THE way a dragonfly remains stable in flight is being mimicked to develop micro wind turbines that can withstand gale-force winds.
Micro wind turbines have to work well in light winds but must avoid spinning too fast when a storm hits, otherwise their generator is overwhelmed. To get round this problem, large turbines use either specially designed blades that stall at high speeds or computerised systems that sense wind speed and adjust the angle of the blade in response. This technology is too expensive for use with micro-scale turbines, though, because they don鈥檛 produce enough electricity to offset the cost. That鈥檚 where dragonflies come in.
As air flows past a dragonfly鈥檚 thin wings, tiny peaks on their surface create a series of swirling vortices. To find out how these vortices affect the dragonfly鈥檚 aerodynamics, aerospace engineer Akira Obata of Nippon Bunri University in Oita, Japan, filmed a model dragonfly wing as it moved through a large tank of water laced with aluminium powder. He noticed that the water flowed smoothly around the vortices like a belt running over spinning wheels, with little drag at low speeds.
Advertisement
Obata found that the flow of water around the dragonfly wing is the same at varying low current speeds, but, unlike an aircraft wing, its aerodynamic performance falls drastically as either water speed or the wing鈥檚 size increases. As air flow behaves in the same way as water, this would explain the insect鈥檚 stability at low speeds, Obata says.
Obata and his colleagues have used this finding to develop a low-cost model of a micro wind turbine whose 25-centimetre-long paper blades incorporate bumps like a dragonfly鈥檚 wing. In trials in which the wind speed over the blades rose from 24 to 145 kilometres per hour, the flexible blades bent into a cone instead of spinning faster. The prototype generates less than 10 watts of electricity, which would be enough to recharge cellphones or light LEDs, the researchers say.
鈥淚t鈥檚 a clever leap,鈥� says David Alexander, a biomechanics specialist at the University of Kansas. 鈥淚n some ways it鈥檚 more appropriate than using an animal wing model for an airplane. A wind turbine blade is just a wing, only it鈥檚 designed to go in tight circles.鈥�
But Wei Shyy of the Hong Kong University of Science and Technology believes that while the dragonfly-inspired design may be more stable, it will also experience more energy loss in terms of drag.
