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

questions on transient simulation

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

Reply
 
LinkBack Thread Tools Display Modes
Old   August 24, 2010, 05:46
Default questions on transient simulation
  #1
Member
 
Sarah
Join Date: Apr 2010
Location: Munich
Posts: 30
Rep Power: 7
spring is on a distinguished road
Hi everyone,

I have a case that doesn't converge in a steady simulation and so I have to make a transient simulation. But after I read the chapter about time-dependent calculations (users giude) I still have some questions and I hope you can help me.

1. the default value for the "max iterations/time step" is 20. My question is that can't it be sometimes more, e.g 50 , or doesn't it depend on the case? Because sometimes my soultion converges after 25-30 iterations and I think that a value of 20 is too strict. What is your opinion, should I take the default value (20 iterations) and trust fluent's users guide, or can I increase this value? My time step is very small (delta_t=1.7e-06).

2. I want to simulate a discrete phase with my transient continious phase. Do I have to simulate the discrete phase as transient too, and do I have to set the same time step for the continous phase (flow) and the discrete phase?

3. everytime I want to change the time step, the calculation begins to iteratate from the first iterataion (all number of time steps again). How can I make small changes in my case setup and just go on with my iterations and not having to iterate all 1000 iterations again?

Thanks in advance for help.

regards,
spring
spring is offline   Reply With Quote

Old   August 24, 2010, 08:19
Default
  #2
Senior Member
 
xrs333
Join Date: Aug 2010
Posts: 125
Rep Power: 7
xrs333 is on a distinguished road
Hi, Spring,
1 Every time step the residuals jump and then tenuate in the following iterations. The number of max itertions per time steps should be so as to let the residuals drop three orders of magnitude. If it is not, it is not recommended to increase this number, but to decrease the time step.
The time step is not evaluated based on its absolute size, but is according to the characteristic time of the flow phenomenon.

2 Though not obligatory, it is recommended to simulate the discrete phase as transient if contiuous phase in transient calculated. Again the time step for particle tracking is determined by the characteristic time of the particle phenomenon, and it is not necessary same as the for continuous phase.

3 This is not a problem. Every time you restart the iteration is just a continuation to the last step before the the interruption.

i hope these will help but not misleading.
xrs333 is offline   Reply With Quote

Old   August 25, 2010, 08:25
Default
  #3
Member
 
Sarah
Join Date: Apr 2010
Location: Munich
Posts: 30
Rep Power: 7
spring is on a distinguished road
Hi xrs333,

thank you very much for your helpful reply. My problem is that I don't have any experiance in transient simulation and that my simulation won't converge no matter what I do . So I have other questions on transient simulations.

1. in the users guide the PISO solver is recommended for transient simulations (especially for large time steps) and the SIMPLE/C solver is recommended for transient LES simulations with (because of their small time steps). Well I think that a time step in the range of 1.xxxe-06 is small but my model is laminar. Should I PISO anyway?

2. I choose a convergence criteria to three ordes of magnitude, but I "relative" not absolut, because I think absolute doesn't make that sense, right?

regards,
spring
spring 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
Truck brake chamber transient simulation - Moving boundary vatsa STAR-CCM+ 0 February 11, 2010 01:14
Imbalance in transient simulation Roland R CFX 3 December 9, 2009 06:39
Synthetic (pulsatile) jet transient simulation aero CFX 0 November 6, 2009 02:10
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 19:49.