Numerical Investigation of the Influence of Shear and Thermal Stratification on the Wind Turbine Tip-Vortex Stability

The interaction between wind turbine wakes and atmospheric turbulence is characterised by complex dynamics. In this study, two major components of the atmospheric boundary layer dynamics have been isolated, namely, the mean velocity profile shear and the thermal stratification, to examine their impact on the near-wake development by undertaking a series of highly resolved large-eddy simulations. Subsequently, instantaneous flow fields are extracted from the simulations and used to conduct Fourier analysis and proper orthogonal decomposition (POD) and compute the mean kinetic energy fluxes by different POD modes to better understand the tip-vortex instability mechanisms. Our findings indicate that shear can significantly affect the breakup of the wind turbine tip-vortices and the shape and stable length of the wake, whereas thermal stratification seems to only have limited contribution to the spatial development of the near-wake field. Finally, our analysis shows that the applied perturbation frequency determines the tip-vortex breakup location as it controls the onset of the mutual inductance instability.