Effect of a vertically flowing water jet underneath a granular bed

The response of a granular bed to a punctual, vertically flowing water jet underneath it is studied experimentally, theoretically, and numerically. Experiments show that three regimes depending on the flow rate Q appear to outline the bed's behavior. For sufficiently small Q , the bed remains motionless and acts as a rigid porous medium [regime (i)]. It then becomes deformed when Q is sufficiently increased [regime (ii)]. Finally, the bed "explodes" and a locally fluidized bed limited to a domain above the water jet is observed as Q is increased further [regime (iii)]. This fluidization creates a "chimney" in the bed, roughly cylindrical in shape, inside which the grains are in motion. The flow motion in regime (i) is theoretically modeled as a Darcy flow inside an unbounded granular bed while a numerical model accounting for the boundaries is performed. Results from the theory and computations are compared to experimental data and the effects of the boundaries, the bed's thickness, and the size of the jet on the flow motion inside the bed are underlined. The onset for fluidization [regime (iii)] is explained by assuming a stick-slip behavior of the chimney. Despite the simplistic model, the comparison with experimental data show very good agreement for bony sand granules and relatively good agreement for spherical glass beads.