Cavttatton (Influence of Free Gas Content) 
DISCUSSION 
Edward Silberman 
St. Anthony Falls Hydraulte Laboratory 
Minneapolts, Minnesota, U.S.A. 
I want to comment on two points. I would agree that the free 
stream nuclei, which are very important, need not be gas bubbles in 
the usual sense that we may think of them as having uncontaminated 
interfaces between gas and water. I made observations about 25 years 
ago (1), using a microscope, of bubbles being dissolved under pressure 
in a rotating apparatus. This was in connection with resorption pro- 
blems that we were working on at that time to get rid of bubbles in 
water tunnels. I was measuring time rate of change of diameter of 
single bubbles generated by cavitation. In watching these bubbles I 
found that a relatively small number of those generated would not 
disappear but instead would collapse ona thin, wrinkled, opaque skin. 
I recorded this fact in the reference paper, although at that time it did 
not seem important. When the pressure was reduced again these 
bubbles would expand just like normal gas bubbles. I think this is per- 
tinent to what Peterson said, and enables us to treat nuclei as though 
they are gas bubbles. I believe that what is left in the water could be 
these bubbles with skins on them. 
It should be mentioned that these bubbles in the rotating appa- 
ratus, when they appeared to be pure gas bubbles would be near the 
center of rotation, but when they collapsed ona skin, their diameters 
remained constant, and they would wander around in the centrifugal 
pressure field without regard to the pressures indicating mean bubble 
densities near that of water. Such bubbles could sustain themselves in 
a towing tank for a long time - maybe indefinitely. 
My second point refers to the implications in the paper that it 
is necessary for the nuclei to touch the body in order to produce ca- 
vitation. I do not believe that this is correct. If you think about what I 
(1) SILBERMAN, E., ''Air Bubble Resorption", Tech. Paper No. 1, 
Series B, St. Anthony Falls Hydraulic Laboratory, 1949. 
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