885095943

3.-05

7SA3. Flexural intensity measuretnents in a beam using a threc-point measurement. Richard DeJong (Dept. of Eng., Calvin College, Grand Rapids, MI 49506)

A method for obtaining the flexural intensity vector in a vibrating beam has been developed using the measurement of the transverse motion at three closely spaced points. This method is superior to the two-point method in that it accounts for both the shear and bending components of the energy flow in the beam. The intensity vector can be accurately measured near discontinuities in the beam, where near-field components cause errors in the two-point method. The new method Ls also superior to the four- or five-point methods that use a fuli finite difference approximation to the flexural wave equation in that it is morę compact in space and less sensitive to instrumentation errors. The new method does use an approximation that makes it sensitive to phase errors at Iow frequencies. The accuracy of the method and the sensitivity to phase errors are investigated using simulations and measurements on beams with discontinuities.

3-35

7SA4. The effccts of fluid loading on the radiation from piane ribbed panels. G. Maidanik and J. Dickey (David Taylor Res. Ctr., Bethesda, MD 20084-5000)

The impulse responsc of an infinite regularly ribbed fluid-Ioaded panel is used to calculatc the spcctral response and radiation efficiency of these structures when they are excited by mechanical (linę) drives. The linear response quantities, pressure and velocity, are displayed in the wave-number-frequency piane, where the patterns show many interesting features connected with the periodicity of the ribs, the mechanical damping, and the fluid loading. The combination of the linear-response quantities to form the quadratic quantities, energy and intensity, is discussed. The near-field and far-field responses in the fluid are calculated and used to determine the intensity on the panel and in the far field. These quantities are then integrated and used to determine the radiation efficiency of the panel.

Contributed Papers

4:05

7SA5. Vibrational power flow and energy localization in coupled Systems. P. Ezanno (Dept. of Mech. Eng., Virginia Polytech. Inst. and State Univ., Blacksburg, VA 24061), E. K. Dimitriadis (The Catholic Univ. of America, Washington, DC 20064), and R. Burdisso (Virginia Polytech. Inst. and State Univ., Blacksburg, VA 24061)

A system of two, nearly identical Euler-Bemoulli beams, which are coupled through a torsional spring, is here analyzed in terms of power exchangc betwecn and energy localization within this nearly symmetric system. This investigation can be viewed as extending over an ensemble of Systems, one member of which consists of two identical beams (the tuned State) and the rest are similar Systems except for smali perturba-tions introduced to the tuned system to crcate mistuned States. Harmonie excitation of members of the ensemble make it elear that vibra-tion localization can occur for smali length perturbations. Clearly, the localization degree depends strongly on the coupling strength. Here, the ensemble is examined under wide-band stationary random excitation. Computations are performed of the mean power flow belween the two beams over specific frcquency bands. It Ls clearly seen that power flow is strongest in the tuned State, but it is also significant for specific other valucs of perturbation. The naturę of this dependence, along with its possible implications to design methodologies, such as Statistical Energy Analysis, will be discussed.

4:20

7SA6. Eigenmode statistics in large two-dimensional systems. Richard L. Weaver (104 S. Wright St., Dept. of Theor. and Appl. Mech., Univ. of Illinois, Urbana, IL 61801)

A finite difference model is used to numerically investigate the higher-acoustic eigenmodes of membranes with random boundaries. Natural frequencies are found to distribute themselves with level repul-sion and spectral rigidity like those of the “Gaussian orthogonal ensemble” in accord with the predictions of random matrix theory. These statistics have also been observed in recent experiments determining the higher eigenfrequencies of aluminum blocks. The present work goes further in that it also considers the statistics of the eigenmode shapes. In particular, values for the mean fourth power of modal amplitudę (re-lated to the “participation ratio” and to variances of power transfer functions in large systems) are reported, as are results on repulsion between modę shapes. (Work supported by NSF.]

435

7SA7. Holographic measurement of power flow in large immersed structures. Joseph A. Clark, Paul M. Honke, and J. Michael Ellis (David Taylor Res. Ctr., Bethesda, MD 20084-5000)

Simplc harmonie motions of a vibrating surface can be decomposed into two component motions: a standing wave, which corresponds to a characteristic modę shape, and a traveling wave, which is associated with the flow of power along the surface. Random motions of the vi-brating surface can be decomposed into a series of simple harmonie motions, each of which can be decomposed into standing and traveling components. Holographic measurement of the pressure field around an immersed, point excited, vibrating cylinder have been processed to re-veal the series of standing and traveling waves noted above. The results have been visualized in computer-gcncrated movies and used to diag-nosc the structural response of the cylinder and power flow along and out of its surface. In this talk, the measurement, processing, and graphic display systems used to perform the diagnosis and features revealed by the investigalion will be dcscribcd.

1977

J. Acoust. Soc. Am.. Vol. 09. No. 4, Pt. 2. April 1991

121st Meetiog: Acoustical Society of America

1977