Effects of turbulence inhomogeneity and atmosphere stability on the aeroelastic response of the IEA 22 MW wind turbine

The characteristics of turbulence spectra, coherence, and the mean vertical wind speed profile depend on atmospheric stability conditions. Inhomogeneity — defined here as the variation of turbulence characteristics with height — also depends on atmospheric stability. The current wind turbine design standard, IEC 61400-1, recommends using site-specific turbulence spectra and coherence for load calculations. However, the IEC standard assumes homogeneous turbulence fields, where both the velocity spectra and variances remain constant over the rotor-swept area. For the IEA 22 MW turbine, whose blades operate between heights of 28 m and 312 m, this assumption of homogeneity is weak. We propose a theoretical approach to model inhomogeneous turbulence using the Mann spectral tensor model, with parameters adjusted to match turbulence measurements from sonic anemometers mounted on a 250-meter mast at Østerild, Denmark. We then evaluate the effects of turbulence inhomogeneity and atmospheric stability on the aeroelastic response of the IEA 22 MW wind turbine, considering full Design Load Case (DLC) 1.2 specified by the IEC standard. Compared to homogeneous turbulence fields, inhomogeneity generally results in higher tower loads in most mean wind speeds. Atmospheric stability exerts a great influence on the aeroelastic turbine response. Even when turbulence intensity levels are matched, tower loads differ significantly when comparing responses based on site-specific spectra and coherence parameters versus those using the standard IEC Kaimal model. This work highlights the importance of performing site-specific spectra and coherence analyses for accurate load validation of large-rotor wind turbines.