On the influence of turbulent inflow conditions on the wakes of floating offshore wind turbines

Abstract

Floating offshore wind farms are a promising technological development that can help provide large amounts of energy from sites previously not feasible for wind farm development. To suc- cessfully develop floating wind farms, knowledge about the involved aerodynamics are crucial. A first building block of this is the understanding of the wake of a single floating wind turbine un- dergoing floating motion in turbulent wind fields. Using an actuator line model within the Large Eddy Simulation framework YALES2 the wake of a floating wind turbine undergoing surge motion has been investigated at varying amplitudes and frequencies. Special attention was given to the inflow conditions, comparing laminar and low turbulent inflow conditions modeled using Mann turbulence boxes. Varying the surge amplitude at a surge frequency equivalent to St = 0.4, the investigations show that the influence of the motion on the wake is greatly reduced even when low turbulence levels are present. Already at a turbulence intensity of 2.5% the wake deficit and wake turbulence in- tensity were indistinguishable from a fixed turbine’s wake for lower surge amplitudes. Large surge amplitudes were required to significantly change these wake statistics, starting from an amplitude of A/D = 0.16 rotor diameters.Using proper orthogonal decomposition the dynamics in the wake have been analyzed. It was found that the effect of the rotor motion on the wake is lower when the inflow turbulence intensity is increased. Varying the motion frequency in the range of St ∈ [0.2, 0.8] at a amplitude of A/D = 0.04, it was shown that a surge motion at a frequency corresponding to St = 0.6 has the highest impact on the wake in terms of the reduction of modal time series correlations. However, even in that case the wake deficit and turbulence intensity in the wake are not changed significantly.