Model of Thermal EHL Based on Navier-Stokes Equations : Effects of Asperities and Extreme Loads

Sammanfattning: A common approach in numerical studies of elastohydrodynamic lubrication (EHL) is based on solving the Reynolds equation that governs pressure distribution in thin lubricant films. The Reynolds equation is derived from the Navier-Stokes equations by taking assumptions that are considered valid when the thickness of the lubricant film is much smaller than its length. A massive increase in the computing power over the last decades has enabled the use of CFD (computational fluid dynamics) approach, based on the Navier-Stokes equations, in solving the EHL problem. Comparisons between the CFD and Reynolds approach have generally shown very good agreement. Differences can occur when the thin film assumptions of the Reynolds equation are not applicable. In this study, a CFD approach has been chosen with the aim of investigating effects of asperities and rheology at high loads on the behavior of the thin EHL films. A high quality mesh was generated in ANSYS ICEM CFD, while ANSYS Fluent has been employed in solving the Navier-Stokes equation by finite volume method (FVM). For EHL modeling, a set of user-defined functions (UDFs) were used for computing density, viscosity, wall temperature, heat source and elastic deformation of one of the contacting surfaces. Two lubricants were used, a commonly used oil in CFD analyses of EHL and Squalane. Non-Newtonian fluid behavior and thermal effects were considered. For Squalane, the two rheology models, Ree-Eyring and Carreau were compared. Squalane has been chosen in this study since it is one of the rare fluids with known parameters for both rheology models. Finally, the influence of surface roughness was explored for the cases of a single asperity and a completely rough wall. A surface roughness profile is generated in MATLAB by using the Pearson distribution function. In the cases where the surfaces are assumed to be completely smooth, the obtained results at the pressure of about 0.5 GPa closely correspond to literature, both in the case of Newtonian and non-Newtonian fluid behavior. At the pressure of about 1 GPa, severe shearing of the lubricant film has been noticed, characterized by a pronounced shear-band and plug flow. It was found that the choice of viscosity and rheology models has a large influence on the obtained results, especially at the high pressure levels. Finally, it was discovered that the developed CFD model of EHL has a great potential in studying the effects of surface roughness on the lubricant film behavior.