scholarly journals Studies on the Pressure Buildup and Shear Flow Factors in the Cavitation Regime

Lubricants ◽  
2020 ◽  
Vol 8 (8) ◽  
pp. 82
Author(s):  
Michael Müller ◽  
Lukas Stahl ◽  
Georg-Peter Ostermeyer
Keyword(s):  
Shear Flow ◽  
Reynolds Equation ◽  
Fluid Distribution ◽  
Pressure Buildup ◽  
Microscopic Scale ◽  
Valid Method ◽  
Flow Factor ◽  
Partial Filling ◽  
Cavitation Pressure

Modeling tribological contacts is commonly based on the Reynolds equation. This study discusses the validity of conventional, averaged Reynolds simulations for systems including starvation regimes. Two fundamental assumptions that are used as common practice in many elasto-hydrodynamic (EHD) calculations, are debated. First, the use of a cavitation pressure (in most cases assumed to be zero) independent of the microscopic roughness. Second, the application of a shear flow factor, which is determined on a microscopic scale with a fully filled gap. The validity of these two assumptions is analyzed with simulations on the microscopic scale. For this purpose, simulations of partially filled contacts are carried out using the partially filled gaps model developed by the authors. The topographies, the filling level and the fluid distribution were varied. The simulations comply with established models for the fully filled state and show a distinct behavior for partial filling and different fluid distributions. Neglecting the contribution to pressure buildup and shear flow of partially filled domains is a valid method in most cases. However, as this study shows, near the fully filled regime, the domains should be handled with care.

Pressure and Shear Flow in a Rough Hydrodynamic Bearing, Flow Factor Calculation

1997 ◽  
Vol 119 (3) ◽  
pp. 549-555 ◽  
Author(s):  
L. Lunde ◽  
K. To̸nder
Keyword(s):  
Finite Element Method ◽  
Surface Roughness ◽  
Finite Element ◽  
Shear Flow ◽  
Reynolds Equation ◽  
Flow Model ◽  
The Finite Element Method ◽  
Average Flow ◽  
Hydrodynamic Bearing ◽  
Average Flow Model

The lubrication of isotropic rough surfaces has been studied numerically, and the flow factors given in the so-called Average Flow Model have been calculated. Both pressure flow and shear flow are considered. The flow factors are calculated from a small hearing part, and it is shown that the flow in the interior of this subarea is nearly unaffected by the bearing part’s boundary conditions. The surface roughness is generated numerically, and the Reynolds equation is solved by the finite element method. The method used for calculating the flow factors can be used for different roughness patterns.

A Modified Average Reynolds Equation for Rough Bearings With Anisotropic Slip

2015 ◽  
Vol 138 (1) ◽  
Author(s):  
Hsiang-Chin Jao ◽  
Kuo-Ming Chang ◽  
Li-Ming Chu ◽  
Wang-Long Li
Keyword(s):  
Surface Roughness ◽  
Shear Stress ◽  
Shear Flow ◽  
Reynolds Equation ◽  
Slenderness Ratio ◽  
Atomic Scale ◽  
Stress Factors ◽  
Squeeze Film ◽  
Boundary Slip ◽  
Slip Conditions

A lubrication theory that includes the coupled effects of surface roughness and anisotropic slips is proposed. The anisotropic-slip phenomena originate from the microscale roughness at the atomic scale (microtexture) and surface properties of the lubricating surfaces. The lubricant flow between rough surfaces (texture) is defined as the flow in nominal film thickness multiplied by the flow factors. A modified average Reynolds equation (modified ARE) as well as the related factors (pressure and shear flow factors, and shear stress factors) is then derived. The present model can be applied to squeeze film problems for anisotropic-slip conditions and to sliding lubrication problems with restrictions to symmetric anisotropic-slip conditions (the two lubricating surfaces have the same principal slip lengths, i.e., b1x=b2x and b1y=b2y). The performance of journal bearings is discussed by solving the modified ARE numerically. Different slenderness ratios 5, 1, and 0.2 are considered to discuss the coupled effects of anisotropic slip and surface roughness. The results show that the existence of boundary slip can dilute the effects of surface roughness. The boundary slip tends to “smoothen” the bearings, i.e., the derived flow factors with slip effects deviate lesser from the values at smooth cases (pressure flow factors φxxp,φyyp=1; shear flow factors φxxs=0; and shear stress factors φf,φfp=1 and φfs=0) than no-slip one. The load ratio increases as the dimensionless slip length (B) decreases exception case is also discussed or the slenderness ratio (b/d) increases. By controlling the surface texture and properties, a bearing with desired performance can be designed.

