Assessing scale-dependent aeroelastic load responses of 5 MW and 22 MW wind turbines under varying atmospheric stability

Abstract

The upscaling of wind turbines has extended rotor-swept areas into the atmospheric boundary layer, several hundred meters above the surface layer. Traditional wind field models have relied on neutral surface-layer characteristics. Recent research emphasizes incorporating atmospheric stability and dynamics at higher altitudes, as larger turbines experience greater load variations due to eddies comparable in size to the rotor, causing non-uniform inflow. This study evaluates a measurement-validated coherence model for aeroelastic analysis of a 5 MW and a 22 MW turbine. The model characterizes atmospheric stability using tuned coefficients and is applied alongside stability-dependent spectral wind field models. Six seeds of 30 minute simulations were conducted for each wind turbine model ranging from near cut-in to near cut-out wind speed, in intervals of 1 m/s, using on one hand, the proposed coherence model with stability-dependent spectra and on the other hand, the IEC Kaimal coherence model. Aeroelastic results reveal distinct turbine responses to atmospheric stability. The trends of higher tower fore–aft bending moments in unstable conditions and lower moments in stable conditions, together with increased blade-root flapwise moments at above-rated wind speeds under stable, high-shear conditions, are captured. Wavelet analysis confirms that larger turbines face greater load variations because eddies are smaller than the rotor, whereas smaller turbines experience more uniform frequency-time correlations with hub-height velocity. Simulations with flexible and rigid blades and towers indicate that flexibility effects are secondary to eddy size and coherence. Overall, it is demonstrated that simple stability-dependent empirical coherence and spectral models can effectively replicate the commonly observed impact of atmospheric stability on wind turbine loads.