B,19 • EFFECT OF PRESSURE GRADIENT 



d is the momentum thickness. Each curve of the set is characterized by d 

 constant value of the form parameter H, where H = b*/Q and in accord- 

 ance with the usual definitions 



5* = displacement thickness = / I 1 ~ yr) 



6 = momentum thickness = 



dy 



dy 



On the grounds that all suitable boundary layer data available up to 1943 

 could be made to fit one or another of these curves, von Doenhoff and 



1.0 

 0.8 

 JJ 0.6 

 0.4 

 0.2 



y/e 



Fig. B,19a. Velocity profiles corresponding to various 

 values of U, after von Doenhoff and Tetervin {8Ii\. 



Tetervin concluded that H was a suitable form parameter. When the 

 pressure increases with increasing x and the gradient is sufficient to bring 

 about eventual separation, H steadily increases, and each successive pro- 

 file takes a shape similar to one of those in the figure. Separation is 

 imminent when R is above 2 and is likely to occur when H is 2.6 or 2.7. 

 Apparently this family characteristic is only true of nonequilibrium pro- 

 files, for Clauser has recently shown that equilibrium profiles, for which 

 R remains nearly constant with increasing x, do not align themselves 

 with the typical forms of this family (see Art. 20) . However, the general 

 features are preserved. 



There has been a great deal of speculation about the abrupt rise of 

 the curves of Fig. B,19a near the origin. It will be noted that even when 

 the flow is about to separate the steep initial rise is present. Thanks to the 

 recent contributions of Coles [55], to be considered in Art. 21, and to facts 



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