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1

Analytical prediction of friction factors and Nusselt numbers of turbulent forced convection in rod bundles with smooth and rough surfaces  

Microsoft Academic Search

A simple analytical method was developed for the prediction of the friction factor, f, of fully developed turbulent flow and the Nusselt number, Nu, of fully developed turbulent forced convection in rod bundles arranged in square or hexagonal arrays. The friction factor equation for smooth rod bundles was presented in a form similar to the friction factor equation for turbulent

Jian Su; Atila P. Silva Freire

2002-01-01

2

Friction factor for turbulent flow in rough pipes from Heisenberg's closure hypothesis  

E-print Network

We show that the main results of the analysis of the friction factor for turbulent pipe flow reported in G. Gioia and P. Chakraborty (GC), Phys. Rev. Lett. 96, 044502 (1996) can be recovered by assuming the Heisenberg closure hypothesis for the turbulent spectrum. This highlights the structural features of the turbulent spectrum underlying GC's analysis.

Esteban Calzetta

2009-04-17

3

Experimental heat transfer and friction factors in turbulated cooling passages of different aspect ratios, where turbulators are staggered  

NASA Astrophysics Data System (ADS)

Turbulator ribs of different height/passage hydraulic diameter, positioned in different configurations relative to one another, are used in advanced aircraft engines to enhance heat transfer from the blade surface to the internal cooling air. An experimental investigation has been conducted to ascertain the effect that turbulators positioned perpendicular to the direction of flow and in a staggered configuration have on heat transfer coefficients and friction factors in cooling passages of different aspect ratios. Comparisons are made between staggered and in-line turbulator configuration results; higher heat transfer coefficients are obtained for the staggered configuration.

Taslim, M. E.; Spring, S. D.

1988-07-01

4

The friction factor of two-dimensional rough-boundary turbulent soap film flows  

E-print Network

We use momentum transfer arguments to predict the friction factor $f$ in two-dimensional turbulent soap-film flows with rough boundaries (an analogue of three-dimensional pipe flow) as a function of Reynolds number Re and roughness $r$, considering separately the inverse energy cascade and the forward enstrophy cascade. At intermediate Re, we predict a Blasius-like friction factor scaling of $f\\propto\\textrm{Re}^{-1/2}$ in flows dominated by the enstrophy cascade, distinct from the energy cascade scaling of $\\textrm{Re}^{-1/4}$. For large Re, $f \\sim r$ in the enstrophy-dominated case. We use conformal map techniques to perform direct numerical simulations that are in satisfactory agreement with theory, and exhibit data collapse scaling of roughness-induced criticality, previously shown to arise in the 3D pipe data of Nikuradse.

Nicholas Guttenberg; Nigel Goldenfeld

2009-03-25

5

Rotordynamic analysis of annular honeycomb-stator turbulent gas seals using a new friction-factor model based on flat plate tests  

NASA Astrophysics Data System (ADS)

A flat plate tester with various honeycomb geometries has been extended to develop a better understanding of the friction-factor behavior of honeycomb surfaces. The friction-factor-jump phenomenon, which is characterized by the dramatic drop and then rise of the friction-factor with increasing Reynolds number, has been explained by acoustic excitation of a large scale coherent flow structure from pressure fluctuation measurements inside the honeycomb cavities. A new friction-factor model based on the flat-plate-test results has been developed as a function of Mach number, dimensionless pressure, and honeycomb geometry variables. A rotordynamic analysis has been developed for centered, turbulent-annular honeycomb-stator seals incorporating the new empirical friction-factor model for honeycomb-stator surfaces. The validity of the new analysis in predicting the rotordynamic and leakage characteristics has been compared to Moody's friction-factor model analysis and experimental data for a short (L/D = 1/6, 25.4 mm long) seal and a longer (L/D = 1/3, 50.8 mm long) seal. The comparisons show that the new honeycomb friction-factor model greatly improves the predictions of leakage and rotordynamic coefficients compared to Moody's friction-factor model for both the short and longer seal, especially, for direct stiffness and cross-coupled stiffness. The new honeycomb friction-factor model predicts leakage and rotordynamic coefficients better for the short than the longer seals.

Ha, Tae Woong

1992-01-01

6

Factors Affecting Friction  

NSDL National Science Digital Library

Based on what they have already learned about friction, students formulate hypotheses concerning the effects of weight and contact area on the amount of friction between two surfaces. In the Associated Activities (Does Weight Matter? and Does Area Matter?), students design and conduct simple experiments to test their hypotheses, using procedures similar to those used in the previous lesson (Discovering Friction). An analysis of their data will reveal the importance of weight to normal friction (the friction that occurs as a result of surface roughness) and the importance of surface area to the friction that occurs between smooth surfaces due to molecular attraction. Based on their data, students will also be able to calculate coefficients of friction for the materials tested, and compare these to published values for various materials.

Engineering K-Ph.D. Program,

7

Transient Turbulent Friction in Smooth Pipe Flows  

NASA Astrophysics Data System (ADS)

A weighting function model of unsteady skin friction in smooth-walled, one-dimensional ducts is derived using an idealized form of the radial viscosity distribution. The model is an enhancement of earlier work by the authors in which additional simplifying assumptions were made. Important improvements include (1) replacing the assumption of uniform (solid) behaviour in an extensive core region by an assumption of uniform turbulent viscosity and (2) relating the wall shear stress to the mean flow velocity instead of to the maximum velocity. The resulting model can be used directly in numerical analyses of transient flows in pipes. It can also be used to deduce numerical values of an empirical coefficient in a popular alternative model of skin friction in which the unsteady contribution is assumed to be proportional to the instantaneous mean acceleration.

Vardy, A. E.; Brown, J. M. B.

2003-01-01

8

Experimental determination of average turbulent heat transfer and friction factor in stator internal rib-roughened cooling channels.  

PubMed

In gas turbine cooling design, techniques for heat extraction from the surfaces exposed to the hot stream are based on the increase of the inner heat transfer areas and on the promotion of the turbulence of the cooling flow. This is currently obtained by casting periodic ribs on one or more sides of the serpentine passages into the core of the blade. Fluid dynamic and thermal behaviour of the cooling flow have been extensively investigated by means of experimental facilities and many papers dealing with this subject have appeared in the latest years. The evaluation of the average value of the heat transfer coefficient most of the time is inferred from local measurements obtained by various experimental techniques. Moreover the great majority of these studies are not concerned with the overall average heat transfer coefficient for the combined ribs and region between them, but do focus just on one of them. This paper presents an attempt to collect information about the average Nusselt number inside a straight ribbed duct. Series of measurements have been performed in steady state eliminating the error sources inherently connected with transient methods. A low speed wind tunnel, operating in steady state flow, has been built to simulate the actual flow condition occurring in a rectilinear blade cooling channel. A straight square channel with 20 transverse ribs on two sides has been tested for Re of about 3 x 10(4), 4.5 x 10(4) and 6 x 10(4). The ribbed wall test section is electrically heated and the heat removed by a stationary flow of known thermal and fluid dynamic characteristics. PMID:11460662

Battisti, L; Baggio, P

2001-05-01

9

Relationship between frictional coefficient and frictional factor in asymmetrical sheet rolling  

Microsoft Academic Search

Generally, two kinds of surface frictional states can be used, i.e. Coulomb friction (?=?p) and constant shear friction (?=mk) in analyzing asymmetrical sheet rolling. The relationship between frictional coefficient and frictional factor in asymmetrical sheet rolling is worthy of investigation. In this study, a relationship between frictional coefficient and frictional factor is proposed. Researchers can use this developed relationship to

Gow-Yi Tzou

1998-01-01

10

Factors affecting piston ring friction  

E-print Network

The piston ring pack friction is a major contributor to the internal combustion engine mechanical friction loss. The oil control ring decides the oil supply to the top two rings in addition to being the major friction ...

Liao, Kai, Ph. D. Massachusetts Institute of Technology

2013-01-01

11

A weighting function model of transient turbulent pipe friction  

Microsoft Academic Search

A weighting function model of transient turbulent pipe friction at moderate Reynolds Numbers is developed in a similar manner to Zielke's equivalent expression for laminar flow (1968). It is shown that a family of weighting function curves exists and that Zielke's curve is an upper bound asymptote that is valid for low Reynolds numbers, not only for laminar flow.The new

A. E. Vardy; Kuo-Lun Hwang

1993-01-01

12

Tidal friction in rotating turbulent convectivestellar and planetary regions  

NASA Astrophysics Data System (ADS)

Turbulent friction in stellar and planetary convection zones is one of the key physical mechanisms that drive the dissipation of the kinetic energy of tidal flows in stars and planets hosting companions. This friction acting both on the equilibrium tide and on tidal inertial waves thus deeply impacts the dynamics of the spin of the host star/planet and the orbital architecture of the surrounding system. It is thus very important to obtain robust prescription for this friction. In the current state-of-the-art, it is modeled by a turbulent viscosity coefficient using mixing-length theory. However, none of the existing prescriptions take into account the action of the possibly rapid rotation that strongly affects convective flows. In this work, we propose such a new prescription that takes into account rotation and discuss the possible implication for tidal dissipation in rotating stars and planets.

Mathis, S.; Auclair-Desrotour, P.; Guenel, M.; Le Poncin-Lafitte, C.

2014-12-01

13

Predicting friction factor in herbaceous emergent wetlands  

NASA Astrophysics Data System (ADS)

Over 53% of all wetlands in the US have been lost since the mid-1780s; to counteract wetland losses, wetland land area is being replaced through wetland restoration and mitigation. Development of the target wetland hydroperiod is critical to restoration success. For wetlands in which outflow is a component of the water budget, such as in riparian wetlands, surface water stage is controlled all or in part by the hydraulic resistance within the wetland, requiring accurate simulation of hydraulic resistance due to vegetation. Hydraulic models that consider vegetation rely on an accurate determination of a resistance parameter such as a friction factor or drag coefficient. At low Reynolds numbers typical of flows in wetlands, hydraulic resistance is orders of magnitude higher than fully turbulent flows and resistance parameters are functions of the flow regime as well as the vegetation density and structure. The exact relationship between hydraulic resistance, flow regime, and vegetation properties at the low-Reynolds number flows remains unclear. Prior research has typically involved laboratory studies of flow through idealized, individual stems. However, emergent wetland vegetation frequently grows as clumps. The goals of this research were to investigate the effect of clumping vegetation on flow resistance and to develop a prediction equation for use in wetland design. A 6-m by 1-m by 0.4-m recirculating flume was planted with mature common rush, Juncus effusus, a common emergent wetland plant. Three different flow rates (3, 4, and 5 L/s) and three different tailgate heights (0, 2.5, and 5 cm) were used to simulate a range of flow conditions. Plant spacing and clump diameter were varied (20 and 25 cm, 8 and 12 cm, respectively). Friction factors ranged from 9 to 40 and decreased with increasing plant density. Non-dimensional parameters determined through Buckingham Pi analysis were used in a regression analysis to develop a prediction model. Results of the regression analysis showed that the fraction of vegetated occupied area was most significant factor in determining friction factor.

Wynn-Thompson, T.; Hall, K.

2012-12-01

14

TRANSIENT TURBULENT FRICTION IN SMOOTH PIPE FLOWS  

Microsoft Academic Search

A weighting function model of unsteady skin friction in smooth-walled, one-dimensional ducts is derived using an idealized form of the radial viscosity distribution. The model is an enhancement of earlier work by the authors in which additional simplifying assumptions were made. Important improvements include (1) replacing the assumption of uniform (solid) behaviour in an extensive core region by an assumption

A. E. VARDY; J. M. B. BROWN

2003-01-01

15

Transient Turbulent Friction in Smooth Pipe Flows  

Microsoft Academic Search

A weighting function model of unsteady skin friction in smooth-walled, one-dimensional ducts is derived using an idealized form of the radial viscosity distribution. The model is an enhancement of earlier work by the authors in which additional simplifying assumptions were made. Important improvements include (1) replacing the assumption of uniform (solid) behaviour in an extensive core region by an assumption

A. E. Vardy; J. M. B. Brown

2003-01-01

16

A skin friction law for compressible turbulent flow  

NASA Technical Reports Server (NTRS)

An algebraic skin friction law is derived for adiabatic, compressible, equilibrium, turbulent boundary layer flow. An outer solution in terms of the Clauser defect stream function is matched to an inner empirical expression composed of compressible laws of the wall and wake. The modified Crocco temperature-velocity relationship and the Clauser eddy viscousity model are used in the outer solution. The skin friction law pertains for all pressure gradients in the incompressible through supersonic range and for small pressure gradients in the hypersonic range. Excellent comparisons with experiment are obtained in the appropriate parameter ranges. The application to numerical computation is discussed.

Barnwell, Richard W.; Wahls, Richard A.

1989-01-01

17

Friction  

NSDL National Science Digital Library

This web resource was created as a study in friction by students at an Advanced Mechanics class at Davidson College in Davidson. The resource includes a historical background as well as the various factors that affect friction. The analysis includes sections about Rolling Friction, Velocity Dependence, Stick-Slip mechanism, Adhesion, Wear, Lubrication, Viscosity and Wet vs. Dry Friction.

Conatser, Ryan; Kinnaman, Ben; Lassiter, Mike

2011-04-13

18

Turbulence and skin friction modification in channel flow with streamwise-aligned superhydrophobic surface texture  

NASA Astrophysics Data System (ADS)

Direct numerical simulations of turbulent flow in a channel with superhydrophobic surfaces (SHS) were performed, and the effects of the surface texture on the turbulence and skin-friction coefficient were examined. The SHS is modeled as a planar boundary comprised of spanwise-alternating regions of no-slip and free-slip boundary conditions. Relative to the reference no-slip channel flow at the same bulk Reynolds number, the overall mean skin-friction coefficient is reduced by 21.6%. A detailed analysis of the turbulence kinetic energy budget demonstrates a reduction in production over the no-slip phases, which is explained by aid of quadrant analysis of the Reynolds shear stresses and statistical analysis of the turbulence structures. The results demonstrate a significant reduction in the strength of streamwise vortical structures in the presence of the SHS texture and a decrease in the Reynolds shear-stress component ?R12? which has a favorable influence on drag over the no-slip phases. A secondary flow which is set up at the edges of the texture also effects a beneficial change in drag. Nonetheless, the skin-friction coefficient on the no-slip features is higher than the reference levels in a simple no-slip channel flow. The increase in the skin-friction coefficient is attributed to two factors. First, spanwise diffusion of the mean momentum from free-slip to no-slip regions increases the local skin-friction coefficient on the edges of the no-slip features. Second, the drag-reducing capacity of the SHS is further reduced due to additional Reynolds stresses, ?R13?.

Jelly, T. O.; Jung, S. Y.; Zaki, T. A.

2014-09-01

19

Evolutionary optimization of an anisotropic compliant surface for turbulent friction drag  

E-print Network

Evolutionary optimization of an anisotropic compliant surface for turbulent friction drag reduction surface is performed in order to investigate its drag reduction effect in a fully developed turbulent flow in a reduction of the friction drag with a maximum reduction rate of 8%. The primary mechanism for drag reduction

Tokyo, University of

20

A Fresh Approach to Flow Turbulence Towards Reduction of Skin-friction Drag  

E-print Network

RESEARCH HIGHLIGHTS A Fresh Approach to Flow Turbulence Towards Reduction of Skin-friction Drag Turbulence Towards Reduction of Skin-friction Drag Subrahmanyam Duvvuri Research advised by Prof. Beverley Mc energy expenditure sector, accounting for 30% of global consumption. A mere 1% reduction in drag

21

Variable enstrophy flux and energy spectrum in two-dimensional turbulence with Ekman friction  

E-print Network

Experiments and numerical simulations reveal that in the forward cascade regime, the energy spectrum of two-dimensional turbulence with Ekman friction deviates from Kraichnan's prediction of $k^{-3}$ power spectrum. In this letter we explain this observation using an analytic model based on variable enstrophy flux arising due to Ekman friction. We derive an expression for the enstrophy flux which exhibits a logarithmic dependence in the inertial range for the Ekman-friction dominated flows. The energy spectrum obtained using this enstrophy flux shows a power law scaling for large Reynolds number and small Ekman friction, but has an exponential behaviour for large Ekman friction and relatively small Reynolds number.

Mahendra K. Verma

2012-03-23

22

Calculation of skin-friction coefficients for low Reynolds number turbulent boundary layer flows  

Microsoft Academic Search

An analysis is presented of the reliability of various generally accepted empirical expressions for the prediction of the skin-friction coefficient C\\/sub f\\/ of turbulent boundary layers at low Reynolds numbers in zero-pressure-gradient flows on a smooth flat plate. The skin-friction coefficients predicted from these expressions were compared to the skin-friction coefficients of experimental profiles that were determined from a graphical

P. K. Barr

1980-01-01

23

An Experimental Study of Turbulent Skin Friction Reduction in Supersonic Flow Using a Microblowing Technique  

NASA Technical Reports Server (NTRS)

A new turbulent skin friction reduction technology, called the microblowing technique has been tested in supersonic flow (Mach number of 1.9) on specially designed porous plates with microholes. The skin friction was measured directly by a force balance and the boundary layer development was measured by a total pressure rake at the tailing edge of a test plate. The free stream Reynolds number was 1.0(10 exp 6) per meter. The turbulent skin friction coefficient ratios (C(sub f)/C(sub f0)) of seven porous plates are given in this report. Test results showed that the microblowing technique could reduce the turbulent skin friction in supersonic flow (up to 90 percent below a solid flat plate value, which was even greater than in subsonic flow).

Hwang, Danny P.

1999-01-01

24

Review of Research into the Concept of the Microblowing Technique for Turbulent Skin Friction Reduction  

NASA Technical Reports Server (NTRS)

A new technology for reducing turbulent skin friction, called the Microblowing Technique (MBT), is presented. Results from proof-of-concept experiments show that this technology could potentially reduce turbulent skin friction by more than 50% of the skin friction of a solid flat plate for subsonic and supersonic flow conditions. The primary purpose of this review paper is to provide readers with information on the turbulent skin friction reduction obtained from many experiments using the MBT. Although the MBT has a penalty for obtaining the microblowing air associated with it, some combinations of the MBT with suction boundary layer control methods are an attractive alternative for a real application. Several computational simulations to understand the flow physics of the MBT are also included. More experiments and computational fluid dynamics (CFD) computations are needed for the understanding of the unsteady flow nature of the MBT and the optimization of this new technology.

2004-01-01

25

Turbulent Friction in the Boundary Layer of a Flat Plate in a Two-Dimensional Compressible Flow at High Speeds  

NASA Technical Reports Server (NTRS)

In the present report an investigation is made on a flat plate in a two-dimensional compressible flow of the effect of compressibility and heating on the turbulent frictional drag coefficient in the boundary layer of an airfoil or wing radiator. The analysis is based on the Prandtl-Karman theory of the turbulent boundary later and the Stodola-Crocco, theorem on the linear relation between the total energy of the flow and its velocity. Formulas are obtained for the velocity distribution and the frictional drag law in a turbulent boundary later with the compressibility effect and heat transfer taken into account. It is found that with increase of compressibility and temperature at full retardation of the flow (the temperature when the velocity of the flow at a given point is reduced to zero in case of an adiabatic process in the gas) at a constant R (sub x), the frictional drag coefficient C (sub f) decreased, both of these factors acting in the same sense.

Frankl, F.; Voishel, V.

1943-01-01

26

Rotordynamic analysis for stepped-labyrinth gas seals using Moody’s friction-factor model  

Microsoft Academic Search

The governing equations are derived for the analysis of a stepped labyrinth gas seal generally used in high performance compressors,\\u000a gas turbines, and steam turbines. The bulk-flow is assumed for a single cavity control volume set up in a stepped labyrinth\\u000a cavity and the flow is assumed to be completely turbulent in the circumferential direction. The Moody’s wall-friction-factor\\u000a model is

Tae Woong Ha

2001-01-01

27

The FIK Identity and Its Implication for Turbulent Skin Friction Control  

Microsoft Academic Search

The identity equation derived by Fukagata, Iwamoto & Kasagi (Phys. Fluids 14, L73-L76, 2002) leads to a general strategy for accomplishing turbulent skin friction drag reduction. This is demonstrated by referring to several typical examples of recently studied control schemes including local blowing\\/suction and surfactant additives. Based on the FIK identity and numerical experiment of direct numerical simulation, the performance

Nobuhide Kasagi; Koji Fukagata

28

Transient turbulent friction in fully rough pipe flows  

NASA Astrophysics Data System (ADS)

A weighting-function model of unsteady skin friction in fully rough-walled flows in one-dimensional ducts is derived using an idealized radial viscosity distribution. The model complements previous work by the authors for smooth-walled flows. It is assumed that, for sufficiently short-lived transients, the viscosity distribution in the cross-section may be regarded as constant and equal to that in a pre-existing steady flow. The eddy viscosity in an outer annulus is assumed to vary linearly from a minimum at the wall to a maximum at the edge of a central core of uniform viscosity. The resulting weighting-function model for short-lived transients is used to develop a simple formula predicting values of unsteady skin friction coefficients suitable for an instantaneous-acceleration model of unsteady skin friction in fully rough pipe flows.

Vardy, A. E.; Brown, J. M. B.

2004-02-01

29

Dynamical turbulent flow on the Galton board with friction.  

PubMed

We study numerically and analytically the dynamics of charged particles on the Galton board, a regular lattice of disk scatters, in the presence of constant external force, magnetic field, and friction. It is shown that under certain conditions friction leads to the appearance of a strange chaotic attractor. In this regime the average velocity and direction of particle flow can be effectively affected by electric and magnetic fields. We discuss the applications of these results to the charge transport in antidot superlattices and the stream of suspended particles in a viscous flow through scatters. PMID:11461559

Chepelianskii, A D; Shepelyansky, D L

2001-07-16

30

Transient turbulent friction in fully rough pipe flows  

Microsoft Academic Search

A weighting-function model of unsteady skin friction in fully rough-walled flows in one-dimensional ducts is derived using an idealized radial viscosity distribution. The model complements previous work by the authors for smooth-walled flows. It is assumed that, for sufficiently short-lived transients, the viscosity distribution in the cross-section may be regarded as constant and equal to that in a pre-existing steady

A. E. Vardy; J. M. B. Brown

2004-01-01

31

Turbulent Friction in Rough Pipes and the Energy Spectrum of the Phenomenological Theory  

E-print Network

The classical experiments on turbulent friction in rough pipes were performed by J. Nikuradse in the 1930's. Seventy years later, they continue to defy theory. Here we model Nikuradse's experiments using the phenomenological theory of Kolmog\\'orov, a theory that is widely thought to be applicable only to highly idealized flows. Our results include both the empirical scalings of Blasius and Strickler, and are otherwise in minute qualitative agreement with the experiments; they suggest that the phenomenological theory may be relevant to other flows of practical interest; and they unveil the existence of close ties between two milestones of experimental and theoretical turbulence.

G. Gioia; Pinaki Chakraborty

2005-12-20

32

Artificially thickened turbulent boundary layers for studying heat transfer and skin friction on rough surfaces  

NASA Astrophysics Data System (ADS)

A technique has been developed to produce artificially thickened boundary layers on a uniformly rough surface which have two-dimensional, equilibrium properties representative of normal behavior at the level of spectra of the longitudinal velocity fluctuations. Skin surface friction coefficients and Stanton numbers are obtained for rough surface boundary layers much thicker than those previously studied. The results provide additional understanding of turbulent shear flow and a basis for testing engineering calculation schemes and design procedures for situations where thick, rough-wall boundary layers are present. The Stanton numbers and skin friction coefficients are shown to be representative of natural behavior because three higher levels of information, as well as the turbulent transport of momentum and heat, are the same as would have existed in naturally developed layers of the same thickness.

Ligrani, P. M.; Moffat, R. J.; Kays, W. M.

1983-06-01

33

Friction Factor Characterization for High-Porosity Random Fiber Regenerators  

NASA Technical Reports Server (NTRS)

NASA Glenn Research Center, the Department of Energy (DOE), and Stirling Technology Company (STC) of Kennewick, Washington are developing a Stirling convertor for a high-efficiency Stirling Radioisotope Power System to provide electric power for NASA Space Science Missions. STC is developing the 55-We Technology Demonstration Convertor (TDC) under contract to DOE. Steady-flow tests were completed to determine the friction factor for the high-porosity regenerators that are used in the TDC. STC fabricated a flow test fixture and three random fiber regenerator test samples, one each at approximately 80, 88, and 96 percent porosities. The flow tests were then completed by the NASA Glenn Flow Calibration Laboratory, and the data reduced to Reynolds number and friction factor. The results showed that the 80 and 88 percent porosity samples had similar characteristics while the 96 percent porosity sample had significantly higher friction factors for given Reynolds numbers compared to the samples with lower porosities. Comparisons were also made between the test data and existing correlations. STC used this data to derive a modified regenerator friction factor correlation for use in the Stirling design code GLIMPS for porosities greater than 88 percent. Using this new correlation, the final optimized regenerator design porosity was reduced from 96 to 90 percent.

Thieme, Lanny G.

2001-01-01

34

Effects of friction factor and slip factor on the performance of a centrifugal slurry pump  

E-print Network

EFFECTS OF FRICTION FACTOR AND SLIP FACTOR ON THE PERFORMANCE OF A CENTRIFUGAL SLURRY PUMP A Thesis by KETANKUMAR KANTILAL SHETH Submitted to the Graduate College of Texas ASM University in partial fulfillment of the requirements... for the degree of MASTER OF SCIENCE August 1985 Ma]or Sub]ect: Mechanical Engineering EFFECTS OF FRICTION FACTOR AND SLIP FACTOR ON THE PERFORMANCE OF A CENTRIFUGAL SLURRY PUMP A Thesis by KETANKUMAR KANTILAL SHETH Approved as to style and content by...

Sheth, Ketankumar Kantilal

2012-06-07

35

Laser interferometer skin-friction measurements of crossing-shock-wave/turbulent-boundary-layer interactions  

NASA Technical Reports Server (NTRS)

Wall shear stress measurements beneath crossing-shock-wave/turbulent boundary-layer interactions have been made for three interactions of different strengths. The interactions are generated by two sharp fins at symetric angles of attack mounted on a flat plate. The shear stress measurements were made for fin angles of 7 and 11 deg at Mach 3 and 15 deg at Mach 3.85. The measurements were made using a laser interferometer skin-friction meter, a device that determines the wall shear by optically measuring the time rate of thinning of an oil film placed on the test model surface. Results of the measurements reveal high skin-friction coefficients in the vicinity of the fin/plate junction and the presence of quasi-two-dimensional flow separation on the interaction center line. Additionally, two Navier-Stokes computations, one using a Baldwin-Lomax turbulence model and one using a k-epsilon model, are compared with the experimental results for the Mach 3.85, 15-deg interaction case. Although the k-epsilon model did a reasonable job of predicting the overall trend in portions of the skin-friction distribution, neither computation fully captured the physics of the near-surface flow in this complex interaction.

Garrison, T. J.; Settles, G. S.; Narayanswami, N.; Knight, D. D.

1994-01-01

36

Estimating Overwater Turbulence Intensity from Routine Gust-Factor Measurements.  

NASA Astrophysics Data System (ADS)

For overwater diffusion estimates the Offshore and Coastal Dispersion (OCD) model is preferred by the U.S. Environmental Protection Agency. The U.S. Minerals Management Service has recommended that the OCD model be used for emissions located on the outer continental shelf. During southerly winds over the Gulf of Mexico, for example, the pollutants from hundreds of offshore platforms may affect the gulf coasts. In the OCD model, the overwater plume is described by the Gaussian equation, which requires the computation of ?y and ?z, which are, in turn, related to the turbulence intensity, overwater trajectory, and atmospheric stability. On the basis of several air sea interaction experiments [the Barbados Oceanographic and Meteorological Experiment (BOMEX), the Air-Mass Transformation Experiment (AMTEX), and, most recently, the Tropical Ocean and Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE)] and the extensive datasets from the National Data Buoy Center (NDBC), it is shown that under neutral and stable conditions the overwater turbulence intensities are linearly proportional to the gust factor (G), which is the ratio of the wind gust and mean wind speed at height z (Uz) as reported hourly by the NDBC buoys. Under unstable conditions, it is first shown that the popular formula relating the horizontal turbulence intensity (?u,/u, where u is the friction velocity) to the ratio of the mixing height (h) and the buoyancy length (L) (i.e., h/L) suffers from a self-correlation problem and cannot be used in the marine environment. Then, alternative formulas to estimate the horizontal turbulence intensities (?u,/Uz) using G are proposed for practical applications. Furthermore, formulas to estimate u and z/L are fundamentally needed in air sea interaction studies, in addition to dispersion meteorology.


Hsu, S. A.; Blanchard, Brian W.

2004-12-01

37

Engineering prediction of turbulent skin friction and heat transfer in high-speed flow  

NASA Technical Reports Server (NTRS)

A large collection of experimental turbulent-skin-friction and heat-transfer data for flat plates and cones was used to determine the most accurate of six of the most popular engineering-prediction methods; the data represent a Mach number range from 4 to 13 and ratio of wall to total temperature ranging from 0.1 to 0.7. The Spalding and Chi method incorporating virtual-origin concepts was found to be the best prediction method for Mach numbers less than 10; the limited experimental data for Mach numbers greater than 10 were not well predicted by any of the engineering methods except the Coles method.

Cary, A. M., Jr.; Bertram, M. H.

1974-01-01

38

Effects of Riblets on Skin Friction in High-Speed Turbulent Boundary Layers  

NASA Technical Reports Server (NTRS)

Direct numerical simulations of spatially developing turbulent boundary layers over riblets are conducted to examine the effects of riblets on skin friction at supersonic speeds. Zero-pressure gradient boundary layers with an adiabatic wall, a Mach number of M1 = 2.5, and a Reynolds number based on momentum thickness of Re = 1720 are considered. Simulations are conducted for boundary-layer flows over a clean surface and symmetric V- groove riblets with nominal spacings of 20 and 40 wall units. The DNS results confirm the few existing experimental observations and show that a drag reduction of approximately 7% is achieved for riblets with proper spacing. The influence of riblets on turbulence statistics is analyzed in detail with an emphasis on identifying the differences, if any, between the drag reduction mechanisms for incompressible and high-speed boundary layers.

Duan, Lian; Choudhari, Meelan M.

2012-01-01

39

Simple LMFBR axial-flow friction-factor correlation  

SciTech Connect

Complicated LMFBR axial lead-length averaged friction-factor correlations are reduced to an easy, ready-to-use function of bundle Reynolds number for wire-wrapped bundles. The function together with the power curves to calculate the associated constants are incorporated in a computer preprocessor, EZFRIC. The constants required for the calculation of the subchannels and bundle friction factors are derived and correlated into power curves of geometrical parameters. A computer program, FRIC, which can alternatively be used to accurately calculate these constants is also included. The accurate values of the constants and the corresponding values predicted by the power curves and percentage error of prediction are tabulated for a wide variety of geometries of interest.

Chan, Y.N.; Todreas, N.E.

1982-12-01

40

Understanding the friction factor behavior in liquid annular seals with deliberately roughened surfaces, a CFD approach  

E-print Network

seals performance and leakage information indicating that friction factor increases as the seal clearance is increased, contradicting the theory predictions based on Moody's pipe-friction model. A Computational Fluid Dynamics (CFD) code is used...

Villasmil Urdaneta, Larry Alfonso

2012-06-07

41

Design, modeling, and optimization of compliant tensegrity fabrics for the reduction of turbulent skin friction  

NASA Astrophysics Data System (ADS)

In this project, we have designed a new type of flexible surface, which we call a tensegrity fabric, and simulated the interaction of this flexible surface with a near-wall turbulent flow. The fabric is constructed by weaving together both members in tension (tendons) and members in compression (bars) to form a plate-class tensegrity structure, then covering this discrete flexible structure with a continuous flexible membrane. We have modeled the flow/structure interaction by coupling a spectral Direct Numerical Simulation (DNS) code resolving the (continuous) turbulent flow system and an efficient structural dynamics code which simulates direclty the motion of the (discrete) extensive, small-scale, and interconnected tensegrity structure. The structural dynamics code used was developed by Prof. Robert Skelton's lab at UC San Diego. An immersed boundary method is used to capture the effect of the moving boundary in the DNS, and a simple tessellation strategy is used to lump the distributed fluid forces (skin friction and pressure) acting on the membrane onto the nearby nodes of the tensegrity structure. Our ultimate goal is to use this new simulation tool to optimize the design of the tensegrity structure (specifically, the orientation, stiffness, mass, and damping of each of the individual tendons and bars in the unit cell upon which the tensegrity structure is based). Our objective in this optimization is to tune the compliance properties of the fabric in such a way as to reduce the skin-friction drag induced at teh flow/structure interface by weakening the vortices near the wall in the overlying turbulent flow.

Luo, Haoxiang; Bewley, Thomas R.

2003-07-01

42

Friction  

NSDL National Science Digital Library

This website from Kathleen Cummings Dominguez at the Illinois Institute of Technology provides a lesson plan on the concepts of friction. It describes a lesson plan which will engage students in active classroom learning.

2010-03-17

43

Friction  

NASA Astrophysics Data System (ADS)

Friction materials such as disk pads, brake linings, and clutch facings are widely used for automotive applications. Friction materials function during braking due to frictional resistance that transforms kinetic energy into thermal energy. There has been a rudimentary evolution, from materials like leather or wood to asbestos fabric or asbestos fabric saturated with various resins such as asphalt or resin combined with pitch. These efforts were further developed by the use of woven asbestos material saturated by either rubber solution or liquid resin binder and functioned as an internal expanding brake, similar to brake lining system. The role of asbestos continued through the use of chopped asbestos saturated by rubber, but none was entirely successful due to the poor rubber heat resistance required for increased speeds and heavy gearing demands of the automobile industry. The use of phenolic resins as binder for asbestos friction materials provided the necessary thermal resistance and performance characteristics. Thus, the utility of asbestos as the main friction component, for over 100 years, has been significantly reduced in friction materials due to asbestos identity as a carcinogen. Steel and other fibrous components have displaced asbestos in disk pads. Currently, non-asbestos organics are the predominate friction material. Phenolic resins continue to be the preferred binder, and increased amounts are necessary to meet the requirements of highly functional asbestos-free disk pads for the automotive industry. With annual automobile production exceeding 70 million vehicles and additional automobile production occurring in developing countries worldwide and increasing yearly, the amount of phenolic resin for friction material is also increasing (Fig. 14.1). Fig. 14.1 Worldwide commercial vehicle production In recent years, increased fuel efficiency of passenger car is required due to the CO2 emission issue. One of the solutions to improve fuel efficiency is to lower the car body weight. It means that the weight of car components must be decreased. In the case of reduced weight for friction parts, the load applied to the friction parts would be higher (more heat also) and trend would lead to phenolic resins with improved heat resistance.

Matsuo, Yoshihiro; Clarke, Daryl D.; Ozeki, Shinichi

44

Friction  

NSDL National Science Digital Library

The representation demonstrates, through an animated, narrated slide-show, how frictional forces, including air resistance, can affect the motion of an object. This resource also includes an interactive test and review of the material. One is also able to download "myskoool" which allows allows one to download lessons to run offline and use anytime.

45

Charts Adapted from Van Driest's Turbulent Flat-plate Theory for Determining Values of Turbulent Aerodynamic Friction and Heat-transfer Coefficients  

NASA Technical Reports Server (NTRS)

A modified method of Van Driest's flat-plate theory for turbulent boundary layer has been found to simplify the calculation of local skin-friction coefficients which, in turn, have made it possible to obtain through Reynolds analogy theoretical turbulent heat-transfer coefficients in the form of Stanton number. A general formula is given and charts are presented from which the modified method can be solved for Mach numbers 1.0 to 12.0, temperature ratios 0.2 to 6.0, and Reynolds numbers 0.2 times 10 to the 6th power to 200 times 10 to the 6th power.

Lee, Dorothy B; Faget, Maxime A

1956-01-01

46

The Relative Validity of the Concepts of Coefficient of Friction and Interface Friction Shear Factor for Use in Metal Deformation Studies  

Microsoft Academic Search

The relative validity of the concepts of constant coefficient of friction (f) and constant interface friction shear factor (m) as quantitative indices for defining friction stresses existing in metal deformation operations has been investigated by means of the ring compression test technique. Material flow stress values have been determined by ring compression tests using mathematical formulae based upon both concepts

Alan T. Male; Vincent De Pierre; George Saul

1973-01-01

47

Measuring and modelling the frictional velocity u*, turbulence and heat fluxes above the North Sea  

NASA Astrophysics Data System (ADS)

In this study, we analyse the frictional velocity u*, drag coefficient, vertical wind speed and turbulence profiles observed at different met-masts in the German North and Baltic Sea. We present an analysis of different models for the frictional velocity u* in convective, neutral and stable thermal stratification of the atmosphere. Atmospheric turbulent momentum and heat flux measurements performed with ultra-sonic anemometers are compared to profile-derived values and a bulk Richardson number formulation of the atmospheric thermal stability. Modelling: An improved approach to model the vertical wind speed profile is presented and compared against meso-scale model results (WRF, COSMO): Bye-Ekman-Coupling (BEC) describes the flux of momentum from the Ekman layer of the atmosphere through the Prandtl layer down to the air-sea interface by a modified wave boundary layer with enhanced Charnock dynamics (Bye et al. 2010). The BEC model is based on the coupled pair of similarity relations for "aerodynamically rough flow" in both fluids (air and sea). The derived drag law is of Charnock form, almost independent of the wave age and consistent with the transfer of momentum to the wave spectrum - which takes place in the smaller rather than the dominant wavelengths. Measurements: It was found that the frictional velocity u* is considerably smaller than predicted by conventional approaches using the Charnock relation: For wind speeds between 10 m/s and 15 m/s at 40 m height above the sea surface, u*(observed) is 14% smaller than u*(Charnock). Most important, we found unexpected, strong and obviously artificial distortions concerning the three wind speed components in the 10Hz data of the three ultra-sonic anemometers at the offshore met-mast FINO1 at 40 m, 60 m and 80 m height. The pattern of these distortions is independent from different post-processing procedures (planar-fit etc.). We anticipate that these artefacts imply severe problems for the eddy covariance technique. Moreover, these artefacts may be relevant in other (previous and on-going) ultra-sonic measurement campaigns where turbulent parameters such as u* and heat fluxes are derived. A simple, but innovative analysis is proposed to check ultra-sonic measurements with respect to these artefacts, using the original temporal 10Hz resolution of the data: The instantaneous vertical wind speed component w is analysed versus the instantaneous wind direction (called wind.dir in the following), computed from the instantaneous horizontal components u and v. The observational density is then plotted in the (w; wind.dir)-space. We found a pattern of stripes of very strong densities for specific wind direction bins, which are thinner than 1° and which cannot be attributed directly to the geometry of the anemometer (transducers, physical structure etc.). The source of this artificial pattern is still unclear and open for discussion. References: Bye JAT, Ghantous M, Wolff J-O (2010) On the variability of the Charnock constant and the functional dependence of the drag coefficient on wind speed. Ocean Dynamics 60(4) 851-860

Tambke, Jens; Bye, John A. T.; Schmidt, Michael; Wolff, Jörg-Olaf

2014-05-01

48

Friction factor data for flat plate tests of smooth and honeycomb surfaces. M.S. Thesis  

NASA Technical Reports Server (NTRS)

Friction factors for honeycomb surfaces were measured with a flat plate tester. The flat plate test apparatus was described and a method was discussed for determining the friction factor experimentally. The friction factor model was developed for the flat plate test based on the Fanno Line Flow. The comparisons of the friction factor were plotted for smooth surfaces and six-honeycomb surfaces with three-clearances, 6.9 bar to 17.9 bar range of inlet pressures, and 5,000 to 100,000 range of the Reynolds number. The optimum geometries for the maximum friction factor were found as a function of cell width to cell depth and cell width to clearance ratios.

Ha, Tae Woong

1989-01-01

49

Laser Interferometer Skin-Friction measurements of crossing-shock wave/turbulent boundary-layer interactions  

NASA Technical Reports Server (NTRS)

Wall shear stress measurements beneath crossingshock wave/turbulent boundary-layer interactions have been made for three interactions of different strengths. The interactions are generated by two sharp fins at symmetric angles of attack mounted on a flat plate. The shear stress measurements were made for fin angles of 7 and 11 degrees at Mach 3 and 15 degrees at Mach 4. The measurements were made using a Laser Interferometer Skin Friction (LISF) meter; a device which determines the wail shear by optically measuring the time rate of thinning of an oil film placed on the test model surface. Results of the measurements reveal high skin friction coefficients in the vicinity of the fin/plate junction and the presence of quasi-two-dimensional flow separation on the interaction centerline. Additionally, two Navier-Stokes computations, one using a Baldwin-Lomax turbulence model and one using a k- model, are compared to the experimental results for the Mach 4, 15 degree interaction case. While the k- model did a reasonable job of predicting the overall trend in portions of the skin friction distribution, neither computation fully captured the physics of the near surface flow in this complex interaction.

Garrison, T. J.; Settles, G. S.

1993-01-01

50

Friction Factor Evaluation Using Experimental and Finite Element Methods for Al-4%Cu Preforms  

NASA Astrophysics Data System (ADS)

In this study, ring compression tests and finite element (FE) simulations have been utilized to evaluate the friction factor, m, under different lubricating conditions for powder metallurgical (P/M) Al-4%Cu preforms. A series of ring compression tests were carried out to obtain friction factor ( m) for a number of lubricating conditions, including zinc stearate, graphite, molybdenum disulfide powder, and unlubricated condition. FE simulations were used to analyze materials deformation, densification, and geometric changes, and to derive the friction calibration curves. The friction factor has been determined for various initial relative densities and different lubricating conditions, and a proper lubricant for cold forging of P/M Al-4%Cu preforms is found. Studies show that the use of lubricants has reduced the friction. However, increase in the number of pores in the preforms leads to excessive friction. The FE simulation results demonstrate a shift in the neutral plane distance from the axis of ring specimen, which occurred due to variations in the frictional conditions and initial relative densities. The load requirement for deformation, effective stress, and effective strain induced, and bulging phenomena obtained by FE simulations have a good agreement with the experimental data.

Desalegn, Wogaso; Davidson, M. J.; Khanra, A. K.

2014-08-01

51

Entrance and exit region friction factor models for annular seal analysis. Ph.D. Thesis  

NASA Technical Reports Server (NTRS)

The Mach number definition and boundary conditions in Nelson's nominally-centered, annular gas seal analysis are revised. A method is described for determining the wall shear stress characteristics of an annular gas seal experimentally. Two friction factor models are developed for annular seal analysis; one model is based on flat-plate flow theory; the other uses empirical entrance and exit region friction factors. The friction factor predictions of the models are compared to experimental results. Each friction model is used in an annular gas seal analysis. The seal characteristics predicted by the two seal analyses are compared to experimental results and to the predictions of Nelson's analysis. The comparisons are for smooth-rotor seals with smooth and honeycomb stators. The comparisons show that the analysis which uses empirical entrance and exit region shear stress models predicts the static and stability characteristics of annular gas seals better than the other analyses. The analyses predict direct stiffness poorly.

Elrod, David Alan

1988-01-01

52

Direct measurements and analysis of skin friction and cooling downstream of multiple flush-slot injection into a turbulent Mach 6 boundary layer  

NASA Technical Reports Server (NTRS)

Experiments were conducted to determine the reduction in surface skin friction and the effectiveness of surface cooling downstream of one to four successive flush slots injecting cold air at an angle of 10 deg into a turbulent Mach 6 boundary layer. Data were obtained by direct measurement of surface shear and equilibrium temperatures, respectively. Increasing the number of slots decreased the skin friction, but the incremental improvement in skin-friction reduction decreased as the number of slots was increased. Cooling effectiveness was found to improve, for a given total mass injection, as the number of slots was increased from one to four. Comparison with previously reported step-slot data, however, indicated that step slots with tangential injection are more effective for both reducing skin friction and cooling than the present flush-slot configuration. Finite-difference predictions are in reasonable agreement with skin-friction data and with boundary-layer profile data.

Howard, F. G.; Strokowski, A. J.

1978-01-01

53

Friction factors in smooth trapezoidal silicon microchannels with different aspect ratios  

Microsoft Academic Search

An experiment has been conducted to measure the friction factor of laminar flow of deionized water in smooth silicon microchannels of trapezoidal cross-section with hydraulic diameters in the range of 25.9–291.0 ?m. It is shown that the friction constant of these microchannels is greatly influenced by the cross-sectional aspect ratio, Wb\\/Wt. Based on the 334 data points, a correlation equation

H. Y Wu; Ping Cheng

2003-01-01

54

Turbulence and mixing in a tube  

NSDL National Science Digital Library

Using the friction factor correlation, estimate the pressure drop, turbulent viscosity and diffusivity, and large- and small-scale homogenization times for a substance introduced at the tube entrance.

Powell, Adam C., IV

2004-09-26

55

The behavior of the skin-friction coefficient of a turbulent boundary layer flow over a flat plate with differently configured transverse square grooves  

SciTech Connect

Skin-friction coefficient of turbulent boundary layer flow over a smooth-wall with transverse square grooves was investigated. Four grooved-wall cases were investigated. The four grooved-wall configurations are single 5mm square grooved-wall, and 5mm square grooves spaced 10, 20 and 40 element widths apart in the streamwise direction. Laser-Doppler Anemometer (LDA) was used for the mean velocity and turbulence intensity measurements. The skin-friction coefficient determined from the velocity profile increases sharply just downstream of the groove. This overshoot is followed by an undershoot and then relaxation back to the smooth-wall value. This behavior is observed in most grooved-wall cases. Integrating the skin-friction coefficient in the streamwise direction indicates that there is an increase in the overall drag in all the grooved-wall cases.

Wahidi, R.; Chakroun, W.; Al-Fahed, S. [Faculty of Engineering, Mechanical Engineering, Kuwait University, P.O. Box 5969, 13060 Safat (Kuwait)

2005-11-01

56

An annular gas seal analysis using empirical entrance and exit region friction factors  

NASA Technical Reports Server (NTRS)

Wall shear stress results from stationary-rotor flow tests of five annular gas seals are used to develop entrance and exit region friction factor models. The friction factor models are used in a bulk-flow seal analysis which predicts leakage and rotor-dynamic coefficients. The predictions of the analysis are compared to experimental results and to the predictions of Nelson's analysis (1985). The comparisons are for smooth-rotor seals with smooth and honeycomb-stators. The present analysis predicts the destabilizing cross-coupled stiffness of a seal better than Nelson's analysis. Both analyses predict direct damping well and direct stiffness poorly.

Elrod, D. A.; Childs, D. W.; Nelson, C. C.

1990-01-01

57

Selection of relaxation factors for computing steady state turbulent flows  

NASA Astrophysics Data System (ADS)

Two computer programs, TEACH and CHAMPION, are modified and used to simulate two-dimensional steady-state turbulent flows. A criterion is defined for selection of the optimum time step in order to ensure the convergence and accuracy of the method. The method is demonstrated through discretization of the one-dimensional transport equation. A relaxation method is employed to obtain a steady-state solution, and variable relaxation factors are derived with a time-dependent solution. Sample problems are provided for turbulent flow and swirling turbulent flow through a pipe with a sudden enlargement in the diameter. The implementation of techniques involving the iterative solution of steady state flows for the solution of unsteady state equation is shown to lower the required computing time for the solution to converge.

Neuberger, A. W.; Chatwani, A. U.; Eickhoff, H.; Koopman, J.

