A Global POD Comparison of Surge- and Pitch-Induced Wake Dynamics of a Floating Offshore Wind Turbine

Abstract. The motion of floating offshore wind turbine platforms strongly affects wake development. Consequently, understanding motion-induced wake effects has become a key research topic, with implications for wind farm layout optimization and advanced control strategies. This work investigates the wake dynamics of floating offshore wind turbines operating under realistic atmospheric boundary layer inflow using Large-Eddy Simulations with an actuator line model. A comprehensive dataset including surge and pitch motions over a range of motion frequencies and amplitudes is analysed through a global proper orthogonal decomposition, providing a common modal basis for the consistent comparison of wake dynamics across all cases. The results identify the reduced frequency as the dominant parameter governing platform-induced wake dynamics. Although the time-averaged wake is only marginally affected under the investigated operating conditions, the coherent wake response strongly depends on the reduced frequency. Motions with a reduced frequency of f_r = 0.2 produce a negligible coherent contribution, whereas f_r = 0.5 generates energetic coherent structures that persist up to approximately 4–5 rotor diameters downstream. In contrast, f_r = 0.8 leads to a more localized response confined to the near wake. Motion amplitude primarily influences the intensity of the coherent response, while the reduced frequency controls its spatial evolution. Surge and pitch exhibit similar overall trends, although pitch produces slightly stronger coherent energy in the upper wake close to the rotor. Finally, only a limited number of POD modes are required to reconstruct the coherent response in regions where the platform induced wake structures remain well organized and spatially coherent, highlighting the potential of the proposed global POD framework for reduced-order modelling of floating offshore wind turbine wakes.