For shapes such as squares or rectangular ducts and tunnels where the height and width are comparable, the characteristic dimension for internal-flow situations is taken to be the hydraulic diameter, defined as $D=4*A/P$, where A is the cross-sectional area, and P is the wetted perimeter. The wetted perimeter for a channel is the total perimeter of all channel walls that are in contact with the flow.

For circular tunnels, the diameter is an input value.

Symbol
$Di_{\mathrm{t}}$
Unit
m
Formulae
$\frac{4 A_{\mathrm{t}}}{2 h_{\mathrm{t}} + 2 w_{\mathrm{t}}}$
Related
$A_{\mathrm{t}}$
$h_{\mathrm{t}}$
$w_{\mathrm{t}}$
Used in
$A_{\mathrm{t}}$
$Do_{\mathrm{t}}$
$\mathrm{Re}_{\mathrm{air}}$
$T_{\mathrm{tw}}$