Investigation of the dynamic stall characteristics for floating offshore wind turbine during pitch and surge motions

Abstract

Platform motions periodically alter the relative inflow velocity and blade angle of attack of floating offshore wind turbines (FOWT), making the accurate prediction of dynamic stall essential for assessing aerodynamic loads and power fluctuations. This study systematically evaluates the Beddoes-Leishman (BL) and Øye dynamic stall models for the NREL 5 MW FOWT under prescribed pitch, surge, and coupled pitch-surge motions at a frequency of 0.1 Hz. Predictions obtained using the blade element momentum (BEM) and free-vortex wake methods (FVM) were benchmarked against high-fidelity computational fluid dynamics (CFD) results. CFD-derived sectional aerodynamic data were subsequently used as inputs to the dynamic stall models to improve their predictions of sectional aerodynamic characteristics. Under the rated condition, the maximum relative deviations among the investigated models from the reference values were 4.4% for power and 8.63% for thrust. The BL model generally produced power predictions closer to the CFD results than the Øye model. Model discrepancies increased with motion amplitude and were more pronounced under surge motion than under pitch motion. Under small-amplitude pitch and surge motions, noticeable flow separation was mainly confined to approximately the inner 30% of the blade span, whereas a pitch amplitude of 4° induced significant flow separation over the entire blade. Coupled pitch-surge motion produced larger power and thrust fluctuation amplitudes than the corresponding single-degree-of-freedom motions, although the sectional flow-separation response was not a linear superposition of the individual motion effects. Using CFD-derived sectional data improved the predictions of both dynamic stall models in the inner, mid-span, and outer-span regions, while limitations remained near the blade tip. These findings clarify the applicability and limitations of engineering dynamic stall models for FOWT under platform-motion conditions.