100 



FIGURE 31. The physical appear- 

 ance of cavitation on the NSRDC 

 body at two turbulence levels in 

 the LTWT. The Reynolds number 

 is 3.4 X 10^. In (a) the turbu- 

 lence level is 0.05 percent and 

 the cavitation index is 0.44. 

 The turbulence level in the re- 

 maining photographs is 0.65 per- 

 cent and the cavitation index is 

 about 0.35 for all cases. 



(a) 



FIGURE 32. In these photographs 

 500 wppm of polyox solution is 

 injected at the nose of the hemi- 

 sphere body . The cavitation 

 index is 0.59, and the Reynolds 

 number is 6.7 x lo^ (HSWT) . The 

 dimensionless injection rate, G, 

 is zero in (a) 1.9 x 10"^ in (b) , 

 4.4 X 10-^ in (c) , and 5.24 x 

 10~° in (d) . In many instances 

 the attached cavitation would 

 disappear. 



has a definite wave structure and the separation 

 line has become very irregular. Inspection of 

 Schlieren photographs of the fully wetted flow at 

 this injection rate showed that the position of 

 transition on the free shear layer had moved upstream 

 from the no injection case and that the separation 

 region was smaller in size. With a further increase 

 in the injection rate to near critical values, dif- 

 ferent types of cavitation were observed depending 

 upon the facility. In the HSWT, band type inception 

 would occur intermittently in patches with irregular 

 separation lines and surfaces as is shown in Figure 

 32 (c), (d) . At injection rates above the critical 

 value, the same type of behavior took place, but with 

 the flow altering between fully wetted and patchy 

 band type cavitation more rapidly. A decrease in 



the cavitation number at this injection rate would 

 make the cavitation more "violent," but no steady 

 attached cavitation could be obtained. At these 

 near-and-above critical injection rates the fully 

 wetted observations showed the laminar separation 

 had been eliminated with only an occasional short 

 reappearance. That is, the flow in the region of 

 interest was almost always turbulent. If then the 

 injection rate was suddenly reduced to zero, a large 

 steady cavity would quickly form on the body. 



In the LTWT the same sequence of cavitation 

 events with increasing injection rates would occur 

 as in the HSWT. However, near and above critical 

 injection rates, travelling bubble and band type 

 cavitation would occur simultaneously, unlike the 

 HSWT where no bubble type cavitation was observed. 



