A,25 • EFFECT OF MACH NUMBER ON TRANSITION 



the surface. Shock waves influence transition in other ways. A weak shock 

 wave striking a boundary layer produces a small disturbance analogous 

 to that of a small roughness element. Stronger shock waves may induce 

 boundary layer separation followed by transition or produce immediate 

 transition. The observed values of the x Reynolds number based on free 

 stream properties range from about 0.5 million to about 90 million but 

 the analysis of the experimental data to isolate the effects of the several 

 variables is difficult. 



The highest value, 90 million, was observed by Sternberg [91] on a 

 cone of 20° included angle attached to the nose of a V-2 rocket in flight. 

 The thickness of the laminar boundary layer on a cone in subsonic flow is 

 l/\/3 times that on a plate at the same x Reynolds number; hence if 

 transition is assumed to occur at a fixed value of the Reynolds number 

 based on boundary layer thickness, the critical x Reynolds number for 

 the cone will be 3 times that for a plate. There is no reason to expect this 

 relationship in supersonic flow, and the scanty experimental comparisons 

 [92] of data taken on cones and plates in the same air stream show httle 

 differences between transition Reynolds number for plate and cone. In 

 this and succeeding articles no attempt is made to adjust the values for 

 cones and other bodies of revolution to give equivalent flat plate Reynolds 

 numbers. 



Various methods have been used to detect transition, the most com- 

 mon ones being Pitot-tube rakes, surface temperature measurements, 

 schlieren, and shadow photographs. Transition at supersonic speed is 

 intermittent [93,94] just as it is at subsonic speed. Hence the photo- 

 graphic methods require numerous pictures to obtain the average loca- 

 tion of transition. Since the transition process extends over a certain 

 range of Reynolds number, the choice of a single value is somewhat arbi- 

 trary. Wherever possible, the values quoted will refer to the beginning of 

 transition, i.e. the point where turbulent bursts begin to appear for 5 or 

 10 per cent of the time or, ehminating the knee of the curve, where the 

 pressure or the temperature begins to rise. Many authors adopt the value 

 of the Reynolds number at which the pressure or temperature reaches its 

 maximum value as that for transition; in these cases the values quoted 

 herein will be less than those stated in the original papers. 



A,25. EflFect of Mach Number on Transition for Bodies without 

 Heat Transfer at Supersonic Speeds. Art. 2 described the large 

 effect of free stream turbulence on transition at subsonic speed. There 

 is a similar large effect at supersonic speed up to a Mach number 

 of 2, although there are few systematic experiments. The most useful 

 test object at supersonic speed is a cone of small included angle. At super- 

 sonic speed there is no pressure gradient, the leading edge effects are 

 small, and there is no contamination from tunnel wall boundary layers. 



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