One moment you’re snuffling around in the undergrowth; the next you’re skewered on a sharp set of talons, the ground receding beneath you. Life can be precarious for a mouse – and if a silent-swooping owl comes calling, chances are you’d never know what hit you.
Inspired by these formidable hunters’ silent flight, Ian Clark from Virginia Tech in the US and colleagues from around the world have 3D-printed a plastic coating for a wind turbine blade that mimics the owl’s wing feather structure – cutting the turbine’s noise by half. They presented their work at the American Institute of Aeronautics and Astronautics Aeroacoustics conference in Dallas in June.
“In one case – and this never happens – the noise reductions we got were more than we expected from the theory!” says Justin Jaworski, a mechanical engineer at Pennsylvania’s Lehigh University and co-author of the paper. “It was a very good problem to have.”
The secret to the owl’s silent swoop was first described in 1934 when Lieutenant Commander R. R. Graham described three feather adaptations on the owl’s wing in Journal of the Aeronautical Society.
As an owl glides, turbulent air hitting the wing’s leading edge strikes a “stiff, comb-like” array of feathers that forces the air to flow in neat, smooth lines. A soft, downy coating on top of the wing further smooths the passage of air, so when the air eventually drops off the finely feathered fringe on the wing’s trailing edge there are no turbulent patches to create sound waves.
“It could be used for any application with a blade turning through the air,” Jaworski says. Imagine silent computer fans, for example.
Aeroplane wings and wind turbine blades have none of these adaptations; air bumping and bouncing across their surfaces creates the dull roar inside an aeroplane cabin and the swishing noise on wind farms.
So Clark and colleagues looked to the owl for inspiration. High-resolution microscopy showed an owl wing’s downy cover is not a smooth surface. The hairs that stick straight up out of the feather resemble the structure of a forest canopy and lean back towards the trailing edge, helping keep the air flowing in a neat stream over the wing.
That structure would have been too time-consuming and expensive to replicate, so the researchers simplified it, creating 3D-printed plastic “finlets” a few millimetres apart to be attached on to the surface of a full-sized turbine blade. Like the wing’s downy covering, the finlets guide the air over the blade so when the airflow falls off the trailing edge, it merges smoothly and quietly with the air flowing underneath.
In wind-tunnel tests, the finlets reduced noise by 10 decibels, a 50% cut in noise. Best of all, there was no effect on turbine performance.
Jaworski says they didn’t need to put the finlets over the whole blade surface to reduce noise – they were effective when placed only along the back edge. Next the team will try the 3D-printed finlets on a working turbine on a wind farm. And while incorporating the coating on an aeroplane wing would be more complicated, because an aeroplane travels much faster than a turbine blade and so the air racing over it is harder to tame, “it could be used for any application with a blade turning through the air”, Jaworski says. Imagine silent computer fans, for example.
Could the finlets also decrease infrasound, the inaudible noise some blame for so-called wind turbine syndrome? No, says Con Doolan, a University of New South Wales engineer. “The infrasonic sound is caused by the whoosh as they pass the tower,” he says, and by wind gusts bouncing off the face of the blade during blustery weather. Finlets can ease the passage of air passing over the blade, but have no effect on wind striking the blade’s face.
But for audible turbine swishes, Doolan is also looking to the owl for inspiration. Among other things, he’s studying whether serrating a blade’s edge would emulate the quieting effect of the owl wing’s fringe. One day, wind turbine blades might incorporate all three of the owl’s adaptations to silent flight, he says.
Also in Cosmos: Wind farm syndrome and other imaginary ailments