Piezo-viscous Effects Research Articles

A Fully Implicit Coupled Scheme for Mixed Elastohydrodynamic Problems on Co-Allocated Grids

In the modeling of elastohydrodynamic lubrication problems considering mixed friction, strongly coupled dependencies occur due to piezo-viscous effects and asperities, which can make a numerical solution exceptionally difficult. A fully implicit coupled scheme for solving mixed elastohydrodynamic lubrication problems is presented. Our scheme uses finite-volume discretization and co-allocated grids for hydrodynamic pressure and elastic deformation. To provide strong coupling between pressure and deformation even in the highly loaded zone, a correction term that adds numerical diffusion is used. The resulting linear equation system of this scheme can be efficiently solved by Krylov subspace methods. This results in an improved accuracy and computational efficiency compared to the existing methods. This approach was validated and has been shown to be accurate.

Performance of textured spherical thrust hybrid bearing operating with shear thinning and piezoviscous lubricants

Improving the lubricating performance of tribo-pairs using engineering textured surfaces has been the main focus of tribology research in recent years. The use of a suitably designed micro-texture on the bearing surface may have a beneficial effect on the performance of fluid film bearings. In the present paper, a mathematical model of a hybrid spherical thrust bearing is developed considering the effect of shear thinning and piezoviscous behaviour of a lubricant. The modified Reynolds equation for a hybrid spherical thrust bearing configuration together with a restrictor flow equation for a capillary restrictor is solved using a finite-element method. In this work, the effect of various micro-textures shapes (spherical, circular, conical and square) and non-Newtonian lubricant behaviour having shear thinning and piezoviscous effects are analysed. The numerically simulated result shows a strong dependence on the combined effect of shear thinning and piezoviscous lubricant behaviour and a chosen geometric shape of a texture. The frictional power loss is seen to reduce nearly by 24.05%, and the stiffness gets enhanced by 11.08%.

Analysis of couple-stresses and piezo-viscous effects in a layered connecting-rod bearing

In this work, the combined effects of couple-stresses and piezo-viscosity on the dynamic behavior of a compression ignition engine big-end connecting-rod bearing with elastic layer are investigated using the V. K. Stokes micro-continuum theory. It is assumed that the journal (crankpin) is rigid and the big-end bearing consists of a thin compressible elastic liner fixed in an infinitely stiff housing. The governing Reynolds' equation and the viscous dissipation term appearing on the RHS of energy equation are modified using the V. K. Stokes micro-continuum theory. The non-Newtonian effect is introduced by a new material constant η, which is responsible for couple-stress property, and the piezo-viscosity effect by the pressure–viscosity coefficient α appearing in the well-known Barus' law. In the proposed model, the nonlinear transient modified Reynolds equation is discretized by the finite difference method, and the resulting system of algebraic equations is solved by means of the subrelaxed successive substitutions method to obtain the fluid-film pressure field as well as the film thickness distribution. The crankpin center trajectories for a given load diagram are determined iteratively by solving the nonlinear equilibrium equations of the journal bearing system with the improved and damped Newton–Raphson method for each time step or crankshaft rotation angle. According to the obtained results, the effects of couple-stresses and piezo-viscosity on the nonlinear dynamic behavior of dynamically loaded bearings with either stiff or compliant liners are significant and cannot be overlooked.

Piezo-Viscous and Surface Roughness Effects on the Squeeze Film Characteristics of Micropolar Fluid in Convex Curved Plates

Piezo-Viscous and Surface Roughness Effects on the Squeeze Film Characteristics of Micropolar Fluid in Convex Curved Plates

Influence of the Thermo-Piezo-Viscous Effect on the THD Lubrication in Porous Self- Lubricating Bearings

A numerical study of the behavior for a finite porous journal bearing lubricated with Newtonian fluid is undertaken considering both thermal and piezo-viscous effects. The modified Reynolds equation is obtained by using the thermo-hydrodynamic aspect to account for the viscous variation versus pressure and temperature where Barus law has been used for the visous formulation. To determine the flow in the porous media, the Darcy’s law has considered. The governing equations were solved numerically using a finite difference approach. Obtained results compared with the case of a thermo-viscous aspect, the piezo-viscous parameter increases the load carrying capacity significantly and improves the characteristics of the journal bearings.

