444 



22 



Reynolds Number x 10 



FIGURE 22. Comparison of cavitation inception (or 

 desinence) number with pressure coefficient at tran- 

 sition, C , for blunt nose. 



best choice would seem the pressure coefficient at 

 the location of cavitation inception. However, 

 this location can not precisely be indicated. For 

 bodies with attached boiindary layers, Arakeri (1973) 

 suggested correlating o^ with the pressure coeffi- 

 cient at transition, Cp . For a 1.5 caliber ogive 

 a close correlation was found between measured 

 values of Q^ and computed values of Cp . The same 

 comparison can be made for the blunt nose. In 

 Figure 22, o- ^ and Cp , derived from Figures 4 and 

 14, are plotted against the Reynolds number. In 

 this case it may be assumed that the real (or viscous) 

 values of Cp_ are the same as those for irrotational 

 flow. It is evident from Figure 22 that Oi (or 

 Ofj) cannot be correlated with Cp_. The location 

 where Cp„ =0.46 (=5^ ^ is well in the laminar 

 region of the botmdary layer for the Reynolds numbers 

 considered. 



The influence of polymer additives on cavitation 

 inception is a rather new phenomenon. Darner (1970) 

 investigated the addition of polymers to water on 

 acoustically induced cavitation inception. Ellis 

 et al. (1970) reported on the effect of polymer 



O 100 200 300 400 500 600 



Polyox WSR-301 Concentration . pom 



FIGURE 23. Surface tension as a function of Polyox 

 WSR-'^SOl concentration in water, as measured in surface 

 tensionmeter. 



solutions on flow-generated cavitation inception. 

 The effect of the polymer was to suppress cavitation 

 inception. An explanation for the effect could, 

 as yet, not be given. Ting and Ellis (1974) studied 

 the growth of individual gas bubbles in dilute 

 polymer solutions but concluded that the polymers 

 hardly affected bubble growth. From Figure 2 3 it 

 is found that the surface tension is slightly 

 reduced by small additions of Polyox WSR-301, but 

 according to Hoyt (1973) this effect should cause 

 earlier cavitation instead of cavitation suppression. 

 From Figure 18, a considerable effect on a^ and 

 0(3 is found when a 500 ppm Polyox solutuion is 

 injected from the nose of the SST hemispherical 

 model. For Re above 1.2 x 10^, the reduction 

 amounts to 30 percent. For the mean value of a-;^ 

 and 0(3 we have Oj^ ^j = 0.445. The a^^- and Otj-values 

 are independent of Re. From Figure 21 it is fotmd 

 that 0£ and o^ are hardly affected by the injection 

 of a 500 ppm Polyox solution from the nose of the 

 blunt model. For Re above 1.2 x 10^, the mean 

 value of Oj^ and o^j in the absence of polymers is 

 Oi (3 = 0.45. Hence, inception on the SST hemispher- 

 ical nose with polymer injection takes place at the 

 same cavitation number as inception on the blunt 

 nose in the absence of polymers. 



As found in Section 3, the influence of the 

 polymer is to suppress the laminar boundary layer 

 separation on the hemispherical nose. Hence, the 

 strong pressure fluctuations, occurring at the 

 position of transition and reattachment of the 

 separated shear layer [Arakeri (1975a) ] and being 

 the principal mechanism for cavitation inception, 

 are eliminated and cavitation will start at a much 

 lower cavitation number. The flow visualization 

 studies described in Section 3 do not only explain 

 the suppression of cavitation inception by polymer 

 injection, but also by having a polymer ocean 

 [Ellis et al. (1970)]. Earlier studies by Van der 

 Meulen (1973, 1974b) showed that polymer injection 

 had hardly any effect on cavitation inception on a 

 Teflon hemispherical nose. The reason for this 

 finding is clear now, since cavitation inception on 

 a Teflon hemispherical nose is related to surface 

 effects and not to viscous effects. 



Appearance on Hemispherical Models 



The appearance of cavitation on the SST hemispher- 

 ical nose is closely related to the occurrence of 

 laminar boundary layer separation. Arakeri (1973) 

 showed that cavitation bubbles are first observed 

 at the location of transition and reattachment of 

 the separated shear layer. This type of cavitation 

 is usually called bubble cavitation. An example 

 is shown in Figure 24a. The larger bubbles at the 

 location of transition are preceded by smaller ones 

 which, according to Arakeri (1973), are travelling 

 upstream with the reverse flow in the separated 

 region. With a reduction in o, the larger bubbles 

 create a single cavity as shown in Figure 24b. 

 With a further reduction in a, the cavity is filling 

 the separated region, and a smooth attached cavity 

 is observed (Figure 24c) . This type of cavitation 

 is usually called sheet cavitation. When o is 

 further reduced, the length and the height of the 

 cavity extend, but the first part of the cavity 

 remains smooth (Figure 24d, e) . By analyzing 

 double exposure holograms made of developed cavita- 

 tion, it could be established that the first smooth 

 part of the cavity is stable. 



