442 



*&Ci^];^>-i 



FIGURE 17. Photographs showing boundary layer flow about SST hemispherical nose. The flow is from right to 

 left. V = 4 m/s. (a) Injection of 50 ppm Polyox in water, (b) Injection of water in 50 ppm Polyox. (c) Injec- 

 tion of 50 ppm Polyox in 50 ppm Polyox. (d) Injection of 500 ppm Polyox in 50 ppm Polyox. 



reduction display a positive Weissenbe-rg effect for 

 which destabilization is predicted analytically. 

 Destabilization is also predicted by the numerical 

 analysis of Kiimmerer (1976) on the stability of 

 boundary layers in an idealized viscoelastic fluid. 

 Experiments by Forame et al. (1972) and Paterson 

 and Abernathy (1972) also suggest destabilization. 

 On the other hand, Castro and Squire (1967) and 

 White and McEligot (1970) found that polymer solu- 

 tions in water cause a delay in transition to 

 turbulence. According to Lumley (1973), drag- 

 reducing polymers tend to increase the thickness 

 of the viscous sublayer. Experimental evidence 



t) 



'■'0 6 



08 1 2 16 20 24 



Reynolds Number x 10'^ 



FIGURE 18. Cavitation inception and desinence number 

 as a function of Reynolds number for SST hemispherical 

 nose with and without polymer injection. 



for this phenomenon has been provided by Rudd (1972) , 

 who measured velocity profiles in a polymer solution 

 by using a laser dopplermeter . By examining the 

 expansion behavior of isolated polymer molecules 

 in a flow field, Lumley (1973) postulated a mech- 

 anism which predicted a decreased intensity of 

 small-scale turbulence in the buffer layer and which 

 also predicted that, in the maximum drag reduction 

 regime, the turbulence should consist primarily of 

 larger eddies. The present observations of waves 

 and streaks along the surfaces of the models seem 

 in agreement with the above predictions. They also 

 agree with the observations made by Hoyt et al. 

 (1974) on the structure of jets of polymer solution 

 discharged in air. 



4. CAVITATION STUDIES 



Inception 



Cavitation inception data for the SST hemispherical 

 nose are plotted in Figure 18. Inception was 

 measured by gradually lowering the pressure until 

 the first appearance of cavitation was observed. 

 Desinence was measured by starting from developed 

 cavitation and gradually raising the pressure until 

 cavitation just disappeared. The type of cavitation 

 mostly observed at inception was sheet cavitation. 

 Also plotted in Figure 18 are cavitation inception 

 data when a 500 ppm Polyox WSR-301 solution was 

 injected from the nose of the model. The type of 

 cavitation observed in this case was travelling 

 bubble cavitation. Cavitation inception data for 

 the Teflon hemispherical nose are plotted in Figure 



