Noise reduction using combined trailing edge and leading edge serrations in a tandem airfoil experiment

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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Prediction of noise from serrated trailing edges

Journal of Fluid Mechanics ◽

10.1017/jfm.2016.132 ◽

2016 ◽

Vol 793 ◽

pp. 556-588 ◽

Cited By ~ 100

Author(s):

B. Lyu ◽

M. Azarpeyvand ◽

S. Sinayoko

Keyword(s):

Noise Reduction ◽

Analytical Model ◽

Leading Edge ◽

Experimental Investigations ◽

Destructive Interference ◽

Trailing Edge ◽

Trailing Edges ◽

Mach Numbers ◽

Trailing Edge Serrations ◽

High Mach

A new analytical model is developed for the prediction of noise from serrated trailing edges. The model generalizes Amiet’s trailing-edge noise theory to sawtooth trailing edges, resulting in a complicated partial differential equation. The equation is then solved by means of a Fourier expansion technique combined with an iterative procedure. The solution is validated through comparison with the finite element method for a variety of serrations at different Mach numbers. The results obtained using the new model predict noise reduction of up to 10 dB at 90$^{\circ }$ above the trailing edge, which is more realistic than predictions based on Howe’s model and also more consistent with experimental observations. A thorough analytical and numerical analysis of the physical mechanism is carried out and suggests that the noise reduction due to serration originates primarily from interference effects near the trailing edge. A closer inspection of the proposed mathematical model has led to the development of two criteria for the effectiveness of the trailing-edge serrations, consistent but more general than those proposed by Howe. While experimental investigations often focus on noise reduction at 90$^{\circ }$ above the trailing edge, the new analytical model shows that the destructive interference scattering effects due to the serrations cause significant noise reduction at large polar angles, near the leading edge. It has also been observed that serrations can significantly change the directivity characteristics of the aerofoil at high frequencies and even lead to noise increase at high Mach numbers.

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Acoustic scattering by a finite rigid plate with a poroelastic extension

Journal of Fluid Mechanics ◽

10.1017/jfm.2016.59 ◽

2016 ◽

Vol 791 ◽

pp. 414-438 ◽

Cited By ~ 36

Author(s):

Lorna J. Ayton

Keyword(s):

Noise Reduction ◽

Acoustic Field ◽

Wind Turbines ◽

Acoustic Scattering ◽

Leading Edge ◽

Chord Length ◽

Far Field ◽

Rigid Plate ◽

Acoustic Source ◽

Trailing Edge

The scattering of sound by a finite rigid plate with a finite poroelastic extension interacting with an unsteady acoustic source is investigated to determine the effects of porosity, elasticity and the length of the extension when compared to a purely rigid plate. The problem is solved using the Wiener–Hopf technique, and an approximate Wiener–Hopf factorisation process is implemented to yield reliable far-field results quickly. Importantly, finite chord-length effects are taken into account, principally the interaction of a rigid leading-edge acoustic field with a poroelastic trailing-edge acoustic field. The model presented discusses how the poroelastic trailing-edge property of owls’ wings could inspire quieter aeroacoustic designs in bladed systems such as wind turbines, and provides a framework for analysing the potential noise reduction of these designs.

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Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review

Applied Sciences ◽

10.3390/app9112224 ◽

2019 ◽

Vol 9 (11) ◽

pp. 2224 ◽

Cited By ~ 11

Author(s):

Yong Wang ◽

Kun Zhao ◽

Xiang-Yu Lu ◽

Yu-Bao Song ◽

Gareth J. Bennett

Keyword(s):

Noise Reduction ◽

Noise Control ◽

Experimental Studies ◽

Morphological Characteristics ◽

Leading Edge ◽

Aerodynamic Noise ◽

Trailing Edge ◽

Reduction Techniques ◽

Noise Measurements ◽

Underlying Mechanisms

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.

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Numerical study on the airfoil self-noise of three owl-based wings with the trailing-edge serrations

Proceedings of the Institution of Mechanical Engineers Part G Journal of Aerospace Engineering ◽

10.1177/0954410020988229 ◽

2021 ◽

pp. 095441002098822

Author(s):

Dian Li ◽

Xiaomin Liu ◽

Lei Wang ◽

Fujia Hu ◽

Guang Xi

Keyword(s):

Flow Field ◽

Noise Reduction ◽

Numerical Study ◽

Barn Owl ◽

Aerodynamic Noise ◽

Frequency Noise ◽

Trailing Edge ◽

Reduction Mechanisms ◽

Trailing Edge Serrations ◽

Bionic Airfoil

Previous publications have summarized that three special morphological structures of owl wing could reduce aerodynamic noise under low Reynolds number flows effectively. However, the coupling noise-reduction mechanism of bionic airfoil with trailing-edge serrations is poorly understood. Furthermore, while the bionic airfoil extracted from natural owl wing shows remarkable noise-reduction characteristics, the shape of the owl-based airfoils reconstructed by different researchers has some differences, which leads to diversity in the potential noise-reduction mechanisms. In this article, three kinds of owl-based airfoils with trailing-edge serrations are investigated to reveal the potential noise-reduction mechanisms, and a clean airfoil based on barn owl is utilized as a reference to make a comparison. The instantaneous flow field and sound field around the three-dimensional serrated airfoils are simulated by using incompressible large eddy simulation coupled with the FW-H equation. The results of unsteady flow field show that the flow field of Owl B exhibits stronger and wider-scale turbulent velocity fluctuation than that of other airfoils, which may be the potential reason for the greater noise generation of Owl B. The scale and magnitude of alternating mean convective velocity distribution dominates the noise-reduction effect of trailing-edge serrations. The noise-reduction characteristic of Owl C outperforms that of Barn owl, which suggests that the trailing-edge serrations can suppress vortex shedding noise of flow field effectively. The trailing-edge serrations mainly suppress the low-frequency noise of the airfoil. The trailing-edge serration can suppress turbulent noise by weakening pressure fluctuation.

