84 



VELOCITY HATIO, 



0.5 1.0 



DISTANCE FROM WALL, y/8 



FIGURE 8. Normalized velocity profile with zero over- 

 heat, compared with Blasius profile. 



normalized and plotted with a curve representing 

 the Blasius profile for a zero pressure gradient 

 boundary layer. Actually, the agreement shown 

 here is better than it should be due to the positive 

 3 of the flow tube boundary layer. 



The most surprising result that has been obtained 

 with the instrumented section is the large deviation 

 from axisymmetry in the profiles, even with no wall 

 heat. Figure 9 shows a plot of 6* (displacement 

 thickness) , 6 (momentum thickness) , and H (shape 

 factor) versus azimuthal angle (j) for no wall heat 

 at a free stream velocity of 1.60 m/sec. The 

 dashed lines indicate the calculated values for 

 6=0 and 6 = 0.16, which is the value of B at the 

 downstream end of the test section. The variations 

 in 6* and 9 are more than 50 percent, which was 

 totally unexpected. Figure 10 shows an azimuthal 

 velocity profile, that is, u versus (j) at a fixed y. 

 Here we see that the departure from axisymmetry is 

 wave-like in nature, and that significant changes 

 in velocity occur over a 15° change in (|). 



This behavior suggests that the asymmetries may 

 be caused by streamwise vortices within the boundary 

 layer, which would have a cross-stream length scale 

 on the order of the boundary layer thickness. 

 Such vortices could be caused by the Goertler 

 instability in the contraction section, as described 

 above. To test this hypothesis, a new contraction 

 section is presently being built which will avoid 



<cml 



.2 



.1 



.2 

 .18 



.16 



e I" 



(cm) 

 .12 



.10 



.08 



2.8 



26, 

 H 2.4 



2.2 

 2.0 



= 0.16 



5 = 016 



B --0 



180 270 



AZIMUTHAL ANGLE i^ 



FIGURE 9. Displacement thickness, momentum thickness, 

 and shape factor vs. azimuthal angle for zero overheat, 

 Ueo = 155 cm/sec. 



the Goertler instability entirely. This new con- 

 traction will have a fully convex inlet section 

 surrounded by an annular bleed flow. All fluid 

 from the settling chamber boundary layer will be 

 removed by the bleed flow. 



Variations in mean velocity profiles due to 

 heating have in fact been measured, but they are 

 small relative to the changes with azimuthal angle 

 shown in Figures 9 and 10. The shape factor H 

 tends to decrease with increasing overheat as 

 expected. However, no firm evidence of buoyancy- 

 driven instabilities has yet been seen, even at 

 low flow velocities and high overheats. 



FIGURE 10. Velocity, u, vs. 

 if, at y = 0.51 cm (0.2 in.). 



azimuthal angle. 



AZIMUTHAL ANGLE <#> 



