CFD Online Discussion Forums - Get access to the coordinates of a cell for post-processing of rising bubble

Dear all, thank you very much in advance for your support.
I am trying to implement a function in my controlDict to calculate the center of mass of my bubble rising (solver interFOAM, version OF7). I managed to implement some functions for the computation of Volume integrals and weighted volume integrals, but since the quantity of interest in my case is:

$c_{x}=\frac{\int \alpha x dV}{\int \alpha dV}$

$c_{y}=\frac{\int \alpha y dV}{\int \alpha dV}$

I would like to get access to the x and y coordinates of the cells in order to obtain this quantity directly in my postProcessing folder.
So far, the changes I made are the implementation of these functions in my controlDict case:

Code:

functions

{

    volumeAverage

    {

        type            volFieldValue;

        libs            ("libfieldFunctionObjects.so");



        log             true;

        writeControl    writeTime;

        writeFields     true;



        regionType      all;

        operation       volAverage;



        fields

        (

            p_rgh

            U

        );

    }



    weightedVolAverage

    {

        type            volFieldValue;

        libs            ("libfieldFunctionObjects.so");



        log             true;

        writeControl    writeTime;

        writeFields     true;



        regionType      all;

        operation       weightedVolAverage;



        weightField     alpha.air;



        fields

        (

            U

        );

    }



 }

Do you know how to do it? If I go in the createFields.H directory of my interFoam solver and introduce new volScalarField I cannot get compile it for several errors trying different expressions for the positions (pol.(), cellI[1], etc). Any advice?

An alternative approach for the computation of the rising velocity is to calculate directly:

$u_{xb}=\frac{\int \alpha u_{x} dV}{\int \alpha dV}$
$u_{yb}=\frac{\int \alpha u_{y} dV}{\int \alpha dV}$

But in this case the output of the routine are not save as ASCII-files. Any suggestion on how to modify the outputs in order to make them post-processable in matlab for instance?

Thanks!!