Observation of Waves Radiated from Circular Cylinders Caused by an Incident Pulse

Abstract

Schlieren visualization and hydrophone measurements are used to observe the radiated wavefronts which result when an acoustic pulse is incident on a metal cylinder in water. The range of size parameter ka from 138 to 1419 is considered. The wavefront positions are traced by the refraction, internal reflection, and radiation of shear and compressional waves. In the case of solid cylinders, many wavefronts display an apparent circumferential property derived from the incidence of energy from the normal to the appropriate critical angle. Identification of one of these wavefronts as resulting from previously identified “Rayleigh-type” wave propagation and a single incident angle is denied, although the circumferential property is verified. A previously identified faster circumferential wave is attributed to a composite wavefront resulting from direct compressional transmission and an increasing number of its internal reflections. Other wavefronts depending on mode conversions are also identified. A mechanism is presented to account for the apparent dispersion indicated by an increasing circumferential wave speed determined from previous hydrophone measurements below ka = 200. Simultaneous schlieren and hydrophone receptions that show the wavefront incident on a hydrophone as well as the resultant hydrophone output at ka values of 355 and 138 are presented. For shells, a prominent wave is seen to exist, in the same position, for both air- and water-filled cases. The wave is shown to be a characteristic of thin shells, originating from the transmitted compressional wave and subsequent mode conversion to a shear wave. The radiated wavefronts disclose a circumferential property for the waves that cause them. One such wavefront, believed to be the same described elsewhere as an antisymmetric Lamb wave, is proven in our case to be caused by direct transmission through the interior of a water-filled shell. The wave previously attributed to a Rayleigh-type circumferential propagation is shown to exist in aluminum shells as thin as those with a 0.9 ratio of inner to outer radius.