scholarly journals Spectral broadening of acoustic waves by convected vortices

2018 ◽  
Vol 841 ◽  
pp. 50-80 ◽  
Author(s):  
Vincent Clair ◽  
Gwénaël Gabard
Keyword(s):  
Scattered Field ◽  
Acoustic Waves ◽  
Acoustic Scattering ◽  
Two Dimensions ◽  
Wind Tunnels ◽  
Spectral Broadening ◽  
Sound Fields ◽  
Physical Mechanisms ◽  
Doppler Shifts ◽  
Acoustic Spectra

The scattering of acoustic waves by a moving vortex is studied in two dimensions to bring further insight into the physical mechanisms responsible for the spectral broadening caused by a region of turbulence. When propagating through turbulence, a monochromatic sound wave will be scattered over a range of frequencies, resulting in typical spectra with broadband sidelobes on either side of the tone. This spectral broadening, also called ‘haystacking’, is of importance for noise radiation from jet exhausts and for acoustic measurements in open-jet wind tunnels. A semianalytical model is formulated for a plane wave scattered by a vortex, including the influence of the convection of the vortex. This allows us to perform a detailed parametric study of the properties and evolution of the scattered field. A time-domain numerical model for the linearised Euler equations is also used to consider more general sound fields, such as that radiated by a point source in a uniform flow. The spectral broadening stems from the combination of the spatial scattering of sound due to the refraction of waves propagating through the vortex, and two Doppler shifts induced by the motion of the vortex relative to the source and of the observer relative to the vortex. The fact that the spectrum exhibits sidebands is directly explained by the directivity of the scattered field which is composed of several beams radiating from the vortex. The evolution of the acoustic spectra with the parameters considered in this paper is compared with the trends observed in previous experimental work on acoustic scattering by a jet shear layer.

Acoustic scattering by a finite nonlinear elastic plate. I Primary, secondary and combination resonances

1987 ◽  
Vol 414 (1846) ◽  
pp. 237-253 ◽  
Keyword(s):  
Incident Wave ◽  
Elastic Plate ◽  
Scattered Field ◽  
Acoustic Waves ◽  
Multiple Scales ◽  
Acoustic Scattering ◽  
Scattered Wave ◽  
Thin Elastic Plate ◽  
Finite Width ◽  
Wave Angle

A thin elastic plate of finite width is irradiated by time-harmonic acoustic waves. The fluid is assumed light compared with the plate mass, and the forcing term is of sufficient amplitude to necessitate the inclusion of a nonlinear term (due to mid-plane stretching) in the plate equation. The order-one scattered field is determined by the method of multiple scales when the forcing frequency approaches a free oscillation frequency (eigenfrequency) of the plate. This solution is shown to agree with previous work, for the linear problem, and can be multivalued for particular values of the plate-fluid parameters. The scattered wave may also exhibit jumps in its amplitude and phase angle as it varies with frequency, incident-wave angle or incident-wave amplitude. The non-linear term further allows the possibility of secondary and combination resonances. These are investigated and the scattered field is shown to contain terms of different frequencies to those of the incident waves. Multivalued solutions and the associated jump phenomenon are again found for these resonant cases.

A stable method for an inverse problem in acoustic scattering by an obstacle with an impedance boundary condition

1984 ◽  
Vol 98 (3-4) ◽  
pp. 355-364 ◽  
Author(s):  
Robert T. Smith
Keyword(s):  
Boundary Condition ◽  
Inverse Problem ◽  
Acoustic Waves ◽  
Acoustic Scattering ◽  
Two Dimensions ◽  
Impedance Boundary Condition ◽  
Compact Set ◽  
Stable Method ◽  
Low Frequencies ◽  
Impedance Boundary

