Scattering of Acoustic Energy by Solid and Air-Filled Cylinders in Water

Abstract

This paper presents the results of an experimental investigation of the scattering of acoustic energy from finite circular cylinders in water. Solid and hollow brass cylinders were used as targets. Distribution-in-angle diagrams of acoustic energy scattered in the plane perpendicular to the longitudinal axis of the cylinders are shown for changes in finite cylinder length, changes in pulse length, and changes in frequency. Pulse photographs are presented to describe the echo structure of solid and hollow cylinders. Photographs are shown of the pulses scattered in the backward direction in the plane normal to the longitudinal axis of the cylinder and in the plane containing the longitudinal axis of the cylinder. The change in echo structure is presented as a function of pulse length and frequency. The echo structure exhibits characteristics dependent upon the sharp corners of the cylinder where there is a large change in slope and a discontinuity between brass and water. The photographs of pulses returned from the cylinders in the plane normal to the longitudinal axis describe the existence of surface waves which travel around the circumferences of the cylinders. (This work was partially supported by the Bureau of Ships.)

Acoustic Resonance Scattering by Submerged Elastic Shells

“…This completed the classical theory, which was always concerned with the determination of form-functions, such as Eq (2), or their squares, which yielded (normalized) backscattering (or monostatic) cross sections (<r B …”

“…The resonance scattering theory (RST) [135] has always been concerned with the residuals that result when either rigid or soft modal backgrounds, found from Eqs (4), are subtracted from the elastic partial waves of Eq (2); that is, with combinations of the form…”

“…Thus, Eq (33b) is the generalization of the eigenfrequency condition for Lamb waves, usually given for flat plates, in vacuo to fluid-loaded spherical shells. In the limit of large a and b (but such that a -b = 2d is fixed), Eq (33b) splits into the symmetric and antisymmetric conditions for Lamb waves in a plate, viz [11,97,148] (2(c 2 /c 2 ) -l) 2 4(rf/c 2 )Vl -c 2 /c 2 -Jc 2 /c 2 d ~ c 2 /c 2 (37) for the antisymmetric branches (aj,j = 0, 1,2,...).…”

“…There is an initial transient regime (region 1), then a steady-state or forced vibration region (2), then a second transient regime, finally followed by the free exponential vibratory decay (3) (ie, the "ringing") of the stored energy once the pulse excitation stops. If we consider sound scattering at normal incidence from an elastic cylindrical body excited by a long pulse that has a carrier frequency corresponding to any of the natural resonances of the (fluid-loaded) body, then one can notice a substantial difference between the DSCS as it is extracted from the forced, steady-state regime (2), and as it is obtained past the final transient region, just as the free ringing (3) begins. The first is a complicated angular pattern because of the forcing of many angular cylindrical harmonics by the incident wave.…”

“…Classical techniques, many of which are particularly suited for high frequencies, have been used theoretically [32,38,39,52,68,75,76,79,93,108,119] and experimentally [2,22,59,61,107,109,110,113,120,131] to study scattering situations dealing with cylindrical shells.…”