Rarefied gas
 

Hi!

I'm modelling a reentry vehicle in CFX and I'm working with low density gas. I'm facing a problem with overflow in all my simulations: there are few peaks of pressure near the geometry (a blunted body) and 3 steps later, the simulation crashs. I read somewhere that Navier-Stokes does not work anymore for these conditions. Does someone have experience with this kind of simulation?

Thanks in advance

CFX should work fine until the gas can no longer be considered a continuum or until the gas disassociates. But for rarefied gasses you will probably need to use a different EOS.

But to start off I would just use an ideal gas at the temperature and pressure you are using, and once that is working reliably I would swap to a more accurate EOS.

Thank you for you answer Glenn.
Do you have any idea when the continuum equation fails? I'm working with density of 10^-5 kg/m3 and I was suposing that this value could be the reason that causes overflow.

Thank you Glenn. I'm gonna check this.

Hi again!
My Knudsen number is about 0.004 (M=10 and Re=4000). My conditions are calculated based on air ideal gas. So the solver should work fine but in the fifth step, the solver always returns this error:

Parallel run: Received message from slave
-----------------------------------------
Slave partition : 3
Slave routine : ErrAction
Master location : RCVBUF,MSGTAG=1012
Message label : 001100279
Message follows below - :

+--------------------------------------------------------------------+
| ERROR #001100279 has occurred in subroutine ErrAction. |
| Message: |
| Floating point exception: Overflow

Any idea?
Thank you!

Your Knudsen number looks OK, so CFX should be good to model this.

You are diverging numerically. Need to improve stability - smaller timesteps, double precision numerics, better mesh quality, better initial condition.