No abstract

The authors have recently undertaken a theoretical simulation of experiments similar to those performed recently at NRL by Earl G. Williams and his colleagues, in which a submerged cylinder is transiently excited and normal displacements are inferred using the generalized nearfield acoustical holography (GENAH) technique. In the present paper, the cylinder is modeled as a thin-walled shell with rigid immovable end caps and with the end conditions taken as being that of simply supported. A short duration point force f(t) is applied normally to the surface and one seeks to describe the resulting normal displacement field using a high-frequency (and short wavelength) asymptotic theory of propagating and evanescent flexural wave pulses on a curved plate. Fluid loading is taken into account approximately with modifications derived using the assumption that the flexural wavelengths are short compared to the length and radius of the cylinder. One asks for the time and surface position dependence of the normal surface displacement within a local region of the cylindrical surface before and after a simulated wave vector filtering has been applied. The filtering process is such that one seeks to isolate only that contribution from a narrow frequency band and from a localized region of wavenumber space. The exercise is intended to demonstrate that one can isolate and follow traveling wavelets propagating away from the source, spiraling around the cylinder, and undergoing successive reflections at the end caps. [Work supported by ONR.]

No abstract

A computer-actuated acoustic research tank facility designed for facilitating precise acoustic measurement of sonar transducers, small arrays, and structural models has been installed at the Georgia Institute of Technology. The apparatus consists of a wooden tank, having dimensions of 7-m diameter and 5-m depth, integrated to a computer actuated positioning and data retrieval system. A resident minicomputer controls positioning and signal generation, as well as providing temporary data storage and signal processing capabilities. Permanent data storage and post-processing are provided by a remote minicomputer. The multiaxial positioning and scanning system, which extends over the entire active tank area, provides accurate triaxial and rotational positioning of the transducer. Component hardware, positioning system design features, computer software and facility capabilities are presented. One current use for the facility is to perform a series of experimental measurements exploring nonlinear effects encountered in sound beams generated by a high-power transducer operating at a moderately high ka value. Waveforms are measured at a variety of locations in the near and farfields, both on and off axis. Signal processing then provides amplitude and phase information for the fundamental and higher harmonics.