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Nov 1999

Volume 106, Issue 5, pp. 2321-L52

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Free vibration of a fluid-filled hollow sphere of a functionally graded material with spherical isotropy

W. Q. Chen, X. Wang, and H. J. Ding

J. Acoust. Soc. Am. Volume 106, Issue 5, pp. 2588-2594 (1999); (7 pages) | Cited 1 time

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An exact, three-dimensional method is developed in the paper to analyze the free vibration of a spherically isotropic hollow sphere made of a functionally graded material and filled with a compressible fluid medium. The material is assumed to be inhomogeneous along the radial direction. By introducing three displacement functions and employing the function expansion method, the governing equations are simplified to an uncoupled second-order ordinary differential equation, and a coupled system of two such equations. Solutions to these equations are given when the elastic constants and the mass density are power functions of the radial direction. To investigate the effect of material gradient on the natural frequencies, numerical calculations are finally performed.© 1999 Acoustical Society of America.‘
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43.40.Ey Vibrations of shells
43.20.Ks Standing waves, resonance, normal modes

Meridional ray contributions to scattering by tilted cylindrical shells above the coincidence frequency: ray theory and computations

Scot F. Morse and Philip L. Marston

J. Acoust. Soc. Am. Volume 106, Issue 5, pp. 2595-2600 (1999); (6 pages) | Cited 8 times

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High-frequency scattering enhancements from a submerged infinite cylindrical shell are studied by means of partial wave series (PWS) calculations and a ray approximation. For certain oblique angles of incidence it is possible to launch a generalized leaky Lamb wave which propagates strictly in the axial direction on the shell. This meridional leaky wave radiates with a locally flat wavefront curvature and produces large far-field amplitudes over a range of tilt angles. When combined with a reflection from a discontinuity in the shell, this process can give rise to large backscattering enhancements at large tilt angles for thick finite cylindrical shells [S. F. Morse et al., J. Acoust. Soc. Am. 103, 785–794 (1998)] and is present in sonar images. A convolution formulation ray theory for the far-field meridional ray amplitude is compared with the exact PWS solution in the region where enhancements are observed for both the a0 and s0 Lamb waves. The ray theory correctly predicts both the peak enhancement amplitude and the general shape of the enhancement curve. © 1999 Acoustical Society of America.
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43.40.Yq Instrumentation and techniques for tests and measurement relating to shock and vibration, including vibration pickups, indicators, and generators, mechanical impedance
43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.20.Fn Scattering of acoustic waves

A high-resolution spectral estimation approach for evaluating the resonances of scattering returns from submerged targets

I-Tai Lu and Fu-Po Wu

J. Acoust. Soc. Am. Volume 106, Issue 5, pp. 2601-2607 (1999); (7 pages)

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The principal aim of this work is to estimate the complex resonant frequencies of scattering returns from the Mock-0 target. The experiments were conducted at the Naval Undersea Warfare Center. First, the singular value decomposition approach is employed for noise suppressing prefiltering where the initial estimate of the system order is obtained. Second, the eigen-matrix pencil method is employed to approximate the complex resonant frequencies. Finally, the system order and the complex resonances are determined by a systematic procedure in which the estimated results obtained from various system orders and aspect angles are compared. Echoes observed at the same aspect angle but excited by different incident pulses with overlapping frequency spectra can also be employed to obtain reliable estimations of resonant frequencies. The key feature of the proposed approach is robustness. © 1999 Acoustical Society of America.
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43.40.Fz Acoustic scattering by elastic structures
43.60.Gk Space-time signal processing, other than matched field processing
43.40.Ey Vibrations of shells

Influence of edge boundary conditions on the free vibrations of cross-ply laminated circular cylindrical panels

Arcangelo Messina and Kostas P. Soldatos

J. Acoust. Soc. Am. Volume 106, Issue 5, pp. 2608-2620 (1999); (13 pages) | Cited 1 time

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This paper deals with the influence of different sets of edge boundary conditions on the dynamic characteristics of shear deformable, cross-ply laminated circular cylindrical panels. The analysis is based on the conjunction of the Ritz method with an appropriate, complete functional basis and its subsequent application on the energy functional of the Love-type version of a unified shear-deformable shell theory [T. Timarci and K. P. Soldatos, J. Sound Vib. 187, 609–624 (1995); J. Eng. Math (to appear January 2000)]. As a result, two different kinds of shear deformable Love-type shell theories are employed and tested, including versions that either fulfill or violate the continuity of the interlaminar stresses along the shell thickness. Although the main theoretical model employed makes use of a certain type of continuous interlaminar stress, the results of the present analysis are initially tested and validated through appropriate comparisons with corresponding numerical results obtained in the relevant literature, mostly on the basis of discontinuous interlaminar plate and shell models. The present approach and analysis are then used for a more thorough investigation of the influence of the edge boundary conditions on the dynamic characteristics of circular cylindrical panels.© 1999 Acoustical Society of America.
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43.40.Ey Vibrations of shells

Feedback control of structurally radiated sound into enclosed spaces using structural sensing

Steven Griffin, Colin Hansen, and Ben Cazzolato

J. Acoust. Soc. Am. Volume 106, Issue 5, pp. 2621-2628 (1999); (8 pages) | Cited 3 times

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A technique is developed that addresses sensor and actuator placement and feedback control of structural/acoustic problems that can be described as a flexible structure surrounding an acoustic cavity. Specifically, this work is directed at the space launch vehicle problem, where it is assumed that it is not possible to obtain, in advance of a required control output, a coherent measurement of the disturbance or to directly measure the quantity to be controlled. These assumptions necessitate the use of structural sensing to predict the sound pressure in the cavity and of feedback control to reduce the radiated sound. A method for selecting sensor and actuator positions based on a transformation of the problem into radiation modes is covered as well as an optimal feedback control approach which allows the control of radiated pressure into a defined subvolume of the cavity using only structural actuators and sensors. Finally, an example problem is completed which draws on all of the theoretical development to suppress radiated sound within a subvolume. © 1999 Acoustical Society of America.  
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43.40.Yq Instrumentation and techniques for tests and measurement relating to shock and vibration, including vibration pickups, indicators, and generators, mechanical impedance
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