The Three Characteristic Frequency Responses to the Nonsteady Forcing of Wind Turbines by Atmospheric Turbulence*

Strongly time-varying spatially-nonuniform motions within the atmospheric winds drive large time changes in the force and moment vectors acting at the rotor hub on the wind turbine main shaft. In turn, these drive strong time changes in force acting on the main bearing, potentially underlying premature failure. The strongest nonsteady responses arise from the continual passage of energetic streak-like turbulence eddies in the atmospheric surface layer (ASL). With high-fidelity LES and advanced actuator line representation of rotor blade aerodynamics within a convective daytime ASL, we analyze the detailed frequency responses of the out-of-plane bending moment vector acting on the rotor hub (M_H) and the corresponding force vector acting on the main bearing of a rigid NREL 5 MW rotor (F_B). We find that the strongest responses result from combined interactions of three frequency ranges, each with different forcing mechanisms and response characteristics. The three-blade-per-revolution (3P) response results from blade rotation through a nonuniform velocity distribution. This strong quasi-periodic response is modulated by the advection of concentrations of high and low speed horizontal velocity fluctuations (“streaks”) at advection time scales (10s of seconds) much higher than the 3P period (seconds), creating lower-frequency responses in magnitude and direction of M_H and F_B. In contrast, we observe large peak-to-peak responses at small sub-second time scales created by the passage of rotor blades through local strong gradients in the velocity field over the rotor plane. These, we find, occur in bursts that are modulated by the 3P frequencies. Thus all three characteristic frequency responses interact in the generation of the strongest, potentially most detrimental, forcings on the main bearing.