A • TRANSITION FROM LAMINAR TO TURBULENT FLOW 



ber (based on peak surface temperature) rises from about 1 million to 

 about 4.5 million as the leading edge Reynolds number increases from 

 200 to 10,000. 



There are no data on the effects of pressure gradient and surface 

 curvature on two-dimensional boundary layers except for measurements 

 [110] on airfoils, mainly at high subsonic speed. Fig. A,25a shows the 

 position of transition as a function of Mach number for a slightly modi- 

 fied NACA 0012 airfoil. This figure indicates a large effect of compressi- 

 bility, transition moving forward with increasing Mach number up to 

 M = 0.65, then a slight rearward movement to ilf = 0.7, a forward move- 

 ment to M = 0.76, and finally a continuing rearward movement up to the 

 highest Mach number of about M = 0.9. 



Xt 



c 



0.4- 



0.6 



-1.0 



0.2 



0.4 



0.6 



M 



0.8 



1.0 



Fig. A,25a. Transition position on modified NACA 0012 

 airfoil as a function of Mach number. 



The corresponding pressure distributions are shown in Fig. A, 25b. 

 It is seen that the final forward movement is associated with a change 

 in the pressure distribution from a predominantly adverse gradient to a 

 favorable gradient. It seems probable therefore that the principal effect 

 of compressibility is the modification of the pressure distribution which 

 in turn affects the position of transition. Whether identical pressure dis- 

 tribution in compressible and incompressible flow would yield the same 

 position of transition has, however, not been determined. 



Some observations of transition on airfoils at supersonic speeds have 

 been made in connection with other investigations. Thus some of the 

 photographs of Stalder and Slack [67] show wedges of turbulence behind 

 dust particles, behind air jets from pressure orifices and from the side 

 wall similar to those observed at low speeds. No systematic basic studies 

 have been made. 



(58) 



