Dear Sang.Le,
Actually, you enter the initial rotor speed in FAST. If the generator and drivetrain DOFs are disabled, the initial rotor speed becomes the constant rotor speed used throughout the simulation.
If you’re trying to find optimal values of the blade-pitch and rotor speed, you should output e.g. the power coefficient (RotCp). To calculate the maximum (optimal) RotCp for a given rotor, you could run FAST with the torque and pitch controllers disabled (with the generator DOF disabled), and run a series of simulations at different fixed rotor speeds and pitch angles at a fixed wind speed (say 8 m/s) in order to sweep a range of tip-speed ratios (TSR) and pitch angles. The maximum RotCp should be obvious by viewing the resulting surface of RotCp versus TSR and pitch.
You should only run a simulation with turbulence with a proper operational torque and pitch control system enabled (with the generator DOF enabled).
Best regards,