Rob
Ansys Employee

As the rotation speed increases so does the axial effect on the generated vortex: look up toroidal vorticies.  Is the tip speed really Mach 2? Or do the operating conditions keep it subsonic? 

Cell count is just that, how many cells between the rotor tip and the casing? I can model something that's well refined with 10k cells, or excessively coarse with 60M: it's all a question of scale and how I place my sizing & elements. 

Reverse flow is common, but you need to be careful if it alters the result. Ideally, the domain is extended to prevent this happening but otherwise the domain is extended far enough that the reverse flow can't alter the flow in area we're interested in.