̽��ֱ��owl and the wind turbine: how stealth feathers could help reduce noise pollution

amb206 — Wed, 09 Sep 2015 13:54:14 +0000

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Owls fly silently: not all species of owl but those species that rely on stealth in hunting small animals. People have known this for hundreds of years but until recently no-one has understood quite how these magical birds manage to swoop undetected on their scurrying prey.

̽��ֱ��puzzle of how the wings of certain species of owls are adapted to minimise the sound that their wings make has been solved by a partnership between researchers at the ̽��ֱ�� of Cambridge and two institutions in the USA.

̽��ֱ��key to the puzzle lies in the intricate structure of owls’ feathers – and especially the plumage on the trailing edge of their wings.

̽��ֱ��researchers have now been able to replicate this structure by producing a prototype surface (patented in 2014) which has potential applications in wind turbines and a wide range of fans. Its use could significantly reduce the noise generated by these products.

An especially promising end-use for the surface is for on-shore wind turbines which are heavily ‘braked’ to reduce noise pollution. ̽��ֱ��braking makes the turbines less efficient.

̽��ֱ��story began in 2010 when Dr Justin Jaworksi, then a researcher in the Department of Applied Mathematics and Theoretical Physics (DAMTP) at Cambridge ̽��ֱ��, decided to look in detail at the structure of owls’ wings.

At DAMTP, Jaworski (who is now at Lehigh ̽��ֱ��) worked with Professor Nigel Peake, a specialist in aeroacoustics known for his work on aircraft, to identify how owls’ wings differed from those of other birds.

They found three key differences. ̽��ֱ��first difference, unrelated to silent flight, is that owls’ wings are have a serrated leading edge in a way that enables them to plunge steeply downwards and then take off again.

̽��ֱ��other two differences combine to enable owls to fly stealthily so that they can hear their prey without it hearing them. “ ̽��ֱ��feathers on the upper wing surface have a particularly detailed and complex micro-structure with layer upon layer of interleaved barbs and hairs,” said Peake.

Much of the noise from wings – whether the wing of bird, plane or fan – originates at the trailing edge where the air passing over the wing surface becomes suddenly turbulent.

Owls have a neat solution to this problem. “At the trailing edge of their wings, owl feathers produce a flexible and porous fringe which reduces air turbulence by smoothing the passage of air,” said Peake.

No other species of bird possesses these features. Even more significantly, species of owl (such as fish owls) not requiring an acoustic stealth advantage do not possess them either.

To understand how the features unique to owl wings contribute to soundlessness, and in order to replicate the surfaces created, Jaworski and Peake have been collaborating with Professors William Devenport at Virginia Tech and Stewart Glegg at Florida Atlantic ̽��ֱ�� in a project funded by the US Office of Naval Research.

“We used advanced mathematical tools in a wind tunnel to show that the role of the fringe on owls’ wings is to negate something called the ‘Flowes Williams and Hall effect’. ̽��ֱ��porous elastic fringe filaments are a much softer ‘sound scatterer’ than a sharp rigid edge,” said Peake.

̽��ֱ��role of the complex feather structure is more of a mystery but the collaborators have been able, to some extent, to replicate its effect in the laboratory. “What appears to be crucial is the way that the fine hairs form a ‘canopy’ perhaps shielding the basal surface of the wing from pressure fluctuations in the turbulent air flow,” said Peake.

" ̽��ֱ��whole project has been very exciting. We’ve been able to use advanced mathematics to understand an amazing natural phenomenon, which then inspired us to develop a practical engineering solution to a really challenging noise pollution problem."

̽��ֱ��intricate structure of owl’s wings was noted more than 300 years ago by Francis Willughby (1635 to 1672), the polymath who compiled one of the world’s first comprehensive and analytical ornithologies. Several species of owl feature in Willughby’s Ornithologia libri tres which was published by his more famous friend and colleague John Ray (1627 to 1705).

Willughby studied at Trinity College, Cambridge, where the Wren Library holds a copy of the ornithology he authored. ̽��ֱ��lavishly produced volume contains dozens of plates showing birds categorised by their characteristics. ̽��ֱ��accompanying text reveals Willughby’s passionate interest in the wonders of the natural world.

Of the eagle-owl, Willughby writes: “ … in the great feathers of the Wings and Tail distinguished with broad, transverse, blackish lines or bars; which lines are so formed, especially in the Tail, that each of the broader are terminated above and below by other narrower ones, like borders or fringes, disposed in a triple order, and at certain intervals distant from each other, as in Hawks.”

