A7 – Interaction of atmospheric boundary layers and oceanic mixed layers with very large turbines in offshore wind farms

For the safe, economical, system friendly and ecologically compatible operation of future offshore wind turbines and wind farms in the multi-GW range, a thorough understanding of their interaction with the atmosphere and the ocean becomes crucial. Thus, the design and operation of very large offshore wind energy systems require a thorough characterisation and modelling of ‘challenging’ atmospheric conditions and the interaction of wind farms with the atmosphere and the ocean. ‘Challenging’ atmospheric conditions refer to phenomena and potential load situations, which might become important simply due to the giant diameter of future wind turbine rotors and the spatial extension of wind farm clusters. At present, they are not considered in the design of either wind turbines or wind farms, and their interactions with wind energy systems are not sufficiently investigated or understood.

Subproject A7 addresses atmospheric and partly oceanographic phenomena at different scales that become important for large offshore wind turbines and wind farm clusters. Low-level jets (LLJ) and coherent structures such as roll convection might directly affect the design of large wind turbines. Non-stationarity of the mesoscale atmospheric flow (i.e., several tens of kilometres to several hundreds of kilometres) could alter and deflect wind farm cluster wakes, which constitute the inflow and design conditions for megastructures. Furthermore, wind farm wake effects can potentially affect the ocean surface currents and feedback to the atmosphere via the sea surface-wave interface. Hence, the magnitude and the consequences of these effects on power performance and loads, on the wake effects in wind farm clusters, and the potential impact on the ecosystem (e.g., change of stratification in the ocean) warrant further investigation. Analysing the research questions, enhanced experimental and numerical approaches will be integrated with lidar (light detection and ranging) measurements in an offshore wind farm cluster and state-of-the-art turbulence-resolving large-eddy simulations (LES). This includes a volumetric, highly temporally and spatially resolved wind field reconstruction technique from scanning lidar and coupled LES-LES simulations of the atmospheric boundary layer and oceanic mixed layer, as well as their interaction with very large wind farm.

During the project period a comprehensive offshore multi-lidar measurement campaign will be carry out and analysed. At the same time, the dynamic volumetric wind field reconstruction will be developed and validated by virtual measurements in LES and accurate measurements in the DLR research wind farm WiValdi. In the numerical part of the project, the interaction of the atmospheric boundary layer and oceanic mixed layer with wind turbine structures ranging from the seabed to the blade tips will be modelled by coupled LES-LES simulations of large wind farms in which the exchange between atmosphere and ocean via the sea-surface-wave interface is also explicitly considered. After that, the ‘challenging’ atmospheric conditions and their interaction with wind turbines will be investigated using lidar measurements, operational wind farm (SCADA) data and comprehensive LES.

The expected outcome of the subproject is a characterisation, statistical extrapolation, and assessment of the relevance of the investigated phenomena for offshore megastructures, exemplified at the hybrid-lambda reference wind turbine and wind farm clusters. Measurement and LES data related to ‘challenging’ atmospheric conditions will be synthesised for improved inflow and wake flow characteristics. Parameterised wind fields will be provided to subproject A1 for further use in the digital twin (Z1). The subproject  A7 is part of the cluster "Wind and wave interactions" and sees major interactions with the clusters "Rotor design" and "Digital twin".