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Author: Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
A new turbulence model is used to describe the acoustical scattering from atmospheric turbulence. A complete set of fluid equations, including the heat flow equation with zero conductivity, is presented for an ideal gas atmosphere. From this set, a complete set of coupled linear differential equation is derived for the acoustic pressure, temperature, mass density, and velocity in the presence of stationary turbulence. From these acoustic wave equations, expressions for acoustic scattering cross sections are derived for individual localized stationary scalable turbules of arbitrary morphology and orientation. Averages over random turbule orientations are also derived. Criteria for comparability of orientationally averaged turbules with different envelope functions are presented and applied, and cross sections for Gaussian and exponential envelopes are compared. The azimuthal dependence of the velocity scattering cross section for a spherically symmetric nonuniformly rotating turbule is illustrated. It is shown that, for incoherent scattering, a collection of randomly oriented turbules of arbitrary morphology may be replaced by an 'equivalent' collection of spherically symmetric, nonuniformly rotating turbules with randomly directed rotation axes.
Author: Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
A new turbulence model is used to describe the acoustical scattering from atmospheric turbulence. A complete set of fluid equations, including the heat flow equation with zero conductivity, is presented for an ideal gas atmosphere. From this set, a complete set of coupled linear differential equation is derived for the acoustic pressure, temperature, mass density, and velocity in the presence of stationary turbulence. From these acoustic wave equations, expressions for acoustic scattering cross sections are derived for individual localized stationary scalable turbules of arbitrary morphology and orientation. Averages over random turbule orientations are also derived. Criteria for comparability of orientationally averaged turbules with different envelope functions are presented and applied, and cross sections for Gaussian and exponential envelopes are compared. The azimuthal dependence of the velocity scattering cross section for a spherically symmetric nonuniformly rotating turbule is illustrated. It is shown that, for incoherent scattering, a collection of randomly oriented turbules of arbitrary morphology may be replaced by an 'equivalent' collection of spherically symmetric, nonuniformly rotating turbules with randomly directed rotation axes.
Author: Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
From a complete set of fluid equations, a complete set of coupled linear differential equations for the acoustic pressure, temperature, mass density, and velocity in the presence of stationary turbulence may be derived. To first order in the turbulent temperature variation and flow velocity, these coupled acoustic equations yield an acoustic wave equation given in the literature. Further reduction of this wave equation results in a second equation given in the literature which is good for turbulent length scales alpha much greater than the acoustic wavelength lambda. The length scale alpha(s) of the scattering volume is found to be just as important as alpha and lambda in predicting the general behavior of acoustic scattering by turbulence. In particular, if alpha “alpha(s), then the first Born temperature and velocity scattering amplitudes for any ratio alpha/lambda are the usual ones predicted by the first equation, and both the forward and backward velocity scattering are essentially zero for solenoidal turbulent flow velocity. The latter is not true if alpha> alpha(s). If a>/= alpha(s)” lambda, then the first Born scattering amplitudes are those predicted by the second equation. If lambda>/= alpha>/= alpha(s), other forms result for the scattering amplitudes. Implications of these findings for predicting results of acoustical scattering experiments where the scattering volume is often ill defined are discussed.
Author: Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
The objective of the ASL research effort in acoustic propagation is to provide the Army with a multi-stream model for investigating acoustic detection systems. The first step in developing this model is to account for turbulent scattering. Five elements are necessary to accomplish this step: (1) model the turbulent region as a collection of vortices with a distribution of characteristic sizes/velocities; (2) characterize each vortex (turbule) as a known (or assumed) velocity distribution in three space; (3) solve the fluid equations to determine the scattering from each turbule; (4) sum the contributions to the scattered sound pressure level at the detector location of all turbules accounting for the propagation characteristics of the atmospheric medium; and (5) incorporate the algorithms devised above into existing (or appropriately modified) propagation models. Progress in these five areas will be reported.
Author: Allan D. Pierce Publisher: Academic Press ISBN: 1483257762 Category : Technology & Engineering Languages : en Pages : 395
Book Description
Underwater Scattering and Radiation describes the relevant theoretical foundations of underwater scattering and radiation. Acoustic scattering from elastic solids is discussed, and variational formulations in acoustic radiation and scattering are presented. Surface waves and quasi-cylindrical modes are also explored, along with the Helmholtz-Kirchhoff integral corollaries. Comprised of two chapters, this volume begins with a comprehensive account of scattering by elastic objects, focusing on the classic idealized shapes of spheres and infinite cylinders. The reader is introduced to important concepts such as normal modes, the S-matrix, and the T-matrix as well as resonances, whispering gallery modes, Franz modes, and Stoneley waves. Subsequent sections describe methods for treating scattering by elastic bodies of more general shapes. The T-matrix formalism is discussed and then applied to spheroidal scatterers and finite cylinders. The second chapter analyzes how variational principles can be used in acoustics, with the choice of topics directed toward applications to underwater acoustic radiation and scattering. This book will be of interest to physicists.
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Author: Acoustical Society of America
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Author: Stanford University. Ginzton Laboratory
Author: Allan D. Pierce
Author: Robert Peter Banaugh
Author: K. H. Chen
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Author: Institute for Computer Applications in Science and Engineering