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Unformatted text preview: ydraulic diameter as
Dh 4A 4 area
wetted perimeter 4Rh (6.74) We should stress that the wetted perimeter includes all surfaces acted upon by the shear
stress. For example, in a circular annulus, both the outer and the inner perimeter should
be added. The fact that Dh equals 4Rh is just one of those things: Chalk it up to an engineer’s sense of humor. Note that for the degenerate case of a circular pipe, Dh
4 R2/(2 R) 2R, as expected.
We would therefore expect by dimensional analysis that this friction factor f, based
upon hydraulic diameter as in Eq. (6.72), would correlate with the Reynolds number
and roughness ratio based upon the hydraulic diameter
f F VDh , (6.75) Dh and this is the way the data are correlated. But we should not necessarily expect the
Moody chart (Fig. 6.13) to hold exactly in terms of this new length scale. And it does
not, but it is surprisingly accurate: f 64 ReDh fMoody ReDh,
Dh 40% laminar flow 15% turbulent flow (6.76) Now let us look at some particular cases.  v v 358  eText Main Menu  Textbook Tab...
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 Spring '08
 Sakar

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