Effect of coupled surge–pitch motions on the wake characteristics of floating offshore wind turbines under atmospheric boundary layer inflows

This study presents a numerical investigation of floating offshore wind turbines (FOWTs) subjected to coupled surge–pitch motions under atmospheric boundary layer (ABL) inflows. Large eddy simulation combined with the actuator line model is used to simulate FOWTs, with a focus on analyzing aerodynamic performance and wake characteristics. Results indicate that platform motion has a limited impact on time-averaged power and thrust but causes pronounced fluctuations in instantaneous power and thrust. Compared to fixed-bottom wind turbines, FOWTs exhibit faster wake recovery under ABL conditions, particularly within 6D (D represents the rotor diameter) downstream. Gaussian fitting shows that FOWTs with larger platform motion amplitudes and higher frequencies exhibit smaller standard deviations and larger centerline displacements. Phase-averaged vorticity analysis reveals that platform motion induces periodic vortex structures near the shear layer, which remain coherent within the first 4D downstream. Dynamic mode decomposition analysis is performed to examine the unsteady evolution of FOWT wakes. The results show that the dominant modes are primarily influenced by inflow turbulence, while platform motion excites response modes with periodic spatial structures. Although the energy contribution of these motion-induced modes is generally limited, low-frequency motions tend to induce higher-energy modes and more pronounced wake responses. These findings provide valuable insight for analytical wake models, reduced-order modeling, and future studies on complex platform motions and multi-turbine interactions.