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

oscillation

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

Reply
 
LinkBack Thread Tools Display Modes
Old   May 3, 2009, 10:49
Default oscillation
  #1
New Member
 
Jin Chen
Join Date: Mar 2009
Location: sweden
Posts: 27
Rep Power: 8
swe704 is on a distinguished road
Hi everyone,

I am simulating a case using standard k-e model with standrad wall function.From the residual monitor,all the parameters perform well druning 1,0000 interations,however after that only the continuity exhibites oscillation with small scale range goes up and down and maintain till now(1,3000),others keep steady in the height of 1e-05.

Does anyone can tell me where is the problem and how can i check whcih part is wrong?

Thanks very much for your reply!
swe704 is offline   Reply With Quote

Old   May 3, 2009, 13:36
Default
  #2
Member
 
Ralf Schmidt
Join Date: Mar 2009
Location: Austria
Posts: 67
Rep Power: 8
Ralf Schmidt is on a distinguished road
Hi!

there might be a problem with the grid. Check the cell skewness (e.g. in Gambit) - it should not be to high (below 0.6) ...
Also, in areas, with high gradients, the cell size should be fine.

Another problem might be, that your yplus is not in the right range for the std wall function ( 3 < yplus < 300)

Best wishes
Ralf
__________________
CFD - nothing but Colourful Fluid Dynamics
Ralf Schmidt is offline   Reply With Quote

Old   May 4, 2009, 04:14
Default
  #3
New Member
 
Jin Chen
Join Date: Mar 2009
Location: sweden
Posts: 27
Rep Power: 8
swe704 is on a distinguished road
Hi Ralf,

Thanks very much for your answer, i chech the y plus and all them are within the range 0.3-120,maybe it is better to use Non equilibrium wall functions instead of SWF.Like you said,the cell skewness is also a factor.

By the way,i am sorry to mention that the continuity has already decreased to 1e-08,does this is low enough to say the results is good even there is small osillation?

Best wishes

Huijuan
swe704 is offline   Reply With Quote

Old   May 4, 2009, 05:26
Default
  #4
Member
 
Ralf Schmidt
Join Date: Mar 2009
Location: Austria
Posts: 67
Rep Power: 8
Ralf Schmidt is on a distinguished road
Hi!

turbulence modeling and the choice of the right turbulence model is a big thing.

It is difficult to guess, what model will be appropriate for your case - depends on fluid properties, Re number, .....
I suggest near wall treatment models for resolving the BL and correctly displaying heat and momentum transfer. ....

For convergence: the residuals give you a good idea, whether your simulation is "finished" or not - and 10e-08 is a very low value...
I would suggest to look at one additional thing: use a surface monitor of the parameter, that is interesting.

e.g. you want to simulate integral heat transfer, monitor that value. When it is (almost) constant for a number of iterations, your simulation is good enough.
Further iterating might lower the residuals, but not change the heat transfer any more and thus will not contribute to solution accuracy!

If you are interested in the flow field, create a number of points in areas with high velocity gradients and monitor the velocities on that points...

Best wishes
Ralf
__________________
CFD - nothing but Colourful Fluid Dynamics

Last edited by Ralf Schmidt; May 4, 2009 at 09:47.
Ralf Schmidt is offline   Reply With Quote

Old   May 4, 2009, 06:31
Default
  #5
New Member
 
Prasad Dudhgaonkar
Join Date: Mar 2009
Location: Chennai
Posts: 9
Rep Power: 8
prasad.ichal is on a distinguished road
Hello...

You may restart the case after smoothing-swapping the grid just in case you haven't done it before. Your solution appears to have reached convergence. You may also go for the second order discretization schemes.

It won't say 'solution converged' if you have set unreachable criterion. In any case, to comment further some solution details are required.
prasad.ichal is offline   Reply With Quote

Old   May 10, 2009, 11:03
Default
  #6
Member
 
Ralf Schmidt
Join Date: Mar 2009
Location: Austria
Posts: 67
Rep Power: 8
Ralf Schmidt is on a distinguished road
Hi!

i am just searching the old fluent forum entries... and i found an interesting discussion about that issue:

Oscillations in Results and Residuals
__________________
CFD - nothing but Colourful Fluid Dynamics
Ralf Schmidt is offline   Reply With Quote

Old   May 11, 2009, 05:41
Default
  #7
New Member
 
Jin Chen
Join Date: Mar 2009
Location: sweden
Posts: 27
Rep Power: 8
swe704 is on a distinguished road
Thanks very much for all the kind help and information!
swe704 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
Must periodic oscillation give periodic results? zonexo Main CFD Forum 6 May 13, 2007 15:36
pressure oscillation!! DJ Main CFD Forum 1 August 19, 2004 11:59
Oscillation problem Pavel FLUENT 0 August 17, 2004 13:38
jet oscillation at low inlet Re wood Main CFD Forum 1 October 21, 1999 13:45
Oscillation arief hariyanto Main CFD Forum 2 July 16, 1998 17:42


All times are GMT -4. The time now is 17:29.