# what reasons may make fluctuations in residuals?

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March 7, 2013, 07:46
what reasons may make fluctuations in residuals?
#1
Senior Member

Mostafa Mahmoudi
Join Date: Jan 2012
Posts: 319
Rep Power: 8
Hi everybody,

what reasons may make fluctuations in residuals?
residuals are attached.

I work with a modified viscoelasticFluidFoam solver that combined whit buoyantBoussinesqSimpleFoam.

thanks
Mostafa
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 March 7, 2013, 09:13 #2 Member   Nikesh Bhattarai Join Date: Nov 2011 Location: Sacheon, South Korea Posts: 82 Rep Power: 6 Such a small fluctuation in residuals is not a problem. You should take care to see whether ur residual reaches below the convergence tolerance or not! Generally we would set the convergence tolerance to be 10^-5 or 10^-6 or less which depends on user. In your case, the residuals are not decreasing. They are fluctuating around a mean value and that mean value is of the order 10^-3 for the p-residual and 10^-1 for Uy-residual. As we can see, these are pretty high values. We want our residuals to be as small as possible, thats the general concept.

March 7, 2013, 09:35
#3
Senior Member

Mostafa Mahmoudi
Join Date: Jan 2012
Posts: 319
Rep Power: 8
Quote:
 Originally Posted by nikesh Such a small fluctuation in residuals is not a problem. You should take care to see whether ur residual reaches below the convergence tolerance or not! Generally we would set the convergence tolerance to be 10^-5 or 10^-6 or less which depends on user. In your case, the residuals are not decreasing. They are fluctuating around a mean value and that mean value is of the order 10^-3 for the p-residual and 10^-1 for Uy-residual. As we can see, these are pretty high values. We want our residuals to be as small as possible, thats the general concept.
I meant that what might cause this problem????!?

 March 7, 2013, 10:11 #4 Member   Nikesh Bhattarai Join Date: Nov 2011 Location: Sacheon, South Korea Posts: 82 Rep Power: 6 There could be numerous reasons for your residuals to be so high. We can't point out one reason for sure. However, you might want to recheck your boundary conditions and schemes used and also the mesh criteria first.

March 7, 2013, 10:34
#5
Senior Member

Mostafa Mahmoudi
Join Date: Jan 2012
Posts: 319
Rep Power: 8
Quote:
 Originally Posted by nikesh There could be numerous reasons for your residuals to be so high. We can't point out one reason for sure. However, you might want to recheck your boundary conditions and schemes used and also the mesh criteria first.
in buoyantboussinesqsimplefoam i just replace the turbulent model with some constants.
alpha, kappa, nu ...

 March 7, 2013, 10:47 #6 Member   Nikesh Bhattarai Join Date: Nov 2011 Location: Sacheon, South Korea Posts: 82 Rep Power: 6 I might not be able to help you much with this since I don't have experience working on this case. Maybe somebody with experience in this might!!

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