Features of owl wings that promote silent flight

Owls are an order of birds of prey that are known for the development of a silent flight. We review here the morphological adaptations of owls leading to silent flight and discuss also aerodynamic properties of owl wings. We start with early observations (until 2005), and then turn to recent advances. The large wings of these birds, resulting in low wing loading and a low aspect ratio, contribute to noise reduction by allowing slow flight. The serrations on the leading edge of the wing and the velvet-like surface have an effect on noise reduction and also lead to an improvement of aerodynamic performance. The fringes at the inner feather vanes reduce noise by gliding into the grooves at the lower wing surface that are formed by barb shafts. The fringed trailing edge of the wing has been shown to reduce trailing edge noise. These adaptations to silent flight have been an inspiration for biologists and engineers for the development of devices with reduced noise production. Today several biomimetic applications such as a serrated pantograph or a fringed ventilator are available. Finally, we discuss unresolved questions and possible future directions.

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Trailing-edge noise reduction of a wing by a surface modification

INTER-NOISE and NOISE-CON Congress and Conference Proceedings ◽

10.3397/in-2021-2326 ◽

2021 ◽

Vol 263 (3) ◽

pp. 3194-3201

Author(s):

Varun Bharadwaj Ananthan ◽

R.A.D. Akkermans ◽

Dragan Kozulovic

Keyword(s):

Noise Reduction ◽

Stochastic Approach ◽

Slight Reduction ◽

Noise Sources ◽

Trailing Edge ◽

Sound Sources ◽

Airframe Noise ◽

Trailing Edge Noise ◽

Random Particle ◽

Noise Production

There is an increased emphasis on reducing airframe noise in the last decades. Airframe noise is sound generated by the interaction of a turbulent flow with the aircraft geometry, and significantly contributes to the overall noise production during the landing phase. One examples of airframe noise is the noise generated at a wing's trailing edge, i.e., trailing-edge noise. In this contribution, we numerically explore the local application of riblets for the purpose of trailing-edge noise reduction. Two configurations are studied: i) a clean NACA0012 wing section as a reference, and ii) the same configuration with riblets installed at the wing's aft part. The numerical investigation follows a hybrid computational aeroacoustics approach, where the time-average flow is studied by means of RANS. Noise sources are generated by means of a stochastic approach called Fast Random Particle Mesh method. The results show a deceleration of the flow behind the riblets. Furthermore, the turbulent kinetic energy indicates increased unsteadiness behind the riblets which is shifted away from the wall due to the presence of the riblets. Lastly, the sound sources are investigated by means of the 3D Lamb-vector, which indicates a slight reduction in magnitude near the trailing edge.

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Concave serrations on broadband trailing edge noise reduction

23rd AIAA/CEAS Aeroacoustics Conference ◽

10.2514/6.2017-4174 ◽

2017 ◽

Cited By ~ 2

Author(s):

Daniele Ragni ◽

Francesco Avallone ◽

Wouter C. van der Velden

Keyword(s):

Noise Reduction ◽

Trailing Edge ◽

Trailing Edge Noise

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Benefits of curved serrations on broadband trailing-edge noise reduction

Journal of Sound and Vibration ◽

10.1016/j.jsv.2017.04.007 ◽

2017 ◽

Vol 400 ◽

pp. 167-177 ◽

Cited By ~ 32

Author(s):

F. Avallone ◽

W.C.P. van der Velden ◽

D. Ragni

Keyword(s):

Noise Reduction ◽

Trailing Edge ◽

Trailing Edge Noise

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An experimental investigation of trailing-edge noise reduction due to elasticity

2018 AIAA/CEAS Aeroacoustics Conference ◽

10.2514/6.2018-2802 ◽

2018 ◽

Author(s):

Maurício M. Nilton ◽

Yasir A. Malik ◽

André V. Cavalieri ◽

Leandro D. de Santana ◽

Mauricio V. Donadon ◽

...

Keyword(s):

Experimental Investigation ◽

Noise Reduction ◽

Trailing Edge ◽

Trailing Edge Noise

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Airfoil Trailing Edge Noise Reduction Using a Boundary-Layer Bump

Acta Acustica united with Acustica ◽

10.3813/aaa.919362 ◽

2019 ◽

Vol 105 (5) ◽

pp. 814-826 ◽

Cited By ~ 6

Author(s):

Yuejun Shi ◽

Seongkyu Lee

Keyword(s):

Boundary Layer ◽

Noise Reduction ◽

Suction Side ◽

Reynolds Numbers ◽

Numerical Approach ◽

Trailing Edge ◽

Velocity Deficit ◽

Trailing Edge Noise ◽

Field Noise ◽

High Reynolds

This paper presents a new idea of reducing airfoil trailing edge noise using a small bump in the turbulent boundary layer. First, we develop and validate a new computational approach to predict airfoil trailing edge noise using steady RANS CFD, an empirical wall pressure spectrum model, and Howe's diff raction theory. This numerical approach enables fast and accurate predictions of trailing edge noise, which is used to study the noise reduction from the bump for various airfoil geometries and flow conditions at high Reynolds numbers. Three types of bumps, the suction-side bump, pressure-side bump, and both-side bumps, are studied. The results show that all types of bumps are able to reduce far-field noise up to 10 dB compared to clean airfoils, but their impacts are diff erent in terms of the eff ective frequency range. Also, bumps with four diff erent heights are compared with each other to investigate the eff ect of the height of bumps on noise reduction. It is demonstrated that a bump causes velocity deficit within the boundary layer near the wall. This velocity deficit results in reduced turbulence kinetic energy near the wall, which is responsible for trailing edge noise reduction. Overall, this paper demonstrates the potential of a boundary-layer bump in trailing edge noise reduction and sheds light on the physical mechanism of noise reduction with boundary-layer bumps.

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Novel Three-Dimensional Surface Treatments for Trailing-Edge Noise Reduction

AIAA Journal ◽

10.2514/1.j058586 ◽

2019 ◽

Vol 57 (10) ◽

pp. 4527-4535 ◽

Cited By ~ 4

Author(s):

Abbas Afshari ◽

Ali A. Dehghan ◽

Mahdi Azarpeyvand

Keyword(s):

Noise Reduction ◽

Three Dimensional ◽

Surface Treatments ◽

Trailing Edge ◽

Trailing Edge Noise ◽

Dimensional Surface

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Uniform flow injection into a turbulent boundary layer for trailing edge noise reduction

Physics of Fluids ◽

10.1063/5.0013461 ◽

2020 ◽

Vol 32 (8) ◽

pp. 085104 ◽

Cited By ~ 2

Author(s):

Máté Szőke ◽

Daniele Fiscaletti ◽

Mahdi Azarpeyvand

Keyword(s):

Boundary Layer ◽

Turbulent Boundary Layer ◽

Noise Reduction ◽

Flow Injection ◽

Uniform Flow ◽

Trailing Edge ◽

Trailing Edge Noise

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Shockwave and Noise Abatement of Transonic Fans

Volume 5: Turbo Expo 2004, Parts A and B ◽

10.1115/gt2004-53545 ◽

2004 ◽

Cited By ~ 2

Author(s):

Liping Xu

Keyword(s):

Shock Wave ◽

Noise Reduction ◽

Three Dimensional ◽

Leading Edge ◽

Aerodynamic Performance ◽

Test Results ◽

Shock Strength ◽

Noise Levels ◽

Noise Abatement ◽

Blade Passage

The aerodynamic sources of the forward tone noise of transonic civil fans are analysed. The leading edge shockwave near the rotor tip section is identified as the main source of tone noise. By comparing the tone noise levels of the same fan operating at two different working lines, numerical calculations show that on the lower working line, the main passage shockwave is swallowed and locked into the blade passage, and the fan blades act as a shield to prevent the strong passage shock wave from propagating upstream. The calculations show that, by running the fan at a lower working line, up to 6 db abatement in the blade passing frequency (BPF) tone can be achieved through shielding the shockwave. With three dimensional CFD it is possible to design swept rotors which have desired shockwave structures near the tip region. Fan rotors with different swept leading edges have been designed to study this effect and comparisons in aerodynamics performances as well as the tone noise levels are made. It is predicted that in a swept rotor the leading edge shock strength can be further weakened and up to 5db further reduction in tone noise is possible. With a more secure shockwave shielding, a forward swept rotor has the combination of better aerodynamic performance and better noise abatement feature. The design and test results of a three dimensional fan rotor LNR2, featuring localised forward swept rotor are presented. Rig test results show that although the noise reduction through shock shielding has been demonstrated, the aerodynamics and noise are complicated by the problems specific to such localised forward swept fan.

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