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 Industrial_CFD January 26, 2013 13:09

Large Eddy Simulation - Residuals

Hi Foamers,

I have a question regarding the residuals I am getting for my initial testing on a LES problem. The mesh is very high quality (pure hexahedral), and the cells nearest the wall are designed such that y+ <1. In addition the timestep is sufficiently small so that the max Co number should be less than 0.5, but the log is indicating that it is much higher, but the solution is relatively stable.

Here is a snippet of the log file:

"Time = 0.000943

Courant Number mean: 0.0301863 max: 45.9277
DILUPBiCG: Solving for Ux, Initial residual = 0.0153451, Final residual = 1.39222e-06, No Iterations 3
DILUPBiCG: Solving for Uy, Initial residual = 0.0154658, Final residual = 1.3237e-06, No Iterations 3
DILUPBiCG: Solving for Uz, Initial residual = 0.0141484, Final residual = 1.36949e-06, No Iterations 3
GAMG: Solving for p, Initial residual = 0.521525, Final residual = 0.00047048, No Iterations 10
GAMG: Solving for p, Initial residual = 0.499248, Final residual = 0.000347157, No Iterations 10
time step continuity errors : sum local = 4.13254e-07, global = 9.54636e-08, cumulative = -7.48025e-05
GAMG: Solving for p, Initial residual = 0.495827, Final residual = 0.00037971, No Iterations 10
GAMG: Solving for p, Initial residual = 0.487864, Final residual = 0.000286617, No Iterations 10
time step continuity errors : sum local = 3.37066e-07, global = 7.99683e-08, cumulative = -7.47226e-05
GAMG: Solving for p, Initial residual = 0.484981, Final residual = 0.000339209, No Iterations 10
GAMG: Solving for p, Initial residual = 0.477989, Final residual = 0.000258817, No Iterations 10
time step continuity errors : sum local = 3.01685e-07, global = 7.23369e-08, cumulative = -7.46502e-05
ExecutionTime = 53404.1 s ClockTime = 54141 s

Time = 0.000944"

I am having a hard time understanding why this could be occurring. Here is my solution dict:
"

solvers
{
p
{
solver GAMG;
preconditioner DIC;
agglomerator faceAreaPair;
tolerance 1e-05;
relTol 0;
nCellsInCoarsestLevel 25;
mergeLevels 1;
smoother GaussSeidel;
nPreSweeps 0;
nPostSweeps 2;
nFinestSweeps 2;
maxIter 10;

}

pFinal
{
solver GAMG;
preconditioner DIC;
agglomerator faceAreaPair;
tolerance 1e-05;
relTol 0;
nCellsInCoarsestLevel 25;
mergeLevels 1;
smoother GaussSeidel;
nPreSweeps 0;
nPostSweeps 2;
nFinestSweeps 2;
maxIter 10;
}

U
{
solver PBiCG;
preconditioner DILU;
tolerance 1e-05;
relTol 0;
}

k
{
solver PBiCG;
preconditioner DILU;
tolerance 1e-05;
relTol 0;
}

B
{
solver PBiCG;
preconditioner DILU;
tolerance 1e-05;
relTol 0;
}

nuTilda
{
solver PBiCG;
preconditioner DILU;
tolerance 1e-05;
relTol 0;
}
}

PISO
{
nCorrectors 3;
nNonOrthogonalCorrectors 1;
}

relaxationFactors
{
fields
{
p 0.3;
}
equations
{
U 0.7;
k 0.7;
B 0.7;
nuTilda 0.7;
}
}
"
All of my discretization schemes are 2nd order (Gauss Linear). Similar settings seem fine in FLUENT (my experience is mostly using commercial solvers). I know I must be missing something.... Anyone have any ideas? Any other information I should post?

Thanks a lot!

 Industrial_CFD January 26, 2013 18:50

Update

I just realized I set the maxIter to 10, which probably isn't letting pressure converge properly. Can anyone comment on this?

BTW, when judging convergence should I monitor the initial or final residuals?

Thanks a lot.

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