438 



maxi mum 



y///,' 'y/////. 



inflexion point 



FIGURE 8. Observed shapes of laminar separation bubble 

 on hemispherical nose (schematically) and definitions 

 of length and maximum height of bubble. 



hemispherical nose is given in Figure 9. In this 

 figure the angular location of separation, yg , is 

 plotted against the Reynolds number. Results on 

 the length, L, the height, H, and the length to 

 height ratio, L/H, of the separation biibble are 

 presented in Figures 10, 11 and 12. Each data 

 point refers to one hologram (values for the upper 

 and lower side of the model are averaged) . Most 

 data points refer to the SST hemispherical nose, 

 a few refer to the Teflon hemispherical nose. 



The present observations are in agreement with 

 those obtained earlier by Arakeri (1973) and Arakeri 

 and Acosta (1973) . From Figure 9 it follows that 

 the boundary layer separation angle is independent 

 on the Reynolds number, which is consistent with 

 theory (Schlichting, 1965) . For the SST hemispher- 

 ical nose the average value of yg is 85.43°. This 

 value is claimed to be quite accurate . To compare 

 this experimental value with the theoretically 

 predicted one, laminar bovindary layer calculations 

 were made using the method derived by Thwaites 

 (1949) . With this method the parameter m is cal- 

 culated, where m is defined as 



dU 

 ds 



(2) 



and where 6 is the momentum thickness, U the velocity 

 at the edge of the boundary layer, v the kinematic 

 viscosity, and s the distance along the surface. 



9 0' 



8 2' 



-! 1 1 1 1 r- 



^^crt 



^ooi „o o„cP o ° o 



o SST 

 • Teflon 



2 04 06 08 1 2 3 



Reynolds Number xlO"^ 



FIGURE 9. Boundary layer separation angle, Yg' 

 as a function of Reynolds number for hemispherical 

 nose. 



5 



i; 04 



4 06 8 10 2 



Reynolds Number x10~-* 



30 40 



FIGURE 10. Length of separation bubble to diameter, 

 L/D, as a function of Reynolds number for hemispheri- 

 cal nose. The solid lines refer to theoretical pre- 

 dictions. 



Laminar boundary layer separation is said to occur 

 for m = 0.09. The computations of Yg were made 

 with the accurate pressure distributions obtained 

 earlier (Figure 3) . For the actual case (with 

 tunnel walls) yg was found to be 85.57°, and thus, 

 in excellent agreement with the experimental value. 

 The theoretical value of yg is hardly affected by 

 the presence of the tunnel walls, since in the 

 absence of tunnel walls we found yg = 85.53°. 

 Arakeri (1973) found experimental and theoretical 

 values of 87°. However, his computations were 

 based on the experimental pressure distribution 

 data by Rouse and McNown (1948), as shown in Figure 3. 



The length and the height of the separation 

 bubbles decrease gradually with increasing Reynolds 

 number. The variations in length and height for a 

 given Reynolds number are partly due to the different 



05 



0,2 4 6 08 1 



Reynolds Number x 10"-^ 



FIGURE 11. Height of separation bubble to diameter, 

 H/D, as a function of Reynolds number for hemispheri- 

 cal nose. 



