Wetting front instability in layered unsaturated porous media.

摘要

The physics of wetting front instability in layered unsaturated porous media and its effect on solute transport is explored. A theoretical framework for experimentation is developed through classical dimensional analysis. Relationships between system parameters, initial/boundary conditions and unstable flow field behavior, as denoted by finger width and finger velocity, are derived. Miller scaling of these relationships allows their generalization to similar porous media with different mean grain sizes and shows the effects of coarseness and fluid properties.; The first experiments examine an initially dry, fine over coarse textured, two dimensional, layered sand system. Relationships between finger width, velocity, moisture content and flow through individual fingers are found and linked to the properties of the bottom layer and system flow rate. The observation that fingers move at different velocities motivates a reinterpretation of the analysis of Parlange and Hill allowing agreement between analysis and experimental results. Further experiments in large columns where full three dimensional fingers form extend the above relationships to three dimensions.; To assess the effect of wetting front instability on solute transport, laboratory experiments are carried out in a two dimensional, fine over coarse, layered sand system. The effect of repeated long term ponded infiltration cycles, intermittent ponding events and uniform initial moisture content at field capacity on the flow field structure and solute breakthrough curves is studied. A simple lumped dispersion coefficient model that includes additional mixing processes due to instability is explored and implications for field solute transport monitoring practices are discussed.; Finally, a physically based theory for the mechanism of finger persistence over repeated infiltration cycles is presented and verified with the use of a new experimental technique which allows rapid visualization of moisture content in thin slabs of porous media. Fingers are shown to be nearly saturated at their tips and to dry a distance behind. Hysteresis in the moisture characteristic relation causes the formation of a two zoned moisture content structure within the homogeneous porous media. This structure persists for long and perhaps indefinite periods of steady infiltration and over subsequent infiltration events.