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 (L and _{1}L= sides)_{2 } |
$latex \frac{2L_1L_2}{L_1 + L_2}&s=1$ |

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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