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1

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

Microsoft Academic Search

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

Jian Su; Atila P. Silva Freire

2002-01-01

2

Friction factor estimation for turbulent flows in corrugated pipes with rough walls  

Microsoft Academic Search

The motivation of the investigation is critical pressure loss in cryogenic flexible hoses used for LNG transport in offshore installations. Our main goal is to estimate the friction factor for the turbulent flow in this type of pipes. For this purpose, two- equation turbulence models (k e and k w) are used in the computations. First, fully developed turbulent flow

Maxim Pisarenco; Bas van der Linden; Arris Tijsseling; Emmanuel Ory; Jacques Dam

2009-01-01

3

Friction factor of two-dimensional rough-boundary turbulent soap film flows.  

PubMed

We use momentum-transfer arguments to predict the friction factor f in two-dimensional turbulent soap film flows with rough boundaries (an analog 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 proportional, variant Re{-1/2} in flows dominated by the enstrophy cascade, distinct from the energy cascade scaling of Re{-1/4} . For large Re, f approximately 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 three-dimensional pipe data of Nikuradse. PMID:19658555

Guttenberg, Nicholas; Goldenfeld, Nigel

2009-06-25

4

PIPE. Heat Transfer and Friction-Factor Methods Turbulent Flow Inside Pipes 3d Rough  

SciTech Connect

Three-dimensional roughened internally enhanced tubes have been shown to be one of the most energy efficient for turbulent, forced convection applications. However, there is only one prediction method presented in the open literature and that is restricted to three-dimensional sand-grain roughness. Other roughness types are being proposed: hemispherical sectors, truncated cones, and full and truncated pyramids. There are no validated heat-transfer and friction-factor prediction methods for these different roughness shapes that can be used in the transition and fully rough region. This program calculates the Nusselt number and friction factor values, for a broad range of three-dimensional roughness types such as hemispherical sectors, truncated cones, and full and truncated pyramids. Users of this program are heat-exchangers designers, enhanced tubing suppliers, and research organizations or academia who are developing or validating prediction methods.

Taylor, R.P.; Hodge, B.K. [Mississippi State University, MS (United States)

1992-02-01

5

Friction factor correlations for laminar, transition and turbulent flow in smooth pipes  

NASA Astrophysics Data System (ADS)

In this paper we derive an accurate composite friction factor vs. Reynolds number correlation formula for laminar, transition and turbulent flow in smooth pipes. The correlation is given as a rational fraction of rational fractions of power laws which is systematically generated by smoothly connecting linear splines in log-log coordinates with a logistic dose curve algorithm. This kind of correlation seeks the most accurate representation of the data independent of any input from theories arising from the researchers’ ideas about the underlying fluid mechanics. As such, these correlations provide an objective metric against which observations and other theoretical correlations may be applied. Our correlation is as accurate, or more accurate, than other correlations in the range of Reynolds numbers in which the correlations overlap. However, our formula is not restricted to certain ranges of Reynolds numbers but instead applies uniformly to all smooth pipe flow data for which data is available. The properties of the classical logistic dose response curve are reviewed and extended to problems described by multiple branches of power laws. This extended method of fitting which leads to rational fractions of power laws is applied to data of Marusic and Perry (1995) [1] for the velocity profile in a boundary layer on a flat plate with an adverse pressure gradient, to data of Nikuradse (1932) [2] and McKeon et al. (2004) [3] on friction factors for flow in smooth pipes and to the data of Nikuradse [4] for effectively smooth pipes.

Joseph, Daniel D.; Yang, Bobby H.

2010-07-01

6

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

NASA Astrophysics Data System (ADS)

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

Ha, Tae Woong

1992-01-01

7

A power-law approximation of the turbulent flow friction factor useful for the design and simulation of urban water networks  

Microsoft Academic Search

An approximation of the friction factor of the Colebrook–White equation is proposed, which is expressed as a power-law function of the pipe diameter and the energy gradient and is combined with the Darcy–Weisbach equation, thus yielding an overall power-law equation for turbulent pressurised pipe flow. This is a generalised Manning equation, whose exponents are not unique but depend on the

D. Koutsoyiannis

2008-01-01

8

PIPE. Heat Transfer and Friction-Factor Methods Turbulent Flow Inside Pipes 3d Rough  

Microsoft Academic Search

Three-dimensional roughened internally enhanced tubes have been shown to be one of the most energy efficient for turbulent, forced convection applications. However, there is only one prediction method presented in the open literature and that is restricted to three-dimensional sand-grain roughness. Other roughness types are being proposed: hemispherical sectors, truncated cones, and full and truncated pyramids. There are no validated

R. P. Taylor; B. K. Hodge

1992-01-01

9

The roughness effects on friction and heat transfer in the fully developed turbulent flow in pipes  

Microsoft Academic Search

Convective heat transfer coefficient is strongly influenced by the mechanism of flow during forced convection. In this paper, the effect of pipe roughness on friction factor and convective heat transfer in fully developed turbulent flow are briefly discussed. A correlation for the friction factor applicable in the region of transition to the fully developed turbulent flow regime is proposed. Using

Kadim Ceylan; Gudret Kelbaliyev

2003-01-01

10

Turbulent friction in flows over permeable walls  

NASA Astrophysics Data System (ADS)

The experimental results of Nikuradse and the concept of hydraulically smooth, transitional, and rough flow regimes are commonly used as a benchmark for data interpretation and modeling of hydraulic resistance. However, Nikuradse's experiments were carried out in pipes with impermeable rough-walls whereas many geophysical flows occur over permeable walls and thus the permeability effects need to be quantified and accounted for. On the basis of our own experimental results, it is shown that wall permeability influences flow resistance dramatically and that the conventional ‘hydraulically-rough regime’, for which the friction factor depends only on the ratio of the roughness size to the flow thickness, does not apply to flows over permeable walls. Indeed, even at high Reynolds number (Re), the friction factor progressively increases with increasing Re. Possible mechanisms that explain this behavior, as well as the implications of these results for modeling of the friction factors and hyporheic exchange in porous-bed rivers are discussed.

Manes, C.; Pokrajac, D.; Nikora, V. I.; Ridolfi, L.; Poggi, D.

2011-02-01

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

Turbulent friction drag reduction over electroactive polymer smart surfaces  

Microsoft Academic Search

Both experiments and numerical simulations have provided evidence that an initially fully devel- oped two-dimensional boundary layer, subjected to a sudden spanwise forcing, exhibits a decrease in turbulent friction drag as well as turbulent quantities such as the Reynolds shear stress and turbulent kinetic energy. In past experiments, such forcing has traditionally been in the form of cam-shaft driven spanwise

Kevin Gouder; Jonathan F. Morrison

13

Wave friction factor rediscovered  

NASA Astrophysics Data System (ADS)

The wave friction factor is commonly expressed as a function of the horizontal water particle semi-excursion ( A wb) at the top of the boundary layer. A wb, in turn, is normally derived from linear wave theory by {{U_{{wb}}/T_{{w}}}}{{2? }} , where U wb is the maximum water particle velocity measured at the top of the boundary layer and T w is the wave period. However, it is shown here that A wb determined in this way deviates drastically from its real value under both linear and non-linear waves. Three equations for smooth, transitional and rough boundary conditions, respectively, are proposed to solve this problem, all three being a function of U wb, T w, and ?, the thickness of the boundary layer. Because these variables can be determined theoretically for any bottom slope and water depth using the deepwater wave conditions, there is no need to physically measure them. Although differing substantially from many modern attempts to define the wave friction factor, the results coincide with equations proposed in the 1960s for either smooth or rough boundary conditions. The findings also confirm that the long-held notion of circular water particle motion down to the bottom in deepwater conditions is erroneous, the motion in fact being circular at the surface and elliptical at depth in both deep and shallow water conditions, with only horizontal motion at the top of the boundary layer. The new equations are incorporated in an updated version (WAVECALC II) of the Excel program published earlier in this journal by Le Roux et al. Geo-Mar Lett 30(5): 549-560, (2010).

Le Roux, J. P.

2012-02-01

14

Skin friction measurements following manipulation of a turbulent boundary layer  

Microsoft Academic Search

Results are presented from three experiments in which direct, local measurements of the skin friction reduction due to flat-plate turbulence manipulators for overall viscous drag reduction were obtained. The results suggest that only a very small overall net drag reduction will be possible for such devices at moderate momentum thickness-derived Reynolds number values, since maximum skin friction drag is neither

V. D. Nguyen; A. M. Savill; R. V. Westphal

1987-01-01

15

Review of research into the concept of the microblowing technique for turbulent skin friction reduction  

Microsoft Academic Search

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

Danny Hwang

2004-01-01

16

Theoretical prediction of turbulent skin friction on geometrically complex surfaces  

NASA Astrophysics Data System (ADS)

This article can be considered as an extension of the paper of Fukagata et al. [Phys. Fluids 14, L73 (2002)] which derived an analytical expression for the constituent contributions to skin friction in a turbulent channel, pipe, and plane boundary layer flows. In this paper, we extend the theoretical analysis of Fukagata et al. (formerly limited to canonical cases with two-dimensional mean flow) to a fully three-dimensional situation allowing complex wall shapes. We start our analysis by considering arbitrarily shaped surfaces and then formulate a restriction on a surface shape for which the current analysis is valid. A theoretical formula for skin friction coefficient is thus given for streamwise and spanwise homogeneous surfaces of any shape, as well as some more complex configurations, including spanwise-periodic wavy patterns. The theoretical analysis is validated using the results of large eddy simulations of a turbulent flow over straight and wavy riblets with triangular and knife-blade cross-sections. Decomposition of skin friction into different constituent contributions allows us to analyze the influence of different dynamical effects on a skin friction modification by riblet-covered surfaces.

Peet, Yulia; Sagaut, Pierre

2009-10-01

17

Magnetosphere-ionosphere coupling through E region turbulence: 2. Anomalous conductivities and frictional heating  

NASA Astrophysics Data System (ADS)

Global magnetospheric MHD codes using ionospheric conductances based on laminar models systematically overestimate the cross-polar cap potential during storm time by up to a factor of 2. At these times, strong DC electric fields penetrate to the E region and drive plasma instabilities that create turbulence. This plasma density turbulence induces nonlinear currents, while associated electrostatic field fluctuations result in strong anomalous electron heating. These two effects will increase the global ionospheric conductance. On the basis of the theory of nonlinear currents developed by Dimant and Oppenheim [2011], this paper derives the correction factors describing turbulent conductivities and calculates turbulent frictional heating rates. Estimates show that during strong geomagnetic storms the inclusion of anomalous conductivity can double the total Pedersen conductance. This may help explain the overestimation of the cross-polar cap potentials by existing MHD codes. The turbulent conductivities and frictional heating presented in this paper should be included in global magnetospheric codes developed for predictive modeling of space weather.

Dimant, Y. S.; Oppenheim, M. M.

2011-09-01

18

An experimental study of turbulent friction on surfaces with discontinuous longitudinal ribbing  

NASA Astrophysics Data System (ADS)

A strain-gauge balance with a floating element was used to measure turbulent friction on surfaces with two- and three-dimensional microribbing. Data are obtained on the effectiveness of drag reduction by ribbed surfaces of different geometrical shapes. It is shown that, under conditions of zero-gradient low-turbulence subsonic flow, surfaces with discontinuous ribbing as well as surfaces with continuous ribbing reduce turbulent friction by 8-10 percent in comparison with smooth surfaces.

Eniutin, G. V.; Lashkov, Iu. A.; Samoilova, N. V.; Fadeev, I. V.; Shumilkina, E. A.

19

Combined effect of longitudinal riblets and LEBU-devices on turbulent friction on a plate  

Microsoft Academic Search

The possibility of reducing turbulent friction with the help of large-eddy-breakup devices (LEBUs) and riblets is studied experimentally. The tests were conducted in a low-turbulence wind tunnel on a flat plate for 2·106 Re 7·106. The local friction coefficient was measured using internal strain-gauge balances, and the total drag was estimated by the momentum-transfer method. It is shown that a

I. V. Gudilin; Yu. A. Lashkov; V. G. Shumilkin

1995-01-01

20

A computational study of turbulent flow separation for a circular cylinder using skin friction boundary conditions  

Microsoft Academic Search

\\u000a In this paper we present a computational study of turbulent flow separation for a circular cylinder at high Reynolds numbers.\\u000a We use a stabilized finite element method together with skin friction boundary conditions, where we study flow separation\\u000a with respect to the decrease of a friction parameter. In particular, we consider the case of zero friction corresponding to\\u000a pure slip

Johan Hoffman; Niclas Jansson

21

Preliminary numerical assessment of turbulent skin friction control with plasma actuators  

Microsoft Academic Search

Plasma actuators (PA) introduce a body force in the near-wall region of a fluid flow. This body force has already been successfully used for separation and transition flow control. We investigate the possibility of applying PAs to turbulent skin friction drag reduction by testing the effect of a modelled PA's body force in a numerically simulated turbulent channel flow. The

Bettina Frohnapfel; Antonio Criscione; Cameron Tropea; Yosuke Hasegawa; Nobuhide Kasagi

2009-01-01

22

The Effects of Superhydrophobic Surfaces on Turbulent Skin Friction and Flow Structure  

Microsoft Academic Search

The application of superhydrophobic surfaces to the reduction of skin friction in turbulent flows is examined through experiments conducted in two facilities: the low-speed turbulent water channel at Brown University and the moderate speed (U = 8m\\/s) boundary layer facility at the Naval Undersea Warfare Center in Newport, RI (NUWC). High resolution PIV measurements are taken in the water channel

Charles Peguero; Charles Henoch; Kenneth Breuer

2007-01-01

23

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

24

Effects of friction on forced two-dimensional Navier-Stokes turbulence  

NASA Astrophysics Data System (ADS)

Large-scale dissipation mechanisms have been routinely employed in numerical simulations of two-dimensional turbulence to absorb energy at large scales, presumably mimicking the quasisteady picture of Kraichnan in an unbounded fluid. Here, “side effects” of such a mechanism—mechanical friction—on the small-scale dynamics of forced two-dimensional Navier-Stokes turbulence are elaborated by both theoretical and numerical analysis. Given a positive friction coefficient ?, viscous dissipation of enstrophy has been known to vanish in the inviscid limit ??0. This effectively renders the scale-neutral friction the only mechanism responsible for enstrophy dissipation in that limit. The resulting dynamical picture is that the classical enstrophy inertial range becomes a dissipation range in which the dissipation of enstrophy by friction mainly occurs. For each ?>0, there exists a critical viscosity ?c, which depends on physical parameters, separating the regimes of predominant viscous and frictional dissipation of enstrophy. It is found that ?c=[?'1/3/(Ckf2)]exp[-?'1/3/(C?)], where ?' is half the enstrophy injection rate, kf is the forcing wave number, and C is a nondimensional constant (the Kraichnan-Batchelor constant). The present results have important theoretical and practical implications. Apparently, mechanical friction is a poor choice in numerical attempts to address fundamental issues concerning the direct enstrophy transfer in two-dimensional Navier-Stokes turbulence. Furthermore, as relatively strong friction naturally occurs on the surfaces and at lateral boundaries of experimental fluids as well as at the interfaces of shallow layers in geophysical fluid models, the frictional effects discussed in this study are crucial in understanding the dynamics of these systems.

Blackbourn, Luke A. K.; Tran, Chuong V.

2011-10-01

25

Microelectromechanical Systems - Based Feedback Control of Turbulence for Skin Friction Reduction  

Microsoft Academic Search

This article focuses on the feedback control of turbulence for skin friction reduction and reviews the state of the art of control algorithms and distributed microsensors and microactuators. From a viewpoint of possible practical applications, we discuss only the control schemes based on the wall-surface sensing of shear stress and pressure fluctuations with their assessment in direct numerical simulation. The

Nobuhide Kasagi; Yuji Suzuki; Koji Fukagata

2009-01-01

26

Toward cost-effective Control of Wall Turbulence for Skin Friction Drag Reduction  

Microsoft Academic Search

This paper discusses the active control of turbulence for skin friction reduction with an emphasis on cost effectiveness. By introducing performance indices such as the net energy saving rate and the control gain, we assess existing control algorithms for true energy saving.We review recent attempts to reduce costs accompanying practical applications, and discuss remaining issues in developing more practically applicable

Nobuhide Kasagi; Yosuke Hasegawa; Koji Fukagata

2009-01-01

27

Heat transfer and friction resistance at the turbulent flow in a flat curvilinear channel  

SciTech Connect

The effect of body (Coriolis) forces, occurring under conditions of flow of liquid along a curvilinear surface, on turbulent transfer is investigated on the basis of the model for turbulent stresses and heat fluxes. The effect of temperature and density fluctuations in the field of body forces is also included. The relations for turbulent viscosity, turbulent stresses, and heat fluxes as a function of the Richardson number or thermal Richardson number are derived. The numberical finite-difference solution of the equations of motion and energy for the case of stabilized turbulent flow and heat transfer in a plane channel is given. The results of the calculation of heat transfer and resistance are in good agreement with the available experimental data and show a considerable increase of the Nusselt number and friction resistance coefficient on a concave wall as compared with a convex one, as well as an increase of hydraulic resistance.

Valueva, E.P.; Popov, V.N. [Moscow Power Institute (Russian Federation)

1995-05-01

28

Velocities, turbulence, and skin friction in a deep-sea logarithmic layer  

Microsoft Academic Search

Speed, turbulence, skin friction, and drag measurements made with metal-clad hot wires, epoxy-coated hot films, and Savonius rotors are reported for a deep-sea boundary layer at a water depth of ~5000 m. They include data from heights z<30 cm, a region hitherto only investigated in detail by Chriss and Caldwell (1982) for a shelf site. A mean speed logarithmic layer

Giselher Gust; Georges L. Weatherly

1985-01-01

29

Effect of Free-Stream Turbulence on the Reduction of Surface Friction in a Turbulent Boundary Layer Behind LEBU-Devices  

Microsoft Academic Search

The effect of increased free-stream turbulence on the reduction of the surface friction coefficient c\\u000a \\u000a f\\u000a in a turbulent boundary layer behind large-eddy break-up (LEBU) devices is investigated using a gravimetric method. The turbulence\\u000a level was ? ? 1.9–4.9 % and the turbulence scale L\\u000a \\u000a e\\u000a ? 40–110 mm. The boundary layer Reynolds number Re** was varied from 2300 to

N. V. Samoilova; V. G. Shumilkin

2005-01-01

30

Friction Factor of Water Flow Through Rough Rock Fractures  

NASA Astrophysics Data System (ADS)

Fluid flow through rock joints occurs in many rock engineering applications. As the fluid flows through rough-walled rock fractures, pressure head loss occurs due to friction drag of the wall and local aperture changes. In this study, the friction factor was experimentally investigated by performing flow tests through sandstone fractures with joint roughness coefficient ranging from 5.5 to 15.4 under changing normal stress from 0.5 to 3.5 MPa. According to the experimental results, the friction factor was formulated as a function of two-independent variables—Reynolds number and relative roughness. Relative roughness is defined as the ratio of maximum asperity height to equivalent hydraulic aperture. The experimental results show that the proposed predictor of the friction factor fits the data with a coefficient of determination R 2 > 0.93. Sensitivity analyses indicate that in general, the proposed friction factor increases with the relative roughness of confined fractures. The large difference of friction factor induced by relative roughness occurs when the Reynolds number is lower than unity, especially for Re < 0.2. For Reynolds numbers greater than unity, the difference of friction factor induced by relative roughness is smaller. Inclusion of joint roughness in calculating the friction resistance to fluid flow in rough rock joints and the influence of normal stress to the joints is a major step towards more accurate predictions for fluid flow in underground joint networks. This study provides a significant improvement in fundamental understanding of fluid flow in the jointed strata.

Zhang, Z.; Nemcik, J.

2013-09-01

31

Friction  

Microsoft Academic Search

\\u000a Friction materials such as disk pads, brake linings, and clutch facings are widely used for automotive applications. Friction\\u000a materials function during braking due to frictional resistance that transforms kinetic energy into thermal energy. There has\\u000a been a rudimentary evolution, from materials like leather or wood to asbestos fabric or asbestos fabric saturated with various\\u000a resins such as asphalt or resin

Yoshihiro Matsuo; Daryl D. Clarke; Shinichi Ozeki

2010-01-01

32

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)] 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

33

A friction factor bound for transitional pipe flow  

NASA Astrophysics Data System (ADS)

An upper bound ?max on the friction factor ? is found for transitional pipe flow up to Reynolds numbers of Re=4000. This bound corresponds to the laminar Hagen-Poiseuille value until the energy stability point of Re=81.52 (where ?max=0.7851) after which it decreases monotonically to approach a viscosity-independent asymptote of 0.27+/-0.01 as Re-->?. Comparison is made with the friction factors associated with recently discovered, finite-amplitude traveling waves in rotating and nonrotating pipe flow as well as experimental data.

Plasting, S. C.; Kerswell, R. R.

2005-01-01

34

Friction factors and roughness measurements of tubular mineral membranes  

NASA Astrophysics Data System (ADS)

No direct measurement of the relative roughness is available for mineral porous media because of the low mechanical resistance of such materials. In this study a method for the experimental determination of the internal diameter and the equivalent roughness is proposed for different commercial membranes used in ultrafiltration and microfiltration processes. The use of classical friction factor correlations is also discussed. The main results are the estimation of the hydraulic diameter of tubular membranes and the use of a quadratic form in order to predict friction factors and the equivalent roughness with an accuracy better than 15%.

René, F.; Leuliet, J. C.; Delplace, F.

1993-08-01

35

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

36

Friction Factor Measurements in an Equally Spaced Triangular Tube Array.  

National Technical Information Service (NTIS)

Friction factor data for adiabatic cross-flow of water in a staggered tube array was obtained over a Reynolds number range (based on hydraulic diameter and gap velocity) of about 10,000 to 250,000. The tubes were 12.7mm (0.5 inch) outer diameter, in a uni...

P. Symolon P. Vassallo

2007-01-01

37

Experimental research on friction factor of end faces of contacting mechanical seals  

NASA Astrophysics Data System (ADS)

The friction of the seal faces is the most important phenomenon in working process of contacting mechanical seals. The friction factor f is a key parameter for expressing the friction regime of the seal faces, the frictional power, the wearing capacity, the friction heat productivity, the temperature distortion of the end face and the temperature of the end face. The relationship between the friction factor f and the friction regime of the end faces of contacting mechanical seals was discussed from a microscopic point of view. The friction factor is usually worked out by the friction torque which is measured in the test. In the computer aided testing device of the mechanical seal system, the experimental investigations on the basic performance of the B104a-70 contacting mechanical seal was carried out. The test results indicate that the bigger the spring pressure of B104a-70 contacting mechanical seal, the bigger the friction factor. When the spring pressure is less, the bigger the rotational speed, the bigger the friction factor. But when the spring pressure is equal to 0.0866 MPa, the friction factor is not almost influenced by the rotational speed. When the rotational speed and spring pressure are less, the medium pressure has a less influence on the friction factor. When the rotational speed or spring pressure is bigger, the bigger the medium pressure, the less the friction factor.

Wei, Long; Gu, Bo-Qin; Feng, Xiu; Sun, Jian-Jun

2008-11-01

38

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.

39

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

40

Simple LMFBR axial-flow friction-factor correlation  

SciTech Connect

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

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

1982-12-01

41

Friction Factor Measurements in an Equally Spaced Triangular Tube Array  

SciTech Connect

Friction factor data for adiabatic cross-flow of water in a staggered tube array was obtained over a Reynolds number range (based on hydraulic diameter and gap velocity) of about 10,000 to 250,000. The tubes were 12.7mm (0.5 inch) outer diameter, in a uniformly spaced triangular arrangement with a pitch-to-diameter ratio of 1.5. The friction factor was compared to several literature correlations, and was found to be best matched by the Idelchik correlation. Other correlations were found to vary significantly from the test data. Based on the test data, a new correlation is proposed for this tube bundle geometry which covers the entire Reynolds number range tested.

Vassallo P, Symolon P

2007-03-19

42

Integral method of calculating friction and heat transfer in the turbulent flow of incompressible media in pipes with a rough permeable wall  

Microsoft Academic Search

A method of calculating friction and heat transfer for a flow of an incompressible liquid over a rough permeable surface is developed on the basis of universal profiles of velocity and temperature in the turbulent boundary layer. A relatively simple transcendental equation is obtained for the friction coefficient c\\/sub f\\/ in the case of uniform injection. This equation can be

B. I. Nigmatulin; Yu. V. Vasilev

1987-01-01

43

The effect of wall friction in single-phase natural circulation stability at the transition between laminar and turbulent flow  

Microsoft Academic Search

The present paper is focused on the prediction of stability of single-phase natural circulation in the range of Reynolds numbers characterizing the transition between laminar and turbulent flow. In particular, the predictions obtained by one-dimensional models making use of different assumptions for evaluating wall friction at this transition are discussed, also in front of experimental information from previous investigations.The starting

W. Ambrosini; N. Forgione; J. C. Ferreri; M. Bucci

2004-01-01

44

Effects of the toe shape of the flash on stress concentration factor in friction welded joints  

Microsoft Academic Search

The authors have previously reported that the fatigue strength of friction welded joints with a flash is markedly affected by the toe shape of the flash caused by friction welding conditions. Accordingly, in this report, the effects of the toe shape of the flash upon the stress concentration factor ? of friction welded joints were examined by means of the

M. Hasegawa; T. Ieda; T. Asada; N. Taki

1997-01-01

45

Friction pressure correlations for turbulent flow of drag reducing polymer solutions in straight and coiled tubing  

Microsoft Academic Search

Accurate determination of friction pressure losses of dilute drag reducing polymer solutions remains to be a challenge in many practical applications. These include a wide variety of hydraulic operations performed on a daily basis in the oil and gas industry. Most drilling, completions, and stimulation jobs require pumping fluids at high flow rates, which in turn generates high frictional pressure

Felipe Gallego; Subhash N. Shah

2009-01-01

46

Skin-friction drag reduction in laminar and turbulent boundary layers  

NASA Astrophysics Data System (ADS)

The skin-friction drag in a constant, mass-flux plane channel flow is sustained below that corresponding to laminar flow when wall-normal blowing and suction at the upper and lower walls are applied as upstream traveling waves. The control is distributed such that the net mass-flux of the channel is not modified. Direct numerical simulations demonstrate that these upstream traveling waves can induce sublaminar viscous drag in fully developed laminar and turbulent flows. Furthermore, it is found that the observed phenomena can be characterized by the linearized governing equations. The Navier-Stokes equations can be linearized by describing the flow velocities as perturbations about a mean, resulting in equations that describe the dynamics of the perturbations. A spectral decomposition, involving a two-dimensional Fourier expansion in the streamwise and spanwise directions and a Galerkin projection of Chebyshev polynomials in the wall-normal direction, is used to spatially discretize these equations, leading to a state space model. The traveling wave control is then introduced as a temporally, phase-shifting, wall-normal velocity wall boundary condition at a particular Fourier wavenumber. This linear model was used to develop reduced-order linear-quadratic-Gaussian (LQG) controllers. While these controls could achieve significant drag reduction, they appeared to be a performance limit. However, they could induce transient sublaminar drag under certain conditions. A nonlinear minimization using full-order nonlinear simulations was attempted to capture this transient behavior on a periodic basis. The upstream traveling wave was discovered in the course of that study. The same linear models can represent the sustained sublaminar drag. Using a recent formulation which expresses the viscous drag of a fully developed channel flow as a sum of laminar drag and the Reynolds shear stress, it is shown that the Reynolds shear stress calculated from the controlled linear model state provides good predictions of how the nonlinear flow will respond. This provides a computationally efficient environment in which to find the optimal amplitude and speed to set a traveling wave to gain the most predicted drag reduction for a specific level of input power.

Kang, Sung Moon

47

Structure of Atmospheric Turbulence in the Friction Layer Below 500 Meters.  

National Technical Information Service (NTIS)

Measurements of wind direction and speed, vertical velocity, and temperature were made from several levels of the 444 m tower near Oklahoma City. Turbulence quantities were calculated from the covariances between observed variables over periods ranging fr...

S. J. Maas J. R. Scoggins

1976-01-01

48

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

49

Determining turbulent flow friction coefficient using adaptive neuro-fuzzy computing technique  

Microsoft Academic Search

In the analysis of water distribution networks, the main required design parameters are the lengths, diameters, and friction coefficients of rough-pipes, as well as nodal demands and water levels in the reservoirs. Although some of these parameters such as the pipe lengths are precisely known and would remain the same at different points of the networks whereas some parameters such

Mehmet Özger; Gürol Yildirim

2009-01-01

50

Improvement in the calculation of turbulent friction in smooth concentric annuli  

Microsoft Academic Search

Data and calculation techniques for frictional pressure loss in smooth, concentric annuli are reviewed. It is shown that the accepted methods of Meter and Bird, and of Rothfus, Monrad, and Senecal deviate by 4% and 8%, respectively, from the correct limit for small gaps. Further, neither correctly predicts the data trends with decreasing radius ratio. It is further shown that

O. C. Jr. Jones; J. C. M. Leung

1979-01-01

51

Turbulent skin-friction reduction by electroactive polymer surfaces with in-plane and out-of-plane motion  

NASA Astrophysics Data System (ADS)

An electroactive polymer (EAP) surface capable of in-plane and out-of-plane deformations is used to apply periodic near-wall forcing to a fully-developed turbulent boundary layer to reduce friction drag. The in-plane wall deformation consists of a flat surface with streamwise-oriented narrow strips of electrode which cause the EAP to expand and contract in the spanwise direction in response to an applied voltage. The out-of-plane wall deformation takes the form of a spanwise travelling harmonic surface wave with amplitude of the order of the viscous sublayer height. Direct measurements of friction drag by means of a drag balance (with accuracy ±2%) and hot-wire measurements of mean and higher-order moments are presented for a range of frequencies, amplitudes and wavelengths. It is expected that the imposition of a spatially and temporally varying Stokes-like layer will enable the direct manipulation of the quasi-streamwise near-wall structures and the reduction of the frequency and intensity of sweep events. While the effectiveness of this form of near-wall forcing has been demonstrated in simulations, the development of EAP surfaces allows experimental assessment of the technique at laboratory Reynolds numbers.

