[Phil_]

Hi Foamers,

I'm having trouble with the directionMixed bc at the outlet.

The intention was to get a "straight out" velocity outlet. For that the directionMixed bc was configured to (hopefully) set the normal velocites at the outlet to a zeroGradient and the tangential velocities to a zero fixedValue. The pressure is fixedValue 0.

With the normal patch vector being n = (0 -1 0):

        type            directionMixed;
        refValue        uniform         (0 0 0);
        refGradient     uniform         (0 0 0);
        valueFraction   uniform   (1 0 0 0 0 1); //(XX XY XZ YY YZ ZZ)
        value           uniform         (0 0 0);

This setup leads to strange velocities adjacent to the boundary (see pictures). Any ideas?

The case is a simple 2D box with laminar flow, solved by simpleFoam.

Best regards
Philip

[Phil_]

Hey,

I have still no idea what's causing this effect

As a test I set the inlet and outlet BC's for the velocity to a fixedValue (I know that's not correct), and the result looks the same (strange high velocities near the outlet).

So maybe the combination of boundary conditions with directionMixed is wrong?

/*--------------------------------*- 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
{
    inlet
    {
        type            fixedValue;
        value           uniform (0.1 0 0);  
}
    
    outlet
    {
        type            directionMixed;
        refValue        uniform       (0 0 0);
        refGradient     uniform       (0 0 0);
        valueFraction   uniform (1 0 0 0 0 1);
        value           uniform       (0 0 0);
    }
    
    wall
    {
        type            fixedValue;
        value           uniform (0 0 0);
    }
    
    frontAndBack
    {
        type            empty;
    }
}


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

/*--------------------------------*- 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       volScalarField;
    object      p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField   uniform 0;

boundaryField
{
    inlet
    {
        type            zeroGradient;
    }

    outlet
    {
        type            fixedValue;
        value           uniform 0;
    }

    wall
    {
        type            zeroGradient;
    }

    frontAndBack
    {
        type            empty;
    }
}

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

Best regards

Philip