Jets and Shock Waves from Cavitation 



Fig. 8 - Rebound of a bubble in 

 a pressure gradient showing the 

 resultant jet {4X) 



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formed and impacted the opposite side of the growing rebounding cavity. This 

 behavior leads to speculation that rebounding cavities previously thought harm- 

 less might indeed be capable of damage if conditions were just right. Some evi- 

 dence for damage during rebound has been cited by Shutler and Mesler (26), but 

 they ascribed it to other causes than a jet. 



To confirm that the jet was due to a pressure gradient pictures were taken 

 by Dr. T. Brooke -Benjamin and the author when the entire liquid and container 

 was in free fall. No jet was observed in this case. 



The effect of a plane solid boundary at various distances was also investi- 

 gated and yielded results as shown in Figs. 9, 10, and 11. In Fig. 9 the cavity 

 has developed an elongated shape as predicted theoretically by Rattray (18), but 

 in addition a jet is seen to develop at the end farthest away from the rigid wall 

 and to travel toward the wall. Obviously the wall was too far away to be dam- 

 aged. Figure 10 shows a bubble closer to the boundary. The jet is seen to de- 

 velop at an angle both to the wall and to the vertical as should be expected since 

 both effects were present. (The container was not in free fall in this case.) 

 Figure 11 shows a bubble still nearer the boundary, which is of interest because 

 of the relatively thin and rapid jet which has formed and impacted the wall be- 

 tween the two consecutive photographs at the left in the bottom sequence. A 

 lower bound on the impact pressure in this case is about 8000 psi, but if the 

 collapsing pressure had been as high as one atmosphere, which is conservative 

 in practice, then pressures of at least 40,000 psi should be expected. This is of 

 the same order as the pressures estimated from the previously mentioned ex- 

 perimental piezoelectric data of Jones and Ellis for hemispherical collapse of a 

 spark generated bubble. 



The experimental work of Benjamin and Ellis at Cambridge thus revealed 

 some new modes of bubble collapse, notably that jets could occur in rebounding 



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