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Nov 1980

Volume 68, Issue S1, pp. S1-S116

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back to top Session GGG. Shock and Vibration V: Education in the Field of Vibrations and Acoustics; Structural Vibration
Invited Papers
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Structural vibrations in an acoustic medium: An amateur teacher's experience (A)

M. C. Junger

J. Acoust. Soc. Am. Volume 68, Issue S1, pp. S115-S115 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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This speaker describes the experience of a consultant and researcher who spent ten years teaching this subject to mechanical, civil, and ocean engineers largely unfamiliar with acoustics. It is indicative of this gap that the acoustics branch germane to structural vibrations was founded by fluid mechanicists (Stokes, Lamb) rather than structural mechanicists. A central concept acceptable to the vibrations engineer models the fluid as a distributed load formulated in terms of an impedance. A second essential concept is that this load's nature (masslike reactance in the hydrodynamic limiting case, or resistance when fluid compressibility predominates) is determined by one dimensionless number: The ratio of acoustic to structural wavenumber. Radiation loading is finally formulated as generalized forces proportional to the structure's modal response or as a distributed load proportional to the wavenumber transform of the structural response.
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Teaching acoustics (A)

E. J. Skudrzyk

J. Acoust. Soc. Am. Volume 68, Issue S1, pp. S115-S115 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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Teaching acoustics is a special experience. Young students are not yet trained to think, and they have great difficulty grasping the basics of any new field they encounter, despite having been taught much of the mathematics in preceding courses. However, once they have learned the basics, they can easily handle the most difficult derivations that are presented to them. In my 400 level course, considerable attention is placed on the harmonic oscillator, on electromechanical analogies, and on circuit theory. About two million highly trained electrical engineers have worked on network theory during the last five decades. They have developed very powerful methods of dealing with linear systems that have no equivalent in classical theory. Examples presented in my acoustics class will convince the strongest adversary of circuit methods that not being acquainted with the theorems of circuit theory and not using them is like being a mathematician and not knowing calculus. Mathematics is a crutch that facilitates an understanding of complex sound phenomena, and much attention is placed on interpreting acoustic phenomena. Examples will be given of sound radiation by complex vibrators and of the relationship between abstract theory and physical understanding.
Contributed Papers
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Theoretical acoustics at the doctoral level (A)

J. Eisler

J. Acoust. Soc. Am. Volume 68, Issue S1, pp. S115-S115 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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A course on the radiation and scattering of acoustic waves is described as taught at the Catholic University. It is intended to serve as a foundation for other doctoral level courses with emphasis and critical scrutiny of the basic principles of acoustics. A further purpose is to introduce the mathematical methods used in theoretical acoustics with an appropriate degree of rigor. By restricting boundary conditions to idealized cases, reasonable coverage may be given both to the mathematical toolbox and to the important theories of the low and high frequency regimes. Thus the student is prepared for a critical understanding of technical problems at the research level.
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Free transverse vibrations of a uniform circular plate with an eccentric (A)

W. H. Lin and M. W. Wambsganss

J. Acoust. Soc. Am. Volume 68, Issue S1, pp. S115-S115 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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This paper deals with the problem of free transverse vibrations of a uniform circular plate with an eccentric hole. Using the method of trasforming cylindrical wave functions from one circle to another, the wave equation governing the transverse displacement of the plate is exactly solved to satisfy the boundary conditions at both inner and outer edges. The frequency equations for the vibrations of the plate with different boundary conditions are derived. This analysis also includes the vibrations of a uniform circular plate with a concentric hole as a special case. Numerical results are presented to show the dependence of the eigenwavenumber on eccentricity and the ratio of inner radius to outer radius. These results are in very good agreement with those obtained by other methods published in the literature. What distinguishes this study from others is that the exact analytical solution of the vibrations of an eccentric plate is obtained. Method of this study can be also applied to the studies of any eccentric waveguides and vibrations.
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Vibration analysis of laminated composites (A)

