[jvc]

I need to compute the mass flow rate for a compressible unsteady flow through an arbitrary plane in my computational domain at each time step. I can compute the mass flow rate at a boundary fairly easily using

patchName = runTime.controlDict().lookupOrDefault<word>("inlet Name","inlet");
label patchIn = mesh.boundaryMesh().findPatchID(patchName);
scalar massflow = Foam::sum(rhoPhi.boundaryField()[patchID]);
Foam::reduce(massflow, sumOp<scalar>());

My problem is how to find the cells in the domain that I want without having a patch. Is there a way to define a "fake" patch just for post-processing? I'm running a solver that is based on interPhaseChangeFoam.

Thanks for the help.