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Under-Relaxation for Temperature Equartion

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Old   November 24, 2010, 14:12
Default Under-Relaxation for Temperature Equartion
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Hagen Müller
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Hello Everybody,

I try to run a calculation for the straight channel on different Reynolds numbers with a modified simpleFoam solver. It solves the energy equation and models the scalar flux with a differential approach. The part where the equation is solved looks like this:

//Energy equation ---------------------------------------------------------------------------------------------------------------------------
fvScalarMatrix TEqn
fvm::div(phi, T)
- fvm::Sp(fvc::div(phi), T)
- fvm::laplacian(kappaEff, T)
- fvc::div(Dp & fvc::grad(p))


eqnResidual = TEqn.solve().initialResidual();
maxResidual = max(eqnResidual, maxResidual);

Unfortunately the solver becomes unstable for Reynolds numbers higher than 25000. I guess under-relaxation for T would solve the problem.
When I define for example T=0.7 in the fvSolutions dictionary, the solver makes only one iteration every timestep.
Does anybody know where the bug is? Do you have different suggestions to make the calculation stable?
Thank you for your help

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Old   November 25, 2010, 03:54
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Claus Meister
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When you have modified something then it is hard to recognize it without seeing the code. However, instead of using relaxation factors, you can reduce the bound of the maxCo number. According to theory, see Versteeg p. 265 (I think), has it the same effect as using RF.
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