Cavttatton (Influence of Free Gas Content) 
DISCUSSION 
Carl-Anders Johnsson 
Statens Skeppsprovnitngsanstalt 
Goteborg, Sweden 
In this very interesting paper the author discusses the diffe- 
rent mechanisms proposed throughout the years as responsible for 
cavitation inception. One of his conclusions seems to be that, for the 
type of experiments referred to, nuclei trapped in crevices of hydro- 
phobic particles could play the main role in the nucleation process 
leading to cavitation inception, 
A consequence of this reasoning would be that the role of free 
stream nuclei is not so important as has been assumed during the last 
years. The dissolved gas content is important however as it can af- 
fect the amount of gas trapped in the hydrophobic particles. 
I will show a slide, which may give some support to this con- 
clusion. The slide shows photographs of the free bubbles in the test 
sections of the two tunnels at SSPA during tests similar to those de- 
scribed in the paper. 
The same body was tested in the two cases and can be used 
for estimating the bubbles sizes. The test conditions are the same in 
the two cases: 
water speed 7.5 m/s, G'S ues. a/a,= 0.1 
and the water taken from the same storage tank. The upper photograph 
shows -the test section of the new large tunnel ; the lower that of the 
smaller tunnel. The difference in bubble spectrum is quite large and is 
of course due to the large difference in the pressure history for the 
water entering the test section in the two cases. 
The point is that the same inception cavitation number of 
6, = 0.45 was observed visually in the two cases. This might 
indicate that the influence of the free bubbles is not important. 
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