Quantification and Modelling of the Dynamic Wake Effect for Floating Offshore Wind Turbines

Abstract Modelling the aerodynamic forces on floating offshore wind turbines (FOWTs) is a challenging task due to the motion of the floating platform, which result in flow transients and associated aerodynamic effects. Each of these needs to be modelled and implemented, before a complete aerodynamic model of the FOWT can be presented. Of special interest is the dynamic wake effect, a result of the time lag between rotor forces and the air flow deceleration within the wake. The dynamic wake effect may present itself significantly in several scenarios including gust loads, changing wind speeds and direction, and the oscillatory motion of the rotor due to platform motion from wave forces. The complexity of the problem is substantiated by the added mass effect, which must be accounted for when considering rotor motions through the air. Computational fluid dynamics (CFD) simulations and blade element momentum (BEM) computations have been performed for the NREL 5MW virtual wind turbine in axial flow and surge motions at different frequencies and amplitudes, to quantify and model the dynamic wake effect, including the added mass effect for rotor motion. This dynamic wake model is then implemented in an unsteady blade element momentum (uBEM) code, for a more complete model of wind turbine aerodynamics.