The principal reason for employing a moving reference frame is to render a problem which is unsteady in the stationary (inertial) frame steady with respect to the moving frame. For a steadily rotating frame (i.e., the rotational speed is constant), it is possible to transform the equations of fluid motion to the rotating frame such that steady-state solutions are possible. By default, FLUENT permits the activation of a moving reference frame with a steady rotational speed. If the rotational speed is not constant, the transformed equations will contain additional terms which are not included in FLUENT's formulation (although they can be added as source terms using user-defined functions). It should also be noted that you can run an unsteady simulation in a moving reference frame with constant rotational speed. This would be necessary if you wanted to simulate, for example, vortex shedding from a rotating fan blade. The unsteadiness in this case is due to a natural fluid instability (vortex generation) rather than induced from interaction with a stationary component.
In Section 10.2.2, the equations for a steadily rotating frame will be described.