Interaction of barn owl leading edge serrations with freestream turbulence

Abstract

The silent flight of barn owls is associated with wing and feather specialisations. Three special
features are known: a serrated leading edge that is formed by free-standing barb tips which
appears as a comb-like structure, a soft dorsal surface, and a fringed trailing edge. We used
a model of the leading edge comb with 3D-curved serrations that was designed based on 3D
micro-scans of rows of barbs from selected barn-owl feathers. The interaction of the flow with
the serrations was measured with Particle-Image-Velocimetry in a flow channel at uniform steady
inflow and was compared to the situation of inflow with freestream turbulence, generated from
the turbulent wake of a cylinder placed upstream. In steady uniform flow, the serrations caused
regular velocity streaks and a flow turning effect. When vortices of different size impacted the
serrations, the serrations reduced the flow fluctuations downstream in each case, exemplified
by a decreased root-mean-square value of the fluctuations in the wake of the serrations. This
attenuation effect was stronger for the spanwise velocity component, leading to an overall flow
homogenization. Our findings suggest that the serrations of the barn owl provide a passive flow
control leading to reduced leading-edge noise when flying in turbulent environments.