The secrets of owls' near noiseless wings

Nov 24, 2013
The three unique wing features believed to make owls fly silently are shown in this graphic. Credit: J. W. Jaworski, I. Clark

Many owl species have developed specialized plumage to effectively eliminate the aerodynamic noise from their wings – allowing them to hunt and capture their prey in silence.

A research group working to solve the mystery of exactly how owls achieve this acoustic stealth will present their findings at the American Physical Society's (APS) Division of Fluid Dynamics meeting, held Nov. 24 – 26, in Pittsburgh, Pa.—work that may one day help bring "silent owl technology" to the design of aircraft, wind turbines, and submarines.

"Owls possess no fewer than three distinct physical attributes that are thought to contribute to their silent flight capability: a comb of stiff feathers along the leading edge of the wing; a flexible fringe a the trailing edge of the wing; and a soft, downy material distributed on the top of the wing," explained Justin Jaworski, assistant professor in Lehigh University's Department of Mechanical Engineering and Mechanics. His group is exploring whether owl stealth is based upon a single attribute or the interaction of a combination of attributes.

For conventional wings, the sound from the hard trailing edge typically dominates the . But prior theoretical work carried out by Jaworski and Nigel Peake at the University of Cambridge revealed that the porous, compliant character of the owl wing's trailing edge results in significant aerodynamic reductions.

"We also predicted that the dominant edge-noise source could be effectively eliminated with properly tuned porous or elastic edge properties, which implies that that the noise signature from the wing can then be dictated by otherwise minor noise mechanisms such as the 'roughness' of the wing surface," said Jaworski.

The velvety down atop an owl's wing creates a compliant but rough surface, much like a soft carpet. This down material may be the least studied of the unique owl noise attributes, but Jaworski believes it may eliminate sound at the source through a novel mechanism that is much different than those of ordinary sound absorbers.

"Our current work predicts the sound resulting from air passing over the downy material, which is idealized as a collection of individual flexible fibers, and how the aerodynamic noise level varies with fiber composition," Jaworski said.

The researchers' results are providing details about how a fuzzy – compliant but rough – surface can be designed to tailor its acoustic signature.

A photographic study of actual owl feathers, carried out with Ian Clark of Virginia Tech, has revealed a surprising 'forest-like' geometry of the down material, so this will be incorporated into the researchers' future theoretical and experimental work to more faithfully replicate the down structure. Preliminary experiments performed at Virginia Tech show that a simple mesh covering, which replicates the top layer of the 'forest' structure, is effective in eliminating some sound generated by rough surfaces.

"If the noise-reduction mechanism of the owl down can be established, there may be far-reaching implications to the design of novel sound-absorbing liners, the use of flexible roughness to affect trailing-edge noise and vibrations for aircraft and , and the mitigation of underwater noise from naval vessels," said Jaworski.

Explore further: New experiments offer insight into how insects fly and how to design tiny flying robots

More information: The presentation "Vortex Noise Reductions from a Flexible Fiber Model of Owl Down," is at 8:39 a.m. on Sunday, November 24, 2013 in the David L. Lawrence Convention Center, Room 319. ABSTRACT: http://meeting.aps.org/Meeting/DFD13/Event/202030

Related Stories

Noise research to combat 'wind turbine syndrome'

Jun 01, 2011

(PhysOrg.com) -- University of Adelaide acoustics researchers are investigating the causes of wind turbine noise with the aim of making them quieter and solving 'wind turbine syndrome'.

Researchers develop 'anti-noise' panel for quiet aircraft

May 08, 2013

Researchers from the University of Twente's CTIT research institute have developed a prototype lightweight panel that uses anti-noise to reduce noise levels inside aircraft. The panels can be used as a replacement for the ...

If you cut down a tree in the forest, can wildlife hear it?

Dec 14, 2012

A new tool developed by the Wildlife Conservation Society (WCS) and its partners is being used by scientists and land managers to model how noise travels through landscapes and affects species and ecosystems— a major factor ...

Recommended for you

What time is it in the universe?

Aug 29, 2014

Flavor Flav knows what time it is. At least he does for Flavor Flav. Even with all his moving and accelerating, with the planet, the solar system, getting on planes, taking elevators, and perhaps even some ...

Watching the structure of glass under pressure

Aug 28, 2014

Glass has many applications that call for different properties, such as resistance to thermal shock or to chemically harsh environments. Glassmakers commonly use additives such as boron oxide to tweak these ...

Inter-dependent networks stress test

Aug 28, 2014

Energy production systems are good examples of complex systems. Their infrastructure equipment requires ancillary sub-systems structured like a network—including water for cooling, transport to supply fuel, and ICT systems ...

Explainer: How does our sun shine?

Aug 28, 2014

What makes our sun shine has been a mystery for most of human history. Given our sun is a star and stars are suns, explaining the source of the sun's energy would help us understand why stars shine. ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

Macrocompassion
not rated yet Nov 25, 2013
As I have been claiming for years, this soft downy surface effect when applied to large aircraft wings will stabilize the boundary layers and both delay the transition of laminar flow to turbulence and also damp the degree of turbulence afyer it has occured in the flow on the rear part of the hard surface. The latter trend for covering the wing with a "sea of grass" is the first that should be examined because it is easier to design and use.