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Old   September 25, 2012, 10:56
Default buoyantBoussinesqSimpleFoam convergence problem.
  #1
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Eskil Aursand
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I am trying to get a buoyantBoussinesqSimpleFoam simulation with a k-eps turbulence model to converge, but it never does. The output states that the residuals are smaller than the SIMPLE tolerances, but it still doesn't stop.
Here is an example output:

Code:
DILUPBiCG:  Solving for Ux, Initial residual = 0.00776199, Final residual = 3.05559e-05, No Iterations 1
DILUPBiCG:  Solving for Uy, Initial residual = 0.0033988, Final residual = 1.59953e-05, No Iterations 1
DILUPBiCG:  Solving for T, Initial residual = 0.000345836, Final residual = 1.40283e-05, No Iterations 1
DICPCG:  Solving for p_rgh, Initial residual = 0.0300916, Final residual = 0.0002878, No Iterations 15
time step continuity errors : sum local = 7.9083e-05, global = -1.09848e-05, cumulative = 0.00106109
DILUPBiCG:  Solving for epsilon, Initial residual = 0.000168969, Final residual = 2.31597e-06, No Iterations 1
DILUPBiCG:  Solving for k, Initial residual = 0.001979, Final residual = 3.89602e-05, No Iterations 1
Here is my fvSolution:

Code:
solvers
{
    p_rgh
    {
        solver          PCG;
        preconditioner  DIC;
        tolerance       1e-08;
        relTol          0.01;
    }

    "(U|T|k|epsilon|R)"
    {
        solver          PBiCG;
        preconditioner  DILU;
        tolerance       1e-05;
        relTol          0.1;
    }
}

SIMPLE
{
    nNonOrthogonalCorrectors 0;
    pRefCell        0;
    pRefValue       0;

    residualControl
    {
        p_rgh           1e-2;
        U               1e-4;
        T               1e-2;

        // possibly check turbulence fields
        "(k|epsilon|omega)" 1e-3;
    }
}

relaxationFactors
{
    fields
    {
        p_rgh           0.5;
    }
    equations
    {
        U               0.2;
        T               0.5;
        "(k|epsilon|R)" 0.5;
    }
}
As seen the final residuals are smaller than the tolerances set in residualControl. If i set the SIMPLE tolerances to 1e-1 it converges however, but I don't understand why they have to be so low.

If it matters: The case is 2D with a cold circle in the center, hot walls on the sides and below, and an insulating boundary above. Gravity points down. Solver is buoyantBoussinesqSimpleFoam.

Any help and tips will be appreciated!
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Old   August 5, 2014, 01:25
Default Exact same problem
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Srivathsan N
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Hi all,

I am facing a similar problem. My last time step residuals are
Code:
DILUPBiCG:  Solving for Ux, Initial residual = 1.13167e-09, Final residual = 1.13167e-09, No Iterations 0
DILUPBiCG:  Solving for Uy, Initial residual = 3.7544e-10, Final residual = 3.7544e-10, No Iterations 0
DILUPBiCG:  Solving for Uz, Initial residual = 0.000492656, Final residual = 4.17232e-06, No Iterations 1
DILUPBiCG:  Solving for T, Initial residual = 9.9924e-06, Final residual = 9.9924e-06, No Iterations 0
DICPCG:  Solving for p_rgh, Initial residual = 1.05994e-08, Final residual = 7.33015e-09, No Iterations 1
time step continuity errors : sum local = 3.02676e-11, global = 9.96442e-20, cumulative = -1.45381e-16
However my fvSolution file reads as follows,

Code:
solvers
{
    p_rgh
    {
        solver          PCG;
        preconditioner  DIC;
        tolerance       1e-08;
        relTol          0.01;
    }

    "(U|T|k|epsilon|R)"
    {
        solver          PBiCG;
        preconditioner  DILU;
        tolerance       1e-05;
        relTol          0.1;
    }
}

SIMPLE
{
    nNonOrthogonalCorrectors 0;
    pRefCell        0;
    pRefValue       0;

    residualControl
    {
        p_rgh           1e-4;
        U               1e-5;
        T               1e-2;
    }
The residuals also seem to fluctuate very much except temperature that seems to flatten out.
Q: Why hasn't the solution converged even though the SIMPLE tolerances have been met?

Any help would be great!
Attached Images
File Type: jpg Residuals.jpg (38.7 KB, 13 views)
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Regards,

Srivaths

Last edited by Sherlock_1812; August 20, 2014 at 08:30.
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Old   August 20, 2014, 08:59
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Cong
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Hi,

I remember I was faced with the same problems with the buoyantSimpleFoam. The residual never goes down below 1e-6. I had no idea why.
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Old   August 20, 2014, 09:16
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Srivathsan N
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I have tried increasing the pseudo time step in the controlDict. There are no oscillation if I do that but the solution diverges after a few iterations. Again, have no idea why.

I'm using OF 2.2.2, by the way.
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Srivaths
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