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Three-Dimensional and Rotational Aerodynamics on the NREL Phase VI Wind Turbine Blade

Journal of Solar Energy Engineering ◽

10.1115/1.2931506 ◽

2008 ◽

Vol 130 (3) ◽

Cited By ~ 10

Author(s):

Alvaro Gonzalez ◽

Xabier Munduate

Keyword(s):

Wind Turbine ◽

Turbine Blade ◽

Three Dimensional ◽

Separated Flow ◽

Leading Edge ◽

Wind Turbine Blade ◽

Trailing Edge ◽

Rotating Blade ◽

Nrel Phase Vi ◽

Edge Separation

This work undertakes an aerodynamic analysis over the parked and the rotating NREL Phase VI wind turbine blade. The experimental sequences from NASA Ames wind tunnel selected for this study respond to the parked blade and the rotating configuration, both for the upwind, two-bladed wind turbine operating at nonyawed conditions. The objective is to bring some light into the nature of the flow field and especially the type of stall behavior observed when 2D aerofoil steady measurements are compared to the parked blade and the latter to the rotating one. From averaged pressure coefficients together with their standard deviation values, trailing and leading edge separated flow regions have been found, with the limitations of the repeatability of the flow encountered on the blade. Results for the parked blade show the progressive delay from tip to root of the trailing edge separation process, with respect to the 2D profile, and also reveal a local region of leading edge separated flow or bubble at the inner, 30% and 47% of the blade. For the rotating blade, results at inboard 30% and 47% stations show a dramatic suppression of the trailing edge separation, and the development of a leading edge separation structure connected with the extra lift.

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Trailing edge flap influence on leading edge vortex flap aerodynamics

10.2514/6.1982-128 ◽

1982 ◽

Cited By ~ 2

Author(s):

J. MARCHMAN, III ◽

A. GRANTZ

Keyword(s):

Leading Edge ◽

Trailing Edge ◽

Leading Edge Vortex ◽

Edge Vortex ◽

Trailing Edge Flap

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Effect of kinematics of orogenic wedge on kinematic evolutionary paths and deformation profiles of major shear zones: An example from the eastern Himalaya

10.5194/egusphere-egu21-3824 ◽

2021 ◽

Author(s):

Pritam Ghosh ◽

Kathakali Bhattacharyya

Keyword(s):

Shear Zone ◽

Strain Softening ◽

Leading Edge ◽

Shear Zones ◽

Deformation History ◽

Progressive Deformation ◽

Trailing Edge ◽

Orogenic Wedge ◽

Transport Direction ◽

Evolutionary Paths

We examine how the deformation profile and kinematic evolutionary paths of two major shear zones with prolonged deformation history and large translations differ with varying structural positions along its transport direction in an orogenic wedge. We conduct this analysis on multiple exposures of the internal thrusts from the Sikkim Himalayan fold thrust belt, the Pelling-Munsiari thrust (PT), the roof thrust of the Lesser Himalayan duplex (LHD), and the overlying Main Central thrust (MCT). These two thrusts are regionally folded due to growth of the LHD and are exposed at different structural positions. The hinterlandmost exposures of the MCT and PT zones lie in the trailing parts of the duplex, while the foreland-most exposures of the same studied shear zones lie in the leading part of the duplex, and thus have recorded a greater connectivity with the duplex. The thicknesses of the shear zones progressively decrease toward the leading edge indicating variation in deformation conditions. Thickness-displacement plot reveals strain-softening from all the five studied MCT and the PT mylonite zones. However, the strain-softening mechanisms varied along its transport direction with the hinterland exposures recording dominantly dislocation-creep, while dissolution-creep and reaction-softening are dominant in the forelandmost exposures. Based on overburden estimation, the loss of overburden on the MCT and the PT zones is more in the leading edge (~26km and ~15km, respectively) than in the trailing edge (~10km and ~17km, respectively), during progressive deformation. Based on recalibrated recrystallized quartz grain thermometer (Law, 2014), the estimated deformation temperatures in the trailing edge are higher (~450-650&#176;C) than in the leading edge (350-550&#176;C) of the shear zones. This variation in the deformation conditions is also reflected in the shallow-crustal deformation structures with higher fracture intensity and lower spacing in the leading edge exposures of the shear zones as compared to the trailing edge exposures.The proportion of mylonitic domains and micaceous minerals within the exposed shear zones increase and grain-size of the constituent minerals decreases progressively along the transport direction. This is also consistent with progressive increase in mean Rs-values toward leading edge exposures of the same shear zones. Additionally, the &#945;-value (stretch ratio) gradually increases toward the foreland-most exposures along with increasing angular shear strain. Vorticity estimates from multiple incremental strain markers indicate that the MCT and PT zones generally record a decelerating strain path. Therefore, the results from this study are counterintuitive to the general observation of a direct relationship between higher Rs-value and higher pure-shear component. We explain this observation in the context of the larger kinematics of the orogen, where the leading edge exposures have passed through the duplex structure, recording the greatest connectivity and most complete deformation history, resulting in the weakest shear zone that is also reflected in the deformation profiles and strain attributes. This study demonstrates that the same shear zone records varying deformation profile, strain and kinematic evolutionary paths due to varying deformation conditions and varying connectivity to the underlying footwall structures during progressive deformation of an orogenic wedge.

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