Unformatted text preview: Dh for the particular geometry of
the duct.
For laminar flow in rectangles and triangles, the wall friction varies greatly, being largest near the midpoints of the sides and zero in the corners. In turbulent flow
through the same sections, the shear is nearly constant along the sides, dropping off
sharply to zero in the corners. This is because of the phenomenon of turbulent secondary flow, in which there are nonzero mean velocities v and w in the plane of the
cross section. Some measurements of axial velocity and secondaryflow patterns are
shown in Fig. 6.16, as sketched by Nikuradse in his 1926 dissertation. The secondaryflow “cells” drive the mean flow toward the corners, so that the axialvelocity contours are similar to the cross section and the wall shear is nearly constant. This is
why the hydraulicdiameter concept is so successful for turbulent flow. Laminar flow
in a straight noncircular duct has no secondary flow. An accurate theoretical prediction of turbulent secondary flow has...
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 Spring '08
 Sakar
 Fluid Dynamics, eText Main Menu, Osborne Reynolds

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