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1

New explicit correlations for turbulent flow friction factor  

Microsoft Academic Search

Two new correlations of single-phase friction factor for turbulent pipe flow are shown in this paper. These two formulas are actually explicit approximations of iterative Colebrook's relation for calculation of flow friction factor. Calculated friction factors are valid for whole turbulent flow including hydraulically smooth and rough pipes with special attention on transient zone of turbulence between them. Hydraulically smooth

Dejan Brki?

2011-01-01

2

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

NASA Astrophysics Data System (ADS)

We show that the main results of the analysis of the friction factor for turbulent pipe flow reported by Gioia and Chakraborty [Phys. Rev. Lett. 96, 044502 (2006)] can be recovered by assuming the Heisenberg closure hypothesis for the turbulent spectrum. This highlights the structural features of the turbulent spectrum underlying the analysis of Gioia and Chakraborty.

Calzetta, Esteban

2009-05-01

3

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

2009-01-02

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

2008-08-11

5

A turbulent approach to unsteady friction  

Microsoft Academic Search

This paper discusses the existing approaches to unsteady friction and presents some new ideas on the modelling of unsteady friction in turbulent pipe flows. This paper aims to contribute to a physically-based unsteady friction model that captures the known physical characteristics of friction phenomena in transient turbulent pipe flows. For practical applicability, a new model should not require extensive computer

Ivo Pothof

2008-01-01

6

Transient, turbulent, smooth pipe friction  

Microsoft Academic Search

Two of the most promising analytical models of unsteady friction in turbulent pipe flows are based on sharply contrasting hypotheses. One uses the history of the flow; the other uses instantaneous conditions. The purposes of this paper are to present an analysis using the former approach and to indicate how to determine which of the two methods is appropriate.A weighting

Alan E. Vardy; Jim M. B. Brown

1995-01-01

7

SEARCH. Single-Phase, Turbulent Heat-Transfer Friction-Factor Data Base Flow Enhanced Tb  

Microsoft Academic Search

Heat-exchanger designers need to know what type of performance improvement can be obtained before they will consider enhanced tubes. In particular, they need access to the heat-transfer coefficients and friction-factor values of enhanced tube types that are commercially available. To compile these data from the numerous publications and reports in the open literature is a formidable task that can discourage

T. S. Ravigurguran; T. J. Rabas

1993-01-01

8

Quick identification of gases for enhancing heat transfer in turbulent pipe flows using standard correlation equations for the convective coefficient and the friction factor  

Microsoft Academic Search

This paper addresses a simple algebraic procedure for the quick identification of gases (different than air) that are capable of enhancing heat transfer in turbulent pipe flows while causing small-to-moderate accretions in pressure drops. Relying on standard correlation equations for the convective coefficient and the friction factor, the algebraic procedure is centered on the existing competition between the four intervening

Antonio Campo

2005-01-01

9

Turbulent friction in a boundary layer of compressible gas  

NASA Astrophysics Data System (ADS)

The main processes of turbulent momentum and heat transfer in the developed flow of a compressible gas are considered. Results are presented on the dependence of the relative friction coefficient on the temperature factor and on the Mach number. An analysis is made of the theoretical and experimental distributions of air velocity in the turbulent boundary layer on a cylinder. Calculations using the proposed turbulent-viscosity model agree well with experimental data.

Dobrocheev, O. V.; Motulevich, V. P.

1987-10-01

10

Heat Transfer Through Turbulent Friction Layers  

NASA Technical Reports Server (NTRS)

The "general Prandtl number" Pr(exp 1) - A(sub q)/A Pr, aside from the Reynolds number determines the ratio of turbulent to molecular heat transfer, and the temperature distribution in turbulent friction layers. A(sub q) = exchange coefficient for heat; A = exchange coefficient for momentum transfer. A formula is derived from the equation defining the general Prandtl number which describes the temperature as a function of the velocity. For fully developed thermal boundary layers all questions relating to heat transfer to and from incompressible fluids can be treated in a simple manner if the ratio of the turbulent shear stress to the total stress T(sub t)/T in the layers near the wall is known, and if the A(sub q)/A can be regarded as independent of the distance from the wall. The velocity distribution across a flat smooth channel and deep into the laminar sublayer was measured for isothermal flow to establish the shear stress ratio T(sub t)/T and to extend the universal wall friction law. The values of T(sub t)/T which resulted from these measurements can be approximately represented by a linear function of the velocity in the laminar-turbulent transition zone. The effect of the temperature relationship of the material values on the flow near the wall is briefly analyzed. It was found that the velocity at the laminar boundary (in contrast to the thickness of the laminar layer) is approximately independent of the temperature distribution. The temperature gradient at the wall and the distribution of temperature and heat flow in the turbulent friction layers were calculated on the basis of the data under two equations. The derived formulas and the figures reveal the effects of the Prandtl number, the Reynolds number, the exchange quantities and the temperature relationship of the material values.

Reichardt, H.

1943-01-01

11

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

12

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

13

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

14

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

15

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

16

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

17

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

18

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

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 in a reduction of the friction drag with a maximum reduction rate of 8%. The primary mechanism for drag reduction attention as potential passive mechanisms to reduce turbulent friction drag. For laminar

Tokyo, University of

20

Reconnection dynamics and normal fluid mutual friction in superfluid turbulence  

E-print Network

We investigate the forcing of the normal fluid via mutual friction in finite temperature superfluid turbulence in helium-4, paying particular attention to the role of quantized vortex reconnections. Through the use of the vortex filament model, we produce three experimentally relevant vortex tangles each with different topological properties in steady state conditions. Subsequently we investigate, through statistical analysis, how the mutual friction force upon the normal fluid is affected by the characteristic of each of the vortex tangles. Finally, by monitoring the vortex reconnection events, we show how reconnections produce areas of relatively high curvature and superfluid velocity leading to regions of high normal fluid mutual friction, particularly for the homogeneous and isotropic tangles.

Laurie, Jason

2014-01-01

21

Compliant wall-turbulent skin-friction reduction research  

NASA Technical Reports Server (NTRS)

Previous compliant-wall experiments successful in reducing skin-friction drag probably have had a (unplanned) membrane resonance at a favorable frequency, amplitude, wave shape, length, and speed. The most probable drag reduction mechanism involves a direct coupling between the fluid and the moving wall when the wall natural resonance frequencies are near the fundamental turbulent burst frequency. Local skin-friction reductions of 61% were measured with mylar/PVC plastisol compliant surfaces. These reductions were observed only at certain flow conditions, indicating that changing tunnel total temperature may have altered the substrate dynamic modulus, damping, and coupled mylar tension. Apparently, the coupled membrane/substrate must be excited in compatible narrow-band natural frequency modes. An accelerated effort is required to develop practical durable compliant surfaces optimized for maximum drag reduction. Application of compliant walls to other transportation modes appears feasible with liquid flows offering the greatest skin-friction drag reduction potential.

Fischer, M. C.; Weinstein, L. M.; Bushnell, D. M.; Ash, R. L.

1975-01-01

22

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

23

Influence of turbulent pr number on friction and heat transfer at a plate immersed in a turbulent gas stream  

Microsoft Academic Search

A solution is offered to the problem of determining the friction and heat transfer coefficients for a plate immersed in a turbulent gas stream, using the approximate dependence of heat content on velocity given in reference [1]. The influence of PrT on friction and heat transfer is evaluated.

I. P. Ginzburg; I. V. Korneva

1965-01-01

24

A phenomenological model to describe turbulent friction in permeable-wall flows  

NASA Astrophysics Data System (ADS)

Describing the canonical properties of turbulent flows over rough-permeable walls such as gravel beds, vegetated- or snow-covered surfaces have, to date, resisted complete theoretical treatment. The major complication in describing such geophysical flows is that the friction factor - Reynolds number relationships significantly deviate from their conventional Nikuradse curves or Moody diagrams derived over impermeable rough boundaries. A novel phenomenological model that describes such anomalous behavior is proposed. It expands the approach in Gioia and Chakraborty (2006) developed for rough-impermeable pipes to include finite velocity effects within the porous wall and canonical length scales governing the momentum exchanges between interstitial and superficial flows.

Manes, C.; Ridolfi, L.; Katul, G.

2012-07-01

25

FRICTION BLISTERS: PATHOPHYSIOLOGY, RISK FACTORS, AND PREVENTION  

Microsoft Academic Search

Friction blisters are one of the most common injuries an active individual can experience. They are usually a minor annoyance, generally requiring only simple first aid and a short period of limited activity. However, it is possible for blisters to develop into more serious problems such as cellulitis or sepsis (1). This article reviews the pathogenesis of blisters, examines factors

Joseph J. Knapik

26

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

27

Analysis of turbulent skin friction generated in flow along a cylinder  

Microsoft Academic Search

This paper presents an extension of FIK identity [K. Fukagata et al., Phys. Fluids 14, L73 (2002)] to turbulent axial flow along a cylinder. This relation gives the contributions of both the mean flow and the turbulent fluctuating flow to the skin friction coefficient. The later contribution is then further decomposed more precisely as proposed by B. Frohnapfel, Y. Hasegawa,

Stephane Monte; Pierre Sagaut; Thomas Gomez

2011-01-01

28

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

29

Experimental investigations of the unsteady turbulent friction behavior with pressure impulses in a circular pipe  

Microsoft Academic Search

Transient pressure changes are studied to analyze unsteady wall tension with turbulent nonperiodic flow changes and for comparison with oscillating flow parameters in a 80 m long, 0.0092 m internal diameter pipe filled with water. The unsteady friction impact is reflected in the deformation of the pressure wave. The weighting function depends on the transition time and on the maximum

H. Arlt

1983-01-01

30

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

31

Flow friction of the turbulent coolant flow in cryogenic porous cables  

NASA Technical Reports Server (NTRS)

Considered are cryogenic power transmission cables with porous cores. Calculations of the turbulent coolant flow with injection or suction through the porous wall are presented within the framework of a two-layer model. Universal velocity profiles were obtained for the viscous sublayer and flow core. Integrating the velocity profile, the law of flow friction in the pipe with injection has been derived for the case when there is a tangential injection velocity component. The effect of tangential velocity on the relative law of flow friction is analyzed. The applicability of the Prandtl model to the problem under study is discussed. It is shown that the error due to the acceptance of the model increases with the injection parameter and at lower Reynolds numbers; under these circumstances, the influence of convective terms in the turbulent energy equation on the mechanism of turbulent transport should be taken into account.

Hendricks, R. C.; Yeroshenko, V. M.; Zaichik, L. I.; Yanovsky, L. S.

1979-01-01

32

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

33

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

2005-07-08

34

A probe for measuring skin friction in disturbed turbulent boundary layers  

NASA Astrophysics Data System (ADS)

In order to alleviate difficulties encountered in obtaining accurate measurement and detailed surveys of skin friction in turbulent boundary layers, a new probe has been developed, similar to a Stanton tube in design. The probe incorporates a hinge system that allows it to be lifted and replaced repeatedly at different locations for flat plate surveys. Tests were performed to measure probe sensitivity to length of delay and sampling times, angular displacement, surface roughness and leading edge geometry, to provide guidelines for assuring maximum possible accuracy and repeatability. The probe was calibrated against a Preston tube in an undisturbed turbulent boundary layer and performed successfully in streamwise skin friction surveys downstream of drag reduction ribbons.

Fulcher, Karen L.; Haritonidis, Joseph H.

1992-01-01

35

A skin friction model for axisymmetric turbulent boundary layers along long thin circular cylinders  

NASA Astrophysics Data System (ADS)

Only a few engineering design models are presently available that adequately depict the axisymmetric skin friction (Cf) maturity along long thin turbulent cylinders. This deficit rests essentially on the experimental and numerical difficulties of measuring (or computing) the spatial evolution of the thin cylinder turbulence. Consequently, the present axisymmetric Cf models have questionable accuracy. Herein, we attempt to formulate a more robust Cf model that owns acceptable error. The formulation is founded on triple integration of the governing equation system that represents a thin cylinder turbulent boundary layer (TBL) at statistical steady-state in appropriate dimensionless units. The final model requires only the radius-based Reynolds number (Rea) and transverse curvature (?/a) as input parameters. We tuned the accompanying coefficients empirically via an expanded statistical database (over 60 data points) that house new Cf values from large-eddy simulations (LES). The LES computations employed a turbulence inflow generation procedure that permits spatial resolution of the TBL at low-high Reynolds numbers and transverse curvatures. Compared to the new skin friction database, the Cf model revealed averaged predictive errors under 5% with a 3.5% standard deviation. Apart from owning higher values than the flat plate TBL, the most distinguishing characteristic of the axisymmetric skin friction is its rising levels when the boundary layer thickness exceeds the cylinder radius. All Cf levels diminish with increasing Reynolds number. These unique features differentiate the axisymmetric TBL along thin cylinders as a separate canonical flow when compared to the turbulent wall shear-layers of channels, pipes, and planar-type geometries.

Jordan, Stephen A.

2013-07-01

36

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

NASA Astrophysics Data System (ADS)

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.

1994-06-01

37

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

E-print Network

) smooth wall to bulk temperature difference in fully developed region x rw in a, cha. nnel, K average air velocity, m/sec streamivise coordinate, m expansion coefficient, equation (2) rib angle-of-attack, degrees orifice diameter to pipe diameter... REDUCTION PROGRAM, PRESSURE DROP 70 81 C HEAT TRANSFER RAW DATA . . 85 D HEAT TRANSFER RESULTS 106 E PRESSURE DROP RAW DATA AND FRICTION RESULTS . . 112 VITA 118 LIST OF TABLES Table Pa, ge 1 Description of Test Run Cases . . 2 Coeffrcients...

McMillin, Robert Dale

2012-06-07

38

Statistically Steady Turbulence in Soap Films: Direct Numerical Simulations with Ekman Friction  

E-print Network

We present a detailed direct numerical simulation (DNS) designed to investigate the combined effects of walls and Ekman friction on turbulence in forced soap films. We concentrate on the forward-cascade regime and show how to extract the isotropic parts of velocity and vorticity structure functions and thence the ratios of multiscaling exponents. We find that velocity structure functions display simple scaling whereas their vorticity counterparts show multiscaling; and the probability distribution function of the Weiss parameter $\\Lambda$, which distinguishes between regions with centers and saddles, is in quantitative agreement with experiments.

Prasad Perlekar; Rahul Pandit

2008-11-09

39

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

40

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

41

Friction factor for isothermal and nonisothermal flow through porous media  

NASA Technical Reports Server (NTRS)

Measurements were performed to determine the pressure drops for gaseous flow through porous materials of different microstructures, porosities, and thickness under isothermal and nonisothermal conditions at various temperature levels. Results were satisfactorily correlated by a simple equation relating the friction factor to the Reynolds number and porosities.

Koh, J. C.; Dutton, J. L.; Benson, B. A.; Fortini, A.

1977-01-01

42

Friction law and turbulent properties in a laboratory Ekman boundary layer  

NASA Astrophysics Data System (ADS)

We use spin-up/spin-down laboratory experiments to study the neutrally stratified Ekman boundary layer. The experiments are performed in the 13 m diameter, 1 m deep Coriolis rotating tank of the LEGI in Grenoble, France. A global flow rotation is produced by an initial change in the tank rotation speed. It then slowly decays under the effect of Ekman friction, evolving from the turbulent state to the laminar state. It is checked that the Ekman layer itself remains in a quasi-steady state during this decay. The velocity is measured by Particle Imaging Velocimetry (PIV) at two scales: the global rotation in a horizontal plane, and the vertical profile inside the boundary layer, where the three velocity components are obtained by stereoscopic PIV. The friction law is obtained by relating the decay rate of the bulk velocity to the velocity itself. This method is justified by the fact that this bulk velocity is independent of height beyond the top of the boundary layer (a few cm), as expected from the Taylor-Proudman theorem for rotating fluids. The local measurements inside the boundary layer provide profiles of the mean velocity and Reynolds stress components, in particular the cross-isobar angle between the interior and near surface velocities. In the laminar regime, good agreement is obtained with the classical Ekman's theory, which validates the method. In the turbulent regime, the results are found consistent with the classical Atmospheric Boundary Layer (ABL) model based on the von Karman logarithmic layer. Our experiments therefore indicate that this theory, in principle valid for very large Reynolds numbers, is already relevant close to the transitional regimes. A fit of the empirical coefficients A and B appearing in this theory yields A = 3.3 and B = 3.0. Extrapolating the results to the atmospheric case gives a friction velocity u* about 12% higher than the traditional fit for the ABL. We may safely deduce that for the oceanic bottom boundary layer, corresponding to lower Reynolds numbers than the atmosphere, our result provides a correct estimate within 10%. The previous laboratory results of Caldwell et al. ["A laboratory study of the turbulent Ekman layer," Geophys. Fluid Dyn. 3, 125-160 (1972), 10.1080/03091927208236078] provided frictions velocities about 20% higher than in our experiments, and slightly higher cross-isobar angles. We attribute this difference to the higher vortical Rossby number Rot in those experiments, and maybe also to roughness effects. We take into account the effect of this vortical Rossby number within the framework of the Ekman layer (Rot ? 0) by replacing the tank rotation rate by the fluid rotation rate.

Sous, D.; Sommeria, J.; Boyer, D.

2013-04-01

43

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

44

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

45

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.

Dominguez, Kathleen C.

2010-03-17

46

Correlation of turbulence factor and hot wire turbulence measurements in subsonic flows of air  

E-print Network

CORRELATION OF TURBULENCE FACTOR AND HOT WIRE TURBULENCE MEASUREMENTS IN SUBSONIC FLOWS OF AIR by CURTIS SINCIAIR WELISp JR. A Thesis Submitted to the Graduate School oi' the Agricultural and Mechanical College of Texas In Partial... Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE August, 1959 Department of Aeronautical Engineering Smgor Sub/est: Aeronautical Engineering CORRELATION OF TURBULENCE FACTOR AND HOT WIRE TURBULENCE NEASUREMENI'S IN SUBSONIC FLOWS...

Wells, Curtis Sinclair

2012-06-07

47

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

48

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.

49

Influence of velocity gradient and wall proximity on the readings of a pitot tube in measurement of surface friction and velocity distribution in a turbulent boundary layer  

Microsoft Academic Search

The authors present results of an experimental investigation of the correlations and readings of a Pitot tube in measuring velocity in a turbulent boundary layer. A new calibration relation is proposed for measuring surface friction.

E. U. Repik; V. K. Kuzenkov; N. P. Mikhailova

1985-01-01

50

Influence of velocity gradient and wall proximity on the readings of a pitot tube in measurement of surface friction and velocity distribution in a turbulent boundary layer  

NASA Astrophysics Data System (ADS)

The authors present results of an experimental investigation of the correlations and readings of a Pitot tube in measuring velocity in a turbulent boundary layer. A new calibration relation is proposed for measuring surface friction.

Repik, E. U.; Kuzenkov, V. K.; Mikhailova, N. P.

1985-06-01

51

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

52

Friction Factor for Flow in Rectangular Ducts with One Side Rib-Roughened  

NASA Technical Reports Server (NTRS)

Numerical simulations of incompressible turbulent flow through rectangular ducts with one side rib-roughened were performed to determine pressure drop. The "PHOENICS " software package was used for the computations, which required provision of a wall function for transverse rib-roughened surfaces. The present study was conducted in the range of 10(exp 5) less than or equal to Reynolds number less than or equal to 10(exp 7), 0.01 less than or equal to rib height to hydraulic diameter ratio less than or equal to 0.04, 10 less than or equal to pitch to rib height ratio less than or equal to 40. Using the numerical results, friction factor charts for various aspect ratios were generated. The numerical results agreed well with experimental data that was obtained for 10(exp 5) less than Reynolds less than 2 x 10(exp 5). In addition, a scheme for predicting friction factor using existing correlations for smooth and rough walls was developed.

Youn, B.; Yuen, C.; Mills, A. F.

1994-01-01

53

Friction factor data for flat plate tests of smooth and honeycomb surfaces  

E-print Network

And Honeycomb Surfaces. (May 1989) Tae Woong Ha, B.S., Han Yang University in Korea Chair of Advisory Committee: Dr. Dara Childs Friction factors for honeycomb surfaces are measured with a flat plate tester. The flat plate test apparatus is described and a... method is discussed for determining the friction factor experimentally. The friction factor model is developed for the flat plate test based on the Fanno Line Flow. The comparisons of the friction factor are plotted for smooth surface and six-honeycomb...

Ha, Tae Woong

2012-06-07

54

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

55

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

56

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

NASA Astrophysics Data System (ADS)

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.

1993-07-01

57

Mach and Reynolds number effects on turbulent skin friction reduction by injection  

NASA Technical Reports Server (NTRS)

The investigation reported is concerned with questions regarding a possible Mach number influence on skin friction reduction caused by injection. The investigation shows that data considered by Danberg (1967) for the no-blowing skin friction coefficient are in error. Accurate profiles and local skin friction coefficient values are obtained when the influence of low Reynolds number amplification in the outer region of the boundary layer is included in a calculation method.

Bushnell, D. M.; Watson, R. D.; Holley, B. B.

1975-01-01

58

Heat transfer and friction in a square channel with one-wall or two-wall rib turbulators  

E-print Network

of Commit tee) D. Rhode (Member) . A. Hassan (Member) I W. Bradley (Head of Department) December 1991 ABSTRACT Heat Transfer and Friction in a Square Channel with One-Wall or Two-Wall Rib Turbulators. (December 1991) Jie Joy Huang, B. S...-shaped broken ribs on two walls with q, /q, =- oo ( ) ~ Re-80528. 3 D 4 Re=49646. 0 WHITE: SMOOTH SIDES + Re=30539. 9 ' V Re=20591. 2 BLACK: ROUGH SIDES ?) 4 Re=i4Tr8. 0 e 3 z Z k 0 j 0 2 4 6 8 10 12 14 16 18 20 X/D II Yrgurr I I Nuaatll nn...

Huang, Jie Joy

2012-06-07

59

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

E-print Network

.2. Preliminary 3-D modeling of several roughened seal experiments ?.?.. 3.3. Friction-factor-to-clearance behavior predictions, isogrid pattern of Iwatsubo [41] ???????????????..... 3.4 Friction-factor-to-clearance behavior predictions, honeycomb... ???.?????????????????????????.... 235 VITA ????????.??????????????????????..? 239 ix LIST OF FIGURES FIGURE Page 1 Honeycomb-stator seal with a smooth-rotor [1] ????????????... 2 2 Numerical friction factors in 2-D knurl geometries ???????????.. 13 3 Numerical friction...

Villasmil Urdaneta, Larry Alfonso

2006-10-30

60

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

Microsoft Academic Search

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

R. Wahidi; W. Chakroun; S. Al-Fahed

2005-01-01

61

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

E-print Network

Bulk flow theory has been widely used to estimate annular seals dynamic coefficients. To predict the flow behavior through the seal, this theory relies on empirical friction factor correlations based on pipe data. Several experiments have gathered...

Villasmil Urdaneta, Larry Alfonso

2012-06-07

62

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

63

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

64

Calculation of skin-friction coefficients for low Reynolds number turbulent boundary layer flows. M.S. Thesis - California Univ. at Davis  

NASA Technical Reports Server (NTRS)

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 method formulated from the law of the wall. These expressions are found to predict values that are consistently different than those obtained from the graphical method over the range 600 Re/sub theta 2000. A curve-fitted empirical relationship was developed from the present data and yields a better estimated value of C/sub f/ in this range. The data, covering the range 200 Re/sub theta 7000, provide insight into the nature of transitional flows. They show that fully developed turbulent boundary layers occur at Reynolds numbers Re/sub theta/ down to 425. Below this level there appears to be a well-ordered evolutionary process from the laminar to the turbulent profiles. These profiles clearly display the development of the turbulent core region and the shrinking of the laminar sublayer with increasing values of Re/sub theta/.

Barr, P. K.

1980-01-01

65

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

66

FRICTION FACTOR IN HIGH PRESSURE NATURAL GAS PIPELINES FROM ROUGHNESS MEASUREMENTS  

E-print Network

of the relative roughness and dividing it by the slope of the power spectrum of the measured roughness, a greatly number. The reported studies have focused on flow over smooth surfaces. In industrial applications-White friction factor correlation [3] predicts that minute irregularities (in the order of 1 µm) on the pipe wall

Gudmundsson, Jon Steinar

67

NON-NEWTONIAN FLUID FLOW IN DUCTS: FRICTION FACTOR AND LOSS COEFICIENTS  

Microsoft Academic Search

The knowledge of the head loss in the flow of non-Newtonian fluids is very important for the execution of pipelines and pumping systems designs, common in plants of almost all kinds of industries. The determination of the total head loss involves the establishment of the friction factor corresponding to pressure drop in the straight section and the loss coefficients caused

Adelson Belizário Leal; Luis Américo Calçada; Cláudia Mirim Scheid

68

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

69

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

70

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

71

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 ...

Benson, Carrie

2011-12-06

72

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

73

What factors does friction depend on? A socio-cognitive teaching intervention with young children  

NASA Astrophysics Data System (ADS)

The objective of this study was to investigate the effect of a socio-cognitive teaching strategy on young children. It tests their understanding of the factors that friction depends on when an object is projected across a horizontal surface. The study was conducted in three phases: pre-test, teaching intervention, and post-test. The sample consisted of 68 preschool children who were assigned to two groups according to age and cognitive ability, based on their responses to a pre-test. The children in the experimental group participated in activities that were approached from a socio-cognitive perspective while the children in the control group participated in the same activities but from a Piagetian perspective. A statistically significant difference was found (Mann-Whitney U-test), between the pre-test and the post-test, providing evidence for the effect of the socio-cognitive strategy on children's understanding of a 'precursor model' for the concept of friction.

Ravanis, Konstantinos; Koliopoulos, Dimitris; Hadzigeorgiou, Yannis

2004-08-01

74

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

75

Laminar heat transfer and friction factor characteristics of carbon nano tube/water nanofluids.  

