A Numerical Study of Surface Roughness Effects on Ultra-Thin Gas Films

1986 ◽  
Vol 108 (2) ◽  
pp. 171-177 ◽  
Author(s):  
J. W. White ◽  
P. E. Raad ◽  
A. H. Tabrizi ◽  
S. P. Ketkar ◽  
P. P. Prabhu
Keyword(s):  
Surface Roughness ◽  
Rough Surface ◽  
Reynolds Equation ◽  
Numerical Study ◽  
Surface Film ◽  
Maximum Load ◽  
Roughness Effects ◽  
Number Range ◽  
Pressure Diffusion ◽  
Bearing Number

A wedge bearing with transverse sinusoidal roughness pattern is studied numerically in order to predict the effect of surface roughness on compressible fluid films. A variable grid implicit finite difference scheme is used to provide steady-state solutions of the Reynolds equation over a bearing number range of five orders of magnitude. At a fixed bearing geometry and orientation, the bearing load is found to increase to a maximum as the bearing number increases, then to decrease and asymptotically approach a limiting value as the bearing number increases further. This is quite unlike the behavior of an incompressible fluid bearing. Analysis indicates that the maximum load occurs at a condition where pressure diffusion and Couette effects of the fluid film are of an equal order of magnitude. The increased emphasis of the pressure diffusion physics is due to the short length scales of the rough surfaces which “trigger” the higher derivative diffusion terms in the Reynolds equation. The criterion required for validity of an infinite bearing number solution with a rough surface is found to be much more restrictive than that of a smooth surface bearing. Last, the type of rough surface film clearance averages used in incompressible lubrication are shown to be incorrect for analysis of very thin gas films. It would appear that one application of this information would be the design of an artificially roughened surface for the take-off and landing of magnetic head sliders so as to minimize contact and wear of the magnetic media.

On the effects of two-dimensional Reynolds roughness in hydrodynamic lubrication

1978 ◽  
Vol 364 (1716) ◽  
pp. 89-106 ◽  
Keyword(s):  
Surface Roughness ◽  
Numerical Computation ◽  
Autocorrelation Function ◽  
Reynolds Equation ◽  
Hydrodynamic Lubrication ◽  
The Other ◽  
Roughness Height ◽  
Two Dimensional ◽  
Roughness Effects ◽  
Dimensional Surface

The hydrodynamic lubrication of rough surfaces is analysed with the Reynolds equation, whose application requires the roughness spacing to be large, and the roughness height to be small, compared with the thick­ness of the fluid film. The general two-dimensional surface roughness is considered, and results applicable to any roughness structure are obtained. It is revealed analytically that two types of term contribute to roughness effects: one depends on the shape of the autocorrelation function and the other does not. The former contribution was neglected by previous workers. The numerical computation of an example shows that these two contributions are comparable in magnitude.

An Averaging Technique for the Analysis of Rough Surface High Bearing Number Gas Flows

1999 ◽  
Vol 121 (2) ◽  
pp. 333-340 ◽  
Author(s):  
James W. White
Keyword(s):  
Surface Roughness ◽  
Rough Surface ◽  
Numerical Solutions ◽  
Scale Effects ◽  
Wavy Surface ◽  
Universal Property ◽  
Flying Height ◽  
Lubrication Equation ◽  
Product Term ◽  
Bearing Number

Earlier analytical solutions by White (1980, 1983, 1992, 1993) included Couette effects, transverse diffusion, and mass storage in a model lubrication equation for narrow width wavy surface high bearing number gas films. The model lubrication equation did not include longitudinal diffusion effects due to the high bearing number restriction. Crone et al. (1991), however, reported numerical solutions of the full Reynolds equation for a gimbal mounted slider subject to wavy surface roughness. The first objective of this work is to reconcile the differences observed between the reported results of White and those of Crone et al. for moving and stationary roughness. The second objective is to describe how to best apply what appears to be a universal property of a high bearing number gas film subjected to a rough surface. Each solution of the model lubrication equation by White (1980, 1983, 1992, 1993) produced a product term based on local gas pressure and clearance (Z = Ph) that is independent of roughness details but which is dependent on the statistical properties of the roughness. In the present work, this characteristic is treated as a universal property of all high bearing number rough surface gas films. The product variable Z = Ph is introduced into the generalized full lubrication equation, and the resulting lubrication equation is ensemble averaged before a solution is attempted. This removes the short length and time scale effects due to the surface roughness. Solution of the ensemble averaged equation for Z(x, y, t) then follows by standard analytical or numerical methods. The unaveraged pressure is then given by P(x, y, t) = Z(x, y, t)/h(x, y, t) and the ensemble averaged or mean pressure at a point is computed from Pm(x, y, t) = Z(x, y, t)E(1/h(x, y, t)), where E(1/h) represents the ensemble average of 1/h. Using this technique, numerical solutions of the full generalized lubrication equation based on kinetic theory were obtained for a low flying gimbal mounted slider. Results indicate that the nominal flying height increases and the minimum flying height decreases as surface roughness increases.

Calculation of Surface Roughness Effects on Air-Riding Seals

2002 ◽  
Author(s):  
C. Guardino ◽  
J. W. Chew ◽  
N. J. Hills
Keyword(s):  
Surface Roughness ◽  
Compressible Flows ◽  
Reynolds Equation ◽  
Incompressible Flows ◽  
Numerical Solutions ◽  
Cfd Analysis ◽  
Roughness Effects ◽  
Stationary Wall ◽  
Comparative Results ◽  
Effect Of Surface

The effects of surface roughness on air-riding seals are investigated here using the Rayleigh-pad as an example. Both incompressible and compressible flows are considered using both CFD analysis and analytical/numerical solutions of the Reynolds equation for various 2D or 3D roughness patterns on the stationary wall. A ‘unit-based’ approach for incompressible flows has also been employed and is shown to be computationally much less expensive than the full-geometry solution. Results are presented showing the effect of surface roughness on the net lift force. The effects of varying the Reynolds number are demonstrated, as well as comparative results for static stiffness.

Surface Roughness Effects on Radiant Energy Interchange

1971 ◽  
Vol 93 (1) ◽  
pp. 88-96 ◽  
Author(s):  
R. G. Hering ◽  
T. F. Smith
Keyword(s):  
Surface Roughness ◽  
Rough Surface ◽  
Roughness Element ◽  
Strong Dependence ◽  
Radiant Energy ◽  
Radiant Heat ◽  
Absorption Factor ◽  
Surface Absorption ◽  
Roughness Effects ◽  
Property Model

Radiant interchange between opaque interacting surfaces is formulated for unequal temperature adjoint plates with a one-dimensional surface roughness profile. Rough surface bidirectional reflectance and directional emittance depend on material emittance, roughness element slope, and roughness element specularity. Numerical solution to simultaneous integral equations governing the dimensionless radiant intensities lead to local and overall absorption factors for selected values of the influencing parameters. Absorption factor results show a strong dependence on surface roughness and indicate that roughness effects are more important in the evaluation of radiant interchange than radiant heat loss. Absorption factor values differing by a factor of two to four are commonly observed for identical emittance materials as a result of roughness. The diffuse emission-diffuse reflection property model employing rough surface apparent hemispherical emittance gave the closest agreement to rough surface absorption factor results. Simple property model results, however, were often significantly in error.

scholarly journals Surface-roughness effects on the mean flow past circular cylinders

1980 ◽  
Vol 98 (4) ◽  
pp. 673-701 ◽  
Author(s):  
O. Güven ◽  
C. Farell ◽  
V. C. Patel
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Pressure Rise ◽  
Boundary Layer Separation ◽  
Circular Cylinders ◽  
Mean Flow ◽  
Roughness Effects ◽  
Number Range ◽  
Mean Pressure ◽  
The Mean

Measurements of mean-pressure distributions and boundary-layer development on rough-walled circular cylinders in a uniform stream are described. Five sizes of distributed sandpaper roughness have been tested over the Reynolds-number range 7 × 104to 5·5 × 105. The results are examined together with those of previous investigators, and the observed roughness effects are discussed in the light of boundary-layer theory. It is found that there is a significant influence of surface roughness on the mean-pressure distribution even at very large Reynolds numbers. This observation is supported by an extension of the Stratford–Townsend theory of turbulent boundary-layer separation to the case of circular cylinders with distributed roughness. The pressure rise to separation is shown to be closely related, as expected, to the characteristics of the boundary layer, smaller pressure rises being associated with thicker boundary layers with greater momentum deficits. Larger roughness gives rise to a thicker and more retarded boundary layer which separates earlier and with a smaller pressure recovery.

