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Strange Residuals in CCM+

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Old   July 26, 2007, 08:47
Default Strange Residuals in CCM+
  #1
PeterGG
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Hello,

I have something strange happening to my simulations in ccm+ - I will be asking support about them after my vacation, however I'd like to know if anyone else experienced this as well or have any clue on what I do wrong - this is past my experience and current knowledge

I have a polyhedra mesh of 5+ million poly's, with MRF-zone, and running it is fine, residuals calmes down nicely after a couple of thousand iterations, force monitors converge as well nicely.. everthing is fine

but then after say 5000 iterations, sometimes less sometimes more depending on what urf I have set, the standard monitored residuals starts growing - at a constant rate, I see it first in one, but the otherones later tag along - and basically what happens is that after 10 000 - 15 000 iterations they have reached the roof and it crashes

but even more interesting is that my force monitors (case interesting monitors) show nothing - they stay constant- except maybe when the residuals are up in the region 1e12 they oscillate a bit..

I've used k-e-realizable, k-w-sst, standard k-e, all with enhanced wall treatment - I see the same thing, tried with different boundaries as well..

Anyone sharing any idea or experience will be appreciated

Regards Peter
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Old   July 27, 2007, 04:54
Default Re: Strange Residuals in CCM+
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Ahmed
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Accumalation of rounding errors, I guess
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Old   July 27, 2007, 06:06
Default Re: Strange Residuals in CCM+
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PeterGG
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Is there any cure or fix? as it is now the residuals starts growing 500 iterations before my flow interesting variables converge - so I'm not completely confident in my results

Use more under relaxation? or rethink my setup?

Anyhow thanks, it gave me some more ideas

Regards Peter
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Old   July 27, 2007, 12:52
Default Re: Strange Residuals in CCM+
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Ahmed
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Not at all, My experience with both FLUENT and StarCD has taught me to start a simulation with a very small URF for pressure (Or the Courant Number if I am using the coupled solver) may be 0.01 or less, Initialize every thing with the value 0.0, start the simulation and paying attention to how the continuity residual changes, Normally it takes longer to converge, but you can start increasing the URF for pressure after a reasonable number of iterations (300 or more). Hope that will help and Good Luck.
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