Wind turbine wake characteristics in various atmospheric conditions investigated with lidar measurements at WiValdi

Wind turbine wakes can significantly impact the performance of downstream turbines, reducing power generation and increasing loads. The characteristics of these wakes are heavily influenced by conditions within the atmospheric boundary layer (ABL). We investigate the interaction between wind turbines and the atmosphere with focus on the near wake region, up to 4 rotor diameters downstream. A large dataset of inflow conditions and wake characteristics comprises measurements from a nacelle-mounted Doppler wind lidar, a meteorological mast and turbine operational data. The data are collected at the research wind farm WiValdi in northern Germany. The lidar scans multiple horizontal planes to derive wake characteristics and near wake lengths, which are then analyzed across a range of atmospheric conditions. The results show that wake velocity deficits are reduced in turbulent conditions and enhanced under stable conditions. Wind veering across the rotor layer is found to correlate with increased wake deflection and vertical tilting, while a high shear exponent and potential temperature gradient which are both characteristic of the stable ABL are associated with increased lateral asymmetry of the velocity deficit. The near wake length is observed to extend on average around 2.01 rotor diameters downstream and exhibits greater sensitivity to atmospheric conditions than to turbine operational parameters. In stable conditions with low turbulence, near wake lengths can be particularly long. Further analysis will explore the asymmetry of the near wake and its vertical tilting in more detail, with complementary measurements from a second, ground-based lidar scanning vertically through the wake during a campaign to improve understanding of the three-dimensional wake dynamics.