CFD Online URL
[Sponsors]
Home > Forums > CFX

Time step for unsteady problem

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

Reply
 
LinkBack Thread Tools Display Modes
Old   January 8, 2004, 03:48
Default Time step for unsteady problem
  #1
Jackie
Guest
 
Posts: n/a
I'm simulating an unsteady problem and have a problem in setting time step. Some value made anerror and cause the program can't create tha result file at the final step. How should we know the suitable value of time step for the unsteady problem ( Grid generation for my domain are the unstructure mesh so delta X , Y and Z are not equal)

Sincerely,

Jackie
  Reply With Quote

Old   January 8, 2004, 07:30
Default Re: Time step for unsteady problem
  #2
Jan Rusås
Guest
 
Posts: n/a
I only have experience with 5.5.1, I don't know if you are using 5.6. Please in the future always specify which verison you are using. The question about selecting time step for transient problems have been raised a couple of times. Try to do a search also at the cummunity pages. If you have an idea about the period of your transient problem, then you could select a timestep that is 30 -100 smaller than the period. Pay some attention to the number of iterations within each timestep. If you have convergence problem - try to increase the number of iteration within each timestep, it is default 3 and/or decrease the timestep Are you sure that your problem is related to the timestep? Jan
  Reply With Quote

Old   January 9, 2004, 01:19
Default Re: Time step for unsteady problem
  #3
Jackie
Guest
 
Posts: n/a
Thank a lot Jan.
  Reply With Quote

Old   January 16, 2004, 13:58
Default Re: Time step for unsteady problem
  #4
Bob
Guest
 
Posts: n/a
Hi Jan and Jackie, Jan said that if convergence is a problem that you should increase your number of iterations, however I've herd that it is better to reduce your time step rather than increase iterations. Why ? well I'm not 100% sure, may be robin could answer that, but I got the information from the Help desk some time ago. Really you should do a time step dependency study, in the same way that you should do a grid dependency study. A good site to look at is the flow mania web site where they look at DES simulations and URANS simulations (which is what you are interested in). the address is: http://cfd.me.umist.ac.uk/flomania/index2.html

Have a look at the publications, particularly one on "Persistance and Effect of 3 dimensionality in URANS sims of 2D Bluff Bodies" there is some useful info their - heighlighting the effects of mesh and time step.

Bob
  Reply With Quote

Old   January 19, 2004, 00:51
Default Re: Time step for unsteady problem
  #5
jackie
Guest
 
Posts: n/a
Thank you for your kind answer Bob.
  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
Time step size and max iterations per time step pUl| FLUENT 24 January 16, 2014 07:35
Hydrostatic Pressure and Gravity miliante OpenFOAM Running, Solving & CFD 132 October 7, 2012 23:50
Time step sizing for 3D sliding mesh problem happyman Main CFD Forum 0 July 1, 2011 10:19
DPM UDF particle position using the macro P_POS(p)[i] dm2747 FLUENT 0 April 17, 2009 02:29
Transient simulation not converging skabilan OpenFOAM Running, Solving & CFD 12 September 17, 2007 18:48


All times are GMT -4. The time now is 23:49.