Bernd 



With regard to the hypothesis that stabilized gas nuclei could be formed 

 with some protein-hydrocarbon shell, such nuclei can aid cavitation only to the 

 extent they provide an unrestrained gas bubble. K the shell were impervious, 

 then its efficacy in aiding bubble formation on the exterior would not differ 

 whether gas were held internally or not. On the other hand, if gas is readily 

 released by rupture or rapid penetration of the shell, it will be rapidly depleted. 



REPLY TO DISCUSSION 



Leslie H. Bernd 



CAVITATION INCEPTION LEVEL OF 

 WATER TUNNELS 



Mr. Johnson's data on the cavitation inception levels obtained in different 

 water tunnels are most interesting. It shows a difference in the cavitation lev- 

 els. It implies that up to a few psi of tensile strength may exist in the water 

 in these tunnels. If so, the trend is to increasing tensile strength with increas- 

 ing water speed. The implication of the data is that comparative tests are desir- 

 able and necessary. But, one expects that considerable research will be neces- 

 sary before the means of producing uniform test results become obvious and 

 before the role of gas nuclei is adequately delineated. 



Assuming the tensile strength in the tunnels reported upon is controlled 

 by gas nuclei carried in the water, investigations we have made on the dissolv- 

 ing of gas nuclei in turbulently flowing water would lead one (as a possibility) to 

 attribute an increase in tensile strength with increasing water speed to a gen- 

 eral decrease in the size of the nuclei produced by the cavitation occurring dur- 

 ing the tests. One would be interested, as pertinent factors, in the sequence in 

 which water speed was varied, as well as the duration of the cavitation tests. 

 Also, for a small test body, one might wish to invoke as a cause of apparent 

 tensile strength the time required to expand a nucleus against the inertia of the 

 surrounding water, as a nucleus passes into the low pressure region at the body 

 to form a cavitation void. 



As relevant information, our investigations on the dissolving of gas nuclei 

 in turbulently flowing water showed the following: 



1. After the formation of gas nuclei by cavitation, rapid dissolving of these 

 nuclei takes place. 



2. The resultant change in the diameter of a nucleus due to dissolving, for 

 a given distance traveled downstream, is generally a constant approximately 

 independent of water speed. The time rate of dissolving increases proportion- 

 ally as the water speed is increased. 



112 



