Reported by Winifred Bird in New Scientist, 02 February 2011.
The future of energy production (Image: Luis Louro/Alamy).
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’t produce enough electricity to offset the cost. That’s where dragonflies come in.
As air flows past a dragonfly’s thin wings, tiny peaks on their surface create a series of swirling vortices. To find out how these vortices affect the dragonfly’s 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.
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’s size increases. As air flow behaves in the same way as water, this would explain the insect’s 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’s 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.
“It’s a clever leap,” says David Alexander, a biomechanics specialist at the University of Kansas. “In some ways it’s more appropriate than using an animal wing model for an airplane. A wind turbine blade is just a wing, only it’s designed to go in tight circles.”
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.