| 01 Aug 2025
Investigation of onshore wind farm wake recovery with in-situ aircraft measurements during AWAKEN
Abstract. The generation of power from wind farms is crucial for achieving sustainability goals. To enhance power output and ensure network stability with an increasing share of variable renewable energy sources, improving the prediction of power output is essential. The interaction between wind farm wakes and the atmospheric boundary layer (ABL) introduces uncertainties in power production that warrant detailed investigation. The flow downwind of wind farms is characterized by a reduction in wind speed and an increase in turbulence which both vary with atmospheric conditions. During the American Wake Experiment (AWAKEN), the Technische Universität Braunschweig conducted measurement flights with a research aircraft upwind and downwind of onshore wind farms in the Southern Great Plains in Oklahoma in the USA. This study utilizes data from twenty flights conducted at approximately hub height in September 2023 to investigate the wind field variability downwind of the wind farms, and vertical profiles to observe atmospheric stratification. The flights were aligned perpendicular to the main wind direction downwind of the wind farms King Plains and Armadillo Flats. Additionally, LIDAR data from both upwind and downwind ground-based measurement sites and sonic anemometer data were used for comprehensive analysis.
Results indicate that under stable ABL conditions, the wake persists until greater downwind distances with a higher velocity deficit within the wake relative to the undisturbed flow compared to unstable stratification. In homogeneous terrain under stable conditions, wake recovery to 95 % occurs between a distance of 4.5 km and 9 km downwind of the wind farm. In the semi complex terrain characterized by shallow hills, slopes, and valleys, the wake exhibits a higher velocity deficit compared to homogeneous terrain while in some cases the wake was amplified by the terrain resulting in higher velocity deficit 10 km downwind of the wind farm compared to the measurements closer to the wind farm. The turbulent kinetic energy (TKE) and "TKE deficit" was found to be a valuable measure in understanding wakes in a semi-complex terrain, showing a clear wake recovery and formation depending on the stratification of the ABL.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Wind Energy Science. The authors also have no other competing interests to declare.
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