Chapt06

70 this is identical to eq 627 except that 1 the shear

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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 | e-Text Main Menu | Textbook Tab...
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