Next in the Cambridge Animal Alphabet: P is for critters that are part of one of the most significant of all human-animal relationships.

Have you missed the series so far? Catch up on Medium here.

Inset images: Barn owl in flight (Chris Thompson); Barn Owl (Pat Gaines); Owls from Ornithologia libri tres by Francis Willughby (Wren Library, Trinity College).

The Cambridge Animal Alphabet series celebrates Cambridge's connections with animals through literature, art, science and society. Here, O is for Owl, the researchers using their wing structure to inspire aeroacoustic developments, and the lavish drawings of them found in one of the world's first ornithologies.

We’ve been able to use advanced mathematics to understand an amazing natural phenomenon, which then inspired us to develop a practical engineering solution

Nigel Peake

Wren Library, Trinity College

Small Owl from Ornithologia libri tres by Francis Willughby

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes

How wings really work

lw355 — Wed, 25 Jan 2012 09:00:22 +0000

It’s one of the most tenacious myths in physics and it frustrates aerodynamicists the world over. Now, ̽��ֱ�� of Cambridge’s Professor Holger Babinsky has created a 1-minute video that he hopes will finally lay to rest a commonly used yet misleading explanation of how wings lift.

“A wing lifts when the air pressure above it is lowered. It’s often said that this happens because the airflow moving over the top, curved surface has a longer distance to travel and needs to go faster to have the same transit time as the air travelling along the lower, flat surface. But this is wrong,” he explained. “I don’t know when the explanation first surfaced but it’s been around for decades. You find it taught in textbooks, explained on television and even described in aircraft manuals for pilots. In the worst case, it can lead to a fundamental misunderstanding of some of the most important principles of aerodynamics.”

To show that this common explanation is wrong, Babinsky filmed pulses of smoke flowing around an aerofoil (the shape of a wing in cross-section). When the video is paused, it’s clear that the transit times above and below the wing are not equal: the air moves faster over the top surface and has already gone past the end of the wing by the time the flow below the aerofoil reaches the end of the lower surface.

“What actually causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes – lower pressure on the upper surface and higher pressure on the lower surface,” clarified Babinsky, from the Department of Engineering. “This is why a flat surface like a sail is able to cause lift – here the distance on each side is the same but it is slightly curved when it is rigged and so it acts as an aerofoil. In other words, it’s the curvature that creates lift, not the distance.”

Babinsky is quick to stress that he is far from the only aerodynamicist who is frustrated by the perpetuation of the myth: colleagues have in the past expressed their concerns in print and online. Where he hopes his video will help debunk the myth once and for all is by providing a quick and visual demonstration to show that the most commonly used explanation cannot possibly be correct. ̽��ֱ��original video, created by Babinsky a few years ago using a wind tunnel, has now been re-edited in high quality with a voice-over in which he explains the phenomenon as it happens.

Babinsky’s research focuses on the fundamental aspects of aerodynamics as they relate to aircraft wings, Formula I racing cars, articulated lorries and wind turbines. One of his visions is to design a wing that will enable aircraft to fly faster and more efficiently. Using a massive wind tunnel within the Department of Engineering, Babinsky and his team have been modelling the shockwaves that are created on aircraft wings and that restrict the plane’s top speed.

̽��ֱ��newly released video will support lectures Babinsky will be giving as part of a series of ̽��ֱ�� of Cambridge Subject Masterclasses aimed at Year 12 school children: “It’s important to put out this video because when I give this lecture to school kids I start by giving the wrong explanation and asking who has heard it and every time 95% of the audience puts their hand up. Only a handful will know that it is wrong.”

A 1-minute video released by the ̽��ֱ�� of Cambridge sets the record straight on a much misunderstood concept – how wings lift.

I start by giving the wrong explanation and asking who has heard it and every time 95% of the audience puts their hand up. Only a handful will know that it is wrong.

Professor Holger Babinsky

Airflow across a wing

Holger Babinsky

Air flow across a wing

This work is licensed under a Creative Commons Licence. If you use this content on your site please link back to this page.

Yes

̽��ֱ�� of Cambridge - wing

̽��ֱ��owl and the wind turbine: how stealth feathers could help reduce noise pollution

How wings really work

Airflow across a wing