Toward a solid boundary, the mean and turbulent velocities approach zero, so 

 t' t decreases rapidly with decreasing y/8. Figure 13 displays this behavior in terms of 

 dimensionless length coordinate 



yU* 



where 



V* = 



ff* = 



V 



(43) 



To 

 p 



8 

 Fig. 13 



20 24 28 32 36 40 44 



On this same figure is a very rough estimate of the laminar shear stress fluctua- 

 tion level, 



/, W 1 



du dv 



— + — 

 dy dx 



T 



(44) 



617 \ 



i — 1 

 dy /„=<> 



This dashed curve was sketched by its estimated value at y — 0, the skin friction fluctua- 

 tion, and the measured mean square derivatives at large values of y*. The indicated 

 monotonic decrease of T f L with increasing y is an assumption based upon its apparent 



— du u 



flatness at the wall: both u' and U are linear in y as y— >0, and — ~ — in this vicinity, 



dy y 

 u / u'\ 



so To ~ t lo ~ }i — and t' ~ /.<. ( — ) It is, of course, conceivable that / L 



could have a maximum away from the wall. 



391 



