Transitional aerodynamics, operating states and intermittency in floating offshore wind turbines under wind-wave conditions

Floating offshore wind turbines experience platform motions that can trigger aerodynamic-state transitions. This study develops an operating-state-centred, high-fidelity computational fluid dynamics (CFD) framework by establishing an angle-of-attack (AOA) evaluation model and direct velocity-based criteria to identify the vortex ring state (VRS) and propeller working state (PWS). The framework is applied under representative coupled wind-wave conditions to examine operating-state evolution, intermittency, aerodynamic responses, and wake behaviour. The results show that: (i) the proposed AOA-velocity framework enables direct identification of transient VRS and PWS under coupled platform motions; (ii) adverse behaviour is strongly spanwise non-uniform and concentrates over the mid- to outer-blade region, where the probability of transient AOA exceeding the critical stall angle reaches approximately 0.5 and intermittency is most pronounced; (iii) integral responses are not synchronous with state transitions, with thrust exhibiting a near-invariant lag of approximately −1s and tangential-force intermittency exceeding that of the normal force; and (iv) stronger excitation drives more severe wake reorganisation, with intensified deficit fluctuations, a hub-height fluctuation intensification up to 0.44, and an upstream shift of vortex breakdown from x/D=4.7–5.5 to x/D=3.7–4.7. These findings provide a clearer physical picture of aerodynamic-state transitions, their non-uniform development, and their delayed reflection in integral loads and wake behaviour under realistic offshore conditions.