SynopsisWe examine the case of plane, time-harmonic acoustic waves in two dimensions, scattered by an obstacle on the surface of which an impedance boundary condition is imposed. A stable method is developed for solving the inverse problem ofdetermining both the shape of the scatterer and the surface impedance from measurements of the asymptotic behaviour of the scattered waves at low frequencies. We accomplish this by minimizing an appropriate functional over a compact set of admissible boundary curves and admissible impedances.

scholarly journals Numerical Solution of Acoustic Scattering by an Adaptive DtN Finite Element Method

2013 ◽  
Vol 13 (5) ◽  
pp. 1277-1244 ◽  
Author(s):  
Xue Jiang ◽  
Peijun Li ◽  
Weiying Zheng
Keyword(s):  
Boundary Condition ◽  
Finite Element ◽  
Boundary Value Problem ◽  
Boundary Value ◽  
Scattering Problem ◽  
Acoustic Scattering ◽  
Adaptive Method ◽  
Two Dimensions ◽  
Adaptive Finite Element ◽  
Acoustic Wave Scattering

AbstractConsider the acoustic wave scattering by an impenetrable obstacle in two dimensions, where the wave propagation is governed by the Helmholtz equation. The scattering problem is modeled as a boundary value problem over a bounded domain. Based on the Dirichlet-to-Neumann (DtN) operator, a transparent boundary condition is introduced on an artificial circular boundary enclosing the obstacle. An adaptive finite element based on a posterior error estimate is presented to solve the boundary value problem with a nonlocal DtN boundary condition. Numerical experiments are included to compare with the perfectly matched layer (PML) method to illustrate the competitive behavior of the proposed adaptive method.

INVERSE ACOUSTIC SCATTERING PROBLEMS IN OCEAN ENVIRONMENTS

1999 ◽  
Vol 07 (02) ◽  
pp. 111-132 ◽  
Author(s):  
YONGZHI XU
Keyword(s):  
Acoustic Waves ◽  
Three Dimensional ◽  
Acoustic Scattering ◽  
Stratified Medium ◽  
Computational Results ◽  
Scattering Problems ◽  
Numerical Examples ◽  
Layered Waveguide ◽  
Shape Determination

This paper presents theoretical and computational results from our research on inverse scattering problems for acoustic waves in ocean environments. In particular, we discuss the determination of a three-dimensional (3-D) distributed inhomogeneity in a two-layered waveguide from scattered sound and the shape determination of an object in a stratified medium. Numerical examples are presented.

scholarly journals Acoustic tweezers via sub–time-of-flight regime surface acoustic waves

2016 ◽  
Vol 2 (7) ◽  
pp. e1600089 ◽  
Author(s):  
David J. Collins ◽  
Citsabehsan Devendran ◽  
Zhichao Ma ◽  
Jia Wei Ng ◽  
Adrian Neild ◽  
...  
Keyword(s):  
Optical Tweezers ◽  
Acoustic Waves ◽  
Optical Waveguides ◽  
Surface Acoustic Waves ◽  
Time Of Flight ◽  
Microfluidic Channel ◽  
Two Dimensions ◽  
Acoustic Standing Wave ◽  
Spatial Dimensions ◽  
Acoustic Tweezers

Micrometer-scale acoustic waves are highly useful for refined optomechanical and acoustofluidic manipulation, where these fields are spatially localized along the transducer aperture but not along the acoustic propagation direction. In the case of acoustic tweezers, such a conventional acoustic standing wave results in particle and cell patterning across the entire width of a microfluidic channel, preventing selective trapping. We demonstrate the use of nanosecond-scale pulsed surface acoustic waves (SAWs) with a pulse period that is less than the time of flight between opposing transducers to generate localized time-averaged patterning regions while using conventional electrode structures. These nodal positions can be readily and arbitrarily positioned in two dimensions and within the patterning region itself through the imposition of pulse delays, frequency modulation, and phase shifts. This straightforward concept adds new spatial dimensions to which acoustic fields can be localized in SAW applications in a manner analogous to optical tweezers, including spatially selective acoustic tweezers and optical waveguides.

