The hydraulic diameter is a term used in fluid mechanics and thermodynamics. It is a measure of the efficiency of a section to pass flow by relating the cross-sectional area of flow to the wetted perimeter of the cross-section.
The hydraulic diameter of a round pipe is simply its diameter. The equations for hydraulic diameter for other cross-sections are given in the table below.
| Flowing Full | |
|---|---|
| Circle | $latex D$ |
| Annulus | $latex D_{outside} – D_{inside}$ |
| Square (L = side) | $latex L$ |
| Rectangle (L1 and L2 = sides) | $latex \frac{2L_1L_2}{L_1 + L_2}&s=1$ |
| Flowing partially full | |
| Half-filled circle (d = diameter) | $latex d&s=1$ |
| Rectangle (h = height, w = width) | $latex \frac{4hw}{w + 2h}&s=2$ |
| Wide, shallow stream (d = depth) | $latex 4d&s=1$ |
| Triangle (d = depth, t = top, s = side length) | $latex \frac{d\cdot t}{s}&s=2$ |
| Trapezoid (d = depth, t = top, b = bottom, s = side length) | $latex \frac{2d(t + b)}{b + 2s}&s=2$ |
The hydraulic diameter should not to be confused with the hydraulic radius term in the Manning equation, which is not half of the hydraulic diameter, but rather one quarter.
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