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Author: Sumedh Mohan Joshi Publisher: ISBN: Category : Languages : en Pages : 172
Book Description
Interface roughness can have a significant effect on the scattering of sound energy, and therefore an understanding of the effects of roughness is essential to making predictions of sound propagation and transmission underwater. Many models of roughness scattering currently in use are two dimensional (2D) in nature; three dimensional (3D) modeling requires significantly more time and computational resources. In this work, an effort is made to quantify the effects of 3D scattering in order to assess whether or under what conditions 3D modeling is necessary. To that end, an exact 3D roughness scattering model is developed based on a commercially available finite element package. The finite element results are compared with two approximate scattering models (the Kirchhoff approximation and first order perturbation theory) to establish the validity and regimes of applicability of each. The rough surfaces are realizations generated from power spectra measured from the sea floor. However, the surfaces are assumed to be pressure release (as on an air-water interface). Such a formulation is nonphysical, but allows the assessment of the validity of the various modeling techniques which is the focus of this work. The comparison between the models is made by calculating the ensemble average of the scattering from realizations of randomly rough surfaces. It is shown that a combination of the Kirchhoff approximation and perturbation theory models recovers the 3D finite element solution.
Author: Sumedh Mohan Joshi Publisher: ISBN: Category : Languages : en Pages : 172
Book Description
Interface roughness can have a significant effect on the scattering of sound energy, and therefore an understanding of the effects of roughness is essential to making predictions of sound propagation and transmission underwater. Many models of roughness scattering currently in use are two dimensional (2D) in nature; three dimensional (3D) modeling requires significantly more time and computational resources. In this work, an effort is made to quantify the effects of 3D scattering in order to assess whether or under what conditions 3D modeling is necessary. To that end, an exact 3D roughness scattering model is developed based on a commercially available finite element package. The finite element results are compared with two approximate scattering models (the Kirchhoff approximation and first order perturbation theory) to establish the validity and regimes of applicability of each. The rough surfaces are realizations generated from power spectra measured from the sea floor. However, the surfaces are assumed to be pressure release (as on an air-water interface). Such a formulation is nonphysical, but allows the assessment of the validity of the various modeling techniques which is the focus of this work. The comparison between the models is made by calculating the ensemble average of the scattering from realizations of randomly rough surfaces. It is shown that a combination of the Kirchhoff approximation and perturbation theory models recovers the 3D finite element solution.
Author: Bryant Minh Tran Publisher: ISBN: Category : Languages : en Pages : 128
Book Description
Rough surface scattering is a problem of interest in underwater acoustic remote sensing applications. To model this problem, a fully three-dimensional (3D) finite element model has been developed, but it requires an abundance of time and computational resources. Two-dimensional (2D) models that are much easier to compute are often employed though they don't natively represent the physical environment. Three quantities have been developed that, when applied, allow 2D rough surface scattering models to be used to predict 3D scattering. The first factor, referred to as the spreading factor, adopted from the work of Sumedh Joshi [1], accounts for geometrical differences between equivalent 2D and 3D model environments. A second factor, referred to as the perturbative factor, is developed through the use of small perturbation theory. This factor is well-suited to account for differences in the scattered field between a 2D model and scattering from an isotropically rough 2D surface in 3D. Lastly, a third composite factor, referred to as the combined factor, of the previous two is developed by taking their minimum. This work deals only with scattering within the plane of the incident wave perpendicular to the scatterer. The applicability of these factors are tested by comparing a 2D scattering model with a fully three-dimensional Monte Carlo finite element method model for a variety of von Karman and Gaussian power spectra. The combined factor shows promise towards a robust method to adequately characterize isotropic 3D rough surfaces using 2D numerical simulations.
