Large eddy simulation of offshore wind turbines interacting with waves using the spectral element method*

Offshore wind is attracting increasing attention as a renewable energy source to meet the decarbonization goal. Winds offshore typically feature higher speeds than on-land and thus possess a higher power generating potential. However, offshore wind turbine aerodynamics is usually complicated by ocean waves and the coupling of ocean waves and surface winds. Computational fluid dynamics (CFD) simulations provide a unique tool to directly probe complex flow structure and dynamics in offshore wind turbine systems, thus improving the physical understanding and designs of turbines with higher stability and efficiency. In this study, we perform large eddy simulations (LES) using a spectral element CFD code Nek5000 for an offshore wind turbine interacting with prescribed ocean waves. The use of the high-order spectral element method ensures capturing the highly transient vortex structures. An actuate line model (ALM) is used to reproduce the aerodynamic forces from the turbine blades without the need to actually resolve the boundary layers around the blades. A log-law based wall model is used for the atmospheric boundary layers (ABL) over the ocean surface. The spectral element LES-ALM modeling framework is first validated on the NREL-5MW wind turbine. The spectral element simulation shows good agreement with the reference data even with a coarser grid size. The validated code is then applied to simulations with flat terrain and moving wave, respectively. Effects of both ABL and moving wave are investigated in detail, focusing on aerodynamic properties along the actuate line, turbine efficiency, and wake dynamics. The results from this study will shed lights on wind/wave/turbine coupling in offshore wind turbine systems. The high-fidelity data generated can further be used to inform low-order models for design optimization.