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

Reason for oscillating residuals

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

Reply
 
LinkBack Thread Tools Display Modes
Old   June 21, 2011, 13:40
Unhappy Reason for oscillating residuals
  #1
New Member
 
Fayaaz Hussain
Join Date: Dec 2010
Posts: 12
Rep Power: 6
fayaazhussain is on a distinguished road
Dear all,
I would like to know the reason behind oscillating residuals. I am trying to simulate flow through a CD nozzle. I had mesh which gave me a converged solution for a pressure ratio of 6. The same mesh was useless with other pressure ratios. I got a error code 255 or error code 1 in ansys cfx. I remeshed by making a little coarser mesh. It works fine with pressure ratio 5. But my residuals started oscillating at the end but somehow managed to converge. The same mesh is being used for a pressure ratio 0f 4 but my residuals are oscillating badly. I heard that a very fine mesh could be one of the reasons for oscillations but my mesh seems to be coarse in my point of view. Can anybody help me in understanding the reasons behind the oscillations? Thanks in advance
fayaazhussain is offline   Reply With Quote

Old   June 21, 2011, 18:56
Default
  #2
Senior Member
 
Julien de Charentenay
Join Date: Jun 2009
Location: Australia
Posts: 229
Rep Power: 9
julien.decharentenay is on a distinguished road
Send a message via Skype™ to julien.decharentenay
Hi Fayaaz,

There are large numbers of reasons why residuals may oscillate, below are a couple of ideas/reasons:

1) The physic of the flow dictates that there is no steady-state solution, only transient ones. Therefore once the solution is within the transient behavior, the residuals oscillates. Coarsening of the mesh may help with this problem. The "appropriate" way is to conduct a transient simulation and time-average it - but it is time expensive;
2) The code implementation (some random if statement) means that the flow is switching between 2 (or more) different models/limiters and the like. This creates oscillation in residuals.

You should look at convergence beyond residuals, i.e. monitoring of the flow field and variable of interest, and look at the area of large residuals (by plotting the residuals distribution).

Regards,
Julien
__________________
---
Julien de Charentenay
julien.decharentenay is offline   Reply With Quote

Old   June 27, 2011, 12:51
Default
  #3
Member
 
Casey
Join Date: Jun 2009
Posts: 96
Rep Power: 8
meangreen is on a distinguished road
lower your under relaxation facrors (URFs). Osciallting residuals (genearlly) means the solution is changing too much between iterations and overshooting the solution.
meangreen is offline   Reply With Quote

Old   August 21, 2014, 10:31
Default
  #4
New Member
 
Marcel Jay
Join Date: Jun 2014
Location: Berlin
Posts: 14
Rep Power: 3
marcel_jay is on a distinguished road
Quote:
Originally Posted by fayaazhussain View Post
Dear all,
I heard that a very fine mesh could be one of the reasons for oscillations but my mesh seems to be coarse in my point of view. Can anybody help me in understanding the reasons behind the oscillations? Thanks in advance
I would like to know how you define a coarse or fine mesh.
My mesh for example has about 700k elements in a quite simple geometry. Would that be too fine?


Cheers
marcel_jay 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
Tutorial of how to plot residuals ! wolle1982 OpenFOAM Running, Solving & CFD 112 July 30, 2015 04:09
what to monitor besides residuals? franzdrs FLUENT 5 March 21, 2013 04:59
Discussion: Reason of Turbulence!! Wen Long Main CFD Forum 3 May 15, 2009 09:52
Mesh size and solver residuals... Scott CFX 5 December 15, 2008 18:10
Convergence - scaled vs unscaled residuals HS FLUENT 1 November 7, 2005 06:45


All times are GMT -4. The time now is 20:26.