Relieving loading on tidal turbines operating in a turbulent environment: proposal for load alleviation device for a marine turbine

Tidal turbines operate in marine currents characterised by strong turbulence. This environment impacts on the life of the device, blades, transmission and running train ancillaries, bearings and seals, for example. The reduction of the transient load on the turbine blade can be achieved through a range of measures; in this investigation, modifications to the blade comprising blowing actuators will be modelled using the FLUENT CFD RANS solver with the k-ω SST turbulence model. Different locations on the suction side for the blowing are explored. Different angles and pressures of the blowing are explored for steady state cases. The final chosen location for the ejection was near the trailing edge (TE) for unsteady simulations with sinusoidal jet excitation and an oscillating-flow inlet boundary condition. The most obvious effectiveness is at low actuation frequency and mid-to-high jet-strength. This results in reduction of lift at the higher angles of attack, α, and reduction of ∆L ∆α . The action of the unsteady ejection re-shapes the hysteresis lift curve with little change in drag and reduces drag in some cases. A counter-rotating pair of vortices is formed due to the TE ejection, which altered the direction of flow leaving the TE due to a small region of induced recirculating flow behind the TE. One of the vortices takes the form of a curtailed TE vortex. It is thought that this could, under the right conditions, be caused to merge with the wake and shed to result in a more docile stall. The CFD validation study performed first of all, consisted of a comparison between the two relative types of motion - a pitching aerofoil and static aerofoil subjected to flow oscillation. This study uncovered substantial differences in the dynamic stall mechanics between wind tunnel experiments on a pitching aerofoil and the kind experienced by a turbine blade due to oscillating loads in highly unsteady flow. Many wind turbine data currently depends on the relative motion between aerofoil and flow.