Air bearing modelling

Dave Larkham

Hi,

I'm interested in using CFX to model a linear air bearing (aerostatic bearing); but I'm having quite a lot of trouble.

Air bearing dimensions: Outer dims: 60x60 mm Inlet dia: 2 mm Orifice dia: 0.4 mm

The bearing is supposed to float on a cushion of air above a flat and smooth surface at anywhere between 5 and 50 microns.

I've modelled one quadrant of the fluid volume. The orifice is located at the centre of the bearing (i.e. at 0,0) and the air bearing is parallel to the smooth surface. I'm currently investigating the bearing properties at a 50 micron separation.

I'm modelling the fluid domain as an ideal gas at 300K and using the standard turbulence model.

Here are my questions:

1. Ideally, I'd like to specify an inlet pressure of 5 bar and an outlet pressure of 0 bar; this has been problematic; so instead, I've been applying a mass flow rate to the inlet (whilst keeping a 0 bar outlet); if I determine the pressure at the inlet (for a given mass flow rate), would I have got the same mass flow rate if I'd specified that inlet pressure in the first place?

2. In order to achieve a model with a reasonable number of elements, I've split the model into three parts and I'm meshing them individually. This allows me to use a very fine mesh for the air gap, whilst using larger elements for the inlet and outlet regions. Is this the best strategy because I've read that "knitting" different meshes together can cause inaccuracies?

3. My main problem is that the vertical thrust predicted by CFX acting on the bearing is very low. I think this is due to large negative pressures existing around the orifice; from what I've read this does not occur in the real world. What could cause it?

We have test results for a very similar bearing and some antique bearing analysis software (that works) that we're comparing ANSYS against.

Thanks for your help.