Three dimensional flow structures in journal bearings

Abstract

In general, the fluid flow in journal bearings can be described by the Navier-Stokes Equations and the conservation of mass. The application of the small gap criterion allows a simplification of these equations yielding the Reynolds Equation, which links the local gap size with the pressure gradient resulting in a powerful tool for the designing process of journal bearings. Typically, the Reynolds Equation is used in EHD-design software based on FE-methods, which is used to compute pressure distributions, forces, deformations and many more parameters needed for the selection of the right bearing geometry. However, there are regions in the journal bearing where the Reynolds Equation must fail, because either the small gap criterion or the Couette flow assumption is violated. There are pockets, grooves and holes, which are necessary to distribute the oil supply across the gap. Moreover, the oil feed represents a cross flow perpendicular to the circumferential main flow. In these regions three dimensional flow structures replace the undisturbed Couette flow, which are strongly affected by vortices, but are non-turbulent due to the Re-scale. This work presents experimental data obtained from a cylinder apparatus with moderate gap sizes, which features independently rotating cylinders and a cross flow through a hole in the sidewall. LDV-measurements of velocity profiles and visualization methods to animate the three dimensional nature of the flow are presented. The experimental data are used to validate 3D-CFD calculations, which are expanded towards smaller gap sizes in the range of typical journal bearings in automotive applications.