Air jet vortex generators for wind turbines
Air jet vortex generators are a boundary layer control device and as such can effect a significant delay to the stall of an aerofoil. They can also reattach a separated flow, as long as the angle of attack is less than the air jets 'on' stall angle. A wind tunnel model was built and tested and then modified to incorporate air jets. The air jets increased CL max by 55% and reduced the drag throughout the incidence range, a, for the chosen blowing pressure of 1 p. s. i. (6900Pa). By varying the air supply so that the jets operate between 'off' and fully 'on' a whole family bf aerofoil L/D characteristics could be derived. The blade set of a 150 kW stall regulated wind turbine was then modified with air jet vortex generators and tested at full scale. This was done primarily to increase its energy yield by reducing energy loss in the region of the power curve 'knee', but also to allow a degree of power regulation in high wind speeds. The air supply in this demonstration was supplied by a fan mounted on, and rotating with, the rotor. Full-scale trials demonstrated that air jets can indeed be used to straighten the power curve prior to rated wind speed. This important result means that a stall regulated machine can be made to behave in the same way as a pitch controlled machine without having to pitch the blades. Of the two air jet configurations tested in the wind tunnel, the second had an improved L/D characteristic and required less mass flow, even though the physical change was minor. This indicates that further optimisation of air jets may be achieved. Further wind tunnel testing showed that it should be possible to eliminate the fan used in the full scale trials and use one of two 'passive' blowing techniques. It was shown that the air jets can be supplied by air at atmospheric pressure due to the suction on the upper surface of the aerofoil which is sufficient to pull air through the jet exits and create vortices. However, to achieve an optimised air jet configuration and to investigate the possibilities of using passive blowing, air jet vortex generators will need to be modelled numerically, so that this innovative concept can be brought to market quickly.