How we can determine convergence by the imbalances of the equations?

AliMadayen · October 1, 2014, 17:52

Hi Everyone

I remember reading some posts regarding the problem that CFX has some problems with very fine meshes. right now i'm working on a problem and I'm checking grid independency of the results.
In the coarse meshes, solution easily converges. But as I make the mesh finer, residuals do not fall below a certain value and begin to alter around that specific value, without decreasing. As I make the mesh finer, this certain value Increases.
However, the domain Imbalances in all cases are very small for all equations. Like 0.3%, at most.

now the question is, can I consider my solution converged, when the residuals are about 1e-4 but imbalances smaller than 1?

P.S. I searched the from as much as I could. I hope I'm not posting a repeated question.
P.S. 2: The problem is cavitation in a minichannel. i've used physical timescales as small as 1e-6, yet the residuals do not decrease more than a certain amount. I use Rayleigh-Plesset model. and these results have good consistency with experimental images of the cavitation region. I just wanna make sure i'm not faking the validation.

ghorrocks · October 5, 2014, 08:33

As you refine the mesh the result is probably becoming transient. So you will probably need a transient simulation to fully converge it.

AliMadayen · October 12, 2014, 19:51

Tnx for your reply.
So you mean that the imbalances alone are not reliable? I just want to grab the concept of that.
I heard someone saying that the sum of cut-off error in fine mesh prevents the solution from being fully converged. Is that true?

ghorrocks · October 12, 2014, 20:01

It is not as simple as that. It is discussed in this FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

The imbalances alone are generally not a reliable measure of convergence. That is why the equation residuals is the default convergence criteria - it is more generally applicable, but even it is not universally applicable.

Yes, round off error can prevent tight convergence. But so can many other things too.

AliMadayen · October 13, 2014, 07:36

I got what I wanted.
Thanks a lot.

October 1, 2014, 17:52	How we can determine convergence by the imbalances of the equations?	#1
AliMadayen New Member Ali Madayen Join Date: Apr 2014 Posts: 22 Rep Power: 12	Hi Everyone I remember reading some posts regarding the problem that CFX has some problems with very fine meshes. right now i'm working on a problem and I'm checking grid independency of the results. In the coarse meshes, solution easily converges. But as I make the mesh finer, residuals do not fall below a certain value and begin to alter around that specific value, without decreasing. As I make the mesh finer, this certain value Increases. However, the domain Imbalances in all cases are very small for all equations. Like 0.3%, at most. now the question is, can I consider my solution converged, when the residuals are about 1e-4 but imbalances smaller than 1? P.S. I searched the from as much as I could. I hope I'm not posting a repeated question. P.S. 2: The problem is cavitation in a minichannel. i've used physical timescales as small as 1e-6, yet the residuals do not decrease more than a certain amount. I use Rayleigh-Plesset model. and these results have good consistency with experimental images of the cavitation region. I just wanna make sure i'm not faking the validation.

October 5, 2014, 08:33		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	As you refine the mesh the result is probably becoming transient. So you will probably need a transient simulation to fully converge it. AliMadayen likes this.

October 12, 2014, 20:01		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	It is not as simple as that. It is discussed in this FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria The imbalances alone are generally not a reliable measure of convergence. That is why the equation residuals is the default convergence criteria - it is more generally applicable, but even it is not universally applicable. Yes, round off error can prevent tight convergence. But so can many other things too. AliMadayen likes this.

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October 12, 2014, 19:51		#3
AliMadayen New Member Ali Madayen Join Date: Apr 2014 Posts: 22 Rep Power: 12	Tnx for your reply. So you mean that the imbalances alone are not reliable? I just want to grab the concept of that. I heard someone saying that the sum of cut-off error in fine mesh prevents the solution from being fully converged. Is that true?

October 13, 2014, 07:36		#5
AliMadayen New Member Ali Madayen Join Date: Apr 2014 Posts: 22 Rep Power: 12	I got what I wanted. Thanks a lot.