摘要:

Alternative large‐scale computational approaches are described and reviewed for predicting the dynamic response of submerged elastic structures subjected to time‐harmonic mechanical or incident pressure loadings. Such problems can be solved by combining a finite element model of the structure with a fluid‐loading model computed using finite element, boundary element, infinite element, or doubly asymptotic techniques. For each of these approaches, a variety of other choices must be made concerning the details of the formulation. Also discussed are the trade‐offs between accuracy and efficiency in the calculations.

ISSN:0001-4966    

DOI:10.1121/1.2028260

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The acoustic scattering that results when penetrable underwater targets are insonified by transient waveforms is studied. The targets are various elastic shells—and also a rigid sphere for comparison—of several thicknesses that are described either exactly by the equations of three‐dimensional elasticity, or approximately by means of Donnell's shell theory. Significant signal processing parameters are outlined that are useful at the source, at the target, and at the receiver to predict quantitatively the nature of the entire scattering process taking place around a submerged structure. These parameters play an important role in the ultimate goal of active target identification [cf. G. Gaunaurd, J. Acoust. Soc. Am. Suppl. 185, S134 (1989)]. This approach, which exploits similarities with the radar literature, is first introduced here for targets away from environmental boundaries and in noiseless media. One should first know what type of echoshouldone receive from a sonartarget, before such return is corrupted and distorted by the environment. The idealized situation described here already uses much of the machinery of statistical communication theory (viz., matched filters, correlations and ambiguity functions, etc.) and is well suited for later extensions to cases accounting for high noise levels and for proximity to boundaries. The pertinent quantities are illustrated with many computed displays related to: (a) the incident pulses; (b) the targets themselves; and (c) the backscattered pulses.

ISSN:0001-4966    

DOI:10.1121/1.2028262

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The asymptotic results deduced from the rigorous formal theory [L. B. Felsen and J. M. Ho, preceding abstract] are examined here within the framework of ray acoustics. It is shown how the saddle points in the two‐dimensional complex spectral domain systematically furnish the azimuthal and meridional invariants that appear less directly in previously constructed ray acoustic forms, and how this parametrization also applies to shell‐guided nonresonant and resonant interactions. These ray interpretations take place most generally in thenonperiodicinfinite azimuthal and meridional domains so as to describetravelingwaves that can subsequently synthesize effects due to truncations or other perturbations on the shell. The relation between ray fields in the infinite nonphysical and the physical domains is explored, especially for ray congruences that synthesize resonances on the shell by self‐consistent closure. By its very nature, the presentation here emphasizes the intimate connection between ray fields and spectra (wavenumbers), and thereby may furnish some guidelines for spectral processing techniques. [Work supported by ONR and DTRC.]

ISSN:0001-4966    

DOI:10.1121/1.2028267

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A salient feature of nonlinear systems is that several distinct steady‐state solutions can coexist: Different initial conditions can lead asymptotically to different attractors. The way in which parameter variations affect the geometry of the basins of attraction is central to an understanding of stability transition phenomena, including the onset of chaotic vibrations. A computer program is described that allows steady‐state solutions and their basins of attraction to be rapidly obtained and visualized for given parameter values. The code was implemented on a vector‐parallel architecture supercomputer. The results of analyses carried out on several mechanical systems are presented, including movies of basin‐boundary evolution as the parameters are varied over a curve in the parameter space. Of particular interest is the way in which animation reveals phenomena not readily seen in still images, such as rapid transitions between smooth and fractal basin boundaries, or the rapid creation and annihilation of entire basins of attraction.

ISSN:0001-4966    

DOI:10.1121/1.2028271

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Ray dynamics in a number of simple range‐dependent underwater acoustic waveguides was examined previously. It has been shown that, in such environments, at least some ray trajectories exhibit chaotic behavior, i.e., exponential sensitivity to initial conditions. This phenomenon is called ray chaos. In the present study, properties of the solution to the parabolic wave equation are examined in a bottom interacting shallow water environment in which ray trajectories are known to be predominantly chaotic. An attempt is made to determine whether the exponential sensitivity associated with ray chaos carries over to finite frequency wave fields. This phenomenon, whose existence is in question, is called wave chaos. In the search for wave chaos, 2‐, 4‐, 8‐, and 16‐kHz wave fields have been used to investigate the spreads in angle and depth of an initially narrow sound beam (these spreads grow exponentially in range under chaotic conditions according to ray theory if the initial beam is sufficiently narrow); the feasibility of back‐propagating sound fields (at ranges beyond some “predictability horizon” chaotic ray trajectories cannot be traced backwards to recover their initial conditions); and several measures of wave field complexity versus range (the complexity of a chaotic geometric wave field grows exponentially in range). In no case was exponential sensitivity, or any associated lack of predictability, observed in the finite frequency wave fields. In other words, no evidence that wave chaos exists was found.

