OF-extend: area-averaged pressure drop across one patch and one user defined surface

jgross · February 19, 2018, 15:03

Hi people,

I am trying to calculate the head of a 2D pump simulation and therefore need to calculate the pressure drop between two surfaces. One of the surfaces is a patch and the other will be a user defined surface, which is some distance from the outlet (see the first figure).

As I said in the title, I am using OF-extend, and therefore libFieldFunctionObjects doesn't seem to be available i.e. surfaceRegion isn't available.

Originally, I had split the casing into two regions, one for the main volute and another for the extension and there was a grid generalised interface (GGI) that connected the two (see the second figure). This meant that calculating the pressure difference was incredibly easy as I could use patchAverage over both of the desired patches. However, this is not a good way to proceed as there is no need for a GGI there, as both regions are stationary.

I could attempt to create an empty patch in the same region so I could perform the calculation the same, however this doesn't seem like the correct way to proceed and indeed might make the results a little less accurate.

I have tried two different methods:

1. Create a faceZone in that region in order to use a swakExpression to calculate it.
This was done by specifying a faceZone (see the third figure) in the following way:

Code:

faceSet outlet new boxToFace (0.09 0.11 0)(0.2 0.1 0.002)
setsToZones -noFlipMap

but this faceZone doesn't contain any points

Code:

outlet
{
    type faceZone;
    faceLabels      0();
    flipMap         0();
}

and so the calculation obviously doesn't work.

2. Create a cuttingPlane surface there and get the pressure field on that "surface" (it's actually a line in the case of a 2D simulation).
This was done by defining a function in the controlDict

Code:

    // Output the pressure at the inlet and the outlet
    cuttingPlane
    {
        type surfaces;
        functionObjectLibs ("libsampling.so");
        outputControl timeStep;
        outputInterval 100;

        setFormat foamFile;
        surfaceFormat vtk;
        interpolationScheme cellPointFace;

        fields (p);

        surfaces
        (
              outlet
              {
                  type cuttingPlane;
                  planeType pointAndNormal;
                  pointAndNormalDict
                  {
                      basePoint (0.15 0.2 0);
                      normalVector (0 1 0);
                  }
                  interpolation true;
              }
        );
    }

The output from this is a number of pressure values along the cuttingPlane. However, I would like the output to be an area-averaged pressure.

Does anyone know an easier way of doing this?

Thanks.
J