PubMed

This paper presents an experimental investigation on the convective heat transfer and friction factor characteristics of CNT/water nanofluid through a circular tube fitted with helical screw tape inserts with constant heat flux under laminar flow condition. Nanofluids of 0.1% and 0.2% volume fractions are prepared by two step method. Thermo-physical properties like thermal conductivity and viscosity are measured by using KD2 thermal property analyzer and Brooke field cone and plate viscometer respectively. From the measurements, it is found that the viscosity increase is substantially higher than the increase in the thermal conductivity. The helical screw tape insets with twist ratios Y = 3, 2.44 and 1.78 are used to study the convective heat transfer and friction factor characteristics under laminar flow in the Reynolds number range of 520-2500. It is observed that, in a plain tube, maximum enhancement in Nusselt number for 0.1% and 0.2% volume fractions of nanofluids compared to pure water is 15% and 32% respectively. With the use of inserts, maximum enhancement in Nusselt number corresponding to twist ratios of 1.78, 2.44 and 3 are obtained as 8%, 16% and 4.6% for 0.1% volume fraction of nanofluid and 5%, 4% and 12% for 0.2% volume fraction of nanofluid when compared with water in plain tube. Thermal performance factor evaluation revealed that the values at all Reynolds number for all twist ratios and both concentration of CNT nanofluid are greater than unity which indicates that helical screw tape inserts with twist ratios considered are feasible in terms of energy saving in laminar flow. PMID:24745238

Rathnakumar, P; Mayilsamy, K; Suresh, S; Murugesan, P

2014-03-01

76

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

77

Experimental determination of solids friction factors and minimum volumetric requirements in air or gas drilling  

SciTech Connect

There are so many variables involved in an air or gas drilling situation that one general correlation has not been developed to predict appropriate volumetric requirements in the field. The results of previous attempts to calculate minimum volumetric requirement are valid over limited ranges of drilling conditions. This research has developed new empirical correlations for solids friction factors for sandstone, limestone, and shale. A new and more realistic technique for predicting minimum volumetric requirements is presented. This method includes a complete analysis of pressures throughout the system. It also offers flexibility in the selection of standpipe pressure and air flow rate, and thus, compressor specifications. The method is primarily graphical so that it can be used at the well site without access to a computer. Results are presented in tabular and graphical forms. These results confirm that existing methods may predict too low or too high a volumetric flow rate.

Machado V, C.J.; Ikoku, C.U.

1980-11-01

78

Experimental determination of solids friction factor and minimum volumetric requirements in air and gas drilling  

SciTech Connect

This research has developed new empirical correlations for solids friction factor for sandstone, limestone, and shale. A new and more realistic technique for predicting minimum volumetric requirements is presented. This method includes a complete analysis of pressures throughout the system. It also offers flexibility in the selection of standpipe pressure and air flow rate and, thus, compressor specification. The method is primarily graphical so that it can be used at the well site without access to a computer. Results are presented in tabular and graphical forms. These results confirm that existing methods may predict too low or too high a volumetric flow rate. This work is pertinent to both oil and natural gas well drilling. 30 refs.

Machado, C.J.; Ikoku, C.U.

1981-01-01

79

Turbulence  

Microsoft Academic Search

This chapter addresses the small-scale, random motions (i.e., turbulence) in clouds. Because the smallest scales of motion in clouds are many orders of magnitude smaller than the characteristic length scale of clouds, then the governing equations must be filtered (i.e., averaged) to make simulation possible. The Reynolds averaging approach is examined first, and several closure models for this system are

William R. Cotton; George Bryan; Susan C. van den Heever

2011-01-01

80

Turbulent heat transfer and friction in a segmental channel that simulates leading-edge cooling channels of modern turbine blades  

E-print Network

cooling channels are modeled as straight segmental channels with 90' rib turbulators on the curved wall only. The ribs are square in cross section and have a height-to-hydraulic diameter ratio of 0.0625. The rib array has a pitch-to-height ratio of 10...

Spence, Rodney Brian

2012-06-07

81

Effects of unsteady free-stream velocity and free-stream turbulence at a stagnation point  

NASA Technical Reports Server (NTRS)

The combined effects of transient free stream velocity and turbulence at a stagnation point on a cylinder situated in a crossflow is investigated analytically, and a model is formulated for the eddy diffusivity induced by free-stream turbulence. The steepest descent method is used to integrate the governing momentum expression, and numerical solutions are given for the unsteady wall shear stress function for specific free-stream transients. It is found after correlation of the results by means of a new turbulence parameter that wall friction increases with increasing free-stream turbulence intensity, and that the friction factor increases with increasing reduced frequency of oscillation values.

Gorla, R. S. R.

1982-01-01

82

Modeling of unsteady friction and viscoelastic damping in piping systems  

NASA Astrophysics Data System (ADS)

In real systems, the phenomena, such as pipe-wall viscoelasticity, unsteady friction or fluid structure interaction induce additional damping and dispersion of transient pressure waves than that defined by classical waterhammer. In this paper, unsteady friction models and viscoelastic damping models will be presented and a theoretical formulation of the viscoelastic damping in piping systems without cavitation will be developed. Firstly, the friction factor will be presented as the sum of the quasi-steady part and the unsteady part related to the instantaneous local acceleration and instantaneous convective acceleration. This unsteady friction model has been incorporated into the method of characteristic algorithm (MOC). Secondly, the damping will be defined in terms of viscoelastic effect attributed to a second viscosity ?'. This model is solved using the Finite Difference Method. Finally, numerical results from the unsteady friction and viscoelastic models are compared with results of laboratory measurements for waterhammer cases with low Reynolds number turbulent flows. This comparison validates the new viscoelastic model.

Landry, C.; Nicolet, C.; Bergant, A.; Müller, A.; Avellan, F.

2012-11-01

83

Propagation factors of cosine-Gaussian-correlated Schell-model beams in non-Kolmogorov turbulence.  

PubMed

Based on the extended Huygens-Fresnel principle and second-order moments of the Wigner distribution function (WDF), we have studied the relative root-mean-square (rms) angular width and the propagation factor of cosine-Gaussian-correlated Schell-model (CGSM) beams propagating in non-Kolmogorov turbulence. It has been found that the CGSM beam has advantage over the Gaussian Schell-model (GSM) beam for reducing the turbulence-induced degradation, and this advantage will be more obvious for the beams with larger parameter n and spatial coherence ? or under the condition of stronger fluctuation of turbulence. The CGSM beam with larger parameter n or smaller spatial coherence ? will be less affected by the turbulence. In addition, the effects of the slope-parameter ?, inner and outer scale and the refractive-index structure constant of the non-Kolmogorov's power spectrum on the propagation factor are also analyzed in detailed. PMID:25321718

Xu, Hua-Feng; Zhang, Zhou; Qu, Jun; Huang, Wei

2014-09-22

84

Edge gradient and safety factor effects on electrostatic turbulent transport in tokamaks  

SciTech Connect

Electrostatic turbulence and transport measurements are performed on the Tokapole-II tokamak at the University of Wisconsin-Madison, as the safety-factor and the edge equilibrium gradients and varied substantially. Tokapole-II is a poloidal divertor tokamak capable of operating at a wide range of safety factors due to its unique magnetic limiter configuration. It also has retractable material limiters in a large scrape-off region, which permits the study of edge boundary conditions like density and temperature gradients. The turbulence is independent of safety factor, but strongly sensitive to the local density gradient, which itself depends upon the limiter configuration. When a material limiter is inserted in a high discharge, the density gradient is increased locally together with a local increase of the turbulence. On the other hand, limiter insertion in low discharges did not increase the density gradient as much and the turbulence properties are unchanged with respect to the magnetic limiter case. It is conducted then, that electrostatic turbulence is caused by the density gradient. Although the electrostatic fluctuation driven transport is enhanced in the large density gradient case, it is in all cases to small to explain the observed energy confinement times. To explore instabilities with small wavelengths, a 0.5 mm diameter shperical Langmuir probe was constructed, and its power compared with the power measured by larger cylindrical probes.

Tan, Ing Hwie

1992-05-01

85

An entrance region friction factor model applied to annular seal analysis - Theory versus experiment for smooth and honeycomb seals  

NASA Technical Reports Server (NTRS)

A friction factor model is developed for the entrance-region of a duct. The model is used in an annular gas seal analysis similar to Nelson's (1984). Predictions of the analysis are compared to experimental results for a smooth-stator/smooth-rotor seal and three honeycomb-stator/smooth-rotor seals. The model predicts a leakage and direct damping well. The model overpredicts the dependence of cross-coupled stiffness on fluid prerotation. The model predicts direct stiffness poorly.

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

1989-01-01

86

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

87

Flow and Heat Transfer Characteristics of Turbulent Gas Flow in Microtube with Constant Heat Flux  

NASA Astrophysics Data System (ADS)

Local friction factors for turbulent gas flows in circular microtubes with constant wall heat flux were obtained numerically. The numerical methodology is based on arbitrary-Lagrangian-Eulerian method to solve two-dimensional compressible momentum and energy equations. The Lam-Bremhorst's Low-Reynolds number turbulence model was employed to calculate eddy viscosity coefficient and turbulence energy. The simulations were performed for a wide flow range of Reynolds numbers and Mach numbers with different constant wall heat fluxes. The stagnation pressure was chosen in such a way that the outlet Mach number ranged from 0.07 to 1.0. Both Darcy friction factor and Fanning friction factor were locally obtained. The result shows that the obtained both friction factors were evaluated as a function of Reynolds number on the Moody chart. The values of Darcy friction factor differ from Blasius correlation due to the compressibility effects but the values of Fanning friction factor almost coincide with Blasius correlation. The wall heat flux varied from 100 to 10000 W/m2. The wall and bulk temperatures with positive heat flux are compared with those of incompressible flow. The result shows that the Nusselt number of turbulent gas flow is different from that of incompressible flow.

Hong, Chungpyo; Asako, Yutaka; Matsushita, Shinichi; Ueno, Ichiro

2012-05-01

88

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, an...

Gallavotti, Giovanni

2014-01-01

89

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

2014-04-26

90

Influence of pressure gradient on streamwise skewness factor in turbulent boundary layer  

NASA Astrophysics Data System (ADS)

The paper shows an effect of favourable and adverse pressure gradients on turbulent boundary layer. The skewness factor of streamwise velocity component was chosen as a measure of the pressure gradient impact. It appears that skewness factor is an indicator of convection velocity of coherent structures, which is not always equal to the average flow velocity. The analysis has been performed based upon velocity profiles measured with hot-wire technique in turbulent boundary layer with pressure gradient corresponding to turbomachinery conditions. The results show that the skewness factor decreases in the flow region subjected to FPG and increases in the APG conditions. The changes of convection velocity and skewness factor are caused by influence of large-scale motion through the mechanism called amplitude modulation. The large-scale motion is less active in FPG and more active in APG, therefore in FPG the production of vortices is random (there are no high and low speed regions), while in the APG the large-scale motion drives the production of vortices. Namely, the vortices appear only in the high-speed regions, therefore have convection velocity higher than local mean velocity. The convection velocity affects directly the turbulent sweep and ejection events. The more flow is dominated by large-scale motion the higher values takes both the convection velocity of small-scale structures and sweep events induced by them.

Dró?d?, Artur

2014-08-01

91

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

92

Overflow: Facts on Friction  

NASA Technical Reports Server (NTRS)

This paper presents results of three minor studies into the behavior of the OVERFLOW with respect to the prediction of skin friction drag on wing bodies at cruise Mach number and wind tunnel Reynolds number. The studies include a preliminary assessment of the behavior of the two new 2-equation turbulence models introduced with the latest version of OVERFLOW (v. 1.8f), an investigation into potential improvements in the matrix dissipation scheme currently implemented in OVERFLOW, and an analysis of the observed sensitivity of the code's skin friction predictions to grid stretching at solid surface boundaries.

Lawrence, Scott

1999-01-01

93

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

94

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

95

Factors controlling threshold friction velocity in semiarid and arid areas of the United States  

USGS Publications Warehouse

A physical model was developed to explain threshold friction velocities u*t for particles of the size 60a??120 I?m lying on a rough surface in loose soils for semiarid and arid parts of the United States. The model corrected for the effect of momentum absorption by the nonerodible roughness. For loose or disturbed soils the most important parameter that controls u*t is the aerodynamic roughness height z 0. For physical crusts damaged by wind the size of erodible crust pieces is important along with the roughness. The presence of cyanobacteriallichen soil crusts roughens the surface, and the biological fibrous growth aggregates soil particles. Only undisturbed sandy soils and disturbed soils of all types would be expected to be erodible in normal wind storms. Therefore disturbance of soils by both cattle and humans is very important in predicting wind erosion as confirmed by our measurements.

Marticorena, Beatrice; Bergametti, G.; Belnap, Jayne

1997-01-01

96

Factors controlling threshold friction velocity in semiarid and arid areas of the United States  

NASA Astrophysics Data System (ADS)

A physical model was developed to explain threshold friction velocities u*t for particles of the size 60-120 ?m lying on a rough surface in loose soils for semiarid and arid parts of the United States. The model corrected for the effect of momentum absorption by the nonerodible roughness. For loose or disturbed soils the most important parameter that controls u*t is the aerodynamic roughness height z0. For physical crusts damaged by wind the size of erodible crust pieces is important along with the roughness. The presence of cyanobacteriallichen soil crusts roughens the surface, and the biological fibrous growth aggregates soil particles. Only undisturbed sandy soils and disturbed soils of all types would be expected to be erodible in normal wind storms. Therefore disturbance of soils by both cattle and humans is very important in predicting wind erosion as confirmed by our measurements.

Marticorena, Beatrice; Bergametti, Gilles; Gillette, Dale; Belnap, Jayne

1997-10-01

97

Near-bed turbulence and relict waveformed sand ripples: Observations from the inner shelf  

Microsoft Academic Search

structure function within the inertial subrange, and ranged from 0.1 ? 10? 6 to 3 ? 10? 6 W\\/kg. The friction velocity, u*, at the bed ranged from 0.3 to 0.5 cm\\/s, and the wave friction factor, fw, from 0.017 to 0.02. The nearbed turbulence intensities and consequently the estimated values of u2* and fw are likely too small by

Alex E. Hay

2008-01-01

98

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

99

Experimental studies on heat transfer and friction factor characteristics of forced circulation solar water heater system fitted with helical twisted tapes  

SciTech Connect

Experimental investigation of heat transfer, friction factor and thermal performance of twisted tape solar water heater with various twist ratios has been conducted and the results are compared with plain tube collector for the same operating conditions with Reynolds number varied from 3000 to 23,000. Experimental data from plain tube collector is validated with the fundamental equations and found that the discrepancy is less than {+-}5.35% and {+-}8.80% for Nusselt number and friction factor, respectively. Correlations have been developed for Nusselt number and friction factor with various twist ratios (Y = 3, 4, 5, 6) and are compared with the experimental values. Results conclude that, heat transfer and pressure drop are higher in twisted tape collector compared to the plain one. Among the various twist ratios, the minimum twist ratio 3 is found to enhance the heat transfer and pressure drop due to swirl generation. As the twist ratio increases, the swirl generation decreases and minimizes the heat transfer and friction factor. (author)

Jaisankar, S. [Department of Mechanical Engineering, Oxford Engineering College, Tiruchirappalli 620009, Tamil Nadu (India); Radhakrishnan, T.K.; Sheeba, K.N. [Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli 620015, Tamil Nadu (India)

2009-11-15

100

Experimental studies on heat transfer and friction factor characteristics of thermosyphon solar water heater system fitted with spacer at the trailing edge of twisted tapes  

Microsoft Academic Search

Experimental investigation of heat transfer and friction factor characteristics of thermosyphon solar water heater system with full- length twist, twist fitted with rod and spacer fitted at the trailing edge for lengths of 100, 200 and 300mm for twist ratio 3 and 5 has been carried out and compared with plain tube collector for the same operating conditions. The experimental

S. Jaisankar; T. K. Radhakrishnan; K. N. Sheeba

2009-01-01

101

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

102

Direct Measurements of Skin Friction  

NASA Technical Reports Server (NTRS)

A device has been developed to measure local skin friction on a flat plate by measuring the force exerted upon a very small movable part of the surface of the flat plate. These forces, which range from about 1 milligram to about 100 milligrams, are measured by means of a reactance device. The apparatus was first applied to measurements in the low-speed range, both for laminar and turbulent boundary layers. The measured skin-friction coefficients show excellent agreement with Blasius' and Von Karman's results. The device was then applied to high-speed subsonic flow and the turbulent-skin-friction coefficients were determined up to a Mach number of about 0.8. A few measurements in supersonic flow were also made. This paper describes the design and construction of the device and the results of the measurements.

Dhawan, Satish

1953-01-01

103

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

104

Semiempirical methods for computing turbulent flows  

NASA Technical Reports Server (NTRS)

Two semiempirical theories which provide a basis for determining the turbulent friction and heat exchange near a wall are presented: (1) the Prandtl-Karman theory, and (2) the theory utilizing an equation for the energy of turbulent pulsations. A comparison is made between exact numerical methods and approximate integral methods for computing the turbulent boundary layers in the presence of pressure, blowing, or suction gradients. Using the turbulent flow around a plate as an example, it is shown that, when computing turbulent flows with external turbulence, it is preferable to construct a turbulence model based on the equation for energy of turbulent pulsations.

Belov, I. A.; Ginzburg, I. P.

1986-01-01

105

EXPERIMENTAL MEASUREMENTS OF TURBULENT HEAT TRANSFER COEFFICIENTS, FRICTION FACTORS AND VELOCITY PROFILES FOR AIR FLOWING PARALLEL TO CLOSELY SPACED RODS  

Microsoft Academic Search

Heat transfer and fluid dynamic studies were made with air flowing ; parallel to the rods of a seven rod cluster. The radial centers of the rods were ; equally spaced in a triangular array with the ratio of the center-to-center ; distance divided by the rod diameter of 1.015. Pressure drops, velocity ; profiles, and average and local (with

L. D. Palmer; L. L. Swanson

1960-01-01

106

Navier-Stokes Computations With One-Equation Turbulence Model for Flows Along Concave Wall Surfaces  

NASA Technical Reports Server (NTRS)

This report presents the use of a time-marching three-dimensional compressible Navier-Stokes equation numerical solver with a one-equation turbulence model to simulate the flow fields developed along concave wall surfaces without and with a downstream extension flat wall surface. The 3-D Navier- Stokes numerical solver came from the NASA Glenn-HT code. The one-equation turbulence model was derived from the Spalart and Allmaras model. The computational approach was first calibrated with the computations of the velocity and Reynolds shear stress profiles of a steady flat plate boundary layer flow. The computational approach was then used to simulate developing boundary layer flows along concave wall surfaces without and with a downstream extension wall. The author investigated the computational results of surface friction factors, near surface velocity components, near wall temperatures, and a turbulent shear stress component in terms of turbulence modeling, computational mesh configurations, inlet turbulence level, and time iteration step. The computational results were compared with existing measurements of skin friction factors, velocity components, and shear stresses of the developing boundary layer flows. With a fine computational mesh and a one-equation model, the computational approach could predict accurately the skin friction factors, near surface velocity and temperature, and shear stress within the flows. The computed velocity components and shear stresses also showed the vortices effect on the velocity variations over a concave wall. The computed eddy viscosities at the near wall locations were also compared with the results from a two equation turbulence modeling technique. The inlet turbulence length scale was found to have little effect on the eddy viscosities at locations near the concave wall surface. The eddy viscosities, from the one-equation and two-equation modeling, were comparable at most stream-wise stations. The present one-equation turbulence model is an effective approach for turbulence modeling in the near solid wall surface region of flow over a concave wall.

Wang, Chi R.

2005-01-01

107

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

108

The effect of safety factor and magnetic shear on turbulent transport in nonlinear gyrokinetic simulations  

SciTech Connect

This paper reports on over 100 nonlinear simulations used to systematically study the effects of safety factor q and magnetic shear s on turbulent energy and particle transport due to ion temperature gradient (ITG) modes and trapped electron modes (TEM) for several reference cases using the GYRO gyrokinetic code. All the simulations are collisionless, electrostatic, and utilize shifted circle geometry. The motivation is to create a database for benchmarking and testing of turbulent transport models. In simulations varying q, it is found that the ion and electron energy transport exhibit an offset linear dependence on q for 1{<=}q{<=}4. This result is valid for cases in which the spectrum is dominated by either TEM or ITG modes. The particle transport also follows a linear q dependence if the diffusivity D is positive (outward). If a particle pinch is predicted, however, then D is found to be insensitive to q. In kinetic electron simulations varying the magnetic shear s, the particle transport can exhibit a null flow at a particular value of s. In the vicinity of the null flow point, the transport spectrum shows that some modes drive an inward flow while others drive an outward flow. For negative magnetic shear, the magnetohydrodynamic {alpha} parameter is shown to be stabilizing for both the energy and particle transport but can be destabilizing for large positive shear. Compared to the ITG dominated case, the TEM cases show the same linear q dependence, but a weaker s dependence is exhibited for positive magnetic shear values when TEM modes dominate the spectrum. In general, the q, s, and {alpha} dependence of the transport including kinetic electrons is consistent with ITG adiabatic electron simulation results.

Kinsey, J.E.; Waltz, R.E.; Candy, J. [Lehigh University, Bethlehem, Pennsylvania 18015 (United States); General Atomics, P.O. Box 85608, San Diego, California 92186-5608 (United States)

2006-02-15

109

Turbulent drag reduction research  

NASA Technical Reports Server (NTRS)

Research in the area of turbulent drag reduction for attached flows is summarized. The most promising passive techniques utilize non-planar geometry. Of particular interest is the suitability of these devices for retrofit of existing vehicles. Five methods for reducing turbulent skin friction drag on bodies/fuselages are discussed. They are: (1) large-eddy breakup devices; (2) riblets; (3) slot injection optimization; (4) control of Emmons spot generation; and (5) relaminarization through massive suction. Except for the Emmons spot work these methods all indicate the possibility of sizable net reductions in skin friction for laboratory conditions.

Bushnell, D. M.; Anders, J. B.; Walsh, M. J.; Mcinville, R. V.

1984-01-01

110

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

111

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

112

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; Schafer, Christof; Morozov, Alexander N.; Wagner, Christian; Hof, Bjorn

2013-01-01

113

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

114

Investigation of twisted tape inserted solar water heaters—heat transfer, friction factor and thermal performance results  

Microsoft Academic Search

Heat transfer in a solar water heater could be enhanced by means of twisted tapes, inserted inside the fluid flow tubes, which induce swirl flow and act as turbulence promoters. Experimental investigations for a solar water heater with twisted tape inserts having twist pitch to tube diameter ratio ranging from 3–12 have been carried out for varying mass flow rates.

A Kumar; B. N Prasad

2000-01-01

115

Heat Transfer Coefficient and Friction Factor Prediction of Corrugated Tubes Combined With Twisted Tape Inserts Using Artificial Neural Network  

Microsoft Academic Search

In the research described here, artificial neural network (ANN) approach has been utilized to characterize the thermohydraulic behavior of corrugated tubes combined with twisted tape inserts in a turbulent flow regime. The experimental data sets were extracted from 57 tubes, 9 and 3 spirally corrugated tubes with varying geometries combined with 5 and 4 twisted tapes with different pitches. The

Mohammad Reza Jafari Nasr; Ali Habibi Khalaj

2010-01-01

116

Skin-Friction Measurements in Incompressible Flow  

NASA Technical Reports Server (NTRS)

Experiments have been conducted to measure the local surface-shear stress and the average skin-friction coefficient in Incompressible flow for a turbulent boundary layer on a smooth flat plate having zero pressure gradient. Data were obtained for a range of Reynolds numbers from 1 million to 45 million. The local surface-shear stress was measured by a floating-element skin-friction balance and also by a calibrated total head tube located on the surface of the test wall. The average skin-friction coefficient was obtained from boundary-layer velocity profiles.

Smith, Donald W.; Walker, John H.

1959-01-01

117

Experimental Heat Transfer Coefficients and Friction Factors in a Rib-Roughened Leading-Edge Cooling Cavity of a Gas Turbine Airfoil  

NASA Astrophysics Data System (ADS)

A gas turbine airfoil contains multiple coolant passageways. These passages usually have rib roughened wall surfaces in order to increase the heat transfer from the blade to the cooling air. Auxiliary power and compressed air is very valuable in a gas turbine, therefore low pumping power requirements are crucial. The thermal performance of three different coolant channel geometries with three different rib sizes was investigated. Heat transfer calculations were performed for Reynolds numbers ranging from 6,000 to 40,000. The performance characteristics were calculated through the use of the convective heat transfer coefficient and the friction factor. In this study, the most desirable characteristics are a high heat transfer coefficient and minimal pumping power requirements. The thermal performance of each case was determined by comparing the average Nusselt numbers to the friction factor ratio. The resulting value was then plotted against the Reynolds number for each case. The trending data indicated thermal efficiency decreases with an increasing Reynolds number for all cases. The picture data shows increased thermal efficiency at larger distances from the nose portion of the cavity. In addition, thermal efficiency was higher at the half distance of the rib pitch while areas close to the ribs saw a lower thermal efficiency. The following experimental data will show that Rig 2 and 3 are the most thermally efficient geometries, with Rig 2 requiring lower pumping power and Rig 3 having a higher average Nusselt number.

Hagan, Peter

118

Numerical investigation of heat transfer and friction factor characteristics in a circular tube fitted with V-cut twisted tape inserts.  

PubMed

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

119

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

120

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.

121

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

2012-03-12

122

Quantum Friction  

NASA Astrophysics Data System (ADS)

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 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.

Tsekov, Roumen

2012-12-01

123

Predicting Turbulent Convective Heat Transfer in Three-Dimensional Duct Flows  

NASA Technical Reports Server (NTRS)

The performance of an explicit algebraic stress model is assessed in predicting the turbulent flow and forced heat transfer in straight ducts, with square, rectangular, trapezoidal and triangular cross-sections, under fully developed conditions over a range of Reynolds numbers. Iso-thermal conditions are imposed on the duct walls and the turbulent heat fluxes are modeled by gradient-diffusion type models. At high Reynolds numbers (>/= 10(exp 5)), wall functions are used for the velocity and temperature fields; while at low Reynolds numbers damping functions are introduced into the models. Hydraulic parameters such as friction factor and Nusselt number are well predicted even when damping functions are used, and the present formulation imposes minimal demand on the number of grid points without any convergence or stability problems. Comparison between the models is presented in terms of the hydraulic parameters, friction factor and Nusselt number, as well as in terms of the secondary flow patterns occurring within the ducts.

Rokni, M.; Gatski, T. B.

