摘要:

The scattering of acoustic waves by an infinitely long cylindrical shell immersed in a fluid is analyzed. A new approach has been proposed using thin shell theory incorporating the impedance of the shell in theT‐matrix formulation. Numerical results are presented for the farfield backscattering amplitude as a function of frequency. Calculations were made for infinitely long circular and elliptic cylindrical shells immersed in water for various shell thicknesses for waves incident at an arbitrary angle in the plane normal to the axis of the cylinder. The impedance resulting from the use of various shell theory approximations for circular cylindrical shells is presented. Love–Timoshenko and Flügge–Byrne–Lurye theories for circular cylindrical shells have been compared with the full elasticity solution using the separation of variables approach. Comments have been made regarding the range of applicability of various shell theories.

ISSN:0001-4966    

DOI:10.1121/1.390906

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

We present a study of the resonance scattering undergone by an air‐filled hollow elastic cylinder excited by an incident plane acoustic wave. We construct the boundary value problem, obtain its classical solution, the solution based on the Resonance Scattering Theory (RST), and generate a variety of useful computed results, some of which are later compared to experimental observations recently performed in France. We present highly accurate expressions for the phase and group velocities and for the phase and group attenuations of the first few surface waves circumnavigating (the extreme cases) of rigid and soft cylinders, and display these dispersion plots in all instances. We analyze the modal backgrounds and modal resonances of the shell, display them in a wide spectral band, determine the SEM‐type pole‐position diagram in the complexk1aplane, and obtain and display the background‐suppressed cross section of the tube. This result serves to generate the acoustic spectrogram of the shell as well as to show the excellent agreement of this theoretical prediction with the experimental observations carried on in France. We analyze cross‐sectional poles and cross‐sectional dips, and reduce many of the present shell results to particular cases for impenetrable cylinders and solid elastic cylinders. For these latter ones, we obtain the dispersion plots for the phase and group velocities of the internal surface waves revolving around them. We determine expressions for the nearfield shell cross sections at different ranges, and compare them to the usual farfield results. We determine the sound pressure levels transmitted into the shell’s interior, and exhibit the controlling role the tube resonances have on the isobaric contours. We display extensive computerized calculations to illustrate all these points. Comparisons with experimental observations are shown to be quite favorable, particularly for the background‐suppressed shell cross section, and for its acoustic spectrogram.

ISSN:0001-4966    

DOI:10.1121/1.390907

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A rigorous theory of the diffraction of time‐harmonic elastic waves by anarbitrarilyoriented, cylindrical, stress‐free crack of finite width embedded in a semi‐infinite elastic medium is presented. The incident wave is taken to be either aPwave, anSVwave, or a Rayleigh wave. The resulting boundary‐value problems for the unknown jump in the particle displacement across the crack are solved by employing the integral‐equation method in combination with the Galerkin method. Numerical results are presented in the form of scattering cross sections, normalized power scattering characteristics, dynamic stress intensity factors, and Rayleigh wave transmission and reflection coefficients, for a range of geometrical parameters.

ISSN:0001-4966    

DOI:10.1121/1.390908

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The smoothing method is used to find the first two moments, i.e., the mean and the two‐point two‐time correlation function, of the field scattered by a rough surface. The results are expressed in terms of a reflection coefficient and a differential scattering coefficient. They are compared with those found by several other methods.

ISSN:0001-4966    

DOI:10.1121/1.390972

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The space–time acoustic wave motion generated by an impulsive monopole point source in a fluid/solid configuration with a plane boundary is calculated with the aid of the modified Cagniard technique. The source is located in the fluid, and numerical results are presented for the reflected‐wave acoustic pressure, especially in those regions of space where head‐wave contributions occur. There is a marked difference in time response in the different regimes that exist for the wave speed in the fluid in relation to the different wave speeds (compressional, shear, Rayleigh) in the solid. These differences are of importance to the situation where the reflected wave in the fluid is used to determine experimentally the elastic properties of the solid.

