摘要:

In this paper a time-domain numerical model for describing the presence of vorticity and entropy boundary layers is introduced. The boundary model can be inserted in acoustic time-domain models such as the finite-difference time-domain (FDTD) model for studying the acoustic behavior of high-quality resonator structures. The main focus in deriving the model is on numerical efficiency. Boundary layer theory is reviewed to obtain a three-dimensional formulation of its influence on FDTD equations that must be applied in boundary discretization cells. Theωdependence that is encountered is approximated in time domain in such a way that no numerical convolution is required. Although this approximation is rather inaccurate, it is sufficient for most applications and introduces almost no extra computational effort. Several numerical examples illustrate the applicability of the model.

ISSN:0001-4966    

DOI:10.1121/1.419780

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

In a previous paper, a lumped parameter model for the acoustic radiation from a vibrating structure was defined by dividing the surface of the structure into elements, expanding the acoustic field from each of the elements in a multipole expansion, and truncating all but the lowest-order terms in the expansion. Here, the lumped parameter model is implemented numerically by requiring the boundary condition for the normal surface velocity to be satisfied in a lumped parameter sense. This alleviates the difficulties typically encountered in enforcing the boundary condition, leading to a relatively simple numerical solution with well-defined convergence properties. The basis functions for the numerical analysis are taken as the acoustic fields of discrete simple, dipole, and tripole sources located at the geometrical centers of the surface elements. The different source types are used to represent the radiation from different kinds of surface elements: simple sources for elements in the plane of an infinite baffle, dipole sources for very thin structures which deform only in bending, and tripole sources for elements associated with parts of a structure enclosing a finite volume. The convergence of the numerical solution for the power output as a function of both frequency and element size is demonstrated through several example problems.

ISSN:0001-4966    

DOI:10.1121/1.419781

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The Hamiltonian approach to the calculation of vibrational modes of elastic cubic objects is exploited, including the effect of higher-order elastic constants. A technique is presented for solving the inverse problem, i.e., the determination of second- and third-order elastic constants for face cubic centered crystals by inversion of natural frequencies data. An unconstrained minimization algorithm, using gradient methods, is used to numerically solve the inverse problem. The method is applied to the case of Al alloys, since they are particularly suitable for applications of the acoustoelastic effect, which requires a knowledge of the third-order elastic constants. The efficiency and the accuracy of the method and the influence of measurement errors are discussed.

ISSN:0001-4966    

DOI:10.1121/1.419792

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

An existing model, the Bergen code, is used to simulate the propagation of ultrasonic beams of finite amplitude, due to rectangular and square sources. It is found that the edges of the source give rise to peaks in the diffraction pattern at the fundamental frequency, which can cause the strongest shockwaves to form off-axis. These tend to be sharply bounded, causing rapid variation in heat deposition and streaming pressure across even an unfocused beam, and suppressing axial maxima in the pressure field.

ISSN:0001-4966    

DOI:10.1121/1.419793

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The operation of the sound amplification by stimulated emission of radiation in the nonlinear mode is considered. Liquid with gas bubbles serves as an active medium. Pumping is produced by alternating electric field. The amplification of the useful wave in the plane resonator is investigated analytically.

ISSN:0001-4966    

DOI:10.1121/1.419802

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The acoustic signal from an overflying aircraft, as heard by a stationary observer, is used to estimate an aircraft’s constant height, ground speed, range, and acoustic frequency. Central to the success of this flight parameter estimation scheme is the need for an accurate estimate of the instantaneous frequency of the observed acoustic signal. In this paper, the polynomial Wigner–Ville distribution is used in this application as the instantaneous frequency estimator. Its performance and the issue of the optimal time domain window length are addressed.

