A • TRANSITION FROM LAMINAR TO TURBULENT FLOW 



ents are concentrated through the action of viscous forces. Hence the 

 simplest situation for the study of transition is the simplest boundary- 

 layer that can be investigated both theoretically and experimentally. 

 This is the flow along a smooth thin flat plate, parallel to the flow in a 

 stream of uniform velocity and hence without longitudinal pressure gradi- 

 ent, for which the theoretical laminar flow solution was given by Blasius 



120 



100 



o 



X 60 



(D 



12 16 



RexX 10-^ 



20 



24 



Fig. A,2a. Velocity distribution in the boundary layer of a plate. Contours of equal 

 local mean speed as function of x and y Reynolds number. Turbulence of free stream, 

 0.5 per cent. 



in 1908. Many measurements have been made on the velocity distribu- 

 tion near such a plate and transition is easily recognized by a typical 

 departure from the Blasius distribution of the type illustrated in Fig. 

 A,2a. Here for measurements in air [1] contour lines of equal values of 

 u/ U are plotted on abscissas and ordinates of Rex and Rcx, u being the 

 local velocity at the point where distances from the leading edge and the 

 surface of the plate are x and y, U the free stream speed, and Re^ and 

 Rcy the Reynolds number formed from x and ij, respectively. Transition 

 accelerates the fluid close to the plate. 



(4 ) 



