Ellis 



oscillating in water at 25 kHz. This is, of course, a rather familiar photograph 

 to some of you but it gives, I believe, a possible indication of the density of cen- 

 ters of high compression that may exist within a cavitation cloud. 



If we accept that such centers of high compression exist, do they occur close 

 enough to damage an adjacent solid surface with shock waves emanating from the 

 rebound? Could a highly compressive collapse occur on a surface? It would be 

 interesting, I believe, to look at the emission from a collapse on or near a solid 

 surface, of single bubbles that contain a sufficient proportion of monatomic or 

 diatomic gas. 



In the calculation of the intensities of shock waves emanating from a spher- 

 ical collapse center, by Professor Plesset and myself, a fairly elaborate proce- 

 dure had to be followed for each example. We were satisfied, therefore, to es- 

 tablish order -of -magnitude results for an adiabatic gas inside the bubble without 

 considering more realistic models that would allow for heat conduction and vapor 

 condensation. It was pointed out in our paper, however, that these effects would 

 induce a much more violent collapse than was shown in the calculated examples, 

 and that the resulting shock waves under these more realistic conditions should 

 certainly be strong enough to account for any known type of cavitation damage. 



These results coupled with the apparently high density of centers of high 

 compression in a cavitation cloud would make it appear reasonable to believe 

 that the shock-wave mechanism of cavitation damage is not a negligible possi- 

 bility. I believe that the centers of high compression and the liquid impact ef- 

 fects discussed by Professor Ellis both have a role to play in the cavitation dam- 

 age process. Other mechanisms have also been discussed. It seems clear that 

 cavity collapse is a rather complicated process. I believe that it would be an 

 error at this time to single out any one hydrodynamic effect as the principal 

 cause of cavitation damage. 



DISCUSSION 



D. C. Gibson 



Department of Applied Mathematics and Theoretical Physics 



University of Cambridge 



(presented at the symposium by T. B. Benjamin) 



During recent experiments with spark- generated cavities in water, a curi- 

 ous phenomenon was observed which apparently has not before been put on 

 record. The phenomenon can be seen in the accompanying sequence of frames 

 from a high-speed cine film, showing a cavity at four instants during the re- 

 bound that followed its first collapse (Fig. D4). To generate this cavity, a 

 0.725-/if capacitor at 4.5 kV was discharged through a spark between the aligned 

 thin electrodes that appear in the photographs. The cavity was formed about 



158 



