A • TRANSITION FROM LAMINAR TO TURBULENT FLOW 



A, 8. Transition of Shear Layers with Reattachment Following 

 Laminar Separation. It has been noted that the laminar boundary- 

 layer of a cylinder appears to undergo transition immediately following 

 separation when the cylinder Reynolds number is greater than 20,000. 

 When the curvature of the body is not large, it is observed that the 

 turbulent shear layer may reattach to the surface, leaving a localized 

 region of separation usually referred to as a separation "bubble." The 

 attached layer continues as a turbulent boundary layer along the surface 

 to the traihng edge or until separation of the turbulent boundary layer 



>3 



>A u 







0.60 0.62 0.64 0.66 



0.68 0.70 



x/c 



0.72 0.74 0.76 0.78 



Fig. A,8a. Velocity distribution in the boundary layer of an airfoil exhibiting a 

 separation bubble. Airfoil NACA 663-OI8, Re^ = 1.7 X 10^ Contours of equal local 

 mean speed. 



occurs. This reattachment is due to the much greater lateral diffusion of 

 momentum in a turbulent as compared to a laminar layer. 



Separation bubbles on airfoils were noted by Jones [30] some years 

 ago and there have been a number of studies of the detailed structure of 

 separation bubbles. In addition, various attempts have been made to 

 develop semiempirical theories. 



Fig. A, 8a shows the velocity distribution in the localized region of 

 laminar separation behind the position of minimum pressure on an NACA 

 663-OI8 airfoil section at zero angle of attack taken from measurements 

 by Bursnall and Loftin [31] in the NACA Langley low turbulence wind 

 tunnel. The corresponding pressure distribution on the airfoil is given in 



(24 > 



