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An analysis of silent flight of owl
Abstract
The present research studies the various parameters such as its special feather materials, the serrated wings feather, long velvet down materials causing the nearly silent flight of an Owl (Barn Owl). An Owl is a bird that is quite common and is well known for its nearly silent flight. The owl contains the long velvet down feathers found on wings and legs absorb the sound frequencies. The elongated distal barbules of the Owl form a multi-layer grid porous structure which also has a positive effect on the sound absorption quality of the Owl. The leading edge serration and trailing edge fringe improves the pressure fluctuation of turbulence boundary, distribute the air rushing and accumulation of air at the end of the wing which suppress the generation of vortex sound and also solves the problem of fluttering in the wings. The owls can perform this phenomenon even at great heights because of their aerodynamically designed wings and feathers. Silent flight gives Owl ability to capture prey by stealth. The main focus and Interest of this paper is to concentrate on the silent flight of Owl and built a basic of a bionic air foil, which is extracted from the Owl wings... This research not only can give the inspiration for solving the aerodynamic noise of aircraft and engineering machine, but also can provide a new idea for the design of low-noise devices Inspired by the noise absorption structure of Owl wings. The results may be further used to build an Unmanned Air Vehicle (UAV), capable of silent flight like Owls.
... At present, the biology of the Strigiformes has been studied in some detail [1][2][3][4][5][6][7][8][9][10][11][12]. Nevertheless, in modern works that provide detailed descriptions of distinctive species-specific morphological features in the body structure and plumage of the Strigiformes [13][14][15], including an exhaustive description of the main aerodynamically advantageous macro-morphological characteristics of the contour feather of individual representatives of the Strigiformes [16][17][18][19][20][21][22], there is almost no information about microstructural features of the feather cover of the Strigiformes. ...
... The presence of the velvet-like dorsal surface of the vane was recorded only in the Barn Owl [17][18][19][20][21][22]. According to our data, the thick velvet-like dorsal surface of the vane is characteristic of all studied species of Strigiformes, including a Snowy Owl, a Northern Hawk-Owl, a Boreal Owl, a Little Owl, and a Eurasian Pygmy Owl and significantly differs from the smoothed dorsal surface of the vane in representatives of raptors (Fig. 7, H). ...
... The presence of the cleft edge of the primary remex inner vane of Strigiformes is a well-known fact. The adaptive nature of the cleft edge of the inner vane of Strigiformes, associated with their noiseless flight in the night-time, is proved [17][18][19][20][21][22]. Nevertheless, the microstructure of the cleft edge of the inner vane -at the SEM level -has not yet been examined in detail. ...
Conducted electron microscopic investigation of the primary remex fine structure of thirteen species of Owls (Strigiformes), using a scanning electron microscope (SEM). It is shown that Owls (Strigiformes) have a number of specific primary remex microstructural characteristics. First of all, these are the features of the structure of the pennaceous barb: a cross section configuration, a pith architectonics on the cross section and longitudinal sections, a cuticular structur of the barb. A number of the unique features in the microstructure of the vanules of the pennaceous barb have been found for the first time (at the scanning electron microscope level, at a large SEM magnification). First of all, these are the structural features of the distal barbules and the structure of the apical portion of the barb with the elongated proximal barbules and the distal barbules tightly contiguous to the ramus and closed with each other. Mentioned characteristics make for the thick velvet-like dorsal surface of the vane and the presence of a complex of peculiar “bunches” (fringes) forming the cleft edge (a fringed edge) of the inner vane – exceptionally specific adaptive characteristics in Strigiformes. Рresentenced original research results suggest that Owls (Strigiformes) have a number specific microstructural characteristics of the primary remex and also a number of the unique features in the microstructure of the primary remex which reflecting the ecological and morphological adaptations conditioned by the flight specificity.
... For instance, many species have asymmetrical ears and a facial disc, which improves their ability to find prey in darkness by hearing (Payne 1971). Additionally, the feathers of owls have a serrated leading edge, a fringe trailing edge, and very fine barbules compared to other birds (Sagar et al. 2017). These features make the feathers softer and allow silent flight (Kopania 2016;Sagar et al. 2017), which presumably also improves hunting success. ...
... Additionally, the feathers of owls have a serrated leading edge, a fringe trailing edge, and very fine barbules compared to other birds (Sagar et al. 2017). These features make the feathers softer and allow silent flight (Kopania 2016;Sagar et al. 2017), which presumably also improves hunting success. ...
... The feathers of owls have a special noise absorption structure that allows them to fly silently while hunting, and this feature has been studied morphologically and acoustically (Kopania 2016;Sagar et al. 2017;Weger & Wagner 2017). However, the genetic correlates of this adaptation in owls remain unclear. ...
