Incompressible vs. Perfect Gas Air Models

[tibich72]

I'm running a CFD simulation on a system consisting of a few solids immersed in a box of air. There's a slow airflow through the box; some of the solids consume power. Elmer solves both the Heat equation and Navier-Stokes.

I'm having problems with how the properties of Air are defined. If the material called "Air" has Compressibility Model set to 'Incompressible', then the max temperature is 128C. If the model is changed to 'Perfect Gas', then the max simulated temperature drops to 47C. This is a bit too much, I think.

This is the entire material properties section for Air:

Code: Select all

Material 7
  Name = "Air"
  Heat Capacity = 1005
  Density = 1.1831
  Heat Conductivity = 0.02605
  Compressibility Model = Perfect Gas
  Viscosity = 1.54075E-05
  Heat Expansion Coefficient = 0.00343
  Specific Heat Ratio = 1.4
  Reference Temperature = 25
  Reference Pressure = 101300
  KE Cmu = 0.09
  KE C1 = 1.44
  KE C2 = 1.92
  KE SigmaK = 1
  KE SigmaE = 1.3
  KE Clip = 1E-06
  Viscosity Model = KE
  Heat Conductivity Model = KE
End

I'm also attaching the .sif file, in case there's something else wrong.

What is the difference between 'Incompressible' and 'Perfect Gas'? More generally, which model corresponds more closely to the air that's outside (or the air used to cool things)? Is there another model that should be used? (The documentation mentions on 'Perfect Gas', 'Incompressible' and 'ArtificiallyCompressible' -- the last one seems to be 'Incompressible' with a better convergence).

Any advice is really appreciated.