I use pressure far field BC's for my problem, but when I initialize, it says "bc type not handled Update_Fluid_Density". What does this error message mean? Thanks.

Is pressure far field BC just for single phase? I use multiphase.

Must use Ideal gas

Yes, I used ideal gas. My model is multiphase/Mixture

Pressure Far Field was designed for aerospace. Its uses are VERY limited because it is a very sensitive BC. I don't think it can handle multiphase/mixture at all... when you say multiphase, I think of two phases being gas and liquid... since Fluent considers liquids to be incompressible, the Pressure Far Field shouldn't even initialize. I highly recommend switching to pressure inlet / outlet conditions. Pressure Far Field was originally designed for the aerospace industry to give a simple BC for analyzing wings and such. It has to be far enough from walls and any other inlets or outlets to avoid a gradient across it. Also, since it assumes constant Mach and Static Pressure, it has to be able to adjust the densities accordingly (which is why an incompressible fluid, whether gasseous or liquid, will not work). I don't believe it can handle gas mixtures either, but I could be wrong on that one.

Hope this helps, and goodluck, Jason