On the basis of photographic studies, it is indicated that the normal collapse mode for a bubble in an ultrasonically induced cavitation field is asymmetrical. In most cases, the bubble involutes into a torus, the direction of the involution being dependent on perturbations of the bubble surface due to pressure gradients, etc. A vortex ring is formed producing a central microjet, the impact of which upon a solid surface is believed to be the predominant damaging mechanism. Three collapse models are hypothesized to explain the existence of the various types and sizes of cavitation bubbles that were observed and to also suggest the mode of a possible damaging collapse. An attempt is made to correlate the known pit damage on an individual specimen to the approximate number of observed bubbles of the type that are believed to be indicative of such a jet impingement on the specimen surface, and it is found that the order of magnitude is correct. Visible evidence of the effect of the shock waves that apparently accompany rebounding bubbles is presented. * The information reported here is contained in a dissertation submitted by the first-named author to the Graduate School, Univ. of Michigan in Sept. 1966, in partial fulfillment of the requirements for the PhD degree. } Now Associate Professor of Mech. Eng. at Colorado State Univ., Fort Collins, Colo. •t Now Professor in charge, Cavitation and Multiphase Flow Lab., Mech. Eng. Dep., Univ. of Michigan. 1L. E. Robinson, B. A. Holmes, and W. C. Leith, "Progress Report on Standardization of the Vibratory Cavitation Test," Trans. Amer. Soc. Mech. Eng. 80, 103-107 (1958). •ASTM Committee G-2 on Cavitation and Impingement Erosion. 1272 Volume 46 Number 5 (Part 2) 1969 I. EXPERIMENTAL APPARATUSThe ultrasonic transducer unit used in this experiment (Fig. 1)
was developed, constructed, and originally used by Garcia. 3,•The remaining parts of the system include an audiooscillator, a power amplifier, an oscilloscope, a frequency counter, an accelerometer, and a container for the water.The signal supplied by the variable-frequency audiooscillator is amplified and applied to the piezoelectric crystals. The standing waves that are generated in the axial direction are amplified by the exponential shape of the horn.The metal specimens to be tested were selected from materials with widely differing mechanical properties in an attempt to correlate damage with specimen properties. This was another goal of the over-all experiment. The specimens were metallographically polished before exposure. One specimen was lightly abraded with emery cloth and was used to examine, by photographic means, the effects of minor surface roughness on the bubble-field population. Also, half the polished surface of one specimen was lightly abraded to test the effect of such a surface on the cavitation-bubble field.