TABLE III CAVITATION EVENT RATES 



105 



Facility 



Model 

 Mat'l. 



'1 



U 

 oo 



(ft/sec) 



R Cavitatable Calculated Measured 



c "^ 



Nuclei/cm Events /sec Events per sec 



(microns) (est) 



NSRDC CU 0.62 29.86 12 



CU 0.66 29.86 15 



AU-Plated 0.65 29.86 14 



DELRIN 0.69 29.86 18 



DELRIN 0.71 29.86 21 



0. 5 

 1.8 

 2. 1 

 0.5 

 2.4 



0.9 

 3.2 

 3.8 

 0.9 

 4.3 



1.0 

 1.0 

 1.0 

 1. 

 1. 



LTWT 



are also shown for the Schiebe body in Figure 37; 

 again the correlation is suggestive but not con- 

 clusive. 



Further evidence of the difference between a 

 resorber facility and a recirculating tunnel is 

 given by the nuclei distributions of the flow in the 

 California Institute of Technology HSWT. These data 

 are averaged in the graph of Figure 42. Following 

 Peterson (1972) it is possible to distinguish par- 

 ticulate matter from gaseous microbubbles down to 

 about 10 micrometers. Thus we identify solid par- 

 ticulates on the one hand and microbubbles on the 

 other. All of the nuclei reported in Figure 42 

 for the LTWT are microbubbles. It is significant 

 that the HSWT shows a very similar distribution of 

 solid particulates , but very few microbubbles. In 

 about ten holograms made of the HSWT flow, within 

 the various sample volumes that were counted, about 

 100 particles/cm on the average were found. How- 

 ever, of these, less than one on the average was a 

 microbubble , too few even to hazard a guess as to 

 the distribution. This finding certainly tends to 

 explain the experimental trends in this facility if 

 it is assumed (as appears evident) that the solid 

 particulates do not act as nucleating sources. 



In closing this section we have perhaps come 

 full circle in inception research to re-emphasize 

 the important role of the cavitation nuclei. The 

 influence of laminar inception on cavitation is now 

 much clearer as are the effects of the processes 

 that cause stimulation of the boundary layer. If 

 there are many nuclei present (so that a large ten- 

 sion on the body does not exist prior to cavitation) 

 it is likely that travelling bubble cavitation will 

 predominate, then the notion of a "standard body" 

 to deduce cavitation susceptibility appears to be 

 useful. However, with only a few nuclei other more 

 complex forms of cavitation are seen at inception. 



Comparison of Nuclei Distributions 



Data from several other investigations, reduced to 

 the number density distribution function, N(R), by 

 the following approximation 



+ R 



number of nuclei per unit 

 with radii between R, and R, 



(Ro- RJ 



are shown in Figure 43. A tabulation of the mea- 

 suring techniques and test conditions for each in- 

 vestigation is given in Table 4. All the data have 

 approximately the same slope, but the values of the 

 distribution function can differ by several orders 

 of magnitude, i.e., although the nuclei population 

 changed by several orders of magnitude, the dis- 

 tribution of the nuclei sizes remains constant. 

 The large differences in populations is undoubtedly 

 a consequence of the large variation in conditions 

 which existed in the water when the data was col- 

 lected and is no doubt one of the contributing fac- 

 tors to the lack of repeatability seen in cavitation 

 tests. 



A goal of cavitation research is to be able to 

 predict the inception of cavitation and thus be able 

 to scale laboratory results to prototype conditions. 

 It is interesting then to compare nuclei populations 

 in water tunnels to those in the ocean. Medwin 

 acoustically measured bubble populations in the 

 ocean near Monterey, California, and in Figure 43 

 two of his measured distributions are presented. 

 The summer distribution agrees reasonably well with 

 the distributions obtained under cavitating con- 

 ditions in strongly deaerated water. However, in 

 the LTWT there are considerably more bubbles than 

 found in the ocean for radii greater than about 30 