58

Structure of atmospheric turbulence in the friction layer below 500 meters  

NASA Technical Reports Server (NTRS)

Measurements of wind direction and speed, vertical velocity, and temperature were made from several levels of the 444 m tower near Oklahoma City. Turbulence quantities were calculated from the covariances between observed variables over periods ranging from 5 min to 1 hr. It was found that in some cases parameters such as mean wind speed, shearing stress, and vertical heat flux could be expressed by simple equations for periods of 15 min to 1 hr. Changes in these quantities with time are related to changes in vertical motion and stability. Power spectra were calculated for sequential 15 min, 30 min, and 1 hr periods. The effects of stability, wind speed, and surface roughness on the spectra of longitudinal and lateral velocity were examined, along with the effect of height on the spectrum of vertical velocity. This region was shown to be composed of a lower region in which mechanical turbulence dominates and an upper region dominated by convective turbulence.

Maas, S. J.; Scoggins, J. R.

1976-01-01

59

Friction factor and heat transfer of nanofluids containing cylindrical nanoparticles in laminar pipe flow  

NASA Astrophysics Data System (ADS)

Numerical simulations of polyalphaolefins-Al2O3 nanofluids containing cylindrical nanoparticles in a laminar pipe flow are performed by solving the Navier-Stokes equation with term of cylindrical nanoparticles, the general dynamic equation for cylindrical nanoparticles, and equation for nanoparticle orientation. The distributions of particle number and volume concentration, the friction factor, and heat transfer are obtained and analyzed. The results show that distributions of nanoparticle number and volume concentration are non-uniform across the section, with larger and smaller values in the region near the pipe center and near the wall, respectively. The non-uniformity becomes significant with the increase in the axial distance from the inlet. The friction factor decreases with increasing Reynolds number. The relationships between the friction factor and the nanoparticle volume concentration as well as particle aspect ratio are dependent on the Reynolds number. The Nusselt number of nanofluids, directly proportional to the Reynolds number, particle volume concentration, and particle aspect ratio, is higher near the pipe entrance than at the downstream locations. The rate of increase in Nusselt number at lower particle volume concentration is more than that at higher concentration. Finally, the expressions of friction factor and Nusselt number as a function of particle volume concentration, particle aspect ratio, and Reynolds number are derived based on the numerical data.

Lin, Jianzhong; Xia, Yi; Ku, Xiaoke

2014-10-01

60

Assessments of fluid friction factors for use in leak rate calculations  

SciTech Connect

Leak before Break procedures require estimates of leakage, and these in turn need fluid friction to be assessed. In this paper available data on flow rates through idealized and real crack geometries are reviewed in terms of a single friction factor k It is shown that for {lambda} < 1 flow rates can be bounded using correlations in terms of surface R{sub a} values. For {lambda} > 1 the database is less precise, but {lambda} {approx} 4 is an upper bound, hence in this region flow calculations can be assessed using 1 < {lambda} < 4.

Chivers, T.C. [Berkeley Technology Centre, Glos (United Kingdom)

1997-04-01

61

Biomechanical risk factors and flexor tendon frictional work in the cadaveric carpal tunnel.  

PubMed

Pathological changes in carpal tunnel syndrome patients include fibrosis and thickening of the subsynovial connective tissue (SSCT) adjacent to the flexor tendons in the carpal tunnel. These clinical findings suggest an etiology of excessive shear-strain force between the tendon and SSCT, underscoring the need to assess tendon gliding characteristics representative of repetitive and forceful work. A mechanical actuator moved the middle finger flexor digitorum superficialis tendon proximally and distally in eight fresh frozen cadaver arms. Eighteen experimental conditions tested the effects of three well-established biomechanical predictors of injury, including a combination of two wrist postures (0° and 30° flexion), three tendon velocities (50, 100, 150mm/sec), and three forces (10, 20, 40N). Tendon gliding resistance was determined with two light-weight load cells, and integrated over tendon displacement to represent tendon frictional work. During proximal tendon displacement, frictional work increased with tendon velocity (58.0% from 50-150mm/sec). There was a significant interaction between wrist posture and tendon force. In wrist flexion, frictional work increased 93.0% between tendon forces of 10 and 40N. In the neutral wrist posture, frictional work only increased 33.5% (from 10-40N). During distal tendon displacement, there was a similar multiplicative interaction on tendon frictional work. Concurrent exposure to multiple biomechanical work factors markedly increased tendon frictional work, thus providing a plausible link to the pathogenesis of work-related carpal tunnel syndrome. Additionally, our study provides the conceptual basis to evaluate injury risk, including the multiplicative repercussions of combined physical exposures. PMID:25553671

Kociolek, Aaron M; Tat, Jimmy; Keir, Peter J

2015-02-01

62

Microstructural factors governing hardness in friction-stir welds of solid-solution-hardened Al alloys  

Microsoft Academic Search

Microstructural factors governing hardness in friction-stir welds of the solid-solution-hardened Al alloys 1080 and 5083 were\\u000a examined by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effect\\u000a of grain boundary on the hardness was examined in an Al alloy 1080 which did not contain any second-phase particles. The weld\\u000a of Al alloy 1080 had a slightly

Yutaka S. Sato; Seung Hwan C. Park; Hiroyuki Kokawa

2001-01-01

63

Microstructures forming in friction welding of Inconel 718 alloy – joint performance and its controlling factors in friction welding of Inconel 718 alloy  

Microsoft Academic Search

Microstructures forming in the friction welding of Inconel 718 alloy have been investigated in order to understand the phenomena occurring during the welding process and to determine the factor controlling the joint performance from a metallographic point of view. In the interfacial zone, liquation microstructures characterized by a eutectic structure consisting of ? and Laves phases, and Nb-rich microstructures along

Hongjie Wang; Kenji Ikeuchi; Masatoshi Aritoshi; Makoto Takahashi; Akio Ikeda

2009-01-01

64

The effect of a turbulent wake on the stagnation point. I - Skin friction results  

NASA Technical Reports Server (NTRS)

The response of a boundary layer in the stagnation region of a two-dimensional body to fluctuations in the freestream is examined. The analysis is restricted to laminar incompressible flow. The assumed form of the velocity distribution at the edge of the boundary layer represents both a pulsation of the incoming flow, and an oscillation of the stagnation point streamline. Both features are essential in accurately representing the effect which freestream spatial and temporal nonuniformities have upon the unsteady boundary layer. Finally, a simple model is proposed which relates the characteristic parameters in a turbulent wake to the unsteady boundary-layer edge velocity. Numerical results are presented for both an arbitrary two-dimensional geometry and a circular cylinder.

Wilson, Dennis E.; Hanford, Anthony J.

1990-01-01

65

A comparison of rotordynamic-coefficient predictions for annular honeycomb gas seals using different friction-factor models  

E-print Network

Predictions of rotordynamic-coefficients for annular honeycomb gas seals are compared using different friction-factor models. Analysis shows that the fundamental improvement in predicting the rotordynamic-coefficients accurately is the two...

D'Sousa, Rohan Joseph

2012-06-07

66

A comparison of rotordynamic-coefficient predictions for annular honeycomb gas seals using different friction-factor models.  

E-print Network

??Predictions of rotordynamic-coefficients for annular honeycomb gas seals are compared using different friction-factor models. Analysis shows that the fundamental improvement in predicting the rotordynamic-coefficients accurately… (more)

D'Sousa, Rohan Joseph

2012-01-01

67

Correlations for Predicting the Air-Side Nusselt Numbers and Friction Factors in Chilled-Water Cooling Coils  

Microsoft Academic Search

An experimental study was conducted to determine Nusselt numbers and friction factors on the air side of wavy-finned, chilled-water cooling coils. General correlations of the dry-surface Nusselt numbers and friction factors were developed from the data obtained from five different cooling coils. A comparison of the Nusselt number correlation to data from the literature revealed that the correlation was generally

D. R. Mirth; S. Ramadhyani

1994-01-01

68

New heat transfer and friction factor design data for perforated plate heat exchangers  

NASA Astrophysics Data System (ADS)

Perforated plate heat exchangers have been found to have inherently low axial conduction and are therefore excellent candidates for cryogenic applications where an all-metal design is required. A total of three plate cores were tested; two were chemically etched and the other mechanically punched. Hole size, percent open area and plate thickness parameters were varied among the plates. Experimental results were compared to analytical projections and found to differ significantly. The single-blow, transient test technique was used to determine the heat transfer coefficients and the isothermal pressure drop test was used to determine friction factors, as a function of Reynolds number.

Hubbell, Richard H.; Cain, Christina L.

69

Acoustics of friction.  

PubMed

This article presents an overview of the acoustics of friction by covering friction sounds, friction-induced vibrations and waves in solids, and descriptions of other frictional phenomena related to acoustics. Friction, resulting from the sliding contact of solids, often gives rise to diverse forms of waves and oscillations within solids which frequently lead to radiation of sound to the surrounding media. Among the many everyday examples of friction sounds, violin music and brake noise in automobiles represent the two extremes in terms of the sounds they produce and the mechanisms by which they are generated. Of the multiple examples of friction sounds in nature, insect sounds are prominent. Friction also provides a means by which energy dissipation takes place at the interface of solids. Friction damping that develops between surfaces, such as joints and connections, in some cases requires only microscopic motion to dissipate energy. Modeling of friction-induced vibrations and friction damping in mechanical systems requires an accurate description of friction for which only approximations exist. While many of the components that contribute to friction can be modeled, computational requirements become prohibitive for their contemporaneous calculation. Furthermore, quantification of friction at the atomic scale still remains elusive. At the atomic scale, friction becomes a mechanism that converts the kinetic energy associated with the relative motion of surfaces to thermal energy. However, the description of the conversion to thermal energy represented by a disordered state of oscillations of atoms in a solid is still not well understood. At the macroscopic level, friction interacts with the vibrations and waves that it causes. Such interaction sets up a feedback between the friction force and waves at the surfaces, thereby making friction and surface motion interdependent. Such interdependence forms the basis for friction-induced motion as in the case of ultrasonic motors and other examples. Last, when considered phenomenologically, friction and boundary layer turbulence exhibit analogous properties and, when compared, each may provide clues to a better understanding of the other. PMID:12002837

Akay, Adnan

2002-04-01

70

Friction WebQuest  

NSDL National Science Digital Library

Let's review what friction is and what it does! 1. Try this interactive game to see what factors make more or less friction. Sid Friction Experiment 2. Read the information on this page, then click the purple box at the bottom to take the quiz and see how much you remember about friction. Friction Info and Quiz 3. Here is another interactive game that lets you experiment with ...

Carrie Benson

2011-12-06

71

Correlation equations for friction factors and convective coefficients in tubes containing bundles of internal, longitudinal fins  

NASA Astrophysics Data System (ADS)

Finned tubes are commonly employed in tubular heat exchangers to augment the heat transfer rates between two dissimilar fluid streams. The goal of this study is two-fold: a) to determine the laminar, fully developed velocity and temperature fields for long, internal, longitudinal finned tubes; and b) to construct empirical correlation equations applicable for the asymptotic friction factor and the asymptotic Nusselt numbers as a function of the number of fins and the relative fin height in the bundle. Neither the fluid dynamics nor the heat transfer literature provides information for the second goal, which can be extremely useful to thermal engineers for the implementation of Computer-Aided Design (CAD) of internal, longitudinal finned tubes in heat exchange devices. Moreover, for the validity of the correlation equations, a seldom adopted criterion for the critical Reynolds number has also been introduced as an indispensable guideline to warrant laminar regimes.

Campo, A.; Chang, J.

72

New friction factor derived from study of Egyptian gas-field pipeline  

SciTech Connect

Evaluations of a newly developed friction-factor equation (and hence a new gas-flow formula) indicate that it produces higher accuracy than all equations currently in use. Further, the new equation can be conveniently sued to optimize operating conditions of any existing gas pipeline or aid in designing new, similar gas pipelines. The evaluation of the new equation showed it to be more flexible in accommodating many ranges of flow rate by developing the relevant new constant related to gas flow rate. The proposed new equation can be used to estimate the effect of any variable on gas flow in a horizontal or inclined system in addition to facilitating the optimum design of any new gas pipeline.

El-Emam, N. [Al-Azhar Univ., Cairo (Egypt); Gad, F.K.; Nafey, A.S. [Suez Canal Univ. (Egypt); Zoghaib, N. [Balayim Petroleum Co., Cairo (Egypt)

1997-11-10

73

Heat-transfer and friction factor design data for all-metal compact heat exchangers  

NASA Astrophysics Data System (ADS)

To improve system life, an effort was undertaken to develop all-metal compact heat exchangers to replace existing heat exchangers which contain organic materials. Eliminating organic materials increases system life because outgassing organics can contaminate the helium working fluid and reduce system life. Perforated plate heat exchangers were found to have inherently low axial conduction and are therefore excellent candidates for cryogenic applications where an all-metal design is required. A total of 11 plate cores were tested; 2 were chemically etched, 1 was mechanically punched, and 8 were manufactured using electron beam drilling. Hole size, percent open area, and plate thickness parameters were varied among the plates. Experimental results were compared to analytical projections and found to differ significantly. The single-blow transient test technique was used to determine the heat transfer coefficients and the isothermal pressure drop test was used to determine friction factors, as a function of Reynolds number.

Cain, Christina L.

1989-03-01

74

Vortex avalanches and the onset of superfluid turbulence  

Microsoft Academic Search

Quantized circulation, absence of Galilean invariance due to a clamped normal\\u000acomponent, and the vortex mutual friction are the major factors that make\\u000asuperfluid turbulence behave in a way different from that in classical fluids.\\u000aThe model is developed for the onset of superfluid turbulence that describes\\u000athe initial avalanche-like multiplication of vortices into a turbulent vortex\\u000atangle.

N. B. Kopnin; L. D. Landau

2003-01-01

75

In-flight compressible turbulent boundary layer measurements on a hollow cylinder at a Mach number of 3.0. [supersonic heat transfer and skin friction  

NASA Technical Reports Server (NTRS)

Skin temperatures, shearing forces, surface static pressures, and boundary layer pitot pressures and total temperatures were measured on a hollow cylinder 3.04 meters long and 0.437 meter in diameter mounted beneath the fuselage of the YF-12A airplane. The data were obtained at a nominal free stream Mach number of 3.0 and at wall-to-recovery temperature ratios of 0.66 to 0.91. The free stream Reynolds number had a minimal value of 4.2 million per meter. Heat transfer coefficients and skin friction coefficients were derived from skin temperature time histories and shear force measurements, respectively. Boundary layer velocity profiles were derived from pitot pressure measurements, and a Reynolds analogy factor of 1.11 was obtained from the measured heat transfer and skin friction data. The skin friction coefficients predicted by the theory of van Driest were in excellent agreement with the measurements. Theoretical heat transfer coefficients, in the form of Stanton numbers calculated by using a modified Reynolds analogy between skin friction and heat transfer, were compared with measured values. The measured velocity profiles were compared to Coles' incompressible law-of-the-wall profile.

Quinn, R. D.; Gong, L.

1978-01-01

76

Friction Factor Measurement, Analysis, and Modeling for Flat-Plates with 12.15 mm Diameter Hole-Pattern, Tested with Air at Different Clearances, Inlet Pressures, and Pressure Ratios  

E-print Network

Friction factor data are important for better prediction of leakage and rotordynamic coefficients of gas annular seals. A flat-plate test rig is used to determine friction factor of hole-pattern/honeycomb flat-plate surfaces representing annular...

Deva Asirvatham, Thanesh

2011-02-22

77

Turbulence  

NASA Astrophysics Data System (ADS)

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.

Frisch, Uriel

1996-01-01

78

Internal friction quality-factor Q under confining pressure. [of lunar rocks  

NASA Technical Reports Server (NTRS)

It has been found in previous studies that small amounts of adsorbed volatiles can have a profound effect on the internal friction quality-factor Q of rocks and other porous media. Pandit and Tozer (1970) have suggested that the laboratory-measured Q of volatile-free rocks should be similar to the in situ seismic Q values of near-surface lunar rocks which according to Latham et al. (1970) are in the range of 3000-5000. Observations of dramatic increases in Q with outgassing up to values approaching 2000 in the seismic frequency range confirm this supposition. Measurements under confining pressures with the sample encapsulated under hard vacuum are reported to aid in the interpretation of seismic data obtained below the lunar surface. It has been possible to achieve in the experiments Q values just under 2000 at about 1 kbar for a terrestrial analog of lunar basalt. It was found that a well-outgassed sample maintains a high Q whereas one exposed to moisture maintains a low Q as the confining pressure is raised to 2.5 kbar. This result suggests that volatiles can indeed affect Q when cracks are partially closed and the high lunar seismic Q values reported are concomitant with very dry rock down to depths of at least 50 km.

Tittmann, B. R.; Ahlberg, L.; Nadler, H.; Curnow, J.; Smith, T.; Cohen, E. R.

1977-01-01

79

The influence of free-stream turbulence on turbulent boundary layers with mild adverse pressure gradients  

NASA Technical Reports Server (NTRS)

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.

Hoffmann, Jon A.

1988-01-01

80

The influence of free-stream turbulence on turbulent boundary layers with mild adverse pressure gradients  

NASA Technical Reports Server (NTRS)

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.

Hoffmann, J. A.; Kassir, S. M.; Larwood, S. M.

1989-01-01

81

Is internal friction friction?  

USGS Publications Warehouse

Mogi [1974] proposed a simple model of the incipient rupture surface to explain the Coulomb failure criterion. We show here that this model can plausibly be extended to explain the Mohr failure criterion. In Mogi's model the incipient rupture surface immediately before fracture consists of areas across which material integrity is maintained (intact areas) and areas across which it is not (cracks). The strength of the incipient rupture surface is made up of the inherent strength of the intact areas plus the frictional resistance to sliding offered by the cracked areas. Although the coefficient of internal friction (slope of the strength versus normal stress curve) depends upon both the frictional and inherent strengths, the phenomenon of internal friction can be identified with the frictional part. The curvature of the Mohr failure envelope is interpreted as a consequence of differences in damage (cracking) accumulated in prefailure loading at different confining pressures.

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

1996-01-01

82

Equivalence of Non-Equilibrium Ensembles and Representation of Friction in Turbulent Flows: The Lorenz 96 Model  

E-print Network

We construct different equivalent non-equilibrium statistical ensembles in a simple yet instructive $N$-degrees of freedom model of atmospheric turbulence, introduced by Lorenz in 1996. The vector field can be decomposed into an energy-conserving, time-reversible part, plus a non-time reversible part, including forcing and dissipation. We construct a modified version of the model where viscosity varies with time, in such a way that energy is conserved, and the resulting dynamics is fully time-reversible. For each value of the forcing, the statistical properties of the irreversible and reversible model are in excellent agreement, if in the latter the energy is kept constant at a value equal to the time-average realized with the irreversible model. In particular, the average contraction rate of the phase space of the time-reversible model agrees with that of the irreversible model, where instead it is constant by construction. We also show that the phase space contraction rate obeys the fluctuation relation, and we relate its finite time corrections to the characteristic time scales of the system. A local version of the fluctuation relation is explored and successfully checked. The equivalence between the two non-equilibrium ensembles extends to dynamical properties such as the Lyapunov exponents, which are shown to obey to a good degree of approximation a pairing rule. These results have relevance in motivating the importance of the chaotic hypothesis. in explaining that we have the freedom to model non-equilibrium systems using different but equivalent approaches, and, in particular, that using a model of a fluid where viscosity is kept constant is just one option, and not necessarily the only option, for describing accurately its statistical and dynamical properties.

Giovanni Gallavotti; Valerio Lucarini

2014-06-06

83

CFD analysis of heat transfer and friction factor charaterstics in a circular tube fitted with horizontal baffles twisted tape inserts  

NASA Astrophysics Data System (ADS)

Swirl/vortex flow generator is an important form of passive augmentation techniques. Twisted-tape is one of the most important members of this form which is used extensively in different type heat exchangers. This paper reports the effect of twisted tape inserts on heat transfer and friction factor characteristics in circular tube under constant heat flux and laminar flow conditions using CFD simulation. Plain twisted tape inserts with twist ratios (y = 2.93, 3.91) and baffled twisted tape inserts with twist ratio (y = 2.93) have been used for the simulation using Fluent version 6.3.26. The results obtained by simulation matched with the literature correlations for plain tube with the discrepancy of less than ± 8% for Nusselt number and ± 6.25% for friction factor. The results have also revealed that the heat transfer in term of the Nusselt number enhanced with increases of Reynolds number, decreases of twist ratio and baffle insert. Among the various twist ratios, the twisted tape with twist ratio of y=2.93 and baffle is offered a maximum heat transfer enhancement.

Salman, Sami D.; Kadhum, Abdul Amir H.; Takriff, Mohd S.; Bakar Mohamad, Abu

2013-12-01

84

Heat transfer and friction factor correlations for a solar air heater duct roughened artificially with multiple v-ribs  

SciTech Connect

The use of artificial roughness on the underside of the absorber plate is an effective and economic way to improve the thermal performance of a solar air heater. Several experimental investigations, involving different types of roughness elements, have been carried out to improve the heat transfer from the absorber plate to air flowing in solar air heaters. This paper presents an experimental investigation carried out to study the effect of multiple v-rib roughness on heat transfer coefficient and friction factor in an artificially roughened solar air heater duct. The experiment encompassed Reynolds number (Re) from 2000 to 20000, relative roughness height (e/D) values of 0.019-0.043, relative roughness pitch (P/e) range of 6-12, angle of attack ({alpha}) range of 30-75 and relative roughness width (W/w) range of 1-10. Extensive experimentation has been conducted to collect data on heat transfer and fluid flow characteristics of a rectangular duct roughened with multiple v-ribs. Using these experimental data, correlations for Nusselt number and friction factor in terms of roughness geometry and flow parameters have been developed. (author)

Hans, V.S. [Department of Mechanical Engineering, PAU, Ludhiana (Punjab) 141004 (India); Saini, R.P. [Alternate Hydro Energy Centre, Indian Institute of Technology, Roorkee (UA) 247667 (India); Saini, J.S. [Mechanical and Industrial Engineering Department, Indian Institute of Technology, Roorkee (UA) 247667 (India)

2010-06-15

85

Drag reduction by polymer additives from turbulent spectra  

NASA Astrophysics Data System (ADS)

We extend the analysis of the friction factor for turbulent pipe flow reported by G. Gioia and P. Chakraborty [Phys. Rev. Lett. 96, 044502 (2006)]10.1103/PhysRevLett.96.044502 to the case where drag is reduced by polymer additives.

Calzetta, Esteban

2010-12-01

86

Interaction between a normal shock wave and a turbulent boundary layer at high transonic speeds. Part 1: Pressure distribution. Part 2: Wall shear stress. Part 3: Simplified formulas for the prediction of surface pressures and skin friction  

NASA Technical Reports Server (NTRS)

An asymptotic description is derived for the interaction between a shock wave and a turbulent boundary layer in transonic flow, for a particular limiting case. The dimensionless difference between the external flow velocity and critical sound speed is taken to be much smaller than one, but large in comparison with the dimensionless friction velocity. The basic results are derived for a flat plate, and corrections for longitudinal wall curvature and for flow in a circular pipe are also shown. Solutions are given for the wall pressure distribution and the shape of the shock wave. Solutions for the wall shear stress are obtained, and a criterion for incipient separation is derived. Simplified solutions for both the wall pressure and skin friction distributions in the interaction region are given. These results are presented in a form suitable for use in computer programs.

Adamson, T. C., Jr.; Liou, M. S.; Messiter, A. F.

1980-01-01

87

Friction Factors for Flow of Drag Reducing Solutions of Micellar Surfactant Additives in Straight Circular Pipes and Conventional Globe Valves  

NASA Astrophysics Data System (ADS)

Experiments were carried out in order to investigate pressure drops due to friction and form for flow of water with micellar drag reducing additives Arquad S-50, CTAB and CTAC, which were prepared in various different concentrations. The rheological behavior of these solutions was studied on a coaxial cylinder rheometer. Standard experimental data evaluation in the form of flow curves disclosed a pattern in which, in the low shear rate range, a nonlinear shear stress decrease with increasing shear rates was observed. For still higher shear rates, shear stress values approached asymptotically constant, shear-independent, values. Therefore, in the low shear rate region, experimental data were described with sufficient accuracy by means of the simple power-law model. A closed-loop experimental stand consisted of a 2700 mm long stabilization section followed either by a 3000 mm long straight, stainless circular tube, (15, 20, and 25 mm I.D.) or with a conventional globe valve. Results are presented in the form of Darcy-Weissbach friction factor ? vs. the generalized Reynolds number Reg. Also, it is worth to note that the pressure loss reduction in the lower shear-rate range in pipes occurs due to the pseudoplastic behavior of the solutions rather than due to the drag-reduction mechanism in the proper (Toms) sense of the word.

Dostál, Martin; Šesták, Ji?í; Mík, Václav; Myška, Ji?í; Toman, Ji?í

2008-07-01

88

Turbulent urinary flow in the urethra could be a causal factor for benign prostatic hyperplasia.  

PubMed

Benign prostatic hyperplasia (BPH) is the most common form of prostate disease in middle-aged and elderly men, and leads to severe impairment later in life. Despite its significant impact on public health, the underlying cause of BPH is yet to be determined. This hypothesis proposes a new causal factor for BPH. Applying concepts of dynamics of fluids to the process of urination it could be stated that a turbulent urinary flow through the passage of the urethra. The turbulent urinary flow in the urethra applies pressure and stretch forces to the surrounding tissue. This stimulus repeated over time and coupled with age-related changes of the urethral tissue could contribute to the development of BPH. In support of this hypothesis, several mechanotransduction studies have shown that vibration and pressure forces applied to different cell tissues can provoke cellular and molecular changes. Another supportive data is the presence of the hyperplasic reaction surrounding the urethra specifically located in the transition zone, the only zone where BPH develops. It is crucial to identify causal factors to understanding the disease and to determine effective primary prevention strategies. Future studies of the dynamics of fluids in the urethra are warranted. The finding of significant forces transmitted to the periurethral tissue from a turbulent urinary flow could give us the clue to the underlying cause of BPH. If this hypothesis proves to be valid there are several primary prevention measures that could be implemented to impede the development of BPH. Educational intervention measures in younger populations to avoid frequent urinary retention and active urination (process of forcing the urine through the urethra at a high velocity) could be considered. Studies of the impact of turbulence in the standing vs. sitting position during urination in men should also be considered. PMID:16764996

Martinez-Borges, Anibal R

2006-01-01

89

Factors Influencing Pitot Probe Centerline Displacement in a Turbulent Supersonic Boundary Layer  

NASA Technical Reports Server (NTRS)

When a total pressure probe is used for measuring flows with transverse total pressure gradients, a displacement of the effective center of the probe is observed (designated Delta). While this phenomenon is well documented in incompressible flow and supersonic laminar flow, there is insufficient information concerning supersonic turbulent flow. In this study, three NASA Lewis Research Center Supersonic Wind Tunnels (SWT's) were used to investigate pitot probe centerline displacement in supersonic turbulent boundary layers. The relationship between test conditions and pitot probe centerline displacement error was to be determined. For this investigation, ten circular probes with diameter-to-boundary layer ratios (D/delta) ranging from 0.015 to 0.256 were tested in the 10 ft x 10 ft SWT, the 15 cm x 15 cm SWT, and the 1 ft x 1 ft SWT. Reynolds numbers of 4.27 x 10(exp 6)/m, 6.00 x 10(exp 6)/in, 10.33 x 10(exp 6)/in, and 16.9 x 10(exp 6)/m were tested at nominal Mach numbers of 2.0 and 2.5. Boundary layer thicknesses for the three tunnels were approximately 200 mm, 13 mm, and 30 mm, respectively. Initial results indicate that boundary layer thickness, delta, and probe diameter, D/delta play a minimal role in pitot probe centerline offset error, Delta/D. It appears that the Mach gradient, dM/dy, is an important factor, though the exact relationship has not yet been determined. More data is needed to fill the map before a conclusion can be drawn with any certainty. This research provides valuable supersonic, turbulent boundary layer data from three supersonic wind tunnels with three very different boundary layers. It will prove a valuable stepping stone for future research into the factors influencing pitot probe centerline offset error.

Grosser, Wendy I.

1997-01-01

90

Prediction of friction coefficients for gases  

NASA Technical Reports Server (NTRS)

Empirical relations are used for correlating laminar and turbulent friction coefficients for gases, with large variations in the physical properties, flowing through smooth tubes. These relations have been used to correlate friction coefficients for hydrogen, helium, nitrogen, carbon dioxide and air.

Taylor, M. F.

1969-01-01

91

Diminishing friction of joint surfaces as initiating factor for destabilising permafrost rocks?  

Microsoft Academic Search

Degrading alpine permafrost due to changing climate conditions causes instabilities in steep rock slopes. Due to a lack in process understanding, the hazard is still difficult to asses in terms of its timing, location, magnitude and frequency. Current research is focused on ice within joints which is considered to be the key-factor. Monitoring of permafrost-induced rock failure comprises monitoring of

Daniel Funk; Michael Krautblatter

2010-01-01

92

Use of a Stanton Tube for Skin-Friction Measurements  

NASA Technical Reports Server (NTRS)

A small total-pressure tube resting against a flat-plate surface was used as a Stanton tube and calibrated as a skin-friction meter at various subsonic and supersonic speeds. Laminar flow was maintained for the supersonic runs at a Mach number M(sub infinity) of 2. At speeds between M(sub infinity) = 1.33 and M(sub infinity) = 1.87, the calibrations were carried-out in a turbulent boundary layer. The subsonic flows were found to be in transition. The skin-friction readings of a floating-element type of balance served as the reference values against which the Stanton tube was calibrated. A theoretical model was developed which, for moderate values of the shear parameter tau, accurately predicts the performance of the Stanton tube in subsonic and supersonic flows. A "shear correction factor" was found to explain the deviations from the basic model when T became too large. Compressibility effects were important only in the case of turbulent supersonic flows, and they did not alter the form of the calibration curve. The test Reynolds numbers, based on the distance from the leading edge and free-stream conditions, ranged from 70,000 to 875,000. The turbulent-boundary-layer Reynolds numbers, based on momentum thickness, varied between 650 and 2,300. Both laminar and turbulent velocity profiles were taken and the effect of pressure gradient on the calibration was investigated.

Abarbanel, S. S.; Hakkinen, R. J.; Trilling, L.

1959-01-01

93

Fascinating Friction!  

NSDL National Science Digital Library

Students use wood, wax paper and oil to investigate the importance of lubrication between materials and to understand the concept of friction. Using wax paper and oil placed between pieces of wood, the function of lubricants between materials is illustrated. Students extend their understanding of friction to bones and joints in the skeletal system and become aware of what engineers can do to help reduce friction in the human body as well as in machines.

Integrated Teaching and Learning Program,

94

Joint strength of Inconel 718 alloy and its improvement by post-weld heat treatment – joint performance and its controlling factors in friction welding of Inconel 718 alloy  

Microsoft Academic Search

The influences of welding parameters on tensile properties of friction-welded joints of Inconel 718 alloy (subjected to a post-weld heat treatment (PWHT) consisting of a solution treatment at 1253 K and double ageing treatments at 993 and 893 K) have been investigated to reveal the controlling factor of the joint performance. All joints obtained were fractured near the bond interface at smaller

Hongjie Wang; Kenji Ikeuchi; Masatoshi Aritoshi; Makoto Takahashi; Akio Ikeda

2009-01-01

95

Turbulence models and Reynolds analogy for two-dimensional supersonic compression ramp flow  

NASA Technical Reports Server (NTRS)

Results of the application of turbulence models and the Reynolds analogy to the Navier-Stokes computations of Mach 2.9 two-dimensional compression ramp flows are presented. The Baldwin-Lomax eddy viscosity model and the kappa-epsilon turbulence transport equations for the turbulent momentum flux modeling in the Navier-Stokes equations are studied. The Reynolds analogy for the turbulent heat flux modeling in the energy equation was also studied. The Navier-Stokes equations and the energy equation were numerically solved for the flow properties. The Reynolds shear stress, the skin friction factor, and the surface heat transfer rate were calculated and compared with their measurements. It was concluded that with a hybrid kappa-epsilon turbulence model for turbulence modeling, the present computations predicted the skin friction factors of the 8 deg and 16 deg compression ramp flows and with the turbulent Prandtl number Pr(sub t) = 0.93 and the ratio of the turbulent thermal and momentum transport coefficients mu(sub q)/mu(sub t) = 2/Prt, the present computations also predicted the surface heat transfer rates beneath the boundary layer flow of the 16 compression ramp.

Wang, Chi R.; Bidek, Maleina C.

1994-01-01

96

Measurement of Heat Transfer in the Turbulent Boundary Layer on a Flat Plate in a Supersonic Flow and Comparison with Skin Friction Results  

NASA Technical Reports Server (NTRS)

Local heat-transfer rates on the surface of a heated flat plate at zero incidence to an air stream flowing at Mach numbers of 1.69 and 2.27 are presented. The Reynolds number range for both Mach numbers was 1 million to 10 million. Surface temperatures were maintained near recovery temperature. It was found that the variation of heat transfer with Mach number was in agreement with previously reported variations of directly measured skin friction with Mach number on unheated bodies. The variation with Mach number of the average skin coefficient, as determined from impact-pressure surveys, was in agreement with that from other momentum loss measurements but differed from the variation obtained from directly measured skin friction as reported by others. (author)

Pappas, C C

1954-01-01

97

Parameters defining flow resistance and the friction factor behavior in liquid annular seals with deliberately roughened surfaces  

E-print Network

is increased. Simplified models based on bulk flow theory and Moody�s approach to characterize wall friction in pipes cannot explain this outcome. This research is an extension of a 2-D numerical analysis of flat plate experiments with water which found...

Villasmil Urdaneta, Larry Alfonso

2006-10-30

98

Discovering Friction  

NSDL National Science Digital Library

With a simple demonstration activity, students are introduced to the concept of friction as a force that impedes motion when two surfaces are in contact. Then, in the Associated Activity (Sliding and Stuttering), they work in teams to use a spring scale to drag an object such as a ceramic coffee cup along a table top or the floor. The spring scale allows them to measure the frictional force that exists between the moving cup and the surface it slides on. By modifying the bottom surface of the cup, students can find out what kinds of surfaces generate more or less friction. They also discover that both static and kinetic friction are involved when an object initially at rest is caused to slide across a surface.

Engineering K-Ph.d. Program

99

Drag reduction of turbulent pipe flows by circular-wall oscillation  

NASA Astrophysics Data System (ADS)

An experimental study on turbulent pipe flows was conducted with a view to reduce their friction drag by oscillating a section of the pipe in a circumferential direction. The results indicated that the friction factor of the pipe is reduced by as much as 25% as a result of active manipulation of near-wall turbulence structure by circular-wall oscillation. An increase in the bulk velocity was clearly shown when the pipe was oscillated at a constant head, supporting the measured drag reduction in the present experiment. The percentage reduction in pipe friction was found to be better scaled with the nondimensional velocity of the oscillating wall than with its nondimensional period, confirming a suggestion that the drag reduction seem to be resulted from the realignment of longitudinal vortices into a circumferential direction by the wall oscillation.

Choi, Kwing-So; Graham, Mark

1998-01-01

100

Heat transfer coefficient and friction factor correlations for the transitional flow regime in rib-roughened rectangular ducts  

Microsoft Academic Search

This paper presents results of an experimental investigation of heat transfer and friction for the flow of air in rectangular ducts with repeated chamfered rib-roughness on one broad wall. The aspect ratios of the rectangular duct investigated are 4.8, 6.1, 7.8, 9.66 and 12.0. The roughened wall is uniformly heated while the remaining three walls are insulated. These boundary conditions

R. Karwa; S. C. Solanki; J. S. Saini

1999-01-01

101

Experimental Analysis of the Single-Phase Heat Transfer and Friction Factor inside the Horizontal Internally Micro-Fin Tube  

E-print Network

of tube is widely used in high flow rate applications because the heat transfer enhancement in high flow rates (turbulent region) is more pronounced than that in the low flow rates (laminar region). Khanpara and shell-side condensers to increase heat transfer. This enables water chillers to reach high efficiency

Ghajar, Afshin J.

102

Free-stream turbulence and concave curvature effects on heated, transitional boundary layers  

NASA Technical Reports Server (NTRS)

An experimental investigation of the transition process on flat-plate and concave curved-wall boundary layers for various free-stream turbulence levels was performed. Results show that for transition of a flat-plate, the two forms of boundary layer behavior, identified as laminar-like and turbulent-like, cannot be thought of as separate Blasius and fully-turbulent profiles, respectively. Thus, simple transition models in which the desired quantity is assumed to be an average, weighted on intermittency, of the theoretical laminar and fully turbulent values is not expected to be successful. Deviation of the flow identified as laminar-like from theoretical laminar behavior is shown to be due to recovery after the passage of a turbulent spot, while deviation of the flow identified as turbulent-like from the full-turbulent values is thought to be due to incomplete establishment of the fully-turbulent power spectral distribution. Turbulent Prandtl numbers for the transitional flow, computed from measured shear stress, turbulent heat flux and mean velocity and temperature profiles, were less than unity. For the curved-wall case with low free-stream turbulence intensity, the existence of Gortler vortices on the concave wall within both laminar and turbulent flows was established using liquid crystal visualization and spanwise velocity and temperature traverses. Transition was found to occur via a vortex breakdown mode. The vortex wavelength was quite irregular in both the laminar and turbulent flows, but the vortices were stable in time and space. The upwash was found to be more unstable, with higher levels of u' and u'v', and lower skin friction coefficients and shape factors. Turbulent Prandtl numbers, measured using a triple-wire probe, were found to be near unity for all post-transitional profiles, indicating no gross violation of Reynolds analogy. No evidence of streamwise vortices was seen in the high turbulence intensity case.

Kim, J.; Simon, T. W.

1991-01-01

103

Friction Force  

NSDL National Science Digital Library

Students use LEGO® MINDSTORMS® robotics to help conceptualize and understand the force of friction. Specifically, they observe how different surfaces in contact result in different frictional forces. A LEGO robot is constructed to pull a two-wheeled trailer made of LEGO parts. The robot is programmed to pull the trailer 10 feet and trial runs are conducted on smooth and textured surfaces. The speed and motor power of the robot is kept constant in all trials so students observe the effect of friction between various combinations of surfaces and trailer wheels. To apply what they learn, students act as engineers and create the most effective car by designing the most optimal tires for given surface conditions.

2014-09-18

104

Exploring Friction  

NSDL National Science Digital Library

The following resource is from Lessonopoly, which has created student activities and lesson plans to support the video series, Science of the Olympic Winter Games, created by NBC Learn and the National Science Foundation. Featuring exclusive footage from NBC Sports and contributions from Olympic athletes and NSF scientists, the series will help teach your students valuable scientific concepts. In this activity, Students will learn several important characteristics about friction. Students will also learn why athletes who curl in the Winter Olympics try to understand and control friction.

2010-01-01

105

Numerical Investigation of Heat Transfer and Friction Factor Characteristics in a Circular Tube Fitted with V-Cut Twisted Tape Inserts  

PubMed Central

Numerical investigation of the heat transfer and friction factor characteristics of a circular fitted with V-cut twisted tape (VCT) insert with twist ratio (y = 2.93) and different cut depths (w = 0.5, 1, and 1.5?cm) were studied for laminar flow using CFD package (FLUENT-6.3.26). The data obtained from plain tube were verified with the literature correlation to ensure the validation of simulation results. Classical twisted tape (CTT) with different twist ratios (y = 2.93, 3.91, 4.89) were also studied for comparison. The results show that the enhancement of heat transfer rate induced by the classical and V-cut twisted tape inserts increases with the Reynolds number and decreases with twist ratio. The results also revealed that the V-cut twisted tape with twist ratio y = 2.93 and cut depth w = 0.5?cm offered higher heat transfer rate with significant increases in friction factor than other tapes. In addition the results of V-cut twist tape compared with experimental and simulated data of right-left helical tape inserts (RLT), it is found that the V-cut twist tape offered better thermal contact between the surface and the fluid which ultimately leads to a high heat transfer coefficient. Consequently, 107% of maximum heat transfer was obtained by using this configuration. PMID:24078795

Salman, Sami D.; Kadhum, Abdul Amir H.; Takriff, Mohd S.; Mohamad, Abu Bakar

2013-01-01

106

Active nematic materials with substrate friction  

NASA Astrophysics Data System (ADS)

Active turbulence in dense active systems is characterized by high vorticity on a length scale that is large compared to that of individual entities. We describe the properties of active turbulence as momentum propagation is screened by frictional damping. As friction is increased, the spacing between the walls in the nematic director field decreases as a consequence of the more rapid velocity decays. This leads to, first, a regime with more walls and an increased number of topological defects, and then to a jammed state in which the walls deliminate bands of opposing flow, analogous to the shear bands observed in passive complex fluids.

Thampi, Sumesh P.; Golestanian, Ramin; Yeomans, Julia M.

2014-12-01

107

Particle image velocimetry characterization of turbulent channel flow with rib patterned superhydrophobic walls  

NASA Astrophysics Data System (ADS)

This paper reports particle image velocimetry (PIV) measurements characterizing turbulent flow in a channel with superhydrophobic surfaces, structured and wetting surfaces, and smooth bottom surfaces. The superhydrophobic and structured surfaces are fabricated with alternating ribs and cavities. Both longitudinal and transverse rib/cavity orientations were considered and the surfaces were made superhydrophobic by application of a Teflon coating. The widths of the ribs and cavities were 8 and 32?m, respectively, and the depth of the cavities was 15?m. PIV measurements were acquired for all surfaces considered over the Reynolds numbers range from 4800 to 10 000. Results from the smooth bottom wall measurements were used as a basis for comparison. The hydraulic diameter of the channel was nominally 8.2mm with an aspect ratio of 8.9. The PIV data captured aggregate velocities over multiple rib/cavity modules, such that a spanwise-averaged (over the width of the laser beam) velocity profile was obtained at the channel centerline. The time-averaged velocity profiles reveal no discernible time-mean slip velocity at the superhydrophobic wall. However, the different surfaces are shown to exhibit a systematic influence on the turbulence intensities, total and turbulent shear stress distributions, turbulence production in the channel, and local friction factors. Superhydrophobic surfaces with the ribs and cavities aligned with the flow are shown to yield an 11% decrease in the friction factor while the same surfaces aligned in the transverse direction are shown to cause a modest increase in the friction factor.

Woolford, B.; Prince, J.; Maynes, D.; Webb, B. W.

2009-08-01

108

Elasto-inertial turbulence  

PubMed Central

Turbulence is ubiquitous in nature, yet even for the case of ordinary Newtonian fluids like water, our understanding of this phenomenon is limited. Many liquids of practical importance are more complicated (e.g., blood, polymer melts, paints), however; they exhibit elastic as well as viscous characteristics, and the relation between stress and strain is nonlinear. We demonstrate here for a model system of such complex fluids that at high shear rates, turbulence is not simply modified as previously believed but is suppressed and replaced by a different type of disordered motion, elasto-inertial turbulence. Elasto-inertial turbulence is found to occur at much lower Reynolds numbers than Newtonian turbulence, and the dynamical properties differ significantly. The friction scaling observed coincides with the so-called “maximum drag reduction” asymptote, which is exhibited by a wide range of viscoelastic fluids. PMID:23757498

Samanta, Devranjan; Dubief, Yves; Holzner, Markus; Schäfer, Christof; Morozov, Alexander N.; Wagner, Christian; Hof, Björn

2013-01-01

109

Elasto-inertial turbulence.  

PubMed

Turbulence is ubiquitous in nature, yet even for the case of ordinary Newtonian fluids like water, our understanding of this phenomenon is limited. Many liquids of practical importance are more complicated (e.g., blood, polymer melts, paints), however; they exhibit elastic as well as viscous characteristics, and the relation between stress and strain is nonlinear. We demonstrate here for a model system of such complex fluids that at high shear rates, turbulence is not simply modified as previously believed but is suppressed and replaced by a different type of disordered motion, elasto-inertial turbulence. Elasto-inertial turbulence is found to occur at much lower Reynolds numbers than Newtonian turbulence, and the dynamical properties differ significantly. The friction scaling observed coincides with the so-called "maximum drag reduction" asymptote, which is exhibited by a wide range of viscoelastic fluids. PMID:23757498

Samanta, Devranjan; Dubief, Yves; Holzner, Markus; Schäfer, Christof; Morozov, Alexander N; Wagner, Christian; Hof, Björn

2013-06-25

110

Friction and Wear  

NASA Technical Reports Server (NTRS)

From the practical point of view, this analysis shows that each problem of friction or wear requires its particular solution. There is no universal solution; one or other of the factors predominates and defines the choice of the solution. In certain cases, copper alloys of great thermal conductivity are preferred; in others, plastics abundantly supplied with water. Sometimes, soft antifriction metals are desirable to distribute the load; at other times, hard metals with high resistance to abrasion or heat.

Pomey, Jacques

1952-01-01

111

Introduction Rolling and Friction  

E-print Network

Introduction Kinematics Solutions Rolling and Friction in Discrete Element Simulations Matthew R of rolling resistance Creep-friction definition Creep-friction vs. Cattaneo-Mindlin friction Classification / papers / EMI2011.pdf #12;Introduction Kinematics Solutions Classification of rolling resistance Creep-friction

Kuhn, Matthew R.

112

Quantum friction  

E-print Network

The Brownian motion of a light quantum particle in a heavy classical gas is theoretically described and a new expression for the friction coefficient is obtained for arbitrary temperature. At zero temperature it equals to the de Broglie momentum of the mean free path divided by the mean free path. Alternatively, the corresponding mobility of the quantum particle in the classical gas is equal to the square of the mean free path divided by the Planck constant. The Brownian motion of a quantum particle in a quantum environment is also discussed.

R. Tsekov

2012-12-05

113

Modified gauge for time-resolved skin-friction measurements  

NASA Technical Reports Server (NTRS)

A new gauge for skin-friction measurements in turbulent flows involving a wire close to the surface and a second, flush-mounted film which serves as a constant temperature guard heater is described and evaluated. It is shown that a laminar calibration of the gauge applies to measurements in turbulent flow and that the frequency response of the gauge is appropriate for time-resolved data in such a flow. Initial results from a turbulent channel flow are presented.

Ajagu, C. O.; Libby, P. A.; Larue, J. C.

1982-01-01

114

Elastomeric friction  

NASA Astrophysics Data System (ADS)

This dissertation examines the tribology of PDMS (polydimethylsiloxane) elastomers from a practical and a fundamental perspective. We examine the adhesive, energetic, and tribological properties of several commercial biofouling release coatings, and show that adhesive (and bioadhesive) release from an elastomer depends on the friction of its surface. Having shown that friction is an obstacle to release, we lubricate a model PDMS network by incorporating linear unreactive PDMS oils varying in molecular weight (0.8--423 kg/mol). Surface segregation upon curing depends on molecular weight and mass percentage. Atomic Force Microscopy (AFM) is used to detect the thickness of the lubricant layer. Surprisingly, high-viscosity oils lubricate better than low-viscosity oils, indicating a non-hydrodynamic lubrication. Applying this technology to a commercial elastomer, we see an improvement in bioadhesive release capabilities, as evidenced by a reduced tenacity of mussel adhesive protein. In comparing entangled polymer melts to crosslinked elastomers, we encountered an opportunity to study the tribology of the latter. We studied the effects of molecular weight, velocity, and temperature on the friction of crosslinked PDMS elastomers sliding against two model surfaces: a self-assembled monolayer (SAM) of n-hexadecylsilane, and a thin (˜100mum) film of polystyrene (PS). The change from smooth to stick-slip (unstable) interfacial sliding occurs at a distinct velocity on each surface, implying that it's not necessarily attributable to a bulk glass transition of the PDMS, as popularly believed. The peak shear stress attained immediately before stick-slip sliding is found to be linear with the shear modulus raised to an exponent n of ¾, in contrast with the predictions of Chernyak and Leonov ( n = 1). Low-velocity behavior differs greatly between the SAM and the PS, implying a mechanistic difference. Whereas on the SAM, sliding likely proceeds purely by stochastic adsorption and desorption of polymer chains, on the PS it may also proceed by disentanglement of PDMS from the PS. Temperature-variant measurements allow us to estimate the activation energy of sliding, implying strongly that the velocity of instability depends on the van der Waals interfacial interaction.