Stability of plane Poiseuille–Couette flows of a piezo-viscous fluid

We examine stability of fully developed isothermal unidirectional plane Poiseuille–Couette flows of an incompressible fluid whose viscosity depends linearly on the pressure as previously considered in Hron et al. [J. Hron, J. Málek, K.R. Rajagopal, Simple flows of fluids with pressure-dependent viscosities, Proc. R. Soc. Lond. A 457 (2001) 1603–1622] and Suslov and Tran [S.A. Suslov, T.D. Tran, Revisiting plane Couette–Poiseuille flows of a piezo-viscous fluid, J. Non-Newtonian Fluid Mech. 154 (2008) 170–178]. Stability results for a piezo-viscous fluid are compared with those for a Newtonian fluid with constant viscosity. We show that piezo-viscous effects generally lead to stabilisation of a primary flow when the applied pressure gradient is increased. We also show that the flow becomes less stable as the pressure and therefore the fluid viscosity decrease downstream. These features drastically distinguish flows of a piezo-viscous fluid from those of its constant-viscosity counterpart. At the same time the increase in the boundary velocity results in a flow stabilisation which is similar to that observed in Newtonian fluids with constant viscosity.

Analysis of non-Newtonian and piezoviscous effects in dynamically loaded connecting-rod bearings

The dynamic behaviour of three elastic connecting-rod big-end bearings is studied. Non-Newtonian and piezoviscous behaviour of the lubricant is included. The present algorithm allows accurate analysis of film rupture and reformation during actual operation of an engine and is based on finite element discretization. The non-Newtonian effect is introduced by modifying the viscosity for each point of a three-dimensional mesh of the film and for each iteration. Using a convenient iterative solution procedure, converged solutions for lubricant flow and elastic deformation fields are obtained. The present algorithm is extended to a global thermo-elastohydrodynamic model, to take into account the effects of viscosity variation with temperature. It is shown that the non-Newtonian shear-thinning and oil piezoviscous effects cannot be neglected in a complete connecting-rod bearing analysis. Furthermore, thermo-elastic behaviour can lead to different conclusions, than obtained by the elastohydrodynamic model.

Elastohydrodynamic Lubrication in Extended Parameter Ranges — Part II: Load Effect

The elastohydrodynamic lubrication (EHL) analysis has been extended to wide parameter ranges in order to satisfy the needs in design optimization of heavy-duty mechanical components, such as gears, bearings, cams and traction drives, especially when operated under severe conditions. Following the first part of the present study that investigates the effect of speed on the EHL film thickness, this paper is focused on the effect of load in an extended load parameter range that covers six orders of magnitude. Obtained results show that, when the load is moderate within a relatively narrow range, the present results agree well with the conventional EHL theory. If the load is light and the speed is high, the film thickness may sometimes go up and down then up again with continuously decreasing load due to the transition from EHL to the pure hydrodynamic lubrication, in which piezo-viscous and elastic deformation effects are negligible. When the load is heavy and the speed is not high, the EHL film may drop quickly as the load increases, and commonly used EHL formulae may sometimes overestimate the film thickness. At a certain constant speed there appears to be a critical load, beyond which the EHL film breaks down. The higher the speed, the heavier the critical load would be. In the extended speed and load ranges the relationship between the film thickness and the load may no longer obey the simple power law described by the conventional theory, but rather a complicated function of speed, load and contact ellipticity. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Cancun, Mexico October 27–30, 2002

The behaviour of low-amplitude surface roughness under line contacts: Non-Newtonian fluids

As a rough surface passes under an elastohydrodynamically lubricated (EHL) contact the surface profile is modified, with long-wavelength components being reduced in amplitude. The behaviour of the shorter wavelengths is complex. Where both surfaces have the same velocity-pure rolling-the roughness passes through the contact unaltered. Where there is relative slip and the fluid is assumed to be piezoviscous and Newtonian, analysis predicts that the roughness will be flattened. It is replaced by a surface profile with a different wavelength travelling through the contact at the entrainment velocity. The amplitude of this complementary wave may be smaller or larger than the roughness it replaces. This behaviour has not been found experimentally and does not occur in soft contacts where piezoviscous effects are absent. This paper extends a previous analysis for Newtonian fluids to include shear rate effects. It shows that while these effects are relatively unimportant for pure rolling they have a major influence where there is relative sliding of the surfaces. The formation of the complementary wave is largely eliminated and, at short wavelengths, instead of being modified the original roughness passes unchanged through the contact.

