摘要:

A method is presented for the analysis of the transmitted and reflected transient elastodynamic wave field at a fracture that may be modeled as a linear slip interface. With this method, the space-time domain problem is first carried over to the transform domain by means of a temporal Laplace transformation and a two-dimensional, horizontal spatial Fourier transformation. Subsequently, for both theSHand theP/SVwave systems, expressions are determined for the transmitted and reflected wave fields at the interface. Afterward, a transformation back to the space-time domain is carried out. The method is capable of providing the exact transmitted and reflected waves (including the body wave, interface wave, and head wave contributions) that are caused by a spatially curved, incident wave field due to a point source. Moreover, it is especially designed to yield the time domain waveforms. Various exact numerical results are given. For theSHcase, the directionally dependent filter behavior of the fracture is shown. For theP/SVcase, first the reflection, transmission, and conversion of body waves is presented. The next set of results concerns the interface wave contribution and includes pictures of the particle movement on both sides of the fracture when either the transversal or the normal specific compliance of the fracture is equal to zero. Finally, the presence of head waves in case ofSV→PandSV→SVreflection is shown.

ISSN:0001-4966    

DOI:10.1121/1.418489

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A computational scheme that integrates the advantages of both the electronic computer and DNA computer (DNA experiment) is proposed to solve the ray acoustic problems at high frequencies. More explicitly, a conventional electronic computer is used to perform most of the sequential operations, such as solving differential equations, designing the DNA experiment, and verifying the final experimental solutions; whereas DNA annealing reaction is used to provide huge parallel operations to assemble the solutions to the scattered field for all the specified incident directions. To illustrate this idea, an example is given where the DNA reaction, rather than carrying out experimentally, is tested and simulated on an electronic computer.

ISSN:0001-4966    

DOI:10.1121/1.418490

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

According to the resonance scattering theory, a process of background subtraction is always required to obtain the resonance spectrum of a target. A new formulation of the theory is proposed here in which the background subtraction is not necessary and the resonance spectrum can be obtained directly. Contrary to the previous theory, this formulation suggests that, within each partial wave, there are three contributions: the resonance, the background, and their correlation. Also, it is shown that the resonance contribution (at least for empty elastic shells) can be synthesized by considering the resonances of Breit–Wigner form and their pair correlation.

ISSN:0001-4966    

DOI:10.1121/1.418491

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The dispersion behavior of circumferential creeping waves around a fluid-filled cylindrical cavity in an infinite elastic medium is analyzed by computational methods. Phase and group velocity as well as attenuation curves are constructed by numerically solving the dispersion equation. A comparison of the corresponding modes for elastic and rigid hosts is presented. The modes in both cases exhibit essentially the same series of cutoff frequencies corresponding to radial resonances at which the phase velocity of the associated modes becomes infinite and the group velocity assumes a limiting value ofπcf/2,wherecfis the compressional wave velocity in the fluid. Attenuation of the circumferential creeping modes in a cylindrical cavity is caused solely by losing energy to the surrounding elastic bulk. Therefore, for all modes, the attenuation diminishes at high frequencies as leakage into the surrounding solid becomes negligible. This is in sharp contrast with the case of leaky Rayleigh wave propagation along the plane surface of a solid–fluid interface where attenuation is caused solely by radiation of energy into the fluid, which causes the frequency to have an opposite effect on the degree of leakage in these situations.

ISSN:0001-4966    

DOI:10.1121/1.418492

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The full derivation of the equations governing the generalized impedance matrixZ,the pressure, and the velocity were presented in Part I of this series [Pagneux et al., J. Acoust. Soc. Am.100, 2034–2048 (1996)]. Here only the results of that paper, i.e., the final set of equations which needed to be solved are repeated. Other factors influencing the solution are the boundary conditions at the end of the waveguide: Source and radiation conditions are also presented. Finally, the details of the numerical implementation are also relevant, and will be discussed in some detail.

ISSN:0001-4966    

DOI:10.1121/1.419306

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The parabolic equation (PE) method has been applied to approximate the wave equation, construct radiation conditions, solve scattering problems, construct initial conditions, and derive energy flux conditions. A new application of the PE method, filters based on rational approximations of depth operators, is described and tested. The rational approximation is designed to act as the identity operator on the desired part of the spectrum of the depth separated wave equation and to annihilate other parts of the spectrum. This approach does not require explicit knowledge of the spectrum. The applications of the filter include directly solving eigenvalue problems, annihilating components of the wave-number spectrum, generating initial conditions at the source range, and eliminating Gibbs oscillations that arise in energy-conserving PE solutions.