Pressure and Shear Flow Factors Calculation for Orthotropic Rough Surfaces

2007 ◽  
Author(s):  
Ramona Dragomir ◽  
Dominique Bonneau ◽  
Patrick Ragot ◽  
Franc¸ois Robbe-Valloire
Keyword(s):  
Surface Roughness ◽  
Shear Flow ◽  
Reynolds Equation ◽  
Rough Surfaces ◽  
Cross Flow ◽  
Lubricated Contacts ◽  
General Average ◽  
The Cross

In general, average Reynolds equation is defined in terms of shear flow factors in order to determine the effects of surface roughness on partially lubricated contacts. This paper is essentially devoted to the application of flow factors model to real shaft and bearing surfaces, obtained by metrological measures. Additionally, the average Reynolds equation is completed by “cross” flow factors. The “cross” flow factors may have an important role if model is applied on either longitudinally or transversely oriented surfaces (surfaces with directional patterns oriented with an angle).

Thermal Elastohydrodynamic Lubrication of Non-Newtonian Fluids With Rough Surfaces

1997 ◽  
Vol 119 (4) ◽  
pp. 605-612 ◽  
Author(s):  
J. M. Wang ◽  
V. Aronov
Keyword(s):  
Surface Roughness ◽  
Shear Flow ◽  
Reynolds Equation ◽  
Rough Surfaces ◽  
Elastohydrodynamic Lubrication ◽  
Operating Conditions ◽  
Newtonian Fluids ◽  
Perturbation Approach ◽  
Correction Factors ◽  
Power Law Fluids

The characteristics of thermal elastohydrodynamic lubrication by non-Newtonian fluids for rough surfaces is investigated theoretically. The general Reynolds equation for two-sided striated roughness lubricated by power law fluids is established using a perturbation approach. New correction factors for the pressure flow and the shear flow are derived; these factors integrate both lubricant rheology and surface roughness characteristics. A more effective numerical algorithm is adopted to obtain the solutions for wide ranges of operating conditions. Observations and discussion lead to further understanding of the various interactions among different factors in a elastohydrodynamic lubrication process.

Application of Average Flow Model to Lubrication Between Rough Sliding Surfaces

1979 ◽  
Vol 101 (2) ◽  
pp. 220-229 ◽  
Author(s):  
Nadir Patir ◽  
H. S. Cheng
Keyword(s):  
Shear Stress ◽  
Shear Flow ◽  
Flow Model ◽  
Stress Factors ◽  
Surface Pattern ◽  
Roughness Effects ◽  
Local Pressure ◽  
Average Flow ◽  
Flow Factor ◽  
Average Flow Model

The Average Flow Model introduced in an earlier paper is extended to include sliding contacts by deriving the shear flow factor for various roughness configurations. Similar to the pressure flow factors, the shear flow factor is obtained through numerical flow simulation on a model bearing having numerically generated roughness. The flow factors for isotropic and directional surfaces are expressed as empirical relationships in terms of h/σ, a surface pattern parameter γ defined as the ratio of x and y correlation lengths, and the variance ratio Vr1 which is the ratio of variance of surface 1 to that of the composite roughness. Expressions for the mean shear stress and horizontal force components due to local pressure in rough bearings are derived through the definition of shear stress factors, also obtained through simulation. The application of the average Reynolds equation to analyze roughness effects in bearings is demonstrated on a finite slider. The effects of the operating parameters as well as the roughness parameters on mean hydrodynamic load, mean viscous friction and mean bearing inflow are illustrated.

Some Discussions on the Flow Factor Tensor—Considerations of Roughness Orientation and Flow Rheology

2000 ◽  
Vol 122 (4) ◽  
pp. 869-872 ◽  
Author(s):  
Wang-Long Li
Keyword(s):  
Power Law ◽  
Reynolds Equation ◽  
Hydrodynamic Lubrication ◽  
Flow Behavior ◽  
Mapping Function ◽  
Power Law Fluid ◽  
Flow Behavior Index ◽  
Random Roughness ◽  
Flow Factor ◽  
Behavior Index

Relations expressing the effects of roughness orientations θi and flow behavior index n of the power-law fluid on the flow factors of area-distributed random roughness in hydrodynamic lubrication are derived. By using a mapping function, the generalized average Reynolds equation contains non-diagonal terms, and the flow factor tensor is symmetrical but not necessarily diagonal according to the coordinate system. Finally, the conditions that two rough surfaces act as though they were perfectly smooth are discussed for some particular combinations. [S0742-4787(00)01604-0]

