Morphodynamics during air injection into water-saturated movable spherical granulates

Laboratory air injection experiments in a two-dimensional vertically placed cell filled with a water-saturated packing of spherical glass beads show a particular transition of air flow patterns at different length scales, tree-like fingering with pore-scale drainage, a local fluidization of beads with finger-scale channeled air flow, and an oscillating irregular channel with finger-scale air flow, where the channel is formed by pushing beads aside. The tree-like pattern shows a parabola-like growth with vertical height, while both the top and the bottom lateral width of the pattern increase linearly for the small injection rate, then stay more or less constant for further increasing injection rates. By tracing the vertical position of maximum advance of the fluidized pattern with time, the evolution of the fluidized pattern is characterized by two dynamic regimes, the pattern not reaching or reaching the injection orifice, respectively, using small and medium injection rates. As the injection rate increases further, the fluidized pattern shows up immediately after air is injected. Via physical conceptual models using local forces or pressure gradient balances, we derive a series of characteristics that describe the width of the tree-like air plume, the width of a single channel, and the starting vertical position of the fluidized pattern.