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Soaring
silently above the landscape, owls search out their prey utilizing acoustic
stealth. University of Cambridge, England researchers led by Dr. Justin
Jaworski are studying the owl’s wing structure and mechanics to better understand
how it mitigates noise to apply that information conventional aircraft design.
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“Many
owl species have developed specialized plumage to effectively eliminate the
aerodynamic noise from their wings, which allows them to hunt and capture their
prey using their ears alone,” said Justin Jaworski with the department of
applied mathematics and theoretical physics at the University of Cambridge. “No
one knows exactly how owls achieve this acoustic stealth, and the reasons for
this feat are largely speculative based on comparisons of owl feathers and
physiology to other not-so-quiet birds such as pigeons.”
All
wings, either natural or engineered, create turbulent eddies as they cut
through the air. When these eddies hit the trailing edge of the wing, they are
amplified and scattered 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 capability: 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 unravel this mystery by developing a theoretical basis
for the owl’s ability to mitigate sound from the trailing edge of its wing,
which is typically an airfoil’s dominant 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 elastic and porous
properties of a trailing edge could be tuned 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 dictated by otherwise minor noise mechanisms such as the
roughness of the wing surface.”
Read
more at the University
of Cambridge.
Owl in flight image via Shutterstock.
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