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An object that is partially insonified by a collimated sound beam may have a scattering cross section sometimes much larger than when the object is totally covered by the incident beam. We quantitatively study this partial insonification problem here, under the classical method of physical optics. The importance of this study stems from the fact that partial coverage of the target by the beam is the situation most likely to occur in many cases of practical importance. We consider several basic target shapes partially insonified by finite beams. These shapes include the spherical, the infinite and finite cylinder, the flat plate, and the capped sphere. High-frequency approximations of the resulting integrals, obtained by means of the saddle-point method, show the relative importance of the scattering centers located at the beam's specular reflection points, or at the edges of the spots that the beam shines on the scatterers. The physical-optics method is extended to obtain formulas for the bistatic cross sections of partially insonified objects. The results are numerically evaluated and graphically displayed in many pertinent instances and compared to the predictions of approaches, such as the Fresnel-zones method and the Geometrical Theory of Diffraction (GTD). The predictions of the physical-optics method have all the advantages and deficiencies of this method and, with very minor modifications, hold equally well for the partial illumination of objects by beams of electromagnetic radiation. 相似文献
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We describe bistatic and mostly monostatic techniques that are useful for target classification by means of active sonar. The echoes returned by any submerged elastic body, exemplified here by a wolfram carbide (WC) sphere, contain features caused by the poles of the scattering amplitude of the problem. We have found these poles and shown that they naturally split into two large sets from which one can separately extract shape or material composition information. The composition information seems easier to determine than details about the shape. Together these sets of poles unambiguously characterize any scatterer. 相似文献
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If the air bubble is near, and strongly interacting, with the (flat) surface of a liquid half-space, then the scattering cross section (SCS) of the bubble differs substantially from its value far-away from the interface. We present the exact solution for this scattering problem which is valid for any incidence direction of the (plane) sound waves, and for any bubble depth, obtained by the general method of images. The bubble is described as a continuous, spatially extended body, having an infinity of modes and resonances. The interaction with the boundary, or the image, is treated by a general, extended continuum approach. This benchmark solution makes use of the addition theorems for the spherical wavefunctions. The resulting SCS contains contributions from the bubble, its image, the incident wave, and the reflected wave from the boundary. It is expressed in terms of coupling coefficients, bmn, which contain products of Wigner-3j symbols. The formulation is illustrated with many computed plots for bubbles at various depths, and the results compare favorably with earlier experimental observations. The approach also serves to describe scattering by fish near the sea surface, in an exact fashion 相似文献
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The authors study the scattering of monochromatic plane acoustic waves incident at 90° angles relative to the axis of symmetry (i.e. broadside or beam aspect) on solid elastic spheroids. In this analysis, the aspect ratios of the spheroids vary in the range 2⩽L / D ⩽5 in steps of one. The nondimensional frequency kL /2 is kept within the band 2⩽kL /2⩽24. A numerical solution based on a modification of the T -matrix method is generated. The authors generate predictions for the backscattered echoes and graphically display their frequency dependence in order to study the resonance features present within them. In this three-dimensional study, the authors identify the (leaky) Rayleigh-type resonances consistent with those present in infinite cylinders 相似文献
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A methodology is presented for the spectral analysis of the echo returns from elastic spheres submerged in a fluid half-space. The influence of a target depth on its resonances as manifested in the scattered field is assessed. Results are expressed in terms of Debye potentials in the fluid and in the solid. The unknown coefficients arising in these potentials are determined by the application of appropriate boundary conditions at the surface of the elastic sphere and at the free boundary of the fluid. These boundary conditions are satisfied exactly through the use of some known transformations of the basic wave functions. As the depth increases, our results degenerate to earlier results for spheres in unbounded media. In spite of the spherical symmetry of the scatterer, the field is found to be azimuthally dependent because of the influence of the plane free-surface of the fluid. The saddle-point method is used in the asymptotic evaluation of certain coefficients which exhibit the influence of depth on the complex eigenfrequencies of the scatterer. We demonstrate that as the sphere's depth increases, the influence of the boundary decreases. The rate of decrease increases as the azimuthal wavenumberm increases. Thus the influence of the boundary is greatest for eitherm = 0 , or at shallow depths, or both. 相似文献
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The author comments on the paradoxical result of Gaunaurd and Huang (see ibid., vol.20, p.285, 1992) that the scattering form function of a bubble increases with decreasing distance from a pressure-release surface. Gaunaurd et al. agree with the comments and explanation made by Strasberg (see J. Acoust. Soc. Amer., vol.25, p.536, 1953), adding that they had only accounted for radiation damping in their analysis. Although radiation damping suffices to accurately predict the resonance shift of the bubble as it approaches the free surface, the resonance amplitudes differ from those intuitively expected 相似文献
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