15 



the Pohlhausen type 6 which are given by 



■■•-(■ 



12 + M y X/yf ik -X 

 T 2\T) ' V 



6 

 dU dU 



Hi) + ' 



-)'(*) 



[3 



being the velocity gradient, and 6 is the so-called 

 Pohlhausen boundary layer thickness. 6 



where x ■■ IT ta' dx" 



Lower Critical 

 Reynolds Number 

 R,,*=420 I 



, Data from experiments of Schubouer 

 and Skramstad (from Reference 3) 



400 800 1200 1600 



Boundary Layer Reynolds Number ^ 



Figure 10 - Curve of Neutral Stability Calculated 



By Lin for Blasius 1 Plat Plate Plow 



(From Reference 4) 



The stability calculations 

 for various X have produced the rela- 

 tionship between lower critical Reyn- 

 olds number and X, as shown in Figure 

 11. Here it may be seen that decreas- 

 ing pressure (increasing velocity, 

 X > 0) in the direction of flow are 

 favorable to the stability of the lami- 

 nar layer and that increasing pressures 

 (X < 0) are unfavorable. 



It must be pointed out that 

 the theory does not predict the po- 

 sition of transition. The instabil- 

 ities which may arise take time to 

 amplify to the state at which they 

 produce transition. Hence it may 

 be expected that the transition Reyn- 

 olds number is much greater than the 



-8 



-6-4-2024 6 

 Karman - Pohlhausen Parameter X 



Figure 11 - Critical Values of R,* 

 as a Function of the Velocity 



Gradient Parameter X 

 (Computed by Schllchting and 

 Ulrich, see Reference 5) 



