Blade-resolved CFD analysis of a floating wind turbine: new insights on unsteady aerodynamics, loads, and wake

The impact of platform motion on rotor aerodynamics and wake dynamics of a floating wind turbine is not yet completely understood. While recent analyses using model-scale wind tunnel experiments have provided a useful benchmark, experimental limitations do exist in the analysis of blade aerodynamics in the spanwise direction as well as of near wake dynamics. To capture these phenomena, engineering methods used to date for wake studies are not adequate and blade-resolved computational fluid dynamics (CFD) methods are needed, despite their significant calculation cost. In the study, an innovative meshing strategy is first developed in order to simulate a model wind turbine with imposed pitching motion, replicating conditions from a recent wind tunnel experiment. Then, highly refined unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations are used to perform an in-depth analysis not only of common turbine performance figures, but specifically of spanwise load oscillations, which, particularly in the outer blade region, are caused by unsteady aerodynamic response driven by rotor tilt rather than platform motion. An unprecedented analysis of the effective angle of attack is also provided, which allowed the reconstruction of the major 3D spanwise phenomena. State-of-the-art free-vortex wake simulations of the same test conditions are also performed with the software QBlade to analyze and discuss the limitations of engineering methods with respect to a blade-resolved approach. Regarding wake dynamics, URANS overpredicts velocity deficits in the near wake, possibly due to limitations in capturing the effect of free-stream turbulence; nevertheless, blade-resolved simulations effectively capture tip vortices and can serve as a benchmark for lower-fidelity models, when no experimental data is available.