Air invasion in a granular layer immersed in a fluid: Morphology and dynamics

We investigate the morphology and dynamics of the region invaded by air injected at the bottom of an immersed granular bed. Previous experimental results point out the formation of a fluidized zone with a parabolic shape which does not depend, in the stationary regime, on the injection flow-rate. By tilting the experiment, we can tune the effective gravity in the system. We show that it does not affect significantly the morphology either. A numerical study made it possible to access the typical height and width of the structure, which are governed by the relative effects of gravity and capillarity. After a brief review on this subject, we propose first, new experimental observations on the air invasion regimes and on the morphology of the fluidized zone, in particular its growth dynamics; then, we complement the previous numerical study by considering the influence of the bottom boundary condition. In particular, we quantify the morphology of the invaded region when the gas is injected in the bulk, thus when air is likely to propagate downwards. These results are of practical importance in the prediction of the morphology of gas invasion in soils, from CO₂ sequestration to pollutant propagation.