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May 1990

Volume 87, Issue S1, pp. S1-S164

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back to top Session DD. Structural Acoustics and Vibration IV: Radiation Models
Contributed Papers
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Sound transmission through a double‐walled cylindrical shell with radial elastic links (A)

C. Cacciolati

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S74-S74 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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A theoretical model is established to study the direct transmission of sound through a finite cylindrical double shell. Shells comply with Donnells' equations and the theory includes constantly spaced stiffeners in axial and circumferential directions. However, this is limited in the case of a large number of stiffeners. The acoustical excitation is a plane wave located in an infinite medium outside the shell. The two internal media are homogeneous. Absorption is taken into consideration by a complex wavenumber. The direct transmission calculation includes the fluid structure coupling with light fluid assumption and mechanical links between shells. There is a distributed stiffness of constant value per unit length in the radial direction and along circumferential lines. The solution of the equations is obtained by a modal method. The acoustical pressure in each medium and the vibratory behavior of shells is given. An application to aircraft design is presented. Numerical results show the comparison between structural linkage and air gap effects on the transmission of sound inside the inner cavity. [Work supported by Aerospatiale Soc., Toulouse, France.]
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Point load response of a finite cylindrical shell with a convected internal fluid (A)

G. Leyrat and J. M. Cuschieri

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S74-S74 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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In this paper, the response of a simply supported baffled cylindrical shell excited by a point force, with an internal constant flow speed, is investigated. The solution to this problem is obtained by using the normal modes of the in vacuo finite shell and the Kirchhoff‐Helmholtz equation derived for a fluid moving at a constant flow speed and bounded by a perfectly rigid cylinder. Numerical results for the input accelerance of a copper pipe of radius 0.025 m, wall thickness 1.5 mm, and various lengths are presented. It is found that the decrease of the natural frequencies of the shell due to the flow is dependent on the circumferential and axial mode numbers. Also investigated are the effects of the circumferential mode number, length to radius ratio, and thickness to radius ratio on the critical flow velocity at which the motion of the pipe becomes unstable.
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Acoustic radiation loading on cylindrical vibrators in an inviscid fluid with axial flow (A)

D. D. Ebenezer and Peter R. Stepanishen

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S74-S74 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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A wave‐vector‐time domain (k̄‐t) method is presented to evaluate the acoustic radiation loading on cylindrical shells vibrating in an inviscid fluid with axial flow. The method is based on the use of the in vacuo modes of vibration of the shell that has infinite rigid extensions. The solution to the differential form of the acoustic wave equation for the pressure is first obtained in wave‐vector‐time space. The time‐dependent modal forces are then expressed as a sum of the modal radiation impulse responses of the finite cylinder convolved with the time‐dependent modal velocities. The modal radiation impulse responses are obtained by using the impulse responses of an infinite cylinder and the modes in wave‐vector space. The approach can thus be easily used to obtain the radiation impulse responses for various boundary conditions, length to radius ratios, and arbitrary space‐ and time‐dependent surface velocities. Numerical results are presented for a wide range of Mach numbers and length to radius ratios of simply supported cylindrical shells.
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Two fast methods for the calculation of sound radiation: Multipol radiator synthesis and boundary element multigrid method (A)

Martin Ochmann

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S74-S74 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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The sound radiation of oscillating structures of arbitrary shape into three‐dimensional space will be calculated by representing the normal velocity on the surface as a superposition of multipoles. Depending on the geometry of the radiator, one or more source locations in the interior volume are selected. Different variants of this technique (null field method, weighted residual methods, etc.) and their interdependence will be discussed. The advantage of the method is a strongly reduced calculation time for complex structures. A disadvantage is the presently still incomplete investigation of the convergence behavior. For this reason a boundery element method will alternatively be applied to the discretized Kirchhoff‐Helmholtz integral equation, which will be solved on grids of varying size (multigrid method). Both methods will be used to calculate the radiation from spheres, cylinders, cubes, etc. Results will be compared to analytical solutions, numerical results obtained by other techniques, and experimental data. [Work supported by the Department of Research and Technology of the Federal Republic of Germany (BMFT).]
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Acoustic radiation from rib‐reinforced infinite cylindrical shell (A)

Courtney B. Burroughs and Kenneth J. Becker

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S74-S74 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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The acoustic radiation from rib‐reinforced cylindrical shells is effected by the interaction between the shell and reinforcing ribs. This interaction includes normal, shear, and moment reactions of the rib to shell motion. An analytic model for the farfield acoustic radiation from a fluid‐loaded infinite circular cylindrical shell with periodic rib supports is developed. The ribs are modeled as circular rings where the reactive forces, moments, and shears are applied by the shell at the outer edge of the ring. Approximate solutions for the force, moment, and shear impedances of the ribs are developed and incorporated into the acoustic radiation model to explore the dependence of the acoustic radiation from ribbed cylindrical shells on rib/shell interaction.
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An in vacuo modal method to analyze fluid‐loaded shells of revolution (A)

