Calculation of turbine rotating speed
 

Hi,

How should I set a simulation in order to be able to see what is the resulting rotational speed of a wind turbine at a certain wind velocity?

I would measure the velocity at on point on the turbine blade, say tip, for example, due to the non-slip condition, the velocity (not averaged velocity) of the fluid on the first prism layer on the blade surface would be the velocity of your blade. That would be your linear velocity and then you can convert that into rotational velocity and thus RPM.

Set the wind speed at the inlet boundary and the pressure at the outlet boundary or outlet at the outlet boundary. Generate a fan angular speed report and display it on the working area. Add loft velocity and pressure fill. :)
Set the initial wind speed in the block itself equal to the wind speed at the entrance border. Use a rate constant if you don't understand the other option.