hydraulic radius

Noun

 * 1)  For a porous material, the ratio of porosity (ratio of volume of interstitial space filled with fluid to volume of material) to specific surface area (ratio of surface area in contact with fluid to volume of material); for a pipe or channel, the ratio of cross-sectional area to wetted perimeter.

Usage notes
The hydraulic radius of a full pipe turns out to be half the radius of the pipe.