Flow SImulation's rotating regions are defined as user-input. In other words, the rotational speed is set by you.
It in no ways works in the opposite order. In fact, for a turbine model as you've mentioned, putting boundary conditions such as a prescribed mass flow rate too close to the RRF will adversely affect the RRF and invalidate your study's results.
Instead, you can set up a design study of varying prescribed rotational speeds and measure your outputs, generating a type of performance curve if you will.
You can set up a parametric study with the input parameter RPM varying and the target goal the mass flow you want.
Flow simulation will iterate the solutions until it meets the target required.
Here is an example at min 30:00