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

On the convergence of unsteady problems.

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

Reply
 
LinkBack Thread Tools Display Modes
Old   March 11, 2005, 07:34
Default On the convergence of unsteady problems.
  #1
mini
Guest
 
Posts: n/a
The time must be 5s or 10s before the convergent solution obtained? If the computation time is very long, what should I do? Thanks.
  Reply With Quote

Old   March 11, 2005, 14:37
Default Re: On the convergence of unsteady problems.
  #2
Melaku Habte
Guest
 
Posts: n/a
Hi,

Unsteady problems (in general) tend to take more time than the same geometry and flow but steady state flows. This is because you have additional parameter (namely time,t) in addition to velocity, pressure, temp..etc. The computation time also depends on your particluar flow geometry (mesh size) and flow type (e.g natural convection problems tend to take more time). I don't know your particlular problem but there is a general rule that I follow i.e your time step should be less or equal to the smallest time-scale of the flow. So, first calculate this time-scale and set your time step based on that....this could be an eastimate not an exact calculation since you don't know the flow before hand. But you still have a feel what to expect etc.. Also set the time step to be as small as possible during the initial step then keep on increasing or let Fluent do this for you by switching to "adaptive time steping". Last, your mesh size directly affects your time scale.

regards

Melaku Habte

  Reply With Quote

Old   March 12, 2005, 02:37
Default Re: On the convergence of unsteady problems.
  #3
mini
Guest
 
Posts: n/a
Thanks for your help. But I doní»t know how long the computation will take before the convergent solution obtained, in general, 5s or 10s? I doní»t know.
  Reply With Quote

Old   March 12, 2005, 03:16
Default Re: On the convergence of unsteady problems.
  #4
Melaku Habte
Guest
 
Posts: n/a
Hi,

I still didn't get your question. How did you pick 5s or 10 sec - is there any physics behind. you must have a reason to pick a certain time step. I also whant to know how long is your actual flow time and mesh size. People would be able to help you when u give a little more clarification of ur problem. If for some reason you have to pick either 5 or 10 s then I would say pick 5s.

regards

Melaku Habte
  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
conditions for unsteady problems dvdromnu OpenFOAM Running, Solving & CFD 5 November 9, 2009 03:15
Convergence of CFX field in FSI analysis nasdak CFX 2 June 29, 2009 01:17
unsteady flow problems Daniel FLUENT 0 January 26, 2009 00:54
convergence for an Unsteady simulation Palani Velladurai FLUENT 1 March 19, 2007 11:56
Unsteady simulation convergence Tomislav Main CFD Forum 1 December 6, 2006 08:53


All times are GMT -4. The time now is 11:03.