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The hydraulic diameter, DH, is a commonly used term when handling flow in non-circular tubes and channels. Using this term, one can calculate many things in the same way as for a round tube. It is defined as
The need for the hydraulic diameter arises due to the use of a single dimension in case of dimensionless quantity such as Reynolds number, which prefer a single variable for flow analysis rather than the set of variables as listed in the table. The Manning formula contains a quantity called the hydraulic radius. Despite what the name may suggest, the hydraulic diameter is not twice the hydraulic radius, but four times larger.
Hydraulic diameter is mainly used for calculations involving turbulent flow. Secondary flows can be observed in non-circular ducts as a result of turbulent shear stress in the turbulent flow. Hydraulic diameter is also used in calculation of heat transfer in internal-flow problems.
List of hydraulic diameters
|Circular tube||For a circular tube the hydraulic diameter is simply the diameter of the tube.|
|Rectangular duct (fully filled). The duct is closed so that the wetted perimeter consists of the 4 sides of the duct.||For the limiting case of a very wide duct, i.e. a slot of width b, where b ≫ a, then DH = 2a.|
|Channel of water or partially filled rectangular duct. Open from top by definition so that the wetted perimeter consists of the 3 sides of the duct (2 on the side and the base).||For the limiting case of a very wide duct, i.e. a slot of width b, where b ≫ a, and a is the water depth, then DH = 4a.|