396 



than kdi, in both cases of no grid and the grid No. 

 1. Little difference between sizes of the bubbles 

 can be noticed, although bubbles can hardly be 

 found on the low-speed side in the case of the grid 

 No. 1. As cavitation numbers are reduced, however, 

 the bubbles grow larger and are fewer in the case 

 of no grid, due to the difference in size of 

 cavitation nuclei as stated above. Bubbles defoirming 

 to generate projections like those on the Clark Y 

 11.7 profile can barely be found. Instead, cavities 

 collapsing to clusters of small bubbles appear. 

 The discrepancy of the collapse aspect between the 

 two hydrofoils can be considered to be caused by 

 the difference of pressure distributions. Cavita- 

 tions of the above type become more than traveling 

 bubbles with the decrease of kd, in the case of 

 the grid No. 1. 



At a = 0.052 rad, only fixed cavitations occurred 

 at the leading edge in cases of both no grid and 

 grid No. 1. The fixed cavitations grown without 

 changing the front edges of cavitation zones from 

 the leading edge and develop their lengths slowly 

 until the kd's are reduced to about the second 

 Icpminl's, being about equal to each other and 

 existing at the mid-chord in both cases. Nonuniform- 

 ity of lengths can be found in the case of grid 

 No. 1. When kd's reach the second |cpmin|'s, how- 



ever, a remarkable difference in the aspects of 

 cavitations between the two cases occurs in spite of 

 only a small difference in the measured pressure dis- 

 tribution. In the case .of no grid, fixed cavitation 

 develops beyond the position of minimum pressure, 

 whereas in the case of grid No. 1, the rear edge 

 of the zone of fixed cavitation does not reach the 

 position of minimum pressure. Instead, another 

 cavitation of the traveling type appears around the 

 position of minimum pressure, and bubbles of the 

 traveling cavitation are found more on the high- 

 speed side. The mechanism of this difference can 

 be surmised as follows : a free shear layer on an 

 interface between cavity and water may be laminar 

 near the point of inception in either case, but 

 the distance necessary for its transition in the 

 case of no grid is larger than in the case of grid 

 No. 1 because of the difference of the turbulence 

 level in the free stream between the two cases, 

 and the distance necessary for a cavity surface to 

 reattach the hydrofoil surface might be the same. 

 The fact that the cavity surfaces in Figure 13(b) 

 at kd = 0.7 are clear in the case of no grid but 

 wavy in the other case may show this. Furthermore, 

 the effect of rolling up the cavity surface caused 

 by the secondary flow may be expected in shear flow. 

 At a = 0.105 rad, only fixed cavitations can be 



'^mm 





^HKm'm ~^^BP' ■ ^^'^P 



FIGURE 12 (f). Behavior of 

 traveling cavitation on the 

 hydrofoil of the Clark Y 11.7 

 profile, ce = rad, flow up to 

 down, 0.3 ms between frames, 

 2ps exposure. 



200mm 



kd = 0.8 



