# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 2 Biological Studies of the Owl's Silent Flight
[PERSON] [20] emphasized that the leading edge serrations are the main apparatus in reducing the noise radiation, which gradually slows down the incoming flow through the serrations and over the upper surface of the wings, smoothing out the local pressure gradient, and thus reducing the emission of any associated noise. The trailing edge fringes allow a partial mixing of the upper and lower airflow, which prevents the formation of the noise-producing vortices. The fluffy down-like texture of the feathers allows a reduction of the noise by reducing the friction between the interlinked feathers [38] and absorbing the flight noise. [PERSON] et al. [28] indicated that the flow on the surface of the owl's primary flying feather is essentially laminar, while the upper and lower surfaces of mallard duck's wings have a strong turbulent flow. When the leading edge serrations of the owl's wing are removed, the laminar flow becomes turbulent, leading to an early flow separation closer to the leading edge, thus making no noise reduction. Therefore, they believed that the leading edge serrated structure suppresses the noise generation by controlling the flow pattern. [PERSON] et al. [24] conducted extensive aerodynamics, acoustics and bionics research in an attempt to reveal the underlying mechanisms which are responsible for the silent flight of the owl, where three mechanisms were discovered: (1) The leading edge serrations behave as vortex sheet generators (not classical vortex generators), which work together with the leading edge slot and the tip feathers to keep the flow laminar and attached to the entire outer half of the wing, which could aid in suppressing turbulent boundary layer noise. (2) Compliant surfaces attenuate the turbulent boundary layer and shift the noise spectrum of the owl towards lower frequency ranges. (3) Distributed wing porosity produced by the soft downy surface thickens the chordwise boundary layer of the flow between the feathers and reduces the velocity gradients at the trailing edge, and thus reduces the trailing edge noise. [PERSON] et al. [24] also used flow visualization technology on two prepared owl wings to obtain the boundary layer streamline pattern with and without the leading edge serrations. As illustrated by the results, after removing the leading edge sawtooth structure, the owl wings had a clear flow  
Figure 3: Feather position in the barn owl and the pigeon (scale bar represents 10 cm), as provided by [PERSON] et al. [21]. separation immediately aft of the leading edge and the flow reattached near the trailing edge, causing considerable turbulence. Based on the work of [PERSON] [20] and the flyover test data of [PERSON] et al. [24], [PERSON] [22; 39] gave some tentative but plausible explanations to the three special feather adaptations: (1) The leading edge serrations act as a set of nearly equi-spaced co-rotating vortex generators, which stabilizes the flow over the upper surface of the owl's wing (evidently \"pseudo-turbulent\") and prevents laminar separation. Streamwise vortices generated by each tooth drastically reduce the boundary layer thickness by providing an attached flow up to the trailing edge of the owl wing, and thus reduce the trailing edge noise since the emitted noise is proportional to the turbulent volume passing through the trailing edge. (2) The fringes at the trailing edge of the owl wings reduce or even eliminate the trailing edge noise scattering at the speed owls fly. (3) However, only with leading-edge serrations and trailing edge fringes, [PERSON] argued that the owl can only fly quietly but not silently compared to all other birds. Therefore, the fluffy down on the wings and legs is important to the silent flight of the owl, although it is possibly the most difficult to explain. [PERSON] speculated that the downy surface of the owl does not act as a sound absorber but similar to a compliant surface, which dampens the turbulent boundary layer of the airflow passing over it.