Spectral broadening of an acoustic pulse propagating through turbulence

Brown, Edmund H.; Clifford, S. F.

doi:10.1121/1.1913572

Cited by 20 publications

(8 citation statements)

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“…This model is able to consider multiple scattering, whereas previous models were restricted to the hypothesis of single scattering. Turbulent scattering was also studied by Brown 2 and Brown & Clifford 3,4 in the context of propagation through a turbulent atmosphere.…”

Section: Introductionmentioning

confidence: 99%

Numerical assessment of the scattering of acoustic waves by turbulent structures

Clair

Gabard

2015

21st AIAA/CEAS Aeroacoustics Conference

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The scattering of harmonic sound waves by a single vortex is studied to provide some insight into the more complex problem of sound scattering by a turbulent layer. Two different methods are used to model the scattering by a steady vortex or by a vortex convected in a uniform mean flow. The first method is based on the linearized Euler equations in the frequency domain and is used to study the scattering by a steady vortex. The speed of this method allows to perform a study of the spatial scattering of a plane wave over a wide range of frequencies, where analytical models previously developed are restricted either to low or high frequencies. The second method uses a finite difference solver, also based on the linearized Euler equations but in the time domain. This method is used to study the scattering by a vortex convected in a uniform mean fow, where a spectral scattering occurs in addition to the spatial scattering. A parametric study is realized, focusing on four parameters including the source frequency and the convection velocity. The spectra deduced from this study show a pattern similar to the haystacking observed in existing studies of the scattering by a turbulent shear layer. Some of the trends deduced from the parametric study are also in agreement with these previous observations. Thus, the detailled study of the scattering by a single eddy is able to provide further understanding of the turbulence scattering.

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Section: Introductionmentioning

confidence: 99%

Numerical assessment of the scattering of acoustic waves by turbulent structures

Clair

Gabard

2015

21st AIAA/CEAS Aeroacoustics Conference

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“…This model includes the effect of multiple scattering that can appear if the turbulent volume is large enough so that acoustic waves are scattered successively by numerous eddies, while previous models were restricted to the hypothesis of single scattering (usually using the Born approximation). Brown (1974) and Brown & Clifford (1973, 1976) developed an analytical model for the scattered intensity based on an inhomogeneous Helmholtz equation with a source term involving velocity and temperature fluctuations. The parabolic equation has been used to derive both analytical (Ostashev et al 2001) and numerical (Dallois et al 2001) models for the evolution of the sound pressure level with the propagation distance through turbulence.…”

Section: Introductionmentioning

confidence: 99%

Spectral broadening of acoustic waves by convected vortices

Clair¹,

Gabard²

2018

J. Fluid Mech.

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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.

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“…The amplitude of received acdar signals depends upon the intensity of scattering or partial reflection of sound in regions of turbulent temperature and wind fluctuations. The altitude of a scatterin• region is determined by using the elapsed signal travel time, and atmospheric velocities are inferred from the properties of the Doppler spectrum of the received signal [see, e.g., Brown, .1972;Brown and Clifford, 1973;Thomson and Coulter, 1974].…”

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A method for measurement of temperature profiles in inversions from refractive transmission of sound

Greenfield¹,

Teufel²,

Thomson³

et al. 1974

J. Geophys. Res.

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A method is described for estimating temperature profiles in the lower troposphere during conditions including a surface-based or elevated inversion layer. The method uses acoustic energy transmitted over paths of the order of 10 km in length. Measurements are made at approximately l-kin intervals extending radially outward from the transmitter. The vertical temperature profile is modeled as two constant temperature gradient layers. The first layer extending from the surface to height H1 has a temperature gradient T•' (usually negative upward). The second-layer temperature gradient T2' is strongly positive upward. For temperature profiles of this type, ray paths arrive with a high intensity at a caustic, and no rays return to earth between the source and the caustic. The method requires that H1 be determined by some other means such as vertical acdar sounding. The Ti' and T2' are then simultaneously determined by measuring the range to the caustic and the wave propagation time. Even if the propagation time cannot be measured, useful estimates of T•' can be obtained from observations of H1 and the caustic distance. For a

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Spectral broadening of an acoustic pulse propagating through turbulence

Cited by 20 publications

References 0 publications

Numerical assessment of the scattering of acoustic waves by turbulent structures

Numerical assessment of the scattering of acoustic waves by turbulent structures

Spectral broadening of acoustic waves by convected vortices

A method for measurement of temperature profiles in inversions from refractive transmission of sound

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