Fluid material definition for supersonic real gas simulations

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Hello All,

I'm trying to define a new Fluid cyclopentan as a compressible gas for real gas analysis in the supersonic speed range 2 to 4. Most of the properties e.g. density I have defined as a 3rd order polynomal function. I´ve done this with the REFPROP.DLL from the NIST.

But I wonder why I can define the compressibility Cp/Cv (Isentropic Expansion Coefficient) only as a constant! The compressibility should be as well a non constant and non linear function which I have got out of REFPROP. Does someone know the solution or is there probably a tutorial out for this?

Best regards

Stefan