Vorvolakos, Katherine

115

Experimental study of boundary layer transition with elevated freestream turbulence on a heated flat plate  

NASA Technical Reports Server (NTRS)

A detailed investigation to document momentum and thermal development of boundary layers undergoing natural transition on a heated flat plate was performed. Experimental results of both overall and conditionally sampled characteristics of laminar, transitional, and low Reynolds number turbulent boundary layers are presented. Measurements were acquired in a low-speed, closed-loop wind tunnel with a freestream velocity of 100 ft/s and zero pressure gradient over a range of freestream turbulence intensities (TI) from 0.4 to 6 percent. The distributions of skin friction, heat transfer rate and Reynolds shear stress were all consistent with previously published data. Reynolds analogy factors for R(sub theta) is less than 2300 were found to be well predicted by laminar and turbulent correlations which accounted for an unheated starting length. The measured laminar value of Reynolds analogy factor was as much as 53 percent higher than the Pr(sup -2/3). A small dependence of turbulent results on TI was observed. Conditional sampling performed in the transitional boundary layer indicated the existence of a near-wall drop in intermittency, pronounced at certain low intermittencies, which is consistent with the cross-sectional shape of turbulent spots observed by others. Non-turbulent intervals were observed to possess large magnitudes of near-wall unsteadiness and turbulent intervals had peak values as much as 50 percent higher than were measured at fully turbulent stations. Non-turbulent and turbulent profiles in transitional boundary layers cannot be simply treated as Blasius and fully turbulent profiles, respectively. The boundary layer spectra indicate predicted selective amplification of T-S waves for TI is approximately 0.4 percent. However, for TI is approximately 0.8 and 1.1 percent, T-S waves are localized very near the wall and do not play a dominant role in transition process.

Sohn, Ki-Hyeon; Reshotko, Eli

1991-01-01

116

Continuum modeling of crowd turbulence  

NASA Astrophysics Data System (ADS)

With the growth in world population, the density of crowds in public places has been increasing steadily, leading to a higher incidence of crowd disasters at high densities. Recent research suggests that emergent chaotic behavior at high densities—known collectively as crowd turbulence—is to blame. Thus, a deeper understanding of crowd turbulence is needed to facilitate efforts to prevent and plan for chaotic conditions in high-density crowds. However, it has been noted that existing algorithms modeling collision avoidance cannot faithfully simulate crowd turbulence. We hypothesize that simulation of crowd turbulence requires modeling of both collision avoidance and frictional forces arising from pedestrian interactions. Accordingly, we propose a model for turbulent crowd simulation, which incorporates a model for interpersonal stress and acceleration constraints similar to real-world pedestrians. Our simulated results demonstrate a close correspondence with observed metrics for crowd turbulence as measured in known crowd disasters.

Golas, Abhinav; Narain, Rahul; Lin, Ming C.

2014-10-01

117

Friction, wear, and lubrication in vacuum  

NASA Technical Reports Server (NTRS)

A review of studies and observations on the friction, wear, and lubrication behavior of materials in a vacuum environment is presented. The factors that determine and influence friction and wear are discussed. They include topographical, physical, mechanical, and the chemical nature of the surface. The effects of bulk properties such as deformation characteristics, fracture behavior, and structure are included.

Buckley, D. H.

1971-01-01

118

CFD Modeling of ITER Cable-in-Conduit Superconductors. Part I: Friction in the Central Channel  

NASA Astrophysics Data System (ADS)

In this paper, the first of a series, we propose a novel approach, based on Computational Fluid Dynamics (CFD), to understand the complex transverse thermal-hydraulic processes in the dual-channel cable-in-conduit conductors (CICC), which are used for the superconducting magnets of the International Thermonuclear Experimental Reactor (ITER). Advanced 2D and 3D CFD, including sophisticated turbulence models, is used to compute the mass flow rate corresponding to an imposed pressure drop in rib-roughened pipes, including spirals mimicking the central channel of an ITER CICC and used in several experiments. The results of the calculation are validated against measured data and can be used to deduce the friction factor fH in the central channel, throwing at the same time some light on the role played by the different parameters (Reynolds number, spiral geometry, etc.) in the central channel friction process for an ITER CICC.

Zanino, R.; Giors, S.; Mondino, R.

2006-04-01

119

Heat transfer performance of internally finned tubes in turbulent flow  

NASA Astrophysics Data System (ADS)

The heat transfer performance for heating water in turbulent flow was experimentally determined for 11 tubes having integral internal spiral and longitudinal fins. Individual tube performance is presented and compared with a smooth tube datum at constant pumping power. A longitudinal and spiral fin tube from within the 11 tubes and a smooth tube were also tested heating a 50% w/w ethylene glycol-water solution, and these data are included. These data are compared with relevant data available in the literature. Correlating equations are presented for heat transfer and Fanning friction factor that describe the air, water, and ethylene glycol-water data within + or - 10%.

Carnavos, T. C.

1980-06-01

120

VOLUME 87, NUMBER 3 P H Y S I C A L R E V I E W L E T T E R S 16 JULY 2001 Dynamical Turbulent Flow on the Galton Board with Friction  

E-print Network

on the Galton Board with Friction A. D. Chepelianskii1 and D. L. Shepelyansky2 1 Lycée Pierre de Fermat, Parvis des Jacobins, 31068 Toulouse Cedex 7, France 2 Laboratoire de Physique Quantique, UMR 5626 du CNRS

Shepelyansky, Dima

121

Estimation of heat transfer coefficient and friction factor in the transition flow with low volume concentration of Al 2O 3 nanofluid flowing in a circular tube and with twisted tape insert  

Microsoft Academic Search

Experiments to evaluate heat transfer coefficient and friction factor for flow in a tube and with twisted tape inserts in the transition range of flow with Al2O3 nanofluid are conducted. The results showed considerable enhancement of convective heat transfer with Al2O3 nanofluids compared to flow with water. It is observed that the equation of Gleninski applicable in transitional flow range

K. V. Sharma; L. Syam Sundar; P. K. Sarma

2009-01-01

122

A skin friction gauge for impulsive flows  

NASA Astrophysics Data System (ADS)

A new skin friction gauge has been designed for use in impulsive facilities. The gauge was tested in the T4 free piston shock tunnel, at the University of Queensland, using a 1.5 m long plate that formed one of the inner walls of a rectangular duct. The test gas was fair and the test section free stream flow had a stagnation enthalpy of 4.7 MJ/kg. Measurements were conducted in a laminar and turbulent boundary layer. The measurements compared well with laminar and turbulent analytical theory.

Goyne, C. P.; Paull, A.; Stalker, R. J.

123

Frictional Widgets: Enhancing Touch Interfaces with Programmable Friction  

E-print Network

Frictional Widgets: Enhancing Touch Interfaces with Programmable Friction Abstract Touch the design possibilities offered by augmenting touchscreens with programmable surface friction. Four exemplar of touch interactions can be enhanced when using a touchscreen with dynamically varied surface friction. We

Levesque, Vincent

124

DNS of turbulent channel flow at very low Reynolds numbers  

E-print Network

Direct numerical simulations (DNS) of fully-developed turbulent channel flows for very low Reynolds numbers have been performed with a larger computational box sizes than those of existing DNS. The friction Reynolds number was decreased down to 60, where the friction Reynolds number is based on the friction velocity and the channel half width. When the Reynolds number was decreased to 60 with small computational box size, the flow became laminar. Using a large box, we found that a localized turbulence was observed to sustain in the form of periodic oblique band. This type of locally disordered flow is similar to a equilibrium turbulent puff in a transitional pipe flow. Various turbulence statistics such as turbulence intensities, vorticity fluctuations and Reynolds stresses are provided. Especially, their near-wall asymptotic behavior and budget terms of turbulence kinetic energy were discussed with respect to the Reynolds-number dependence and an influence of the computational box size. Other detailed charac...

Tsukahara, Takahiro

2014-01-01

125

On adaptive friction compensation  

Microsoft Academic Search

A method of compensating for friction in control systems is presented. The method entails the use of an observer to estimate the friction which is modeled as a constant times the sign of the velocity. The purpose of the observer is to estimate this constant. The observer model is selected to ensure that the error in estimation of the friction

Bernard Friedland; Young-Jin Park

1992-01-01

126

Friction in Our Lives  

NSDL National Science Digital Library

Students will understand the following: 1. Friction is a force that opposes motion, or makes it difficult for an object to move across a surface. 2. The amount of friction depends on the surface type and the force pressing two surfaces together. 3. Everyday life provides examples of how friction both helps and hinders everything we do.

Discovery

2012-01-17

127

Influence of tube-entrance configuration on average heat-transfer coefficients and friction factors for air flowing in an Inconel tube  

NASA Technical Reports Server (NTRS)

A heat-transfer investigation was conducted with air flowing through an electrically heated Inconel tube having either a long-approach or a right-angle-edge entrance, an inside diameter of 0.402 inch, and a length of 24 inches over a range of Reynolds numbers up to 375,000 and average inside-tube-wall temperatures up to 2000 degrees R. Good correlation of heat-transfer data was obtained for both entrances, which substantiates work previously reported. A fair correlation of friction data was obtained for both entrances. The entrance configuration had little effect on the average heat-transfer and friction coefficients.

Lowdermilk, Warren H; Grele, Milton D

1950-01-01

128

Rotational Quantum Friction  

E-print Network

We investigate the frictional forces due to quantum fluctuations acting on a small sphere rotating near a surface. At zero temperature, we find the frictional force near a surface to be several orders of magnitude larger than that for the sphere rotating in vacuum. For metallic materials with typical conductivity, quantum friction is maximized by matching the frequency of rotation with the conductivity. Materials with poor conductivity are favored to obtain large quantum frictions. For semiconductor materials that are able to support surface plasmon polaritons, quantum friction can be further enhanced by several orders of magnitude due to the excitation of surface plasmon polaritons.

Rongkuo Zhao; Alejandro Manjavacas; F. Javier García de Abajo; J. B. Pendry

2012-09-25

129

Nanotribology and Nanoscale Friction  

SciTech Connect

Tribology is the science and technology of contacting solid surfaces in relative motion, including the study of lubricants, lubrication, friction, wear, and bearings. It is estimated that friction and wear cost the U.S. economy 6% of the gross national product (Persson, 2000). For example, 5% of the total energy generated in an automobile engine is lost to frictional resistance. The study of nanoscale friction has a technological impact in reducing energy loss in machines, in microelectromechanical systems (MEMS), and in the development of durable, low-friction surfaces and ultra-thin lubrication films.

Guo, Yi [Stevens Institute of Technology, Hoboken, New Jersey; Qu, Zhihua [University of Central Florida, Orlando; Braiman, Yehuda [ORNL; Zhang, Zhenyu [ORNL; Barhen, Jacob [ORNL

2008-01-01

130

Analytical and experimental study of fluid friction and heat transfer in low Reynolds number flow heat exchangers  

NASA Astrophysics Data System (ADS)

Analysis of fluid friction and heat transfer in low Reynolds number flow heat exchangers is undertaken. Three configurations typically utilized in compact heat exchangers are examined. These are: the plain non- circular duct of constant cross-sectional area, the offset or interrupted strip fin, and the turbulator strip. Analytical models for each of these geometries are developed by combining asymptotic solutions using simple non-linear superposition. Models for predicting the friction factor-Reynolds number product, f Re, and Nusselt number, Nu, in non-circular ducts for hydrodynamically fully developed flow (HFDF), hydrodynamically developing flow (HDF), thermally fully developed flow (TFDF), thermally developing flow (TDF), and simultaneously developing flow (SDF) are developed. Thermal and hydrodynamic entrance models are developed by combining the asymptotic solutions for small and large values of the dimensionless duct length. Through the use of a novel characteristic length, the square root of the cross-sectional flow area, scatter in the dimensionless data for fully developed laminar flows is considerably reduced. Most numerical and analytical data are predicted within +/-10% for HFDF and TFDF, +/-12% for HDF and TDF, and +/-15% for SDF for most non-circular ducts. Simple analytic models for predicting the Fanning friction factor, f, and Colburn j factor of two common enhancement devices, the offset strip fin and the turbulator strip are developed from fundamental solutions of fluid dynamics and heat transfer. Models for the offset strip fin are valid over the full range of Reynolds numbers for rectangular and other non-circular sub-channel cross- sections. Model predictions for the offset strip fin agree with published experimental data within +/-20%. Models for the turbulator strip are valid over the full Reynolds number range for both straight and curved turbulator profiles. Model predictions for the turbulator strip agree with new experimental data to within +/-20%. Finally, a detailed experimental study of the thermal and hydraulic characteristics of turbulator strips is undertaken. Simple design correlations are presented along with a performance evaluations of each device using the constant mass flow rate and constant pumping power criteria.

Muzychka, Yuri Stephan

1999-11-01

131

Microblowing Technique Demonstrated to Reduce Skin Friction  

NASA Technical Reports Server (NTRS)

One of the most challenging areas of research in aerodynamics is the reduction of skin friction, especially for turbulent flow. Reduced skin friction means less drag. For aircraft, less drag can lead to less fuel burned or to a greater flight range for a fixed amount of fuel. Many techniques and methods have been tried; however, none of them has significantly reduced skin friction in the flight environment. An innovative skin-friction reduction technique, the Microblowing Technique (MBT), was invented in 1993. This is a unique concept in which an extremely small amount of air is blown vertically at a surface through very small holes. It can be used for aircraft or marine vehicles, such as submarines (where water is blown through the holes instead of air). As shown in the figure, the outer layer, which controls vertical flow, is a plate with high-vertical holes. The inner layer, which produces evenly distributed flow, is a low-permeability porous plate. Microblowing reduces the surface roughness and changes the flow velocity profile on the surface, thereby reducing skin friction.

Hwang, Danny P.; Biesiadny, Tom J.

1998-01-01

132

Microstructures of Inconel 718 alloy subjected to rapid thermal and stress cycle – joint performance and its controlling factors in friction welding of Inconel 718 alloy  

Microsoft Academic Search

The microstructures of an Inconel 718 alloy subjected to rapid thermal and stress cycles have been investigated to explain those observed in the friction welded joint of the alloy. The thermal and stress cycles were simulated with a Gleeble thermal and mechanical simulator. It turned out that the microstructural changes caused by the rapid heating cycle at peak temperatures of

Hongjie Wang; Kenji Ikeuchi; Makoto Takahashi; Akio Ikeda

2009-01-01

133

Heat transfer coefficient and friction factor correlations for rectangular solar air heater duct having transverse wedge shaped rib roughness on the absorber plate  

Microsoft Academic Search

As is well known, the heat transfer coefficient of a solar air heater duct can be increased by providing artificial roughness on the heated wall (i.e. the absorber plate). Experiments were performed to collect heat transfer and friction data for forced convection flow of air in solar air heater rectangular duct with one broad wall roughened by wedge shaped transverse

J. L Bhagoria; J. S Saini; S. C Solanki

2002-01-01

134

The normal stress effect and equilibrium friction coefficient of articular cartilage under steady frictional shear.  

PubMed

During creep or stress relaxation, articular cartilage exhibits a time-dependent friction coefficient which has been shown to reach an equilibrium value, mu eq, as the tissue deformation equilibrates. This study investigates the frictional properties of articular cartilage explants under steady frictional shear and constant compressive strain after the tissue reaches stress-relaxation equilibrium. The two parameters measured are the normal force and frictional torque, from which the friction coefficient was then calculated. It is shown in this experimental study that: (1) Under a prescribed infinitesimal compressive strain, cartilage supports higher compressive normal stress under steady shear than it does in the absence of frictional shear. Furthermore, the normal stress increases with increasing sliding velocity, resulting in a velocity-dependent value of mu eq. The observed normal stress effectively increases the compressive stiffness of cartilage by a factor up to 3.1. (2) Under a prescribed steady frictional shear both the normal stress and frictional shear stress increase, though not proportionally, with increasing compressive strain, producing a decreasing friction coefficient. (3) This velocity-dependent normal stress effect is also shown to result, at least partly, from intrinsic properties of cartilage. The normal stress effect has not been previously reported for articular cartilage, and represents an intriguing mechanical response not commonly encountered in solids, though common in non-Newtonian fluids. PMID:9239561

Wang, H; Ateshian, G A

1997-08-01

135

Testing a Missing Spectral Link in Turbulence  

NASA Astrophysics Data System (ADS)

Although the cardinal attribute of turbulence is the velocity fluctuations, these fluctuations have been ignored in theories of the frictional drag of turbulent flows. Our goal is to test a new theory that links the frictional drag to the spectral exponent ?, a property of the velocity fluctuations in a flow. We use a soap-film channel wherein for the first time the value of ? can be switched between 3 and 5/3, the two theoretically possible values in soap-film flows. To induce turbulence with ?=5/3, we make one of the edges of the soap-film channel serrated. Remarkably, the new theory of the frictional drag holds in both soap-film flows (for either value of the spectral exponent ?) and ordinary pipe flows (where ?=5/3), even though these types of flow are governed by different equations.

Kellay, Hamid; Tran, Tuan; Goldburg, Walter; Goldenfeld, Nigel; Gioia, Gustavo; Chakraborty, Pinaki

2012-12-01

136

Numerical investigation of pressure drop and heat transfer in developing laminar and turbulent nanofluid flows  

NASA Astrophysics Data System (ADS)

This paper concerns the study of laminar and turbulent force convection heat transfer and pressure drop between horizontal parallel plates with a nanofluid composed of Al2O3 and water. A set of governing equations are solved using a non-staggered SIMPLE procedure for the velocity-pressure coupling. For the convection-diffusion terms a power-law scheme is employed. A modified k-? model with a two-layer technique for the near-wall region has been used to predict the turbulent viscosity. The effects of nanoparticle volume fraction in the base fluid on laminar and turbulent flow variables are presented and discussed. The velocity and temperature profiles, friction factor, pressure coefficient and Nusselt number at different Reynolds numbers in the entrance region for both the laminar and turbulent flow regimes are reported under different thermal boundary conditions. The results show that the effect of the presence of nanoparticles in the base fluid on hydraulic and thermal parameters for the turbulent flow is not very significant, while the rate of heat transfer for the laminar flow with nanoparticles is greater than that of the base liquid. Furthermore, the thermal boundary layer and consequently the Nusselt number more quickly reach their fully developed values by increasing the percentage of nanoparticles in the base fluid for the laminar flow regime, while no changes in the trend are observed for the turbulent flow.

Ziaei-Rad, Masoud

2013-07-01

137

Friction of rocks  

USGS Publications Warehouse

Experimental results in the published literature show that at low normal stress the shear stress required to slide one rock over another varies widely between experiments. This is because at low stress rock friction is strongly dependent on surface roughness. At high normal stress that effect is diminished and the friction is nearly independent of rock type. If the sliding surfaces are separated by gouge composed of Montmorillonite or vermiculite the friction can be very low. ?? 1978 Birkha??user Verlag.

Byerlee, J.

1978-01-01

138

Polymer/riblet combination for hydrodynamic skin friction reduction  

NASA Technical Reports Server (NTRS)

A process is disclosed for reducing skin friction and inhibiting the effects of liquid turbulence in a system involving the flow of a liquid along the surface of a body, e.g. a marine vehicle. This process includes injecting a drag reducing polymer into the valleys of adjacent, evenly spaced, longitudinal grooves extending along the length of the surface of the body, so that the rate of diffusion of the polymer from individual grooves into the liquid flow is predictably controlled by the groove dimensions. When the polymer has diffused over the tips of the grooves into the near wall region of the boundary layer, the polymer effectively reduces the turbulent skin friction. A substantial drag reducing effect is achieved with less polymer than must be used to lower skin friction when the surface of the body is smooth.

Reed, Jason C. (inventor.); Bushnell, Dennis M. (inventor.)

1995-01-01

139

Theory of Quantum Friction  

E-print Network

Here, we develop a comprehensive quantum theory for the phenomenon of quantum friction. Based on a theory of macroscopic quantum electrodynamics for unstable systems, we calculate the quantum expectation of the friction force, and link the friction effect to the emergence of system instabilities related to the Cherenkov effect. These instabilities may occur due to the hybridization of particular guided modes supported by the individual moving bodies, and selection rules for the interacting modes are derived. It is proven that the quantum friction effect can take place even when the interacting bodies are lossless and made of nondispersive dielectrics.

Mario G. Silveirinha

2014-06-09

140

Eddy fluxes in baroclinic turbulence  

NASA Astrophysics Data System (ADS)

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 m2 s -1. Wind-driven near-inertial waves, mesoscale eddies and thermohaline intrusions are discussed as possible factors contributing to observed mixing patterns.

Thompson, Andrew F.

141

Progress in modeling hypersonic turbulent boundary layers  

NASA Technical Reports Server (NTRS)

A good knowledge of the turbulence structure, wall heat transfer, and friction in turbulent boundary layers (TBL) at high speeds is required for the design of hypersonic air breathing airplanes and reentry space vehicles. This work reports on recent progress in the modeling of high speed TBL flows. The specific research goal described here is the development of a second order closure model for zero pressure gradient TBL's for the range of Mach numbers up to hypersonic speeds with arbitrary wall cooling requirements.

Zeman, Otto

1993-01-01

142

Effect of heat transfer on turbulent boundary layers  

NASA Astrophysics Data System (ADS)

We use direct numerical simulation to gather a database of hypersonic turbulent boundary layers at different flow conditions varying heat transfer. A statistical description of the data is given, including the effect of wall-temperature condition on fluctuation levels, Reynolds stresses, energy and vorticity budgets, Reynolds analogies, skin friction, wall- pressure loading, and entrainment. Additionally, the turbulence structure is visualized and characterized.

Beekman, Izaak; Pino Martin, M.

2008-11-01

143

An analytical model of capped turbulent oscillatory bottom boundary layers  

Microsoft Academic Search

An analytical model of capped turbulent oscillatory bottom boundary layers (BBLs) is proposed using eddy viscosity of a quadratic form. The common definition of friction velocity based on maximum bottom shear stress is found unsatisfactory for BBLs under rotating flows, and a possible extension based on turbulent kinetic energy balance is proposed. The model solutions show that the flow may

Kenji Shimizu

2010-01-01

144

Extended ion pumped vacuum friction test  

NASA Technical Reports Server (NTRS)

Boundary layer friction data under ion pumped vacuum was taken for sixteen material couples. The test series was an extension of a previous study of the effects of modified ion pumped environments. Sliding distances imposed in the present effort greatly exceeded any studied in the previous contiguous, flight or ground tests. Wear out of specific couples, in particular, thin film lubricants was noted. The behavior of the test hardware including wear out of the mechanisms was noted. As a result, the impact of test interruption was observed for several test couples. Recovery of the friction upon re-establishing sliding in vacuum was generally rapid. The results of the extended sliding study reinforce the previous conclusion that sliding distance (mechanical history) is the primary factor in establishing the force limiting boundary layer friction. General friction value under the extended sliding confirm those observed in previous orbital and the related ground test studies.

Hammel, R. L.

1971-01-01

145

Active feedback control achieving sub-laminar skin friction drag  

NASA Astrophysics Data System (ADS)

Effects of an idealized feedback control are studied by means of direct numerical simulation (DNS). The control input is a body force directly suppressing the Reynolds shear stress near the wall. Namely, the body force is applied in the wall-normal direction and in phase with the streamwise velocity fluctuation. The DNS of turbulent pipe flow at constant flow rate at Re_b=5300 (i.e, Re_?? 180 for uncontrolled flow) shows that the skin friction can be reduced even to a sub-laminar level. This is caused by the reversal of the sign of Reynolds shear stress, which results in a negative value of ``the turbulent contribution to skin friction'' [Fukagata et al., Phys. Fluids 14, L73 (2002)]. The turbulence structure is also drastically modified with this control. The quasi-streamwise vortices completely vanished and alternating spanwise roller-like structures formed instead.

Fukagata, Koji; Kasagi, Nobuhide

2004-11-01

146

FRICTIONAL EQUILIBRIUM POSTURES FOR ROBOTIC  

E-print Network

FRICTIONAL EQUILIBRIUM POSTURES FOR ROBOTIC LOCOMOTION - COMPUTATION, GEOMETRIC CHARACTERIZATION, AND STABILITY ANALYSIS YIZHAR OR #12;FRICTIONAL EQUILIBRIUM POSTURES FOR ROBOTIC LOCOMOTION - COMPUTATION grasping and manipulation . . . . . . . . . . . . . . 6 1.1.3 Dynamic stability of frictional equilibrium

Rimon, Elon

147

Heat Transfer in a Turbulent Liquid or Gas Stream  

NASA Technical Reports Server (NTRS)

The,theory of heat.transfer from a solid body to a liquid stream could he presented previously** only with limiting assumptions about the movement of the fluid (potential flow, laminar frictional flow). (See references 1, 2, and 3). For turbulent flow, the most important practical case, the previous theoretical considerations did not go beyond dimensionless formulas and certain conclusions as to the analogy between the friction factor and the unit thermal conductance, (See references 4, 5, 6, and 7,) In order to obtain numerical results, an experimental treatment of the problem was resorted to, which gave rise to numerous investigations because of the importance of this problem in many branches of technology. However, the results of these investigations frequently deviate from one another. The experimental results are especially dependent upon the overall dimensions and the specific proportions of the equipment. In the present work, the attempt will be made to develop systematically the theory of the heat transfer and of the dependence of the unit thermal conductance upon shape and dimensions, using as a basis the velocity distribution for turbulent flow set up by Prandtl and Von Karman.

Latzko, H.

1944-01-01

148

Frictional drag reduction by bubble injection  

NASA Astrophysics Data System (ADS)

The injection of gas bubbles into a turbulent boundary layer of a liquid phase has multiple different impacts on the original flow structure. Frictional drag reduction is a phenomenon resulting from their combined effects. This explains why a number of different void-drag reduction relationships have been reported to date, while early works pursued a simple universal mechanism. In the last 15 years, a series of precisely designed experimentations has led to the conclusion that the frictional drag reduction by bubble injection has multiple manifestations dependent on bubble size and flow speed. The phenomena are classified into several regimes of two-phase interaction mechanisms. Each regime has inherent physics of bubbly liquid, highlighted by keywords such as bubbly mixture rheology, the spectral response of bubbles in turbulence, buoyancy-dominated bubble behavior, and gas cavity breakup. Among the regimes, bubbles in some selected situations lose the drag reduction effect owing to extra momentum transfer promoted by their active motions. This separates engineers into two communities: those studying small bubbles for high-speed flow applications and those studying large bubbles for low-speed flow applications. This article reviews the roles of bubbles in drag reduction, which have been revealed from fundamental studies of simplified flow geometries and from development of measurement techniques that resolve the inner layer structure of bubble-mixed turbulent boundary layers.

Murai, Yuichi

2014-07-01

149

DNS of turbulent heat transfer in channel flow with respect to Reynolds and Prandtl number effects  

Microsoft Academic Search

The direct numerical simulation (DNS) of turbulent heat transfer in a channel flow has been carried out to investigate the Reynolds and Prandtl number effects on the turbulent heat transport. The configuration is a fully developed turbulent channel flow with uniform heating from both walls. The Reynolds numbers based on the friction velocity and the channel half width are 180

Hiroshi Kawamura; Hiroyuki Abe; Yuichi Matsuo

1999-01-01

150

Deposition rates of unattached and attached radon progeny in room with turbulent airflow and ventilation  

Microsoft Academic Search

In this paper deposition rate coefficients for unattached and attached radon progeny were estimated according to a particle deposition model for turbulent indoor airflow described by Zhao and Wu [2006. Modeling particle deposition from fully developed turbulent flow in ventilation duct. Atmos. Environ. 40, 457–466]. The parameter which characterizes turbulent indoor airflow in this model is friction velocity, u*. Indoor

N. Stevanovic; V. M. Markovic; D. Nikezic

2009-01-01

151

Experimental studies on heat transfer and friction factor characteristics of Al{sub 2}O{sub 3}/water nanofluid in a circular pipe under laminar flow with wire coil inserts  

SciTech Connect

In this paper, fully developed laminar flow convective heat transfer and friction factor characteristics of Al{sub 2}O{sub 3}/water nanofluid flowing through a uniformly heated horizontal tube with and without wire coil inserts is presented. For this purpose, Al{sub 2}O{sub 3} nanoparticles of 43 nm size were synthesized, characterized and dispersed in distilled water to form stable suspension containing 0.1% volume concentration of nanoparticles. The Nusselt number in the fully developed region were measured and found to increase by 12.24% at Re = 2275 for plain tube with nanofluid compared to distilled water. Two wire coil inserts made of stainless steel with pitch ratios 2 and 3 were used which increased the Nusselt numbers by 15.91% and 21.53% respectively at Re = 2275 with nanofluid compared to distilled water. The better heat transfer performance of nanofluid with wire coil insert is attributed to the effects of dispersion or back-mixing which flattens the temperature distribution and make the temperature gradient between the fluid and wall steeper. The measured pressure loss with the use of nanofluids is almost equal to that of the distilled water. The empirical correlations developed for Nusselt number and friction factor in terms of Reynolds/Peclet number, pitch ratio and volume concentration fits with the experimental data within {+-}15%. (author)

Chandrasekar, M.; Suresh, S. [Department of Mechanical Engineering, National Institute of Technology, Tiruchirappalli 620015 (India); Chandra Bose, A. [Nanomaterials Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli 620015 (India)

2010-02-15

152

Friction of rocks  

Microsoft Academic Search

Experimental results in the published literature show that at low normal stress the shear stress required to slide one rock over another varies widely between experiments. This is because at low stress rock friction is strongly dependent on surface roughness. At high normal stress that effect is diminished and the friction is nearly independent of rock type. If the sliding

J. Byerlee

1978-01-01

153

The Force of Friction  

NSDL National Science Digital Library

In the first of two lessons of this curricular unit, students are introduced to the concept of friction as a force that impedes motion when two surfaces are in contact. Student teams use spring scales to drag objects, such as a ceramic coffee cup, along a table top or the floor, measuring the frictional force that exists between the moving object and the surface it slides on. By modifying the bottom surface of the object, students find out what kinds of surfaces generate more or less friction. They also discover that both static and kinetic friction are involved when an object initially at rest is caused to slide across a surface. In the second lesson of the unit, students design and conduct experiments to determine the effects of weight and surface area on friction. They discover that weight affects normal friction (the friction that results from surface roughness), but for very smooth surfaces, the friction due to molecular attraction is affected by contact area.

Engineering K-Phd Program

154

NTNU Java: Frictional force  

NSDL National Science Digital Library

This applet illustrates the effect of frictional forces on the motion of masses. In this experiment, two masses, resting one on top of the other, are pulled by a hanging mass connected by an ideal rope. Masses and friction can be changed. Different force vectors are color-coded.

Hwang, Fu-Kwun

2004-12-18

155

Experimental and numerical investigation of transition to turbulent flow and heat transfer inside a horizontal smooth rectangular duct under uniform bottom surface temperature  

NASA Astrophysics Data System (ADS)

In this study, steady-state turbulent forced flow and heat transfer in a horizontal smooth rectangular duct both experimentally and numerically investigated. The study was carried out in the transition to turbulence region where Reynolds numbers range from 2,323 to 9,899. Flow is hydrodynamically and thermally developing (simultaneously developing flow) under uniform bottom surface temperature condition. A commercial CFD program Ansys Fluent 12.1 with different turbulent models was used to carry out the numerical study. Based on the present experimental data and three-dimensional numerical solutions, new engineering correlations were presented for the heat transfer and friction coefficients in the form of {{Nu}} = {{C}}2 {{Re}}^{{{{n}}_{ 1} }} and {{f}} = {{C}}3 {{Re}}^{{{{n}}3 }} , respectively. The results have shown that as the Reynolds number increases heat transfer coefficient increases but Darcy friction factor decreases. It is seen that there is a good agreement between the present experimental and numerical results. Examination of heat and mass transfer in rectangular cross-sectioned duct for different duct aspect ratio (?) was also carried out in this study. Average Nusselt number and average Darcy friction factor were expressed with graphics and correlations for different duct aspect ratios.

Arslan, Kamil; Onur, Nevzat

2013-07-01

156

Imagine Life without Friction  

NSDL National Science Digital Library

Students are introduced to the concept of inertia and its application to a world without the force of friction acting on moving objects. When an object is in motion, friction tends to be the force that acts on this object to slow it down and eventually come to a stop. By severely limiting friction through the use of hover pucks, students learn that the energy of one moving puck is transferred directly to another puck at rest when they collide. Students learn the concept of the conservation of energy via a "collision," and come to realize that with friction, energy is converted primarily to heat to slow and stop an object in motion. In the associated activity, "The Puck Stops Here," students investigate the frictional force of an object when different materials are placed between the object and the ground. They apply this understanding to the challenge to design a new hockey puck for the National Hockey League.

Engineering K-PhD Program,

157

Friction plug welding  

NASA Technical Reports Server (NTRS)

Friction plug welding (FPW) usage is advantageous for friction stir welding (FSW) hole close-outs and weld repairs in 2195 Al--Cu--Li fusion or friction stir welds. Current fusion welding methods of Al--Cu--Li have produced welds containing varied defects. These areas are found by non-destructive examination both after welding and after proof testing. Current techniques for repairing typically small (<0.25) defects weaken the weldment, rely heavily on welders' skill, and are costly. Friction plug welding repairs increase strength, ductility and resistance to cracking over initial weld quality, without requiring much time or operator skill. Friction plug welding while pulling the plug is advantageous because all hardware for performing the weld can be placed on one side of the workpiece.

Takeshita, Riki (Inventor); Hibbard, Terry L. (Inventor)

2001-01-01

158

Friction in hip prostheses.  

PubMed

Although the reduction of frictional torques was the driving force behind the design of the Charnley prosthesis, later concerns about wear and subsequent loosening of this and other hip replacements have dominated debate within the bioengineering community. To stimulate discussion on the role of friction in loosening, a review of the frictional characteristics of different prostheses was undertaken. The use of simple laboratory screening-type machines in the frictional assessment of different material combinations is discussed together with experiments performed on single axis simulators using both conventional and experimental prostheses. In particular, recent developments in the use of soft layer components are highlighted. Further, the possible link between excessively high frictional torques and loosening is discussed in the light of current results obtained from explanted prostheses. PMID:9239463

Hall, R M; Unsworth, A

1997-08-01

159

Statistics of Frictional Families  

NASA Astrophysics Data System (ADS)

We develop a theoretical description for mechanically stable frictional packings in terms of the difference between the total number of contacts required for isostatic packings of frictionless disks and the number of contacts in frictional packings, m=Nc0-Nc. The saddle order m represents the number of unconstrained degrees of freedom that a static packing would possess if friction were removed. Using a novel numerical method that allows us to enumerate disk packings for each m, we show that the probability to obtain a packing with saddle order m at a given static friction coefficient ?, Pm(?), can be expressed as a power series in ?. Using this form for Pm(?), we quantitatively describe the dependence of the average contact number on the friction coefficient for static disk packings obtained from direct simulations of the Cundall-Strack model for all ? and N.

Shen, Tianqi; Papanikolaou, Stefanos; O'Hern, Corey S.; Shattuck, Mark D.

2014-09-01

160

Statistics of frictional families.  

PubMed

We develop a theoretical description for mechanically stable frictional packings in terms of the difference between the total number of contacts required for isostatic packings of frictionless disks and the number of contacts in frictional packings, m=Nc0 - Nc. The saddle order m represents the number of unconstrained degrees of freedom that a static packing would possess if friction were removed. Using a novel numerical method that allows us to enumerate disk packings for each m, we show that the probability to obtain a packing with saddle order m at a given static friction coefficient ?, Pm(?), can be expressed as a power series in ?. Using this form for Pm(?), we quantitatively describe the dependence of the average contact number on the friction coefficient for static disk packings obtained from direct simulations of the Cundall-Strack model for all ? and N. PMID:25279647

Shen, Tianqi; Papanikolaou, Stefanos; O'Hern, Corey S; Shattuck, Mark D

2014-09-19

161

Kinetic Turbulence  

NASA Astrophysics Data System (ADS)

The weak collisionality typical of turbulence in many diffuse astrophysical plasmas invalidates an MHD description of the turbulent dynamics, motivating the development of a more comprehensive theory of kinetic turbulence. In particular, a kinetic approach is essential for the investigation of the physical mechanisms responsible for the dissipation of astrophysical turbulence and the resulting heating of the plasma. This chapter reviews the limitations of MHD turbulence theory and explains how kinetic considerations may be incorporated to obtain a kinetic theory for astrophysical plasma turbulence. Key questions about the nature of kinetic turbulence that drive current research efforts are identified. A comprehensive model of the kinetic turbulent cascade is presented, with a detailed discussion of each component of the model and a review of supporting and conflicting theoretical, numerical, and observational evidence.

Howes, Gregory G.

162

Action of friction Frictional processes are not often considered in  

E-print Network

Action of friction Frictional processes are not often considered in any detail in studies are switched off, the pressure falls to just 921mb. Frictional processes can be thought of in terms of changes. The first term on the right­hand­side represents barotropic damping by friction, and the second

Plant, Robert

163

Static Friction Phenomena The following static friction phenomena have a direct dependency on velocity.  

E-print Network

Coulomb Friction Viscous Friction Stribeck Friction Static Friction Phenomena The following static friction phenomena have a direct dependency on velocity. Static Friction Model: Friction force opposes the direction of motion when the sliding velocity is zero. Coulomb Friction Model: Friction force

Simpkins, Alex

164

Friction-Induced Fluid Heating in Nanoscale Helium Flows  

SciTech Connect

We investigate the mechanism of friction-induced fluid heating in nanoconfinements. Molecular dynamics simulations are used to study the temperature variations of liquid helium in nanoscale Poiseuille flows. It is found that the fluid heating is dominated by different sources of friction as the external driving force is changed. For small external force, the fluid heating is mainly caused by the internal viscous friction in the fluid. When the external force is large and causes fluid slip at the surfaces of channel walls, the friction at the fluid-solid interface dominates over the internal friction in the fluid and is the major contribution to fluid heating. An asymmetric temperature gradient in the fluid is developed in the case of nonidentical walls and the general temperature gradient may change sign as the dominant heating factor changes from internal to interfacial friction with increasing external force.

Li Zhigang [Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon (Hong Kong)

2010-05-21

165

Semiactive control of friction dampers  

Microsoft Academic Search

Quite a few papers have appeared dealing with how to minimize the effects of friction in machines. A recurring theme has been that friction is a dynamic phenomenon. As such, much attention has been paid to obtaining the appropriate friction model structure, and to the identification of model parameters. For semi-active damping, friction dampers hold promise in many applications. In

Pierre Dupont; Ann Stokes

1995-01-01

166

Combined Devices for Turbulent-Drag Reduction  

NASA Technical Reports Server (NTRS)

Aircraft skin-friction drag reduced as much as 15 percent. One effective drag-reduction technique involves use of riblets. Riblets are longitudinal striations or grooves machined on originally smooth surface. Grooves alined with flow. Grooves have depths and spacings on order of turbulent wallstreak and burst dimensions and designed to change near-wall structure of turbulent boundary layer. Another approach, using large-eddy-breakup (LEBU) devices, or turbulence manipulators or ribbons also demonstrated reductions in local skin friction and net drag in air. LEBU device consists of thin, ribbonlike strips or airfoils suspended parallel to test surface and positioned within turbulent boundary layer. Technique potentially reduce net skin-friction drag by at least 15 percent on turbulent boundary layer of aircraft, representing possible annual savings in fuel costs of $300 to $400 million for U.S. commercial fleet. Also applicable to frictionloss reduction inside pipes and ducts, contributing to increased efficiency of pumps, heat exchangers, air conditioners, and other devices involving fluid flow.

Walsh, M. J.; Anders., John B., Jr.; Hefner, J. N.

1986-01-01

167

Friction stir welding tool  

DOEpatents

A friction stir welding tool is described and which includes a shank portion; a shoulder portion which is releasably engageable with the shank portion; and a pin which is releasably engageable with the shoulder portion.

Tolle; Charles R. (Idaho Falls, ID), Clark; Denis E. (Idaho Falls, ID), Barnes; Timothy A. (Ammon, ID)

2008-04-15

168

Friction stir weld tools  

NASA Technical Reports Server (NTRS)

A friction stir weld tool sleeve is supported by an underlying support pin. The pin material is preferably selected for toughness and fracture characteristics. The pin sleeve preferably has a geometry which employs the use of an interrupted thread, a plurality of flutes and/or eccentric path to provide greater flow through. Paddles have been found to assist in imparting friction and directing plastic metal during the welding process.

Carter, Robert W. (Inventor); Payton, Lewis N. (Inventor)

2007-01-01

169

Friction Stir Weld Tools  

NASA Technical Reports Server (NTRS)

A friction stir weld tool sleeve is supported by an underlying support pin. The pin material is preferably selected for toughness and fracture characteristics. The pin sleeve preferably has a geometry which employs the use of an interrupted thread, a plurality of flutes and/or eccentric path to provide greater flow through. Paddles have been found to assist in imparting friction and directing plastic metal during the welding process.

Carter, Robert W. (Inventor); Payton, Lewis N. (Inventor)

2007-01-01

170

The influence of freestream turbulence spectrum on boundary layer transition  

NASA Astrophysics Data System (ADS)

The flow in turbomachines is characterized by a high turbulent activity. Its spectrum frequently reveals energy peaks at privileged frequencies. They generally have an influence upon the boundary layer transition onset. This type of forced transition was studied at the von Karman Institute in a low speed wind tunnel along a flat test surface. Discrete frequency energy peaks were generated into the mainstream flow by acoustic means. The receptivity of the boundary layer flow to the acoustic excitation frequency is put in evidence both by the frequency-dependent transition Reynolds number and the streamwise intermittency factor distributions. The intermittency measurements were performed with the help of a conditional sampling technique; the latter includes a new approach to the positioning of the laminar/turbulent status decision threshold. The growths of selected oscillation modes are compared in a natural and a forced transition situation. A model providing an estimation, within the transition region, of the turbulence level profiles and the skin friction coefficient is proposed.

Costa, Jorge; Arts, Tony

171

Isolating Curvature Effects in Computing Wall-Bounded Turbulent Flows  

NASA Technical Reports Server (NTRS)

The flow over the zero-pressure-gradient So-Mellor convex curved wall is simulated using the Navier-Stokes equations. An inviscid effective outer wall shape, undocumented in the experiment, is obtained by using an adjoint optimization method with the desired pressure distribution on the inner wall as the cost function. Using this wall shape with a Navier-Stokes method, the abilities of various turbulence models to simulate the effects of curvature without the complicating factor of streamwise pressure gradient can be evaluated. The one-equation Spalart-Allmaras turbulence model overpredicts eddy viscosity, and its boundary layer profiles are too full. A curvature-corrected version of this model improves results, which are sensitive to the choice of a particular constant. An explicit algebraic stress model does a reasonable job predicting this flow field. However, results can be slightly improved by modifying the assumption on anisotropy equilibrium in the model's derivation. The resulting curvature-corrected explicit algebraic stress model possesses no heuristic functions or additional constants. It lowers slightly the computed skin friction coefficient and the turbulent stress levels for this case (in better agreement with experiment), but the effect on computed velocity profiles is very small.

Rumsey, Christopher L.; Gatski, Thomas B.

2001-01-01

172

Friction Stir Welding Development  

NASA Technical Reports Server (NTRS)

The research of this summer was a continuation of work started during the previous summer faculty fellowship period. The Friction Stir Welding process (FSW) patented by The Welding Institute (TWI), in Great Britain, has become a popular topic at the Marshall Space Flight Center over the past year. Last year it was considered a novel approach to welding but few people took it very seriously as a near term solution. However, due to continued problems with cracks in the new aluminum-lithium space shuttle external tank (ET), the friction stir process is being mobilized at full speed in an effort to mature this process for the potential manufacture of flight hardware. It is now the goal of NASA and Lockheed-Martin Corporation (LMC) to demonstrate a full-scale friction stir welding system capable of welding ET size barrel sections. The objectives this summer were: (1) Implementation and validation of the rotating dynamometer on the MSFC FSW system; (2) Collection of data for FSW process modeling efforts; (3) Specification development for FSW implementation on the vertical weld tool; (4) Controls and user interface development for the adjustable pin tool; and (5) Development of an instrumentation system for the planishing process. The projects started this summer will lead to a full scale friction stir welding system that is expected to produce a friction stir welded shuttle external tank type barrel section. The success of this could lead to the implementation of the friction stir process for manufacturing future shuttle external tanks.

Romine, Peter L.

1998-01-01

173

Acting on Frictions: Learning Blocks and Flows in Knowledge Intensive Organizations  

ERIC Educational Resources Information Center

Purpose: It is argued in this paper that opportunities for learning manifest themselves in the form of frictions between the structure-as-experienced by actors and the structure-as-preferred. These frictions are considered as potential triggers of learning processes. The concept of friction promises to contribute to our understanding of factors

Bijlsma-Frankema, Katinka; Rosendaal, Bastiaan; Taminiau, Yvette

2006-01-01

174

Impact of Friction and Scale-Dependent Initial Stress on Radiated Energy-Moment Scaling  

E-print Network

271 Impact of Friction and Scale-Dependent Initial Stress on Radiated Energy-Moment Scaling Bruce E from an event depends on a number of factors, including the friction and, crucially, the initial stress distribution of initial stresses consistent with the dynamics and a given friction. We examine a variety

Shaw, Bruce E.

175

Evaluation of friction condition in cold forging by using T-shape compression test  

Microsoft Academic Search

Friction plays an important role in metal forming, and numerical simulation of forging processes requires precise informations about the material properties and the value of the friction factor m or coefficient ?. This paper describes the T-shape compression, a new friction testing method by combined compression and extrusion of a cylinder between a flat punch and a V-grooved die. It

Q. Zhang; E. Felder; S. Bruschi

2009-01-01

176

Turbulent film condensation in a tube with concurrent and countercurrent vapor flow  

NASA Astrophysics Data System (ADS)

The Nusselt analysis for laminar film condensation was extended for the turbulent film condensation with countercurrent and concurrent vapor flow inside a vertical tube. This was done for constant flux by accounting for the interfacial shear due to vapor friction and suction. The decrease in the stream flow rate due to condensation is considered. The best eddy diffusivity model to describe the flow and temperature distribution is the Van Driest model with a damping factor as well as some modification which takes into account the effect of variable shear stress. A numerical solution of the problem is then required. This analysis can predict the average film thickness, local heat transfer coefficient and average heat transfer coefficients in the laminar and turbulent flow region of liquid and vapor flow for both or without interfacial shear.

Faghri, A.

