Small fins on owl feathers show the way to reduce aircraft noise

A recent research study conducted by City, by Professor Christoph Bruecker of the University of London and his team revealed how the microstructured fins on owl feathers allow for silent flight and can indicate the way forward to reduce aircraft noise in future.

Professor Bruecker is president of the city’s Royal Academy of Engineering Research for nature-inspired flow sensing and control for sustainable transportation and Sir Richard Olver BAE Systems Chair for Aeronautical Engineering.

His team published their findings in the Institute of Physics journal, Bioinspiration and biomimicry in a paper entitled “Flow rotation effect and laminar control thanks to the three-dimensional curvature of the serrations of the leading edge of the owl’s wing”.

Their research outlines their translation of the detailed 3-D geometric data of typical owl feather examples provided by Professor Hermann Wagner of the RWTH University of Aachen (Germany) into a biomimetic airfoil to study the aerodynamic effect on special filaments on the leading edge of the feathers.

The results show that these structures function as arrays of finlets that consistently rotate the flow direction close to the aerodynamic wall and maintain the flow longer and with greater stability, avoiding turbulence.

The City’s research team was inspired by the complex 3-D geometry of the extensions along the front of the owl’s feathers, reconstructed by Professor Wagner and his team in previous studies using high-resolution micro-CT scans.

After being transferred into a digital shape model, flow simulations around those structures (using computational fluid dynamics) clearly indicated the aerodynamic function of these extensions as finlets, which rotate the flow direction in a coherent fashion.

This effect is known to stabilize the flow over an airfoil, typical of owls as they flap their wings and glide.

Using flow studies in a water tunnel, Professor Bruecker also demonstrated the reverse flow hypothesis in experiments with an enlarged finlet model.

His team was surprised that, instead of producing swirls, the fins act as thin guide fins due to their special 3D curvature. The regular arrangement of these fins on the wingspan then transforms the flow direction near the wall in a uniform and consistent manner.

The team plans to use a technical realization of such a large airfoil in an anechoic wind tunnel for further acoustic testing. The result of this research will prove important for future laminar wing design and has the potential to reduce aircraft noise.