Abstract
It is well-known that many species of owl have the unique ability to fly silently, which can be attributed to their distinctive and special feather adaptations. Inspired by the owls, researchers attempted to reduce the aerodynamic noise of aircraft and other structures by learning their noise reduction features from different viewpoints and then using the gained knowledge to develop a number of innovative noise reduction solutions. Although fruitful results have been achieved in the bio-inspired aerodynamic noise control, as far as the authors know, comparatively little work has been done to summarize the main findings and progresses in this area. In this bibliographic survey, we systematically review the progresses and trends of the bio-inspired aerodynamic noise control, including the macroscopic and microscopic morphological characteristics of the owl wing feathers, the noise measurements on both flying birds in the field and prepared wings in the wind tunnel, as well as theoretical, numerical and experimental studies that explored the feasibility, parameter influence, aerodynamic effects and underlying mechanisms of the four main bio-inspired noise reduction techniques, i.e., leading edge serrations, trailing edge serrations, fringe-type trailing edge extensions and porous material inspired noise reduction. Finally, we also give some suggestions for future work.
Highlights
In recent years, the problem of aerodynamic noise and its control have become a matter of serious concern and attracted a number of researchers’ interest
Kroeger 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, 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
The results indicate that the leading edge serrations could effectively reduce both rotational noise and vortex noise components under some specific configurations and running conditions
Summary
The problem of aerodynamic noise and its control have become a matter of serious concern and attracted a number of researchers’ interest. Graham [20] compared the wing feathers of the owls and other non-silent flying birds, and suggested that the owl’s wings have three distinctive and unique characteristics that can reduce noise (see Figure 1), namely, the serrated feathers on the leading edges, the fringes formed at the trailing edges, and the soft downy coating on the surface of wings and legs. This motivates the present paper, which aims to go one step into that direction In this bibliographic survey, the progresses and trends of the bio-inspired aerodynamic noise control are systematically reviewed, and some suggestions for future work are discussed, in an effort to advance the state of the art in these important areas and try to focus future studies into the most promising directions or directions that have not yet been perfectly understood.
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