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

Transient Simulation

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

Reply
 
LinkBack Thread Tools Display Modes
Old   August 22, 2005, 12:24
Default Transient Simulation
  #1
Cathy Lee
Guest
 
Posts: n/a
For a transient simulation, the residual outcomes showed fluctuations and did not decrease down- Droped down to convergence criteria and increased and do iteration again-and droped down to the criteria....it kept cycling like this and did not stop. Would you please help me on this?
  Reply With Quote

Old   August 22, 2005, 15:09
Default Re: Transient Simulation
  #2
helper
Guest
 
Posts: n/a
Try to use less aggressive (smaller) under-relaxation factors or CFL number. Or, see if reducing time-step size helps.
  Reply With Quote

Old   August 22, 2005, 16:55
Default Re: Transient Simulation
  #3
Cathy Lee
Guest
 
Posts: n/a
Thank you so much for your advice. However, initially, I set the under-relaxation factors 0.3 for pressure and 0.7 for momentum/ 20 for the time-step size. I changed to 0.3 for pressure and 0.2 for momentum and kept the time-step size;however it did not help at all. Are there any other potential reasons cause these problems?

Thanks again.
  Reply With Quote

Old   August 22, 2005, 18:44
Default Re: Transient Simulation
  #4
helper
Guest
 
Posts: n/a
Try a smaller time-step size or keep reducing time-step size, as I said.
  Reply With Quote

Old   August 23, 2005, 12:16
Default Re: Transient Simulation
  #5
Cathy Lee
Guest
 
Posts: n/a
I reduced the time-step size from 20 to 5. It hit the convergence criteria once and a message telling "it is converged". However, it did the next iteration and the redisuals increased again; it does not stop and keeps running . Can I use the outcome for postprocessing? Or I have to change something and try again. If I have to change, would you please guide me what else I can do? Also, I would like to know whether the accuracy changes if I reduce under-relaxation factor and take smaller time-step size.
  Reply With Quote

Old   August 24, 2005, 02:04
Default Re: Transient Simulation
  #6
Suvash
Guest
 
Posts: n/a
I think you should keep the default under-reluxation factor and decrease the time step size. Time step size 5 is also very large for some cases. try it for 1(one) or less than 1(one)..
  Reply With Quote

Old   August 24, 2005, 04:29
Default Re: Transient Simulation
  #7
edi
Guest
 
Posts: n/a
On accuracy and URFs: this issue was faced many times in this forum, you can also find analytical explanations in the manual. (Very) roughly speaking:

-low URFs: less convergence problems, more iterations required for a "converged" solution;

-higher URFs: growing convergence problems, less iterations required.

In theory, once you get your solution "converged", it doesn't matter what URFs you chose, so the goal should be finding the higher URFs that allow a converged solution... . The moral of the story is that you better keep on with your default URFs...

On your time step: time step 5 is very large for MOST of the cases!! I agree with Suvash: try it for less than 1. To speed up your calculation you should consider the NITA algorythm, but the time must be further reduced and it affects accuracy in some cases...

Hope this can help

Good luck

Edi.
  Reply With Quote

Old   August 24, 2005, 12:32
Default Re: Transient Simulation
  #8
breathless
Guest
 
Posts: n/a
i have the same problem with breathing, i inhale and then later i exhale, ... it keeps repeating ... inhale followed by exhale ... what should i do?

the code is doing what it should ... converging at each time-step, advancing time, and then converging again at the next time step.
  Reply With Quote

Old   August 24, 2005, 13:30
Default Re: Transient Simulation
  #9
Cathy Lee
Guest
 
Posts: n/a
Many thanks to all!!
  Reply With Quote

Old   August 27, 2005, 11:42
Default Re: Transient Simulation
  #10
Swarup
Guest
 
Posts: n/a
There is no problem in this. The code is trying to obtain converged solution per time step & obtaining it. Residuals increase suddenly because they are calculated using values between two time steps. Some problems converge periodically. If you are using transient simulation for time accurate predictions, then you must achieve convergence per time step, otherwise you can afford to obtain convergence at higher times, when steady state is achieved. There are some problems which have NO steady state solution as is the case with natural convection heat transfer.

Swarup.
  Reply With Quote

Old   September 29, 2010, 21:49
Exclamation
  #11
New Member
 
hoaiphuong
Join Date: Sep 2010
Posts: 8
Rep Power: 6
alac1407 is on a distinguished road
Hi everyone!
I have a problem about simulating venturi tube
In my simulation, Inlet velocity is sin function and it varies on over time.
So should I use Steady State or Transient !
And if I use Transient model, please help me set parameters in Transient tab!
Thank you very much in advance !
alac1407 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
about transient radiations simulation waradevivek FLUENT 0 October 17, 2011 10:16
Beginning a Transient Simulation Norflow CFX 2 October 7, 2011 00:38
initial conditions for transient simulation littlelz CFX 2 February 5, 2009 06:56
Transient simulation - pressure shifted Eric CFX 1 February 14, 2006 13:42
Charts for transient simulation Anurag CFX 2 March 28, 2005 12:09


All times are GMT -4. The time now is 01:44.