CFD Online Logo CFD Online URL
Home > Forums > Main CFD Forum

How many timesteps in the sub-iteration

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

LinkBack Thread Tools Display Modes
Old   March 14, 2008, 07:29
Default How many timesteps in the sub-iteration
wil tub
Posts: n/a
Hi, guys, in your expereience, how many timesteps should be set in the sub-iteration in the dual-time calculation?
  Reply With Quote

Old   March 14, 2008, 10:17
Default Re: How many timesteps in the sub-iteration
Posts: n/a
Dear Wil Turb,

I believe you are asking for the number of subiterations per physical iteration in an unsteady solver employing dual time stepping. Instead, I think you should be looking at options to terminate the pseudo--steady state solution in every physcial iteration. The points to be noted are as follows.

1. You can fix an arbitrary number of subiterations. This could be 10 or 20, and you could use it and get your answers. This works mostly, but you would not know to the extent your pseudo--steady state problem is being converged, it may not be actually achieving steady state and you are compromising heavily on time accuracy. If temporal accuracy is a concern, I believe that a fixed number of subiterations is never an option.

2. Another possibility which is more reasonable is to set a tolerance for the dual iterations. Well, again the tolerance is a user-choice, making it subjective but atleast the user has a better control on the convergence of the inner iterations. A typical preferred value is 1e-6, and some people may go upto machine precision.

3. Engineering applications look at an optimal tradeoff between accuracy and effort. Clearly, going for a low tolerance or high number of subiterations helps ensure design accuracy but then this could be non-optimal and lead to a larger effort. Going for a smaller effort means that you could end up with a lower time accuracy. A practical engineering solution is to employ a temporal error estimator (Look into the works of Kennedy, Carpenter and Vatsa) which can help decide a suitable termination criterion for the dual loop. Well, here too there is a need for some user intervention, but with some experiments a reasonably good termination criterion can be built.

It is therefore clear that all the three options to terminate the inner iterations are not completely universal, but the third choice seems to a practically viable and promising option. And in any case, you are looking to use (1) without really looking at temporal accuracy, my answer would be that the subiterations may depend on the problem and domain at hand, but 20 would not be a bad choice .....

Hope this helps



  Reply With Quote


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
how to increase "Newton Pressure Iteration Limit" kus CFX 9 April 21, 2013 01:54
snappyHexMesh aborting Tobi OpenFOAM Native Meshers: snappyHexMesh and Others 0 November 10, 2010 04:23
Parallel runs slower with MTU=9000 than MTU=1500 Javier Larrondo FLUENT 0 October 28, 2007 23:30
Creating, loop or iteration using expression edito Jan CFX 3 July 28, 2003 11:01
Heat exchanger problem chiseung FLUENT 16 October 20, 2001 04:36

All times are GMT -4. The time now is 10:47.