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May 1988

Volume 83, Issue S1, pp. S1-S122

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back to top Session JJ. Structural Acoustics and Vibration III: Plates and Shells and Radiation Problems
Contributed Papers
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An analytic model for vibration of turbomachinery rotor blades (A)

James P. Charpie and Courtney B. Burroughs

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S77-S77 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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An analytic model for the vibration response of turbomachinery rotor blades that includes the effects of variations in blade thickness, curvature of the blade edges, and variations in the curvature of the blade middle surface in two dimensions is developed. Mathematical fits to both surfaces of the blade are generated in terms of polynomial equations with constant coefficients. The polynomial equations for the blade surfaces are then used in shell equations, based on Love's thin shell assumptions, to generate differential equations governing the vibration motion of the blade. The coefficients in the blade‐fitting polynomials act as parameters in the equations of motion for the blade, so that equations for a new blade can be developed easily by changing the coefficients in the governing equations to those generated for the new blade. An approximate method of solution for the resonant response of a sample blade is presented.
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Modification of Lamb wave spectrum in fluid coupled plate (A)

S. L Rokhlin, D. E. Chimenti, and A. H. Nayfeh

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S77-S77 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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It has been accepted previously that for most practical eases when a plate is immersed in a liquid the Lamb wave spectrum is only slightly perturbed due to energy loss related to leakage of the wave from the plate to the liquid. In contrast it was recently discovered by D. E. Chimenti and A. H. Nayfeh [Appl. Phys. Lett. 49 (9) (1986)] that for leaky Lamb waves in a graphite epoxy composite plate in water a strong anomaly appears in the topology of the S0 mode as a function of frequency. Although it was caused by the fluid, the reason for this anomaly was still not clear. The nature of the anomalous behavior of leaky Lamb waves is discussed in this paper. It is shown that for an understanding of the spectrum topology, the full complex spectrum of propagating and nonpropagating modes should be considered. It is demonstrated that the topological changes in the S0 mode are related to its nearness to the first complex branch of the spectrum. Increasing liquid density leads to interaction between these modes and mutual interchange between their branches in the complex plane. It is shown that, in general, as the density of the immersion liquid increases from zero to infinity the complex spectrum of plate modes gradually transforms from a spectrum of Lamb waves in a free plate to a spectrum of waves in a plate clamped on its surfaces with slip (boundary conditions σxy = 0, Uy = 0).
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The solution of fluid‐structure interaction problem using quadratic, doubly curved finite elements (A)

Joseph J. Shirron and Luise Schuetz

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S77-S77 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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A model to solve coupled fluid—structure interaction problems, such as acoustic scattering from an elastic body, is described. This model uses doubly curved, nine‐noded, isoparametric finite elements over which the pressure is taken to vary quadratically. Results from this model are compared with another computer model (NASHUA) that is based on planar finite elements over which the acoustic pressure is assumed to remain constant. Several examples are presented to demonstrate the reduction in the number of elements needed for accurate modeling. Comparison of solution time and accuracy is also given.
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Acoustical power radiation from cylindrical shells, with or without stiffeners: Understanding of phenomena and practical aspects (A)

B. Laulagnet, J. L. Guyader, and C. Lesueur

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S77-S77 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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An analytical study of radiation by finite, stiffened cylindrical shells is presented. The model of stiffener utilized allows one to treat hollow cross‐section cases encountered in industry. The point driving force is on the stiffener or directly on the shell and can be radial, tangential, or longitudinal. Fluid and structure equations are solved with the modal analysis, using the in vacuo nonstiffened basis. This leads to the calculation of radiation impedances and mass and stiffness generalized terms of stiffeners. Theoretical results are presented both in air and in water, in radiated power, radial quadratic velocity, and radiation factor. Mechanical modal cross‐coupling influence is shown as well as the stiffening influence on the in water vibroacoustic shell behavior. Parameters like rigidity, number and distribution of stiffeners, shell and stiffener structural damping, location, and force type are studied in air. Finally means for noise reducing in air are deduced. [Work supported by the Ministère de la Défense, Contrat D.R.E.T. No. 85065.]
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Sound radiation from a vibrating body in motion (A)

Xiao‐Feng Wu and Adnan Akay

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S77-S78 (1988); (2 pages)

Online Publication Date: 13 Aug 2005

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Past studies of acoustic radiation problems have been concerned with sound waves generated by either a vibrating body or a moving point source with no dimensions of its own. This paper presents a method for determining the acoustic field radiated from an arbitrary object undergoing, simultaneously, a harmonic oscillation and a spatial motion with respect to the surrounding fluid medium. At this stage of the investigation, the turbulence and flow distortion caused by the motion of the vibrating body are neglected. The method is based on the assumption that the surface of the vibrating body can be described in terms of a distribution of simple monopole and dipole sources. Utilizing a linear coordinate transformation, one is then able to reduce the problem to that of radiation from a stationary source for which the analytical solution has been obtained [Morse and Ingard, Theoretical Acoustics (Princeton U.P., Princeton, NJ, 1986)]. The total acoustic field is the sum of contributions from all the simple sources on the surface of the moving object. The method is illustrated using a vibrating sphere moving with either constant or time‐dependent velocity profiles relative to the surrounding medium. Extension of this method to impulsive motions is discussed. [Work supported by NSF and WSU Institute for Manufacturing Research.]
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Leaky Lamb wave inspection of thin plates (A)

Peter B. Nagy and Laszlo Adler

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S78-S78 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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A single‐transducer immersion technique is introduced to study dispersion of leaky Lamb waves in different plates, such as fiber‐reinforced composite laminates and adhesive lap joints. The plate under study is interrogated by a broadband pulse from both sides when placed between the transmitting‐receiving transducer and a perpendicularly aligned mirror. The transmitted peaks in the spectrum of the received echo from this mirror correspond to double mode‐converted leaky Lamb wave resonances. These peaks are measured at different angles of incidence, i.e., at given phase velocities to get velocity dispersion curves. Experimental results are presented to demonstrate the accuracy and versatility of this very simple measuring technique
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Transient structural vibration created by a laser beam (A)

Yousef A. Abo‐Mossallam

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S78-S78 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Experimental work is conducted to investigate the transient vibration of a structure by an argon laser beam. The conversion of laser light into mechanical energy has been studied through focusing laser beam to vaporize material from a target attached to the structure. The rapid ejection of target material results in an impulsive reaction transmitted to the structure. Different target materials are tested to enhance the efficiency of energy conversion. A clamped beam and a cantilever beam are used in the experiments and the measured natural frequencies are compared favorably with the calculated ones. Beam splitters and multiple targets are used to test the concept of multipoint simultaneous impulsive loading.
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Sound radiation control for two plates assembled in an L shape (A)

Pascal F. Millot and Jean‐Louis Guyader

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S78-S78 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Reduction of sound radiated in a cavity by an L‐shape structure excited by mechanical forces, can—in some instances—be achieved by taking advantage of the phenomena that are unique to that geometrical shape. Instead of relying only on the control of the velocity of the structure to reduce the sound radiated, it will be shown that it is possible to select modes of the assembled plates that have a low‐radiation impedance, by positioning correctly the mechanical excitation forces. At low frequencies, improvements can reach several decades for the mean‐squared pressure inside the cavity for identical plates, while above the limit frequency of the cavity no improvement should be expected with this method. Below the limit frequency, improvements will greatly depend on the respective dimensions of the plates, as well as on the values for the first resonant frequency of the plates and of the cavity. Theoretical results, derived from a modal approach for the cavity and the plates, as well as experimental results obtained for plaster plates in laboratory, will be presented.
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