Gouder, Kevin; Morrison, Jonathan

2008-11-01

52

Turbulent heat transfer and friction in pin fin channels with lateral flow ejection  

Microsoft Academic Search

Experiments were conducted to study the effects of lateral flow ejection on the overall heat transfer and pressure drops for turbulent flow through pin fin channels. The two test sections of the investigation were rectangular channels with staggered arrays of six and eight streamwise rows of pins, respectively. The pin length-to-diameter ratio was one and both the streamwise and spanwise

S. C. Lau; J. C. Han; Y. S. Kim

1989-01-01

53

Skin friction Reduction by Introduction of Micro-bubbles into Turbulent Boundary Layer  

Microsoft Academic Search

The phenomenon of drag reduction by the injection of micro- bubbles into turbulent boundary layer has been investigated using an Eulerian-Eulerian two-fluid model. Two variants namely the Inhomogeneous and MUSIG (MUltiple SIze Group) based on Population balance models are investigated. The simulated results are compared against the experimental findings of Madavan et al (1). The model employed in the investigation

K. Mohanarangam; C. P. Cheung

54

A theoretical prediction of friction drag reduction in turbulent flow by superhydrophobic surfaces  

Microsoft Academic Search

We present a theoretical prediction for the drag reduction rate achieved by superhydrophobic surfaces in a turbulent channel flow. The predicted drag reduction rate is in good agreement with results obtained from direct numerical simulations at Retau~=180 and 400. The present theory suggests that large drag reduction is possible also at Reynolds numbers of practical interest (Retau~105-106) by employing a

Koji Fukagata; Nobuhide Kasagi; Petros Koumoutsakos

2006-01-01

55

Effect of local heat supply to a turbulent boundary layer on the friction  

Microsoft Academic Search

The results of calculating a supersonic turbulent boundary layer on a flat plate in the presence of thermal energy supply\\u000a to the boundary layer are presented. Two methods of energy supply are considered: heating a local interval of the surface,\\u000a which is otherwise thermally insulated and using a local volume heat source. It is shown that for the same amount

A. V. Kazakov; M. N. Kogan; A. P. Kuryachii

1997-01-01

56

An artificial neural network for non-iterative calculation of the friction factor in pipeline flow  

Microsoft Academic Search

A non-iterative procedure was developed, using an artificial neural network (ANN), for calculating the friction factor, f, in the Darcy-Weisbach equation when estimating head losses due to friction in closed pipes. The Regula-Falsi method was used as an implicit solution procedure to estimate the f values for a range of Reynolds numbers, Re, and relative roughness e\\/D values (where e

Walid H Shayya; Shyam S Sablani

1998-01-01

57

Turbulent friction drag reduction using electroactive polymer and electromagnetically driven surfaces  

NASA Astrophysics Data System (ADS)

This work reports aerodynamic testing of two spanwise-oscillating surfaces fabricated out of electroactive polymers (EAPs) in the dielectric form of actuation, and of an electromagnetic-driven linear motor. Hot-wire and PIV measurements of velocity and direct measurement of friction drag using a drag balance are presented. A maximum of 16 % surface friction reduction, as calculated by the diminution of the wall-normal streamwise velocity gradient, was obtained. Among other quantities, the spatial dependence of the drag reduction was investigated. When this spatial transient and portions which are static are accounted for, the direct drag measurements complement the hot-wire data. PIV measurements, where the laser beam was parallel to the oscillating surface at y + ? 15, support the hot-wire data. The two actuators are original in design, and significant contributions have been made to the development of EAPs. This experiment is the first to aerodynamically test EAP actuators at such a large scale and at a relatively moderate Re.

Gouder, Kevin; Potter, Mark; Morrison, Jonathan F.

2013-01-01

58

Improvement in the calculation of turbulent friction in smooth concentric annuli  

SciTech Connect

Data for smooth concentric annuli having an overall -25 to +35 percent scatter about the Colbrook prediction for smooth tubes, along with calculation techniques, are reviewed. It is shown that the accepted methods of Meter and Bird, and of Rothfus, Monrad, and Senecal, deviate substantially from the correct limit for small gaps. Further, neither correctly predicts the data trends with decreasing radius ratio. It is demonstrated that the theoretically determined laminar equivalent diameter which provides similarity in laminar flow for round tubes and concentric annuli also provides similarity in turbulent flow. 36 refs.

Jones, O.C. Jr.; Leung, J.C.M.

1981-12-01

59

The Effect of Hydrodynamic Unsteadiness on the Flow Structure and on the Coefficients of Heat Transfer and Skin Friction under Conditions of Turbulent Pipe Flow of Heat-Transfer Agent  

Microsoft Academic Search

Critical analysis is made of the current status of investigations of heat transfer and skin friction under conditions of unsteady-state turbulent pipe flow. This analysis is based on the experimental data on the structure of turbulent flows and heat transfer. Comparison is made of experimental and theoretical results.

G. A. Dreitser; V. M. Kraev

2004-01-01

60

Update on the evaluation of different correlations for the flow friction factor and heat transfer of Stirling engine regenerators  

Microsoft Academic Search

This paper is intended to update the overview of different correlations for the friction factor and heat transfer of Stirling engine regenerators given by the Oscillating-Flow Regenerator Test Rig data. For that purpose the data was recalculated using the general applicable set of equations for Reynolds number, friction factor, pressure drop, Nusselt number and heat transfer area. Based on these

Bernd Thomas; Deborah Pittman

2000-01-01

61

A friction factor correlation for the offset strip-fin matrix  

NASA Astrophysics Data System (ADS)

This work deals with the offset strip fin surface geometry used in parallel plate fin 'compact' heat exchangers. Previous attempts to correlate existing data have been impaired by the possibility that burrs may exist on the leading and trailing edges of the fins. Such burrs are believed to affect the heat transfer and friction characteristics. Patankar and Prakash (1981) have numerically predicted the heat transfer and friction characteristics of one geometry, but were unable to validate their results because of lack of accurate data on the geometry modeled. The experimental objective of this work was to obtain friction data on scaled-up matrix geometries that had precisely known dimensions, and no burred fin-edges. Data were taken on eight surface geometries that span a wide range of geometric parameters. These data were correlated within + or - 10 percent to provide a generalized equation for the friction factor as a function of the geometric parameters and Reynolds number. The resulting correlation predicts actual heat exchanger friction data with good accuracy. This work further suggests that the effect of burrs on the published data for actual heat exchanger geometries is insignificant.

Webb, R. L.; Joshi, H. M.

62

Fluid Friction  

NSDL National Science Digital Library

Students use wood blocks and model boats to investigate how the shape of an object determines the friction it encounters as it is drawn through a towing tank. They learn the advantages of streamlining and increased waterline length and observe that the shape of an object affects its stability. Students are also introduced to the terms power, turbulence, hydrodynamics, work, and boat design, and explore the relationship of waterline length to speed.

63

Acoustics of friction  

NASA Astrophysics Data System (ADS)

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

Akay, Adnan

2002-04-01

64

Numerical approach to laser beam propagation through turbulent atmosphere and evaluation of beam quality factor  

Microsoft Academic Search

In this paper, propagation of a Gaussian laser beam through turbulent atmosphere is evaluated numerically. The beam quality factor for the propagated beam has been estimated for different turbulent conditions that are characterized by parameter Cn. The calculations show that the beam quality can be affected dramatically by atmospheric turbulence and the laser beam size and wavelength have major role

M. H. Mahdieh

2008-01-01

65

Laminar flow in chevron-type plate heat exchangers: CFD analysis of tortuosity, shape factor and friction factor  

Microsoft Academic Search

Laminar or low Reynolds number flows are usually obtained when liquid foods with high viscosity are processed in plate heat exchangers (PHEs). The tortuosity coefficient is a key parameter used by PHEs manufacturers to estimate Fanning friction factors and convective heat transfer coefficients. Using the finite-element computational fluid dynamics program POLYFLOW®, fully developed laminar flows in double-sine chevron-type PHEs passages

Carla S. Fernandes; Ricardo P. Dias; João M. Nóbrega; João M. Maia

2007-01-01

66

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

67

Turbulence  

SciTech Connect

Current theories for material mixing include multiphase interpenetration and single-field turbulence transport with large density variations. Neither approach by itself is adequate for current problem-solving needs, but in combination they offer tremendous opportunities for the analysis of complex material dynamics. Multiphase theory contributes the ''ordered'' jets or particulate trajectories that penetrate in wave-like fashion; turbulence transport superimposes the important nonlinear diffusive component to the mixing. Shear impedance and energy transport arise naturally in this combined analysis.

Harlow, F.H.

1987-04-01

68

The effects of spatially complex inner shelf roughness on boundary layer turbulence and current and wave friction: Tairua embayment, New Zealand  

NASA Astrophysics Data System (ADS)

A field study of bedforms, associated hydraulic roughness, and turbulence was conducted on the inner shelf off the east coast of New Zealand's North Island under conditions that included two significant storm events. Sharply contrasting rough and smooth beds were characterized via field mapping and deployment of instrumented benthic tripods. Rough areas of coarse sand exhibited ripples with heights and lengths of ˜25 and ˜100 cm, while smooth areas supported smaller ripples with heights and lengths of ˜5 cm and ˜20 cm. Contacts between the two surfaces were sharp and maintained their position. Roughness contrasts were enhanced significantly during storms, which simultaneously accentuated migrating orbital ripples over the coarse bed and replaced ripples on the fine sediment bed with smoother hummocky features. Spectra of the fluctuating vertical velocity components, w', from both smooth and rough sites showed good fits to -5/3 slopes within the inertial sub range enabling independent estimates of wave-averaged bed stress to be made via the inertial dissipation method (IDM). We also utilized the vertical fluctuation data to obtain alternative estimates of the wave friction factor, fw, following Smyth and Hay (J. Phys. Oceanography 32 (2002) 3490); SH. These two methods yielded generally similar results. Under high wave conditions, fw estimated via IDM averaged 0.027 at the rough site and 0.0045 at the smooth site while the SH method gave respective values of 0.027 and 0.013. Under low-energy conditions, fw from IDM averaged 0.0082 at the rough site and 0.012 at the smooth site, while the SH method yielded mean values of 0.0080 and 0.016. Thus, fw was much larger at the rough site than at the smooth site during storms but smaller at the rough site during fair weather. During storms, structured vortices with frequencies at the first harmonic of the swell waves formed over the rough surface and penetrated above the wave current boundary layer causing retardation of mean currents. Such storm-induced vortices were only intermittently present over the smooth surface. The application of Nielsen's (J. Geophys. Res. 86 (1981) 6467) roughness model produced some qualitatively similar trends in fw, although predicted fw was larger than observed values at the rough site. Additionally, low modelled fw over the smooth bed during high energy was based on plane bed theory rather than the inferred hummocky bed.

Trembanis, A. C.; Wright, L. D.; Friedrichs, C. T.; Green, M. O.; Hume, T.

2004-08-01

69

The use of the limiting laws of turbulent friction and heat transfer to construct wall functions on permeable surfaces  

Microsoft Academic Search

The limiting laws of turbulent transfer of momentum and heat at high values of the Reynolds number are used to derive wall\\u000a functions (“wall laws”) under conditions of a flow of gas with variable physical properties along a permeable surface.

L. I. Zaichik; A. I. Leont’ev

2000-01-01

70

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

71

Performance loss factors for optical communication through clear air turbulence  

Microsoft Academic Search

Atmospheric turbulence can significantly degrade the bit error rate performance of a free-space laser communication link. We describe an approach for calculating the average bit error rate of a direct detection binary optical communication link in clear-air atmospheric turbulence, and discuss strategies for optimizing system performance.

Jennifer C. Ricklin; Stephane Bucaille; Frederic M. Davidson

2004-01-01

72

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

73

Effect of void fraction and friction factor models on the prediction of pressure drop of R134a during downward condensation in a vertical tube  

NASA Astrophysics Data System (ADS)

The theoretical flow models of homogeneous and separated flow are applied to in-tube condensation to predict the pressure drop characteristics of R134a. The homogeneous flow model is modified by ten different dynamic viscosity correlations and various alternative correlations of total, frictional and momentum pressure drops to take account of the partial condensation inside the tube. Numerical analyses were performed to determine the average and local homogeneous wall shear stresses and friction factors by means of a CFD program. The equivalent Reynolds number model is modified by six different two-phase friction factors to determine the total condensation pressure drop in the separated flow model. The refrigerant side total pressure drops, frictional pressure drops, friction factors and wall shear stresses are determined within a ±30% error band. The importance of using the alternative total, momentum and frictional pressure drop correlations for the homogeneous flow model is also shown.

Dalkilic, A. S.; Kürekci, N. A.; Wongwises, S.

2012-01-01

74

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

75

Differential Advection as a Factor in Clear-Air Turbulence  

Microsoft Academic Search

Clear-air turbulence observations made by accelerometer measurements have been correlated with six-hour forecasts of the change in vertical stability. The change in stability has been forecast by computing the six-hour horizontal temperature advection at two levels. By means of the Kolmogorov-Smirnov non-parametric test for cumulative distributions, a significant correlation has been shown to exist between the observations of turbulence and

Edwin L. Keitz

1959-01-01

76

One-dimensional, steady compressible flow with friction factor and uniform heat flux at the wall specified  

SciTech Connect

The purpose of this work is to present generalized graphical results to readily permit passage design for monatomic gases, the results including accommodation of any independently specified friction factor, heat transfer coefficient, and wall heat flux. Only constant area passages are considered, and the specified wall heat flux is taken to be uniform.

Landram, C.S.

1997-10-27

77

Experimental and CFD studies on heat transfer and friction factor characteristics of a tube equipped with modified twisted tape inserts  

Microsoft Academic Search

This paper reports experimental and Computational Fluid Dynamics (CFD) investigations on the friction factor, Nusselt number and thermal–hydraulic performance of a tube equipped with the classic and three modified twisted tape inserts. The results showed that the Nusselt number and performance of the jagged insert were higher than other ones. Maximum increase of 31% and 22% were observed in the

Masoud Rahimi; Sayed Reza Shabanian; Ammar Abdulaziz Alsairafi

2009-01-01

78

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

79

Numerical predictions and measurements of Reynolds normal stresses in turbulent pipe flow of polymers  

Microsoft Academic Search

An anisotropic low Reynolds number k–? turbulence model has been developed and its performance compared with experimental data for fully-developed turbulent pipe flow of four different polymer solutions. Although the predictions of friction factor, mean velocity and turbulent kinetic energy show only slight improvements over those of a previous isotropic model [Cruz, D.O.A., Pinho, F.T., Resende, P.R., 2004. Modeling the

P. R. Resende; M. P. Escudier; F Presti; F. T. Pinho; D. O. A. Cruz

2006-01-01

80

Characteristics of turbulent flow in the annuli with smooth and rough surfaces  

Microsoft Academic Search

The structure of turbulence of fully developed flow through three concentric annuli with rough walls shown in Fig. 1 was investigated\\u000a experimentally for Reynolds number range of Re =15000?66000. Time mean velocity distribution, friction factor, turbulence\\u000a intensity and turbulent kinetic energy in annuli of three radius ratios of ?=0.26, 0.39 and 0.56 for four cases of roughness\\u000a type were measured.

Soo Whan Ahn; Kyung Chun Kim

1999-01-01

81

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

82

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

NASA Astrophysics Data System (ADS)

Degrading alpine permafrost due to changing climate conditions causes instabilities in steep rock slopes. Due to a lack in process understanding, the hazard is still difficult to asses in terms of its timing, location, magnitude and frequency. Current research is focused on ice within joints which is considered to be the key-factor. Monitoring of permafrost-induced rock failure comprises monitoring of temperature and moisture in rock-joints. The effect of low temperatures on the strength of intact rock and its mechanical relevance for shear strength has not been considered yet. But this effect is signifcant since compressive and tensile strength is reduced by up to 50% and more when rock thaws (Mellor, 1973). We hypotheisze, that the thawing of permafrost in rocks reduces the shear strength of joints by facilitating the shearing/damaging of asperities due to the drop of the compressive/tensile strength of rock. We think, that decreasing surface friction, a neglected factor in stability analysis, is crucial for the onset of destabilisation of permafrost rocks. A potential rock slide within the permafrost zone in the Wetterstein Mountains (Zugspitze, Germany) is the basis for the data we use for the empirical joint model of Barton (1973) to estimate the peak shear strength of the shear plane. Parameters are the JRC (joint roughness coefficient), the JCS (joint compressive strength) and the residual friction angle (?r). The surface roughness is measured in the field with a profile gauge to create 2D-profiles of joint surfaces. Samples of rock were taken to the laboratory to measure compressive strength using a high-impact Schmidt-Hammer under air-dry, saturated and frozen conditions on weathered and unweathered surfaces. Plugs where cut out of the rock and sand blasted for shear tests under frozen and unfrozen conditions. Peak shear strength of frozen and unfrozen rocks will be calculated using Barton's model. First results show a mean decrease of compressive strength of around 40% when frozen water-saturated rock is exposed to thawing. The friction of sand-blasted rock-plugs decreases by a mean value of 32% considering degradation of rocks by freeze-thaw cycles. Surface roughness could be measured succesfully with the profile gauge and the results show a significant difference between untouched and sheared joint surfaces in the field. Here we show, that shear resistance of rock joints will be diminshed just by the thawing of intact rock. This study will help to establish a sound concept for the destabilization of rocks in permafrost and provide the data for first stability modelling. This will be crucial for predict rock instability in permafrost regions. References: Barton, N. (1973): Review of new shear strength criterion for rock jonts. Engineering Geology 7: 287-332 Mellor, M. (1973): Mechanical Properties of Rocks at Low Temperatures. 2nd International Conference on Permafrost, Yakutsk, Siberia, 334-343.

Funk, Daniel; Krautblatter, Michael

2010-05-01

83

A semi-empirical correlation for adiabatic interfacial friction factor in horizontal air-water countercurrent stratified flow  

SciTech Connect

A semi-empirical correlation for an adiabatic interfacial friction factor (f{sub ia}) in a stratified wavy flow, based on the simultaneous measurements of the main flow parameters in air-water countercurrent stratified flows and the concept of surface roughness in a wavy flow, has been developed. The functional form for f{sub ia} in particular, has been obtained by making an analogy between the effect of surface roughness on the gas-to-wall friction factor (f{sub g}) in a wavy flow. A total of 186 data points from the present countercurrent flow test and 15 from the concurrent pipe flow reported by others have been used in the present analysis. Comparisons between the data and the predictions of the present correlation shows that the agreement is within {+-}30%.

Chun, M.H.; Kim, Y.S. [Korea Advanced Institute of Science and Technology, Taejon (Korea, Republic of). Dept. of Nuclear Engineering

1995-09-01

84

Experimental and numerical study of turbulent flow and heat transfer inside hexagonal duct  

NASA Astrophysics Data System (ADS)

Flow and heat transfer characteristics in transition and turbulent regions are studied experimentally and numerically in a horizontal smooth regular hexagonal duct under constant wall temperature boundary condition covering a range of Reynolds number from 2.3 × 103 to 52 × 103. Two types of k-omega (standard and shear stress transport (SST)) and three types of k- ? (standard, renormalization (RNG), and realizable) turbulence model are employed for transition and turbulent regions, respectively. Both average and fully developed Darcy friction factor and Nusselt number are presented as a function of Reynolds number. It is seen that k-omega SST and k- ? realizable turbulence models gave the best agreement with the experimental data in transition and turbulent regions, respectively. All the experimental results are correlated within an accuracy of ±13 % and ±7 % for Nusselt number and Darcy friction factor, respectively. Results obtained in this study are compared with circular duct results using hydraulic diameter.

Turgut, O?uz; Sar?, Mehmet

2013-04-01

85

Electrokinetic aspects of turbulent drag reduction in surfactant solutions  

Microsoft Academic Search

Drag reduction is a phenomenon by which small amounts of additives can greatly reduce the turbulent friction factor of a fluid. It can be used to reduce energy consumption of pumping, increase flow rate, and decrease the sizes of pumps, pipes, and fittings in flow systems. Very wide review of the phenomenon was done by Zakin, Lu, and Bewersdorff1 and

M. Fichman; G. Hetsroni

2005-01-01

86

Relation between elongation factor and angle of friction from various outcrops  

NASA Astrophysics Data System (ADS)

The study of granular materials, and more particularly their mechanical behaviour, has made it possible to highlight, the influence of the grains shape on their mechanical behaviour. Recently when being concerned with till formations (glacial deposits) that cover mountain slopes and govern natural hazards, Lebourg (2000) has shown that there exists a simple linear regression of the internal angle of friction (phi) on a shape parameter (elongation factor IA ) based on the analysis of six tills formations coming from a paleoglacial valley (Vallée d Aspe, Pyrénées Occidentales : IGN map 1547 OT, 1/25 000 ; geological map URDOS 1/80 000). These results are in agreement with previous works where relation between shape and mechanical properties is assumed. Till-forming materials look like a heap of unsorted very heterogeneous material characterised by rock debris of all sizes from angular blocks of metric size to very fine rock. In addition to the block sizes, lithology, petrography and the spatial distribution of the blocks are also heterogeneous. Then it is hard, if not impossible, to collect a large sample of mechanical and physical data from the till in order to execute good simulations while running numerical programs. The validation of such a relation would be very interesting for other till formations and in any case of natural hazards such as landslides. So we propose new results based on the analysis of a set of samples collected on the site of La Clapière, a rocky landslide. The landslide at La Clapière, in south-eastern France, is located on the east side of the steep La Tinée river valley upstream of the village of Saint Etienne de Tinée. On the one hand, triaxial compression (test with simultaneous compression of a cylindrical sample and application of axisymetric confining pressure) has been performed on four sets of samples collected at La Clapière, then mechanical properties (E : Young modulus, phi: angle of internal friction, C : cohesion) were estimated. Sample forming materials are coarse to fine particles (gouge and weathered and crushed gneiss). On the other side samples were screened with a set of 10 from 0.08 mm up to 25 mm, then grey level images of a series of individual particles belonging to each granulometric interval have been acquired (particles lie on a stage in a natural way) for 2D parameters measurements. Two dimensional size and shape parameters (A: area, P, perimeter, RI, RC : radii of the largest inner respectively the smallest outer circle of each particle and the ratio IA= RI/RC) have been measured by the mean of an image analyser and computed and statiscally analysed (statistical parameters, normality evaluation, …). This paper is an attempt to show how the simple linear regression inferred for various tills can be extended to weathered, crushed materials.

Martins-Campina, B.; Lebourg, T.; Riss, J.; Benabderrazik, A.; Fabre, R.

2003-04-01

87

On the vertical large eddy breakup device capability to decrease the turbulent drag  

NASA Astrophysics Data System (ADS)

The possibility of decreasing the turbulent friction with the use of streamwise-aligned vertical large eddy breakup devices installed normal to the surface of a flat plate in an incompressible equilibrium turbulent boundary layer with a nominally gradientless flow past this plate is studied experimentally. The Reynolds number based on the boundary-layer momentum thickness is 1099 in the cross section where these vertical devices are mounted. It is shown that elements of this geometry are effective tools for modification of the turbulent boundary layer from the viewpoint of both the gain in friction drag and the suppression of turbulent fluctuations of velocity and, hence, can be considered as one of the most important factors of controlling the structure of the near-wall turbulence in the flow past a flat surface.

Boiko, A. V.; Kornilov, V. I.

2009-12-01

88

Effect of artificial roughness on heat transfer and friction factor in a solar air heater  

Microsoft Academic Search

Convective heat transfer coefficient between absorber plate and air in a flat-plate solar air heater can be enhanced by providing the absorber plate with artificial roughness. An investigation of fully developed turbulent flow in a solar air heater duct with small diameter protrusion wires on the absorber plate has been carried out and expressions for prediction of average Stanton number

B. N. Prasad; J. S. Saini

1988-01-01

89

Quantum friction.  

PubMed

We investigate the van der Waals friction between graphene and an amorphous SiO(2) substrate. We find that due to this friction the electric current is saturated at a high electric field, in agreement with experiment. The saturation current depends weakly on the temperature, which we attribute to the quantum friction between the graphene carriers and the substrate optical phonons. We calculate also the frictional drag between two graphene sheets caused by van der Waals friction, and find that this drag can induce a voltage high enough to be easily measured experimentally. PMID:21405628

Volokitin, A I; Persson, B N J

2011-03-02

90

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

91

Adiabatic two-phase frictional pressure drops in microchannels  

SciTech Connect

Two-phase pressure drops were measured over a wide range of experimental test conditions in two sizes of microchannels (sight glass tubes 0.509 and 0.790 mm) for two refrigerants (R-134a and R-245fa). Similar to the classic Moody diagram in single-phase flow, three zones were distinguishable when plotting the variation of the two-phase friction factor versus the two-phase Reynolds number: a laminar regime for Re{sub TP} < 2000, a transition regime for 2000 {<=} Re{sub TP} < 8000 and a turbulent regime for Re{sub TP} {>=} 8000. The laminar zone yields a much sharper gradient than in single-phase flow. The transition regime is not predicted well by any of the prediction methods for two-phase frictional pressure drops available in the literature. This is not unexpected since only a few data are available for this region in the literature and most methods ignore this regime, jumping directly from laminar to turbulent flow at Re{sub TP} = 2000. The turbulent zone is best predicted by the Mueller-Steinhagen and Heck correlation. Also, a new homogeneous two-phase frictional pressure drop has been proposed here with a limited range of application. (author)

Revellin, Remi; Thome, John R. [EPFL, STI ISE LTCM, ME Gl 464, Station 9, CH-1015 Lausanne (Switzerland)

2007-07-15

92

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

NASA Astrophysics Data System (ADS)

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

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

2008-07-01

93

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; Belnap, Jayne

1997-01-01

94

Radar Turbulence Estimates. Effects of Wind Shear and Reflectivity Factor Gradients.  

National Technical Information Service (NTIS)

Analysis of Doppler radar spectral width provides an estimate of the turbulence eddy dissipation rate (epsilon), which in turn can be correlated to atmospheric turbulence. The width of the Doppler spectrum provides an overestimate of the turbulence eddy d...

S. J. Sycuro

1985-01-01

95

Fascinating Friction!  

NSDL National Science Digital Library

In this activity, 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

96

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

PubMed

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

Martinez-Borges, Anibal R

2006-06-09

97

Simultaneous Investigation of Marine Factors Effect on Corrosion Rate of SS 304 in Turbulent Condition  

NASA Astrophysics Data System (ADS)

Seawater is generally considered to be the most corrosive of the natural environments which threatens all the steel structures in their life time. The effect of environmental marine factors is extremely important in its corrosive behaviour. In this article, the influence of different marine parameters including pH, temperature, salinity and velocity is evaluated on the corrosion rate of SS 304 samples under turbulent condition. Since in actual marine environment, the entire parameters act simultaneously and as there is the possibility of differences between individual and synergistic influences of effecting factors, the influence of each considered parameter on the rate of corrosion has been studied under the combined effect of other factors. Finally, the results are presented via the graphs of dual factor effects which illustrate two-by-two influences of environmental parameters on corrosion rate variation. This research presents the difference between the qualitative and quantitative effects of a parameter under different ranges of considered parameter and also under the simultaneous effect of other combined acting factors, which emphasizes the importance of synergistic effects. In each case, the results obtained in turbulent condition are compared with previous study which had investigated the effect of marine parameters in laminar medium.

Atashin, S.; Toloei, A. S.; Pakshir, M.

2013-07-01

98

Calculation of the turbulent boundary layer in the initial section of pipe  

SciTech Connect

This article constructs a flow model for the turbulent boundary layer in a pipe operating under conditions of pressure and gravitation encountered in a hydroelectric power plant. Pipe roughness and friction factors are taken into account as are hydraulic conductivity and pipe dimension considerations. Continuity equations are given and the accuracy of the model is compared against experimental data.

Temirkhanov, A.M.; Spivak, V.M.

1987-11-01

99

Turbulent boundary layer at the initial portion of a pipe with rough walls  

Microsoft Academic Search

The development of a turbulent boundary layer at the initial portion of a pipe with rough walls is considered in the framework of the boundary-layer theory. It is shown that the consideration of roughness can be carried out by introducing into the “standard” law of friction a function which takes into account this factor. An experimental investigation is carried out

V. V. Kuz'min; A. A. Tupichenkov; A. V. Fafurin

1971-01-01

100

Calculation of the turbulent boundary layer in the initial section of pipe  

Microsoft Academic Search

This article constructs a flow model for the turbulent boundary layer in a pipe operating under conditions of pressure and gravitation encountered in a hydroelectric power plant. Pipe roughness and friction factors are taken into account as are hydraulic conductivity and pipe dimension considerations. Continuity equations are given and the accuracy of the model is compared against experimental data.

A. M. Temirkhanov; V. M. Spivak

1987-01-01

101

Drag reduction of turbulent boundary layers by means of grooved surfaces  

Microsoft Academic Search

The phenomenon that the friction factor of a flat plate can be decreased by coating the surface with small longitudinal grooves was studied. This phenomenon occurs in a turbulent boundary layer flow and apparently the grooves influence the eddies in the flow in such a way that the stress transport shows a 7 percent decrease. Water channel and wind tunnel

Cornelis Johannes Adrianus Pulles

1988-01-01

102

Experimental studies on heat transfer and friction factor characteristics of laminar flow through a circular tube fitted with regularly spaced helical screw-tape inserts  

SciTech Connect

Experimental investigation of heat transfer and friction factor characteristics of circular tube fitted with full-length helical screw element of different twist ratio, and helical screw inserts with spacer length 100, 200, 300 and 400mm have been studied with uniform heat flux under laminar flow condition. The experimental data obtained are verified with those obtained from plain tube published data. The effect of spacer length on heat transfer augmentation and friction factor, and the effect of twist ratio on heat transfer augmentation and friction factor have been presented separately. The decrease in Nusselt number for the helical twist with spacer length is within 10% for each subsequent 100mm increase in spacer length. The decrease in friction factor is nearly two times lower than the full length helical twist at low Reynolds number, and four times lower than the full length helical twist at high Reynolds number for all twist ratio. The regularly spaced helical screw inserts can safely be used for heat transfer augmentation without much increase in pressure drop than full length helical screw inserts. (author)

Sivashanmugam, P.; Suresh, S. [Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli 620 015, Tamil Nadu (India)

2007-02-15

103

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

104

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

105

Principles of Dry Friction.  

National Technical Information Service (NTIS)

This program explains the advantages and disadvantages of friction, the forces involved in friction, static and kinetic friction, and the calculation of the coefficients of static and kinetic friction.

1994-01-01

106

Exploring Friction  

NSDL National Science Digital Library

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

2010-01-01

107

New Empirical Relationship between Thrust Coefficient and Induction Factor for the Turbulent Windmill State  

SciTech Connect

Wind turbines sometimes experience the turbulent windmill state during startup or shutdown. This rarely happens during normal operation, so it has little effect on power curves or energy production. However, for completeness we need to be able to model situations where the axial induction factor exceeds 0.5. Classical momentum theory, which shows a relationship between the thrust coefficient and the axial induction factor, is not valid in this region. Glauert plotted some experimental data taken by Lock in the 1920s against this parabolic relationship and found very poor agreement for operation in this high-induction state. He proposed a new empirical relationship to fit the experimental data. Unfortunately, the new empirical curve does not account for tip or hub losses. Others have proposed multiplying the axial induction factor by the loss factor to correct the curve, but this still leaves a mathematical no-man's-land between the classical curve and the modified version of Glauert's empirical curve. The purpose of this paper is to document the derivation of a new curve that accounts for tip and hub losses and eliminates the numerical problems of the previous approaches.

Buhl, M.L., Jr.

2005-08-01

108

Turbulence management: Application aspects  

NASA Astrophysics Data System (ADS)

Turbulence management for the reduction of turbulent friction drag is an important topic. Numerous research programs in this field have demonstrated that valuable net drag reduction is obtainable by techniques which do not involve substantial, expensive modifications or redesign of existing aircraft. Hence, large projects aiming at short term introduction of turbulence management technology into airline service are presently under development. The various points that have to be investigated for this purpose are presented. Both design and operational aspects are considered, the first dealing with optimizing of turbulence management techniques at operating conditions, and the latter defining the technical problems involved by application of turbulence management to in-service aircraft. The cooperative activities of Airbus Industrie and its partners are cited as an example.

Hirschel, E. H.; Thiede, P.; Monnoyer, F.