1999-01-01

124

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

125

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

126

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

127

Prediction of Very High Reynolds Number Compressible Skin Friction  

NASA Technical Reports Server (NTRS)

Flat plate skin friction calculations over a range of Mach numbers from 0.4 to 3.5 at Reynolds numbers from 16 million to 492 million using a Navier Stokes method with advanced turbulence modeling are compared with incompressible skin friction coefficient correlations. The semi-empirical correlation theories of van Driest; Cope; Winkler and Cha; and Sommer and Short T' are used to transform the predicted skin friction coefficients of solutions using two algebraic Reynolds stress turbulence models in the Navier-Stokes method PAB3D. In general, the predicted skin friction coefficients scaled well with each reference temperature theory though, overall the theory by Sommer and Short appeared to best collapse the predicted coefficients. At the lower Reynolds number 3 to 30 million, both the Girimaji and Shih, Zhu and Lumley turbulence models predicted skin-friction coefficients within 2% of the semi-empirical correlation skin friction coefficients. At the higher Reynolds numbers of 100 to 500 million, the turbulence models by Shih, Zhu and Lumley and Girimaji predicted coefficients that were 6% less and 10% greater, respectively, than the semi-empirical coefficients.

Carlson, John R.

1998-01-01

128

Friction surfacing and linear friction welding  

NASA Astrophysics Data System (ADS)

The paper describes the development of the friction-surfacing and linear-friction welding technologies, with particular attention given to the equipment evolution and the application of the processes and advanced materials (such as intermetallics, metal-matrix composites (MMCs), ODS alloys, and powder metallurgy alloys) for the aerospace industry. The use of friction surfacing to modify the surface material with MMCs, to repair defects by plugging, and manufacture/reprocess materials is described.

Nicholas, E. D.

129

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

130

Dahl friction modeling  

E-print Network

The drive behind improved friction models has been better prediction and control of dynamic systems. The earliest model was of classical Coulomb friction; however, the discontinuity during force reversal of the Coulomb ...

Chou, Danielle, 1981-

2004-01-01

131

Friction in hip prostheses  

Microsoft Academic Search

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

R. M. Hall; A. Unsworth

1997-01-01

132

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

133

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

134

Correlations of velocity and temperature fluctuations in the stagnation-point flow of circular cylinder in turbulent flow  

NASA Technical Reports Server (NTRS)

The present analyses of boundary layer flow and turbulence transport attempt to characterize the influence of freestream turbulence on the surface heat-transfer rate and stagnation point region skin friction of a circular cross-section cylinder in turbulent flow. The Reynolds stress-transport equations and k-epsilon two-equation turbulence modeling are used, yielding time-averaged turbulence double-correlations, mean-flow properties, surface heat-transfer rate, and skin-friction with freestream isotropic turbulence. A comparison of analytical results with experimental data indicates that large Reynolds normal stresses are induced at the boundary layer edge by the kinetic energy of the turbulence.

Wang, C. R.

1988-01-01

135

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

2012-08-21

136

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

137

Scale-invariant cascades in turbulence and evolution  

NASA Astrophysics Data System (ADS)

In this dissertation, I present work addressing three systems which are traditionally considered to be unrelated: turbulence, evolution, and social organization. The commonality between these systems is that in each case, microscopic interaction rules give rise to an emergent behavior that in some way makes contact with the macroscopic scale of the problem. The open-ended evolution of complexity in evolving systems is analogous to the scale-free structure established in turbulent flows through local transportation of energy. In both cases, an invariance is required for the cascading behavior to occur, and in both cases the scale-free structure is built up from some initial scale from which the behavior is fed. In turbulence, I examine the case of two-dimensional turbulence in order to support the hypothesis that the friction factor and velocity profile of turbulent pipe flows depend on the turbulent energy spectrum in a way unpredicted by the classic Prandtl theory. By simulating two-dimensional flows in controlled geometries, either an inverse energy cascade or forward enstrophy cascade can be produced. The friction factor scaling of the flow changes depending on which cascade is present, in a way consistent with momentum transfer theory and roughness-induced criticality. In the problem of evolution, I show that open-ended growth of complexity can be obtained by ensuring that the evolutionary dynamics are invariant with respect to changes in complexity. Finite system size, finite point mutation rate, and fixed points in the fitness landscape can all interrupt this cascade behavior, producing an analogue to the integral scale of turbulence. This complexity cascade can exist both for competing and for symbiotic sets of organisms. Extending this picture to the qualitatively-different levels of organization of real lifeforms (viruses, unicellular, biofilms, multicellular) requires an understanding of how the processes of evolution themselves evolve. I show that a separation of spatial or temporal scales can enhance selection pressure on parameters that only matter several generations down the line. Because of this, I conclude that the prime candidates for the emergence of novel evolutionary mechanisms are biofilms and things living in oscillating environments. Finally, in the problem of social organization, I show that different types of control hierarchies - leaders or communal decision making - can emerge depending on the relationship between the environment in which members of the social group act and the development and exchange of information.

Guttenberg, Nicholas Ryan

138

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

139

Predicting Turbulent Convective Heat Transfer in Fully Developed Duct Flows  

NASA Technical Reports Server (NTRS)

The performance of an explicit algebraic stress model (EASM) is assessed in predicting the turbulent flow and forced heat transfer in both straight and wavy ducts, with rectangular, trapezoidal and triangular cross-sections, under fully developed conditions. A comparison of secondary flow patterns. including velocity vectors and velocity and temperature contours, are shown in order to study the effect of waviness on flow dynamics, and comparisons between the hydraulic parameters. Fanning friction factor and Nusselt number, are also presented. In all cases. isothermal conditions are imposed on the duct walls, and the turbulent heat fluxes are modeled using gradient-diffusion type models. The formulation is valid for Reynolds numbers up to 10(exp 5) and this minimizes the need for wall functions that have been used with mixed success in previous studies of complex duct flows. In addition, the present formulation imposes minimal demand on the number of grid points without any convergence or stability problems. Criteria in terms of heat transfer and friction factor needed to choose the optimal wavy duct cross-section for industrial applications among the ones considered are discussed.

Rokni, Masoud; Gatski, Thomas B.

2001-01-01

140

Influence of fundamental force factors on the transverse velocity of fine particles moving in a turbulent gas stream  

Microsoft Academic Search

On the basis of a numerical solution of the equations of fine particle motion in a turbulent vertical stream forces of a different nature, acting in the radial direction and governing the average transverse particle velocity, are analyzed.

Z. R. Gorbis; F. E. Spokoinyi; R. V. Zagainova

1976-01-01

141

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

142

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

143

Couette Longitudinal Turbulent Flow Between Coaxial Cylinders  

NASA Astrophysics Data System (ADS)

Within the framework of the model of anisotropic wall turbulence, an analytical solution is obtained for the problem on turbulent incompressible-fluid flow between two coaxial circular cylinders due to the motion of the internal cylinder along the common axis (Couette flow). The velocity profile and the surface-friction coefficient of the internal cylinder are determined. The solution is compared to the existing solutions and to experimental data.

Babkin, V. A.

2014-01-01

144

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

145

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

146

Estimation of turbulence intensity and shear factor for diurnal and nocturnal periods with an URANS flow solver coupled with WRF  

NASA Astrophysics Data System (ADS)

Mesoscale results using the WRF model were downscaled from 3 km to 250 m resolution in a one-way coupling with VENTOS®/M. The results were compared against field measurements at one site comprising 4 meteorological masts, each with two sets of cup anemometers and wind vanes. The results showed that the addition of VENTOS®/M to the model chain improved the wind speed RMSE. Regarding the prediction of wind direction ambivalent results were obtained. Special attention was given to the prediction of turbulence intensity, particularly in reproducing its inverse proportionality with increasing wind speed (cf. IEC 61400-1 standard). The typical use of computational models in wind resource assessment, i.e., relying on decoupled methodologies and neutrally-stratified regimes, does not allow the representation of turbulence intensity for all wind speeds. The results obtained with VENTOS®/M were in agreement with the measured turbulence characteristics at both high and low wind speeds. Such was achieved without the coupling of any turbulence related field, relying solely on the turbulence model embedded in VENTOS®/M and its respective wall boundary conditions, based on Monin-Obukhov similarity theory. The behaviour under different stratification regimes was verified by analysing diurnal and nocturnal events separately.

Veiga Rodrigues, C.; Palma, J. M. L. M.

2014-06-01

147

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

148

Enhanced nanoscale friction on fluorinated graphene.  

PubMed

Atomically thin graphene is an ideal model system for studying nanoscale friction due to its intrinsic two-dimensional (2D) anisotropy. Furthermore, modulating its tribological properties could be an important milestone for graphene-based micro- and nanomechanical devices. Here, we report unexpectedly enhanced nanoscale friction on chemically modified graphene and a relevant theoretical analysis associated with flexural phonons. Ultrahigh vacuum friction force microscopy measurements show that nanoscale friction on the graphene surface increases by a factor of 6 after fluorination of the surface, while the adhesion force is slightly reduced. Density functional theory calculations show that the out-of-plane bending stiffness of graphene increases up to 4-fold after fluorination. Thus, the less compliant F-graphene exhibits more friction. This indicates that the mechanics of tip-to-graphene nanoscale friction would be characteristically different from that of conventional solid-on-solid contact and would be dominated by the out-of-plane bending stiffness of the chemically modified graphene. We propose that damping via flexural phonons could be a main source for frictional energy dissipation in 2D systems such as graphene. PMID:22720882

Kwon, Sangku; Ko, Jae-Hyeon; Jeon, Ki-Joon; Kim, Yong-Hyun; Park, Jeong Young

2012-12-12

149

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

150

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

151

Contribution of Reynolds stress distribution to the skin friction in wall-bounded flows  

NASA Astrophysics Data System (ADS)

A simple expression is derived of the componential contributions that different dynamical effects make to the frictional drag in turbulent channel, pipe and plane boundary layer flows. The local skin friction can be decomposed into four parts, i.e., laminar, turbulent, inhomogeneous and transient components, the second of which is a weighted integral of the Reynolds stress distribution. It is reconfirmed that the near-wall Reynolds stress is primarily important for the prediction and control of wall turbulence. As an example, the derived expression is used for an analysis of the drag modification by the opposition control and by the uniform wall blowing/suction.

Fukagata, Koji; Iwamoto, Kaoru; Kasagi, Nobuhide

2002-11-01

152

Deficiencies in Tidal Friction  

Microsoft Academic Search

There exists a continually slowing of the Earth's rotation. The friction of the ocean tides against shallow basins has long been attributed for this anomaly. There is however shortcomings in the tidal friction theory. In addition, the Moon is apparently gaining angular momentum as its orbit increases by 3.8 cm\\/year. This gain seems to exceed what is attributed to the

Jack Hohner

2004-01-01

153

A Friction Experiment  

Microsoft Academic Search

Force-sensor friction experiments such as the one in Vernier Software's Physics with Computers depend on an object moving with constant velocity in order to measure the force of kinetic friction; when a block is pulled by a horizontal force sensor and the block moves along a horizontal surface with constant velocity, Newton's first law ensures that the force-sensor reading is

Leo Takahashi

2002-01-01

154

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

155

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

156

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

157

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

158

Influence of the main force factors on the transverse velocity of small particles moving in a turbulent gas stream  

Microsoft Academic Search

Radially oriented forces which define the mean transverse velocity of suspended particles are analyzed on the basis of a numerical solution of the equations of motion of small particles in a turbulent vertical gas stream. The changes across the stream cross-section of resistance, inertia, and electrostatic repulsion forces, thermo- and photophoresis forces, and migration effects in nonuniform fields are identified.

Z. R. Gorbis; F. E. Spokoinyi; R. V. Zagainova

1976-01-01

159

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

160

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

161

Emmons spot forcing for turbulent drag reduction  

NASA Technical Reports Server (NTRS)

An Emmons spot-generation wind tunnel system has been designed to trigger closely spaced Emmons spots in the spanwise and longitudinal directions of an aerodynamic surface. For certain combinations of generator frequencies and amplitude, hole size, and hole spacing, experimental results indicate smaller turbulence scales and a reduction in skin friction of about 15 percent.

Goodman, W. L.

1985-01-01

162

Refilling Probe Hole of Friction Spot Joints by Friction Forming  

Microsoft Academic Search

Friction stir spot welding (FSSpW) is a variant of friction stir welding (FSW) that has wider industrial application. However, Probe hole left after the welding is the main limitation of their process. In this present study, modified FSSpW has been developed and the refilling is achieved by friction forming process. This process has been called Friction Stir Spot Welding with

S. John Prakash; S. Muthukumaran

2011-01-01

163

Alpha models and boundary-layer turbulence  

NASA Astrophysics Data System (ADS)

We study boundary-layer turbulence using the Navier-Stokes-alpha model obtaining an extension of the Prandtl equations for the averaged flow in a turbulent boundary layer. In the case of a zero pressure gradient flow along a flat plate, we derive a nonlinear fifth-order ordinary differential equation, an extension of the Blasius equation. We study it analytically and prove the existence of a two-parameter family of solutions satisfying physical boundary conditions. From this equation we obtain a theoretical prediction of the skin-friction coefficient in a wide range of Reynolds numbers based on momentum thickness, and deduce the maximal value of the skin-friction coefficient in the turbulent boundary layer. The two-parameter family of solutions to the equation matches experimental data in the transitional boundary layers with different free stream turbulence intensity. A one-parameter sub-family of solutions, obtained using our skin-friction coefficient law, matches experimental data in the turbulent boundary layer for moderately large Reynolds numbers.

Cheskidov, Alexey

164

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

165

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

166

Science 101: What Causes Friction?  

ERIC Educational Resources Information Center

Defining friction and asking what causes it might seem like a trivial question. Friction seems simple enough to understand. Friction is a force between surfaces that pushes against things that are moving or tending to move, and the rougher the surfaces, the greater the friction. Bill Robertson answers this by saying, "Well, not exactly".…

Robertson, Bill

2014-01-01

167

Estimation of SO{sub 2} dry deposition using turbulence parameters observed by sonic anemometer-thermometer  

SciTech Connect

The dry deposition of pollutants can be calculated from the concentration of pollutants in the atmosphere and deposition velocity. To calculate deposition velocity, turbulence parameters such as friction velocity and Monin-Obukhov length are used. However, due to the difficulties in observation of turbulence parameters, usually mean values of wind speed and temperature observed using conventional meteorological instruments are used to estimate the dry deposition. The dry deposition velocity is the function of aerodynamic resistance (R{sub a}), sublayer resistance (R{sub b}), surface resistance (R{sub c}). R{sub a} and R{sub b} are calculated from turbulence parameters and R{sub c} is related to surface characteristics. The purpose of the present study is to compare the dry deposition obtained using the data sets of mean values and turbulence parameters measured by sonic anemometer-thermometer. The field observation was performed for 30 days from October 27 to November 25, 1995. The turbulence parameters were measured by 3 dimensional sonic anemometer-thermometer and mean meteorological variables are obtained at two heights, 2.5 m and 10 m. The results show that the dry deposition velocity is large, in daytime and small in nighttime. The major factor of diurnal variation is Ra. In the daytime the dry deposition velocity calculated using mean meteorological data show relatively similar to the dry deposition velocity calculated using the turbulence data, however there are big differences at night.

Lee, Chong Bum; Kim, Jeong, Sik; Kim, Yong Goog; Cho, Chang Rae [Kangwon National Univ. (Korea, Republic of); Byun, D.W. [National Oceanic and Atmospheric Administration, Research Triangle Park, NC (United States)

1996-12-31

168

PIV experiments in rough-wall, laminar-to-turbulent, oscillatory boundary-layer flows  

NASA Astrophysics Data System (ADS)

Exploratory measurements of oscillatory boundary layers were conducted over a smooth and two different rough beds spanning the laminar, transitional and turbulent flow regimes using a multi-camera 2D-PIV system in a small oscillatory-flow tunnel (Admiraal et al. in J Hydraul Res 44(4):437-450, 2006). Results show how the phase lag between bed shear stress and free-stream velocity is better defined when the integral of the momentum equation is used to estimate the bed shear stress. Observed differences in bed shear stress and phase lag between bed shear stress and free-stream velocity are highly sensitive to the definition of the bed position ( y = b). The underestimation of turbulent stresses close to the wall is found to explain such differences when using the addition of Reynolds and viscous stresses to define both the bed shear stress and the phase lag. Regardless of the flow regime, in all experiments, boundary-layer thickness reached its maximum value at a phase near the flow reversal at the wall. Friction factors in smooth walls are better estimated using a theoretical equation first proposed by Batchelor (An introduction to fluid dynamics. Cambridge University Press, Cambridge, 1967) while the more recent empirical predictor of Pedocchi and Garcia (J Hydraul Res 47(4):438-444, 2009a) was found to be appropriate for estimating friction coefficients in the laminar-to-turbulent transition regime.

Mujal-Colilles, Anna; Mier, Jose M.; Christensen, Kenneth T.; Bateman, Allen; Garcia, Marcelo H.

2014-01-01

169

Turbulent diffusion of chemically reacting gaseous admixtures  

NASA Astrophysics Data System (ADS)

We study turbulent diffusion of chemically reacting gaseous admixtures in a developed turbulence. In our previous study [Phys. Rev. Lett. 80, 69 (1998), 10.1103/PhysRevLett.80.69] using a path-integral approach for a delta-correlated in a time random velocity field, we demonstrated a strong modification of turbulent transport in fluid flows with chemical reactions or phase transitions. In the present study we use the spectral ? approximation that is valid for large Reynolds and Peclet numbers and show that turbulent diffusion of the reacting species can be strongly depleted by a large factor that is the ratio of turbulent and chemical times (turbulent Damköhler number). We have demonstrated that the derived theoretical dependence of a turbulent diffusion coefficient versus the turbulent Damköhler number is in good agreement with that obtained previously in the numerical modeling of a reactive front propagating in a turbulent flow and described by the Kolmogorov-Petrovskii-Piskunov-Fisher equation. We have found that turbulent cross-effects, e.g., turbulent mutual diffusion of gaseous admixtures and turbulent Dufour effect of the chemically reacting gaseous admixtures, are less sensitive to the values of stoichiometric coefficients. The mechanisms of the turbulent cross-effects differ from the molecular cross-effects known in irreversible thermodynamics. In a fully developed turbulence and at large Peclet numbers the turbulent cross-effects are much larger than the molecular ones. The obtained results are applicable also to heterogeneous phase transitions.

Elperin, T.; Kleeorin, N.; Liberman, M.; Rogachevskii, I.

2014-11-01

170

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

171

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

172

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

173

Fused metallic friction materials  

SciTech Connect

A friction material member particularly useful in high load and temperature applications and a method for making the same are disclosed. The friction material member is made by preparing a mixture of sinterable metal particles, carbonaceous material, friction enhancers and curable polymeric resin; the mixture comprising at least about 50 percent by weight of metal particles and sufficient resin to coat and bond the metal particles, carbonaceous material and friction enhancers. The mixture is molded and the resin cured to form a piece in a solid preformed shape corresponding to the shape of the mold. Thereafter, the preformed piece is heated in a substantially oxygen free atmosphere for a time and at temperatures and pressures sufficient to fuse at least a portion of the metal particles into a sintered metal matrix. The preformed piece may be sintered to a metal backing plate during the heating step, or may be welded, brazed or soldered to the plate after heating. The resulting product may be classified as a hybrid friction material, i.e., being a cross between semi-metallic and sintered full metallic friction materials.

Myers, P.A.

1984-03-20

174

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

175

Discrete scale invariance in turbulence?  

Microsoft Academic Search

Based on theoretical argument and experimental evidence, we conjecture that structure functions of turbulent times series exhibit log-periodic modulations decorating their power law dependence. In order to provide ironclad experimental evidence, we stress the need for novel methods of averaging and propose a novel ``canonical'' averaging scheme for the analysis of structure factors of turbulent flows. The strategy is to

D. Sornette

1998-01-01

176

Topological complexity of frictional interfaces: friction networks  

NASA Astrophysics Data System (ADS)

Through research conducted in this study, a network approach to the correlation patterns of void spaces in rough fractures (crack type II) was developed. We characterized friction networks with several networks characteristics. The correlation among network properties with the fracture permeability is the result of friction networks. The revealed hubs in the complex aperture networks confirmed the importance of highly correlated groups to conduct the highlighted features of the dynamical aperture field. We found that there is a universal power law between the nodes' degree and motifs frequency (for triangles it reads T(k) ∝ k? (? ? 2 ± 0.3)). The investigation of localization effects on eigenvectors shows a remarkable difference in parallel and perpendicular aperture patches. Furthermore, we estimate the rate of stored energy in asperities so that we found that the rate of radiated energy is higher in parallel friction networks than it is in transverse directions. The final part of our research highlights 4 point sub-graph distribution and its correlation with fluid flow. For shear rupture, we observed a similar trend in sub-graph distribution, resulting from parallel and transversal aperture profiles (a superfamily phenomenon).

Ghaffari, H. O.; Young, R. P.

2012-03-01

177

Topological Complexity of Frictional Interfaces: Friction Networks  

E-print Network

Through research conducted in this study, a network approach to the correlation patterns of void spaces in rough fractures (crack type II) was developed. We characterized friction networks with several networks characteristics. The correlation among network properties with the fracture permeability is the result of friction networks. The revealed hubs in the complex aperture networks confirmed the importance of highly correlated groups to conduct the highlighted features of the dynamical aperture field. We found that there is a universal power law between the nodes' degree and motifs frequency (for triangles it reads T(k)\\proptok{\\beta} ({\\beta} \\approx2\\pm0.3)). The investigation of localization effects on eigenvectors shows a remarkable difference in parallel and perpendicular aperture patches. Furthermore, we estimate the rate of stored energy in asperities so that we found that the rate of radiated energy is higher in parallel friction networks than it is in transverse directions. The final part of our research highlights 4 point sub-graph distribution and its correlation with fluid flow. For shear rupture, we observed a similar trend in sub-graph distribution, resulting from parallel and transversal aperture profiles (a superfamily phenomenon).

H. O. Ghaffari; R. P. Young

2011-05-21

178

Interfacial friction damping properties in magnetorheological elastomers  

NASA Astrophysics Data System (ADS)

In this study, the interfacial friction damping properties of magnetorheological elastomers (MREs) were investigated experimentally. Two kinds of carbonyl iron particles, with sizes of 1.1 µm and 9.0 µm, were used to fabricate four MRE samples, whose particle weight fractions were 10%, 30%, 60% and 80%, respectively. Their microstructures were observed using an environmental scanning electron microscope (SEM). The dynamic performances of these samples, including shear storage modulus and loss factor were measured with a modified dynamic mechanical analyzer (DMA). The experimental results indicate that MRE samples fabricated with 1.1 µm carbonyl iron particles have obvious particle agglomeration, which results in the fluctuation of loss factor compared with other MRE samples fabricated with large particle sizes. The analysis implies that the interfacial friction damping mainly comes from the frictional sliding at the interfaces between the free rubber and the particles.

Fan, Yanceng; Gong, Xinglong; Xuan, Shouhu; Zhang, Wei; Zheng, Jian; Jiang, Wanquan

2011-03-01

179

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

2012-10-29

180

Theory and numerical computation of the von Karman constant in two-dimensional turbulent flows  

NASA Astrophysics Data System (ADS)

We present a calculation of the velocity profile in two-dimensional (2D) turbulent flows. The method is based upon the momentum-transfer theory for the friction factor, proposed by Gioia and Chakraborty, and when fitted to a putative law of the wall profile yields a value for the von Kármán constant which is in satisfactory agreement with direct numerical simulations at width Reynolds numbers between 20,000 and 80,000. We compare the theoretical results with experimental results on turbulent 2D soap films, taking into account the effects of air resistance. Our findings indicate that the von Kármán constant in 2D is significantly less than the accepted value in 3D.

Guttenberg, Nicholas; Goldenfeld, Nigel; Larkin, Jason; Prescott, Alisia; Kellay, Hamid; Goldburg, Walter

2008-11-01

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

Turbulence and diffusion: fossil turbulence  

E-print Network

Fossil turbulence processes are central to turbulence, turbulent mixing, and turbulent diffusion in the ocean and atmosphere, in astrophysics and cosmology, and in most other natural flows. George Gamov suggested in 1954 that galaxies might be fossils of primordial turbulence produced by the Big Bang. John Woods showed that breaking internal waves on horizontal dye sheets in the interior of the stratified ocean form highly persistent remnants of these turbulent events, which he called fossil turbulence. The dark mixing paradox of the ocean refers to undetected mixing that must exist somewhere to explain why oceanic scalar fields like temperature and salinity are so well mixed, just as the dark matter paradox of galaxies refers to undetected matter that must exist to explain why rotating galaxies don't fly apart by centrifugal forces. Both paradoxes result from sampling techniques that fail to account for the extreme intermittency of random variables involved in self-similar, nonlinear, cascades over a wide range of scales; turbulent vorticity for dark mixing, and accreting small-planetary-mass MACHO number density for dark matter.

Carl H. Gibson

2000-03-10

183

Vibration and noise in oil-immersed friction couples - A basic discussion  

SciTech Connect

Noise has been an annoyance since man began to use friction as an engineering tool. With the advent of lubricated friction devices, many believed that noise would disappear. However, noise in oil-immersed (or ''wet'') friction couples continues to be a constant and difficult problem for designers of clutches, brakes, and other frictional devices. While prediction of frictional noise remains difficult, understanding basic hydrodynamic factors that lead to noise can provide insight into this phenomenon. Factors that affect the hydrodynamic state within the friction couple, such as torque, speed, geometry, lubricant, friction material, and temperature, must be understood and controlled to eliminate noise and vibration in these couples. Field and lab experience may be used to provide empirical guidelines for control and prevention of noise and vibration in wet friction couples.

Anleitner, M.A.

1986-01-01

184

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

185

Presentation of KI-COF, a phenomenological model of variable friction in fretting contact  

E-print Network

Presentation of KI-COF, a phenomenological model of variable friction in fretting contact M. Cheikh of the contact and the amount of slip. Key words: Fretting wear, fretting fatigue, variable friction, coefficient with fretting is the coef- ficient of friction [2]. This factor is influenced by the surface quality, the wear

Paris-Sud XI, Université de

186

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.

