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

Imbalances in Output file Warning

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 10, 2017, 04:03
Default Imbalances in Output file Warning
  #1
Senior Member
 
Join Date: Mar 2013
Location: Germany
Posts: 357
Rep Power: 14
AS_Aero is on a distinguished road
Hi All

Have anyone faced this issue and what is it behind it ?

I did a RANS computation for simple pipe flow with a diffuser at the end of the pipe.
Inlet is given as Mass flow BC'n
Outlet is given Avg Static Pressure =0
Reference Pressure =1 atm

And the case was ran for 5000 iterations

And I was running it on a cluster. So when I checked the .out file it showed that Domain Imbalances

+--------------------------------------------------------------------+
| Normalised Imbalance Summary |
+--------------------------------------------------------------------+
| Equation | Maximum Flow | Imbalance (%) |
+--------------------------------------------------------------------+
| U-Mom | 1.2845E+01 | -0.0282 |
| V-Mom | 1.2845E+01 | 0.0633 |
| W-Mom | 1.2845E+01 | -0.0661 |
| P-Mass | 5.8000E-02 | 0.0090 |
+----------------------+-----------------------+---------------------+

Does it has anything to do with the simulation or should we consider this for some convergence issue or something related to it ! Kindly let me know.
AS_Aero is offline   Reply With Quote

Old   March 10, 2017, 06:00
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,665
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Most simulations are converged by the time the imbalances reach that figure. But you should check in your case whether your simulation is converged by doing a sensitivity analysis on convergence criteria.

Also: CFX rarely requires 5000 iterations to obtain convergence. Something like 200 is more normal. You probably don't have the solver set up very well.
ghorrocks is offline   Reply With Quote

Old   March 10, 2017, 06:11
Default
  #3
Senior Member
 
Join Date: Mar 2013
Location: Germany
Posts: 357
Rep Power: 14
AS_Aero is on a distinguished road
Thats true you are right ! By sensitivity analysis what do you mean ? my residual was given 1e-5 and the RMS values seems to be stable and within a 10 % deflection. Also Yes I ran 5000 just to see how the changes are from 500 !! And since its a small job and in cluster, it takes less than a couple of hours
But as per your suggestion this imbalances are fine ?
AS_Aero is offline   Reply With Quote

Old   March 10, 2017, 07:07
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,665
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
So a simulation at either 1e-4 or 1e-6 RMS and compare it to your 1e-5 result. If they are the same then you have sufficiently converged. If they are different you need to use a tighter convergence criteria.

I said for most simulations your convergence looks fine. But there are exceptions which require tighter or looser convergence. That is why you do the sensitivity check to determine what your case requires.
ghorrocks is offline   Reply With Quote

Old   March 10, 2017, 07:37
Default
  #5
Senior Member
 
Join Date: Mar 2013
Location: Germany
Posts: 357
Rep Power: 14
AS_Aero is on a distinguished road
Perfect Solution !! Thanks a lot for your valuable comments !!
Respect !!
AS_Aero is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
[swak4Foam] swak4foam building problem GGerber OpenFOAM Community Contributions 54 April 24, 2015 17:02
[swak4Foam] swak4Foam-groovyBC build problem zxj160 OpenFOAM Community Contributions 18 July 30, 2013 14:14
[swak4Foam] funkySetFields compilation error tayo OpenFOAM Community Contributions 39 December 3, 2012 06:18
[blockMesh] error message with modeling a cube with a hold at the center hsingtzu OpenFOAM Meshing & Mesh Conversion 2 March 14, 2012 10:56
"parabolicVelocity" in OpenFoam 2.1.0 ? sawyer86 OpenFOAM Running, Solving & CFD 21 February 7, 2012 12:44


All times are GMT -4. The time now is 07:47.