Owls (Strigiformes) evolved specific adaptations to their nocturnal predatory lifestyle, such as asymmetrical ears, a facial disc, and a feather structure allowing silent flight. Owls also share some traits with diurnal raptors and other nocturnal birds, such as cryptic plumage patterns, reversed sexual size dimorphism and acute vision and hearing. The genetic basis of some of these adaptations to a nocturnal predatory lifestyle has been studied by candidate gene approaches, but rarely with genome-wide scans. Here, we used a genome-wide comparative analysis to test for selection in the early history of the owls. We estimated the substitution rates in the coding regions of twenty bird genomes, including eleven owls of which five were newly sequenced. Then, we tested for functional overrepresentation across the genes that showed signals of selection. In the ancestral branch of the owls, we found traces of positive selection in the evolution of genes functionally related to visual perception, especially to phototransduction, and to chromosome packaging. Several genes that have been previously linked to acoustic perception, circadian rhythm and feather structure also showed signals of an accelerated evolution in the origin of the owls. We discuss the functions of the genes under positive selection and their putative association with the adaptation to the nocturnal predatory lifestyle of the owls.
... Owls, the pinnacle of stealth flight can hunt even the most adept of prey and have long attracted interests from the aeroacoustics research community. For example, the porous structure of feathers observed in the owl's wing have long been established as one of the facilitators of silent flight [5,6]. This feature has been adapted for the engineering lifting surface to reproduce the low-noise characteristics. ...
... Whilst the level of noise reduction continues to increase, it eventually reaches its peak at a non-dimensional frequency at, (4) SrΔPWL,Max, achieving the (5) maximum level of spectral noise reduction, ΔPWLMax. The slope of this spectral noise increase, (6) ΔPWLSlope is the variation of noise reduction level as a function of the frequency, i.e. ΔPWLMax/ΔSr, where the ΔSr is the Strouhal range defined by (SrΔPWL,Max -SrL). ...
Extensive research efforts in the aeroacoustics community have firmly established the benefits of porous trailing edges to achieve low-noise radiation. However, most studies of porous treatment are based on the use of very complex, open-cell structures to manipulate turbulent flow. Although this implementation has been shown to improve the aeroacoustics performance, the exact physical mechanisms that can be drawn from such a geometry are limited due to their complex topology. This study aims to draw from previous works and to develop an optimised experimental method that utilises a 3D-printed array of rectilinear, structured permeable trailing edges on a NACA-0012 aerofoil based on a Box-Behnken experimental design. The essence of the work is to isolate individual porous parameters, and investigate the interdependencies of these parameters on target values such as the overall sound power level, the Strouhal number of the maximum noise reduction and many other characteristics of the far field. Twenty-eight porous trailing edges were produced based on the initial experimental design. Each is unique with the combination of streamwise and spanwise separation distance between the pores, pore size and porous coverage. The experiment was conducted over various angles of attack and Reynolds numbers. The results show that many of these trailing edges can indeed achieve low-noise radiation, and acceptable prediction accuracies are obtained for all the response variables except the total sound power reduction, ΔOAPWL, and the lower Strouhal limit of the noise reduction. This paper will establish the findings, discuss the results and detail the next stage of the experiment for the improvement of the statistical model.
... From the microscopic point of view, understanding the role of individual feather morphology plays in noise reduction is also an important way to understand owl flight. The long velvet down feathers found on owl wings can absorb the sound frequencies, and the elongated distal barbules of owls belong to multilayer grid porous structures which also has an effect on the sound absorption [8,9]. Graham [10] suggested that these feathers act as a sound absorber. ...
This study mainly focuses on the structural design, acoustic performance prediction, and experimental validation of a novel composite sound absorber that is similar to the feather surface structure of barn owls. The composite sound absorber is composed of an ultra-light nanofibrous membrane backed with a substrate porous materials layer (NMSPM). A theoretical NMSPM model is proposed for predicting the sound absorption coefficient (SAC) of NMSPM. The experimental results demonstrate that the nanofibrous membrane can effectively improve the acoustic performance of substrate porous materials (SPM) based on specific material parameters conditions, which also compare well with the numerical results of the theoretical NMSPM model. The applicability of the theoretical NMSPM model for different sorts of SPMs is explained based on both the experimental and numerical results. A suitable thickness range of nanofibrous membrane for effectively improving the SAC of SPM is given through a series of numerical analyses. This study may provide new thinking for a novel composite sound absorber design without weight sacrifices.
... This design breaks down turbulence into smaller currents called micro-turbulences. These soft feathers allow air to pass through which eliminates sound (Hoppitt, 2000). Some people suspect that, as the owls flies, these feathers may also shift sound energy created by the owl's wing to a higher frequency that prey can't hear. ...
