B,19 • EFFECT OF PRESSURE GRADIENT 

 12-18. Their final expression, written in the form most commonly used, is 



2 



c/ 



= [5.890 log (4.0752?e9)]2 



(18-10) 



On the experimental side, measurements using the floating-element 

 technique, wherein the shear stress on an element of the wall is deter- 

 mined from a direct force measurement, are now believed to be the most 

 reliable. The best known examples of results employing this technique 



X 



u 



105 



10* 



10^ 



Ree 



103 



10' 



Fig. B,18. Local skin friction coefficient for smooth wall, zero pressure gradient. 

 Experimental values represented by points. Curve 1: Eq. 18-5; curve 2: Eq. 18-8; 

 curve 3: Eq. 15-10. 



are those of Kempf [82] and the more recent results of Schultz-Grunow 

 [74] and Dhawan [37]. These are given in Fig. B,18. Represented for 

 comparison are the curves corresponding to the power formula (Eq. 15-10) 

 and the logarithmic formulas (Eq. 18-5 and 18-8). 



B,19. Effect of Pressure Gradient. When a body moves through a 

 fluid, the pressure in the neighborhood of the body is different from that 

 in the undisturbed fluid in ways that are too well known to be recounted 

 here. It suffices merely to point out that pressure gradients are the rule 

 rather than the exception. The present discussion will be limited to two- 



< 129) 



