Abstract:
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.
