can postulate a highly efficient protection system if the hydrogen is introduced directly 

 into the cavities to form a cushion. This may actually occur if hydrogen bubbles (even 

 though microscopic in size) are introduced into the low pressure region since cavities 

 will tend to form preferentially about such bubbles acting as the required nuclei. 



Determination of the mechanism of such cathodic protection is complicated 

 by the protection provided against electrochemical corrosion in a corrosive medium, 

 and Petracchi's experiments are not definitive in this respect. It seems clear that 

 mechanical damage will be reduced if attack occurs only on non-corroded material 

 as it would be in a cathodically-protected system. 



To attempt to separate the effects of cathodic protection for mechanical vs. 

 electrochemical corrosion and to determine the validity of the mechanism suggested 

 in the foregoing is the goal of work at the California Institute of Technology. 



G. K. Batchelor 



I have a question, sir. I am not sure whether it should be addressed to the 

 old cock or the the young cock. Perhaps I can address it to the two of them collectively. 



The question is this. Can they give us an authoritative statement on the impor- 

 tance of radiation damping during the collapse of a vapour-filled cavity? I refer to the 

 conversion of some of the kinetic energy of the water into sound waves, which are 

 lost from the neighborhood of the cavity. 



As a layman to the literature, I have got the impression it is believed to be 

 a small effect. As against that, some calculations by Proudman at Cambridge con- 

 cerning the collapse of a cavity containing some permanent gas have shown that 

 acoustic radiation might account for as much as 50 per cent of the total kinetic energy 

 in one cycle, under conditions which would be typical for cavities occurring in sea 

 water in the neighborhood of propellers. 



M. S. Plesset 



I should like to refer Professor Batchelor's question to Dr. Gilmore. He has 

 given this problem some attention. 



Concerning the problem of cavitation damage which has been discussed by 

 Mr. Gawn, this is indeed a large subject as Mr. Gawn has mentioned. I appreciate his 

 comments as well as those of Mr. Eisenberg and Mr. Preiser. 



I shall now defer to Dr. Gilmore for a discussion of the question posed by 

 Professor Batchelor. 



F. R. Gilmore 



I really do not know what fraction of the bubble's energy is radiated away 

 acoustically during this period, but I suspect it is not a large fraction. There are, how- 

 ever, basic difficulties in defining the acoustic energy radiated during such a period, 

 because most of the flow energy is still in the neighborhood of the bubble, where 

 amplitudes are so large that the acoustic relations do not apply. 



Whether this flow energy is ultimately radiated away acoustically or remains 

 to re-expand the bubble may well depend upon processes occurring near the very end 

 of collapse. For example, experimental evidence indicates that a bubble containing no 

 permanent gases may collapse completely and not rebound at all. In such a case, one 

 might say that at the time of complete collapse all of the bubble energy has been 

 radiated away acoustically and/or dissipated viscously (for example, by a shock wave), 

 unless one prefers to say that the subsequent production of a pressure pulse by the 

 liquid which is still moving inward at the collapse time represents acoustic energy 

 produced after the bubble has vanished. On the other hand, evidence indicates that 

 presence of only a small amount of air in a bubble can cause the bubble to rebound 

 to a considerable fraction of its original size. Moreover, only a part of this loss in 

 energy may be due to acoustic radiation, since dissipation in the compressed gas and 



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