Ansys Fluent | Blower | Mesh Motion (Unsteady)

Mesh Motion actually rotates the cell zone at each time step, unlike a Moving Reference Frame which leaves the mesh still and adds source terms instead. The two zones then slide against each other across a shared interface.

That difference decides what you can see in the results. A steady MRF gives you a performance point, averaged as if the blades were frozen. An unsteady sliding mesh captures the blade passing frequency, the pressure fluctuations at the outlet and the interaction between the rotor and the casing, which is what you need for noise or fatigue work.

The cost is the time step. It has to be small enough that the rotating zone advances only a fraction of a cell per step, so a full revolution takes many steps, and several revolutions are usually needed before the signal becomes periodic.