[rheise]

Hello everyone,

I have a problem running my multi-phase model (water, air) with two inlets using two different fixed fluid-velocities (water) and the interFoam solver (and I'm relatively new to openFoam). The turbulence model used is RAS - kOmegaSST.

I want to simulate a constant water flow through those two inlets until more or less steady state conditions are reached. This works quite well for 0.2 seconds of simulation time and then the time steps decrease constantly (stopped the simulation at deltaT = XXe-80). I tried a lot of things like increasing Courant numbers up to exorbitant high values, coarsening my mesh, or applying different initial water conditions through setFieldsDict. I also played around with different PIMPLE-values (nOuterCorrectors, nCorrectors, nNonOrthogonalCorrectors) but nothing helped.

Since DeltaT = (deltaX/U)*C, I suspect that the inlet velocities seem to be too high (even though they are really low compared to the mesh size) until flow within the channel has been developed to a stable state.

The velocity boundaries are:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.3.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volVectorField;
    location    "0";
    object      U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 1 -1 0 0 0 0];

internalField   uniform (0 0 0);

boundaryField
{
    wall
    {
        type            fixedValue;
        value           uniform (0 0 0);
    }

    inlet1
    {
        type            fixedValue;
        value           uniform (0.4572 0 0);
    }
    inlet2
    {
        type            fixedValue;
        value           uniform (0.762 0 0);
    }
    outlet
    {
        type            zeroGradient;
    }

    atmosphere
    {
        type            pressureInletOutletVelocity;
        value           uniform (0 0 0);
    }
}


// ************************************************************************* //

As you can see, there are two different relatively small inlet velocities of 0.45 and 0.76 m/s. The mesh that I used for this model should definitely be coarse enough so that those velocities should not lead to any problem.

A file with the base geometry is attached. My base model consists of two inlets (red, black), atmospheric top (yellow), one outlet (green) and a geometry which serves as a wall (blue).



Another assumption is that this problem might be solvable with different solvers or schemes. I tried to improve both, fvSolution and fvSchemes without any success. I hope that someone using this forum might have suggestions on how to improve my model in order to fix this problem.

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      fvSolution;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

solvers
{

    "alpha.water.*"
    {
        nAlphaCorr      2;
        nAlphaSubCycles 1;
        alphaOuterCorrectors yes;
        cAlpha          1;

        MULESCorr       yes;
        nLimiterIter    3;

        solver          smoothSolver;
        smoother        symGaussSeidel;
        tolerance       1e-8;
        relTol          0;
    }

    pcorr
    {
        solver          PCG;
        preconditioner  DIC;
        tolerance       1e-10;
        relTol          0;
    }

    p_rgh
    {
        solver          PCG;
        preconditioner  DIC;
        tolerance       1e-07;
        relTol          0.05;
    }

    p_rghFinal
    {
        solver          PCG;
        preconditioner  DIC;
        tolerance       1e-07;
        relTol          0;
    }

    "(U|k|omega)"
    {
        solver          PBiCG;
        preconditioner  DILU;
        tolerance       1e-06;
        relTol          0;
    }

    "(U|k|omega)Final"
    {
        solver          PBiCG;
        preconditioner  DILU;
        tolerance       1e-08;
        relTol          0;
    }
}

PIMPLE
{
// momentumPredictor                yes;
    nOuterCorrectors            2;
    nCorrectors                1;
    nNonOrthogonalCorrectors    1;
    pRefCell                    0;
    pRefValue                  0;

    residualControl
    {
        "(p|U|k|omega)"
        {
            tolerance 1e-2;
            relTol 0;
        }
    }
}


// ************************************************************************* //

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      fvSchemes;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

ddtSchemes
{
    default         Euler;
}

gradSchemes
{
    default         Gauss linear;
}

divSchemes
{
    div(rhoPhi,U)  Gauss linear;
    div(phi,alpha)  Gauss vanLeer;
    div(phirb,alpha) Gauss interfaceCompression;
    div(phi,k)      Gauss upwind;
    div(phi,omega)  Gauss upwind;
    
    div(phi,R)      Gauss upwind;
    div(R)          Gauss linear;
    div(phi,nuTilda) Gauss upwind;
    div((nuEff*dev(T(grad(U))))) Gauss linear;

    div(((rho*nuEff)*dev2(T(grad(U))))) Gauss linear;
}

laplacianSchemes
{
    default         Gauss linear corrected;
}

interpolationSchemes
{
    default         linear;
}

snGradSchemes
{
    default         corrected;
}

fluxRequired
{
    default         no;
    p_rgh;
    pcorr;
    alpha;
}

wallDist
{
    method meshWave;
}

// ************************************************************************* //

Thanks,
Robert