395 



200mm 



Kd 



2.75 



FIGURE 12 (e) . Behavior of 

 fixed cavitation on the hydro- 

 foil of the Clark Y 11 . 7 pro- 

 file, a = 0.157 rad, flow up 

 to down, 12 ms between frames, 

 2\ss exposure. 



tions , which seem to be little affected by either 

 kd or the shear of the free stream. The projections, 

 in the case of no grid, are supposed to be generated 

 in cores of trailing vortices and adhere to the 

 hydrofoil surface, because velocities of the babbles 

 exceed those of surrounding water in regions down- 

 stream from positions of minimum pressure. It can 

 be seen in high speed motion pictures shown in 

 Figure 12(f) that the main body of bubbles, having 

 generated projections, decay, leave behind them 

 projections of two string-like biibbles, and then 

 collapse. In cases of the two shear grids, bubbles 

 are inclined upward toward the high-speed side due 

 to the secondary flow caused by the flow shears . 

 Trailing vortices on the low-speed side reach the 

 hydrofoil surface easier than those on the high- 

 speed side. Bubbles which generate projections be- 

 come fewer as kd is reduced in the case of the 

 shear grids. Several bubbles can be found which 

 seem to collapse and generate micro jets. 



At a = 0.105 rad, cavitations of both types, 

 traveling and fixed, appear, though the former are 



fewer than the latter. Front edges of fixed cavi- 

 tation zones are round compared with tips of the 

 above-mentioned projections. At a = 0.157 rad, only 

 fixed cavitations occur. A cycle of formation of 

 the break off of a fixed cavity is shown in high 

 speed motion pictures in Figure 12(e). At first, 

 a clear bubble is generated, like those observed in 

 our laboratory on the surface of an axisymmetrical 

 body with a hemispherical nose. The bubble develops 

 in both streamwise and spanwise directions. The 

 middle part of the spanwise breadth of the bubble 

 becomes bubbly, then wavy, and after the development 

 of the middle part breaks off in pieces of micro- 

 bubble clouds which are transported downstream al- 

 though a few remaining small parts grow and 

 disappear. 



08 Profile 



At angle of attack tiny bubbles of traveling 

 cavitation can be found at a kd a little smaller 