M. C. Dökmeci

J. Acoust. Soc. Am. Volume 68, Issue S1, pp. S116-S116 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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This study presents a macromechanical analysis of vibration of laminated composites within the concept of the effective stiffnesses of composites. The laminated composite may possess arbitrary number of matrix and reinforcing layers, each with a distinct but uniform thickness, curvature, and mechanical properties. Starting with a linearized displacement field for each constituent layer and using an unconstrained, dynamic variational principle due to the author [J. Ship Res. 23, 115–122 (1979)], the set of two‐dimensional governing equations of the prestressed laminated composite is systematically and consistently established. The continuity of tractions and displacements at the interfaces is maintained, and all the effects of transverse shear and normal strains and the rotary inertia of layers are included. The governing equations accommodate all the types of small incremental motions as well as those of initial finite motions of the laminated composite. Further, special cases of interest are pointed out, and in particular, the linearized version of the governing equations is considered. Then a theorem which enumerates the sufficient conditions for its solutions is constructed by means of the logarithmic convexity arguments. [Supported by AEK Tek: 2940‐3‐77.]
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Flow induced vibrations of long, flexible cylinders (A)

F. C. DeMetz

J. Acoust. Soc. Am. Volume 68, Issue S1, pp. S116-S116 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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The mechanisms of flow‐induced vibrations in long, flexible cylinders are reviewed. The excitation mechanisms include vortex shedding, turbulent pressure field, fluctuating shear stress, and static and oscillatory instability. The relative amplitudes of the transverse and longitudinal cylinder vibrations are discussed in terms of some elementary models of the forcing functions and the mechanical properties of the cylinder.
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Variational principles for fluid‐solid interactions, with application to ship beams (A)

N. Sarigül and M. C. Bökmeci

J. Acoust. Soc. Am. Volume 68, Issue S1, pp. S116-S116 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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The purpose of this paper is threefold: (1) To derive the unconstrained variational principles which treat initial and boundary value problems of fluid‐solid interactions in a unified manner, then by using it (2) to establish the one‐dimensional governing equations of vibrations of ship beams immersed in an ideal fluid, and also (3) to examine the uniqueness in solutions of these equations. First, a general principle of physics (Hamilton's principle) is transformed so as to derive the variational principles through the dislocation potentials and Lagrange multipliers. Next, the variational principles together with the generalized Mindlin method of structures are used to construct the governing equations by expanding the displacement components and the hydrodynamic pressure in series. The governing equations are coupled with the pressure field and incorporate all the effects of transverse shear and normal strains and the rotatory inertia, and they accommodate all the types of extensional, flexural, and torsional motions of ship beams. Also, the uniqueness is examined, and the sufficient conditions are enumerated for it. [Work supported by TÜBİTAK.]
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Ensemble averages of power flow in randomly excited coupled beams (A)

H. G. Davies and M. A. Wahab

J. Acoust. Soc. Am. Volume 68, Issue S1, pp. S116-S116 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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This paper considers the transmission of transverse vibrational energy between two coupled beams subjected to random excitation. Extensive numerical results are given for the frequency decomposition of power flow between the beams. The power flow is a function of the three nondimensional parameters describing frequency, w/w1, where w1 is the fundamental resonance frequency of beam one, length ratio, L2/L1, and damping or modal overlap ratio. Single frequency and octave band averages are shown for various values of L2/L1, and means and variances are calculated for the ensemble of systems corresponding to a uniform distribution of 1 ⩽ L2/L1 ⩽ 2. Various approximate results based on modal analysis and on SEA are compared with the exact results.
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Acoustic radiation from an impulsively excited elastic plate (A)

Seroj S. Mackertich and Sabih I. Hayek

J. Acoust. Soc. Am. Volume 68, Issue S1, pp. S116-S116 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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The acoustic radiated pressure time signature of a submerged elastic plate which is impulsively loaded is predicted analytically by use of integral transforms on time and space. The first arrival of the acoustic pulse at an observer point in the medium was shown to correspond to the acoustic time of arrival for the normal distance from the observer to the plate. After the first arrival, the time signature oscillates with decreasing amplitude with the passage of time. The period of oscillation was shown to increase with time and the decay rate was shown to decay inversely with elapsed time. [Work supported by NAVSEA.]
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