187

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

188

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

189

Teach Engineering: Discovering Friction  

NSDL National Science Digital Library

This inquiry-based lesson for Grades 6-9 introduces the concept of friction as a force that impedes motion when two surfaces are in contact. Students use a spring scale to measure the frictional force between a moving coffee mug and the surface it slides on. This resource includes background information for teachers, suggestions for lesson introduction and closure, and extension activities. TeachEngineering is a Pathway project of the National Science Digital Library. It provides a large collection of teacher-tested, research-based content for K-12 teachers to connect real-world experiences with curricular content.

2011-10-11

190

Heat release effects on decaying homogeneous compressible turbulence  

E-print Network

.78, Re? = 31.2) in Case 4 . . . . . . . . . . . . . . . 114 x LIST OF FIGURES FIGURE Page 1 Typicalenergyspectrumofturbulenceandmethodologiesforstudy- ing turbulence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 Flow around hypersonic... turbulence. A good understanding of high-enthalpy compressible turbulence is crucial for analyzing the flow around re-entry spacecrafts and hypersonic flight vehicles, and inside scramjet engines. One main factor affecting turbulence in these high...

Lee, Kurn Chul

2009-05-15

191

Burgers Turbulence  

E-print Network

The last decades witnessed a renewal of interest in the Burgers equation. Much activities focused on extensions of the original one-dimensional pressureless model introduced in the thirties by the Dutch scientist J.M. Burgers, and more precisely on the problem of Burgers turbulence, that is the study of the solutions to the one- or multi-dimensional Burgers equation with random initial conditions or random forcing. Such work was frequently motivated by new emerging applications of Burgers model to statistical physics, cosmology, and fluid dynamics. Also Burgers turbulence appeared as one of the simplest instances of a nonlinear system out of equilibrium. The study of random Lagrangian systems, of stochastic partial differential equations and their invariant measures, the theory of dynamical systems, the applications of field theory to the understanding of dissipative anomalies and of multiscaling in hydrodynamic turbulence have benefited significantly from progress in Burgers turbulence. The aim of this revie...

Bec, Jeremie

2007-01-01

192

Cluster Turbulence  

E-print Network

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.

Michael L. Norman; Greg L. Bryan

1998-02-26

193

Pulsatile Newtonian frictional losses in a rigid tube.  

NASA Technical Reports Server (NTRS)

Analytic expressions for the instantaneous average velocity and friction factor have been obtained for pulsatile Newtonian laminar flow with mean flow. Experimental pulsatile pressure gradients were found to be complex wave forms whose characteristics strongly influenced the experimental pulsatile average velocity and friction factor. Experimental and theoretical reduced instantaneous friction factor histories varied in a non-sinusoidal manner throughout the pulse cycle and exhibited extremum points. At high pulsation pressure ratios, substantial deviation occurred in the portion of the pulse cycle where the instantaneous average velocity approached zero. At low pulsation pressure ratios, the experimental and theoretical time average friction factor equaled unity and was independent of pulsation modulus. One potential application of this result is in the study of blood flow through small blood vessels.

Phillips, E. M.; Chiang, S. H.

1973-01-01

194

Nonlinear friction in quantum mechanics  

E-print Network

The effect of nonlinear friction forces in quantum mechanics is studied via dissipative Madelung hydrodynamics. A new thermo-quantum diffusion equation is derived, which is solved for the particular case of quantum Brownian motion with a cubic friction. It is extended also by a chemical reaction term to describe quantum reaction-diffusion systems with nonlinear friction as well.

Roumen Tsekov

2010-03-01

195

Friction drilling of cast metals  

Microsoft Academic Search

This study investigates the friction drilling process, a nontraditional hole-making technique, for cast metals. In friction drilling, a rotating conical tool is applied to penetrate work-material and create a bushing in a single step without generating chip. The cast aluminum and magnesium alloys, two materials studied, are brittle compared to the ductile metal workpiece material used in previous friction drilling

Scott F. Miller; Jia Tao; Albert J. Shih

2006-01-01

196

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 rampâs ratio of rise to run to quantify their results. Relates to the linked video, DragonflyTV GPS: Kinetic Sculpture Challenge.

Twin Cities Public Television, Inc.

2007-01-01

197

Internal friction in gadolinium  

Microsoft Academic Search

The low-temperature internal friction spectrum of gadolinium consists of two peaks, labelled ? and ?. The former is independent and the latter dependent on, the strain amplitude. Domain wall motion, induced by the applied stress, is responsible for both peaks, which are not caused by a relaxation mechanism. The microeddy currents generated by changes in magnetization are responsible for the

C. F. Burdett

1968-01-01

198

Robotic friction stir welding  

Microsoft Academic Search

The forces and torques associated with friction stir welding (FSW) are discussed as they relate to implementation of the welding process with industrial robots. Experimental results are presented that support the conclusions drawn from models developed by others. It is shown that even with heavy-duty industrial robots with high stiffness, force feedback is important for successful robotic FSW. Methods of

George E. Cook; Reginald Crawford; Denis E. Clark; Alvin M. Strauss

2004-01-01

199

Start up and steady state friction of alumina against alumina  

Microsoft Academic Search

Using a pin-on-disc testing machine, the authors investigated the start up and steady state friction of highly polished alumina against itself and studied the factors affecting the friction. The lubricants used in this paper were 1 wt.% water solution of carboxymethyl cellulose sodium salt (CMC-Na 1 wt.% water solution) and distilled water. It is observed that many factors, such as

Y. S. Zhou; M. Ohashi; K. Ikeuchi

1997-01-01

200

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

201

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

202

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

203

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

204

Rubber friction for tire tread compound on road surfaces  

NASA Astrophysics Data System (ADS)

We have measured the surface topography and calculated the surface roughness power spectrum for an asphalt road surface. For the same surface we have measured the friction for a tire tread compound for velocities 10-6 m s-1 < v < 10-3 m s-1 at three different temperatures (at -8?°C, 20?°C and 48?°C). The friction data was shifted using the bulk viscoelasticity shift factor aT to form a master curve. We have measured the effective rubber viscoelastic modulus at large strain and calculated the rubber friction coefficient (and contact area) during stationary sliding and compared it to the measured friction coefficient. We find that for the low velocities and for the relatively smooth road surface we consider, the contribution to friction from the area of real contact is very important, and we interpret this contribution as being due to shearing of a very thin confined rubber smear film.

Lorenz, B.; Persson, B. N. J.; Fortunato, G.; Giustiniano, M.; Baldoni, F.

2013-03-01

205

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

206

Rubber friction for tire tread compound on road surfaces.  

PubMed

We have measured the surface topography and calculated the surface roughness power spectrum for an asphalt road surface. For the same surface we have measured the friction for a tire tread compound for velocities 10(-6) m s(-1) < v < 10(-3) m s(-1) at three different temperatures (at -8 °C, 20 °C and 48 °C). The friction data was shifted using the bulk viscoelasticity shift factor a(T) to form a master curve. We have measured the effective rubber viscoelastic modulus at large strain and calculated the rubber friction coefficient (and contact area) during stationary sliding and compared it to the measured friction coefficient. We find that for the low velocities and for the relatively smooth road surface we consider, the contribution to friction from the area of real contact is very important, and we interpret this contribution as being due to shearing of a very thin confined rubber smear film. PMID:23334507

Lorenz, B; Persson, B N J; Fortunato, G; Giustiniano, M; Baldoni, F

2013-03-01

207

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

208

Superfluid vortex lines in a model of turbulent flow  

NASA Astrophysics Data System (ADS)

Recent experiments have shown that the high Reynolds number turbulent flow of superfluid helium is similar to classical turbulence. To understand this evidence we have developed an idealized model of normal fluid turbulence which is based on vorticity tubes and we have studied numerically the behavior of superfluid quantized vortex lines in this model of turbulent normal flow. We have found that the vortex lines form ordered superfluid vortex bundles in regions of high normal fluid vorticity. A vortex wave instability and mutual friction are responsible for generating a high density of vortex lines such that the resulting macroscopic superfluid vorticity and the driving normal fluid vorticity patterns match. The results are discussed from the point of view of the idea, put forward to explain experiments, that in the isothermal, turbulent flow of He II a high density of vortex lines locks the two fluid components together and the resulting turbulent flow is that of a classical Navier-Stokes fluid.

Barenghi, Carlo F.; Samuels, David C.; Bauer, Gregory H.; Donnelly, Russell J.

1997-09-01

209

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

210

Turbulence in Compressible Flows  

NASA Technical Reports Server (NTRS)

Lecture notes for the AGARD Fluid Dynamics Panel (FDP) Special Course on 'Turbulence in Compressible Flows' have been assembled in this report. The following topics were covered: Compressible Turbulent Boundary Layers, Compressible Turbulent Free Shear Layers, Turbulent Combustion, DNS/LES and RANS Simulations of Compressible Turbulent Flows, and Case Studies of Applications of Turbulence Models in Aerospace.

1997-01-01

211

Influences of peripherally-cut twisted tape insert on heat transfer and thermal performance characteristics in laminar and turbulent tube flows  

SciTech Connect

Effects of peripherally-cut twisted tape insert on heat transfer, friction loss and thermal performance factor characteristics in a round tube were investigated. Nine different peripherally-cut twisted tapes with constant twist ratio (y/W = 3.0) and different three tape depth ratios (DR = d/W = 0.11, 0.22 and 0.33), each with three different tape width ratios (WR = w/W = 0.11, 0.22 and 0.33) were tested. Besides, one typical twisted tape was also tested for comparison. The measurement of heat transfer rate was conducted under uniform heat flux condition while that of friction factor was performed under isothermal condition. Tests were performed with Reynolds number in a range from 1000 to 20,000, using water as a working fluid. The experimental results revealed that both heat transfer rate and friction factor in the tube equipped with the peripherally-cut twisted tapes were significantly higher than those in the tube fitted with the typical twisted tape and plain tube, especially in the laminar flow regime. The higher turbulence intensity of fluid in the vicinity of the tube wall generated by the peripherally-cut twisted tape compared to that induced by the typical twisted tape is referred as the main reason for achieved results. The obtained results also demonstrated that as the depth ratio increased and width ratio decreased, the heat transfer enhancement increased. Over the range investigated, the peripherally-cut twisted tape enhanced heat transfer rates in term of Nusselt numbers up to 2.6 times (turbulent regime) and 12.8 times (laminar regime) of that in the plain tube. These corresponded to the maximum performance factors of 1.29 (turbulent regime) and 4.88 (laminar regime). (author)

Eiamsa-ard, Smith [Department of Mechanical Engineering, Faculty of Engineering, Mahanakorn University of Technology, Bangkok 10530 (Thailand); Seemawute, Panida [Department of Civil Engineering, Faculty of Engineering, Mahanakorn University of Technology, Bangkok 10530 (Thailand); Wongcharee, Khwanchit [Department of Chemical Engineering, Faculty of Engineering, Mahanakorn University of Technology, Bangkok 10530 (Thailand)

2010-09-15

212

Kolmogorov Behavior of Near-Wall Turbulence and Its Application in Turbulence Modeling  

NASA Technical Reports Server (NTRS)

The near-wall behavior of turbulence is re-examined in a way different from that proposed by Hanjalic and Launder and followers. It is shown that at a certain distance from the wall, all energetic large eddies will reduce to Kolmogorov eddies (the smallest eddies in turbulence). All the important wall parameters, such as friction velocity, viscous length scale, and mean strain rate at the wall, are characterized by Kolmogorov microscales. According to this Kolmogorov behavior of near-wall turbulence, the turbulence quantities, such as turbulent kinetic energy, dissipation rate, etc. at the location where the large eddies become Kolmogorov eddies, can be estimated by using both direct numerical simulation (DNS) data and asymptotic analysis of near-wall turbulence. This information will provide useful boundary conditions for the turbulent transport equations. As an example, the concept is incorporated in the standard k-epsilon model which is then applied to channel and boundary flows. Using appropriate boundary conditions (based on Kolmogorov behavior of near-wall turbulence), there is no need for any wall-modification to the k-epsilon equations (including model constants). Results compare very well with the DNS and experimental data.

Shih, Tsan-Hsing; Lumley, John L.

1992-01-01

213

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

214

High Speed Ice Friction  

NASA Astrophysics Data System (ADS)

Many different tribological experiments have been run to determine the frictional behaviour of ice at high speeds, ostensibly with the intention of applying results to everyday fields such as winter tyres and sports. However, experiments have only been conducted up to linear speeds of several metres a second, with few additional subject specific studies reaching speeds comparable to these applications. Experiments were conducted in the cold rooms of the Rock and Ice Physics Laboratory, UCL, on a custom built rotational tribometer based on previous literature designs. Preliminary results from experiments run at 2m/s for ice temperatures of 271 and 263K indicate that colder ice has a higher coefficient of friction, in accordance with the literature. These results will be presented, along with data from further experiments conducted at temperatures between 259-273K (in order to cover a wide range of the temperature dependent behaviour of ice) and speeds of 2-15m/s to produce a temperature-velocity-friction map for ice. The effect of temperature, speed and slider geometry on the deformation of ice will also be investigated. These speeds are approaching those exhibited by sports such as the luge (where athletes slide downhill on an icy track), placing the tribological work in context.

Seymour-Pierce, Alexandra; Sammonds, Peter; Lishman, Ben

2014-05-01

215

Bioinspired orientation-dependent friction.  

PubMed

Spatular terminals on the toe pads of a gecko play an important role in directional adhesion and friction required for reversible attachment. Inspired by the toe pad design of a gecko, we study friction of polydimethylsiloxane (PDMS) micropillars terminated with asymmetric (spatular-shaped) overhangs. Friction forces in the direction of and against the spatular end were evaluated and compared to friction forces on symmetric T-shaped pillars and pillars without overhangs. The shape of friction curves and the values of friction forces on spatula-terminated pillars were orientation-dependent. Kinetic friction forces were enhanced when shearing against the spatular end, while static friction was stronger in the direction toward the spatular end. The overall friction force was higher in the direction against the spatula end. The maximum value was limited by the mechanical stability of the overhangs during shear. The aspect ratio of the pillar had a strong influence on the magnitude of the friction force, and its contribution surpassed and masked that of the spatular tip for aspect ratios of >2. PMID:25178923

Xue, Longjian; Iturri, Jagoba; Kappl, Michael; Butt, Hans-Jürgen; del Campo, Aránzazu

2014-09-23

216

Computer modeling of granular and two-phase turbulent flows  

NASA Astrophysics Data System (ADS)

This thesis focuses on developing a computational model for analyzing rapid granular and turbulent two-phase flows in various regions. In this work, a computational scheme for simulating dry frictional granular chute flows is developed. A kinetic-based model which includes the frictional energy loss effects is used, and the boundary conditions for a bumpy wall with small friction are derived by ensuring the balance of momentum and energy. At the free surface, the condition of vanishing solid volume fraction is used. The mean velocity, the fluctuation kinetic energy and the solid volume fraction profiles are evaluated. It is shown that steady granular gravity flow down a bumpy frictional chute could be achieved at arbitrary inclination angles. The computational results also show that the slip velocity may vary considerably depending on the granular layer height, the surface boundary roughness, the friction coefficient and the inclination angles. A small friction coefficient and a smooth wall lead to a region of low density and high fluctuation energy in the neighborhood of the wall. For high friction coefficients and rough walls, the solid volume fraction increases monotonically up to the wall, while a region of low fluctuation energy is formed near the solid surface. The model predictions are compared with the existing experimental and simulation data, and good agreement is observed. In particular, the model can well predictate the features of the variation of solid volume fraction and fluctuation energy profiles for different particle-wall friction and restitution coefficients and wall roughness. Two-phase gas-particle turbulent flows at various loadings in vertical, horizontal and inclined channels and in a vertical pipe are also analyzed. Thermodynamically consistent two- phase turbulent flow models that account for the particle-particle collisions and the phasic fluctuation kinetic energy interactions are used, and a computational model for analyzing dilute and dense turbulent flows in ducts is developed. The governing equations for the gas-phase turbulence are upgraded to a two-equation low Reynolds number turbulence closure model that can be integrated directly to the wall. Two specific models are used in the analysis. The first model is isotropic and the equations governing the phasic fluctuation kinetic energy and dissipation rates resemble the extended k-? type turbulence model. The second model is rate-dependent and anisotropic that allows capturing the anisotropy of particulate and fluid phase turbulent stresses. A no-slip boundary condition for the fluid phase and slip velocity boundary condition for the particle phase are used in both cases. The computational model is first applied to dilute gas- particle turbulent flows. The predicted mean velocity and turbulence intensity profiles are compared with various experimental data, and good agreement is observed. Examples of additional flow properties such as the phasic fluctuation energy, phasic fluctuation energy production and dissipation, as well as interaction momentum and energy supply terms are also presented and discussed. Application of the model to relatively dense gas-particle turbulent flows are also described. The model predictions are compared with the experimental data of Miller and Gidaspow and reasonable agreement is observed. It is shown that flow behavior is strongly affected by the phasic fluctuation energy, and the momentum and energy transfer between the particulate and the fluid constituents. For aerosol particles, a new two-fluid model for evaluating the particle deposition velocity in turbulent channel flows is described. The rate-dependent model is first used to calculate the components of particle turbulence intensities for gas-particle turbulent flows in a vertical channel. Then the model of Reeks (1983) and Guha (1997) is used for evaluating the particle wall deposition rates. Variations of particle deposition velocity with particle relaxation time are evaluated and the results are compared with the available experimental data and ea

Cao, Jianfa

217

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

218

Turbulence modeling  

NASA Technical Reports Server (NTRS)

The objective of this work is to develop, verify, and incorporate the baseline two-equation turbulence models which account for the effects of compressibility into the three-dimensional Reynolds averaged Navier-Stokes (RANS) code and to provide documented descriptions of the models and their numerical procedures so that they can be implemented into 3-D CFD codes for engineering applications.

Bardina, Jorge E.

1995-01-01

219

Burgers Turbulence  

E-print Network

The last decades witnessed a renewal of interest in the Burgers equation. Much activities focused on extensions of the original one-dimensional pressureless model introduced in the thirties by the Dutch scientist J.M. Burgers, and more precisely on the problem of Burgers turbulence, that is the study of the solutions to the one- or multi-dimensional Burgers equation with random initial conditions or random forcing. Such work was frequently motivated by new emerging applications of Burgers model to statistical physics, cosmology, and fluid dynamics. Also Burgers turbulence appeared as one of the simplest instances of a nonlinear system out of equilibrium. The study of random Lagrangian systems, of stochastic partial differential equations and their invariant measures, the theory of dynamical systems, the applications of field theory to the understanding of dissipative anomalies and of multiscaling in hydrodynamic turbulence have benefited significantly from progress in Burgers turbulence. The aim of this review is to give a unified view of selected work stemming from these rather diverse disciplines.

Jeremie Bec; Konstantin Khanin

2007-04-12

220

Burgers turbulence  

NASA Astrophysics Data System (ADS)

The last decades witnessed a renewal of interest in the Burgers equation. Much activities focused on extensions of the original one-dimensional pressureless model introduced in the thirties by the Dutch scientist J.M. Burgers, and more precisely on the problem of Burgers turbulence, that is the study of the solutions to the one- or multi-dimensional Burgers equation with random initial conditions or random forcing. Such work was frequently motivated by new emerging applications of Burgers model to statistical physics, cosmology, and fluid dynamics. Also Burgers turbulence appeared as one of the simplest instances of a nonlinear system out of equilibrium. The study of random Lagrangian systems, of stochastic partial differential equations and their invariant measures, the theory of dynamical systems, the applications of field theory to the understanding of dissipative anomalies and of multiscaling in hydrodynamic turbulence have benefited significantly from progress in Burgers turbulence. The aim of this review is to give a unified view of selected work stemming from these rather diverse disciplines.

Bec, Jérémie; Khanin, Konstantin

2007-08-01

221

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

222

Effects of unsteady free stream velocity and free stream turbulence on stagnation point heat transfer  

NASA Technical Reports Server (NTRS)

The combined effects of transient free stream velocity and free stream turbulence on heat transfer at a stagnation point over a cylinder situated in a crossflow are studied. An eddy diffusivity model was formulated and the governing momentum and energy equations are integrated by means of the steepest descent method. The numerical results for the wall shear stress and heat transfer rate are correlated by a turbulence parameter. The wall friction and heat transfer rate increase with increasing free stream turbulence intensity.

Gorla, R. S. R.

1984-01-01

223

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

2010-07-16

224

Skin friction reduction by slot injection at Mach 0.8  

NASA Technical Reports Server (NTRS)

Surface skin friction, boundary layer profiles and turbulent intensity due to axially symmetric tangential slot injection into a transonic boundary layer were measured. Effects of slot height, multiple slot injection, and injection mass flow rate on the surface skin friction downstream of the the slot have been investigated. Surface skin friction was a function of the injection mass flow rate for x/s 40. Large normal pressure gradient and relatively large turbulent intensity were found near the slot with small injection mass flow rate; the region the high turbulent intensity moved downstream with increasing injection mass flow rate. The results with two slot injections indicated that the distance between slots should be less than 30 slot heights in order to achieve some benefits from the first slot. Of significant importance in the present investigation is that the skin friction reduction obtained at transonic speed is of the same order as obtained in the hypersonic regime. Additional work is required in order to formulate a correlation between the turbulent intensity and the injection mass flow rate that may be used in future analysis.

Zakkay, V.; Wang, C. R.

1976-01-01

225

Heat transfer, flow friction, noise and vibration studies of perforated surfaces  

Microsoft Academic Search

A systematic investigation was conducted to study perforated plate surfaces which find important application compact heat exchanger design either as primary or secondary heat transfer surfaces. It was concluded that surface perforation results in enhanced heat transfer and flow friction performance only in the transition and turbulent flow regimes. The enhancement is due to the combined effects of boundary layer

C. Y. Liang

1975-01-01

226

Tidal friction for semidiurnal tides  

Microsoft Academic Search

The quadratic law of bottom friction demands an increased frictional coefficient for the S2 and N2 tides with respect to a dominant M2 tidal signal. A numerical model of the semidiurnal tides in the northeast Atlantic gives an increase in friction of ˜35% for the N2, S2 and K2 tides with respect to the M2 tide and this value is

R. D. Pingree; D. K. Griffiths

1987-01-01

227

The Friction of Piston Rings  

NASA Technical Reports Server (NTRS)

The coefficient of friction between piston ring and cylinder liner was measured in relation to gliding acceleration, pressure, temperature, quantity of oil and quality of oil. Comparing former lubrication-technical tests, conclusions were drawn as to the state of friction. The coefficients of friction as figured out according to the hydrodynamic theory were compared with those measured by tests. Special tests were made on "oiliness." The highest permissible pressure was measured and the ratio of pressure discussed.

Tischbein, Hans W

1945-01-01

228

Turbulent Parametrizations of Turbulent flows  

NASA Astrophysics Data System (ADS)

This work is a study about the different turbulent models existing and their role in environmental flows. The code TELEMAC 3D as well as other RANS and LES codes aplicable to fully turbulence flows are compared. We quantify and classify the different models of turbulence that work best in non-homogeneous and complex flows in function of the number of solved equations and boundary conditions. Then we carry out some test cases to compare the results with experiments and field observations. The full DNS resolution of Navier-Stokes equations is possible only for the simple BC cases and low Reynolds numbers. It involves a great power of computing but there exists an alternative to resolving Navier-Stokes equations if we just want the mean value of quantities (velocity, pressure, temperature...). We apply the mean operator in the motion equations; therefore a Reynolds's decomposition on the unknowns. The new equations are called RANS in opposition of the motions equations The comparison shows differences between these models, especially the form and local shape of the turbulence magnitudes. The biggest differences are found for stratified flows with a test case producing a mixing zone between the two different fluids. The channel and the horizontal mesh are the same. For the vertical mesh, we refine the mixing zone and compare it with salt wedges and estuarine mixing results.

Calmet, H.; Furmanek, P.

2009-04-01

229

Correlations of velocity and temperature fluctuations in the stagnation-point flow of circular cylinder in turbulent flow  

NASA Technical Reports Server (NTRS)

Boundary layer flow and turbulence transport analyses to study the influence of the free-stream turbulence on the surface heat transfer rate and the skin friction around the stagnation point of a circular cylinder in a turbulent flow are presented. The analyses are formulated with the turbulent boundary layer equations, the Reynolds stress transport equations and the k - epsilon two-equation turbulence modeling. The analyses are used to calculate the time-averaged turbulence double correlations, the mean flow properties, the surface heat transfer rate and the skin friction with an isotropic turbulence in the freestream. The analytical results are described and compared with the existing experimental measurements. Depending on the free-stream turbulence properties, the turbulence kinetic energy can increase or decrease as the flow moves toward the surface. However, the turbulence kinetic energy induces large Reynolds normal stresses at the boundary layer edge. The Reynolds normal stresses change the boundary layer profiles of the time-averaged double correlations of the velocity and temperature fluctuations, the surface heat transfer rate and the skin friction. The free-stream turbulence dissipation rate can affect the stagnation-point heat transfer rate but the influence of the free-stream temperature fluctuation on the heat transfer rate is insignificant.

Wang, Chi R.

1988-01-01

230

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

231

Action of friction Frictional processes are not often considered in  

E-print Network

. The first term on the right­hand­side represents barotropic damping by friction, and the second are studying baroclinic effects of friction using numerical simulations of fric­ tionally­damped baroclinic­ aly over the low centre, where the vorticity is high. Positive, baroclinically­generated PV is formed

Plant, Robert

232

Filament turbulence  

NASA Astrophysics Data System (ADS)

How much information do you need to distinguish between different mechanisms for spatiotemporal chaos in three-dimensions? In this talk, I will show that the observation of the dynamics on the surface of a medium can be sufficient. Studying mechanisms for filament turbulence in the context of reaction-diffusion media, we found numerically that two major classes of instabilities leave a very different signature on what can be observed on the surface of a three-dimensional medium. These results are of direct relevance in the context of ventricular fibrillation - a turbulent electrical wave activity that destroys the coherent contraction of the ventricular muscle and its main pumping function leading to sudden cardiac death. While it has been proposed that the three-dimensional structure of the heart plays an important role in this type of filament turbulence, only the surface of the heart is currently accessible to experimental observation preventing the study of the full dynamics. Our results suggest that such observations might be sufficient.

