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1

Turbulent flow heat transfer and friction factor characteristics of shrouded fin arrays with uninterrupted fins  

Microsoft Academic Search

Experiments were performed to study the fully developed turbulent flow heat transfer and friction factor characteristics of shrouded, rectangular cross-sectioned longitudinal fin arrays with uninterrupted fins subjected to a uniform heat flux boundary condition at the fin base. The inter-fin spacing was varied from 9.3 to 53 mm, the fin height from 0 to 40 mm, and the fin tip-to-shroud

S. P. Sukhatme

1995-01-01

2

An experimental investigation into the effects turbulator profile and spacing have on heat transfer coefficients and friction factors in small cooled turbine airfoils  

Microsoft Academic Search

An experimental investigation is conducted using liquid crystals to study the effects of turbulator profile and spacing on heat transfer coefficient. Friction factors are also measured and both friction factor and heat transfer results for fifteen turbulator geometries are compared. All test configurations position the turbulators on two opposite walls of a rectangular test section in a staggered arrangement with

M. E. Taslim; S. D. Spring

1991-01-01

3

An experimental investigation into the effects turbulator profile and spacing have on heat transfer coefficients and friction factors in small cooled turbine airfoils  

NASA Astrophysics Data System (ADS)

An experimental investigation is conducted using liquid crystals to study the effects of turbulator profile and spacing on heat transfer coefficient. Friction factors are also measured and both friction factor and heat transfer results for fifteen turbulator geometries are compared. All test configurations position the turbulators on two opposite walls of a rectangular test section in a staggered arrangement with an angle of attack to the mainstream flow of 90 degrees. It is concluded that while turbulators with aspect ratios greater than unity produce higher heat transfer coefficients at the expense of higher pressure losses, 'jersey-barrier' shaped turbulators, properly spaced, are very effective in heat removal with moderate pressure losses.

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

1991-06-01

4

Friction factor in micropipe gas flow under laminar, transition and turbulent flow regime  

Microsoft Academic Search

The present work deals with the compressible flow of nitrogen gas inside microtubes ranging from 30 to 500?m and with different values of the surface roughness (<1%), for different flow regimes. The first part of the work is devoted to a benchmark of friction factor data obtained at DIENCA (University of Bologna) and the ENEA laboratories, using fused silica pipes

G. P. Celata; M. Lorenzini; G. L. Morini; G. Zummo

2009-01-01

5

Heat Transfer Through Turbulent Friction Layers  

NASA Technical Reports Server (NTRS)

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

Reichardt, H.

1943-01-01

6

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

7

Turbulence and skin friction evolutions in an oscillating boundary layer  

Microsoft Academic Search

The paper presents and analyzes experimental results on an oscillating flat plate turbulent boundary layer. Emphasis is placed on quantities, which characterize the evolution of turbulence and of the skin friction. These results have been obtained in a rather large range of reduced frequencies with (omega nu)\\/(U-tau\\/bar\\/)-squared in the interval from 0.0035 to 0.013 and (omega XO)\\/(U-e\\/bar\\/) in the interval

J. Cousteix; R. Houdeville

1985-01-01

8

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

9

Compliant wall-turbulent skin-friction reduction research  

NASA Technical Reports Server (NTRS)

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

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

1975-01-01

10

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

11

Friction  

NSDL National Science Digital Library

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

Conatser, Ryan; Kinnaman, Ben; Lassiter, Mike

2011-04-13

12

Predicting friction factor in herbaceous emergent wetlands  

NASA Astrophysics Data System (ADS)

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

Wynn-Thompson, T.; Hall, K.

2012-12-01

13

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

14

Measurements of local skin friction in a microbubble-modified turbulent boundary layer  

Microsoft Academic Search

The present local skin friction reduction measurements by means of an array of flush-mounted hot film probes in a microbubble-modified, zero-pressure gradient turbulent boundary layer have both qualitatively and quantitatively confirmed earlier integrated skin friction measurements that indicated the reduction to be a function of plate orientation, gas flow rate, and freestream velocity. While skin friction is reduced monotonically for

N. K. Madavan; S. Deutsch; C. L. Merkle

1985-01-01

15

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

16

Dynamical Contributions to the Skin Friction in Polymer Drag Reduced Wall-Bounded Turbulence  

Microsoft Academic Search

The friction drag in polymer drag reduced turbulent boundary layer flow is decomposed into four dynamical contributions. It is found that drag reduction is achieved by either, or both, an attenuation of the Reynolds stress and a reduction in the total stress gradient near the wall. Particle image velocimetry (PIV) measurements made in a turbulent developing boundary layer flow, with

Christopher White; Vijay Somandepalli; Yves Dubief; Godfrey Mungal

2006-01-01

17

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

NASA Technical Reports Server (NTRS)

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

2004-01-01

18

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

NASA Technical Reports Server (NTRS)

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

Hwang, Danny P.

1999-01-01

19

Optimal Control of Heat Transfer and Skin Friction in Wall Turbulence  

Microsoft Academic Search

Optimal control theory based on the Frechet derivative of a cost function was applied to simultaneous control of turbulent heat transfer and skin friction with local blowing\\/suction on the wall, and its performance was evaluated by using a direct numerical simulation of a turbulent channel flow. Spatio-temporal relationship between the near-wall coherent structures and the distribution of control input is

M. Yokoo; N. Kasagi; Y. Suzuki

20

Analysis of turbulent skin friction generated in flow along a cylinder  

NASA Astrophysics Data System (ADS)

This paper presents an extension of FIK identity [K. Fukagata et al., Phys. Fluids 14, L73 (2002)] to turbulent axial flow along a cylinder. This relation gives the contributions of both the mean flow and the turbulent fluctuating flow to the skin friction coefficient. The later contribution is then further decomposed more precisely as proposed by B. Frohnapfel, Y. Hasegawa, and N. Kasagi, ``Reactive Flow Control for Skin Friction Drag Reduction based on Sensing of the Streamwise Wall-Shear Stress,'' Euromech Fluid Mechanics Conference 8 (EFMC8), Bad Reichenhall, Germany, 13-16 Sept. 2010, S4-30. The Reynolds shear stress can be linked to the eigenvalues of the anisotropy tensor, the angle between the principal axis of the Reynolds stress tensor, and the mean flow direction and the turbulent kinetic energy. These eigenvalues and the alignment are important elements of the Reynolds stress profile. The present analysis is based on high-fidelity Reynolds-Stress-Model-based simulations. The results are first validated using available DNS and experimental data. Then, results are used in order to investigate the variations of the skin friction componential contributions with respect to characteristic dimensionless radius a+, Reynolds numbers, Rea (cylinder-radius-based Reynolds number) and Re? (boundary-layer-thickness-based Reynolds number), or curvature ratio ?/a, and anisotropic decomposition of the Reynolds stress. Explicit empirical formula for surface responses of skin friction and its turbulent component is given.

Monte, Stephane; Sagaut, Pierre; Gomez, Thomas

2011-06-01

21

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

22

The FIK Identity and Its Implication for Turbulent Skin Friction Control  

Microsoft Academic Search

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

Nobuhide Kasagi; Koji Fukagata

23

Flow friction of the turbulent coolant flow in cryogenic porous cables  

NASA Technical Reports Server (NTRS)

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

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

1979-01-01

24

Some effects of finite spatial resolution on skin friction measurements in turbulent boundary layers  

NASA Technical Reports Server (NTRS)

The effects of finite spatial resolution often cause serious errors in measurements in turbulent boundary layers, with particularly large effects for measurements of fluctuating skin friction and velocities within the sublayer. However, classical analyses of finite spatial resolution effects have generally not accounted for the substantial inhomogeneity and anisotropy of near-wall turbulence. The present study has made use of results from recent computational simulations of wall-bounded turbulent flows to examine spatial resolution effects for measurements made at a wall using both single-sensor probes and those employing two sensing volumes in a V shape. Results are presented to show the effects of finite spatial resolution on a variety of quantitites deduced from the skin friction field.

Westphal, Russell V.

1988-01-01

25

A model for rough wall turbulent heating and skin friction  

Microsoft Academic Search

A Reynolds stress model for turbulent boundary layers on rough walls is used to investigate the effects of roughness character and compressibility. The flow around roughness elements is treated as form drag. A method is presented for deriving the required roughness shape and spacing from profiometer surface measurements. Calculations based on the model compare satisfactorily with low speed data on

M. L. Finson

1982-01-01

26

Skin-Friction Drag Reduction in Turbulent Channel Flow by a Scale-Dependent Molecular Viscosity  

NASA Astrophysics Data System (ADS)

In prior work, we have proposed that the primary mechanism of drag reduction by dilute polymer solutions is the polymer's extraction of a minute amount of turbulence kinetic energy from the large turbulent scales. Here, we mimic this mechanism by performing DNS with a scale-dependent molecular viscosity in turbulent channel flow. Simulations were performed in channels of size 10h x5h x2h and 40h x10h x2h at a base Reynolds number of Re?˜230. Drag reductions of 50% and higher were observed when the molecular viscosity was artificially raised from ?s to (3-4)?s in a band of large-scale wavenumbers corresponding to 0.01 < ?kx^2 +ky^2/kd< 0.1. Many characteristics of drag reduction by dilute polymer solutions were reproduced by the scale-dependent molecular viscosity, including strong anisotropy in the turbulence structure, interruption of the turbulent energy cascade, a pileup of turbulence kinetic energy at the large scales in the streamwise component of the fluctuating velocity, and a shift of the peak of turbulence production away from the wall. These results open up new possibilities for devising novel turbulent skin-friction drag reduction strategies in wall flows.

Lee, Dong-Hyun; Akhavan, Rayhaneh

2011-11-01

27

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

NASA Astrophysics Data System (ADS)

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 at Brown University for both baseline (hydrophilic) and superhydrophobic surfaces. The mean and fluctuation velocity statistics are compared between the two surfaces. The friction velocity, u*, is estimated from the velocity fields using several independent methods. Direct drag and LDV measurements are taken for both the hydrophilic and superhydrophobic surfaces in the water tunnel at NUWC and will be discussed.

Peguero, Charles; Henoch, Charles; Breuer, Kenneth

2007-11-01

28

Charts for predicting turbulent skin friction from the Van Driest method (2)  

NASA Technical Reports Server (NTRS)

Charts are presented for rapidly estimating local or average turbulent skin friction on flat plates from Reynolds numbers, based on either length or momentum thickness. These charts facilitate the conversion from one Reynolds number to the other. Ranges of variables covered are: Mach number, 0 to 10; length Reynolds number, 10 to the 5th to 10 to the 9th power; and wall-to-adiabatic-wall temperature ratio, 0.2 to 1.0.

Hopkins, E. J.

1972-01-01

29

Bubble friction drag reduction in a high-Reynolds-number flat-plate turbulent boundary layer  

Microsoft Academic Search

Turbulent boundary layer skin friction in liquid flows may be reduced when bubbles are present near the surface on which the boundary layer forms. Prior experimental studies of this phenomenon reached downstream-distance-based Reynolds numbers (Re_{x}) of several million, but potential applications may occur at Re_{x} orders of magnitude higher. This paper presents results for Re_{x} as high as 210 million

Wendy C. Sanders; Eric S. Winkel; David R. Dowling; Marc Perlin; Steven L. Ceccio

2006-01-01

30

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

31

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

32

A numerical study of the effects of superhydrophobic surface on skin-friction drag in turbulent channel flow  

NASA Astrophysics Data System (ADS)

Superhydrophobic surfaces have attracted much attention lately as they present the possibility of achieving a substantial skin-friction drag reduction in turbulent flows. In this paper, the effects of a superhydrophobic surface, consisting of microgrates aligned in the flow direction, on skin-friction drag in turbulent flows were investigated through direct numerical simulation of turbulent channel flows. The superhydrophobic surface was modeled through a shear-free boundary condition on the air-water interface. Dependence of the effective slip length and resulting skin-friction drag on Reynolds number and surface geometry was examined. In laminar flows, the effective slip length depended on surface geometry only, independent of Reynolds number, consistent with an existing analysis. In turbulent flows, the effective slip length was a function of Reynolds number, indicating its dependence on flow conditions near the surface. The resulting drag reduction was much larger in turbulent flows than in laminar flows, and near-wall turbulence structures were significantly modified, suggesting that indirect effects resulting from modified turbulence structures played a more significant role in reducing drag in turbulent flows than the direct effect of the slip, which led to a modest drag reduction in laminar flows. It was found that the drag reduction in turbulent flows was well correlated with the effective slip length normalized by viscous wall units.

Park, Hyunwook; Park, Hyungmin; Kim, John

2013-11-01

33

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

NASA Technical Reports Server (NTRS)

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

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

1994-01-01

34

Free stream turbulence. A unified investigation of its effects on skin friction and heat transfer  

NASA Astrophysics Data System (ADS)

Existing prediction formulas for the effect of freestream turbulence (PST) on skin friction and heat transfer, based on measurements in low-intensity grid turbulence, fail at high intensities typical of the upstream stages of gas turbines. However, the previous evidence for this came from tests with various unconventional turbulence generators. In the present work measurements were made on the downstream-moving surface of a conveyor belt. This increased the effective value of Tu, the ratio of r.m.s. free-stream intensity to the velocity difference across the boundary layer. Thus conventional grids could be used to generate high Tu. Heat transfer was measured by a quasi-transient technique. A fixed heating lamp shone on the belt near its upstream end, and the streamwise decrease of surface temperature was measured using a chordwise strip of liquid crystals. Conductive heat transfer into the belt was calculable, so convective heat transfer into the airstream could be deduced. Skin-friction results collapse fairly well using Hancock & Bradshaw's combination of intensity and length scale but lie well above their low-Tu correlation.

Bradshaw, Peter; Bott, Donald M.

1995-04-01

35

Experimental determination of the friction factor coefficient in microchannels  

NASA Astrophysics Data System (ADS)

Experimental data used to determine the friction factor in microchannels are presented. Glass microchannels with a circular cross section, diameters of 34.5, 33.6, and 24.5 ?m, and different lengths are studied in the experiments. Pure deionized degassed water is used as a working fluid. The range of the Reynolds numbers is 13 ? Re ? 330. A method of two channels is used to calculate the friction factor. The results obtained are in good agreement with theoretical predictions for the case of a developed laminar flow in circular channels, but the use of the method of two channels has some constraints.

Aniskin, V. M.; Adamenko, K. V.; Maslov, A. A.

2011-01-01

36

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

NASA Technical Reports Server (NTRS)

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

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

1974-01-01

37

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

NASA Technical Reports Server (NTRS)

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

Duan, Lian; Choudhari, Meelan M.

2012-01-01

38

Friction Factor Characterization for High-Porosity Random Fiber Regenerators  

NASA Technical Reports Server (NTRS)

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

Thieme, Lanny G.

2001-01-01

39

Direct measurements at Mach 6 of turbulent skin-friction reduction by injection from single and multiple flush slots  

NASA Technical Reports Server (NTRS)

The surface skin-friction reduction downstream of one to four successive flush slots injecting at an angle of 10 deg into a turbulent Mach 6 boundary layer has been determined from direct measurements of surface shear. Increasing the number of succeeding slots increases the skin-friction reduction, but this trend reverses at high injection rates. The incremental improvement in skin-friction reduction decreases with increasing number of slots. Comparison with previously reported step slot data indicates that step slots with tangential injection are more effective in reducing skin friction than the present flush slot configuration. Finite-difference predictions are in reasonable agreement with skin-friction and boundary-layer profile data.

Srokowski, A. J.; Howard, F. G.; Feller, W. V.

1976-01-01

40

Pressure-gradient effects on hypersonic turbulent skin friction and boundary-layer profiles.  

NASA Technical Reports Server (NTRS)

Local skin friction, total-temperature profiles, and pitot-pressure profiles were measured on the wall of a Mach-7.4 wind tunnel. The wall to adiabatic wall temperature ratio was varied from 0.3 to 0.5. Boundary-layer characteristics were compared with those predicted by a finite-difference method. Local skin friction was predicted to within 15%. Pressure-gradient effects on the temperature and Mach number distributions and the shape factor (displacement thickness/momentum thickness) were underpredicted, but the velocity distributions were closely predicted.

Hopkins, E. J.; Keener, E. R.

1972-01-01

41

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

NASA Astrophysics Data System (ADS)

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

Luo, Haoxiang; Bewley, Thomas R.

2003-07-01

42

A method for determining the frictional velocity in a turbulent channel flow with roughness on the bottom wall  

Microsoft Academic Search

A method is proposed for determining the frictional velocity Ut on both walls of a fully developed turbulent channel flow, one smooth and the other rough. This should aid experimentalists in obtaining a reliable estimate of Ut with knowledge of only the pressure drop and location where the Reynolds shear stress is zero. The method is general and does not

S. Leonardi; P. Orlandi; R. A. Antonia

2005-01-01

43

Reduction of skin-friction in a microbubble-laden spatially developing turbulent boundary layer over a flat plate  

Microsoft Academic Search

Experimental evidence during the past three decades indicates that injection of gaseous microbubbles into a liquid turbulent boundary layer over a flat plate or over axisymmetrical bodies can reduce the skin-friction by as much as 80% from its value without bubble injection. However, the basic physical mechanisms responsible for that reduction are not yet fully understood. The present study is

Antonino Ferrante

2004-01-01

44

Evaluation of the effect of discrete fluid injection on turbulent boundary layer skin friction in the presence of wall roughness  

Microsoft Academic Search

The purpose of the present experimental study was to determine if discrete mass injection into a turbulent boundary layer over a rough wall would effectively smooth the surface and reduce the skin friction drag. Accordingly, a test program was planned and a series of tests were conducted in the Free Surface Water Tunnel at the California Institute of Technology. In

W. W. Haigh; D. D. Mantrom; J. E. Lewis

1976-01-01

45

Friction  

NASA Astrophysics Data System (ADS)

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

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

46

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

47

Skin friction and velocity profile family for compressible turbulent boundary layers  

NASA Technical Reports Server (NTRS)

The paper presents a general approach to constructing mean velocity profiles for compressible turbulent boundary layers with isothermal or adiabatic walls. The theory is based on a density-weighted transformation that allows the extension of the incompressible similarity laws of the wall to the compressible regions. The velocity profile family is compared to a range of experimental data, and excellent agreement is obtained. A self-consistent skin friction law, which satisfies the proposed velocity profile family, is derived and compared with the well-known Van Driest II theory for boundary layers in zero pressure gradient. The results are found to be at least as good as those obtained by using the Van Driest II transformation.

Huang, P. G.; Bradshaw, P.; Coakley, T. J.

1993-01-01

48

Friction  

NSDL National Science Digital Library

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

49

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

50

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

NASA Technical Reports Server (NTRS)

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

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

1994-01-01

51

Manipulation of turbulent boundary layers by outer-layer devices - Skin-friction and flow-visualization results  

Microsoft Academic Search

The ability of devices introduced into the outer region of a low-Reynolds-number turbulent boundary layer to reduce surface drag is investigated. Direct measurements of local skin friction (using a floating-plate drag balance) and laser-sheet smoke flow visualization results were obtained for the cases of single flat plates, a cylinder with the same drag as one of these, and two plates

A. M. Savill; J. C. Mumford

1988-01-01

52

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

Microsoft Academic Search

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

Kevin Gouder; Jonathan Morrison

2008-01-01

53

Effectiveness of active flow control for turbulent skin friction drag reduction  

NASA Astrophysics Data System (ADS)

The effectiveness of the opposition control method proposed by Choi et al. [J. Fluid Mech. 262, 75 (1994)] has been studied using direct numerical simulations. In this study, the effects of the amplitude and the phase of wall blowing and suction control input were considered separately. It is found that the amplitude of wall blowing and suction as well as the detection plane location played an important role in active control for skin-friction drag reduction. By changing the amplitude, a substantial drag reduction was achieved for all detection plane locations considered, and the efficiency of the opposition control was also improved. When the control was effective, the drag reduction was proportional to the wall blowing and suction strength. There existed a maximum wall blowing and suction strength, beyond which the opposition control became less effective or even unstable. Turbulence characteristics affected by various wall blowing and suction parameters were analyzed to understand the underlying mechanisms for drag reduction. The wall-normal velocity and vorticity fluctuations showed a strong correlation with drag reduction.

Chung, Yongmann M.; Talha, Tariq

2011-02-01

54

Lattice Boltzmann Simulations of Skin-Friction Drag Reduction in Turbulent Channel Flow with Slip/No Slip Wall Ridges  

NASA Astrophysics Data System (ADS)

To gain a better understanding of the mechanisms at work in skin friction drag reduction with superhydrophobic surfaces, Lattice Boltzmann simulations were performed in turbulent channels with alternating slip/no slip ridges on the walls. Simulations were performed in turbulent channels of size 5h x2.5 x2h and 10h x5h x2h at a base Reynolds number of Re?˜230. Alternating slip/no slip ridges of width 4 <=w+ <=140, aligned in the streamwise direction, all with the same fractional area of slip boundary, were studied. Drag reductions of 4%, 8%, 21%, 33% and 47%, corresponding to slip velocities of Uslip/Ubulk= 0.05, 0.1, 0.26, 0.31 and 0.36 were observed for w+=g+= 4, 8, 40, 70 and 140, respectively. The mean velocity profiles display the characteristics of combined slip described by Min and Kim [Min et al. 2004]. The streamwise and spanwise turbulence intensities show large slips at the wall, the magnitude of which increases with increasing drag reduction. Examination of the anisotropy invariant maps shows a shift of turbulence structure towards the one-dimensional turbulence limit near the wall with increasing drag reduction. For z^+ > 25, the turbulence structure returns to the isotropic limit.

Rastegari, Amirreza; Akhavan, Rayhaneh

2011-11-01

55

Friction Factor Correlation for 217 Pin Wire-Wrap Spaced LMFBR Fuel Assemblies.  

National Technical Information Service (NTIS)

Data from four independent water flow tests of 217 pin wire wrap spaced LMFBR fuel assemblies were analyzed and a friction factor correlation was developed. The data show that the mean friction factor varied little from one test to the next and the test s...

