[hokhay]

Hi all foamers,

I am a new foamer with only 2 months experience, so please forgive my ignorance.

I am now designing a windsurfing board fin for my own use and I would like simulation the performance difference between each fin shape.

My case is very simple. It is based on the steatdy state cavity tutorial with single phase flow of water. The water is flow from the inlet at 8m/s and the fin is submerged vertically from the top wall of the domain at an angle of 5 degrees. The top wall is a moving wall with 8m/s and the other walls are all freestream outlet.

The simulation finish smoothly with good convergence. However when I calculate the force on the fin using paraview, the lift force is only a few Newtons and the drag force is less than 1 Newton. I then check the pressure at the stagnation point on the fin, it only gives a value of 80Pa. It is far too small for 8.5m/s water which the pressure should be 32000.

It is obvious that the simulation is only doing air simulation. The only thing I know that need to be changed from air to water simulation is the viscosity. Is there something else I miss?

Here is the file for the case

fvScheme

ddtSchemes
{
    default         steadystate;
}

gradSchemes
{
    default         Gauss linear;
    grad(p)         cellLimited leastSquares 1;
    grad(U)         cellLimited Gauss linear 1;
    grad(nuTilda)   cellLimited Gauss linear 1;
}

divSchemes
{
    default         none;
    div(phi,U)      bounded Gauss linearUpwindV grad(U);
    div(phi,k)      bounded Gauss upwind;
    div(phi,omega)  bounded Gauss upwind;
    div((nuEff*dev2(T(grad(U))))) Gauss linear;
    div(phi,nuTilda) bounded Gauss upwind;
}

laplacianSchemes
{
    default         Gauss linear corrected;
}

interpolationSchemes
{
    default         linear;
}

snGradSchemes
{
    default         corrected;
}

wallDist
{
    method meshWave;
}

fvSolution

solvers
{
    p
    {
        solver          GAMG;
        tolerance       1e-04;
        relTol          0.1;
        smoother        GaussSeidel;
    nCellsInCoarsestLevel 50;
    nPerSweeps    1;
    nPostSweeps    2;
    nFinestSweeps    3;
    }

    pFinal
    {
        $p;
        tolerance       1e-5;
        relTol          0;
    }

    Phi
    {
        $p;
    }

    "(k|U|nuTilda|omega)"
    {
        solver          smoothSolver;
        smoother        GaussSeidel;
        tolerance       1e-08;
        relTol          0.05;
    }
}

PISO
{
    nNonOrthogonlCorrectors 1;
    pRefValue 0;
    pRefCell 0;
}

SIMPLE
{
    nNonOrthogonlCorrectors 1;
    pRefValue 0;
    pRefCell 0;
}

potentialFlow
{
    nNonOrthogonalCorrectors 10;
    PhiRefCell 0;
    PhiRefValue 0;
}

relaxationFactors
{
    p           0.4;
    k        0.5;
    U           0.4;
    nuTilda     0.6;
    omega    0.6;
}

cache
{
    grad(U);
}

transportProperties

transportModel  Newtonian;

nu              [0 2 -1 0 0 0 0] 1e-6;
rho             [1 -3 0 0 0 0 0] 1000;

Thank you very much

[akidess]

How did you check the pressure? The default unit in Paraview is likely not Pa.

[hokhay]

By plotting the P value in paraview. The unit should be the same as in OpenFoam, right?

Sent from my LG-H818 using CFD Online Forum mobile app

[akidess]

Yes. OpenFOAM does not always use Pascal though (constant density solvers).

[hokhay]

Thanks akidess,

I have checked with the unit of p value in my case, it is m^2 s^-2 which I suppose it is dynamic pressure per unit density. Am I right?

In this case, if I would like to calculate the exact force acting on my fin, I just need to integrate the dynamic pressure around the fin and the multiple with the water density. Am I correct?

Thanks a lot

Jason

[akidess]

Yes. And if you get tired of doing it manually in Paraview you can also get it on the fly: http://www.openfoam.com/documentatio...es-forces.html

[hokhay]

Thanks you very much for your information

Sent from my LG-H818 using CFD Online Forum mobile app