ISSN:0001-4966    

DOI:10.1121/1.2028276

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Ice moduli may be determined from acoustic velocities measured via techniques such as resonance vibration of ice rods, seismic and flexural wave measurement, and propagation of high‐frequency pulses in core samples. However, due to the porous nature of sea ice, handling procedures can alter the brine volume of, and introduce air into, the saline ice. This in turn changes velocities from theirin situvalues. This is demonstrated with velocities measured during temperature cycling of an ice core sample. These results indicate the advantages of a pulse‐type experiment designed to allow rapid measurement of velocities and related ice properties in the field. One such design is presented, including the equipment used, and then experimental results obtained are related to the elastic moduli of a sealed‐pore Biot porous media model of sea ice. The expressions for the longitudinal and shear velocities in ice, derived assuming a sealed‐pore structure, most generally apply to sea ice away from the growing ice/water interface. The results from the type of analysis presented here ultimately lead to prediction of the acoustic impedance layers in sea ice, a crucial factor in understanding and predicting acoustic propagation processes involving sea ice. [Work supported by ONT with technical management by NOARL.]

ISSN:0001-4966    

DOI:10.1121/1.2028278

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Over the past 5 years, four acoustic field programs have been conducted by the Applied Research Laboratory (ARL) at Penn State Univ. These tests have been conducted from both stationary and moving platforms under the Arctic pack ice. The measurements include multiple signal waveforms to study the reverberation generated by both smooth and rough ice. The forward scattering versus ice type has been measured as a function of the depth of the source and the receiver. The basis of these measurements is to quantify the acoustic medium in the Arctic to ascertain what information about the environment can be extracted using state‐of‐the‐art acoustic sensors within certain physical limitations. The effects of multipath and out‐of‐plane scattering will be discussed along with its effects upon direct path propagation. Ambient noise effects on acoustic processing will be discussed.

ISSN:0001-4966    

DOI:10.1121/1.2028283

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Primary and secondary production in the oceans and hence abundance of macrofauna is limited by light or more often by nutrients. Any oceanic system that injects new or regenerated nutrients into a body of water creates a center of high biological productivity. While classical oceanographers have identified such production areas and depth zones by measurements of physical parameters and net sampling, the fishermen have done it empirically with sonar and other acoustic devices available to them after the end of World War II. The fisheries biologists started extensively to employ acoustical techniques in their abundance estimations, first with analog and later with numerous digital integrators and echo counters using dual or split beam transducers. This development is illustrated with examples from the Peruvian and Northwest Africa upwelling systems, krill abundance at the ice edge, and mapping of superswarms in Antarctica, and salmon and herring populations in Alaska. A next generation of acoustical systems will be used to map the patchiness of herbivores and carnivores foraging on primary and secondary producers. Once patches have been identified, it is possible to measure the physical characteristics of the water masses with patches and hence study the microstructure of front and boundary layers.

ISSN:0001-4966    

DOI:10.1121/1.2028285

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Acoustic Doppler current profilers, ADCPs, arrived on the oceanographic scene a decade ago as a development of ship‐mounted Doppler speed logs to measure vertical profiles of horizontal water velocities. Backscattered acoustic intensity is one of the parameters that can be recorded and early anecdotal observations indicated that intensity variations had distinctly biological characteristics such as diel zooplankton migrations. These early observations led to an ongoing study of the conditions under which ADCPs might be able to make quantitative estimates of zooplankton biomass. The attraction of ADCPs, and the motivation for the study, is not so much the ADCPs' ability to discern details about zooplankton populations but rather their ease of use and general availability. The results of the study to date, with operating frequencies of 150 and 300 kHz, have shown that ADCPs produce biomass estimates to within ± 15% throughout their acoustic range when the individual transducers have been calibrated and proper care is exercised in processing the results. ADCPs provide the ability to conduct nonintrusive large‐scale surveys, in either time or space, of zooplankton biomass providing input, for example, to ecological models and as guides to more conventional sampling schemes.

ISSN:0001-4966    

DOI:10.1121/1.2028287

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A new, computational approach provides a methodology for specifying how the acoustic power radiated from a complex, three‐dimensional vibrating structure may be controlled. The method uses a superposition principle to transform the surface of a vibrating structure into a discrete distribution of acoustic sources interior to the boundary surface of the structure. The transformation is carried out by requiring that the interior sources produce the same or nearly the same normal velocities as those prescribed on the surface of the vibrating structure. Based on this transformation, the total time‐averaged, acoustic power output from the vibrating structure can be expressed in terms of the strengths of the equivalent sources. To control the radiated acoustic power, one or more secondary point sources is then placed on or near the surface of the vibrating structure. By using the total power radiated from the vibrating structure and the secondary sources as an objective function for an optimization operation, the optimal strengths and locations of the secondary sources are found that lead to a maximum reduction in the acoustic power radiated from the vibrating structure. The significance of this approach is that it provides design guidelines for applying active control techniques to complex vibrating structures.

ISSN:0001-4966    

DOI:10.1121/1.2028294

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP