885095929

9:50

6SA4. Array Processing for wave vectors and modes in structural acoustics. Arthur B. Baggeroer (DepCs. of Ocean and Elec. Eng., MIT, Cambridge. MA 02139)

Array processing mcthods have emphasized beamforming and directional spectrum estimation for cqually spaced, linear arrays with single componcnt sensors that are in a homogeneous signal field with long duration dala. Structural acoustics problcms oftcn have jusl the opposite characteristics with irregular spacing of multicomponcnt sensors that are in inhomogeneous wave fields with transient data. These eliminate many array processing methods and require generalization of others. Recently, array processing mcthods such as matched field and matched modę, slowness-lime spectra, and singular value decomposition have been advanced and used for structural acoustics. This presentation will give an overview of advanced array processing methods with an emphasis on those applicable to problems in structural acoustics. |Work supportcd by ONR.J

10:15

6SA5. Experimental mcasurements of wave propagation on submerged sheils. Earl G. Williams (Naval Res. Ix»b., Codę 5137, Washington. DC 20375-5000)

Various signal processing techniques. combined with cylindrical near-field acoustical holography, are used to study wave propagation on thin, cndcappcd cylindrical sheils excited with a point drivcr, and submerged in an cxternal fluid. The upper frequency limit was doubled compared to previous experiments reported in previous meetings of the ASA. In this new frequency regime (2<Aw<4) k-ó) processing of the dala reveal that an entirely new spectrum of supersonic helical waves iscxcited. Comparisons with infinite shell theory reveal that these waves are the well-known shear and extensional modes of propagation. These dominate the radiation to the far field whereas the subsonic helical waves due to bending are no longer important. The acoustic pressure in the extreme near field is used along with an assortment of A-spacc signal processing techniques to study the propagation on and radiation from these waves on the shell. It is shown how direction filtering in k space of helical wave components can be used to reconstruct the z-t domain representations of these filtered components, and the resulting wave propagation is recordcd in a vidcotape display. With this display the group velocity and other physical parameters can be determined experimen-tally.

Contributed Papers

10:40

6SA6. On wave-vector filier analysis of turbulent flow. R. H. Mellen (Kildare Corp., 95 Trumbull St., New London, CT 06320)

Convective turbulence models are based mainly on mcasurements of space/frequency correlation along the longitudinal and transverse axes and the wave-number/frequency spectrum is derived by Fourier trans-formation of an analytic model. It has been found that the Iow wavc-number “convcctive ridge” resulting from the Corcos model is appar-ently an artifact of the rhombic Fourier window, sińce it does not occur with the smoother elliptic window [R. H. Mellen. J. Acoust. Soc. Am. 88. 2891-2893 (1990)). Wave-vcctor filier arrays are also used for de-lermining wave-numbcr/frequency statistics in this region. Rclative ef-fects of the two window models on the analysis of experimental results are examined.

10:55

6SA7. Calculation of shock spectrum from a discretely sampled velocity time bisiory. Peter K. Kasper, Benjamin A. Słupek, and Rosalyn F. Sawczyszyn (CASDE Corp., 2800 Shirlington Rd., Ste. 600, Arlington, VA 22206)

A shock spectrum corresponds to the maximum response of a me-chanical resonator for a given time history excitation input. With modern computer-bascd instrumcniation, ihc desired time history input is often provided as discretely sampled acceleration or velocity values. Determination of a shock spectrum requires a solution method for cal-culating resonator response to the transient input. Beck and Dowling [J. L. Beck and M. J. Dowling, Earthquake Eng. Struct. Dyn. 16, 245-253 (1988)] present an algorithm for computing resonator response given a discretely sampled input acceleration time history. The current paper extends the application to account for input excitation in terms of a transient base motion velocily. The algorithm is derwed from a closed-form integration of the system response with linear exlrapolation be-tween discrete input values. Verification of the discrete ve!ocity input algorithm is demonstrated by comparison with corresponding results using a continuous time history input excilation.

11:10

6SA8. Theory and experimenl of two concentric submerged cylindrical sheils coupled by Ihe entrained fluid. Shigeru Yoshikawa (5th Res. Ctr., Technol. Res. and Dev. Insi., Japan Defense Agency, Yokosuka 239, Japan), Earl G. Williams, and Karl B. Washbum (Naval Res. Lab., Washington. DC 20375-5000)

In this paper theory and expcriment for the coupled vibration of two concentric cylindrical sheils (double shell) are described; the inner shell contains air, the outer shell is surrounded by water, and water exists in the annular space bctwcen them. A point force is applted to the inner shell, whose displacement produces acoustic pressure in the annular space and in tum this pressure wave drives the outer shell. The naturę of acoustic field in the annular fluid and its coupling effect on the sheils is investigated. Using Fliigge’s shell cquat»ons and the Helmholtz cqua-tion, the normal modes of an infinite double shell are calculated. The theory includes the effect of initial prestress (uniform axiat compres-sion). The results of these numerical calculations in wave-number space will be compared with the results from generaiized near-field acoustical holography (GENAH) experiment on a simply supported finite double shell. The wavc-number/frequency representation of this double shell will be also compared with that of a single submerged shell.

1955

J. Acoust. Soc. Am., Vol. 89, No. 4, Pt. 2, April 1991

121 st Meeting: Acoustical Society of America

1955