Thank you for your reply and conducive advice.
In my humble opinion, the inflow angle would remain 90 degree when the rotor is parked, therefore the lift force is perpendicular to the wind velocity so as to the RotThrust (if the shaft tilt angle is 0), and the lift force will not affect the magnitude of RotThrust.
Following your precious advice, I drew the PSD of RotThrust of a parked wind turbine under 35m/s wind speed and different blade pitch angles (blue curves in the following Figures).
For a comparison purpose, I investigated the RotThrust of a rotor with rigid blade (flap and edge-wise mode disabled), whose PSD (red curves) is plotted together with the previous set of PSD.
As shown in the following graphs:
* Both the red and blue curves are peaked around the 1st and 2nd tower frequency, which is 0.32Hz and 2.9Hz (marked by 2 vertical red lines through all the graphs)
* In addition to the aforementioned two peaks, the PSD of a flexible rotor (blue curves) are significantly excited around 1.10Hz when the pitch angle is larger than 50 degree.
By checking the natural frequency of a 5MW turbine in the manual, the blade may be excited in a mode of '1st Blade Asymmetric Edgewise Pitch (1.07Hz)' or '1st Blade Asymmetric Edgewise Yaw (1.08Hz)', which is understandable because it corresponds to a high blade pitch angle and the blade may vibration edgewise.
However, I still have some doubts and want to hear your advice~
* Why the blade mode would be significantly excited only when blade pitch angle is high?
(Since the incoming turbulence for all cases are unchanged, and the 'Kaimal Spectrum' seems to have no peak around 1.10Hz, so I would expect all the PSD bears similar trends).
* Have I ignored anything that is significant to account for this phenomenon? And through what theory can I derived similar results as shown in the bottom 2 pictures?