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

Search idea for solving a transient problem

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

Reply
 
LinkBack Thread Tools Display Modes
Old   May 13, 2011, 03:36
Default Search idea for solving a transient problem
  #1
Senior Member
 
Join Date: Mar 2011
Posts: 155
Rep Power: 7
tH3f0rC3 is on a distinguished road
Hi there,

I need a good idea for solving a case transient.
The problem is that the case consists of about nearly 2 mio elements. And I need to solve 285 seconds transient.
The courant criteria calculated time steps from ~10^(-5) seconds.

Now I need a good idea how to solve the case transient.

What I still have tried:
- I still have reduced the element number from 9mio to 2mio elements.
- I have tried to run the case with a fixed time step 0.0001s, without success.
-> I have a question to that. If I choose to let the solver use the courant criteria for calculating the time steps, does the solver use constant time steps or can it be that sometimes he calculated a time step of deltat=10^(-5)s and sometimes deltat=10^(-3) seconds?
I want to know this because if the solver calculates the courant criteria only at time t=0 and uses than constant time steps, I think I can halve the time calculated by courant criteria. Because I think that the implementation of courant criteria in the solvers is configured conservative.
I think that on every time step the courant criteria is used to calculate the next time step, but I'm nut quite sure.

Maybe you need the information that I use chtMultiRegionFoam.

Best Regards,
tH3f0rC3
tH3f0rC3 is offline   Reply With Quote

Old   May 13, 2011, 05:45
Default
  #2
Senior Member
 
Bernhard
Join Date: Sep 2009
Location: Delft
Posts: 790
Rep Power: 13
Bernhard is on a distinguished road
You can easily see from the output of the solver, whether your timestep changes or not.
You can use adjustTimeStep in controlDict to recalculate the timestep based on the Courant number limit.
Bernhard is offline   Reply With Quote

Old   May 13, 2011, 10:22
Default
  #3
Senior Member
 
Join Date: Mar 2011
Posts: 155
Rep Power: 7
tH3f0rC3 is on a distinguished road
Quote:
Originally Posted by Bernhard View Post
You can easily see from the output of the solver, whether your timestep changes or not.
You can use adjustTimeStep in controlDict to recalculate the timestep based on the Courant number limit.
Thanks for your answer, Bernhard,
so adjustTimeStep is for the recalculation after every time step?
So if I switch the setting to "no;" the courant number is only calculated at time t=0 and this time step is than used for every following time step?

Best Regards,
tH3f0rC3
tH3f0rC3 is offline   Reply With Quote

Old   May 13, 2011, 15:01
Default
  #4
Senior Member
 
Bernhard
Join Date: Sep 2009
Location: Delft
Posts: 790
Rep Power: 13
Bernhard is on a distinguished road
If you set it to no, then the deltaT that you set in controlDict is used. The Courant number is only calculated, but not used later on.
Bernhard is offline   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
Making a video from different snap shots of a Transient Problem Fascal FLUENT 1 December 22, 2010 07:19
Making a video from different snap shots of a Transient Problem Fascal FLUENT 0 December 19, 2010 02:03
time step and iterations in steady state problem using transient solver jing113cn FLUENT 2 January 15, 2010 04:18
transient heat transfer problem kamal FLUENT 2 June 29, 2008 14:55
transient problem run in steady state luigi FLUENT 4 March 13, 2008 07:54


All times are GMT -4. The time now is 14:46.