1989-04-01

109

Turbulent structures and statistics in turbulent channel flow with two-dimensional slits  

Microsoft Academic Search

Direct numerical simulation (DNS, hereafter) of turbulent channel flow with periodic two-dimensional slits has been performed in order to investigate the turbulent statistics and the turbulent structures behind the slits. The Reynolds numbers based on the friction velocity and the channel half width are 10–1500. In the wake region, the mean flow becomes asymmetric with respect to the centerline of

Soichiro Makino; Kaoru Iwamoto; Hiroshi Kawamura

2008-01-01

110

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

111

Factors Affecting Bond and Friction Losses in Multi-Strand Post-Tensioning Tendons Including the Effects of Emulsifiable Oils.  

National Technical Information Service (NTIS)

Emulsifiable oils are often used in post-tensioned construction to reduce friction losses and to provide temporary corrosion protection for tendons during the period of time between stressing and grouting. In the past, oils were flushed from the ducts wit...

T. Luthi J. Diephuis J. J. Icaza J. E. Breen M. E. Kreger

2005-01-01

112

Generalized model of friction losses in flow along smooth and rough surfaces  

Microsoft Academic Search

Turbulent flows past smooth and rough surfaces involve essentially different friction resistance laws. In the case of a smooth surface, friction resistance is largely determined by the Reynolds number, whereas in the case of rough surfaces, the determining parameter is the relative roughness. Here, an approach is developed whereby friction resistance in both cases can be treated in a unified

A. D. Al'Tshul; B. V. Serebro

1989-01-01

113

Drag Reduction in Liquid-Liquid Friction  

Microsoft Academic Search

We study the effect of the addition of long flexible polymers in stratified flows. A two-layer stratification is used, in which the lower layer is brought into motion and drives a two-dimensional turbulent flow in the upper layer through viscous friction. Upon polymer addition in either of the two layers the kinetic energy in the upper layer is reduced by

Osanne Paireau; Daniel Bonn

1999-01-01

114

Active wall motions for skin-friction drag reduction  

Microsoft Academic Search

In the present study we investigate a possibility of reducing skin-friction drag in a turbulent channel flow with active wall motions. The wall is locally deformed according to two successful control strategies [J. Fluid Mech. 262, 75 (1994); J. Fluid Mech. 358, 245 (1998)]. Results show that overall 13-17% drag reductions are obtained with the active wall motions, and turbulence

Sangmo Kang; Haecheon Choi

2000-01-01

115

Acoustics of friction  

Microsoft Academic Search

This article presents an overview of the acoustics of friction by covering friction sounds, friction-induced vibrations and waves in solids, and descriptions of other frictional phenomena related to acoustics. Friction, resulting from the sliding contact of solids, often gives rise to diverse forms of waves and oscillations within solids which frequently lead to radiation of sound to the surrounding media.

Adnan Akay

2002-01-01

116

Nonlinear ion turbulent heating in electron drift wave turbulence  

NASA Astrophysics Data System (ADS)

The total turbulent heat transfer is composed of quasilinear electron cooling, quasilinear ion heating, nonlinear ion heating and zonal flow frictional heating. In a previous paper [1], we discussed quasilinear turbulent heating and zonal flow frictional heating. Here we apply weak turbulence theory to calculate the nonlinear ion turbulent heating via the beat mode resonance in electron drift wave turbulence [2]. The nonlinear diffusion in velocity space, affected by E x B motion and by the parallel velocity scattering, is further analyzed. This calculation proposes and analyzes a new collisionless turbulent energy transfer channel through nonlinear Landau damping. This process enters the electron-ion energy coupling. We estimate it by using the saturation balance. The results show that the collisionless turbulent energy transfer through the nonlinear Landau damping and the zonal flow frictional damping can both be important in a low collisionality, electron heated plasma, such as ITER. [4pt] [1] L. Zhao and P. H. Diamond, submitted to Phys. Plasma, 2012.[0pt] [2] W.M. Manheimer and T.H. Dupree, Phys. Fluid, 11, 2709 (1968).

Zhao, Lei; Diamond, Patrick H.

2012-10-01

117

Computation of Turbulent Flow Through Parallel Plates with Streamwise Periodic Ribs by a Method with Self-Adjusted Relaxation Factor  

Microsoft Academic Search

In this study, the flow-field and forced-convection characteristics of turbulent flow through plates with streamwise periodic ribs is simulated by a faster converged numerical method. The method based on self-adjusted under relaxation factors is motivated from the concept of Bi-CGSTAB algorithm, which updates the solution vectors by the 2-norm minimization of the residual vectors of the governing equations, thus the

Wen-bin Tsai

2006-01-01

118

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

119

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

120

Friction and anchorage loading revisited.  

PubMed

Contemporary concepts of sliding mechanics explain that friction is inevitable. To overcome this frictional resistance, excess force is required to retract the tooth along the archwire (ie, individual retraction of canines, en masse retraction of anterior teeth), in addition to the amount of force required for tooth movement. The anterior tooth retraction force, in addition to excess force (to overcome friction), produces reciprocal protraction force on molars, thereby leading to increased anchorage loading. However, this traditional concept was challenged in recent literature, which was based on the finite element model, but did not bear correlation to the clinical scenario. This article will reinforce the fact that clinically, friction increases anchorage loading in all three planes of space, considering the fact that tooth movement is a quasistatic process rather than a purely continuous or static one, and that conventional ways of determining the effects of static or dynamic friction on anchorage load cannot be applied to clinical situations (which consist of anatomical resistance units and a complex muscular force system). The article does not aim to quantify friction and its effect on the amount of anchorage load. Rather, a new perspective regarding the role of various additional factors (which is not explained by contemporary concept) that may influence friction and anchorage loading is provided.. PMID:22567633

Dholakia, Kartik D

2012-01-01

121

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

PubMed Central

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

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

2013-01-01

122

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

2013-09-01

123

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

124

Hydrodynamic skin-friction reduction  

NASA Astrophysics Data System (ADS)

A process for reducing skin friction, inhibiting the effects of liquid turbulence, and decreasing heat transfer in a system involving flow of a liquid along a surface of a body includes applying a substantially integral sheet of a gas, e.g., air, immediately adjacent to the surface of the body; a marine vehicle, which has a longitudinally grooved surface in proximity with the liquid and with a surface material having high contact angle between the liquid and said wall to reduce interaction of the liquid; water, with the surface of the body; and the hull of the marine vehicle.

Reed, Jason C.; Bushnell, Dennis M.; Weinstein, Leonard M.

1989-10-01

125

Turbulent friction drag reduction: Boundary layer manipulators  

Microsoft Academic Search

Internal manipulation by slant of thin longitudinal riblets is addressed experimentally in three dimensional flow. An ONERA D profile mounted between the lateral walls of a subsonic wind tunnel is maintained at an angle sweep of 22.5 degrees and at incidence 0 degrees. When the profile is recovered by different models of ribleted walls the variations on the drag coefficient

E. Coustols

1990-01-01

126

Turbulent friction drag reduction: Boundary layer manipulators  

NASA Astrophysics Data System (ADS)

Internal manipulation by slant of thin longitudinal riblets is addressed experimentally in three dimensional flow. An ONERA D profile mounted between the lateral walls of a subsonic wind tunnel is maintained at an angle sweep of 22.5 degrees and at incidence 0 degrees. When the profile is recovered by different models of ribleted walls the variations on the drag coefficient are estimated by wake explorations and compared to the drag of the profile encased in a smooth sheet of equivalent thickness.

Coustols, E.

1990-03-01

127

Correlations predict friction pressures of fracturing gels  

Microsoft Academic Search

Friction pressure calculations are very important in the design of any hydraulic fracturing treatment. They are useful in determining horsepower requirements, bottomhole treating pressure, and maximum wellhead pressure. This article presents a classical approach in which flow data of various gels in multiple pipes are presented as two dimensionless quantities, Fanning friction factor and generalized Reynolds number. These dimensionless quantities

Shah

1984-01-01

128

Semimetallic Brake Friction Materials Containing ZrSiO4: Friction Performance and Friction Layers Evaluation  

Microsoft Academic Search

The effect of ZrSiO4 (zircon) content on friction performance and friction surfaces of semimetallic brake friction materials is discussed. The experimental results indicate that the varying content of zircon affects the friction performance as well as plays crucial role in the iron film formation on the friction surfaces. The friction layers, formed during friction process, were carefully characterized using scanning

Vlastimil Mat?jka; Gražyna Simha Martynková; Yuning Ma; Yafei Lu

2009-01-01

129

Physics of ice friction  

NASA Astrophysics Data System (ADS)

Although the study of friction has a long history, ice friction has only been investigated during the last century. The basic physical concepts underlying the different friction regimes, such as boundary, mixed, and hydrodynamic friction are also relevant to ice friction. However, these friction regimes must be described with respect to the thickness of the lubricating liquidlike layer on ice. In this review the state of knowledge on the physics of ice friction is discussed. Surface melting theories are introduced. These theories attempt to explain the existence and nature of the liquidlike surface layer on ice at any temperature and without any load applied. Pressure melting, as the long-time explanation for the ease of ice friction, is discussed, together with the prevailing theory of frictional heating. The various laboratory setups for ice friction measurements are presented as well as their advantages and disadvantages. The individual influence of the different parameters on the coefficient of ice friction is discussed; these include the effects of temperature, sliding velocity, normal force exerted by the sliding object, the contact area between ice and slider, relative humidity, and also properties of the slider material such as surface roughness, surface structure, wettability, and thermal conductivity. Finally, the most important ice friction models based on the frictional heating theory are briefly introduced and research directions on the subject of ice friction are discussed.

Kietzig, Anne-Marie; Hatzikiriakos, Savvas G.; Englezos, Peter

2010-04-01

130

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.

131

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

SciTech Connect

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

Zanino, R.; Giors, S.; Mondino, R. [Dipartimento di Energetica, Politecnico, Turin, 10129 (Italy)

2006-04-27

132

Drag Reduction in Liquid-Liquid Friction  

NASA Astrophysics Data System (ADS)

We study the effect of the addition of long flexible polymers in stratified flows. A two-layer stratification is used, in which the lower layer is brought into motion and drives a two-dimensional turbulent flow in the upper layer through viscous friction. Upon polymer addition in either of the two layers the kinetic energy in the upper layer is reduced by roughly one-half: The polymer reduces the liquid-liquid friction that drives the flow in the upper layer. This drag reduction can be attributed to a change in the nonsteady viscous boundary layer. The main characteristics of the quasi-two-dimensional turbulence that results in the upper layer are found to remain unaffected by the addition of the polymer.

Paireau, Osanne; Bonn, Daniel

1999-12-01

133

Friction Drag Variation via Spanwise Transversal Surface Waves  

Microsoft Academic Search

The introduction of spanwise velocity is a promising technique to effect the near-wall turbulent flow field to influence friction\\u000a drag. However, the essential physical mechanism which significantly reduces friction drag has not been understood, yet. It\\u000a is the objective of this numerical study to improve the fundamental knowledge on the drag reduction mechanism. The investigation\\u000a is based on spanwise traveling

Stephan Klumpp; Matthias Meinke; Wolfgang Schröder

2011-01-01

134

Bubble Friction Drag Reduction at High Reynolds Number  

Microsoft Academic Search

Turbulent boundary layer (TBL) skin friction in liquid flows may be reduced when bubbles are present near the surface on which the TBL forms. Prior experimental studies of this phenomenon have been limited to downstream-based-Reynolds numbers of several million, while full-scale applications of this phenomenon lie orders of magnitude higher. This presentation describes experimental results from bubble friction drag reduction

Wendy Sanders; Elizabeth Ivy; Eric Winkel; Jinhyun Cho; Steven Ceccio; David Dowling; Marc Perlin

2003-01-01

135

The Friction Blister.  

National Technical Information Service (NTIS)

Friction blisters rank high as causes of military disability. Controlled studies show that: Friction blisters are not burns but are due to shearing forces within the epidermis. These produce a cleft into which fluid passes, provided there is a sufficient ...

W. A. Akers M. B. Sulzberger

1970-01-01

136

Correlation of Transonic-Cone Preston-Tube Data and Skin Friction.  

National Technical Information Service (NTIS)

Preston-tube measurements obtained on the Arnold Engineering Development Center (AEDC) Transition Cone have been correlated with theoretical skin friction coefficients in transitional and turbulent flow. This has been done for the NASA Ames 11-Ft Transoni...

A. S. Abu-mostafa T. D. Reed

1984-01-01

137

Comparison of turbulence schemes for prediction of wave-induced near-bed sediment suspension above a plane bed  

NASA Astrophysics Data System (ADS)

Based on a wave bottom boundary layer model and a sediment advection-diffusion model, seven turbulence schemes are compared regarding their performances in prediction of near-bed sediment suspension beneath waves above a plane bed. These turbulence algorithms include six empirical eddy viscosity schemes and one standard two-equation k-? model. In particular, different combinations of typical empirical formulas for the eddy viscosity profile and for the wave friction factor are examined. Numerical results are compared with four laboratory data sets, consisting of one wave boundary layer hydrodynamics experiment and three sediment suspension experiments under linear waves and the Stokes second-order waves. It is shown that predictions of near-bed sediment suspension are very sensitive to the choices of the empirical formulas in turbulence schemes. Simple empirical turbulence schemes are possible to perform equally well as the two-equation k-? model. Among the empirical schemes, the turbulence scheme, combining the exponential formula for eddy viscosity and Swart formula for wave friction factor, is the most accurate. It maintains the simplicity and yields identically good predictions as the k-? model does in terms of the wave-averaged sediment concentration.

Zhang, Chi; Zheng, Jin-Hai; Wang, Yi-Gang; Zhang, Meng-Tao; Jeng, Dong-Sheng; Zhang, Ji-Sheng

2011-09-01

138

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

139

Earthquakes and friction laws  

Microsoft Academic Search

Earthquakes have long been recognized as resulting from a stick–slip frictional instability. The development of a full constitutive law for rock friction now shows that the gamut of earthquake phenomena—seismogenesis and seismic coupling, pre- and post-seismic phenomena, and the insensitivity of earthquakes to stress transients—all appear as manifestations of the richness of this friction law.

Christopher H. Scholz

1998-01-01

140

Turbulent drag reduction by additives  

NASA Astrophysics Data System (ADS)

Turbulent drag reduction by additives Daniel Bonn ENS/LPS Paris, France and WZI Amsterdam, The netherlands A spectacular reduction of energy losses in turbulent flows can be achieved by the addition of small amounts of certain polymers or surfactants. I will discuss a few of our recent experiments that shed some light on the different possible mechanisms of drag reduction. Polymer drag reduction is believed to be due to the large elongational viscosity of the polymer solution; this stabilizes the turbulent boundary layer, leading to less turbulent energy generation, and hence less dissipation. For flexible polymers, indeed a correlation between drag reduction and elongational viscosity can be found. We show however that solutions of rigid polymers that exhibit no measurable elastic effects, drag reduction is as efficient as for flexible polymer solutions with an elevated elongational viscosity. This drag reduction can be attributed to the existence of a spatial viscosity gradient in the boundary layer. For surfactants, we show that drag reduction is due to an effective wall slip. Due to the wall slip, the bulk of the turbulent system does not experience a large wall friction. Since it is the wall friction that generates the turbulence, this naturally leads to a smaller dissipation.

Bonn, Daniel

2005-03-01

141

Rotational quantum friction.  

PubMed

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. PMID:23005949

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

2012-09-21

142

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

143

Probing fundamental friction mechanisms on polymer surfaces  

NASA Astrophysics Data System (ADS)

Despite its significance, there exists little fundamental understanding of the factors that control friction, especially on polymer surfaces. The role of sliding rate, temperature, water absorption, capillary forces, and chemical composition of the polymer, all contribute to the overall frictional behavior of a system. With friction force microscopy (FFM), the amount of energy dissipated into the polymer by fiction can be gauged. Not only does this provide us with a tool to differentiate various morphologies and microstructures but quantitative measurements can reveal the fundamental mechanism(s) of energy dissipation on polymer surfaces. The strong dependence of shearing rate on friction is explored in chapter 3. A contact length scale is developed which correlates similar time scales of measured frictional response to polymer molecular relaxations. The ability to image this frictional dissipation is also demonstrated. The dependence of friction on temperature is the subject of chapter 4. The distribution of frictional forces is obtained by collecting lateral force images and subsequently converted them to "friction histograms". The maximum of the distribution of polymer sliding friction as a function of temperature also shows a correspondence to bulk dynamic mechanical measurements, suggesting a major contribution to friction is polymer relaxations i.e. energy dissipation modes of bond torsions, rotations, and translational motion within the polymer chains. The scanning velocity dependence of these measurements has been found to follow time-temperature superposition. The results further suggest that greater free volume may be present at polymer surfaces. Chapter 5 deals with the effect humidity has on several hydrophilic polymers, and the effect adsorbed and absorbed water has on friction, adhesion, and morphology. Sharp increases in both friction and adhesion are measured when the glass to rubber transition occurs. The time dependent behavior of the glass transition was also evident, displaying the interplay between solvent content and relaxation rate. In chapter 6 a simple contact mechanics model is developed in an attempt to explain the frictional dependence on viscoelastic relaxation, the contribution of interfacial adhesion in FFM measurements, what the true tip-polymer contact zone may be, and provide some means of predicting polymer frictional forces.

Hammerschmidt, Jon Andrew

144

Testing a Missing Spectral Link in Turbulence  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

145

Turbulent Flow Around an Oscillating Body in Superfluid Helium: Dissipation Characteristics of the Nonlinear Regime  

NASA Astrophysics Data System (ADS)

By examining the resonance curves of an oscillator submerged in superfluid liquid helium, it is found that their shape is affected by two distinct dissipation regimes when the amplitude is large enough to generate turbulence in the liquid. In a resonance curve, the central part close to resonance, may be in a turbulent regime, but the response is of much lower amplitude away from the resonance frequency, so that the oscillation can still be in the linear regime for frequencies not exactly at resonance. This introduces an ambiguity in estimating the inverse quality factor Q -1 of the oscillator. By analyzing experimental data we consider a way of matching the two ways of estimating Q -1 and use the information to evaluate the frictional force as a function of velocity in a silicon paddle oscillator generating turbulence in the superfluid.

Zemma, E.; Luzuriaga, J.

2013-08-01

146

Magnon Turbulence in Ferromagnetic Nanocontact  

NASA Astrophysics Data System (ADS)

We theoretically examine the minimal factors for the emergence of magnon turbulence under current. We show that the turbulence emerges even in the nanocontact, where only the exchange term and the spin-transfer torque contribute in the determination of the magnetic structure and dynamics therein. Namely, the sufficient factors for the emergence of the turbulence are only the exchange and the spin-transfer torque. We also show that the critical spin current density of the magnon turbulence is related to a current-induced exchange length. The emergence of the magnon turbulence is characterized by a crossover of the length and the system size.

Matsushita, Katsuyoshi; Sasaki, Munetaka; Sato, Jun; Imamura, Hiroshi

2013-03-01

147

Micromachine friction test apparatus  

DOEpatents

A microelectromechanical (MEM) friction test apparatus is disclosed for determining static or dynamic friction in MEM devices. The friction test apparatus, formed by surface micromachining, is based on a friction pad supported at one end of a cantilevered beam, with the friction pad overlying a contact pad formed on the substrate. A first electrostatic actuator can be used to bring a lower surface of the friction pad into contact with an upper surface of the contact pad with a controlled and adjustable force of contact. A second electrostatic actuator can then be used to bend the cantilevered beam, thereby shortening its length and generating a relative motion between the two contacting surfaces. The displacement of the cantilevered beam can be measured optically and used to determine the static or dynamic friction, including frictional losses and the coefficient of friction between the surfaces. The test apparatus can also be used to assess the reliability of rubbing surfaces in MEM devices by producing and measuring wear of those surfaces. Finally, the friction test apparatus, which is small in size, can be used as an in situ process quality tool for improving the fabrication of MEM devices.

deBoer, Maarten P. (Albuquerque, NM); Redmond, James M. (Albuquerque, NM); Michalske, Terry A. (Cedar Crest, NM)

2002-01-01

148

Turbulent drag reduction by Lorentz force oscillation  

NASA Astrophysics Data System (ADS)

An experimental investigation into an electromagnetic technique to reduce skin-friction drag of turbulent boundary layers was conducted with an electroconductive solution in water, with potential applications to ships and underwater vehicles. We used an array of actuators made of permanent magnets interleaved with copper electrodes, which are set flush with the wall surface across the flow. This setup created the Lorentz force in the cross-flow direction within a thin region near the wall. More than 40% of turbulent skin-friction reductions were observed when the electromagnetic force is oscillated across the flow.

Pang, Junguo; Choi, Kwing-So

2004-05-01

149

Compressible friction coefficients in a simulated heat pipe  

Microsoft Academic Search

Research was performed to verify previously obtained friction coefficients for compressible, laminar and turbulent flow in a simulated heat pipe for intermediate mass injection and suction rates. The research was divided into two main parts: an experimental study and numerical study. In the experimental study, a simulated heat pipe consisting of a long porous tube with air injected into one

Constance A. Holladay

1987-01-01

150

Frictional drag reduction by wavy advection of deformable bubbles  

Microsoft Academic Search

Bubbles can reduce frictional drag in wall turbulence, and its effect is expected to use for ships and pipelines to save their power consumptions. A number of basic experiments have been carried out to date for finding out the best condition for enhancing the drag reduction. One issue that remains at present is the difference of the performance between steady

Yoshihiko Oishi; Yuichi Murai; Yuji Tasaka; Takeda Yasushi

2009-01-01

151

Active Wall Motion for the Skin-Friction Drag Reduction  

Microsoft Academic Search

In this study, we investigate a possibility of reducing the skin-friction drag in a turbulent channel flow with active wall motion using direct numerical simulation at Re_tau = 140. The strategy of moving the wall is based on the successful active control strategy used by Choi, Moin & Kim (1994, JFM). They prescribed the blowing\\/suction at the wall to be

Sangmo Kang; Haecheon Choi

1999-01-01

152

Polymer\\/riblet combination for hydrodynamic skin friction reduction  

Microsoft Academic Search

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

Jason C. Reed; Dennis M. Bushnell

1995-01-01

153

Do growing cumulus experience friction?  

NASA Astrophysics Data System (ADS)

Efforts to tune cumulus parameterisations, notably by adjusting entrainment rates, have yielded conflicting results with no single entrainment representation producing satisfactory representation of both the sensitivity of convection to ambient humidity (which requires a high rate) and accurate representation of the mean state (which may require convection to reach the tropopause more easily). This situation calls for a rethinking of the basic assumptions upon which convection schemes are built. We argue that a crucial target is the assumption that convective plumes or thermals experience heavy drag upon ascent, which is either explicit or implicit in basic models underlying convective schemes. Relaxation of this assumption appears to be justified on the basis of heuristic fluid dynamical arguments and laboratory studies, and would tend to alleviate some basic problems by allowing highly-entraining clouds to penetrate more deeply. We present analyses of the momentum budget of numerically-simulated cumulus updrafts to reassess the role of damping due to friction-like processes, in order to determine more rigorously whether friction should be reduced or eliminated in idealised cumulus models. A fundamental issue that arises in this computation is the ambiguity in defining what exactly constitutes the "parcel" that is represented in the idealised ascending-parcel calculation, in a realistic situation of a strongly deforming and turbulent fluid with rapid phase changes.

Sherwood, S.; Hernandez-Deckers, D.; Colin, M.; Robinson, F.

2012-04-01

154

Joint Agency Turbulence Experiment.  

National Technical Information Service (NTIS)

Doppler radar data acquired during the 1981 Joint Agency Turbulence Experiment are presented. Pulse-pair processed reflectivity factor and Doppler spectrum mean data collected during volume scan periods are displayed over constant height surfaces at aircr...

A. R. Bohne

1983-01-01

155

Polymer/riblet combination for hydrodynamic skin friction reduction  

NASA Astrophysics Data System (ADS)

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.; Bushnell, Dennis M.

1995-08-01

156

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

157

Adaptive wall functions for the v2-f turbulence model  

Microsoft Academic Search

Adaptive wall functions for the v2-f turbulence model have been derived for the flow over a flat plate at zero pressure gradient. These wall functions were implemented via tables for the turbulence quantities and the friction velocity u. A special treatment for the and f boundary conditions is proposed. On fine grids (y+<1) this approach yields results consistent with the

Georgi Kalitzin; Gorazd Medic

2007-01-01

158

A survey of the turbulent drag reduction using passive devices  

Microsoft Academic Search

Turbulent skin friction drag reduction using large eddy break-up devices (LEBU) and riblets is reviewed. Drag reductions of turbulent boundary layers are observed from riblets whose height and width range from 10 to 100 and 10 to 200 viscous lengths, respectively; the maximum reduction is obtained from riblets with cusped peaks and semicircular valleys of 15 viscous lengths in height

K. S. Choi

1984-01-01

159

On Quadratic Bottom Drag, Geostrophic Turbulence, and Oceanic Mesoscale Eddies  

Microsoft Academic Search

Many investigators have idealized the oceanic mesoscale eddy field with numerical simulations of geo- strophic turbulence forced by a horizontally homogeneous, baroclinically unstable mean flow. To date such studies have employed linear bottom Ekman friction (hereinafter, linear drag). This paper presents simu- lations of two-layer baroclinically unstable geostrophic turbulence damped by quadratic bottom drag, which is generally thought to be

Brian K. Arbic; Robert B. Scott

2008-01-01

160

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

Microsoft Academic Search

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

Hiroshi Kawamura; Hiroyuki Abe; Yuichi Matsuo

1999-01-01

161

Feedback Control of Near-Wall Reynolds Shear Stress in Wall-Turbulence  

Microsoft Academic Search

A new idealized control scheme is proposed for drag reduction in wall-turbulence and its effects are studied by means of DNS of turbulent pipe flow. The control input is given as a function of sensed streamwise velocity fluctuation above the wall in order to suppress the near-wall Reynolds shear stress, which is directly related to the turbulent skin friction drag

Koji FUKAGATA; Nobuhide KASAGI

2004-01-01

162

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

2012-06-25

163

Eddy fluxes in baroclinic turbulence  

NASA Astrophysics Data System (ADS)

The eddy heat flux generated by the statistically equilibrated baroclinic instability of a uniform, horizontal temperature gradient is studied using a two-mode quasigeostrophic model. An overview of the dependence of the eddy diffusivity of heat Dtau on the planetary potential vorticity gradient beta, the bottom friction kappa, the deformation radius lambda, the vertical shear of the large-scale flow 2U and the domain size L is provided at 70 numerical simulations with beta = 0 (f-plane) and 110 simulations with beta ? 0 (beta-plane). Strong, axisymmetric, well-separated baroclinic vortices dominate the equilibrated barotropic vorticity and temperature fields of f-plane turbulence. The heat flux arises from a systematic northward (southward) migration of anti-cyclonic (cyclonic) eddies with warm (cold) fluid trapped in the cores. Zonal jets form spontaneously on the beta-plane, and stationary, isotropic, jet-scale eddies align within the strong eastward-flowing regions of the jets. In both studies, the vortices and jets give rise to a strong anti-correlation between the barotropic vorticity zeta and the temperature field tau. The baroclinic mode is also an important contributor to dissipation by bottom friction and energizes the barotropic mode at scales larger than lambda. This in part explains why previous parameterizations for the eddy heat flux based on Kolmogorovian cascade theories are found to be unreliable. In a separate study, temperature and salinity profiles obtained with expendable conductivity, temperature and depth (XCTD) probes within Drake Passage, Southern Ocean are used to analyze the turbulent diapycnal eddy diffusivity kappa rho to a depth of 1000 meters. The Polar Front separates two dynamically different regions with strong, surface-intensified mixing north of the Front. South of the Polar Front mixing is weaker and peaks at a depth of approximately 500 m, near the local temperature maximum. Peak values of kapparho are found to exceed 10-3 m2 s -1. Wind-driven near-inertial waves, mesoscale eddies and thermohaline intrusions are discussed as possible factors contributing to observed mixing patterns.

Thompson, Andrew F.

164

Friction induced rail vibrations  

NASA Astrophysics Data System (ADS)

A model of rail, considered as multiple supported beam, subjected on friction induced vibration is studied in this work using FEM. The model is presented as continuous system and the mass and elastic properties of a real object are taken into account. The friction forces are nonlinear functions of the relative velocity during slipping. The problem is solved using Matlab Simulink.

Kralov, Ivan; Sinapov, Petko; Nedelchev, Krasimir; Ignatov, Ignat

2012-11-01

165

Atomistics of friction  

NASA Astrophysics Data System (ADS)

When two solid bodies contact and slide against each other, a frictional phenomenon occurs. There have been two models for the origin of the friction forces: the surface roughness model and Tomlinson’s model. The surface roughness model explains the origin of the static friction force; contacting solid surfaces are so rough that surface asperities are mechanically locked against the gravitational force. From an atomistic point of view, Tomlinson explained a mechanism of the energy dissipation for the origin of the dynamic friction force. The atomistic mechanisms are described for the origin of the static and the dynamic friction forces, based on the theoretical conclusion that Tomlinson’s mechanism is unlikely to occur in realistic frictional systems. The mechanism for the origin of the static friction force resembles the mechanical locking mechanism in a surface roughness model. The origin of the dynamic friction force is formulated as a problem of how the given translational kinetic energy dissipates into the internal relative motions of constituent atoms of bodies during sliding. From studying the available phase space volume of the translational motion becomes negligibly small for a large system size, compared with that of the internal motions, it is concluded that the energy dissipation occurs irreversibly from the translational motion to the internal motions. The comparison of the atomistic mechanisms with the surface roughness model and Tomlinson’s model is discussed. A phenomenon of superlubricity, where two solid bodies move relatively with no resistance, is discussed.

Hirano, M.

2006-03-01

166

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. 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 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-Ph.d. Program

167

Effects of pipe roughness on friction pressures of fracturing fluids  

SciTech Connect

In this study, the effects of pipe roughness on friction pressures of uncrosslinked and crosslinked fracturing gels are reported from flow data gathered in smooth and rough pipes. Pipe-roughness effects are found to be very significant in the estimation of friction pressures of fracturing fluids. These effects are more pronounced with less viscous fluids than with more viscous fluids and at higher rates with both fluids. From the data gathered with fluids tested in this study, correlations to estimate friction pressures of fracturing gels in turbulent flow in rough pipes are also presented. Though not ideal, these correlations allow better estimates of friction pressures in rough tubular conductors than existing methods do.

Shah, S.N. (Halliburton Services, Duncan, OK (USA))

1990-05-01

168

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 the 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 will 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 will investigate the frictional force of an object when different materials are placed between the object and the ground. This understanding will be used to design a new hockey puck for the National Hockey League.

Engineering K-Ph.d. Program

169

Turbulent drag reduction by Lorentz force oscillation  

Microsoft Academic Search

An experimental investigation into an electromagnetic technique to reduce skin-friction drag of turbulent boundary layers was conducted with an electroconductive solution in water, with potential applications to ships and underwater vehicles. We used an array of actuators made of permanent magnets interleaved with copper electrodes, which are set flush with the wall surface across the flow. This setup created the

Junguo Pang; Kwing-So Choi

2004-01-01

170

Rise of total pressure in frictional flow  

NASA Astrophysics Data System (ADS)

Examination of total pressure transport equation derived from Navier-Stokes equation reputed the assumption that total pressure can only decrease everywhere in frictional flow. Two classical flow cases with exact solutions demonstrated the local rise in total pressure due to stress application through the mechanical energy redistribution mechanism. The observed proportionality of the rise in total pressure with viscosity implied the capability of total pressure to increase more effectively in turbulent flows than in laminar flows due to higher effective viscosity and stress.

