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Old   August 9, 2014, 11:30
Default problem with convergence!
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Hey!

I have a problem with convergence in my project!
I'working on a centrifugal compressor and its diffuser!
The solver specifications is:
Density based solver
Air-ideal gas
K-epsilon turbulence model

My problem is non-convergent the solution!
As appear in the attached picture, the residuals of continuty and energy is not convergent!
Do you have a suggestion for me to help the residuals convergence!?

Tnx
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Old   August 11, 2014, 04:28
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Hi
please check your mesh quality. Maximum cell skewness and orthogonal quality are important parameter for solution converge.
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Old   August 11, 2014, 05:51
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Quote:
Originally Posted by sara22 View Post
Hi
please check your mesh quality. Maximum cell skewness and orthogonal quality are important parameter for solution converge.
Slm!

Tnx for your reply!
Because the number of elements was too great (about 6,500,000 elements),I had to reduce them (to abaut 2,500,000).
As you referred, probably the mesh quality is cause of the problem!
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Old   August 11, 2014, 06:35
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In your residual plot you just show a few hundred iterations. I would not expect such a large mesh (2.5 mio) to converge that fast. You probably just need more iterations (except your intialization already is really close to the final result).
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Old   August 11, 2014, 07:41
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Quote:
Originally Posted by RodriguezFatz View Post
In your residual plot you just show a few hundred iterations. I would not expect such a large mesh (2.5 mio) to converge that fast. You probably just need more iterations (except your intialization already is really close to the final result).
Hi!
Oh!realy? Do you believe if I continue the solution it can be converge?
Because I don't see any convergence sign for energy and continuity, I guess it could not to convergence and I stopped the process!

Tnx dear Philipp for your help!
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Old   August 11, 2014, 07:42
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Well, this could be the reason. But before you didn't try, I would not try to change anything else.
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Old   August 11, 2014, 07:57
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Quote:
Originally Posted by RodriguezFatz View Post
Well, this could be the reason. But before you didn't try, I would not try to change anything else.
Because the solution time was too long ( 15 min for 10 iterations! ) I decided to no more iteration! But as you said, I will continue the solution and I hope that the problem would converge!
How many iterations you're guessing would be required!??
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Old   August 11, 2014, 08:48
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Good question. I would try 500 more and see if there is any development...
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Old   August 11, 2014, 09:55
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Quote:
Originally Posted by RodriguezFatz View Post
Good question. I would try 500 more and see if there is any development...
I will continue the solution for more than 500 iterations
And
According your experience, for a turbocharger with centrifugal compressor which type will be better in solution control for flow? 1st order or 2nd order or 3rd order MUSCL or QUICK? (Current solution was based on 1st order!)

Very grateful!
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Old   August 11, 2014, 09:56
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Sorry, I don't have experience with that. It was more a general hint, that you should not expect convergence too soon with such a large grid.
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Old   August 11, 2014, 11:53
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Quote:
Originally Posted by RodriguezFatz View Post
Sorry, I don't have experience with that. It was more a general hint, that you should not expect convergence too soon with such a large grid.
Ok!
Anyway, thank you dear friend!
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Old   August 12, 2014, 02:51
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Everything can cause a divergence.
Mesh quality and the number of iteration which Phlilpp and Sara suggest are strong candidates.

If you have enough time, I recommend to start from the simplest model first.
If you are using MRF, moving wall, or whatever, shut all of them off and start just with turbulence model. If it looks like behaving favorably, then change the scheme from 1st order to 2nd order, and add more complex models step by step.

In my experience(of course it can be different issue in your case) especially energy equation is important. I never begin the first calculation with energy equation.
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Old   August 12, 2014, 04:40
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Quote:
Originally Posted by swtbkim View Post
Everything can cause a divergence.
Mesh quality and the number of iteration which Phlilpp and Sara suggest are strong candidates.

If you have enough time, I recommend to start from the simplest model first.
If you are using MRF, moving wall, or whatever, shut all of them off and start just with turbulence model. If it looks like behaving favorably, then change the scheme from 1st order to 2nd order, and add more complex models step by step.

In my experience(of course it can be different issue in your case) especially energy equation is important. I never begin the first calculation with energy equation.
Dear friend! Thanx for your reply!

I'm using mesh motion and the model has a grid interface!
The compressor is rotating with 20,000 rpm and its output exits from 2 outlets!
Also I'm checked this model with increasing angular velocity step by step (5000 & 10,000 & 15,000 & 20,000 rpm) but the convergence issue for energy and continuity exists!

Based on your suggestion, I will try the solution as non-rotating fluid and only with turbulence model!

And in your oponion, is it possible that "no existing of boundary layer mesh" and "great y-plus" be reason of this divergence!??

Best regards
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