445 



Wi^f>^0'' 



'F. 



.l^-isi'iCLM?: 



IX =90 



(b) 





Is =90 



h(=90 



FIGURE 24. Photographs showing progressive 

 development of cavitation on SST hem- 

 ispherical nose. The flow is from left to 

 right. V^ = 13.2 m/s. (a) a = 0.60; 

 (b) a = 0.59; (c) a = 0.56; (d) a = 0.47; 

 (e) a = 0.39. 



The appearance of cavitation on the Teflon 

 hemispherical nose is closely related to the presence 

 of weak spots on the surface. Discrete cavities 

 originate from points located on the hemisphere. 

 The cavities develop cone-shaped in the downstream 

 direction. The first part of the cavity surface 

 is smooth; the cavity leaves the wall at a very 

 small angle. Some of these features can be observed 

 on the photographs presented in Figure 25. The 

 cavitation separation angle Yes ^'-'^ both hemispher- 

 ical models is plotted in Figure 26. For the Teflon 

 model it is found that the cavities start upstream 

 of the minimum pressure point (Ycg < Yp ■ ) , when 

 o is sufficiently low. For the SST model it is 

 found that the cavities always start downstream 

 of the minimum pressure point (Yes *" '''p ■ ' • ''^CS 

 is both a function of a and Re. For a given Re, 

 Y(-.„ decreases with decreasing a and for a given 

 a, Yes 'i^'^^s^ses with increasing Re. These tenden- 

 cies for the SST model are in agreement with the 

 observations by Arakeri (1975b) . 



The shape of the cavity nose on the SST model 

 has been analyzed further. A schematic drawing of 

 the geometry of the cavity nose is presented in 

 Figure 27. From a detailed study of the holograms 

 it could be established that the cavity nose was 

 circularly shaped. It was found that the nose 

 angle g varied between 70° and 120°, but was 

 independent of a or Re. An average value of 90° 

 was obtained from 28 cavity noses. Since the cavity 

 nose is immersed in the separation bubble and the 

 flow comes to a standstill near the cavity nose. 



it is to be expected that the nose angle equals 

 the contact angle for the present liguid-gas-solid 

 system. This is confirmed by the fact that, accord- 

 ing to Adamson (1966) , the contact angle for a 

 water-air-steel system is 70°-90°. The nose radius 

 r was independent of a but, as shown in Figure 28, 

 the radius decreases with increasing Re. The length 

 of the sheet cavity (the smooth part preceding the 

 developed cavity) is more or less independent of 

 o but decreases with increasing Re. In Table 2, 

 mean values of Lg^/D are compared with corresponding 

 values of L/D, obtained from Figure 10 (with 

 Og = 7.5). From this table it can be concluded 

 that transition to turbulence on the cavity surface 

 is closely related to transition to turbulence on 

 the fully wetted separated shear layer. The shape 

 of the developed cavity is determined by the total 

 length to maximum height ratio of the cavity, 

 Lq/Hq, (in most cases the cavity reached its maxi- 

 mum height close to the trailing edge of the cavity) . 

 Values of this ratio are given in Figure 29. The 

 mean value of Lq/Hj-, is 10.2. Since the mean value 

 of the length to height ratio of the separation 

 bubble is 10.8, it may be concluded that the shape 

 of the developed cavity appearing on the SST hemis- 

 pherical nose is strongly governed by the shape of 

 the separation bubble . 



With polymer injection, the cavities on the SST 

 hemispherical nose are either attached or may show 

 the appearance of traveTling bubbles, resembling 

 the type of cavitation observed on the blunt nose. 

 Details are given by Van der Meulen (1976b) . 