scholarly journals Optical and acoustical measurement of ballistic noise signatures

2021 ◽  
Author(s):  
Matthew Blevins ◽  
Gregory Lyons ◽  
Carl Hart ◽  
Michael White
Keyword(s):  
Acoustic Waves ◽  
Preliminary Analysis ◽  
Imaging System ◽  
Pressure Sensors ◽  
Physical Mechanisms ◽  
Ballistic Projectiles ◽  
Localization And Tracking ◽  
Ongoing Project ◽  
Experimental Effort ◽  
The U.S

Supersonic projectiles in air generate acoustical signatures that are fundamentally related to the projectile’s shape, size, and velocity. These characteristics influence various mechanisms involved in the generation, propagation, decay, and coalescence of acoustic waves. To understand the relationships between projectile shape, size, velocity, and the physical mechanisms involved, an experimental effort captured the acoustic field produced by a range of supersonic projectiles using both conventional pressure sensors and a schlieren imaging system. The results of this ongoing project will elucidate those fundamental mechanisms, enabling more sophisticated tools for detection, classification, localization, and tracking. This paper details the experimental setup, data collection, and preliminary analysis of a series of ballistic projectiles, both idealized and currently in use by the U.S. Military.

Acoustic Scattering

2017 ◽  
Author(s):  
John A. Adam
Keyword(s):  
Acoustic Waves ◽  
Acoustic Radiation ◽  
Scattering Amplitude ◽  
Medical Physics ◽  
Plane Waves ◽  
Acoustic Scattering ◽  
Radiation Condition ◽  
Optical Theorem ◽  
Rigid Sphere ◽  
Symmetric Geometry

This chapter focuses on the mathematics underlying the scattering of acoustic waves. Scattering of waves and/or particles is a common phenomenon. The scattering of plane waves from spheres is applied in a wide array of fields, from optics and acoustics to meteorology, elasticity, seismology, medical physics, quantum mechanics, and biochemistry. With respect to the problem of electromagnetic wave scattering from a sphere, Lorenz found the complete mathematical solution in 1890 in terms of an infinite series of so-called partial waves. The solution is known as the Mie or Debye-Mie solution. The chapter first considers scattering by a cylinder and time-averaged energy flux before discussing spherically symmetric geometry, taking into account the scattering amplitude, the optical theorem, and the Sommerfeld radiation condition. It also examines the case of a rigid sphere, acoustic radiation from a rigid pulsating sphere, the sound of mountain streams, and mathematical bubbles.

Coupled Finite Element/Boundary Element Method for the Analysis of the Acoustic Scattering From Elastic Structures

1995 ◽  
Author(s):  
Bertrand Dubus ◽  
Antoine Lavie ◽  
Dominique Decultot ◽  
Gérard Maze
Keyword(s):  
Finite Element ◽  
Boundary Element ◽  
Acoustic Waves ◽  
Acoustic Scattering ◽  
Resonant Mode ◽  
Mode Shapes ◽  
Thin Cylindrical Shell ◽  
Isolation And Identification ◽  
Fluid Medium ◽  
Physical Phenomena

Abstract Considerable interest has been expressed recently in the scattering of acoustic waves from elastic targets. For structures of arbitrary shapes, the numerical method relying upon a finite element description of the solid part and a boundary element description of the waves propagating in the infinite fluid medium is the most commonly used. This paper presents a coupling between the ATILA finite element code and the EQI boundary element code performed using a solid variable methodology. Results are presented for a thin cylindrical shell bounded by hemispherical endcaps which are insonified at axial and normal incidences. Comparisons are made with measurements of backscattered pressure spectra and angular patterns obtained with the quasi-harmonic MIIR (Method of Isolation and Identification of Resonances). Emphasis is put on post-processing techniques contributing to the interpretation of physical phenomena such as the extraction of resonant mode shapes.

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