Calculating forces on an internal faces
 

I need to calculate the forces on an internal face. for example, for flow around a cylinder with diameter D, I need to calculate the force on a cylinder with diameter 2D. However, by using the forceCoeffs library we can calculate the force on the patches only. I can convert my internal face to a faceset or zoneset. However, I do not know how I can calculate the force on the faceset or facezone.
any help will be appreciated.

Best Regards