10 ) and that laminar separations were present on the ITTC test body and 

 the modified ellipsoidal head form previously tested by Peterson at the 

 David W. Taylor Naval Ship Research and Development Center (see Arakeri and 

 Acosta 1973, 1976). 



At first, it was surprising to realize that laminar separation could 

 exist on these well-known test bodies to comparatively high Reynolds 

 numbers because of contrary assumptions found in some of the cavitation 

 literature. Yet, even the simplest approximate laminar boundary-layer 

 calculations* showed that, on these bodies, a separation was to be expect- 

 ed. It then seemed evident that the presence or absence of this laminar 

 separation would be extremely important in cavitation inception. 



Inception Observations within the Laminar 

 Separation 



With the schlieren system in operation, it became possible to carry 

 out simultaneous observations of inception and the thermal boundary layer. 

 The hemisphere body proved to be a good candidate for this purpose. The 

 development of cavitation on this body may be seen in Figure 12. Cavi- 

 tation is seen to commence within the reattachment region of the laminar 

 separation. Many small bubbles are seen within the recirculation region 

 of the separation; these are presumably the same small bubbles seen in 

 Parkin and Kermeen's photographs. Motion pictures were also taken of the 

 flow in this region, at the same magnification of the schlieren photographs. 

 From these, it was inescapable that these small bubbles were in the re- 

 circulation region of the separation, but it was not possible to establish a 

 direct relation between these very small bubbles and the larger macroscopic 

 cavitation patches seen further downstream. 



With continued reduction in pressure, the macroscopic region of cavi- 

 tation grows to eventually result in the smooth, clear viscous-cavitation 

 separation seen in the last of Figure 12. This is a new type of separation 

 phenomenon, somewhat related to separation in lubrication. (Arakeri 

 (1975a) was able to make semi-empirical correlations for the location of 

 this separation. ) A similar sequence of events was found for cavitation 



*For example the axisymmetric version of Thwaites method, see "Laminar 

 Boundary Layers," Oxford, Rosenhead (ed.) pp. 430-432. 



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