Issa, R. I.

1995-04-01

171

Computational fluid dynamics investigation of turbulence models for non-newtonian fluid flow in anaerobic digesters.  

PubMed

In this paper, 12 turbulence models for single-phase non-newtonian fluid flow in a pipe are evaluated by comparing the frictional pressure drops obtained from computational fluid dynamics (CFD) with those from three friction factor correlations. The turbulence models studied are (1) three high-Reynolds-number k-? models, (2) six low-Reynolds-number k-? models, (3) two k-? models, and (4) the Reynolds stress model. The simulation results indicate that the Chang-Hsieh-Chen version of the low-Reynolds-number k-? model performs better than the other models in predicting the frictional pressure drops while the standard k-? model has an acceptable accuracy and a low computing cost. In the model applications, CFD simulation of mixing in a full-scale anaerobic digester with pumped circulation is performed to propose an improvement in the effective mixing standards recommended by the U.S. EPA based on the effect of rheology on the flow fields. Characterization of the velocity gradient is conducted to quantify the growth or breakage of an assumed floc size. Placement of two discharge nozzles in the digester is analyzed to show that spacing two nozzles 180° apart with each one discharging at an angle of 45° off the wall is the most efficient. Moreover, the similarity rules of geometry and mixing energy are checked for scaling up the digester. PMID:21047058

Wu, Binxin

2010-11-03

172

Iliotibial band friction syndrome  

PubMed Central

Published articles on iliotibial band friction syndrome have been reviewed. These articles cover the epidemiology, etiology, anatomy, pathology, prevention, and treatment of the condition. This article describes (1) the various etiological models that have been proposed to explain iliotibial band friction syndrome; (2) some of the imaging methods, research studies, and clinical experiences that support or call into question these various models; (3) commonly proposed treatment methods for iliotibial band friction syndrome; and (4) the rationale behind these methods and the clinical outcome studies that support their efficacy.

2010-01-01

173

Friction mechanism in industrial brakes  

Microsoft Academic Search

Mechanical brakes are acting not stochastically but surprisingly determined as the presented results measured on industrial brake systems will show. They help understanding these complex friction systems. An important mechanism is thereby the periodically change of the local friction intensity during the continuous friction process. Another essential influence on the friction properties are the metal particles that originate from the

D. Severin; S. Dörsch

2001-01-01

174

Critical instability and friction scaling of fluid flows through pipes with rough inner surfaces.  

PubMed

It has been shown experimentally over nearly 80 years that surface fine roughness of circular pipes has a crucial effect on the natural transition to turbulence. In this Letter, a theoretical explanation is suggested for the roughness-induced instability. Once the nonlinear effect of roughness is introduced (through a pipe with fine corrugation surface), the mean velocity profile becomes unstable to three-dimensional, asymmetric, and helical traveling waves at moderate Reynolds numbers. The threshold of the aspect ratio or shape factor of the roughness element required to cause instability scales as Re-2. Inspired by the current model, a scaling form is proposed and the scaled friction factor measurements in rough pipes collapse onto a universal curve. PMID:20366316

Tao, Jianjun

2009-12-28

175

Critical Instability and Friction Scaling of Fluid Flows through Pipes with Rough Inner Surfaces  

NASA Astrophysics Data System (ADS)

It has been shown experimentally over nearly 80 years that surface fine roughness of circular pipes has a crucial effect on the natural transition to turbulence. In this Letter, a theoretical explanation is suggested for the roughness-induced instability. Once the nonlinear effect of roughness is introduced (through a pipe with fine corrugation surface), the mean velocity profile becomes unstable to three-dimensional, asymmetric, and helical traveling waves at moderate Reynolds numbers. The threshold of the aspect ratio or shape factor of the roughness element required to cause instability scales as Re-2. Inspired by the current model, a scaling form is proposed and the scaled friction factor measurements in rough pipes collapse onto a universal curve.

Tao, Jianjun

2009-12-01

176

Turbulent modeling for low speed compressible flow  

NASA Astrophysics Data System (ADS)

An investigation of turbulence models at high Reynolds numbers is conducted. The numerical code uses the Roe scheme and preconditioning matrix and dual time stepping are adopted for economizing the computational time and improving convergence properties. In order to validate the code, DNS of the turbulent channel flow are performed at Reynolds numbers, based on the friction velocity, of 180 and 500. The results for the mean velocity profiles and turbulent intensities are in good agreement with the benchmark DNS data obtained by spectral codes. The same code is used to perform LES with different models, among them the classical Smagorinsky model and the Truncated Navier Stokes (TNS) method, and comparisons are made with databases for high friction velocity Reynolds numbers of 1000 and 2000.

Li, Chunggang; Domaradzki, J. A.; Fu, Wushung

2010-11-01

177

Friction stir welding tool  

DOEpatents

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

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

2008-04-15

178

Isostaticity at Frictional Jamming  

NASA Astrophysics Data System (ADS)

Amorphous packings of frictionless, spherical particles are isostatic at jamming onset, with the number of constraints (contacts) equal to the number of degrees of freedom. Their structural and mechanical properties are controlled by the interparticle contact network. In contrast, amorphous packings of frictional particles are typically hyperstatic at jamming onset. We perform extensive numerical simulations in two dimensions of the geometrical asperity (GA) model for static friction to further investigate the role of isostaticity. In the GA model, interparticle forces are obtained by summing up purely repulsive central forces between periodically spaced circular asperities on contacting grains. We compare the packing fraction, contact number, mobilization distribution, and vibrational density of states (in the harmonic approximation) using the GA model to those generated using the Cundall-Strack approach. We find that static packings of frictional disks obtained from the GA model are mechanically stable and isostatic when we consider interactions between asperities on contacting particles. The crossover in the structural and mechanical properties of static packings from frictionless to frictional behavior as a function of the static friction coefficient coincides with a change in the type of interparticle contacts and the disappearance of a peak in the density of vibrational modes for the GA model. These results emphasize that mesoscale features of the model for static friction play an important role in determining the properties of granular packings.

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

2013-05-01

179

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

180

Turbulent shear flow in a rapidly rotating spherical annulus  

NASA Astrophysics Data System (ADS)

This dissertation presents experimental measurements of torque, wall shear stress, pressure, and velocity in the boundary-driven turbulent flow of water between concentric, independently rotating spheres, commonly known as spherical Couette flow. The spheres' radius ratio is 0.35, geometrically similar to that of Earth's core. The measurements are performed at unprecedented Reynolds number for this geometry, as high as fifty-six million. The role of rapid overall rotation on the turbulence is investigated. A number of different turbulent flow states are possible, selected by the Rossby number, a dimensionless measure of the differential rotation. In certain ranges of the Rossby number near state borders, bistable co-existence of states is possible. In these ranges the flow undergoes intermittent transitions between neighboring states. At fixed Rossby number, the flow properties vary with Reynolds number in a way similar to that of other turbulent flows. At most parameters investigated, the large scales of the turbulent flow are characterized by system-wide spatial and temporal correlations that co-exist with intense broadband velocity fluctuations. Some of these wave-like motions are identifiable as inertial modes. All waves are consistent with slowly drifting large scale patterns of vorticity, which include Rossby waves and inertial modes as a subset. The observed waves are generally very energetic, and imply significant inhomogeneity in the turbulent flow. Increasing rapidity of rotation as the Ekman number is lowered intensifies those waves identified as inertial modes with respect to other velocity fluctuations. The turbulent scaling of the torque on inner sphere is a focus of this dissertation. The Rossby-number dependence of the torque is complicated. We normalize the torque at a given Reynolds number in the rotating states by that when the outer sphere is stationary. We find that this normalized quantity can be considered a Rossby-dependent friction factor that expresses the effect of the self-organized flow geometry on the turbulent drag. We predict that this Rossby-dependence will change considerably in different physical geometries, but should be an important quantity in expressing the parameter dependence of other rapidly rotating shear flows.

Zimmerman, Daniel S.

181

Measuring anisotropic friction on WTe2 using atomic force microscopy in the force-distance and friction modes.  

PubMed

Layered materials which can be easily cleaved have proved to be excellent samples for the study of atomic scale friction. The layered transition metal dichalcogenides have been particularly popular. These materials exhibit a number of interesting properties ranging from superconductivity to low frictional coefficients. In this paper we have investigated the tribology of the dichalcogenide-WTe2. The coefficient of friction is less than 0.040 along the Te rows and increases to over 0.045 across the rows. The frictional forces almost doubled at normal loads of 5000 nN when scanning in the [010] direction in comparison to the [100] direction. The frictional responses of the AFM probe have been monitored in the frictional force and force-versus-distance (f-d) mode. A comparison between the outcomes using the two different modes demonstrates the factors which need to be considered for accurate measurements. PMID:20355449

Watson, Gregory S; Myhra, Sverre; Watson, Jolanta A

2010-04-01

182

Examinations of the friction behavior in nonstationary flow processes in pipes  

NASA Astrophysics Data System (ADS)

A calculation procedure is presented in order to use an improved unsteady friction term. The method of characteristics for the water hammer calculation along the pipe axis and a finite difference calculation of the unsteady radial velocity distribution were combined. Various turbulence models, which are used for the calculation of the velocity distribution along the radius for laminar and turbulent flow, were considered. The turbulence models are as follows: the Prandtl mixture length model; a one equation model in which kinetic turbulent energy is calculated with a transport equation; and the k-epsilon model. The procedure was validated with experimental results of fluctuating single pressure peaks and with examples from the literature.

Eichinger, Peter

1992-01-01

183

Astrophysical Turbulence  

NASA Astrophysics Data System (ADS)

It is difficult to exaggerate the importance of turbulence in astrophysics, or the challenges which it poses. Turbulence is responsible for dynamical pressure support, energy transport, angular momentum transport, chemical mixing, and magnetic field generation and evolution in a host of astrophysical settings. Turbulent astrophysical flows differ from terrestrial forms of turbulence which have been studied traditionally by virtue of their inherent compressibility, strong radiative cooling, self-gravity, and the importance in many environments of magnetic fields. Recent years have seen important advances in several distinct areas of astrophysical turbulence theory -- including modeling of turbulence in stars, accretion disks, and the interstellar medium, as well basic studies of MHD turbulence which provide the framework for all these applications. In one star, the Sun, helioseismologic data are allowing increasingly sophisticated comparison of observations with the theory of turbulent stellar interiors. Attempts to model the solar differential rotation has shown clearly that turbulent angular momentum transport is an essential ingredient, and attempts to model it are improving. Solar dynamo calculations are only slightly behind. Models of turbulent accretion disks are becoming increasingly realistic, with the dynamical role of magnetic fields a crucial element, and global, time-dependent modeling now feasible. It seems likely that the alpha viscosity parameter will be soon be supplanted by ab initio calculations of the accretion rate. Although the presence of interstellar turbulence has long been recognized, recent theoretical studies have significantly increased our understanding of its effects, particularly in the cold ISM where it plays a dominant role. Self-consistent dynamical studies will soon be able to identify how strong turbulence evolves and shapes the internal structure of magnetized interstellar clouds. Basic studies in MHD turbulence have made substantial recent progress in such longstanding problems as the nature of the turbulent cascade, dynamo generation of fields, and the process of magnetic reconnection. In all of these studies, a crucial new ingredient has been computational advances that now make possible direct hydrodynamic/MHD simulations of three-dimensional, time-dependent turbulence with inertial dynamic ranges of more than two orders of magnitude. These advances in numerical experimentation are inspiring new analytical work, new comparisons between models and observations, and advances in observations and data analysis themselves. The ITP program on Astrophysical Turbulence will provide a forum for intensive interaction among analytical theorists, computational physicists, and observers from all of the subspecialties, with prospects for major research progress through interdisciplinary discussions and collaborations.

Ostriker, Eve C.; Zweibel, Ellen G.

2000-04-01

184

Astrophysical Turbulence  

NASA Astrophysics Data System (ADS)

It is difficult to exaggerate the importance of turbulence in astrophysics, or the challenges which it poses. Turbulence is responsible for dynamical pressure support, energy transport, angular momentum transport, chemical mixing, and magnetic field generation and evolution in a host of astrophysical settings. Turbulent astrophysical flows differ from terrestrial forms of turbulence which have been studied traditionally by virtue of their inherent compressibility, strong radiative cooling, self-gravity, and the importance in many environments of magnetic fields. Recent years have seen important advances in several distinct areas of astrophysical turbulence theory -- including modeling of turbulence in stars, accretion disks, and the interstellar medium, as well basic studies of MHD turbulence which provide the framework for all these applications. In one star, the Sun, helioseismologic data are allowing increasingly sophisticated comparison of observations with the theory of turbulent stellar interiors. Attempts to model the solar differential rotation has shown clearly that turbulent angular momentum transport is an essential ingredient, and attempts to model it are improving. Solar dynamo calculations are only slightly behind. Models of turbulent accretion disks are becoming increasingly realistic, with the dynamical role of magnetic fields a crucial element, and global, time-dependent modeling now feasible. It seems likely that the alpha viscosity parameter will be soon be supplanted by ab initio calculations of the accretion rate. Although the presence of interstellar turbulence has long been recognized, recent theoretical studies have significantly increased our understanding of its effects, particularly in the cold ISM where it plays a dominant role. Self-consistent dynamical studies will soon be able to identify how strong turbulence evolves and shapes the internal structure of magnetized interstellar clouds. Basic studies in MHD turbulence have made substantial recent progress in such longstanding problems as the nature of the turbulent cascade, dynamo generation of fields, and the process of magnetic reconnection. In all of these studies, a crucial new ingredient has been computational advances that now make possible direct hydrodynamic/MHD simulations of three-dimensional, time-dependent turbulence with inertial dynamic ranges of more than two orders of magnitude. These advances in numerical experimentation are inspiring new analytical work, new comparisons between models and observations, and advances in observations and data analysis themselves. The ITP program on Astrophysical Turbulence will provide a forum for intensive interaction among analytical theorists, computational physicists, and observers from all of the subspecialties, with prospects for major research progress through interdisciplinary discussions and collaborations.

Ostriker, Eve C.; Zweibel, Ellen G.

2000-05-01

185

Chemical origins of frictional aging.  

PubMed

Although the basic laws of friction are simple enough to be taught in elementary physics classes and although friction has been widely studied for centuries, in the current state of knowledge it is still not possible to predict a friction force from fundamental principles. One of the highly debated topics in this field is the origin of static friction. For most macroscopic contacts between two solids, static friction will increase logarithmically with time, a phenomenon that is referred to as aging of the interface. One known reason for the logarithmic growth of static friction is the deformation creep in plastic contacts. However, this mechanism cannot explain frictional aging observed in the absence of roughness and plasticity. Here, we discover molecular mechanisms that can lead to a logarithmic increase of friction based purely on interfacial chemistry. Predictions of our model are consistent with published experimental data on the friction of silica. PMID:23215300

Liu, Yun; Szlufarska, Izabela

2012-11-02

186

Active wall motions for skin-friction drag reduction  

NASA Astrophysics Data System (ADS)

In the present study we investigate a possibility of reducing skin-friction drag in a turbulent channel flow with active wall motions. The wall is locally deformed according to two successful control strategies [J. Fluid Mech. 262, 75 (1994); J. Fluid Mech. 358, 245 (1998)]. Results show that overall 13-17% drag reductions are obtained with the active wall motions, and turbulence intensities and near-wall streamwise vortices are significantly weakened. It is remarkable that instantaneous wall shapes are elongated in the streamwise direction and resemble riblets in appearance. However, the mechanism of the present drag reduction is essentially different from that of riblets.

Kang, Sangmo; Choi, Haecheon

2000-12-01

187

Turbulent acceleration and heating in toroidal magnetized plasmas  

NASA Astrophysics Data System (ADS)

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 Z2/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.; Smolyakov, A.

2013-07-01

188

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

189

Plasma turbulence  

SciTech Connect

The origin of plasma turbulence from currents and spatial gradients in plasmas is described and shown to lead to the dominant transport mechanism in many plasma regimes. A wide variety of turbulent transport mechanism exists in plasmas. In this survey the authors summarize some of the universally observed plasma transport rates.

Horton, W. [Univ. of Texas, Austin, TX (United States). Inst. for Fusion Studies; Hu, G. [Globalstar LP, San Jose, CA (United States)

1998-07-01

190

Frictional ignition of methane by continuous-mining machines in underground coal mines  

Microsoft Academic Search

This report presents information on frictional methane ignitions associated with continuous miners. Data indicate a gradual increase in frictional methane ignitions over the 6-year period 1971 through 1976. Ninety percent of these ignitions are attributed to continuous-mining machines. Factors affecting such frictional ignitions are analyzed and recommendations are submitted for reducing their incidence: make proper use of diffuser fans, line

E. M. Kawenski; G. C. Price; C. R. Stephan

1979-01-01

191

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

192

Does the sailfish skin reduce the skin friction like the shark skin?  

Microsoft Academic Search

The shape of shark skin - riblet - reduces the skin friction up to 8% in a turbulent boundary layer, as compared to a smooth surface. The sailfish is the fastest sea animal, reaching its maximum speed of 110km\\/h. On the sailfish skin, we observe a number of V-shaped protrusions pointing downstream. So, we investigate the possibility of skin-friction reduction

Woong Sagong; Sangho Choi; Chulkyu Kim; Woo-Pyung Jeon; Haecheon Choi

2007-01-01

193

Turbulent flow in smooth and rough pipes.  

PubMed

Recent experiments at Princeton University have revealed aspects of smooth pipe flow behaviour that suggest a more complex scaling than previously noted. In particular, the pressure gradient results yield a new friction factor relationship for smooth pipes, and the velocity profiles indicate the presence of a power-law region near the wall and, for Reynolds numbers greater than about 400x103 (R+>9x103), a logarithmic region further out. New experiments on a rough pipe with a honed surface finish with krms/D=19.4x10-6, over a Reynolds number range of 57x103-21x106, show that in the transitionally rough regime this surface follows an inflectional friction factor relationship rather than the monotonic relationship given in the Moody diagram. Outer-layer scaling of the mean velocity data and streamwise turbulence intensities for the rough pipe show excellent collapse and provide strong support for Townsend's outer-layer similarity hypothesis for rough-walled flows. The streamwise rough-wall spectra also agree well with the corresponding smooth-wall data. The pipe exhibited smooth behaviour for ks+ < or =3.5, which supports the suggestion that the original smooth pipe was indeed hydraulically smooth for ReD< or =24x106. The relationship between the velocity shift, DeltaU/utau, and the roughness Reynolds number, ks+, has been used to generalize the form of the transition from smooth to fully rough flow for an arbitrary relative roughness krms/D. These predictions apply for honed pipes when the separation of pipe diameter to roughness height is large, and they differ significantly from the traditional Moody curves. PMID:17244585

Allen, J J; Shockling, M A; Kunkel, G J; Smits, A J

2007-03-15

194

Friction of Quartz in High Vacuum.  

National Technical Information Service (NTIS)

The purpose of this investigation was to determine the fundamental factors controlling the frictional properties of quartz surfaces, with emphasis on the effects of surface cleanliness. Ultra-high vacuums (to 10 to the minus 10th power torr) and high temp...

L. G. Bromwell

1966-01-01

195

Hysteresis Losses in Rolling and Sliding Friction  

Microsoft Academic Search

Previous work on the mechanism of rolling friction has shown that it is mainly due to elastic hysteresis losses in the rolling elements. Under conditions of uniform tension or torsion it is generally assumed that the energy dissipated by hysteresis is a constant fraction (the hysteresis loss factor) of the elastic energy introduced during the cycle. This elastic input energy

J. A. Greenwood; H. Minshall; D. Tabor

1961-01-01

196

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

197

Friction Sensitivity of Primary Explosives.  

National Technical Information Service (NTIS)

A Bundesanstalt fur Materialprufung (BAM) small friction tester, manufactured in West Germany, has been used to rank primary explosives in their order of friction sensitivity. Primary explosives RD 1333 lead azide, dextrinated lead styphnate, polyvinyl-al...

J. Harris

1982-01-01

198

Investigation of the turbulent boundary layer on a permeable rough surface with heat transfer  

Microsoft Academic Search

The present paper reports on the integral method of the skin friction and heat transfer calculation for nonadiabatic compressible turbulent boundary layer (TBL) on a permeable rough surface in the presence of the heat transfer and pressure gradient. The influence of the roughness is presented as relative variations of the skin friction coefficient and Stanton number. The roughness geometry is

G. F. Sivykh; E. G. Zaulichnii

1978-01-01

199

Low temperature friction force microscopy  

Microsoft Academic Search

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

Christopher Gregory Dunckle

2010-01-01

200

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

201

Quantum theory of friction  

SciTech Connect

We present a Markovian quantum theory of friction. Our approach is based on the idea that collisions between a Brownian particle and single molecules of the surrounding medium constitute, as far as the particle is concerned, instantaneous simultaneous measurements of its position and momentum.

Barnett, Stephen M. [Department of Physics, University of Strathclyde, Glasgow G4 0NG (United Kingdom); Cresser, James D. [Department of Physics, Macquarie University, New South Wales 2109 (Australia); Australian Centre for Quantum Computer Technology, Macquarie University, Sydney, New South Wales 2109 (Australia)

2005-08-15

202

High-Temperature Friction.  

National Technical Information Service (NTIS)

Analysis of the work conducted in areas of study of the processes of friction and wear at elevated temperatures shows that along with the determined successes in the question of designating antifriction materials for these conditions there is a number of ...

M. M. Krasnoshchekov O. D. Lysyak

1970-01-01

203

Collets and friction  

NASA Astrophysics Data System (ADS)

Conical sleeves, known as collets, have long been used for clamping sliding bodies to cylindrical columns in any desired position. Quantitative expressions for their properties are presented. Of particular interest is the ability of a collet to hold a shaft stationary against axial forces in either direction, and yet be able to release its grip with a minimum of additional force. This treatment relates the firmness of grip, the ease of release and the interface stresses to the cone angle, the coefficients of friction and the applied axial forces. The central part played by hysteresis is revealed. Experiments employing a test model give good support to the theoretical conclusions. They show that, up to the interface pressures reached, about 250 MPa, the friction between lubricated hardened steel and mild steel surfaces is independent of pressure. The strong dependence of friction on surface roughness is demonstrated. The possible adaptation of the collet test equipment to the measurement of friction at high interface pressures is touched upon. It could be a complementary alternative to the more flexible pin-on-disc method. With much larger working areas, it should have the advantages of better defined areas of contact, reduced ploughing effects and less leakage of lubricant.

McIlraith, A. H.

2005-07-01

204

Friction: Terms of detachment  

NASA Astrophysics Data System (ADS)

Experience with friction is as old as human history, but the subject remains surprisingly subtle. Now the first experimental observations of the initial fleeting instants of sliding have been made. The results have implications that range from the smallest sliding surfaces in nanomachines to seismic signals in earthquakes.

Marder, Michael

2004-09-01

205

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

206

Friction in rail guns  

Microsoft Academic Search

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

P. K. Kay

1984-01-01

207

Laminar and weakly turbulent oceanic gravity currents performing inertial oscillations  

NASA Astrophysics Data System (ADS)

The small scale dynamics of a weakly turbulent oceanic gravity current is determined. The gravity current considered is initially at rest and adjusts by performing inertial oscillations to a geostrophic mean flow. The dynamics is explored with a hierarchy of mathematical models. The most involved are the fully 3-D Navier-Stokes equations subject to the Boussinesq approximation. A 1-D and 0-D mathematical model of the same gravity current dynamics are systematically derived. Using this hierarchy and the numerical solutions of the mathematical models, the turbulent dynamics at the bottom and the interface is explored and their interaction investigated. Three different regimes of the small scale dynamics of the gravity current are identified, they are characterised by laminar flow, coherent roll vortices and turbulent dynamics with coherent streaks and bursts. The problem of the rectification of the turbulent fluxes, that is, how to average out the fluctuations and calculate their average influence on the flow, is considered. It is shown that two different regimes of friction are superposed, an Ekman friction applies to the average geostrophic flow and a linear friction, not influenced by rotation, to the inertial oscillations. The combination of the two makes the bulk friction non-local in time for the 0-D model. The implications of the results for parametrisations of the Ekman dynamics and the small scale turbulent fluxes in the planetary boundary layer are discussed.

Wirth, A.

2012-05-01

208

Investigation of the Frictional Resistance of Seal Coat Pavement Surfaces.  

National Technical Information Service (NTIS)

Numerous factors, including aggregate characteristics, construction variables, traffic volume, and environment, are believed to be affecting the frictional performance of highway pavements. The objective of this phase of the study was to investigate the e...

M. A. U. Abdul-Malak C. G. Papaleontiou D. W. Fowler A. H. Meyer

1988-01-01

209

Wall Turbulence.  

ERIC Educational Resources Information Center

This paper gives an account of research on the structure of turbulence close to a solid boundary. Included is a method to study the flow close to the wall of a pipe without interferring with it. (Author/JN)

Hanratty, Thomas J.

1980-01-01

210

Easy Turbulence.  

National Technical Information Service (NTIS)

It seems a safe bet that the understanding of developed turbulence, a long standing challenge for theoretical and mathematical physics, will enter into the third millennium as an unsolved problem. This is an introductory course on the subject. The authors...

K. Gawedzki

1999-01-01

211

Turbulent transition  

NASA Astrophysics Data System (ADS)

Clustering of dark and grey solitons in fibre lasers has been found to induce a turbulent regime similar to that of a rapidly flowing fluid in a pipe. Sergei Turitsyn from the University of Aston, UK, explains.

2013-10-01

212

Quantifying Bottom Friction over Rippled Beds with High Resolution Observations  

NASA Astrophysics Data System (ADS)

Previous investigations of turbulent flow over irregular topography have shown that the physical roughness provided by the presence of ripples decreases as wave forcing increases. Contributions by both wave radiation stress and turbulent Reynolds stress during periods of high waves are counteracted by reduced bedform-induced stresses from the flattened topography. These individual contributions combine to affect the total bottom friction and accompanying drag coefficient. In this study, laboratory observations of the flow field over active ripple fields (obtained with Particle Imaging Velocimetry techniques) are used to evaluate the stresses induced by regular surface waves propagating over the seabed. The turbulent signal (contributing to the Reynolds stress) is estimated by ensemble averaging over multiple wave periods, and a bedform disturbance signal is defined by spatially averaging over individual ripples. The observations are compared with the Doubly Averaged (in both time and space) Navier Stokes equations discussed by Rodriguez-Abudo and Foster in the companion presentation. The formulation provides an excellent opportunity to evaluate the drag partitioning in active and relic rippled beds. For a range of ripple and regular wave conditions, the relative contributions by bedform induced drag, wave radiation stress, turbulent Reynolds stress, and viscous stresses are examined. In each case, we evaluate the total bottom friction with the acceleration deficit through the boundary layer.

Foster, D. L.; Rodriguez-Abudo, S.; Henry, L. W.

2010-12-01

213

Anisotropic turbulence of pseudo Alfvèn waves  

NASA Astrophysics Data System (ADS)

In this work we are considering the weak turbulent regime of two dimensional MHD system by applying the wave-kinetic formalism developed in the works [1,2]. Due to the geometry constraints in the two dimensional case only the Pseudo Alfvèn Waves(PAW) can exist. We find that the turbulent behavior of the MHD system into the two dimensional case is crucially different from its behavior in three dimensional one previously considered in [3]. We show that there is no Kolmogorov-like solutions, no energy cascade and therefore no turbulence universality. We also show that the triad interactions of the PAW are not empty and lead to the derivation of the wave-kinetic equation for the energy spectrum. The important property of this equation is its simplicity and therefore the possibility to treat it analytically. In particular we consider two cases: with uniform friction and viscous friction. In the first case the analysis can fully be done analytically, at the second one we proceed with qualitative and numerical tools. The main results of this work are summarized in [4].[4pt] [1] Zakharov V.E.et al,Kolmogorov spectra of turbulence1,Springer 1992[0pt] [2] Nazarenko S.V. Wave turbulence, Springer 2010[0pt] [3] Galtier et al,J.Plasma Phys., 2000,63(5), pp.447-488[0pt] [4] Tronko N.et al,in preparation

Tronko, Natalia; Galtier, Sébastien; Nazarenko, Sergey

2012-03-01

214

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

NASA Astrophysics Data System (ADS)

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

Hunsaker, Douglas F.

215

Supersonic, turbulent flow computation and drag optimization for axisymmetric afterbodies  

Microsoft Academic Search

The compressible, turbulent flow about an axisymmetric body was numerically studied using the MacCormack unsplit explicit algorithm applied to the mass-average Navier-Stokes equations solved in conjunction with the k-? turbulence model of Jones and Launder. Numerical predictions of total body drag (pressure drag, skin friction drag, and base drag) were made for an axisymmetric body six diameters in length, with

R. M. Cummings; H. T. Yang; Y. H. Oh

1995-01-01

216

Supersonic, turbulent flow computation and drag optimization for axisymmetric afterbodies  

Microsoft Academic Search

The compressible, turbulent flow about an axisymmetric body was numerically studied using the MacCormack unsplit explicit algorithm applied to the mass-average Navier-Stokes equations solved in conjunction with the k- epsilon turbulence model of Jones and Launder. Numerical predictions of total body drag (pressure drag, skin friction drag, and base drag) were made for an axisymmetric body six diameters in length,

R. M. Cummings; H. T. Yang; Y. H. Oh

1995-01-01

217

Suboptimal control of turbulent channel flow for drag reduction  

Microsoft Academic Search

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

Changhoon Lee; John Kim; Haecheon Choi

1998-01-01

218

DNS of turbulent channel flow with a higher Reynolds number  

Microsoft Academic Search

With an aid of recent developments in the super and parallel computers, the direct numerical simulation (DNS) of turbulence is now being increasingly performed. It is already more than 10 years ago when Kim-Moin-Moser published their DNS on the turbulent channel flow. Their Reynolds number based on the friction velocity and channel half width was Re_tau=180. In a couple of

Hiroshi Kawamura; Hiroyuki Abe; Yuichi Matsuo

1998-01-01

219

Origin of Subglacial Debris-bed Friction  

NASA Astrophysics Data System (ADS)

Numerical models of glaciers sliding on hard beds assume that basal flow resistance is controlled entirely by viscous drag on bedrock bumps. However, observations and measurements indicate that basal ice can contain large concentrations of rock debris that exert significant frictional resistance: for example, locally high shear stress (˜500 kPa) was measured below 200 m of ice on a smooth rock tablet at the bed of Engabreen, Norway. This value of shear stress is an order of magnitude greater than estimated by leading theories. To better understand the origin of debris-bed friction, we built a new laboratory apparatus that recorded the contact force between a clast and a hard bed as a function of ice velocity toward the bed. An independent experiment with the same apparatus in which the clast is isolated from the bed was used to obtain the ice viscosity. After correcting for cavity formation and ice flow geometry, results indicate that the contact force between a clast and a hard bed is about twice the drag force on the same clast estimated using Stokes's law. This value is insufficient to explain the high debris-bed friction measured beneath Engabreen. An alternative explanation is that longitudinal ice extension caused by ice flowing over the rough topography near the smooth rock tablet increased the rate of ice convergence with the bed by a factor of 5. Our measurements confirm that debris-bed friction is controlled by contact forces caused by flow of ice towards the bed due to basal melting and longitudinal ice extension. This form of frictional drag has yet to be included in models of ice flow. Inclusion of debris-bed friction may prove important to properly estimating rates of basal sliding, energy dissipation and meltwater production at the bed, and, more importantly, to quantifying the stick-slip behavior of hard-bedded glaciers.

