Specifying Hydraulic Diameter

What is hydraulic diameter?

When considering the pressure loss with respect to the flow in a duct, hydraulic diameter refers to the diameter of the equivalent circular tube. With respect to the flow through a non-circular cross-section, it is the diameter of a circular tube that would have same pressure loss.

Hydraulic diameter: Dh = 4A / s
where A is the cross-sectional area, s is the circumferential length of the cross section.

For more details on hydraulic diameter, refer to the attached PDF.