Experimental unveiling of the governing mechanisms of platform motion on floating wind turbine wakes: Analysis of turbulence, spectra, and coherent structures

Floating offshore wind turbines (FOWTs) represent a critical technology for harnessing deep-sea wind energy resources. To elucidate the complex effects of platform motions on FOWT wakes, this study simulates typical isolated (pitch/surge) and coupled platform motions through structured wind tunnel experiments, systematically revealing wake modulation mechanisms under different motion patterns. The study demonstrates that pitch motion predominantly drives large-scale lateral sweeping of the wake, while surge motion governs periodic pulsations of the velocity deficit in the wake center. Low-frequency (0.1 Hz) pitch and coupled motions significantly increase turbulence integral time scales to 0.92 s and 1.03 s in the wake core region, inducing larger and longer-lasting coherent turbulent structures. Through modulation of tip/shear layer flows, shorter pitch motion periods (5 s) generate higher peak frequencies of 58.8 Hz, whereas coupled motions produce the highest peak frequencies of 65.3 Hz. Platform motions amplify wake meandering by enhancing dominant antisymmetric modes. In the near-wake region, pitch-involved cases require only approximately 5 modes to capture 90% of the total turbulent kinetic energy, which is crucial for developing reduced-order models and predicting wake evolution under different motion patterns of FOWTs. These findings provide a critical theoretical foundation for optimizing floating wind farm design and operational control strategies.