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

Negative densities found

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

Like Tree1Likes
  • 1 Post By Fengwei

Reply
 
LinkBack Thread Tools Display Modes
Old   June 1, 2006, 23:53
Default Negative densities found
  #1
vadivel
Guest
 
Posts: n/a
While solving compressible high speed flow over an airplane, after firing the job i am getting negative densities found in more than 100 cells after few iterations then run stopped. i have meshed using Proam(trimmed cell mesh) and prostar(cartesian mesh),Boundry conditons are input-inlet flow,output-pressure. i also have tried with reducing relaxation factor.

how to solve this problem......
  Reply With Quote

Old   June 2, 2006, 09:59
Default Re: Negative densities found
  #2
css
Guest
 
Posts: n/a
Reduce the relaxation factor of pressure upto 0.05 and reduce other terms considerably and restart the analysis. Let the flow field to develop and later on you can keep on increase the U.R.Factors (pressure up to 0.7). Apply the boundary conditions as per the guidence given in user guide capter 6,7 (useful points may help you in some extent like extrude one layer of cells at the boundary when you are applying pressure B.C on the trimmed cells)for compressible flows.
  Reply With Quote

Old   June 3, 2006, 03:31
Default Re: Negative densities found
  #3
Piet
Guest
 
Posts: n/a
Actually I would recommend to use another cfd-solver. Star-CD is vulnerable of negative densities. I had also enormous problems with neg. den. and the combination of inlet and pressure boundary seems to be accountable for it. A colleage of mine works with Fluent, and so we decided to generate a benchmark with pressure as inlet and a inlet boundary as outlet.

The same model (mesh) with same boundaries. Fluent without a problem , Star crashes after 30 iterations. And by way I am by far not a Star beginner, I know a lot of possible tricks to get a solution converged, but that's not the way it should be. It means always additional work and TIME.

To be honest I prefer Star and the Star environment but if you don't have time and you need a converged solution use Fluent. The Fluent solver is more robust.

Piet

  Reply With Quote

Old   June 3, 2006, 14:23
Default Re: Negative densities found
  #4
Ant
Guest
 
Posts: n/a
Are you using densit.f (the density user subroutine)?

I have experienced getting negative densities, but managed to solve that by using densit.f to provide the density.
  Reply With Quote

Old   June 8, 2006, 11:40
Default Re: Negative densities found
  #5
Zamir Rashid
Guest
 
Posts: n/a
Have you tried turning on the environmental pressure and mean value options for the outlet pressure boundary? That might help.
  Reply With Quote

Old   June 12, 2006, 21:54
Default Re: Negative densities found
  #6
Fengwei
Guest
 
Posts: n/a
set real constant 89 and 90 maybe useful.
  Reply With Quote

Old   June 19, 2006, 05:25
Default Re: Negative densities found
  #7
Ant
Guest
 
Posts: n/a
What do constants 89 and 90 do?
  Reply With Quote

Old   June 19, 2006, 20:12
Default Re: Negative densities found
  #8
Fengwei
Guest
 
Posts: n/a
RCONS 89 X: specify mininum temperature limit X (default 54.) RCONS 90 X: specify maximum temperature limit X (default 20000.)

for normal air the mininum temperature limit should be 293 I think. I have solved the negative problem by set RCONS 89=293 RCONS 90=4000

lizhi likes this.
  Reply With Quote

Old   June 20, 2006, 09:54
Default Re: Negative densities found
  #9
Ant
Guest
 
Posts: n/a
Thanks, I will try these out.
  Reply With Quote

Old   June 21, 2006, 19:20
Default Re: Negative densities found
  #10
azmir
Guest
 
Posts: n/a
This is a common problem and has been dicussed in previous forums; try the search button above. Further, please read this one:

www.cfd-online.com/Forum/starcd_archive_2005.cgi/read/8106
  Reply With Quote

Old   June 22, 2006, 06:41
Default Re: Negative densities found
  #11
Ant
Guest
 
Posts: n/a
It is always advisable to search; however, during my search I did not find the advice which Fengwei provided about constants 89 and 90. I don't think it hurts to ask again in case someone else has found a new solution to share.

Regards, Ant.
  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
negative densities Riyaz Ahamed CD-adapco 2 August 14, 2007 01:47
Negative Densities Chris CD-adapco 4 June 25, 2007 08:39
Negative densities!!! Nalleli CD-adapco 5 August 31, 2005 06:38
Negative densities found ina an I.C engine problem varun CD-adapco 5 June 2, 2005 02:03
Help, nagative densities found? rosary CD-adapco 1 May 14, 2001 04:12


All times are GMT -4. The time now is 18:36.