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Negative Pressures at water/air Interface?

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Old   March 29, 2019, 12:05
Question Negative Pressures at water/air Interface?
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Hello,

I need help troubleshooting why there would be negative pressure within the water column for an interFoam simulation of water flowing over a weir. See attached image.

1. What would cause the pressure at the interface to be negative? There are ~7 mesh cells between 0Pa and the alpha interface so it isn't negative in just 1 cell (see the line plot in attached image)

2. Is there a way to have the interface correspond to 0Pa? If I change the alpha phase to 0.99 the interface is still negative.
Attached Images
File Type: jpg negative_pressure.jpg (43.0 KB, 16 views)
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Old   March 29, 2019, 12:08
Default Boundary Conditions
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0/U

Code:
dimensions      [0 1 -1 0 0 0 0];

internalField   uniform (0 0 0);

boundaryField
{

    inlet_water
    {
        type                      flowRateInletVelocity;
        volumetricFlowRate        constant 30;
        value                     uniform (0 0 0);
    }

    outlet_air
    {
        type            pressureInletOutletVelocity;
        value           uniform (0 0 0);
    }
    outlet_water
    {
        type            pressureInletOutletVelocity;
        value           uniform (0 0 0);
    }

    "(inlet_wall|conc)"
    {
        type            noSlip;
    }

    "(wall1|wall2)"
    {
        type            symmetry;
    }



    "(bed1|bed2)"
    {
        type            noSlip;
    }


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

    defaultFaces
    {
        type            empty;
0/alpha.water
Code:
dimensions      [0 0 0 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    inlet_water
    {
        type            fixedValue;
        value           uniform 1;

    outlet_air
    {
        type            inletOutlet;
        inletValue      uniform 0;
        value           uniform 0;
    }

    outlet_water
    {
        type            inletOutlet;
        inletValue      uniform 1;
        value           uniform 1;
    }


    "(inlet_wall|conc)"
    {
        type            zeroGradient;
    }

    "(wall1|wall2)"
    {
        type            symmetry;
    }


    "(bed1|bed2)"
    {
        type            zeroGradient;
    }


    top
    {
        type            inletOutlet;
        inletValue      uniform 0;
        value           uniform 0;
    }

    defaultFaces
    {
        type            empty;
    }
}
0/p_rgh
Code:
dimensions      [1 -1 -2 0 0 0 0];

internalField   uniform $pressure;

boundaryField
{
    inlet_water
    {
        type            fixedFluxPressure;
    }

    outlet_water
    {
        type            totalPressure;
        rho             rho;
        psi             none;
        gamma           1;
        p0              uniform 4286970;
        value           uniform 4286970;
    }

    outlet_air
    {
        type            totalPressure;
        rho             rho;
        psi             none;
        gamma           1;
        p0              uniform 0;
        value           uniform 0;
    }


    "(inlet_wall|conc)"
    {
        type            fixedFluxPressure;
    }

    "(wall1|wall2)"
    {
        type            symmetry;
    }


    "(bed1|bed2)"
    {
            type            fixedFluxPressure;
    }


     top
    {
        type            totalPressure;
        p0              uniform 0;
    }


    defaultFaces
    {
        type            empty;
    }
}
fvSchemes
Code:
ddtSchemes
{
    default         Euler;
}

gradSchemes
{
    default         Gauss linear;
}

divSchemes
{
    div(rhoPhi,U)  Gauss upwind; //Gauss limitedLinearV 1;
    div(phi,alpha)  Gauss upwind; //Gauss vanLeer;
    div(phirb,alpha) Gauss interfaceCompression;
    div(((rho*nuEff)*dev2(T(grad(U))))) Gauss linear;
    div(phi,omega) Gauss upwind;
    div(phi,p_rgh)  Gauss upwind;
    div(phi,k)      Gauss upwind;
    div(phi,epsilon) Gauss upwind;
}

laplacianSchemes
{
    default         Gauss linear corrected;
}

interpolationSchemes
{
    default         linear;
}

snGradSchemes
{
    default         corrected;
}

fluxRequired
{
    default         no;
    p_rgh;
    pcorr;
    alpha.water;
}
fvSolution
Code:
solvers
{
    "alpha.water.*"
    {
        isoFaceTol      1e-6;
        surfCellTol     1e-6;
        nAlphaBounds    3;
        snapTol         1e-12;
        clip            true;

        nAlphaSubCycles 1;
        cAlpha          1; // Note: cAlpha is not used by isoAdvector but must
                           // be specified because interfacePropertes object
                           // reads it during construction.
    }

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

    p_rgh
    {
        solver          GAMG;
        smoother        DICGaussSeidel;
        tolerance       1e-07;
        relTol          0.05;
    }

    p_rghFinal
    {
        $p_rgh;
        tolerance       1e-07;
        relTol          0;
    }



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



    "(U|k|epsilon)Final"
    {
        $U;
        relTol          0;
    }


}

PIMPLE
{
    momentumPredictor no; 
    nCorrectors     3;
    nOuterCorrectors 1;
    nNonOrthogonalCorrectors 0;
    pRefCell        0;
    pRefValue       0;
}


relaxationFactors
{
    fields
    {
    }
    equations
    {
        ".*" 0.1;
    }
}
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