Davidsen, Joern

2010-03-01

233

Vortex Lines in a model of Turbulent Flow  

NASA Astrophysics Data System (ADS)

Recent experiments have showed that the isothermal, high Reynolds number flow of Helium II is similar to classical turbulence. The evidence comes from measurements of mass flow rates and pressure drops in pipe flow, high Taylor number Couette flow, turbulent vortex rings and grid turbulence. Motivated by these phenomena, we have studied numerically the behaviour of quantized vortex lines in a model of normal fluid turbulence based on the vortex tube structures of ABC flows. We have found that the vortex lines, driven by a vortex wave instability and mutual friction, concentrate in the regions of high normal vorticity. Thus bundles of vortex lines are created which mimic the vorticity of the normal fluid. We have investigated the time scale of this effect and time dependent ABC models. The results support the idea that a high density of vortex lines creates a state of vortex - coupled turbulence.

Barenghi, Carlo F.

1996-11-01

234

Alteration of outer flow structures for turbulent drag reduction  

NASA Technical Reports Server (NTRS)

Paper reviews large eddy behavior in turbulent boundary layers and presents a survey of turbulent wall layers subjected to abrupt changes in boundary conditions. From this survey several methods have been identified with potential for altering the turbulence production process in the outer region of the boundary layer and reducing the turbulent skin-friction drag over relatively long downstream distances; these include convex longitudinal surface curvature, large-eddy breakup devices, and Emmons spot alteration. Paper also presents results of an ongoing experimental study to parametrically evaluate large-eddy breakup devices as a turbulent drag reduction concept and reproduce the 20 percent net drag reductions found in large-eddy breakup experiments at Illinois Institute of Technology.

Hefner, J. N.; Anders, J. B.; Bushnell, D. M.

1983-01-01

235

Finger pad friction and its role in grip and touch  

PubMed Central

Many aspects of both grip function and tactile perception depend on complex frictional interactions occurring in the contact zone of the finger pad, which is the subject of the current review. While it is well established that friction plays a crucial role in grip function, its exact contribution for discriminatory touch involving the sliding of a finger pad is more elusive. For texture discrimination, it is clear that vibrotaction plays an important role in the discriminatory mechanisms. Among other factors, friction impacts the nature of the vibrations generated by the relative movement of the fingertip skin against a probed object. Friction also has a major influence on the perceived tactile pleasantness of a surface. The contact mechanics of a finger pad is governed by the fingerprint ridges and the sweat that is exuded from pores located on these ridges. Counterintuitively, the coefficient of friction can increase by an order of magnitude in a period of tens of seconds when in contact with an impermeably smooth surface, such as glass. In contrast, the value will decrease for a porous surface, such as paper. The increase in friction is attributed to an occlusion mechanism and can be described by first-order kinetics. Surprisingly, the sensitivity of the coefficient of friction to the normal load and sliding velocity is comparatively of second order, yet these dependencies provide the main basis of theoretical models which, to-date, largely ignore the time evolution of the frictional dynamics. One well-known effect on taction is the possibility of inducing stick–slip if the friction decreases with increasing sliding velocity. Moreover, the initial slip of a finger pad occurs by the propagation of an annulus of failure from the perimeter of the contact zone and this phenomenon could be important in tactile perception and grip function. PMID:23256185

Adams, Michael J.; Johnson, Simon A.; Lefevre, Philippe; Levesque, Vincent; Hayward, Vincent; Andre, Thibaut; Thonnard, Jean-Louis

2013-01-01

236

Finger pad friction and its role in grip and touch.  

PubMed

Many aspects of both grip function and tactile perception depend on complex frictional interactions occurring in the contact zone of the finger pad, which is the subject of the current review. While it is well established that friction plays a crucial role in grip function, its exact contribution for discriminatory touch involving the sliding of a finger pad is more elusive. For texture discrimination, it is clear that vibrotaction plays an important role in the discriminatory mechanisms. Among other factors, friction impacts the nature of the vibrations generated by the relative movement of the fingertip skin against a probed object. Friction also has a major influence on the perceived tactile pleasantness of a surface. The contact mechanics of a finger pad is governed by the fingerprint ridges and the sweat that is exuded from pores located on these ridges. Counterintuitively, the coefficient of friction can increase by an order of magnitude in a period of tens of seconds when in contact with an impermeably smooth surface, such as glass. In contrast, the value will decrease for a porous surface, such as paper. The increase in friction is attributed to an occlusion mechanism and can be described by first-order kinetics. Surprisingly, the sensitivity of the coefficient of friction to the normal load and sliding velocity is comparatively of second order, yet these dependencies provide the main basis of theoretical models which, to-date, largely ignore the time evolution of the frictional dynamics. One well-known effect on taction is the possibility of inducing stick-slip if the friction decreases with increasing sliding velocity. Moreover, the initial slip of a finger pad occurs by the propagation of an annulus of failure from the perimeter of the contact zone and this phenomenon could be important in tactile perception and grip function. PMID:23256185

Adams, Michael J; Johnson, Simon A; Lefèvre, Philippe; Lévesque, Vincent; Hayward, Vincent; André, Thibaut; Thonnard, Jean-Louis

2013-03-01

237

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

238

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

239

Dynamics, Friction, And Complementarity Problems  

Microsoft Academic Search

. An overview is presented here of recent work and approaches to solving dynamicproblems in rigid body mechanics with friction. It begins with the differential inclusion approachto Coulomb friction where the normal contact force is known. Numerical methods for these differentialinclusions commonly lead to linear or nonlinear complementarity problems. Then the measuredifferential inclusion formulation of rigid body dynamics due to

D. e. Stewart; J. c. Trinkle

1995-01-01

240

ORIGINAL PAPER Thermally Activated Friction  

E-print Network

provided friction coef- ficient data down to ~77 K under He [3, 4] and N2 [2] gas streams. Contamination with water has been a particular concern for these studies; 1 ppb of H20 in the gas stream is the equilibrium the interface and influenc- ing the nature of sliding and wear. Of these, frictional forces play a central role

Sawyer, Wallace

241

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

242

Characteristics of turbulence in boundary layer with zero pressure gradient  

NASA Technical Reports Server (NTRS)

The results of an experimental investigation of a turbulent boundary layer with zero pressure gradient are presented. Measurements with the hot-wire anemometer were made of turbulent energy and turbulent shear stress, probability density and flattening factor of u-fluctuation (fluctuation in x-direction), spectra of turbulent energy and shear stress, and turbulent dissipation. The importance of the region near the wall and the inadequacy of the concept of local isotropy are demonstrated. Attention is given to the energy balance and the intermittent character of the outer region of the boundary layer. Also several interesting features of the spectral distribution of the turbulent motions are discussed.

Klebanoff, P S

1955-01-01

243

Eliminating friction with friction: 2D Janssen effect in a friction-driven system.  

PubMed

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. PMID:24856724

Karim, M Yasinul; Corwin, Eric I

2014-05-01

244

Magnetohydrodynamic Turbulence  

NASA Astrophysics Data System (ADS)

Magnetohydrodynamic (MHD) turbulence theory is modeled on neutral fluid (Navier-Stokes) turbulence theory, but with some important differences. There have been essentially no repeatable laboratory MHD experiments wherein the boundary conditions could be controlled or varied and a full set of diagnostics implemented. The equations of MHD are convincingly derivable only in the limit of small ratio of collision mean-free-paths to macroscopic length scales, an inequality that often goes the other way for magnetofluids of interest. Finally, accurate information on the MHD transport coefficients-and thus, the Reynolds-like numbers that order magnetofluid behavior-is largely lacking; indeed, the algebraic expressions used for such ingredients as the viscous stress tensor are often little more than wishful borrowing from fluid mechanics. The one accurate thing that has been done extensively and well is to solve the (strongly nonlinear) MHD equations numerically, usually in the presence of rectangular periodic boundary conditions, and then hope for the best when drawing inferences from the computations for those astrophysical and geophysical MHD systems for which some indisputably turbulent detailed data are available, such as the solar wind or solar prominences. This has led to what is perhaps the first field of physics for which computer simulations are regarded as more central to validating conclusions than is any kind of measurement. Things have evolved in this way due to a mixture of the inevitable and the bureaucratic, but that is the way it is, and those of us who want to work on the subject have to live with it. It is the only game in town, and theories that have promised more-often on the basis of some alleged ``instability''-have turned out to be illusory.

Montgomery, David C.

2004-01-01

245

Friction drive position transducer  

NASA Astrophysics Data System (ADS)

A spring force loaded contact wheel mounted in a stationary position relative to a reciprocating shaft is disclosed. The apparatus of the present invention includes a tensioning assembly for maintaining absolute contact between the contact wheel and the reciprocating shaft wherein the tensioning assembly urges the contact wheel against the shaft to maintain contact therebetween so that the wheel turns as the shaft is linearly displaced. A rotary encoding device is coupled to the wheel for translating the angular and rotational movement thereof into an electronic signal for providing linear displacement information and derivative data with respect to displacement of the shaft. Absolute friction contact and cooperative interaction between the shaft and the contact wheel is further enhanced in the preferred embodiment by advantageously selecting the types of surface finish and the amount of surface area of the contact wheel relative to the surface condition of the shaft as well as by reducing the moment of inertia of the contact wheel.

Waclawik, Ronald E.; Cayer, James L.; Lapointe, Kenneth M.

1991-10-01

246

Rolling friction robot fingers  

NASA Technical Reports Server (NTRS)

A low friction, object guidance, and gripping finger device for a robotic end effector on a robotic arm is disclosed, having a pair of robotic fingers each having a finger shaft slideably located on a gripper housing attached to the end effector. Each of the robotic fingers has a roller housing attached to the finger shaft. The roller housing has a ball bearing mounted centering roller located at the center, and a pair of ball bearing mounted clamping rollers located on either side of the centering roller. The object has a recess to engage the centering roller and a number of seating ramps for engaging the clamping rollers. The centering roller acts to position and hold the object symmetrically about the centering roller with respect to the X axis and the clamping rollers act to position and hold the object with respect to the Y and Z axis.

Vranish, John M. (inventor)

1992-01-01

247

Parameter influence on friction welding of dissimilar surface-carburized\\/volume-hardened alloyed steels  

Microsoft Academic Search

The influence of the main friction-welding parameters, such as the axial pressure, the friction stroke and the upsetting temperature on the compositional, structural and hardness gradient is shown for dissimilar joints made out of carburized and volume-hardened steels. The expulsion of the carbon-enriched layer in the burr as a qualitative factor is analyzed. Low axial pressure and long friction time

I. Mitelea; C. M. Craciunescu

2010-01-01

248

Aspects of Turbulent / Non-Turbulent Interfaces  

NASA Technical Reports Server (NTRS)

A distinct boundary between turbulent and non-turbulent regions in a fluid of otherwise constant properties is found in many laboratory and engineering turbulent flows, including jets, mixing layers, boundary layers and wakes. Generally, the flow has mean shear in at least one direction within t he turbulent zone, but the non-turbulent zones have no shear (adjacent laminar shear is a different case, e.g. transition in a boundary layer). There may be purely passive differences between the turbulent and non-turbulent zones, e.g. small variations in temperature or scalar concentration, for which turbulent mixing is an important issue. The boundary has several major characteristics of interest for the present study. Firstly, the boundary advances into the non-turbulent fluid, or in other words, nonturbulent fluid is entrained. Secondly, the change in turbulence properties across the boundary is remarkably abrupt; strong turbulent motions come close to the nonturbulent fluid, promoting entrainment. Thirdly, the boundary is irregular with a continually changing convoluted shape, which produces statistical intermittency. Its shape is contorted at all scales of the turbulent motion.

Bisset, D. K.; Hunt, J. C. R.; Rogers, M. M.; Koen, Dennis (Technical Monitor)

1999-01-01

249

Flow Friction or Spontaneous Ignition?  

NASA Technical Reports Server (NTRS)

"Flow friction," a proposed ignition mechanism in oxygen systems, has proved elusive in attempts at experimental verification. In this paper, the literature regarding flow friction is reviewed and the experimental verification attempts are briefly discussed. Another ignition mechanism, a form of spontaneous combustion, is proposed as an explanation for at least some of the fire events that have been attributed to flow friction in the literature. In addition, the results of a failure analysis performed at NASA Johnson Space Center White Sands Test Facility are presented, and the observations indicate that spontaneous combustion was the most likely cause of the fire in this 2000 psig (14 MPa) oxygen-enriched system.

Stoltzfus, Joel M.; Gallus, Timothy D.; Sparks, Kyle

2012-01-01

250

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

251

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

252

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

253

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

254

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

255

Turbulent eddy viscosity modeling in transonic shock/boundary-layer interactions  

NASA Technical Reports Server (NTRS)

The treatment of turbulence effects on transonic shock/turbulent boundary layer interaction is addressed within the context of a triple deck approach valid for arbitrary practical Reynolds numbers between 1000 and 10 billion. The modeling of the eddy viscosity and basic turbulent boundary profile effects in each deck is examined in detail using Law-of-the-Wall/Law-of-the-Wake concepts as the foundation. Results of parametric studies showing how each of these turbulence model aspects influences typical interaction zone property distributions (wall pressure, displacement thickness and local skin friction) are presented and discussed.

Inger, G. R.

1989-01-01

256

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

257

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.

258

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

259

A Method of Normalizing Dynamic Friction Data  

Microsoft Academic Search

A method was described to normalize wear-induced changes in the dynamic friction of surfaces lubricated with an additive in oil solution. Changing dynamic friction data were normalized by comparing them at equivalent degrees of boundary lubrication, determined by measuring the coefficient of friction of the surfaces in neat white oil. Friction tests were made with three different additives to demonstrate

Clarence Albertson; George Wolfram

1969-01-01

260

Evaluation of friction loss in flexible and galvanized duct  

E-print Network

or similar exterior material. 3. When exterior abrasion of the duct can be a factor, a vinyl scuff strip can be glued or plastic welded in spiral fashion on the outside cover. 4. Reduction in flow because of Internal friction can be minimized by using... or similar exterior material. 3. When exterior abrasion of the duct can be a factor, a vinyl scuff strip can be glued or plastic welded in spiral fashion on the outside cover. 4. Reduction in flow because of Internal friction can be minimized by using...

Zimmermann, Carlos Michael Alberto

2012-06-07

261

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

262

Observations of the vertical structure of turbulent oscillatory boundary layers above fixed roughness using a prototype wideband coherent Doppler profiler: 2. Turbulence and stress  

NASA Astrophysics Data System (ADS)

Measurements of turbulence and shear stress in oscillatory boundary layers are reported from experiments carried out with a prototype wideband coherent Doppler profiler above fixed roughness beds of 0.37 mm diameter sand and 3.9 mm diameter gravel. The 10 s oscillation period and 0.75 m to 1.5 m oscillation excursions correspond to roughness Reynolds numbers for the gravel bed in the 290 to 490 range, assuring fully rough turbulent conditions. Bottom stress was estimated via the law-of-the-wall, the vertical integral of the defect acceleration, and the Reynolds stress. The Reynolds stress was obtained from the second moment of the beam-coordinate velocities. Bed friction factors, fw, from the defect stresses are in reasonable agreement with predictions based on Swart's empirical relation as modified by Nielsen (1992) and with values determined using Laser Doppler Anemometry (LDA) by Sleath (1987) via the defect method and by Jensen (1988) via the law-of-the-wall. The fw values determined here from the law-of-the-wall are higher than predicted (ca. 50% higher for the gravel bed), likely due to background vertical shear associated with residual motions in the tank. The Reynolds stresses are lower than the predictions by a factor of 3 to 4, compared to the factor of 5 to 10 obtained by Sleath (1987). Beam coordinate turbulent kinetic energy spectra indicate that the vertical momentum flux is mostly associated with fluctuations between the forcing frequency and the inertial subrange, the latter contributing typically less than 10% of the total observed Reynolds stress.

Hay, Alex E.; Zedel, Len; Cheel, Richard; Dillon, Jeremy

2012-03-01

263

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

264

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

265

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

266

Rubber friction and tire dynamics.  

PubMed

We propose a simple rubber friction law, which can be used, for example, in models of tire (and vehicle) dynamics. The friction law is tested by comparing numerical results to the full rubber friction theory (Persson 2006 J. Phys.: Condens. Matter 18 7789). 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 accurately calculate ?-slip curves (and the self-aligning torque) 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. PMID:21406818

Persson, B N J

2011-01-12

267

Rubber friction and tire dynamics  

NASA Astrophysics Data System (ADS)

We propose a simple rubber friction law, which can be used, for example, in models of tire (and vehicle) dynamics. The friction law is tested by comparing numerical results to the full rubber friction theory (Persson 2006 J. Phys.: Condens. Matter 18 7789). 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 accurately calculate ?-slip curves (and the self-aligning torque) 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.

Persson, B. N. J.

2011-01-01

268

Size scaling of static friction.  

PubMed

Sliding friction across a thin soft lubricant film typically occurs by stick slip, the lubricant fully solidifying at stick, yielding and flowing at slip. The static friction force per unit area preceding slip is known from molecular dynamics (MD) simulations to decrease with increasing contact area. That makes the large-size fate of stick slip unclear and unknown; its possible vanishing is important as it would herald smooth sliding with a dramatic drop of kinetic friction at large size. Here we formulate a scaling law of the static friction force, which for a soft lubricant is predicted to decrease as f(m)+?f/A(?) for increasing contact area A, with ?>0. Our main finding is that the value of f(m), controlling the survival of stick slip at large size, can be evaluated by simulations of comparably small size. MD simulations of soft lubricant sliding are presented, which verify this theory. PMID:23473164

Braun, O M; Manini, Nicola; Tosatti, Erio

2013-02-22

269

Subsonic and turbulent reactive flows  

Microsoft Academic Search

The FAST2D code which is a two dimensional hydrocode was developed, it was modified and used to model homogeneous flows. The speed of the transport algorithm which advances in time the density, momentum, and energy was improved by a factor of two. This improved code was used to model the transition to turbulence for shear flow. To model such a

K. Kailasanath; M. Fry; E. Hyman

1983-01-01

270

Radiation friction vs ponderomotive effect  

E-print Network

The concept of ponderomotive potential is upgraded to a regime in which radiation friction becomes dominant. The radiation friction manifests itself in novel features of long-term capturing of the particles released at the focus and impenetrability of the focus from the exterior. We apply time scales separation to the Landau-Lifshitz equation splitting the particle motion into quivering and slow drift of a guiding center. The drift equation is deduced by averaging over fast motion.

Fedotov, A M; Gelfer, E G; Narozhny, N B; Ruhl, H

2014-01-01

271

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

272

The influence of high temperatures on the tribological properties of automotive friction materials  

NASA Astrophysics Data System (ADS)

Temperatures of over 800C can be generated at the frictional interface within the brake systems of large vehicles, such high temperatures result in severe wear at the frictional interface, and can also lead to a very dangerous condition known as brake fade, characterised by a sharp fall in the coefficient of friction between the pad and disc, resulting in a catastrophic loss of braking efficiency. Common friction materials are very specialised composites often containing up to 15 components bound together within a phenolic resin matrix. The high temperature behaviour of the various constituents of friction materials were investigated using thermogravimetric analysis, focusing in particular on the thermal decomposition of the phenolic resin matrix material, where it has been firmly established that the thermal decomposition products of phenolic resin are the primary cause of brake fade. This has lead to the development of a novel approach for reducing fade in conventional resin based friction materials, involving a partial carbonisation to 400C. The high temperature wear characteristics of both modified and conventional friction materials were examined using standard dynamometer tests, as well as a 'continuous drag' type test machine, equipped with a heating facility. During this study a number of factors were identified as the main influences on the overall wear behaviour of friction materials. These included test temperature, sample test history, and the various effects of friction films, which were the subject of a detailed analysis. The formation of friction films was found to be an important facet of a successful friction material, producing a reduction in wear at the frictional interface. Films were examined and analysed using EDX, SEM, and X-ray diffraction techniques, which revealed the presence of a high proportion of magnetite (Fe3O4), containing iron which originated from the disc surface. It was established that the incorporation of iron in friction material formulations encouraged film formation, thereby reducing disc wear substantially.

Savage, Luke

273

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

2014-08-01

274

Process parameters optimization for friction stir welding of RDE40 aluminium alloy using Taguchi technique  

Microsoft Academic Search

Taguchi approach was applied to determine the most influential control factors which will yield better tensile strength of the joints of friction stir welded RDE-40 aluminium alloy. In order to evaluate the effect of process parameters such as tool rotational speed, traverse speed and axial force on tensile strength of friction stir welded RDE-40 aluminium alloy, Taguchi parametric design and

A. K. LAKSHMINARAYANAN; V. BALASUBRAMANIAN

2008-01-01

275

Adhesion energy between mica surfaces: Implications for the frictional coefficient under dry and wet conditions  

NASA Astrophysics Data System (ADS)

frictional strength of faults is a critical factor that contributes to continuous fault slip and earthquake occurrence. Frictional strength can be reduced by the presence of sheet-structured clay minerals. In this study, two important factors influencing the frictional coefficient of minerals were quantitatively analyzed by a newly developed computational method based on a combination of first-principles study and thermodynamics. One factor that helps reduce the frictional coefficient is the low adhesion energy between the layers under dry conditions. Potassium ions on mica surfaces are easily exchanged with sodium ions when brought into contact with highly concentrated sodium-halide solutions. We found that the surface ion exchange with sodium ions reduces the adhesion energy, indicating that the frictional coefficient can be reduced under dry conditions. Another factor is the lubrication caused by adsorbed water films on mineral surfaces under wet conditions. Potassium and sodium ions on mica surfaces have a strong affinity for water molecules. In order to remove the adsorbed water molecules confined between mica surfaces, a differential compressive stress of the order of tens of gigapascals was necessary at room temperature. These water molecules inhibit direct contact between mineral surfaces and reduce the frictional coefficient. Our results imply that the frictional coefficient can be modified through contact with fluids depending on their salt composition. The low adhesion energy between fault-forming minerals and the presence of an adsorbed water film is a possible reason for the low frictional coefficient observed at continuous fault slip zones.

Sakuma, Hiroshi

2013-12-01

276

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

277

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

278

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

279

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

280

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

281

Rate and state sea ice friction  

NASA Astrophysics Data System (ADS)

The distribution of ice thicknesses in the Arctic is a function of the ice deformation which occurs through ridging, rafting and sliding of ice floes. To determine the relative importance of each of these forms of deformation, it is crucial to have a good model of ice friction. We present data on ice-ice friction from a series of large ice-tank experiments, undertaken at the HSVA ship testing facility in Hamburg, Germany. We focus on the impact of varying the sliding rate, and the hold-time before commencing sliding, following the work of Ruina (1983). We move a 2m square floating ice block, of thickness 25cm, under horizontal normal stress, and detail the force required to move the block and hence the implied friction coefficient ?. Loads are of the order 1kN. We find that the time-averaged friction coefficient shows slight rate-dependence, and ? = 0.3-0.4 for a variety of rates (from 1-10cm/s). However, the detailed sliding mechanism varies with rate, and stick-slip behaviour is observed at low rates. The state-dependence is found to be a crucial factor in determining the load required to initiate movement of the ice block. To test for state dependence we apply the side load for a given time interval (the hold time) before starting to move the block. With a hold time of 1000s, the forces are an order of magnitude greater than with a hold time of 10s, and we present data for hold times from 1- 1000s. This work has important implications for sea ice rheology components within global climate models, particularly given that recent satellite observations show that almost all the deformation of Arctic sea ice is due to in-plane frictional sliding. The results outlined above suggest that the static contact time between ice floes may be the key parameter influencing the ensemble movement of sea ice. The work also has value for smaller-scale modelling of sea ice for engineering purposes, for example in predicting forces on offshore structures. As well as presenting our results, we will discuss possible further experiments to extend the range of validity of the work.

Lishman, B.; Sammonds, P.

2009-04-01

282

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

283

Light propagation through anisotropic turbulence.  

PubMed

A wealth of experimental data has shown that atmospheric turbulence can be anisotropic; in this case, a Kolmogorov spectrum does not describe well the atmospheric turbulence statistics. In this paper, we show a quantitative analysis of anisotropic turbulence by using a non-Kolmogorov power spectrum with an anisotropic coefficient. The spectrum we use does not include the inner and outer scales, it is valid only inside the inertial subrange, and it has a power-law slope that can be different from a Kolmogorov one. Using this power spectrum, in the weak turbulence condition, we analyze the impact of the power-law variations ? on the long-term beam spread and scintillation index for several anisotropic coefficient values ?. We consider only horizontal propagation across the turbulence cells, assuming circular symmetry is maintained on the orthogonal plane to the propagation direction. We conclude that the anisotropic coefficient influences both the long-term beam spread and the scintillation index by the factor ?(2-?). PMID:21383832

Toselli, Italo; Agrawal, Brij; Restaino, Sergio

2011-03-01

284

A note on the use of surface pitot tubes for the measurement of skin friction in laminar boundary layers  

Microsoft Academic Search

A calibration is deduced for the measurement of skin friction by means of a small circular pitot tube attached to a surface. The analysis is primarily concerned with laminar boundary layer but limits are suggested for which the calibration is expected to be approximately valid for both laminar and turbulent flows.

D. I. A. Poll

1983-01-01

285

A note on the use of surface pitot tubes for the measurement of skin friction in laminar boundary layers  

NASA Astrophysics Data System (ADS)

A calibration is deduced for the measurement of skin friction by means of a small circular pitot tube attached to a surface. The analysis is primarily concerned with laminar boundary layer but limits are suggested for which the calibration is expected to be approximately valid for both laminar and turbulent flows.

Poll, D. I. A.

1983-03-01

286

Wave Turbulence  

E-print Network

In this paper we review recent developments in the statistical theory of weakly nonlinear dispersive waves, the subject known as Wave Turbulence (WT). We revise WT theory using a generalisation of the random phase approximation (RPA). This generalisation takes into account that not only the phases but also the amplitudes of the wave Fourier modes are random quantities and it is called the ``Random Phase and Amplitude'' approach. This approach allows to systematically derive the kinetic equation for the energy spectrum from the the Peierls-Brout-Prigogine (PBP) equation for the multi-mode probability density function (PDF). The PBP equation was originally derived for the three-wave systems and in the present paper we derive a similar equation for the four-wave case. Equation for the multi-mode PDF will be used to validate the statistical assumptions about the phase and the amplitude randomness used for WT closures. Further, the multi-mode PDF contains a detailed statistical information, beyond spectra, and it finally allows to study non-Gaussianity and intermittency in WT, as it will be described in the present paper. In particular, we will show that intermittency of stochastic nonlinear waves is related to a flux of probability in the space of wave amplitudes.

