The schlieren image of two-dimensional ultrasonic fields and cavity resonances

Abstract

The schlieren technique can be used to visualize the two-dimensional ultrasonic standing wave fields associated with circular cylindrical shells under resonance conditions. However, the interpretation of the schlieren image is not simple due to the complex relationship between the acoustic and optical fields. A model for predicting the optical image in an ideal schlieren system is presented and used to investigate the influence of the acoustic pressure and optical spatial filtering on the resultant image, demonstrating the conditions under which the image is a meaningful representation of the acoustic wave field. A low-frequency (>100 kHz), wide-aperture, laboratory schlieren system is used to image the fluid column resonances of a circular cylindrical shell. Experimental results agree well with the predictions, validating the theory. Although the schlieren image is two-dimensional, limiting investigations to targets having translational symmetry, the technique is noninvasive and can potentially provide greater insight into the acoustic resonance behavior of more complex scattering geometries.