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

about convergence

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

Reply
 
LinkBack Thread Tools Display Modes
Old   September 2, 2002, 22:08
Default about convergence
  #1
lingo
Guest
 
Posts: n/a
Hi: I am modeling a minisize centrifugal pump.I set the boundary conditions as following ressure inlet and outlet.I select segregated,RNGk-e model,standard wall function.Now I meet convergence problem.I think my mesh(unstructural )is good,the problem is that:when the residual reach 2e-3(the default is 1e-3),it never drops again.When I choose 1st order solution,it works well.But i think i shoud choose 2nd order and i don't want to low the under-relaxation( maybe I don't know how to change the under-relaxation appropriately) .Expecting your help!

regards
  Reply With Quote

Old   September 3, 2002, 05:04
Default Re: about convergence
  #2
Jin-Wook LEE
Guest
 
Posts: n/a
Why are you hesitating to give small the uner-relaxation as one of the possible approach ?

In general, residual can be decreased relatively easily with lower under-relaxation, even though not always so and even though it converges more slowly.

Sincerely, Jinwook

  Reply With Quote

Old   September 3, 2002, 07:40
Default Re: about convergence
  #3
Sridhar
Guest
 
Posts: n/a
Hi Ling,

I also do think that when you switch onto the 2nd order one will have to go for lower under relaxation factors. After a considerable no. of iterations when you see that the residuals are coming down you can again gradually increase your relax params.

Another pt worth mentioning is: with 1st order you may not have much of a problem with residuals. You will not see a dramatic reduction of residuals with 2nd order. Moreover, residuals are not the only thing by which we can judge convergence. You can monitor some properties like pstat etc at some critical locns to see if there are changing substantially with iterns. Hope this helps.

Regards,

Sridhar
  Reply With Quote

Old   September 5, 2002, 12:38
Default Re: about convergence
  #4
sri
Guest
 
Posts: n/a
Hi Lingo,

Monitoring the mass imbalance would also be a good option in this case.Reducing urfs to some extent is fine.But giving an urf=0.01 will simply converge your solution which doesn't make sense.

sri
  Reply With Quote

Old   September 8, 2002, 08:23
Default Re: about convergence
  #5
lingo
Guest
 
Posts: n/a
Thank you very much.I will follow your advice.Now I know I can monitor other properties like surfaces,forces and so on,but how can i judge convergence by them?Maybe my engish is too poor,so I want to know the meaning of "pstat" and "locns" in Sridhar's response ,"urfs" in sri' response.Expecting you help!
  Reply With Quote

Old   September 9, 2002, 02:56
Default Re: about convergence
  #6
Sridhar
Guest
 
Posts: n/a
Hello lingo,

pstat: Static Pressure; locns: locations; urf's: Under Relaxation Factors.

For judging convergence while monitoring properties: Select some "critical" surfaces where you think certain properties may be varying/ are important from a physical point of a view, put monitors there. If you see that there is no variation (<1% or so) in those properties on the selected surfaces, you can more or less take that the solution is converged.

if you see that the residuals are widely oscillating then, more likely than not, either your Bound. conds. are not correct or your mesh quality might not be good enough.

Hope this helps. All the best.

Regards, Sridhar.
  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
Convergence Centurion2011 FLUENT 24 May 9, 2015 08:02
problem with Min/max rho tH3f0rC3 OpenFOAM 7 February 23, 2013 06:37
Force can not converge colopolo CFX 13 October 4, 2011 22:03
Convergence of CFX field in FSI analysis nasdak CFX 2 June 29, 2009 01:17
Defect correction and convergence ganesh Main CFD Forum 4 June 30, 2006 14:20


All times are GMT -4. The time now is 14:46.