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

Time Step Size and Number of Iterations per Time Step

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

Like Tree1Likes
  • 1 Post By ghorrocks

Reply
 
LinkBack Thread Tools Display Modes
Old   August 12, 2015, 11:27
Default Time Step Size and Number of Iterations per Time Step
  #1
spl
New Member
 
spl
Join Date: Nov 2014
Posts: 28
Rep Power: 3
spl is on a distinguished road
Hi

I'm running a transient simulation for a radial turbine and I would like your advice on time step size and number of iterations per time step. A number of studies have used times steps range from 0.6 degrees per time step to 6.5 degree per time step but very few have stated how many iterations are used per time step. My understanding is that less iterations are required when using smaller time steps. So I have a few questions -

Is there a general rule for number of iterations per time for CFX?

What is the best method of judging convergence within a time step?

Is it best to select a number of iterations per time step (say 20) and then reduce the time step until the transient results are consistent or is there a better approach?

Thanks
spl is offline   Reply With Quote

Old   August 12, 2015, 19:31
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 11,582
Rep Power: 90
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
Quote:
Is there a general rule for number of iterations per time for CFX?
Yes. 3-5 for most simulations, 5-10 for simulations with tricky coupling (eg some mutiphase simulations)

Quote:
What is the best method of judging convergence within a time step?
The residuals, and in some cases imbalances. In very rare cases other criteria are needed as well.

Quote:
Is it best to select a number of iterations per time step (say 20) and then reduce the time step until the transient results are consistent or is there a better approach?
There is a better approach. Use adaptive time stepping to home in on 3-5 coeff loops per time step. Then it will automatically find the right time step, and change it as the simulation progresses.
spl likes this.
ghorrocks is offline   Reply With Quote

Old   August 13, 2015, 04:23
Default
  #3
Senior Member
 
JuPa's Avatar
 
Mr CFD
Join Date: Jun 2012
Location: Britain
Posts: 311
Rep Power: 6
JuPa is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
The residuals, and in some cases imbalances. In very rare cases other criteria are needed as well.
It's extremely difficult to converge every time step within 1% imbalance.
JuPa is offline   Reply With Quote

Old   August 13, 2015, 06:02
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 11,582
Rep Power: 90
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
That depends on the simulation. Some are easy some are hard. But the bigger question is whether it is required for the simulation at hand. If it is needed then you have to do it regardless of it being hard.
ghorrocks is offline   Reply With Quote

Old   August 13, 2015, 11:56
Default
  #5
spl
New Member
 
spl
Join Date: Nov 2014
Posts: 28
Rep Power: 3
spl is on a distinguished road
Thanks for your help.
spl 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
interDyMFoam with VOF + 6DOF instable pbalz OpenFOAM Running, Solving & CFD 7 January 18, 2016 22:42
Time step size and max iterations per time step pUl| FLUENT 31 August 21, 2015 04:46
Superlinear speedup in OpenFOAM 13 msrinath80 OpenFOAM Running, Solving & CFD 18 March 3, 2015 06:36
pisoFoam with k-epsilon turb blows up - Some questions Heroic OpenFOAM Running, Solving & CFD 26 December 17, 2012 04:34
Transient simulation not converging skabilan OpenFOAM Running, Solving & CFD 12 September 17, 2007 17:48


All times are GMT -4. The time now is 13:17.