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

oscillations in residual/iterations plot

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 6, 2013, 04:36
Default oscillations in residual/iterations plot
  #1
Member
 
Thomas Hoffmann
Join Date: Oct 2012
Posts: 67
Rep Power: 14
Thomashoffmann is on a distinguished road
Hi.
I'm simulation a pipe flow with a non newtonian fluid. After about 2700 iterations my residual starts to oscillate (see attached).

Do you know if this is something that I should be worried about? And why it happends?


Thanks

Thomas
Attached Images
File Type: jpg Capture.jpg (21.5 KB, 32 views)
Thomashoffmann is offline   Reply With Quote

Old   March 6, 2013, 05:18
Default
  #2
Senior Member
 
OJ
Join Date: Apr 2012
Location: United Kindom
Posts: 473
Rep Power: 20
oj.bulmer will become famous soon enough
Perhaps, there is a need for a smaller under relaxation factor.
oj.bulmer is offline   Reply With Quote

Old   March 6, 2013, 05:35
Default
  #3
Member
 
Thomas Hoffmann
Join Date: Oct 2012
Posts: 67
Rep Power: 14
Thomashoffmann is on a distinguished road
ok, I just tried to run it again with a under relaxation of 0.8 and this is what I got - see attached..
It starts oscillating again at momentum residual=3E-4 - the same value as before... I think my result is accurate at this value, I just dont like that it is doing this oscillation thing...
Maybe I just shouldn't worry?
Attached Images
File Type: jpg Capture.jpg (20.2 KB, 24 views)
Thomashoffmann is offline   Reply With Quote

Old   March 6, 2013, 06:01
Default
  #4
Senior Member
 
OJ
Join Date: Apr 2012
Location: United Kindom
Posts: 473
Rep Power: 20
oj.bulmer will become famous soon enough
Typically such small oscillations are indicative of numerical noise amd all other artifacts, which exist even after the solution has converged. To be honest, though I suggested the URFs, or any other change for that matter to make the residual more smoother, there won't be a significant change in the results beyond a point.

I would plot the parameters of interest on important locations, along with the volumetric mass imbalance, to make sure the solution is converged, rather than worrying about residuals. It may be possible that the resiruals are gradually decreasing but monitors remain flatter, indicating convergence. The measure of convergence is the result you are interested in, not residual!

Regards
OJ
oj.bulmer is offline   Reply With Quote

Old   March 6, 2013, 06:28
Default
  #5
Member
 
Thomas Hoffmann
Join Date: Oct 2012
Posts: 67
Rep Power: 14
Thomashoffmann is on a distinguished road
I'll try studying the results..
Thank you for the helpful answer!
Thomashoffmann is offline   Reply With Quote

Reply


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
I want to plot residual U: vahid.najafi OpenFOAM Post-Processing 11 July 1, 2016 03:54
Plot on a curve Monica FLUENT 1 July 11, 2012 01:24
Transfer low-res graphic file plot to excel plot quarkz Main CFD Forum 3 April 9, 2009 14:54
graph plot anindya Main CFD Forum 2 September 17, 2003 13:00
why can't I plot Y+ value? Jiaying Xu Siemens 2 August 18, 2001 11:56


All times are GMT -4. The time now is 12:57.