Author: E. R. Pike Publisher: Elsevier ISBN: 0080540732 Category : Science Languages : en Pages : 1831
Book Description
Scattering is the collision of two objects that results in a change of trajectory and energy. For example, in particle physics, such as electrons, photons, or neutrons are "scattered off" of a target specimen, resulting in a different energy and direction. In the field of electromagnetism, scattering is the random diffusion of electromagnetic radiation from air masses is an aid in the long-range sending of radio signals over geographic obstacles such as mountains. This type of scattering, applied to the field of acoustics, is the spreading of sound in many directions due to irregularities in the transmission medium. Volume I of Scattering will be devoted to basic theoretical ideas, approximation methods, numerical techniques and mathematical modeling. Volume II will be concerned with basic experimental techniques, technological practices, and comparisons with relevant theoretical work including seismology, medical applications, meteorological phenomena and astronomy. This reference will be used by researchers and graduate students in physics, applied physics, biophysics, chemical physics, medical physics, acoustics, geosciences, optics, mathematics, and engineering. This is the first encyclopedic-range work on the topic of scattering theory in quantum mechanics, elastodynamics, acoustics, and electromagnetics. It serves as a comprehensive interdisciplinary presentation of scattering and inverse scattering theory and applications in a wide range of scientific fields, with an emphasis, and details, up-to-date developments. Scattering also places an emphasis on the problems that are still in active current research. The first interdisciplinary reference source on scattering to gather all world expertise in this technique Covers the major aspects of scattering in a common language, helping to widening the knowledge of researchers across disciplines The list of editors, associate editors and contributors reads like an international Who's Who in the interdisciplinary field of scattering
Author: Chengxi Li (Ph. D.) Publisher: ISBN: Category : Languages : en Pages : 152
Book Description
The objective of this thesis is to develop and apply efficient three-dimensional (3D) direct simulation capabilities for underwater sound field predictions in shallow water environments. Despite the large number of theoretical and experimental studies, direct numerical simulation of the shallow water acoustic field is still challenging due to environmental complexities and large computation cost involved. In this thesis, we develop a highly efficient O(NlogN) multi-layer boundary-element method, PFFT-BEM, for direct numerical simulation of acoustic propagation and scattering in shallow water environment. This method utilizes a Pre-corrected Fast Fourier Transform (PFFT) approach to accelerate the boundary-element method and reduce the computational efforts from O(N2~3) to O(NlogN) where N is the total number of boundary unknowns. PFFT-BEM is capable of accounting for complex topography, inhomogeneity of water properties, and dynamic environments associated with realistic coastal conditions. With the O(NlogN) efficiency and linear scalability on massively parallel high-performance computing platforms, we first conduct multilayer 3D simulations benchmarking low-mid frequency acoustics over kilometer ranges against available theoretical results and field experiments. We then apply largescale PFFT-BEM simulations to investigate two underwater acoustics problems which are of scientific interest and practical importance: (1) 3D sound scattering from rough ocean surface; (2) 3D sound propagation and scattering around underwater seamount(s). For the 3D rough surface scattering problem, several approximation models have been proposed such as the perturbation theory and Kirchhoff approximation. These approximation models provide fast predictions of statistics for the acoustics scattering necessary for predicting the scattering effects and reverberations from the rough surfaces. The validities of these models need to be assessed by direct numerical methods. However, most existing direct numerical studies regarding the validity regions of the approximation models are limited to the 2D rough surface scattering problem. We apply direct PFFT-BEM computations to study the 3D rough surface scattering problem with a Gaussian roughness spectrum. We examine the accuracy of the approximation model results through comparisons with direct numerical simulation results by 3D PFFT-BEM with a Monte Carlo technique. We identify and quantify the 3D validity regions of the approximation models as a function of the surface roughness and correlation length. We characterize and quantify the importance of 3D scattering effects on the validities of different approximation models. Moreover, we find that both perturbation theory and Kirchhoff approximation become inaccurate for 3D scattering problems with low grazing angles. For the problem of 3D sound propagation/scattering around underwater seamount(s), we investigate the effects of seamount geometry and sound source frequencies on the sound scatterings by the seamount using 3D PFFT-BEM simulations. In particular, we investigate the backscattering, blocking and 3D scattering effects due to the presence of the seamount. We find that the acoustics scattering effects by the seamount have a strong dependence on the source frequency, and small variations in seamount geometry (such as seamount height and cross section shape) can induce significant changes in the acoustics scattering field.
Author: Sumedh Mohan Joshi
Author: Sumedh Mohan Joshi
Author: Bryant Minh Tran
Author: Charles Edward Shepherd
Author: Acoustical Society of America
Author: Andreas Kleefeld
Author: 
Author: E. R. Pike
Author: John A. Fawcett
Author: Chengxi Li (Ph. D.)
Author: