87 



1 . Rome 



4. NPL 



5. Caltech 



3 . Delft 



7. SSPA 



8. SSPA 



SSPA 



FIGURE 2. Photographs of dif- 

 ferent types of cavitation ob- 

 served in the ITTC tests, 

 Lindgren and Johnsson (1966) . 



Viscous Effects 



Parkin and Kermeen (1953) appear to be the first 

 investigators to appreciate the influence of the 

 boundary layer on the inception process. However, 

 even though their interpretations of the experi- 

 mental results were used in many subsequent incep- 

 tion theories [e.g. van der Walle (1962) , Holl and 

 Kornhauser (1969) to name only two] further experi- 

 mental investigations of these viscous effects 

 were carried out only much later. 



Among these, Arakeri and Acosta (1963), by using 

 the schlieren flow visualization technique, were 

 able to observe cavitation inception within the 

 structure of the flow. A primary feature of the flow 

 observed by them was a laminar separation in which 

 the cavitation was seen to occur first. There was 

 further some suggestion by them that the laminar- 

 to-turbulent transition itself may promote cavita- 

 tion, perhaps through a mechanism similar to that 

 for inception in turbulent pipe flow [Arndt and 

 Daily (1969)]. In any case, it should be expected 

 then, that any factor which could influence the 

 presence of separation or even transition may also 

 influence the inception of cavitation. One such 

 well-known factor is freestream turbulence. [For 

 recent accounts of these effects on transition see, 

 e.g., Spangler and Wells (1968), Hall and Gibbings 

 (1972), and Mack (1977)]. Unfortunately, the mea- 

 surement of turbulence in water is more difficult 

 than its aerodynamic counterpart and, until recent 



times, there has been no great demand for determin- 

 ing the freestream turbulence in water tunnels. For 

 reference we tabulate in Table I the turbulence 

 levels for a few water tunnels for which this inform- 

 ation is available (12th ITTC Cavitation Committee) . 



Polymer Effects 



It was inevitable that the much-heralded, drag- 

 reducing polymer solutions would be the subject 

 of cavitation experiments also. Very early in the 

 course of this work Hoyt (1966) and Ellis et al. 

 (1970) found that the inception index was reduced 

 by as much as a factor of two for hemisphere-nosed 

 bodies. There was, furthermore, a pronounced change 

 in the physical appearance of the cavitation, once 

 it was well developed, as stibsequently illustrated 

 by the beautiful photographs of Brennan (1970) . Two 

 possible explanations for the cavitation-suppression 

 effect were then advanced: in the first, it was 

 speculated that the dynamics of individual bubbles 

 were changed by the presence of the polymer, and 

 in the second, it was assumed that the basic viscous 

 flow about the model was altered by the presence of 

 the polymer. Ting and Ellis (1974) could find no 

 difference in the collapse time of spark-generated 

 bubbles in either water or polymer solutions weak- 

 ening the idea that the bubble mechanics are impor- 

 tant for this process. Later, however, Holl and 

 co-workers (1974) in commenting on experiments 



