The hydraulic diameter, DH, is a commonly used term when handling flow in noncircular tubes and channels. Using this term one can calculate many things in the same way as for a round tube. It is defined as
For a round tube, this checks as:
For an annulus the hydraulic diameter is
and for a rectangular duct, if completely filled with fluid:
And for a rectangular duct, if partly filled with fluid:
For the special case of a square duct, where L=W, then DH = L. For the other limiting case of a very wide duct, i.e. a slot of width W where W ≫ L', then DH = 2L.
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. When the flow is laminar, secondary flows do not occur.
Hydraulic diameter is also used in calculation of heat transfer in internal flow problems.