Impacts of upstream wind turbine wakes over hilly terrain on the fatigue loads and power output of downstream wind turbines

This study employs high-fidelity offline-coupled SOWFA-OpenFAST simulations to investigate the aerodynamic loads, fatigue loads, and power performance of downstream wind turbines operating under the combined influence of terrain-induced flows and upstream wind turbine wakes. A range of terrain conditions, characterized by different terrain-to-turbine scale ratios and surface roughness, is considered herein to elucidate the governing mechanisms of terrain-turbine wake interactions and their influence on downstream turbine performance. The results show that terrain significantly alters inflow characteristics through flow acceleration, separation, and wake deflection, thereby amplifying aerodynamic load fluctuations with increasing terrain-to-turbine scale ratio. Conversely, higher surface roughness enhances turbulent mixing and attenuates wake flow deflection, leading to reduced load fluctuations of the downstream turbine. The downstream turbine experiences pronounced fatigue load amplification in the near-wake region, while elevated surface roughness promotes wake recovery and mitigates fatigue in the far wake. Moreover, the tower exhibits greater sensitivity to unsteady terrain-turbine wake coupling effects compared with the blades. Terrain-induced acceleration near hilltops partially offsets the upstream wake deficit, but increased terrain-to-turbine scale ratio results in reduced power outputs. Overall, the findings highlight that terrain-to-turbine scale ratio and surface roughness jointly modulate the aerodynamic performance of downstream turbines, providing valuable insights for optimized turbine siting and fatigue mitigation strategies in complex terrains.