We study the effect of local wettability reversal on remobilizing immobile fluid clusters in steady-state two-phase flow in porous media. We consider a two-dimensional network model for a porous medium and introduce a wettability alteration mechanism. A qualitative change in the steady-state flow patterns, destabilizing the percolating and trapped clusters, is observed as the system wettability is varied. When capillary forces are strong, a finite wettability alteration is necessary to move the system from a single-phase to a two-phase flow regime. When both phases are mobile, we find a linear relationship between fractional flow and wettability alteration.

A gravity-driven flow of grains through a narrow pipe in vacuum is studied by means of a onedimensional model with two coefficients of restitution. Numerical simulations show clearly how density waves form when a strikingly simple criterion is fulfilled: that dissipation due to collisions between the grains and the walls of the pipe is greater per collision than that which stems from collisions between particles. Counterintuitively, the highest flow rate is observed when the number of grains per density wave grows large. We find strong indication that the number of grains per density wave always approaches a constant as the particle number tends to infinity, and that collapse to a single wave, which was often observed also in previous simulations, occurs because the number of grains is insufficient for multiple wave formation.

A recent experiment has considered the effective permeability of two-phase flow of air and a water-glycerol solution under steady-state conditions in a two-dimensional model porous medium, and found a power law dependence with respect to capillary number. Running simulations on a two-dimensional network model similar power law behavior is found, for high viscosity contrast as in the experiment and also for viscosity matched fluids. Two states are found, one with stagnant clusters and one without. For the stagnant cluster state, a power law exponent 0.50 is found for viscosity matched fluids and 0.54 for large viscosity contrast.

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