1986-06-01

177

Turbulence modeling  

NASA Technical Reports Server (NTRS)

The performance of existing two-equation eddy viscosity models was examined. An effort was made to develop better models for near-wall turbulence using direct numerical simulations of plane channel and boundary layer flows. The asymptotic near-wall behavior of turbulence was used to examine the problems of current second order closure models and develop new models with the correct near-wall behavior. Rapid Distortion Theory was used to analytically study the effects of mean deformation on turbulence, obtain analytical solutions for the spectrum tensor, Reynolds stress tensor, anisotropy tensor and its invariants, which can be used in the turbulence model development. The potential of the renormalization group theory in turbulence modeling was studied, as well as compressible turbulent flows, and modeling of bypass transition.

Shih, Tsan-Hsing

1991-01-01

178

Chemical Origins of Frictional Aging  

NASA Astrophysics Data System (ADS)

Although the basic laws of friction are simple enough to be taught in elementary physics classes and although friction has been widely studied for centuries, in the current state of knowledge it is still not possible to predict a friction force from fundamental principles. One of the highly debated topics in this field is the origin of static friction. For most macroscopic contacts between two solids, static friction will increase logarithmically with time, a phenomenon that is referred to as aging of the interface. One known reason for the logarithmic growth of static friction is the deformation creep in plastic contacts. However, this mechanism cannot explain frictional aging observed in the absence of roughness and plasticity. Here, we discover molecular mechanisms that can lead to a logarithmic increase of friction based purely on interfacial chemistry. Predictions of our model are consistent with published experimental data on the friction of silica.

Liu, Yun; Szlufarska, Izabela

2012-11-01

179

Turbulence revisited  

Microsoft Academic Search

A new approach to turbulence based upon the Stabilization Principle is introduced. Onset of turbulence is interpreted as loss of stability of solutions to the Navier–Stokes equations in the class of differentiable functions. Developed turbulence is considered as postinstability motion in the class of non-differentiable functions. A non-linear version of the Liouville equation is proposed for describing postinstability motions of

Michail Zak

2009-01-01

180

Friction of Aviation Engines  

NASA Technical Reports Server (NTRS)

The first portion of this report discusses measurements of friction made in the altitude laboratory of the Bureau of Standards between 1920 and 1926 under research authorization of the National Advisory Committee for Aeronautics. These are discussed with reference to the influence of speed, barometric pressure, jacket-water temperature, and throttle opening upon the friction of aviation engines. The second section of the report deals with measurements of the friction of a group of pistons differing from each other in a single respect, such as length, clearance, area of thrust face, location of thrust face, etc. Results obtained with each type of piston are discussed and attention is directed particularly to the fact that the friction chargeable to piston rings depends upon piston design as well as upon ring design. This is attributed to the effect of the rings upon the thickness and distribution of the oil film which in turn affects the friction of the piston to an extent which depends upon its design.

Sparrow, S W; Thorne, M A

1928-01-01

181

Turbulent acceleration and heating in toroidal magnetized plasmas  

SciTech Connect

It is shown that turbulence is responsible for a source of momentum, which cannot be recast as a divergence of a momentum flux. This process is similar to turbulent heating, with similar properties. The sum over all species vanishes up to polarization contributions. Hence, toroidal momentum is transferred from species to species, mediated by turbulence. As for momentum flux, symmetry breaking is needed. Flow shear is investigated as a source of symmetry breaking, leading to a source of momentum proportional to the shear rate. Turbulent acceleration is significant for ion species. It is found that it is proportional to the charge number Z, while turbulent heating scales as Z{sup 2}/A, where A is the mass number. It is maximum in the edge, where the E × B flow shear rate and turbulence intensity are maximum. When both are large enough, the turbulent torque may overcome the collisional friction between impurities and main ions, thus leading to different toroidal velocities.

Garbet, X.; Esteve, D.; Sarazin, Y.; Abiteboul, J.; Bourdelle, C.; Dif-Pradalier, G.; Ghendrih, P.; Grandgirard, V.; Latu, G. [CEA, IRFM, F-13108 St. Paul-lez-Durance cedex (France)] [CEA, IRFM, F-13108 St. Paul-lez-Durance cedex (France); Smolyakov, A. [Department of Physics and Engineering Physics, University of Saskatchewan, 116 Science Place, Saskatoon, Saskatchewan S7N 5E2 (Canada)] [Department of Physics and Engineering Physics, University of Saskatchewan, 116 Science Place, Saskatoon, Saskatchewan S7N 5E2 (Canada)

2013-07-15

182

Memory function of turbulent fluctuations in soft-mode turbulence  

E-print Network

Modal relaxation dynamics has been observed experimentally to clarify statistical-physical properties of soft-mode turbulence, the spatiotemporal chaos observed in homeotropically aligned nematic liquid crystals. We found a dual structure, dynamical crossover associated with violation of time-reversal invariance, the corresponding time scales satisfying a dynamical scaling law. To specify the origin of the dual structure, the memory function due to non-thermal fluctuations has been defined by a projection-operator method and obtained numerically using experimental results. The results of the memory function suggest that the non-thermal fluctuations can be divided into Markov and non-Markov contributions, the latter is called the turbulent fluctuation (TF). Consequently, the relaxation dynamics is separated into three characteristic stages: bare-friction, early, and late stages. If the dissipation due to TFs dominates over that of the Markov contribution, the bare-friction stage contracts; the early and late stages then configure the dual structure. The memory effect due to TFs results in the time-reversible relaxation at the early stage, and the disappearance of the memory by turbulent mixing leads to a simple exponential relaxation at the late stage. Furthermore, the memory effect due to TFs is shown to originate from characteristic spatial coherency called the patch structure.

Takayuki Narumi; Junichi Yoshitani; Masaru Suzuki; Yoshiki Hidaka; Fahrudin Nugroho; Tomoyuki Nagaya; Shoichi Kai

2013-01-21

183

Skin friction balance  

NASA Technical Reports Server (NTRS)

A skin friction balance uses a parallel linkage mechanism to avoid inaccuracies in skin friction measurement attributable to off-center normal forces. The parallel linkage mechanism includes a stationary plate mounted in a cage, and an upper and lower movable plate which are linked to each other and to the stationary plate throught three vertical links. Flexure pivots are provided for pivotally connecting the links and the plates. A sensing element connected to the upper plate moves in response to skin friction, and the lower plate moves in the opposite direction of the upper plate. A force motor maintains a null position of the sensing element by exerting a restoring force in response to a signal generated by a linear variable differential transformer (LVDT).

Ping, Tcheng (inventor); Supplee, Frank H., Jr. (inventor)

1989-01-01

184

Micro Surface Texturing for Friction Control  

NASA Astrophysics Data System (ADS)

Recently, surface texturing has gained momentum as a way to control the friction which is involved in various applications and systems, such as components of internal combustion engines, dies and punches of the metal forming processes and Micro-electrical-mechanical Systems (MEMS). This dissertation demonstrates that under dry sliding, there is a specific significant surface texturing parameter at which the coefficient of friction should be at a minimum. This dissertation met this objective through an extensive study of the relevant literature on surface texturing and friction, analysing the friction mechanisms involved in dry sliding, and highlighting the key factors that control friction as the real area of contact and material properties. An analytical proof is derived demonstrating that a minimum friction force exists if the two components of the friction force, adhesion and mechanical deformation, are differentiated with respect to the real contact area. In addition, numerical simulations and experimental work were performed to test this hypothesis. In the two and three dimensional finite element models, normal and sliding contact between a rigid indenter and elastic-plastic surfaces, which are textured by circular and hexagonal dimples of different sizes and densities, are simulated and analysed. Circular craters of different sizes and densities, are fabricated using laser ablation on hardened tool steel samples, while the hexagonal dimples are fabricated using photo-lithography. The dimples are arranged in adjacent equilateral triangles layout. Coefficients of friction were measured using a scratch tester under dry sliding conditions and constant load. In addition, adhesion forces were estimated using an Atomic Force Microscope (AFM). The adhesion force is found to be exponentially decreasing with the increase of the spatial texture density. The dimensionless quantity, spatial texture density (D/L) was identified as the most significant texturing parameter. It is equal to the ratio of the size of the crater (D) to the distance between the centres of two consecutive craters (L). A minimum coefficient of friction exits at values of spatial texture densities that lie between 0.25 and 0.5.

Rashwan, Ola

185

An investigation of turbulence models  

NASA Technical Reports Server (NTRS)

The accuracy to which a turbulent boundary layer or wake can be predicted numerically depends on the validity of the turbulence closure model used. The modeling of turbulence physics is one of the most difficult problems in computational fluid dynamics (CFD). In fact, it is one of the pacing factors in the development of CFD. In general, there are three main approaches to the description of trubulence physics. First is turbulence modeling in which the Reynolds averaged Navier-Stokes equations are used and some closure approximation is made for the Reynolds stresses. A second approach to turbulence is large eddy simulation (LES) in which the computational mesh is taken to be fine enough that the large scale structure of the turbulence can be calculated directly. An empirical assumption must be made for the small scale sub-grid turbulence. The third approach is direct simulation. In this technique the Navier-Strokes equations are solved directly on a mesh which if fine enough to resolve the smallest length scale of the turbulence. The Reynolds averaged equations are not used and no closure assumption is required. These last two approaches require extensive computer resources and as such are not engineering tools. The purpose of the work was to investigate the various engineering turbulence models for accuracy and ease of programming. This involved comparison of the models with each other and with experimental data.

Doria, Michael L.

1988-01-01

186

Atomic scale friction of molecular adsorbates during diffusion.  

PubMed

Experimental observations suggest that molecular adsorbates exhibit a larger friction coefficient than atomic species of comparable mass, yet the origin of this increased friction is not well understood. We present a study of the microscopic origins of friction experienced by molecular adsorbates during surface diffusion. Helium spin-echo measurements of a range of five-membered aromatic molecules, cyclopentadienyl, pyrrole, and thiophene, on a copper(111) surface are compared with molecular dynamics simulations of the respective systems. The adsorbates have different chemical interactions with the surface and differ in bonding geometry, yet the measurements show that the friction is greater than 2 ps(-1) for all these molecules. We demonstrate that the internal and external degrees of freedom of these adsorbate species are a key factor in the underlying microscopic processes and identify the rotation modes as the ones contributing most to the total measured friction coefficient. PMID:23697433

Lechner, B A J; de Wijn, A S; Hedgeland, H; Jardine, A P; Hinch, B J; Allison, W; Ellis, J

2013-05-21

187

Interfacial friction between semiflexible polymers and crystalline surfaces  

NASA Astrophysics Data System (ADS)

The results obtained from molecular dynamics simulations of the friction at an interface between polymer melts and weakly attractive crystalline surfaces are reported. We consider a coarse-grained bead-spring model of linear chains with adjustable intrinsic stiffness. The structure and relaxation dynamics of polymer chains near interfaces are quantified by the radius of gyration and decay of the time autocorrelation function of the first normal mode. We found that the friction coefficient at small slip velocities exhibits a distinct maximum which appears due to shear-induced alignment of semiflexible chain segments in contact with solid walls. At large slip velocities, the friction coefficient is independent of the chain stiffness. The data for the friction coefficient and shear viscosity are used to elucidate main trends in the nonlinear shear rate dependence of the slip length. The influence of chain stiffness on the relationship between the friction coefficient and the structure factor in the first fluid layer is discussed.

Priezjev, Nikolai V.

2012-06-01

188

Interfacial friction between semiflexible polymers and crystalline surfaces.  

PubMed

The results obtained from molecular dynamics simulations of the friction at an interface between polymer melts and weakly attractive crystalline surfaces are reported. We consider a coarse-grained bead-spring model of linear chains with adjustable intrinsic stiffness. The structure and relaxation dynamics of polymer chains near interfaces are quantified by the radius of gyration and decay of the time autocorrelation function of the first normal mode. We found that the friction coefficient at small slip velocities exhibits a distinct maximum which appears due to shear-induced alignment of semiflexible chain segments in contact with solid walls. At large slip velocities, the friction coefficient is independent of the chain stiffness. The data for the friction coefficient and shear viscosity are used to elucidate main trends in the nonlinear shear rate dependence of the slip length. The influence of chain stiffness on the relationship between the friction coefficient and the structure factor in the first fluid layer is discussed. PMID:22713064

Priezjev, Nikolai V

2012-06-14

189

Evaluation of an incompressible energy-vorticity turbulence model for fully rough pipe flow  

NASA Astrophysics Data System (ADS)

Traditional methods of closing the Boussinesq-based Reynolds-averaged Navier-Stokes equations are considered, and suggestions for improving two-equation turbulence models are made. The traditional smooth-wall boundary conditions are shown to be incorrect, and the correct boundary conditions are provided along with sample solutions to traditional models. The correct boundary condition at a smooth wall for dissipation-based turbulence models is that which forces both the turbulent kinetic energy and its first derivative to zero. Foundations for an energy-vorticity model suggested by Phillips are presented along with the near-smooth-wall behavior of the model. These results show that at a perfectly smooth wall, the turbulent kinetic energy may approach the wall at a higher order than is generally accepted. The foundations of this model are used in the development of a k-lambda model for fully rough pipe flow. Closure coefficients for the model are developed through gradient-based optimization techniques. Results of the model are compared to results from the Wilcox 1998 and 2006 k-o models as well as four eddy-viscosity models. The results show that the Phillips k-lambda model is much more accurate than other models for predicting the relationship between Reynolds number and friction factor for fully rough pipe flow. However, the velocity profiles resulting from the model deviate noticeably from the law of the wall.

Hunsaker, Douglas F.

190

Interfacial friction and adhesion of polymer brushes.  

PubMed

A bead-probe lateral force microscopy (LFM) technique is used to characterize the interfacial friction and adhesion properties of polymer brushes. Our measurements attempt to relate the physical structure and chemical characteristics of the brush to their properties as thin-film, tethered lubricants. Brushes are synthesized at several chain lengths and surface coverages from polymer chains of polydimethylsiloxane (PDMS), polystyrene (PS), and a poly(propylene glycol)-poly(ethylene glycol) block copolymer (PPG/PEG). At high surface coverage, PDMS brushes manifest friction coefficients (COFs) that are among the lowest recorded for a dry lubricant film (? ? 0.0024) and close to 1 order of magnitude lower than the COF of a bare silicon surface. Brushes synthesized from higher molar mass chains exhibit higher friction forces than those created using lower molar mass polymers. Increased grafting density of chains in the brush significantly reduces the COF by creating a uniform surface of stretched chains with a decreased surface viscosity. Brushes with lower surface tension and interfacial shear stresses manifest the lowest COF. In particular, PDMS chains exhibit COFs lower than PS by a factor of 3.7 and lower than PPG/PEG by a factor of 4.7. A scaling analysis conducted on the surface coverage (?) in relation to the fraction (?) of the friction force developing from adhesion predicts a universal relation ? ~ ?(4/3), which is supported by our experimental data. PMID:21696203

Landherr, Lucas J T; Cohen, Claude; Agarwal, Praveen; Archer, Lynden A

2011-08-01

191

Static Friction Prediction  

NSDL National Science Digital Library

In this activity (page 2 of PDF), learners will predict which objects have to overcome the most static friction to slide down a ramp. Learners then test their prediction as well as measure and calculate the ramp’s ratio of rise to run to quantify their results. Relates to the linked video, DragonflyTV GPS: Kinetic Sculpture Challenge.

2012-05-09

192

PEBBLES Simulation of Static Friction and New Static Friction Benchmark  

SciTech Connect

Pebble bed reactors contain large numbers of spherical fuel elements arranged randomly. Determining the motion and location of these fuel elements is required for calculating certain parameters of pebble bed reactor operation. This paper documents the PEBBLES static friction model. This model uses a three dimensional differential static friction approximation extended from the two dimensional Cundall and Strack model. The derivation of determining the rotational transformation of pebble to pebble static friction force is provided. A new implementation for a differential rotation method for pebble to container static friction force has been created. Previous published methods are insufficient for pebble bed reactor geometries. A new analytical static friction benchmark is documented that can be used to verify key static friction simulation parameters. This benchmark is based on determining the exact pebble to pebble and pebble to container static friction coefficients required to maintain a stable five sphere pyramid.

Joshua J. Cogliati; Abderrafi M. Ougouag

2010-05-01

193

A laboratory study of friction-velocity estimates from scatterometry - Low and high regimes  

NASA Technical Reports Server (NTRS)

Measurements from scatterometers pointing at wind-waves in three large wave tanks are examined to study fetch effects and the correlation with wind friction velocity. Time-series measurements were made at 13, 35, and 95 m with a Ka-band scatterometer aimed upwind at 30 deg incidence angle and vertical polarization. Average normalized radar cross-section (sigma-0) values from all fetches follow a common trend for sigma-0 as a function of wind friction velocity, so the fetch dependence is negligible. An empirical power-law model yields a high correlation between sigma-0 and wind friction velocity, but, because systematic anomalies arise, we reexamine a turbulence approach that delineates low and high regimes with a transition at a wind friction velocity of approximately 25 cm/s. Using this criteria, the data are well represented by a two-section power-law relationship between sigma-0 and wind friction velocity.

Bliven, L. F.; Giovanangeli, J.-P.; Wanninkhof, R. H.; Chapron, B.

1993-01-01

194

Internal rotor friction instability  

NASA Technical Reports Server (NTRS)

The analytical developments and experimental investigations performed in assessing the effect of internal friction on rotor systems dynamic performance are documented. Analytical component models for axial splines, Curvic splines, and interference fit joints commonly found in modern high speed turbomachinery were developed. Rotor systems operating above a bending critical speed were shown to exhibit unstable subsynchronous vibrations at the first natural frequency. The effect of speed, bearing stiffness, joint stiffness, external damping, torque, and coefficient of friction, was evaluated. Testing included material coefficient of friction evaluations, component joint quantity and form of damping determinations, and rotordynamic stability assessments. Under conditions similar to those in the SSME turbopumps, material interfaces experienced a coefficient of friction of approx. 0.2 for lubricated and 0.8 for unlubricated conditions. The damping observed in the component joints displayed nearly linear behavior with increasing amplitude. Thus, the measured damping, as a function of amplitude, is not represented by either linear or Coulomb friction damper models. Rotordynamic testing of an axial spline joint under 5000 in.-lb of static torque, demonstrated the presence of an extremely severe instability when the rotor was operated above its first flexible natural frequency. The presence of this instability was predicted by nonlinear rotordynamic time-transient analysis using the nonlinear component model developed under this program. Corresponding rotordynamic testing of a shaft with an interference fit joint demonstrated the presence of subsynchronous vibrations at the first natural frequency. While subsynchronous vibrations were observed, they were bounded and significantly lower in amplitude than the synchronous vibrations.

Walton, J.; Artiles, A.; Lund, J.; Dill, J.; Zorzi, E.

1990-01-01

195

In-Flight Capability for Evaluating Skin-Friction Gages and Other Near-Wall Flow Sensors  

NASA Technical Reports Server (NTRS)

An 8-in.-square boundary-layer sensor panel has been developed for in-flight evaluation of skin-friction gages and other near-wall flow sensors on the NASA Dryden Flight Research Center F-15B/Flight Test Fixture (FTF). Instrumentation on the sensor panel includes a boundary-layer rake, temperature sensors, static pressure taps, and a Preston tube. Space is also available for skin-friction gages or other near-wall flow sensors. Pretest analysis of previous F-15B/FTF flight data has identified flight conditions suitable for evaluating skin-friction gages. At subsonic Mach numbers, the boundary layer over the sensor panel closely approximates the two-dimensional (2D), law-of-the-wall turbulent boundary layer, and skin-friction estimates from the Preston tube and the rake (using the Clauser plot method) can be used to evaluate skin-friction gages. At supersonic Mach numbers, the boundary layer over the sensor panel becomes complex, and other means of measuring skin friction are needed to evaluate the accuracy of new skin-friction gages. Results from the flight test of a new rubber-damped skin-friction gage confirm that at subsonic Mach numbers, nearly 2D, law-of-the-wall turbulent boundary layers exist over the sensor panel. Sensor panel data also show that this new skin-friction gage prototype does not work in flight.

Bui, Trong T.; Pipitone, Brett J.; Krake, Keith L.; Richwine, Dave (Technical Monitor)

2003-01-01

196

Quantum friction and fluctuation theorems  

E-print Network

We use general concepts of statistical mechanics to compute the quantum frictional force on an atom moving at constant velocity above a planar surface. We derive the zero-temperature frictional force using a non-equilibrium fluctuation-dissipation relation, and show that in the large-time, steady-state regime quantum friction scales as the cubic power of the atom's velocity. We also discuss how approaches based on Wigner-Weisskopf and quantum regression approximations fail to predict the correct steady-state zero temperature frictional force, mainly due to the low frequency nature of quantum friction.

F. Intravaia; R. O. Behunin; D. A. R. Dalvit

2014-05-19

197

Journal of Energy, Heat and Mass Transfer, Vol. 16,01-14 (1994) The Ultimate Asymptotes and Possible Causes of Friction Drag and Heat  

E-print Network

and Possible Causes of Friction Drag and Heat Transfer Reduction Phenomena M. KOSTIC Department of Mechanical the fascinating friction drag and heat transfer reduction, associated with turbulent flows of so-called "drag to these phenomena, up to an extreme to evaluate the fluid elasticity on the basis ofthe corresponding drag reduction

Kostic, Milivoje M.

198

Friction and wear of plasma-deposited diamond films  

NASA Technical Reports Server (NTRS)

Reciprocating sliding friction experiments in humid air and in dry nitrogen and unidirectional sliding friction experiments in ultrahigh vacuum were conducted with a natural diamond pin in contact with microwave-plasma-deposited diamond films. Diamond films with a surface roughness (R rms) ranging from 15 to 160 nm were produced by microwave-plasma-assisted chemical vapor deposition. In humid air and in dry nitrogen, abrasion occurred when the diamond pin made grooves in the surfaces of diamond films, and thus the initial coefficients of friction increased with increasing initial surface roughness. The equilibrium coefficients of friction were independent of the initial surface roughness of the diamond films. In vacuum the friction for diamond films contacting a diamond pin arose primarily from adhesion between the sliding surfaces. In these cases, the initial and equilibrium coefficients of friction were independent of the initial surface roughness of the diamond films. The equilibrium coefficients of friction were 0.02 to 0.04 in humid air and in dry nitrogen, but 1.5 to 1.8 in vacuum. The wear factor of the diamond films depended on the initial surface roughness, regardless of environment; it increased with increasing initial surface roughness. The wear factors were considerably higher in vacuum than in humid air and in dry nitrogen.

Miyoshi, Kazuhisa; Wu, Richard L. C.; Garscadden, Alan; Barnes, Paul N.; Jackson, Howard E.

1993-01-01

199

Holographic Turbulence  

E-print Network

We construct turbulent black holes in asymptotically AdS[subscript 4] spacetime by numerically solving Einstein’s equations. Using the AdS/CFT correspondence we find that both the dual holographic fluid and bulk geometry ...

Adams, Allan

200

Wall Turbulence.  

ERIC Educational Resources Information Center

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)

Hanratty, Thomas J.

1980-01-01

201

Suboptimal control of turbulent channel flow for drag reduction  

Microsoft Academic Search

Two simple feedback control laws for drag reduction are derived by applying a suboptimal control theory to a turbulent channel flow. These new feedback control laws require pressure or shear-stress information only at the wall, and when applied to a turbulent channel flow at Re[tau]=110, they result in 16 22% reduction in the skin-friction drag. More practical control laws requiring

Changhoon Lee; John Kim; Haecheon Choi

1998-01-01

202

GEOPOLITICAL FACTORS AND INCREASINGLY TURBULENT SUPPLY AND DEMAND SCENARIOS IN ENERGY MARKETS: MODELING REJUVENATED INTEREST IN BIOMASS ENERGY SOURCES  

Microsoft Academic Search

As the peak of global oil production approaches, increasing competition resulting from increasing demand in emerging economies challenges traditional energy market relationships. Recent events underscore additional disruptions and uncertainty in energy markets, resulting from random fluctuations introduced by wars and natural disasters. This paper proposes a modeling approach to understanding and predicting the impacts of these combined factors in the

Ronald F. Farina

203

Cluster Turbulence  

NASA Astrophysics Data System (ADS)

We report on results of recent, high resolution hydrodynamic simulations of the formation and evolution of X-ray clusters of galaxies carried out within a cosmological framework. We employ the highly accurate piecewise parabolic method (PPM) on fixed and adaptive meshes which allow us to resolve the flow field in the intracluster gas. The excellent shock capturing and low numerical viscosity of PPM represent a substantial advance over previous studies using SPH. We find that in flat, hierarchical cosmological models, the ICM is in a turbulent state long after turbulence generated by the last major merger should have decayed away. Turbulent velocites are found to vary slowly with cluster radius, being $\\sim 25%$ of $\\sigma_{vir}$ in the core, increasing to $\\sim 60%$ at the virial radius. We argue that more frequent minor mergers maintain the high level of turbulence found in the core where dynamical times are short. Turbulent pressure support is thus significant throughout the cluster, and results in a somewhat cooler cluster ($T/T_{vir} \\sim .8$) for its mass. Some implications of cluster turbulence are discussed.

Norman, Michael L.; Bryan, Greg L.

204

On the friction coefficient of straight-chain aggregates  

E-print Network

A methodology to calculate the friction coefficient of an aggregate in the continuum regime is proposed. The friction coefficient and the monomer shielding factors, aggregate-average or individual, are related to the molecule-aggregate collision rate that is obtained from the molecular diffusion equation with an absorbing boundary condition on the aggregate surface. Calculated friction coefficients of straight chains are in very good agreement with previous results, suggesting that the friction coefficients may be accurately calculated from the product of the collision rate and an average momentum transfer,the latter being independent of aggregate morphology. Langevin-dynamics simulations show that the diffusive motion of straight-chain aggregates may be described either by a monomer-dependent or an aggregate-average random force, if the shielding factors are appropriately chosen.

Lorenzo Isella; Yannis Drossinos

2011-01-31

205

Friction in rail guns  

NASA Technical Reports Server (NTRS)

The influence of friction is included in the present equations describing the performance of an inductively driven rail gun. These equations, which have their basis in an empirical formulation, are applied to results from two different experiments. Only an approximate physical description of the problem is attempted, in view of the complexity of details in the interaction among forces of this magnitude over time periods of the order of milisecs.

Kay, P. K.

1984-01-01

206

Turbulent heat transfer in a trapezoidal channel with transverse and v-shaped ribs on two opposite walls  

E-print Network

This study investigates the turbulent heat transfer and friction in a trapezoidal channel with opposite walls roughened with transverse and v-shaped ribs. The roughened channel depicts the internal cooling passage of an aerofoil near the trailing...

Subramanian, Karthik

2006-04-12

207

Reduction of friction stress of ethylene glycol by attached hydrogen ions.  

PubMed

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

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

2014-01-01

208

Reduction of friction stress of ethylene glycol by attached hydrogen ions  

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

209

Estimation of the Friction Velocity in Stably Stratified Boundary-Layer Flows Over Hills  

Microsoft Academic Search

A method is suggested for the calculation of the friction velocity for stable turbulent boundary-layer flow over hills. The method is tested using a continuous upstream mean velocity profile compatible with the propagation of gravity waves, and is incorporated into the linear model of Hunt, Leibovich and Richards with the modification proposed by Hunt, Richards and Brighton to include the

José Luis Argaín; Pedro M. A. Miranda; Miguel A. C. Teixeira

2008-01-01

210

Friction drag reduction through damping of the near-wall spanwise velocity fluctuation  

Microsoft Academic Search

Direct numerical simulation (DNS) of a turbulent channel flow is performed in order to explore the possibility of reducing wall friction drag and saving energy through damping of the spanwise velocity fluctuation. Starting from an idealized feedback control, we extend the idea to a practically realizable control, in which only the streamwise wall shear stress is required as sensor input,

Bettina Frohnapfel; Yosuke Hasegawa; Nobuhide Kasagi

2010-01-01

211

FRICTION DRAG REDUCTION THROUGH DAMPING OF THE NEAR-WALL SPANWISE VELOCITY FLUCTUATION  

Microsoft Academic Search

Direct numerical simulation (DNS) of a turbulent channel flow is performed in order to explore the possibility of reducing wall friction drag and saving energy through damping of the spanwise velocity fluctuation. Starting from an idealized feedback control, we extend the idea to a practically realizable control in which wall information is used as sensor input and actuators of finite

Bettina Frohnapfel; Yosuke Hasegawa; Nobuhide Kasagi

2009-01-01

212

Turbulent drag reduction through oscillating discs  

NASA Astrophysics Data System (ADS)

The changes of a turbulent channel flow subjected to oscillations of wall flush-mounted rigid discs are studied by means of direct numerical simulations. The Reynolds number is $R_\\tau$=$180$, based on the friction velocity of the stationary-wall case and the half channel height. The primary effect of the wall forcing is the sustained reduction of wall-shear stress, which reaches a maximum of 20%. A parametric study on the disc diameter, maximum tip velocity, and oscillation period is presented, with the aim to identify the optimal parameters which guarantee maximum drag reduction and maximum net energy saving, computed by taking into account the power spent to actuate the discs. This may be positive and reaches 6%. The Rosenblat viscous pump flow is used to predict the power spent for disc motion in the turbulent channel flow and to estimate localized and transient regions over the disc surface subjected to the turbulent regenerative braking effect, for which the wall turbulence exerts work on the discs. The FIK identity is employed to show that the wall-friction reduction is due to two distinguished effects. One effect is linked to the direct shearing action of the near-wall oscillating disc boundary layer on the wall turbulence, which causes the attenuation of the turbulent Reynolds stresses. The other effect is due the additional disc-flow Reynolds stresses produced by the inter-disc structures. The contribution to drag reduction due to turbulent Reynolds stress attenuation depends on the penetration thickness of the disc-flow boundary layer, while the contribution due to the elongated structures scales linearly with a simple function of the maximum tip velocity and oscillation period for the largest disc diameter tested, a result suggested by the Rosenblat flow solution. A brief discussion on the future applicability of the oscillating-disc technique is also presented.

Wise, Daniel J.; Ricco, Pierre

2014-05-01

213

Friction surfaced Stellite6 coatings  

SciTech Connect

Solid state Stellite6 coatings were deposited on steel substrate by friction surfacing and compared with Stellite6 cast rod and coatings deposited by gas tungsten arc and plasma transferred arc welding processes. Friction surfaced coatings exhibited finer and uniformly distributed carbides and were characterized by the absence of solidification structure and compositional homogeneity compared to cast rod, gas tungsten arc and plasma transferred coatings. Friction surfaced coating showed relatively higher hardness. X-ray diffraction of samples showed only face centered cubic Co peaks while cold worked coating showed hexagonally close packed Co also. - Highlights: Black-Right-Pointing-Pointer Stellite6 used as coating material for friction surfacing. Black-Right-Pointing-Pointer Friction surfaced (FS) coatings compared with casting, GTA and PTA processes. Black-Right-Pointing-Pointer Finer and uniformly distributed carbides in friction surfaced coatings. Black-Right-Pointing-Pointer Absence of melting results compositional homogeneity in FS Stellite6 coatings.

Rao, K. Prasad; Damodaram, R. [Department of Metallurgical and Materials Engineering - Indian Institute of Technology Madras, Chennai 600 036 (India); Rafi, H. Khalid, E-mail: khalidrafi@gmail.com [Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600 036 (India); Ram, G.D. Janaki [Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600 036 (India); Reddy, G. Madhusudhan [Metal Joining Group, Defence Metallurgical Research Laboratory (DMRL) Kanchanbagh, Hyderabad 500 058 (India); Nagalakshmi, R. [Welding Research Institute, Bharat Heavy Electricals Limited, Tiruchirappalli 620 014 (India)

2012-08-15

214

Soliton turbulence  

NASA Technical Reports Server (NTRS)

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.

Tchen, C. M.

1986-01-01

215

Internal friction in enzyme reactions.  

PubMed

The empirical concept of internal friction was introduced 20 years ago. This review summarizes the results of experimental and theoretical studies that help to uncover the nature of internal friction. After the history of the concept, we describe the experimental challenges in measuring and interpreting internal friction based on the viscosity dependence of enzyme reactions. We also present speculations about the structural background of this viscosity dependence. Finally, some models about the relationship between the energy landscape and internal friction are outlined. Alternative concepts regarding the viscosity dependence of enzyme reactions are also discussed. PMID:23281036

Rauscher, Anna; Derényi, Imre; Gráf, László; Málnási-Csizmadia, András

2013-01-01

216

Experimental assessment of turbulent drag reduction by wall traveling waves  

NASA Astrophysics Data System (ADS)

Waves of spanwise velocity applied at the wall of a turbulent channel flow and traveling in the streamwise direction have been recently discovered in a numerical study by Quadrio et al. [1] to alter the natural turbulent friction significantly. In particular, depending on the parameters that define the waves, drag can be either increased or decreased, and at low Re full relaminarization has been achieved. Large drag reductions are obtained at the expense of very limited expenditure in energy, thus resulting in a largely positive overall energy budget. This paper presents the first experimental assessment of the performance of the streamwise-traveling waves in a turbulent pipe flow.

Quadrio, M.; Auteri, F.; Baron, A.; Belan, M.; Bertolucci, A.

217

Holographic turbulence.  

PubMed

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

Adams, Allan; Chesler, Paul M; Liu, Hong

2014-04-18

218

Practical calculator programs. Part 9. Calculator program determines turbulence criteria  

SciTech Connect

A program developed for the TI-59 hand-held calculator determines whether the flow pattern for a given mud system is laminar or turbulent, calculating the Reynolds number for each pattern. Applicable to Newtonian, Bingham plastic, and power-law fluids, the program contributes to determining the frictional pressure loss in the pipe and annulus.

Chenevert, M.E.; Hollo, R.

1981-11-16

219

Practical calculator programs. Part 9. Calculator program determines turbulence criteria  

Microsoft Academic Search

A program developed for the TI-59 hand-held calculator determines whether the flow pattern for a given mud system is laminar or turbulent, calculating the Reynolds number for each pattern. Applicable to Newtonian, Bingham plastic, and power-law fluids, the program contributes to determining the frictional pressure loss in the pipe and annulus.

M. E. Chenevert; R. Hollo

1981-01-01

220

Application of neural networks to turbulence control for drag reduction  

Microsoft Academic Search

A new adaptive controller based on a neural network was constructed and applied to turbulent channel flow for drag reduction. A simple control network, which employs blowing and suction at the wall based only on the wall-shear stresses in the spanwise direction, was shown to reduce the skin friction by as much as 20% in direct numerical simulations of a

Changhoon Lee; John Kim; David Babcock; Rodney Goodman

1997-01-01

221

Application of generalized wall function for complex turbulent flows  

Microsoft Academic Search

This paper describes a generalized wall function for three-dimensional turbulent boundary layer flows. Since the formulation is valid for various pressure gradients including those associated with zero skin friction, it can be applied to wall bounded complex flows with acceleration, deceleration and recirculation. This generalized wall function is extended to the whole surface layer (or inner layer), covering the viscous

Tsan-Hsing Shih; Louis A. Povinelli; Nan-Suey Liu

2003-01-01

222

Improved Skin Friction Interferometer  

NASA Technical Reports Server (NTRS)

An improved system for measuring aerodynamic skin friction which uses a dual-laser-beam oil-film interferometer was developed. Improvements in the optical hardware provided equal signal characteristics for each beam and reduced the cost and complexity of the system by replacing polarization rotation by a mirrored prism for separation of the two signals. An automated, objective, data-reduction procedure was implemented to eliminate tedious manual manipulation of the interferometry data records. The present system was intended for use in two-dimensional, incompressible flows over a smooth, level surface without pressure gradient, but the improvements discussed are not limited to this application.

Westphal, R. V.; Bachalo, W. D.; Houser, M. H.

1986-01-01

223

Analysis Method of Friction Torque and Weld Interface Temperature during Friction Process of Steel Friction Welding  

NASA Astrophysics Data System (ADS)

This paper describes an analysis method of the friction torque and weld interface temperature during the friction process for steel friction welding. The joining mechanism model of the friction welding for the wear and seizure stages was constructed from the actual joining phenomena that were obtained by the experiment. The non-steady two-dimensional heat transfer analysis for the friction process was carried out by calculation with FEM code ANSYS. The contact pressure, heat generation quantity, and friction torque during the wear stage were calculated using the coefficient of friction, which was considered as the constant value. The thermal stress was included in the contact pressure. On the other hand, those values during the seizure stage were calculated by introducing the coefficient of seizure, which depended on the seizure temperature. The relationship between the seizure temperature and the relative speed at the weld interface in the seizure stage was determined using the experimental results. In addition, the contact pressure and heat generation quantity, which depended on the relative speed of the weld interface, were solved by taking the friction pressure, the relative speed and the yield strength of the base material into the computational conditions. The calculated friction torque and weld interface temperatures of a low carbon steel joint were equal to the experimental results when friction pressures were 30 and 90 MPa, friction speed was 27.5 s-1, and weld interface diameter was 12 mm. The calculation results of the initial peak torque and the elapsed time for initial peak torque were also equal to the experimental results under the same conditions. Furthermore, the calculation results of the initial peak torque and the elapsed time for initial peak torque at various friction pressures were equal to the experimental results.

Kimura, Masaaki; Inoue, Haruo; Kusaka, Masahiro; Kaizu, Koichi; Fuji, Akiyoshi

224

Numerical Estimation of Frictional Torques with Rate and State Friction  

E-print Network

In this paper, numerical estimation of frictional torques is carried out of a rotary elastic disc on a hard and rough surface under different rotating conditions. A one dimensional spring- mass rotary system is numerically solved under the quasistatic condition with the rate and state dependent friction model. It is established that torque of frictional strength as well as torque of steady dynamic stress increases with radius and found to be maximum at the periphery of the disc. Torque corresponding to frictional strength estimated using the analytical solution matches closely with the simulation only in the case of high stiffness of the connecting spring. In steady relaxation simulation, a steadily rotating disc is suddenly stopped and relaxational angular velocity and corresponding frictional torque decreases with both steady angular velocity and stiffness of the connecting spring in the velocity strengthening regime. In velocity weakening regime, in contrast, torque of relaxation stress deceases but relaxation velocity increases. The reason for the contradiction is explained.

Arun K. Singh; T. N. Singh

2015-01-20

225

Observations of the space-time structure of flow, turbulence, and stress over orbital-scale ripples  

NASA Astrophysics Data System (ADS)

spatial and temporal structure of flow, turbulence, and stress over equilibrium orbital-scale sand ripples are investigated at turbulence-resolving scales with a wide-band coherent Doppler profiler (MFDop) and an oscillating tray apparatus. The oscillation period and horizontal excursion were 10 s and 0.5 m. A single trial was also executed at 0.6 m excursion. Ripple wavelength and amplitude were 25 and 2.2 cm. Ensemble-averaged velocity profiles were acquired with 3 mm vertical resolution at 42 Hz. The spatial pattern of flow as a function of oscillation phase was determined by combining the phase-averaged velocity measurements from trials with the MFDop at different positions relative to a particular ripple crest. The MFDop measurements are used to investigate the coevolution of the lee vortex, turbulent kinetic energy, Reynolds stress, and turbulence production as a function of phase. Shear stress is determined from the vertically integrated vorticity equation and the double-averaged momentum equations. Friction factors obtained from the two methods are comparable and range from 0.1 to 0.2.

Hare, Jenna; Hay, Alex E.; Zedel, Len; Cheel, Richard

2014-03-01

226

Turbulent combustion  

SciTech Connect

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.

Talbot, L.; Cheng, R.K. [Lawrence Berkeley Laboratory, CA (United States)

1993-12-01

227

Friction Stir Welding  

NASA Technical Reports Server (NTRS)

Friction stir welding (FSW) is a solid state welding process invented in 1991 at The Welding Institute in the United Kingdom. A weld is made in the FSW process by translating a rotating pin along a weld seam so as to stir the sides of the seam together. FSW avoids deleterious effects inherent in melting and promises to be an important welding process for any industries where welds of optimal quality are demanded. This article provides an introduction to the FSW process. The chief concern is the physical effect of the tool on the weld metal: how weld seam bonding takes place, what kind of weld structure is generated, potential problems, possible defects for example, and implications for process parameters and tool design. Weld properties are determined by structure, and the structure of friction stir welds is determined by the weld metal flow field in the vicinity of the weld tool. Metal flow in the vicinity of the weld tool is explained through a simple kinematic flow model that decomposes the flow field into three basic component flows: a uniform translation, a rotating solid cylinder, and a ring vortex encircling the tool. The flow components, superposed to construct the flow model, can be related to particular aspects of weld process parameters and tool design; they provide a bridge to an understanding of a complex-at-first-glance weld structure. Torques and forces are also discussed. Some simple mathematical models of structural aspects, torques, and forces are included.

Nunes, Arthur C., Jr.

2008-01-01

228

Elastic model of dry friction  

SciTech Connect

Friction of elastic bodies is connected with the passing through the metastable states that arise at the contact of surfaces rubbing against each other. Three models are considered that give rise to the metastable states. Friction forces and their dependence on the pressure are calculated. In Appendix A, the contact problem of elasticity theory is solved with adhesion taken into account.

Larkin, A. I.; Khmelnitskii, D. E., E-mail: dekl2@cam.ac.uk [Landau Institute for Theoretical Physics (Russian Federation)

2013-09-15

229

Orbital Friction Stir Weld System  

NASA Technical Reports Server (NTRS)

This invention is an apparatus for joining the ends of two cylindrical (i.e., pipe-shaped) sections together with a friction stir weld. The apparatus holds the two cylindrical sections together and provides back-side weld support as it makes a friction stir weld around the circumference of the joined ends.

Ding, R. Jeffrey (Inventor); Carter, Robert W. (Inventor)

2001-01-01

230

Orbital friction stir weld system  

NASA Technical Reports Server (NTRS)

This invention is an apparatus for joining the ends of two cylindrical (i.e., pipe-shaped) sections together with a friction stir weld. The apparatus holds the two cylindrical sections together and provides back-side weld support as it makes a friction stir weld around the circumference of the joined ends.

Ding, R. Jeffrey (Inventor); Carter, Robert W. (Inventor)

2001-01-01

231

Friction in a Moving Car  

ERIC Educational Resources Information Center

Describes an out-of-doors, partially unstructured experiment to determine the coefficient of friction for a moving car. Presents the equation which relates the coefficient of friction to initial velocity, distance, and time and gives sample computed values as a function of initial speed and tire pressure. (GS)

Goldberg, Fred M.

1975-01-01

232

Effect of coating material on heat transfer and skin friction due to impinging jet onto a laser producedhole  

NASA Astrophysics Data System (ADS)

Jet impingement onto a two-layer structured hole in relation to laser drilling is investigated. The hole consists of a coating layer and a base material. The variations in the Nusselt number and the skin friction are predicted for various coating materials. The Reynolds stress turbulent model is incorporated to account for the turbulence effect of the jet flow and nitrogen is used as the working fluid. The study is extended to include two jet velocities emanating from the conical nozzle. It is found that coating material has significant effect on the Nusselt number variation along the hole wall. In addition, the skin friction varies considerably along the coating thickness in thehole.

Shuja, S. Z.; Yilbas, B. S.

2013-07-01

233

Eliminating Friction with Friction: 2D Janssen Effect in a Friction-Driven System  

NASA Astrophysics Data System (ADS)

The Janssen effect is a unique property of confined granular materials experiencing gravitational compaction in which the pressure at the bottom saturates with an increasing filling height due to frictional interactions with side walls. In this Letter, we replace gravitational compaction with frictional compaction. We study friction-compacted 2D granular materials confined within fixed boundaries on a horizontal conveyor belt. We find that even with high-friction side walls the Janssen effect completely vanishes. Our results demonstrate that gravity-compacted granular systems are inherently different from friction-compacted systems in at least one important way: vibrations induced by sliding friction with the driving surface relax away tangential forces on the walls. Remarkably, we find that the Janssen effect can be recovered by replacing the straight side walls with a sawtooth pattern. The mechanical force introduced by varying the sawtooth angle ? can be viewed as equivalent to a tunable friction force. By construction, this mechanical friction force cannot be relaxed away by vibrations in the system.

Karim, M. Yasinul; Corwin, Eric I.

2014-05-01

234

Towards Petascale DNS of High Reynolds-Number Turbulent Boundary Layer  

NASA Astrophysics Data System (ADS)

In flight vehicles, a large portion of fuel consumption is due to skin-friction drag. Reduction of this drag will significantly reduce the fuel consumption of flight vehicles and help our nation to reduce CO 2 emissions. In order to reduce skin-friction drag, an increased understanding of wall-turbulence is needed. Direct numerical simulation (DNS) of spatially developing turbulent boundary layers (SDTBL) can provide the fundamental understanding of wall-turbulence in order to produce models for Reynolds averaged Navier-Stokes (RANS) and large-eddy simulations (LES). DNS of SDTBL over a flat plate at Retheta = 1430 - 2900 were performed. Improvements were made to the DNS code allowing for higher Reynolds number simulations towards petascale DNS of turbulent boundary layers. Mesh refinement and improvements to the inflow and outflow boundary conditions have resulted in turbulence statistics that match more closely to experimental results. The Reynolds stresses and the terms of their evolution equations are reported.

Webster, Keegan R.

235

DNS of turbulent heat transfer in channel flow with low to medium-high Prandtl number fluid  

Microsoft Academic Search

The direct numerical simulation (DNS) of the turbulent heat transfer for various Prandtl numbers ranging from 0.025 to 5 are performed to obtain statistical quantities such as turbulent heat flux, temperature variance and their budget terms. The configuration is the fully developed channel flow with uniform heating from both walls. The Reynolds number based on the friction velocity and the

Hiroshi Kawamura; Kouichi Ohsaka; Hiroyuki Abe; Kiyoshi Yamamoto

1998-01-01

236

Exploring a flight deck based wake turbulence situational awareness tool  

Microsoft Academic Search

As NextGen concepts move toward increasing en route and terminal throughput, wake turbulence separation may become a limiting factor in the pursuit of capacity improvements. Better knowledge of the probable location of wakes (for air traffic controllers as well as pilots) could help provide safe separation from wake turbulence while avoiding unnecessary restrictions to operations. The Wake Turbulence Avoidance Automation

Clark Lunsford; Marshall Koch; H. Peter Stassen; Steven Estes; Brendan Hogan

2012-01-01

237

Compressible Turbulence  

NASA Astrophysics Data System (ADS)

We present a model to treat fully compressible, nonlocal, time-dependent turbulent convection in the presence of large-scale flows and arbitrary density stratification. The problem is of interest, for example, in stellar pulsation problems, especially since accurate helioseismological data are now available, as well as in accretion disks. Owing to the difficulties in formulating an analytical model, it is not surprising that most of the work has gone into numerical simulations. At present, there are three analytical models: one by the author, which leads to a rather complicated set of equations; one by Yoshizawa; and one by Xiong. The latter two use a Reynolds stress model together with phenomenological relations with adjustable parameters whose determination on the basis of terrestrial flows does not guarantee that they may be extrapolated to astrophysical flows. Moreover, all third-order moments representing nonlocality are taken to be of the down gradient form (which in the case of the planetary boundary layer yields incorrect results). In addition, correlations among pressure, temperature, and velocities are often neglected or treated as in the incompressible case. To avoid phenomenological relations, we derive the full set of dynamic, time-dependent, nonlocal equations to describe all mean variables, second- and third-order moments. Closures are carried out at the fourth order following standard procedures in turbulence modeling. The equations are collected in an Appendix. Some of the novelties of the treatment are (1) new flux conservation law that includes the large-scale flow, (2) increase of the rate of dissipation of turbulent kinetic energy owing to compressibility and thus (3) a smaller overshooting, and (4) a new source of mean temperature due to compressibility; moreover, contrary to some phenomenological suggestions, the adiabatic temperature gradient depends only on the thermal pressure, while in the equation for the large-scale flow, the physical pressure is the sum of thermal plus turbulent pressure.

