3D-CFD Flow Structures in Journal Bearings 2009-01-2688
Hydrodynamic radial journal bearings under unsteady load, which are common for automotive applications, are exposed to cavitation, e.g. flow, suction, shock and exit cavitation. The fluid mechanic description of the flow in journal bearings takes advantage of the small bearing clearance, which allows the reduction of the Navier-Stokes equations and leads to the Reynolds equation. The Reynolds equation is two-dimensional, the radial pressure gradient and the radial velocity component are neglected. However, the equation includes the surface velocities, oil density and viscosity and describes the relation between hydrodynamic pressure and local clearance. With the introduction of a cavitation index or a mass flow coefficient a powerful method to carry out numerical studies can be created, which allows the calculation of flow properties and the prediction of regions where the lubrication film disintegrates.
Thus the onset of cavitation, the generation of vapor bubbles and the formation of a two-phase flow can be described correctly, as long as the small clearance criterion is valid. There are, however, forms of cavitation failures, which are locally confined. In these cases the vapor bubbles collapse in the vicinity of the wall and cause erosion, which can lead to the failure of the bearing. These forms of cavitation damage are found typically adjacent to feed holes and other structures in the journal, which are three-dimensional and where the small clearance criterion is not valid. Consequently, the application of a two-dimensional model based on the Reynolds equation must fail.
This current work presents 3D computational flow calculations and experimental results of the flow structures inside the flow film between a rotating inner cylinder and a fixed outer cylinder simulating the flow inside a journal bearing. In particular, the surrounding region adjacent to a feedhole in the outer cylinder was targeted, where the geometry is truly three-dimensional. The local flow field is also three-dimensional including a radial pressure gradient and a strong radial velocity due to the incoming flow orthogonal to the cylinder wall.
The numerical results, which are supported by Laser-Doppler-Velocimetry (LDV) and visualization results, show complex three-dimensional vortex patterns interacting with the naturally reversed flow in the wide gap region of the bearing. A variation of the normalized clearance, the Reynolds number and the ratio of main and side (or feed) flow yields the effect on the local pressure distribution. Flow structure and pressure distribution indicate a good correlation to typical cavitational erosion marks. Within this work great emphasis has been given to the careful generation of orthogonal grids in the calculation domain, which are needed to reduce the calculation time. Moreover, the minimal number of cells across the gap shall not be lower than 8, which has been verified by extensive pre-studies. Hence, a large number of cells is required to fill the gap throughout the bearing without violating the recommended aspect ratio.
Future activities target the incorporation of cavitation theory into the CFD code, which will allow the direct modeling of generation, transport and collapse of vapor bubbles.