Yeontaek Choi; Yuri V. Lvov; Sergey Nazarenko

2004-12-14

287

The First Turbulence and First Fossil Turbulence  

Microsoft Academic Search

A model is proposed connecting turbulence, fossil turbulence and the big-bang origin of the universe. While details are incomplete, the model is consistent with our knowledge of these processes and is supported by observations. Turbulence arises in a hot big-bang quantum gravitational dynamics scenario at Planck scales. Chaotic, eddy-like motions produce an exothermic Planck particle cascade from 10-35 m at

Carl H. Gibson

2004-01-01

288

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

289

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

290

Frictional characteristics and heat transfer of antimisting fuels in tubes. Final report Aug 80-Sep 81  

SciTech Connect

Experiments have been performed to determine the skin friction and heat transfer behavior of antimisting kerosene (AMK) in pipe flows. The additive used in the AMK was FM-9 developed by Imperial Chemical Industries. AMK has been developed as an aviation safety fuel to reduce post-crash fires. The principle aim of the present investigation was to determine the modification in flow and heat transfer behavior caused by the presence of the antimisting polymer additive in jet fuel. The present study indicates that the AMK skin friction versus Reynolds number, or Nusselt number versus Reynolds number behavior, can be divided into three regions: (1) Newtonian laminar region, (2) shear-thickening transition region, and (3) drag-reducing turbulent region. At low flow rates, AMK has Newtonian behavior, i.e., constant viscosity. At a certain critical wallshear rate which depends on the fuel temperature and additive concentration, shear thickening occurs and causes a large increase in skin friction and heat transfer rates. In the third region, the skin friction and heat transfer rates drop rapidly and fall below the predicted Newtonian flow skin friction and heat transfer values; e.g., for 0.3 percent FM-9 AMK at a temperature of 20 C, 22,000 and 10,000. Beyond these points, there is a reduction in skin friction and heat transfer rates.

Wat, J.; Sarohia, V.

1982-08-01

291

Stagnation point heat transfer augmentation due to free stream turbulence  

NASA Technical Reports Server (NTRS)

A model has been proposed for the momentum eddy diffusivity induced by free stream turbulence intensity and integral length scale. The eddy diffusivity model is applied to the stagnation point of a cylinder situated in a uniform crossflow in the presence of free stream turbulence. A numerical solution of the governing momentum and energy equations with the proposed eddy diffusivity model yielded results for the skin friction coefficient and the Nusselt number. The numerical predictions of the present work are compared with experimental data and the agreement between the two is seen to be very good.

Gorla, R. S. R.

1982-01-01

292

Slow rupture of frictional interfaces  

NASA Astrophysics Data System (ADS)

The failure of frictional interfaces and the spatiotemporal structures that accompany it are central to a wide range of geophysical, physical and engineering systems. Recent geophysical and laboratory observations indicated that interfacial failure can be mediated by slow slip rupture phenomena which are distinct from ordinary, earthquake-like, fast rupture. These discoveries have influenced the way we think about frictional motion, yet the nature and properties of slow rupture are not completely understood. We show that slow rupture is an intrinsic and robust property of simple non-monotonic rate-and-state friction laws. It is associated with a new velocity scale cmin, determined by the friction law, below which steady state rupture cannot propagate. We further show that rupture can occur in a continuum of states, spanning a wide range of velocities from cmin to elastic wave-speeds, and predict different properties for slow rupture and ordinary fast rupture. Our results are qualitatively consistent with recent high-resolution laboratory experiments and may provide a theoretical framework for understanding slow rupture phenomena along frictional interfaces.

Bar Sinai, Yohai; Brener, Efim A.; Bouchbinder, Eran

2012-02-01

293

TURBULENT SHEAR ACCELERATION  

SciTech Connect

We consider particle acceleration by large-scale incompressible turbulence with a length scale larger than the particle mean free path. We derive an ensemble-averaged transport equation of energetic charged particles from an extended transport equation that contains the shear acceleration. The ensemble-averaged transport equation describes particle acceleration by incompressible turbulence (turbulent shear acceleration). We find that for Kolmogorov turbulence, the turbulent shear acceleration becomes important on small scales. Moreover, using Monte Carlo simulations, we confirm that the ensemble-averaged transport equation describes the turbulent shear acceleration.

Ohira, Yutaka, E-mail: ohira@phys.aoyama.ac.jp [Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara 252-5258 (Japan)

2013-04-10

294

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

295

Turbulent drag reduction through oscillating discs  

E-print Network

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...

Wise, Daniel J

2014-01-01

296

Studies on the frictional behavior of magnetic recording tapes  

NASA Technical Reports Server (NTRS)

Methods were developed for measuring frictional forces acting on a magnetic tape in motion or at rest, as well as the extent of stick slip. The effects of factors such as temperature, humidity, kind of gaseous atmosphere, and tape speed on the frictional interaction between various tapes and heads were investigated. Results were instrumental in the selection of a tape for the Mariner Mars 1971 spacecraft. Studies are reported on the stick slip behavior of tapes and the performance of a metallic tape, compared with the usual plastic tapes.

Kalfayan, S. H.; Silver, R. H.; Hoffman, J. K.

1972-01-01

297

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

298

Low temperature friction force microscopy  

NASA Astrophysics Data System (ADS)

The application of friction force techniques within atomic force microscopy (AFM) allows for direct measurements of friction forces at a sliding, single-asperity interface. The temperature dependence of such single-asperity contacts provides key insight into the comparative importance of dissipative mechanisms that result in dry sliding friction. A variable temperature (VT), ultrahigh vacuum (UHV) AFM was used with an interface consisting of a diamond coated AFM tip and diamond-like carbon sample in a nominal sample temperature range of 90 to 275K. The results show that the coefficient of kinetic friction, mu k, has a linear dependence that is monotonically increasing with temperature varying from 0.28 to 0.38. To analyze this data it is necessary to correlate the sample temperature to the interface temperature. A detailed thermal model shows that the sample temperature measured by a macroscopic device can be very different from the temperature at the contact point. Temperature gradients intrinsic to the design of VT, UHV AFMs result in extreme, non-equilibrium conditions with heat fluxes on the order of gigawatts per squared meter through the interface, which produce a discontinuous step in the temperature profile due to thermal boundary impedance. The conclusion from this model is that measurements acquired by VT, UHV AFM, including those presented in this thesis, do not provide meaningful data on the temperature dependence of friction for single-asperities. Plans for future work developing an isothermal AFM capable of the same measurements without the introduction of temperature gradients are described. The experimental results and thermal analysis described in this thesis have been published in the Journal of Applied Physics, "Temperature dependence of single-asperity friction for a diamond on diamondlike carbon interface", J. App. Phys., 107(11):114903, 2010.

Dunckle, Christopher Gregory

299

Estimation of the frictional dissipation of lunisolar tide  

NASA Astrophysics Data System (ADS)

Lunisolar tidal friction and variation in the earth's moment of inertia are the major factors for the long-term change in the length of day (lod). This article uses the latest geophysical and paleontological clock data to study and compare numerically the lunisolar tidal friction, variation in the terrestrial moment of inertia and long-term change in the lod for the past 1500 Ma. Two important conclusions are obtained: 1) the variation in J2 can not be explained solely by the rotational deformation of the earth, which shows that gravitational differentiation still exists in the earth; 2) tidal friction was much greater a few hundred million years ago than it is now. If it is assumed that tidal dissipation is inversely proportional to the cube of the distance between the earth and the moon, then the theoretical results of the numbers of days in a tropical year and in a synodic month accord rather well with the paleontological clock data.

Gao, Bu-xi

300

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

301

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

302

Accurate and Independent Measurements of Wall-Shear Stress in Turbulent Flows  

NASA Astrophysics Data System (ADS)

Oil Film Interferometry (OFI) is used to directly measure the wall-shear stress in the high Reynolds number turbulent boundary layers from three facilities used for ICET. Various optical arrangements were utilized to collect the digital images generated on transparent plugs integrated into the boundary layer surface. Test-section free stream velocities ranging from 10 to 60 m/s and development lengths from 5.5m to 21 m, resulted in friction velocities varying from 0.35 to 1.65 m/s, corresponding to boundary layer thicknesses varying by factors of nearly four. Silicon oils with viscosities from 20 to 1000 cSt were employed in the measurements, with multiple oils used for several of the test conditions. A reference temperature measurement was used in all three facilities and for the calibration of the oils as a function of temperature in four different laboratories using two types of viscometers. The processing of the images was carried out using several approaches and compared for consistency of the results. Results of the skin friction coefficient from the three wind tunnels are examined and compared as a function of the displacement thickness Reynolds number, as determined from hot-wire and Pitot probe profiles at comparable conditions, and are found to be accurately represented by the logarithmic Rotta relation. The various uncertainties and the final accuracy of this type of measurement are discussed.

R"Uedi, J.-D.; Duncan, R.; Imayama, S.; Chauhan, K.

2009-11-01

303

On the impact of wave-like disturbances on turbulent fluxes and turbulence statistics in nighttime conditions: a case study  

NASA Astrophysics Data System (ADS)

The interpretation of flux measurements in nocturnal conditions is typically fraught with challenges. This paper reports on how the presence of wave-like disturbances in a time series, can lead to an overestimation of turbulence statistics, errors when calculating the stability parameter, erroneous estimation of the friction velocity u* used to screen flux data, and errors in turbulent flux calculations. Using time series of the pressure signal from a microbarograph, wave-like disturbances at an AmeriFlux site are identified. The wave-like disturbances are removed during the calculation of turbulence statistics and turbulent fluxes. Our findings suggest that filtering eddy-covariance data in the presence of wave-like events prevents both an~overestimation of turbulence statistics and errors in turbulent flux calculations. Results show that large-amplitude wave-like events, events surpassing three standard deviations, occurred on 18% of the nights considered in the present study. Remarkably, on flux towers located in a very stably stratified boundary-layer regime, the presence of a gravity wave can enhance turbulence statistics more than 50%. In addition, the presence of the disturbance modulates the calculated turbulent fluxes of CO2 resulting in erroneous turbulent flux calculations of the order of 10% depending on averaging time and pressure perturbation threshold criteria. Furthermore, the friction velocity u* was affected by the presence of the wave, and in at least one case, a 10% increase caused u* to exceed the arbitrary 0.25 m s-1 threshold used in many studies. This results in an unintended bias in the data selected for analysis in the flux calculations. The impact of different averaging periods was also examined and found to be variable specific. These early case study results provide an insight into errors introduced when calculating "purely" turbulent fluxes. These results could contribute to improving modeling efforts by providing more accurate inputs of both turbulent kinetic energy, and isolating the turbulent component of u* for flux selection in the stable nocturnal boundary layer.

Durden, D. J.; Nappo, C. J.; Leclerc, M. Y.; Duarte, H. F.; Zhang, G.; Parker, M. J.; Kurzeja, R. J.

2013-12-01

304

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

305

Dynamics of sliding mechanisms in nanoscale friction  

E-print Network

Nanotribology is the study of friction and wear at the nanoscale, with relevance to such applications as micromechanical systems (MEMS) and thin, hard coatings. For these systems, classical laws of friction are inappropriate ...

Yim, Shon W., 1973-

2002-01-01

306

Turbulent-Laminar Patterns in Pipes and Channels  

NASA Astrophysics Data System (ADS)

When fluid flows through a channel, pipe, or duct, there are two basic forms of motion: smooth laminar motion and complex turbulent motion. The discontinuous transition between these states is a fundamental problem that has been studied for more than 100 years. What has received far less attention is the large-scale nature of the turbulent flows near transition once they are established. We have carried out extensive numerical computations in pipes and channels to investigate the nature of transitional turbulence in these flow. We show the existence of three fundamentally different turbulent states separated by two distinct Reynolds numbers. In the case of pipe flow for example, below Re approximately 2200, turbulence takes the form of familiar equilibrium (or long-time transient) puffs. The turbulence is intensive -- puffs are localized and the ratio of turbulent to laminar flow is not dictated by system size but by factors such as initial conditions. At Re=2200 the flow makes a striking transition to extensive turbulence where the amount of turbulent flow scales with pipe length. The asymptotic state is an irregular (intermittent) alternation of turbulent and laminar flow whose complexity is inherent and does not result from random initial disturbances. Intermittency continues until Re=2500 where the intermittency factor, and other measures, reveal a continuous transition to a state of uniform turbulence along the pipe. We argue that these states are a manifestation of universal large-scale structures in transitional shear flows.

Barkley, Dwight

2010-03-01

307

The Power of Friction: Quantifying the ``Goodness'' of Frictional Grasps \\Lambda  

E-print Network

The Power of Friction: Quantifying the ``Goodness'' of Frictional Grasps \\Lambda Marek Teichmann of fingers, coefficient of friction and the the goodness of a grasp. In particular, we give a general framework for defining a grasp metric that takes friction into account. Our approach rectifies a flaw

Mishra, Bud

308

Re-Filling Probe Hole of Friction Spot Joints by Friction Forming  

Microsoft Academic Search

Friction stir spot welding (FSSpW) is a variant of friction stir welding (FSW) and having wider industrial application. However, Probe hole left after the welding is the main limitation of their process. In this present study, modified FSSpW has been developed and the refilling is achieved by friction forming process. This process has been named as Friction Stir Spot Welding

S. John Prakash; S. Muthukumaran

2011-01-01

309

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

310

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

311

Low-Friction Joint for Robot Fingers  

NASA Technical Reports Server (NTRS)

Mechanical linkage allows adjacent parts to move relative to each other with low friction and with no chatter, slipping, or backlash. Low-friction joint of two surfaces in rolling contact, held in alinement by taut flexible bands. No sliding friction or "stick-slip" motion: Only rolling-contact and bending friction within bands. Proposed linkage intended for finger joints in mechanical hands for robots and manipulators.

Ruoff, C. F.

1985-01-01

312

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.

313

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

314

Skin Friction Measurements by a Dual-Laser-Beam Interferometer Technique  

NASA Technical Reports Server (NTRS)

A portable dual-laser-beam interferometer that nonintrusively measures skin friction by monitoring the thickness change of an oil film subject to shear stress is described. The method is an advance over past versions in that the troublesome and error-introducing need to measure the distance to the oil leading edge and the starting time for the oil flow has been eliminated. The validity of the method was verified by measuring oil viscosity in the laboratory, and then using those results to measure skin friction beneath the turbulent boundary layer in a low speed wind tunnel. The dual-laser-beam skin friction measurements are compared with Preston tube measurements, with mean velocity profile data in a "law-of-the-well" coordinate system, and with computations based on turbulent boundary-layer theory. Excellent agreement is found in all cases. (This validation and the aforementioned improvements appear to make the present form of the instrument usable to measure skin friction reliably and nonintrusively in a wide range of flow situations in which previous methods are not practical.)

Monson, D. J.; Higuchi, H.

1981-01-01

315

Transitional and turbulent boundary layer with heat transfer  

NASA Astrophysics Data System (ADS)

We report on our direct numerical simulation of an incompressible, nominally zero-pressure-gradient flat-plate boundary layer from momentum thickness Reynolds number 80-1950. Heat transfer between the constant-temperature solid surface and the free-stream is also simulated with molecular Prandtl number Pr=1. Skin-friction coefficient and other boundary layer parameters follow the Blasius solutions prior to the onset of turbulent spots. Throughout the entire flat-plate, the ratio of Stanton number and skin-friction St/Cf deviates from the exact Reynolds analogy value of 0.5 by less than 1.5%. Mean velocity and Reynolds stresses agree with experimental data over an extended turbulent region downstream of transition. Normalized rms wall-pressure fluctuation increases gradually with the streamwise growth of the turbulent boundary layer. Wall shear stress fluctuation, ?w,rms'+, on the other hand, remains constant at approximately 0.44 over the range, 800Turbulent Prandtl number Prt peaks at around 1.9 at the wall, and decreases monotonically toward the boundary layer edge with no near-wall secondary peak, in good agreement with previous boundary layer heat transfer experiments. In the transitional region, turbulent spots are tightly packed with numerous hairpin vortices. With the advection and merging of turbulent spots, these young isolated hairpin forests develop into the downstream turbulent region. Isosurfaces of temperature up to Re?=1900 are found to display well-resolved signatures of hairpin vortices, which indicates the persistence of the hairpin forests.

Wu, Xiaohua; Moin, Parviz

2010-08-01

316

Vertical velocity and turbulence aspects during Mistral events as observed by UHF wind profilers  

NASA Astrophysics Data System (ADS)

The general purpose of this paper is to experimentally study mesoscale dynamical aspects of the Mistral in the coastal area located at the exit of the Rhône-valley. The Mistral is a northerly low-level flow blowing in southern France along the Rhône-valley axis, located between the French Alps and the Massif Central, towards the Mediterranean Sea. The experimental data are obtained by UHF wind profilers deployed during two major field campaigns, MAP (Mesoscale Alpine Program) in autumn 1999, and ESCOMPTE (Expérience sur Site pour COntraindre les Modèles de Pollution atmosphériques et de Transports d'Emission) in summer 2001. Thanks to the use of the time evolution of the vertical profile of the horizontal wind vector, recent works have shown that the dynamics of the Mistral is highly dependent on the season because of the occurrence of specific synoptic patterns. In addition, during summer, thermal forcing leads to a combination of sea breeze with Mistral and weaker Mistral due to the enhanced friction while, during autumn, absence of convective turbulence leads to substantial acceleration as low-level jets are generated in the stably stratified planetary boundary layer. At the exit of the Rhône valley, the gap flow dynamics dominates, whereas at the lee of the Alps, the dynamics is driven by the relative contribution of "flow around" and "flow over" mechanisms, upstream of the Alps. This paper analyses vertical velocity and turbulence, i.e. turbulent dissipation rate, with data obtained by the same UHF wind profilers during the same Mistral events. In autumn, the motions are found to be globally and significantly subsident, which is coherent for a dry, cold and stable flow approaching the sea, and the turbulence is found to be of pure dynamical origin (wind shears and mountain/lee wave breaking), which is coherent with non-convective situations. In summer, due to the ground heating and to the interactions with thermal circulation, the vertical motions are less pronounced and no longer have systematic subsident charateristics. In addition, those vertical motions are found to be much less developed during the nighttimes because of the stabilization of the nocturnal planetary boundary layer due to a ground cooling. The enhanced turbulent dissipation-rate values found at lower levels during the afternoons of weak Mistral cases are consistent with the installation of the summer convective boundary layer and show that, as expected in weaker Mistral events, the convection is the preponderant factor for the turbulence generation. On the other hand, for stronger cases, such a convective boundary layer installation is perturbed by the Mistral.

Caccia, J.; Guénard, V.; Benech, B.; Campistron, B.; Drobinski, P.

2004-11-01

317

Large eddy simulation of controlled transition to turbulence  

NASA Astrophysics Data System (ADS)

Large eddy simulation of H- and K-type transitions in a spatially developing zero-pressure-gradient boundary layer at Ma? = 0.2 is investigated using several subgrid scale (SGS) models including constant coefficient Smagorinsky and Vreman models and their dynamic extensions, dynamic mixed scale-similarity, dynamic one-equation kinetic energy model, and global coefficient Vreman models. A key objective of this study is to assess the capability of SGS models to predict the location of transition and the skin friction throughout the transition process. The constant coefficient models fail to detect transition, but the dynamic procedure allows for a negligible turbulent viscosity in the early transition region. As a result, the "point" of transition is estimated correctly. However, after secondary instabilities set in and result in the overshoot in the skin friction profile, all models fail to produce sufficient subgrid scale shear stress required for the correct prediction of skin friction and the mean velocity profile. The same underprediction of skin friction persists into the turbulent region. Spatially filtered direct numerical simulation data in the same boundary layers are used to provide guidelines for SGS model development and validation.

Sayadi, Taraneh; Moin, Parviz

2012-11-01

318

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

319

Friction stir welding of aluminium plate  

Microsoft Academic Search

Friction stir welding is a novel welding process which is a solid state butt welding process for sheet or plate using the friction phenomenon.This paper describes the effect of the rotation speed of the stir rod and the plate traverse speed during the friction stir welding process on the quality of welded joints. It was found that aluminium plates could

T. Shinoda; Y. Kondo

1997-01-01

320

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.

321

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

322

Valvetrain Friction Reduction Through Surface Engineering  

Microsoft Academic Search

ABSTRACTIn a direct acting mechanical bucket tappet type valvetrain, the cam and tappet contact is responsible for about 85% of the total valvetrain frictional losses. Since this contact operates primarily in mixed lubrication regime, it offers an opportunity for friction reduction through surface engineering. The friction reduction potential of thin Mn-phosphate coating, diamond-like carbon coating, and polishing on bucket surface

Arup Gangopadhyay; Douglas G. McWatt; Robert J. Zdrodowski; Steve J. Simko; Steve Matera; Kirk Sheffer; Robert S. Furby

2011-01-01

323

Dynamical Friction in Stellar Systems: an introduction  

Microsoft Academic Search

An introductory exposition of Chandrasekhar's gravitational dynamical friction, appropriate for an undergraduate class in mec hanics, is presented. This friction results when a massive particle moving through a ``sea'' of much lighter star particles experiences a retarding force du to an exchange of energy and momentum. General features of dynamical friction are presented, both in an elementary and in a

Hector Aceves; Maria Colosimo

2006-01-01

324

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

325

Development of an enstrophy-based two-equation turbulence closure model  

NASA Astrophysics Data System (ADS)

The development of a new two-equation turbulence closure model based on the exact turbulent kinetic energy, k and the variance of vorticity, or enstrophy, zeta is presented. The primary motivation was to develop a model, applicable to complex three-dimensional flowfields, that employs one set of model constants and does not use damping functions or geometrical factors. Development begins by considering a number of two-dimensional and axisymmetric flowfields in order to determine the appropriate closure coefficients. First, similarity solutions of a variety of both planar and axisymmetric free shear flows are considered. Next, a variety of wall bounded flows are examined beginning with a boundary layer solution of a flat plate and proceeding to the Navier-Stokes solutions for a variety of two-dimensional airfoils. The airfoils considered range from a low speed stalled airfoil to a transonic airfoil with shock induced separation. Final model validation was performed by considering a supersonic three-dimensional Cylinder-Offset flare. In general, good agreement with experiment is indicated. Moreover, the k-zeta model performed, in most cases, as well as or better than the other models. The above objective has been achieved. The current model is shown to accurately predict growth rates as well as similarity profiles of velocity, turbulent kinetic energy, and shear stress for a variety of both planar and axisymmetric free shear flows. Moreover, the model predicts skin-friction, pressure distribution, and shock position with good accuracy for a variety of wall bounded flows, including flows with large adverse pressure gradients and shock induced separation. Also, the current model solves both the free shear and wall bounded flows using only one set of closure coefficients and boundary conditions. Furthermore, the current model is free of wall damping functions and geometrical factors in both the governing equations and in the definition of eddy viscosity. This makes the model highly suited for three-dimensional applications.

Robinson, David Franklin

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

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

2005-11-22

328

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

329

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

330

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

331

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 geodesi...

Bini, Donato; Gregoris, Daniele; Succi, Sauro

2014-01-01

332

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

333

Dynamical friction in colliding galaxies  

Microsoft Academic Search

Effects of dynamical friction have been studied in a head-on collision between two galaxies. The galaxies are represented by Plummer's model of various masses and scale lengths. Analytical expressions are obtained for changes in the binding energies of the galaxies due to the tidal effects and numerical results are given for various pairs of colliding galaxies.

Farooq Ahmed

1979-01-01

334

Application of Navier-Stokes code PAB3D with kappa-epsilon turbulence model to attached and separated flows  

NASA Technical Reports Server (NTRS)

A three-dimensional Navier-Stokes solver was used to determine how accurately computations can predict local and average skin friction coefficients for attached and separated flows for simple experimental geometries. Algebraic and transport equation closures were used to model turbulence. To simulate anisotropic turbulence, the standard two-equation turbulence model was modified by adding nonlinear terms. The effects of both grid density and the turbulence model on the computed flow fields were also investigated and compared with available experimental data for subsonic and supersonic free-stream conditions.

Abdol-Hamid, Khaled S.; Lakshmanan, B.; Carlson, John R.

1995-01-01

335

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

336

Friction Networks: Network-Configurations of Dynamic Friction Patterns  

NASA Astrophysics Data System (ADS)

The complex configurations of dynamic friction patterns-regarding real time contact areas- are transformed into appropriate networks. With this transformation of a system to network space, many properties can be inferred about the structure and dynamics of the system. Here, we analyze the dynamics of static friction, i.e. nucleation processes, with respect to "friction networks". We show that networks can successfully capture the crack-like shear ruptures and possible corresponding acoustic features. We found that the fraction of triangles remarkably scales with the detachment fronts. There is a universal power law between nodes' degree and motifs frequency . We confirmed the obtained universality in aperture-based friction networks. Based on the achieved results, we extracted a possible friction law in terms of network parameters and compared it with the rate and state friction laws. In particular, the evolutions of loops are scaled with power law, indicating the aggregation of cycles around hub nodes. Also, the transition to slow rupture is scaled with the fast variation of local heterogeneity. Furthermore, the motif distributions and modularity space of networks -in terms of within-module degree and participation coefficient-show non-uniform general trends, indicating a universal aspect of energy flow in shear ruptures. As a conclusion to our study, we introduced friction networks over dynamics of different real time contact areas. Based on our solid observations, we formulated a probabilistic frame for the evolution of the state variable in terms of friction networks. Moreover, we confirmed that slow ruptures generally hold small localization, while regular ruptures carry a high level of energy localization. We also introduced two new universalities with respect to the evolution of dry frictional interfaces: the scaling of local and global characteristics and the occupation of certain regions of modularity parameter space. Our results showed how the relatively highly correlated "elements" of an interface can reveal more features of the underlying dynamics. We proposed that assortativity as an index to correlation of node's degree can completely uncover acoustic features of the interfaces. Our formulation can be coupled with elasto-dynamic equations to complete our understanding of the interface's more realistic features. [1] E.Bouchbinder, E. A. Brener, I. Barel, and M. Urbakh, arXiv: 1103.3942(2011). [2] Y. Bar Sinai, E.A. Brener, E. Bouchbinder, arXiv: 1111.3246(2011). [3] Y. Kaneko, J.-P. Ampuero, geophysical research letters, 38, l21307 (2011) [4] O. M. Braun, I. Barel, and M. Urbakh, Phys. Rev. Lett. 103, 194301 (2009). [5] A.Ruina J. Geophys. Res., 88, 10359-10370 (1983). [6] H.O.Ghaffari, and R.P Young, http://arxiv.org/abs/1105.4265 (2011). [7] H.O.Ghaffari, and R.P. Young, http://arxiv.org/abs/1108.5078 (2011).

