101 



1x10= 2 3 4 5 6 7 8 



BODY REYNOLDS NUMBER - UD/!/ 



FIGURE 33. Cavitation inception with polymer 

 injection on the hemisphere body. 



This difference will be discussed later. These 

 inception data have been simiraarized in Figure 33. 



WSRDC Body 



The NSRDC body was tested only in the LTWT and it 

 too was observed to go through a sequence of cavi- 

 tation development similar to that of the hemisphere 

 nose body in the LTWT; namely, that the injection 

 of polymer at sub-critical rates changed the orig- 

 inal band type inception to simultaneously occurring 

 intermittent band and travelling bubble type incep- 

 tion. At above critical injection rates the inter- 

 mittency became more rapid but still no steady 

 attached cavitation could be obtained. Examples 

 of these types of cavitation are shown in Figure 

 34. Notice in particular Figure 34(d) where only 

 one cavitation bubble is visible at a cavitation 

 number of 0.34. Values of the inception index 



versus body Reynolds number are presented in Fig- 

 ure 35. 



Schiebe Body 



The Schiebe body was tested in both the LTWT and the 

 HSWT, but the influence of polymer was only studied 

 in the LTWT. Again as for the hemisphere nose body, 

 the type of cavitation depended upon the facility. 

 In the LTWT, travelling bubble type inception always 

 occurred and the presence of polymer was found to 

 have no significant effect on either the type of 

 cavitation or the inception index. Lowering of the 

 tunnel pressure below the inception value produced 

 a steady, attached cavity of the type normally as- 

 sociated with the presence of a laminar separation. 

 On the other hand, in the HSWT, travelling bubble 

 type cavitation events were extremely rare. In- 

 ception occurred with the sudden appearance of an 

 unsteady attached cavity occasionally preceded by 

 one or two travelling bubble events. Examples of 

 these types of cavitation on the Schiebe body are 

 given in Figure 36 and a summary of the inception 

 data is given in Figure 37. A unique location of 

 inception could not be accurately determined in 

 either facility for this body. 



7. DISCUSSION 



Freestream Turbulence Level 



The main purpose of the investigation of freestream 

 turbulence level upon cavitation inception was to 

 determine if it could be a contributing factor to 

 the differences in cavitation results on identical 

 bodies tested in different facilities. In particu- 

 lar, could the differences in cavitation inception 

 on the same NSRDC test body between the CIT HSWT and 

 the NSRDC 12-inch tunnel be explained by different 



■< ^-'^ 



FIGURE 34. The physical appear- 

 ance of cavitation on the NSRDC 

 body at a Reynolds number of 

 3.4 X 10^ in the LTWT with (a) 

 no injection, (b) G = 3.4 x 10"'', 

 cavitation index = 0.45 [same as 

 in (a)], (c) 3.4 K 10"'', cavita- 

 tion index = 0.34, and (d) 7.1 x 

 10~^ at the same index! 



