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Convergence Problem in Star-CCM+

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Old   May 16, 2008, 06:00
Default Convergence Problem in Star-CCM+
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mk_mat
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Hi guys,

I have a heat transfer model consist of fluid and solid. I used the segregated solid and fluid models for simulation - solution converged but temperature would increase if I continued the simulation! in other words, Energy residual is a straight line and if I continue the simulation temperature of solid and fluid will increase slowly. They should stop after convergence, don't they?!

Does this make sense?!

I switched to Coupled solver and solution doesn't converge at all

any help/comment?!
Rodriguez Arthurs likes this.
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Old   May 16, 2008, 07:13
Default Re: Convergence Problem in Star-CCM+
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Will
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Well don't rely on residuals for convergence, they aren't the be all and end all of judging convergence. They are a good indicator of numerical convergence but it doesn't necessarily mean that your engineering values are converged, think about the simulation as an experiment, you wouldn't stop the test until the readouts you are interested in are stable would you....
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Old   May 16, 2008, 08:19
Default Re: Convergence Problem in Star-CCM+
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mk_mat
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Yes, you are absolutely right but in that case I have to wait for about 8 days to get the results that I expect!

Don't you think it's a long period of time for a model with about 0.5 million cells?!

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Old   May 16, 2008, 08:24
Default Re: Convergence Problem in Star-CCM+
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Will
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Try changing the underelaxation for the solid to 1.0 that should make it converge quicker
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Old   May 16, 2008, 08:27
Default Re: Convergence Problem in Star-CCM+
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mk_mat
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In the case of Segregate solver yes - what if I use Coupled Solver?!
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Old   May 16, 2008, 09:01
Default Re: Convergence Problem in Star-CCM+
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Will
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Well unless it is buoyancy driven flow (natural convection) or high speed you shouldn't need to use the coupled solver, if you do need to use the coupled solver make sure your courant number is set correctly.
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Old   May 16, 2008, 09:37
Default Re: Convergence Problem in Star-CCM+
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Joern Beilke
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When you think that the fluid flow doesn't change any more (velocity, pressure and turbulence) just freeze these solvers and increase the time step to 1 second. It should help to speed up the calculation.

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