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Author: Publisher: ISBN: Category : Languages : en Pages : 79
Book Description
The objective of the ARL research effort in acoustic propagation is to provide the Army with a multistream model for investigating acoustic detection systems operating in a variety of realistic battlefield conditions. The first step is to account for scattering from turbulent regions of the atmosphere with an explicit algorithm. The turbulent region is considered to be a collection of vortices with a distribution of characteristic sizes and velocities and random orientation and position. Each vortex (turbule) is characterized as a known (or assumed) velocity distribution in three dimensional space. The scattering from each turbule is determined by solving the fluid equations. The contribution to the sound pressure level at the detector location of all turbules is summed up accounting for the propagation characteristics of the atmospheric medium. The algorithms devised to model the above picture of the turbulent region are then used in existing (or appropriately modified) propagation models. Progress in incorporating turbulence scattering into the Past Field Program (PFP) acoustic propagation model is recounted in this report. (MM).
Author: Publisher: ISBN: Category : Languages : en Pages : 79
Book Description
The objective of the ARL research effort in acoustic propagation is to provide the Army with a multistream model for investigating acoustic detection systems operating in a variety of realistic battlefield conditions. The first step is to account for scattering from turbulent regions of the atmosphere with an explicit algorithm. The turbulent region is considered to be a collection of vortices with a distribution of characteristic sizes and velocities and random orientation and position. Each vortex (turbule) is characterized as a known (or assumed) velocity distribution in three dimensional space. The scattering from each turbule is determined by solving the fluid equations. The contribution to the sound pressure level at the detector location of all turbules is summed up accounting for the propagation characteristics of the atmospheric medium. The algorithms devised to model the above picture of the turbulent region are then used in existing (or appropriately modified) propagation models. Progress in incorporating turbulence scattering into the Past Field Program (PFP) acoustic propagation model is recounted in this report. (MM).
Author: Publisher: ISBN: Category : Languages : en Pages : 14
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: Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
The foundations of the modern theory of sound propagation and scattering in a homogeneous and isotropic atmospheric turbulence are developed: The sound scattering cross-section for von Karman spectra of temperature and wind velocity fluctuations is calculated; the rigouros theory of line of sight sound propagation in an atmosphere with Kolmogorov, Gaussian and von Karman spectra of temperature and wind velocity fluctuations is developed; a new theoretical formulation of the interference of the direct wave from source to receiver and that reflected from the ground in a turbulent atmosphere is presented; the sound scattering cross section in an atmosphere with arbitrary profiles of temperature and wind velocity is calculated; some predictions of the modern theory are verified experimentally; correct wideangle parabolic equations for sound waves in a turbulent atmosphere are derived and used for numerical simulations of sound propagation. The modern theory has already been adopted by scientists for calculations of sound propagation in turbulent media and as a basis for development of new acoustic remote sensing techniques of the atmosphere and ocean in several countries and organizations including the U.S. Army Research Laboratory.
Author: Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
Sound waves propagating near the ground are scattered by random fluctuations in the velocity of temperature fields. We revisit the problem of scattering of sound by turbulence using an improved von Karman-type model for the atmospheric turbulence spectrum. The new model incorporates large boundary-layer scale eddies generated by atmospheric convection, as well as smaller height-scale eddies generated by surface-layer shear. We show that velocity fluctuations- ions from the large convective eddies are typically the cause of random signal behavior for low acoustical frequencies and line-of-sight propagation. For higher frequencies and scattering angles, the shear turbulence becomes more important, with the relative importance of scattering by temperature and velocity fluctuations depending on the degree of atmospheric convection. By applying the new model to monostatic solar systems, we find that solar measurements of the temperature structure parameter can be systematically contaminated by the velocity structure parameter in strong wind conditions. We also discuss how the new model can be used to determine appropriate baselines for direction-finding arrays when there is significant degradation of signal coherence caused by turbulence.
Author: D. F. Gordon Publisher: ISBN: Category : Languages : en Pages : 56
Book Description
An underwater sound propagation program has been adapted to Arctic use. The program enables computation of the coherent sound field with normal modes and the incoherent surface scattered field by integrating over ray paths from the surface to the source and receiver. For Arctic use, ice scattering and ice loss functions have been developed by comparing computed losses with Arctic propagation loss data. The sensitivity of the computed losses to the form of the scattering function is determined for one frequency. A difference in ice loss between summer and winter is hypothesized, but the data are insufficient to check the hypothesis. (Author).
Author: Publisher: ISBN: Category : Languages : en Pages : 10
Book Description
Three-dimensional models of anisotropic and inhomogeneous spectra of temperature and wind velocity fluctuations in unstable atmospheric boundary layers have been developed, modified or extended. Furthermore, theories of sound propagation through anisotropic, inhomogeneous, and intermittent atmospheric turbulence have been developed. These theories allow consideration of different geometries of sound propagation: line-of-sight sound propagation, interference of the direct and ground reflected waves, sound scattering into a refractive shadow zone, and waveguide sound propagation. Using the spectra developed or adopted from the literature, it was shown that anisotropy, inhomogeneity, and intermittency of atmospheric turbulence can significantly affect the statistical moments of a sound field for these geometries. Some of the theoretical results obtained have been verified experimentally. Two related tasks have also been accomplished. First, a new scheme of source localization in the atmosphere by means of acoustic tomography was proposed. The scheme accounts for sound refraction in the atmosphere and allows retrieval of vertical profiles of temperature and wind velocity. Secondly, it was shown that, in many cases, the effects of sound refraction on acoustic remote sensing of wind velocity and the structure parameters of temperature and velocity fluctuations are significant. Algorithms were developed to account for these effects in acoustic remote sensing of the atmosphere.
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Author: Acoustical Society of America
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Author: D. F. Gordon
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Author: C. A. Boyles