Aeroacoustics of Silent Owl Flight

Abstract

The ability of some species of owl to fly in effective silence is unique among birds and provides a distinct hunting advantage, but it remains a mystery as to exactly what aspects of the owl and its flight are responsible for this dramatic noise reduction. Crucially, this mystery extends to how the flow physics may be leveraged to generate noise-reduction strategies for wider technological application. We review current knowledge of aerodynamic noise from owls, ranging from live owl noise measurements to mathematical modeling and experiments focused on how owls may disrupt the standard routes of noise generation. Specialized adaptations and foraging strategies are not uniform across all owl species: Some species may not have need for silent flight, or their evolutionary adaptations may not be effective for useful noise reduction for certain species. This hypothesis is examined using mathematical models and borne out where possible by noise measurements and morphological observations of owl feathers and wings.

Acoustic scattering by cascades with complex boundary conditions: compliance, porosity and impedance

“…A popular choice is a poroelastic extension, which was originally inspired by the silent flight of owls (Graham 1934), and the corresponding experimental support of Geyer, Sarradj & Fritzsche (2010). A detailed review of research into the silent flight of owls is available in Jaworski & Peake (2020). Poroelastic extensions have been applied to semi-infinite (Jaworski & Peake 2013) and finite (Ayton 2016) blades and have demonstrated considerable noise reductions.…”

Reducing aerofoil–turbulence interaction noise through chordwise-varying porosity

“…Negative values indicate the owl is quieter than the buzzard by that many dB. These results differ greatly from the porous aerofoil results in Geyer & Sarradj (2014), particularly at high frequencies since there are numerous other silent-flight designs on the owl's wing which are not modelled by porosity alone, such as leading-edge combs, serrations and canopies, all of which may alter the owl's noise reduction at different frequencies (Jaworski & Peake 2020). At high frequencies, serrations are particularly effective for noise reduction, with Moreau & Doolan (2013) observing up to 13 dB of noise reduction.…”

“…Porosity is a known feature that promotes the silent flight of owls. Therefore, we expect the owl-like distribution to outperform the buzzard-like distribution acoustically, although we note there are many other features we do not consider in our model, such as serrations and canopies (Jaworski & Peake 2020), which aid the owl's silent flight. We also note these two species have different flight speeds; the owl flies at speeds 6-10 ms −1 (Neuhaus, Bretting & Schweizer 1973) and the buzzard at a mean speed of 11.6 ms −1 (Alerstam et al 2007).…”

Unsteady aerodynamics of porous aerofoils

“…The numerical approach advocated in the present research is sufficiently fast and accurate to be integrated into design optimisation routines. In particular, it is often desirable to reduce aeroacoustic emissions with a minimal aerodynamic penalty (Jaworski & Peake 2013, 2020). Initial assessment of this performance trade-off was explored by Weidenfeld & Arad (2018) in the case of an elastic aerofoil, and optimisation of elastic aerofoil effects on unsteady propulsion by Moore (2015) found that the limiting case of a torsional spring at the leading edge of the wing led to optimal thrust conditions.…”