- Hydraulic diameter
The hydraulic diameter, D_h , is a commonly used term when handling
flow in noncirculartube s and channels. Using this term one cancalculate many things in the same way as for a round tube.Definition:
D_h = frac {4A}{U}
where "A" is the cross sectional
area and "U" is thewetted perimeter of the cross-section.For a round tube, this checks as:
:D_h = frac{4 frac {pi D^2}{4{pi D} = D
The
Manning formula contains a quantity called thehydraulic radius . Despite what the name may suggest, the hydraulic diameter is not twice the hydraulic radius.For an annulus the hydraulic diameter is
:D_h = frac{4 cdot 0.25 pi (D_o^2 - D_i^2)} {pi (D_o + D_i)} = D_o - D_i
and for a rectangular duct
:D_h = frac {4 L W} {2 (L + W)} = frac{2LW}{L+W}
For the special case of a square duct, where L = W , then D_h = L . For the other limiting case of a very wide duct, i.e. a slot of width W where L gg W, then D_h = 2W.
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