ISSN:0001-4966    

DOI:10.1121/1.419803

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A time-domain model that describes wide-angle paraxial propagation of acoustic pulses in shallow water is developed. This model incorporates weak nonlinear effects and depth variability in both ambient density and sound speed. Derivations of paraxial approximations are based on an iterative approach, in which the wide-angle approximation is obtained by using a narrow-angle equation to approximate the second range derivative in the two-way equation. Scaling arguments are used to obtain a more tractable simplification of the equation. The wide-angle equation is solved numerically by splitting into components representing distinct physical processes and using a modified version of the time-domain parabolic equation (TDPE) code [M. D. Collins, J. Acoust. Soc. Am.84, 2114–2125 (1988)]. A high-order upwind flux-correction method is modified to handle the nonlinear component. Numerical results are presented for adiabatic propagation in a shallow, isospeed channel. It is demonstrated that nonlinear effects are significant, even at small ranges, if the peak source pressure is high enough. Both nonlinear and wide-angle effects are illustrated, and their differences are discussed.

ISSN:0001-4966    

DOI:10.1121/1.419815

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The coupled mode parabolic equation (PE) is a generalization of the adiabatic mode PE that includes mode coupling terms. It is practical to apply this approach to large-scale problems involving coupling of energy between both modes and azimuths. The solution is expressed in terms of the normal modes and mode coefficients, which satisfy coupled horizontal wave equations. The coupled mode PE may be solved efficiently with the splitting method. The first step is equivalent to solving the adiabatic mode PE over one range step. The second step involves the integration of the coupling term. The coupling mode PE solution conserves energy, which is an important aspect of a range-dependent propagation model. The derivation of the coupled mode PE, which involves completing the square of an operator, is related to the derivation of an adiabatic mode PE that accounts for ambient flow. Examples are presented to illustrate the accuracy of the coupled mode PE.

ISSN:0001-4966    

DOI:10.1121/1.419819

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This work presents an analysis of the effects of ocean internal waves on long-range acoustic pulse propagation from the geometrical-optics point of view. The chaotic behavior of rays and the microfolding of timefronts are investigated. The extent of the region of the timefront in which strongly chaotic rays appear, and the strength of the rays’ sensitivity to initial conditions, are found to depend on the average (range-independent) sound-speed profile, on the range from the source to the receiver, and on the internal-wave spectral model, but not on the specific realization of the internal waves. For a particular experiment (SLICE89), it is concluded that the observed depth diffusion of energy in the late-arriving portion of the timefront is a result of refraction (of geometrical-optics rays), not diffraction. It is found that internal-wave effects cause an upper turning point of a ray to be spread to the extent of 10 km horizontally and 100 m vertically, which affects the resolution of ocean-acoustic tomography. The validity and usefulness of ray-based, semi-classical (WKB) waveforms to represent received arrivals are evaluated by comparing with waveforms generated with multifrequency, parabolic-equation simulations. Center frequencies of 250 Hz and 1000 Hz with 100-Hz bandwidth are used. The semi-classical waveforms reproduce the correct arrival time and temporal shape of almost all arrivals, even those that are made of dozens of microrays induced by the internal waves. The overall intensities of the 1000-Hz arrivals are reasonably accurate, while the 250-Hz intensities show differences of order 5 dB.

ISSN:0001-4966    

DOI:10.1121/1.419820

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Scattering of sound from an infinite plate is significantly altered by the presence of an impedance discontinuity. Here, an approximate analytical solution is developed for the scattering of sound from a three-dimensional protuberance on a thin, infinite, submerged elastic plate. The protuberance is modeled as a solid elastic sphere of dimensions comparable to the incident acoustic wave[ka∼O(1)],and is attached to the plate along its equatorial plane. The plate is modeled using thin plate theory and supports the lowest-order flexural, compressional, and horizontally polarized shear(SH)modes, while the solid sphere is modeled with the full three-dimensional elastodynamic equations of motion. This idealized environmental model provides an understanding of the underlying physics of 3-D scattering, which is shown to be strongly frequency dependent. The analysis demonstrates that the attachment of the plate manifests itself in the scattered field in a frequency selective manner, and that the resonances of the elastic modes in the sphere leave their imprint on the elastic waves coupled into the plate. It is also shown that since the attached plate primarily restrains the motions of the sphere in the horizontal plane, the dominant contribution to the scattered field arises from the in-plane coupling forces.

ISSN:0001-4966    

DOI:10.1121/1.419828

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP