[simpomann]

Hey,

I am sorry to bring this up again, but I experience problems with the described way of calculating the average mag(U) over a plane.


I have a simple pipe (flow is 12m/s with air, laminar, no turb model yet) and added the following to my controlDict:

functions
{
    makeMagU {
       type expressionField;
         functionObjectLibs
        (
            "libswakFunctionObjects.so"
        );      autowrite false;
       fieldName magU;
       expression "mag(U)";
    }
    makerho {
       type expressionField;
         functionObjectLibs
        (
            "libswakFunctionObjects.so"
        );      autowrite false;
       fieldName rho;
       expression "1.199";
    }

messung_40d
    {
        type            faceSource;
        functionObjectLibs ("libfieldFunctionObjects.so");

        enabled         true;
        outputControl   outputTime;

        // Output to log&file (true) or to file only
        log             true;

        // Output field values as well
        valueOutput     true;  //true;

        // Type of source: patch/faceZone/sampledSurface
        source          sampledSurface;

        sampledSurfaceDict
        {
                type cuttingPlane;
                planeType           pointAndNormal;
                pointAndNormalDict
                {
                        basePoint       (0.185 0 0);  
                        normalVector    (1 0 0);
                }
                source cells; // sample cells or boundaryFaces
                interpolate true;
        }
        // Operation: areaAverage/sum/weightedAverage ...
        operation       areaAverage;

        fields
        (
            p
            U
        magU
        );
    }

messung_48d
    {
        type            faceSource;
        functionObjectLibs ("libfieldFunctionObjects.so");

        enabled         true;
        outputControl   outputTime;

        // Output to log&file (true) or to file only
        log             true;

        // Output field values as well
        valueOutput     true;  //true;

        // Type of source: patch/faceZone/sampledSurface
        source          sampledSurface;

        sampledSurfaceDict
        {
                type cuttingPlane;
                planeType           pointAndNormal;
                pointAndNormalDict
                {
                        basePoint       (0.281 0 0);
                        normalVector    (1 0 0);
                }
                source cells; // sample cells or boundaryFaces
                interpolate true;
        }
        // Operation: areaAverage/sum/weightedAverage ...
        operation       areaAverage;

        fields
        (
            p
            U
        magU
        );
    }


}

And I get an output over my plane (cut through the pipe after 40 diameters, flow is straight x-directional):

# Source : sampledSurface sampledSurface
# Faces  : 204
# Time    sum(magSf)    areaAverage(p)    areaAverage(U)    areaAverage(magU)
100    0.000104042    -0.511471    (14.8039 0.00143989 0.0096608)    14.804
200    0.000104042    -1.07848    (15.1898 -0.002704 0.00862669)    15.1898
300    0.000104042    -1.10052    (15.2711 -0.002448 0.00478738)    15.2711
400    0.000104042    -1.13157    (15.2688 -0.00338944 0.00642759)    15.2688
500    0.000104042    -1.05324    (15.2643 -0.00676656 0.00863005)    15.2643
600    0.000104042    -1.30854    (15.2804 -0.00189827 0.00590263)    15.2804
700    0.000104042    -1.30885    (15.2774 -0.00181508 0.00626128)    15.2774
800    0.000104042    -1.29363    (15.2785 -0.00386213 0.00488485)    15.2785
900    0.000104042    -1.29393    (15.278 -0.00271772 0.00544641)    15.2781
1000    0.000104042    -1.28934    (15.2778 -0.00274002 0.00545235)    15.2781

If I load the case into paraView, slice at the same distance, extract and integrate variables (cell values) I end up with a more reasonable mag(U) average of 12.009.

What did I do wrong here?
I thought my lines to the control dict would just create a field mag(U) out of the calculated U vector field and then place a slice through it and calculate an average.
Is it possible that the SWAK-code leaves out cells with a mag(U) value of 0 for some reason? My mesh is pretty rough, so a high percentage of the cells actually has the wall boundary condition of U = (0 0 0).

I found out that my average values for pressure seem to be perfectly fine (SWAK sampling and manual paraView pp lead to the exact same numbers)!
So its only a mag(U) thing here...