flux through fan duct
 

I'm using actuator disks to simulate fans. My problem is now that I would like to get the flux through the fan during the calculation in order to change the forces of the actuator disk so to say "on the fly". For the moment I have an inlet (red surface in the image) and outlet, so it would be easy to calculate the flux from these boundaries, but later the fan will be situated in a closed room and then I do not have any more this possibility.
So, I think about using cuttingPlane to cut through the middle of the duct (perpendicular to the flow direction) and from U and the face areas of the triangles the calculation of the flux should be possible. sampling seems to work (sampleDict is attached), but how can I implement this in my solver (the simpleFoam solver modified to be able to use actuator disks; it's also attached)?
I already managed to get at each iteration step a new time folder in the surfaces directory containing the vectorfield data of U and the faceCentres, faces and points of the sampled surface (cuttingPlane through the fan duct).
But how can I tell OpenFOAM to read this data? Or even better, is there a possibility to implement the cuttingPlane and the flux calculation without writing timefolders in the surfaces folder?
I'm sure there's a simple solution for my problem and I would really appreciate any help.