Ghaffari, H.; Young, R. P.

2012-04-01

337

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

338

Friction induced hunting limit cycles: A comparison between the LuGre and switch friction model  

Microsoft Academic Search

In this paper, friction induced limit cycles are predicted for a simple motion system consisting of a motor-driven inertia subjected to friction and a PID-controlled regulator task. The two friction models used, i.e., (i) the dynamic LuGre friction model and (ii) the static switch friction model, are compared with respect to the so-called hunting phenomenon. Analysis tools originating from the

Ron H. A. Hensen; M. J. G. van de Molengraft; Maarten Steinbuch

2003-01-01

339

Prediction of dynamic friction forces in spur gears using alternate sliding friction formulations  

NASA Astrophysics Data System (ADS)

In this communication, several sliding friction formulations used in spur gear dynamics are examined and compared in terms of the predictions of interfacial friction forces and off-line-of-action motions. Competing friction formulations include Coulomb models with time-varying friction coefficients and empirical expressions based on elasto-hydrodynamic and/or boundary lubrication regime principles. Predicted results compare well with friction force measurements.

He, Song; Cho, Sungmin; Singh, Rajendra

2008-01-01

340

Modeling Compressed Turbulence  

SciTech Connect

From ICE to ICF, the effect of mean compression or expansion is important for predicting the state of the turbulence. When developing combustion models, we would like to know the mix state of the reacting species. This involves density and concentration fluctuations. To date, research has focused on the effect of compression on the turbulent kinetic energy. The current work provides constraints to help development and calibration for models of species mixing effects in compressed turbulence. The Cambon, et al., re-scaling has been extended to buoyancy driven turbulence, including the fluctuating density, concentration, and temperature equations. The new scalings give us helpful constraints for developing and validating RANS turbulence models.

Israel, Daniel M. [Los Alamos National Laboratory

2012-07-13

341

Introduction to quantum turbulence  

PubMed Central

The term quantum turbulence denotes the turbulent motion of quantum fluids, systems such as superfluid helium and atomic Bose–Einstein condensates, which are characterized by quantized vorticity, superfluidity, and, at finite temperatures, two-fluid behavior. This article introduces their basic properties, describes types and regimes of turbulence that have been observed, and highlights similarities and differences between quantum turbulence and classical turbulence in ordinary fluids. Our aim is also to link together the articles of this special issue and to provide a perspective of the future development of a subject that contains aspects of fluid mechanics, atomic physics, condensed matter, and low-temperature physics. PMID:24704870

Barenghi, Carlo F.; Skrbek, Ladislav; Sreenivasan, Katepalli R.

2014-01-01

342

Introduction to quantum turbulence.  

PubMed

The term quantum turbulence denotes the turbulent motion of quantum fluids, systems such as superfluid helium and atomic Bose-Einstein condensates, which are characterized by quantized vorticity, superfluidity, and, at finite temperatures, two-fluid behavior. This article introduces their basic properties, describes types and regimes of turbulence that have been observed, and highlights similarities and differences between quantum turbulence and classical turbulence in ordinary fluids. Our aim is also to link together the articles of this special issue and to provide a perspective of the future development of a subject that contains aspects of fluid mechanics, atomic physics, condensed matter, and low-temperature physics. PMID:24704870

Barenghi, Carlo F; Skrbek, Ladislav; Sreenivasan, Katepalli R

2014-03-25

343

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

344

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.

345

Building blocks of turbulence  

E-print Network

Turbulence is ubiquitous in nature and although the equations governing fluid flow are well known, there are no analytical expressions that describe the complexity of turbulent motion. The nonlinear nature and the large number of spatial and temporal degrees of freedom turn this into one of the most challenging problems in mathematics and the physical sciences alike. We here report the discovery of unstable localised solutions for pipe flow that share key spatial characteristics of turbulence in the intermittent regime. While their temporal dynamics are very simple, much of the spatial complexity found in low Reynolds number turbulence is already encoded in them. We furthermore demonstrate how turbulent transients arise from one such solution branch. Our observations shed light on the origin of turbulence and link the localised structures commonly observed in turbulent flows to invariant solutions of the Navier-Stokes equations.

Avila, Marc; Roland, Nicolas; Hof, Bjoern

2013-01-01

346

Planck-Kerr Turbulence  

E-print Network

A quantum gravitational instability is identified at Planck scales between non-spinning extreme Schwarzschild black holes and spinning extreme Kerr black holes, which produces a turbulent Planck particle gas. Planck inertial vortex forces balance gravitational forces as the Planck turbulence cascades to larger scales and the universe expands and cools. Turbulent mixing of temperature fluctuations and viscous dissipation of turbulent kinetic energy provide irreversibilities necessary to sustain the process to the strong force freeze out temperature where inflation begins. Turbulent temperature fluctuations are fossilized when they are stretched by inflation beyond the horizon scale of causal connection. As the horizon of the expanding universe grows, the fluctuations seed patterns of nucleosynthesis, and these seed the formation of structure in the plasma epoch. Fossil big bang turbulence is supported by extended self similarity coefficients computed for cosmic microwave background temperature anisotropies that match those for high Reynolds number turbulence.

Carl H. Gibson

2003-04-24

347

A two-equation multidimensional model of turbulent bubbly flows  

NASA Astrophysics Data System (ADS)

A new model of turbulence in bubbly flows was developed based on the two-phase extension of single-phase k-3 model. Phase indicator function approach together with ensemble averaging was applied to the single-phase equations of fluid motion to receive a two-fluid model. An exact equation of turbulent kinetic energy for the two-phase system was derived. This equation contained single-phase and unknown interfacial terms. A closure was proposed for the turbulent interfacial terms. The proposed closure was based on the assumption of high density ratio typical for the most of bubbly flows. The interfacial turbulence terms account for an additional turbulence in liquid created by the bubble wakes. The modeled form of liquid dissipation rate balance contained two distinct turbulence dissipation time scales: one for the single-phase shear induced turbulence and the other for the bubble induced turbulence. The proposed turbulence model contains unknown empirical constants. To estimate the values of these constants, the model was implemented in CFX4.2 commercial CFD solver. Comparing numerical prediction to experiment, constant values were estimated by trial and error method. To verify universality of found constants, model's predictions were compared to other experiments. The comparison showed, that the model constants have certain generality. In particular, experimentally observed phenomenon of bubble induced liquid turbulence reduction was predicted and elucidated. Model was also able to qualitatively predict bubble size effect on the liquid turbulence. However, it was found that for some downward flows eddy diffusivity assumption is not valid. A new logarithmic wall law was derived for bubbly flows. The derivation of the law was based on the assumption of additional turbulent viscosity associated with bubble wakes in the boundary layer. The new wall law contained empirical constant accounting for non-linearity of bubble and shear induced turbulence interaction. The value of this constant was deduced from experimental data. An improved wall friction prediction was achieved with the new wall law over conventional single-phase wall law. The improvement was especially noticeable for the low liquid flow rates when bubble induced turbulence plays a significant role. The model was also able to predict bubble size effect on the wall shear stress.

Troshko, Andrey Arturovich

348

Turbulent heat transfer and pressure drop characteristics of dilute water based Al2O3-Cu hybrid nanofluids.  

PubMed

A study on fully developed turbulent convective heat transfer and pressure drop characteristics of Al2O3-Cu/water hybrid nanofluid flowing through a uniformly heated circular tube is presented in this paper. For this, Al2O3-Cu nanocomposite powder was synthesized in a thermo chemical route using hydrogen reduction technique and dispersed the hybrid nano powder in deionised water to form a stable hybrid nanofluid of 0.1% volume concentration. The prepared powder was characterized by X-ray Diffraction (XRD) and Scanning Electron Microscope (SEM) to confirm the chemical composition, determine the particle size and study the surface morphology. Stability of the nanofluid was ensured by pH and zeta potential measurements. The average heat transfer enhancement for Al2O3-Cu/water hybrid nanofluid is 8.02% when compared to pure water. The experimental results also showed that 0.1% Al2O3-Cu/water hybrid nanofluids have slightly higher friction factor compared to 0.1% Al2O3/water nanofluid. The empirical correlations proposed for Nusselt number and friction factor were well agreed with the experimental data. PMID:24745264

Suresh, S; Venkitaraj, K P; Hameed, M Shahul; Sarangan, J

2014-03-01

349

Stick-slip friction and wear of articular joints.  

PubMed

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

2013-02-12

350

Hypersonic laminar–turbulent transition on circular cones and scramjet forebodies  

Microsoft Academic Search

Laminar–turbulent transition in hypersonic boundary layers has a dramatic effect on heat transfer, skin friction, and separation. This effect is critical to reentry vehicles and airbreathing cruise vehicles, yet the physics of the transition process is not yet well enough understood to be used for predictive purposes. The literature for transition on circular cones and scramjet forebodies is reviewed, from

Steven P. Schneider

2004-01-01

351

PIV Measurements of a Shock Wave\\/Turbulent Boundary Layer Interaction  

Microsoft Academic Search

Particle Image Velocimetry is used to investigate the interaction between an incident planar shock wave and turbulent boundary layer developing on a flat plate at Mach 2.1. The mean velocity profile and deduced skin friction coefficient of the undisturbed boundary layer show good agreement with theory. A particle response assessment establishes the fidelity of the tracer particles. The interaction region

R. A. Humble; F. Scarano; B. W. van Oudheusden; M. Tuinstra

2006-01-01

352

Evaluation of the RAMS model for estimating turbulent fluxes over the Chesapeake Bay  

Microsoft Academic Search

This study has examined the ability of a mesoscale model to compute pollutant deposition velocity and surface fluxes over water with the accuracy needed for air pollution studies. The Regional Atmospheric Modeling System (RAMS) was evaluated against buoy data over the Chesapeake Bay. Turbulence and surface layer variables (e.g., surface fluxes, friction velocity) and ultimately deposition velocities were primarily examined

J. T. McQueen; R. A. Valigura; B. J. B. Stunder

1997-01-01

353

Langmuir turbulence and deeply penetrating jets in an unstratified mixed layer  

E-print Network

and by diagnosing Rt and the Eulerian mean and Stokes shears. When either Lat or ds are sufficiently small. In the latter, tke production is from the mean shear and is locally balanced by dissipation. We define the turbulent Langmuir number Lat = (v*/Us)0.5 (v* is the ocean's friction velocity and Us is the surface Stokes

Polton, Jeff

354

MEASUREMENT OF WALL SHEAR STRESS IN TURBULENT BOUNDARY LAYERS USING AN OPTICAL INTERFEROMETRY METHOD  

E-print Network

. The wall shear stress is calculated by measuring the velocity from a Pitot tube xed on the wall and a wall turbulent boundary layers. A calibration chart is required and the most commonly used is that of Pa- tel not depend on the law of the wall and preferably one which does not require calibration. A skin friction

Marusic, Ivan

355

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

356

Effect of anisotropic turbulence on aerodynamic noise. [Lighthill theory mathematical model for axisymmetric turbulence  

NASA Technical Reports Server (NTRS)

A model based on Lighthill's theory for predicting aerodynamic noise from a turbulent shear flow is developed. This model is a generalization of the one developed by Ribner. It does not require that the turbulent correlations factor into space and time-dependent parts. It replaces his assumption of isotropic turbulence by the more realistic one of axisymmetric turbulence. In the course of the analysis, a hierarchy of equations is developed wherein each succeeding equation involves more assumptions than the preceding equation but requires less experimental information for its use. The implications of the model for jet noise are discussed. It is shown that for the particular turbulence data considered anisotropy causes the high-frequency self-noise to be beamed downstream.

Goldstein, M.; Rosenbaum, B.

1973-01-01

357

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

358

Study of strong turbulence effects for optical wireless links  

NASA Astrophysics Data System (ADS)

Strong turbulence measurements that are taken using real time optical wireless experimental setups are valuable when studying the effects of turbulence regimes on a propagating optical beam. In any kind of FSO system, for us to know the strength of the turbulence thus the refractive index structure constant, is beneficial for having an optimum bandwidth of communication. Even if the FSO Link is placed very well-high-above the ground just to have weak enough turbulence effects, there can be severe atmospheric conditions that can change the turbulence regime. Having a successful theory that will cover all regimes will give us the chance of directly processing the image in existing or using an additional hardware thus deciding on the optimum bandwidth of the communication line at firsthand. For this purpose, Strong Turbulence data has been collected using an outdoor optical wireless setup placed about 85 centimeters above the ground with an acceptable declination and a path length of about 250 meters inducing strong turbulence to the propagating beam. Variations of turbulence strength estimation methods as well as frame image analysis techniques are then been applied to the experimental data in order to study the effects of different parameters on the result. Such strong turbulence data is compared with existing weak and intermediate turbulence data. Aperture Averaging Factor for different turbulence regimes is also investigated.

Yuksel, Heba; Meric, Hasim; Kunter, Fulya

2012-10-01

359

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

360

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

361

Advances in Wave Turbulence Optical Wave Turbulence  

E-print Network

nonlinear phenomenon which occurs in a variety of nonlinear wave- bearing physical systems. The experimental impediments and the computationally intensive nature of simulating of hydrodynamic or plasma wave turbulence

Turitsyn, Sergei K.

362

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

363

Nonstationarity of turbulent heat fluxes at Summit, Greenland  

Microsoft Academic Search

Turbulence data collected over a total of 25 days during two summers are used to describe processes responsible for the nonstationarity\\u000a of turbulent sensible heat fluxes at Summit, Greenland. A stationarity test shows that about 40% of the data are classified\\u000a as nonstationary. Three main factors are explored to account for the large fraction of nonstationary runs: (1) intermittency\\u000a of turbulence

Nicolas J. Cullen; Konrad Steffen; Peter D. Blanken

2007-01-01

364

Inhomogeneous distribution of droplets in cloud turbulence  

E-print Network

We solve the problem of spatial distribution of inertial particles that sediment in turbulent flow with small ratio of acceleration of fluid particles to acceleration of gravity $g$. The particles are driven by linear drag and have arbitrary inertia. The pair-correlation function of concentration obeys a power-law in distance with negative exponent. Divergence at zero signifies singular distribution of particles in space. Independently of particle size the exponent is ratio of integral of energy spectrum of turbulence times the wavenumber to $g$ times numerical factor. We find Lyapunov exponents and confirm predictions by direct numerical simulations of Navier-Stokes turbulence. The predictions include typical case of water droplets in clouds. This significant progress in the study of turbulent transport is possible because strong gravity makes the particle's velocity at a given point unique.

Itzhak Fouxon; Yongnam Park; Roei Harduf; Changhoon Lee

2014-09-24

365

A model for fully developed turbulence  

NASA Technical Reports Server (NTRS)

A model for stationary, fully developed turbulence is presented in which the turbulent spectral energy function is completely determined once the time scale for the energy fed into the eddy interaction is known. The form of the eddy correlation time scale determining the turbulent viscosity is suggested by the basic equation of the model itself, up to a dimensionless constant that is fixed by demanding that the coefficient of the spectrum in the Heisenberg-Kolmogoroff inertial range of wavenunmbers be the experimental value. The model makes quantitative predictions that are compared with data on turbulent convection; the k-epsilon and Smagorinsky relations; the spectral function, transfer term, and dissipation term; the skewness factor; the Kolmogoroff and Batchelor constants; and the inertial-conductive and inertial-convective ranges.

Canuto, V. M.; Goldman, I.; Chasnov, J.

1987-01-01

366

Friction Forces during Sliding of Various Brackets for Malaligned Teeth: An In Vitro Study  

PubMed Central

Aims. To measure the friction force generated during sliding mechanics with conventional, self-ligating (Damon 3 mx, Smart Clip, and Time 3) and low-friction (Synergy) brackets using different archwire diameters and ligating systems in the presence of apical and buccal malalignments of the canine. Methods. An experimental setup reproducing the right buccal segment of the maxillary arch was designed to measure the friction force generated at the bracket/wire and wire/ligature interfaces of different brackets. A complete factorial plan was drawn up and a three-way analysis of variance (ANOVA) was carried out to investigate whether the following factors affect the values of friction force: (i) degree of malalignment, (ii) diameter of the orthodontic wire, and (iii) bracket/ligature combination. Tukey post hoc test was also conducted to evaluate any statistically significant differences between the bracket/ligature combinations analyzed. Results. ANOVA showed that all the above factors affect the friction force values. The friction force released during sliding mechanics with conventional brackets is about 5-6times higher than that released with the other investigated brackets. A quasilinear increase of the frictional forces was observed for increasing amounts of apical and buccal malalignments. Conclusion. The Synergy bracket with silicone ligature placed around the inner tie-wings appears to yield the best performance. PMID:23533364

Crincoli, Vito; Di Bisceglie, Maria Beatrice; Balsamo, Antonio; Serpico, Vitaliano; Chiatante, Francesco; Pappalettere, Carmine; Boccaccio, Antonio

2013-01-01

367

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

368

Modification in drag of turbulent boundary layers resulting from manipulation of large-scale structures  

NASA Technical Reports Server (NTRS)

The effects of placing a parallel-plate turbulence manipulator in a boundary layer are documented through flow visualization and hot wire measurements. The boundary layer manipulator was designed to manage the large scale structures of turbulence leading to a reduction in surface drag. The differences in the turbulent structure of the boundary layer are summarized to demonstrate differences in various flow properties. The manipulator inhibited the intermittent large scale structure of the turbulent boundary layer for at least 70 boundary layer thicknesses downstream. With the removal of the large scale, the streamwise turbulence intensity levels near the wall were reduced. The downstream distribution of the skin friction was also altered by the introduction of the manipulator.

Corke, T. C.; Guezennec, Y.; Nagib, H. M.

1981-01-01

369

An Assessment of Oil Film Interferometry to Measure Skin Friction  

NASA Astrophysics Data System (ADS)

In recent years, the independent measurement of wall shear stress with oil film interferometry has led to a step increase in the understanding of turbulent boundary layers. However, while many arguments depend critically on a precise knowledge of the skin friction, the systematic errors of the oil film technique are not well known. In particular the basic theory underlying the technique has essentially not evolved since it was first proposed by Tanner & Blows (J. Phys. E: Sci. Instrum., vol. 9, 1976, p. 194). The purpose of this study is to elucidate the dominant systematic error of the classical oil film method. We derive the corrections to the basic Tanner & Blows similarity solution for the film development in zero pressure gradient boundary layers and validate the analysis experimentally. This allows to formulate "best practice guidelines" for the oil film technique that help push uncertainties below 1%.

Monkewitz, Peter A.; Segalini, Antonio; Rüedi, Jean-Daniel

2010-11-01

370

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

371

Fossil turbulence revisited  

E-print Network

A theory of fossil turbulence presented in the 11th Liege Colloquium on Marine turbulence is "revisited" in the 29th Liege Colloquium "Marine Turbulence Revisited". The Gibson (1980) theory applied universal similarity theories of turbulence and turbulent mixing to the vertical evolution of an isolated patch of turbulence in a stratified fluid as it is constrained and fossilized by buoyancy forces. Towed oceanic microstructure measurements of Schedvin (1979) confirmed the predicted universal constants. Universal constants, spectra, hydrodynamic phase diagrams (HPDs) and other predictions of the theory have been reconfirmed by a wide variety of field and laboratory observations. Fossil turbulence theory has many applications; for example, in marine biology, laboratory and field measurements suggest phytoplankton species with different swimming abilities adjust their growth strategies differently by pattern recognition of several days of turbulence-fossil-turbulence dissipation and persistence times above threshold values, signaling a developing surface layer sea change. In cosmology, self-gravitational structure masses are interpreted as fossils of primordial hydrodynamic states.

Carl H. Gibson

1999-04-19

372

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

373

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 ...

Clark, Mr

2012-10-31

374

Collisionless inter-species energy transfer and turbulent heating in drift wave turbulence  

SciTech Connect

We reconsider the classic problems of calculating 'turbulent heating' and collisionless inter-species transfer of energy in drift wave turbulence. These issues are of interest for low collisionality, electron heated plasmas, such as ITER, where collisionless energy transfer from electrons to ions is likely to be significant. From the wave Poynting theorem at steady state, a volume integral over an annulus r{sub 1}=-S{sub r}|{sub r{sub 1}{sup r{sub 2}}}{ne}0. Here S{sub r} is the wave energy density flux in the radial direction. Thus, a wave energy flux differential across an annular region indeed gives rise to a net heating, in contrast to previous predictions. This heating is related to the Reynolds work by the zonal flow, since S{sub r} is directly linked to the zonal flow drive. In addition to net heating, there is inter-species heat transfer. For collisionless electron drift waves, the total turbulent energy source for collisionless heat transfer is due to quasilinear electron cooling. Subsequent quasilinear ion heating occurs through linear ion Landau damping. In addition, perpendicular heating via ion polarization currents contributes to ion heating. Since at steady state, Reynolds work of the turbulence on the zonal flow must balance zonal flow frictional damping ({approx}{nu}{sub ii}{sup 2}{approx}|(e{phi}(tilde sign)/T)|{sup 4}), it is no surprise that zonal flow friction appears as an important channel for ion heating. This process of energy transfer via zonal flow has not previously been accounted for in analyses of energy transfer. As an application, we compare the rate of turbulent energy transfer in a low collisionality plasma with the rate of the energy transfer by collisions. The result shows that the collisionless turbulent energy transfer is a significant energy coupling process for ITER plasma.

Zhao, L. [Center for Astrophysics and Space Sciences and Department of Physics, University of California at San Diego, La Jolla, California 92093-0424 (United States); Diamond, P. H. [Center for Astrophysics and Space Sciences and Department of Physics, University of California at San Diego, La Jolla, California 92093-0424 (United States); WCI Center for Fusion Theory, National Fusion Research Institute, Gwahangno113, Yuseong-gu, Daejeon 305-333 (Korea, Republic of)

2012-08-15

375

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

376

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

377

Micron-Scale Friction and Sliding Wear of Polycrystalline Silicon Thin Structural Films in Ambient Air  

Microsoft Academic Search

Micron-scale static friction and wear coefficients, surface roughness, and resulting wear debris have been studied for sliding wear in polycrystalline silicon in ambient air at micro- Newton normal loads using on-chip sidewall test specimens, fabricated with the Sandia SUMMiT VTM process. With increasing number of wear cycles friction coefficients increased by a factor of two up to a steady-state regime,

Daan Hein Alsem; Michael T. Dugger; Eric A. Stach; Robert O. Ritchie

2008-01-01

378

The Effect of Frequency and Amplitude of Vibration on the Coefficient of Friction for Metals  

Microsoft Academic Search

Experiments were conducted to determine the effects of frequency and amplitude of vibration on friction. The experimental analysis also seeks to take into account a variety of factors influencing the coefficient of friction such as normal load and surface roughness. An in-house pin-on-disc apparatus was constructed with a spindle speed control and applied forced-feedback to perform the tests. The response

JAMIL ABDO; MAHMOUD TAHAT

379

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

380

Passive vibration absorber with dry friction  

Microsoft Academic Search

Summary  ?The properties of a passive vibration absorber with dry friction significantly differ from those of the classical linear\\u000a absorber. The exceptional phenomenon is the possibility of suppressing all excited modes. This effect is influenced to a small\\u000a extent by a special shape of the friction characteristic, but mainly by an appropriately adjusted threshold of the static\\u000a friction. The theoretical predictions

A. Hartung; H. Schmieg; P. Vielsack

2001-01-01

381

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

382

Quantum friction-fact or fiction?  

NASA Astrophysics Data System (ADS)

Two parallel dielectric plates separated by vacuum interact through zero-point charge fluctuations and experience friction when the plates are in relative motion and the vacuum is sheared. Even at the absolute zero of temperature, residual quantum fluctuations remain because the zero-point energy gives rise to 'quantum friction'. In a recent paper, the reality of these fluctuations is questioned and the existence of quantum friction is called into question. Here we refute this assertion.

Pendry, J. B.

2010-03-01

383

Turbulence simulation in diverse conditions for FSO Links  

NASA Astrophysics Data System (ADS)

Simulation of beam propagation through turbulent media has always been a tricky subject when it comes to moderate-to-strong turbulent regimes. Creating a well controlled turbulent environment is beneficial as a fast and a practical approach when it comes to testing the optical wireless communication systems in diverse atmospheric conditions. Turbulent media is created using multiple phase screens each having controlled random variations in its frequency and power while the propagated beam is calculated using Fresnel diffraction method. The effect of the turbulent media is added to the propagated beam using modified Von Karman spectrum. Created scintillation screens are tested and compared with the experimental data which are gathered in different turbulence regimes within various atmospheric conditions. We believe that the general drawback of the beam propagation simulation is the difference in terms of spatial distribution and sequential phase textures. To overcome these two challenges we calculate the Aperture Averaging Factors to create more realistic results. In this manner, it is possible create more viable turbulent like scintillations thus the relationship between the turbulence strength and the simulated turbulence parameters are distinctly available. Our simulation gives us an elusive insight on the real atmospheric turbulent media. It improves our understanding on parameters that are involved in real time intensity fluctuations that occur in every wireless optical communication system.

Yuksel, Heba; Meric, Hasim

2012-10-01

384

Experimental investigation on mechanical behavior of friction welded AL7075  

Microsoft Academic Search

Friction welding is widely used as a mass production method in various industries. In friction welding, the joints are formed in the solid state by utilizing the heat generated by friction. Friction welding can be used to join different types of ferrous metals and non-ferrous metals that cannot be welded by traditional welding processes. The process parameters such s friction

M Rajendran

2012-01-01

385

Tailoring the frictional properties of granular media  

E-print Network

A method of modifying the roughness of soda-lime glass spheres is presented, with the purpose of tuning inter-particle friction. The effect of chemical etching on the surface topography and the bulk frictional properties of grains is systematically investigated. The surface roughness of the grains is measured using white light interferometry and characterised by the lateral and vertical roughness length scales. The underwater angle of repose is measured to characterise the bulk frictional behaviour. We observe that the co-efficient of friction depends on the vertical roughness length scale. We also demonstrate a bulk surface roughness measurement using a carbonated soft drink.