Canuto, V. M.

1997-06-01

238

Frictional slip of granite at hydrothermal conditions  

USGS Publications Warehouse

To measure the strength, sliding behavior, and friction constitutive properties of faults at hydrothermal conditions, laboratory granite faults containing a layer of granite powder (simulated gouge) were slid. The mechanical results define two regimes. The first regime includes dry granite up to at least 845?? and wet granite below 250??C. In this regime the coefficient of friction is high (?? = 0.7 to 0.8) and depends only modestly on temperature, slip rate, and PH2O. The second regime includes wet granite above ~350??C. In this regime friction decreases considerably with increasing temperature (temperature weakening) and with decreasing slip rate (velocity strengthening). These regimes correspond well to those identified in sliding tests on ultrafine quartz. The results highlight the importance of fluid-assisted deformation processes active in faults at depth and the need for laboratory studies on the roles of additional factors such as fluid chemistry, large displacements, higher concentrations of phyllosilicates, and time-dependent fault healing. -from Authors

Blanpied, M.L.; Lockner, D.A.; Byerlee, J.D.

1995-01-01

239

Reduction of turbulence in wind tunnels  

NASA Technical Reports Server (NTRS)

A brief nonmathematical outline is given of modern views as to the nature of the effect of turbulence, and their bearing on the desirability of designing wind tunnels for small or large turbulence. Experiments made on a particular wind tunnel for the purpose of reducing the turbulence are described, to illustrate the influence of certain factors on the magnitude of the turbulence. Moderate changes in the size, shape, and wall thickness of cells of the honeycomb were found to have little effect. The addition of a room honeycomb at the entrance was also of little value in reducing the turbulence. The turbulence decreased with increasing distance between the honeycomb and the measuring station. A further decrease was obtained by using a large area reduction in the entrance cone, with the honeycomb at the extreme entrance end. (author)

Dryden, Hugh L

1931-01-01

240

Improved detection of atmospheric turbulence with SLODAR  

E-print Network

We discuss several improvements in the detection of atmospheric turbulence using SLOpe Detection And Ranging (SLODAR). Frequently, SLODAR observations have shown strong ground-layer turbulence, which is beneficial to adaptive optics. We show that current methods which neglect atmospheric propagation effects can underestimate the strength of high altitude turbulence by up to ~ 30%. We show that mirror and dome seeing turbulence can be a significant fraction of measured ground-layer turbulence, some cases up to ~ 50%. We also demonstrate a novel technique to improve the nominal height resolution, by a factor of 3, called Generalized SLODAR. This can be applied when sampling high-altitude turbulence, where the nominal height resolution is the poorest, or for resolving details in the important ground-layer.

Goodwin, Michael; Lambert, Andrew

2007-01-01

241

Improved detection of atmospheric turbulence with SLODAR  

E-print Network

We discuss several improvements in the detection of atmospheric turbulence using SLOpe Detection And Ranging (SLODAR). Frequently, SLODAR observations have shown strong ground-layer turbulence, which is beneficial to adaptive optics. We show that current methods which neglect atmospheric propagation effects can underestimate the strength of high altitude turbulence by up to ~ 30%. We show that mirror and dome seeing turbulence can be a significant fraction of measured ground-layer turbulence, some cases up to ~ 50%. We also demonstrate a novel technique to improve the nominal height resolution, by a factor of 3, called Generalized SLODAR. This can be applied when sampling high-altitude turbulence, where the nominal height resolution is the poorest, or for resolving details in the important ground-layer.

Michael Goodwin; Charles Jenkins; Andrew Lambert

2007-06-19

242

Occurrence of turbulent flow conditions in supercritical fluid chromatography.  

PubMed

Having similar densities as liquids but with viscosities up to 20 times lower (higher diffusion coefficients), supercritical CO2 is the ideal (co-)solvent for fast and/or highly efficient separations without mass-transfer limitations or excessive column pressure drops. Whereas in liquid chromatography the flow remains laminar in both the packed bed and tubing, except in extreme cases (e.g. in a 75 ?m tubing, pure acetonitrile at 5 ml/min), a supercritical fluid can experience a transition from laminar to turbulent flow in more typical operation modes. Due to the significant lower viscosity, this transition for example already occurs at 1.3 ml/min for neat CO2 when using connection tubing with an ID of 127 ?m. By calculating the Darcy friction factor, which can be plotted versus the Reynolds number in a so-called Moody chart, typically used in fluid dynamics, higher values are found for stainless steel than PEEK tubing, in agreement with their expected higher surface roughness. As a result turbulent effects are more pronounced when using stainless steel tubing. The higher than expected extra-column pressure drop limits the kinetic performance of supercritical fluid chromatography and complicates the optimization of tubing ID, which is based on a trade-off between extra-column band broadening and pressure drop. One of the most important practical consequences is the non-linear increase in extra-column pressure drop over the tubing downstream of the column which leads to an unexpected increase in average column pressure and mobile phase density, and thus decrease in retention. For close eluting components with a significantly different dependence of retention on density, the selectivity can significantly be affected by this increase in average pressure. In addition, the occurrence of turbulent flow is also observed in the detector cell and connection tubing. This results in a noise-increase by a factor of four when going from laminar to turbulent flow (e.g. going from 0.5 to 2.5 ml/min for neat CO2). PMID:25145564

De Pauw, Ruben; Choikhet, Konstantin; Desmet, Gert; Broeckhoven, Ken

2014-09-26

243

Rubber friction: comparison of theory with experiment.  

PubMed

We have measured the friction force acting on a rubber block slid on a concrete surface. We used both unfilled and filled (with carbon black) styrene butadiene (SB) rubber and have varied the temperature from -10 °C to 100 °C and the sliding velocity from 1 ?m/s to 1000 ?m/s. We find that the experimental data at different temperatures can be shifted into a smooth master-curve, using the temperature-frequency shifting factors obtained from measurements of the bulk viscoelastic modulus. The experimental data has been analyzed using a theory which takes into account the contributions to the friction from both the substrate asperity-induced viscoelastic deformations of the rubber, and from shearing the area of real contact. For filled SB rubber the frictional shear stress ?(f) in the area of real contact results mainly from the energy dissipation at the opening crack on the exit side of the rubber-asperity contact regions. For unfilled rubber we instead attribute ?(f) to shearing of a thin rubber smear film, which is deposited on the concrete surface during run in. We observe very different rubber wear processes for filled and unfilled SB rubber, which is consistent with the different frictional processes. Thus, the wear of filled SB rubber results in micrometer-sized rubber particles which accumulate as dry dust, which is easily removed by blowing air on the concrete surface. This wear process seams to occur at a steady rate. For unfilled rubber a smear film forms on the concrete surface, which cannot be removed even using a high-pressure air stream. In this case the wear rate appears to slow down after some run in time period. PMID:22139094

Lorenz, B; Persson, B N J; Dieluweit, S; Tada, T

2011-12-01

244

Flexure Bearing Reduces Startup Friction  

NASA Technical Reports Server (NTRS)

Design concept for ball bearing incorporates small pieces of shim stock, wire spokes like those in bicycle wheels, or other flexing elements to reduce both stiction and friction slope. In flexure bearing, flexing elements placed between outer race of ball bearing and outer ring. Elements flex when ball bearings encounter small frictional-torque "bumps" or even larger ones when bearing balls encounter buildups of grease on inner or outer race. Flexure of elements reduce high friction slopes of "bumps", helping to keep torque between outer ring and inner race low and more nearly constant. Concept intended for bearings in gimbals on laser and/or antenna mirrors.

Clingman, W. Dean

1991-01-01

245

REDUCED ENGINE FRICTION AND WEAR  

SciTech Connect

This Final Technical Report discusses the progress was made on the experimental and numerical tasks over the duration of this project regarding a new technique for decreasing engine friction and wear via liner rotation. The experimental subtasks involved quantifying the reduction in engine friction for a prototype rotating liner engine relative to a comparable baseline engine. Both engine were single cylinder conversions of nominally identical production four-cylinder engines. Hot motoring tests were conducted initially and revealed that liner rotation decreased engine friction by 20% under motoring conditions. A well-established model was used to estimate that liner rotation should decrease the friction of a four-cylinder engine by 40% under hot motoring conditions. Hot motoring tear-down tests revealed that the crankshaft and valve train frictional losses were essentially the same for the two engines, as expected. However, the rotating liner engine had much lower (>70%) piston assembly friction compared to the conventional engine. Finally, we used the Instantaneous IMEP method to compare the crank-angle resolved piston assembly friction for the two engines. Under hot motoring conditions, these measurements revealed a significant reduction in piston assembly friction, especially in the vicinity of compression TDC when the lubrication regime transitions from hydrodynamic through mixed and into boundary friction. We have some remaining problems with these measurements that we expect to solve during the next few weeks. We will then perform these measurements under firing conditions. We also proposed to improve the state-of-the-art of numerical modeling of piston assembly friction for conventional engines and then to extend this model to rotating liner engines. Our research team first modeled a single ring in the Purdue ring-liner test rig. Our model showed good agreement with the test rig data for a range of speeds and loads. We then modeled a complete piston assembly in an engine. The model appears to produce the correct behavior, but we cannot quantify its strengths or weaknesses until our crank-angle-resolved measurements have been completed. Finally, we proposed and implemented a model for the effects of liner rotation on piston assembly friction. Here, we propose that the rotating liner design is analogous to the shaft-bushing mechanism. Therefore, we used the side-slip rolling friction model to simulate the effects of liner rotation. This model appears to be promising, but final analysis of its strengths and/or weaknesses must await our crank-angle-resolved measurements.

Ron Matthews

2005-05-01

246

Illinois PER Interactive Examples: Friction  

NSDL National Science Digital Library

This is an interactive homework problem for introductory physics students relating to kinetic friction on a moving block of known mass. Given the coefficient of kinetic friction for the situation, the student must determine the force of kinetic friction acting on the mass. A user-activated "help" sequence is provided for each step of the problem-solving, from conceptual analysis through quantitative calculation. To promote critical thinking, immediate feedback is received for both correct and incorrect responses. This item is part of a larger collection of interactive homework problems for introductory physics.

Gladding, Gary

2008-09-10

247

Friction characteristics of graphite and graphite-metal combinations at various temperatures  

NASA Technical Reports Server (NTRS)

Characteristics of the coefficient of friction of graphite and of graphitic combinations between 70 and 4000 degrees F are given. Graphite's good frictional quality is attributed to a gas film on surface platelets. Major factors investigated are, surface finish and ''wear in'', surface conditioning, temperature, and interfacial pressure.

Manjoine, M. J.

1970-01-01

248

Turning behaviour depends on frictional damping in the fruit fly Drosophila  

Microsoft Academic Search

Turning behaviour in the fruit fly Drosophila depends on several factors including not only feedback from sensory organs and muscular control of wing motion, but also the mass moments of inertia and the frictional damping coefficient of the rotating body. In the present study we evaluate the significance of body friction for yaw turning and thus the limits of visually

Thomas Hesselberg; Fritz-Olaf Lehmann

2007-01-01

249

Frictional Properties of Fault Gouges Recovered from NanTroSEIZE Expedition 316 Drilling  

Microsoft Academic Search

Along subduction interfaces, the updip limit of the seismogenic zone is an important issue, notably regarding tsunami generation. One of the factors controlling this transition is the frictional properties of rock assemblages formed along active faults crossing accretionary prisms. We have thus begun a broad experimental survey of the frictional properties of fault materials retrieved from splay faults cored during

D. L. Goldsby; O. Fabbri

2008-01-01

250

Nanotribology fundamentals: Predicting the viscous coefficient of friction  

NASA Astrophysics Data System (ADS)

In this work, I have used the Quartz Crystal Microbalance (QCM) to study nanoscale friction of monolayer adsorbates on (111) metals. The friction of these systems is viscous friction, defined as Ff = etanu = ( mt )nu. Here, eta is the viscous coefficient of friction, nu is the velocity of the adsorbate, m is adsorbate mass, and tau is the slip time, which is the time required for the film's speed to fall to 1/e of its original value. The main focus of this dissertation is to determine the factors that control eta, the viscous coefficient of friction. I have examined three different parameters in order to determine their effect on eta. An equation for predicting the viscous coefficient of friction has been proposed: eta = etasubs + aU2o . Here, etasubs is the damping of adsorbate sliding energy within the substrate, a is a constant depending on mainly temperature and adsorbate film coverage, and Uo is the atomic-scale surface corrugation. I have examined the sliding friction of n-octane on Cu(111) vs. Pb(11I) surfaces, which have gamma = 0.45 meV and gamma = 0.26 meV, respectively. I have observed that the slip time for a monolayer of n-octane/Cu(111) is 0.94 ns +/- 0.36 ns, and the slip time of noctane/Pb(111) is 0.59 ns +/- 0.13 ns. I therefore observe no direct evidence of a link between the damping of perpendicular FT modes and sliding friction. It is still possible, however, that the damping of the parallel FT phonon mode affects sliding friction. Finally, I studied the slippage of monolayer methanol films at room temperature on rotating, rigid, and slowly ratcheting C60 substrates, to examine the effect that the molecular rotation of the substrate surface has on the sliding friction of an adsorbate. I found that at all coverages, the slip time for methanol on rigid and slowly ratcheting C60 was longer (hence lower friction) than the slip time for methanol on rotating C 60, defying the ball bearing analogy. (Abstract shortened by UMI.)

Coffey, Tonya S.

251

Implications of Strong-Rate-Weakening Friction  

E-print Network

Implications of Strong-Rate- Weakening Friction for the Length-Scale Dependence of the Strength · Rapid transitions between high static friction and very low dynamic friction · Leads to slip-pulse rupture · Slip pulses are extremely localized and have strong positive feedback between friction and slip

Greer, Julia R.

252

Energy concept of the coefficient of friction  

Microsoft Academic Search

Friction is a complex energy process. The traditional ‘laws’ of friction cannot reflect its energetic nature and interdependent phenomena comprising the friction process. The friction force is one of the responses of a tribosystem on the external excitation\\/initiation of the dissipative flows (degradation of macromechanical energy-entropy production). The mechanical and thermal energy dissipation is accompanied by the production of dissipative

Z. Rymuza

1996-01-01

253

Cloud Dispersal in Turbulent Flows  

E-print Network

Cold clouds embedded in warm media are very common objects in astrophysics. Their disruption timescale depends strongly on the dynamical configuration. We discuss the evolution of an initially homogeneous cold cloud embedded in warm turbulent gas. Within a couple of dynamical timescales, the filling factor of the cold gas within the original cloud radius drops below 50%. Turbulent diffusivities estimated from the time evolution of radial filling factor profiles are not constant with time. Cold and warm gas are bodily transported by turbulence and mixed. This is only mildly indicated by column density maps. The radiation field within the cloud, however, increases by several orders of magnitudes due to the mixing, with possible consequences for cloud chemistry and evolution within a few dynamical timescales.

F. Heitsch; A. D. Slyz; J. E. G. Devriendt; A. Burkert

2006-10-31

254

Slab Analysis of Ring Rolling Assuming Constant Shear Friction  

NASA Astrophysics Data System (ADS)

In this article, an analytic solution for ring rolling process based on the slab method theory is presented, in which the non-uniformity of the normal and shear stresses across the section of the deforming material throughout the plastic region is considered. The friction factor multiplied by the shear yield strength (? = mk) is used to present friction between the main roll and the ring. The influence of the process parameters such as friction factor, feed speed, main roll rotational speed, and radii of the main roll and mandrel on process outputs is investigated. Complete expressions for the ring rolling pressure, force, and torque are obtained, and the position of neutral point is predicted. Comparison of the analytic results of this model with the experimental results of other investigators and FEM analysis show that they are in good agreement.

Parvizi, A.; Abrinia, K.; Salimi, M.

2011-12-01

255

Probing the Mechanisms of Rock Friction at the Nanometer Scale with Atomic Force Microscopy  

NASA Astrophysics Data System (ADS)

In spite of the success of rate- and state-variable friction laws in describing laboratory data and reproducing a rich variety of earthquake phenomena when employed in models, these ‘laws’ lack a physical basis. To identify mechanisms underlying the time dependence of friction, i.e., the ’evolution effect’, atomic force microscopy (AFM) is employed to probe friction and adhesion for single asperity nanometer-scale contacts between Si AFM tips and wafers which have been thermally oxidized. 'lide-hold-slide’ (SHS) friction experiments in the AFM reveal a linear increase in friction (healing) with the log of the hold time th at given relative humidities (RH). Incrementally increasing RH in finite steps of 20% - from 0 to 40%, then from 40 to 80% - causes 1) an increase and decrease in adhesion force, respectively, 2) an increase and decrease in steady-state friction (?ss), respectively, and 3) an increase and decrease in frictional healing (?? after a hold), respectively, for any given th. Interestingly, the normalized friction healing factor ??/?ss increases monotonically with RH for any given th. Healing is substantially suppressed when the AFM tip is pulled out of contact and exposed to air for ~1800 s before a SHS test; healing can be revitalized by scanning the tip over the sample for a few millimeters. The experiments demonstrate that the time dependence of friction at nanometer-sized contacts is qualitatively similar to that observed in rock friction experiments: increasing humidity increases healing and healing depends on the history and chemistry of the tip surface. The data suggest that condensed water at the contact interface plays important roles, both physical and chemical, in dictating friction and healing in these experiments. Effects of tip history and chemistry on friction also suggest that mechanical creep of the tip and/or specimen contributes negligibly to frictional healing in these tests.

Li, Q.; Carpick, R.; Goldsby, D.; Tullis, T. E.

2009-12-01

256

Identification of maximum road friction coefficient and optimal slip ratio based on road type recognition  

NASA Astrophysics Data System (ADS)

The identification of maximum road friction coefficient and optimal slip ratio is crucial to vehicle dynamics and control. However, it is always not easy to identify the maximum road friction coefficient with high robustness and good adaptability to various vehicle operating conditions. The existing investigations on robust identification of maximum road friction coefficient are unsatisfactory. In this paper, an identification approach based on road type recognition is proposed for the robust identification of maximum road friction coefficient and optimal slip ratio. The instantaneous road friction coefficient is estimated through the recursive least square with a forgetting factor method based on the single wheel model, and the estimated road friction coefficient and slip ratio are grouped in a set of samples in a small time interval before the current time, which are updated with time progressing. The current road type is recognized by comparing the samples of the estimated road friction coefficient with the standard road friction coefficient of each typical road, and the minimum statistical error is used as the recognition principle to improve identification robustness. Once the road type is recognized, the maximum road friction coefficient and optimal slip ratio are determined. The numerical simulation tests are conducted on two typical road friction conditions(single-friction and joint-friction) by using CarSim software. The test results show that there is little identification error between the identified maximum road friction coefficient and the pre-set value in CarSim. The proposed identification method has good robustness performance to external disturbances and good adaptability to various vehicle operating conditions and road variations, and the identification results can be used for the adjustment of vehicle active safety control strategies.

Guan, Hsin; Wang, Bo; Lu, Pingping; Xu, Liang

2014-09-01

257

Friction microprobe investigation of particle layer effects on sliding friction  

SciTech Connect

Interfacial particles (third-bodies), resulting from wear or external contamination, can alter and even dominate the frictional behavior of solid-solid sliding in the absence of effective particle removal processes (e.g., lubricant flow). A unique friction microprobe, developed at Oak Ridge National Laboratory, was used to conduct fine- scale friction studies using 1.0 mm diameter stainless steel spheres sliding on several sizes of loose layers of fine aluminum oxide powders on both aluminum and alumina surfaces. Conventional, pin-on-disk experiments were conducted to compare behavior with the friction microprobe results. The behavior of the relatively thick particle layers was found to be independent of the nature of underlying substrate, substantiating previous work by other investigators. The time-dependent behavior of friction, for a spherical macrocontact starting from rest, could generally be represented by a series of five rather distinct phases involving static compression, slider breakaway, transition to steady state, and dynamic layer instability. A friction model for the steady state condition, which incorporates lamellar powder layer behavior, is described.

Blau, P.J.

1993-01-01

258

Reynolds number and pressure gradient effects on compressible turbulent boundary layers  

NASA Technical Reports Server (NTRS)

A detailed investigation of attached supersonic turbulent boundary layers over an extensive range of Reynolds numbers (12 x 10 to the 6th to 314 x 10 to the 6th) is presented. Experimental measurements were obtained for adverse pressure gradients ranging in magnitude from those of previous investigations to those approaching separation. The measurements include mean values of surface pressure and skin-friction, mean-flow profiles, and profiles of the three turbulent velocity fluctuation components and turbulent shear stress. Numerical solutions, employing three turbulence models of various degrees of complexity have been compared with the details of the measured flow fields. Generally, it was found that the more sophisticated turbulence models are superior to a mixing length model for predicting the Reynolds number and pressure gradient effects. However, some details of the turbulent fluctuations as well as the exact Reynolds number trends indicated by the data were not accurately predicted with any of the turbulence models considered.

Acharya, M.; Kussoy, M. I.; Horstman, C. C.

1978-01-01

259

Using measurements in urban areas to estimate turbulent velocities for modeling dispersion  

NASA Astrophysics Data System (ADS)

This study extends a study [Princevac, M., Venkatram, A., 2007. Estimating micrometeorological inputs for modeling dispersion in urban areas during stable conditions. Atmospheric Environment, doi:10.1016/j.atmosenv.2007.02.029.] in which mean winds and temperatures measured at one or two levels on towers located in urban areas were fitted to Monin-Obukhov similarity equations to obtain estimates of micrometeorological variables required in modeling dispersion in the stable boundary layer. This study shows that such methods are also useful in unstable conditions: measurements of the mean wind speed and the standard deviation of temperature fluctuations, ?T, at one level on a tower yield estimates of surface heat flux, surface friction velocity, and standard deviations of turbulent velocities that are within a factor of two of values observed at two urban sites over 80% of the time.

Venkatram, Akula; Princevac, Marko

2008-05-01

260

Multimodal Friction Ignition Tester  

NASA Technical Reports Server (NTRS)

The multimodal friction ignition tester (MFIT) is a testbed for experiments on the thermal and mechanical effects of friction on material specimens in pressurized, oxygen-rich atmospheres. In simplest terms, a test involves recording sensory data while rubbing two specimens against each other at a controlled normal force, with either a random stroke or a sinusoidal stroke having controlled amplitude and frequency. The term multimodal in the full name of the apparatus refers to a capability for imposing any combination of widely ranging values of the atmospheric pressure, atmospheric oxygen content, stroke length, stroke frequency, and normal force. The MFIT was designed especially for studying the tendency toward heating and combustion of nonmetallic composite materials and the fretting of metals subjected to dynamic (vibrational) friction forces in the presence of liquid oxygen or pressurized gaseous oxygen test conditions approximating conditions expected to be encountered in proposed composite material oxygen tanks aboard aircraft and spacecraft in flight. The MFIT includes a stainless-steel pressure vessel capable of retaining the required test atmosphere. Mounted atop the vessel is a pneumatic cylinder containing a piston for exerting the specified normal force between the two specimens. Through a shaft seal, the piston shaft extends downward into the vessel. One of the specimens is mounted on a block, denoted the pressure block, at the lower end of the piston shaft. This specimen is pressed down against the other specimen, which is mounted in a recess in another block, denoted the slip block, that can be moved horizontally but not vertically. The slip block is driven in reciprocating horizontal motion by an electrodynamic vibration exciter outside the pressure vessel. The armature of the electrodynamic exciter is connected to the slip block via a horizontal shaft that extends into the pressure vessel via a second shaft seal. The reciprocating horizontal motion can be chosen to be random with a flat spectrum over the frequency range of 10 Hz to 1 kHz, or to be sinusoidal at any peak-to-peak amplitude up to 0.8 in. (.2 cm) and fixed or varying frequency up to 1 kHz. The temperatures of the specimen and of the vessel are measured by thermocouples. A digital video camera mounted outside the pressure vessel is aimed into the vessel through a sapphire window, with its focus fixed on the interface between the two specimens. A position transducer monitors the displacement of the pneumatic-cylinder shaft. The pressure in the vessel is also monitored. During a test, the output of the video camera, the temperatures, and the pneumatic-shaft displacement are monitored and recorded. The test is continued for a predetermined amount of time (typically, 10 minutes) or until either (1) the output of the position transducer shows a sudden change indicative of degradation of either or both specimens, (2) ignition or another significant reaction is observed, or (3) pressure in the vessel increases beyond a pre-set level that triggers an automatic shutdown.

Davis, Eddie; Howard, Bill; Herald, Stephen

2009-01-01

261

Rubber friction and tire dynamics  

E-print Network

We propose a simple rubber friction law, which can be used, e.g., in models of tire (and vehicle) dynamics. The friction law is tested by comparing numerical results to the full rubber friction theory (B.N.J. Persson, J. Phys.: Condensed Matter 18, 7789 (2006)). Good agreement is found between the two theories. We describe a two-dimensional (2D) tire model which combines the rubber friction model with a simple mass-spring description of the tire body. The tire model is very flexible and can be used to calculate accurate mu-slip (and the self-aligning torque) curves for braking and cornering or combined motion (e.g., braking during cornering). We present numerical results which illustrate the theory. Simulations of Anti-Blocking System (ABS) braking are performed using two simple control algorithms.

B. N. J. Persson

2010-07-16

262

Modelling of friction stir welding  

E-print Network

This thesis investigates the modelling of friction stir welding (FSW). FSW is a relatively new welding process where a rotating non-consumable tool is used to join two materials through high temperature deformation. The aim of the thesis...

Colegrove, Paul Andrew

263

Friction Stir Welding  

NSDL National Science Digital Library

Probably the best resource to learn about friction stir welding (FSW) comes from the entity that developed the technology. The Welding Institute (1) offers a thorough overview of FSW and its advantages over other types of welding. The University of Cambridge (2) maintains another informative Web site about FSW. This is a more visual resource, allowing the visitor to view images and video clips that show FSW equipment and how the process works. Three introductory slide presentations are also available. For those who are unfamiliar with other types of welding, the Joining Technologies company (3) has an online welding reference center. Of particular interest is the Weld Defects section, which describes many of the problems of conventional welding that FSW solves. The American Welding Society published this research paper (4) in the January 2003 issue of the Welding Journal. The nine-page document presents experimental results of FSW tests, showing that defect-free welds can be achieved with a material such as mild steel. Automobile design is a prime application area for FSW, as is noted in a fact sheet from the National Transportation Research Center (5). It states that while other welding methods are suitable for standard metals in automobiles, new lightweight materials cannot be effectively joined unless a technique like FSW is used. A research paper that will be presented at an international conference in July 2003 (6) discusses the residual stresses resulting from a weld created with the FSW process. While welds of this type are typically much stronger than others, it is important to note how the performance of a weld is degraded by such residual stresses. NASA has devised a new technique, called thermal stir welding, that improves upon FSW. This breakthrough is presented in a two-page summary (7) that briefly explains the differences between thermal stir welding and other advanced methods. A new friction stir welding center was announced in November 2002, and it will be used in the construction of a new jet airplane. FSW will replace over half of the rivets traditionally used to hold planes together. This development, and its importance for jet manufacturing, are outlined in a press release from Eclipse Aviation (8).

Leske, Cavin.

264

Friction measurement in zero and adverse pressure gradient boundary layer using oil droplet interferometric method  

NASA Astrophysics Data System (ADS)

The oil droplet interferometric technique has been used to investigate the skin friction distribution along a zero and adverse pressure gradient boundary layer developing in the Laboratoire de Mécanique de Lille wind tunnel. This experimental task was a part of the WALLTURB project, funded by the European Community, in order to bring significant progress in the understanding of near wall turbulence in boundary layers. Skin friction values close to 0.01 Pa have been measured with this optical method. A comparison with the results obtained with hot-wire anemometry and macro-PIV demonstrates the great potential of the oil droplet technique.

Pailhas, Guy; Barricau, P.; Touvet, Y.; Perret, L.

2009-08-01

265

Tire/runway friction interface  

NASA Technical Reports Server (NTRS)

An overview is given of NASA Langley's tire/runway pavement interface studies. The National Tire Modeling Program, evaluation of new tire and landing gear designs, tire wear and friction tests, and tire hydroplaning studies are examined. The Aircraft Landing Dynamics Facility is described along with some ground friction measuring vehicles. The major goals and scope of several joint FAA/NASA programs are identified together with current status and plans.

Yager, Thomas J.

1990-01-01

266

Modeling of Instabilities and Self-organization at the Frictional Interface  

NASA Astrophysics Data System (ADS)

The field of friction-induced self-organization and its practical importance remains unknown territory to many tribologists. Friction is usually thought of as irreversible dissipation of energy and deterioration; however, under certain conditions, friction can lead to the formation of new structures at the interface, including in-situ tribofilms and various patterns at the interface. This thesis studies self-organization and instabilities at the frictional interface, including the instability due to the temperature-dependency of the coefficient of friction, the transient process of frictional running-in, frictional Turing systems, the stick-and-slip phenomenon, and, finally, contact angle (CA) hysteresis as an example of solid-liquid friction and dissipation. All these problems are chosen to bridge the gap between fundamental interest in understanding the conditions leading to self-organization and practical motivation. We study the relationship between friction-induced instabilities and friction-induced self-organization. Friction is usually thought of as a stabilizing factor; however, sometimes it leads to the instability of sliding, in particular when friction is coupled with another process. Instabilities constitute the main mechanism for pattern formation. At first, a stationary structure loses its stability; after that, vibrations with increasing amplitude occur, leading to a limit cycle corresponding to a periodic pattern. The self-organization is usually beneficial for friction and wear reduction because the tribological systems tend to enter a state with the lowest energy dissipation. The introductory chapter starts with basic definitions related to self-organization, instabilities and friction, literature review, and objectives. We discuss fundamental concepts that provide a methodological tool to investigate, understand and enhance beneficial processes in tribosystems which might lead to self-organization. These processes could result in the ability of a frictional surface to exhibit "self-protection" and "self-healing" properties. Hence, this research is dealing with the fundamental concepts that allow the possibility of the development of a new generation of tribosystem and materials that reinforce such properties. In chapter 2, we investigate instabilities due to the temperature-dependency of the coefficient of friction. The temperature-dependency of the coefficient of friction can have a significant effect on the frictional sliding stability, by leading to the formation of "hot" and "cold" spots on the contacting surfaces. We formulate a stability criterion and perform a case study of a brake disk. In chapter 3, we study frictional running-in. Running-in is a transient period on the onset of the frictional sliding, in which friction and wear decrease to their stationary values. In this research, running-in is interpreted as friction-induced self-organization process. We introduce a theoretical model of running-in and investigate rough profile evolution assuming that its kinetics is driven by two opposite processes or events, i.e., smoothening which is typical for the deformation-driven friction and wear, and roughening which is typical for the adhesion-driven friction and wear. In chapter 4, we investigate the possibility of the so-called Turing-type pattern formation during friction. Turing or reaction-diffusion systems describe variations of spatial concentrations of chemical components with time due to local chemical reactions coupled with diffusion. During friction, the patterns can form at the sliding interface due to the mass transfer (diffusion), heat transfer, various tribochemical reactions, and wear. In chapter 5, we investigate how interfacial patterns including propagating trains of stick and slip zones form due to dynamic sliding instabilities. These can be categorized as self-organized patterns. We treat stick and slip as two phases at the interface, and study the effects related to phase transitions. Our results show how interfacial patterns form, how

Mortazavi, Vahid

267

Turbulent Jets?  

NASA Astrophysics Data System (ADS)

Over the last few years we have fielded numerous supersonic jet experiments on the NOVA and OMEGA lasers and Sandia's pulsed-power Z-machine in a collaboration between Los Alamos National Laboratory, the Atomic Weapons Establishment, Lawrence Livermore National Laboratory, and Sandia National Laboratory. These experiments are being conducted to help validate our radiation-hydrodynamic codes, especially the newly developing ASC codes. One of the outstanding questions is whether these types of jets should turn turbulent given their high Reynolds number. Recently we have modified our experiments to have more Kelvin-Helmholtz shear, run much later in time and therefore have a better chance of going turbulent. In order to diagnose these large (several mm) jets at very late times ( 1000 ns) we are developing point-projection imaging on both the OMEGA laser, the Sandia Z-Machine, and ultimately at NIF. Since these jets have similar Euler numbers to jets theorized to be produced in supernovae explosions, we are also collaborating with the astrophysics community to help in the validation of their new codes. This poster will present a review of the laser and pulsed-power experiments and a comparison of the data to simulations by the codes from the various laboratories. We will show results of simulations wherein these jets turn highly 3-dimensional and show characteristics of turbulence. With the new data, we hope to be able to validate the sub-grid-scale turbulent mix models (e. g. BHR) that are being incorporated into our codes.*This work is performed under the auspices of the U. S. Department of Energy by the Los Alamos National Laboratory Laboratory under Contract No. W-7405-ENG-36, Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48, the Laboratory for Laser Energetics under Contract No. DE-FC03-92SF19460, Sandia National Laboratories under Contract No. DE-AC04-94AL85000, the Office of Naval Research, and the NASA Astrophysical Theory Grant.

Wilde, B. H.; Rosen, P. A.; Foster, J. M.; Perry, T. S.; Steinkamp, M. J.; Robey, H. F.; Khokhlov, A. M.; Gittings, M. L.; Coker, R. F.; Keiter, P. A.; Knauer, J. P.; Drake, R. P.; Remington, B. A.; Bennett, G. R.; Sinars, D. B.; Campbell, R. B.; Mehlhorn, T. A.

2003-10-01

268

Factoring  

NSDL National Science Digital Library

Test your factoring skills Factors and Multiples Jeopardy How much do you know about factoring and multiples? Play Jeopardy and find out! Prime Factoring Turkey Shoot Blast these turkeys using your factoring skills. Help the Professor Super save the planet by "cooking" the Giant Frozen Turkeys of Destruction. Math Lines 12 X-Factor Shoot the ball at the other factors to get a product of 12. You can also ...

Mr Clark

2012-10-31

269

The Effect of Friction on Penetration in Friction Stir Welding  

NASA Technical Reports Server (NTRS)

"Friction stir butt welding," as it was originally termed by Wayne Thomas and Christopher Dawes, in the early 1990s, but now commonly called "friction stir welding," has made great progress as a new welding technique. Marshall Space Flight Center has been investigating the use of FSW for assembly of the Shuttle's external fuel tank since the late 1990s and hopes to have the process in use by the summer of 2002. In FSW, a cylindrical pin tool of hardened steel, is rotated and plunged into the abutting edges of the parts to be joined. The tool is plunged into the weldment to within about .050 in of the bottom to assure full penetration. As the tool moves along the joint, the tool shoulder helps produce frictional heating, causing the material to plasticize. The metal of the two abutting plates flows from the front of the tool to the back where it cools and coalesces to form a weld in the solid phase. One quarter inch thick plates of aluminum alloy 2219 were used in this study. Two samples, each consisting of two 4 in x 12 in plates, were friction stir welded. The anvil for one sample was coated with molybdenum sulfide, while for the other sample a sheet of roughened stainless steel was placed between the anvil and the sample. The retractable pin tool was used so that the depth of the pin tool penetration could be varied. As welding proceeded, the length of the pin tool was gradually increased from the starting point. The purpose of this investigation is to find out at what point, in the down ramp, penetration occurs. Differences in root structure of the friction stir weld due to differences in anvil friction will be observed. These observations will be analyzed using friction stir weld theory.

Rapp, Steve

2002-01-01

270

Turbulence and turbulent mixing in natural fluids  

NASA Astrophysics Data System (ADS)

Turbulence and turbulent mixing in natural fluids begin with big bang turbulence powered by spinning combustible combinations of Planck particles and Planck antiparticles. Particle prograde accretions on a spinning pair release 42% of the particle rest mass energy to produce more fuel for turbulent combustion. Negative viscous stresses and negative turbulence stresses work against gravity, extracting mass-energy and space-time from the vacuum. Turbulence mixes cooling temperatures until strong-force viscous stresses freeze out turbulent mixing patterns as the first fossil turbulence. Cosmic microwave background temperature anisotropies show big bang turbulence fossils along with fossils of weak plasma turbulence triggered as plasma photon-viscous forces permitting gravitational fragmentation on supercluster to galaxy mass scales. Turbulent morphologies and viscous-turbulent lengths appear as linear gas-protogalaxy-clusters in the Hubble ultra-deep field at z~7. Protogalaxies fragment into Jeans mass clumps of primordial-gas planets at decoupling: the dark matter of galaxies. Shortly after the plasma-to-gas transition, planet mergers produce stars that explode on overfeeding to fertilize and distribute the first life.

Gibson, Carl H.

2010-12-01

271

Finite Element Analysis of the Amontons-Coulomb's Model using Local and Global Friction Tests  

SciTech Connect

In spite of the abundant number of experimental friction tests that have been reported, the contact with friction modeling persists to be one of the factors that determine the effectiveness of sheet metal forming simulation. This difficulty can be understood due to the nature of the friction phenomena, which comprises the interaction of different factors connected to both sheet and tools' surfaces. Although in finite element numerical simulations friction models are commonly applied at the local level, they normally rely on parameters identified based on global experimental tests results. The aim of this study is to analyze the applicability of the Amontons-Coulomb's friction coefficient identified using complementary tests: (i) load-scanning, at the local level and (ii) draw-bead, at the global level; to the numerical simulation of sheet metal forming processes.

Oliveira, M. C.; Menezes, L. F.; Ramalho, A. [CEMUC, Department of Mechanical Engineering, University of Coimbra, Polo II, Rua Luis Reis Santos, Pinhal de Marrocos, 3030-788 Coimbra (Portugal); Alves, J. L. [Department of Mechanical Engineering, University of Minho, Campus de Azurem, 4800-058, Guimaraes (Portugal)

2011-05-04

272

Characterization of frictional pressure drop for liquid flows through microchannels  

Microsoft Academic Search

This paper investigates pressure driven liquid flow through round and square microchannels fabricated from fused silica and stainless steel. Pressure drop data are used to characterize the friction factor for channel diameters in the range 15–150 ?m and over a Reynolds number range 8–2300. Distilled water, methanol, and isopropanol were used in this study based on their distinct polarity and

J. Judy; D. Maynes; B. W. Webb

2002-01-01

273

High speed friction microscopy and nanoscale friction coefficient mapping  

NASA Astrophysics Data System (ADS)

As mechanical devices in the nano/micro length scale are increasingly employed, it is crucial to understand nanoscale friction and wear especially at technically relevant sliding velocities. Accordingly, a novel technique has been developed for friction coefficient mapping (FCM), leveraging recent advances in high speed AFM. The technique efficiently acquires friction versus force curves based on a sequence of images at a single location, each with incrementally lower loads. As a result, true maps of the coefficient of friction can be uniquely calculated for heterogeneous surfaces. These parameters are determined at a scan velocity as fast as 2?mm?s?1 for microfabricated SiO2 mesas and Au coated pits, yielding results that are identical to traditional speed measurements despite being ~1000 times faster. To demonstrate the upper limit of sliding velocity for the custom setup, the friction properties of mica are reported from 200?µm?s?1 up to 2?cm?s?1. While FCM is applicable to any AFM and scanning speed, quantitative nanotribology investigations of heterogeneous sliding or rolling components are therefore uniquely possible, even at realistic velocities for devices such as MEMS, biological implants, or data storage systems.

Bosse, James L.; Lee, Sungjun; Andersen, Andreas Sø; Sutherland, Duncan S.; Huey, Bryan D.

2014-11-01

274

Turbulence structure of open channel flows over permeable and impermeable beds: A comparative study  

NASA Astrophysics Data System (ADS)

The behavior of turbulent open channel flows over permeable surfaces is not well understood. In particular, it is not clear how the surface and the subsurface flow within the permeable bed interact and influence each other. In order to clarify this issue we carried out two sets of experiments, one involving velocity measurements in open channel flows over an impermeable bed composed of a single layer of spheres, and another one where velocities were measured over and within a permeable bed made of five such layers. Comparison of surface flow velocity statistics between the two sets of experiments confirmed that bed permeability can significantly affect flow resistance. It was also confirmed that even in the hydraulically rough regime, the friction factors for the permeable bed increase with increasing Reynolds number. Such an increase in flow resistance implies a different distribution of normal form-induced stress between the permeable and impermeable bed cases. Subsurface flow measurements performed within the permeable bed revealed that there is an intense transport of turbulent kinetic energy (TKE) occurring from the surface to the subsurface flow. We provide evidence that the transport of TKE toward the lower bed levels is driven mainly by pressure fluctuations, whereas TKE transport due to turbulent velocity fluctuations is limited to a thinner layer placed in the upper part of the bed. It was also confirmed that the turbulence imposed by the surface flow gradually dissipates while penetrating within the porous medium. Dissipation occurs faster for the small scales than for the large ones, which instead are persistent, although weak, even at the lowest bed levels.

Manes, Costantino; Pokrajac, Dubravka; McEwan, Ian; Nikora, Vladimir

2009-12-01

275

BOOK REVIEW: Turbulent Combustion  

Microsoft Academic Search

The book Turbulent Combustion by Norbert Peters is a concise monograph on single-phase gaseous low Mach number turbulent combustion. It is compiled from the author's review papers on this topic plus some additional material. Norbert Peters characterizes turbulent combustion both by the way fuel and air are mixed and by the ratio of turbulent and chemical time scales. This approach

Norbert Peters

2001-01-01

276

Statistical turbulence theory and turbulence phenomenology  

NASA Technical Reports Server (NTRS)

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.

Herring, J. R.

1973-01-01

277

Dynamical Friction on Satellites  

NASA Astrophysics Data System (ADS)

Deienno et al 2011 (A&A, v.536, A57) investigated the effects of the planetary migration on the satellites of Uranus. We concluded that Uranus might have had more satellites than those observed today. However, due to the Late Heavy Bombardment (LHB) phenomenon, those satellites beyond Oberon were destabilized mostly by collisions involving themselves or with some regular ones. In this work we apply the same methodology for the Saturnian system. We found that the satellites with orbits inside Titan's orbit are immune to the LHB phenomenon. On the other hand, Hyperion, Iapetus, and even Titan, in some cases are strongly affected by the LHB, and depending on the value of Saturn's obliquity, Iapetus might not have resisted to the LHB event. We also found that, the final orbital elements of the surviving satellites differ from what we see today, mainly in inclination. While eccentricity and orbital semi-major axis can be easily damped by tides, for orbital inclinations, tidal effects are not so efficient. Thus, considering that eccentricity and orbital semi-major axis will still evolve by tides, to study the problem of the orbital inclination we consider that: according to our simulations, during the LHB event, collisions between planetesimals and satellites are a common event, causing in some cases destruction of satellites. So, we hypothesized that the material delivered by these catastrophic events could form a disc of particles around the remaining of satellites' orbits. This disc interacts with the remaining satellites and by dynamical friction phenomenon the orbital inclination can be damped. Some preliminary results have shown that, indeed, this tentative can be a viable way to damp conveniently the inclination of some satellites. Acknowledgement: FAPESP-CNPq

Deienno, Rogerio; Yokoyama, T.; Prado, A. F. B. A.

2012-10-01

278

Numerical study of axial turbulent flow over long cylinders  

NASA Astrophysics Data System (ADS)

The effects of transverse curvature are investigated by means of direct numerical simulations of turbulent axial flow over cylinders. Two cases of Reynolds number of about 3400 and layer-thickness-to-cylinder-radius ratios of 5 and 11 were simulated. All essential turbulence scales were resolved in both calculations, and a large number of turbulence statistics were computed. The results are compared with the plane channel results of Kim et al. (1987) and with experiments. With transverse curvature the skin friction coefficient increases and the turbulence statistics, when scaled with wall units, are lower than in the plane channel. The momentum equation provides a scaling that collapses the cylinder statistics, and allows the results to be interpreted in light of the plane channel flow. The azimuthal and radial length scales of the structures in the flow are of the order of the cylinder diameter. Boomerang-shaped structures with large spanwise length scales were observed in the flow.

Neves, J. C.; Moin, P.; Moser, R. D.

279

Numerical study of axial turbulent flow over long cylinders  

NASA Technical Reports Server (NTRS)

The effects of transverse curvature are investigated by means of direct numerical simulations of turbulent axial flow over cylinders. Two cases of Reynolds number of about 3400 and layer-thickness-to-cylinder-radius ratios of 5 and 11 were simulated. All essential turbulence scales were resolved in both calculations, and a large number of turbulence statistics were computed. The results are compared with the plane channel results of Kim et al. (1987) and with experiments. With transverse curvature the skin friction coefficient increases and the turbulence statistics, when scaled with wall units, are lower than in the plane channel. The momentum equation provides a scaling that collapses the cylinder statistics, and allows the results to be interpreted in light of the plane channel flow. The azimuthal and radial length scales of the structures in the flow are of the order of the cylinder diameter. Boomerang-shaped structures with large spanwise length scales were observed in the flow.

Neves, J. C.; Moin, P.; Moser, R. D.

1991-01-01

280

A One-Dimensional Global-Scaling Erosive Burning Model Informed by Blowing Wall Turbulence  

NASA Technical Reports Server (NTRS)

A derivation of turbulent flow parameters, combined with data from erosive burning test motors and blowing wall tests results in erosive burning model candidates useful in one-dimensional internal ballistics analysis capable of scaling across wide ranges of motor size. The real-time burn rate data comes from three test campaigns of subscale segmented solid rocket motors tested at two facilities. The flow theory admits the important effect of the blowing wall on the turbulent friction coefficient by using blowing wall data to determine the blowing wall friction coefficient. The erosive burning behavior of full-scale motors is now predicted more closely than with other recent models.

Kibbey, Timothy P.

2014-01-01

281

Atmospheric turbulence MTF for optical waves' propagation through anisotropic non-Kolmogorov atmospheric turbulence  

NASA Astrophysics Data System (ADS)

The conventional investigations for atmospheric turbulence have assumed that the refractive-index fluctuations of atmosphere are statistically homogeneous and isotropic. Developments of experimental and theoretical investigations have shown that the isotropic turbulence generally exists near the ground, and in the free atmosphere layer above the ground the anisotropic turbulence appears. Hence, deviations from the previously published results obtained with the isotropic turbulence assumption are possible. In this study, new analytic expressions for the anisotropic atmospheric turbulence modulation transfer function (MTF) are derived for optical plane and spherical waves propagating through anisotropic non-Kolmogorov turbulence. They consider both an anisotropic coefficient and a general spectral power law value in the range 3 to 4. When the anisotropic coefficient equals one (corresponding to the isotropic turbulence), the new results obtained in this work can reduce correctly to the previously published analytic expressions under isotropic non-Kolmogorov turbulence. The derived MTF models physically describe the turbulence anisotropic property of high atmospheric layer. Numerical calculations show that with the increase of anisotropic factor which is proportional to the atmospheric layer altitude, the atmospheric turbulence produces less effect on the imaging system.

