The coupling of tower-shadow effect and surge motion intensifies aerodynamic load variability in downwind floating offshore wind turbines

Abstract

As the deployment of large-scale wind turbines expands into deeper waters, the downwind floating offshore wind turbine (FOWT) becomes an increasingly attractive prospect. Yet, the aerodynamic performance of these turbines, which is significantly impacted by both the tower-shadow effect and the complex six degrees of freedom (6 DOFs) platform motions, exhibits a complexity that surpasses that of traditional upwind floating or downwind fixed wind turbines. To thoroughly explore this complexity, this study employs computational fluid dynamics (CFD) using the unsteady, non-compressible Reynolds-averaged Navier-Stokes (RANS) method on a full-scale model of a downwind FOWT. Our investigation primarily focuses on the intertwining impacts of the tower-shadow effect and the platform's surge motion on the aerodynamic behavior of downwind FOWTs. The study reveals several key findings: the downwind FOWT exhibits comparable, if not superior, power-generation capabilities to prevailing upwind FOWTs; the surge motion significantly dictates both the fluctuation periods and amplitudes of rotor thrust and torque; the tower-shadow effect induces sudden reductions in rotor thrust and torque at a frequency three times that of the rotor rotation; and intriguingly, the magnitude of these sudden drops can be amplified or diminished by the surge motion. The maximum drop amplitude of torque reaches 120–150% the magnitude of the minimum drop amplitude during selected surge motions. This demonstrates that the coupling effects of the tower-shadow effect and surge motion add another layer of variability to the turbine's performance. In conclusion, downwind FOWTs present heightened variability in aerodynamic loads, underscoring the need to consider the potential consequences on power fluctuation and structural fatigue in turbine design and operation.