Dear Amirhossein,
We’ve had a discussion on this forum regarding how FAST calculates RotCp. Please see the forum topic found here: Betz law and RotCp. Hopefully this clarifies things a bit.
If you are interested in deriving the power coefficient curve (say as a function of TSR or pitch), you should ensure that the solution is in a steady-state condition. You should use steady/uniform wind input, not turbulent wind input.
Best regards,