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

Epsilon Convergence Trouble

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

Reply
 
LinkBack Thread Tools Display Modes
Old   August 26, 2007, 19:04
Default Epsilon Convergence Trouble
  #1
Carlos
Guest
 
Posts: n/a
Hello,

I am simulating airflow over a vehicle using the realizable k-epsilon turbulence model on a grid of ~ 8 million cells. I am using a hybrid grid of hex and tet cells and the simulations are second order.

For slow air speeds I get satisfactory convergence, however for faster speeds I have trouble with the residual levels for epsilon. All other residuals are converged ok (below 10^-5) and everything levels off after 6000 iterations. Epsilon does level off too, but there is a medium amplitude oscillation present and the average residual value is around 10^-4.

I have tried relaxing the solution which helps generally but epsilon is largely insensitive. I tried lowering the under relaxation factor for epsilon from 0.8 to 0.6 but this has little or no effect.

Any ideas or secret recipies for improved convergence with this turbulence model? Many thanks,

Carlos.

  Reply With Quote

Old   August 27, 2007, 01:55
Default Re: Epsilon Convergence Trouble
  #2
AAA
Guest
 
Posts: n/a
Hi

I think you've nailed the problem Carlos. With 8 million cells and such low residuals after 6000 iterations, you are my man. But what velocities are we talking here?
  Reply With Quote

Old   August 27, 2007, 07:40
Default Re: Epsilon Convergence Trouble
  #3
Carlos
Guest
 
Posts: n/a
Hi Triple A,

In terms of the appearance of the residuals yes I do have a converged solution, however I'm not entirely happy with the residual level for epsilon. I have read many times that the default convergence limit is 10^-3 and this is not enough for 3D problems. As an absolute bare minimum these days I try to get everything below 10^-4. Usually with k-e models everything plays ball except for epsilon.

I am using a velocity of approx 22m/s which is about 50Mph. The flow is both internal and external as I am looking at ventilation in livestock trailers. Does anybody know of any other method other than altering the relaxation of the solution to get lower residuals? Is it worth looking at Multigrid methods to try and lower things? It just seems like something which takes a long time to set up. And am I correcting in thinking that Multigrid simply SPEEDS UP convergence and doesn't actually help the convergence levels?

Maybe I am just being paranoid here but I am trying to be as rigorous as possible in writing a paper for a good journal. Thanks for your time. All the best,

Carlos.

  Reply With Quote

Old   August 27, 2007, 08:20
Default Re: Epsilon Convergence Trouble
  #4
Dusan
Guest
 
Posts: n/a
it is truth, it has to be at least half of cells, i think
  Reply With Quote

Old   August 27, 2007, 11:22
Default Re: Epsilon Convergence Trouble
  #5
AAA
Guest
 
Posts: n/a
Hi Carlos,

I usually have slightly lower residuals when I perform adaption. Try performing adaption on velocity gradients and see if this helps.

Regards
  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
SimpleFoam k and epsilon bounded nedved OpenFOAM Running, Solving & CFD 13 November 4, 2013 15:13
Force can not converge colopolo CFX 13 October 4, 2011 22:03
Orifice Plate with a fully developed flow - Problems with convergence jonmec OpenFOAM Running, Solving & CFD 3 July 28, 2011 05:24
NACA 0012 Convergence trouble mecarlg FLUENT 6 September 3, 2009 19:31
SimpleFoam k and epsilon bounded nedved OpenFOAM Running, Solving & CFD 1 November 25, 2008 21:21


All times are GMT -4. The time now is 02:08.