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andreis July 31, 2002 14:55

Gas compression
Hi there

I have a cylinder-piston arrangement with a gas inside. I am trying to model the pressure increase as the piston is plunged into the cylinder. I have set up the model in TascFlow with the moving grid function.

My problem is that as the piston decreases the volume the pressure stays constant (at the reference pressure).

The model has been set up as transient, compressible, ideal gas, adiabatic wall boundaries....

What am i doing wrong? Any help would be greatly appreciated.

Thanks in advance andreis

Robin July 31, 2002 15:52

Re: Gas compression
Hi Andreis,

1. When defining your material, are you sure you defined it as an Ideal Gas?

2. Do you have any openings?


cfd guy August 1, 2002 10:00

Re: Gas compression
Hi andreis,
I'll assume that you don't have any outflow or opening boundary condition in your piston. If you don't have a reference pressure in any location of your model TASCflow will set a ambient(reference) pressure at node [2,2,2]:MAIN and your pressure variable will stay constant along compression/expansion. To bypass this restriction set LCHEKP = F in your PRM file and you'll notice the pressure changing.
Good Luck, cfd guy

andreis August 1, 2002 10:35

Re: Gas compression
Excellent, the pressure does change. Then how is the initial pressure set? thru Pref?

Also, if I need to compress a liquid, is it only necessary to provide a linearised eqation of state in the PROPR.F file or is there more to it than that?

(using TascFlow 2.11.1)

Thanks in advance andreis

cfd guy August 1, 2002 10:48

Re: Gas compression
The initial pressure will be your initial guess. Pref will be desconsidered with LCHEKP = f.
I don't know if TASCflow allows you to input your own equation of state.
Regards, cfd guy

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