Cohen, D. O.; Byers, J.; Iverson, N. R.

2011-12-01

220

Near-wall behavior of RANS turbulence models and implications for wall functions  

Microsoft Academic Search

The paper proposes a novel wall-function formulation applicable to any RANS turbulence model. It is based on the assumption of wall layer universality, applied to the entire model. The approach is implemented via tables for the turbulence quantities and the friction velocity u?. The influence of numerical errors on the wall-function solution is investigated and improvements are proposed. Numerical results

Georgi Kalitzin; Gorazd Medic; Gianluca Iaccarino; Paul Durbin

2005-01-01

221

Turbulent motion in the wall boundary layer of a viscoelastic fluid  

Microsoft Academic Search

The experimentally observed effect (Toms effect) of additions of aqueous polymer solutions to reduce frictional resistance is interpreted with the aid of Maxwell's model and Oldroyd's model of a viscoelastic fluid. An equation describing the mean turbulent motion of a viscoelastic fluid at an infinite wall is derived, which differs from the Reynolds equation in that it contains turbulent stresses

Ia. S. Khodorkovskii

1976-01-01

222

Turbulent boundary layers developing over compliant surfaces  

NASA Astrophysics Data System (ADS)

The problem of reducing drag due to skin friction remains of interest. This is the case because of the significant benefits that would result from an application of a drag reducing scheme on airplanes, ships or underwater vehicles. One of the techniques that have been proposed for such a scheme is wall compliance. Wall compliance could, in principle, work in two ways: either it could delay transition, or it could modify the inner part of a turbulent boundary layer so that reduced skin friction would result. The objective of this research program was to develop prediction techniques for high Reynolds number turbulent flows over compliant surfaces. This objective was pursued by evaluating the wall induced Reynolds stresses using solutions of the liner momentum equations.

Lekoudis, S. G.; Sengupta, T.

1983-05-01

223

Friction-reducing device  

SciTech Connect

This patent describes a sucker rod coupling adapted to reduce friction within production tubing in a well bore. It comprises: a substantially cylindrical body member and roller assemblies; the body member comprising means at each end thereof for attaching the coupling to a sucker rod, and axially and circumferentially spaced recesses, each recess containing a roller guide connected to the body, and each recess being further adapted to receive and support a roller assembly around the roller guide in such manner that the roller assembly can revolve around the roller guide; the roller assemblies each comprising rollers rotatably mounted on and linked by a chain, the rollers being adapted to reduce frictional contact between the body member and the tubing by rotating between the roller guide and the tubing while the chain revolves around the roller guide.

Dollison, W.W.

1990-04-24

224

Highly turbulent Taylor-Couette flow  

Microsoft Academic Search

The research issues addressed in this mostly experimental thesis concern highly\\u000aturbulent Taylor-Couette (TC) flow (Re>105, implying Ta>1011). We study it on\\u000aa fundamental level to aid our understanding of (TC) turbulence and to make predictions towards astrophysical disks, and at a practical level as applications can be found in bubble-induced skin-friction drag reduction on ships. In PART I we

Gils van Dennis Paulus Maria

2011-01-01

225

Synthetic turbulence  

NASA Astrophysics Data System (ADS)

A family of schemes is outlined for constructing stochastic fields that are close to turbulence. The fields generated from the more sophisticated versions of these schemes differ little in terms of one-point and two-point statistics from velocity fluctuations in high-Reynolds-number turbulence; we shall designate such fields as synthetic turbulence. All schemes, implemented here in one dimension, consist of the following three ingredients, but differ in various details. First, a simple multiplicative procedure is utilized for generating an intermittent signal which has the same properties as those of the turbulent energy dissipation rate ?. Second, the properties of the intermittent signal averaged over an interval of size r are related to those of longitudinal velocity increments ?u(r), evaluated over the same distance r, through a stochastic variable V introduced in the spirit of Kolmogorov's refined similarity hypothesis. The third and final step, which partially resembles a well-known procedure for constructing fractional Brownian motion, consists of suitably combining velocity increments to construct an artificial velocity signal. Various properties of the synthetic turbulence are obtained both analytically and numerically, and found to be in good agreement with measurements made in the atmospheric surface layer. A brief review of some previous models is provided.

Juneja, A.; Lathrop, D. P.; Sreenivasan, K. R.; Stolovitzky, G.

1994-06-01

226

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

227

Skin friction reduction by large air bubbles in a horizontal channel flow  

Microsoft Academic Search

Microbubble and air film methods are believed to be applicable to skin friction reduction in ships. Small bubbles are dispersed into the turbulent boundary layer in the former case, and wide air layers cover the wall surface in the latter case. Previous studies did not specifically address the intermediate case between the microbubble and air film conditions. This study is

Yuichi Murai; Hiroshi Fukuda; Yoshihiko Oishi; Yoshiaki Kodama; Fujio Yamamoto

2007-01-01

228

Quantum turbulence  

NASA Astrophysics Data System (ADS)

We review physical properties of quantum fluids He II and 3He-B, where quantum turbulence (QT) has been studied experimentally. Basic properties of QT in these working fluids are discussed within the phenomenological two-fluid model introduced by Landau. We consider counterflows in which the normal and superfluid components flow against each other, as well as co-flows in which the direction of the two fluids is the same. We pay special attention to the important case of zero temperature limit, where QT represents an interesting and probably the simplest prototype of three-dimensional turbulence in fluids. Experimental techniques to explore QT such as second sound attenuation, Andreev reflection, NMR, ion propagation are briefly introduced and results of various experiments on so-called Vinen QT and Kolmogorov QT both in He II and 3He are discussed, emphasizing similarities and differences between classical and quantum turbulence.

Skrbek, L.

2011-12-01

229

Rubber friction and tire dynamics  

Microsoft Academic Search

We propose a simple rubber friction law, which can be used, e.g., in models\\u000aof tire (and vehicle) dynamics. The friction law is tested by comparing\\u000anumerical results to the full rubber friction theory (B.N.J. Persson, J. Phys.:\\u000aCondensed Matter 18, 7789 (2006)). Good agreement is found between the two\\u000atheories. We describe a two-dimensional (2D) tire model which combines

B. N. J. Persson

2010-01-01

230

Friction loss in straight pipes of unplasticized polyvinyl chloride.  

PubMed

In order to design proper ductwork for a local exhaust system, airflow characteristics were investigated in straight pipes of unplasticized polyvinyl chloride (PVC). A linear decrease in static pressure was observed downstream at points from the opening of the VU pipes (JIS K 6741) located at distances greater than 10 times the pipe diameter, for velocities ranging between 10.18-36.91 m/s. Roughness inside pipes with small diameters was found to be 0.0042-0.0056 mm and the friction factor was calculated on the basis of Colebrook's equation for an airflow transition zone. An extended friction chart was then constructed on the basis of the roughness value and the friction factor. This chart can be applied when designing a local exhaust system with the ducts of diameters ranging from 40 to 900 mm. The friction loss of the PVC pipe was found to be approximately 2/3 of that of a galvanized steel pipe. PMID:8768669

Iwasaki, T; Ojima, J

1996-01-01

231

Application of generalized wall function for complex turbulent flows  

Microsoft Academic Search

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

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

2003-01-01

232

Turbulent breaking of overturning gravity waves below a critical level  

Microsoft Academic Search

The interaction of an internal gravity wave with its evolving critical layer and the subsequent generation of turbulence by overturning waves are studied by three-dimensional numerical simulations. The simulation describes the flow of a stably stratified Boussinesq fluid between a bottom wavy surface and a top flat surface, both without friction and adiabatic. The amplitude of the surface wave amounts

Andreas Dörnbrack; Thomas Gerz; Ulrich Schumann

1995-01-01

233

Laminar-turbulent transition in a bayonet tube  

Microsoft Academic Search

This paper provides the details of a numerical study of the bayonet tube during the laminar-turbulent transition. Attention is focused on the frictional characteristics of the tube. The results constitute a systematic investigation of the effect of the principal parameters on hydraulic resistance represented by an Euler number. Specifically discussed are the effects of fluid properties, tube length, and inner

Harpal Minhas; G. S. H. Lock

1996-01-01

234

Application of neural networks to turbulence control for drag reduction  

Microsoft Academic Search

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

Changhoon Lee; John Kim; David Babcock; Rodney Goodman

1997-01-01

235

Investigation of Oscillating Riblets for Turbulent Drag Reduction  

Microsoft Academic Search

Results from direct numerical simulations show that stationary, blade-shaped riblets reduce turbulent friction drag by 8.6%. Tilting the riblets sinusoidally at an oscillation period of T + = 35, improves the drag reduction up to 11.1% compared to a smooth reference wall. The main improvement is found at the positions of maximal riblet tilting. Mean flow and fluctuation profiles are

F. Kramer; F. Thiele; E. Wassen

236

Delaying Transition to Turbulence by a Passive Mechanism  

Microsoft Academic Search

Reducing skin friction is important in nature and in many technological applications. This reduction may be achieved by reducing stresses in turbulent boundary layers, for instance tailoring biomimetic rough skins. Here we take a second approach consisting of keeping the boundary layer laminar as long as possible by forcing small optimal perturbations. Because of the highly non-normal nature of the

Jens H. Fransson; Alessandro Talamelli; Luca Brandt; Carlo Cossu

2006-01-01

237

Microbubble Drag Reduction in Liquid Turbulent Boundary Layers  

Microsoft Academic Search

The interactions between a dense cloud of small bubbles and a liquid turbulent boundary layer are reviewed on the basis of available experimental observations to understand and quantify their capability for reducing skin friction. Gas bubbles are generally introduced into the boundary layer by injection through a porous surface or by electrolysis. After injection, the bubbles stay near the wall

Charles L. Merkle; Steven Deutsch

1992-01-01

238

A computational technique for turbulent flow of wastewater sludge.  

PubMed

A computational fluid dynamics (CFD) technique applied to the turbulent flow of wastewater sludge in horizontal, smooth-wall, circular pipes is presented. The technique uses the Crank-Nicolson finite difference method in conjunction with the variable secant method, an algorithm for determining the pressure gradient of the flow. A simple algebraic turbulence model is used. A Bingham-plastic rheological model is used to describe the shear stress/shear rate relationship for the wastewater sludge. The method computes velocity gradient and head loss, given a fixed volumetric flow, pipe size, and solids concentration. Solids concentrations ranging from 3 to 10% (by weight) and nominal pipe sizes from 0.15 m (6 in.) to 0.36 m (14 in.) are studied. Comparison of the CFD results for water to established values serves to validate the numerical method. The head loss results are presented in terms of a head loss ratio, R(hl), which is the ratio of sludge head loss to water head loss. An empirical equation relating R(hl) to pipe velocity and solids concentration, derived from the results of the CFD calculations, is presented. The results are compared with published values of Rhl for solids concentrations of 3 and 6%. A new expression for the Fanning friction factor for wastewater sludge flow is also presented. PMID:16121510

Bechtel, Tom B

239

Contact-crack problem with friction in spruce  

Microsoft Academic Search

This analysis presents of how friction in wood-to-metal interfaces can affect data from wedge-splitting tests on Double Cantilever Beam (DCB) spruce specimens. Cases evaluated were short cracks, and cracks emanating from a long notch. A numerical analysis is performed to clarify the influence of frictional characteristics at the wood-aluminium wedge interface on stress intensity factors. This demonstrates a correlation between

S. Vasic; I. Smith

2003-01-01

240

Direct numerical simulation of polymer-induced drag reduction in turbulent boundary layer flow of inhomogeneous polymer solutions  

Microsoft Academic Search

Skin-friction drag reduction in turbulent boundary layer flow of inhomogeneous polymer solutions is investigated using direct numerical simulations. A continuum constitutive model (FENE-P) accounting for the effects of polymer microstructure and concentration is used to describe the effect of viscoelasticity. The evolution of wall friction along the streamwise direction is a function of the dynamics of the polymer distribution in

Costas D. Dimitropoulos; Yves Dubief; Eric S. G. Shaqfeh; Parviz Moin

2006-01-01

241

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

NASA Astrophysics Data System (ADS)

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

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

242

Frictional action at lower limb/prosthetic socket interface.  

PubMed

The frictional action at stump/socket interface is discussed by a simplified model and finite element model analyses and clinical pressure measurements. The friction applied to the stump skin produces stresses within tissue and these stresses may damage the tissues and affect their normal functions. The combination of normal and shear stresses is considered to be a critical factor leading to amputee's discomfort and tissue damage. However, friction at the stump/socket interface has a beneficial action. A simplified residual limb model and a finite element model using real geometry have been developed to analyse the support action of friction. Both results show that the friction plays a critical role both in supporting the load of the amputee's body during the support phase of the gait cycle and in preventing the prosthesis from slipping off the limb during swing phase. Pressure at the below-knee socket during walking were measured with conditions of different friction. The results reveal that a larger pressures was produced at the lubricated interface than at the normal interface. A proper choice of coefficient of friction will balance the requirements of relief of load stress and reduction of slip with the general ability to support loads. PMID:8718946

Zhang, M; Turner-Smith, A R; Roberts, V C; Tanner, A

1996-04-01

243

Wall friction measurement in the absence of mean shear  

NASA Astrophysics Data System (ADS)

The dimensionless frictional force f between a pipe wall and a flowing turbulent fluid is f=?s /U^2, where U is mean flow speed in the x-direction, ? is kinematic viscosity, and s=u y , where the y axis is perpendicular to the flow direction. The derivative is evaluated at the wall, y = 0. Described here a scheme for measuring f in a turbulent fluid where s is close to zero. Hence the source of frictional dissipation is from fluctuations in the shear about its mean, namely s^2. This type of shear is encountered in turbulence in a closed container such as a food mixer. The scheme, which involves photon correlation spectroscopy, averages the shear rate over a laser spot size w ˜ 100 ?m or smaller. The scheme yields the probability density function (PDF) of components of the shear rate tensor and the moments of of the PDF. The theory will be described briefly and measurements will be presented where s ˜ 0. In that limit f is redefined to be f =u' sij/?, where sij is the dominant component being measured, and u' is the rms fluctuations of the velocity.

Stefanus, Stefanus; Castiglione, James; Cerbus, Rory; Goldburg, Walter

2012-02-01

244

Micromechanics of rock friction. 3: Quantitative modeling of base friction  

NASA Astrophysics Data System (ADS)

A micromechanical model for friction of brittle elastic solids is introduced and applied to rock friction. It is assumed that contacts are characterized by an adhesive shear strength, a material property that is only the free variable in the formulation. The net contact state of rough surfaces is calculated from elastic contact theory based on the measured topography of the surfaces. The model is tested by comparing it with friction measured with the same surfaces. Good agreement between theory and experiment was obtained from the initial application of shear stress to the onset of steady-state friction. The early stages of friction were predicted by the model based on the initial surface topographies and including the contributions of both normal and oblique contacts. Slip hardening was successfully modeled by including the evolution of the surface topography and contact state due to wear.

Wang, Weibin; Scholz, C. H.

1995-03-01

245

An experimental study of changes in the structure of a turbulent boundary layer due to surface geometry changes  

NASA Astrophysics Data System (ADS)

This investigation attempts to observe changes in the flow structure of a nominally zero pressure gradient turbulent boundary layer by altering the surface of a smooth plate with small triangular riblets oriented in the direction of flow and protruding just above the viscous sublayer. Hot-wire measurements show a reduction in the local skin friction over the riblet valleys by about 40% compared to the smooth surface near the end of the plate. Local skin friction increases by about 10% over the riblet peaks. Results showing the effects of the riblets on the mean, root-mean-square and skewness factor distributions of the streamwise velocity are presented. The mean profile displays characteristics similar to those of drag reducing additives; the streamwise turbulence intensity is reduced by approximately 10-15% near the surface by the riblets; the skewness factor distribution is virtually unchanged. A comparison of results from an application of the VITA 'burst' detection technique to smooth and riblet surface data is also presented although the results are rather inconclusive. Flow visualization of 'bursting' over both surfaces, while dramatically revealing the structure, reveals little difference resulting from the presence of the riblet.

1983-01-01

246

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

NASA Astrophysics Data System (ADS)

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

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

2013-07-01

247

The Friction of Lubricated Metals  

Microsoft Academic Search

An analysis has been made of the kinetic friction between metals sliding under conditions of boundary lubrication. With mineral oils and many other lubricants an intermittent clutching and breaking away of the surfaces still occurs through the oil film. The friction, the surface temperature and the area of contact all show violent fluctuations and the behaviour may be essentially the

F. P. Bowden; L. Leben

1940-01-01

248

REDUCED ENGINE FRICTION AND WEAR  

Microsoft Academic Search

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

Ron Matthews

2005-01-01

249

Friction of Polymeric Brake Materials.  

National Technical Information Service (NTIS)

P.T.F.E. (Teflon) in brake material formulations produces a positive slope of coefficient of friction versus speed, on a Chase dynamometer. This sliding speed dependence is different from temperature dependence of friction. Teflon added to the brake formu...

K. C. Ludema

1974-01-01

250

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

251

Friction laws at the nanoscale.  

PubMed

Macroscopic laws of friction do not generally apply to nanoscale contacts. Although continuum mechanics models have been predicted to break down at the nanoscale, they continue to be applied for lack of a better theory. An understanding of how friction force depends on applied load and contact area at these scales is essential for the design of miniaturized devices with optimal mechanical performance. Here we use large-scale molecular dynamics simulations with realistic force fields to establish friction laws in dry nanoscale contacts. We show that friction force depends linearly on the number of atoms that chemically interact across the contact. By defining the contact area as being proportional to this number of interacting atoms, we show that the macroscopically observed linear relationship between friction force and contact area can be extended to the nanoscale. Our model predicts that as the adhesion between the contacting surfaces is reduced, a transition takes place from nonlinear to linear dependence of friction force on load. This transition is consistent with the results of several nanoscale friction experiments. We demonstrate that the breakdown of continuum mechanics can be understood as a result of the rough (multi-asperity) nature of the contact, and show that roughness theories of friction can be applied at the nanoscale. PMID:19242472

Mo, Yifei; Turner, Kevin T; Szlufarska, Izabela

2009-02-26

252

Elastic model of dry friction  

NASA Astrophysics Data System (ADS)

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

Larkin, A. I.; Khmelnitskii, D. E.

2013-09-01

253

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

254

Advances in turbulence studies  

Microsoft Academic Search

Important contemporary trends in both experimental and theoretical turbulence research are reported. Particular attention is given to vortex reconnection, cascade, and mixing in turbulent flows; intermittent turbulence from closures; tearing instabilities in 2D MHD turbulence; axisymmetric hydromagnetic dynamo; bifurcations in MHD flow generated by electric current discharge; renormalization group analysis of MHD turbulence with low magnetic Reynolds number; Solution for

Herman Branover; Yeshajahu Unger

1993-01-01

255

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

Microsoft Academic Search

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

Hiroshi Kawamura; Kouichi Ohsaka; Hiroyuki Abe; Kiyoshi Yamamoto

1998-01-01

256

Numerical solutions of turbulent convection over a flat plate with angle of attack  

SciTech Connect

As evident from the literature, the knowledge of turbulent convection over flat plates with an angle of attack is limited principally to empirical results of overall heat transfer coefficients (possibly in the form of Nusselt numbers or Colburn j factors) in some ranges of the Reynolds number. This study was motivated in part by the lack of a conclusive method to determine heat loss due to wind effects on the exterior of inclined solar collectors and photovoltaic panels, as also evident from the widespread use of the Jurges correlation which does not incorporate any dimensions or fluid properties and is thus strictly correct only for the size of plate and thermal conditions of Jurges' experiment. The purpose of this investigation is to examine the turbulent boundary layer flow corresponding skin friction and convection heat loss as a function of free-stream velocity and angle of attack, by using a combined analytical/numerical approach. Many such approaches exist for the computation of turbulent flow. The approach taken in this study was to establish and solve two modelsm a zeroth-order one and a first-order one, determine their validity by comparison to existing experimental results and correlations, and compare the effort required for thier solution. Since it is generally accepted that the zero-order model is a reasonable approximation, it serves here as a base with width the first-order model solution accuracy and effort are compared.

Truncellito, N.T. (Univ. of Pennsylvania, Philadelphia (USA) General Electric Co., Valley Forge, PA (USA)); Yeh, H.; Lior, N. (Univ. of Pennsylvania, Philadelphia (USA))

1987-02-01

257

Premixed flame effects on turbulence and pressure-related terms  

SciTech Connect

Direct numerical simulations (DNS) were carried out for premixed, planar, turbulent flames. Heat release effects are accounted for by inclusion of variable density. The simulated flames are thin in the sense that the reaction progress variable is bi-modal and consistent with BML theory. The DNS data were used for detailed study of flame effects on turbulence within the turbulent flame brush by examining the turbulent kinetic energy budget. The flame effects on turbulent kinetic energy were found to depend strongly on the heat release. Both mean and fluctuation pressure terms were found to be the main factors responsible for increases in turbulent kinetic energy. The main sinks for turbulence are dissipation and mean dilatation. Pressure diffusion was found to dominate the other turbulent kinetic energy diffusion terms. A model was developed for pressure dilatation that matches the DNS results very closely. The model indicates that pressure dilatation will remain an important source of turbulence even as heat release increases.

Zhang, S.; Rutland, C.J. [Univ. of Wisconsin, Madison, WI (United States). Dept. of Mechanical Engineering

1995-09-01

258

Fault Wear and Friction Evolution: Experimental Analysis  

NASA Astrophysics Data System (ADS)

Wear is an inevitable product of frictional sliding of brittle rocks as evidenced by the ubiquitous occurrence of fault gouge and slickenside striations. We present here experimental observations designed to demonstrate the relationship between wear and friction and their governing mechanisms. The experiments were conducted with a rotary shear apparatus on solid, ring-shaped rock samples that slipped for displacements up to tens of meters. Stresses, wear and temperature were continuously monitored. We analyzed 86 experiments of Kasota dolomite, Sierra White granite, Pennsylvania quartzite, Karoo gabbro, and Tennessee sandstone at slip velocities ranging from 0.002 to 0.97 m/s, and normal stress from 0.25 to 6.9 MPa. We conducted two types of runs: short slip experiments (slip distance < 25 mm) primarily on fresh, surface-ground samples, designed to analyze initial wear mechanisms; and long slip experiments (slip distance > 3 m) designed to achieve mature wear conditions and to observe the evolution of wear and friction as the fault surfaces evolved. The experiments reveal three wear stages: initial, running-in, and steady-state. The initial stage is characterized by (1) discrete damage striations, the length of which is comparable to total slip , and local pits or plow features; (2) timing and magnitude of fault-normal dilation corresponds to transient changes of normal and shear stresses; and (3) surface roughness increasing with the applied normal stress. We interpret these observations as wear mechanisms of (a) plowing into the fresh rock surfaces; (b) asperity breakage; and (c) asperity climb. The running-in stage is characterized by (1) intense wear-rate over a critical wear distance of Rd = 0.3-2 m; (2) drop of friction coefficient over a weakening distance of Dc = 0.2-4 m; (3) Rd and Dc display positive, quasi-linear relation with each other. We interpret these observations as indicating the organizing of newly-created wear particles into a 'three-body' structure that acts to lubricate the fault (Reches & Lockner, 2010). The steady-state stage is characterized by (1) relatively low wear-rate (approximately 10% of running-in wear-rate) and (2) quasi-constant friction coefficient. These observations suggest only small changes in the gouge layer in term of thickness (100 to 200 microns) and strength in this final stage. The present study indicates that (1) wear by plowing and asperity failure initiate early, during the first few millimeters of slip; and (2) wear and associated gouge formation appear as the controlling factors of friction evolution and fault weakening.

Boneh, Y.; Chang, J. C.; Lockner, D. A.; Reches, Z.

2011-12-01

259

Observations of turbulence in a tidal beam and across a coastal ridge  

NASA Astrophysics Data System (ADS)

During a microstructure survey off California in Monterey Bay, we found a midwater beam of strong turbulence emanating from the shelf break along the ray path of the semidiurnal M2 internal tide. Within the 50-m-thick beam the turbulence kinetic energy dissipation rate ? exceeded 10-6 W kg-1, and the diapycnal eddy diffusivity K? was >0.01 m2 s-1. The beam extended 4 km off the shelf break. Several factors suggest that this beam of strong turbulence resulted from the breaking of semidiurnal internal tides: the beam appeared to originate from the shelf break, which is a potential generation site for semidiurnal internal tides; the beam closely followed the ray path of the semidiurnal internal tide; the average ? off the shelf break varied by a factor of 100 with a semidiurnal tidal periodicity; the isopycnal displacement confirmed the presence of semidiurnal internal tides. Processes associated with the breaking of internal tides are intermittent and sporadic. At the same location we also observed equally intense turbulence in a ˜100-m-thick layer of stratified water across the ridge of a sea fan. This layer of strong turbulence was separated from the bottom and was clearly not generated by bottom friction. Although less well resolved in time, the strong turbulence above the bottom seemed to vary with the semidiurnal tide and existed at the lee of the ridge, where the isopycnal surface dipped and rebounded in a pattern resembling that of internal hydraulic jumps. On the basis of the behavior of the density field, we believe that the deep mixing was most likely produced by the across-ridge current of internal tides. The breaking of internal tides at middepth, where the Richardson number is close to the critical value, is likely due to shear instability. The presence of the coastal ridge provides an alternative pathway for converting energy from internal tides to turbulence via internal hydraulics. Multiplying the average ? in the midwater beam by the length of the global coastline gives 31 GW, only a small fraction of the estimated 360 GW dissipated globally by M2 internal tides. Our observations suggest that either most internal tides are generated away from shelf breaks or most internal tides generated at shelf breaks propagate away from their generation sites, rather than dissipate locally, and eventually contribute to pelagic mixing.

Lien, R.-C.; Gregg, M. C.

2001-03-01

260

Explosive turbulent magnetic reconnection.  

PubMed

We report simulation results for turbulent magnetic reconnection obtained using a newly developed Reynolds-averaged magnetohydrodynamics model. We find that the initial Harris current sheet develops in three ways, depending on the strength of turbulence: laminar reconnection, turbulent reconnection, and turbulent diffusion. The turbulent reconnection explosively converts the magnetic field energy into both kinetic and thermal energy of plasmas, and generates open fast reconnection jets. This fast turbulent reconnection is achieved by the localization of turbulent diffusion. Additionally, localized structure forms through the interaction of the mean field and turbulence. PMID:23829741

Higashimori, K; Yokoi, N; Hoshino, M

2013-06-17

261

Explosive Turbulent Magnetic Reconnection  

NASA Astrophysics Data System (ADS)

We report simulation results for turbulent magnetic reconnection obtained using a newly developed Reynolds-averaged magnetohydrodynamics model. We find that the initial Harris current sheet develops in three ways, depending on the strength of turbulence: laminar reconnection, turbulent reconnection, and turbulent diffusion. The turbulent reconnection explosively converts the magnetic field energy into both kinetic and thermal energy of plasmas, and generates open fast reconnection jets. This fast turbulent reconnection is achieved by the localization of turbulent diffusion. Additionally, localized structure forms through the interaction of the mean field and turbulence.

Higashimori, K.; Yokoi, N.; Hoshino, M.

2013-06-01

262

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

263

FRICTION IDENTIFICATION AND COMPENSATION ON NANOMETER SCALE  

Microsoft Academic Search

This work concerns the modelling and experimental verification of the highly nonlinear friction behavior in positioning on the nanometer scale. The main goal of this work is to adjust and identify a simple dynamic friction model which allows a model-based estimation of the friction force in combination with the system inertia against displacement. Experiments in the pre-sliding and sliding friction

A. Amthor; T. Hausotte; Ch. Ament; P. Li; G. Jaeger

264

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

265

Friction model for the velocity dependence of nanoscale friction  

NASA Astrophysics Data System (ADS)

The velocity dependence of nanoscale friction is studied for the first time over a wide range of velocities between 1 µm s-1 and 10 mm s-1 on large scan lengths of 2 and 25 µm. High sliding velocities are achieved by modifying an existing commercial atomic force microscope (AFM) setup with a custom calibrated nanopositioning piezo stage. The friction and adhesive force dependences on velocity are studied on four different sample surfaces, namely dry (unlubricated), hydrophilic Si(100); dry, partially hydrophobic diamond-like carbon (DLC); a partially hydrophobic self-assembled monolayer (SAM) of hexadecanethiol (HDT); and liquid perfluoropolyether lubricant, Z-15. The friction force values are seen to reverse beyond a certain critical velocity for all the sample surfaces studied. A comprehensive friction model is developed to explain the velocity dependence of nanoscale friction, taking into consideration the contributions of adhesion at the tip-sample interface, high impact velocity-related deformation at the contacting asperities and atomic scale stick-slip. A molecular spring model is used for explaining the velocity dependence of friction force for HDT.

Tambe, Nikhil S.; Bhushan, Bharat

2005-10-01

266

Friction and wear of friction materials containing two different phenolic resins reinforced with aramid pulp  

Microsoft Academic Search

Friction and wear characteristics of automotive friction materials containing two different phenolic resins (a straight novolac resin and a modified novolac resin) were investigated using a pad-on-disk type friction tester. Six different friction materials with different relative amounts of the phenolic resins and aramid pulp were manufactured and tested. Two different test modes were employed to examine the friction characteristics

Seong Jin Kim; Ho Jang

2000-01-01

267

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

268

Exploring a flight deck based wake turbulence situational awareness tool  

Microsoft Academic Search

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

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

2012-01-01

269

Turbulence stabilization  

NASA Astrophysics Data System (ADS)

We recently developed a new approach to get a stabilized image from a sequence of frames acquired through atmospheric turbulence. The goal of this algorihtm is to remove the geometric distortions due by the atmosphere movements. This method is based on a variational formulation and is efficiently solved by the use of Bregman iterations and the operator splitting method. In this paper we propose to study the influence of the choice of the regularizing term in the model. Then we proposed to experiment some of the most used regularization constraints available in the litterature.

Mao, Yu; Gilles, Jérôme

2012-05-01

270

Rubber friction: comparison of theory with experiment.  

PubMed

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

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

2011-12-06

271

Sliding friction in viscous hydrodynamics  

SciTech Connect

Basset's collective friction coefficient for a viscous fluid flowing past a rough solid surface is obtained - analytically - as an intrinsic consequence of the Navier-Stokes equations by treating the surface as a source of vorticity.

Dekker, H.

1989-04-10

272

Low Friction Coatings for Icebreakers.  

National Technical Information Service (NTIS)

Part I of the first phase of development of low friction coatings for icebreakers involved the evaluation of State-of-the-Art coatings available from industry. Laboratory simulation tests were conducted to determine materials' ability to withstand ice imp...