Thermal effects on film thickness and traction in rolling/sliding EHL line contacts—an accurate inlet zone analysis

Thermal analysis in an EHL line contact has been obtained using a computational method developed by Elrod and Brewe ( NASA Tech. Mem. 88845). Piezoviscous effects have been incorporated using Barus and Roelands' viscosity models for the P-150 oil used in the present analysis. A significant reduction in film thickness and traction at high rolling speeds relative to isothermal conditions has been obtained. Results are presented through figures and tables in the text. The results of the present analysis have been compared with the results of Murch and Wilson ( ASMEJ. Lubr. Technol., 97 (1975) 212), Goksem and Hargreaves ( Trans. ASME, 100 (1978) 346) and Ghosh and Hamrock ( NASA Tech. Mem. 83424). The thermal effect on film thickness reduction factor C, is expressed in terms of the dimensionless thermal loading parameter Q and slip S as: C = 1.0 [ 1.0 + 0.133 Q 0.71 ( 1.0 + 5.65 S 0.96 ) ]

Elastohvdrodvnamic Lubrication of Elliptical Contacts with Pure Spin

In a lubricated, non-conformal conjunction piezoviscous effects associated with the lubricant and elastic distortion of the bounding surfaces can have a significant influence upon the generation of the lubricant film and consequently upon the oil pressure distribution. In many widely studied cases which have been concerned with surface motions of rolling/sliding, it has been shown that these effects enhance the load-generating capability of the oil-film. However, some machine elements experience more complicated bearing surface motions. For example, spinning as well as entraining occurs in the elliptical contact regions of angular contact bearings and some forms of continuously variable speed drives. In this paper, results from a study on the elastohydrodynamic lubrication of elliptical contacts with pure spin are reported. In contrast to the former situation, an unfavourable influence of the surface deflection upon the generation of the fluid film has been observed. Comparison with experimental observation demonstrated encouraging agreement.

On Reciprocating Elastomeric Seals: Calculation of Film Thicknesses Using the Inverse Hydrodynamic Lubrication Theory

The elastohydrodynamic lubrication (EHL) problem of an axi-symmetric, elastomeric, reciprocating seal may be considered as one of a heavily loaded line contact with only small piezo-viscous effects. A method is presented to calculate the overall film thickness for the stationary, isothermal situation with sufficient lubricant supply and negligible surface roughness effects. It applies the inverse theory to the frictionless dry contact pressure distribution, considering the influence of the boosting action in the entry zone of the film. Its validity is shown for the isoviscous EHL of the Hertzian contact. The finite element method (FEM) may be used to calculate the contact situation of mounted and pressurized seals. Important aspects of such calculations are discussed. To calculate film thicknesses using such FEM results, a computer program PROGRES was written. Good correspondence between the predictions of PROGRES and experimental outstroke film thicknesses are obtained for a rectangular rod seal with rou...

Piezoviscous Effects in Nonconformal Contacts Lubricated Hydrodynamically

The analysis is concerned with the piezoviscous-rigid regime of lubrication for the general case of elliptical contacts. In this regime several formulas of the lubricant film thickness have been proposed by Hamrock and Dowson, by Dowson et al., and more recently by Houpert. However, either they do not include the load parameter W, which has a strong effect on film thickness, or they overestimate the film thickness by using the Barus formula for pressure-viscosity characteristics. The Roelands formula was used for the pressure-viscosity relationship. The effects of the dimensionless load, speed, and materials parameters, the radius ratio, and the lubricant entrainment direction were investigated. The dimensionless load parameter was varied over a range of one order of magnitude. The dimensionless speed parameter was varied by 5.6 times the lowest value. Conditions corresponding to the use of solid materials of steel, bronze, and silicon nitride and lubricants of paraffinic and naphthenic mineral oil were considered in obtaining the exponent in the dimensionless materials parameter. The radius ratio was varied from 0.2 to 64 (a configuration approaching a line contact). Forty-one cases were used in obtaining a minimum film thickness formula. Contour plots indicate in detail the pressure developed between the contacting solids.