Cui, Linyan; Xue, Bindang; Cao, Xiaoguang; Zhou, Fugen

2014-11-01

282

Gravito-Turbulent Disks in 3D: Turbulent Velocities vs. Depth  

E-print Network

Characterizing turbulence in protoplanetary disks is crucial for understanding how they accrete and spawn planets. Recent measurements of spectral line broadening promise to diagnose turbulence, with different lines probing different depths. We use 3D local hydrodynamic simulations of cooling, self-gravitating disks to resolve how motions driven by "gravito-turbulence" vary with height. We find that gravito-turbulence is practically as vigorous at altitude as at depth: even though gas at altitude is much too rarefied to be itself self-gravitating, it is strongly forced by self-gravitating overdensities at the midplane. The long-range nature of gravity means that turbulent velocities are nearly uniform vertically, increasing by just a factor of 2 from midplane to surface, even as the density ranges over nearly three orders of magnitude. The insensitivity of gravito-turbulence to height contrasts with the behavior of disks afflicted by the magnetorotational instability (MRI); in the latter case, non-circular ve...

Shi, Ji-Ming

2014-01-01

283

Versatile Friction Stir Welding/Friction Plug Welding System  

NASA Technical Reports Server (NTRS)

A proposed system of tooling, machinery, and control equipment would be capable of performing any of several friction stir welding (FSW) and friction plug welding (FPW) operations. These operations would include the following: Basic FSW; FSW with automated manipulation of the length of the pin tool in real time [the so-called auto-adjustable pin-tool (APT) capability]; Self-reacting FSW (SRFSW); SR-FSW with APT capability and/or real-time adjustment of the distance between the front and back shoulders; and Friction plug welding (FPW) [more specifically, friction push plug welding] or friction pull plug welding (FPPW) to close out the keyhole of, or to repair, an FSW or SR-FSW weld. Prior FSW and FPW systems have been capable of performing one or two of these operations, but none has thus far been capable of performing all of them. The proposed system would include a common tool that would have APT capability for both basic FSW and SR-FSW. Such a tool was described in Tool for Two Types of Friction Stir Welding (MFS- 31647-1), NASA Tech Briefs, Vol. 30, No. 10 (October 2006), page 70. Going beyond what was reported in the cited previous article, the common tool could be used in conjunction with a plug welding head to perform FPW or FPPW. Alternatively, the plug welding head could be integrated, along with the common tool, into a FSW head that would be capable of all of the aforementioned FSW and FPW operations. Any FSW or FPW operation could be performed under any combination of position and/or force control.

Carter, Robert

2006-01-01

284

Instantaneous engine frictional torque, its components and piston assembly friction  

SciTech Connect

The overall goal of this report is to document the work done to determine the instantaneous frictional torque of internal combustion engine by using a new approach known as (P-[omega]) method developed at Wayne State University. The emphasis has been to improve the accuracy of the method, and apply it to both diesel and gasoline engines under different operating conditions. Also work included an investigation to determine the effect of using advanced materials and techniques to coat the piston rings on the instantaneous engine frictional torque and the piston assembly friction. The errors in measuring the angular velocity, [omega], have been determined and found to be caused by variations in the divisions within one encoder, encoder-to-encoder variations, misalignment within the encoder itself and misalignment between the encoder and crankshaft. The errors in measuring the cylinder gas pressure, P, have been determined and found to be caused by transducer-to-transducer variations, zero drift, thermal stresses and lack of linearity. The ability of the (P-[omega]) method in determining the frictional torque of many engine components has been demonstrated. These components include valve train, fuel injection pump with and without fuel injection, and piston with and without different ring combinations. The emphasis in this part of the research program has been on the piston-ring assembly friction. The effects of load and other operating variables on IFT have been determined. The motoring test, which is widely used in industry to measure engine friction has been found to be inaccurate. The errors have been determined at different loads.

Nichols, F.A. (ed.) (Argonne National Lab., IL (United States)); Henein, N.A. (Wayne State Univ., Detroit, MI (United States). Center for Automotive Research)

1992-05-01

285

Instantaneous engine frictional torque: Its components and piston assembly friction  

NASA Astrophysics Data System (ADS)

The overall goal of this report is to document the work done to determine the instantaneous frictional torque of an internal combustion engine by using a new approach known as the P-(omega) method developed at Wayne State University. The emphasis has been to improve the accuracy of the method, and apply it to both diesel and gasoline engines under different operating conditions. Also work included an investigation to determine the effect of using advanced materials and techniques to coat the piston rings on the instantaneous engine frictional torque and the piston assembly friction. The errors in measuring the angular velocity, omega, have been determined and found to be caused by variations in the divisions within one encoder, encoder-to-encoder variations, misalignment within the encoder itself, and misalignment between the encoder and crankshaft. The errors in measuring the cylinder gas pressure, P, have been determined and found to be caused by transducer-to-transducer variations, zero drift, thermal stresses and lack of linearity. The ability of the (P-(omega)) method in determining the frictional torque of many engine components has been demonstrated. These components include valve train, fuel injection pump with and without fuel injection, and piston with and without different ring combinations. The emphasis in this part of the research program has been on the piston-ring assembly friction. The effects of load and other operating variables on IFT have been determined. The motoring test, which is widely used in industry to measure engine friction has been found to be inaccurate. The errors have been determined at different loads.

Nichols, Fred A.; Henein, N. A.

1992-05-01

286

Dynamic evaluation on friction torque of bearing for space use  

NASA Astrophysics Data System (ADS)

As the important index of evaluating the operational sensitivity of the rolling bearings, the friction torque of bearing for space using influences the signal transferring accuracy and the system stability of the spacecraft directly. It is difficult to do the large number of tests based on the particularity of the bearing for space using. Therefore, considering the limitation of the statistical principle, a dynamic evaluation model called the grey bootstrap modeling (GBM) is proposed in the paper to dynamically evaluate the friction torque of bearing for space using. The test shows under the condition of the minimum of the mean uncertainty, choosing the suitable rolling factor, the number of resampling and the confidence level, the GBM can describe the fluctuation interval of the friction torque on-line much better, and also can separate the systematic error. So, the GBM has the better performance of dynamic forecasting and its liability is quite high.

Chen, Shichao; Xia, Xintao; Wang, Zhongyu; Wang, Changxing

2008-10-01

287

Friction between Ring Polymer Brushes  

E-print Network

Friction between ring-polymer brushes at melt densities sliding past each other are studied using extensive course-grained molecular dynamics simulations and scaling arguments, and the results are compared to the friction between linear-polymer brushes. We show that for a velocity range spanning over three decades, the frictional forces measured for ring-polymer brushes are half the corresponding friction in case of linear brushes. In the linear-force regime, the weak inter-digitation of two ring brushes compared to linear brushes also leads to a lower number of binary collisions between the monomers of opposing brushes. At high velocities, where the thickness of the inter-digitation layer between two opposing brushes is on the order monomer size regardless of brush topology, stretched segments of ring polymers take a double-stranded conformation. As a result, monomers of the double-stranded segments collide less with the monomers of the opposing ring brush even though a similar number of monomers occupies the inter-digitation layer for ring and linear-brush bilayers. The numerical data obtained from our simulations is consistent with the proposed scaling analysis. Conformation-dependent frictional reduction observed in ring brushes can have important consequences in non-equilibrium bulk systems.

A. Erbas; J. Paturej

2015-01-07

288

Characteristics of turbulent structures in the unstable atmospheric surface layer  

NASA Astrophysics Data System (ADS)

An atmospheric surface-layer (ASL) experiment conducted at a meteorological site in the Oostelijk-Flevoland polder of the Netherlands is described. Turbulent fluctuations of wind velocity, air temperature and static pressure were measured, using three 10 m towers. Simultaneous turbulent signals at several heights on the towers were used to investigate the properties of the turbulent structures which contribute most significantly to the turbulent vertical transports in the unstable ASL. These turbulent structures produce between 30 and 50% of the mean turbulent vertical transport of horizontal alongwind momentum and they contribute to between 40 and 50% of the mean turbulent vertical heat transport; in both cases this occurs during 15 to 20% of the total observation time. The translation speed of the turbulent structures equals the wind speed averaged over the depth of the ASL, which scales on the surface friction velocity. The inclination angle of the temperature interface at the upstream edge of the turbulent structures to the surface is significantly smaller than that of the internal shear layer, which is associated with the temperature interface. The turbulent structures in the unstable ASL are determined by a large-scale temperature field: Convective motions, which encompass the whole depth of the planetary boundary layer (PBL), penetrate into the ASL. The curvature of the vertical profile of mean horizontal alongwind velocity forces the alignment of the convective cells in the flow direction (Kuettner, 1971), which have an average length of several hundreds of metres and an average width of a few tens of metres. This mechanism leads to the formation of turbulent structures, which extend throughout the depth of the ASL.

Schols, J. L. J.; Jansen, A. E.; Krom, J. G.

1985-10-01

289

Coherent Structure Formation in Turbulent Thermal Superfluids  

SciTech Connect

By means of numerical calculations, we show that in turbulent thermal superfluids the normal fluid induces coherent bundles of quantized line vortices in the superfluid. These filamentary structures are formed in between the normal fluid vortices, acquiring eventually comparable circulation. They are self-stretched and evolve according to self-regulating dynamics. Their spectrum mimics the normal fluid spectrum with the mutual friction force exciting the large scales and damping the small scales. Strongly interacting triads of them merge sporadically into stronger, braided vortex filaments, inducing strong fluctuations in the system's energetics. A theoretical account of the system's statistical mechanics is proposed.

Kivotides, Demosthenes [Department of Mathematics, University of Newcastle, Newcastle upon Tyne, NE1 7RU (United Kingdom)

2006-05-05

290

Rotational effects in turbulence driven by convection  

NASA Technical Reports Server (NTRS)

We have treated turbulence with rotation in a thin Keplerian disk. Highlighting implicit assumptions already existent in the alpha model together with a geometrical but physically reasonable deduction of the degrees of freedom of the largest eddies, which is of paramount importance in our formulation, we were able to obtain relations satisfied by parameters of the turbulence, such as turnover time and alpha. The effects of rotation in the turbulence we have taken implictly through an anisotropy factor (x) which is simply related to the Rossby number. Convection is the process assumed to generate turbulence, and we have used Canuto and Goldman's treatment of convective instability, whose characteristic growth time we have assumed equal to the turnover time. We have also used their procedure to obtain the turbulent viscosity.

Meirellesfilho, C.; Reyes-Ruiz, M.; Luo, C.

1994-01-01

291

Frictional drag reduction by wavy advection of deformable bubbles  

NASA Astrophysics Data System (ADS)

Bubbles can reduce frictional drag in wall turbulence, and its effect is expected to use for ships and pipelines to save their power consumptions. A number of basic experiments have been carried out to date for finding out the best condition for enhancing the drag reduction. One issue that remains at present is the difference of the performance between steady and unsteady status in terms of bubble concentration. All the experiments in the past deal with the steady effect, i.e., the drag reduction is evaluated as a function of mean void fraction or given gas flow rate of continuous injection. Despite to this, the actual phenomena highly depend on local interaction between two phases upon unsteady manner. We focus on this point and elucidate the influence of time-fluctuating void fraction on the total response to the drag reduction. This view is in fact important to estimate the persistency of the bubble-based drag reduction in the flow direction since bubbles formulate wavy advection during their migration. Our experiments are designed to measure the above-mentioned effect from laminar, transitional, and turbulent flows in a horizontal channel. For avoiding the contamination effect that worsens the reproducibility of the experiment, Silicone oil is used as carrier fluid. The oil also simulates the high Weber number bubble condition because of low surface tension. The unsteady interaction between the wavy advection of bubbles and the local skin friction, a synchronized system is constructed to connect the high-speed camera with the shear transducer, which can evaluate the interaction at 1000 fps. From the results, we confirm that the drag reduction is provided at Re>3000 in the turbulent flow regime, and also the total drag reduction is enhanced by the presence of the waves.

Oishi, Yoshihiko; Murai, Yuichi; Tasaka, Yuji; Yasushi, Takeda

2009-02-01

292

Estimation of the Friction Velocity in Stably Stratified Boundary-Layer Flows Over Hills  

Microsoft Academic Search

A method is suggested for the calculation of the friction velocity for stable turbulent boundary-layer flow over hills. The\\u000a method is tested using a continuous upstream mean velocity profile compatible with the propagation of gravity waves, and is\\u000a incorporated into the linear model of Hunt, Leibovich and Richards with the modification proposed by Hunt, Richards and Brighton\\u000a to include the

José Luis Argaín; Pedro M. A. Miranda; Miguel A. C. Teixeira

2009-01-01

293

Turbulent boundary-layer control with plasma spanwise travelling waves  

NASA Astrophysics Data System (ADS)

Arrays of dielectric-barrier-discharge plasma actuators have been designed to generate spanwise travelling waves in the turbulent boundary layer for possible skin-friction drag reductions. Particle image velocimetry was used to elucidate the modifications to turbulence structures created by the plasma spanwise travelling waves. It has been observed that the plasma spanwise travelling waves amalgamated streamwise vortices, lifting low-speed fluid from the near-wall region up and around the peripheries of their cores to form wide ribbons of low-speed streamwise velocity within the viscous sublayer.

Whalley, Richard D.; Choi, Kwing-So

2014-08-01

294

MHD Turbulence, Turbulent Dynamo and Applications  

NASA Astrophysics Data System (ADS)

MHD Turbulence is common in many space physics and astrophysics environments. We first discuss the properties of incompressible MHD turbulence. A well-conductive fluid amplifies initial magnetic fields in a process called small-scale dynamo. Below equipartition scale for kinetic and magnetic energies the spectrum is steep (Kolmogorov -5/3) and is represented by critically balanced strong MHD turbulence. In this Chapter we report the basic reasoning behind universal nonlinear small-scale dynamo and the inertial range of MHD turbulence. We measured the efficiency of the small-scale dynamo C E = 0. 05, Kolmogorov constant C K = 4. 2 and anisotropy constant C A = 0. 63 for MHD turbulence in high-resolution direct numerical simulations. We also discuss so-called imbalanced or cross-helical MHD turbulence which is relevant for in many objects, most prominently in the solar wind. We show that properties of incompressible MHD turbulence are similar to the properties of Alfvénic part of MHD cascade in compressible turbulence. The other parts of the cascade evolve according to their own dynamics. The slow modes are being cascaded by Alfvénic modes, while fast modes create an independent cascade. We show that different ways of decomposing compressible MHD turbulence into Alfvén, slow and fast modes provide consistent results and are useful in understanding not only turbulent cascade, but its interaction with fast particles.

Beresnyak, Andrey; Lazarian, Alex

295

Vacuum friction in rotating particles  

E-print Network

We study the frictional torque acting on particles rotating in empty space. At zero temperature, vacuum friction transforms mechanical energy into light emission and produces particle heating. However, particle cooling relative to the environment occurs at finite temperatures and low rotation velocities. Radiation emission is boosted and its spectrum significantly departed from a hot-body emission profile as the velocity increases. Stopping times ranging from hours to billions of years are predicted for materials, particle sizes, and temperatures accessible to experiment. Implications for the behavior of cosmic dust are discussed.

A. Manjavacas; F. J. García de Abajo

2010-09-21

296

Vacuum Friction in Rotating Particles  

SciTech Connect

We study the frictional torque acting on particles rotating in empty space. At zero temperature, vacuum friction transforms mechanical energy into light emission and produces particle heating. However, particle cooling relative to the environment occurs at finite temperatures and low rotation velocities. Radiation emission is boosted and its spectrum significantly departed from a hot-body emission profile as the velocity increases. Stopping times ranging from hours to billions of years are predicted for materials, particle sizes, and temperatures accessible to experiment. Implications for the behavior of cosmic dust are discussed.

Manjavacas, A.; Garcia de Abajo, F. J. [Instituto de Optica--CSIC, Serrano 121, 28006 Madrid (Spain)

2010-09-10

297

Rubber friction on ice and snow surfaces   

E-print Network

The friction of rubber on ice and snow surfaces is complex. Deeper scientific understanding is important for optimising performance of tyres in winter. Rubber, ice and snow systems exhibit frictional behaviour which ...

Skouvaklis, Gerasimos

2011-06-28

298

Instantaneous engine frictional torque: Its components and piston assembly friction  

Microsoft Academic Search

The overall goal of this report is to document the work done to determine the instantaneous frictional torque of an internal combustion engine by using a new approach known as the P-(omega) method developed at Wayne State University. The emphasis has been to improve the accuracy of the method, and apply it to both diesel and gasoline engines under different

Fred A. Nichols; N. A. Henein

1992-01-01

299

Flames in fractal grid generated turbulence  

NASA Astrophysics Data System (ADS)

Twin premixed turbulent opposed jet flames were stabilized for lean mixtures of air with methane and propane in fractal grid generated turbulence. A density segregation method was applied alongside particle image velocimetry to obtain velocity and scalar statistics. It is shown that the current fractal grids increase the turbulence levels by around a factor of 2. Proper orthogonal decomposition (POD) was applied to show that the fractal grids produce slightly larger turbulent structures that decay at a slower rate as compared to conventional perforated plates. Conditional POD (CPOD) was also implemented using the density segregation technique and the results show that CPOD is essential to segregate the relative structures and turbulent kinetic energy distributions in each stream. The Kolmogorov length scales were also estimated providing values ?0.1 and ?0.5 mm in the reactants and products, respectively. Resolved profiles of flame surface density indicate that a thin flame assumption leading to bimodal statistics is not perfectly valid under the current conditions and it is expected that the data obtained will be of significant value to the development of computational methods that can provide information on the conditional structure of turbulence. It is concluded that the increase in the turbulent Reynolds number is without any negative impact on other parameters and that fractal grids provide a route towards removing the classical problem of a relatively low ratio of turbulent to bulk strain associated with the opposed jet configuration.

Goh, K. H. H.; Geipel, P.; Hampp, F.; Lindstedt, R. P.

2013-12-01

300

Skin-friction measurements in a 3-D, supersonic shock-wave/boundary-layer interaction  

NASA Astrophysics Data System (ADS)

An experimental study has been conducted in a three-dimensional, supersonic shockwave/boundary-layer interaction (3-D SW/BLI) with the intent of providing accurate experimental data for turbulence modeling and computational fluid dynamics (CFD) code validation. The experiment was performed in the High Reynolds Channel 1 (HRCI) wind tunnel at NASA Ames Research Center. The test was conducted at a Mach number of M(sub infinity) = 2.89 and at a Reynolds number of Re = 15 x 106/m. The model consisted of a sting-supported cylinder aligned with the tunnel axis and a 20 deg half-angle conical flare offset 1.27 cm from the cylinder centerline. The generated shock system was verified to be steady by schlieren visualization. The highlight of the study was the acquisition of 3-D skin-friction data by a laser interferometric skin friction (LISF) meter. Surface pressure measurements were obtained in 15 deg intervals around the cylinder and flare. Additional measurements included surface oil flow and laser light sheet illumination which were used to document the flow topology. Skin-friction measurements are proving to be a very challenging test of a CFD code predictive capability. However, at the present time there is a very limited amount of accurate skin-friction data in complex flows such as in 3-D SW/BLI. The LISF technique is advantageous as compared to other skin-friction measurement techniques for application in complex flows like the present since it is non-intrusive and is capable of performing measurements in flows with large shear and pressure gradients where the reliability of other techniques is questionable. Thus, the prevent skin-friction data will prove valuable to turbulence modeling and CFD code validation efforts.

Wideman, Jeffrey Kenneth

301

Selecting the Coefficients of Sliding Friction in the Problem on Frictional Interaction of Three Solids  

Microsoft Academic Search

The choice of the coefficients of sliding friction in the static equilibrium problem for a system of three solids with friction at two points is discussed. This system simulates the mechanism of gravitational seismic isolation of a solid. It is shown that the coefficients of friction must be identical at both points of frictional contact irrespective of the type of

E. Ya. Antonyuk; Nikolai Plakhtienko

2003-01-01

302

Frictional response of a thick gouge sample: 2. Friction law and implications for faults  

E-print Network

not involve any characteristic length scale. The decrease of the gouge friction coefficient m with imposed of the friction law is to prescribe the evolution of the fault effective coefficient of friction as a function-weakening laws prescribe that the coefficient of friction essentially depends on slip displacement [e.g., Palmer

Schmittbuhl, Jean

303

Showing Area Matters: A Work of Friction  

ERIC Educational Resources Information Center

Typically, we teach the simplified friction equation of the form F[subscript s] = [mu][subscript s]N for static friction, where F[subscript s] is the maximum static friction, [mu][subscript s] is the coefficient of static friction, and "N" is the normal force pressing the surfaces together. However, this is a bit too simplified, and doesn't work…

Van Domelen, David

2010-01-01

304

Joint Winter Runway Friction Program Accomplishments  

NASA Technical Reports Server (NTRS)

The major program objectives are: (1) harmonize ground vehicle friction measurements to report consistent friction value or index for similar contaminated runway conditions, for example, compacted snow, and (2) establish reliable correlation between ground vehicle friction measurements and aircraft braking performance. Accomplishing these objectives would give airport operators better procedures for evaluating runway friction and maintaining acceptable operating conditions, providing pilots information to base go/no go decisions, and would contribute to reducing traction-related aircraft accidents.

Yager, Thomas J.; Wambold, James C.; Henry, John J.; Andresen, Arild; Bastian, Matthew

2002-01-01

305

The Science of Speed: Friction & Heat  

NSDL National Science Digital Library

The Science of Speed explains the scientific principles that are so essential to the NASCAR experience. Viewers learn how science makes cars powerful, agile, fast and safe--and how these same principles affect their own cars. Friction always creates heat. Brakes and tires depend on friction to work, but more friction isnât always better. In the engine, friction is never good and engine builders use everything from oil to high-tech coatings to get a little extra horsepower.

Productions, Santa F.

306

Apparatus for measurement of coefficient of friction  

NASA Technical Reports Server (NTRS)

An apparatus designed to measure the coefficient of friction in certain controlled atmospheres is described. The coefficient of friction observed during high-load tests was nearly constant, with an average value of 0.56. This value is in general agreement with that found in the literature and also with the initial friction coefficient value of 0.67 measured during self-mated friction of 440C steel in an oxygen environment.

Slifka, A. J.; Siegwarth, J. D.; Sparks, L. L.; Chaudhuri, Dilip K.

1990-01-01

307

Response of wind shear warning systems to turbulence with implication of nuisance alerts  

NASA Technical Reports Server (NTRS)

The objective was to predict the inherent turbulence response characteristics of candidate wind shear warning system concepts and to assess the potential for nuisance alerts. Information on the detection system and associated signal processing, physical and mathematical models, wind shear factor root mean square turbulence response and the standard deviation of the wind shear factor due to turbulence is given in vugraph form.

Bowles, Roland L.

1988-01-01

308

Turbulent heat transfer and friction in a square channel with discrete rib turbulators  

E-print Network

Re, x 10' Figure 6 Ribbed Wall and Smooth Wall Stanton Numbers vs Reynokls Nnmber, Full Rrb Cases 70 40 30 20 o 10 s K 7 IN 4 Si y+ y Q 6 0 0 11 0 &&8 & + ++ o 0 Case 1 Case 2a + Case 3a Case 2b & Case 3 b 8 10 20 Re, x 10... +$ Case 1 4 Case 2a 6 Case 2b + Case 3a 0 Case 3 b 8 10 20 Re, x 10' 40 60 80 Figure g Thermal Performances of Full Rrbs 27 full ribs The 90 full ribs have a, slightly higher ribbed wall thermal performance than the 60' and 45' crossed full ribs...

McMillin, Robert Dale

1989-01-01

309

The friction of wrinkles Hamid Mohammadi1  

E-print Network

The friction of wrinkles Hamid Mohammadi1 and Martin H. M¨user2 1 Dept. of Applied Mathematics pattern has asymmetries not present in the counterbody. The instabilities then cause Coulomb's friction Likewise, the presence of friction - as observed for the much investigated keratocytes on silicon rubber15

Mueser, Martin

310

Automotive friction-induced noises A. Elmaiana  

E-print Network

Automotive friction-induced noises A. Elmaiana , J.-M. Duffala , F. Gautiera , C. Pezeratb and J, France 3143 #12;Friction-induced noises are numerous in the automotive field. They also involve a large friction-induced noises with simple structures and automotive materials. Qualitative sensitivity studies

Paris-Sud XI, Université de

311

Friction in full view A. P. Merklea  

E-print Network

Friction in full view A. P. Merklea and L. D. Marksb Materials Science and Engineering proposed friction mechanisms explaining the unique tribological properties of graphite. Wear of graphite chemical or struc- tural information from the interface during a friction experi- ment. Examples

Marks, Laurence D.

312

Pulling by Pushing, Slip with Infinite Friction,  

E-print Network

Pulling by Pushing, Slip with Infinite Friction, and Perfectly Rough Surfaces Kevin M. Lynch the two objects even with an infinite coefficient of friction. Thus the common conception that infinite friction prevents slip is in error. This paper shows examples of the phenomena with both quasi

313

Performance of a disc brake friction material  

Microsoft Academic Search

The friction materials used in brakes are required to provide a stable coefficient of friction and a lower wear rate at various operating speeds, pressures, temperatures and environmental conditions. These friction materials must also be compatible with the rotor material in order to reduce its extensive wear, vibration and noise during braking. All of these requirements need to be achieved

Pradnya Kosbe; Chittaranjan More

2010-01-01

314

Friction analysis based on integral quadratic constraints  

Microsoft Academic Search

One of the most important nonlinearities in mechanical control systems is friction. Friction may cause steady state errors, as well as unwanted oscillations. The purpose of this paper is to demonstrate how recently developed tools for nonlinear system analysis can give us a better understanding of these effects. One of the most common approaches to analysis friction systems is passivity

A. Rantzer

1996-01-01

315

Friction Plug Weld Repair Geometric Innovations  

NASA Technical Reports Server (NTRS)

A viewgraph presentation outlines the fundamentals of friction plug welding. A process overview is given for friction push plug welding, including different uses and strengths of push plug welding. Details are given for friction pull plug welding, including welding parameters, details on observed defects, expected benefits, and test results.

Coletta, Edmond R.; Cantrell, Mark A.; McCool, A. (Technical Monitor)

2000-01-01

316

Friction forces in cosmological models  

E-print Network

We investigate the dynamics of test particles undergoing friction forces in a Friedmann-Robertson-Walker (FRW) spacetime. The interaction with the background fluid is modeled by introducing a Poynting-Robertson-like friction force in the equations of motion, leading to measurable (at least in principle) deviations of the particle trajectories from geodesic motion. The effect on the peculiar velocities of the particles is investigated for various equations of state of the background fluid and different standard cosmological models. The friction force is found to have major effects on particle motion in closed FRW universes, where it turns the time-asymptotic value (approaching the recollapse) of the peculiar particle velocity from ultra-relativistic (close to light speed) to a co-moving one, i.e., zero peculiar speed. On the other hand, for open or flat universes the effect of the friction is not so significant, because the time-asymptotic peculiar particle speed is largely non-relativistic also in the geodesic case.

Donato Bini; Andrea Geralico; Daniele Gregoris; Sauro Succi

2014-08-23

317

Dynamical friction on satellite galaxies  

E-print Network

For a rigid model satellite, Chandrasekhar's dynamical friction formula describes the orbital evolution quite accurately, when the Coulomb logarithm is chosen appropriately. However, it is not known if the orbital evolution of a real satellite with the internal degree of freedom can be described by the dynamical friction formula. We performed N-body simulation of the orbital evolution of a self-consistent satellite galaxy within a self-consistent parent galaxy. We found that the orbital decay of the simulated satellite is significantly faster than the estimate from the dynamical friction formula. The main cause of this discrepancy is that the stars stripped out of the satellite are still close to the satellite, and increase the drag force on the satellite through two mechanisms. One is the direct drag force from particles in the trailing tidal arm, a non-axisymmetric force that slows the satellite down. The other is the indirect effect that is caused by the particles remaining close to the satellite after escape. The force from them enhances the wake caused in the parent galaxy by dynamical friction, and this larger wake in turn slows the satellite down more than expected from the contribution of its bound mass. We found these two have comparable effects, and the combined effect can be as large as 20% of the total drag force on the satellite.

Michiko Fujii; Yoko Funato; Junichiro Makino

2006-06-23

318

Modeling and simulation of friction  

Microsoft Academic Search

Two new models for 'slip-stick' friction are presented. One, called the 'bristle model,' is an approximation designed to capture the physical phenomenon of sticking. This model is relatively inefficient numerically. The other model, called the 'reset integrator model,' does not capture the details for the sticking phenomenon, but is numerically efficient and exhibits behavior similar to the model proposed by

David A. Haessig; Bernard Friedland

1991-01-01

319

Pricing and Matching with Frictions  

Microsoft Academic Search

Suppose that n buyers each want one unit and m sellers each have one or more units of a good. Sellers post prices, and then buyers choose sellers. In symmetric equilibrium, similar sellers all post one price, and buyers randomize. Hence, more or fewer buyers may arrive than a seller can accommodate. We call this frictions. We solve for prices

Kenneth Burdett; Shouyong Shi; Randall Wright

2001-01-01

320

Tool Wear in Friction Drilling  

SciTech Connect

This study investigated the wear of carbide tools used in friction drilling, a nontraditional hole-making process. In friction drilling, a rotating conical tool uses the heat generated by friction to soften and penetrate a thin workpiece and create a bushing without generating chips. The wear of a hard tungsten carbide tool used for friction drilling a low carbon steel workpiece has been investigated. Tool wear characteristics were studied by measuring its weight change, detecting changes in its shape with a coordinate measuring machine, and making observations of wear damage using scanning electron microscopy. Energy dispersive spectroscopy was applied to analyze the change in chemical composition of the tool surface due to drilling. In addition, the thrust force and torque during drilling and the hole size were measured periodically to monitor the effects of tool wear. Results indicate that the carbide tool is durable, showing minimal tool wear after drilling 11000 holes, but observations also indicate progressively severe abrasive grooving on the tool tip.

Miller, Scott F [ORNL; Blau, Peter Julian [ORNL; Shih, Albert J. [University of Michigan

2007-01-01

321

Friction anisotropy in boronated graphite  

NASA Astrophysics Data System (ADS)

Anisotropic friction behavior in macroscopic scale was observed in boronated graphite. Depending upon sliding speed and normal loads, this value was found to be in the range 0.1-0.35 in the direction of basal plane and becomes high 0.2-0.8 in prismatic face. Grazing-incidence X-ray diffraction analysis shows prominent reflection of (0 0 2) plane at basal and prismatic directions of boronated graphite. However, in both the wear tracks (1 1 0) plane become prominent and this transformation is induced by frictional energy. The structural transformation in wear tracks is supported by micro-Raman analysis which revealed that 3D phase of boronated graphite converted into a disordered 2D lattice structure. Thus, the structural aspect of disorder is similar in both the wear tracks and graphite transfer layers. Therefore, the crystallographic aspect is not adequate to explain anisotropic friction behavior. Results of X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy shows weak signature of oxygen complexes and functional groups in wear track of basal plane while these species dominate in prismatic direction. Abundance of these functional groups in prismatic plane indicates availability of chemically active sites tends to forming strong bonds between the sliding interfaces which eventually increases friction coefficient.

Kumar, N.; Radhika, R.; Kozakov, A. T.; Pandian, R.; Chakravarty, S.; Ravindran, T. R.; Dash, S.; Tyagi, A. K.

2015-01-01

322

Friction-formed liquid droplets.  

PubMed

The formation of nanoscale liquid droplets by friction of a solid is observed in real-time. This is achieved using a newly developed in situ transmission electron microscope (TEM) triboprobe capable of applying multiple reciprocating wear cycles to a nanoscale surface. Dynamical imaging of the nanoscale cyclic rubbing of a focused-ion-beam (FIB) processed Al alloy by diamond shows that the generation of nanoscale wear particles is followed by a phase separation to form liquid Ga nanodroplets and liquid bridges. The transformation of a two-body system to a four-body solid-liquid system within the reciprocating wear track significantly alters the local dynamical friction and wear processes. Moving liquid bridges are observed in situ to play a key role at the sliding nanocontact, interacting strongly with the highly mobile nanoparticle debris. In situ imaging demonstrates that both static and moving liquid droplets exhibit asymmetric menisci due to nanoscale surface roughness. Nanodroplet kinetics are furthermore dependent on local frictional temperature, with solid-like surface nanofilaments forming on cooling. TEM nanotribology opens up new avenues for the real-time quantification of cyclic friction, wear and dynamic solid-liquid nanomechanics, which will have widespread applications in many areas of nanoscience and nanotechnology. PMID:21289398

Lockwood, A J; Anantheshwara, K; Bobji, M S; Inkson, B J

2011-03-11

323

Deformation During Friction Stir Welding  

NASA Technical Reports Server (NTRS)

Friction Stir Welding (FSW) is a solid state welding process that exhibits characteristics similar to traditional metal cutting processes. The plastic deformation that occurs during friction stir welding is due to the superposition of three flow fields: a primary rotation of a radially symmetric solid plug of metal surrounding the pin tool, a secondary uniform translation, and a tertiary ring vortex flow (smoke rings) surrounding the tool. If the metal sticks to the tool, the plug surface extends down into the metal from the outer edge of the tool shoulder, decreases in diameter like a funnel, and closes up beneath the pin. Since its invention, ten years have gone by and still very little is known about the physics of the friction stir welding process. In this experiment, an H13 steel weld tool (shoulder diameter, 0.797 in; pin diameter, 0.312 in; and pin length, 0.2506 in) was used to weld three 0.255 in thick plates. The deformation behavior during friction stir welding was investigated by metallographically preparing a plan view sections of the weldment and taking Vickers hardness test in the key-hole region.

White, Henry J.

2002-01-01

324

Coulomb Friction Driving Brownian Motors  

NASA Astrophysics Data System (ADS)

We review a family of models recently introduced to describe Brownian motors under the influence of Coulomb friction, or more general non-linear friction laws. It is known that, if the heat bath is modeled as the usual Langevin equation (linear viscosity plus white noise), additional non-linear friction forces are not sufficient to break detailed balance, i.e. cannot produce a motor effect. We discuss two possibile mechanisms to elude this problem. A first possibility, exploited in several models inspired to recent experiments, is to replace the heat bath's white noise by a “collisional noise”, that is the effect of random collisions with an external equilibrium gas of particles. A second possibility is enlarging the phase space, e.g. by adding an external potential which couples velocity to position, as in a Klein—Kramers equation. In both cases, non-linear friction becomes sufficient to achieve a non-equilibrium steady state and, in the presence of an even small spatial asymmetry, a motor effect is produced.

Alessandro, Manacorda; Andrea, Puglisi; Alessandro, Sarracino

2014-10-01

325

GEOMEMBRANE COEFFICIENTS OF INTERFACE FRICTION  

Microsoft Academic Search

The influence of soil particle shape, confining pressure and the amount of relative displacement on the value of the interface friction angle are studied in the laboratory using a ring shear apparatus. Two quartz sands, one angular and the other rounded,withtwogeomembranes,onesoftandtheotherhard,wereusedastheinterfac- ingconstituents. The geomembrane surface texture before and aftershearing isquanti- fied using a profilometer. Test dataindicate thatthe valueof theinterface frictionangle

Y. P. Vaid; N. Rinne

1995-01-01

326

Friction stir welding and processing  

Microsoft Academic Search

Friction stir welding (FSW) is a relatively new solid-state joining process. This joining technique is energy efficient, environment friendly, and versatile. In particular, it can be used to join high-strength aerospace aluminum alloys and other metallic alloys that are hard to weld by conventional fusion welding. FSW is considered to be the most significant development in metal joining in a

R. S. Mishra; Z. Y. Ma

2005-01-01

327

Preface: Friction at the nanoscale  

NASA Astrophysics Data System (ADS)

Interfacial friction is one of the oldest problems in physics and chemistry, and certainly one of the most important from a practical point of view. Everyday operations on a broad range of scales, from nanometer and up, depend upon the smooth and satisfactory functioning of countless tribological systems. Friction imposes serious constraints and limitations on the performance and lifetime of micro-machines and, undoubtedly, will impose even more severe constraints on the emerging technology of nano-machines. Standard lubrication techniques used for large objects are expected to be less effective in the nano-world. Novel methods for control and manipulation are therefore needed. What has been missing is a molecular level understanding of processes occurring between and close to interacting surfaces to help understand, and later manipulate friction. Friction is intimately related to both adhesion and wear, and all three require an understanding of highly non-equilibrium processes occurring at the molecular level to determine what happens at the macroscopic level. Due to its practical importance and the relevance to basic scientific questions there has been major increase in activity in the study of interfacial friction on the microscopic level during the last decade. Intriguing structural and dynamical features have been observed experimentally. These observations have motivated theoretical efforts, both numerical and analytical. This special issue focusses primarily on discussion of microscopic mechanisms of friction and adhesion at the nanoscale level. The contributions cover many important aspects of frictional behaviour, including the origin of stick-slip motion, the dependence of measured forces on the material properties, effects of thermal fluctuations, surface roughness and instabilities in boundary lubricants on both static and kinetic friction. An important problem that has been raised in this issue, and which has still to be resolved, concerns the possibility of controlling frictional response. The ability to control and manipulate frictional forces is extremely important for a variety of applications. These include magnetic storage and recording systems, miniature motors, and more. This special issue aims to provide an overview of current theoretical and experimental works on nanotribology and possible applications. In selecting the papers we have tried to maintain a balance between new results and review-like aspects, so that the present issue is self-contained and, we hope, readily accessible to non-specialists in the field. We believe that the particular appeal of this collection of papers also lies in the fusion of both experiment and theory, thus providing the connection to reality of the sometimes demanding, mathematically inclined contributions. Profound thanks go to all our colleagues and friends who have contributed to this special issue. Each has made an effort not only to present recent results in a clear and lucid way, but also to provide an introductory review that helps the reader to understand the different topics.

Fusc, Claudio; Smith, Roger; Urbakh, Michael; Vanossi, Andrea

2008-09-01

328

Turbulent Drag Reduction with Surfactant Additives — Basic Research and Application to an Air Conditioning System  

NASA Astrophysics Data System (ADS)

It is well known that a small amount of chemicals such as water-soluble polymers or surfactants dramatically suppresses turbulence when they are added to liquid flow at large Reynolds number. In the last two decades, the application of surfactants to heat transportation systems such as district heating and cooling systems has attracted much interest among researchers. It has been revealed that 70% of the pumping power used to drive hot water in primary pipelines or district heating systems was saved by adding only a few hundred ppm of surfactant into the circulating water. The technological achievement requires a new design strategy for pipeline networks and heat exchangers to handle the drag reducing liquid flow. In the case of a Newtonian fluid such as water or air, the knowledge for designing fluid systems has been accumulated and the accuracy of numerical prediction is sufficient. On the other hand, the design system for surfactant solutions is not mature because drag-reducing flow phenomena are much more complicated than for Newtonian flow, for example, the friction factor for a surfactant solution depends not only on Reynolds number but also pipe diameter. In order to provide a design strategy for heat transportation systems using surfactant additives, we are now carrying out both experimental and numerical studies for surfactant solutions. In this lecture, experimental and numerical studies on the turbulence structure in drag reducing flow will be introduced. The result of an application study relating to the air conditioning system will be also shown.

Kawaguchi, Y.; Li, F. C.; Yu, B.; Wei, J. J.

329

Stick-slip friction and wear of articular joints  

PubMed Central

Stick-slip friction was observed in articular cartilage under certain loading and sliding conditions and systematically studied. Using the Surface Forces Apparatus, we show that stick-slip friction can induce permanent morphological changes (a change in the roughness indicative of wear/damage) in cartilage surfaces, even under mild loading and sliding conditions. The different load and speed regimes can be represented by friction maps—separating regimes of smooth and stick-slip sliding; damage generally occurs within the stick-slip regimes. Prolonged exposure of cartilage surfaces to stick-slip sliding resulted in a significant increase of surface roughness, indicative of severe morphological changes of the cartilage superficial zone. To further investigate the factors that are conducive to stick-slip and wear, we selectively digested essential components of cartilage: type II collagen, hyaluronic acid (HA), and glycosaminoglycans (GAGs). Compared with the normal cartilage, HA and GAG digestions modified the stick-slip behavior and increased surface roughness (wear) during sliding, whereas collagen digestion decreased the surface roughness. Importantly, friction forces increased up to 2, 10, and 5 times after HA, GAGs, and collagen digestion, respectively. Also, each digestion altered the friction map in different ways. Our results show that (i) wear is not directly related to the friction coefficient but (ii) more directly related to stick-slip sliding, even when present at small amplitudes, and that (iii) the different molecular components of joints work synergistically to prevent wear. Our results also suggest potential noninvasive diagnostic tools for sensing stick-slip in joints. PMID:23359687

Lee, Dong Woog; Banquy, Xavier; Israelachvili, Jacob N.

2013-01-01

330

Turbulent flow of gas in fractures  

E-print Network

was made to determine the variables which affect turbulent flow in fractures and the range in magnitude of the turbulence factors. Successful results were obtained on forty runs with lithologies of cores xanging from sandstone to shale~ with proppant... sises of 40 - 60, 20 - 40 and 10 - 20 mesh and with varying concentration of proppants . The confining pressure was varied for each core up to $, 000 psi step by step. The proppant concentration in each fracture was varied up to a complete monolayer...

Koh, Wong In

2012-06-07

331

Friction of ice on ice  

NASA Astrophysics Data System (ADS)

New measurements have been made of the friction coefficient of freshwater polycrystalline ice sliding slowly (5 × 10-8 to 1 × 10-3 m s-1) upon itself at temperatures from 98 to 263 K under low normal stresses (?98 kPa). Sliding obeys Coulomb's law: the shear stress is directly proportional to the normal stress across the interface, while cohesion offers little contribution to frictional resistance. The coefficient of kinetic friction of smooth surfaces varies from ?k = 0.15 to 0.76 and, at elevated temperatures (?223 K), exhibits both velocity strengthening at lower velocities (<10-5 to 10-4 m s-1) and velocity weakening at higher velocities. Strengthening and weakening are attributed to creep deformation of asperities and localized melting, respectively. At intermediate temperatures of 173 and 133 K, the kinetic coefficient appears to not exhibit significant dependence upon velocity. However, at the low temperature of 98 K the coefficient of kinetic friction exhibits moderate velocity strengthening at both the lowest and the highest velocities but velocity independence over the range of intermediate velocities. No effect was detected of either grain size or texture. Over the range of roughness 0.4 × 10-6 m ? Ra ? 12 × 10-6 m, a moderate effect was detected, where ?k ? Ra0.08. Slide-hold-slide experiments revealed that the coefficient of static friction increases by an amount that scales logarithmically with holding time. Implications of the results are discussed in relation to shearing across "tiger stripe" faults within the icy crust of Saturn's Enceladus, sliding of the arctic sea ice cover and brittle compressive failure of cold ice.

Schulson, Erland M.; Fortt, Andrew L.

2012-12-01

332

Factorize  

NSDL National Science Digital Library

This interactive applet allows a student to visually explore the concept of factors by creating different rectangular arrays for a number. The user constructs the array by clicking and dragging on a grid. The length and width of the array are factors of the number. A student can elect an option of a randomly selected number or the student selects his own number between 2 and 50. Exploration questions are included to promote student discovery of mathematical concepts with factors.

2000-01-01

333

THE FIRST TURBULENT COMBUSTION  

Microsoft Academic Search

The first turbulent combustion arises in a hot-big-bang cosmological model (Gibson, 2004) where nonlinear exothermic turbulence permitted by quantum mechanics, general relativity, multidimensional superstring theory, and fluid mechanics cascades from Planck to strong-force freeze-out scales with gravity balancing turbulent inertial-vortex forces. Interactions between Planck scale spinning and non-spinning black holes produce high Reynolds number turbulence and temperature mixing with huge

CARL H. GIBSON

2005-01-01

334

Turbulent flow in graphene  

E-print Network

We demonstrate the possibility of a turbulent flow of electrons in graphene in the hydrodynamic region, by calculating the corresponding turbulent probability density function. This is used to calculate the contribution of the turbulent flow to the conductivity within a quantum Boltzmann approach. The dependence of the conductivity on the system parameters arising from the turbulent flow is very different from that due to scattering.

Kumar S. Gupta; Siddhartha Sen

2009-11-03

335

Frictional dissipation in a polymer bilayer system.  

PubMed

Sliding friction between a silicon tip and a polymer bilayer system consisting of a polystyrene (PS) film covered with a few-nanometers-thick capping layer of hard plasma polymer is studied using friction force microscopy. The system was chosen to enable subsurface dissipation channels to be distinguished from surface friction. Frictional energy dissipation in the underlayer can be identified through the kinetics of the polymer relaxation modes that we measured using nanoscale friction experiments as a function of sample temperature, scanning velocity, and applied load. We found a strong nonlinear increase in friction as a function of applied load around the glass-transition temperature of the PS underlayer. This behavior is a clear signature of frictional dissipation occurring in the volume of the polystyrene layer, well below the surface of the sample. The time-temperature kinetics associated with frictional energy dissipation into the PS was found to be in agreement with the known material properties of PS. Moreover, the data was found to support the hypothesis that the observed friction can be understood as the sum of friction resulting from the relaxation process in the polymer underlayer induced by stress due to the sliding of the tip and a second term associated with dissipation due to sliding friction on the capping layer. PMID:24456524

Jansen, Lars; Lantz, Mark A; Knoll, Armin W; Schirmeisen, André; Gotsmann, Bernd

2014-02-18

336

Turbulence and Holography  

E-print Network

We examine the interplay between recent advances in quantum gravity and the problem of turbulence. In particular, we argue that in the gravitational context the phenomenon of turbulence is intimately related to the properties of spacetime foam. In this framework we discuss the relation of turbulence and holography and the interpretation of the Kolmogorov scaling in the quantum gravitational setting.

Vishnu Jejjala; Djordje Minic; Y. Jack Ng; Chia-Hsiung Tze

2008-06-30

337

Quantum Gravity and Turbulence  

E-print Network

We apply recent advances in quantum gravity to the problem of turbulence. Adopting the AdS/CFT approach 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. In the gravitational context, turbulence is intimately related to the properties of spacetime, or quantum, foam.

Vishnu Jejjala; Djordje Minic; Y. Jack Ng; Chia-Hsiung Tze

2010-05-18

338

'Rational' Turbulence Models?  

NASA Astrophysics Data System (ADS)

A procedure to construct turbulence models is outlined beginning with the simplest case, a model for weak time-dependent perturbations of homogeneous isotropic turbulence, and ending with some models for inhomogeneous turbulence. The approach combines features of Yoshizawa's two-scale direct interaction approximation and the Hilbert expansion of kinetic theory.

Rubinstein, Robert; Woodruff, Stephen L.

