I am performing a loads analysis on NREL 5MW machine and I came across this plot for root flapwise moment (for a stochastic wind field generated through Turbsim):
Although, the ‘mean value’ of this bending moment is higher for wind speed of 11.7 m/s (close to rated) but their amplitude is much higher for 20 m/s case.
I don’t understand this trend as I expect rotor thrust to be maximum at rated wind speed and then decrease after this (due to shift in control system from torque to pitch control). So the mean values support this statement, but
why do bending moment’s amplitude not follow the same trend?
Would this convey, higher wind speeds have more impact on fatigue even though the mean value of moment are lower?
Can someone, please help me in understanding this.
Thanks a lot in advance,