24 



were observed. Subsequently, oscillations of such cavities were reported by 

 Briggs et al, 31 Kornfeld and Suvorov, 12 Harvey 11 and Osborne. 4 The first 

 two groups of experimenters used sound fields and magnetostriction oscillators, 

 respectively, so that it is not clear that the oscillations observed were 

 associated with a self -excitation. Harvey and his colleagues, however, using 

 rods withdrawn rapidly from a liquid, observed several oscillations of the re- 

 sultant cavity and attributed them to energy storage in air entrained in the 

 bubble. 



The first definitive results on the oscillations of such cavities 

 were obtained by Knapp and Hollander, 47 who succeeded in photographing cavi- 

 ties in a flowing liquid through several cycles of the motion. A strip from 

 the films obtained by these investigators is reproduced in Figure 15, which 

 clearly shoi^s tvro cycles of the motion — the initial growth and collapse and 

 the subsequent rebound, growth, and collapse. Figure l6 from Reference 47 



shows the time history of the life of 

 a bubble. It will be seen that the 

 bubble radius is greater than the 

 average radius for the largest part of 

 each cycle. Knapp and Hollander 

 showed by a rather convincing argument, 

 that in their experiments the initial 

 air content of the cavities was ex- 

 tremely small for cavities in the 

 range of their observations and that 

 the cavities contain mostly water va- 

 por. They attribute the rebound of 

 the cavity primarily to the storage of 

 energy in the liquid in elastic com- 

 pression, with this stored energy sub- 

 sequently producing the outward radial 

 velocity. As has been suggested by 

 Harvey and others, 48 however, an os- 

 cillating cavity may grow by diffusion 

 of gas into the cavity from the liquid 

 with the cavity continually gaining 

 gas molecules, because of the larger 

 surface area and longer time available 

 during that part of the cycle for 

 which the radius of the cavity is 

 greater than the average radius. 



Figure 15 - Oscillation of a 

 Cavitation Bubble in a Flowing 

 Liquid — From Knapp and 

 Hollander, Reference 47 