F. F. Ling M. B. Peterson S. J. Calabrese

1974-01-01

273

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

2010-11-26

274

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

275

Size Scaling of Static Friction  

NASA Astrophysics Data System (ADS)

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 fm+?f/A? for increasing contact area A, with ?>0. Our main finding is that the value of fm, 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.

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

2013-02-01

276

Molecular Origins of Elastomeric Friction  

Microsoft Academic Search

Frictional properties of soft elastomers have been in question for over half of a century. Early studies [1–3] on natural rubber originated for the sole purpose of tabulating properties for bulk consumer applications, such as viscoelastic adhesives, [4,5] tires, [6] and windshield wipers, [7,8] to name a few. Empirical tabulation of frictional properties persisted until the early 1950’s, when Roth

Scott Sills; Katherine Vorvolakos; Manoj K. Chaudhury; René M. Overney

277

Experimental Friction Blisters in Macaque Monkeys1  

Microsoft Academic Search

Friction blisters were successfully produced experimentally on the palms and soles of two Macaca irus and four Macaca speciosa monkeys. The lesions corresponded to those seen clinically and experimentally in man.Histologically, monkey friction blisters represented intraepidermal vesicles and were identical to human friction blisters.Monkey friction blister fluid qualitatively reflected the composition of serum.The production of experimental friction blisters in laboratory

Thomas A Cortese; Tommy B Griffin; Laurence L Layton; Thomas C Hutsell; Tommy B Griffin Maj MC

1969-01-01

278

[Frictional properties of human skin at different ages, gender and anatomical regions].  

PubMed

A reciprocal sliding skin friction testing apparatus was performed to investigate human skin frictional properties of different physiological factors such as different ages, gender and anatomical regions. Student's t-test for non-paired data and one-way analysis of variance (ANOVA) were used in data analysis. Fisher LSD was used for multiple comparison on the statistical significance of the coefficients. The results showed that the skin frictional properties at the different anatomical regions were strongly dependent on the age. The differences of friction coefficients at the different anatomical regions became negligible with the increase of age. The skin frictional properties of different gender at the same anatomical region and age exhibited no significant differences. The friction coefficients of different ages at the same anatomical regions showed significant differences. PMID:17899754

Li, Wei; Qu, Shuxin; Zhou, Zhongrong

2007-08-01

279

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

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

280

Wind gust and turbulence statistics of typhoons in South China  

NASA Astrophysics Data System (ADS)

The wind data of four typhoons were obtained and analyzed. The wind speeds were measured by sonic anemometers at four observation sites in Guangdong and Hainan provinces. Detailed analysis of the wind data was conducted to investigate the turbulence characteristics of the typhoons. Characteristics of the gust factor and the turbulence integral scale of the typhoons were concluded with high confidence. The relationships among the gust factor, gust duration time, mean wind speed, roughness length, and turbulence intensity were described. The turbulence integral scale was found to be closely related to the segment length and turbulence intensity.

Wang, Binglan; Hu, Fei; Cheng, Xueling

2011-02-01

281

Evaluation of flush mounted hot-film sensors for skin friction reduction measurements in viscoelastic polymer solutions  

Microsoft Academic Search

The purpose of this investigation was to evaluate the performance of flush mounted hot-film sensors for mean wall shear stress measurement in turbulent flows of dilute drag reducing polymer solution. A series of pipe flow expriments were conducted over a range of Reynolds numbers and polymer solution concentrations to compare the level of skin friction drag reduction measured by hot-film

W. L. Harbison; H. L. Petrie

1991-01-01

282

Specific impulse losses due to friction and dispersion in a gas-film cooled liquid rocket engine nozzle  

Microsoft Academic Search

Conclusions The proposed method and program for calculating the compressible turbulent boundary layer in rocket engine nozzles with gas film cooling make it possible to determine the specific impulse losses due to friction, the heat fluxes and other characteristics of the flow. The calculations are based on the numerical solution of the equations of gas dynamics in the boundary layer

V. G. Lushchik; V. I. Sizov; L. E. Sternin; A. E. Yakubenko

1993-01-01

283

A multi-state description of roughness effects in turbulent pipe flow  

NASA Astrophysics Data System (ADS)

Despite persistent efforts over the last century, a theory for predicting the effects of surface roughness on the mean flow is still missing. Here, we extend a recently proposed multi-state theory to incorporate roughness effects. A formula for the roughness function is constructed by applying the Lie-group invariance theory, giving excellent agreement with the fully collapsed data of both the Nikuradze sand-coated and Princeton honed pipes. A major advantage of the theory is its ability to successfully describe the non-universality—either inflectional or monotonic variations—in a transitionally rough regime with a single sharpness parameter. This model then yields an analytic prediction for the friction factor and mean velocity profiles in rough pipes, agreeing with the empirical data. Consistent with classical understanding (e.g. Townsend's similarity hypothesis), our results confirm the multi-layer theory of wall turbulence, regardless of smooth or rough surfaces.

She, Zhen-Su; Wu, You; Chen, Xi; Hussain, Fazle

2012-09-01

284

Application of Taguchi approach to optimize friction stir welding parameters of polyethylene  

Microsoft Academic Search

This paper presents experimental and numerical results of butt friction stir welding of high density polyethylene. The FSW designed tool insulates the welded samples and preserves the heat gained from friction thus avoiding the appearance of blisters and splits after welding. The experimental tests, conducted according to combinations of process factors such as rotation speed, welding speed, pin diameter and

M. A. Rezgui; M. Ayadi; A. Cherouat; K. Hamrouni; A. Zghal; S. Bejaoui

2010-01-01

285

A new formulation for the streamwise turbulence intensity distribution  

NASA Astrophysics Data System (ADS)

Numerical and experimental data from zero pressure-gradient turbulent boundary layers over smooth walls have been analyzed by means of the so called diagnostic plot introduced by Alfredsson & Örlü [Eur. J. Fluid Mech. B/Fluids, 42, 403 (2010)]. In the diagnostic plot the local turbulence intensity is shown as a function of the local mean velocity normalized with a reference velocity scale. In the outer region of the boundary layer a universal linear decay of the turbulence intensity is observed independent of Reynolds number. The deviation from this linear region appears in the buffer region and seems to be universal when normalized with the friction velocity. Therefore, a new empirical fit for the streamwise velocity turbulence intensity distribution is proposed and the results are compared with up to date reliable high-Reynolds number experiments and extrapolated towards Reynolds numbers relevant to atmospherical boundary layers.

Alfredsson, P. Henrik; Örlü, Ramis; Segalini, Antonio

2011-12-01

286

Advances in turbulence studies  

NASA Astrophysics Data System (ADS)

Important contemporary trends in both experimental and theoretical turbulence research are reported. Particular attention is given to vortex reconnection, cascade, and mixing in turbulent flows; intermittent turbulence from closures; tearing instabilities in 2D MHD turbulence; axisymmetric hydromagnetic dynamo; bifurcations in MHD flow generated by electric current discharge; renormalization group analysis of MHD turbulence with low magnetic Reynolds number; Solution for turbulent primary azimuthal velocity in liquid-metal flows in sliding electric contacts; analogies between geophysical and hydromagnetic flows; turbulent electrically-induced vortical flows; dissipation length scale dynamics; two-phase grid turbulence; abridged octave wavenumber ring models for 2D turbulence; rag theory of magnetic fluctuations in turbulent flow; and instabilities of the nonuniform flows of a low-temperature plasma in MHD channels. (No individual items are abstracted in this volume)

Branover, Herman; Unger, Yeshajahu

287

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

288

Refined turbulence models for simulation of IC-engine cylinder flows  

Microsoft Academic Search

Turbulence and turbulent mixing are two of the most important factors that influence the efficiency and emissions level in internal combustion (IC) engines, particularly for diesel engines. This study has been performed with the premise to accurately predict in-cylinder turbulence by employing the large eddy simulation (LES) technique. In order to assess the turbulence scales involved correctly, a review of

Ibrahim Yavuz

2000-01-01

289

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

290

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

291

Instantaneous engine frictional torque: Its components and piston assembly friction  

NASA Astrophysics Data System (ADS)

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

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

1992-05-01

292

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

293

Frictional ignition with coal mining  

SciTech Connect

This paper reviews recent U.S. Bureau of Mine studies of frictional ignition of a methane-air environment by coal mining bits cutting into sandstone and the effectiveness of remedial techniques to reduce the likelihood of frictional ignition. Frictional ignition with a minim bit always involves a worn bit having a wear flat on the tip of the bit. The worn bit forms hot spots on the surface of the sandstone because of frictional abrasion. The hot spots then can ignite the methane-air environment. A small wear flat forms a small hot spot, which does not give ignition, while a large wear flat forms a large hot spot, which gives ignition. The likelihood of frictional ignition can be somewhat reduced by using a mushroom-shaped tungsten-carbide bit tip on the mining bit and by increasing the bit clearance angle; it can be significantly reduced by using a water spray nozzle in back of each bit, which is carefully oriented to direct the water spray onto the sandstone surface directly behind the bit and thereby cool the hot spots formed by the worn bit. A bit replacement schedule must be used to avoid the formation of a dangerously worn bit.

Courtney, W.G.

1990-01-01

294

Ambipolar Drift Heating in Turbulent Molecular Clouds  

NASA Astrophysics Data System (ADS)

We present calculations of frictional heating by ion-neutral drift in three-dimensional simulations of turbulent, magnetized molecular clouds. We show that ambipolar drift heating is a strong function of position in a turbulent cloud, and its average value can be significantly larger than the average cosmic-ray heating rate. The heating rate per unit volume due to ambipolar drift, HAD=|JXB|2/?i?in~ B4/(16?2L2B?i ?in), is found to depend on the rms Alfvénic Mach number, MA, and on the average field strength, as HAD~M2A<|B|>4. This implies that the typical scale of variation of the magnetic field, LB, is inversely proportional to MA, which we also demonstrate.

Padoan, Paolo; Zweibel, Ellen; Nordlund, Åke

2000-09-01

295

Frictional drag reduction by wavy advection of deformable bubbles  

NASA Astrophysics Data System (ADS)

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

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

2009-02-01

296

Generalized wall functions for turbulent flows with strong adverse pressure gradient  

Microsoft Academic Search

The generalized wall functions for turbulent flows with strong adverse pressure gradients are derived on the basis of the\\u000a asymptotic theory of near-wall turbulence. The generalized wall functions have a correct asymptotic behavior in the limit\\u000a of zero friction velocity and can be applied to computations of flows under a strong adverse pressure gradient and with separation\\u000a or reattachment. Calculations

D. Ph. Sikovsky

2011-01-01

297

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

Microsoft Academic Search

Results from a study to assess the accuracy of turbulent heating and skin friction predictiontechniques for hypersonic applications are presented. The study uses the original and a modifiedBaldwin-Lomax turbulence model with a space marching code. Grid converged turbulentpredictions using the wall damping formulation (original model) and local dampingformulation (modified model) are compared with experimental data for several flat plates.The wall

Arthur D. Dilley

2001-01-01

298

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

299

Dogs with heart diseases causing turbulent high-velocity blood flow have changes in platelet function and von Willebrand factor multimer distribution.  

PubMed

The purpose of this prospective study was to investigate platelet function using in vitro tests based on both high and low shear rates and von Willebrand factor (vWf) multimeric composition in dogs with cardiac disease and turbulent high-velocity blood flow. Client-owned asymptomatic, untreated dogs were divided into 4 groups: 14 Cavalier King Charles Spaniels (Cavaliers) with mitral valve prolapse (MVP) and no or minimal mitral regurgitation (MR), 17 Cavaliers with MVP and moderate to severe MR, 14 control dogs, and 10 dogs with subaortic stenosis (SAS). Clinical examinations and echocardiography were performed in all dogs. PFA100 closure times (the ability of platelets to occlude a hole in a membrane at high shear rates), platelet activation markers (plasma thromboxane B2 concentration, platelet surface P-selectin expression), platelet aggregation (in whole blood and platelet-rich plasma with 3 different agonists), and vWf multimers were analyzed. Cavaliers with moderate to severe MR and dogs with SAS had longer closure times and a lower percentage of the largest vWf multimers than did controls. Maximal aggregation responses were unchanged in dogs with SAS but enhanced in Cavaliers with MVP (regardless of MR status) compared with control dogs. No significant difference in platelet activation markers was found among groups. The data suggest that a form of platelet dysfunction detected at high shear rates was present in dogs with MR and SAS, possibly associated with a qualitative vWf defect. Aggregation results suggest increased platelet reactivity in Cavaliers, but the platelets did not appear to circulate in a preactivated state in either disease. PMID:16095168

Tarnow, Inge; Kristensen, Annemarie T; Olsen, Lisbeth H; Falk, Torkel; Haubro, Lotte; Pedersen, Lotte G; Pedersen, Henrik D

300

Internal friction in lunar rocks and terrestrial rocks  

Microsoft Academic Search

Our recent work has shown that the removal of small traces of absorbed volatiles remaining in nominally dry terrestrial analogs of lunar basalt drastically increase the internal friction quality factor Q. Now room temperature Q values above 2000 have been obtained by removal of these volatiles in high vacuum at elevated temperatures. Removal of trace volatiles increases Q in a

B. R. Tittmann

1974-01-01

301

SOCIAL AND ECONOMIC EFFECTS OF REDUCING FRICTIONS IN SOCIAL NETWORKING  

Microsoft Academic Search

Can we use the underlying technologies of virtual social networking in a (more) productive way? If we reduce frictions within social networks, caused by distance, inaccessibility, and other factors, can we detect social and economic effects, such as a reduction of transportation costs and a faster spread of knowledge? We want to know how a virtual social network is organised

Mark Schonewille

302

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

303

Vacuum friction in rotating particles.  

PubMed

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. PMID:20867571

Manjavacas, A; García de Abajo, F J

2010-09-08

304

Surface roughness and dry friction  

NASA Astrophysics Data System (ADS)

Persson's multiscale contact mechanics theory combined with a multiscale Brillouin-Prandtl-Tomlinson model is used to show that on the basis of these models “dry friction” [i.e., kinetic friction that remains at exceedingly small velocities (but still above the creep range) close to its value at higher velocities] should almost always occur for self-affine surfaces when the dominant interaction between two surfaces in contact is due to interatomic hard core repulsion, except for extremely smooth surfaces (i.e., surfaces with a Hurst index very close to 1).

Sokoloff, J. B.

2012-02-01

305

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

306

Seismology: The roller coaster of fault friction  

NASA Astrophysics Data System (ADS)

During an earthquake, friction is a key control on the initiation, propagation and termination of fault motion. Laboratory experiments that use variable slip rates suggest that friction evolves in a more complex fashion than generally assumed.

Lapusta, Nadia

2009-10-01

307

Turbulent boundary-layer control with spanwise travelling waves  

Microsoft Academic Search

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

Richard D Whalley; Kwing-So Choi

2011-01-01

308

Turbulent drag reduction by traveling wave of flexible wall  

Microsoft Academic Search

A direct numerical simulation of channel turbulent flow and its control by spanwise traveling waves of flexible wall shows that such an on-wall open-loop vorticity creation control can effectively reduce the friction drag for any incompressible Newtonian fluid. The control effect is similar to that of existing transverse-wave controls by spanwise near-wall oscillating flow\\/wall oscillation and traveling body-force wave. The

H. Zhao; J.-Z. Wu; J.-S. Luo

2004-01-01

309

The effect of metal fibers on the friction performance of automotive brake friction materials  

Microsoft Academic Search

This study investigates the effect of different metallic fibers upon friction and wear performance of various brake friction couples. Based on a simple experimental formulation, friction materials with different metal fibers (Cu, steel, or Al) were fabricated and then evaluated using a small-scale friction tester. Two different counter disks (gray cast iron and aluminum metal matrix composite (Al-MMC)) were employed

H Jang; K Ko; S. J Kim; R. H Basch; J. W Fash

2004-01-01

310

Coulomb constitutive laws for friction: Contrasts in frictional behavior for distributed and localized shear  

Microsoft Academic Search

We describe slip-rate dependent friction laws based on the Coulomb failure criteria. Frictional rate dependence is attributed to a rate dependence of cohesionc and friction angle ?. We show that differences in the stress states developed during sliding result in different Coulomb friction laws for distributed shear within a thick gouge layer versus localized shear within a narrow shear band

Chris Marone; B. E. Hobbs; A. Ord

1992-01-01

311

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

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

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

314

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

315

Dynamic models of friction wedge dampers  

Microsoft Academic Search

Friction wedges play a central role in the vertical dynamics of railroad freight cars. They also play a role in lateral dynamics and stability. While friction wedges are mechanically very simple, their inherent nonlinearities lead to complications in modeling and dynamics. In this paper we present two simplified models of the fundamental physics of friction wedge dampers in the geometry

John F. Gardner; Joseph P. Cusumano

1997-01-01

316

Friction Modeling and Compensation for Haptic Interfaces  

Microsoft Academic Search

Friction cancellation and high gain force feedback are studied for their relative beneìts in mitigating the effects of friction in haptic interfaces. Although either technique alone is capable of signiìcant improvements, we ìnd that a combination of approximate cancellation coupled with variable-gain low-bandwidth force feedback provides ex- cellent friction reduction and is more robust. This improves the feel of the

Nicholas L. Bernstein; Dale A. Lawrence; Lucy Y. Pao

2005-01-01

317

Atomic scale friction and wear of mica  

Microsoft Academic Search

The frictional behavior of mica surfaces with silicon nitride tips has been investigated systematically with the AFM as a function of load, tip geometry, mica lattice orientation and humidity. Frictional forces are found to be proportional to loads between 10 and 80 nN. The friction coefficient is quite reproducible for different samples, tip radii, scanning speed and direction. At low

J. Hu; X.-d. Xiao; D. F. Ogletree; M. Salmeron

1995-01-01

318

Studying the frictional force directions via bristles  

NASA Astrophysics Data System (ADS)

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 effective in teaching frictional force directions.

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

2010-11-01

319

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

320

Mechanical properties of copper to titanium joined by friction welding  

Microsoft Academic Search

This paper describes a fundamental investigation of friction welding pure copper to titanium. Friction welding was performed using a brake type friction welder. The effect of friction time and upset pressure on the mechanical and metallurgical properties were evaluated. Under constant upset pressure, the tensile strength made little difference with an increase in friction time, whereas at the constant friction

Seong-Yeon Kim; Seung-Boo Jung; Chang-Chae Shur; Yun-Mo Yeon; Dae-Up Kim

2003-01-01

321

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

322

An analytical model of capped turbulent oscillatory bottom boundary layers  

NASA Astrophysics Data System (ADS)

An analytical model of capped turbulent oscillatory bottom boundary layers (BBLs) is proposed using eddy viscosity of a quadratic form. The common definition of friction velocity based on maximum bottom shear stress is found unsatisfactory for BBLs under rotating flows, and a possible extension based on turbulent kinetic energy balance is proposed. The model solutions show that the flow may slip at the top of the boundary layer due to capping by the water surface or stratification, reducing the bottom shear stress, and that the Earth's rotation induces current and bottom shear stress components perpendicular to the interior flow with a phase lag (or lead). Comparisons with field and numerical experiments indicate that the model predicts the essential characteristics of the velocity profiles, although the agreement is rather qualitative due to assumptions of quadratic eddy viscosity with time-independent friction velocity and a well-mixed boundary layer. On the other hand, the predicted linear friction coefficients, phase lead, and veering angle at the bottom agreed with available data with an error of 3%-10%, 5°-10°, and 5°-10°, respectively. As an application of the model, the friction coefficients are used to calculate e-folding decay distances of progressive internal waves with a semidiurnal frequency.

Shimizu, Kenji

2010-03-01

323

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

324

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

325

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

326

Tool Wear in Friction Drilling  

SciTech Connect

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

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

2007-01-01

327

Stable Marriages and Search Frictions  

Microsoft Academic Search

We embed a two-sided matching market with non-transferable utility, a marriage market, into a random search model. We study steadystate equilibria and characterize the limit of the corresponding equilibrium matchings as exogenous search frictions become small. The central question is whether the set of such limit matchings coincides with the set of stable matchings for the underlying marriage market. We

Stephan Lauermann; Georg Nöldeke

2012-01-01

328

Turbulence and Flame Propagation.  

National Technical Information Service (NTIS)

The report contains five subreports which all together represent the authors section's recent research in turbulence and flame propagation. Part 1 contains calculations of a turbulent nitrogen diffusion flame based on 'Eddy Dissipation Concept' (EDC), whi...

M. Brostroem S. Byggstoeyl B. Grimsmo J. Holen N. I. Lilleheie

1987-01-01

329

Wake Turbulence Training Aid.  

National Technical Information Service (NTIS)

The goal of the Wake Turbulence Training Aid is to reduce the number of wake-turbulence related accidents and incidents by improving the pilot's and air traffic controller's decision making and situational awareness through increased and shared understand...

G. C. Hay R. H. Passman

1995-01-01

330

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

331

Friction of ice on ice  

NASA Astrophysics Data System (ADS)

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

Schulson, Erland M.; Fortt, Andrew L.

2012-12-01

332

Premixed Turbulent Flame Propagation.  

National Technical Information Service (NTIS)

An experimental study has been conducted of turbulence-flame interactions in premixed turbulent flames and their effect on flame-generated turbulence, flame structure and flame propagation. The flame configuration used for this study is that of a freely p...

D. A. Santavicca

1989-01-01

333

Atomic roughness enhanced friction on hydrogenated graphene  

NASA Astrophysics Data System (ADS)

Atomic friction on hydrogenated graphene is investigated using molecular dynamics simulations. Hydrogenation is found to increase friction significantly, and the atomic-level information provided by the simulations reveals that atomic roughness induced by hydrogenation is the primary cause of the friction enhancement. Other proposed mechanisms, specifically adhesion and rigidity, are excluded based on the simulation results and analyses performed using the Prandtl-Tomlinson model. In addition, it is found that friction does not monotonically increase with hydrogen coverage on the graphene surface; instead, a maximum friction is observed at a hydrogen coverage between 5 and 10%.

Dong, Yalin; Wu, Xiawa; Martini, Ashlie

2013-09-01

334

Friction in the Zero Sliding Velocity Limit  

NASA Astrophysics Data System (ADS)

Using an adiabatic approximation method, which searches for Tomlinson model-like instabilities for a simple but still realistic model for two crystalline surfaces, with mobile molecules present at the interface, sliding relative to each other, we are able to account for the virtually universal occurrence of ``dry friction'' at zero temperature. A modified version of this method allows us to calculate the kinetic friction at non-zero temperature as well. We have also considered the static friction, and have demonstrated that the model is able to account for static friction being larger than kinetic friction.

Daly, Christopher; Sokoloff, Jeffrey B.; Zhang, Jian

2004-03-01

335

Atomic roughness enhanced friction on hydrogenated graphene.  

PubMed

Atomic friction on hydrogenated graphene is investigated using molecular dynamics simulations. Hydrogenation is found to increase friction significantly, and the atomic-level information provided by the simulations reveals that atomic roughness induced by hydrogenation is the primary cause of the friction enhancement. Other proposed mechanisms, specifically adhesion and rigidity, are excluded based on the simulation results and analyses performed using the Prandtl-Tomlinson model. In addition, it is found that friction does not monotonically increase with hydrogen coverage on the graphene surface; instead, a maximum friction is observed at a hydrogen coverage between 5 and 10%. PMID:23965631

Dong, Yalin; Wu, Xiawa; Martini, Ashlie

2013-08-21

336

Friction in the zero sliding velocity limit  

NASA Astrophysics Data System (ADS)

Using an adiabatic approximation method, which searches for Tomlinson model-like instabilities for a simple but still realistic model for two crystalline surfaces, with mobile molecules present at the interface, sliding relative to each other, we are able to account for the virtually universal occurrence of “dry friction” at zero temperature. A modified version of this method allows us to calculate the kinetic friction at nonzero temperature as well. We have also considered the static friction, and have demonstrated that the model is able to account for static friction being larger than kinetic friction.

Daly, C.; Zhang, J.; Sokoloff, J. B.

2003-12-01

337

Turbulent drag reduction by the seal fur surface  

NASA Astrophysics Data System (ADS)

The drag-reducing ability of the seal fur surface was tested in a rectangular channel flow using water and a glycerol-water mixture to measure the pressure drop along the channel in order to evaluate friction factors in a wide range of Reynolds number conditions, and the drag reduction effect was confirmed quantitatively. The maximum reduction ratio was evaluated to be 12% for the glycerol-water mixture. The effective range of the Reynolds number, where the drag reduction was remarkable, was wider for the seal fur surface compared to that of a riblet surface measured in this channel and in previous studies. It was also found that for the seal fur surface, unlike riblets, any drag increase due to the effect of surface roughness was not found up to the highest Reynolds number tested. Measurements of the seal fur surface using a 3D laser microscope revealed that there were riblet-like grooves, composed of arranged fibers, of which spacings were comparable to that of effective riblets and were distributed in various wavelengths. Using LDV measurements, it was found that the difference in the mean velocity scaled by the outer variable among the smooth, riblet, and seal fur surfaces did not appear at any spanwise locations. Streamwise turbulence intensity for the seal fur surface was found to be about 5% smaller than those for smooth and riblet surfaces.

Itoh, Motoyuki; Tamano, Shinji; Iguchi, Ryo; Yokota, Kazuhiko; Akino, Norio; Hino, Ryutaro; Kubo, Shinji

2006-06-01

338

End to end loop formation in a single polymer chain with internal friction  

NASA Astrophysics Data System (ADS)

We use Rouse and Zimm models with solvent viscosity independent internal friction to study the end to end loop formation of a single polymer chain within the Wilemski–Fixmann theoretical framework. Our calculation shows internal friction makes loop formation between two ends of a polymer chain slower but has a weaker dependence on the chain length as compared to no internal friction. The average looping time shows not linear but fractional dependence on the solvent viscosity. Also the effect of the internal friction to the looping time is neither additive nor multiplicative but always additive to the reconfiguration time. Our numerical results show internal friction can reduce the looping rate by a factor of two to an order of magnitude depending on the time scale associated with it.

Samanta, Nairhita; Chakrabarti, Rajarshi

2013-09-01

339

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

340

Influence of particle characteristics on granular friction  

NASA Astrophysics Data System (ADS)

We report on laboratory experiments designed to illuminate grain-scale deformation mechanisms within fault gouge. We vary particle size distribution, grain and surface roughness, and gouge layer thickness to better understand how grain sliding, rolling, dilation, and compaction affect the strength and stability of granular fault gouge. The experiments employed the double direct shear testing geometry and were run at room temperature, controlled humidity, and shearing rates from 0.1 to 3000 ?m/s. Experiments were carried out under constant normal stress of 5 and 10 MPa and thus within a nonfracture loading regime where sliding friction for smooth, spherical particles is measurably lower than for rough, angular particles. We compare results from shear between smooth boundaries, where we hypothesize that grain boundary sliding is the dominant deformation mechanism, and roughened surfaces, where rolling and granular dilation contribute to shear deformation. We find that particle angularity and bounding surface roughness increase the frictional strength within sheared layers, indicating differences in particle reorganization due to these factors. In gouge material composed of <30% angular grains we observe repetitive stick-slip sliding where stress drop decreases while preinstability creep increases with increasing gouge layer thickness. Our data show significant differences in stick-slip characteristics as a function of gouge layer thickness and particle size, which we interpret in terms of the mechanics of grain bridges that support forces on the layers. We suggest that force chains exhibit qualitative differences as a function of grain angularity and bounding surface roughness.

Anthony, Jennifer L.; Marone, Chris

2005-08-01

341

Formation of low-speed ribbons in turbulent channel flow subject to a spanwise travelling wave  

NASA Astrophysics Data System (ADS)

Turbulent flow control with skin-friction drag reduction subject to spanwise travelling wave (STW) induced by Lorentz force is investigated by direct numerical simulation (DNS) in a channel. The results show that STW produces a set of distinct longitudinal vortices, suppressing the regeneration of near-wall turbulence structures. It is also shown that the formation of low-speed ribbons by STW is associated with these longitudinal vortices, which weaken the sweep and ejection events in the near-wall region. At the same time, the production of counter-gradient Reynolds stresses is increased, leading to up to 30% of turbulent drag reduction by STW.

Huang, L. P.; Choi, K. S.; Fan, B. C.

2011-12-01

342

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

343

Direct Optical Measurements of Vorticity Vectors in Turbulent Taylor Vortex Flow with the Vorticity Optical Probe.  

National Technical Information Service (NTIS)

Vorticity measurements with the Vorticity Optical Probe in the turbulent Taylor vortex regime of Couette-Taylor Flow are described. Preliminary data near the outer boundary (y/g=0.1) at Reynolds numbers above and below a skin friction scaling transition a...

R. D. Ferguson M. B. Frish

1992-01-01

344

Turbulent flow in a machine honed rough pipe for large Reynolds numbers: General roughness scaling laws  

Microsoft Academic Search

An alternate inner wall variable, for flow over a transitional rough pipe surface, is defined as the ratio of normal coordinate measured above the mean roughness level to the wall roughness scale. The Reynolds equations for mean turbulent flow in a transitional rough pipe, in two layers (inner and outer) are considered. The predictions of the mean velocity and friction

Noor Afzal; Abu Seena; A. Bushra

345

Turbulent Drag Reduction: Studies of Feedback Control and Flow Over Riblets  

Microsoft Academic Search

The objective of this study is to explore concepts for control of turbulent boundary layers leading to skin -friction reduction using the direct numerical simulation technique. This report is divided into three parts where three different control methods are investigated; a passive control by longitudinal riblets, an active control by sensing and perturbing structures near the wall, and a feedback

Haecheon Choi

1993-01-01

346

Turbulent boundary layer over a ribleted surface with tandem manipulators using surface drag balances  

Microsoft Academic Search

Low speed wind tunnel tests have been carried out to assess the combined drag reduction effects of longitudinally grooved or 'ribleted' surfaces and upstream tandem turbulence manipulators of LEBUs (Large Eddy Break-Up devices). Skin friction and surface drag measurements are presented covering the regions upstream, over and downstream of the ribleted plate, both with and without LEBUs. Surface drag surveys

V. D. Nguyen; J. Dickinson; Y. Jean; Y. Chalifour; A. Smaili

1990-01-01

347

Active turbulence control in wall bounded flows using direct numerical simulation  

Microsoft Academic Search

The objective of the study is to explore concepts for control of turbulent boundary layers leading to skin-friction reduction using the direct numerical simulation technique. This report is divided into three parts where different control methods are investigated: an active control by sensing and perturbing structures near the wall, a feedback control procedure guided by control theory, and a passive

Parviz Moin

1992-01-01

348

Injection into a turbulent boundary layer through porous surfaces with different surface geometries  

Microsoft Academic Search

The turbulent boundary layer over a perforated titanium wall is analyzed with and without injection, and the results are compared to previous work on a smooth, solid wall and a porous, sintered-metal wall at the same conditions. Results for the case of no injection show that the perforated titanium and porous, sintered metal walls have local skin friction coefficients that

F. S. Collier Jr.; J. A. Schetz

1983-01-01

349

A Model for Static and Dry Friction  

NASA Astrophysics Data System (ADS)

It will be shown that the Muser-Robbins (MR) model, consisting of mobile molecules trapped between two incommensurate crystalline solids, exhibits many of the qualitative features of friction between macroscopic solids, such as the result that the static friction is greater than the kinetic friction, stick-slip motion and a force of static friction which increases as a function of the time that the two solids are in contact and stationary. At zero temperature, the kinetic friction is highly sensitive to the direction of sliding, but this sensitivity decreases markedly as the temperature rises. At low temperatures (with the surfaces stationary for a relatively long time), the model gives a static friction approximately 3 times larger than the kinetic friction for sufficiently slow velocities, but this ratio decreases steadily as the temperature is increased.

