nuclei, larger than about 5 micrometers diameter, are not very abundant in 

 this facility, a conclusion supported by the data of Figure 32. Further- 

 more, it points out that the range of sizes important in cavitation testing 

 work should extend downwards at least that far. 



There is, then, a direct influence of the nuclei density distribution 

 on the rate of travelling -bubble cavitation events; when there are very few 

 freestream nuclei, these events will of course be correspondingly few. 

 And, as we have seen, appreciable tensions can be readily achieved, even in 

 comparatively "dirty" tunnel water. From the physical evidence of tests 

 in several tunnels, attached forms of cavitation may occur at inception 

 even in the absence of laminar separation. Again, we return to the Caltech 

 facilities in Figure 35 to show these different types of inception on ; * 

 identical test bodies. Not surprisingly, it was found that the inception 

 indices differ markedly with travelling-bubble cavitation, showing the 



higher values (although still considerably below |-c |). In fact, in 



min 

 the HSWT there was a closer correlation with -c than any other parameter 



P tr 

 (Gates and Acosta ibid) . That attached forms of cavitation can sometimes 



occur first even on nonseparating bodies shows how difficult it is to have 

 a single universal threshold measure of inception. 



We can now recognize that a single kind of phenomenon that can lead 

 to a uniform scaling law in cavitation inception does not occur. It is 

 worthwhile, however, to draw together, in Table 4, the following "expecta- 

 tions" or trends in inception as we now understand it based on water 

 tunnel tests (from Arakeri and Acosta 1979). It would be extremely 

 valuable to extend these observations and even trends to prototype 

 Reynolds numbers; this, however, must await a future day. 



FULLY DEVELOPED CAVITATION IN INTERNAL 

 FLOWS 



We now leave inception to consider more developed states of cavita- 

 tion. This is a subject for which vast literature exists. Our present 

 purpose is, however, a more modest one; namely to consider developed 



68 



