The aerodynamics of a blade pitch, rotor speed, and surge step for a wind turbine regarding dynamic inflow

Abstract

Floater motions introduce unsteadiness in the aerodynamics of floating offshore wind turbines. The aerodynamics of a wind turbine after three perturbations are studied: a blade pitch step, a rotor speed step for which dynamic inflow is expected, and a surge velocity step. The free vortex wake method and an analytical helical vortex model based on the Joukowsky rotor model are used to study the dynamic behavior of the induced velocity at the blades. As expected, the dynamic inflow effect is clear for the blade pitch and rotor speed changes, but for a surge velocity step, the models show that very little dynamic inflow effect takes place because the velocity induced by the vortex helix is not significantly modified: the tip vortex helix circulation change is partially compensated by the geometry change of the helix. For the rate of change, the velocities induced on the rotor by the vortex helix for the pitch and rotor speed changes show a rapid adjustment at the blade tip, with a slower change throughout the rest of the blade and at the center of the rotor. The convection velocity of the tip vortices is shown to be the main variable of the temporal evolution of the dynamic inflow effect.