Understanding owls’ naturally stealthy wing design could help engineers design quieter aircraft, wind turbines and submarines.
Many owl species have developed specialised plumage to effectively eliminate the aerodynamic noise from their wings allowing them to swoop down on prey silently – a feature that could have broad applications for engineers working in fluid dynamics.
A group of researchers from the Lehigh University, USA, lead by Justin Jaworski, assistant professor at the university's Department of Mechanical Engineering and Mechanics are exploring whether owl stealth is based upon a single attribute or the interaction of a combination of attributes.
"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," said Jaworski.
For conventional wings, the sound from the hard trailing edge typically dominates the acoustic signature, 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 noise 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 very different from those of ordinary sound absorbers.
"Our current work predicts the sound resulting from air passing over the downy material, which is idealised as a collection of individual flexible fibres, 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 wind turbines, and the mitigation of underwater noise from naval vessels," said Jaworski.
The team is presenting its research at a meeting of the American Physical Society that began yesterday.