Cavitation (Influence of Free Gas Content) 
large difference in acoustic impedence between the materials as pre- 
viously discussed in Section A, Brockett [9] ; [10] has also obser- 
ved, during studies with the same shape headform, that not all visible 
cavities produce noise. It should again be noted that the estimated rate 
at which bubbles struck the headform did not include bubbles less than 
25 mum in diameter. Therefore, the estimates are to be considered 
low. In any event, it certainly appears that there were a sufficient 
number of bubbles available to account for the number of visible cavi- 
ties observed photographically. When the dissolved gas content of the 
water was reduced, some filtering of the water also occurred. Howe- 
ver, it can be seen from the particle size distributions in Figure 13 
that when the gas bubble content of the water was reduced by over a 
factor of 10, little change occurred in the number of solid particles 
present. At this point no conclusions can be made concerning the 
effect of the presence of the solid particles in the water on the nucle- 
ation process. If they have a density greater than that of water, then 
they may have a trajectory which tends to direct them away from the 
low pressure region of the body. If their density is approximately that 
of water, then one would expect that these solid particles would have 
trajectories corresponding to the streamlines. In any event, it is 
clear that a large number of solid particles were always present and 
for the sake of completeness their size distributions are presented 
here, 
When the dissolved gas content, a/a Ts » was reduced below 
approximately 0.6, the number of gas bubbles were so few it became 
impractical to manually scan the image volume with a microscope. 
However, inception measurements were made and these are given in 
Table 1. The results indicate that the addition of new water into the 
tunnel may have had some effect, but the statistics are inadequate to 
verify this point. From the data one can also see that the typical de- 
crease in cavitation inception number, go, , occurs as the dissolved 
gas content is reduced. Based on the previous discussion, however, 
it is not readily apparent what type of nuclei are most affected by this 
change and further discussion of this result must also be deferred. 
C. Modifications to the Surface Characteristics of the Headforms 
Nuclei originating from gas trapped in crevices on the body 
surface have been postulated as one source of cavitation nuclei. There 
is sufficient experimental data available to show that under certain 
circumstances this type of nucleus can be a significant factor in cavity 
formation on a body [1] : [2] ; [22] . Therefore, the possibility 
exists that this nucleus source may have been a factor in the experi- 
ments reported here. Studies were carried out to evaluate this factor 
in two ways. First, headforms were plated with gold to minimize sur- 
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