Characteristics of displacement fluid fronts - from rapid interfacial jumps to steady capillary flows

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Fluid displacement in porous media takes place as a sequence of rapid interfacial jumps with fluid redistributions triggered by pressure relaxation and force balance. These rapid dynamics are often followed by a period of interfacial pinning before a new cascade of fluid invasion occurs. These discontinuous processes cannot be represented by standard macroscopic formulation of water flow based on Richards equation with flow driven by Buckingham-Darcy law. Therefore, we propose to develop and test alternative modeling strategies (modified invasion percolation in a pore network; foam drainage equation) to reproduce phase distribution and displacement processes at pore scale. In the region behind a displacement front, fluid flow may take place along stable interfaces exhibiting flow regimes described by the Buckingham-Darcy flow and the Richards equation. For different processes (infiltration, imbibition, evaporation and drainage) and porous media, we plan to investigate conditions where Richards equation fails, providing estimates for transition length and times and thresholds for applicability of Richards equation. These limits will be determined in experimental series using high speed camera and neutron imaging and theoretical considerations. In collaboration with subproject SP1, our modeling strategy will be tested by comparison with Lattice-Boltzmann simulations. Experimental results will be provided for subprojects SP6 and SP7 to define modified upscaling schemes, and SP1 and SP2 to validate the coupling approaches between porous medium and atmosphere.