D. R. Spencer R. A. Markley

1981-01-01

56

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

NASA Technical Reports Server (NTRS)

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

Ha, Tae Woong

1989-01-01

57

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

NASA Technical Reports Server (NTRS)

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

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

1975-01-01

58

Friction factor correlation with application to the central cooling channel of cable-in-conduit super-conductors for fusion magnets  

Microsoft Academic Search

A correlation has been developed for the turbulent friction factor f of a circular channel with a helical rib roughness of rectangular cross section, which is relevant to the central channel (hole) in two-channel cable-in-conduit conductors. The correlation is based on data we measured on a pipe with three different types of helix. It relates f with the Reynolds number

Roberto Zanino; P. Santagati; L. Savoldi; A. Martinez; S. Nicollet

2000-01-01

59

Analysis of leakage and friction factors in coal mine ventilation systems. Special research report number SR99  

Microsoft Academic Search

A critical review of previous studies in the area of leakage and friction factors has been made. The need for new friction factors and leakage characteristics is emphasized and theoretical considerations for their determination discussed. Results of preliminary investigations carried out are analyzed and discussed there is a general decrease in the value of friction factors as compared with McElroy's

R. Kharkar; R. V. Ramani; R. Stefanko

1974-01-01

60

A Brief Examination of Factors Affecting Tractive Friction Coefficients of Spheres Rolling on Flat Plates  

Microsoft Academic Search

The use of steel ball-bearing balls in a new variable speed friction drive has given impetus to the study of factors governing their tractive capacity. The coefficient of tractive friction, defined as the ratio of tractive force to normal force at a specified slip rate, was observed to depend on lubricant properties and ball diameter, but to be relatively independent

W. S. Rouverol; R. I. Tanner

1960-01-01

61

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

NASA Technical Reports Server (NTRS)

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

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

1978-01-01

62

On the manner in which outer layer disturbances affect turbulent boundary layer skin friction  

Microsoft Academic Search

This paper presents results, from bth experimental studies and turbulence modelling, of the manipulated flow behind flat plate devices introduced into the outer region of turbulent boundary layers. An analysis of mean velocity profiles, in conjunction with drag balance and other measurements for u sub tau, reveals only small changes in the log law and little influence of plate thickness

A. M. Savill

1987-01-01

63

Modified friction factor correlation for CICC's based on a porous media analogy  

NASA Astrophysics Data System (ADS)

A modified correlation for the bundle friction factor in CICC's based on a porous media analogy is presented. The correlation is obtained by the analysis of the collected pressure drop data measured for 23 CICC's. The friction factors predicted by the proposed correlation are compared with those resulting from the pressure drop data for two CICC's measured recently using cryogenic helium in the SULTAN test facility at EPFL-CRPP.

Lewandowska, Monika; Bagnasco, Maurizio

2011-09-01

64

Friction Factor Determination for Horizontal Two-Phase Flow Through Fully Eccentric Annuli  

Microsoft Academic Search

In this study, empirical friction factor correlations were developed for two-phase stratified- and intermittent-flow patterns through horizontal fully eccentric annuli. Two-phase flow hydraulics were investigated, and a flow pattern prediction model is proposed. The friction factor correlations were validated using experimental data collected at the multiphase flow loop METU-PETE-CTMFL. Two different geometrical configurations were used during experiments—that is, 0.1143 m

C. Omurlu Metin; M. E. Ozbayoglu

2009-01-01

65

Investigation of the Factors Influencing Skid Resistance and the International Friction Index  

Microsoft Academic Search

This dissertation is compiled of the findings of several phases of a detailed research study\\u000athat was aimed at investigating the Skid Resistance phenomenon.\\u000aIn the first phase of the dissertation research a study was performed to evaluate the\\u000adifferent factors that influence frictional measurements obtained using the Dynamic\\u000aFriction Tester (DFT). A temperature calibration factor that would account for

Luis G. Fuentes

2009-01-01

66

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

67

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

Microsoft Academic Search

Experiments were conducted to determine the reduction in surface skin friction and the effectiveness of surface cooling downstream of one to four successive flush slots injecting cold air at an angle of 10 deg into a turbulent Mach 6 boundary layer. Data were obtained by direct measurement of surface shear and equilibrium temperatures, respectively. Increasing the number of slots decreased

F. G. Howard; A. J. Strokowski

1978-01-01

68

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

69

Analysis of countercurrent liquid - vapor interactions and the effect on the liquid friction factor  

Microsoft Academic Search

A closed mathematical model of the liquid friction factor for flow occurring in triangular grooves is presented. The model considers the interfacial shear stresses due to liquid-vapor frictional interactions for countercurrent flow. Using a coordinate transformation and the Nachtsheim-Swigert iteration scheme, a method is developed by which the asymptotic, two-point boundary value problem with one point-value varying can be solved.

G. P. Peterson; H. B. Ma

1996-01-01

70

Structure of atmospheric turbulence in the friction layer below 500 meters  

NASA Technical Reports Server (NTRS)

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

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

1976-01-01

71

High-Reynolds-number turbulent-boundary-layer wall pressure fluctuations with skin-friction reduction by air injection.  

PubMed

The hydrodynamic pressure fluctuations that occur on the solid surface beneath a turbulent boundary layer are a common source of flow noise. This paper reports multipoint surface pressure fluctuation measurements in water beneath a high-Reynolds-number turbulent boundary layer with wall injection of air to reduce skin-friction drag. The experiments were conducted in the U.S. Navy's Large Cavitation Channel on a 12.9-m-long, 3.05-m-wide hydrodynamically smooth flat plate at freestream speeds up to 20 ms and downstream-distance-based Reynolds numbers exceeding 200 x 10(6). Air was injected from one of two spanwise slots through flush-mounted porous stainless steel frits (approximately 40 microm mean pore diameter) at volume flow rates from 17.8 to 142.5 l/s per meter span. The two injectors were located 1.32 and 9.78 m from the model's leading edge and spanned the center 87% of the test model. Surface pressure measurements were made with 16 flush-mounted transducers in an "L-shaped" array located 10.7 m from the plate's leading edge. When compared to no-injection conditions, the observed wall-pressure variance was reduced by as much as 87% with air injection. In addition, air injection altered the inferred convection speed of pressure fluctuation sources and the streamwise coherence of pressure fluctuations. PMID:18529171

Winkel, Eric S; Elbing, Brian R; Ceccio, Steven L; Perlin, Marc; Dowling, David R

2008-05-01

72

Wall mass transfer and pressure gradient effects on turbulent skin friction  

NASA Technical Reports Server (NTRS)

The effects of mass injection and pressure gradients on the drag of surfaces were studied theoretically with the aid of boundary-layer and Navier-Stokes codes. The present investigation is concerned with the effects of spatially varying the injection in the case of flat-plate drag. Effects of suction and injection on wavy wall surfaces are also explored. Calculations were performed for 1.2 m long surfaces, one flat and the other sinusoidal with a wavelength of 30.5 cm. Attention is given to the study of the effect of various spatial blowing variations on flat-plate skin friction reduction, local skin friction coefficient calculated by finite difference boundary-layer code and Navier-Stokes code, and the effect of phase-shifting sinusoidal mass transfer on the drag of a sinusoidal surface.

Watson, R. D.; Balasubramanian, R.

1984-01-01

73

Skin-friction drag reduction in laminar and turbulent boundary layers  

Microsoft Academic Search

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

Sung Moon Kang

2006-01-01

74

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

75

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

76

Modelling turbulent skin-friction control using linearised Navier-Stokes equations  

NASA Astrophysics Data System (ADS)

A linear model has been used to make qualitative predictions of turbulent drag-reduction resulting from open-loop control using streamwise travelling waves of spanwise wall velocity. There is very good agreement, over a wide parameter space, between our qualitative prediction of drag decrease/increase, and the DNS results of Quadrio et. al. (2009) at Re? = 200.

Duque, C. A.; Baig, M. F.; Lockerby, D. A.; Chernyshenko, S. I.; Davies, C.

2011-12-01

77

An extension of the transpired skin-friction equation to compressible turbulent boundary layers  

Microsoft Academic Search

W INJECTION or suction of fluid into the turbulent boundary layer has proved over the years to be an effective means of controlling the properties of fluid flowing over a surface. The result of this injection or suction of fluid is to modify the velocity and temperature distributions through the boundary layer so that the drag and the heat transfer

ATILA P. SILVA-FREIRE

78

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

Microsoft Academic Search

In this project, we have designed a new type of flexible surface, which we call a tensegrity fabric, and simulated the interaction of this flexible surface with a near-wall turbulent flow. The fabric is constructed by weaving together both members in tension (tendons) and members in compression (bars) to form a plate-class tensegrity structure, then covering this discrete flexible structure

Haoxiang Luo; Thomas R. Bewley

2003-01-01

79

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

NASA Astrophysics Data System (ADS)

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 Re?~=180 and 400. The present theory suggests that large drag reduction is possible also at Reynolds numbers of practical interest (Re?~105-106) by employing a hydrophobic surface, which induces a slip length on the order of ten wall units or more.

Fukagata, Koji; Kasagi, Nobuhide; Koumoutsakos, Petros

2006-05-01

80

Ethylene Oxide - Styrene Block Copolymer Surfaces for Turbulent Drag Reduction.  

National Technical Information Service (NTIS)

A drag reduction test facility was constructed to evaluate the effects of hydrophilic coatings on the turbulent friction factor for nylon tubes. A cobalt 60 gamma irradiation cell was reactivated. Procedures were developed to use this source to graft copo...

C. C. Gryte

1976-01-01

81

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

NASA Technical Reports Server (NTRS)

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

Wilson, Dennis E.; Hanford, Anthony J.

1990-01-01

82

Evaluation of thermophysical properties, friction factor, and heat transfer of alumina nanofluid flow in tubes  

NASA Astrophysics Data System (ADS)

Various thermophysical properties, fluid flow parameter and heat transfer characteristics were measured for nanofluid with 6% volume concentration of solid Al2O3 nanoparticles in water. Thermal conductivity measurements showed that there is a definite enhancement in thermal conductivity of the nanofluid compared to that of water. At 7°C, the enhancement was 16% which decreased to 6.96% at 50°C. The viscosity measurements of the 6% volume concentration Al2O 3/water nanofluid showed that its viscosity is higher by a factor of 1.25 to 10.24 than the viscosity of water. Also the measurements of the viscosity of different volume concentration of Al2O3/water nanofluid showed that, the viscosity decreases as the volume concentration decreases. The plot between the shear stress and strain rate for the 6% volume concentration Al2O3/water nanofluid showed that it is a Newtonian fluid for the range of strain rate between 6-122 s-1. Several readings of viscosity were taken by subjecting the nanofluid to heating and cooling cycle. It was found that above 62.65°C, the 6% volume concentration Al2O3/water nanofluid experiences an irrecoverable increase in viscosity and when cooled from beyond this temperature, a hysteresis effect on the viscosity is seen. The friction factor results for laminar flow for the 6% volume concentration Al2O3/water nanofluid showed that it matches the value given by the Hagen-Poiseulle equation (f = 64/Re ). The transition from laminar flow to turbulent was found to occur at a Reynolds number of approximately 1500. The convective heat transfer results were in agreement with that proposed by the Lienhard correlation (Lienhard and Lienhard, 2008). For fully developed laminar flow, the Nusselt number under constant heat flux condition was found to be within ±7% of 4.36. In the laminar flow regime, the Nusselt numbers for thermally developing flow were within ±10% of the value calculated from the Lienhard correlation.

Tiwari, Sanjib

83

Heat transfer and friction characteristics in rectangular channels with rib turbulators  

SciTech Connect

The effect of the channel aspect ratio on the distribution of the local heat transfer coefficient in rectangular channels with two opposite ribbed walls (to simulate turbine airfoil cooling passages) was determined for a Reynolds number range of 10,000 to 60,000. The channel width-to-height ratios (W/H, ribs on side W) were 1/4, 1/2, 1, 2, and 4. The test channels were heated by passing current through thin, stainless steel foils instrumented with thermocouples. The local heat transfer coefficients on the ribbed side wall and on the smooth side wall of each test channel from the channel entrance to the fully developed regions were measured for two rib spacings (P/e = 10 and 20). The rib angle-of-attack was kept at 90 deg. The local data in the fully developed region were averaged and correlated, based on the heat transfer and friction similarity laws developed for ribbed channels, to cover the ranges of channel aspect ratio, rib spacing, rib height, and Reynolds number. The results compare well with the published data for flow in a square channel with two opposite ribbed walls. The correlations can be used in the design of turbine airfoil cooling passages.

Han. J.C. (Texas A and M Univ., College Station (USA))

1988-05-01

84

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

Microsoft Academic Search

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

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

2009-01-01

85

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

Microsoft Academic Search

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

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

2001-01-01

86

Friction factor and heat transfer coefficient of R134a liquid flow in mini-channels  

Microsoft Academic Search

This article presents an experimental study of friction factor and heat transfer coefficient for a vertical liquid up-flow of R134a. A flat aluminium multiport extruded tube composed of 11 parallel rectangular channels (3.28mm×1.47 mm) with hydraulic diameter of 2.01 mm was used. Mass flux ranges from 28 to 800 kgm?2s?1 and heat flux from 0.84 to 22 kWm?2. Working pressure

Bruno Agostini; Barbara Watel; André Bontemps; Bernard Thonon

2002-01-01

87

Friction factors of power-law fluids in chevron-type plate heat exchangers  

Microsoft Academic Search

In the present work laminar flows of Newtonian and power-law fluids through cross-corrugated chevron-type plate heat exchangers (PHEs) are numerically studied in terms of the geometry of the channels. The plates area enlargement factor was a typical one (1.17), the corrugation angle, ?, varied between 30° and 60° and the flow index behaviour, n, between 0.25 and 1. Single friction

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

2008-01-01

88

Friction factor correlation for CICC’s based on a porous media analogy  

NASA Astrophysics Data System (ADS)

We use an analogy to porous media to derive a correlation for the friction factor in CICC's. We start with a review of selected literature work on this topic. We then recall the principles of the porous medium analogy, and present the main body of our work on the analysis of pressure drop data. We finally conclude with the proposed correlation. The main novelty of our work is to propose predictive correlations for the permeability and drag factor that include the dependency on void fraction of the CICC's.

Bagnasco, M.; Bottura, L.; Lewandowska, M.

2010-11-01

89

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.

90

Acoustics of friction.  

PubMed

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

Akay, Adnan

2002-04-01

91

Refinement of the dependences of the contact friction stresses in the deformation zone during cold rolling on the technological factors  

NASA Astrophysics Data System (ADS)

The regression equation used to calculate the friction coefficient between a strip and rolls in the working stands of cold-rolling mills is refined using a database containing 580 friction coefficients. The reliability of this equation and the importance of the factors entering into it are proved by mathematical statistics methods. As compared to the calculations performed by alternative equations, the calculation of the rolling force by the refined formula decreases the difference between the calculated and experimental results more than twofold.

Garber, E. A.; Yagudin, I. V.; Traino, A. I.

2013-07-01

92

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

93

An investigation on friction factors and heat transfer coefficients in a rectangular duct with surface roughness  

Microsoft Academic Search

An investigation on the fully developed heat transfer and friction factor characteristics has been made in rectangular ducts\\u000a with one-side roughened by five different shapes. The effects of rib shape geometries as well as Reynolds numbers are examined.\\u000a The rib height-to-duct hydraulic diameter, pitch-to-height ratio, and aspect ratio of channel width to height are fixed ate\\/De=0.0476,P\\/e=8, andW\\/H=2.33, respectively. To understand

Soo Whan Ahn; Kang Pil Son

2002-01-01

94

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

95

Turbulent flow in a channel with transverse rib heat transfer augmentation  

NASA Technical Reports Server (NTRS)

Turbulent flow in a 2D channel with repeated rectangular rib roughness was numerically simulated using a low Reynolds number form of the k-epsilon turbulence model. Friction factors and average Stanton numbers were calculated for various pitch to rib height ratios and bulk Reynolds numbers. Comparisons with experiment were generally adequate, with the predictions of friction superior to those for heat transfer. The effect of variable properties for channel flow was investigated, and the results showed a greater effect for friction than for heat transfer. Comparison with experiment yielded no clear conclusions. The turbulence model was also validated for a related problem, that of flow downstream of an abrupt pipe expansion.

Chang, B. H.; Mills, A. F.

1993-01-01

96

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

97

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

PubMed

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

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

2014-03-01

98

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

99

Measurements of heat transfer coefficients and friction factors in passages rib-roughened on all walls  

SciTech Connect

A liquid crystal technique was used to measure heat transfer coefficients in twelve test sections with square and trapezoidal cross-sectional areas representing blade midchord cooling cavities in a modern gas turbine. Full-length ribs were configured on suction side as well as pressure side walls while half-length ribs were mounted on partition walls between adjacent cooling cavities. Ribs were in staggered arrangements with a nominal blockage ratio of 22% and an angle of attack to the mainstream flow, {alpha}, of 90 deg. Heat transfer measurements were performed on the roughened walls with full-length as well as half-length ribs. Nusselt numbers, friction factors, and thermal performances of all geometries are compared. The most important conclusion of this study is that the roughening of the partition walls enhances the heat transfer coefficients on those walls but, more importantly, enhances heat transfer coefficients on the primary walls considerably.

Taslim, M.E.; Li, T. [Northeastern Univ., Boston, MA (United States). Dept. of Mechanical Engineering; Spring, S.D. [GE Aircraft Engines, Lynn, MA (United States)

1998-07-01

100

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

SciTech Connect

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

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

1981-01-01

101

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

102

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

NASA Astrophysics Data System (ADS)

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

Campo, A.; Chang, J.

103

Laboratory Study of Borehole Friction Factor With a Dynamic-Filtration Apparatus  

Microsoft Academic Search

This paper discusses a medium-scale borehole friction tester with dynamic-filtration capabilities that showed that friction coefficients are affected by mud quality, mudcake, and lubricant addition in low-solids, water-based muds (WBM's). Most measured values, including those from oil-based muds (OBM's), were between 0.2 and 0.3. This paper also reports on new effects of filter cake and solids deposition on borehole friction.

Eric Maidla; Andrew Wojtanowicz

1990-01-01

104

Laboratory study of borehole friction factor with a dynamic-filtration apparatus  

SciTech Connect

This paper discusses a medium-scale borehole friction tester with dynamic-filtration capabilities that showed that friction coefficients are affected by mud quality, mudcake, and lubricant addition in low-solids, water-based muds (WBM's). Most measured values, including those from oil-based muds (OBM's), were between 0.2 and 0.3. This paper also reports on new effects of filter cake and solids deposition on borehole friction.

Maidla, E.E.; Wojtanowicz, A.K. (Louisiana State Univ., LA (US))

1990-09-01

105

Friction reduction and heat transfer enhancement in turbulent pipe flow of non-Newtonian liquid-solid mixtures  

NASA Astrophysics Data System (ADS)

Argonne National Laboratory (ANL) has identified two concepts for developing advanced energy transmission fluids for thermal systems, in particular district heating and cooling systems. A test series was conducted at ANL to prove these concepts. This paper presents experimental results and discusses the degradation behavior of linear polymer additives and the flow and heat transfer characteristics of non-melting slurry flows. The test results furnished strong evidence that the use of friction reducing additives and slurries can yield improved thermal-hydraulic performance of thermal systems.

Choi, U. S.; Liu, K. V.

1988-02-01

106

Analysis of turbulent heat transfer, mass transfer, and friction in smooth tubes at high Prandtl and Schmidt numbers  

NASA Technical Reports Server (NTRS)

The expression for eddy diffusivity from a previous analysis was modified in order to account for the effect of kinematic viscosity on the turbulence in the region close to a wall. By using the modified expression, good agreement was obtained between predicted and experimental results for heat and mass transfer at Prandtl and Schmidt numbers between 0.5 and 3000. The effects of length-to-diameter ratio and of variable viscosity were also investigated for a wide range of Prandtl numbers.

Deissler, Robert G

1955-01-01

107

Skin-friction reduction by multiple cavities  

Microsoft Academic Search

A possibility of reducing the skin-friction drag in a turbulent boundary layer with a two-dimensional cavity (elongated in the spanwise direction) has been investigated in the literature. In our previous numerical study (Hahn et al. 1999), we have shown that a single cavity indeed reduces the skin friction by about 10% in a turbulent boundary layer. Now, the question is

Chulkyu Kim

2005-01-01

108

A new friction factor correlation for laminar, single-phase flows through rock fractures  

SciTech Connect

Single-phase flow through fractured media occurs in various situations, such as transport of dissolved contaminants through geological strata, sequestration of carbon dioxide in depleted gas reservoirs, and in primary oil recovery. In the present study, fluid flows through a rock fracture were simulated. The fracture geometry was obtained from the CT scans of a rock fracture produced by the Brazilian method in a sandstone sample. A post-processing code using a CAD package was developed and used to generate the three-dimensional fracture from the CT scan data. Several sections along the fracture were considered and the GambitTM code was used to generate unstructured grids for flow simulations. FLUENTTM was used to analyze the flow conditions through the fracture section for different flow rates. Because of the small aperture of the fractures, the gravitational effects could be neglected. It was confirmed that the pressure drop was dominated by the smallest aperture passages of the fracture. The accuracy of parallel plate models for estimating the pressure drops through fractures was studied. It was shown that the parallel plate flow model with the use of an appropriate effective fracture aperture and inclusion of the tortuosity factor could provide reasonable estimates for pressure drops in the fracture. On the basis of the CFD simulation data, a new expression for the friction factor for flows through fractures was developed. The new model predictions were compared with the simulation results and favorable agreement was found. It was shown that when the length of the fracture and the mean and standard deviation of the fracture are known, the pressure loss as a function of the flow rate could be estimated. These findings may prove useful for design of lab experiments, computational studied of flows through real rock fractures, or inclusions in simulators for large-scale flows in highly fractured rocks.

Nazridoust, K. (Clarkson Univ., Potsdam, NY); Ahmadi, G. (Clarkson Univ., Potsdam, NY); Smith, D.H.

