Gravity-driven fingering in unsaturated fractures

Gravity-driven wetting-front instability is known to occur in both porous media and Hele-Shaw cells. In this paper a systematic investigative procedure for studying gravity-driven fingering in unsaturated, rough-walled fractures is described. As a first step toward understanding this system, experiments were performed in an analogue fracture consisting of two roughened glass plates held in close contact. Results from preliminary experiments in both initially dry and wet analogue fractures are presented, including measurements taken from individual fingers within a fully unstable flow field. For initially dry fractures, increasing the volume of fluid contained in the front leads to increases in both finger width and velocity