Investigation of Wind Turbine Wakes in Complex Terrain at the WINSENT Test Site Using UAS Measurements

Wind energy plays a key role in achieving carbon-neutral power generation, yet its deployment in complex terrain remains challenging. The WINSENT (Wind Science and Engineering Test Site in Complex Terrain) research facility addresses these challenges by operating research wind turbines in complex terrain.
The test site is located on the Swabian Alb near Stuttgart, Germany, in close proximity to a steep, forested escarpment that influences the local flow conditions. It is equipped with two research wind turbines (RWTs) and four meteorological masts with heights of 100 m. Unlike purely commercial turbines, the research turbines are operated under full experimental control, permitting deliberate activation and shutdown of the turbine and enabling wake studies under well-defined operating conditions.
Additional observation is provided by the University of Tübingen through campaign-based in-situ measurements using multicopter uncrewed aircraft systems (UAS). The UAS are simultaneously deployed at strategic locations, including the upstream inflow and multiple horizontal distances downstream of the turbines. They resolve turbulent structures down to sub-metre scales, allowing detailed investigation of flow variability, terrain-induced influences, flux measurements, turbulent kinetic energy (TKE), and mean wind statistics.
An extensive investigation of RWTs’ wake formation and horizontal and vertical structure is presented during multiple simultaneous UAS measurements. Despite the high surface roughness and the strongly heterogeneous flow conditions induced by the present complex terrain, turbine wakes can be clearly identified from the ultra-near-wake region at distances as close as 20 m downstream of the rotor, as well as at downstream locations corresponding to one-, two-, and three-rotor-diameter distances, with maximum observed wind-speed deficits reaching approximately one third of the inflow wind speed. Measurements acquired during turbine operation and under powered-off conditions are compared, revealing pronounced differences in wake structure, turbulence levels, and wake recovery, and confirming that the observed wind-speed deficits are primarily turbine-induced.