Continuous evolution in nature has created optimum solutions for creature survival that have inspired many innovative engineering designs. Riblet geometries, passive flow control devices, have been studied, which were inspired by the skin of fast-swimming sharks. Turbulent boundary layer research reveals the positive effect of riblets in reducing drag by manipulating turbulent structures. Reducing drag is an important topic for the aviation industry, as it directly relates to fuel savings and reductions in carbon footprints. Aircraft noise represents another significant area of concern. When riblet designs modify turbulent structures, they can also impact pressure sources within the boundary layer, consequently influencing the generation of self-noise. Earlier research studies have demonstrated the favorable outcomes of riblet configurations on the variations in wall pressure, resulting in reduced levels of noise propagation. The current review paper is mainly devoted to the application of riblets in the aviation industry, focusing on studies that are performed in wind tunnels, flight tests, and using numerical techniques. Proving the desired performance of micro-grooves, their method of fabrication and implementation on aircraft surfaces are important topics that are also discussed. In addition, the effect of durability on the performance and required maintenance intervals was previously investigated and is also presented. Finally, recommendations for future activities in the relevant fields of study are provided.
View Video Presentation: https://doi.org/10.2514/6.2021-2109.vid Extensive research efforts in the aeroacoustics community have firmly established the benefits of porous trailing edges to achieve low-noise radiation for aerofoil. This paper investigates a novel and simplistic pore-arrangement at the trailing edge for noise reduction. The research entails several 3D-printed interchangeable trailing edges, each of which has only a single-row of porous-array at a pre-determined distance upstream of the trailing edge. This type of treatment is expected to facilitate a frequency-targeting capability based on the wave interference mechanism. More specifically, it can cause a phase-cancellation between the pressure waves emanating from two sources (i.e. the porous-array and the trailing edge) that are physically displaced in a longitudinal direction. If the acoustic sources are 180 degrees out of phase, destructive interference occurs between the two scattering sources leading to a cancellation of noise. Eighty-one trailing edges that have different combinations of streamwise separation distance between the two sources, pore size and spanwise separation distance between the pores were tested. The results show that many of these trailing edges can indeed achieve low-noise radiation. Near wake measurements have established that the single-row of porous array will not alter the boundary layer significantly. Therefore, the observed noise reduction is solely based on the acoustic interference mechanism only.
High-resolution Particle-Image Velocimetry (PIV) and time-resolved force measurements were performed to analyze the impact of the comb-like structure on the leading edge of barn owl wings on the flow field and overall aerodynamic performance. The Reynolds number was varied in the range of 40,000 to 120,000 and the range of angle of attack was 0° to 6° for the PIV and −15° to +20° for the force measurements to cover the full flight envelope of the owl. As a reference, a wind-tunnel model which possessed a geometry based on the shape of a typical barn owl wing without any owl-specific adaptations was built, and measurements were performed in the aforementioned Reynolds number and angle of attack range. This clean wing model shows a separation bubble in the distal part of the wing at higher angles of attack. Two types of comb-like structures, i.e., artificial serrations, were manufactured to model the owl's leading edge with respect to its length, thickness, and material properties. The artificial structures were able to reduce the size of the separation region and additionally cause a more uniform size of the vortical structures shed by the separation bubble within the Reynolds number range investigated, resulting in stable gliding flight independent of the flight velocity. However, due to increased drag coefficients in conjunction with similar lift coefficients, the overall aerodynamic performance, i.e., lift-to-drag ratio is reduced for the serrated models. Nevertheless, especially at lower Reynolds numbers the stabilizing effect of the uniform vortex size outperforms the lower aerodynamic performance.
Many species of owls are able to fly noiselessly, and their wing feathers play an important role for the silent flight. In this paper, we studied the sound suppression mechanism of the eagle owl (Bubo bubo) by Stereo Microscope (SM), Scanning Electron Microscopy (SEM) and Laser Scanning Confocal Microscope (LSCM). To investigate the effects of special characteristics of wing feather on owl silent flight, the acoustic properties, including the sound absorption coefficient and flight noise, were compared between the eagle owl and common buzzard (Buteo buteo). The results show that the eagle owl generates lower noise than common buzzard during flight, and its wing feather has better sound absorption properties. The leading edge serration and trailing edge fringe can improve the pressure fluctuation of turbulence boundary, and suppress the generation of vortex sound. The elongated distal barbules form a multi-layer grid porous structure which also has an effect on sound absorption. This research not only can give the inspiration for solving the aerodynamic noise of aircraft and engineering machine, but also can provide a new idea for the design of low-noise devices.
Flow control can effectively reduce the aerodynamic noise radiated from a circular cylinder. As one of the flow control methods, a bionic method, inspired by the serrations at the leading edge of owls' wing, was proposed in this paper. The effects of bionic serrated structures arranged on the upper and lower sides of a cylinder on the aerodynamic and aeroacoustic performance of the cylinder were numerically investigated. At a free stream speed of 24.5 m·s−1, corresponding to Reynolds number of 1.58×104, the simulation results indicate that the bionic serrated structures can decrease the frequency of the vortex shedding and control the fluctuating aerodynamic force acting on the cylinder, thus reduce the aerodynamic noise. A qualitative view of the vorticity in the wake of the cylinder suggest that the serrated structures reduce aerodynamic sound by suppressing the unsteady motion of vortices