392 



symbols to fixed, kd's indicated in the figure are 

 based on the velocity at the mid- span. 



tions and movements of cavitation zones cannot be 

 found. 



Clark y 11.7 Profile 



In the case of no grid, traveling cavitations oc- 

 curred a little downstream from positions of minimum 

 pressure at a = and 0.052 rad. Front edges of 

 average zones of cavitation move forward beyond posi- 

 tions of minimum pressure as kd is reduced, uni- 

 formly in the core of the free stream. At a = 0.105 

 rad, in the core of the free stream, cavitations, 

 mainly traveling mixed with fixed, occurred just 

 downstream from positions of minimum pressure. How- 

 ever, with a small decrease of kd from the incipient, 

 the type of cavitation changes to fixed and the 

 front edges of cavitation zones move backward from 

 positions of inception and forward with a further 

 decrease of kd. In the boundary layers on both 

 side walls, fixed cavitations occurred very close 

 to the leading edge of the profile and to the side 

 walls, and front edges of cavitation zones move 

 little as kd is reduced. At a = 0.157 rad, fixed 

 cavitations occurred just downstream from positions 

 of minimum pressure and front edges of cavitation 

 zones moved forward just a little and never ex- 

 ceeded positions of minimum pressure, in the core 

 of free stream. In the boundary layers on both 

 side walls, fixed cavitations occurred just down- 

 stream from the leading edge of the profile and al- 

 most attached to the side walls, and front edges of 

 cavitation zones moved little as kd was reduced. 



At all attack angles , lines of rear edges of 

 cavitation zones have shapes similar to the velocity 

 profile at kd's a little smaller than the incipient. 

 But rear edges move backward with a further decrease 

 of kd to be almost uniform in the spanwise direction. 



In cases of grids No. 1 and No. 2, positions of 

 inception are closer to positions of minimum 

 pressure than in the case of no grid, in correspon- 

 dence with size distributions of cavitation nuclei: 

 Front edges of cavitation zones move forward beyond 

 positions of minimum pressure in the cores of free 

 streams at a = 0, 0.052, and 0.105 rad. At a = 0, 

 0.052, and 0.105 rad, incipient cavitations are of 

 the traveling type, but at a = 0.105 rad, in the 

 cores of the free stream, cavitations sometimes 

 change their type from traveling to fixed as kd is 

 reduced, and in those cases front edges of zones 

 of fixed cavitations move backward from the inception 

 position. In the boundary layer on the low-speed 

 side wall a fixed cavitation occurred very close 

 to the leading edge of the profile and to the side 

 wall, but no inception of cavitation of any type 

 can be detected in the boundary layer on the other 

 side wall, in the range of kd in this experiment. 

 At a = 0.157 rad, fixed cavitations occurred at 

 positions of minimum pressure, including the boundary 

 layers on both side walls, and front edges of 

 cavitation zones move little. 



At kd's a little smaller than the incipient, 

 lengths of cavitations are larger on the high-speed 

 side than on the other side at a = and 0.052 rad. 

 At a = 0.105 and 0.157 rad, however, they are larger 

 near the wall on the low-speed side than on zones 

 more distant from the wall. Rear edges of cavita- 

 tion zones have a tendency to be uniform in the 

 spanwise direction at all attack angles as cavita- 

 tions develop. 



Much difference between the two grids in the loca- 



08 Profile 



At angle of attack, positions of cavitation 

 inception and movements of front and rear edges of 

 cavitation zones with a decrease of kd, compared 

 with positions of minimum pressure, are quite 

 similar to those of the Clark Y 11.7 profile in 

 the cases of no grid and grid No. 1. However, at 

 a's larger than 0, fixed cavitations always occurred 

 at the leading edge over the whole span, irrespective 

 of the existence of the shear grid. Front edges of 

 cavitation zones never moved from the leading edge 

 as kd's were reduced. Lengths of cavitation zones 

 do not grow much, owing to the steep negative- 

 pressure zones just behind the leading edge, until 

 kd's are reduced to about the second |cpmin|'s. 

 But in the case of no grid, once kd's increase, 

 they develop suddenly beyond positions of minimum 

 pressure and tend to be uniform in the spanwise 

 direction as can be seen in Figure 13(b) at a = 

 0.052 and kd = 0.7. In the case of grid No. 1, 

 however, lengths of the fixed cavitation do not 

 grow enough to reach positions of minimum pressure. 

 Instead cavitations of the traveling type appear 

 around positions of the second minimum pressure, as 

 can be seen in Figure 13(b) at a = 0.052 and kd = 

 0.7 and as shown in Figure 11(b) by the symbols A. 

 The length of the cavitation zone is about the same 

 as that in the case of no grid in the free stream 

 core but smaller than that in the boundary layers 

 on both sides, at the beginning of development. 

 At a = 0.105 rad, the length of the cavitation zone 

 is much larger than that in the case of no grid at 

 the beginning of development, but becomes about the 

 same as the others with a further decrease of kd. 



Aspect and Behavior of Cavitation Bubbles and 

 Cavities 



Figures 12 and 13 show several examples among the 

 Sys-exposure photographs and an example of high- 

 speed motion pictures of cavitations taken at the 

 inception and each stage of development occurring 

 on the Clark Y 11.7 and 08 profiles, respectively. 

 Cavitation niimbers indicated in the figure on the 

 left hand side are based on the velocity at the 

 mid- span. 



Clark ¥11.7 Profile 



At a = and 0.052 rad, incipient cavitations are 

 of the traveling type in all cases, and in general, 

 the bubble radius and number of bubbles in the case 

 of no grid were the largest and the smallest, 

 respectively, of the three cases, followed by the 

 case of grid No. 1, which agrees with the size 

 distributions of cavitation nuclei given previously. 

 Each bubble is circular when observed perpendicular 

 to the hydrofoil surface, but as the cavitation 

 number is reduced, two, in the case of no grid, or 

 one, in both cases of two shear grids, horn-like 

 projections are projected behind each bubble from 

 the downstream or both sides. The groups of plots 

 lying second from the bottom in Figures 10(a) (b) 

 show positions of the upstream tips of the projec- 