2006-09-30

109

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

110

Overflow: Facts on Friction  

NASA Technical Reports Server (NTRS)

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

Lawrence, Scott

1999-01-01

111

Skin-friction measurements in high-enthalpy hypersonic boundary layers  

NASA Astrophysics Data System (ADS)

Skin-friction measurements are reported for high-enthalpy and high-Mach-number laminar, transitional and turbulent boundary layers. The measurements were performed in a free-piston shock tunnel with air-flow Mach number, stagnation enthalpy and Reynolds numbers in the ranges of 4.4 6.7, 3 13 MJ kg(-1) and 0.16× 10(6) 21× 10(6) , respectively. Wall temperatures were near 300 K and this resulted in ratios of wall enthalpy to flow-stagnation enthalpy in the range of 0.1 0.02. The experiments were performed using rectangular ducts. The measurements were accomplished using a new skin-friction gauge that was developed for impulse facility testing. The gauge was an acceleration compensated piezoelectric transducer and had a lowest natural frequency near 40 kHz. Turbulent skin-friction levels were measured to within a typical uncertainty of ± 7%. The systematic uncertainty in measured skin-friction coefficient was high for the tested laminar conditions; however, to within experimental uncertainty, the skin-friction and heat-transfer measurements were in agreement with the laminar theory of van Driest (1952). For predicting turbulent skin-friction coefficient, it was established that, for the range of Mach numbers and Reynolds numbers of the experiments, with cold walls and boundary layers approaching the turbulent equilibrium state, the Spalding & Chi (1964) method was the most suitable of the theories tested. It was also established that if the heat transfer rate to the wall is to be predicted, then the Spalding & Chi (1964) method should be used in conjunction with a Reynolds analogy factor near unity. If more accurate results are required, then an experimentally observed relationship between the Reynolds analogy factor and the skin-friction coefficient may be applied.

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

2003-06-01

112

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

113

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

114

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

115

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

116

Turbulent Flow through a Microchannel with Superhydrophobic Walls  

NASA Astrophysics Data System (ADS)

A growing amount of research has focused on the reduction of drag in microfluidic transport. One approach is to fabricate micro-ribs and cavities in the channel wall that are then treated with a hydrophobic coating. Such treatment reduces the surface contact area between the flowing liquid and the solid wall, thus yielding walls with no-slip and nearly shear-free regions at the microscale. Most of the previous work has focused on the laminar flow where reductions in the frictional resistance as large as 87% have been observed. Little research, however, has explored the potential drag reduction associated with turbulent flow through such microchannels. Results of an investigation of the turbulent fully developed flow in a parallel plate microchannel with microengineered surfaces will be discussed. A k-? turbulence modeling scheme is implemented for closure to the turbulent RANS equations. Results are presented for the friction factor as a function of the relevant governing dimensionless parameters. The Reynolds number was varied from 2,000 to 10,000 and compared to previously obtained laminar flow data. Results show, as with the laminar flow case, that as the shear-free region increases the friction factor decreases. The observed reduction, however, was found to be significantly greater in the turbulent flow cases than in the case of laminar flow.

Jeffs, Kevin; Maynes, Daniel; Brent, Webb

2007-11-01

117

Microblowing Technique Demonstrated to Reduce Skin Friction.  

National Technical Information Service (NTIS)

One of the most challenging areas of research in aerodynamics is the reduction of skin friction, especially for turbulent flow. Reduced skin friction means less drag. For aircraft, less drag can lead to less fuel burned or to a greater flight range for a ...

D. P. Hwang T. J. Biesiadny

1998-01-01

118

Turbulent Heat Transfer Performance of Newtonian Fluids in Asymmetrically Heated Rectangular Channels  

Microsoft Academic Search

The fully established friction factors and heat transfer coefficients of the turbulent flow of water in a 150-hydraulic-diameters-long, 5:1 rectangular duct were measured experimentally. The bottom wider wall was heated by passing direct electric current through it, yielding constant wall temperature (spanwise) and uniform heat input per unit length along the bottom wall. The other surfaces were adiabatic.The measured turbulent

B. K. Rao

1989-01-01

119

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

120

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

Microsoft Academic Search

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

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

2009-01-01

121

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

NASA Technical Reports Server (NTRS)

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

Grosser, Wendy I.

1997-01-01

122

Hydraulic Model of the Skin Friction Reduction with Surface Grooves  

Microsoft Academic Search

\\u000a The reduction of skin friction in turbulent flows holds considerable promise for energy savings. The present work shows how\\u000a and why skin friction and the dissipation are interrelated in turbulent channel flows. A hydraulic model formulation is presented\\u000a for the skin friction reduction that can be obtained with a surface structure recently proposed for flow control. The model\\u000a predictions are

Bettina Frohnapfel; Peter Lammers; Jovan Jovanovi?; Antonio Delgado

123

Hydrodynamic skin-friction reduction  

Microsoft Academic Search

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

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

1989-01-01

124

Direct measurements of skin friction  

NASA Technical Reports Server (NTRS)

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

Dhawan, Satish

1953-01-01

125

Fascinating Friction!  

NSDL National Science Digital Library

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

Integrated Teaching And Learning Program

126

Use of a Stanton Tube for Skin-Friction Measurements  

NASA Technical Reports Server (NTRS)

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

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

1959-01-01

127

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

NASA Technical Reports Server (NTRS)

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

Pappas, C C

1954-01-01

128

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

NASA Technical Reports Server (NTRS)

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

Wang, Chi R.; Bidek, Maleina C.

1994-01-01

129

Effect of clamping force and friction coefficient on stress intensity factor of cracked lapped joints  

Microsoft Academic Search

Fatigue failure of steel connections is a well-known failure mechanism, especially for structures which sustain heavy cyclic loads like steel bridges. Reliable determination of stress intensity factors (SIFs) for cracks in bolted joints is required to evaluate their safety and fatigue life. The SIF is a traditional topic in mechanics and there have been many solutions for many different cases.

H. E. M. Sallam; A. E. A. El-Sisi; E. B. Matar; O. M. El-Hussieny

2011-01-01

130

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

131

Friction welding.  

NASA Technical Reports Server (NTRS)

Results of an exploratory study of the structure and properties of friction welds in Udimet 700 (U-700) and TD-nickel (TD-Ni) bar materials, as well as dissimilar U-700/TD-Ni friction welds. Butt welds were prepared by friction welding 12.7-mm-diam U-700 bars and TD-Ni bars. Specimens for elevated temperature tensile and stress rupture testing were machined after a postweld heat treatment. Friction welding of U-700 shows great potential because the welds were found to be as strong as the parent metal in stress rupture and tensile tests at 760 and 980 C. In addition, the weld line was not detectable by metallographic examination after postheating. Friction welds in TD-Ni or between U-700 and TD-Ni were extremely weak at elevated temperatures. The TD-Ni friction welds could support only 9% as much stress as the base metal for 10-hour stress rupture life at 1090 C. The U-700/TD-Ni weld could sustain only 15% as much stress as the TD-Ni parent metal for a 10-hour stress rupture life at 930 C. Thus friction welding is not a suitable joining method for obtaining high-strength TD-Ni or U-700/TD-Ni weldments.

Moore, T. J.

1972-01-01

132

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

Microsoft Academic Search

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

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

2009-01-01

133

A New Method for Predicting Friction Pressures and Rheology of Proppant-Laden Fracturing Fluids  

Microsoft Academic Search

The prediction of fiction pressures for proppant-laden fracturing fluids requires estimations of both the base-gel friction factor and the effect of proppant on fluid rheology. This paper introduces two new expressions, each theoretically based with constants determined from data, that address these two issues for hydroxpropyl guar (HPG)-based fracturing fluids in laminar and turbulent flow. The paper first introduces a

Richard Keck; Warren Nehmer; Gary Strumolo

1992-01-01

134

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

135

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

136

Velocity profiles for turbulent boundary layers under freestream turbulence  

NASA Astrophysics Data System (ADS)

Correlations for changes of skin friction coefficients and wake parameters, relative to the low freestream turbulence condition, are presented for the case of turbulent boundary layers under freestream turbulence with zero and adverse pressure gradients. The turbulent boundary layers were evaluated on a plate in a wind tunnel using a monoplane rod set turbulence generator; comparisons were also made using the data of several other investigators. The results, which define the velocity profiles within the boundary layers, were found to collapse for a large range of the pressure gradient parameter.

Hoffmann, J. A.; Mohammadi, K.

1991-09-01

137

Friction Force  

NSDL National Science Digital Library

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

Applying Mechatronics to Promote Science (AMPS) Program GK-12,

138

Welding Metals by Friction.  

National Technical Information Service (NTIS)

Contents: Features of friction welding; Mechanism of formation of a welded joint in solid phase; Basic information from the friction theory; Heat liberation during friction welding; Basic parameters and recommended regimes in friction welding process; Fri...

V. I. Vill

1972-01-01

139

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

140

Skin-Friction Measurements in Incompressible Flow  

NASA Technical Reports Server (NTRS)

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

Smith, Donald W.; Walker, John H.

1959-01-01

141

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

142

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

PubMed

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

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

2013-01-01

143

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

144

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

NASA Astrophysics Data System (ADS)

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

Hagan, Peter

145

The Role of Surface Friction in Downslope Windstorms  

Microsoft Academic Search

Numerical simulations of the 11 January 1972 windstorm in Boulder, Colorado, were carded out using a hydrostatic model with a turbulent kinetic energy parameterization to investigate the role of fictional effects in the development of nonlinear mountain waves. Sensitivity tests to the roughness length specification and to the turbulent mixing and dissipation length formulations show that surface friction delays the

Evelyne Richard; Patrick Mascart; Everett C. Nickerson

1989-01-01

146

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

147

Friction and wear  

NASA Technical Reports Server (NTRS)

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

Pomey, Jacques

1952-01-01

148

Active and inactive motions in a turbulent boundary layer - Interactions with free-stream turbulence  

Microsoft Academic Search

The ideas of active and inactive motions are used to study the effects of grid-generated turbulence on flat plate turbulent boundary layers. Three-component laser doppler velocimetry measurements were made in flows with 5 percent and 7.5 percent freestream turbulence intensity. Skin friction measurements indicate that increases in shear stress production may be limited even as freestream turbulence levels are increased.

P. L. Johnson; J. P. Johnston

1989-01-01

149

Predicting Turbulent Convective Heat Transfer in Three-Dimensional Duct Flows  

NASA Technical Reports Server (NTRS)

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

Rokni, M.; Gatski, T. B.

1999-01-01

150

Influence of interface geometry on rubber friction  

Microsoft Academic Search

While the mechanics of rubber friction are still debated, it is widely accepted that there are two principal factors, adhesion and hysteresis, commonly considered to give rise to a frictional force during the sliding of a rigid surface on a flexible elastomer. The experiments described here, validated by finite element analysis (FEA), confirm an additional (third) geometric factor, which had

P. Gabriel; A. G. Thomas; J. J. C. Busfield

2010-01-01

151

Turbulent pipe flow of a drag-reducing rigid “rod-like” polymer solution  

Microsoft Academic Search

Fully developed turbulent pipe flow of an aqueous solution of a rigid “rod-like” polymer, scleroglucan, at concentrations of 0.005% (w\\/w) and 0.01% (w\\/w) has been investigated experimentally. Fanning friction factors were determined from pressure-drop measurements for the Newtonian solvent (water) and the polymer solutions and so levels of drag reduction for the latter. Mean axial velocity u and complete Reynolds

A. Japper-Jaafar; M. P. Escudier; R. J. Poole

2009-01-01

152

The mechanical properties of guidewires. Part III: Sliding friction.  

PubMed

Stiffness is always one of two factors that determine frictional resistance. The other factor, the coefficient of friction, depends on the surface characteristics. Teflon coating reduces the sliding friction to one-half, and a hydrophilic plastic coat to about one-sixth compared with steel. PMID:8485751

Schröder, J

1993-01-01

153

Hurricane Formation in Diabatic Ekman Turbulence  

Microsoft Academic Search

This study numerically examines the evolution of Diabatic Ekman Turbulence (DET) under various conditions. DET is quasi 2D turbulence that is modified by surface friction and parameterized cumulus convection. The self-organization of DET is here simulated in a 3-layer troposphere. In our primary model, winds over the ocean elevate the moist entropy of boundary layer air, whose convergence may then

David Schecter; Timothy Dunkerton

2008-01-01

154

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

NASA Technical Reports Server (NTRS)

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

Sohn, Ki-Hyeon; Reshotko, Eli

1991-01-01

155

Measurements of heat transfer coefficients and friction factors in rib-roughened channels simulating leading-edge cavities of a modern turbine blade  

SciTech Connect

Leading edge cooling cavities in modern gas turbine blades play an important role in maintaining the leading edge temperature at levels consistent with air foil design life. These cavities often have a complex cross-sectional shape to be compatible with the external contour of the blade at the leading edge. A survey of many existing geometries shows that, for analytical as well as experimental analyses, such cavities can be simplified in shape by a four-sided polygon with one curved side similar to the leading edge curvature, a rectangle with one semicircular side (often the smaller side) or a trapezoid, the smaller base of which is replaced by a semicircle. Furthermore, to enhance the heat transfer coefficient in these cavities, they are mostly roughened on three sides with ribs of different geometries. Experimental data on friction factors and heat transfer coefficients in such cavities are rare if not nonexistent. A liquid crystal technique was used in this experimental investigation to measure heat transfer coefficients in six test sections representing the leading-edge cooling cavities. Both straight and tapered ribs were configured on the two opposite sidewalls in a staggered arrangement with angles of attack to the mainstream flow, {alpha}, of 60 and 90 deg. The ribs on the curved surface were of constant cross section with an angle of attack 90 deg to the flow. Heat transfer measurements were performed on the straight sidewalls, as well as on the round surface adjacent to the blade leading edge. Effects such as rib angle of attack to the mainstream flow and constant versus tapered rib cross-sectional areas were also investigated. Nusselt numbers, friction factors, and thermal performances are reported for nine rib geometries in six test sections.

Taslim, M.E.; Li, T. [Northeastern Univ., Boston, MA (United States). Dept. of Mechanical Engineering; Spring, S.D. [GE Aircraft Engines, Lynn, MA (United States)

1997-07-01

156

Hydrodynamic skin-friction reduction  

NASA Technical Reports Server (NTRS)

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, e.g., 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, e.g., water, with the surface of the body, e.g., the hull of the marine vehicle.

Reed, Jason C. (Inventor); Bushnell, Dennis M. (Inventor); Weinstein, Leonard M. (Inventor)

1991-01-01

157

Prediction of Very High Reynolds Number Compressible Skin Friction  

NASA Technical Reports Server (NTRS)

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

Carlson, John R.

1998-01-01

158

Friction, wear, and lubrication in vacuum  

NASA Technical Reports Server (NTRS)

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

Buckley, D. H.

1971-01-01

159

Stirring turbulence with turbulence  

NASA Astrophysics Data System (ADS)

We stir wind--tunnel turbulence with an active grid that consists of rods with attached vanes. The time--varying angle of these rods is controlled by random numbers. We study the response of turbulence on the statistical properties of these random numbers. The random numbers are generated by the Gledzer--Ohkitani--Yamada shell model, which is a simple dynamical model of turbulence that produces a velocity field displaying inertial--range scaling behavior. The range of scales can be adjusted by selection of shells. We find that the largest energy input and the smallest anisotropy are reached when the time scale of the random numbers matches that of the large eddies in the wind--tunnel turbulence. A large mismatch of these times creates a flow with interesting statistics, but it is not turbulence.

van de Water, Willem; Ergun Cekli, Hakki; Joosten, Rene

2011-11-01

160

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

NASA Astrophysics Data System (ADS)

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

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

2006-04-01

161

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

162

A skin friction gauge for impulsive flows  

NASA Technical Reports Server (NTRS)

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

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

1995-01-01

163

Wall Turbulence.  

National Technical Information Service (NTIS)

This research was aimed at understanding turbulence generated at a wall. The goals were to investigate the structure of turbulence in the viscous wall region, to understand how production of turbulence is related to measured structural parameters, to deve...

L. P. Purtell

1992-01-01

164

Turbulence Reynolds number and the turbulent kinetic energy balance in the atmospheric surface layer  

Microsoft Academic Search

The relation between the turbulence Reynolds numberR? and a Reynolds numberz* based on the friction velocity and height from the ground is established using direct measurements of the r.m.s. longitudinal velocity and turbulent energy dissipation in the atmospheric surface layer. Measurements of the relative magnitude of components of the turbulent kinetic energy budget in the stability range 0 >z\\/L ?

E. F. Bradley; R. A. Antonia; A. J. Chambers

1981-01-01

165

Polymer flexibility and turbulent drag reduction  

NASA Astrophysics Data System (ADS)

Polymer-induced drag reduction is the phenomenon by which the friction factor of a turbulent flow is reduced by the addition of small amounts of high-molecular-weight linear polymers, which conformation in solution at rest can vary between randomly coiled and rodlike. It is well known that drag reduction is positively correlated to viscous stresses, which are generated by extended polymers. Rodlike polymers always assume this favorable conformation, while randomly coiling chains need to be unraveled by fluid strain rate in order to become effective. The coiling and stretching of flexible polymers in turbulent flow produce an additional elastic component in the polymer stress. The effect of the elastic stresses on drag reduction is unclear. To study this issue, we compare direct numerical simulations of turbulent drag reduction in channel flow using constitutive equations describing solutions of rigid and flexible polymers. When compared at constant ?r2 , both simulations predict the same amount of drag reduction. Here ? is the polymer volume fraction and r is the polymer aspect ratio, which for flexible polymers is based on average polymer extension at the channel wall. This demonstrates that polymer elasticity plays a marginal role in the mechanism for drag reduction.

Gillissen, J. J. J.

2008-10-01

166

Field Evaluation of Subbase Friction Characteristics.  

National Technical Information Service (NTIS)

An important factor to be considered in the design of prestressed concrete pavements is the friction characteristics of the slab-support interface. Due to the slab's length, detrimental tensile stresses may develop as the slab movements caused by temperat...

W. S. Chia B. F. McCullough N. H. Burns

1986-01-01

167

Exploratory Calibration of Adjustable-Protrusion Surface-Obstacle (APSO) Skin Friction Vector Gage.  

National Technical Information Service (NTIS)

The design of an adjustable-protrusion surface-obstacle (APSO) skin friction vector gage is presented. Results from exploratory calibrations conducted in laminar and turbulent boundary layers at the Washington University Low-Speed Wind Tunnel and for turb...

R. J. Hakkinen J. S. Neubauer P. J. Hamory T. T. Bui G. K. Noffz

2003-01-01

168

Friction of plastic webs  

Microsoft Academic Search

This paper is about the measurement of the friction of plastic and similar flexible webs in contact with cylindrical surfaces. There are numerous ways for measuring friction coefficients of materials in relative contact, but to be meaningful, the testing technique should simulate the tribosystem of interest. The subject of this paper is the capstan friction test, which simulates the tribosystem

K. G Budinski

2001-01-01

169

Wall shear stress measurement in a turbulent pipe flow using ultrasound Doppler velocimetry  

NASA Astrophysics Data System (ADS)

A turbulent boundary layer of a water flow is investigated by means of pulsed ultrasound Doppler velocimetry. The advantage of this method is the acquisition of complete velocity profiles along the sound propagation line within very short time intervals. The shear stress velocity, used for normalizing the velocity profiles, was determined by fitting the profiles to the universal profiles in a turbulent boundary layer obtained from Prandtl's mixing length theory. A coordinate transformation in the near-wall region is proposed to allocate the velocity data to "true" wall distances. From the experimental values of the wall shear stress velocity, the friction factors for a turbulent pipe flow are calculated and compared to the Blasius law. The overall error in measurement was estimated to +/-8.4%.

Nowak, M.

2002-05-01

170

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

171

On the possibility of skin-friction reduction using d-type roughness  

Microsoft Academic Search

Contrary to our physical intuition, some experimental results suggested that skin-friction drag may be reduced with d-type roughness or a chain of small-scale cavities. In order to examine the possibility of skin-friction reduction using d-type roughness, direct numerical simulations of turbulent flows over a single or multiple cavities are performed in the present study. Both the turbulent channel and boundary

Seonghyeon Hahn; Haecheon Choi

2004-01-01

172

Intermittency and rough-pipe turbulence.  

PubMed

Recently, by analyzing the measurement data of Nikuradze [NACA Tech. Memo No. 1292 (1950)], it has been proposed [N. Goldenfeld, Phys. Rev. Lett. 96, 044503 (2006)] that the friction factor, f , of rough-pipe flow obeys a scaling law in the turbulent regime. Here, we provide a phenomenological scaling argument to explain this law and demonstrate how intermittency modifies the scaling form, thereby relating f to the intermittency exponent, eta . By statistically analyzing the measurement data of f , we infer a satisfactory estimate for eta ( approximately 0.02) , the inclusion of which is shown to improve the data-collapse curve. This provides empirical evidence for intermittency other than the direct measurement of velocity fluctuations. PMID:18643125

Mehrafarin, Mohammad; Pourtolami, Nima

2008-05-01

173

Intermittency and rough-pipe turbulence  

NASA Astrophysics Data System (ADS)

Recently, by analyzing the measurement data of Nikuradze [NACA Tech. Memo No. 1292 (1950)], it has been proposed [N. Goldenfeld, Phys. Rev. Lett. 96, 044503 (2006)] that the friction factor, f , of rough-pipe flow obeys a scaling law in the turbulent regime. Here, we provide a phenomenological scaling argument to explain this law and demonstrate how intermittency modifies the scaling form, thereby relating f to the intermittency exponent, ? . By statistically analyzing the measurement data of f , we infer a satisfactory estimate for ? (?0.02) , the inclusion of which is shown to improve the data-collapse curve. This provides empirical evidence for intermittency other than the direct measurement of velocity fluctuations.