339

Control of Turbulence  

Microsoft Academic Search

We discuss a few applications of active control of turbulent fluid flow and their implications for the economy and the environment. We outline a conceptual basis for control, sketching sensors, actuators, and the algorithm. The control of turbulence requires an understanding of turbulent flows beyond our present capabilities, but we describe the physical basis for control of the boundary layer:

John Lumley; Peter Blossey

1998-01-01

340

Entrainment due to turbulent shear flow at the interface of a stably stratified fluid  

Microsoft Academic Search

The entrainment rates of a two-layer fluid system and a linearly stratified system are investigated. The generation of turbulent shear flow using the disk pump apparatus of Odell and Kovasznay (1971) is described. The salinity distributions, deepening of the mixed layer, and mean velocity of the mixed layer are measured; the equation for calculating friction velocity is provided. The entrainment

SIAVASH NARIMOUSA; ROBERT R. LONG; S. A. Kitaigorodskii

1986-01-01

341

Frictional constraints on crustal faulting  

USGS Publications Warehouse

We consider how variations in fault frictional properties affect the phenomenology of earthquake faulting. In particular, we propose that lateral variations in fault friction produce the marked heterogeneity of slip observed in large earthquakes. We model these variations using a rate- and state-dependent friction law, where we differentiate velocity-weakening behavior into two fields: the strong seismic field is very velocity weakening and the weak seismic field is slightly velocity weakening. Similarly, we differentiate velocity-strengthening behavior into two fields: the compliant field is slightly velocity strengthening and the viscous field is very velocity strengthening. The strong seismic field comprises the seismic slip concentrations, or asperities. The two "intermediate" fields, weak seismic and compliant, have frictional velocity dependences that are close to velocity neutral: these fields modulate both the tectonic loading and the dynamic rupture process. During the interseismic period, the weak seismic and compliant regions slip aseismically, while the strong seismic regions remain locked, evolving into stress concentrations that fail only in main shocks. The weak seismic areas exhibit most of the interseismic activity and aftershocks but can also creep seismically. This "mixed" frictional behavior can be obtained from a sufficiently heterogenous distribution of the critical slip distance. The model also provides a mechanism for rupture arrest: dynamic rupture fronts decelerate as they penetrate into unloaded complaint or weak seismic areas, producing broad areas of accelerated afterslip. Aftershocks occur on both the weak seismic and compliant areas around a fault, but most of the stress is diffused through aseismic slip. Rapid afterslip on these peripheral areas can also produce aftershocks within the main shock rupture area by reloading weak fault areas that slipped in the main shock and then healed. We test this frictional model by comparing the seismicity and the coseismic slip for the 1966 Parkfield, 1979 Coyote Lake, and 1984 Morgan Hill earthquakes. The interevent seismicity and aftershocks appear to occur on fault areas outside the regions of significant slip: these regions are interpreted as either weak seismic or compliant, depending on whether or not they manifest interevent seismicity.

Boatwright, J.; Cocco, M.

1996-01-01

342

Friction, Wear, and Surface Damage of Metals as Affected by Solid Surface Films  

NASA Technical Reports Server (NTRS)

As predicted by friction theory, experiments showed that friction and surface damage of metals can be reduced by solid surface films. The ability of materials to form surface films that prevent welding was a very important factor in wear of dry and boundary lubricated surfaces. Films of graphitic carbon on cast irons, nio on nickel alloys, and feo and fe sub 3 o sub 4 on ferrous materials were found to be beneficial. Abrasive films such as fe sub 2 o sub 3 or moo sub 3 were definitely detrimental. It appears that the importance of oxide films to friction and wear processes has not been fully appreciated.

Bisson, Edmond E; Johnson, Robert L; Swikert, Max A; Godfrey, Douglas

1956-01-01

343

TURBULENCE EFFECTS ON THE CHARGE CAPTURE PROCESS IN WEAK TURBULENT PLASMAS  

SciTech Connect

The turbulence effects on the charge capture process are investigated in weak turbulent plasmas. The effective interaction potential taking into account the correction factor to the nonlinear dielectric function due to the fluctuation of the electric fields and Bohr-Lindhard model are employed in order to obtain the electron capture radius and electron capture cross section in turbulent plasmas. It is shown that the influence of the fluctuating electric fields in the plasma considerably decreases the electron charge capture radius and electron capture probability. Hence, we have found that the turbulence effect strongly suppresses the electron capture cross section in weak turbulent plasmas. In addition, it is found that the electron capture radius and electron cross section decrease with an increase of the projectile energy.

Na, Sang-Chul; Jung, Young-Dae, E-mail: ydjung@hanyang.ac.k [Department of Applied Physics, Hanyang University, Ansan, Kyunggi-Do 426-791 (Korea, Republic of)

2009-12-10

344

Turbulence structures associated with fire-atmosphere interactions  

NASA Astrophysics Data System (ADS)

Wildland fires radically modify the atmospheric boundary layer by emitting large sensible and latent heat fluxes. These fluxes drive fire-atmosphere interactions at multiple scales resulting in fire-induced circulations in and around the fire front. During the fire front passage, FFP, turbulence kinetic energy increases due to increased heating and wind shear that develops in response to both free convection and fire-induced winds. New field observations from multiple fire experiments have shown that turbulence spectral energy increases during the FFP as a result of small eddies being shed from the fire front and that that normalized velocity spectra using the friction velocity collapse into a narrow band in the inertial subrange, suggesting that Monin-Obukhov scaling is a valid scaling parameter that can be used for wildfire prediction systems. Additionally, during FFP the mean profiles of winds and sensible heat flux change compared to ambient conditions due to the fire-atmosphere interactions. These profiles are also different during different environmental conditions such as grass fires in open field and fires within a forest canopy. This presentation will discuss new turbulence observations from the FireFlux II field experiment conducted in 2013 which indicate that during FFP there are also an increases in horizontal mean winds, friction velocity, horizontal and vertical velocity variances and a decrease in anisotropy in turbulence kinetic energy and are similar to lower intensity fires.

Clements, C. B.; Seto, D.; Heilman, W. E.

2013-12-01

345

Numerical simulation of premixed turbulent methane combustion  

SciTech Connect

In this paper we study the behavior of a premixed turbulent methane flame in three dimensions using numerical simulation. The simulations are performed using an adaptive time-dependent low Mach number combustion algorithm based on a second-order projection formulation that conserves both species mass and total enthalpy. The species and enthalpy equations are treated using an operator-split approach that incorporates stiff integration techniques for modeling detailed chemical kinetics. The methodology also incorporates a mixture model for differential diffusion. For the simulations presented here, methane chemistry and transport are modeled using the DRM-19 (19-species, 84-reaction) mechanism derived from the GRIMech-1.2 mechanism along with its associated thermodynamics and transport databases. We consider a lean flame with equivalence ratio 0.8 for two different levels of turbulent intensity. For each case we examine the basic structure of the flame including turbulent flame speed and flame surface area. The results indicate that flame wrinkling is the dominant factor leading to the increased turbulent flame speed. Joint probability distributions are computed to establish a correlation between heat release and curvature. We also investigate the effect of turbulent flame interaction on the flame chemistry. We identify specific flame intermediates that are sensitive to turbulence and explore various correlations between these species and local flame curvature. We identify different mechanisms by which turbulence modulates the chemistry of the flame.

Bell, John B.; Day, Marcus S.; Grcar, Joseph F.

2001-12-14

346

Federal Aviation Administration Wake Turbulence Program- Recent Highlights  

E-print Network

Aircraft-generated wake turbulence has for years been a major factor in the air-traffic-control-imposed separations between aircraft during departure, transit and arrival operations conducted at airports and air corridors of high volume. Applied research at a global level aimed to mitigate the adverse effect of wake turbulence traces back to the 1970s, although fundamental research related to the wake turbulence dates

Jeffery A. Tittsworth; Steven R. Lang; Edward J. Johnson; Stephen Barnes

347

Astrophysical Hydromagnetic Turbulence  

NASA Astrophysics Data System (ADS)

Recent progress in astrophysical hydromagnetic turbulence is being reviewed. The physical ideas behind the now widely accepted Goldreich-Sridhar model and its extension to compressible magnetohydrodynamic turbulence are introduced. Implications for cosmic ray diffusion and acceleration is being discussed. Dynamo-generated magnetic fields with and without helicity are contrasted against each other. Certain turbulent transport processes are being modified and often suppressed by anisotropy and inhomogeneities of the turbulence, while others are being produced by such properties, which can lead to new large-scale instabilities of the turbulent medium. Applications of various such processes to astrophysical systems are being considered.

Brandenburg, A.; Lazarian, A.

2013-10-01

348

Astrophysical Hydromagnetic Turbulence  

NASA Astrophysics Data System (ADS)

Recent progress in astrophysical hydromagnetic turbulence is being reviewed. The physical ideas behind the now widely accepted Goldreich-Sridhar model and its extension to compressible magnetohydrodynamic turbulence are introduced. Implications for cosmic ray diffusion and acceleration is being discussed. Dynamo-generated magnetic fields with and without helicity are contrasted against each other. Certain turbulent transport processes are being modified and often suppressed by anisotropy and inhomogeneities of the turbulence, while others are being produced by such properties, which can lead to new large-scale instabilities of the turbulent medium. Applications of various such processes to astrophysical systems are being considered.

Brandenburg, A.; Lazarian, A.

349

Friction Stir Process Mapping Methodology  

NASA Technical Reports Server (NTRS)

In FSW (friction stir welding), the weld process performance for a given weld joint configuration and tool setup is summarized on a 2-D plot of RPM vs. IPM. A process envelope is drawn within the map to identify the range of acceptable welds. The sweet spot is selected as the nominal weld schedule. The nominal weld schedule is characterized in the expected manufacturing environment. The nominal weld schedule in conjunction with process control ensures a consistent and predictable weld performance.

Kooney, Alex; Bjorkman, Gerry; Russell, Carolyn; Smelser, Jerry (Technical Monitor)

2002-01-01

350

Prediction of High-Lift Flows using Turbulent Closure Models  

NASA Technical Reports Server (NTRS)

The flow over two different multi-element airfoil configurations is computed using linear eddy viscosity turbulence models and a nonlinear explicit algebraic stress model. A subset of recently-measured transition locations using hot film on a McDonnell Douglas configuration is presented, and the effect of transition location on the computed solutions is explored. Deficiencies in wake profile computations are found to be attributable in large part to poor boundary layer prediction on the generating element, and not necessarily inadequate turbulence modeling in the wake. Using measured transition locations for the main element improves the prediction of its boundary layer thickness, skin friction, and wake profile shape. However, using measured transition locations on the slat still yields poor slat wake predictions. The computation of the slat flow field represents a key roadblock to successful predictions of multi-element flows. In general, the nonlinear explicit algebraic stress turbulence model gives very similar results to the linear eddy viscosity models.

Rumsey, Christopher L.; Gatski, Thomas B.; Ying, Susan X.; Bertelrud, Arild

1997-01-01

351

Turbulent boundary-layer control with spanwise travelling waves  

NASA Astrophysics Data System (ADS)

It has been demonstrated through numerical simulations using Lorentz forcing that spanwise travelling waves on turbulent wall flows can lead to a skin-friction drag reduction on the order of 30%. As an aeronautical application of this innovative flow control technique, we have investigated into the use of Dielectric-Barrier-Discharge (DBD) plasma actuators to generate spanwise travelling waves in air. The near-wall structures modified by the spanwise travelling waves were studied using the PIV technique in a wind tunnel, while the associated turbulence statistics were carefully documented using hot-wire anemometry. We observed the spreading of low-speed fluid by the spanwise travelling streamwise vortices, which seems to have greatly attenuated the turbulence production process. This is very much in line with the finding of DNS studies, where wide low-speed ribbons replaced the low-speed streaks.

Whalley, Richard D.; Choi, Kwing-So

2011-12-01

352

Turbulence Spectra in the Surface Layer with a Steady Surface Thermal Inversion  

NASA Astrophysics Data System (ADS)

the EBEX-2000 (International Energy Balance Experiment, 2000, EBEX-2000) was carried out over a flood- irrigated cotton field with very strong evaporation and transpiration. And thus the latent heat flux took most part of the solar radiation and the sensible heat flux, which would directly heat the atmosphere, was very small and even became negative in mid-afternoon. Therefore, the thermal turbulence was suppressed and there always existed a surface thermal inversion during the observation. The temperatures measured at 8.7 m were always higher than that at 2.7 m, which further restrained the development of the turbulence in the lower part of the surface layer, and the turbulence exchanges for the momentum, energy and other were restrained too. Owing to strong action of the underlying surface, there is distinct wind shear, and the nearer the distance to the ground, the stronger the wind shear. Moreover, the surface thermal inversion makes the wind shear sustainable and stable. On the other hand, due to the strong blocking and friction action of the underlying surface, eddies would be strongly impacted when they came close to the ground, in particular for small eddies. That is to say, the nearer the distance to the ground, the stronger the influence of the ground on small eddies and the larger the range of eddy-size that can be directly influenced by the ground. Both the above factors contribute to the differences between the spectra at intermediate frequencies at the two heights: the horizontal power spectrum at 8.7 m does not obey -1 power law at intermediate frequencies, but it does at 2.7 m. The vertical power spectra at 8.7 m are somewhat flatter and broader at the spectral peak, while they are sharper and narrower at 2.7 m.

Peng, Z.; Hu, F.; Ma, X.; Liu, S.

2007-12-01

353

Rubber friction on smooth surfaces  

E-print Network

We study the sliding friction for viscoelastic solids, e.g., rubber, on hard flat substrate surfaces. We consider first the fluctuating shear stress inside a viscoelastic solid which results from the thermal motion of the atoms or molecules in the solid. At the nanoscale the thermal fluctuations are very strong and give rise to stress fluctuations in the MPa-range, which is similar to the depinning stresses which typically occur at solid-rubber interfaces, indicating the crucial importance of thermal fluctuations for rubber friction on smooth surfaces. We develop a detailed model which takes into account the influence of thermal fluctuations on the depinning of small contact patches (stress domains) at the rubber-substrate interface. The theory predicts that the velocity dependence of the macroscopic shear stress has a bell-shaped f orm, and that the low-velocity side exhibits the same temperature dependence as the bulk viscoelastic modulus, in qualitative agreement with experimental data. Finally, we discuss the influence of small-amplitude substrate roughness on rubber sliding friction.

B. N. J. Persson; A. I. Volokitin

2006-07-04

354

Notes on rotating turbulence  

NASA Technical Reports Server (NTRS)

This work investigates the turbulent constitutive relation when turbulence is subjected to solid body rotation. Laws regarding spectra and asymptotic decay of rotating homogeneous turbulence were confirmed through large-eddy simulation (LES) computations. Rotating turbulent flows exist in many industrial, geophysical, and astrophysical applications. From Lagrangian analysis a relation between turbulent stress and strain in rotating homogeneous turbulence was inferred. This relation was used to derive the spectral energy flux and, ultimately, the energy spectrum form. If the rotation wavenumber k(sub Omega) lies in the inertial subrange, then for wavenumbers less than k(sub Omega) the turbulence motions are affected by rotation and the energy spectrum slope is modified. Energy decay laws inferred in other reports and the present results suggest a modification of the epsilon model equation and eddy viscosity in k-epsilon models.

Zeman, Otto

1994-01-01

355

Early turbulence in von Karman swirling flow of polymer solutions  

NASA Astrophysics Data System (ADS)

We present quantitative experimental results on the transition to early turbulence in von Karman swirling flow of water- and water-sugar-based polymer solutions compared to the transition to turbulence in their Newtonian solvents by measurements of solely global quantities as torque ?(t) and pressure p(t) with large statistics as a function of Re. For the first time the transition values of Re_c\\textit{turb} to fully developed turbulence and turbulent drag reduction regime Re_c\\textit{TDR} are obtained as functions of elasticity El by using the solvents with different viscosities and polymer concentrations ?. Two scaling regions for fundamental turbulent characteristics are identified and they correspond to the turbulent and TDR regimes. Both Re_c\\textit{turb} and Re_c\\textit{TDR} are found via the dependence of the friction coefficient Cf and Cp, defined through scaled average torque \\bar? and rms pressure fluctuations p\\textit{rms} , respectively, on Re for different El and ? and via the limits of the two scaling regions.

Burnishev, Yuri; Steinberg, Victor

2015-01-01

356

The evaluation of a turbulent loads characterization system  

SciTech Connect

In this paper we discuss an on-line turbulent load characterization system that has been designed to acquire loading spectra from turbines of the same design operating in several different environments and from different turbine designs operating in the same environment. This System simultaneously measures the rainflow-counted alternating and mean loading spectra and the hub-height turbulent mean shearing stress and atmospheric stability associated with the turbulent inflow. We discuss the theory behind the measurement configuration and the results of proof-of-concept testing recently performed at the National Wind Technology Center (NWTC) using a Bergey EXCEL-S 10-kW wind turbine. The on-line approach to characterizing the load spectra and the inflow turbulent scaling parameter produces results that are consistent with other measurements. The on-line approximation of the turbulent shear stress or friction velocity u* also is considered adequate. The system can be used to characterize turbulence loads during turbine deployment in a wide variety of environments. Using the WISPER protocol, we found that a wide-range, variable-speed turbine will accumulate a larger number of stress cycles in the low-cycle, high-amplitude (LCHA) region when compared with a constant speed rotor under similar inflow conditions.

Kelley, N.D.; McKenna, H.E. [National Renewable Energy Lab., Golden, CO (United States)

1996-01-01

357

Comparison between kinetic-ballooning-mode-driven turbulence and ion-temperature-gradient-driven turbulence  

SciTech Connect

Electromagnetic turbulence driven by kinetic ballooning modes (KBMs) in high-? plasma is investigated based on the local gyrokinetic model. Analysis of turbulent fluxes, norms, and phases of fluctuations shows that KBM turbulence gives narrower spectra and smaller phase factors than those in ion-temperature-gradient (ITG)-driven turbulence. This leads to the smaller transport fluxes in KBM turbulence than those in ITG turbulence even when they have similar linear growth rates. From the analysis of the entropy balance relation, it is found that the entropy transfer from ions to electrons through the field-particle interactions mainly drives electron perturbations, which creates radial twisted modes by rapid parallel motions of electrons in a sheared magnetic geometry. The nonlinear coupling between the dominant unstable mode and its twisted modes is important for the saturation of KBM turbulence, in contrast to the importance of zonal flow shearing in ITG turbulence. The coupling depends on the flux-tube domain with the one-poloidal-turn parallel length and on the torus periodicity constraint.

Maeyama, S., E-mail: maeyama.shinya@jaea.go.jp; Nakata, M.; Miyato, N.; Yagi, M. [Japan Atomic Energy Agency, Rokkasho, Aomori 039-3212 (Japan)] [Japan Atomic Energy Agency, Rokkasho, Aomori 039-3212 (Japan); Ishizawa, A.; Watanabe, T.-H. [National Institute for Fusion Science, Toki, Gifu 509-5292 (Japan)] [National Institute for Fusion Science, Toki, Gifu 509-5292 (Japan); Idomura, Y. [Japan Atomic Energy Agency, Kashiwa, Chiba 277-8587 (Japan)] [Japan Atomic Energy Agency, Kashiwa, Chiba 277-8587 (Japan)

2014-05-15

358

Comparison between kinetic-ballooning-mode-driven turbulence and ion-temperature-gradient-driven turbulence  

NASA Astrophysics Data System (ADS)

Electromagnetic turbulence driven by kinetic ballooning modes (KBMs) in high-? plasma is investigated based on the local gyrokinetic model. Analysis of turbulent fluxes, norms, and phases of fluctuations shows that KBM turbulence gives narrower spectra and smaller phase factors than those in ion-temperature-gradient (ITG)-driven turbulence. This leads to the smaller transport fluxes in KBM turbulence than those in ITG turbulence even when they have similar linear growth rates. From the analysis of the entropy balance relation, it is found that the entropy transfer from ions to electrons through the field-particle interactions mainly drives electron perturbations, which creates radial twisted modes by rapid parallel motions of electrons in a sheared magnetic geometry. The nonlinear coupling between the dominant unstable mode and its twisted modes is important for the saturation of KBM turbulence, in contrast to the importance of zonal flow shearing in ITG turbulence. The coupling depends on the flux-tube domain with the one-poloidal-turn parallel length and on the torus periodicity constraint.

Maeyama, S.; Ishizawa, A.; Watanabe, T.-H.; Nakata, M.; Miyato, N.; Yagi, M.; Idomura, Y.

2014-05-01

359

Environmental effects on friction and wear of diamond and diamondlike carbon coatings  

NASA Technical Reports Server (NTRS)

Reciprocating sliding friction experiments were conducted with a natural diamond flat, diamond film, and low and high density diamondlike carbon (DLC) films in contact with pin specimens of natural diamond and silicon nitride (Si3N4) both in humid air and dry air nitrogen. The results indicated that for natural diamond pin contacts the diamond films and the natural diamond flat were not susceptible to moisture but that moisture could increase both the coefficient of friction and the wear factors of the DLC films. The coefficients of friction and wear factors of the diamond films were generally similar to those of the natural diamond flat both in humid air and dry air nitrogen. In dry nitrogen the coefficients of friction of the high density DLC films in contact with pin specimens of both diamond and Si3N4 were generally low (about 0.02) and similar to those of the natural diamond flat and the diamond films. The wear factors of the materials in contact with both natural diamond and Si3N4 were generally in the ascending order of natural diamond flat, diamond film, high density DLC film, and low density DLC film. The moisture in the environment increased the coefficients of friction for Si3N4 pins in contact with all the materials. This increase in friction is due to the silicon oxide film produced on the surface of Si3N4 pins in humid air.

Miyoshi, Kazuhisa; Wu, Richard L. C.; Garscadden, Alan

1992-01-01

360

Ris-R-1188(EN) Turbulence and turbulence-  

E-print Network

turbulence intensity inside the wind farm and direct-wake turbulence intensity are being devised and a methodRisø-R-1188(EN) Turbulence and turbulence- generated structural loading in wind turbine clusters in English Turbulence ­ in terms of standard deviation of wind speed fluctuations ­ and other flow

361

Effective medium theory for drag-reducing micro-patterned surfaces in turbulent flows.  

PubMed

Many studies in the last decade have revealed that patterns at the microscale can reduce skin drag. Yet, the mechanisms and parameters that control drag reduction, e.g. Reynolds number and pattern geometry, are still unclear. We propose an effective medium representation of the micro-features, that treats the latter as a porous medium, and provides a framework to model turbulent flow over patterned surfaces. Our key result is a closed-form expression for the skin friction coefficient in terms of frictional Reynolds (or Kármán) number in turbulent regime, the viscosity ratio between the fluid in and above the features, and their geometrical properties. We apply the proposed model to turbulent flows over superhydrophobic ridged surfaces. The model predictions agree with laboratory experiments for Reynolds numbers ranging from 3000 to 10000. PMID:24671449

Battiato, Ilenia

2014-03-01

362

Friction coefficient dependence on electrostatic tribocharging  

NASA Astrophysics Data System (ADS)

Friction between dielectric surfaces produces patterns of fixed, stable electric charges that in turn contribute electrostatic components to surface interactions between the contacting solids. The literature presents a wealth of information on the electronic contributions to friction in metals and semiconductors but the effect of triboelectricity on friction coefficients of dielectrics is as yet poorly defined and understood. In this work, friction coefficients were measured on tribocharged polytetrafluoroethylene (PTFE), using three different techniques. As a result, friction coefficients at the macro- and nanoscales increase many-fold when PTFE surfaces are tribocharged, but this effect is eliminated by silanization of glass spheres rolling on PTFE. In conclusion, tribocharging may supersede all other contributions to macro- and nanoscale friction coefficients in PTFE and probably in other insulating polymers.

Burgo, Thiago A. L.; Silva, Cristiane A.; Balestrin, Lia B. S.; Galembeck, Fernando

2013-08-01

363

Friction Anisotropy with Respect to Topographic Orientation  

PubMed Central

Friction characteristics with respect to surface topographic orientation were investigated using surfaces of different materials and fabricated with grooves of different scales. Scratching friction tests were conducted using a nano-indentation-scratching system with the tip motion parallel or perpendicular to the groove orientation. Similar friction anisotropy trends were observed for all the surfaces studied, which are (1) under a light load and for surfaces with narrow grooves, the tip motion parallel to the grooves offers higher friction coefficients than does that perpendicular to them, (2) otherwise, equal or lower friction coefficients are found under this motion. The influences of groove size relative to the diameter of the mating tip (as a representative asperity), surface contact stiffness, contact area, and the characteristic stiction length are discussed. The appearance of this friction anisotropy is independent of material; however, the boundary and the point of trend transition depend on material properties. PMID:23248751

Yu, Chengjiao; Wang, Q. Jane

2012-01-01

364

Granular Brownian motion with dry friction  

NASA Astrophysics Data System (ADS)

The interplay between Coulomb friction and random excitations is studied experimentally by means of a rotating probe in contact with a stationary granular gas. The granular material is independently fluidized by a vertical shaker, acting as a “heat bath” for the Brownian-like motion of the probe. Two ball bearings supporting the probe exert nonlinear Coulomb friction upon it. The experimental velocity distribution of the probe, autocorrelation function, and power spectra are compared with the predictions of a linear Boltzmann equation with friction, which is known to simplify in two opposite limits: at high collision frequency, it is mapped to a Fokker-Planck equation with nonlinear friction, whereas at low collision frequency, it is described by a sequence of independent random kicks followed by friction-induced relaxations. Comparison between theory and experiment in these two limits shows good agreement. Deviations are observed at very small velocities, where the real bearings are not well modeled by Coulomb friction.

Gnoli, A.; Puglisi, A.; Touchette, H.

2013-04-01

365

SRM propellant, friction/ESD testing  

NASA Technical Reports Server (NTRS)

Following the Pershing 2 incident in 1985 and the Peacekeeper ignition during core removal in 1987, it was found that propellant can be much more sensitive to Electrostatic Discharges (ESD) than ever before realized. As a result of the Peacekeeper motor near miss incident, a friction machine was designed and fabricated, and used to determine friction hazards during core removal. Friction testing with and electrical charge being applied across the friction plates resulted in propellant ignitions at low friction pressures and extremely low ESD levels. The objective of this test series was to determine the sensitivity of solid rocket propellant to combined friction pressure and electrostatic stimuli and to compare the sensitivity of the SRM propellant to Peacekeeper propellant. The tests are fully discussed, summarized and conclusions drawn.

Campbell, L. A.

1989-01-01

366

Switching friction with thermal- responsive gels.  

PubMed

The thermosensitive graphene oxide (GO)/poly(N-isopropyl acrylamide) (pNIPAM) composite hydrogels are prepared, and their tribological properties in response to external stimuli are evaluated. The frictional coefficient of the hydrogels is closely related to the gel composition and ambient temperature. When the gel is in swelling state below the low critical solution temperature (LCST), it shows ultra-low friction and exhibits high friction at a shrunk state above the LCST. The huge difference of frictional coefficient under two states can be reversibly switched many times by altering the temperature. The incorporation of a nonthermal sensitive monomer into pNIPAM could change the LCST and thus the transformation point of frictional coefficient can be altered. These reversible and tunable frictional hydrogels have potential application in the design of intelligent control equipment. PMID:24249089

Wu, Yang; Cai, Meirong; Pei, Xiaowei; Liang, Yongmin; Zhou, Feng

2013-11-01

367

Single-crystal-silicon-based microinstrument to study friction and wear at MEMS sidewall interfaces  

NASA Astrophysics Data System (ADS)

Since the advent of microelectromechanical systems (MEMS) technology, friction and wear are considered as key factors that determine the lifetime and reliability of MEMS devices that contain contacting interfaces. However, to date, our knowledge of the mechanisms that govern friction and wear in MEMS is insufficient. Therefore, systematically investigating friction and wear at MEMS scale is critical for the commercial success of many potential MEMS devices. Specifically, since many emerging MEMS devices contain more sidewall interfaces, which are topographically and chemically different from in-plane interfaces, studying the friction and wear characteristics of MEMS sidewall surfaces is important. The microinstruments that have been used to date to investigate the friction and wear characteristics of MEMS sidewall surfaces possess several limitations induced either by their design or the structural film used to fabricate them. Therefore, in this paper, we report on a single-crystal-silicon-based microinstrument to study the frictional and wear behavior of MEMS sidewalls, which not only addresses some of the limitations of other microinstruments but is also easy to fabricate. The design, modeling and fabrication of the microinstrument are described in this paper. Additionally, the coefficients of static and dynamic friction of octadecyltrichlorosilane-coated sidewall surfaces as well as sidewall surfaces with only native oxide on them are also reported in this paper.

Ansari, N.; Ashurst, W. R.

2012-02-01

368

Rubber friction on (apparently) smooth lubricated surfaces  

E-print Network

We study rubber sliding friction on hard lubricated surfaces. We show that even if the hard surface appears smooth to the naked eye, it may exhibit short wavelength roughness, which may give the dominant contribution to rubber friction. That is, the observed sliding friction is mainly due to the viscoelastic deformations of the rubber by the substrate surface asperities. The presented results are of great importance for rubber sealing and other rubber applications involving (apparently) smooth surfaces.

M. Mofidi; B. Prakash; B. N. J. Persson; O. Albohl

2007-10-18

369

High-velocity frictional properties of gabbro  

Microsoft Academic Search

High-velocity friction experiments have been performed on a pair of hollow-cylindrical specimens of gabbro initially at room temperature, at slip rates from 7.5 mm\\/s to 1.8 m\\/s, with total circumferential displacements of 125 to 174 m, and at normal stresses to 5 MPa, using a rotary-shear high-speed friction testing machine. Steady-state friction increases slightly with increasing slip rate at slip

Akito Tsutsumi; Toshihiko Shimamoto

1997-01-01

370

Friction-stir welding: microstructural characterization  

Microsoft Academic Search

The microstructures of friction-stir welded aluminum alloys (1100 and 6061) are dominated by dynamic recrystallization. Friction-stir\\u000a welding of 6061 aluminum to copper produced a complex, intercalated microstructure which was also dominated by dynamic recrystallization.\\u000a The friction-stir welding process is characterized by extreme plastic deformation in the solid state; there is no associated\\u000a melting.

L. E. Murr; R. D. Flores; O. V. Flores; J. C. McClure; G. Liu; D. Brown

1998-01-01

371

Coefficient of friction of composite delamination surfaces  

Microsoft Academic Search

The coefficient of friction of delamination surfaces from graphite fiber\\/epoxy matrix (IM7\\/8552) composites has been measured in reciprocating sliding. The worn surfaces were examined with a scanning electron microscope and the friction and wear mechanisms were studied. The results show that after an initial wear-in of the surfaces the coefficient of friction will decrease with increasing number of reciprocal cycles

Joakim Schön

2000-01-01

372

High-temperature friction of refractory compounds  

Microsoft Academic Search

The paper overviews long-term studies into the behavior of metallic (carbides, borides, and nitrides of transition metals),\\u000a and nonmetallic (boron and silicon carbides, aluminum nitride) refractory compounds as well as composite materials based on\\u000a them in high-temperature friction in vacuum and air. The friction characteristics (wear rate and friction coefficient) are\\u000a indicated as a function of temperature in the range

Yu. G. Tkachenko; D. Z. Yurchenko; M. S. Koval’chenko

2008-01-01

373

Friction stir welding for the transportation industries  

Microsoft Academic Search

This paper will focus on the relatively new joining technology—friction stir welding (FSW). Like all friction welding variants, the FSW process is carried out in the solid-phase. Generically solid-phase welding is one of the oldest forms of metallurgical joining processes known to man. Friction stir welding is a continuous hot shear autogenous process involving a non-consumable rotating probe of harder

W. M Thomas; E. D Nicholas

1997-01-01

374

Generalized similarity in finite range solar wind magnetohydrodynamic turbulence.  

PubMed

Extended or generalized similarity is a ubiquitous but not well understood feature of turbulence that is realized over a finite range of scales. The ULYSSES spacecraft solar polar passes at solar minimum provide in situ observations of evolving anisotropic magnetohydrodynamic turbulence in the solar wind under ideal conditions of fast quiet flow. We find a single generalized scaling function characterizes this finite range turbulence and is insensitive to plasma conditions. The recent unusually inactive solar minimum--with turbulent fluctuations down by a factor of approximately 2 in power--provides a test of this invariance. PMID:20366193

Chapman, S C; Nicol, R M

2009-12-11

375

Generalized Similarity in Finite Range Solar Wind Magnetohydrodynamic Turbulence  

SciTech Connect

Extended or generalized similarity is a ubiquitous but not well understood feature of turbulence that is realized over a finite range of scales. The ULYSSES spacecraft solar polar passes at solar minimum provide in situ observations of evolving anisotropic magnetohydrodynamic turbulence in the solar wind under ideal conditions of fast quiet flow. We find a single generalized scaling function characterizes this finite range turbulence and is insensitive to plasma conditions. The recent unusually inactive solar minimum - with turbulent fluctuations down by a factor of approx2 in power - provides a test of this invariance.

Chapman, S. C.; Nicol, R. M. [Centre for Fusion, Space and Astrophysics, Department of Physics, University of Warwick, Coventry, CV4 7AL (United Kingdom)

2009-12-11

376

Tailoring the frictional properties of granular media.  

PubMed

A method of modifying the roughness of soda-lime glass spheres is presented, with the purpose of tuning interparticle friction. The effect of chemical etching on the surface topography and the bulk frictional properties of grains are systematically investigated. The surface roughness of the grains is measured using white-light interferometry and characterized by the lateral and vertical roughness length scales. The underwater angle of repose is measured to characterize the bulk frictional behavior. We observe that the coefficient of friction depends on the vertical roughness length scale. PMID:22060360

Utermann, Sonia; Aurin, Philipp; Benderoth, Markus; Fischer, Cornelius; Schröter, Matthias

2011-09-01

377

Rolling friction on a wheeled laboratory cart  

NASA Astrophysics Data System (ADS)

A simple model is developed that predicts the coefficient of rolling friction for an undriven laboratory cart on a track that is approximately independent of the mass loaded onto the cart and of the angle of inclination of the track. The model includes both deformation of the wheels/track and frictional torque at the axles/bearings. The concept of rolling friction is contrasted with the static or kinetic friction that in general is also present, such as for a cylinder or ball rolling along a horizontal or inclined surface.

Mungan, Carl E.

2012-05-01

378

Effect of grafted oligopeptides on friction.  

PubMed

Frictional and normal forces in aqueous solution at 25 °C were measured between a glass particle and oligopeptide films grafted from a glass plate. Homopeptide molecules consisting of 11 monomers of either glutamine, leucine, glutamic acid, lysine, or phenylalanine and one heteropolymer were each "grafted from" an oxidized silicon wafer using microwave-assisted solid-phase peptide synthesis. The peptide films were characterized using X-ray photoelectron spectroscopy and secondary ion mass spectrometry. Frictional force measurements showed that the oligopeptides increased the magnitude of friction compared to that on a bare hydrophilic silicon wafer but that the friction was a strong function of the nature of the monomer unit. Overall we find that the friction is lower for more hydrophilic films. For example, the most hydrophobic monomer, leucine, exhibited the highest friction whereas the hydrophilic monomer, polyglutamic acid, exhibited the lowest friction at zero load. When the two surfaces had opposite charges, there was a strong attraction, adhesion, and high friction between the surfaces. Friction for all polymers was lower in phosphate-buffered saline than in pure water, which was attributed to lubrication via hydrated salt ions. PMID:23594080

Iarikov, Dmitri D; Ducker, William A

2013-05-14

379

Skin friction for steel piles in sand  

E-print Network

Skin Friction Versus Pile Movement. Test Pile 1 Loaded in Tension 17 10. Skin Friction-Soil Shear Strength Ratio Versus Pile Movement. Compression Test Pile 1 21 Skin Friction-Soil Sheer Strength Ratio Versus Pile Movement. Tension Test Pile 1 22... Data 35 16. 17. Mohr Envelope for Firm Saturated Sand. Mohr Envelope for Dense Saturated Sand. 37 LIST OF FIGURES (Continued) Figure Skin Friction-Soil Shear Strength Ratio Versus Pile Movement for Firm Saturated Sand 39 19. Skin F iction...

Sulaiman, Ibrahim Hikmat

2012-06-07

380

Measurement of Gear Tooth Dynamic Friction  

NASA Technical Reports Server (NTRS)

Measurements of dynamic friction forces at the gear tooth contact were undertaken using strain gages at the root fillets of two successive teeth. Results are presented from two gear sets over a range of speeds and loads. The results demonstrate that the friction coefficient does not appear to be significantly influenced by the sliding reversal at the pitch point, and that the friction coefficient values found are in accord with those in general use. The friction coefficient was found to increase at low sliding speeds. This agrees with the results of disc machine testing.

Rebbechi, Brian; Oswald, Fred B.; Townsend, Dennis P.

1996-01-01

381

Turbulent Inflow Measurements  

NASA Technical Reports Server (NTRS)

In the present research, tilt rotor aeroacoustics have been studied experimentally and computationally. Experimental measurements were made on a 1/12.5 scale model. A dimensional analysis showed that the model was a good aeroacoustic approximation to the full-scale aircraft, and scale factors were derived to extrapolate the model measurements to the full-scale XV-15. The experimental measurements included helium bubble flow visualization, silk tuft flow visualization, 2-component hot wire anemometry, 7-hole pressure probe measurements, vorticity measurements, and outdoor far field acoustic measurements. The hot wire measurements were used to estimate the turbulence statistics of the flow field into the rotors, such as length scales, velocity scales, dissipation, and turbulence intermittency. To date, these flow measurements are the only ones in existence for a hovering tilt rotor. Several different configurations of the model were tested: (1) standard configurations (single isolated rotor, two rotors without the aircraft, standard tilt rotor configuration); (2) flow control devices (the 'plate', the 'diagonal fences'); (3) basic configuration changes (increasing the rotor/rotor spacing, reducing the rotor plane/wing clearance, operating the rotors out of phase). Also, an approximation to Sikorsky's Variable Diameter Tilt Rotor (VDTR) configuration was tested, and some flow measurements were made on a semi-span configuration of the model. Acoustic predictions were made using LOWSON.M, a Mathematica code. This hover prediction code, from HOVER.FOR, used blade element theory for the aerodynamics, and Prandtl's Vortex theory to model the wake, along with empirical formulas for the effects of Reynolds number, Mach number, and stall. Aerodynamic models were developed from 7-hole pressure probe measurements of the mean velocity into the model rotors. LOWSON.M modeled a rotor blade as a single force and source/sink combination separated in the chordwise direction, at an effective blade radius. Spanwise, Mach-weighted integrals were used to find the equivalent forces and equivalent source strengths.

George, Albert R.

1996-01-01

382

Market Assessment of Forward-Looking Turbulence Sensing Systems  

NASA Technical Reports Server (NTRS)

In recognition of the importance of turbulence mitigation as a tool to improve aviation safety, NASA's Aviation Safety Program developed a Turbulence Detection and Mitigation Sub-element. The objective of this effort is to develop highly reliable turbulence detection technologies for commercial transport aircraft to sense dangerous turbulence with sufficient time warning so that defensive measures can be implemented and prevent passenger and crew injuries. Current research involves three forward sensing products to improve the cockpit awareness of possible turbulence hazards. X-band radar enhancements will improve the capabilities of current weather radar to detect turbulence associated with convective activity. LIDAR (Light Detection and Ranging) is a laser-based technology that is capable of detecting turbulence in clear air. Finally, a possible Radar-LIDAR hybrid sensor is envisioned to detect the full range of convective and clear air turbulence. To support decisions relating to the development of these three forward-looking turbulence sensor technologies, the objective of this study was defined as examination of cost and implementation metrics. Tasks performed included the identification of cost factors and certification issues, the development and application of an implementation model, and the development of cost budget/targets for installing the turbulence sensor and associated software devices into the commercial transport fleet.

Kauffmann, Paul; Sousa-Poza, Andres

2001-01-01

383

Abstract--Friction modeling is essential for joint dynamic identification and control. Joint friction is composed of a  

E-print Network

Abstract--Friction modeling is essential for joint dynamic identification and control. Joint friction is composed of a viscous and a dry friction force. According to Coulomb law, dry friction depends linearly on the load in the transmission. However, in robotics field, a constant dry friction is frequently

Paris-Sud XI, Université de

384

Turbulent-viscosity reduction mechanism based on anisotropic turbulence effects  

NASA Astrophysics Data System (ADS)

The relationship between turbulent viscosity and turbulence anisotropy is investigated on the basis of a statistical theory of inhomogeneous turbulence. An anisotropic turbulent viscosity is derived with the axisymmetric anisotropy subject to an enhanced streamwise turbulence intensity taken into account. The result paves the way for explaining a mechanism of drag reduction due to the change of turbulence anisotropy in a developed pipe flow. The drag reduction by polymer additives is discussed in light of this mechanism.

Yoshizawa, Akira

2003-12-01

385

Some Hamiltonian Models of Friction  

E-print Network

Mathematical results on some models describing the motion of a tracer particle through a Bose-Einstein condensate are described. In the limit of a very dense, very weakly interacting Bose gas and for a very large particle mass, the dynamics of the coupled system is determined by classical non-linear Hamiltonian equations of motion. The particle's motion exhibits deceleration corresponding to friction (with memory) caused by the emission of Cerenkov radiation of gapless modes into the gas. Precise results are stated and outlines of proofs are presented. Some technical details are deferred to forthcoming papers.

Juerg Froehlich; Zhou Gang; Avy Soffer

2010-11-15

386

A review of the physics of ice surface friction and the development of ice skating.  

PubMed

Our walking and running movement patterns require friction between shoes and ground. The surface of ice is characterised by low friction in several naturally occurring conditions, and compromises our typical locomotion pattern. Ice skates take advantage of this slippery nature of ice; the first ice skates were made more than 4000 years ago, and afforded the development of a very efficient form of human locomotion. This review presents an overview of the physics of ice surface friction, and discusses the most relevant factors that can influence ice skates' dynamic friction coefficient. It also presents the main stages in the development of ice skating, describes the associated implications for exercise physiology, and shows the extent to which ice skating performance improved through history. This article illustrates how technical and materials' development, together with empirical understanding of muscle biomechanics and energetics, led to one of the fastest forms of human powered locomotion. PMID:24950115

Formenti, Federico

2014-01-01

387

Effect of friction stir processing on the tribological performance of high carbon steel.  

SciTech Connect

Friction stir processing (FSP) was applied to 1080 carbon steel as a means to enhance the near-surface material properties. The process transformed the original pearlite microstructure to martensite, resulting in significant increase in surface hardness. This surface hardening produced a significant benefit for friction and wear behavior of the steel as measured by unidirectional sliding ball-on-flat testing. Under dry sliding, FSP reduced friction coefficient by approximately 25% and wear rate by an order of magnitude. Under oil lubrication, FSP had only a marginal effect on friction, but it reduced wear rates by a factor of 4. The improvement in tribological performance of 1080 steel by FSP technique is attributed to reduced plasticity of the near-surface material during sliding contact

Aldajah, S. H.; Ajayi, O. O.; Fenske, G. R.; David, S. (Energy Systems); (United Arab Emirates Univ.); (ORNL)

2009-06-15

388

Wear and friction of oxidation-resistant mechanical carbon graphites at 650 C in air  

NASA Technical Reports Server (NTRS)

Studies were conducted to determine the friction and wear properties of experimental carbon-graphites. Hemispherically tipped carbon-graphite rider specimens were tested in sliding contact with rotating Inconel X-750 disks in air. A surface speed of 1.33 m/sec, a load of 500 g, and a specimen temperature of 650 C were used. Results indicate: (1) hardness is not a major factor in determining friction and wear under the conditions of these studies. (2) Friction and wear as low as or lower than those observed for a good commercial seal material were attained with some of the experimental materials studied. (3) The inclusion of boron carbide (as an oxidation inhibitor) has a strong influence on wear rate. (4) Phosphate treatment reduces the friction coefficient when boron carbide is not present in the base material.

Allen, G. P.; Wisnader, D. W.

1975-01-01

389

Frictional behavior of large displacement experimental faults  

USGS Publications Warehouse

The coefficient of friction and velocity dependence of friction of initially bare surfaces and 1-mm-thick simulated fault gouges (400 mm at 25??C and 25 MPa normal stress. Steady state negative friction velocity dependence and a steady state fault zone microstructure are achieved after ???18 mm displacement, and an approximately constant strength is reached after a few tens of millimeters of sliding on initially bare surfaces. Simulated fault gouges show a large but systematic variation of friction, velocity dependence of friction, dilatancy, and degree of localization with displacement. At short displacement (<10 mm), simulated gouge is strong, velocity strengthening and changes in sliding velocity are accompanied by relatively large changes in dilatancy rate. With continued displacement, simulated gouges become progressively weaker and less velocity strengthening, the velocity dependence of dilatancy rate decreases, and deformation becomes localized into a narrow basal shear which at its most localized is observed to be velocity weakening. With subsequent displacement, the fault restrengthens, returns to velocity strengthening, or to velocity neutral, the velocity dependence of dilatancy rate becomes larger, and deformation becomes distributed. Correlation of friction, velocity dependence of friction and of dilatancy rate, and degree of localization at all displacements in simulated gouge suggest that all quantities are interrelated. The observations do not distinguish the independent variables but suggest that the degree of localization is controlled by the fault strength, not by the friction velocity dependence. The friction velocity dependence and velocity dependence of dilatancy rate can be used as qualitative measures of the degree of localization in simulated gouge, in agreement with previous studies. Theory equating the friction velocity dependence of simulated gouge to the sum of the friction velocity dependence of bare surfaces and the velocity dependence of dilatancy rate of simulated gouge fails to quantitatively account for the experimental observations.

Beeler, N.M.; Tullis, T.E.; Blanpied, M.L.; Weeks, J.D.

1996-01-01

390

String Theory and Turbulence  

E-print Network

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.

Vishnu Jejjala; Djordje Minic; Y. Jack Ng; Chia-Hsiung Tze

2009-12-14

391

RNG and turbulence modeling  

NASA Astrophysics Data System (ADS)

The ReNormalization Group (RNG) theory involves approximations which neglect much of the complexity of turbulent flow. The Yakhot and Orszag method is explained in detail. The strengths and weaknesses of RNG turbulence models are outlined. The basic RNG theory of the forced Navier-Stokes equations and the calculation of the Kolmogorov constant are studied. The derivation of the RNG kappa epsilon equations, which should serve as a guide for the derivation of other turbulence models is examined. The asymptotic theory of infinite Reynolds number turbulence is described. Extensions of RNG models to finite Reynolds number flows are dealt with. The practical application of RNG results is given.

Smith, Leslie A.

392

40 CFR 1066.260 - Parasitic friction compensation evaluation.  

Code of Federal Regulations, 2012 CFR

... 2012-07-01 false Parasitic friction compensation evaluation. 1066.260...Specifications § 1066.260 Parasitic friction compensation evaluation. (a) Overview...verify the accuracy of the dynamometer's friction compensation. (b) Scope and...

2012-07-01

393

30 CFR 56.19014 - Friction hoist overtravel protection.  

Code of Federal Regulations, 2012 CFR

...2012-07-01 2012-07-01 false Friction hoist overtravel protection. 56.19014...Personnel Hoisting Hoists § 56.19014 Friction hoist overtravel protection. In a friction hoist installation, tapered guides or...

2012-07-01

394

30 CFR 57.19008 - Friction hoist synchronizing mechanisms.  

Code of Federal Regulations, 2014 CFR

...2014-07-01 2014-07-01 false Friction hoist synchronizing mechanisms. 57...Personnel Hoisting Hoists § 57.19008 Friction hoist synchronizing mechanisms. ...effective position of safety devices, friction hoists shall be equipped with...

2014-07-01

395

40 CFR 1066.260 - Parasitic friction compensation evaluation.  

Code of Federal Regulations, 2014 CFR

... 2014-07-01 false Parasitic friction compensation evaluation. 1066.260...Specifications § 1066.260 Parasitic friction compensation evaluation. (a) Overview...verify the accuracy of the dynamometer's friction compensation. (b) Scope and...

2014-07-01

396

30 CFR 57.19014 - Friction hoist overtravel protection.  

Code of Federal Regulations, 2012 CFR

...2012-07-01 2012-07-01 false Friction hoist overtravel protection. 57.19014...Personnel Hoisting Hoists § 57.19014 Friction hoist overtravel protection. In a friction hoist installation, tapered guides or...

2012-07-01

397

30 CFR 57.19014 - Friction hoist overtravel protection.  

Code of Federal Regulations, 2013 CFR

...2013-07-01 2013-07-01 false Friction hoist overtravel protection. 57.19014...Personnel Hoisting Hoists § 57.19014 Friction hoist overtravel protection. In a friction hoist installation, tapered guides or...