ISSN:0001-4966    

DOI:10.1121/1.390970

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

An analytical expression for the impulse response of a focused transducer with an axisymmetric nonuniform surface velocity distribution is derived using a finite polynomial expansion of the velocity distribution function. A computing scheme is presented for the numerical calculation of the transient pressure at an observation point in front of the transducer. The effect of various nonuniform velocity distributions on the characteristics of the pressure field of a medium focused transducer is shown with grey‐scale pictures of calculated continuous‐wave and pulsed pressure distributions.

ISSN:0001-4966    

DOI:10.1121/1.390973

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A theoretical model is presented for evaluating the transient field radiated on the axis of a thick piezoelectric disk, by vibrations propagating radially on the circular transmitting face, from its rim towards its center. We had previously considered these vibrations to account for two unexpected signals (denotedS1andS2), which were observed by measuring, with a miniature probe, the field produced in a liquid by a barium titanate disk (25 mm in diameter, 20 mm in thickness). These signalsS1andS2, not described by the well‐known piston model, are due to vibrations which propagate radially on the disk face at two different speeds (5.0 × 103and 2.6 × 103m/s); we have called them radial vibrations as a simplification. Here, a simulation is undertaken to explain the complex changes observed in the amplitude and in the shape of signalsS1andS2, when the distance from the disk face varies. In this theoretical approach, it is assumed that the disturbance initially located on the rim of the disk face remains unchanged during its propagation along a disk radius. With these simplifying hypotheses, different analytical expressions for the impulse velocity potential φi(t) are obtained, each of them valid in a limited area of the disk axis. The resulting transient pressurep(t) is then used to predict the signals detected by the miniature probe. For this simulation we have taken account of the time dependence of the initial motion, together with the response of the measuring device. Several plots are chosen to illustrate the influence of different parameters such as the speed of the radial vibration or the duration of the input excitation signal. Finally, the comparison with the experimental results proves the efficiency of our model to describe the main characteristics of the signals produced by the radial vibrations: their contribution is important only close to the disk face and the position of a particular point on axis, called focus, is correctly predicted. Different possible improvements in the modeling are also discussed.

ISSN:0001-4966    

DOI:10.1121/1.390974

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A generalized impulse response formulation to evaluate the harmonic pressure field of ultrasonic planar vibrators having axisymmetric nonuniform surface velocity distributions is presented. The harmonic pressure is expressed as a Fourier transform of a generalized impulse response which is a function of the spatially nonuniform velocity of the vibrator. A backward projection method is then developed to reconstruct the normal surface velocity of axisymmetric vibrators from harmonic field pressures using an angular spectrum or Hankel transform formulation. The numerical accuracy of the backward projection technique is evaluated using the impulse response formulation to evaluate the pressure fields for several velocity distributions on disk vibrators. Experiments were performed to reconstruct the velocity distributions over the surface of a uniformly driven piezoelectric ceramic disk and ceramic ring using farfield measurements of the complex pressure. The experimental results were in good agreement with theoretical results based on the electrode patterns of the transducers.

ISSN:0001-4966    

DOI:10.1121/1.390975

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The acoustoelastic response of a transversely isotropic body loaded in plane stress is computed for arbitrarily severe anisotropy. Relations for the propagation speeds of longitudinal and shear waves propagating normal to the plane of stress are given as functions of the stress. The shear wave results are combined to obtain expressions for the polarization direction, which in general differs from the principal directions of stress or strain, and the magnitude of the ultrasonic birefringence as given by the difference in transit time per unit length of the two shear waves. Similar expressions derived previously for slightly anisotropic bodies are recalled and are compared to the present results. It is shown that the earlier results are special cases of the more general model discussed here and a numerical investigation of the acoustoelastic response of several materials indicates that the assumptions leading to these more restricted models may not be valid, even in materials which at first glance appear to be slightly anisotropic.

ISSN:0001-4966    

DOI:10.1121/1.390903

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Using a three‐scale expansion, we solved the equation governing the propagation of the coherence function in a random medium with a mean index of refraction profile. An expression for the intensity distribution near a caustic was obtained including both diffraction and scattering effects. The expression may be readily evaluated in the absence of scattering and in the case of weak or strong scattering. Numerical examples are presented.

ISSN:0001-4966    

DOI:10.1121/1.390904

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP