93 



FIGURE 14. The effect of freestream turbulence level 

 on the flow past the hemisphere body at a body Reynolds 

 number of 2.6 x 10^. (Same turbulence values as in 

 Figure 13.) 



approximately lOmm. As can be seen in the first 

 photograph of each of Figures 13 and 14, the NSRDC 

 and the hemisphere nose bodies respectively have a 

 laminar separation. Transition on these bodies oc- 

 curs on the resulting free shear layer and the flow 

 subsequently reattaches as a turbulent boundary 

 layer. With increasing turbulence intensities the 

 point of transition on the NSRDC body moved upstream 

 on the free shear layer. As the position of tran- 

 sition moved forward, the size of the separation 

 bubble decreased until finally it disappeared when 

 the position of transition and separation coincided. 

 Once the point of transition moved upstream of the 

 point of separation, no further observations of the 

 thermal boundary layer could be made with the pres- 

 ent schlieren system. Unlike the NSRDC model, the 

 increasing turbulence level seemed to have no ef- 

 fect upon the viscous flow about the hemisphere 

 nose body — as can readily be seen in Figure 14. 

 This rather surprising result will be returned to 

 later. 



As is shown in the first photograph of Figure 15, 

 the Schiebe body has no laminar separation and tran- 

 sition occurs on the model surface rather than on a 

 free shear layer. With increasing freestream tur- 

 bulence level two effects were noted; first, as can 

 be seen in Figure 15, the position of transition 

 moves substantially upstream and secondly, the ap- 

 pearance of the disturbance appears to change. This 

 change is not quite so evident in only a few pic- 

 tures, but we believe v;e observe more-or-less peri- 

 odic and highly amplified boundary layer waves in 

 Figure 15a and even b. However, for the higher 



turbulence levels frequent "bursts" interspersed 

 with a periodic phenomenon seemed to be more common. 

 A random collection of schlieren photographs of the 

 same body (Figure 16) at an intermediate turbulence 

 level shows these various forms more clearly. 



Discussion 



To quantify the effects of turbulence level, the 

 position, length, and maximum height of the separa- 

 tion bubble were measured for the NSRDC and hemi- 

 sphere nose bodies. For the Schiebe body, which 

 has no separation, the position of transition was 

 recorded — the position of transition being defined 

 as that point at which the first noticeable dis- 

 turbance occurs in the laminar boundary layer. These 

 quantities are defined in Figure 17 and were mea- 

 sured directly from the negatives of the schlieren 

 photographs with the aid of a scale or reference 



FIGURE 15. The effect of freestream turbulence level 

 on the flow past the Schiebe body at a body Reynolds 

 number of 2.5 '^ 10". The turbulence levels are those 

 in figure 13 and the regions shown are. at arc-length 

 diameter ratios of (a) 0.82-1.07, (b) 0.76-1.01, 

 (c) 0.60-0.85, (d) 0.61-0.86, (e) 0.47-0.63. 