2013-07-01

398

40 CFR 1066.260 - Parasitic friction compensation evaluation.  

Code of Federal Regulations, 2013 CFR

... 2013-07-01 false Parasitic friction compensation evaluation. 1066.260...Specifications § 1066.260 Parasitic friction compensation evaluation. (a) Overview...verify the accuracy of the dynamometer's friction compensation. (b) Scope and...

2013-07-01

399

30 CFR 57.19014 - Friction hoist overtravel protection.  

Code of Federal Regulations, 2010 CFR

...2010-07-01 2010-07-01 false Friction hoist overtravel protection. 57.19014...Personnel Hoisting Hoists § 57.19014 Friction hoist overtravel protection. In a friction hoist installation, tapered guides or...

2010-07-01

400

30 CFR 56.19014 - Friction hoist overtravel protection.  

Code of Federal Regulations, 2011 CFR

...2011-07-01 2011-07-01 false Friction hoist overtravel protection. 56.19014...Personnel Hoisting Hoists § 56.19014 Friction hoist overtravel protection. In a friction hoist installation, tapered guides or...

2011-07-01

401

30 CFR 56.19008 - Friction hoist synchronizing mechanisms.  

Code of Federal Regulations, 2012 CFR

...2012-07-01 2012-07-01 false Friction hoist synchronizing mechanisms. 56...Personnel Hoisting Hoists § 56.19008 Friction hoist synchronizing mechanisms. ...effective position of safety devices, friction hoists shall be equipped with...

2012-07-01

402

30 CFR 57.19014 - Friction hoist overtravel protection.  

Code of Federal Regulations, 2011 CFR

...2011-07-01 2011-07-01 false Friction hoist overtravel protection. 57.19014...Personnel Hoisting Hoists § 57.19014 Friction hoist overtravel protection. In a friction hoist installation, tapered guides or...

2011-07-01

403

30 CFR 57.19008 - Friction hoist synchronizing mechanisms.  

Code of Federal Regulations, 2012 CFR

...2012-07-01 2012-07-01 false Friction hoist synchronizing mechanisms. 57...Personnel Hoisting Hoists § 57.19008 Friction hoist synchronizing mechanisms. ...effective position of safety devices, friction hoists shall be equipped with...

2012-07-01

404

30 CFR 57.19008 - Friction hoist synchronizing mechanisms.  

Code of Federal Regulations, 2010 CFR

...2010-07-01 2010-07-01 false Friction hoist synchronizing mechanisms. 57...Personnel Hoisting Hoists § 57.19008 Friction hoist synchronizing mechanisms. ...effective position of safety devices, friction hoists shall be equipped with...

2010-07-01

405

30 CFR 57.19008 - Friction hoist synchronizing mechanisms.  

Code of Federal Regulations, 2013 CFR

...2013-07-01 2013-07-01 false Friction hoist synchronizing mechanisms. 57...Personnel Hoisting Hoists § 57.19008 Friction hoist synchronizing mechanisms. ...effective position of safety devices, friction hoists shall be equipped with...

2013-07-01

406

30 CFR 56.19014 - Friction hoist overtravel protection.  

Code of Federal Regulations, 2014 CFR

...2014-07-01 2014-07-01 false Friction hoist overtravel protection. 56.19014...Personnel Hoisting Hoists § 56.19014 Friction hoist overtravel protection. In a friction hoist installation, tapered guides or...

2014-07-01

407

30 CFR 56.19008 - Friction hoist synchronizing mechanisms.  

Code of Federal Regulations, 2013 CFR

...2013-07-01 2013-07-01 false Friction hoist synchronizing mechanisms. 56...Personnel Hoisting Hoists § 56.19008 Friction hoist synchronizing mechanisms. ...effective position of safety devices, friction hoists shall be equipped with...

2013-07-01

408

30 CFR 57.19008 - Friction hoist synchronizing mechanisms.  

Code of Federal Regulations, 2011 CFR

...2011-07-01 2011-07-01 false Friction hoist synchronizing mechanisms. 57...Personnel Hoisting Hoists § 57.19008 Friction hoist synchronizing mechanisms. ...effective position of safety devices, friction hoists shall be equipped with...

2011-07-01

409

30 CFR 56.19014 - Friction hoist overtravel protection.  

Code of Federal Regulations, 2013 CFR

...2013-07-01 2013-07-01 false Friction hoist overtravel protection. 56.19014...Personnel Hoisting Hoists § 56.19014 Friction hoist overtravel protection. In a friction hoist installation, tapered guides or...

2013-07-01

410

30 CFR 56.19008 - Friction hoist synchronizing mechanisms.  

Code of Federal Regulations, 2010 CFR

...2010-07-01 2010-07-01 false Friction hoist synchronizing mechanisms. 56...Personnel Hoisting Hoists § 56.19008 Friction hoist synchronizing mechanisms. ...effective position of safety devices, friction hoists shall be equipped with...

2010-07-01

411

30 CFR 56.19014 - Friction hoist overtravel protection.  

Code of Federal Regulations, 2010 CFR

...2010-07-01 2010-07-01 false Friction hoist overtravel protection. 56.19014...Personnel Hoisting Hoists § 56.19014 Friction hoist overtravel protection. In a friction hoist installation, tapered guides or...

2010-07-01

412

30 CFR 57.19014 - Friction hoist overtravel protection.  

Code of Federal Regulations, 2014 CFR

...2014-07-01 2014-07-01 false Friction hoist overtravel protection. 57.19014...Personnel Hoisting Hoists § 57.19014 Friction hoist overtravel protection. In a friction hoist installation, tapered guides or...

2014-07-01

413

30 CFR 56.19008 - Friction hoist synchronizing mechanisms.  

Code of Federal Regulations, 2014 CFR

...2014-07-01 2014-07-01 false Friction hoist synchronizing mechanisms. 56...Personnel Hoisting Hoists § 56.19008 Friction hoist synchronizing mechanisms. ...effective position of safety devices, friction hoists shall be equipped with...

2014-07-01

414

30 CFR 56.19008 - Friction hoist synchronizing mechanisms.  

Code of Federal Regulations, 2011 CFR

...2011-07-01 2011-07-01 false Friction hoist synchronizing mechanisms. 56...Personnel Hoisting Hoists § 56.19008 Friction hoist synchronizing mechanisms. ...effective position of safety devices, friction hoists shall be equipped with...

2011-07-01

415

Sensible heat and friction characteristics of plate fin-and-tube heat exchangers having plane fins  

Microsoft Academic Search

In the present study, 15 samples of plate fin heat exchangers with different geometrical parameters, including the number of tube rows, fin spacing and fin thickness are tested and compared in an induced flow open wind tunnel. Results are presented in the form of friction factor and Colburn j-factor against Reynolds number based on the tube collar diameter in the

Yi-Chung Hsieh; Yur-Tsai Lin

1996-01-01

416

Effect of friction in wedging of elastic solids  

NASA Technical Reports Server (NTRS)

In this paper the contact problem for an elastic wedge of arbitrary angle is considered. It is assumed that the external load is applied to the medium through a rigid wedge and the coefficient of friction between the loading wedge and the elastic solid is constant. The problem is reduced to a singular integral equation of the second kind with the contact pressure as the unknown function. An effective numerical solution of the integral equation is described and the results of three examples are presented. The comparison of these results with those obtained from the frictionless wedge problem indicates that generally friction has the tendency of reducing the peak values of the stress intensity factors calculated at the wedge apex and at the end points of the contact area.

Erdogan, F.; Arin, K.

1976-01-01

417

Multiscale physics-based modeling of friction  

NASA Astrophysics Data System (ADS)

Frictional contacts between solids exist in nature and in a wide range of engineering applications. Friction causes energy loss, and it is the main source of wear and surface degradation which limits the lifetime of mechanical systems. Yet, friction is needed to walk, run, accelerate, slow down or stop moving systems. Whether desirable or not, friction is a very complex physical phenomenon. The behavior of systems with friction is nonlinear, and the physical mechanisms governing friction behavior span a wide range of spatial and temporal scales. A thorough study of friction should employ experimentalists and theoreticians in chemistry, materials science, tribology, mechanics, dynamics, and structural engineering. High spatial and temporal resolutions are required to capture and model essential physics of a frictional contact. However, such a detailed model is impractical in large-scale structural dynamics simulations; especially since frictional contacts can be numerous in a given application. Reduced-order models (ROMs) achieve broader applicability by compromising several aspects and accounting for the important physics. Hence, rather simple Coulomb friction is still the most ubiquitous model in the modeling and simulation literature. As an alternative, a reduced-order friction model built-up from micromechanics of surfaces is proposed in this work. Continuum-scale formulation of pre-sliding friction behavior is combined with material-strength-based friction coefficients to develop a physics-based friction model at asperity-scale. Then, the statistical summation technique is utilized to build a multiscale modeling framework. A novel joint fretting setup is designed for friction experiments in a practical setting, and the developed models are tested. Both asperity and rough surface friction models show good agreement with experimental data. The influences of materials, surface roughness and contact contamination on the friction are also studied. Finally, the developed models are incorporated in to a simple dynamical system to illustrate broader applicability. The models proposed in this work account for loading-history dependence, partial slip, gross slip, nonlinear stiffness and energy dissipation characteristics of frictional contacts. In doing so, the models require no curve-fit or look-up parameters. Instead, the formulations are developed from continuum mechanics, and the required parameters can be determined from simple tension/compression and surface roughness tests. In this sense, the developed models are physics-based and predictive. The parameters employed in the models depend on the contact conditions, surface roughness and material properties. For instance, the developed models use a dry-contact formulation at asperity-scale, and the macroscale friction predictions are tested only for dry contacts. However, the multiscale modeling approach can be applied to model the effect of lubrication provided that asperity-scale contact accounts for it. Besides, frictional contacts of certain materials exhibit severe adhesion, cold welding and galling behavior as demonstrated experimentally in this work. If these behaviors are modeled at asperity-scale, then the friction at rough contact scale can be obtained by the same procedure presented in this work. This flexibility is another significant advantage of the proposed modeling approach.

Eriten, Melih

418

Evaluation of CFD Turbulent Heating Prediction Techniques and Comparison With Hypersonic Experimental Data  

NASA Technical Reports Server (NTRS)

Results from a study to assess the accuracy of turbulent heating and skin friction prediction techniques for hypersonic applications are presented. The study uses the original and a modified Baldwin-Lomax turbulence model with a space marching code. Grid converged turbulent predictions using the wall damping formulation (original model) and local damping formulation (modified model) are compared with experimental data for several flat plates. The wall damping and local damping results are similar for hot wall conditions, but differ significantly for cold walls, i.e., T(sub w) / T(sub t) < 0.3, with the wall damping heating and skin friction 10-30% above the local damping results. Furthermore, the local damping predictions have reasonable or good agreement with the experimental heating data for all cases. The impact of the two formulations on the van Driest damping function and the turbulent eddy viscosity distribution for a cold wall case indicate the importance of including temperature gradient effects. Grid requirements for accurate turbulent heating predictions are also studied. These results indicate that a cell Reynolds number of 1 is required for grid converged heating predictions, but coarser grids with a y(sup +) less than 2 are adequate for design of hypersonic vehicles. Based on the results of this study, it is recommended that the local damping formulation be used with the Baldwin-Lomax and Cebeci-Smith turbulence models in design and analysis of Hyper-X and future hypersonic vehicles.

Dilley, Arthur D.; McClinton, Charles R. (Technical Monitor)

2001-01-01

419

Laminar, turbulent, and inertial shear-thickening regimes in channel flow of neutrally buoyant particle suspensions.  

PubMed

The aim of this Letter is to characterize the flow regimes of suspensions of finite-size rigid particles in a viscous fluid at finite inertia. We explore the system behavior as a function of the particle volume fraction and the Reynolds number (the ratio of flow and particle inertia to viscous forces). Unlike single-phase flows, where a clear distinction exists between the laminar and the turbulent states, three different regimes can be identified in the presence of a particulate phase, with smooth transitions between them. At low volume fractions, the flow becomes turbulent when increasing the Reynolds number, transitioning from the laminar regime dominated by viscous forces to the turbulent regime characterized by enhanced momentum transport by turbulent eddies. At larger volume fractions, we identify a new regime characterized by an even larger increase of the wall friction. The wall friction increases with the Reynolds number (inertial effects) while the turbulent transport is weakly affected, as in a state of intense inertial shear thickening. This state may prevent the transition to a fully turbulent regime at arbitrary high speed of the flow. PMID:25554885

Lashgari, Iman; Picano, Francesco; Breugem, Wim-Paul; Brandt, Luca

2014-12-19

420

Laminar, Turbulent, and Inertial Shear-Thickening Regimes in Channel Flow of Neutrally Buoyant Particle Suspensions  

NASA Astrophysics Data System (ADS)

The aim of this Letter is to characterize the flow regimes of suspensions of finite-size rigid particles in a viscous fluid at finite inertia. We explore the system behavior as a function of the particle volume fraction and the Reynolds number (the ratio of flow and particle inertia to viscous forces). Unlike single-phase flows, where a clear distinction exists between the laminar and the turbulent states, three different regimes can be identified in the presence of a particulate phase, with smooth transitions between them. At low volume fractions, the flow becomes turbulent when increasing the Reynolds number, transitioning from the laminar regime dominated by viscous forces to the turbulent regime characterized by enhanced momentum transport by turbulent eddies. At larger volume fractions, we identify a new regime characterized by an even larger increase of the wall friction. The wall friction increases with the Reynolds number (inertial effects) while the turbulent transport is weakly affected, as in a state of intense inertial shear thickening. This state may prevent the transition to a fully turbulent regime at arbitrary high speed of the flow.

Lashgari, Iman; Picano, Francesco; Breugem, Wim-Paul; Brandt, Luca

2014-12-01

421

Comparison of Frictional Heating Models  

SciTech Connect

The purpose of this work was to compare the predicted temperature rises using four well-known models for frictional heating under a few selected conditions in which similar variable inputs are provided to each model. Classic papers by Archard, Kuhlmann-Wilsdorf, Lim and Ashby, and Rabinowicz have been examined, and a spreadsheet (Excel ) was developed to facilitate the calculations. This report may be used in conjunction with that spreadsheet. It explains the background, assumptions, and rationale used for the calculations. Calculated flash temperatures for selected material combinations, under a range of applied loads and sliding speeds, are tabulated. The materials include AISI 52100 bearing steel, CDA 932 bronze, NBD 200 silicon nitride, Ti-6Al-4V alloy, and carbon-graphite material. Due to the assumptions made by the different models, and the direct way in which certain assumed quantities, like heat sink distances or asperity dimensions, enter into the calculations, frictional hearing results may differ significantly; however, they can be similar in certain cases in light of certain assumptions that are shared between the models.

Davies, Nicholas R [ORNL; Blau, Peter Julian [ORNL

2013-10-01

422

Rolling friction of adhesive microspheres  

NASA Astrophysics Data System (ADS)

The rolling friction of adhesive microspheres is an important quantity as it determines the strength and stability of larger aggregates. Current models predict rolling forces that are 1 to 2 orders of magnitude smaller than observed experimentally. Starting from the well-known Johnson-Kendall-Roberts (JKR) contact description, we derive an analytical theory for the rolling friction based on the concept of adhesion hysteresis, e.g. a difference in apparent surface energies for opening/closing cracks. We show how adhesion hysteresis causes the pressure distribution within the contact to become asymmetrical, leading to an opposing torque. Analytical expressions are derived relating the size of the hysteresis, the rolling torque, and the rolling displacement, ?. We confirm the existence of a critical rolling displacement for the onset of rolling, the size of which is set by the amount of adhesion hysteresis and the size of the contact area. We demonstrate how the developed theory is able to explain the large rolling forces and particle-size dependence observed experimentally. Good agreement with experimental results is achieved for adhesion hysteresis values of (??/?) ? 3 for polystyrene, and (??/?) ? 0.5 for silicates, at crack propagation rates of 0.1 µm s-1 and 1-10 µm s-1, respectively.

Krijt, S.; Dominik, C.; Tielens, A. G. G. M.

2014-04-01

423

Friction-Maintained Dynamic Stability  

NASA Astrophysics Data System (ADS)

Mechanical systems relying on Coulomb friction to maintain dynamic stability may suffer a dynamic instability if exposed to an initial displacement exceeding a system-specific threshold. In fluid systems, even small values of negative damping are sufficient to drive the dynamic instability with sufficiently large initial displacement. The Tainter gate failures at the Folsom dam in 1995 and at the Wachi dam in 1967 are two well-known failures. To aid in preventing a recurrence, the authors engaged in a decade long research program that provided evidence that both gates failed due to an essential dynamic instability mechanism that all Tainter-gates may possess. This paper presents measurements suggesting "friction-maintained dynamic stability" of a full-scale 50-ton Tainter gate. Accompanying gate model studies showed that the gate can fail when exposed to an initial displacement exceeding a threshold value. The present study should serve to alert gate designers, owners and operators that many Tainter gates which have not yet failed may, nonetheless, have a high susceptibility to failure if and when they are exposed to a sudden input of energy resulting in an initial displacement exceeding the gate-specific threshold displacement.

Anami, Keiko; Ishii, Noriaki; Knisely, Charles W.; Tsuji, Takuma; Oku, Tatsuya; Sato, Shigeki

424

High Reynolds number analysis of flat plate and separated afterbody flow using non-linear turbulence models  

NASA Technical Reports Server (NTRS)

The ability of the three-dimensional Navier-Stokes method, PAB3D, to simulate the effect of Reynolds number variation using non-linear explicit algebraic Reynolds stress turbulence modeling was assessed. Subsonic flat plate boundary-layer flow parameters such as normalized velocity distributions, local and average skin friction, and shape factor were compared with DNS calculations and classical theory at various local Reynolds numbers up to 180 million. Additionally, surface pressure coefficient distributions and integrated drag predictions on an axisymmetric nozzle afterbody were compared with experimental data from 10 to 130 million Reynolds number. The high Reynolds data was obtained from the NASA Langley 0.3m Transonic Cryogenic Tunnel. There was generally good agreement of surface static pressure coefficients between the CFD and measurement. The change in pressure coefficient distributions with varying Reynolds number was similar to the experimental data trends, though slightly over-predicting the effect. The computational sensitivity of viscous modeling and turbulence modeling are shown. Integrated afterbody pressure drag was typically slightly lower than the experimental data. The change in afterbody pressure drag with Reynolds number was small both experimentally and computationally, even though the shape of the distribution was somewhat modified with Reynolds number.

Carlson, John R.

1996-01-01

425

Fossil turbulence and fossil turbulence waves can be dangerous  

E-print Network

Turbulence is defined as an eddy-like state of fluid motion where the inertial-vortex forces of the eddies are larger than any other forces that tend to damp the eddies out. By this definition, turbulence always cascades from small scales where vorticity is created to larger scales where turbulence fossilizes. Fossil turbulence is any perturbation in a hydrophysical field produced by turbulence that persists after the fluid is no longer turbulent at the scale of the perturbation. Fossil turbulence patterns and fossil turbulence waves preserve and propagate energy and information about previous turbulence. Ignorance of fossil turbulence properties can be dangerous. Examples include the Osama bin Laden helicopter crash and the Air France 447 Airbus crash, both unfairly blamed on the pilots. Observations support the proposed definitions, and suggest even direct numerical simulations of turbulence require caution.

Carl H Gibson

2012-11-25

426

Rolling Friction on a Wheeled Laboratory Cart  

ERIC Educational Resources Information Center

A simple model is developed that predicts the coefficient of rolling friction for an undriven laboratory cart on a track that is approximately independent of the mass loaded onto the cart and of the angle of inclination of the track. The model includes both deformation of the wheels/track and frictional torque at the axles/bearings. The concept of…

Mungan, Carl E.

2012-01-01

427

Gimbaled-shoulder friction stir welding tool  

NASA Technical Reports Server (NTRS)

A gimbaled-shoulder friction stir welding tool includes a pin and first and second annular shoulders coupled to the pin. At least one of the annular shoulders is coupled to the pin for gimbaled motion with respect thereto as the tool is rotated by a friction stir welding apparatus.

Carter, Robert W. (Inventor); Lawless, Kirby G. (Inventor)

2010-01-01

428

Friction Coefficient for Quarks in Supergravity Duals  

E-print Network

We study quarks moving in strongly-coupled plasmas that have supergravity duals. We compute the friction coefficient of strings dual to such quarks for general static supergravity backgrounds near the horizon. Our results also show that a previous conjecture on the bound has to be modified and higher friction coefficients can be achieved.

E. Antonyan

2006-11-22

429

Slip instability and state variable friction laws  

Microsoft Academic Search

The dependence of the friction on slip history is described by an experimentally motivated constitutive law where the friction is dependent on slip rate and state variables. The state variables are defined macroscopically by evolution equations for their rates of change in terms of their present values and slip rate. Experiments may strongly suggest that one state variables is adequate

Andy Ruina

1983-01-01

430

Wiping Metal Transfer in Friction Stir Welding  

NASA Technical Reports Server (NTRS)

Much evidence suggests that as the friction stir pin-tool moves along a weld seam the displacement of metal takes place by a wiping action at the surface of a plug of metal that rotates with the tool. The wiping model is explained and some consequences for the friction stir welding process are drawn.

Nunes, Arthur C., Jr.; Whitaker, Ann F. (Technical Monitor)

2001-01-01

431

The Nature of the Coefficient of Friction  

Microsoft Academic Search

The modern theory of the friction between dry metal surfaces ascribes it to local minute welds or adhesions between the surfaces and suggests that for a given pair of surfaces the friction force is uniquely defined by the normal load alone. Herein it is demonstrated that this cannot in general be true and that some further condition of operation must

J. T. Burwell; E. Rabinowicz

1953-01-01

432

Friction and wear of synthetic diamond  

Microsoft Academic Search

The friction and wear properties of a number of commercially available super hard tool materials have been studied over a range of temperatures. Friction and wear data were collected for these materials sliding one on another, and in self-sliding combinations. The sliding configuration was a pin on flat arrangement and all the measurements were made in vacuum. Generally, the slider

E. J Brookes; P Greenwood; G Xing

1999-01-01

433

Friction Compensation in Robotics: an Overview  

Microsoft Academic Search

Friction effects are particularly critical for industrial robots, since they can induce large positioning errors, stick-slip motions, and limit cycles. This paper offers a reasoned overview of the main friction compensation techniques that have been developed in the last years, regrouping them according to the adopted kind of control strategy. Some experimental results are reported, to show how the control

Basilio Bona; Marina Indri

2005-01-01

434

Single State ElastoPlastic Friction Models  

Microsoft Academic Search

For control applications involving small displace- ments and velocities, friction modeling and compensation can be very important. In particular, the modeling of presliding displacement (motion prior to fully developed slip) can play a pivotal role. In this paper, it is shown that existing single-state friction models exhibit a nonphysical drift phenomenon which results from modeling presliding as a combination of

Pierre Dupont; Vincent Hayward; Brian Armstrong; Friedhelm Altpeter

2002-01-01

435

Frictional Characteristics of Quasicrystals at High Temperatures  

Microsoft Academic Search

AlCuFe quasicrystal coatings were deposited by unbalanced magnetron sputtering for study of their friction and wear properties at high temperatures. It has been shown that growth and annealing conditions can be controlled to produce icosahedral quasicrystal or the approximant cubic phase. The comparison of friction and wear properties between quasicrystal and an approximant with nearly the same stoichiometry permits assessment

B. S. Phillips; J. S. Zabinski

2003-01-01

436

Novel Friction Law for the Static Friction Force based on Local Precursor Slipping  

PubMed Central

The sliding of a solid object on a solid substrate requires a shear force that is larger than the maximum static friction force. It is commonly believed that the maximum static friction force is proportional to the loading force and does not depend on the apparent contact area. The ratio of the maximum static friction force to the loading force is called the static friction coefficient µM, which is considered to be a constant. Here, we conduct experiments demonstrating that the static friction force of a slider on a substrate follows a novel friction law under certain conditions. The magnitude of µM decreases as the loading force increases or as the apparent contact area decreases. This behavior is caused by the slip of local precursors before the onset of bulk sliding and is consistent with recent theory. The results of this study will develop novel methods for static friction control. PMID:25205283

Katano, Yu; Nakano, Ken; Otsuki, Michio; Matsukawa, Hiroshi

2014-01-01

437

Novel Friction Law for the Static Friction Force based on Local Precursor Slipping  

E-print Network

The sliding of a solid object on a solid substrate requires a shear force that is larger than the maximum static friction force. It is commonly believed that the maximum static friction force is proportional to the loading force and does not depend on the apparent contact area. The ratio of the maximum static friction force to the loading force is called the static friction coefficient mM, which is considered to be a constant. Here, we conduct experiments demonstrating that the static friction force of a slider on a substrate follows a novel friction law under certain conditions. The magnitude of mM decreases as the loading force increases or as the apparent contact area decreases. This behavior is caused by the slip of local precursors before the onset of bulk sliding and is consistent with recent theory. The results of this study will develop novel methods for static friction control.

Yu Katano; Ken Nakano; Michio Otsuki; Hiroshi Matsukawa

2014-09-11

438

Novel friction law for the static friction force based on local precursor slipping.  

PubMed

The sliding of a solid object on a solid substrate requires a shear force that is larger than the maximum static friction force. It is commonly believed that the maximum static friction force is proportional to the loading force and does not depend on the apparent contact area. The ratio of the maximum static friction force to the loading force is called the static friction coefficient µM, which is considered to be a constant. Here, we conduct experiments demonstrating that the static friction force of a slider on a substrate follows a novel friction law under certain conditions. The magnitude of µM decreases as the loading force increases or as the apparent contact area decreases. This behavior is caused by the slip of local precursors before the onset of bulk sliding and is consistent with recent theory. The results of this study will develop novel methods for static friction control. PMID:25205283

Katano, Yu; Nakano, Ken; Otsuki, Michio; Matsukawa, Hiroshi

2014-01-01

439

Friction forces on phase transition fronts  

SciTech Connect

In cosmological first-order phase transitions, the microscopic interaction of the phase transition fronts with non-equilibrium plasma particles manifests itself macroscopically as friction forces. In general, it is a nontrivial problem to compute these forces, and only two limits have been studied, namely, that of very slow walls and, more recently, ultra-relativistic walls which run away. In this paper we consider ultra-relativistic velocities and show that stationary solutions still exist when the parameters allow the existence of runaway walls. Hence, we discuss the necessary and sufficient conditions for the fronts to actually run away. We also propose a phenomenological model for the friction, which interpolates between the non-relativistic and ultra-relativistic values. Thus, the friction depends on two friction coefficients which can be calculated for specific models. We then study the velocity of phase transition fronts as a function of the friction parameters, the thermodynamic parameters, and the amount of supercooling.

Mégevand, Ariel, E-mail: megevand@mdp.edu.ar [IFIMAR (CONICET–UNMdP), Departamento de Física, Facultad de Ciencias Exactas y Naturales, UNMdP, Deán Funes 3350, (7600) Mar del Plata (Argentina)

2013-07-01

440

Frictional properties of jointed welded tuff  

SciTech Connect

The results of the experiments on simulated joints in welded tuff from the Grouse Canyon Member of the Belted Range Tuff warrant the following conclusions: (1) The coefficient of friction of the joints is independent of normal stress at a given sliding velocity. (2) The coefficient of friction increases with both increasing time of stationary contact and decreasing sliding velocity. (3) Time and velocity dependence of friction is due to an increase in the real area of contact on the sliding surface, caused by asperity creep. (4) Joints in water-saturated tuff show a greater time and velocity dependence of friction than those in dehydrated tuff. (5) The enhanced time and velocity dependence of friction with water saturation is a result of increased creep at asperity contacts, which is in turn due to a reduction in the surface indentation hardness by hydrolytic weakening and/or stress corrosion cracking.

Teufel, L.W.

1981-07-01

441

Frictional microscopy of polymers and nanocomposites  

NASA Astrophysics Data System (ADS)

The mechanical and frictional properties of polystyrene, polymethylmethacrylate and nanocomposites with montmorillonite were studied by using the microindentation technique and frictional microscopy. The micromechanical tests revealed a decrease in the modulus and microhardness of the composite compared with those of a neat polystyrene, with a minimum of their values at 1-3 wt.% of the filler, but a local maximum of the tensile modulus of the filled polymer arose and increased at the same filler concentration. The frictional microscopy revealed anisotropy of the friction coefficient of the nanocomposite and to its noticeable dependence on the content of the filler. The maximum value of the friction coefficient was also reached at 1-3 wt.% of the filler and corresponds to the greatest degree of interplanar distance in the layered silicate and to minimum microhardness and elastic modulus of the composite surface.

Kotomin, S. V.; Ezhov, A. A.; Sollogoub, C.; Yarikov, D.

2014-05-01

442

Numerical simulation of compressible, turbulent, two-phase flow  

NASA Technical Reports Server (NTRS)

A computer program for numerically simulating compressible, turbulent, two-phase flows is described and applied. Special attention is given to flows in which dust is ingested into the turbulent boundary layer behind shock waves moving over the earth's surface. it is assumed that the two phases are interpenetrating continua which are coupled by drag forces and heat transfer. The particle phase is assumed to be dilute, and turbulent effects are modeled by zero- and two-equation eddy viscosity models. An important feature of the turbulence modeling is the treatment of surface boundary conditions which control the ingestion of particles into the boundary layer by turbulent friction and diffusion. The numerical method uses second-order implicit upwind differencing of the inviscid terms of the equations and second-order central differencing of the viscous terms. A diagonal form of the implicit algorithm is used to improve efficiency, and the transformation to a curvilinear coordinate system is accomplished by the finite volume techniques. Applications to a series of representative flows include a two-phase nozzle flow, the steady flow of air over a sand bed, and the air flow behind a normal shock wave in uniform motion over a sand bed. Results of the latter two applications are compared with experimental results.

Coakley, t. J.; Champney, J. M.

1985-01-01

443

Turbulent boundary layer over porous surfaces with different surface geometries  

NASA Technical Reports Server (NTRS)

The turbulent boundary layer over three porous walls with different surface geometries was studied in order to investigate the individual influences of porosity and small roughness, as well as their combined effects, on turbulent boundary layer behavior. The tests were conducted in a 2 m x 2 m tunnel on a large axisymmetric model at speeds corresponding to Re(L) = 5,000,000-6,000,000. The development of the turbulent boundary layer was compared for that of sintered metal, bonded screening, and perforated sheet and then to that for the flow over a solid smooth wall and a solid, sand-roughened wall. The comparisons reveal that the effect of porosity is to shift the logarithmic region of the wall law down by a certain amount from the solid wall results and to increase the skin friction values by about 30-40%. The downward shift of the logarithmic region of the wall law and the increase of the skin friction value by the combined effects of small roughness and porosity are found to be roughly the sum of their individual effects.

Kong, F. Y.; Schetz, J. A.

1982-01-01

444

Atmospheric Turbulence (METR 6103)  

E-print Network

: A First Course in Turbulence, The MIT Press, 300 pp; Garratt, J. R., 1992: The Atmospheric Boundary Layer in the atmospheric boundary and surface layers. Similarity and scaling techniques. Monin-Obukhov similarity theory of turbulence closures. Large eddy simulation of atmospheric boundary layer flows. Note: Any student

Fedorovich, Evgeni

445

Measurements of Turbulence Attenuation by a Dilute Dispersion of Solid Particles in Homogeneous Isotropic Turbulence  

NASA Technical Reports Server (NTRS)

This research addresses turbulent gas flows laden with fine solid particles at sufficiently large mass loading that strong two-way coupling occurs. By two-way coupling we mean that the particle motion is governed largely by the flow, while the particles affect the gas-phase mean flow and the turbulence properties. Our main interest is in understanding how the particles affect the turbulence. Computational techniques have been developed which can accurately predict flows carrying particles that are much smaller than the smallest scales of turbulence. Also, advanced computational techniques and burgeoning computer resources make it feasible to fully resolve very large particles moving through turbulent flows. However, flows with particle diameters of the same order as the Kolmogorov scale of the turbulence are notoriously difficult to predict. Some simple flows show strong turbulence attenuation with reductions in the turbulent kinetic energy by up to a factor of five. On the other hand, some seemingly similar flows show almost no modification. No model has been proposed that allows prediction of when the strong attenuation will occur. Unfortunately, many technological and natural two-phase flows fall into this regime, so there is a strong need for new physical understanding and modeling capability. Our objective is to study the simplest possible turbulent particle-laden flow, namely homogeneous, isotropic turbulence with a uniform dispersion of monodisperse particles. We chose such a simple flow for two reasons. First, the simplicity allows us to probe the interaction in more detail and offers analytical simplicity in interpreting the results. Secondly, this flow can be addressed by numerical simulation, and many research groups are already working on calculating the flow. Our detailed data can help guide some of these efforts. By using microgravity, we can further simplify the flow to the case of no mean velocity for either the turbulence or the particles. In fact 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.

Eaton, John; Hwang, Wontae; Cabral, Patrick

2002-01-01

446

Turbulent Spot Formation  

NASA Astrophysics Data System (ADS)

One of the most interesting pattern forming processes occurs when separated spatial regions of turbulent and laminar flows (turbulent spots) form in shear flows. Examples include the spots in boundary layer flows, pipe flow slugs, and Spiral Turbulence in circular Couette flow. This process was studied in detail in a plane Couette flow apparatus where spots can easily be created through a subcritical (or metastable) transition and can easily be observed in the laboratory frame. The results from this study showed pervasive vortex structures with their rotation axes aligned in the direction of the wall velocity. An interpretation of the visualization evidence from this experiment is presented. This interpretation suggests that the stress on the laminar flow from the turbulence induces vortex stretching and vortex tilting in the laminar flow that result in the before mentioned pervasive vortex structures. These vortex structures and their complex interactions constitute the smaller scale turbulence.

Hegseth, John

1998-03-01

447

THE GEOPHYSICAL TURBULENCE PROGRAM AND THE TURBULENCE NUMERICS TEAM  

E-print Network

THE GEOPHYSICAL TURBULENCE PROGRAM AND THE TURBULENCE NUMERICS TEAM Annick Pouquet, September 22, 2006 1 Introduction The Geophysical Turbulence Program (GTP) and the Turbulence Numerics Team (TNT) are both housed in IMAGe. GTP has been in existence almost since the origin of NCAR and has presently more

Pouquet, Annick

448

RESEARCH ARTICLE Evolution of the turbulent/non-turbulent interface  

E-print Network

. The evolution of the rate of entrainment also confirms the creation and growth of the jump in the meanRESEARCH ARTICLE Evolution of the turbulent/non-turbulent interface of an axisymmetric turbulent; and Anand and Bo- ersma in Exp Fluids 47:995­1007, 2009). The evolution of the turbulence characteristics

Marusic, Ivan

449

Turbulence Decay and Cloud Core Relaxation in Molecular Clouds  

NASA Astrophysics Data System (ADS)

The turbulent motion within molecular clouds is a key factor controlling star formation. Turbulence supports molecular cloud cores from evolving to gravitational collapse and hence sets a lower bound on the size of molecular cloud cores in which star formation can occur. On the other hand, without a continuous external energy source maintaining the turbulence, such as in molecular clouds, the turbulence decays with an energy dissipation time comparable to the dynamic timescale of clouds, which could change the size limits obtained from Jean's criterion by assuming constant turbulence intensities. Here we adopt scaling relations of physical variables in decaying turbulence to analyze its specific effects on the formation of stars. We find that the decay of turbulence provides an additional approach for Jeans' criterion to be achieved, after which gravitational infall governs the motion of the cloud core. This epoch of turbulence decay is defined as cloud core relaxation. The existence of cloud core relaxation provides a more complete understanding of the effect of the competition between turbulence and gravity on the dynamics of molecular cloud cores and star formation.

Gao, Yang; Xu, Haitao; Law, Chung K.

2015-02-01

450

Dynamical Friction on extended perturbers  

E-print Network

Following a wave-mechanical treatment we calculate the drag force exerted by an infinite homogeneous background of stars on a perturber as this makes its way through the system. We recover Chandrasekhar's classical dynamical friction (DF) law with a modified Coulomb logarithm. We take into account a range of models that encompasses all plausible density distributions for satellite galaxies by considering the DF exerted on a Plummer sphere and a perturber having a Hernquist profile. It is shown that the shape of the perturber affects only the exact form of the Coulomb logarithm. The latter converges on small scales, because encounters of the test and field stars with impact parameters less than the size of the massive perturber become inefficient. We confirm this way earlier results based on the impulse approximation of small angle scatterings.

O. Esquivel; B. Fuchs

2008-04-01

451

Macrostructure of Friction Stir Welds  

NASA Technical Reports Server (NTRS)

This paper will discuss two of the well know large scale features of friction stir welds: the "onion rings" seen in transverse sections, and the striations on the surface of the work piece. It will be shown that the surface features (sometimes called "tool marks") are the result of irregularities on the rotating shoulder of the pin tool and disappear when the shoulder is polished. The "onion ring" structure seen in transverse cross sections is formed by parts of the "carousel", the zone of material adjacent to and rotating with the pin tool, that are shed off in each rotation. The relation between the carousel and the "ring vortex", a rotational flow extending both in and out of the carousel and resembling a smoke-ring with the hole centered on the pin tool, will be discussed.

Aloor, S.; Nowak, B.; Vargas, R.; McClure, J. C.; Murr, L. E.; Nunes, A. C.; Munafo, Paul M. (Technical Monitor)

2002-01-01

452

Computation of turbulent flows over backward and forward-facing steps using a near-wall Reynolds stress model  

NASA Technical Reports Server (NTRS)

Separation and reattachment of turbulent shear layers is observed in many important engineering applications, yet it is poorly understood. This has motivated many studies on understanding and predicting the processes of separation and reattachment of turbulent shear layers. Both of the situations in which separation is induced by adverse pressure gradient, or by discontinuities of geometry, have attracted attention of turbulence model developers. Formulation of turbulence closure models to describe the essential features of separated turbulent flows accurately is still a formidable task. Computations of separated flows associated with sharp-edged bluff bodies are described. For the past two decades, the backward-facing step flow, the simplest separated flow, has been a popular test case for turbulence models. Detailed studies on the performance of many turbulence models, including two equation turbulence models and Reynolds stress models, for flows over steps can be found in the papers by Thangam & Speziale and Lasher & Taulbee). These studies indicate that almost all the existing turbulence models fail to accurately predict many important features of back step flow such as reattachment length, recovery rate of the redeveloping boundary layers downstream of the reattachment point, streamlines near the reattachment point, and the skin friction coefficient. The main objectives are to calculate flows over backward and forward-facing steps using the NRSM and to make use of the newest DNS data for detailed comparison. This will give insights for possible improvements of the turbulence model.

Ko, Sung HO

1993-01-01

453

Quantum turbulence in superfluids with wall-clamped normal component.  

PubMed

In Fermi superfluids, such as superfluid (3)He, the viscous normal component can be considered to be stationary with respect to the container. The normal component interacts with the superfluid component via mutual friction, which damps the motion of quantized vortex lines and eventually couples the superfluid component to the container. With decreasing temperature and mutual friction, the internal dynamics of the superfluid component becomes more important compared with the damping and coupling effects from the normal component. As a result profound changes in superfluid dynamics are observed: the temperature-dependent transition from laminar to turbulent vortex motion and the decoupling from the reference frame of the container at even lower temperatures. PMID:24704879

Eltsov, Vladimir; Hänninen, Risto; Krusius, Matti

2014-03-25

454

Comparison of friction produced by two types of orthodontic bracket protectors  

PubMed Central

Introduction Fixed orthodontic appliances have been regarded as a common causative factor of oral lesions. To manage soft tissue discomfort, most orthodontists recommend using a small amount of utility wax over the brackets in order to alleviate trauma. This in vitro study aimed at evaluating friction generated by two types of bracket protectors (customized acetate protector [CAP] and temporary resin protector [TRP]) during the initial stages of orthodontic treatment. Methods An experimental model (test unit) was used to assess friction. In order to measure the friction produced in each test, the model was attached to a mechanical testing machine which simulated maxillary canines alignment. Intergroup comparison was carried out by one-way ANOVA with level of significance set at 5%. Results The friction presented by the TRP group was statistically higher than that of the control group at 6 mm. It was also higher than in the control and CAP groups in terms of maximum friction. Conclusion The customized acetate protector (CAP) demonstrated not to interfere in friction between the wire and the orthodontic bracket slot. PMID:24713564

Mendonça, Steyner de Lima; Praxedes Neto, Otávio José; de Oliveira, Patricia Teixeira; dos Santos, Patricia Bittencourt Dutra; Pinheiro, Fábio Henrique de Sá Leitão

2014-01-01

455

Friction of Compression-ignition Engines  

NASA Technical Reports Server (NTRS)

The cost in mean effective pressure of generating air flow in the combustion chambers of single-cylinder compression-ignition engines was determined for the prechamber and the displaced-piston types of combustion chamber. For each type a wide range of air-flow quantities, speeds, and boost pressures was investigated. Supplementary tests were made to determine the effect of lubricating-oil temperature, cooling-water temperature, and compression ratio on the friction mean effective pressure of the single-cylinder test engine. Friction curves are included for two 9-cylinder, radial, compression-ignition aircraft engines. The results indicate that generating the optimum forced air flow increased the motoring losses approximately 5 pounds per square inch mean effective pressure regardless of chamber type or engine speed. With a given type of chamber, the rate of increase in friction mean effective pressure with engine speed is independent of the air-flow speed. The effect of boost pressure on the friction cannot be predicted because the friction was decreased, unchanged, or increased depending on the combustion-chamber type and design details. High compression ratio accounts for approximately 5 pounds per square inch mean effective pressure of the friction of these single-cylinder compression-ignition engines. The single-cylinder test engines used in this investigation had a much higher friction mean effective pressure than conventional aircraft engines or than the 9-cylinder, radial, compression-ignition engines tested so that performance should be compared on an indicated basis.

Moore, Charles S; Collins, John H , Jr

1936-01-01

456

Experimental Investigation of Average Heat-Transfer and Friction Coefficients for Air Flowing in Circular Tubes Having Square-Thread-Type Roughness  

NASA Technical Reports Server (NTRS)

An investigation of forced-convection heat transfer and associated pressure drops was conducted with air flowing through electrically heated Inconel tubes having various degrees of square-thread-type roughness, an inside diameter of 1/2 inch, and a length of 24 inches. were obtained for tubes having conventional roughness ratios (height of thread/radius of tube) of 0 (smooth tube), 0.016, 0.025, and 0.037 over ranges of bulk Reynolds numbers up to 350,000, average inside-tube-wall temperatures up to 1950deg R, and heat-flux densities up to 115,000 Btu per hour per square foot. Data The experimental data showed that both heat transfer and friction increased with increase in surface roughness, becoming more pronounced with increase in Reynolds number; for a given roughness, both heat transfer and friction were also influenced by the tube wall-to-bulk temperature ratio. Good correlation of the heat-transfer data for all the tubes investigated was obtained by use of a modification of the conventional Nusselt correlation parameters wherein the mass velocity in the Reynolds number was replaced by the product of air density evaluated at the average film temperature and the so-called friction velocity; in addition, the physical properties of air were evaluated at the average film temperature. The isothermal friction data for the rough tubes, when plotted in the conventional manner, resulted in curves similar to those obtained by other investigators; that is, the curve for a given roughness breaks away from the Blasius line (representing turbulent flow in smooth tubes) at some value of Reynolds number, which decreases with increase in surface roughness, and then becomes a horizontal line (friction coefficient independent of Reynolds number). A comparison of the friction data for the rough tubes used herein indicated that the conventional roughness ratio is not an adequate measure of relative roughness for tubes having a square-thread-type element. The present data, as well as those of other investigators, were used to isolate the influence of ratios of thread height to width, thread spacing to width, and the conventional roughness ratio on the friction coefficient. A fair correlation of the friction data was obtained for each tube with heat addition when the friction coefficient and Reynolds number were defined on the basis of film properties; however, the data for each tube retained the curve characteristic of that particular roughness. The friction data for all the rough tubes could be represented by a single line for the complete turbulence region by incorporating a roughness parameter in the film correlation. No correlation was obtained for the region of incomplete turbulence.

Sams, E. W.

1952-01-01

457

Turbulent boundary layer over solid and porous surfaces with small roughness  

NASA Technical Reports Server (NTRS)

The turbulent boundary layer over a smooth, solid wall is examined along with a sandpaper-roughened, solid wall, and a porous wall. Suitable results are obtained with the smooth, solid wall for the Law of the Wall, the Defect Law, and the axial and turbulence intensities. The logarithmic portion of the Wall Law is shifted in the sandpaper-roughened, solid wall and an increase in the normal turbulence intensity and Reynolds stress is observed. An increase in the local skin friction values and all the turbulence values is found with the porous wall. The influence of a slightly rough, porous wall of sintered metal on the boundary layer is compared with a solid sandpaper-roughened wall in the same nominal K(+) range.

Kong, F. Y.; Schetz, J. A.

1981-01-01

458

Counterflow quantum turbulence in a square channel under the normal fluid with a Poiseuille flow  

NASA Astrophysics Data System (ADS)

We perform a numerical analysis of superfluid turbulence produced by thermal counterflow in He II by using the vortex filament model. Counterflow in a low aspect ratio channel is known to show the transition from laminar flow to the two turbulent states TI and TII. The present understanding is that the TI has the turbulent superfluid and the laminar normal fluid but both fluids are turbulent in the TII state. This work studies the vortex tangle in the TI state. Solid boundary condition is applied to walls of a square channel, and the velocity field of the normal fluid is prescribed to be a laminar Poiseuille profile. An inhomogeneous vortex tangle, which concentrates near the solid boundaries, is obtained as the statistically steady state. It is sustained by its characteristic space-time oscillation. The inhomogeneity of the vortex tangle shows the characteristic dependence on temperature, which is caused by two effects, namely the profile of the counterflow velocity and the mutual friction.

Yui, Satoshi; Tsubota, Makoto

2014-12-01

459

Induced velocities of grains embedded in a turbulent gas. [test particle theory application to protostellar clouds  

NASA Technical Reports Server (NTRS)

A theory is presented for the dynamics of dust particles in an incompressible turbulent fluid. Grain-gas coupling occurs through friction forces that are proportional to the mean grain velocity relative to the gas. This test particle theory is applied to the case of Kolmogoroff spectrum in a protostellar cloud. The mean turbulence induced grain velocity and the mean turbulent relative velocity of two grains are calculated. Whereas the former should determine the dust scale height, grain-grain collisions are influenced by the latter. For a reasonable strength of turbulence, the mean induced relative velocity of two particles turns out to be at least as large as the corresponding terminal velocity difference during gravitational settling.

Voelk, H. J.; Morfill, G.; Roeser, S.; Jones, F. C.

1978-01-01

460

Turbulent heat transfer in a channel flow at transitional Reynolds numbers  

E-print Network

Direct numerical simulation of a turbulent channel flow with heat transfer was performed at very low Reynolds numbers. Two different thermal boundary conditions were studied, and temperature was considered as a passive scalar. The computations were carried out with huge computational boxes (up to 327.7 x 2 x 128 in the streamwise, wall-normal, and spanwise directions, respectively). The emphases of this paper are to investigate the large-scale structure (puff) in the intermittent-turbulent flow including the scalar fields and to provide the values of the transitional and critical Reynolds numbers, below which the turbulent flow becomes intermittent and laminar, respectively. The statistics, such as the skin friction and the Stanton number, were also examined: they suggest that the puff should be effective in sustaining turbulence and in heat transfer enhancement.

Tsukahara, Takahiro

2014-01-01