[Tom Lauriks]

Hi foamers,


I'm performing an LES of channel flow. My domain is a box, with the boundaries perpendicular to x or z periodic. The boundaries perpendicular to y are walls with a no slip condition.


I want to calculate derived variables by extracting data with function objects. I need the average x velocity component, u, over planes parallel to the walls. I need, e.g., u_avg on a plane with y=0 (on the wall) and with y=0.00456 (to calculate the gradient). However, u_avg on the plane on the wall is not zero. I integrated u over the same plane in paraview and this results in zero.



I'll paste the code of my function object below. Anyone any ideas what is going on?


In the function dict in my controlDict:

planeBottom1
{
    type            surfaceFieldValue;
    functionObjectLibs ("libfieldFunctionObjects.so");
    enabled         true;
    writeControl   timeStep;
    writeInterval  10;
    log             true;
    writeFields     false;
    regionType          sampledSurface;
    name      dummy;
    sampledSurfaceDict
    {
        type plane;
        planeType       pointAndNormal;
        pointAndNormalDict
        {
            basePoint       (0 0 0);
            normalVector    (0 1 0);
        }
    }
    operation       areaAverage;
    fields
    (
        U
    );
}

The output of this function object:
# Region type : sampledSurface dummy
# Faces : 2048
# Area : 4.500000e+00
# Time areaAverage(U)
0.0311504 (1.868055e-01 1.709084e-17 -1.899840e-29)
0.224026 (1.045627e-01 -4.220888e-14 -6.870517e-22)
0.905977 (5.695619e-02 3.788209e-10 -2.763147e-16)
1.284177612133 (3.842192e-02 1.437856e-11 -4.871026e-14)

[Tom Lauriks]

I found a solution to my problem. I specified to interpolate in the sampledSurfaceDict:

sampledSurfaceDict
{
type plane;
interpolate true;
planeType pointAndNormal;
pointAndNormalDict
{
basePoint (0 0.0 0);
normalVector (0 1 0);
}
}

The output is now:
# Region type : sampledSurface dummy
# Faces : 2048
# Area : 4.500000e+00
# Time areaAverage(U)
0.0012 (5.338272e-19 -4.697408e-40 -1.106682e-40)
0.00264 (5.323610e-19 6.582011e-33 -4.653934e-36)
0.004368 (5.306123e-19 -6.219411e-34 -1.794711e-35)


This is not what I expected though. I expected the cell values adjacent to the wall to be zero, so I didn't expect to need interpolation there. Is this a bug, or am I missing something?

[Krapf]

Since the values are calculated in the cell center, they are not equal to 0.

[Tom Lauriks]

I was assuming that in the finite volume method, there exists only the cell value. And that therefore the value of the boundary condition, is imposed at the cell next to the wall. But it's good to know the boundary condition is imposed at the boundary faces and that the cell values at the boundary cells are still calculated. Thanks for your answer.