[steph79]

Hello all,

I'm using OpenFOAM 2.1.1 (interFoam solver) to model water droplet impacts on superhydrophobic surfaces with very high constantAlphaContactAngle - 163 degrees. In short, it's an extension of the dam break tutorial where there's an initialised droplet of water within the domain which, after a period of time, will impact on a superhydrophobic surface. Here are my BC's;

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
{
    substrate
    {
        type            fixedValue;
        value           uniform (0 0 0);
    }

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

    }
    defaultFaces
    {
        type            empty;
    }
}

FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p_rgh;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [1 -1 -2 0 0 0 0];

internalField   uniform 0;

boundaryField
{

    substrate
    {
        type            totalPressure;  // zeroGradient; has also been attempted
        p0              uniform 0;
        U               U;
        phi             phi;
        rho             rho;
        psi             none;
        gamma           1;
        value           uniform 0; 
    }

    atmosphere
    {
        type            totalPressure;
        p0              uniform 0;
        U               U;
        phi             phi;
        rho             rho;
        psi             none;
        gamma           1;
        value           uniform 0;
    }

    defaultFaces
    {
        type            empty;
    }
}

FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      alpha;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 0 0 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    substrate
    {
        type            constantAlphaContactAngle;
        gradient        uniform 0;
        limit           none;
        theta0          163;
        value           uniform 0;
    }

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

    defaultFaces
    {
        type            empty;
    }
}

Now I've tried this for 90 & 100 degrees and it works fine, but with 163 degrees alpha1 becomes strongly non-physical and this appears to be the case irrespective of Courant number conservativeness, the number of alpha sub cycles and (in the following case) very strict tolerance criteria. See below;

Courant Number mean: 0.00770252 max: 0.487324
Interface Courant Number mean: 0.000653673 max: 0.474258
deltaT = 1.13562e-05
Time = 0.0803864

MULES: Solving for alpha1
Phase-1 volume fraction = 0.00347786  Min(alpha1) = -0.353626  Max(alpha1) = 1.00404
MULES: Solving for alpha1
Phase-1 volume fraction = 0.00347586  Min(alpha1) = -0.350839  Max(alpha1) = 1.00402
MULES: Solving for alpha1
Phase-1 volume fraction = 0.00347386  Min(alpha1) = -0.347173  Max(alpha1) = 1.00401
MULES: Solving for alpha1
Phase-1 volume fraction = 0.00347187  Min(alpha1) = -0.345021  Max(alpha1) = 1.00399
MULES: Solving for alpha1
Phase-1 volume fraction = 0.00346987  Min(alpha1) = -0.346423  Max(alpha1) = 1.00398
DICPCG:  Solving for p_rgh, Initial residual = 0.013328, Final residual = 9.31482e-13, No Iterations 257
time step continuity errors : sum local = 1.35985e-15, global = 4.94506e-17, cumulative = -7.33426e-11
DICPCG:  Solving for p_rgh, Initial residual = 0.0013377, Final residual = 9.42321e-13, No Iterations 235
time step continuity errors : sum local = 1.38612e-15, global = -2.03524e-17, cumulative = -7.33426e-11
DICPCG:  Solving for p_rgh, Initial residual = 0.00022603, Final residual = 9.38786e-10, No Iterations 164
time step continuity errors : sum local = 1.38035e-12, global = 3.02764e-14, cumulative = -7.33123e-11
ExecutionTime = 926.66 s  ClockTime = 1033 s

There are no issues when the droplet is free-falling, only during surface contact and thereafter.


Could anyone advise me on what course of action to take? Am I misplaced in thinking that interFoam is suitable for this problem, and if so, is there a better choice of solver for this case? Incidentally, I have tried more diffusive upwind divergence schemes without success either, below are my present schemes;

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 limitedLinearV 1;
    div(phi,alpha)  Gauss vanLeer;
    div(phirb,alpha) Gauss interfaceCompression;
}

laplacianSchemes
{
    default         Gauss linear corrected;
}

interpolationSchemes
{
    default         linear;
}

snGradSchemes
{
    default         corrected;
}

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


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

Any help would be greatly appreciated. Thanks.