Mehrafarin, Mohammad; Pourtolami, Nima

2008-05-01

174

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

175

Large-Eddy Simulation of the Flat-plate Turbulent Boundary Layer at High Reynolds numbers  

NASA Astrophysics Data System (ADS)

The near-wall, subgrid-scale (SGS) model [Chung and Pullin, "Large-eddy simulation and wall-modeling of turbulent channel flow'', J. Fluid Mech. 631, 281--309 (2009)] is used to perform large-eddy simulations (LES) of the incompressible developing, smooth-wall, flat-plate turbulent boundary layer. In this model, the stretched-vortex, SGS closure is utilized in conjunction with a tailored, near-wall model designed to incorporate anisotropic vorticity scales in the presence of the wall. The composite SGS-wall model is presently incorporated into a computer code suitable for the LES of developing flat-plate boundary layers. This is then used to study several aspects of zero- and adverse-pressure gradient turbulent boundary layers. First, LES of the zero-pressure gradient turbulent boundary layer are performed at Reynolds numbers Retheta based on the free-stream velocity and the momentum thickness in the range Retheta = 103-1012. Results include the inverse skin friction coefficient, 2/Cf , velocity profiles, the shape factor H, the Karman "constant", and the Coles wake factor as functions of Re theta. Comparisons with some direct numerical simulation (DNS) and experiment are made, including turbulent intensity data from atmospheric-layer measurements at Retheta = O (106). At extremely large Retheta , the empirical Coles-Fernholz relation for skin-friction coefficient provides a reasonable representation of the LES predictions. While the present LES methodology cannot of itself probe the structure of the near-wall region, the present results show turbulence intensities that scale on the wall-friction velocity and on the Clauser length scale over almost all of the outer boundary layer. It is argued that the LES is suggestive of the asymptotic, infinite Reynolds-number limit for the smooth-wall turbulent boundary layer and different ways in which this limit can be approached are discussed. The maximum Retheta of the present simulations appears to be limited by machine precision and it is speculated, but not demonstrated, that even larger Retheta could be achieved with quad- or higher-precision arithmetic. Second, the time series velocity signals obtained from LES within the logarithmic region of the zero-pressure gradient turbulent boundary layer are used in combination with an empirical, predictive inner--outer wall model [Marusic et al., "Predictive model for wall-bounded turbulent flow'', Science 329, 193 (2010)] to calculate the statistics of the fluctuating streamwise velocity in the inner region of the zero-pressure gradient turbulent boundary layer. Results, including spectra and moments up to fourth order, are compared with equivalent predictions using experimental time series, as well as with direct experimental measurements at Reynolds numbers Retau based on the friction velocity and the boundary layer thickness, Retau = 7,300, 13,600 and 19,000. LES combined with the wall model are then used to extend the inner-layer predictions to Reynolds numbers Retau = 62,000, 100,000 and 200,000 that lie within a gap in log(Retau) space between laboratory measurements and surface-layer, atmospheric experiments. The present results support a log-like increase in the near-wall peak of the streamwise turbulence intensities with Retau and also provide a means of extending LES results at large Reynolds numbers to the near-wall region of wall-bounded turbulent flows. Finally, we apply the wall model to LES of a turbulent boundary layer subject to an adverse pressure gradient. Computed statistics are found to be consistent with recent experiments and some Reynolds number similarity is observed over a range of two orders of magnitude.

Inoue, Michio

176

Predicting Turbulent Convective Heat Transfer in Fully Developed Duct Flows  

NASA Technical Reports Server (NTRS)

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

Rokni, Masoud; Gatski, Thomas B.

2001-01-01

177

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

178

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

NASA Astrophysics Data System (ADS)

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

Ziaei-Rad, Masoud

2013-07-01

179

Metal-silicate friction in ultrahigh vacuum  

NASA Technical Reports Server (NTRS)

Results of the analysis and experiments on the frictional behavior of materials under conditions of ultrahigh vacuum, as related to kinematic problems in a lunar environment, are presented. Experiments have been performed under varying conditions of cleanliness to determine frictional characteristics at 10 to the minus 7th power to 10 to the minus 8th power torr between flat basalt and spherical 7075-T6 aluminum surfaces. A quantitative analysis has been made of the factors influencing frictional behavior of these materials. The proposed mathematical model incorporated parameters of surface cleanliness, surface roughness, and surface energy.

Ofodile, E. I.; Frisch, J.

1971-01-01

180

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

181

Turbulent drag reduction by transverse wall oscillations  

NASA Astrophysics Data System (ADS)

Skin-friction drag reduction by transverse wall oscillations has received significant attention in the last two decades. Both experiments and simulations have demonstrated that oscillations with properly selected amplitude and frequency can reduce turbulent drag by as much as 40 percent. For a turbulent channel flow, we develop a model-based approach to design oscillations that suppress turbulence. We show that judiciously selected linearization of the flow with control can be used to determine turbulent eddy viscosity in a computationally efficient way. The resulting correction to the turbulent mean velocity is then used to identify optimal frequency of oscillations, which is in close agreement with previously conducted experimental and numerical studies. This demonstrates the predictive power of our simulation-free approach to controlling turbulent flows.

Moarref, Rashad; Jovanovic, Mihailo R.

2011-11-01

182

Microbubble skin friction reduction on an axisymmetric body  

Microsoft Academic Search

A reduction in skin friction drag is shown when gas is introduced into the liquid turbulent boundary layer of a submerged axisymmetric body. The 89 mm diameter, 632 mm long body has a cylindrical balance 273 mm long. Free stream speeds in the 305 mm diameter tunnel are as high as 17 m\\/sec, giving length Reynolds number of up to

S. Deutsch; J. Castano

1986-01-01

183

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

184

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

185

On the theory of skin friction reduction by compliant walls  

Microsoft Academic Search

A theoretical model has been developed for the calculation of the perturbation Reynolds stress induced by a compliant wall which is excited by a turbulent boundary layer. The analysis is based on the technique developed by Lin to determine the Reynolds stresses induced by viscous action near the wall. It is shown that a friction reducing compliant coating should have

D. T. Tsahalis

1977-01-01

186

Cavity as a Passive Device for the Skin-Friction Drag Reduction  

Microsoft Academic Search

A possibility of reducing the skin-friction drag in a turbulent boundary layer with an open cavity is investigated in this study. Large eddy simulations are performed for turbulent boundary layer flows over four different cavity configurations at Re = u_? theta \\/nu =1520, where u_? is the free-stream velocity and theta is the momentum thickness. The cavities considered are combinations

Seonghyeon Hahn; Joonhyuk Choi; Haecheon Choi

1999-01-01

187

Polymer/riblet combination for hydrodynamic skin friction reduction  

NASA Technical Reports Server (NTRS)

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

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

1995-01-01

188

Vibrations in friction welding  

Microsoft Academic Search

For more than 50 years rotational friction welding has been successfully used for joining metallic materials. Since lightweight\\u000a structures became established in industry new challenges emerge to rotational friction welding, that have not yet been studied.\\u000a This report considers vibrations in rotational friction welding in order to identify possibilities and means and to avoid\\u000a damage of the component structure by the

M. F. Zaeh; A. Poehler

2011-01-01

189

Experimental study on drag-reducing channel flow with surfactant additives––spatial structure of turbulence investigated by PIV system  

Microsoft Academic Search

The turbulent frictional drag of water can be reduced dramatically by adding small amounts of drag-reducing materials, such as polymers or surfactants. As a percentage drag reduction of 80% can easily be achieved, this technique is thought to be the most practical method of reducing turbulent frictional drag. In this work, a double pulse particle image velocimetry (PIV) system was

Yasuo Kawaguchi; Takehiko Segawa; Ziping Feng; Peiwen Li

2002-01-01

190

High freestream turbulence effects on the transport of heat and momentum  

Microsoft Academic Search

Understanding high freestream turbulence effects on the transport of heat and momentum is needed for better predictive schemes. This dissertation presents the effects of very high freestream turbulence levels, between Tu equals 10 percent and 20 percent, on surface heat transfer and skin friction in terms of the turbulent transport of heat and momentum. Both the velocity and thermal fields

Karen Ann Thole

1992-01-01

191

Theory of quantum friction  

NASA Astrophysics Data System (ADS)

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

Silveirinha, Mário G.

2014-06-01

192

Turbulence Research.  

National Technical Information Service (NTIS)

Turbulent flows were studied especially those aspects that are dominated by the intrinsic nonlinearity of the governing equations. Theories were developed first for the calculation on quasi-parallel shear flows later for coherent turbulent puffs. Measurem...

L. S. G. Kovasznay

1975-01-01

193

Enhanced nanoscale friction on fluorinated graphene.  

PubMed

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

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

2012-12-12

194

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

Microsoft Academic Search

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

Hongjie Wang; Kenji Ikeuchi; Makoto Takahashi; Akio Ikeda

2009-01-01

195

Experimental determination of solids friction factors and minimum volumetric requirements in foam and mist drilling and well-cleanout operations. Final report  

Microsoft Academic Search

In order to accurately predict minimum air and liquid volumetric requirements for foam drilling operations, the frictional losses due to the solid phase and the settling velocities of such solids must be taken into account. To adequately account for pressure drop across the bits, a new equation which takes the compressibility of foam into consideration is required. Thus, the objectives

G. A. Okpobiri; C. U. Ikoku

1982-01-01

196

Passive and active control of near-wall turbulence  

Microsoft Academic Search

Wall skin friction reduction is the primary focus of this paper. Turbulent boundary layer structure and near-wall turbulence structure are considered in particular along with how passive and active modifications to the structure can lead to reductions in drag. Direct numerical simulation of low Reynolds number turbulent flow over streamwise aligned micro-grooved striations or riblets, particularly V-groove riblets and the

A. Pollard

1998-01-01

197

Turbulence modeling for sharp-fin-induced shock wave/turbulent boundary-layer interactions  

NASA Technical Reports Server (NTRS)

Solutions of the Reynolds averaged Navier-Stokes equations are presented and compared with a family of experimental results for the 3-D interaction of a sharp fin induced shock wave with a turbulent boundary layer. Several algebraic and two equation eddy viscosity turbulence models are employed. The computed results are compared with experimental surface pressure, skin friction, and yaw angle data as well as the overall size of the interaction. Although the major feature of the flow fields are correctly predicted, several discrepancies are noted. Namely, the maximum skin friction values are significantly underpredicted for the strongest interaction cases. These and other deficiencies are discussed.

Horstman, C. C.

1990-01-01

198

Extended ion pumped vacuum friction test  

NASA Technical Reports Server (NTRS)

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

Hammel, R. L.

1971-01-01

199

In vitro friction and lubrication of large bearing hip prostheses.  

PubMed

New material combinations and designs of artificial hip implants are being introduced in an effort to improve proprioception and functional longevity. Larger joints in particular are being developed to improve joint stability, and it is thought that these larger implants will be more satisfactory for younger and more physically active patients. The study detailed here used a hip friction simulator to assess the friction and lubrication properties of large-diameter hip bearings of metal-on-metal and ceramic-on-reinforced-polymer couplings. Joints of different diameters were evaluated to determine what effect, if any, bearing diameter had on lubrication. In addition, the effects of lubricant type are considered, using carboxymethyl cellulose and bovine calf serum, and the physiological lubricant is shown to be considerably more effective at reducing friction. The frictional studies showed that the metal-on-metal joints worked under a mixed lubrication regime, producing similar friction factor values to each other. The addition of bovine calf serum (BCS) reduced the friction. The ceramic-on-reinforced-polymer samples were shown to operate with high friction factors and mixed lubrication. When tested with BCS, the larger-diameter bearings showed a decrease in friction compared with the smaller-size bearings, and the addition of BCS resulted in an increase in friction, unlike the metal-on-metal system. The study demonstrated that the component's diameter had little or no influence on the lubrication and friction of the large bearing combinations tested. PMID:20839653

Flanagan, S; Jones, E; Birkinshaw, C

2010-01-01

200

The Force of Friction  

NSDL National Science Digital Library

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

Engineering K-Ph.d. Program

201

Friction welding of plastics  

Microsoft Academic Search

Using a specially constructed and instrumented machine, the friction welding characteristics of four thermoplastics: nylon 66, acetal, polymethylmethacrylate and polyvinyl chloride (PVC) were investigated. It was found that during the frictioning stage the inter-facial torque and burn-off rate are both constant, but dependent on the axial pressure and rubbing velocity. The rate of heat generation was found to be greatest

R. J. Crawford; Y. Tam

1981-01-01

202

Friction properties of cork  

Microsoft Academic Search

The friction coefficient, µ, of cork sliding on another material (glass and steel in most experiments and also cork) was measured for various compressive stresses and sliding velocities. There is a strong effect of stress and a negligible effect of velocity on the friction coefficient. Values of µ are in the range 0.4 to 1.2. The effect of moisture content

M. Fatima Vaz; M. A Fortes

1998-01-01

203

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

204

Friction of rocks  

Microsoft Academic Search

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

J. Byerlee

1978-01-01

205

Brief communication: The generic skin-friction pattern underneath coherent near-wall structures  

NASA Astrophysics Data System (ADS)

A generic model of the skin-friction pattern associated with the dynamically significant near-wall streamwise vortices is deduced. Coherent vortical structures are first detected by the ?2-criterion and the surrounding flow field is thereafter ensemble-averaged to eliminate incoherent turbulence. The generic model of the footprint of the coherent structures confirms the prevailing conjecture that maximum skin-friction is induced by the downwash of high-speed fluid and occurs aside of the vortex tail. The surface-average of the skin-friction pattern turns out to be practically equal to the global wall-friction.

Solbakken, Stian; Andersson, Helge I.

2004-03-01

206

The prediction of fully developed turbulent pressure drops in a triangular duct for a power law fluid  

NASA Astrophysics Data System (ADS)

A model in which the nondimensional eddy viscosity and velocity distributions in circular pipe flow are applicable to noncircular passage flows was applied to the prediction of fully developed turbulent pressure drops in triangular ducts for Newtonian and power law fluids. The numerical calculations for the predicted turbulent velocity profiles and friction factors in triangular ducts were carried out by utilizing a modified integral transformation after Krajewski and the laminar velocity solutions for power law fluids. Friction factors for Newtonian and power law fluid flows were experimentally investigated over a Reynolds number range of 500 to 40000 in isosceles triangular ducts having apex angles of 10 and 60 degrees. Comparisons were made between the theoretical and experimental results. For the 60 degree duct, the model was successful in predicting friction factor-Reynolds number relations for power law fluids in the range 0.857 n 1. For the 10 degree duct, the model was unable to account for the laminarization of the flow in the apex region and the presence of secondary flows.

Chang, T. D.

207

Imagine Life without Friction  

NSDL National Science Digital Library

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

Engineering K-Phd Program

208

Friction plug welding  

NASA Technical Reports Server (NTRS)

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

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

2001-01-01

209

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

NASA Astrophysics Data System (ADS)

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

Arslan, Kamil; Onur, Nevzat

2013-07-01

210

Drag reduction potentials of turbulence manipulation in adverse pressure gradient flows  

Microsoft Academic Search

It is noted that there is no neccessary correlation between skin friction minimization and drag minimization; the latter requires that account should be taken of both friction and pressure drag, so that in adverse pressure gradient regions, both low skin friction and small shape factor should be pursued. This restricts drag-reduction potentials, since skin friction redictions often entail shape-factor increments.

B. van den Berg

1988-01-01

211

Turbulent heat transfer enhancement in a heat exchanger using helically corrugated tube  

Microsoft Academic Search

The augmentation of convective heat transfer in a single-phase turbulent flow by using helically corrugated tubes has been experimentally investigated. Effects of pitch-to-diameter ratio (P\\/DH=0.18, 0.22 and 0.27) and rib-height to diameter ratio (e\\/DH=0.02, 0.04 and 0.06) of helically corrugated tubes on the heat transfer enhancement, isothermal friction and thermal performance factor in a concentric tube heat exchanger are examined.

S. Pethkool; S. Eiamsa-ard; S. Kwankaomeng; P. Promvonge

2011-01-01

212

Presentation of computer code SPIRALI for incompressible, turbulent, plane and spiral grooved cylindrical and face seals  

NASA Technical Reports Server (NTRS)

A viewgraph presentation is made showing the capabilities of the computer code SPIRALI. Overall capabilities of SPIRALI include: computes rotor dynamic coefficients, flow, and power loss for cylindrical and face seals; treats turbulent, laminar, Couette, and Poiseuille dominated flows; fluid inertia effects are included; rotor dynamic coefficients in three (face) or four (cylindrical) degrees of freedom; includes effects of spiral grooves; user definable transverse film geometry including circular steps and grooves; independent user definable friction factor models for rotor and stator; and user definable loss coefficients for sudden expansions and contractions.

Walowit, Jed A.

1994-01-01

213

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

2009-12-31

214

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

215

Quantum Turbulence  

NASA Astrophysics Data System (ADS)

We examine developments in the study of quantum turbulence with a special focus on clearly defining many of the terms used in the field. We critically review the diverse theoretical, computational, and experimental approaches from the point of view of experimental observers. Similarities and differences between the general properties of classical and quantum turbulence are elucidated. The dynamics and interactions of quantized vortices and their role in quantum turbulence are discussed with particular emphasis on reconnection and vortex ring collapse. A stark distinction between the velocity statistics of quantum and classical turbulence is exhibited and used to highlight a potential analogy between quantum turbulence and magnetohydrodynamic (MHD) turbulence in astrophysical plasmas. Although much of this review pertains to superfluid 4He (He II), the underlying science is broadly applicable to other quantum fluids such as 3He-B, type-II superconductors, Bose-Einstein condensates, Weinberg-Salam fields, and grand-unified-theory (GUT) Higgs fields.

Paoletti, Matthew S.; Lathrop, Daniel P.

2011-03-01

216

Noncontact dielectric friction.  

PubMed

Dielectric fluctuations are shown to be the dominant source of noncontact friction in high-sensitivity scanning probe microscopy of dielectric materials. Recent measurements have directly determined the friction acting on custom-fabricated single-crystal silicon cantilevers whose capacitively charged tips are located 3-200 nm above thin films of poly(methyl methacrylate), poly(vinyl acetate), and polystyrene. Differences in measured friction among these polymers are explained here by relating electric field fluctuations at the cantilever tip to dielectric relaxation of the polymer. PMID:16869550

Kuehn, Seppe; Marohn, John A; Loring, Roger F

2006-08-01

217

Noncontact Dielectric Friction  

PubMed Central

Dielectric fluctuations are shown to be the dominant source of noncontact friction in high-sensitivity scanning probe microscopy of dielectric materials. Recent measurements have directly determined the friction acting on custom-fabricated single-crystal silicon cantilevers whose capacitively charged tips are located 3–200 nm above thin films of poly(methyl methacrylate), poly(vinyl acetate), and polystyrene. Differences in measured friction among these polymers are explained here by relating electric field fluctuations at the cantilever tip to dielectric relaxation of the polymer.

Kuehn, Seppe; Marohn, John A.; Loring, Roger F.

2007-01-01

218

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

219

Glass Fiber Based Friction Materials  

Microsoft Academic Search

Glass fibers have been considered as a reinforcing agent in friction linings as one component of a potential substitute for asbestos. The mechanical, thermal and frictional properties of six different grades of glass fibers have been evaluated in model phenolic resin based friction materials. The Young's modulus, ultimate tensile strength, flexural strength, flexural modulus, coefficient of thermal expansion and friction

N. Subramaniam; Brijnaresh R. Sinha; Frank D. Blum; Yung-Rwei Chen; L. R. Dharani

1991-01-01

220

Modulated Turbulence  

NASA Astrophysics Data System (ADS)

Many turbulent flows are subject to periodic modulation, examples are the pulsatile flow of blood through arteries and geophysical flows driven by periodic tides. When the modulation is slow, the turbulence will adjust adiabatically, but when the modulation period comes close to an internal time scale of the flow, the turbulence may resonate with the driving. The possibility of a resonance is intriguing as one may object that turbulence does not have a single dominant timescale, but a continuum of strongly fluctuating times. In our experiment we periodically modulate a turbulent windtunnel flow with an active grid. An active grid is a regular grid of axes with attached vanes which are rotated by servo motors. By controlling the time-dependent angle of all axes precisely, the grid cycles through a sequence of transparency patterns. Thus we modulate turbulence in space, characterized by these patterns, and time, characterized by the modulation frequency. We consider 3 distinct spatial modes, all share the same transparency sequence. We find a large resonant enhancement of the mean turbulent dissipation rate at a modulation frequency which equals the large-eddy turnover rate. Thus, we find the best frequency to inject energy in a turbulent flow. The resonant enhancement depends on the spatial mode of the grid, but all spatial modes share the same behavior of the response. Modulation only affects the large-scale spatial structure of turbulence, leaving the small-scale motion unaltered.

Ergun Cekli, Hakki; van de Water, Willem

2009-11-01

221

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

222

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

223

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

224

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

SciTech Connect

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

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

2010-02-15

225

Internal Rotor Friction Instability.  

National Technical Information Service (NTIS)

The analytical developments and experimental investigations performed in assessing the effect of internal friction on rotor systems dynamic performance are documented. Analytical component models for axial splines, Curvic splines, and interference fit joi...

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

1990-01-01

226

Dry Friction Bearings.  

National Technical Information Service (NTIS)

Contents: Porous metal ceramic bearings; Mineral ceramic bearings; Self-lubricating pressed wood bearings; Metal materials for special bearings; Operation of rolling surface bearings under dry friction conditions; Rolling surface bearings with solid lubri...

B. D. Voronkov

1970-01-01

227

Friction stir welding tool  

SciTech Connect

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

228

Friction-Stir Processing.  

National Technical Information Service (NTIS)

Friction-stir processing (FSP) is an emerging surface-engineering technology that can locally eliminate casting defects and refine microstructures, thereby improving strength and ductility, increase resistance to corrosion and fatigue, enhance formability...

M. W. Mahoney, S. P. Lynch

2006-01-01

229

Friction Stir Weld Tools  

NASA Technical Reports Server (NTRS)

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

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

2007-01-01

230

Friction stir weld tools  

NASA Technical Reports Server (NTRS)

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

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

2007-01-01

231

Friction Stir Welding Development  

NASA Technical Reports Server (NTRS)

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

Romine, Peter L.

1998-01-01

232

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

233

The effect of journal misalignment on the operation of a turbulent flow hydrostatic bearing  

SciTech Connect

An analysis for calculation of the dynamic force and moment response in turbulent flow, orifice compensated hydrostatic journal bearings is presented. The fully developed flow of a barotropic liquid is described by variable properties, bulk-flow equations and local turbulent friction factors based on bearing surface condition. Bearing load and moments and, dynamic force and moment coefficients are calculated for perturbations in journal center displacements and misaligned journal axis rotations. Numerical results for the effect of static misalignment angles in the plane of the eccentricity vector are presented for a water lubricated hydrostatic bearing. The predictions show that journal axis misalignment causes a reduction in load capacity due to loss in film thickness, increases the flow rate and produces significant restoring moments (couples). Force and moment coefficients due to dynamic journal axis rotations are also discussed. 37 refs.

