Flapwise moment mean and amplitude

Dear all,

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,

Dear Neelabh,

I believe the results you show make sense. For a conventional blade-pitch-to-feather-controlled wind turbine, the mean value of the thrust will peak at rated wind speed and then drop again for higher mean wind speeds. However, the standard deviation of wind speed typically increases with mean wind speed (even though the turbulence intensity – the ratio of the standard deviation of wind speed to mean wind speed – drops with increasing mean wind speed). Thus, the standard deviation of the wind turbine loads/response (and corresponding fatigue) will increase with increasing mean wind speed. This is the reason why wind turbines typically have a cut-out wind speed beyond which the turbine will shut down.

Best regards,

Thanks Jason :slight_smile: