# Transcient response of delta pressure

## Recommended Posts

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 !

##### Share on other sites

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.

##### Share on other sites

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.

##### Share on other sites

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...

##### Share on other sites

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

##### Share on other sites

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..