Owls are commonly known for their quiet flight, enabled by three adaptions of their wings and plumage: leading edge serrations, trailing edge fringes and a soft and elastic downy upper surface of the feathers. In order to gain a better understanding of the aeroacoustic effects of the third property that is equivalent to an increased permeability of the plumage to air, an experimental survey on a set of airfoils made of different porous materials was carried out. Several airfoils with the same shape and size but made of different porous materials characterized by their flow resistivities and one non-porous reference airfoil were subject to the flow in an aeroacoustic open jet wind tunnel. The flow speed has been varied between approximately 25 and 50 m/s. The geometric angle of attack ranged from −16° to 20° in 4°-steps. The results of the aeroacoustic measurements, made with a 56-microphone array positioned out of flow, and of the measurements of lift and drag are given and discussed.

Measurement of the noise generation at the trailing edge of porous airfoils

The interaction of a turbulent eddy with a semi-infinite poroelastic edge is examined with respect to the effects of both elasticity and porosity on the efficiency of aerodynamic noise generation. The scattering problem is solved using the Wiener–Hopf technique to identify the scaling dependence of the resulting aerodynamic noise on plate and flow properties, including the dependence on a characteristic flow velocity . Special attention is paid to the limiting cases of porous-rigid and impermeable–elastic plate conditions. Asymptotic analysis of these special cases reveals parametric limits where the far-field acoustic power scales like for a porous edge, and a new finite range of behaviour is found for an elastic edge, to be compared with the well-known dependence for a rigid impermeable edge. Further numerical results attempt to address how trailing-edge noise may be mitigated by porosity and flexibility and seek to deepen the understanding of how owls hunt in acoustic stealth.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/B014886DFCB950CB1B7FE5AA83ACC963/S0022112013001390a.pdf/div-class-title-aerodynamic-noise-from-a-poroelastic-edge-with-implications-for-the-silent-flight-of-owls-div.pdf

Aerodynamic noise from a poroelastic edge with implications for the silent flight of owls

Many species of owl, including the barn and barred owl, use both visual and bi-aural location to search for prey around dusk and at night. Their bi-aural location system has a maximum sensitivity between 3-6kHz although the hearing of the owl has an upper limit in excess of 20kHz. Its prey, typically voles and mice, squeak and and squeal in the frequency range of 3-6kHz and this range of frequency includes the rustling of leaves made by prey. The hearing of these prey is acute between 2-20kHz. The owl in both gliding and flapping flight generates noise at low frequencies below 2kHz, but is almost totally silent at frequencies above 2kHz. Hence the flight of the owl is almost silent to its prey. When an owl attacks its prey from its perch from a height of 6-10m the prey are unaware of its approach before they are captured by its long talons. The noise suppression devices developed by owls during their long evolution period of millions of years have been identified. They comprise (i) leading edge feathers in the form of a comb, (ii) trailing edge feathers in the form of a fringe, and (iii) fluffy down on the wings and legs. The paper discusses the aerodynamic characteristics of each of these devices and suggests tentative explanations as to how these flow characteristics lead to a large reduction in the noise generated by the owl in flight and especially in its critical range of frequencies above 2kHz. Many species of owl have perfected their use of their habitat and their means of survival through the capture of their prey, in the form of small animals such as voles and mice. The owl as we observe it today has been in existence for over 12 million years. It is suggested it developed its strategy of a silent predator when the small animals were in glut as a result of an abundance of bushes carrying red berries some 10 million years ago. The strategy used by many species of owls involves capturing their prey by day and night and especially at dusk from a perch at a height of 3-6m by gliding steeply towards the prey in daylight or in flapping flight after dark. Some owls today use different techniques for capturing prey. Here we will discuss the 'noiseless' flight of owls such as the barn owl (Tyto alba) and the barred owl (Strix varia), both of which use the strategies discussed above. In the aural response of the prey the owl's flight is completely silent to within 1m of the ground. At this height, even if the approach of the owl is heard, the prey have no time to escape before being struck by the owl's talons. The owl has a highly sensitive bi-aural hearing and location system enabling it to both hear the movements of the prey and its squeaks and squeals together with the rustling of leaves as well as its location. The frequency of these squeaks and squeals and rustling of leaves is in the range 2-10kHz. The maximum hearing response of the bi-aural and location

A Study of the Silent Flight of the Owl

This paper presents and interprets a series of extensive studies of the application of leading-edge serrations as a device for reducing the vortex noise radiated from stationary and rotating airfoils in low Reynolds number flow. In these studies, a variety of serrations were attached at selected locations near the leading edge of stationary and rotating airfoils. The noise levels of the airfoils were reduced considerably with the serrations attached. An explanation of the aeroacoustic flow mechanisms involved is given.

Investigation of Acoustic Effects of Leading-Edge Serrations on Airfoils

Owls are known for their silent flight. Even though there is some information available on the mechanisms that lead to a reduction of noise emission, neither the morphological basis, nor the biological mechanisms of the owl's silent flight are known. Therefore, we have initiated a systematic analysis of wing morphology in both a specialist, the barn owl, and a generalist, the pigeon. This report presents a comparison between the feathers of the barn owl and the pigeon and emphasise the specific characteristics of the owl's feathers on macroscopic and microscopic level. An understanding of the features and mechanisms underlying this silent flight might eventually be employed for aerodynamic purposes and lead to a new wing design in modern aircrafts. A variety of different feathers (six remiges and six coverts), taken from several specimen in either species, were investigated. Quantitative analysis of digital images and scanning electron microscopy were used for a morphometric characterisation. Although both species have comparable body weights, barn owl feathers were in general larger than pigeon feathers. For both species, the depth and the area of the outer vanes of the remiges were typically smaller than those of the inner vanes. This difference was more pronounced in the barn owl than in the pigeon. Owl feathers also had lesser radiates, longer pennula, and were more translucent than pigeon feathers. The two species achieved smooth edges and regular surfaces of the vanes by different construction principles: while the angles of attachment to the rachis and the length of the barbs was nearly constant for the barn owl, these parameters varied in the pigeon. We also present a quantitative description of several characteristic features of barn owl feathers, e.g., the serrations at the leading edge of the wing, the fringes at the edges of each feather, and the velvet-like dorsal surface. The quantitative description of the feathers and the specific structures of owl feathers can be used as a model for the construction of a biomimetic airplane wing or, in general, as a source for noise-reducing applications on any surfaces subjected to flow fields.

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Morphometric characterisation of wing feathers of the barn owl Tyto alba pratincola and the pigeon Columba livia

: A combined aerodynamics, acoustics and bionics study was conducted in an attempt to discover novel mechanisms to reduce the noise associated with aircraft flight. The strigiformes order of birds, selected in the bionics effort as possessing characteristics of silent flight, was studied extensively. Three mechanisms producing the potential for acoustic quieting were discovered as a result of this study. These are: vortex sheet generators, compliant surfaces, and distributed wing porosity. An experimental program aimed at initiating full scale flight evaluation of these concepts was outlined. The detailed results of these studies are included. The report develops design concepts which may be applied to obtain ultra quiet flight for convert/reconnaissance/surveillance aircraft such as the YO-3.

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Tibor C. Helvey

Papers

Low Speed Aerodynamics for Ultra-Quiet Flight