Hydraulic Diameter
Hydraulic diameter, abbreviated as \(d_h\), is normally used when the flow is in noncircular pipe or tubes and channels. Circular pipe has the same pressure drop of a rectangular channel but a greater average velocity. Square or rectangular pipes have a greater weight and a greater pressure drop compared with a circular pipe with the same section. For established shapes, various calculations are linked below.
\(\large{ d_h = \frac { 4 \;A } { P } }\) Where: \(\large{ d_h }\) = hydraulic diameter \(\large{ A }\) = cross section area \(\large{ P }\) = cross section wetting perimeter 
Hydraulic Diameters for Specific Flow Profiles

Tags: Equations for Hydraulic