389 



10' 



FIGURE 6. Spanwise variation of size distribution of 

 cavitation nuclei. 



Cavitation Inception 



Spanwise variations of local incipient cavitation 

 numbers are plotted in Figure 7 for the Clark Y 

 11.7 profile, and in Figure 8 for the Os profile. 

 Also, spanwise variations of positions of minimijin 

 pressure for the case of no grid, grid No. 1, and 

 grid No. 2, are shown. 



Clark Y 11.7 Profile 



In the case of no grid incipient cavitation 

 numbers, kdi , are a little smaller than absolute 

 values of minimum pressure coefficients, | Cpmin | ' s 

 over the whole span at the attack angles, a, of 

 and 0.052 rad, and in the core of free stream at 

 a's of 0.105 and 0.157 rad. Differences between 

 kdi's and | Cpmin | ' s increase as a increases until 



within the accuracy of this measurement. u'/U was 

 6.8 and 6.2^ in the case of grids No. 1 and No. 2, 

 respectively, and w'/U was 3.6% in the case of both 

 two shear grids. It has been reported by Harris 

 et al. (1977) that in a shear flow generated by a 

 shear grid, w' and the other lateral component of 

 turbulent velocity, say v' , are almost the same. 

 If it is also assumed that v' = w' in this experi- 

 ment, the resultant turbulence levels were 8.5 and 

 8.0^ in the case of the grids No. 1 and No. 2, 

 respectively, in the core of the free stream. In 

 the case of no grid both u'/U and w'/U were 0.1^, 

 and the turbulence can be regarded as isotropic 

 at a level of 0.17^, in the core of free stream. 



Spanwise Variation of Size Distribution of Cavita- 

 tion Nuclei 



Attenuations of sound pressures were measured at 

 four positions in the spanwise direction ( 12.5, 

 37.5, 62.5, and 87.5^ span) from the low-speed side 

 at the position of mid-chord in the absence of the 

 hydrofoil, and at the cavitation numbers of 2.75 

 and 0.65. Because the levels of attenuated sound 

 pressures were not calibrated for micro bubbles 

 of known sizes, sound pressure levels in the shear 

 flows at each measuring position were compared with 

 one in the uniform flow in which any spanwise 

 variation was not noticed. Frequencies and 

 differences of sound pressure levels were related 

 to equivalent radii, and to differences of the 

 numbers of cavitation nuclei from those in the 

 uniform flow by using the formulae presented by 

 Richardson (1947) and Gavrilov (1964). 



At a cavitation number of 2.75, any noticeable 

 difference of size distributions between the shear 

 flows and the uniform flow was not found. At a 

 cavitation number of 0.65, however, remarkable 

 differences were noticed as can be seen in Figure 

 6. Numbers of nuclei with radii smaller than 24ijm 

 in both shear flows are considerably larger than 

 those in xiniform flow, and the larger the numbers 

 of nuclei the smaller the nuclei radii are. Size 

 distributions in the two shear flows were not so 

 different from each other in the high-speed sides 

 of free streams, but in the low-speed sides, the 

 shear flow made by the grid No. 2 is richer in 

 nuclei, especially in the range of small radii, than 

 the other. 



0.2 4 



IS 



14 



12 



10 



,•-+-. 



By Numachi 11947) ^~^ 







2.4 



02 04 



06 



I 



(c) a = lOSrad 



0.2 0.4 



FIGURE 7. Spanwise variation of incipient cavitation 

 numbers for the Clark Y 11.7 profile. 



