F,19 • EFFECT OF SURFACE ROUGHNESS 



of the transition Reynolds number with variable turbulence Re^^ to that 

 with 0.4 per cent turbulence Re^^, and the abscissa is the percentage ratio 

 of the root-mean-square velocity fluctuation u' to the mean velocity u in 

 the supply chamber. 



F,19. Eflfect of Surface Roughness. Transition promoted by surface 

 roughness can still be controlled by cooling, depending, however, upon 

 the roughness size. Data [56] obtained for wire rings 3 inches from the tip 

 of a 10-degree smooth cone at local Mach number 1.90, 2.70, and 3.65 



Trip size 

 O No trip 



X 



n 



A 



0.0005 in. 

 0.001 in. 

 0.002 in. 

 0.004 



ReeQttrip=2.0lXl0'5 

 Ree/in. = 0.67 X 1 0' 

 Te = 228°R 



Xj 



Cooling 



N 



2.3 



Fig. F,19b. Effect of roughness on transition with cooling. 

 Me = 2.70. Wire trips at Re^ = 2.01 X 10^; 10° cone. 



are shown in Fig. F,19a, F,19b, and F,19c, respectively. These data show 

 that: (1) for sufficiently small two-dimensional roughness, cooling can 

 still delay transition as though the body were smooth, (2) sufficiently 

 large roughnesses disrupt the flow to such an extent that cooling is no 

 longer effective, (3) for intermediate roughnesses, a reversal in transition 

 is apparently possible, during which transition is first delayed and then 

 promoted by cooling, and (4) the effect of roughness in promoting tran- 

 sition decreases as Mach number increases. The reversal may be explained 

 by the argument that cooling first tends to stabilize the flow until the 



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