Rarefied gas

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 August 11, 2011, 12:00 Rarefied gas #1 New Member   Join Date: Dec 2010 Posts: 10 Rep Power: 9 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

 August 11, 2011, 19:16 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,201 Rep Power: 110 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.

 August 11, 2011, 21:54 #3 New Member   Join Date: Dec 2010 Posts: 10 Rep Power: 9 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.

 August 11, 2011, 22:00 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,201 Rep Power: 110

 August 12, 2011, 17:20 #5 New Member   Join Date: Dec 2010 Posts: 10 Rep Power: 9 Thank you Glenn. I'm gonna check this.

 August 15, 2011, 09:28 #6 New Member   Join Date: Dec 2010 Posts: 10 Rep Power: 9 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!

 August 15, 2011, 18:57 #7 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,201 Rep Power: 110 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.

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