Huo‐Wang Chen and Peter R. Stepanishen

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S75-S75 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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A general approach is presented to evaluate the acoustic field and vibratory response of harmonically excited fluid‐loaded shells of revolution. The excitation force and velocity of shells of revolution are expressed as a modal sum of the in vacuo eigenvectors. The eigenvectors for shells of interest are evaluated by the finite element method. A new method is presented to determine the acoustic loading on the shells. This method is based on the use of internal source distributions to obtain acoustic radiation impedances. In the present study, the modal velocity coefficients are obtained from a set of algebraic equations and the Helmholtz integral equation is used to evaluate the acoustic pressure in the fluid. Numerical results for spherical and spheroidal shells will be presented and discussed. [Work supported by ONR.]
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Low‐frequency sound radiation from a line force excited, fluid‐loaded, baffled strip (A)

Chris Kauffmann

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S75-S75 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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The two‐dimensional problem of a fluid‐loaded, baffled elastic strip (width 2a) excited by a time‐harmonic line force (circular frequency ω) is analyzed theoretically. A Green's integral representation for the plate velocity distribution is used and boundary conditions are met by a system of two integral equations along the plate edges [P. Filippi, J. Sound Vib. 100, 69–81 (1985)]. For the case of clamped edges, results for the farfield pressure are presented for several values of koa, where k0 = ω/co is the wavenumber in the fluid. It is shown that at low frequencies, i.e., for koa ≪ 1, the strip radiates like a monopole with a strength equal to the net volume velocity of the strip. This monopole behavior of the finite strip differs from the well‐known dipole radiation at low frequencies for a line force excited, infinite plate. The results are compared with similar results published by Filippi. The connection is discussed of the present problem to the case of an infinite plate with two parallel clamped line joints. [Work supported by the TNO Institute of Applied Physics, Delft, The Netherlands.]
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Sound radiation from structural discontinuities (A)

Richard F. Keltie

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S75-S75 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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A useful method for analyzing the vibration response of structures with abrupt changes in property or dimension is to separate the vibration field into the incident, reflected, and transmitted components. These components will, in general, include both propagating waves and nonpropagating nearfield constituents. Utilizing this method, the specific contributions of each portion of the vibratory response to the resulting sound radiation are quantified for planar radiators. The general methodology is developed and results are given for the particular cases of subsonic bending waves incident upon elastic supports and rigid masses. The relative role of each portion of the vibratory response in scattering energy into the supersonic wavenumber domain, and contributing to the sound field, is discussed as a function of the wavenumber ratio, k/kB, of the incident bending wave.
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A boundary element approach to weak radiator analysis—Preliminary results for a plate (A)

Kenneth A. Cunefare and Gary Koopmann

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S75-S75 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Weak radiator analysis attempts to answer the question how should a structure respond at a given frequency such that it radiates sound least efficiently? Use of the boundary element method leads to a quadratic expression for the total radiated power from a structure formulated in terms of the surface velocity distribution on the structure. This quadratic power expression, formulated at the surface, represents the basis for the technique. Constrained optimization applied to the expression for power leads to that response shape that radiates sound least efficiently. The constraint considered here corresponds to a nonzero velocity at some point on the surface, equivalent to a drive point. More general constraints are possible, such as nonzero mean‐square velocity. Weak radiator analysis is demonstrated through its application to a centrally driven, 760 × 610 mm plate, forming one side of a box. The resulting weak radiator shapes are unique, i.e., they do not correspond to the mode shapes of either a simply supported or clamped plate.
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Development of a multidiffraction device for improving the noise barrier performance (A)

Joon Eun Hee and Sung Jun Oh

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S75-S75 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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A multidiffraction device has been studied, both theoretically and experimentally, based on an expanded concept of the double barrier principle. The device, which has four diffraction leaves, is intended to be used on top of an existing noise barrier to improve the barrier efficiency. The multiple diffractions provided by the four diffraction leaves was found to be the major contributor to the improved performance of the barrier. However, the interference of the primary rays with the reflected ones from the bottom surfaces between the diffraction leaves was also found to play a role, indicating the possibility of designing an optimum device for a given situation. The performance tests, carried out both in an anechoic chamber using a scale model and in the field using an actual device, indicate about 5‐dB improvement in noise reduction on the average. [Work supported by Hyosung Al. Co.]
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