CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Rarefied gas

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   August 11, 2011, 12:00
Default Rarefied gas
  #1
New Member
 
Join Date: Dec 2010
Posts: 10
Rep Power: 6
thais_b is on a distinguished road
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
thais_b is offline   Reply With Quote

Old   August 11, 2011, 19:16
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,820
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
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.
ghorrocks is offline   Reply With Quote

Old   August 11, 2011, 21:54
Default
  #3
New Member
 
Join Date: Dec 2010
Posts: 10
Rep Power: 6
thais_b is on a distinguished road
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.
thais_b is offline   Reply With Quote

Old   August 11, 2011, 22:00
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,820
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
http://en.wikipedia.org/wiki/Knudsen_number
ghorrocks is offline   Reply With Quote

Old   August 12, 2011, 17:20
Default
  #5
New Member
 
Join Date: Dec 2010
Posts: 10
Rep Power: 6
thais_b is on a distinguished road
Thank you Glenn. I'm gonna check this.
thais_b is offline   Reply With Quote

Old   August 15, 2011, 09:28
Default
  #6
New Member
 
Join Date: Dec 2010
Posts: 10
Rep Power: 6
thais_b is on a distinguished road
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!
thais_b is offline   Reply With Quote

Old   August 15, 2011, 18:57
Default
  #7
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,820
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
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.
ghorrocks is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Constant velocity of the material Sas CFX 15 July 13, 2010 08:56
mass flow in is not equal to mass flow out saii CFX 2 September 18, 2009 08:07
Two-Phase Buoyant Flow Issue Miguel Baritto CFX 4 August 31, 2006 12:02
Rarefied Gas Dynamics Andrea Panizza Main CFD Forum 3 July 15, 2003 07:45
Gas pressure question Dan Moskal Main CFD Forum 0 October 24, 2002 22:02


All times are GMT -4. The time now is 04:14.