Control Volume analysis, change in momentum
 

I'm attempting to calculate values of different fields and functionObjects through an arbitrary internal surface.

For example, if I had flow around a cylinder, and I wanted to calculate the change in momentum (or any field, for that matter) from an upstream slice or surface to a downstream surface, how would I go about that in OpenFOAM's post-processing?

I've looked into faceSet, sample, and createPatch, but I'm not sure I've approached it correctly.

I've used a "power calculation" in createFields by using a surfaceScalarField: (0.5*(linearInterpolate(U)&linearInterpolate(U))+l inearInterpolate(p))*(linearInterpolate(U) & mesh.Sf()) which I believe would calculate the power at every cell. I can use patchIntegrate to use this at the outlet and inlet of the domain, but patchIntegrate can't be used on internal surfaces.

Any suggestions or tips on how to implement this would be greatly appreciated. Thanks.