Scientists studying the acoustics of owl flight have designed a 3D printed wing attachment that reduces wind turbine noise by a remarkable 10 decibels – without impacting aerodynamics. 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.
The team has succeeded – through physical experiments and theoretical modelling – in using the downy canopy of owl feathers as a model to inspire the design of a 3D printed, wing attachment that reduces wind turbine noise by a remarkable 10 decibels – without impacting aerodynamics.
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.
The researchers specifically looked at the velvety down that makes up the upper wing surface of many large owls – a unique physical attribute, even among birds, that contributes to owls’ noiseless flight. 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). The finlet invention may be retrofitted to an existing wing design and used in conjunction with other noise-reduction strategies to achieve even greater noise suppression.
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