air content, a "hysteresis" effect In the inception of cavita- 



tionc As the static pressure is reduced in the working section 



of the High Speed Water Tunnel at C.IoT,, one can go to 



pressures below those at which cavitation usually begins = This 



state of tension in the flow is unstable and cavitation can be 



initiated by a small disturbance in water tunnel conditions. 



The same phenomenon has been observed by Parkin in the Garfield Thomas 



Tunnel at Penn State « Other workers have of course been 



aware of the fact that tensions can be produced in ordinary 



water, but it is of interest to note that the effect can be 



found in the gross conditions obtaining in a water tunnels 



We at the California Institute of Technology are particularly 

 interested in the observations both here and at the Taylor 

 Model Basin on the nonappearance of rebound in the collapse 

 of cavitation bubbles « One might expect that the reversal of 

 the liquid motion following collapse would lead to such ten- 

 sions in the water that a cavity would reopeno Our observa- 

 tions at low air content are, however, unambiguous in showing 

 no rebound. We have here very possibly another example of 

 ordinary water withstanding tensions for a short- period of 

 time. As compared with the observations mentioned above on 

 the state of tension in a water tunnel flow, one would expect 

 that the tensions following a vapor bubble collapse are much 

 higher but the duration of the tension is no doubt much 

 shorter., 



Eisenberg also reviews the theoretical results of M, Rattray 

 on the collapse of a cavitation bubble near a solid boundary o 

 Rattray showed that the effect of the boundary was to intro- 

 duce large distortions in a bubble which begins its collapse 

 in a spherical shape o He also foujid that the collapse time 

 was lengthened o I should like to point out here the possi- 

 bility of the appearance of distortions in an initially 

 spherical bubble which collapses in an infinite liquids These 

 distortions would appear now, not as a consequence of an 

 adjacent boundary, but because the collapse motion of a 

 spherical cavity in a liquid is unstable o These deviations 

 from the simple spherical shape would be of importance for 

 the further development of the theory of cavitation bubbles » 



In concluding my discussion, I should like to say that the 

 relatively small group of workers in this field are grateful 

 to Kt^ Eisenberg for his excellent review of the progress in 

 the mechanics of cavitationc He contributes to this field not 

 only by his own work at the Taylor Model Basin, but hj his 

 critical examination in his review reports of all the work 

 being carried on in this complex and difficult field o 



