Microgravity Effects on Water Flow and Distribution in Unsaturated Porous Media

University of Utah

Several aspects of the physical processes of liquid flow and distribution within partially saturated porous media are altered in the reduced gravity conditions (microgravity) of orbiting spacecraft. The objectives of this study were to simulate and test measured flow and distribution in porous media from a microgravity environment using conventional capillary flow theory. Two past microgravity experiments studying water supply and uptake in porous media took place on a U.S. space shuttle, titled ASC-1, and on the Russian space station Mir, titled Greenhouse-2. Data from microgravity and ground experiments were simulated using similar physical flow models by elimination of the gravitational contribution to flow in microgravity as a first order approximation. Simulations of flow in the ASC-1 system were matched to data by adjusting media hydraulic parameters to account for apparent pronounced hysteresis and modifications of the substratewater characteristic (SWC) under microgravity. Qualitative analysis of media parameters indicated narrower pore size distributions and inactive or non-participating large pores in microgravity. Although hysteretic effects were accounted for in the Greenhouse-2 simulations, a medium unsaturated hydraulic conductivity reduction of four orders of magnitude was most effective in matching simulated and measured water content dynamics. Evidence of accentuated hysteresis, altered SWC. and reduced hydraulic conductivity from microgravity simulations may be attributable to proposed mechanisms of air- and liquid-entrapment, fingering flows, particle capturing, separation, and rearrangement, Haines jumps and hydraulic discontinuity. These are likely spawned by enhanced interfacial flows and altered hydrostatic and hydrodynamic forces occurring through sorption and desorption processes.