CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > Siemens > STAR-CCM+

Current number and time step for an implicit unsteady simulation

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   August 4, 2014, 10:54
Default Current number and time step for an implicit unsteady simulation
  #1
New Member
 
Join Date: May 2014
Posts: 12
Rep Power: 12
Clémence is on a distinguished road
Hello everyone,

I have a serious question for my simulation, and a few time left to make it.

I am making a simulation with the implicit unsteady solver. For that solver, I have been told that the CFL condition were not needed. To find the time step, the following condition is needed:
dt < 0.5 * (mesh size)²/viscosity
So I could find that the time step has to be lower than 54.94. For now it is ok, I took 0.01s for the time step, and I could increase it during the simulation.

But I don't know what to choose for the current number. Do I have to follow the CFL condition which say that the CFL is lower than 10??? But in this case the time step cannot be determined the way I did...

Thank you for your help!!!
Clémence is offline   Reply With Quote

Old   November 29, 2014, 20:21
Default
  #2
Member
 
Join Date: Mar 2013
Posts: 42
Rep Power: 13
SB123 is on a distinguished road
Quote:
Originally Posted by Clémence View Post
Hello everyone,

I have a serious question for my simulation, and a few time left to make it.

I am making a simulation with the implicit unsteady solver. For that solver, I have been told that the CFL condition were not needed. To find the time step, the following condition is needed:
dt < 0.5 * (mesh size)²/viscosity
So I could find that the time step has to be lower than 54.94. For now it is ok, I took 0.01s for the time step, and I could increase it during the simulation.

But I don't know what to choose for the current number. Do I have to follow the CFL condition which say that the CFL is lower than 10??? But in this case the time step cannot be determined the way I did...

Thank you for your help!!!
clemence i'm kinda looking for a similar answer, but your question is not clear. where did you get that relationship for your dt?
secondly if by current number you mean courant number, then that's what i'm looking for as an answer too, what should the courant number be when working in implicit unsteady RANS eqns?

i also have heard its not specifcally an issue like it is with explicit. I imagine its to be used more as an estimate as opposed to 'be all truth' as with explicit. .... any thoughts would be appreciated
SB123 is offline   Reply With Quote

Old   September 30, 2016, 09:30
Default Time Step determination for implicit unsteady Solver
  #3
New Member
 
Teena T.
Join Date: Mar 2013
Location: Germany
Posts: 14
Rep Power: 13
tiatabasco is on a distinguished road
I think one should try with different time steps for the same mesh and come to a conclusion. But I am also wondering based on what I can conclude. My guess wud be check the courant number for each case and take the one which has courant greater than one but not very large say under 50. Anybody got an update on this one please let me know.
tiatabasco is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Micro Scale Pore, icoFoam gooya_kabir OpenFOAM Running, Solving & CFD 2 November 2, 2013 13:58
How to write k and epsilon before the abnormal end xiuying OpenFOAM Running, Solving & CFD 8 August 27, 2013 15:33
same geometry,structured and unstructured mesh,different behaviour. sharonyue OpenFOAM Running, Solving & CFD 13 January 2, 2013 22:40
Upgraded from Karmic Koala 9.10 to Lucid Lynx10.04.3 bookie56 OpenFOAM Installation 8 August 13, 2011 04:03
IcoFoam parallel woes msrinath80 OpenFOAM Running, Solving & CFD 9 July 22, 2007 02:58


All times are GMT -4. The time now is 12:32.