Surge motion of a floating wind turbine can lead to conditions where the rotor moves backwards faster than the wind, leading to propeller-like conditions or vortex ring state (VRS). The effect of surge on the thrust of a floating turbine was investigated with OpenFOAM for conditions favourable to propeller and vortex ring state. Due to lower blade velocities and larger blade twists, a region of negative thrust is shown to extend spanwise from the blade root towards the tip signifying propeller state. Predictions that strong waves with low/moderate wind speeds leads to propeller-like conditions were confirmed for a representative surging simulation with a 9.4 m amplitude in waves with an 8.1 s period and 7 m/s wind speed. A negative thrust for the entire rotor, through the combination of an inboard region of negative and outboard region of small but still positive thrust, was observed during the expected part of the surging cycle. VRS was observed with blade tip-vortex interaction and root vortex recirculation due to the duration with a negative relative rotor velocity being similar to the blade passing period, inhibiting vortex advection downstream. This work explains and demonstrates the causes of propeller state and VRS for floating turbines.

Armour EDGE is a novel shield developed to protect the leading edge of wind turbine blades from erosion. The aerodynamic impact on aerofoils of National Renewable Energy Laboratory (NREL) 5MW wind turbine has been investigated using 2D fully turbulent computational fluid dynamics (CFD), with three profiles at critical locations along the blade simulated both with and without the shield to compare aerodynamic performance. Two wind speeds were investigated that reflect regular operating conditions: at rated speed of 11.4 m/s and a below rated speed of 7 m/s. The results showed that the presence of the shield during rated wind speed reduced the drag by as much as 4.5%, where the lift‐to‐drag ratio increased by a maximum of 4%. At the below rated wind speeds, the shield had negligible impact on the performance of all but one National Advisory Committee for Aeronautics (NACA) 64‐618 profile, which resulted in an increase in the drag coefficient of 7%. It was also found that the suction side of the aerofoil is much more sensitive to leading edge protection placement than the pressure side. It was concluded that the erosion shield as a method of leading edge protection, with a gradual transition from shield to blade, will not have a major impact on the aerodynamic performance of a multi‐megawatt wind turbine blade and could slightly increase aerofoil efficiency at high wind speeds.

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