Computational Analysis of Tandem Floating Offshore Wind Turbines under Coupled Pitch-Surge Motion: A Comparative Study of the NREL 5 MW and the IEA 22 MW Turbines

This study conducts a comparative CFD analysis of tandem-configured floating offshore wind turbines, in which the upstream National Renewable Energy Laboratory (NREL) 5 Megawatt (MW) and the International Energy Agency (IEA) 22 MW turbines are under coupled pitch-surge motion. Increasing pitch-surge amplitudes suppress the mean thrust of upstream turbines but enhance the thrust of stationary downstream turbines. The upstream IEA 22 MW turbine uniquely exhibits increased mean power generation with larger motion amplitudes, despite transient power losses during the downstroke phases. Compared to the upstream NREL 5 MW turbine, the upstream IEA 22 MW turbine operates at a higher angle of attack, exceeding the static stall angle, and undergoes a more severe and prolonged dynamic stall, marked by a substantially expanded flow separation zone and elevated reverse flow velocity magnitudes, particularly in the wingtip region. In contrast, downstream turbines do not show detectable dynamic stall. Although divergent wake velocity distributions are observed between the NREL 5 MW and IEA 22 MW turbines, increased pitch-surge amplitudes enhance flow velocity recovery, expanding the high-speed region and reducing the low-speed zone. Turbulent kinetic energy (TKE) levels in the wake of the IEA 22 MW turbine are decreased relative to the NREL 5 MW turbine, suggesting that dynamic blade kinematics associated with pitch-surge amplitudes improve velocity recovery through enhanced wake mixing. Furthermore, wingtip vortices coalesce into thicker three-dimensional (3-D) vortex rings as the motion amplitudes increase, with increasing downstream skewness. In the two-dimensional (2-D) planes, the vortex stripe of upstream IEA 22 MW turbine undergoes an early breakdown, interacting with the vorticity stripe of the downstream turbine to form a meandering topology. These results elucidate the physical mechanisms that govern the flow dynamics and turbine performance and provide a foundational framework for refined aerodynamic designs, the unified similarity wake model, and improved spatial configuration of wind farm arrays.