On the wings of an owl

There’s a lot of learning going on about how owls fly – and what we can learn from that.

Last month, as reported in Cosmos, researchers in the UK revealed how Lily the barn owl could cope with winds that gust as fast as her flight speed by changing the shape and posture of her wings.

Now a new study has shown how micro-structured finlets on owl feathers enable silent flight – which, the researchers say, may provide clues to reducing aircraft noise.

In a paper in the journal Bioinspiration & Biomimetics, they describe how these arrays of finlets coherently turn the flow direction near the aerodynamic wall and keep the flow for longer and with greater stability, avoiding turbulence.

“This work describes a novel mechanism of laminar flow control of straight and backward swept wings with a comb-like leading edge device,” they write. “It is inspired by the leading-edge comb on owl feathers and the special design of its barbs, resembling a cascade of complex 3D-curved thin finlets.”

The project started with Hermann Wagner and colleagues at RWTH Aachen University in Germany, who had captured the complex geometry of the extensions along the front of the owl’s feathers using high-resolution micro-CT scans.

Christoph Bruecker and his team at City University London then used this to create experimental models and carry out flow simulations around them using computational fluid dynamics. This clearly indicated the aerodynamic function of these extensions as finlets, they say, which turn the flow direction in a coherent way.

During flow studies in a water tunnel, Bruecker was surprised to find that instead of producing vortices, the finlets acted as thin guide vanes due to their 3D curvature. The regular array of finlets over the wingspan turns the flow direction near the wall in a smooth and coherent manner.

The plan now is to create a technical realisation of such a swept wing aerofoil pattern and run some acoustic tests in anechoic wind-tunnel.