Hydraulic diameter
The hydraulic diameter, <math> D_h <math>, is 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.
Definition:
<math> D_h = \frac {4A}{U} <math>
where A is the cross-sectional area and U is the wetted perimeter of the cross-section.
For a round tube, this checks as:
<math> D_h = \frac{\frac {4 \pi D^2}{4}}{\pi D} = D <math>