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Transcient response of delta pressure

Altair Forum User

Hi every one !

I have difficulties resolving a tank decompression problem...

I have a tank full of air at pressure P1 at t=0.

At t=0.2, the tank is open to the outside at pressure P2<<P1.

Analytically, after 1 sec, pressure of the tank is still far from outside pressure...

So, on Acusolve, I did one volume with one outlet only. I put P1 value on pressure in Nodal Initial Condition and I created a multiplier function in order to get my outlet pressure equal to P1 before t=0.2sec, then drop down to P2<<P1 after t=0.2sec. I run the calculation for 1.2 sec.

But the result show me that pressure inside the tank is folowing instantaneously the outlet pressure, which is absolutely not physically correct...
I think that I miss some parameters somewhere, but I don't know where...

Is somebody has any idea to help me ?

Thanks a lot !

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ydigit

The phenomenon you are trying to simulate requires a compressible solver. AcuSolve currently does not have a compressible solver. You could try with Altair RADIOSS, that is also used for airbag simulations.

acupro

Or at least use air properties that vary with absolute pressure - such as isentropic density - instead of constant density. You could also use ideal gas, where the density depends on absolute pressure AND temperature. There you'll have to include the temperature solve as well.

Altair Forum User

Thanks to both of you !

I will try both proposals ! I need to take charge of Radioss...

Acupro, I tryed to run a simulation with ideal gas, but Acusolve told me about negative absolute pressure... I tried to run before a steady calculation with constant density air to use it as restart for the transient one but I had the same problem...

acupro

Good idea to start from a steady, constant-properties solution. Make sure you use absolute pressure in your boundary conditions for both cases.

Altair Forum User

I use only one outlet and I put P1 in pressure field... there is no problem to run steady state solution but still negative absolute pressure by running the transient one..

acupro

You may want to contact your local Altair support office. Or - you could upload the two .inp files here - the original with constant properties, plus the 'restart' with ideal gas. When you switch to ideal gas, what do you do with the temperature field? If you're not interested in the temperature field, you could also try isentropic density instead of ideal gas.