Cavitation (Influence of Free Gas Content) 
well known, separation can occur in both laminar and turbulent 
boundary layers. In both cases, the separation will take place down - 
stream of the minimum pressure point on the surface in the region of 
a positive pressure gradient. A relatively crude experiment was per- 
formed using a fluorescent oil film on the headform surface [9] ; 
[16] . The result indicated that separation occurred at an X/D = 0.5, 
for velocities below 4.2 meters per second and a water temperature 
of 10°C. This is in agreement with tests performed at the California 
Institute of Technology with a hemispherical nose headform [23] : 
There, laminar boundary layer separation was also found to occur 
downstream from the minimum pressure point. From the high speed 
movies of the headform in the 12 inch water tunnel it was found that 
many bubbles were already visible at the minimum pressure point, 
This same result was apparent in the experiments run in the high 
speed basin and reported earlier [1 6] . On the basis of this discus- 
sion itis felt that boundary layer separation, if present, occurred 
sufficiently downstream to be of negligible influence on the inception 
observed in the experiments. 
E. Comparison Between Water Tunnel and High Speed Towing Basin 
Cavitation Studies 
In order to clarify the role of the free stream gas bubble in 
the cavitation nucleation process occurring in the water tunnel tests, 
it is worth while to compare results with those obtained in the high 
speed towing basin at NSRDC. As previously reported {1 6} , these 
same headforms were mounted on a strut and tested in the towing 
basin, The prodedure was to wait at the end of the basin for 45 mi- 
nutes prior to each run, In this period of time the basin water became 
very smooth and high speed photography was possible through its sur- 
face. It was found that the incipient cavitation number varied between 
0.6 and 0.8. The higher values were again typical of the plastic 
(DELRIN) headforms and this is attributed to the difference in acou- 
stic impedence between the metal and plastic. In the towing basin the 
inception velocity was also determined with a hydrophone inside the 
body. Unlike the water tunnel tests, these acoustic results were found 
to agree with the high speed photography. 
From the data on the buoyant rise of bubbles in water [14] 3 
it can be estimated that a bubble 4 wm in diameter will rise 270 mm 
in the 45 minute period. Larger bubbles will rise correspondingly 
faster. By any one of a number of theories for gas bubbles in water 
(e.g., [24] , [25] ) it can be shown that bubbles smaller than 4 um 
in diameter should have dissolved completely in a matter of minutes. 
This is supported by the experimental evidence of Liebermann [26] . 
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