Could owl wings hold the key to quieter aircraft and wind turbines?
Wind turbines, helicopters, and other devices that use rotors have a big noise problem, but the solution may come from a not-so little bird. Scientists from Chiba University in Japan and Shanghai Jiao Tong University in China claim that the answer could be in the wings of the owl, which allows the nocturnal hunting bird to fly almost silently and may lead to quieter wind turbines, aircraft, and drones.
A wind turbine turning in the distance may look graceful, but up close that seemingly effortless sweep is accompanied by a whooshing noise that often makes them difficult to site near populated areas. Meanwhile, helicopters are many times noisier with their characteristic thumping sound and quad-copters make a noise like a flying bandsaw.
Making these devices quieter is one of the key goals of aeronautical engineers and birds seem like a good place to look for the answer. But not all birds are the same. Pigeons in flight make an absolute racket and even falcons aren't much quieter. This is because with their small wings and large bodies, pigeons flap their wings a lot and create significant turbulence, which means noise. And falcons and other raptors may have larger wings in proportion to their bodies, but they push large amounts of air to build up speed and that means turbulence and therefore noise.
But owls are different. Their flight is famously silent. Indeed, if one measures an owl in flight with highly sensitive microphones, almost no sound is recorded as they flap their wings. Part of the reason is that owl wings are designed to generate a lot of lift with very little flapping, but there's more to it than that.
"Owls are known for silent flight, owing to their unique wing features, which are normally characterized by leading-edge serrations, trailing-edge fringes and velvet-like surfaces," says Hao Liu, from Chiba University. "We wanted to understand how these features affect aerodynamic force production and noise reduction, and whether they could be applied elsewhere."
To see how these features can cut down on turbulence, the scientists created feather wing models based on the owl's and studied them in a low-speed wind tunnel using large-eddy computer simulation models and Particle-Image Velocimetry (PIV), which is an optical method for studying how air flows.
They found that the leading-edge serrations on owl wings passively control how air flows over the wing. Specifically, it controls the transition between how air flows in a streamlined fashion close to the wing, or laminar flow, and how this breaks up into turbulence on the upper wing surface.
This transition and the turbulence is caused by the angle of attack (AoA), or at what angle the wing is to its direction of flight. If it's parallel, there should be very little turbulence. If it's at a steep angle, there will be much more. But the serrations can cut down this turbulence – at a price.
"We found, however, that a trade off exists between force production and sound suppression," says Liu. "Serrated leading-edges reduce aerodynamic performance at lower AoAs than 15° compared to clean leading-edges, but can achieve noise reduction and aerodynamic performance at AoAs above 15°, which owl wings often reach in flight.
"These owl-inspired leading edge serrations, if applied to wind turbine blades, aircraft wings or drone rotors, could provide a useful biomimetic design for flow control and noise reduction. At a time when issues of noise are one of the main barriers to the building of wind turbines, for example, a method for reducing the noise they generate is most welcome."
The research was published in Bioinspiration and Biomimetics.