San Andres, L. (Texas A M Univ., College Station (United States))

1993-07-01

234

Topological complexity of frictional interfaces: friction networks  

NASA Astrophysics Data System (ADS)

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

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

2012-03-01

235

Turbulent acceleration and heating in toroidal magnetized plasmas  

SciTech Connect

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

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

2013-07-15

236

Some characteristics of turbulent boundary layers in rapidly accelerated flows  

NASA Technical Reports Server (NTRS)

An analysis of time-mean-turbulent boundary layer velocity profiles measured in a rapidly accelerating flow suggests that the outer region of the velocity profiles consists of essentially inviscid, rotational flow. The extent of this inviscid outer region was observed in some cases to exceed 90 percent of what is ordinarily thought of as the turbulent boundary layer thickness. On the other hand, the inner frictional region of these velocity profiles appears to have turbulent characteristics similar to those of more conventional turbulent boundary layers. Hence, the outer edge boundary condition for this inner region is more properly the external rotational flow region than the free stream.

Brinich, P. F.; Neumann, H. E.

1971-01-01

237

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

Microsoft Academic Search

The friction coefficient between a railway wheel and rail surface is a crucial factor in maintaining high acceleration and braking performance of railway vehicles thus monitoring this friction coefficient is important. Restricted by the difficulty in directly measuring the friction coefficient, the creep force or creepage, indirect methods using state observers are used more frequently. This paper presents an approach

Y Zhao; B Liang; S Iwnicki

2012-01-01

238

RIBS. Turbulent Flow Inside Pipes with Two-Dimensional Rib Roughness  

SciTech Connect

A commonly used internal enhancement for single-phase forced-convective turbulent flow applications is tranverse and/or near tranverse ribs. These enhanced surfaces consist of a uniform inside diameter with periodic and discrete disruption of ribs. Enhanced tubes of this type are made by an extrusion process and are used in some condensers and evaporators in refrigeration systems. Tubes of this type fall into an enhancement category called separation and reattachment that has been identified as one of the most energy efficient. Lacking are prediction methods that are mechanistic based that can be used to calculate the heat-transfer coefficients and friction-factors for tubes with this enhancement type. This program calculates the Nusselt number and friction factor for enhanced tubes with tranverse, rectangular ribs with a spacing exceeding the reattachment length. The input quantities are the enhancement height, spacing, and the width. The Nusselt number and friction factor are calculated for a specific Reynolds number or for a range of Reynolds numbers. Users of the program are heat-exchanger designers, enhanced tubing suppliers, and research organizations or academia who are developing or validating prediction methods. The manufacturers of refrigeration heat exchangers and enhanced tube suppliers are potential users of this software.

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

1993-05-01

239

Research on Frictional Wear Properties of C\\/C Composite with Electric Current  

Microsoft Academic Search

\\u000a Using C\\/C composite and chrome bronze as a friction couple, the frictional wear properties of C\\/C composite with electric\\u000a current is studied in this paper. The results have shown that current, velocity and load are important factors to affect the\\u000a frictional wear properties of C\\/C composite with electric current. The coefficient of friction and the wear rate increase\\u000a with the

L. X. Jia; Y. Z. Zhang; J. Li; L. M. Sun; Y. W. Zhao

240

Friction of Aviation Engines  

NASA Technical Reports Server (NTRS)

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

Sparrow, S W; Thorne, M A

1928-01-01

241

Downstream influence of swept slot injection in hypersonic turbulent flow  

NASA Technical Reports Server (NTRS)

Results of an experimental and numerical investigation of tangential swept slot injection into a thick turbulent boundary layer at Mach 6 are presented. Film cooling effectiveness, skin friction, and flow structure downstream of the swept slot injection were investigated. The data were compared with that for unswept slots, and it was found that cooling effectiveness and skin friction reductions are not significantly affected by sweeping the slot.

Hefner, J. N.; Cary, A. M., Jr.; Bushnell, D. B.

1977-01-01

242

Recent advances in active control of turbulent boundary layers  

NASA Astrophysics Data System (ADS)

In this article, we review the recent progress in active control of a turbulent boundary layer for skin-friction drag reduction. Near-wall coherent structures, which are closely associated with large skin-friction drag and are thus often the target to be manipulated, are discussed briefly, providing a rationale of various control strategies. Open- and closed-loop controls are extensively reviewed, largely focusing on techniques and drag-reduction mechanisms. Finally, some concluding remarks are given.

Zhou, Yu; Bai, HongLei

2011-07-01

243

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

244

Simple Lagrangian formulation of bubbly flow in a turbulent boundary layer (bubbly boundary layer flow)  

Microsoft Academic Search

For the theoretical consideration of a system for reducing skin friction, a mathematical model was derived to represent, in a two-phase field, the effect on skin friction of the injection of micro air bubbles into the turbulent boundary layer of a liquid stream. Based on the Lagrangian method, the equation of motion governing a single bubble was derived. The random

Yuki Yoshida; Yoshiaki Takahashi; Hiroharu Kato; Akira Masuko; Osamu Watanabe

1997-01-01

245

Friction enhancement in concertina locomotion of snakes  

PubMed Central

Narrow crevices are challenging terrain for most organisms and biomimetic robots. Snakes move through crevices using sequential folding and unfolding of their bodies in the manner of an accordion or concertina. In this combined experimental and theoretical investigation, we elucidate this effective means of moving through channels. We measure the frictional properties of corn snakes, their body kinematics and the transverse forces they apply to channels of varying width and inclination. To climb channels inclined at 60°, we find snakes use a combination of ingenious friction-enhancing techniques, including digging their ventral scales to double their frictional coefficient and pushing channel walls transversely with up to nine times body weight. Theoretical modelling of a one-dimensional n-linked crawler is used to calculate the transverse force factor of safety: we find snakes push up to four times more than required to prevent sliding backwards, presumably trading metabolic energy for an assurance of wall stability.

Marvi, Hamidreza; Hu, David L.

2012-01-01

246

Effect of frictional heating on brake materials  

NASA Technical Reports Server (NTRS)

An exploratory study of the properties of aircraft brake materials was made to determine ways of improving friction and wear behavior while minimizing surface temperatures. It is found that frictional variation at high temperature involves material softening and metal transfer, formation of oxides, and surface melting. The choice of proper materials to combat these effects is discussed. Minimum surface temperatures are found to result from use of materials with large density-specific heat and density-specific heat-conductivity factors, use of a higher load-lower friction system, and maximization of the contact area. Some useful trade-off criteria for the size of brake disks against weight considerations are suggested. Additional information on material behavior and peak braking temperatures was gathered from an inspection of used brake pads and rotor disks.

Ho, T.-L.; Peterson, M. B.; Ling, F. F.

1974-01-01

247

Skin friction balance  

NASA Astrophysics Data System (ADS)

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

Ping, Tcheng; Supplee, Frank H., Jr.

1989-06-01

248

Skin friction balance  

NASA Technical Reports Server (NTRS)

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

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

1989-01-01

249

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

250

Frictional cooling: Experimental results  

NASA Astrophysics Data System (ADS)

The classical methods used in beam cooling are hard to be adapted for a beam of short-lived elementary particles. A novel method, the so-called frictional cooling - that is cooling a beam of low-energy charged particles by moderation in matter and acceleration in an electrostatic field - has been shown to be feasible. In our experiments performed in 1994/1995 a beam of short-lived particles was cooled for the first time ever. Utilizing frictional cooling on a beam of slow negative muons we observed increase in phase space density by about one order of magnitude.

Mühlbauer, M.; Daniel, H.; Hartmann, F. J.; Hauser, P.; Kottmann, F.; Petitjean, C.; Schott, W.; Taqqu, D.; Wojciechowski, P.

1999-06-01

251

Nonlinear internal friction, chaos, fractal and musical instruments.  

National Technical Information Service (NTIS)

Nonlinear and structure sensitive internal friction phenomena in materials are used for characterizing musical instruments. It may be one of the most important factors influencing timbre of instruments. As a nonlinear dissipated system, chaos and fractals...

Z. Q. Sun C. W. Lung

1995-01-01

252

Studies in turbulence  

NASA Technical Reports Server (NTRS)

Various papers on turbulence are presented. Individual topics addressed include: modeling the dissipation rate in rotating turbulent flows, mapping closures for turbulent mixing and reaction, understanding turbulence in vortex dynamics, models for the structure and dynamics of near-wall turbulence, complexity of turbulence near a wall, proper orthogonal decomposition, propagating structures in wall-bounded turbulence flows. Also discussed are: constitutive relation in compressible turbulence, compressible turbulence and shock waves, direct simulation of compressible turbulence in a shear flow, structural genesis in wall-bounded turbulence flows, vortex lattice structure of turbulent shear slows, etiology of shear layer vortices, trilinear coordinates in fluid mechanics.

Gatski, Thomas B. (editor); Sarkar, Sutanu (editor); Speziale, Charles G. (editor)

1992-01-01

253

Cermet Anti-Friction Material.  

National Technical Information Service (NTIS)

The nickel-based anti-friction cermet material containing barium fluoride is distinguished by the fact that iron and boron are introduced in order to increase its wear-resistance under dry friction conditions.

G. G. Lvova, I. A. Panfilova, I. M. Fedorchenko, L. I. Pugina, N. E. Ponomarenko

1973-01-01

254

Magnetic Low-Friction Track  

NASA Astrophysics Data System (ADS)

The standard low-friction tracks used to test Newton's laws are the air track and the low-friction cart track. Both are commercially available and provide low-friction environments to test various physics concepts. At a recent science fair, one of the authors (JG) presented a magnetically levitated cart and track. A literature search found no previous testing of magnetically levitated carts.1 This paper compares a magnetically levitated cart against the two standard low-friction tracks.

Paetkau, Mark; Bahniwal, Manpreet; Gamblen, James

2008-05-01

255

Micro Surface Texturing for Friction Control  

NASA Astrophysics Data System (ADS)

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

Rashwan, Ola

256

Friction drilling of cast metals  

Microsoft Academic Search

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

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

2006-01-01

257

Interfacial friction between semiflexible polymers and crystalline surfaces  

NASA Astrophysics Data System (ADS)

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

Priezjev, Nikolai V.

2012-06-01

258

Friction Stir Weld Tools.  

National Technical Information Service (NTIS)

A friction stir weld tool sleeve is supported by an underlying support pin. The pin material is preferably selected for toughness and fracture characteristics. The pin sleeve preferably has a geometry which employs the use of an interrupted thread, a plur...

L. N. Payton R. W. Carter

2007-01-01

259

Polymer Friction Research.  

National Technical Information Service (NTIS)

Earlier work in this laboratory has shown that the friction and wear properties of the poly(n-alkyl methacrylates) are related to the side chain crystallinity in the polymer. In both thick specimens and thin films deposited on solid substrates, the coeffi...

H. Gisser J. Illuminati S. Portnoy

1974-01-01

260

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

261

Friction: Let it slip  

NASA Astrophysics Data System (ADS)

Friction involves a complex set of phenomena spanning a large range of length scales, but experiments assessing the evolution of the slip-front between two dry sliding bodies now reveal that slip can be reasonably well described by linear fracture mechanics theory.

Carpick, Robert W.; Bennewitz, Roland

2014-06-01

262

Coulomb Friction Damper  

NASA Technical Reports Server (NTRS)

Standard hydraulic shock absorber modified to form coulomb (linear friction) damper. Device damps very small velocities and is well suited for use with large masses mounted on soft springs. Damping force is easily adjusted for different loads. Dampers are more reliable than fluid dampers and also more economical to build and to maintain.

Appleberry, W. T.

1983-01-01

263

Low Friction Servo Valve.  

National Technical Information Service (NTIS)

The low friction servo valve has a housing which contains therein an input shaft, a pair of slidably mounted valve spools, and an actuator. Mechanical input is received by the input shaft in order to provide rotational displacement thereof. This displacem...

R. F. Richter

1979-01-01

264

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

NASA Technical Reports Server (NTRS)

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

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

1993-01-01

265

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

266

Turbulent boundary layer with injection and surface roughness  

Microsoft Academic Search

Experimental studies were conducted to measure directly the skin friction on an axisymmetric body with and without injection through the surface in a turbulent, low-speed flow. The axisymmetric model, with interchangeable solid, smooth and porous, slightly rough walls, was tested at length Re from 4.96 to 6.11 million. For both the law-of-the-wall and axial turbulent intensities, good correlation between the

J. A. Schetz; B. Nerney

1977-01-01

267

Internal rotor friction instability  

NASA Technical Reports Server (NTRS)

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

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

1990-01-01

268

Aspects of Turbulent / Non-Turbulent Interfaces.  

National Technical Information Service (NTIS)

A distinct boundary between turbulent and non-turbulent regions in a fluid of otherwise constant properties is found in many laboratory and engineering turbulent flows, including jets, mixing layers, boundary layers and wakes. Generally, the flow has mean...

D. K. Bisset J. C. R. Hunt M. M. Rogers

1999-01-01

269

Effects of uniform blowing or suction from a spanwise slot on a turbulent boundary layer flow  

Microsoft Academic Search

Effects of uniform blowing or suction from a spanwise slot on a turbulent boundary layer flow are investigated using the direct numerical simulation technique. The magnitudes of blowing or suction are less than 10% of the free-stream velocity. The skin friction and near-wall turbulence intensities are significantly changed by blowing and suction. In the case of uniform blowing, the skin

Haecheon Choi

1999-01-01

270

Quantum friction and fluctuation theorems  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

271

Aqueous turbulence structure immediately adjacent to the air - water interface and interfacial gas exchange  

NASA Astrophysics Data System (ADS)

Air-sea interaction and the interfacial exchange of gas across the air-water interface are of great importance in coupled atmospheric-oceanic environmental systems. Aqueous turbulence structure immediately adjacent to the air-water interface is the combined result of wind, surface waves, currents and other environmental forces and plays a key role in energy budgets, gas fluxes and hence the global climate system. However, the quantification of turbulence structure sufficiently close to the air-water interface is extremely difficult. The physical relationship between interfacial gas exchange and near surface turbulence remains insufficiently investigated. This dissertation aims to measure turbulence in situ in a complex environmental forcing system on Lake Michigan and to reveal the relationship between turbulent statistics and the CO2 flux across the air-water interface. The major objective of this dissertation is to investigate the physical control of the interfacial gas exchange and to provide a universal parameterization of gas transfer velocity from environmental factors, as well as to propose a mechanistic model for the global CO2 flux that can be applied in three dimensional climate-ocean models. Firstly, this dissertation presents an advanced measurement instrument, an in situ free floating Particle Image Velocimetry (FPIV) system, designed and developed to investigate the small scale turbulence structure immediately below the air-water interface. Description of hardware components, design of the system, measurement theory, data analysis procedure and estimation of measurement error were provided. Secondly, with the FPIV system, statistics of small scale turbulence immediately below the air-water interface were investigated under a variety of environmental conditions. One dimensional wave-number spectrum and structure function sufficiently close to the water surface were examined. The vertical profiles of turbulent dissipation rate were intensively studied. Comparison between the turbulence structures measured during the wind wave initiation period and those obtained during the growing period was presented. Significant wave effects on near surface turbulence were found. A universal scaling law was proposed to parameterize turbulent dissipation rate immediately below the air-water interface with friction velocity, significant wave height and wave age. Finally, the gas transfer velocity was measured with a floating chamber (FC) system, along with simultaneously FPIV measurements. Turbulent dissipation rate both at the interface and at a short distance away from the interface (~ 10 cm) were analyzed and used to examine the small scale eddy model. The model coefficient was found to be dependent on the level of turbulence, instead of being a constant. An empirical relationship between the model coefficient and turbulent dissipation rate was provided, which improved the accuracy of the gas transfer velocity estimation by more than 100% for data acquired. Other data from the literature also supported this empirical relation. Furthermore, the relationship between model coefficient and turbulent Reynolds number was also investigated. In addition to physical control of gas exchange, the disturbance on near surface hydrodynamics by the FC was also discussed. Turbulent dissipation rates are enhanced at the short distance away from the interface, while the surface dissipation rates do not change significantly.

Wang, Binbin

272

Static-dynamic friction transition of FRP esthetic orthodontic wires on various brackets by suspension-type friction test.  

PubMed

A new testing apparatus for the measurement of frictional properties was designed and the frictional coefficients were obtained and compared with each other in various combinations of brackets and orthodontic wires, including esthetic fiber-reinforced plastic (FRP) wire that was especially designed and manufactured. Three kinds of wires (stainless steel, nickel-titanium, and FRP) and four brackets (single-crystal alumina, polycrystalline alumina, polycarbonate, and stainless steel) were used. The testing was done under dry and wet conditions. The friction testing equipment was designed to attach the bracket to a C-shaped bar suspended with a variable mass, and sliding along a fixed wire. The transition between static and dynamic friction was measured as a breakaway force, with the use of a universal test machine. In addition to material properties, this testing fixture eliminates geometrical factors, such as the rotational moment at the edge of the bracket slot, deflection of the orthodontic wire, and tension of the ligature wire. Nearly ideal frictional properties between materials are obtained. The frictional properties of FRP wire were similar to those of metal wires on all brackets, except the polycrystalline alumina bracket. The frictional coefficient between the polycrystalline ceramic bracket and FRP wire was larger than that of other combinations. There was little difference in frictional coefficients between dry and wet conditions. PMID:14598404

Suwa, N; Watari, F; Yamagata, S; Iida, J; Kobayashi, M

2003-11-15

273

Turbulence in Compressible Flows  

NASA Technical Reports Server (NTRS)

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

1997-01-01

274

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

SciTech Connect

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

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

2010-09-15

275

Turbulent drag reduction through oscillating discs  

NASA Astrophysics Data System (ADS)

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

Wise, Daniel J.; Ricco, Pierre

2014-05-01

276

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

NASA Technical Reports Server (NTRS)

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

Shih, Tsan-Hsing; Lumley, John L.

1992-01-01

277

Holographic turbulence.  

PubMed

We construct turbulent black holes in asymptotically AdS4 spacetime by numerically solving Einstein's equations. Using the AdS/CFT correspondence we find that both the dual holographic fluid and bulk geometry display signatures of an inverse cascade with the bulk geometry being well approximated by the fluid-gravity gradient expansion. We argue that statistically steady-state black holes dual to d dimensional turbulent flows have horizons whose area growth has a fractal-like structure with fractal dimension D=d+4/3. PMID:24785028

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

2014-04-18

278

Holographic Turbulence  

NASA Astrophysics Data System (ADS)

We construct turbulent black holes in asymptotically AdS4 spacetime by numerically solving Einstein's equations. Using the AdS/CFT correspondence we find that both the dual holographic fluid and bulk geometry display signatures of an inverse cascade with the bulk geometry being well approximated by the fluid-gravity gradient expansion. We argue that statistically steady-state black holes dual to d dimensional turbulent flows have horizons whose area growth has a fractal-like structure with fractal dimension D =d+4/3.

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

2014-04-01

279

Friction and wear of plasma-deposited diamond films  

NASA Technical Reports Server (NTRS)

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

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

1993-01-01

280

Friction at small displacement.  

NASA Technical Reports Server (NTRS)

Low contact resistance between metal surfaces is often observed in spite of interposed lubricant and/or oxide films. To study this effect an apparatus is used with which normal force and tangential microdisplacement are applied between a small lead rider and a gold flat with various surface film conditions. Under nonoxidized and nonlubricated conditions, and with either oxide or stearic acid lubricant film alone, friction is high and contact resistance is low. With oxide and lubricant together, friction is much lower and slide is smooth, but contact resistance remains low and Ohm's law is obeyed. The results are consistent with Greenwood's theory of contact resistance for a cluster of minute metallic contact spots within the load-supporting area. The contact resistance of such a cluster is indistinguishable, for practical purposes, from that given by complete metallic contact.

Campbell, W. E.; Aronstein, J.

1972-01-01

281

The theory of homogeneous turbulence in superfluid 3He B  

NASA Astrophysics Data System (ADS)

Recent studies have shown that the structure of homogeneous grid turbulence in superfluid 4He is quite similar to that of its classical counterpart. Both the normal component and the superfluid component are turbulent. On length scales greater than the spacing, ?, of the quantized vortex lines that allow the superfluid component to undergo turbulent (rotational) motion, the velocity fields in the two components are practically identical and are consistent with the existence an inertial regime with an energy spectrum of the Kolmogorov form. This type of turbulence is possible because the normal component has a very small viscosity, so that the Reynolds number for the normal fluid can be very high. In the case of superfluid 3He B, the superfluid component can be turbulent in much the same way as in 4He, but turbulence in the normal fluid is inhibited by a very large viscosity. An outline is given of a theory of homogeneous grid turbulence in this case. Because the velocity fields in the two fluids can no longer be the same, a force of mutual friction must act between the two fluids. It is shown that this frictional force damps turbulent motion, by an amount that depends on the dimensionless parameter, ?, that characterizes the magnitude of the mutual friction. If ?>1 turbulence on all length scales greater than ? is strongly damped, but for smaller values of ? the damping is confined to larger eddies, the smaller eddies still exhibiting an inertial regime with a Kolmogorov spectrum. Comparison is made with recent theoretical work by Volovik and his co-workers and with the experiments of Finne et al on the way in which 3He B is brought into rotation with a containing vessel for different values of ?. The presentation emphasizes the physical principles involved, the mathematical details having been set out in another paper.

Vinen, W. F.

2005-08-01

282

Anisotropic surface friction  

NASA Astrophysics Data System (ADS)

Contrary to the literature, we find that altering the surface roughness has a large effect on intruder drag in the quasi-static regime. Moreover we pattern the surface with a sawtooth texture and observe anisotropic drag: when the texture is comparable in size to the bead diameter the frictional force is 1/3rd greater for the flow directed against the sawtooth versus the opposite flow. We present a systematic study showing the dependence of the anisotropy on the texture's orientation.

Koehler, Stephan; Delacruz, Ibar; Nesbitt, Nathan; Morse, Ian

2011-11-01

283

Friction in rail guns  

NASA Technical Reports Server (NTRS)

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

Kay, P. K.

1984-01-01

284

Improved skin friction interferometer  

Microsoft Academic Search

An improved system for measuring aerodynamic skin friction which uses a dual-laser-beam oil-film interferometer was developed. Improvements in the optical hardware provided equal signal characteristics for each beam and reduced the cost and complexity of the system by replacing polarization rotation by a mirrored prism for separation of the two signals. An automated, objective, data-reduction procedure was implemented to eliminate

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

1986-01-01

285

High Speed Ice Friction  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

286

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

287

Turbulence modeling  

NASA Technical Reports Server (NTRS)

A survey designed to provide an introduction to the subject of turbulence modeling, and to explain the need for such models is given. The subject is developed along chronological lines since this provides a logical development plan and also because it then moves from relatively simple phenomenological models through more complicated procedures and ultimately to the subject of large-eddy simulation.

