Dynamic process and dynamic-stall phenomenon on blade sections of a floating horizontal-axis wind turbine caused by the platform's pitch motion

Abstract

Floating horizontal-axis wind turbines (FHAWTs) experience six degrees of freedom motion in space with their floating platforms. Such dynamic process may induce complex dynamic flow and variable local load on the blades, however, still insufficiently understood from a perspective of unsteady aerodynamics. In this study, the local aerodynamic load in the dynamic process induced by the platforms’ pitch motion is investigated by computational fluid dynamics. Our results indicate that the axial force of each blade section fluctuates significantly over time, with varying levels of fluctuation among the blade sections. The pressure distributions around the blade sections also vary significantly. Additionally, we observe the light dynamic stall phenomenon that cause stronger lift in upstroke and weaker lift in downstroke, and the flow separation and reattachment process during the dynamic stall demonstrates a delayed response relative to instantaneous changes in the angle of attack. The load variations due to the dynamic process and dynamic stall can lead to severe fatigue and extreme loads on the surface and structure of the blades. These findings suggest that quasi-steady analysis, which neglects the dynamic process, is not sufficient for accurately predicting the blade loads under pitch motion. Furthermore, we find that the fatigue-load properties vary with location on a blade section, with the leading and trailing edges being more susceptible to fatigue problems. These results highlight the importance of considering the potential harmful effects of pitch motion in the design of FHAWTs.