Since Lift and Drag forces are, by definition, required to be perpendicular/parallel to the oncoming air, THEN non-perpendicular inlet air NECESSITATES a rotation of OpenFoam force results IF you want those forces to truly represent Lift and Drag forces …

That statement pretty much sums up why some of us have had trouble understanding the force results of simulation runs where we have changed the Inlet Air Angle in order to evaluate the efficiencies of our geometries at different Angles of Attack…

I have come up with three methods that allow rotation of force values from/during Simulation Runs in order that they truly represent Lift and Drag forces.

Here is a project that demonstrates those three methods of OpenFoam force results rotation.

** EDIT:** Here is a project I just finished where I was having extreme problems until I rotated the forces as above

** EDIT:** And here is an example where I think results from a simulation that varies the Inlet Air Angle have been mis-interpreted