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Noise in velocity graph

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Old   April 15, 2015, 09:57
Default Noise in velocity graph
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Sasa Goran
Join Date: Feb 2015
Location: Japan
Posts: 23
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Hi all!

i am simulating a continuous ink-jet in OpenFOAM 2.1.1 using compressibleInterFoam

The geometry is basically a cilinder filled with ink that converges into a small nozzle and then exits into the atmosphere. The inlet is defined as a complex harmonical function of the pressure which in the first few time steps gradually increases from 0 to the required harmonical function (if the first time step is more than zero, the velocity experiences a sharp jump at the beginning).

The issue i am currently dealing with is that the velocity, which is induced by the pressure difference, seems to have some noice at the first time steps. later the graph becomes smooth and stable but for these first few time steps some areas with spikes can be seen. Did any of you experience something similar? Do you know where the problem might be?

I am using slightly modified files from the compressibleInterFoam tutorial cases:



fvSolution:

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      fvSolution;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

solvers
{
    pcorr
    {
        solver          PCG;
        preconditioner
        {
            preconditioner  DIC;

            tolerance       1e-13;
            relTol          0;
            smoother        DICGaussSeidel;
            nPreSweeps      0;
            nPostSweeps     2;
            nFinestSweeps   2;
            cacheAgglomeration false;
            nCellsInCoarsestLevel 10;
            agglomerator    faceAreaPair;
            mergeLevels     1;
        }
        tolerance       1e-13;
        relTol          0;
        maxIter         100;
    }

    "(rho|rhoFinal)"
    {
        solver          diagonal;
    }

    p_rgh
    {
        solver          GAMG;
        tolerance       1e-13;
        relTol          0.01;
        smoother        DIC;
        nPreSweeps      0;
        nPostSweeps     2;
        nFinestSweeps   2;
        cacheAgglomeration true;
        nCellsInCoarsestLevel 10;
        agglomerator    faceAreaPair;
        mergeLevels     1;
    }

    p_rghFinal
    {
        solver          PCG;
        preconditioner
        {
            preconditioner  DIC;

            tolerance       1e-13;
            relTol          0;
            nVcycles        2;
            smoother        DICGaussSeidel;
            nPreSweeps      2;
            nPostSweeps     2;
            nFinestSweeps   2;
            cacheAgglomeration true;
            nCellsInCoarsestLevel 10;
            agglomerator    faceAreaPair;
            mergeLevels     1;
        }
        tolerance       1e-13;
        relTol          0;
        maxIter         20;
    }



     U
    {
        solver          GAMG;
        tolerance       1e-13;
        relTol          0.01;
        smoother        DIC;
        nPreSweeps      0;
        nPostSweeps     2;
        nFinestSweeps   2;
        cacheAgglomeration true;
        nCellsInCoarsestLevel 10;
        agglomerator    faceAreaPair;
        mergeLevels     1;
    }


    "(k|B|nuTilda)"
    {
        solver          PBiCG;
        preconditioner  DILU;
        tolerance       1e-13;
        relTol          0;
    }


 UFinal
    {
        solver          PBiCG;
        preconditioner
        {
            preconditioner  DILU;

            tolerance       1e-13;
            relTol          0;
            nVcycles        2;
            smoother        DICGaussSeidel;
            nPreSweeps      2;
            nPostSweeps     2;
            nFinestSweeps   2;
            cacheAgglomeration true;
            nCellsInCoarsestLevel 10;
            agglomerator    faceAreaPair;
            mergeLevels     1;
        }
        tolerance       1e-13;
        relTol          0;
        maxIter         20;
    }

}

PIMPLE
{
    momentumPredictor yes;
    nCorrectors     3;
    nNonOrthogonalCorrectors 1;
    nAlphaCorr      1;
    nAlphaSubCycles 2;
    cAlpha          1;
}


// ************************************************************************* //
fvSchemes:

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.1                                 |
|   \\  /    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(rho*phi,U)  Gauss SFCD; 

    div(phi,alpha)  Gauss vanLeer;
    div(phirb,alpha) Gauss interfaceCompression 1;
    div(phi,p_rgh)  Gauss SFCD;      div(phi,k)      Gauss vanLeer;
    div((nuEff*dev(T(grad(U))))) Gauss linear;
}

laplacianSchemes
{
    default         Gauss linear corrected;
}

interpolationSchemes
{
    default         linear;
}

snGradSchemes
{
    default         corrected;
}

fluxRequired
{
    default         no;
    p_rgh;
    pcorr;
    alpha1;
}


// ************************************************************************* //
i can not use an upwind scheme as this gives me constant noise. it needs to be second order.



Anyway, thanks for the help!
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compressibleinterfoam, noise, pressure and velocity, viscous

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