Schlieren Visualization of Radiation Caused by Illumination of Plates with Short Acoustical Pulses

Abstract

Schlieren visualization of the radiated field in water above and below aluminum plates, illuminated by short acoustic pulses, is presented. At certain angles of incidence of the incoming pulse, radiated wavefronts are seen to progress along the plates with specific angles of inclination and associated speeds of progression. In particular, strong effects are observed near the longitudinal and shear critical angles. The measured angles of incidence, at which these effects were generated, are in good agreement with predictions of Lamb theory for plates whose thicknesses are 0.58 wavelengths and larger. For thinner plates, the effects of water loading appear to become significant. A path trace analysis gives good agreement with the group velocity of observed effects when obvious trapped propagation paths are suggested by the reflection coefficient curves for a water-aluminum boundary. Specific radiated wavefronts on plates are related to waves which were observed on cylindrical shells. Relationship of the shell results to Lamb modes is discussed. Plates with thicknesses from 0.002 to 0.241 cm, and frequencies of 5 and 7.2 MHz, are used.