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August 9, 2014, 12:30 |
problem with convergence!
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#1 |
Senior Member
Amin
Join Date: Oct 2013
Location: Germany
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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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August 11, 2014, 05:28 |
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#2 |
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Sa.Baraty
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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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thanks |
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August 11, 2014, 06:51 |
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#3 | |
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Amin
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Quote:
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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August 11, 2014, 07:35 |
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#4 |
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Philipp
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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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August 11, 2014, 08:41 |
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#5 | |
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Amin
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Quote:
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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August 11, 2014, 08:42 |
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#6 |
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Philipp
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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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August 11, 2014, 08:57 |
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#7 | |
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Amin
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Quote:
How many iterations you're guessing would be required!?? |
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August 11, 2014, 09:48 |
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#8 |
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Philipp
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Good question. I would try 500 more and see if there is any development...
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August 11, 2014, 10:55 |
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#9 | |
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Amin
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Quote:
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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August 11, 2014, 10:56 |
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#10 |
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Philipp
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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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August 11, 2014, 12:53 |
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#11 |
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Amin
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August 12, 2014, 03:51 |
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#12 |
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Sangwoo Kim
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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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Best regards |
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August 12, 2014, 05:40 |
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#13 | |
Senior Member
Amin
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Quote:
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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