Evolution within the Atmospheric Boundary Layer of Coherent Structures Generated by Wind Turbines: LiDAR Measurements and Modal Decomposition Techniques

Wind turbine wakes undergo complex dynamics varying as a function of the incoming wind conditions, which are modulated by the atmospheric static stability and terrain characteristics. Accurate evaluation of the wind velocity components over the rotor disc represent the relevant input for the wind turbine aerodynamics. Power extraction exerted by the turbines on the atmospheric boundary layer leads to the generation of wind turbine wakes, which are flow regions characterized by a velocity deficit and the presence of coherent vorticity structures evolving downstream. Therefore, wind turbulence and dynamics of these vorticity structures produced by utility-scale wind turbines need to be characterized accurately. In this work, a field measurement campaign is described where different remote sensing instruments were assessed to retrieve the wind velocity components with different levels of accuracy. This campaign provided an unique opportunity to retrieve the wind velocity components with different technologies and perform inter-comparison studies in terms of both first and second order turbulent statistics. The results show that Doppler LiDARs have capability to retrieve mean wind speed with high accuracy and turbulent fluctuations with certain limitations. Besides the development of experimental measurement techniques for probing vorticity dynamics within the atmospheric boundary layer, research endeavors were focused in performing analyses based on Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) to identify coherent structures within wind turbine wakes. Different data-sets, which include Large Eddy Simulation (LES) of single wind turbine with and without tower and nacelle were analyzed. Careful analysis enabled the detection of different instability modes of the helicoidal tip vortices, namely elliptical instability and leap-frogging due to the self rotation and mutual induction among the vortex cores.