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

time step selection

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

Reply
 
LinkBack Thread Tools Display Modes
Old   January 10, 2004, 16:05
Default time step selection
  #1
Jackie
Guest
 
Posts: n/a
Hi all,

I'm simulating an unsteady problem by using Upwind scheme. Could anybody please suggest me, how to know the suitable time step value (delta t) that made the program give the accurate result. For the value I'm using now is normally generate an overflow in every running time. Jackie
  Reply With Quote

Old   January 11, 2004, 06:54
Default Re: time step selection
  #2
James Date
Guest
 
Posts: n/a
Hi

Try and make sure the time step is much less than the period of the unsteady problem. Try and make a guess at the period of the unsteadiness from experimental data if you have it to hand i.e. Strouhal number.

The more time steps you can have within the period the more accurate the approximation of the CFD solution. If the time step is greater than the natural period of the unsteady problem, you will fail to model the unsteadiness correctly.

James
  Reply With Quote

Old   January 12, 2004, 02:01
Default Re: time step selection
  #3
Jackie
Guest
 
Posts: n/a
Thank you for your kind answer James. But i have more questions.

Do the time step selection have to relate to size of grid? I'm now using upwind scheme and in order to get the stable result. We have to fine the stability criterion (CFL value). For example in 1D simulation upwind mehod with c=u*delta(X)/delta(t), in this case c should less than or equal 1. However this case is for 1D struture grid that delta(X) must be unity.

My problem is the mesh generating in domain is 3D unstructure grid. Delta X, Y and Z are not equal. So, how to calculate delta t (time step) for my problem is my question. Do you have any idea or sugesstion please.

Sincerely,

Jackie
  Reply With Quote

Old   January 12, 2004, 05:04
Default Re: time step selection
  #4
Rami
Guest
 
Posts: n/a
Jackie,

You should calculate a local CFL number at any cell based on the cells dimensions and velocity.

For steady problems, you can advance the solution using local time steps. However, as your problem is time-dependent, you should use a single (global) time step to maintain your solution time-accurate. To guarantee stability, you should ensure that this global time step does not violate the CFL stability condition in ANY cell.
  Reply With Quote

Old   January 12, 2004, 06:11
Default Re: time step selection
  #5
Anton Lyaskin
Guest
 
Posts: n/a
IMHO time step selection has little in common with your spatial differencing scheme - it depends upon temporal differencing scheme. For example, you can use implicit temporal differencing which doesn't become unstable with CFL>1.0
  Reply With Quote

Old   January 12, 2004, 13:26
Default Re: time step selection
  #6
James Date
Guest
 
Posts: n/a
Yep, the CFL < 1.0 must be satisfied at all locations in the fluid domain. Thus the global time step must equal to that needed for the smallest cell. You can make a quick guess of the time step using the v (fluid velocity) and d (smallest cell size) to give t. s/v=t then make it an order smaller to be on the safe side, you can always make it bigger later on.

A book that explains this simply is:

Using Computational Fluid Dynmaics, An introductory CFD book by C. T. Shaw. Available in full text directly online.

http://www.eng.warwick.ac.uk/staff/cts/cfdbook/

Hope this helps.

James

  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
Multiple floating objects CKH OpenFOAM 10 September 21, 2011 23:13
DPM UDF particle position using the macro P_POS(p)[i] dm2747 FLUENT 0 April 17, 2009 01:29
Computational time sunnysun OpenFOAM Running, Solving & CFD 5 March 16, 2009 04:32
AMG versus ICCG msrinath80 OpenFOAM Running, Solving & CFD 2 November 7, 2006 16:15
VOF özgür FLUENT 8 January 6, 2004 09:23


All times are GMT -4. The time now is 11:45.