Cavttatton (Influence of Free Gas Content) 
in the natural existing environment of standard test facilities with 
emphasis on the measurement of flow conditions and the control of 
headform surface condition. This simple body produced only discrete 
cavities translating along the surface. From these results and a com- 
parison with the pertinent literature certain conclusions can be in- 
ferred. 
The results substantiate what other investigators have found 
in that a very precise definition of inception is necessary. When noi- 
se is used as an inception criteria, then it was shown that free gas 
bubbles were not specifically needed for the nucleation of the noise 
producing transient cavities under discussion in this paper. Hydro- 
phobic particles can function as an adequate source of nuclei. 
The dissolved gas is important because it can affect, through 
the mechanism of diffusion, the amount of gas trapped in hydrophobic 
particles for a given pressure history of the water. Conversely fora 
given dissolved gas content, changes in the normal pressure history 
of the water will also affect the ability of these hydrophobic particles 
to nucleate cavities. For the experiments reported here, body surface 
nucleation of cavitation was not considered a significant influence. If 
a material such as teflon is used which is hydrophobic and known to 
be porous on a microscopic scale, then surface nuclei could in fact 
be the controlling source. 
New stable nuclei can be generated in the typical test facility 
water whenever a gas/water interface is produced because of local 
adsorption at the interface of dissolved organic material. In a water 
tunnel this could occur during the filling process, by the introduction 
of locally supersaturated water or even during the deaeration process. 
In both water tunnels and towing basins this could occur during the 
actual tests where bubbles of one form or another are produced. In 
any event, the persistance of these hydrophobic particles can be ex- 
pected unless very special water treatment procedures are followed. 
If either 2- or 3- dimensional boundary layer separation 
occurs, g; may be affected but the type of cavity nucleus initially 
responsible may not be important if an attached cavity eventually 
forms. 
For flow fields to cavitate when nuclei mobility across stream- 
lines is required - suchas a vortex - then the free gas content of the 
water can be expected to be of prime importance. But here again, 
care must be taken to specifically define whether cavitation is based 
on visual or acoustic observations. 
1147 