Murphy, J. D.

1984-01-01

288

Direct Numerical Simulations of Turbulent Flows over Superhydrophobic Surfaces  

Microsoft Academic Search

Direct numerical simulations are used to investigate the drag reducing performance of superhydrophobic surfaces in turbulent channel flow. Slip velocities, wall shear stresses, and Reynolds stresses are considered for a variety of superhydrophobic surface micro-feature geometry configurations at a friction Reynolds number of Retau = 180. For the largest micro-feature spacing of 90mum an average slip velocity over 75% of

Michael B. Martell; J. Blair Perot; Jonathan P. Rothstein

2008-01-01

289

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

290

Limiting behavior of turbulent scalar transport close to a wall  

Microsoft Academic Search

A direct numerical simulation of fully developed turbulent flow in a channel is used to study passive scalar transport in the immediate vicinity of a wall. The Reynolds number, based on the channel half-height and friction velocity, is 150 and the Prandtl number is varied from 1 to 10. DNS results and experimental measurements of mass transfer rates at high

Y. Na; T. J. Hanratty

2000-01-01

291

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

292

Frictional stress at the wall in an ascending gas—Liquid flow  

Microsoft Academic Search

The determination of the coefficients of friction and heat and mass exchange in a turbulent two-phase flow is a very important problem today both from the purely scientific point of view and for many technical applications. The main difficulty in solving this problem is the large number of parameters affecting the structure of the two-phase flow. A knowledge of the

A. P. Burdukov; O. N. Kashinskii; V. P. Odnoral

1979-01-01

293

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

Microsoft Academic Search

The sailfish is the fastest sea animal, reaching its maximum speed of 110 km\\/h. On its skin, a number of V-shaped protrusions pointing downstream exist. Thus, in the present study, the possibility of reducing the skin friction using its shape is investigated in a turbulent boundary layer. We perform a parametric study by varying the height and width of the

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

2008-01-01

294

Friction laws for lubricated nanocontacts  

NASA Astrophysics Data System (ADS)

We have used friction force microscopy to probe friction laws for nanoasperities sliding on atomically flat substrates under controlled atmosphere and liquid environment, respectively. A power law relates friction force and normal load in dry air, whereas a linear relationship, i.e., Amontons' law, is observed for junctions fully immersed in model lubricants, namely, octamethylciclotetrasiloxane and squalane. Lubricated contacts display a remarkable friction reduction, with liquid and substrate specific friction coefficients. Comparison with molecular dynamics simulations suggests that load-bearing boundary layers at junction entrance cause the appearance of Amontons' law and impart atomic-scale character to the sliding process; continuum friction models are on the contrary of limited predictive power when applied to lubrication effects. An attempt is done to define general working conditions leading to the manifestation of nanoscale lubricity due to adsorbed boundary layers.

Buzio, R.; Boragno, C.; Valbusa, U.

2006-09-01

295

Friction behavior of quenched and tempered steel in partial and gross slip conditions in fretting point contact  

Microsoft Academic Search

Tangential traction caused by friction in contacting surfaces is a major factor in fretting fatigue that increases stress levels and leads to a reduction in fatigue life. Friction in fretting contact was studied in partial, mixed and gross slip conditions on quenched and tempered steel. Measurements were made with sphere-on-plane contact geometry for polished and ground surfaces. Friction was evaluated

A. Pasanen; A. Lehtovaara; R. Rabb; P. Riihimäki

2009-01-01

296

Anomalous friction in suspended graphene  

NASA Astrophysics Data System (ADS)

Since the discovery of the Amonton's law and with support of modern tribological models, friction between surfaces of three-dimensional materials is known to generally increase when the surfaces are in closer contact. Here, using molecular dynamics simulations of friction force microscopy on suspended graphene, we demonstrate an increase of friction when the scanning tip is retracted away from the sample. We explain the observed behavior and address why this phenomenon has not been observed for isotropic 3D materials.

Smolyanitsky, A.; Killgore, J. P.

2012-09-01

297

The Friction of Piston Rings  

NASA Technical Reports Server (NTRS)

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

Tischbein, Hans W

1945-01-01

298

Internal friction in enzyme reactions.  

PubMed

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

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

2013-01-01

299

A Statistical Analysis of the Effect of Wet Pavement Friction on Highway Traffic Safety  

Microsoft Academic Search

This study focuses on analysis of statistical association between wet pavement friction and roadway crashes aimed at answering these questions: (1) Is wet pavement friction a significant factor for explaining variation in crash history? (2) Is this factor more relevant at locations with high expected braking frequency, such as sharp curves and intersections?The data set includes “found data” locations with

John N. Ivan; Nalini Ravishanker; Eric Jackson; Brien Aronov; Sizhen Guo

2012-01-01

300

Rolling Friction of Metals  

Microsoft Academic Search

Theoretical equations of rolling friction coefficient of metal by the rigid pendulum method were obtained as a function of dynamic mechanical properties of metal for the case of cylinder and sphere as follows, lambdac{=}3k\\/2pi(3\\/2)1\\/3\\\\tandelta(W\\/E1(omega))1\\/3r-2\\/3 and lambdas{=}128k\\/15pi2(4\\/pi)1\\/4\\\\tandelta(W\\/E1(omega))1\\/4r-3\\/4, respectively, where \\\\tandelta is the mechanical loss, E1(omega) the Young's modulus, W the load, r the radius of roller and k the adjusting parameter.

Kikuwo Minato; Chitoshi Nakafuku; Tetuo Takemura

1969-01-01

301

Improved Skin Friction Interferometer  

NASA Technical Reports Server (NTRS)

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

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

1986-01-01

302

Turbulence modeling  

NASA Technical Reports Server (NTRS)

Recent developments at several levels of statistical turbulence modeling applicable to aerodynamics are briefly surveyed. Emphasis is on examples of model improvements for transonic, two-dimensional flows. Experience with the development of these improved models is cited to suggest methods of accelerating the modeling process necessary to keep abreast of the rapid movement of computational fluid dynamics into the computation of complex three-dimensional flows.

Rubesin, Morris W.

1987-01-01

303

Burgers turbulence  

Microsoft Academic Search

The last decades witnessed a renewal of interest in the Burgers equation. Much activities focused on extensions of the original one-dimensional pressureless model introduced in the thirties by the Dutch scientist J.M. Burgers, and more precisely on the problem of Burgers turbulence, that is the study of the solutions to the one- or multi-dimensional Burgers equation with random initial conditions

Jérémie Bec; Konstantin Khanin

2007-01-01

304

Physics and control of wall turbulence for drag reduction.  

PubMed

Turbulence physics responsible for high skin-friction drag in turbulent boundary layers is first reviewed. A self-sustaining process of near-wall turbulence structures is then discussed from the perspective of controlling this process for the purpose of skin-friction drag reduction. After recognizing that key parts of this self-sustaining process are linear, a linear systems approach to boundary-layer control is discussed. It is shown that singular-value decomposition analysis of the linear system allows us to examine different approaches to boundary-layer control without carrying out the expensive nonlinear simulations. Results from the linear analysis are consistent with those observed in full nonlinear simulations, thus demonstrating the validity of the linear analysis. Finally, fundamental performance limit expected of optimal control input is discussed. PMID:21382821

Kim, John

2011-04-13

305

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

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

306

Filament turbulence  

NASA Astrophysics Data System (ADS)

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

Davidsen, Joern

2010-03-01

307

Finger pad friction and its role in grip and touch  

PubMed Central

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

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

2013-01-01

308

Skin Friction following BLADE manipulation in a turbulent pipe flow  

NASA Astrophysics Data System (ADS)

This study attempts to analyze the measured wall shear stress distribution downstream of single and tandem BLADEs in fully developed pipe flow. Previous works have indicated the adverse effect of overall drag increase with the single BLADE in both channel and pipe flows, and an even larger drag increase with the tandem BLADES. This is contrary to that observed for external boundary layer flow. Extensive comparisons are then made to the wall shear stress distribution following BLADEs in boundary layer flow, leading to the conclusion of little or no potential in the application of BLADEs alone to pipe flow.

Khoo, B. C.; Chew, Y. T.; Mah, Y. A.

1993-09-01

309

Friction Stir Welding  

NASA Technical Reports Server (NTRS)

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

Nunes, Arthur C., Jr.

2008-01-01

310

Towards Petascale DNS of High Reynolds-Number Turbulent Boundary Layer  

NASA Astrophysics Data System (ADS)

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

Webster, Keegan R.

311

Friction versus dilation revisited: Insights from theoretical and numerical models  

NASA Astrophysics Data System (ADS)

The stability and dynamics of geological faults are often controlled by the frictional strength of the fault gouge, a granular layer that accumulates between the fault blocks due to wear. One of the main factors controlling the shear strength (as measured by an apparent friction coefficient) of such granular layers is the systems resistance to dilation, a by-process accompanying granular shear. This paper develops theoretical models connecting shear resistance and dilation during the different types of motions that occur in shear of uniform grains and compares theoretical predictions to results from two-dimensional discrete element simulations. We find that minimization of dilation-associated work (and thus minimization of apparent friction) is the factor that determines whether granular deformation will be localized or distributed and whether grains will predominantly slide or roll. Simulations of more realistic systems, with heterogeneous grain sizes, suggest that the apparent friction and dilation rate follow a physical picture consisting of two of the deformation mechanisms identified in shear of uniform grain systems: (1) shear localization onto a few layers of rolling grains and (2) brief compaction events in which dilated regions outside the shear zone compact. The first mechanism provides a nearly linear relationship between apparent friction and dilation rate. The second mechanism produces significant deviations from the linear relationship, with strongly negative dilation rates reflecting compaction events. Because observed compaction events occur outside of shear zones, dilation rate is not always simply related to apparent friction.

Makedonska, N.; Sparks, D. W.; Aharonov, E.; Goren, L.

2011-09-01

312

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

313

Orbital friction stir weld system  

NASA Technical Reports Server (NTRS)

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

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

2001-01-01

314

Elastic model of dry friction  

SciTech Connect

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

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

2013-09-15

315

Orbital Friction Stir Weld System  

NASA Technical Reports Server (NTRS)

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

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

2001-01-01

316

Skin Friction and Transition Location Measurement on Supersonic Transport Models  

NASA Technical Reports Server (NTRS)

Flow visualization techniques were used to obtain both qualitative and quantitative skin friction and transition location data in wind tunnel tests performed on two supersonic transport models at Mach 2.40. Oil-film interferometry was useful for verifying boundary layer transition, but careful monitoring of model surface temperatures and systematic examination of the effects of tunnel start-up and shutdown transients will be required to achieve high levels of accuracy for skin friction measurements. A more common technique, use of a subliming solid to reveal transition location, was employed to correct drag measurements to a standard condition of all-turbulent flow on the wing. These corrected data were then analyzed to determine the additional correction required to account for the effect of the boundary layer trip devices.

Kennelly, Robert A., Jr.; Goodsell, Aga M.; Olsen, Lawrence E. (Technical Monitor)

2000-01-01

317

Bi-directional, buried-wire skin-friction gage  

NASA Technical Reports Server (NTRS)

A compact, nonobtrusive, bi-directional, skin-friction gage was developed to measure the mean shear stress beneath a three-dimensional boundary layer. The gage works by measuring the heat flux from two orthogonal wires embedded in the surface. Such a gage was constructed and its characteristics were determined for different angles of yaw in a calibration experiment in subsonic flow with a Preston tube used as a standard. Sample gages were then used in a fully three-dimensional turbulent boundary layer on a circular cone at high relative incidence, where there were regimes of favorable and adverse pressure gradients and three-dimensional separation. Both the direction and magnitude of skin friction were then obtained on the cone surface.

Higuchi, H.; Peake, D. J.

1978-01-01

318

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

2013-06-21

319

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

PubMed

The Janssen effect is a unique property of confined granular materials experiencing gravitational compaction in which the pressure at the bottom saturates with an increasing filling height due to frictional interactions with side walls. In this Letter, we replace gravitational compaction with frictional compaction. We study friction-compacted 2D granular materials confined within fixed boundaries on a horizontal conveyor belt. We find that even with high-friction side walls the Janssen effect completely vanishes. Our results demonstrate that gravity-compacted granular systems are inherently different from friction-compacted systems in at least one important way: vibrations induced by sliding friction with the driving surface relax away tangential forces on the walls. Remarkably, we find that the Janssen effect can be recovered by replacing the straight side walls with a sawtooth pattern. The mechanical force introduced by varying the sawtooth angle ? can be viewed as equivalent to a tunable friction force. By construction, this mechanical friction force cannot be relaxed away by vibrations in the system. PMID:24856724

Karim, M Yasinul; Corwin, Eric I

2014-05-01

320

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

NASA Astrophysics Data System (ADS)

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

Karim, M. Yasinul; Corwin, Eric I.

2014-05-01

321

Turbulent Jets?  

NASA Astrophysics Data System (ADS)

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

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

2003-10-01

322

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

323

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

324

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

2005-01-01

325

Rolling friction robot fingers  

NASA Technical Reports Server (NTRS)

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

Vranish, John M. (inventor)

1992-01-01

326

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

327

Reynolds number and pressure gradient effects on compressible turbulent boundary layers  

NASA Technical Reports Server (NTRS)

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

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

1978-01-01

328

A LES-Langevin model for turbulence  

NASA Astrophysics Data System (ADS)

We propose a new model of turbulence for use in large-eddy simulations (LES). The turbulent force, represented here by the turbulent Lamb vector, is divided in two contributions. The contribution including only subfilter fields is deterministically modeled through a classical eddy-viscosity. The other contribution including both filtered and subfilter scales is dynamically computed as solution of a generalized (stochastic) Langevin equation. This equation is derived using Rapid Distortion Theory (RDT) applied to the subfilter scales. The general friction operator therefore includes both advection and stretching by the resolved scale. The stochastic noise is derived as the sum of a contribution from the energy cascade and a contribution from the pressure. The LES model is thus made of an equation for the resolved scales, including the turbulent force, and a generalized Langevin equation integrated on a twice-finer grid. We compare the full model with several approximations. In the first one, the friction operator of the Langevin equation is simply replaced by an empirical constant, of the order of the resolved scale correlation time. In the second approximation, the integration is replaced by a condition of instantaneous adjustment to the stochastic force. In this approximation, our model becomes equivalent to the velocity-estimation model of Domaradzki et al. [1-3]. In the isotropic, homogeneous situations we study, both approximations provide satisfactory results, at a reduced computational cost. The model is finally validated by comparison to DNS and is tested against classical LES models for isotropic homogeneous turbulence, based on eddy viscosity. We show that even in this situation, where no walls are present, our inclusion of backscatter through the Langevin equation results in a better description of the flow.

Laval, J.-P.; Dubrulle, B.

2006-02-01

329

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

NASA Technical Reports Server (NTRS)

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

Inger, G. R.

1989-01-01

330

Aspects of Turbulent / Non-Turbulent Interfaces  

NASA Technical Reports Server (NTRS)

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

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

1999-01-01

331

Development of relaxation turbulence models  

NASA Technical Reports Server (NTRS)

Relaxation turbulence models have been intensively studied. The complete time dependent mass averaged Navier-Stokes equations have been solved for flow into a two dimensional compression corner. A new numerical scheme has been incorporated into the developed computed code with an attendant order of magnitude reduction in computation time. Computed solutions are compared with experimental measurements of Law for supersonic flow. Details of the relaxation process have been studied; several different relaxation models, including different relaxation processes and varying relaxation length, are tested and compared. Then a parametric study has been conducted in which both Reynolds number and wedge angle are varied. To assess effects of Reynolds number and wedge angle, the parametric study includes the comparison of computed separation location and upstream extent of pressure rise; numerical results are also compared with the measurements of surface pressure, skin friction and mean velocity field.

Hung, C. M.

1976-01-01

332

Flexure Bearing Reduces Startup Friction  

NASA Technical Reports Server (NTRS)

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

Clingman, W. Dean

1991-01-01

333

Flow Friction or Spontaneous Ignition?  

NASA Technical Reports Server (NTRS)

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

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

2012-01-01

334

Friction Stir Processing for Efficient Manufacturing  

Microsoft Academic Search

Friction at contacting surfaces in relative motion is a major source of parasitic energy loss in machine systems and manufacturing processes. Consequently, friction reduction usually translates to efficiency gain and reduction in energy consumption. Furthermore, friction at surfaces eventually leads to wear and failure of the components thereby compromising reliability and durability. In order to reduce friction and wear in

Christopher B. Smith; Oyelayo Ajayi

2012-01-01

335

Friction measurements on carbon fibre tows  

Microsoft Academic Search

Friction plays an important role in the production of fibre reinforced composite products. The fibrous tows deform during the forming phase. Friction is regarded as a dominant phenomenon in tow deformation mechanisms. The coefficient of friction is a material-interface characteristic which gives a relation between applied deformation loads and frictional forces. A capstan experiment has been performed with carbon fibre

B. Cornelissen; L. Warnet; R. Akkerman

2010-01-01

336

Fabric-skin friction property measurement system  

Microsoft Academic Search

Purpose – Understanding the friction property between fabric and skin is an important issue in both product design and comfort evaluation. The purpose of this paper is to present a new type of tester to evaluate the friction between skin and clothing. Design\\/methodology\\/approach – The proposed fabric-skin friction tester consists of test host, friction equipment, control box, and computer processing

Xu Wang; Ping Liu; Fumei Wang

2010-01-01

337

Friction Reduction From Electrochemically Deposited Films  

Microsoft Academic Search

The lubricated sliding friction characteristics of both electrochemically precharged and uncharged piston ring specimens were determined m a laboratory friction apparatus. The iron phosphate films deposited on precharged piston ring specimens from a specific lubricant blend reduced both static and dynamic friction by as much as 25 to 35 percent. The mechanisms of friction reduction using electrochemically deposited films were

Simon C. Tung; Simon S. Wang

1991-01-01

338

Rubber friction: comparison of theory with experiment.  

PubMed

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

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

2011-12-01

339

Friction force microscopy studies on SiO2 supported pristine and hydrogenated graphene  

NASA Astrophysics Data System (ADS)

A graphene sample supported on SiO2 with pristine and plasma-hydrogenated parts is investigated by friction force microscopy. An initial contrast in friction is apparent between the two regions. A tip induced cleaning of the surface in the course of continuous scanning results in a very clean surface accompanied with a reduction of the friction force by a factor of up to 4. The contamination is adhering stronger to hydrogenated regions, but once cleaned, the frictional behavior is the same on pristine and hydrogenated graphene. Raman imaging demonstrates that the hydrogenation remains intact under the mechanical treatment.

Fessler, Gregor; Eren, Baran; Gysin, Urs; Glatzel, Thilo; Meyer, Ernst

2014-01-01

340

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

341

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

342

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

343

Understanding the frictional response of organic monolayer coatings using Atomic Force Microscopy  

NASA Astrophysics Data System (ADS)

Friction and wear are yet to be fundamentally understood, yet they can be major limiting factors for applications including microelectromechanical systems (MEMS). We use atomic force microscopy to determine frictional constitutive relations for nanoscale contacts designed to represent the asperities in MEMS. Quantitative measurements of friction and contact stiffness are performed using SiO2- and organic monolayer-functionalized tips on organic monolayer-functionalized silicon. Using octadecyltrichrolosilane, octadecene, and perfluorinated monolayers, we find that friction depends on the type of molecule, its packing density, and the surface attachment chemistry. We also find that fluorination increases friction, as in MEMS, and that molecular transfer to the SiO2 tip causes large variation in the measurements. With monolayer-coated tips, this variation, as well as the overall friction and adhesion, are significantly reduced.

Flater, Erin E.; Corwin, Alex D.; Carpick, Robert W.

2005-03-01

344

Turbulence and marine life  

Microsoft Academic Search

SUMMARY: The decay of the mechanical energy of waves, currents and tides generates turbulence that propagates down- wards, diffuses nutrients and influences in different ways the movement of organisms. Light and turbulence combine in cre- ating local conditions for primary production. Mechanical properties of organisms (size, shape, production of mucilages) interact under different regimes of turbulence. Turbulence is a main

RAMON MARGALEF

345

Nanoscopic friction behavior of pharmaceutical materials.  

PubMed

The characteristics of various pharmaceutical dosage forms are influenced by surface properties such as the friction behavior. For example, die wall friction is a key issue in developing a solid dosage form. However, the friction properties are not completely understood mainly because of the lack of fundamental measurements. Herein, the friction behavior of pharmaceutical materials was investigated and compared with their adhesion behavior using atomic force microscopy. The sliding speed causes significant variations in the frictional force. Compared with other materials, lubricant materials showed less distinct differences in friction tests than in adhesion tests, indicating the dependence of the lubricant efficiency on the stress state. The three parameters obtained from the modified Amonton's law, i.e., absolute frictional force, friction coefficient and residual force, showed consistent trends. Overall, the friction behavior was not a direct reflection of the adhesion forces. The intrinsic friction behavior of a single pharmaceutical particle can be quantified using atomic force microscopy. PMID:17442509

Lee, Jonghwi

2007-08-01

346

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

347

Multimodal Friction Ignition Tester  

NASA Technical Reports Server (NTRS)

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

Davis, Eddie; Howard, Bill; Herald, Stephen

2009-01-01

348

Turbulent boundary-layer control with plasma actuators.  

PubMed

This paper reviews turbulent boundary-layer control strategies for skin-friction reduction of aerodynamic bodies. The focus is placed on the drag-reduction mechanisms by two flow control techniques-spanwise oscillation and spanwise travelling wave, which were demonstrated to give up to 45 per cent skin-friction reductions. We show that these techniques can be implemented by dielectric-barrier discharge plasma actuators, which are electric devices that do not require any moving parts or complicated ducting. The experimental results show different modifications to the near-wall structures depending on the control technique. PMID:21382824

Choi, Kwing-So; Jukes, Timothy; Whalley, Richard

2011-04-13

349

Quantum turbulence in superfluids with wall-clamped normal component  

PubMed Central

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

Eltsov, Vladimir; Hanninen, Risto; Krusius, Matti

2014-01-01

350

Viscous Friction of Polymer Brushes  

NASA Astrophysics Data System (ADS)

Polymer brushes are unique soft structures that can exhibit solid-like behaviors, i.e., if they are deformed by an external force, they can relax and take their original conformations when the external force is removed. Despite their solid-like character, tribological behavior of polymer brushes exhibits fluid-like properties: For instance, friction force exerted on two interdigitated brushes sheared in opposite directions goes to zero linearly as the shear velocity vanishes, i.e., no static friction occurs, which is a property observed mostly for fluidic friction. In this talk, we present our simulation result and scaling arguments on the friction of planar brush-on-brush systems. Our theoretical approach and simulation regimes encompass both linear and non-linear regimes. We show that individual brush ends move on well-defined average trajectories. The dissipation in the system can be related to these average trajectories for a wide range of shear velocities.

