Stability of Floating Wind Turbine Wakes
Time: Thu 2021-06-03 10.30 - 11.30
Location: FLOW eSeminar (Zoom)
Participating: Vitor Kleine (KTH)
Abstract. Floating offshore wind turbines (FOWTs) are the next frontier in offshore wind energy, allowing exploration of deep-water regions previously unavailable to fixed-foundation turbines. Since offshore turbines operate in lower turbulence levels, the intrinsic hydrodynamic unstable modes of the tip vortices can have even more relevance than in onshore turbines. For floating turbines, platform motion induced by wind and wave loads can trigger vortex instabilities, modifying the wake structure, possibly influencing the flow reaching downstream wind turbines.
We have been studying those phenomena by the means of numerical simulations and their comparison with analytical studies. In our numerical simulations, the wind turbine blades are modeled as actuator lines in the incompressible Navier–Stokes equations. For heave and surge motion with low frequency, several vortices coalesce to form a large flow structure. High amplitude of oscillations in the streamwise velocity were observed due to these flow structures, which may increase fatigue or induce high amplitude motion on downstream turbines. The number of vortices that interact, as other qualitative phenomena of the numerical simulation, are well predicted by a simple stability model of two-dimensional row of vortices. The disturbances imposed by the platform motion were also compared to the eigenvectors resulting from linear stability theory for helical vortices. The growth rates obtained numerically are compatible with the predictions of the simpler analytical models.