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Sonu December 3, 2007 10:53

Compressible flow analysis
I am new to STARCD. I am working on the Exhaust pipe and compressble flow. I know what should be done in incompressible flow analysis. what are the extra things I need to do for Compressible flow analysis. Thanks a lot in advance

barracuda December 4, 2007 13:36

Re: Compressible flow analysis
Change the fluid density to ideal gas, and you will also need to specify the molecular weight of the gas (28.96 kg/kmol for air).

I assume that the Mach number is fairly low (< 0.3), so you really shouldn't have any convergence problems.

ravi December 5, 2007 01:19

Re: Compressible flow analysis
I am also having a similar problem related to the compressibility. when i run the simulation for an engine intake(Mach = 0.7), it gives an error msg, "negative densities found at more than 1000 locations." I am using ideal(T,P) in the Mol. properties and high reynolds k-e turb model. can sm1 help me , wat extra condition i need to apply to get the results.


barracuda December 5, 2007 08:25

Re: Compressible flow analysis
This can be due to either the temperature or the absolute pressure going negative. In your case, since M=0.7, negative absolute pressures are probably temporarily occing during the solution.

Is your analysis steady-state or transient?

If steady, try ramping up the inlet velocity (or pressure) over the first few hundred iterations. This can be done using a very simple user subroutine.

More advice can be found at:

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