Many species of owl are able to hunt in effective silence by suppressing their noise at sound frequencies above 1.6 kilohertz (kHz) - over the range that can be heard by humans.
A team of researchers including Justin W Jaworski, assistant professor at Lehigh University in the US, are working to pinpoint the mechanisms that accomplish this virtual silence to improve man-made aerodynamic design - of wind turbines, aircraft, naval ships and even automobiles.
They have further investigated how such a design can reduce roughness and trailing-edge noise. In particular, trailing-edge noise is prevalent in low-speed applications and sets their minimum noise level.
The ability to reduce wing noise has implications beyond wind turbines, as it can be applied to other aerodynamic situations such as the noise created by air seeping through automobile door and window spaces.
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As seen under a microscope, the down consists of hairs that form a structure similar to that of a forest. The hairs initially rise almost perpendicular to the feather surface but then bend over in the flow direction to form a canopy with interlocking barbs at the their tops - cross-fibres.
After realising that the use of a unidirectional canopy - with the cross-fibres removed - was the most effective - as it did not produce high-frequency self-noise of the fabric canopies, but still suppressed the noise-producing surface pressure - they created a 3-D-printed, plastic attachment consisting of small "finlets" that can be attached to an airfoil (wing).
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