Cavttatton (Influence of Free Gas Content) 
mination of the region upstream of the body in which the bubbles 
would have to be located in order to produce cavitation. The results 
of the numerical calculations are shown in Figures 2 - 4 as the local 
pressure coefficient, Cp , experienced by the bubbles along the 
bubble trajectory. Figures 2 and 3 show the situation for a typical 
cavitation inception condition experienced in the 12 inch water 
tunnel with a metallic body. The bubble screening effect is easily 
seen, The 25 wm diameter bubble does not experience as lowa 
local pressure as the 50 um diameter bubble when they both start 
at the same point upstream. Correspondingly, the 25 wm _ bubble 
does not pass as close to the body as the 50 wm bubble and does 
not strike the body as soon. 
Figures 5 and 6_ show the variation in bubble diameter for some 
of the bubble sizes considered. None of the bubbles experienced ex- 
tremely rapid growth rates. For the range of bubble trajectories 
considered, all the bubble wall velocities were less than 0.1 meter 
per second. Once a bubble touched the body, the numerical method 
is of course not valid. However, it appears reasonable to assume 
that when the bubble touches the body, its translational velocity may 
decrease sufficiently to permit further volume increase. On this basis 
it was concluded from Figures 2, 3, 5, and 6, that all bubbles would 
have to be initially within the cross-sectional area of radius 3.75 mm 
upstream from the 5cm diameter headform for them to produce 
cavitation. For the purposes of further discussion, the bubbles out- 
side this area are assumed not to contribute to the cavitation on the 
body. 
The question still to be resolved is whether the bubbles that 
strike the body will in fact actually produce a vaporous cavity. Be- 
fore discussing this aspect of the problem, the numerical calcula- 
tions of the bubble trajectories over the same body with the same in- 
ception coefficient, 0, , but ata pressure approximately 3.4 times 
higher, should be considered. Figures 4 and 7 represent a typical 
inception condition when the same headform was tested in the high 
speed towing basin 16] . The experimental results from the basin 
were essentially the same as those obtained in the water tunnel and 
therefore the same og; was used in the calculations. The interesting 
result is that for the higher speeds in the basin, the bubble trajector- 
ies are slightly further from the headform and therefore the bubble 
diameters are correspondingly smaller. If the bubble strikes the 
headform, it strikes further downstream, From this result it can be 
concluded that if given identical bubble size distributions, then the 
rate at which bubbles produced cavities should be directly propor- 
tional to the velocity of the body in the basin or conversely, the up- 
1137 
