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Is that a checker-boarding pattern? How to avoid it?

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Old   December 10, 2009, 07:02
Question Is that a checker-boarding pattern? How to avoid it?
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Robert Castilla
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Hi,

I am simulating internal turbulent flow in complex geometries with simpleFoam and k-Omega SST, and I have found this strange behavior (see attached picture).

The fvSolution and fvSchemes files are:

solvers
{
p
{
solver GAMG;
tolerance 1e-7;
relTol 0.1;
// preconditioner DIC
smoother GaussSeidel;
nPreSweeps 0;
nPostSweeps 2;
cacheAgglomeration on;
agglomerator faceAreaPair;
nCellsInCoarsestLevel 10;
mergeLevels 1;
};

U
{
solver smoothSolver;
smoother GaussSeidel;
tolerance 1e-8;
relTol 0.1;
nSweeps 1;
};

k
{
solver smoothSolver;
smoother GaussSeidel;
tolerance 1e-8;
relTol 0.1;
nSweeps 1;
};

omega
{
solver smoothSolver;
smoother GaussSeidel;
tolerance 1e-8;
relTol 0.1;
nSweeps 1;
};
}

SIMPLE
{
nNonOrthogonalCorrectors 0;
}

relaxationFactors
{
p 0.3;
U 0.7;
k 0.7;
epsilon 0.7;
}

-----------------------------------------------------------------------

ddtSchemes
{
default steadyState;
}

gradSchemes
{
default Gauss linear;
grad(p) Gauss linear;
grad(U) Gauss linear;
// grad(U) Gauss limitedLinear;
}

divSchemes
{
default none;
div(phi,U) Gauss linearUpwindV cellLimited Gauss linear 1;
div(phi,k) Gauss upwind;
div(phi,omega) Gauss upwind;
div((nuEff*dev(grad(U).T()))) Gauss linear;
}

laplacianSchemes
{
default Gauss linear corrected;
// default Gauss linear limited 0.5;
// default Gauss linear limited 0.333;
}

interpolationSchemes
{
// default upwind phi;
// interpolate(U) upwind phi;
default linear;
interpolation linear;
}

snGradSchemes
{
default corrected;
}

fluxRequired
{
default no;
p;
}
-----------------------------------------------------------------------------------


I am not sure if it is the checker-boarding pattern. If it is, it should be avoided with SIMPLE, isn't it? Maybe have I to tune the SIMPLE parameters? Can somebody give me some hint?

Thank you

Robert
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Old   December 10, 2009, 09:45
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Robert Castilla
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I am trying with pisoFoam (transient solver) and results are not better.
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Old   December 11, 2009, 12:50
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Robert Castilla
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It seems so be related to mesh. It has been meshed with snappyHexMesh. If there are sharp transitions of cell size, this kind of patterns arise. I don't know the reason, but I have tried with a regular (not grading) hexahedral mesh, it went fine. Maybe it is something that only happens when meshing with snappyHexMesh inan internal flow.
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