Daly, Christopher

2005-03-01

350

Education in an Age of Social Turbulence (A Roundtable)  

ERIC Educational Resources Information Center

|The latest scheduled Sorokin Readings on "Global Social Turbulence and Russia," a topic whose relevance has been confirmed by events of the past 10 years, were held on 6-7 December at Moscow State University. One key factor that keeps such turbulence in check is the education level as a factor of a high standard of living. The array of problems…

Russian Education and Society, 2012

2012-01-01

351

Astrophysical Hydromagnetic Turbulence  

NASA Astrophysics Data System (ADS)

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

Brandenburg, A.; Lazarian, A.

2013-08-01

352

The effect of surface friction on the development of tropical cyclones  

NASA Astrophysics Data System (ADS)

When tropical cyclones (hereafter referred as TCs) are over the ocean, surface friction plays a dual role in the development of TCs. From the viewpoint of water vapor supply, frictional convergence and Ekman pumping provide a source of moisture for organized cumulus convection and is propitious to the spin-up of TCs. On the other hand, surface friction leads to a dissipation of kinetic energy that impedes the intensification of TCs. Which role is dominant in the developing stage of TCs is a controversial issue. In the present work, the influence of surface friction on the growth of TCs is re-examined in detail by conducting two sets of numerical experiments initialized with different cyclonic disturbances. Results indicate that, because of the inherent complexities of TCs, the impact of surface friction on the evolution of TCs can not be simply boiled down to being positive or negative. In the case that a TC starts from a low-level vortex with a warm core, surface friction and the resultant vertical motion makes an important contribution to the convection in the early developing stage of the TC by accelerating the build-up of convective available potential energy (CAPE) and ensuring moisture supply and the lifting of air parcels. This effect is so prominent that it dominates the friction-induced dissipation and makes surface friction a facilitative factor in the spin-up of the TC. However, for a TC formed from a mesoscale convective vortex (MCV) spawned in a long-lasting mesoscale convective system (MCS), the initial fields, and especially the low-level humidity and cold core, enable the prerequisites of convection (i.e., conditional instability, moisture, and lifting), to be easily achieved even without the help of boundary-layer pumping induced by surface friction. Accordingly, the reliance of the development of TCs on surface friction is not as heavy as that derived from a lowlevel vortex. The positive effect of surface friction on the development of TCs realized through facilitating favorable conditions for convection is nearly cancelled out by the friction-induced dissipation. However, as SST is enhanced in the latter case, the situation may be changed, and different development speeds may emerge between model TCs with and without surface friction considered. In short, owing to the fact that TC development is a complicated process affected by many factors such as initial perturbations, SST, etc., the importance of surface friction to the intensification of TCs may vary enormously from case to case.

Fang, Juan; Tang, Jianping; Wu, Rongsheng

2009-11-01

353

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

354

Frictional constraints on crustal faulting  

NASA Astrophysics Data System (ADS)

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 heterogeneous 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, John; Cocco, Massimo

1996-06-01

355

Wind Turbulence Characteristics Analysis of Near Sea Area, Rudong, Jiangsu Province, China  

Microsoft Academic Search

The wind characteristics of coastal area are analyzed with the whole year observations of 2005 at a wind observation tower locating at Rudong County, Jiangsu Province, China. In this area both turbulence intensity and gust factor vary with time, height and wind speed. Gust factor and turbulence intensity share a strong linear relationship. Turbulence intensity is of medium-level at low

Yan Chen; Xiazhen Xu

2009-01-01

356

The evaluation of a turbulent loads characterization system  

SciTech Connect

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

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

1996-01-01

357

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

358

Stabilizing Stick-Slip Friction  

Microsoft Academic Search

Even the most regular stick-slip frictional sliding is always stochastic, with irregularity in both the intervals between slip events and the sizes of the associated stress drops. Applying small-amplitude oscillations to the shear force, we show, experimentally and theoretically, that the stick-slip periods synchronize. We further show that this phase locking is related to the inhibition of slow rupture modes

Rosario Capozza; Shmuel M. Rubinstein; Itay Barel; Michael Urbakh; Jay Fineberg

2011-01-01

359

Numerical simulation of premixed turbulent methane combustion  

SciTech Connect

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

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

2001-12-14

360

Force-assembly with friction  

SciTech Connect

Previously, force-assembly has been defined as an assembly process for which a single admittance control law (i.e., a single nominal velocity in conjunction with a single mapping of forces to motions) can guarantee the proper assembly of a given pair of mating parts. As a testbed application of force-assembly, the insertion of a workpiece into a fixture consisting of multiple rigid fixture elements (fixels) is addressed. Previous work in this area has shown that, when workpiece/fixture contact is frictionless and positional error is infinitesimal, there always exists an admittance control law that will ensure the proper insertion of a workpiece into a deterministic fixture. When workpiece/fixture contact is frictionless, the workpiece/fixture contact force contains the relative positional information required to identify error-reducing motions. Friction between the workpiece and fixture, however, provides a disturbance to the geometrical information contained in the contact force. This paper addresses: (1) the identification of the conditions that must be satisfied for force-assembly with friction, and (2) the formulation and results of an optimization of the admittance control law to obtain the maximum value of friction that will satisfy the force-assembly conditions for a given workpiece/fixture combination. Results indicate that force-assembly fails when the contact forces are no longer characteristic. Forces are characteristic if the possible contact forces that may occur for one type of misalignment are unique to that type of misalignment.

Schimmels, J.M. (Marquette Univ., Milwaukee, WI (United States)); Peshkin, M.A. (Northwestern Univ., Evanston, IL (United States). Dept. of Mechanical Engineering)

1994-08-01

361

Simulation of the turbulent-energy redistribution in a diluted polymer solution  

NASA Astrophysics Data System (ADS)

A model of transfer of the Reynolds stresses in a turbulent flow of a low-concentration aqueous polymer solution, taking into account the weight of the polymer and its concentration in this solution, is proposed. The influence of a polymer additive in such a solution on the turbulent diffusion and the redistribution of turbulent energy between the Reynolds-stress tensor components was numerically investigated. It is shown that a high-molecular-weight polymer additive in the indicated flow plays a crucial role in changing the intensity of the energy exchange between the turbulent-stress components, which manifests itself as an increase in the anisotropy of the solution and a decrease in the turbulent-energy production and leads to a decrease in the friction drag in the flow.

Voropaev, G. A.; Dimitrieva, N. F.

2013-01-01

362

On the asymptotic state of high Reynolds number, smooth-wall turbulent flows  

NASA Astrophysics Data System (ADS)

It is argued that the extrapolation of the log-wake law for the mean turbulent velocity profile to arbitrarily large Reynolds numbers, and also the similarity scaling for the intensity of stream-wise turbulent velocity fluctuations indicated by recent experimental measurements, are consistent with the hypothesis that smooth-wall turbulence is asymptotically transitory in the sense that these fluctuations almost everywhere decay with respect to the outer velocity scale when 1/log (Re?) << 1, where Re? is the Reynolds number based on the skin-friction velocity u?. The existence of one or more near-wall maxima in these turbulent velocity fluctuations whose value may grow with Re?, does not invalidate the main scaling arguments. At gigantic Re?, this paradigm suggests that nonlinear motions and ``turbulent'' energy production are still present immediately adjacent to the wall, but that their amplitude becomes vanishingly small compared to the outer velocity scale.

Pullin, D. I.; Inoue, M.; Saito, N.

2013-01-01

363

Instabilities and turbulence-like dynamics in an oppositely driven binary particle mixture  

NASA Astrophysics Data System (ADS)

Using extensive particle-based simulations, we investigate out-of-equilibrium pattern dynamics in an oppositely driven binary particle system in two dimensions. A surprisingly rich dynamical behavior including lane formation, jamming, oscillation and turbulence-like dynamics is found. The ratio of two friction coefficients is a key parameter governing the stability of lane formation. When the friction coefficient transverse to the external force direction is sufficiently small compared to the longitudinal one, the lane structure becomes unstable to shear-induced disturbances, and the system eventually exhibits a dynamical transition into a novel turbulence-like phase characterized by random convective flows. We numerically construct an out-of-equilibrium phase diagram. Statistical analysis of complex spatio-temporal dynamics of the fully nonlinear turbulence-like phase suggests its apparent reminiscence to the swarming dynamics in certain active matter systems.

Ikeda, Masahiro; Wada, Hirofumi; Hayakawa, Hisao

2012-09-01

364

Turbulence in MESA-2D.  

National Technical Information Service (NTIS)

The MESA-2D Eulerian, compressible flow code has been extended to include the effects of turbulence. In this turbulence model, transport equations for the turbulent kinetic energy k, the turbulent scale s, and the turbulence mass flux components a(sub i) ...

B. Daly

1992-01-01

365

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

NASA Astrophysics Data System (ADS)

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

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

2007-12-01

366

Interaction of turbulence with oblique MHD shocks  

NASA Astrophysics Data System (ADS)

An analytic model developed by Ao et al.[1] describes the interaction of turbulence with a thin MHD shock. The interaction is governed by a modified Burgers' equation. In this paper, we solve the modified Burgers' equation numerically for arbitrary oblique MHD shocks. Starting with the classical ideal MHD Rankine-Hugoniot condition, we introduce a perturbed upstream and demonstrate the evolution of the oblique MHD shock structure. We also show the dependence of shock-turbulence interaction on different oblique angles. Our results show that turbulence plays a very important role in modifying shock front structure for oblique MHD shocks. When turbulence is modifying shock front structure, the oblique angle is another important factor. Larger oblique angle turns out to be able to creat a wider smoothed ramp in some regions and higher compression ratio in other regions.

Ao, X.; Zank, G. P.

2009-11-01

367

Effect of trace moisture on friction  

NASA Astrophysics Data System (ADS)

Although the effects of humidity on friction have been widely studied, much less is known about friction behavior when only trace level moisture is present on a surface. Trace moisture is particularly important for hydrophilic surfaces because such surfaces will be often terminated with dissociated water even if they are transferred to nominally dry conditions. Here we demonstrate that friction of hydrophilic surfaces increases in the presence of trace moisture due to formation of hydrogen bonds across the interface. Using first principle calculations we discover that frictional instability during sliding is related to rotation of hydroxyl groups, which are bonded across the interface via hydrogen bonding. Using theoretical analysis based on Prandtl-Tomlinson's model we quantify free energy released during such instabilities and we predict that velocity dependence of trace moisture friction follows the same trend as dry friction.

Liu, Yun; Szlufarska, Izabela

2010-03-01

368

Granular Brownian motion with dry friction  

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

369

Switching Atomic Friction by Electrochemical Oxidation.  

PubMed

Friction between the sliding tip of an atomic force microscope and a gold surface changes dramatically upon electrochemical oxidation of the gold surface. Atomic-scale variations of the lateral force reveal details of the friction mechanisms. Stick-slip motion with atomic periodicity on perfect Au(111) terraces exhibits extremely low friction and almost no dependence on load. Significant friction is observed only above a load threshold at which wear of the surface is initiated. In contrast, irregular stick-slip motion and a linear increase of friction with load are observed on electrochemically oxidized surfaces. The observations are discussed with reference to the amorphous structure of the oxo-hydroxide surface and atomic place exchange mechanisms upon oxidation. Reversible, fast switching between the two states of friction has been achieved in both perchloric and sulfuric acid solutions. PMID:21314153

Labuda, Aleksander; Hausen, Florian; Gosvami, Nitya Nand; Gru?tter, Peter H; Lennox, R Bruce; Bennewitz, Roland

2011-02-11

370

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

371

Physically Meaningful Harmonization of Tire\\/Pavement Friction Measurement Devices  

Microsoft Academic Search

Accurate characterization and evaluation of tire\\/pavement friction is critical in assuring runway and highway safety. Historically, Pavement Friction Measurement Devices (PFMDs) employing different measuring mechanisms have been used to evaluate tire\\/pavement friction. They yield significantly disparate friction coefficients under the same contact conditions. Currently, an empirically developed data harmonization method based on a reference device (Dynamic Friction Tester (DFT)) is

Madhura Priyanga Nishshanke Rajapakshe

2011-01-01

372

Frictional behavior of large displacement experimental faults  

Microsoft Academic Search

The coefficient of friction and velocity dependence of friction of initially bare surfaces and 1-mm-thick simulated fault gouges (< 90 gm) of Westerly granite were determined as a func- tion of displacement to >400 mm at 25øC and 25 MPa normal stress. Steady state negative friction velocity dependence and a steady state fault zone microstructure are achieved after --18 mm

N. M. Beeler; T. E. Tullis; M. L. Blanpied

1996-01-01

373

Engagement behavior of degrading wet friction clutches  

Microsoft Academic Search

This paper presents modeling, simulation and experimental validation of the friction material degradation effects on the dynamic engagement behavior of a wet friction clutch system. The clutch system is modeled as a 3 DOF lumped-mass-spri ng-damper incorporating an extension of the Generalized Maxwell Slip (GMS) friction model. The effects of the dynamic sliding velocity (relative velocity between the input and

Agusmian Partogi Ompusunggu; Thierry Janssens; Farid Al-Bender; Paul Sas; Hendrik Van Brussel

2011-01-01

374

Thermal activation in boundary lubricated friction  

Microsoft Academic Search

The friction coefficients for copper pairs lubricated with fatty acids and fluorinated fatty acids have been measured over a wide range of sliding speeds and temperatures. Sliding speeds in the range 10?7?10?2 m s?1 and temperatures in the range 4.2–300 K were used. The friction coefficients near 300 K are generally low and increase with sliding speed, while the friction

P. C. Michael; E. Rabinowicz; Y. Iwasa

1996-01-01

375

On the modelling of Coulomb friction  

NASA Astrophysics Data System (ADS)

This paper analyses two different representations of Coulomb friction in the context of a dynamic simulation of the torsional vibrations of a driven drill-string. A simple model is used to compare the relative merits of a piecewise analytic approach using a discontinuous friction profile to a numerical integration using a smooth nonlinear representation of the Coulomb friction. In both cases the effects of viscous damping on the excitation of torsional relaxation oscillations are exhibited.

Cull, S. J.; Tucker, R. W.

1999-03-01

376

Friction and wear characteristics of polymer-matrix friction materials reinforced by brass fibers  

NASA Astrophysics Data System (ADS)

This study is an investigation of friction materials reinforced by brass fibers, and the influence of the organic adhesion agent, cast-iron debris, brass fiber, and graphite powder on the friction-wear characteristics. Friction and wear testing was performed on a block-on-ring tribometer (MM200). The friction pair consisted of the friction materials and gray cast iron (HT200). The worn surface layers formed by sliding dry friction were examined using scanning electron microscopy (SEM), x-ray energy-dispersive analysis (EDX), and differential thermal analysis-thermogravimetric analysis (DTA-TAG). The experimental results showed that the friction coefficient and the wear loss of the friction materials increased with the increase of cast-iron debris, but decreased with the increase of graphite powder content. The friction coefficient and wear loss also increased slightly when the mass fraction of brass fibers was over 19%. When the mass fraction of organic adhesion agent was about 10 11%, the friction materials had excellent friction-wear performance. Surface heating from friction pyrolyzes the organic ingredient in the worn surface layer of the friction materials, with the pyrolysis depth being about 0.5 mm. The surface layers were rich in iron but poor in copper, and they were formed on the worn surface of the friction material. When the mass fraction of brass fibers was about 16 20%, the friction materials possessed better wear resistance and a copper transfer film formed on the friction surface of counterpart. Fatigue cracks were also found in the worn surface of the gray cast-iron counterpart, with fatigue wear being the prevailing wear mechanism.

Xian, Jia; Xiaomei, Ling

2004-10-01

377

Resistive MHD turbulence  

SciTech Connect

Plasma turbulence studies based on fluid equations are useful to develop an understanding of key concepts in magnetized plasma turbulence theory, and they have the potential to explain the plasma edge fluctuations and anomalous transport. Computers with the numerical resolution required for carrying out the necessary computational studies have only recently become available. The coupling between analytical and computational studies has led in recent years to progress in building up turbulence theory and testing renormalization prescriptions. In this paper, the resistive pressure-gradient-driven turbulence is reviewed to illustrate these points. An extended version of this paper will be published in Fizica Plazmy.

Carreras, B.A.

1990-01-01

378

Active Wall Motion for the Skin-Friction Drag Reduction  

NASA Astrophysics Data System (ADS)

In this study, we investigate a possibility of reducing the skin-friction drag in a turbulent channel flow with active wall motion using direct numerical simulation at Re_? = 140. The strategy of moving the wall is based on the successful active control strategy used by Choi, Moin & Kim (1994, JFM). They prescribed the blowing/suction at the wall to be exactly opposite to the wall-normal velocity at y^+ ~= 10 and obtained about 25% drag reduction. In the present study, the wall is locally deformed such that the induced velocity by the wall motion matches the wall-normal velocity at y^+ ~= 10 with an opposite sign, while the amount of maximum wall deformation is limited to be 5 in wall units. In order to effectively address the small amplitude of wall deformation, the Navier-Stokes equations are coordinate-transformed with an approximation of Taylor-series truncation. Results show that overall 17% drag reduction is obtained with active wall motion, and turbulence intensities and near-wall streamwise vortices are significantly weakened. It is also remarkable that instantaneous wall shapes are elongated in the streamwise direction and resemble riblets in appearance. However, the spanwise spacing of the wall peak-to-peak is about 80 - 90 in wall units, indicating that the mechanism of present drag reduction is essentially different from that of riblets. Wall motions based on open-loop control strategies are underway and the results will be also presented.

Kang, Sangmo; Choi, Haecheon

1999-11-01

379

Frictional Properties of Automatic Transmission Fluids: Part II—Origins of Friction–Sliding Speed Behavior  

Microsoft Academic Search

A wet clutch in an automatic transmission has very specific and unusual friction characteristics. For the clutch to operate efficiently and without stick–slip, friction must both increase with sliding speed and be high over the whole sliding speed range. This is achieved by the use of a very rough friction material, which inhibits fluid film formation, combined with sophisticated design

M. Ingram; J. Noles; R. Watts; S. Harris; H. A. Spikes

2010-01-01

380

Pipe friction head loss in transportation of high-concentration sludge for centralized solids treatment.  

PubMed

The objective of this study was to develop a method of predicting pipe friction head loss, which is a key factor in pipeline transportation of high-concentration sludge, thereby optimizing design for pipeline systems. The study involved both experimentation and analysis of pipeline transportation. Moreover, the study led to the representation of pipe friction head loss as functions of average velocity, transportation distance, pipe diameter, pipe interior roughness, and solids viscosity and temperature. PMID:11765991

Murakami, H; Katayama, H; Matsuura, H

381

Correlation of two-phase friction for refrigerants in small-diameter tubes  

Microsoft Academic Search

Single-phase and adiabatic two-phase flow pressure drop were measured for R-134A, R-22 and R-404A flowing in a multi-port extruded aluminum tube with hydraulic diameter of 2.13 mm, and in two copper tubes having inside diameters of 6.25 and 3.25 mm, respectively. The single-phase friction factor was predicted within ±10% using the Blasius friction correlation. The Friedel correlation did not predict

Ming Zhang; Ralph L. Webb

2001-01-01

382

Dynamic Mechanical Properties of Polymer-Lining and Their Effect on Coefficient of Friction  

Microsoft Academic Search

\\u000a In order to obtain the dynamic and thermomechanical properties of polymer-lining and their effect on coefficient of friction\\u000a (COF) between lining and wire rope, dynamic mechanical analysis (DMA) and friction experiment were performed on the polymer-linings\\u000a of GM-3 and K25. The effects of temperature and frequency on storage modulus (E1), loss modulus (E2) and loss factor (tan?) were obtained by

Yuxing Peng; Zhencai Zhu; Guoan Chen

383

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

384

Fractional trajectories: Decorrelation versus friction  

NASA Astrophysics Data System (ADS)

The fundamental connection between fractional calculus and subordination processes is explored and affords a physical interpretation of a fractional trajectory, that being an average over an ensemble of stochastic trajectories. Heretofore what has been interpreted as intrinsic friction, a form of non-Markovian dissipation that automatically arises from adopting the fractional calculus, is shown to be a manifestation of decorrelations between trajectories. We apply the general theory developed herein to the Lotka-Volterra ecological model, providing new insight into the final equilibrium state. The relaxation time to achieve this state is also considered.

Svenkeson, A.; Beig, M. T.; Turalska, M.; West, B. J.; Grigolini, P.

2013-11-01

385

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

386

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

NASA Astrophysics Data System (ADS)

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

Ansari, N.; Ashurst, W. R.

2012-02-01

387

Influence of epidermal hydration on the friction of human skin against textiles.  

PubMed

Friction and shear forces, as well as moisture between the human skin and textiles are critical factors in the formation of skin injuries such as blisters, abrasions and decubitus. This study investigated how epidermal hydration affects the friction between skin and textiles.The friction between the inner forearm and a hospital fabric was measured in the natural skin condition and in different hydration states using a force plate. Eleven males and eleven females rubbed their forearm against the textile on the force plate using defined normal loads and friction movements. Skin hydration and viscoelasticity were assessed by corneometry and the suction chamber method, respectively.In each individual, a highly positive linear correlation was found between skin moisture and friction coefficient (COF). No correlation was observed between moisture and elasticity, as well as between elasticity and friction. Skin viscoelasticity was comparable for women and men. The friction of female skin showed significantly higher moisture sensitivity. COFs increased typically by 43% (women) and 26% (men) when skin hydration varied between very dry and normally moist skin. The COFs between skin and completely wet fabric were more than twofold higher than the values for natural skin rubbed on a dry textile surface.Increasing skin hydration seems to cause gender-specific changes in the mechanical properties and/or surface topography of human skin, leading to skin softening and increased real contact area and adhesion. PMID:18331977

Gerhardt, L-C; Strässle, V; Lenz, A; Spencer, N D; Derler, S

2008-11-01

388

Numerical analyzing physical essence of sliding friction between molecularly smooth surfaces with zero separations  

NASA Astrophysics Data System (ADS)

Sliding friction at nanometer scale has become an important factor in the miniaturization of moving components in which the empirical laws of friction do not always hold. This paper carries out large-scale parallel molecular dynamics simulation on sliding friction at atomic scale to uncover the special physical essence. The simulation result shows that both of the elastic deformation normal to the sliding direction at the local area and the edge dislocation are generated in the bulk materials. The successive generation and fracture of the molecular junction between a friction pair ought to be the key reason for the variation of friction force. Furthermore, end failure is observed under both larger or smaller sliding velocity although it does not increase linearly by increasing velocity, that is to say, the end failure decreased if the velocity was larger than some critical value. Different kinds of materials transfer mode is observed in the friction process under different sliding velocity because of the different cross-section of the interaction surfaces. The stick-slip feature of this nanoscale sliding friction with zero separation is different from what was observed at a microscale and should be the integral effect of different control mechanism and the corresponding dynamic behavior.

Han, Xuesong

2012-12-01

389

Suppression of electronic friction on Nb films in the superconducting state  

NASA Astrophysics Data System (ADS)

Investigations on the origins of friction are still scarce and controversial. In particular, the contributions of electronic and phononic excitations are poorly known. A direct way to distinguish between them is to work across the superconducting phase transition. Here, non-contact friction on a Nb film is studied across the critical temperature TC using a highly sensitive cantilever oscillating in the pendulum geometry in ultrahigh vacuum. The friction coefficient ? is reduced by a factor of three when the sample enters the superconducting state. The temperature decay of ? is found to be in good agreement with the Bardeen-Cooper-Schrieffer theory, meaning that friction has an electronic nature in the metallic state, whereas phononic friction dominates in the superconducting state. This is supported by the dependence of friction on the probe-sample distance d and on the bias voltage V. ? is found to be proportional to d-1 and V2 in the metallic state, whereas ?˜d-4 and ?˜V4 in the superconducting state. Therefore, phononic friction becomes the main dissipation channel below the critical temperature.

Kisiel, Marcin; Gnecco, Enrico; Gysin, Urs; Marot, Laurent; Rast, Simon; Meyer, Ernst

2011-02-01

390

Generalized Similarity in Finite Range Solar Wind Magnetohydrodynamic Turbulence  

SciTech Connect

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

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

2009-12-11

391

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

392

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

393

Evaluation of flush mounted hot-film sensors for skin friction reduction measurements in viscoelastic polymer solutions  

Microsoft Academic Search

The performance of flush mounted hot-film sensors for mean wall shear stress measurement in turbulent flows of dilute drag reducing polymer solution is evaluated. A series of pipe flow experiments were conducted to compare the level of skin friction reduction measured by hot-film sensors with values determined from the pipe pressure drop. Water calibrated hot-film sensors consistently underestimate the wall

W. L. Harbison; H. L. Petrie

1990-01-01

394

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

395

30 CFR 57.19014 - Friction hoist overtravel protection.  

Code of Federal Regulations, 2010 CFR

...2009-07-01 2009-07-01 false Friction hoist overtravel protection. 57.19014...Personnel Hoisting Hoists § 57.19014 Friction hoist overtravel protection. In a friction hoist installation, tapered guides or...

2009-07-01

396

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

397

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

398

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

399

30 CFR 56.19008 - Friction hoist synchronizing mechanisms.  

Code of Federal Regulations, 2010 CFR

...2009-07-01 2009-07-01 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...

2009-07-01

400

30 CFR 57.19008 - Friction hoist synchronizing mechanisms.  

Code of Federal Regulations, 2010 CFR

...2009-07-01 2009-07-01 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...

2009-07-01

401

30 CFR 56.19014 - Friction hoist overtravel protection.  

Code of Federal Regulations, 2010 CFR

...2009-07-01 2009-07-01 false Friction hoist overtravel protection. 56.19014...Personnel Hoisting Hoists § 56.19014 Friction hoist overtravel protection. In a friction hoist installation, tapered guides or...

2009-07-01

402

40 CFR 1066.260 - Parasitic friction compensation evaluation.  

Code of Federal Regulations, 2013 CFR

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

2013-07-01

403

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

404

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

405

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

406

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

407

Turbulence and heat exchange in condensing vapor-liquid flow  

NASA Astrophysics Data System (ADS)

Turbulence and heat exchange during condensation of a vapor stream countercurrently flowing to a subcooled liquid stream in a slightly inclined channel has been investigated by direct numerical simulation (DNS). Condensation rates and imposed pressure gradients have been varied, and capillary-gravity waves have been allowed to develop at the (deformable) vapor-liquid interface. These simulations extend our previous DNS of turbulence and scalar exchange in stratified gas-liquid flows without condensation. The previous studies indicated that for conditions in which the gas-liquid interface remained continuous, i.e., did not ``break,'' scalar exchange rates on both the gas and liquid sides were largely determined by sweeps which brought high momentum fluid from the bulk flow to the interface. As sweep frequencies were found to scale with interfacial friction velocities, scalar exchange coefficients could be parametrized with a surface renewal theory. The issue addressed in the current work is how these findings are altered by condensation which acts somewhat like suction through a wall on the vapor side and injection through a wall on the liquid side. Both suction and injection have been found to affect shear stresses, turbulence characteristics, and scalar exchange rates, and hence similar effects might be expected during condensation. The present simulations indicate that the turbulence characteristics in both phases are affected, with turbulence intensities and Reynolds stresses being enhanced on the vapor side and attenuated on the liquid side. For a given imposed pressure gradient, the interfacial shear stress decreases as a result of the interfacial momentum exchange due to condensation. Interfacial waves are also found to be damped by condensation and the streamwise vortical structures on the liquid side are attenuated. The frequencies of sweeps and ejections, however, do scale with the interfacial friction velocity, reduced due to condensation, as does the liquid-side heat transfer coefficient. The simulations indicate that the scaling relationship between the interfacial friction velocity and the liquid-side heat transfer coefficient is similar to that in the absence of condensation, although the interfacial friction velocity itself is different, being dependent on condensation rates. As condensation rates depend in turn on the liquid-side heat transfer, their prediction becomes a coupled problem. A procedure for determining condensation rates as a function of imposed pressure gradient and liquid subcooling is derived from the simulations.

Lakehal, Djamel; Fulgosi, Marco; Banerjee, Sanjoy; Yadigaroglu, George

2008-06-01

408

Effects of alumina in nonmetallic brake friction materials on friction performance  

Microsoft Academic Search

The effects of alumina (Al2O3) as an abrasive on brake friction performance and friction layers of nonmetallic brake friction materials were evaluated.\\u000a Five experimental compositions containing from 0 to 14.6 vol% alumina were tested (Al2O3—0, 3.4, 5.6, 9.0, and 14.6 vol%). The experimental results indicated that alumina enhances friction coefficient and improves\\u000a wear performance. The formation and development of friction layers were

Vladimír Tomásek; Gabriela Kratosová; Rongping Yun; Yanli Fan; Yafei Lu

2009-01-01

409

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

410

Global variation of meteor trail plasma turbulence  

NASA Astrophysics Data System (ADS)

We present the first global simulations on the occurrence of meteor trail plasma irregularities. These results seek to answer the following questions: when a meteoroid disintegrates in the atmosphere, will the resulting trail become plasma turbulent? What are the factors influencing the development of turbulence? and how do these trails vary on a global scale? Understanding meteor trail plasma turbulence is important because turbulent meteor trails are visible as non-specular trails to coherent radars. Turbulence also influences the evolution of specular radar meteor trails; this fact is important for the inference of mesospheric temperatures from the trail diffusion rates, and their usage for meteor burst communication. We provide evidence of the significant effect that neutral atmospheric winds and ionospheric plasma density have on the variability of meteor trail evolution and on the observation of non-specular meteor trails. We demonstrate that trails are far less likely to become and remain turbulent in daylight, explaining several observational trends for non-specular and specular meteor trails.

Dyrud, L. P.; Urbina, J.; Fentzke, J. T.; Hibbit, E.; Hinrichs, J.

2011-12-01

411

Wall turbulence manipulation by large-scale streamwise vortices  

NASA Astrophysics Data System (ADS)

This paper describes an experimental study of the manipulation of a fully developed turbulent channel flow through large-scale streamwise vortices originated by vortex generator jets distributed along the wall in the spanwise direction. Apart from the interest in flow management itself, an important aim of the research is to observe the response of the flow to external perturbations as a technique for investigating the structure of turbulence. Considerable mean and fluctuating skin friction reductions, locally as high as 30% and 50% respectively, were measured for an optimal forcing flow intensity. Mean and fluctuating velocity profiles are also greatly modified by the manipulating large-scale vortices; in particular, attenuation of the turbulence intensity was measured. Moreover the flow manipulation caused an increase in longitudinal coherence of the wall organized motions, accompanied by a reduced frequency of burst events, demonstrated by a reduction of the velocity time derivative PDFs and by an higher intermittency. A strong transversal periodic organization of the flow field was observed, including some typical behaviours in each of the periodic boxes originated by the interaction of the vortex pairs. Results are interpreted and discussed in terms of management of the near-wall turbulent structures and with reference to the wall turbulence regeneration mechanisms suggested in the literature.

