Large-eddy simulations of a utility-scale offshore wind farm under neutral atmospheric conditions

Large-eddy simulations of a utility-scale offshore wind farm were performed to study the effect of sea-surface boundary conditions on high-fidelity simulation results of turbine performance and wake recovery. The actuator line and surface models were used to model the turbine blades and nacelle, respectively. Two series of simulations were performed using wave-phase-aware and wave-phase-averaged approaches. Both simulations were compared against field LiDAR and supervisory control and data acquisition (SCADA) measurements. In the wave-phase-aware approach, the kinematics of the ocean waves were obtained using a high-order spectral method and imposed as sea-surface boundary conditions of the large-eddy simulation. This coupling was done by imposing shear stress over the bottom boundary based on wave motion and flow velocity. The wave-phase-averaged approach utilizes the log-law of the wall for sea-surface parametrization, which assumes a uniform roughness across the domain. Comparisons between the large-eddy simulations of the two approaches revealed that the wave-phase-aware model resulted in faster wake recovery compared to the wave-phase-averaged approach. Analysis of the mean kinetic energy flux indicated that the wave-phase-aware model had a flux mean-flow kinetic energy due to convection from the free flow above the wake. Furthermore, comparisons of the results of the two approaches with SCADA measurements demonstrated that the wave-phase-aware model provided greater precision in power production, with a relative error ranging from 3% to 10%. In contrast, the wave-phase-averaged approach exhibited a higher error, between 10% and 20%, attributed to the larger wind shear at the rotor.