Erbas, Aykut; Rubinstein, Michael

2013-03-01

351

Friction and Wear of Ceramics.  

National Technical Information Service (NTIS)

The adhesion, friction, wear, and lubricated behaviors of both oxide and non-oxide ceramics are reviewed. Ceramics are examined in contact with themselves, other harder materials, and metals. Elastic, plastic, and fracture behavior of ceramics in solid st...

D. H. Buckley

1986-01-01

352

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

353

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

354

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

NASA Astrophysics Data System (ADS)

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

Sakuma, Hiroshi

2013-12-01

355

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

356

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

Microsoft Academic Search

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

A. K. LAKSHMINARAYANAN; V. BALASUBRAMANIAN

2008-01-01

357

Shear instability in the subsurface layer of a material in friction  

NASA Astrophysics Data System (ADS)

The shear instability in subsurface layers of a material in friction has been investigated. As the friction surface is approached, several characteristic regions can be distinguished in the bulk of the metal: the region of plastic deformation and texturing (I), the region of severe fragmentation (II), the region of turbulent flow (III), and the region of laminar flow (IV). Regions I and II can be referred to as regions of conventional plastic deformation, whereas regions III and IV correspond to regions of the development of shear instability of the Kelvin-Helmholtz type at the shear boundary. The possibility of implementing this phenomenon within the hydrodynamic approach has been evaluated.

Tarasov, S. Yu.; Rubtsov, V. E.

2011-02-01

358

Analytical skin friction and heat transfer formula for compressible internal flows  

NASA Technical Reports Server (NTRS)

An analytic, closed-form friction formula for turbulent, internal, compressible, fully developed flow was derived by extending the incompressible law-of-the-wall relation to compressible cases. The model is capable of analyzing heat transfer as a function of constant surface temperatures and surface roughness as well as analyzing adiabatic conditions. The formula reduces to Prandtl's law of friction for adiabatic, smooth, axisymmetric flow. In addition, the formula reduces to the Colebrook equation for incompressible, adiabatic, axisymmetric flow with various roughnesses. Comparisons with available experiments show that the model averages roughly 12.5 percent error for adiabatic flow and 18.5 percent error for flow involving heat transfer.

Dechant, Lawrence J.; Tattar, Marc J.

1994-01-01

359

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

NASA Astrophysics Data System (ADS)

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

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

2009-08-01

360

Friction Stir Welding  

NSDL National Science Digital Library

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

Leske, Cavin.

361

Applications of Turbulence Measurements over Lake Flevo.  

National Technical Information Service (NTIS)

Trivane measured fluctuations are reported of wind and temperature over an inland lake. The analysis includes: (1) tilt errors and precipitation effects in trivane measurements of turbulent fluxes over open water, (2) gust factors over open water and over...

J. Wieringa

1973-01-01

362

Tire/runway friction interface  

NASA Technical Reports Server (NTRS)

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

Yager, Thomas J.

1990-01-01

363

Frictional properties of aesthetic brackets  

Microsoft Academic Search

SUMMARY The purpose of this study was to compare the frictional properties of two self-ligating aesthetic brackets, Opal (Ultradent Products) and Oyster (Gestenco Int.), with those of four conventionally ligated aesthetic brackets, Transcend (3M Unitek), Inspire (Ormco), Allure (GAC Int.), and Image (Gestenco Int.). Friction was tested with different wire dimensions and qualities (stainless steel (SS) wire 0.017 × 0.025

C. A. Reicheneder; U. Baumert; T. Gedrange; P. Proff; A. Faltermeier; D. Muessig

2007-01-01

364

Numerical study of axial turbulent flow over long cylinders  

NASA Technical Reports Server (NTRS)

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

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

1991-01-01

365

Groove geometry effects on turbulent heat transfer and fluid flow  

NASA Astrophysics Data System (ADS)

The present work represents a two-dimensional numerical prediction of forced turbulent flow heat transfer through a grooved tube. Four geometric groove shapes (circular, rectangular, trapezoidal and triangular) were selected to perform the study, as well as two aspect ratios of groove-depth to tube diameter ( e/D = 0.1 and 0.2). The study focuses on the influence of the geometrical shapes of grooves and groove-depth on heat transfer and fluid flow characteristics for Reynolds number ranging from 10,000 to 20,000. The characteristics of Nusselt number, friction factor and entropy generation are studied numerically by the aid of the computational fluid dynamics (CFD) commercial code of FLUENT. It is observed that the best performance occurs with the lower depth-groove ratio, whereas it is found that the grooved tube provides a considerable increase in heat transfer at about 64.4 % over the smooth tube and a maximum gain of 1.52 on thermal performance factor is obtained for the triangular groove with ( e/D = 0.1).

Ramadhan, Abdulmajeed A.; Al Anii, Yaser T.; Shareef, Amer J.

2013-02-01

366

The Effect of Friction on Penetration in Friction Stir Welding  

NASA Technical Reports Server (NTRS)

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

Rapp, Steve

2002-01-01

367

Effects of Topographical Slope Angle and Atmospheric Stratification on Surface-Layer Turbulence  

NASA Astrophysics Data System (ADS)

The effect of topographical slope angle and atmospheric stratification on turbulence intensities in the unstably stratified surface layer have been parameterized using observations obtained from a three-dimensional sonic anemometer installed at 8 m height above the ground at the Seoul National University (SNU) campus site in Korea for the years 1999 2001. Winds obtained from the sonic anemometer are analyzed according to the mean wind direction, since the topographical slope angle changes significantly along the azimuthal direction. The effects of the topographical slope angle and atmospheric stratification on surface-layer turbulence intensity are examined with these data. It is found that both the friction velocity and the variance for each component of wind normalized by the mean wind speed decrease with increase of the topographical slope angle, having a maximum decreasing rate at very unstable stratification. The decreasing rate of the normalized friction velocity ( u * / U) is found to be much larger than that of the turbulence intensity of each wind component due to the reduction of wind shear with increase in slope angle under unstable stratification. The decreasing rate of the w component of turbulence intensity (? w / U) is the smallest over the downslope surface whereas that of the u component (? u / U) has a minimum over the upslope surface. Consequently, ? w / u * has a maximum increasing rate with increase in slope angle for the downslope wind, whereas ? u / u * has its maximum for the upslope wind. The sloping terrain is found to reduce both the friction velocity and turbulence intensity compared with those on a flat surface. However, the reduction of the friction velocity over the sloping terrain is larger than that of the turbulence intensity, thereby enhancing the turbulence intensity normalized by the friction velocity over sloping terrain compared with that over a flat surface.

Park, Moon-Soo; Park, Soon-Ung

2006-03-01

368

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

369

Determining factor for the blowoff limit of a flame spreading in an opposed turbulent flow, in a narrow solid-fuel duct  

Microsoft Academic Search

This study clarified the blowoff mechanism for a flame spreading in an opposed turbulent flow in narrow solid fuel ducts. To clarify this mechanism, two experiments were conducted. The first experiment was to investigate the influence of ambient pressure and fuel duct size on the blowoff limit. The results indicated that the flow velocity at the point when blowoff occurred,

Nozomu Hashimoto; Harunori Nagata; Tsuyoshi Totani; Isao Kudo

2006-01-01

370

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

371

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

372

Simultaneous achievement of drag reduction and heat transfer augmentation in wall turbulence by optimal control theory  

NASA Astrophysics Data System (ADS)

The analogy concept between momentum and heat transfer, which is based upon the similarity between the Navier-Stokes and energy transport equations, has been widely used for analyzing turbulent transport phenomena. This fact implies inherent difficulty in enhancing heat transfer with pumping power reduced or not increased as much as heat transfer. We consider a fully developed turbulent channel flow with uniform heat generation in the fluid, so that the averaged transport equations have an identical form. The problem thus posed offers the most difficult challenge to achieve dissimilar momentum and heat transfer. Even under such a difficult condition, we demonstrate that the dissimilar control is possible if we exploit the continuity constraint on the velocity field. In order to optimize the control input, i.e., wall blowing/suction, the optimal control theory developed by Bewley et al. (2001) is applied. By defining the cost functional as a weighted sum of the intensity of the wall blowing/suction and the analogy factor, i.e., the ratio of wall heat flux and skin friction, the analogy factor increases more than double beyond that in the uncontrolled flow. It is also found that the resultant optimized control input exhibits a downstream traveling wave-like property.

Yamamoto, Akira; Hasegawa, Yosuke; Kasagi, Nobuhide

2011-11-01

373

Light propagation through anisotropic turbulence.  

PubMed

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

Toselli, Italo; Agrawal, Brij; Restaino, Sergio

2011-03-01

374

What can friction tell us about shallow megathrust slip behavior?  

NASA Astrophysics Data System (ADS)

In subduction zones, the updip propagation of great earthquake ruptures on plate boundary megathrusts is currently one of the most important questions in earth science, primarily because rupture that approaches the surface causes seafloor displacement, resulting in enormous tsunamis. Moreover, the extent of updip rupture propagation is a key factor in defining the magnitude of the earthquake itself. Within the depth limits of the seismogenic zone, velocity-weakening frictional behavior is essential for the nucleation of large-magnitude earthquake rupture. Results of friction experiments at low slip velocities (~10-6-10-4 m/s) have suggested that velocity-weakening tends to occur in frictionally strong materials (typically non-clay), which may act as asperities on fault surfaces. However, the role of frictional strength and velocity dependence in controlling the extent of rupture propagation beyond the updip limit of the seismogenic zone is still unclear. Low to high-velocity friction experiments have provided insights into fault strength evolution over slip velocities spanning ~10 orders of magnitude, from plate convergence rates to coseismic slip rates. Results using primarily non-clay materials typically exhibit high friction at low velocities that progressively weakens at higher velocities (velocity-weakening), becoming nearly frictionless at coseismic slip rates [Di Toro et al., 2011]. However, the shallow near-trench regions of subduction zones are typically rich in clay minerals which are weak (friction coefficient ? ~0.4) and velocity-strengthening at slip rates < 10-3 m/s. A compilation of friction experiments using samples from the Nankai Trough region offshore Japan obtained by scientific ocean drilling shows that this material exhibits such behavior at low to intermediate slip velocities. However, after reaching peak values at ~10-2 m/s, these materials also exhibit a precipitous drop in friction toward near-zero values at coseismic slip rates. This suggests that all geologic materials, regardless of composition, are extremely weak when coseismic slip rates are enforced. Therefore, the likelihood of near-trench rupture propagation in subduction zones depends critically on whether slip can reach velocities ? ~10-2 m/s, where dynamic weakening becomes dominant. This depends on whether the propagating earthquake rupture can overcome the overall strength of the fault gouge and/or velocity-strengthening behavior at low to intermediate slip rates. We discuss here the possibility of near-trench earthquake rupture at Nankai and other subduction zones on the basis of laboratory friction measurements.

Ikari, M.; Kopf, A.; Hirose, T.

2012-12-01

375

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

376

The First Turbulence and First Fossil Turbulence  

Microsoft Academic Search

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

Carl H. Gibson

2004-01-01

377

INVITED PAPER: Refined cf relation for turbulent channels and consequences for high-Re experiments  

NASA Astrophysics Data System (ADS)

There have been rising concerns regarding the accuracy of measurements in turbulent channel flows, in particular, measurements of the skin-friction results. In the present study, two different methods, namely, mean streamwise pressure gradient (PG) and oil film interferometry (OFI), are used to estimate the wall skin-friction relation, cf = f(Rem), for fully developed turbulent plane-channel flow over a wide range of Reynolds numbers. The channel skin-friction data are then fitted to the well-known logarithmic friction law, providing outstanding agreement with values for the constants of the logarithmic law of the mean velocity profile. A revised logarithmic skin-friction relation is developed, providing good agreement with our skin-friction results and data from the literature, when constants of the logarithmic friction relation adopted from the recent work of Zanoun et al (2003 Phys. Fluids 15 3079-89, 2005 4th Int. Conf. on Heat Transfer, Fluid Mechanics and Thermodynamics, HEAT2005, 19-22 September, Cairo, Egypt) are utilized. A new experimental channel facility is proposed, allowing measurements at high Reynolds numbers well beyond those achieved previously in laboratories, i.e. over five times the highest Rem reached in the present study, while maintaining sufficiently high spatial resolution.

Zanoun, E.-S.; Nagib, H.; Durst, F.

2009-04-01

378

Fundamental studies on friction and wear of engineering plastics  

Microsoft Academic Search

Engineering plastics having good mechanical properties are at present frequently used as materials for various machine elements. In this research, experimental investigations have been carried out regarding tribological factors such as friction and wear for five kinds of fundamental engineering plastics: non-reinforced polycarbonate, polyphenylene oxide, polyamide, polyacetal, and polybutylene terephthalate. It is clarified that the tribological properties, especially the wear

Hiroki Endo; Etsuo Marui

2004-01-01

379

Nonlinear friction model for servo press simulation  

NASA Astrophysics Data System (ADS)

The friction coefficient was measured under an idealized condition for a pulse servo motion. The measured friction coefficient and its changing with both sliding distance and a pulse motion showed that the friction resistance can be reduced due to the re-lubrication during unloading process of the pulse servo motion. Based on the measured friction coefficient and its changes with sliding distance and re-lubrication of oil, a nonlinear friction model was developed. Using the newly developed the nonlinear friction model, a deep draw simulation was performed and the formability was evaluated. The results were compared with experimental ones and the effectiveness was verified.

Ma, Ninshu; Sugitomo, Nobuhiko; Kyuno, Takunori; Tamura, Shintaro; Naka, Tetsuo

2013-12-01

380

Versatile Friction Stir Welding/Friction Plug Welding System  

NASA Technical Reports Server (NTRS)

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

Carter, Robert

2006-01-01

381

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

382

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

383

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

384

Superradiance-tidal friction correspondence  

NASA Astrophysics Data System (ADS)

Since the work of Hartle in the 1970s, and the subsequent development of the membrane paradigm approach to black hole physics it has been widely accepted that superradiant scattering of gravitational waves bears strong similarities with the phenomenon of "tidal friction" (well known from Newtonian gravity) operating in binary systems of viscous material bodies. In this paper we revisit the superradiance-tidal friction analogy within the context of ultracompact relativistic bodies. We advocate that as long as these bodies have nonzero viscosity they should undergo tidal friction that can be construed as a kind of superradiant scattering from the point of view of the dynamics of an orbiting test body. In addition we consider the presence of anisotropic matter, which is required for at least some ultracompact bodies, if they are to sustain a radius very close to the gravitational radius. We find that the tidal friction/superradiance output is enhanced with increasing anisotropy and that strongly anisotropic systems exhibit an unconventional response to tidal and centrifugal forces. Finally, we make contact with the artificial system comprising a black hole with its horizon replaced by a mirror (sometimes used as a proxy for ultracompact material bodies) and discuss superradiance and tidal friction in relation to it.

Glampedakis, Kostas; Kapadia, Shasvath J.; Kennefick, Daniel

2014-01-01

385

Gravito-turbulent Disks in Three Dimensions: Turbulent Velocities versus Depth  

NASA Astrophysics Data System (ADS)

Characterizing turbulence in protoplanetary disks is crucial for understanding how they accrete and spawn planets. Recent measurements of spectral line broadening promise to diagnose turbulence, with different lines probing different depths. We use three-dimensional local hydrodynamic simulations of cooling, self-gravitating disks to resolve how motions driven by "gravito-turbulence" vary with height. We find that gravito-turbulence is practically as vigorous at altitude as at depth. Even though gas at altitude is much too rarefied to be itself self-gravitating, it is strongly forced by self-gravitating overdensities at the midplane. The long-range nature of gravity means that turbulent velocities are nearly uniform vertically, increasing by just a factor of two from midplane to surface, even as the density ranges over nearly three orders of magnitude. The insensitivity of gravito-turbulence to height contrasts with the behavior of disks afflicted by the magnetorotational instability (MRI); in the latter case, non-circular velocities increase by at least a factor of 15 from midplane to surface, with various non-ideal effects only magnifying this factor. The distinct vertical profiles of gravito-turbulence versus MRI turbulence may be used in conjunction with measurements of non-thermal linewidths at various depths to identify the source of transport in protoplanetary disks.

Shi, Ji-Ming; Chiang, Eugene

2014-07-01

386

An intrinsic velocity-independent criterion for superfluid turbulence.  

PubMed

Hydrodynamic flow in classical and quantum fluids can be either laminar or turbulent. Vorticity in turbulent flow is often modelled with vortex filaments. While this represents an idealization in classical fluids, vortices are topologically stable quantized objects in superfluids. Superfluid turbulence is therefore thought to be important for the understanding of turbulence more generally. The fermionic 3He superfluids are attractive systems to study because their characteristics vary widely over the experimentally accessible temperature regime. Here we report nuclear magnetic resonance measurements and numerical simulations indicating the existence of sharp transition to turbulence in the B phase of superfluid 3He. Above 0.60T(c) (where T(c) is the transition temperature for superfluidity) the hydrodynamics are regular, while below this temperature we see turbulent behaviour. The transition is insensitive to the fluid velocity, in striking contrast to current textbook knowledge of turbulence. Rather, it is controlled by an intrinsic parameter of the superfluid: the mutual friction between the normal and superfluid components of the flow, which causes damping of the vortex motion. PMID:12944960

Finne, A P; Araki, T; Blaauwgeers, R; Eltsov, V B; Kopnin, N B; Krusius, M; Skrbek, L; Tsubota, M; Volovik, G E

2003-08-28

387

Kinetic wave turbulence  

NASA Astrophysics Data System (ADS)

We consider a general model of Hamiltonian wave systems with triple resonances, in the standard kinetic limit of a continuum of weakly interacting dispersive waves with random phases. We show in this limit that the leading-order, asymptotically valid dynamical equation for multimode amplitude distributions is not the well-known equation of Peierls (also, Brout & Prigogine and Zaslavskii & Sagdeev), but is instead a reduced equation containing only a subset of the terms in that equation. Our equations are consistent with the Peierls equation in that the additional terms in the latter vanish as inverse powers of volume in the large-box limit. The equations that we derive are the direct analogue of the Boltzmann hierarchy obtained from the BBGKY hierarchy in the low-density limit for gases. We show that the asymptotic multimode equations possess factorized solutions for factorized initial data, which correspond to preservation in time of the property of “random phases & amplitudes”. The factors satisfy the equations for the 1-mode probability density functions (PDFs) previously derived by Choi et al. and Jakobsen & Newell. Analogous to the Klimontovich density in the kinetic theory of gases, we introduce the concepts of the “empirical spectrum” and the “empirical 1-mode PDF”. We show that the factorization of the hierarchy equations implies that these quantities are self-averaging: they satisfy the wave-kinetic closure equations of the spectrum and 1-mode PDF for almost any selection of phases and amplitudes from the initial ensemble. We show that both of these closure equations satisfy an H-theorem for an entropy defined by Boltzmann’s prescription S=kBlogW. We also characterize the general solutions of our multimode distribution equations, for initial conditions with random phases but with no statistical assumptions on the amplitudes. Analogous to a result of Spohn for the Boltzmann hierarchy, these are “super-statistical solutions” that correspond to ensembles of solutions of the wave-kinetic closure equations with random initial conditions or random forces. On the basis of our results, we discuss possible kinetic explanations of intermittency and non-Gaussian statistics in wave turbulence. In particular, we advance the explanation of a “super-turbulence” produced by stochastic or turbulent solutions of the wave kinetic equations themselves.

Eyink, Gregory L.; Shi, Yi-Kang

2012-09-01

388

Direct numerical simulations of turbulent flows over superhydrophobic surfaces  

Microsoft Academic Search

Direct numerical simulations (DNSs) are used to investigate the drag-reducing per- formance of superhydrophobic surfaces (SHSs) in turbulent channel flow. SHSs combine surface roughness with hydrophobicity and can, in some cases, support a shear-free air-water interface. Slip velocities, wall shear stresses and Reynolds stresses are considered for a variety of SHS microfeature geometry configurations at a friction Reynolds number of

MICHAEL B. M ARTELL; LAIR P EROT; JONATHAN P. R OTHSTEIN

2009-01-01

389

Natural laminar-turbulent transition delay by dielectric barrier discharge  

NASA Astrophysics Data System (ADS)

The use dielectric barrier discharge for the delay of laminar turbulent transition excited by natural flow disturbances in a quiet wind-tunnel was investigated experimentally. Optimal electrodes location and the operational regime of high-voltage impulse generator provided maximal downstream shift of transition location were found. It was demonstrated that the 10% increase of the laminar part of boundary layer can be obtained using barrier discharge with the cross-flow electrodes. This gives up to 20% friction drag reduction.

Ustinov, Maxim; Kogan, Mikhail; Litvinov, Vladimir; Uspensky, Alexander

2011-12-01

390

Turbulent Bubbly Channel Flow Investigated by Ultrasound Velocity Profiler  

Microsoft Academic Search

Ultrasound velocity profiler (UVP) is applied to measurements of a horizontal turbulent bubbly channel flow to ascertain the mechanism of bubble-induced frictional drag reduction. Typical parameter regimes of the target flow are Re number of 0.6-6.0 x104, void fraction of 0-3%, and bubble diameter of 10-50mm. Since the UVP system only outputs velocity profiles on an ultrasound beam inside a

Yuichi Murai; Hideki Fujii; Yuji Tasaka; Yasushi Takeda

2006-01-01

391

Turbulent boundary layer control utilizing the Lorentz force  

Microsoft Academic Search

Direct numerical simulations (DNS) of a turbulent channel flow at low Reynolds number (Retau=100,200,400, where Retau is the Reynolds number based on the wall-shear velocity and channel half-width) are carried out to examine the effectiveness of using the Lorentz force to reduce skin friction. The Lorentz force is created by embedding electrodes and permanent magnets in the flat surface over

Timothy W. Berger; John Kim; Changhoon Lee; Junwoo Lim

2000-01-01

392

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

393

On constructing the theory of ductile fracture near friction surfaces  

NASA Astrophysics Data System (ADS)

A criterion of ductile fracture is proposed, which takes into account the singular character of theoretical solutions near the maximum friction surfaces and the emergence of a thin layer with intense plastic strains near surfaces with high friction stresses in real processes of metal forming. The equation for the thickness of the layer with intense plastic strains and the fracture criterion include the strain rate intensity factor, apparently, characterizing the intensity of physical processes that occur in a thin material layer near the friction surfaces. Some experimental data are used to determine the thickness of this layer. The ductile fracture criterion is analyzed by solving the problem of strip extrusion under conditions of plane strain deformation.

