced rotating turbulence. These results and those obtained for inertial particles will be discussed. PP and VL were financially supported by the Foundation for Fundamental Research on Matter (FOM), which is part of NWO. This work was sponsored by NWO-NCF (SH-176).
Microbubbles and Microparticles are Not Faithful Tracers of Turbulent Acceleration
NASA Astrophysics Data System (ADS)
Mathai, Varghese; Calzavarini, Enrico; Brons, Jon; Sun, Chao; Lohse, Detlef
2016-07-01
We report on the Lagrangian statistics of acceleration of small (sub-Kolmogorov) bubbles and tracer particles with Stokes number St ≪1 in turbulent flow. At a decreasing Reynolds number, the bubble accelerations show deviations from that of tracer particles; i.e., they deviate from the Heisenberg-Yaglom prediction and show a quicker decorrelation despite their small size and minute St. Using direct numerical simulations, we show that these effects arise due the drift of these particles through the turbulent flow. We theoretically predict this gravity-driven effect for developed isotropic turbulence, with the ratio of Stokes to Froude number or equivalently the particle drift velocity governing the enhancement of acceleration variance and the reductions in correlation time and intermittency. Our predictions are in good agreement with experimental and numerical results. The present findings are relevant to a range of scenarios encompassing tiny bubbles and droplets that drift through the turbulent oceans and the atmosphere. They also question the common usage of microbubbles and microdroplets as tracers in turbulence research.