Study on combined influence of inter-asperity cavitation and elastic deformation of non-Gaussian surfaces on flow factors

2008 ◽  
Vol 222 (6) ◽  
pp. 1039-1048 ◽  
Author(s):  
F-M Meng ◽  
D-T Qin ◽  
H-B Chen ◽  
Y-Z Hu ◽  
H Wang
Keyword(s):  
Shear Flow ◽  
Film Thickness ◽  
Elastic Deformation ◽  
Computer Code ◽  
Factor Analyses ◽  
Flow Factor ◽  
Oblique Flow ◽  
The One ◽  
Non Gaussian ◽  
Gaussian Surface

The combined influence of inter-asperity cavitation and elastic deformation of non-Gaussian surfaces on flow factors is numerically investigated based on the equations for flow factor analyses, since some engineering surfaces are non-Gaussian. For this task, non-Gaussian surfaces are generated at first through a digital filter technique by using authors’ computer code whose validity is proven. The numerical results show that the pressure flow factor increases whereas the shear flow factor decreases with low film thickness-to-roughness ratio ( h/σ < 3 or so). This is due to the above-said combined influence, if the oblique flow of lubricant is not obvious. But for a high film thickness-to-roughness ratio (approximately h/σ ≥ 3), the combined influence becomes weaker, hence ignored. Therefore, the above-said combined effect similar to the one from Gaussian surface circumstances ought to be considered in flow factor analyses and their applications.

Gas Lubrication Analysis Method of Step-Dimpled Face Mechanical Seals

2012 ◽  
Vol 134 (1) ◽  
Author(s):  
Shaoxian Bai ◽  
Xudong Peng ◽  
Yefeng Li ◽  
Songen Sheng
Keyword(s):  
Finite Difference ◽  
Finite Difference Method ◽  
Shear Flow ◽  
Pressure Distribution ◽  
Difference Method ◽  
Reynolds Equation ◽  
Stokes Equations ◽  
Leakage Rate ◽  
Navier Stokes ◽  
Navier Stokes Equations

In solving Reynolds equation with the conventional finite difference method, keeping the flow continuity has ofen been ignored, which will lead to an analysis error in the pressure distribution and leakage rate, especially for discontinuous clearance caused by step structures such as laser surface texturing sealing surfaces. In this paper, a finite difference method is introduced to satisfy the flow continuity to solve the Reynolds equation. Then, the pressure distribution for a typical rectangular step structure is obtained via two different methods: a numerical solution of the exact full Navier-Stokes equations, and a solution of the Reynolds equation solved by the previously mentioned method. A comparison between the two solution methods illustrates that, for both pressure flow and shear flow, the pressure distribution from the new difference method is in good agreement with that from the Navier-Stokes equations, and the new difference method can reflect the characteristic of the pressure sudden-change of the shear flow at the steps. Finally, the pressure distribution and leakage rate of a step-dimpled seal face are acquired with the presented method. The results show that the presented method allows gas-lubricating analysis of mechanical face seals with discontinuous clearance, and can keep the leakage rate continuous in the radial direction.

Numerical analysis of combined influences of inter-asperity cavitation and elastic deformation on flow factors

2007 ◽  
Vol 221 (7) ◽  
pp. 815-827 ◽  
Author(s):  
F-M Meng ◽  
W-Z Wang ◽  
Y-Z Hu ◽  
H Wang
Keyword(s):  
Numerical Analysis ◽  
Film Thickness ◽  
Elastic Deformation ◽  
Reynolds Equation ◽  
Rough Surfaces ◽  
Cross Flow ◽  
Combined Effects ◽  
Factor Analyses ◽  
Flow Factor ◽  
Small Ratio

Combined influences of inter-asperity cavitation and elastic deformation of rough surfaces on flow factors were investigated based on an extended Reynolds equation for flow factor analyses. The numerical results reveal that when effect of cross-flow of lubricant is not obvious, the pressure flow factor increases, whereas the shear flow factor decreases, for a small ratio of film thickness to roughness, due to influences of inter-asperity cavitation and the elastic deformation of rough surfaces. When the ratio of film thickness to roughness becomes big, however, the influences become weak and can even be negligible. Moreover, the above influences are sensitive to the orientations of rough surfaces. Therefore, combined effects of inter-asperity cavitation and elastic deformation of rough surfaces should be considered in flow factor analyses.

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