A,2 • TRANSITION ON FLAT PLATE 



For the particular measurements illustrated in Fig. A,2a transition 

 occurred at Ret = 1.1 X 10^. However, other measurements have yielded 

 values varying from 9 X 10^ to 2.8 X 10®. The principal variable con- 

 trolUng transition is the turbulence initially present in the air stream 

 (V,B and IX, F). For present purposes we will use as the quantitative 

 measure of the turbulence the quantity 



100 VUu'^ + v'^ + w") 



u 



where u', v', and w' are the root-mean-square values of the three com- 

 ponents of the turbulent velocity fluctuations. Fig. A,2b shows a plot of 



o 



X 



CD 

 Dd 1 



100 



fl^ 



2 -f v'2 4- w'2) 



U^ 



Fig. A,2b. Effect of intensity of free stream turbulence on 

 the transition Reynolds number of a plate. 



Ret against the intensity of turbulence for the available experimental data. 

 While the scale of the turbulence is also known to be of importance, there 

 is insufficient evidence for flat plates to demonstrate its effect. (See Art. 

 11.) The magnitude of the effect of small fluctuations of velocity amount- 

 ing to less than one per cent of the average stream velocity is quite 

 surprising. 



When the free stream turbulence is less than about 0.1 per cent, 

 Schubauer and Skramstad [2] found that transition was preceded by 

 the appearance of the Tollmien-Schlichting oscillations predicted by the 

 theory of instability of laminar boundary layers (IV,F). These oscillations 

 grew in amplitude until transition ensued. The oscillations are not ob- 

 served at higher turbulence, the random turbulent fluctuations apparently 

 imposing random fluctuations within the boundary layer which mask the 

 Tollmien-SchUchting oscillations. 



(5> 



