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Pressure won't converge using SimpleFoam

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Old   June 14, 2021, 08:02
Default Pressure won't converge using SimpleFoam
  #1
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Hi All,

I'm running a case in simpleFoam and my pressure residuals won't converge. A similar case to this run a number of years ago had converged at this stage.

I have checked the mesh, and there are no issues with skewness or nonOrthogonality so I know the error doesn't lie there.

my fvScheme and fvSolution are as follows

fvSolution

solvers
{
p
{
solver GAMG;
tolerance 1e-6;
relTol 0.1;
smoother GaussSeidel;
nPreSweeps 0;
nPostSweeps 2;
cacheAgglomeration on;
agglomerator faceAreaPair;
nCellsInCoarsestLevel 10;
mergeLevels 1;
}

U
{
solver smoothSolver;
smoother GaussSeidel;
tolerance 1e-6;
relTol 0.1;
nSweeps 1;
}

k
{
solver smoothSolver;
smoother GaussSeidel;
tolerance 1e-6;
relTol 0.1;
nSweeps 1;
}

epsilon
{
solver smoothSolver;
smoother GaussSeidel;
tolerance 1e-6;
relTol 0.1;
nSweeps 1;
}
}

SIMPLE
{
nNonOrthogonalCorrectors 0;

residualControl
{
p 1e-3;
U 1e-4;
"(k|epsilon)" 1e-4;
}
}

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

cache
{
grad(U);
}

fvScheme

ddtSchemes
{
default steadyState;
}

gradSchemes
{
default Gauss linear;
}

divSchemes
{
default none;
div(phi,U) bounded Gauss upwind;
div((nuEff*dev(T(grad(U))))) Gauss linear;
div(phi,epsilon) bounded Gauss upwind;
div(phi,k) bounded Gauss upwind;
div((nuEff*dev2(T(grad(U))))) Gauss linear;

}

laplacianSchemes
{
default Gauss linear limited corrected 0.333;
}

interpolationSchemes
{
default linear;
}

snGradSchemes
{
default limited corrected 0.333;
}

fluxRequired
{
default no;
p;
}

If anyone could advise, that would be greatly appreciated
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Old   June 14, 2021, 08:05
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This is the most recent time step of the output


smoothSolver: Solving for Ux, Initial residual = 6.26867549999e-05, Final residual = 4.83802208018e-06, No Iterations 4
smoothSolver: Solving for Uy, Initial residual = 4.91362392681e-05, Final residual = 3.40070029735e-06, No Iterations 4
smoothSolver: Solving for Uz, Initial residual = 0.000164303461286, Final residual = 9.92105112991e-06, No Iterations 4
GAMG: Solving for p, Initial residual = 0.00202976194843, Final residual = 0.000165595182502, No Iterations 2
time step continuity errors : sum local = 8.95914605453e-09, global = 2.94382119613e-10, cumulative = -2.68013954291e-06
smoothSolver: Solving for epsilon, Initial residual = 0.000134760412047, Final residual = 7.63620479172e-06, No Iterations 2
smoothSolver: Solving for k, Initial residual = 0.000571158570201, Final residual = 3.84860947038e-05, No Iterations 4
ExecutionTime = 88904.63 s ClockTime = 90881 s
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Old   June 15, 2021, 08:13
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Some suggestions:
- Check your boundary conditions. Are they identical to the previous set up? Has OF changed in its treatment of those boundary types since you last ran the case?
- do you get the same problem if you change the pressure solver from GAMG to PCG?
- How good is your mesh quality?

Maybe also look at the pressure field in paraview to see if there is a bump/discontinuity/irregularity in the pressure field - that might help you identify where the solution is stalling, and therefore help find the reason.

Good luck & let us know when you find the solution!
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Old   June 15, 2021, 08:31
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Daniel
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Hey Richard,

I think here lies the problem:

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

At first, 0.3 in fields for p is low, try a higher one. Your residuals look good, so this hould help. Also put a factor in Equations for p in, like this:

relaxationFactors
{
fields
{
p 0.7;
}
equations
{
U 0.7;
k 0.7;
p 0.7;
epsilon 0.7;
}
}
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