Owls2 have the
uncanny(神秘的) ability to fly silently, relying on
specialized3 plumage to reduce noise so they can hunt in
acoustic4 stealth(秘密行动). Researchers from the University of Cambridge, England, are studying the
owl1's wing structure to better understand how it
mitigates5 noise so they can apply that information to the design of conventional aircraft. They present their findings at the American Physical Society's (APS) Division of Fluid
Dynamics7 meeting, held Nov. 18 -- 20, in San Diego, Calif.
"Many owl species have developed specialized plumage to effectively eliminate the aerodynamic noise from their wings, which allows them to hunt and capture their
prey8 using their ears alone," said Justin Jaworski with the department of
applied9 mathematics and theoretical physics at the University of Cambridge. "No one knows exactly how owls achieve this acoustic stealth, and the reasons for this
feat10 are largely
speculative11 based on comparisons of owl feathers and
physiology12(生理学) to other not-so-quiet birds such as pigeons."
All wings, either natural or engineered, create turbulent
eddies13 as they cut through the air. When these eddies hit the trailing edge of the wing, they are
amplified14 and
scattered15 as sound. Conventional aircraft, which have hard trailing edges, are particularly noisy in this regard.
Owls, however, possess no fewer than three distinct physical attributes that are thought to contribute to their silent flight
capability16: a comb of stiff feathers along the leading edge of the wing; a soft downy material on top of the wing; and a flexible fringe at the trailing edge of the wing. At present it is not known whether it is a single attribute or the combination of attributes that are the root cause of the noise reduction.
The researchers attempted to
unravel17 this mystery by developing a theoretical basis for the owl's ability to
mitigate6 sound from the trailing edge of its wing, which is typically an airfoil's
dominant18 noise source. Earlier owl noise experiments suggest that their wing noise is much less dependent on air speed and that there is a large reduction of high frequency noise across a range where human ears are most sensitive.
Using mathematical models, the researchers demonstrated that
elastic19(灵活的) and
porous20 properties of a trailing edge could be
tuned21 so that aerodynamic noise would depend on the flight speed as if there were no edge at all. "This implied that the dominant noise source for conventional wings could be eliminated," said Nigel Peake also of the University of Cambridge. "The noise signature from the wing could then be
dictated22 by otherwise
minor23 noise
mechanisms24 such as the roughness of the wing surface."