A,25 • EFFECT OF MACH NUMBER ON TRANSITION 



2.6 million at a Mach number of 1.86 to 4.4 million at a Mach number of 

 2.46, then decreased to 1.6 million at a Mach number of 4.51. 



A systematic study of the effects of single roughness elements has been 

 made by Brinich [106] at a Mach number of 3.12. The basic model was a 

 hollow cylinder with axis along the stream direction as previously de- 

 scribed and the roughness elements were wires encircling the cylinder. 

 The diameter and location of the roughness element were varied. The 

 results were analyzed along the lines discussed in Art. 5, the ratio of the 

 transition Reynolds number for the rough cylinder to that for the smooth 



1.0 



0.8 



(Ret)c 



0.6 



0.4 



0.2 



k/5,* 



Fig. A,25c. Effect of single cylindrical roughness element on the transition Reynolds 

 number of a smooth hollow cylinder at a Mach number of 3.12, the values for smooth 

 and rough cylinder being compared for the same transition position. 



cylinder being plotted against the ratio of the height of the element to the 

 displacement thickness of the boundary layer. The displacement thick- 

 ness was that computed by the compressible flow formula. The scatter 

 band was very wide but it was clear that compressible boundary layers 

 are much less sensitive to roughness than incompressible ones. 



The transition Reynolds number of the smooth cylinder varied with 

 the tunnel stagnation pressure. Brinich compared the rough cylinder value 

 with the smooth cylinder value at the same pressure, which appears to be 

 the natural basis for comparison. However, if the comparison is made 

 with the smooth cylinder value for the same transition position, the 

 scatter is greatly reduced. Fig. A,25c shows the recomputed data, omitting 



< 61 ) 