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

2011-05-25

386

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

387

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

388

Numerical Tokamak Turbulence Project  

SciTech Connect

The primary research objective of the Numerical Tokamak Turbulence Project (NTTP) is to develop a predictive ability in modeling turbulent transport due to drift-type instabilities in the core of tokamak fusion experiments, through the use of three-dimensional kinetic and fluid simulations and the derivation of reduced models.

Cohen, B.; Dawson, J.M.; Reynders, J.V.W.; Decyk, V.K.; Dorland, W.D.; Hammett, G.W.; Kerbel, G.D.; Leboeuf, J.N.; Lee, W.W.; Parker, S.E.; Waltz, R.E.; Dimits, A.M.; Shumaker, D.E.; Nevins, W.M.; Beer, M.; Lin, Z.; Sydora, R.; Lynch V.; Smith, S.; Williams, T.

1999-12-09

389

Turbulence in pulsatile flows.  

PubMed

Turbulence during pulsatile flow has been suggested as a possible mechanism to enhance the transport of gases during high-frequency ventilation. Experimental studies on oscillatory flow in straight, circular tubes have identified three types of flow: (a) laminar; (b) conditionally turbulent, in which high-frequency disturbances occur during the decelerating phase of the flow cycle but relaminarize by the beginning of the subsequent accelerating phase; and (c) fully turbulent flow, in which disturbances occur throughout the flow cycle. Fully turbulent flow has been observed only when a mean flow is present, and only laminar or conditionally turbulent flow has been observed for purely oscillatory flow. A critical Reynolds number based on the Stokes layer can be defined, and transition Reynolds numbers between 400 and 550 have been experimentally determined for purely oscillatory flow in a circular tube, although lower values are expected for physiological flows. There are some indications that the structure of oscillating turbulent flow is similar to steady turbulent flow, and preliminary work in our laboratory shows that the spectral content of flows during high-frequency ventilation is similar to that in steady turbulent flow. PMID:6532271

Winter, D C; Nerem, R M

1984-01-01

390

BOOK REVIEW: Turbulent Flows  

Microsoft Academic Search

It was a pleasure to read this important book. To understand and predict the development of turbulent flows represents both a continuing scientific challenge and also a serious practical problem in many different fields.Professor Pope has based his book on graduate level lecture courses on turbulence that he has presented at MIT and at Cornell University. It is intended for

Stephen B. Pope

2001-01-01

391

Skin friction measurement in complex flows using thin oil film techniques  

NASA Technical Reports Server (NTRS)

The NASA Grant NAG2-261 was initiated to support a program of research to study complex flows that occur in flight and laboratory experiments by building, testing and optimizing an on-board technique for direct measurement of surface shear stress using thin oil film techniques. The program of research has proceeded under the supervision of the NASA Ames Research Center and with further cooperation from the NASA Ames-Dryden and NASA Langley Research Centers. In accordance with the original statement of work, the following research milestones were accomplished: (1) design and testing of an internally mounted one-directional skin friction meter to demonstrate the feasibility of the concept; (2) design and construction of a compact instrument capable of measuring skin friction in two directions; (3) study of transitional and fully turbulent boundary layers over a flat plate with and without longitudinal pressure gradients utilizing the compact two-directional skin friction meter; (4) study of the interaction between a turbulent boundary layer and a shock wave generated by a compression corner using the two-directional meter; and (5) flight qualification of the compact meter and accompanying electronic and pneumatic systems, preliminary installation into flight test fixture.

1994-01-01

392

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

2010-11-04

393

Lubrication and friction prediction in metal-on-metal hip implants  

NASA Astrophysics Data System (ADS)

A general methodology of mixed lubrication analysis and friction prediction for a conforming spherical bearing in hip implants was developed, with particular reference to a typical metal-on-metal hip replacement. Experimental measurement of frictional torque for a similar implant was carried out to validate the theoretical prediction. A ball-in-socket configuration was adopted to represent the articulation between the femoral head and the acetabular cup under cyclic operating conditions of representative load and motion. The mixed lubrication model presented in this study was first applied to identify the contact characteristics on the bearing surfaces, consisting of both fluid-film and boundary lubricated regions. The boundary lubricated contact was assumed to occur when the predicted fluid film thickness was less than a typical boundary protein layer absorbed on the bearing surfaces. Subsequently, the friction was predicted from the fluid-film lubricated region with viscous shearing due to both Couette and Poiseuille flows and the boundary protein layer contact region with a constant coefficient of friction. The predicted frictional torque of the typical metal-on-metal hip joint implant was compared with the experimental measurement conducted in a functional hip simulator and a reasonably good agreement was found. The mixed lubrication regime was found to be dominant for the conditions considered. Although the percentage of the boundary lubricated region was quite small, the corresponding contribution to friction was quite large and the resultant friction factor was quite high.

Wang, F. C.; Brockett, C.; Williams, S.; Udofia, I.; Fisher, J.; Jin, Z. M.

2008-03-01

394

Estimation of the friction coefficient between wheel and rail surface using traction motor behaviour  

NASA Astrophysics Data System (ADS)

The friction coefficient between a railway wheel and rail surface is a crucial factor in maintaining high acceleration and braking performance of railway vehicles thus monitoring this friction coefficient is important. Restricted by the difficulty in directly measuring the friction coefficient, the creep force or creepage, indirect methods using state observers are used more frequently. This paper presents an approach using a Kalman filter to estimate the creep force and creepage between the wheel and rail and then to identify the friction coefficient using the estimated creep force-creepage relationship. A mathematic model including an AC motor, wheel and roller is built to simulate the driving system. The parameters are based on a test rig at Manchester Metropolitan University. The Kalman filter is designed to estimate the friction coefficient based on the measurements of the simulation model. Series of residuals are calculated through the comparison between the estimated creep force and theoretical values of different friction coefficient. Root mean square values of the residuals are used in the friction coefficient identification.

Zhao, Y.; Liang, B.; Iwnicki, S.

2012-05-01

395

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

396

Modeling of friction-induced deformation and microstructures.  

SciTech Connect

Frictional contact results in surface and subsurface damage that could influence the performance, aging, and reliability of moving mechanical assemblies. Changes in surface roughness, hardness, grain size and texture often occur during the initial run-in period, resulting in the evolution of subsurface layers with characteristic microstructural features that are different from those of the bulk. The objective of this LDRD funded research was to model friction-induced microstructures. In order to accomplish this objective, novel experimental techniques were developed to make friction measurements on single crystal surfaces along specific crystallographic surfaces. Focused ion beam techniques were used to prepare cross-sections of wear scars, and electron backscattered diffraction (EBSD) and TEM to understand the deformation, orientation changes, and recrystallization that are associated with sliding wear. The extent of subsurface deformation and the coefficient of friction were strongly dependent on the crystal orientation. These experimental observations and insights were used to develop and validate phenomenological models. A phenomenological model was developed to elucidate the relationships between deformation, microstructure formation, and friction during wear. The contact mechanics problem was described by well-known mathematical solutions for the stresses during sliding friction. Crystal plasticity theory was used to describe the evolution of dislocation content in the worn material, which in turn provided an estimate of the characteristic microstructural feature size as a function of the imposed strain. An analysis of grain boundary sliding in ultra-fine-grained material provided a mechanism for lubrication, and model predictions of the contribution of grain boundary sliding (relative to plastic deformation) to lubrication were in good qualitative agreement with experimental evidence. A nanomechanics-based approach has been developed for characterizing the mechanical response of wear surfaces. Coatings are often required to mitigate friction and wear. Amongst other factors, plastic deformation of the substrate determines the coating-substrate interface reliability. Finite element modeling has been applied to predict the plastic deformation for the specific case of diamond-like carbon (DLC) coated Ni alloy substrates.

Michael, Joseph Richard; Prasad, Somuri V.; Jungk, John Michael; Cordill, Megan J. (University of Minnesota); Bammann, Douglas J.; Battaile, Corbett Chandler; Moody, Neville Reid; Majumdar, Bhaskar Sinha (New Mexico Institure of Mining and Technology)

2006-12-01

397

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

398

Hurricane Formation in Diabatic Ekman Turbulence  

NASA Astrophysics Data System (ADS)

This paper examines a simple representation of turbulent flow in the tropical troposphere, which occasionally produces a hurricane. In this paradigm, the flow is essentially two-dimensional (2D) turbulence under the cooperative influence of Ekman pumping and deep cumulus convection. After an incubation period, diabatic Ekman pumping can supercede ideal 2D mechanisms of self-organization, such as vortex merger. A strong cyclone-anticyclone asymmetry can develop, with very intense convective cyclones dominating the system. Diabatic Ekman Turbulence (DET), as described above, is readily studied with a 3-layer model of the troposphere. The model used here includes a frictional boundary layer (BL), a lower troposphere (LT), and an upper troposphere (UT). The parameterizations of surface fluxes, deep convection and radiative cooling are similar to those used by Ooyama in his seminal study of axisymmetric tropical cyclone intensification [Ooyama, K., J. Atmos. Sci., 26, 3 (1969)]. The results given below are from numerical simulations in a 2000km-by-2000km periodic box. In all simulations, the initial turbulence is concentrated in the BL and LT, whereas the UT starts at rest. In general, we find that DET freely evolves into a Frictional Radiative Convective Equilibrium (FRCE), in which there is an approximate balance between convective energy input and energy output by surface drag and radiation. The predictability of the FRCE that emerges from random noise varies with sea-surface temperature (SST), the Coriolis parameter f, and the ratio CE/CD, in which CE and CD are surface-exchange coefficients for moist entropy and momentum, respectively. At low values of these control parameters, DET tends to dissipate. As the control parameters increase to typical tropical values, the FRCE bifurcates into a metastable synoptic-scale gyre or a hurricane. At higher values of the control parameters, the FRCE is always a hurricane. The hurricane that emerges from DET is realistic in several ways. During rapid intensification, the hurricane typically develops polygonal eyewalls and mesovortices. In the FRCE, the hurricane exhibits moderate intensity oscillations that resemble eyewall breakdown and regeneration cycles. The time-averaged intensity of the hurricane increases with the SST and the ratio of surface-exchange coefficients CE/CD [ibid; Emanuel, K.A., J. Atmos. Sci., 43, 585 (1986)].

Schecter, D. A.; Dunkerton, T. J.

2007-12-01

399

Physical Processes of Interstellar Turbulence  

E-print Network

I discuss the role of self-gravity and radiative heating and cooling in shaping the nature of the turbulence in the interstellar medium (ISM) of our galaxy. The heating and cooling cause it to be highly compressible, and, in some regimes of density and temperature, to become thermally unstable, tending to spontaneously segregate into warm/diffuse and cold/dense phases. On the other hand, turbulence is an inherently mixing process, tending to replenish the density and temperature ranges that would be forbidden under thermal processes alone. The turbulence in the ionized ISM appears to be transonic (i.e, with Mach numbers $\\Ms \\sim 1$), and thus to behave essentially incompressibly. However, in the neutral medium, thermal instability causes the sound speed of the gas to fluctuate by up to factors of $\\sim 30$, and thus the flow can be highly supersonic with respect to the dense/cold gas, although numerical simulations suggest that this behavior corresponds more to the ensemble of cold clumps than to the clumps'...

Vazquez-Semadeni, Enrique

2012-01-01

400

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

401

Turbulence in an almond orchard: Vertical variations in turbulent statistics  

Microsoft Academic Search

Three-dimensional wind velocity components were measured above and within a uniform almond orchard. Turbulent statistics associated with the turbulent flow inside the canopy are examined in detail. Turbulence in an almond orchard is characterized by relatively high turbulent intensities and large skewness and kurtosis values. These results indicate that the frequency distribution of wind velocity components is non-Gaussian. Conditional sampling

Dennis D. Baldocchi; Boyd A. Hutchison

1987-01-01

402

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

403

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

404

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

405

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

406

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

407

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

408

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

409

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

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 56.19008 - 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

412

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

413

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

414

Friction-Induced Vibrations in Railway Transportation Chandra Prakash Sharma  

E-print Network

Friction-Induced Vibrations in Railway Transportation by Chandra Prakash Sharma B. Tech., Sardar . . . . . . . . . . . . . . . . . . . . . . . 9 2.2.2 Friction Management in Railways . . . . . . . . . . . . . 10 2.3 Friction-Induced Vibration Phenomena . . . . . . . . . . . . . . . 16 2.3.1 Disk Brake Squeal

Phani, A. Srikantha

415

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

416

30 CFR 57.19014 - Friction hoist overtravel protection.  

...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

417

30 CFR 56.19008 - Friction hoist synchronizing mechanisms.  

...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

418

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

419

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

420

30 CFR 57.19008 - Friction hoist synchronizing mechanisms.  

...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

421

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

422

30 CFR 56.19014 - Friction hoist overtravel protection.  

...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

423

Physical Processes of Interstellar Turbulence  

E-print Network

I discuss the role of self-gravity and radiative heating and cooling in shaping the nature of the turbulence in the interstellar medium (ISM) of our galaxy. The heating and cooling cause it to be highly compressible, and, in some regimes of density and temperature, to become thermally unstable, tending to spontaneously segregate into warm/diffuse and cold/dense phases. On the other hand, turbulence is an inherently mixing process, tending to replenish the density and temperature ranges that would be forbidden under thermal processes alone. The turbulence in the ionized ISM appears to be transonic (i.e, with Mach numbers $\\Ms \\sim 1$), and thus to behave essentially incompressibly. However, in the neutral medium, thermal instability causes the sound speed of the gas to fluctuate by up to factors of $\\sim 30$, and thus the flow can be highly supersonic with respect to the dense/cold gas, although numerical simulations suggest that this behavior corresponds more to the ensemble of cold clumps than to the clumps' internal velocity dispersion. Finally, coherent large-scale compressions in the warm neutral medium (induced by, say, the passage of spiral arms or by supernova shock waves) can produce large, dense molecular clouds that are subject to their own self-gravity, and begin to contract gravitationally. Because they are populated by nonlinear density fluctuations, whose local free-fall times are significantly smaller than that of the whole cloud, the fluctuations terminate their collapse earlier, giving rise to a regime of hierarchical gravitational fragmentation, with small-scale collapses occurring within larger-scale ones. Thus, the "turbulence" in molecular clouds may be dominated by a gravitationally contracting component at all scales.

Enrique Vazquez-Semadeni

2012-02-20

424

Asymptotic similarity in turbulent boundary layers  

NASA Astrophysics Data System (ADS)

The turbulent boundary layer is one of the most fundamental and important applications of fluid mechanics. Despite great practical interest and its direct impact on frictional drag among its many important consequences, no theory absent of significant inference or assumption exists. Numerical simulations and empirical guidance are used to produce models and adequate predictions, but even minor improvements in modeling parameters or physical understanding could translate into significant improvements in the efficiency of aerodynamic and hydrodynamic vehicles. Classically, turbulent boundary layers and fully-developed turbulent channels and pipes are considered members of the same "family," with similar "inner" versus "outer" descriptions. However, recent advances in experiments, simulations, and data processing have questioned this, and, as a result, their fundamental physics. To address a full range of pressure gradient boundary layers, a new approach to the governing equations and physical description of wall-bounded flows is formulated, using a two variable similarity approach and many of the tools of the classical method with slight but significant variations. A new set of similarity requirements for the characteristic scales of the problem is found, and when these requirements are applied to the classical "inner" and "outer" scales, a "similarity map" is developed providing a clear prediction of what flow conditions should result in self-similar forms. An empirical model with a small number of parameters and a form reminiscent of Coles' "wall plus wake" is developed for the streamwise Reynolds stress, and shown to fit experimental and numerical data from a number of turbulent boundary layers as well as other wall-bounded flows. It appears from this model and its scaling using the free-stream velocity that the true asymptotic form of u'2 may not become self-evident until Retheta ? 275,000 or delta+ ? 105, if not higher. A perturbation expansion made possible by the novel inclusion of the scaled streamwise coordinate is used to make an excellent prediction of the shear Reynolds stress in zero pressure gradient boundary layers and channel flows, requiring only a streamwise mean velocity profile and the new similarity map. Extension to other flows is promising, though more information about the normal Reynolds stresses is needed. This expansion is further used to infer a three layer structure in the turbulent boundary layer, and modified two layer structure in fully-developed flows, by using the classical inner and logarithmic profiles to determine which portions of the boundary layer are dominated by viscosity, inertia, or turbulence. A new inner function for U+ is developed, based on the three layer description, providing a much more simplified representative form of the streamwise mean velocity nearest the wall.

Duncan, Richard D.

425

Kozai Cycles and Tidal Friction  

SciTech Connect

Several studies in the last three years indicate that close binaries, i.e. those with periods of {approx}< 3 d, are very commonly found to have a third body in attendance. We argue that this proves that the third body is necessary in order to make the inner period so short, and further argue that the only reasonable explanation is that the third body causes shrinkage of the inner period, from perhaps a week or more to the current short period, by means of the combination of Kozai cycles and tidal friction (KCTF). In addition, once KCTF has produced a rather close binary, magnetic braking also combined with tidal friction (MBTF) can decrease the inner orbit further, to the formation of a contact binary or even a merged single star. Some of the products of KCTF that have been suggested, either by others or by us, are W UMa binaries, Blue Stragglers, X-ray active BY Dra stars, and short-period Algols. We also argue that some components of wide binaries are actually merged remnants of former close inner pairs. This may include such objects as rapidly rotating dwarfs (AB Dor, BO Mic) and some (but not all) Be stars.

L, K; P.P., E

2009-07-17

426

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

427

Deterministic chaos in frictional wedges.  

NASA Astrophysics Data System (ADS)

A triangular wedge, composed of a frictional material such as sand, and accreting additional material at its front, is the classical prototype for accretionary wedges and fold-and-thrust belts. The Sequential Limit Analysis method is applied to capture the internal deformation to these structures resulting from a large number of faulting events during compression. The method combines the application of the kinematic approach of limit analysis to predict the optimum thrust-fold and a set of geometrical rules to update the geometry accordingly, at each increment of shortening. It is shown that the topography remains planar to first order with an average slope predicted by the critical Coulomb wedge theory. Failure by faulting occurs anywhere within the wedge at criticality and its exact position is sensitive to topographic perturbations resulting from the deformation history. The convergence analysis in terms of the shortening increments and of the topography discretisation reveals that the timing and the position of a single faulting event cannot be predicted. The convergence is achieved nevertheless in terms of the statistics of the distribution of the faulting events throughout the structure and during the entire deformation history. These two convergence properties plus the perturbation sensitivity justify the claim that these compressed frictional wedges are imperfection sensitive, chaotic systems. This fundamental system has to be understood before considering the influence of softening on activated ramps and of erosion which are also discussed.

Mary, Baptiste; Maillot, Bertrand; Leroy, Yves M.

2013-04-01

428

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] [ORNL; Blau, Peter Julian [ORNL] [ORNL

2013-10-01

429

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

430

Turbulence Effect of the Intergalactic Medium  

NASA Astrophysics Data System (ADS)

The turbulence in the intergalactic medium (IGM) and its impact on the clustering of baryonic matter are investigated with the cosmological hydrodynamic simulation in the ?CDM framework. The observational tools that may be used to verify the possibility of the turbulence in the IGM are also discussed. A brief review of modern cosmology is given in chapter 1, mainly focusing on the dynamical equation of the scale factor -- Friedmann equation, and the theory background of the structure formation. Then the method of cosmological numerical simulation is introduced, as well as the cosmological hydrodynamic code WIGEON. After a short review of the turbulence in classic fluid mechanics, the IGM turbulence on large scales is investigated with simulations in chapter 2. The vorticity in the IGM velocity field significantly increases with time, as it can be effectively generated by shocks and complex structures. The vorticity field shows highly non-Gaussian and intermittent features. Its power spectrum is then used to measure the development of turbulence. The relation between the power spectra of vorticity and velocity indicates that the cosmic baryonic and velocity fields are in the state of fully developed turbulence within the scale range of 0.2h^{-1}˜ 3.0h^{-1} Mpc at z?0. The dynamical effect of the IGM turbulence on the baryon clustering is studied in chapter 3. The random motion of the turbulent fluid yields non-thermal turbulent pressure, which would enlarge the Jeans length, and hence delay and partly prevent the IGM from falling into the gravitational well of dark matter halos. Consequently, the baryon fraction f_{b} will deviate from its cosmic mean f_{b}^{cosmic}, and become highly nonuniform on the scales from a few hundred kpc to several Mpc, varying from as low as 1% to a few times of f_{b}^{cosmic}. The turbulence pressure in the IGM is weakly scale-dependent, and comparable to the gravitational energy density of the halos with masses of approximate 10^{11}h^{-1} M_{?}. f_{b} decreases from 0.8f_{b}^{cosmic} in the halos with masses of approximate 10^{12}h^{-1} M_{?} to 0.3f_{b}^{cosmic} in the halos with masses of approximate 10^{11}h^{-1} M_{?}, and shows further decrease when the halo mass is under 10^{11}h^{-1} M_{?}. The trend is similar to the observations, although the simulated f_{b} in halos is higher than the observed value by a factor of 2˜4. The turbulence of the IGM should be an important dynamical factor leading to the remarkable missing of the baryonic matter in the halos with masses of less than 10^{12}h^{-1} M_{?}. The IGM turbulence may introduce uncertainties to the kinetic Sunyaev-Zel'dovich (kSZ) effect and the Lyman alpha forest, and in turn could be constrained by future observations, which are discussed in chapter 4. The kSZ effect is sensitive to the curl component of the motions of the IGM. The structure functions of 2D simulated kSZ maps show strong intermittence, and the intermittent exponents follow a law similar to the She-Leveque scaling of fully developed turbulence. On the other hand, the intermittence is weak in the maps of thermal Sunyaev-Zel'dovich (tSZ) effect. Nevertheless, the superposition of the kSZ and tSZ effects still contains significant intermittence. The turbulent behavior of the IGM may be revealed by the observation of the SZ effect on angular scales equal to or less than 0.5'. The main results in this thesis are summarized in chapter 5, where future works are also discussed.

Zhu, W. S.

2013-07-01

431

The influence of stem design on critical squeaking friction with ceramic bearings.  

PubMed

Ceramic-on-ceramic hip joints have been reported to squeak, a phenomenon that may occur in compromised lubrication conditions. One factor related to the incidence of in vivo squeaking is the stem design. However, it has not yet been possible to relate stem design to squeaking in deteriorating lubrication conditions. The purpose of this study was to determine critical friction factors for different stem designs. A hip simulator was used to measure the friction factor of a ceramic bearing with different stem designs and gradually deteriorating lubrication represented by evaporation of a volatile fluid lubricant. The critical squeaking friction factor was measured at the onset of squeaking for each stem. Critical friction was higher for the long cobalt chrome (0.32?±?0.02) and short titanium stems (0.39?±?0.02) in comparison with a long titanium stem (0.29?±?0.02). The onset of squeaking occurred at a friction factor lower than that measured for dry conditions, in which squeaking is usually investigated experimentally. The results suggest that shorter or heavier stems might limit the possibility of squeaking as lubrication deteriorates. The method developed can be used to investigate the influence of design parameters on squeaking probability. PMID:23813771

Fan, Na; Morlock, Michael M; Bishop, Nicholas E; Huber, Gerd; Hoffmann, Norbert; Ciavarella, Michele; Chen, Guang X; Hothan, Arne; Witt, Florian

2013-10-01

432

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

433

Studying the Frictional Force Directions via Bristles  

ERIC Educational Resources Information Center

We present simple apparatus designed to help Thai high school students visualize the directions of frictional forces. Bristles of toothbrushes, paintbrushes and scrubbing brushes are used to demonstrate the frictional forces acting in a variety of situations. These demonstrations, when followed by discussion of free-body diagrams, were found to be…

Prasitpong, S.; Chitaree, R.; Rakkapao, S.

2010-01-01

434

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

435

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

436

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

437

Developments in unsteady pipe flow friction modelling  

Microsoft Academic Search

This paper reviews a number of unsteady friction models for transient pipe flow. Two distinct unsteady friction models, the Zielke and the Brunone models, are investigated in detail. The Zielke model, originally developed for transient laminar flow, has been selected to verify its effectiveness for \\

Anton Bergant; Angus Ross Simpson; John Vìtkovsk

2001-01-01

438

Frictional Melting Processes in Planetary Materials  

E-print Network

lubrication, shock veins Abstract Frictional melting is the result of the conversion of mechanical deforma surfaces can lubricate earthquake faults, facilitate the post-shock modification of impact craters articles rehensive search FurtherANNUAL REVIEWS #12;1. INTRODUCTION Frictional melting is an everyday

Jellinek, Mark

439

Frictional ageing from interfacial bonding and the origins of rate and state friction.  

PubMed

Earthquakes have long been recognized as being the result of stick-slip frictional instabilities. Over the past few decades, laboratory studies of rock friction have elucidated many aspects of tectonic fault zone processes and earthquake phenomena. Typically, the static friction of rocks grows logarithmically with time when they are held in stationary contact, but the mechanism responsible for this strengthening is not understood. This time-dependent increase of frictional strength, or frictional ageing, is one manifestation of the 'evolution effect' in rate and state friction theory. A prevailing view is that the time dependence of rock friction results from increases in contact area caused by creep of contacting asperities. Here we present the results of atomic force microscopy experiments that instead show that frictional ageing arises from the formation of interfacial chemical bonds, and the large magnitude of ageing at the nanometre scale is quantitatively consistent with what is required to explain observations in macroscopic rock friction experiments. The relative magnitude of the evolution effect compared with that of the 'direct effect'--the dependence of friction on instantaneous changes in slip velocity--determine whether unstable slip, leading to earthquakes, is possible. Understanding the mechanism underlying the evolution effect would enable us to formulate physically based frictional constitutive laws, rather than the current empirically based 'laws', allowing more confident extrapolation to natural faults. PMID:22139421

Li, Qunyang; Tullis, Terry E; Goldsby, David; Carpick, Robert W

2011-12-01

440

The first turbulent combustion  

E-print Network

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. Interac