Iuso, Gaetano; Onorato, Michele; Spazzini, Pier Giorgio; di Cicca, Gaetano Maria

2002-12-01

412

Turbulent Bed Cooling Tower.  

National Technical Information Service (NTIS)

The purpose of this work is to determine whether the turbulent bed contactor (TBC), a relatively new and efficient device commonly used for gas scrubbing, can be proven as a competitive cooling system in electric power generation. The turbulent bed employ...

R. G. Barile

1975-01-01

413

Clear air turbulence detection  

Microsoft Academic Search

Although it is difficult to describe the nature of clear air turbulence (or CAT), we know its effect¿the sudden vibration experienced by an aircraft in flight in a stormfree, cloudfree sky. It is evident that this turbulence is an eddy motion of the air that disrupts its uniform flow, a motion such as that associated with jet streams; however, we

R. T. H. Collis

1966-01-01

414

Century of Turbulence.  

National Technical Information Service (NTIS)

This paper presents a summary of research on fluid dynamics over the past 100 years. From the very beginning, there have been two major threads in turbulence research. The first concerned the calculation of the practical effects of turbulence, primarily t...

J. L. Lumley A. M. Yaglom

2000-01-01

415

Magnetohydrodynamic and Plasma Turbulence.  

National Technical Information Service (NTIS)

A review of the development in turbulence of electrically conducting media is given for the last 4-5 years. Theories of homogeneous turbulence both 'passive' and 'active' are summarized. The 'chemical' approach in hot wakes is also reviewed. The possibili...

L. S. GG. Koovasznay

1964-01-01

416

Designing LES of the High Reynolds Surface Layer to Account for Numerical Friction in the Algorithm.  

NASA Astrophysics Data System (ADS)

Numerical friction stabilizes large-eddy simulation (LES), but also impacts accuracy. We explore this issue using a theory (Brasseur & Wei 2010) where the LES is designed in a 3-parameter space that quantifies the level of friction in the SFS stress model (ReLES), the relative content of resolved to SFS stress (), and surface layer resolution. To achieve law-of-the-wall in the mean, the LES must be in the ``high-accuracy zone'' (HAZ) of the -ReLES parameter space. Using rough-wall channel flow and atmospheric boundary layer LES, we analyze simulations that are identical except for spectral vs. finite volume (FV) algorithms. Numerical friction shifts the LES away from the HAZ in the -ReLES parameter space consistent with changes in mean shear-rate. The effective low pass filter from numerical friction shifts the total stress from resolved to subfilter-scale contributions, and effect that is more apparent when the spectral version of the LES is in the HAZ. A consequence is the enhancement of streamwise coherence in turbulence structure, particularly apparent in the integral scales. We shall discuss the requirements to adjust the FV LES to match a corresponding spectral LES in the HAZ, and differences in efficiency and accuracy. Support: NSF, DOE.

Brasseur, James; Vijayakumar, Ganesh; Churchfield, Matthew; Lavely, Adam; Paterson, Eric; Moriarty, Patrick

2011-11-01

417

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…

Mungan, Carl E.

2012-01-01

418

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

419

Sensitivity studies of friction-induced vibration  

Microsoft Academic Search

Friction-induced vibration is notoriously twitchy. This paper examines the origin of the sensitivity, using a model with two linear systems coupled at a single-point sliding contact where a general linearised model for dynamic frictional force is allowed. Sensitivity and convergence studies show that system uncertainty is significant enough to affect the stability of predictions and that modes neglected from the

Tore Butlin; Jim Woodhouse

2009-01-01

420

Review of automotive brake friction materials  

Microsoft Academic Search

The gradual phasing-out of asbestos in automotive brake friction materials in many parts of the world has sparked the onset of extensive research and development into safer alternatives. As a result, the brake friction industry has seen the birth of different brake pads and shoes in the past decade, each with their own unique composition, yet performing the very same

D Chan; G W Stachowiak

2004-01-01

421

On Linear Friction in Lagrange's Equation  

Microsoft Academic Search

A linear frictional force in the equation of motion of a particle is obtained if one modifies the Lagrange equation of motion by the addition of the frictional term, which may be expressed in terms of the Rayleigh dissipative function. It is shown that one can construct Lagrangians in several simple cases so that the Lagrange equation of motion contains

Harry H. Denman

1966-01-01

422

Monitoring frictional strength with acoustic wave transmission  

Microsoft Academic Search

We monitored acoustic transmissivity of a frictional interface during laboratory experiments where complex slip history was imposed. Frictional strength, which was also tracked continuously, showed complex changes due to various causes such as aging of the interface in quasi-stationary contact and slip weakening that began with miniscule pre-failure slip. Albeit such complexity, acoustic transmissivity was found to be a unique

Kohei Nagata; Masao Nakatani; Shingo Yoshida

2008-01-01

423

Friction of drill bits under Martian pressure  

Microsoft Academic Search

Frictional behavior was investigated for two materials that are good candidates for Mars drill bits: Diamond Impregnated Segments and Polycrystalline Diamond Compacts (PDC). The bits were sliding against dry sandstone and basalt rocks under both Earth and Mars atmospheric pressures and also at temperatures ranging from subzero to over 400 °C. It was found that the friction coefficient dropped from

K. A. Zacny; G. A. Cooper

2007-01-01

424

Friction and Wear of Refractory Compounds  

Microsoft Academic Search

The sliding characteristics of the borides, carbides, nitrides, and silicides and oxides of several metals were investigated in air at temperatures up to 2000 F. Tests were of the crossed-cylinder type and friction and wear effects measured under repetitive sliding conditions.Correlation of the friction and wear characteristics with known or predicted solid solubility was only fair. High hardness coupled with

K. P. Zeman; L. F. Coffin Jr

1960-01-01

425

Anisotropy of frictional forces in muscovite mica  

Microsoft Academic Search

The static and dynamic frictional forces of single-crystal muscovite mica are measured as a function of the lattice misfit angle between the two contacting cleavage lattices at a very light load under both dry and ambient atmospheres. The frictional forces are anisotropic with respect to the lattice misfit angle, i.e., they increase (decrease) when the contacting surfaces approach being commensurate

Motohisa Hirano; Kazumasa Shinjo; Reizo Kaneko; Yoshitada Murata

1991-01-01

426

Metallic Contact and Friction between Sliding Surfaces  

Microsoft Academic Search

A new device has been developed and used to study metallic contact and friction between sliding, lubricated surfaces. The system consists basically of a fixed metal ball loaded against a rotating cylinder. The extent of metallic contact is determined by measuring both the instantaneous and average electrical resistance between the two surfaces. Friction between the ball and cylinder is recorded

M. J. Furey

1961-01-01

427

ANALYSIS OF THE MAGNETIZED FRICTION FORCE.  

SciTech Connect

A comprehensive examination of theoretical models for the friction force, in use by the electron cooling community, was performed. Here, they present their insights about the models gained as a result of comparison between the friction force formulas and direct numerical simulations, as well as studies of the cooling process as a whole.

FEDOTOV, A.V.; BRUHWILER, D.L.; SIDORIN, A.O.

2006-05-29

428

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…

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

2010-01-01

429

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

430

Suppression of Friction by Mechanical Vibrations  

Microsoft Academic Search

Mechanical vibrations are known to affect frictional sliding and the associated stick-slip patterns causing sometimes a drastic reduction of the friction force. This issue is relevant for applications in nanotribology and to understand earthquake triggering by small dynamic perturbations. We study the dynamics of repulsive particles confined between a horizontally driven top plate and a vertically oscillating bottom plate. Our

Rosario Capozza; Andrea Vanossi; Alessandro Vezzani; Stefano Zapperi

2009-01-01

431

Friction compensation for an industrial hydraulic robot  

Microsoft Academic Search

A model based friction compensation scheme using a novel dynamical friction model was implemented on an industrial Schilling Titan II hydraulic robot. Off-line estimation of parameters was carried out, using the results of two kinds of experiments. These experiments were done independently at each joint. A nonlinear PI type controller was used in the inner torque loop to improve its

P. Lischinsky; C. Canudas-de-Wit; G. Morel

1999-01-01

432

ACTIVE CONTROL OF FRICTION-DRIVEN OSCILLATIONS  

Microsoft Academic Search

This paper introduces an active control technique that combats oscillations driven by dry friction forces. Dry friction can act as the excitation mechanism for some high amplitude oscillations; curve squeal from trains is a well known and notorious example. Another, more elementary, example is the oscillation of a mass-spring system sliding on a moving belt. A model which predicts the

Maria A. Heckl; I. D. Abrahams

1996-01-01

433

Numerical study of turbulent bubbly downflows in a vertical channel  

NASA Astrophysics Data System (ADS)

Direct numerical simulations are used to study turbulent bubbly downflows in a vertical channel. All flow scales, including the bubbles and the flow around them, are fully resolved using a front-tracking/finite-volume method. The turbulent bubbly channel flow is driven downward by an imposed constant pressure gradient, and the friction Reynolds number of the flow, based on the friction velocity and half-width of the channel, is 127.3, corresponding to a bulk Reynolds number of 3786 for a flow without bubbles. Three cases with several nearly spherical bubbles are examined. The bubble diameter is 31.8 wall units for all cases but the number of bubbles is varied, giving average void fractions of 1.5%, 3%, and 6%. The lift force on the bubbles drives them away from the walls until the mixture in the center of the channel is in hydrostatic equilibrium. Thus, the flow consists of a core region where the average void fraction and the mean vertical velocity are approximately constant and a bubble-free wall layer. The vertical velocity fluctuations in the wall layer decrease as the void fraction increases and the width of the wall layer decreases, but in the bubble-rich core the velocity fluctuations are higher than for a corresponding single-phase turbulent flow.

Lu, Jiacai; Tryggvason, Gretar

2006-10-01

434

Flat plate turbulent boundary-layer control using vertical LEBUs  

NASA Astrophysics Data System (ADS)

Necessity of aerodynamic drag reduction of aircrafts and other moving objects stimulates researchers for finding out new means of the near-wall turbulence control. In [1] it has been found that the vertical positioning of the LEBUs in boundary layer can be much more efficient compared to the conventional horizontal one, although, according to the same authors, the devices were far from being optimized. Present work is focused upon the study of possibility of turbulent skin-friction reduction using flow-aligned vertical LEBUs, the LEBUs being mounted perpendicular to the flat plate surface in nominally gradient-free incompressible turbulent boundary layer. The Reynolds number based on the momentum thickness of the boundary layer at the LEBUs' position was 1099. All measurements were performed using a computer-controlled automated system of space/time hot wire visualization of mean and fluctuating components of the velocity field. The system provided accuracy not worse than approximately ±2 µm along x, y, and z coordinates. Local skin friction C f in the regular (unmodified) shear flow was determined from the condition of the best correspondence between measured and and classic velocity coefficient profiles in the region of the law of the wall functionality U^+ = A log y+ + B with known coefficients A and B. In the modified boundary layer C f was determined by the mean velocity gradient at the wall (partial U/partial y)_{y=0}. The measurement technique is given in more detail in [2].

Kornilov, V. I.; Boiko, A. V.

435

Simulating video through turbulence  

NASA Astrophysics Data System (ADS)

Atmospheric turbulence can cause significant degradation to video over long horizontal paths. The refractive index fluctuations along the path from scene to camera lead to blur, varying across the frame and from frame to frame. Computationally inexpensive methods model this effect with an MTF blur function; however this technique neglects anisoplanatic effects. Wave optics techniques have been developed taking into account anisoplanatism, but ignoring scintillation, and spatial and temporal effects. Since long horizontal paths over varied surfaces (e.g. water to land or vice versa) will encounter varying turbulence strength along the path, the turbulence strength should be defined independently at each phase screen. Also important, turbulence strength can vary over short time scales (<1s), so a physically accurate simulation must allow time-dependent phase screens. We will present results of a wave optics simulation technique that includes these spatial and temporal variations. The results will provide validation for turbulence removal algorithms.

McBryde, Kevin; Gibson, Kris

2013-09-01

436

Numerical simulation of turbulent flow of hydraulic oil through 90° circular-sectional bend  

NASA Astrophysics Data System (ADS)

Oil flow through pipe bends is found in many engineering applications. However, up to now, the studies of oil flow field in the pipe bend appear to be relatively sparse, although the oil flow field and the associated losses of pipe bend are very important in practice. In this paper, the relationships between the turbulent flow of hydraulic oil in a bend and the Reynolds number Re and the curvature ratio ? are studied by using computational fluid dynamics (CFD). A particular emphasis is put on hydraulic oil, which differs from air or water, flowing through 90° circular-sectional bend, with the purpose of determining the turbulent flow characteristics as well as losses. Three turbulence models, namely, RNG k- ? model, realizable k- ? model, and Reynolds stress model (RSM), are used respectively. The simulation results in the form of contour and vector plots for all the three turbulence models for pipe bends having curvature ratio of ? = 0.5, and the detailed pressure fields and total pressure losses for different Re and ? for RSM are presented. The RSM can predict the stronger secondary flow in the bend better than other models. As Re increases, the pressure gradient changes rapidly, and the pressure magnitude increases at inner and outer wall of the bend. When ? decreases, two transition points or transition zones of pressure gradient arise at inner wall, meanwhile, the transition point moves towards the inlet at outer wall of the bend. Owing to secondary flow, the total pressure loss factor k increases as the bend tightens, on the contrary, as Re increases, factor k decreases due to higher velocity heads, and the rapid change of pressure gradient on the surface of the bend leads to increasing of friction and separation effects, and magnified swirl intensity of secondary flow. A new mathematical model is proposed for predicting pressure loss in terms of Re and ? in order to provide support to the one-dimensional simulation software. The proposed research provides reference for the analysis of oil flow with higher Re in the large bends.

Wang, Liwei; Gao, Dianrong; Zhang, Yigong

2012-09-01

437

Lubricated friction and volume dilatancy are coupled  

NASA Astrophysics Data System (ADS)

Dilation (expansion of film thickness) by approx0.1 A, which is less than one-tenth of the width of confined fluid molecules, was observed when confined films crossed from the resting state ("static friction") to sliding ("kinetic friction"). These measurements were based on using piezoelectric bimorph sensors possessing extremely high resolution for detecting position changes, during the course of sliding molecularly thin films of squalane, a model lubricant fluid, between atomically smooth single crystals of mica. Detailed inspection of energy balance shows that the dilation data and the friction forces satisfied energy conservation of identifiable energies at the slip point, from static to kinetic friction. This shows experimentally, for the first time to the best of our knowledge, a direct coupling between friction forces and decrease in the mean density of the intervening molecularly thin fluid.

Demirel, A. Levent; Granick, S.

2002-10-01

438

Large Friction Anisotropy of a Polydiacetylene Monolayer  

SciTech Connect

Friction force microscopy measurements of a polydiacetylene monolayer film reveal a 300% friction anisotropy that is correlated with the film structure. The film consists of a monolayer of the red form of N-(2-ethanol)- 10,12 pentacosadiynamide, prepared on a Langmuir trough and deposited on a mica substrate. As confirmed by atomic force microscopy and fluorescence microscopy, the monolayer consists of domains of linearly oriented conjugated backbones with pendant hydrocarbon side chains above and below the backbones. Maximum friction occurs when the sliding direction is perpendicular to the backbone. We propose that the backbones impose anisotropic packing of the hydrocarbon side chains which leads to the observed friction anisotropy. Friction anisotropy is therefore a sensitive, optically-independent indicator of polymer backbone direction and monolayer structural properties.

Burns, A.R.; Carpick, R.W.; Sasaki, D.Y.

1999-05-11

439

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.

440

Tidal Friction in the Earth and Ocean  

NASA Astrophysics Data System (ADS)

"Tidal Friction" is a classic subject in geophysics, with ties to some of the great scientists of the Victorian era. The subject has been reinvigorated over the past decade by space geodesy, and particularly by the Topex/Poseidon satellite altimeter mission. In fact, the topic has now taken on some significance in oceanography, with potential implications for problems of mixing, thermocline maintenance, and the thermohaline circulation. Likewise, tidal measurements have become sufficiently precise to reveal new information about the solid earth. In this respect, the tidal force is an invaluable "probe" of the earth, at frequencies well outside the seismic band. This talk will "follow the energy" of tides while noting some important geophysical implications at each stage. In the present earth-moon-sun configuration, energy for tides is extracted from the earth's rotation. Ancient eclipses bear witness to this, and the discrepancy between Babylonian (and other) observations and tidal predictions yields unique information about the mantle and the overlying fluid envelope. Complementary information comes from tidal anelasticity estimates, which are now available at frequencies ranging from semidiurnal to fortnightly, monthly, and 18.6 years. These data, when combined with various kinds of gravity measurements, are relevant to the present-day sea-level problem. Solid-earth tidal dissipation represents less than 5% of the system total. As has long been realized, the largest energy sink is the ocean. About 70% of the oceanic dissipation occurs in shallow seas (the traditional sink) and 30% in the deep ocean, generally near rugged bottom topography. The latter represents a substantial amount of power, roughly 1 gigawatt, available for generation of internal tides and other baroclinic motions. Experiments like HOME are helping unravel the links between barotropic tides, internal tides, turbulence, and mixing. The latter opens possible linkages to climate, and recent work involving 18.6-year oscillations adds intriguing (although not completely convincing) evidence of climate connections from the nodal modulations of diurnal-band tides. Connections at longer periods are quite conceivable, since tides are critically sensitive to sea level, but most ideas along these lines are still speculative and in need of further development.

Ray, R. D.

2006-12-01

441

Effects of ZrSiO 4 in non-metallic brake friction materials on friction performance  

Microsoft Academic Search

The effects of ZrSiO4 (zirconium silicate or zircon) as an abrasive on brake friction performance and friction layers of non-metallic brake friction materials were evaluated. The experimental results indicated that ZrSiO4 enhances friction coefficient, but depresses wear rate. However, ZrSiO4 can improve the negative wear rate of the friction materials. The formation and development of friction layers are complex so

Yuning Ma; Gražyna Simha Martynková; Marta Valášková; Vlastimil Mat?jka; Yafei Lu

2008-01-01

442

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

SciTech Connect

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

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

2009-06-15

443

In-vitro evaluation of the cytocompatibility of wear particles generated by UHMWPE/zirconia friction.  

PubMed

Cytotoxicity of wear products generated by UHMWPE/yttria partially stabilized zirconia (YPSZ) friction and UHMWPE/surface-nitrided Ti-6Al-4V friction in pseudo-extracellular fluid (PECF) at 37 degrees C was evaluated. Though the amount of abraded UHMWPE against YPSZ was almost the same as that against nitrided Ti-6Al-4V, the wear products generated by UHMWPE/YPZ friction significantly inhibited cell growth while those by UHMWPE/nitrided Ti-6Al-4V friction showed no growth inhibition. Dissolved Zr and Y ions were at least 70 times less than the amount causing growth inhibition. The cytotoxicity was caused mainly by the particles less than 0.22 mum in size. Amorphous zirconium-containing particles (5-20 nm) found in the PECF, formed presumably by stress corrosion, would be a responsible factor for the cytotoxicity. PMID:10171688

Ito, A; Tateishi, T; Niwa, S; Tange, S

1993-01-01

444

Experimental study of the wall pressure fluctuations under a turbulent boundary layer downstream of tandem aerofoil external manipulators  

Microsoft Academic Search

The boundary layer alteration downstream of a tandem of aerofoil external manipulators was evaluated by mean and turbulent characteristic measurements. The results show that the manipulated boundary layer structure remains identical to the natural one's. The local skin friction reaches a maximum of 20 percent at 36 boundary layer thicknesses downstream the device and then persists up to 100 boundary

Philippe Olivero

1990-01-01

445

On the effect of turbulence on bubbles: experiments and numerical simulations of bubbles in wall-bounded flows  

Microsoft Academic Search

Previous research has shown that the addition of gas bubbles to a turbulent boundary layer in water can reduce the local skin-friction drag by up to 80%. Application of this technique to ships seems promising, but to date no significant drag reductions are obtained on full-scale ships. More knowledge about the drag reduction mechanism is required to better understand its

M. J. W. Harleman

2012-01-01

446

Laminar turbulent boundary layers; Proceedings of the Energy Sources Technology Conference, New Orleans, LA, February 12-16, 1984  

Microsoft Academic Search

Among the topics discussed are drag and aeroacoustic noise characteristics due to the coupled roughness and blowing of surfaces, skin friction and heat transfer for combined roughness and mass addition, the effect of drag-reducing additives on the development and separation of a turbulent boundary layer with adverse pressure gradient, numerical investigations of the microbubble drag reduction mechanism, the effects of

E. M. Uram; H. E. Weber

1984-01-01

447

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. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31:1627-1632, 2013. PMID:23813771

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

2013-06-27

448

Threshold friction velocities for large pebble gravel beds  

NASA Astrophysics Data System (ADS)

Experiments have been performed in a high-speed wind tunnel to determine threshold friction velocities for the onset of continuous particle motion (the fluid threshold velocity per Bagnold [1941]) for two types of gravel soil beds. The 50% finer-by-weight particle diameter ? for the fine gravel sample was ˜6.35 mm and for the coarse gravel bed was 42 mm. Measured threshold friction velocities (u*t), as determined with rake-mounted Pitot-static probes, were 3.1 and 9.9 m/s for the fine and coarse gravel soils, respectively. Corresponding free-stream threshold velocities (u?t) were 41.8 and 94.3 m/s. Summarized results from the current measurements and other natural soil experiments [Gillette, 1978, 1988] vary approximately with the square root of particle diameter, in agreement with the prediction of Bagnold [1941] but exceed his results by a factor of 2-3.

Batt, R. G.; Peabody, S. A.

1999-10-01

449

Turbulent drag reduction by traveling wave of flexible wall  

NASA Astrophysics Data System (ADS)

A direct numerical simulation of channel turbulent flow and its control by spanwise traveling waves of flexible wall shows that such an on-wall open-loop vorticity creation control can effectively reduce the friction drag for any incompressible Newtonian fluid. The control effect is similar to that of existing transverse-wave controls by spanwise near-wall oscillating flow/wall oscillation and traveling body-force wave. The main physical mechanism is the change of boundary vorticity flux due to wall acceleration, which forces the streamwise vorticity generated at the wall to be confined in a thin generalized Stokes layer and hence alleviates the low-speed streaks, suppresses the streamwise vortices and hairpin vortices in viscous sublayer, associated with a regularized wall skin-friction pattern. A new potential is revealed toward reducing the power input in open-loop control by waves, which is worth further exploring: the optimization of imposed wave pattern.

Zhao, H.; Wu, J.-Z.; Luo, J.-S.

2004-03-01

450

Turbulent scaling in fluids  

SciTech Connect

This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The project was a study of turbulence in fluids that are subject to different body forces and to external temperature gradients. Our focus was on the recent theoretical prediction that the Kolomogorov picture of turbulence may need to be modified for turbulent flows driven by buoyancy and subject to body forces such as rotational accelerations. Models arising from this research are important in global climate modeling, in turbulent transport problems, and in the fundamental understanding of fluid turbulence. Experimentally, we use (1) precision measurements of heat transport and local temperature; (2) flow visualization using digitally- enhanced optical shadowgraphs, particle-image velocimetry, thermochromic liquid-crystal imaging, laser-doppler velocimetry, and photochromic dye imaging; and (3) advanced image- processing techniques. Our numerical simulations employ standard spectral and novel lattice Boltzmann algorithms implemented on parallel Connection Machine computers to simulate turbulent fluid flow. In laboratory experiments on incompressible fluids, we measure probability distribution functions and two-point spatial correlations of temperature T and velocity V (both T-T and V-T correlations) and determine scaling relations for global heat transport with Rayleigh number. We also explore the mechanism for turbulence in thermal convection and the stability of the thermal boundary layer.

Ecke, R.; Li, Ning; Chen, Shiyi; Liu, Yuanming

1996-11-01

451

Modeling turbulent flame propagation  

SciTech Connect

Laser diagnostics and flow simulation techniques axe now providing information that if available fifty years ago, would have allowed Damkoehler to show how turbulence generates flame area. In the absence of this information, many turbulent flame speed models have been created, most based on Kolmogorov concepts which ignore the turbulence vortical structure, Over the last twenty years, the vorticity structure in mixing layers and jets has been shown to determine the entrainment and mixing behavior and these effects need to be duplicated by combustion models. Turbulence simulations reveal the intense vorticity structure as filaments and simulations of passive flamelet propagation show how this vorticity Creates flame area and defines the shape of the expected chemical reaction surface. Understanding how volume expansion interacts with flow structure should improve experimental methods for determining turbulent flame speed. Since the last decade has given us such powerful new tools to create and see turbulent combustion microscopic behavior, it seems that a solution of turbulent combustion within the next decade would not be surprising in the hindsight of 2004.

Ashurst, W.T.

1994-08-01

452

Visualization of plasma turbulence with laser-induced fluorescence (invited)  

SciTech Connect

Turbulence is a key factor limiting the performance of fusion devices. Plasma edge turbulence determines the boundary values of the plasma density and temperature, which in turn determine the internal gradients and controls global plasma transport. In recent years, significant progress has been made in modeling turbulence behavior in plasmas and its effect on transport. Progress has also been made in diagnostics for turbulence measurement; however, there is still a large gap in our understanding of it. An approach to improve this situation is to experimentally visualize the turbulence, that is, a high resolution 2-D image of the plasma density. Visualization of turbulence can improve the connection to theory and help validate theoretical models. One method that has been successfully developed to visualize turbulence in gases and fluids is planar laser-induced fluorescence. We have recently applied this technique to visualize turbulence and structures in a plasma. This was accomplished using an Alexandrite laser that is tunable between 700 and 800 nm, and from 350 to 400 nm with second harmonic generation. The fluorescence light from an argon ion transition has been imaged onto an intensified charged coupled device camera that is gated in synchronization with the laser. Images from the plasma show a rotating structure at 30 kHz in addition to small scale turbulence.

Levinton, Fred M.; Trintchouk, Fedor

2001-01-01

453

Quantized friction across ionic liquid thin films.  

PubMed

Ionic liquids - salts in the liquid state under ambient conditions - are of great interest as precision lubricants. Ionic liquids form layered structures at surfaces, yet it is not clear how this nano-structure relates to their lubrication properties. We measured the friction force between atomically smooth solid surfaces across ionic liquid films of controlled thickness in terms of the number of ion layers. Multiple friction-load regimes emerge, each corresponding to a different number of ion layers in the film. In contrast to molecular liquids, the friction coefficients differ for each layer due to their varying composition. PMID:23942943

Smith, Alexander M; Lovelock, Kevin R J; Gosvami, Nitya Nand; Welton, Tom; Perkin, Susan

2013-08-28

454

Chemical reaction rates and solvent friction  

SciTech Connect

The role of the dynamic solvent friction in influencing the rates of chemical reactions in solution is described. Features considered include (a) the bias of the reaction coordinate toward a direction of lesser friction in the diffusive limit, (b) the importance of frequency-dependent friction in atom transfers, tunneling reactions and isomerizations, (c) the dynamic nonequilibrium solvation in charge transfers which leads to a polar solvent molecule reorientation time dependence for the rate, and (d) the importance of internal degrees of freedom in the location of the Kramers turnover for isomerizations.

Hynes, J.T.

1986-01-01

455

Effect of trace moisture on friction  

NASA Astrophysics Data System (ADS)

We report that even in nominally dry conditions trace moisture present on hydrophilic surfaces of SiC leads to a considerable increase in friction via formation of hydrogen bonds across the sliding interface. We perform quantum mechanical calculations to demonstrate that frictional instabilities on such surfaces are related to formation and breaking of hydrogen bonds between surface hydroxyl groups and to the elastic rotation of these groups. We perform a theoretical analysis based on the modified Prandtl-Tomlinson model and we predict that in the presence of trace moisture friction force will have logarithmic dependence on the sliding velocity.

Liu, Y.; Szlufarska, I.

2010-03-01

456

Measurements of friction injuries in man.  

PubMed

Repetitive frictional insults over years to human skin result in lichenification, callosites, and clavi (corns). No measurements of the forces involved have been made for callosites or corns. Two reports deal with lichenification produced by repeated rubbing over weeks. By contrast, friction blisters have been seriously studied because they can be produced in minutes and because foot blisters can disable soldiers. Three devices to measure the coefficient of friction of materials on man's skin and eight machines to rub man's skin are reviewed. PMID:4073046

Akers, W A

1985-01-01

457

Laser beam propagation through strong turbulence  

Microsoft Academic Search

The effects of strong turbulence (C2n up to 3×10?9 m?2\\/3) on laser beam propagation were measured. Beam broadening of a factor of 2, and wander over several hundred microradians were observed. Long-term beam broadening agrees with values calculated from simplified analytic models.

J. L. Barrett; P. A. Budni

1992-01-01

458

Laser beam propagation through strong turbulence  

Microsoft Academic Search

The effects of strong turbulence (C sub n super 2 up to 3 x 10 exp -9 per m exp 2\\/3) on laser beam propagation were measured. Beam broadening of a factor of 2, and wander over several hundred microradians were observed. Long-term broadening agrees with values calculated from simplified analytic models.

J. L. Barrett; P. A. Budni

1992-01-01

459

Surface roughness and friction coefficient in peened friction stir welded 2195 aluminum alloy  

NASA Astrophysics Data System (ADS)

The tribological properties of friction stir welded 2195 aluminum alloy joints were investigated for several laser- and shot-peened specimens. The first portion of this study assessed the surface roughness changes at different regions of the weld resulting from the various peening processes and included an atomic force microscopy (AFM) study to reveal fine structures. The second portion investigated the friction characteristics for various conditions when slid against a 440C ball slider. Shot peening resulted in significant surface roughness when compared to the unpeened and laser-peened samples. The initial friction for all types of specimens was highly variable. However, long-term friction was shown to be lowest for samples with no peening treatment. Laser peening caused the friction to increase slightly. The shot peening process on the other hand resulted in an increase of the long-term friction effects on both sides of the weld.

Hatamleh, Omar; Smith, James; Cohen, Donald; Bradley, Robert

2009-05-01

460

A Golden Section approach to optimization of automotive friction materials  

Microsoft Academic Search

A Golden Section approach combined with relational grade analysis is proposed as an experimental design tool helpful in the development of new automotive friction materials. Golden Section was used to design the volume fraction of the components systematically. The changes in friction performance (friction coefficient and wear) measured using Friction Assessment and Screening Test (FAST) can be correlated with component

Y. Lu

2003-01-01

461

A systems based experimental approach to tactile friction  

Microsoft Academic Search

This work focuses on the friction in contacts where the human finger pad is one of the interacting surfaces. This ‘tactile friction’ requires a full understanding of the contact mechanics and the behaviour of human skin. The coefficient of friction cannot be considered as a property of the skin alone, but depends on the entire tribo-system. In this work, frictional

M. A. Masen

462

Observer-based control strategies for compensation of dynamic friction  

Microsoft Academic Search

This paper describes and compares different observer-based control strategies for compensating the dynamic friction in controlled motion systems. The once identified system with friction requires usually an observer due to unknown disturbances and time variant friction behavior. The recently developed two-state dynamic friction model with elasto- plasticity is applied within three different types of observer, all involved in the control

Michael Ruderman; Torsten Bertram

2011-01-01

463