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

Convergence Criteria for first Timestep in transient run

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

Reply
 
LinkBack Thread Tools Display Modes
Old   February 14, 2013, 21:11
Default Convergence Criteria for first Timestep in transient run
  #1
New Member
 
Join Date: Jul 2010
Posts: 7
Rep Power: 7
aerost is on a distinguished road
Hello,

Does anyone know if it is possible to have a different convergence criteria for the first timestep of a transient simulation?

I would like to converge the first timestep for a minimum of ~500 iterations, then allow each subsequent timestep to end at a maximum of say ~50 iterations. Is this possible?

THANKS!
aerost is offline   Reply With Quote

Old   February 15, 2013, 06:47
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,830
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
You could try making the max number of iterations a CEL expression.

But 500 coeff loops for an iteration is crazy. This is FAR too much. Even 50 per iteration is too much. A far better way is to use adaptive time stepping homing in on 3-5 coeff loops per iteration. Much faster and more accurate.
ghorrocks is offline   Reply With Quote

Old   February 18, 2013, 20:46
Default
  #3
New Member
 
Join Date: Jul 2010
Posts: 7
Rep Power: 7
aerost is on a distinguished road
Thank you. I think you are right after a bit of research. These transient sims are pretty time intensive so it's not the time to be learning by trial and error.

My specific case is a turbomachinery case and I have found that the solution takes many timesteps to converge towards a realistic and periodic one. As a result I end up throwing out many of the first few timesteps. That works, but I'd prefer to find a way to converge to a useful solution in the first couple of timesteps to save time and space.

At any rate... I am trying an approach with more timesteps and a lower number of iterations to see if the results come out acceptably... thanks again.
aerost is offline   Reply With Quote

Old   February 19, 2013, 05:01
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,830
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
If you do not care about the initial transient then do not bother accurately simulating it. Just do it approximately and quickly and use a more accurate setup when the flow has matured.

But the adaptive timesteps with 3-5 coeff loops per time steps is almost always the best option available. You are wasting your time by doing anything else.
ghorrocks is offline   Reply With Quote

Reply

Tags
cfx, convergence criteria, timestep, transient

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
Transient run taking too long time Alexander Tham Keat Fu CFX 3 June 20, 2012 06:53
Force can not converge colopolo CFX 13 October 4, 2011 22:03
Convergence criteria PrandtlGlawert FLUENT 1 August 13, 2009 18:06
Convergence Criteria edwin FLUENT 1 February 14, 2008 20:24
Error when changing timestep in transient run Tony CFX 4 December 8, 2004 15:13


All times are GMT -4. The time now is 06:37.