Alexandrov, S. E.; Lyamina, E. A.

2011-07-01

394

Smooth-and enhanced-tube heat transfer and pressure drop : effect of Prandtl number and the role of transition to turbulence.  

SciTech Connect

An extensive experimental investigation was carried out to determine the pressure drop and heat transfer characteristics in laminar, transitional, and turbulent flow through one smooth tube and twenty-three enhanced tubes. The working fluids for the experiments were air, water, ethylene glycol, and ethylene glycol/water mixtures; Prandtl number (Pr) ranged from 0.7 to 125.3. The smooth-tube experiments were carried out with Pr values of 0.7, 6.8, 24.8, 39.1, and 125.3; Pr values of 0.7, 6.8, and 24.8 were tested with enhanced tubes. Reynolds number (Re) range (based on the maximum internal diameter of a tube) was 200 to 55,000, depending on Prandtl number and tube geometry. The enhanced-tube friction factors are no more than 5.5 times the smooth-tube values. For some tubes, the friction factors are practically the same for 0.7 {<=} Pr {<=} 25, with moderate differences in laminar, transitional, or turbulent flow for some of the tubes. Consistent with the low-pressure-drop characteristics of these enhanced tubes, the increases in heat transfer coefficient are no more than 2.5 times the smooth-tube values for the Re and Pr ranges covered in the study. For most enhanced tubes, heat-transfer coefficients are insensitive to variations in the Prandtl number for liquids. There is a definite connection between the attainable friction or heat transfer and the transition process.

Obot, N. T.; Das, L.; Rabas, T. J.; Clarkson Univ.

2002-12-01

395

Resonance enhanced turbulent transport  

SciTech Connect

The effect of oscillatory shear flows on turbulent transport of passive scalar fields is studied by numerical computations based on the results provided by E. Kim [Physics of Plasmas 13, 022308 (2006)]. Turbulent diffusion is found to depend crucially on the competition between suppression due to shearing and enhancement due to resonances, depending on the characteristic time and length scales of shear flow and turbulence. Enhancements in transport occur for turbulence with finite memory time either due to Doppler or parametric resonances. Scalings of turbulence amplitude and transport are provided in different parameter spaces. The results suggest that oscillatory shear flows are not only less efficient in regulating turbulence, but also can enhance the value of turbulent diffusion, accelerating turbulent transport.

Newton, Andrew P. L.; Kim, Eun-jin [Department of Applied Mathematics, University of Sheffield, Sheffield, S3 7RH (United Kingdom)

2007-12-15

396

Surface roughness and dry friction.  

PubMed

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). PMID:22463359

Sokoloff, J B

2012-02-01

397

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

398

Design Modifications of a Friction Test Rig for High Vacuum and Elevated Temperature Operation.  

National Technical Information Service (NTIS)

This program deals with the parametric evaluation of wear-life factors, friction coefficients, and a search for dry film lubricant materials for lubrication of bearings operable in vacuum and elevated temperature environments. One of the most critical fac...

L. E. Wieser

1964-01-01

399

Laser Peening Effects on Friction Stir Welding.  

National Technical Information Service (NTIS)

Friction Stir Welding (FSW) is a welding technique that uses frictional heating combined with forging pressure to produce high strength bonds. It is attractive for aerospace applications. Although residual stresses in FSW are generally lower when compared...

O. Hatamleh

2011-01-01

400

Friction Welding of Metals and Alloys.  

National Technical Information Service (NTIS)

The literature survey covers different friction welding processes, and how similar and dissimilar metals successfully can be joined together. The temperature distribution in the heat affected zone during friction welding is described by means of different...

O. T. Midling

1990-01-01

401

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

NASA Technical Reports Server (NTRS)

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

Monson, D. J.; Higuchi, H.

1981-01-01

402

Friction layers and friction films on PMC brake pads  

Microsoft Academic Search

A rather new tool, the focused ion beam (FIB) technique, was used to characterise superficial layers at micro-contact areas of a commercial brake pad. The friction material was a polymer matrix composite (PMC) with approximately 50% metal content (semi-metallic) and the counter part was a cast iron rotor. Though the contact areas were not visible as topographic features, they could

W. Österle; I. Urban

2004-01-01

403

Friction properties of novel PVP/PVA blend hydrogels as artificial cartilage.  

PubMed

In this work, novel polyvinylpyrrolidone (PVP)/polyvinylalcohol (PVA) blend hydrogels were prepared by repeated freezing-thawing cycles. The factors that influenced friction properties of blend hydrogels, such as PVP content, contact load, sliding speed, and lubrication condition, were mainly studied by sliding with stainless steel ball. The results showed that friction coefficients of the PVP/PVA blend hydrogels were definitely dependent on such influence factors. The friction system consisting of blend hydrogel and stainless steel ball nearly exhibited a mixed lubrication regime especially under bovine serum lubrication, and it can be proposed as a promising method to reduce wear of the prosthesis. PMID:19743512

Ma, Ruyin; Xiong, Dangsheng; Miao, Feng; Zhang, Jinfeng; Peng, Yan

2010-06-01

404

Heat transfer enhancement in channels with turbulence promoters  

NASA Technical Reports Server (NTRS)

Repeated rib-roughness elements have been used in advanced turbine cooling designs to enhance the internal heat transfer. Often the ribs are perpendicular to the main flow direction so that they have an angle-of-attack of 90 degrees. The objective of this investigation was to determine the effect of rib angle-of-attack on the pressure drop and the average heat-transfer coefficients in the fully developed turbulent air flow in a square duct with two opposite rib-roughened walls for Reynolds numbers varied from 7,000 to 90,000. The rib height-to-equivalent diameter ratio was kept at a constant value of 0.063, the rib pitch-to-height ratio was varied from 10 to 20, and the rib angle-of-attack (alpha) was varied from 90 to 60 deg to 45 to 30 deg, respectively. The thermal-performance comparison indicated that the increased heat conductance for the rib with an oblique angle to the flow (alpha = 45 deg - 30 deg) was about 10-20 percent higher than the rib with a 90 deg angle to the flow, and the pumping power requirement for the angled rib was about 20-50 percent lower than the transverse rib. Semi-empirical correlations for friction factor and heat-transfer coefficients were developed to account for rib spacing and rib angle. The correlations can be used in the design of turbine-blade cooling passages.

Han, J. C.; Park, J. S.; Lei, C. K.

1984-01-01

405

DNS of Decelerating Turbulent Boundary Layers  

NASA Astrophysics Data System (ADS)

We conduct Direct Numerical Simulation (DNS) of turbulent plane-channel flow subjected to bulk deceleration and to a uniform strain field consisting of streamwise compression (partial U/partial x < 0) and wall-normal stretching (partial V/partial y > 0). This creates a time-developing flow containing most of the essential physics of spatially developing APG boundary layers, particularly in the outer layer. The logistics are much more favorable than those of a true spatial case, both for the DNS and for the testing of turbulence models. The computations are performed at two Reynolds numbers, with initial Re_? = 180 and 395, and advanced past the point when the deceleration causes the mean skin friction to change sign. This parallel-flow analog of ``separation'' isolates outer-layer features of separated boundary layers that are due to mean strain and vanishing surface stress from those caused by streamline curvature. Files are available to allow comparisons at the level of skin friction, velocity and stress profiles, and Reynolds-stress budgets. Changes in the velocity--pressure-gradient term ?_ij are found to dominate the initial evolution of the flow.

Coleman, G. N.; Kim, J.; Spalart, P. R.

1998-11-01

406

Experimental study on the friction and wear performance of new dual friction materials  

Microsoft Academic Search

Three types of palygorskites and three types of NBR-modified phenolic resin are compressed respectively into new dual friction materials by material testing machine. Their friction, wear and impact resistance performances are studied experimentally. The results show that modified friction materials have better performance than unmodified phenolic resin materials. But compared with NBR-modified phenolic resin, the friction and wearing performances of

Du Fei; Ma Tianbing

2011-01-01

407

Observation of Joining Phenomena in Friction Stage and Improving Friction Welding Method  

Microsoft Academic Search

This report describes the observation result of joining phenomena in the friction stage, and an improvement of the conventional friction welding method with similar materials. The materials used were carbon steels and a brake type (direct drive) friction welding machine was used for joining. As the improving friction welding method, relative speed was instantaneously rendered to zero at the end

Masaaki Kimura; Kenji Seo; Masahiro Kusaka; Akiyoshi Fuji

2003-01-01

408

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

409

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

410

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

411

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 in…

Russian Education and Society, 2012

2012-01-01

412

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

413

Apparatus for measurement of coefficient of friction  

NASA Technical Reports Server (NTRS)

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

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

1990-01-01

414

Joint Winter Runway Friction Program Accomplishments  

NASA Technical Reports Server (NTRS)

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

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

2002-01-01

415

Low-Friction Joint for Robot Fingers  

NASA Technical Reports Server (NTRS)

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

Ruoff, C. F.

1985-01-01

416

Turbulence and Turbulence Suppression in the Helimak  

NASA Astrophysics Data System (ADS)

The Helimak is an approximation to the infinite cylindrical slab, but with open field lines of finite length. Radially-segmented isolated end plates allow application of radial electric fields that drive radial currents. Above a threshold in applied voltage (driven current), the fractional turbulent amplitude is greatly reduced, as is the radial turbulent particle transport in the region of applied bias. Stabilization is observed for both positive and negative bias. Concurrent measurements of the ion flow velocity are made by Doppler spectroscopy. The turbulence -- density, potential, and temperature fluctuations and their relations, will be compared with simulations from a fluid model for this geometry. Comparisons of turbulence reduction with changes in radial correlation length and flow shear will be given. Although the radial correlation length is much smaller than the plasma, the turbulent structures have a large spatial scale of short lifetime. The amplitude reduction is associated with shrinkage in size of the most coherent structures.

Gentle, K. W.; Rowan, W. L.; Liao, K.; Li, B.

2010-11-01

417

Impurity transport in temperature gradient driven turbulence  

NASA Astrophysics Data System (ADS)

In the present paper, the transport of impurities driven by trapped electron (TE) mode turbulence is studied. Non-linear (NL) gyrokinetic simulations using the code GENE are compared with results from quasilinear (QL) gyrokinetic simulations and a computationally efficient fluid model. The main focus is on model comparisons for electron temperature gradient driven turbulence regarding the sign of the convective impurity velocity (pinch) and the impurity density gradient R/LnZ (peaking factor) for zero impurity flux. In particular, the scaling of the impurity peaking factors with impurity charge Z and with driving temperature gradient is investigated and compared with results for the more studied ion temperature gradient (ITG) driven turbulence. The question of helium ash removal in TE mode turbulence is also investigated. In addition, the impurity peaking is compared to the main ion peaking obtained by a self-consistent fluid calculation of the density gradients corresponding to zero particle fluxes. For the scaling of the peaking factor with impurity charge Z, a weak dependence is obtained from NL GENE and fluid simulations. The QL GENE results show a stronger dependence for low Z impurities and overestimates the peaking factor by up to a factor of two in this region. As in the case of ITG dominated turbulence, the peaking factors saturate as Z increases, at a level much below neoclassical predictions. The scaling with Z is, however, weak or reversed as compared to the ITG case. The results indicate that TE mode turbulence is as efficient as ITG turbulence at removing He ash, with DHe/?eff>1.0. The scaling of impurity peaking with the background temperature gradients is found to be weak in the NL GENE and fluid simulations. The QL results are also here found to significantly overestimate the peaking factor for low Z values. For the parameters considered, the background density gradient for zero particle flux is found to be slightly larger than the corresponding impurity zero flux gradient.

Skyman, A.; Nordman, H.; Strand, P.

2012-03-01

418

Advanced friction modeling for sheet metal forming  

Microsoft Academic Search

The Coulomb friction model is frequently used for sheet metal forming simulations. This model incorporates a constant coefficient of friction and does not take the influence of important parameters such as contact pressure or deformation of the sheet material into account. This article presents a more advanced friction model for large-scale forming simulations based on the surface changes on the

J. Hol; M. V. Cid Alfaro; M. B. de Rooij; T. Meinders

2011-01-01

419

Multiscale friction modeling for sheet metal forming  

Microsoft Academic Search

The most often used friction model for sheet metal forming simulations is the relative simple Coulomb friction model. This paper presents a more advanced friction model for large scale forming simulations based on the surface change on the micro-scale. The surface texture of a material changes when two surfaces are in contact under a normal load. This is caused by

J. Hol; M. V. Cid Alfaro; Rooij de M. B; V. T. Meinders; Eric Felder; Pierre Montmitonnet

2010-01-01

420

Frictional characteristics of an aerostatic linear bearing  

Microsoft Academic Search

Frictional characteristics of an aerostatic linear bearing are evaluated in detail by means of a developing method. In the method, the resultant force acting on the moving part of the bearing is measured highly accurately as the inertial force using an optical interferometer. The velocity dependence of dynamic friction is focused on in this paper. The component of dynamic frictional

Yusaku Fujii

2006-01-01

421

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

422

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

NASA Technical Reports Server (NTRS)

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

Bowles, Roland L.

1988-01-01

423

Turbulent boundary layers: Inflow effects and cross-validation of simulation and experiment  

NASA Astrophysics Data System (ADS)

A recent assessment of available direct numerical simulation (DNS) data from turbulent boundary layer flows [Schlatter & "Orl"u, J. Fluid Mech. 659, 116 (2010)] showed surprisingly large differences not only in the skin friction coefficient or shape factor, but also in their predictions of mean and fluctuation profiles far into the sublayer. Several DNS of a zero pressure-gradient (ZPG) turbulent boundary layer (TBL) àla Schlatter et al. [Phys. Fluids 21, 051702 (2009)] with physically different inflow conditions and tripping effects were performed. Most of the differences observed when comparing available DNS could thereby be traced back to different initial conditions. It was also found, that if transition is initiated at a low enough Reynolds number (based on the momentum-loss thickness) Re?< 300, all data agree well for both inner and outer layer for Re?>2000; a result that gives a lower limit for meaningful comparisons between numerical and/or wind tunnel experiments. Based on these results a detailed comparison between DNS and experiment of a ZPG TBL flow at Re?=2500 and 4000 is presented. Good agreement is obtained for integral quantities, mean and fluctuating streamwise velocity profiles, but also for the probability distribution and spectral map throughout the boundary layer.

Oerlue, Ramis; Schlatter, Philipp

2011-11-01

424

Microstructures Produced by Dynamic Friction  

NASA Astrophysics Data System (ADS)

An experimental technique in which an explosive charge induces sliding between two metals has been developed as part of a study of dry friction at very high sliding velocities and pressures. Aluminium alloy/stainless steel and pure aluminium/pure copper tribo-pairs have been investigated. Optical studies of cross-sections of the aluminium samples have shown that, depending on the stress/sliding velocity conditions, the sub-surface deformation is either deep, suggesting high friction, or concentrated near the surface suggesting low friction. Recent further studies of the microstructures near the surface of the samples are described. Transmission electron microscopy reveals that a clearly delineated layer of nanocrystalline material ~1 ?m thick is created at the surface of the aluminium samples. Spectroscopic analysis shows evidence of inter-penetration of the two materials with mixing occurring at a very fine scale. The observations support the contention that, in a mechanism akin to adiabatic shear, thermal softening of the material at the sliding interface plays a key role in shock-induced friction.

Winter, R. E.; Keightley, P. T.; Kim, Hong Jin; Rigney, D. A.; Emge, A.

2007-12-01

425

Deformation During Friction Stir Welding  

NASA Technical Reports Server (NTRS)

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

White, Henry J.

2002-01-01

426

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

427

Financial Frictions and Credit Spreads  

Microsoft Academic Search

This paper uses the credit-friction model developed by C´urdia and Woodford, in a series of papers, as the basis for attempting to mimic the behavior of credit spreads in moderate as well as in times of crisis. We are able to generate movements in representative credit spreads that are, at times, both sharp and volatile. We then study the impact

Ke Pang; Pierre L. Siklos

2010-01-01

428

Financial Frictions and Credit Spreads  

Microsoft Academic Search

This paper uses the credit-friction model developed by Curdia and Woodford, in a series of papers, as the basis for attempting to mimic the behavior of credit spreads in moderate as well as crisis times. We are able to generate movements in representative credit spreads that are, at times, both sharp and volatile. We then study the impact of quantitative

Ke Pang; Pierre L. Siklos

2010-01-01

429

Friction-formed liquid droplets.  

PubMed

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

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

2011-03-11

430

Macrostructure of Friction Stir Welds.  

National Technical Information Service (NTIS)

This paper will discuss two of the well know large scale features of friction stir welds: the 'onion rings' seen in transverse sections, and the striations on the surface of the work piece. It will be shown that the surface features (sometimes called 'too...

S. Aloor B. Nowak R. Vargas J. C. McClure L. E. Murr A. C. Nunes

2002-01-01

431

Measuring Adhesion And Friction Forces  

NASA Technical Reports Server (NTRS)

Cavendish balance adapted to new purpose. Apparatus developed which measures forces of adhesion and friction between specimens of solid materials in vacuum at temperatures from ambient to 900 degrees C. Intended primarily for use in studying adhesion properties of ceramics and metals, including silicon carbide, aluminum oxide, and iron-base amorphous alloys.

Miyoshi, Kazuhisa

1991-01-01

432

Turbulence Lidar Development Status  

NASA Technical Reports Server (NTRS)

This viewgraph provides an overview of efforts conducted by Coherent Technologies, Inc. in conjunction with NASA Langley Research Center to develop a robust turbulence detection capability that spans full range of turbulence environments. Topics covered include: general principles of infrared doppler radar (lidar) turbulence measurement, performance simulation, complete detection capability provided by dual wavelength radar, algorithm development, technology development needs and turbulence detection problems. Versions of this turbulence lidar system were flight tested. Data analysis from these flight tests are presented. Future flight tests aboard DC-8 and B-757 are planned as well as continued algorithm development and performance simulation activities.

Clark, Ivan

2003-01-01

433

Introduction to quantum turbulence  

PubMed Central

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

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

2014-01-01

434

Modeling Compressed Turbulence  

SciTech Connect

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

Israel, Daniel M. [Los Alamos National Laboratory

2012-07-13

435

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

PubMed

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

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

2014-03-01

436

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

437

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

438

Calculation of a Turbulent Boundary Layer for Substantial Positive Pressure Gradients.  

National Technical Information Service (NTIS)

In this article is exponded a new method of calculation ofa turbulent boundary layer, which is based on the boundary laws of friction and heat transfer in the diffusion region of a gas flow. There are obtained values of critical parameters at point of abr...

S. S. Kytateladze A. I. Leont'ev

1964-01-01

439

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

440

Scaling laws for fully developed turbulent shear flows. II - Processing of experimental data  

NASA Astrophysics Data System (ADS)

A nonuniversal scaling law for the mean velocity distribution in fully developed turbulent shear flow is verified using classic Nikuradze (1932) data on velocity distribution and skin friction. Results obtained make it possible to determine the necessary constants in the scaling law and to confirm high accuracy.

Barenblatt, G. I.; Prostokishin, V. M.

1993-03-01

441

Experimental Results from a Flat Plate, Turbulent Boundary Layer Modified for the Purpose of Drag Reduction  

Microsoft Academic Search

Recent experiments on a flat plate, turbulent boundary layer at high Reynolds numbers (>10^7) were performed to investigate various methods of reducing skin friction drag. The methods used involved injecting either air or a polymer solution into the boundary layer through a slot injector. Two slot injectors were mounted on the model with one located 1.4 meters downstream of the

Brian R. Elbing

2006-01-01

442

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

443

Turbulence structures associated with fire-atmosphere interactions  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

444

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

2013-09-20

445

Stick-slip friction and wear of articular joints  

PubMed Central

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

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

2013-01-01

446

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

447

Scaling properties of turbulence driven shear flow  

SciTech Connect

The characteristics and scaling properties of the turbulence driven shear flow are investigated in a cylindrical laboratory plasma device. For a given plasma pressure, the density fluctuation amplitude and radial particle flux increase with the applied magnetic field. Strong flow shear is found to coexist at high magnetic fields (>700 G) with approx10 kHz drift wave turbulence, but not at low magnetic fields (<700 G). The absolute value of the divergence of the turbulent Reynolds stress at the shear layer is shown to increase with the magnetic field as well. For a fixed magnetic field, the shear flow is found to decrease as the discharge gas pressure is increased. The density fluctuation amplitude and divergence of the turbulent Reynolds stress also decrease with the plasma pressure. For both situations the cross phase between the radial and azimuthal components of the velocity is found to be a key factor to determine variations in the turbulent Reynolds stress at different magnetic fields and discharge pressures. The results show that the generation of the shear flow is related to the development of specific frequency components of the drift wave turbulence for a variety of plasma conditions. The linear stability analysis shows that the observed variation in the turbulence and shear flow with magnetic field is also consistent with a critical gradient behavior.

Yan, Z. [University of California, San Diego, La Jolla, California 92093 (United States); University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States); Tynan, G. R.; Holland, C.; Xu, M.; Muller, S. H.; Yu, J. H. [University of California, San Diego, La Jolla, California 92093 (United States)

2010-01-15

448

Frictional dissipation in a polymer bilayer system.  

PubMed

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

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

2014-02-18

449

Prediction of High-Lift Flows using Turbulent Closure Models  

NASA Technical Reports Server (NTRS)

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

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

1997-01-01

450

Turbulent boundary-layer control with spanwise travelling waves  

NASA Astrophysics Data System (ADS)

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

Whalley, Richard D.; Choi, Kwing-So

2011-12-01

451

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

452

Turbulence and Turbulence Suppression in the Helimak  

NASA Astrophysics Data System (ADS)

The Helimak is an approximation to the infinite cylindrical slab, b