ISSN:0001-4966    

DOI:10.1121/1.418493

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Vibro-acoustic energy travels through hose walls as longitudinal waves and flexural waves, apart from the sound waves through the fluid medium inside. Longitudinal waves in the hose wall are coupled to the sound waves inside by means of the hose-wall Poisson’s ratio. Both in turn get coupled to bending or flexural waves because of the energy transfer or interaction at the bends. For any of these three types of waves incident on one end of a hose, waves of all the three types may be transmitted on the other end because of their dynamical coupling with one another. Therefore, in the present paper, expressions have been derived for the 3×3 transmission loss matrix for a two-dimensional or planar piping system in terms of elements of the overall 8×8 transfer matrix of the system. These expressions have then been used in a comprehensive computer program to evaluate the vibro-acoustic performance of hoses, with particular application to the automotive climate control systems with gaseous as well as liquid media. Finally, parametric studies have been made that have led to some general design guidelines.

ISSN:0001-4966    

DOI:10.1121/1.418494

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This paper deals with a numerical and experimental study of ultrasound induced cavitation. On the basis of a computational analysis of Gilmore’s equation for the individual bubble dynamics, a slight decrease in the pressure radiated by a single bubble immediately before reaching the transient cavitation regime is predicted. Experiments presented in this paper confirm this observation. Therefore, a new method to detect the inception of transient cavitation is proposed. The numerical analysis of a single bubble behavior, carried out for the different bubble sizes, suggests that two distinct “types” of transient cavitation may occur in a cavitation field in connection with the relative number of bubbles involved in the process. The experiments seem to confirm the theoretical hypothesis. The experimental work consisted mainly of insonifying distilled water, with a gas content of about 75% of saturation, at 20 kHz and analyzing the resulting acoustic emissions.

ISSN:0001-4966    

DOI:10.1121/1.418495

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The vertical directionality of acoustic ambient noise has been a subject of much interest in the past. It is a well-defined physical quantity that can be measured experimentally with a vertical array. It possesses certain deterministic features that can be modeled theoretically with environmental acoustic and source data. Ambient noise in shallow waters, including its vertical directionality, is not very well known and is also difficult to model/predict. This is because the acoustic environment varies with time and is location dependent. Thus arises the question, how does the vertical directionality of the ambient noise depend on the acoustic environments (found in typical coastal waters)? Due to the shallow water depth, it is noted that sound (noise) propagation can be significantly influenced by the bottom. The degree of bottom interaction will depend on the sound-speed profile in the water column: whether it is downward refractive or not. Bottom attenuation will in turn determine how far the sound will propagate in the water column. Using a modal representation, a closed-form expression is obtained which can be used to interpret and predict the distant noise vertical directionality as a function of the environmental acoustic parameters. The nearby (overhead) noise is separately modeled and is found less sensitive to environmental changes as the propagation distance is short. As the deterministic features of the noise vertical directionality is controlled by the sound propagation in the channel, it could be used as an acoustic indicator of the acoustic environment in the area. Time variations of the noise directionality will also be discussed.

ISSN:0001-4966    

DOI:10.1121/1.418496

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The Acoustic Reverberation Special Research Program (ARSRP) of the Office of Naval Research conducted low-frequency acoustic reverberation experiments just west of the Mid-Atlantic Ridge in 1993. Analyses of these data are supported by a bathymetric survey that produced nearly full-coverage bathymetry gridded to a resolution of200×200 m.At this scale direct-path reverberation data has been successfully modeled using Lambert’s law applied to the local grazing angle determined from the high-resolution bathymetry. The analysis suggests that there could be a slightly weaker dependence on grazing angle than sine squared. The good agreement of this simple model with data shows that, for this region, two-dimensional seafloor morphology at the proper scale determines the ability to predict reverberation. For a model, such as Lambert’s law, that glosses over the details of seafloor microroughness or texture at the scale the acoustic wavelength(λ=6 m),the seafloor morphology scale that is critical is shown to be an order of magnitude or two greater than an acoustic wavelength. An average Lambert coefficient for the region is between−14and−17 dB,but it is less than−20 dBfor deeply sedimented areas and around −12 dB for rugged exposed rock areas.

ISSN:0001-4966    

DOI:10.1121/1.418497

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP