Aerodynamic dissipation effects on the rotating blades of floating wind turbines

Due to the flexibility of their mooring systems, floating wind turbines are susceptible to large oscillations, which may compromise their performance and structural stability. Dissipation effects from wind-blade interactions and other sources are thus an important factor. In the design process the aerodynamic effects are often simulated through a quasi-static approach, whose accuracy is not guaranteed. In this study we numerically examine the aerodynamically generated added-mass and damping effects on the blades using a quasi-static blade-element method and an unsteady boundary-element method. The results based on unsteady simulation suggest that there exists a phase shift in the aerodynamic force as frequency increases, causing a switching from dissipation-dominated behavior in low frequency to a mixture of dissipation and inertia effects in high frequency. This is consistent with predictions via a simplified model with Theodorsen's theory. The quasi-static method, on the other hand, cannot predict this potentially important phenomenon. We also show that compared with other dissipation effects such as the wave radiation damping and the damping from the mooring system, the aerodynamic damping is smaller in magnitude and thus negligible in the responses of the platform itself. Nevertheless, its effect on the structural vibration of the tower may still be significant.