The hydraulic diameter, DH, is a commonly used term when handling flow in non-circular tubes and channels.
When the cross-section is uniform along the tube or channel length, it is defined as[1][2] where More intuitively, the hydraulic diameter can be understood as a function of the hydraulic radius RH, which is defined as the cross-sectional area of the channel divided by the wetted perimeter.
Here, the wetted perimeter includes all surfaces acted upon by shear stress from the fluid.
[3] Note that for the case of a circular pipe, The need for the hydraulic diameter arises due to the use of a single dimension in the case of a dimensionless quantity such as the Reynolds number, which prefers a single variable for flow analysis rather than the set of variables as listed in the table below.
The Manning formula contains a quantity called the hydraulic radius.
Hydraulic diameter is mainly used for calculations involving turbulent flow.
Hydraulic diameter is also used in calculation of heat transfer in internal-flow problems.
[4] In the more general case, channels with non-uniform non-circular cross-sectional area, such as the Tesla valve, the hydraulic diameter is defined as:[5] where This definition is reduced to