Surface Roughness Effects in High-Speed Hydrodynamic Journal Bearings

1997 ◽  
Vol 119 (4) ◽  
pp. 776-780 ◽  
Author(s):  
H. Hashimoto
Keyword(s):  
Surface Roughness ◽  
High Speed ◽  
Reynolds Equation ◽  
Computation Time ◽  
Experimental Results ◽  
Roughness Effects ◽  
Modified Reynolds Equation ◽  
Nonlinear Simultaneous Equations ◽  
Theoretical Results ◽  
Good Agreement

This paper describes an applicability of modified Reynolds equation considering the combined effects of turbulence and surface roughness, which was derived by Hashimoto and Wada (1989), to high-speed journal bearing analysis by comparing the theoretical results with experimental ones. In the numerical analysis of modified Reynolds equation, the nonlinear simultaneous equations for the turbulent correction coefficients are greatly simplified to save computation time with a satisfactory accuracy under the assumption that the shear flow is superior to the pressure flow in the lubricant films. The numerical results of Sommerfeld number and attitude angle are compared with the experimental results to confirm the applicability of the modified Reynolds equation in the case of two types of bearings with different relative roughness heights. Good agreement was obtained between theoretical and experimental results.

Calculation of Surface Roughness Effects on Air-Riding Seals

2004 ◽  
Vol 126 (1) ◽  
pp. 75-82 ◽  
Author(s):  
C. Guardino ◽  
J. W. Chew ◽  
N. J. Hills
Keyword(s):  
Surface Roughness ◽  
Compressible Flows ◽  
Reynolds Equation ◽  
Incompressible Flows ◽  
Numerical Solutions ◽  
Three Dimensional ◽  
Roughness Effects ◽  
Stationary Wall ◽  
Comparative Results ◽  
Effect Of Surface

The effects of surface roughness on air-riding seals are investigated here using the Rayleigh pad as an example. Both incompressible and compressible flows are considered using both CFD analysis and analytical/numerical solutions of the Reynolds equation for various two-dimensional or three-dimensional roughness patterns on the stationary wall. A “unit-based” approach for incompressible flows has also been employed and is shown to be computationally much less expensive than the full-geometry solution. Results are presented showing the effect of surface roughness on the net lift force. The effects of varying the Reynolds number are demonstrated, as well as comparative results for static stiffness.

scholarly journals Some Aspects of Gear Tribology

1990 ◽  
Vol 204 (1) ◽  
pp. 13-19
Author(s):  
R W Snidle ◽  
H P Evans ◽  
A Dyson
Keyword(s):  
Surface Roughness ◽  
Recent Work ◽  
Rough Surface ◽  
Gear Tooth ◽  
Lubricant Film ◽  
Physical Explanation ◽  
Roughness Effects ◽  
New Developments ◽  
Ehl Contact

This paper reviews work being carried out at Cardiff on tribological aspects of heavily loaded gear tooth contacts. The contacts occurring in high conformity (Wildhaber-Novikov) gears have been analysed to predict the thickness of the lubricant film, and the work has been extended to include a consideration of surface roughness effects in gears of this type. For conventional gears a different rough surface elastohydrodynamic (EHL) analysis has been developed and this shows that roughness features can become almost flattened where they occur within the main EHL contact. Recent work on scuffing research is described and new developments in the search for a physical explanation of this form of failure are discussed.

Surface Roughness Effects on Fluid Flow between Two Rotating Cylinders

2015 ◽  
Vol 642 ◽  
pp. 275-280
Author(s):  
Sutthinan Srirattayawong ◽  
Shian Gao
Keyword(s):  
Surface Roughness ◽  
Film Thickness ◽  
Reynolds Equation ◽  
Surface Profile ◽  
Fluid Film ◽  
Real Surface ◽  
Contact Center ◽  
Roughness Effects ◽  
Fluid Film Thickness ◽  
Lubricant Viscosity

In general, the thin fluid film problems are explained by the classical Reynolds equation, but this approach has some limitations. To overcome them, the method of Computational Fluid Dynamics (CFD) is used in this study, as an alternative to solving the Reynolds equation. The characteristics of the two cylinders contact with real surface roughness are investigated. The CFD model has been used to simulate the behavior of the fluid flows at the conjunction between two different radius cylinders. The non-Newtonian fluid is employed to calculate the lubricant viscosity, and the thermal effect is also considered in the evaluation of the lubricant properties. The pressure distributions, the fluid film thickness and the temperature distributions are investigated. The obtained results show clearly the significance of the surface roughness on the lubricant flow at the contact center area. The fluctuated flow also affects the pressure distribution, the temperature and the lubricant viscosity in a similar pattern to the rough surface profile. The surface roughness effect will decrease when the film thickness is increased.

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