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International audience; Submerged granular flows through an orifice were investigated numerically in the context of sinkhole occurrences during a flood due to the presence of underground conduits. To account for fluid-solid interaction at the pore-scale, we use a numerical modelling that combines the Discrete Element Method (DEM) for the solid particles with the Lattice Boltzmann Method (LBM) for the fluid dynamics. The numerical setup studied is a submerged granular discharge from a hopper, which is shown to be particularly sensitive to hydraulic boundary conditions. With a given choice of configuration, we performed a parametric study by varying particle diameters, fluid viscosity and hopper orifice size, enabling the exploration of the Archimedes number over five orders of magnitude. The solid discharge rates are shown to have self-similar temporal evolutions and the grains at the orifice display self-similar velocity profiles, normalized by the maximum velocity reached at the center of the orifice. In this paper, we finally propose an extension of the classical Beverloo law that takes into account the effect of fluid entrainment by the downward granular flow.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE [ADD1_IRSTEA]Hydrosystèmes et risques naturels; International audience; The ability of viscoplastic fluids to self-select their flow geometry through the formation of unyielded dead zones has important consequences for flows in wavy channels, flows over obstacles, etc. Yet, the mechanisms controlling the formation and dimensions of the dead zones remain poorly understood. We present a detailed cross-comparison of experimental and numerical results concerning channel flows of a viscoplastic fluid over a rectangular cavity filled by the same material. In all the configurations studied, which correspond to moderate values of the Bingham number, a continuous dead zone forms inside the cavity. Both numerical and experimental data reveal that, unlike at high Bingham numbers, the shear-rate profiles above the dead zone display an asymmetric shape. Accordingly, two different flow zones can be distinguished: a Poiseuille-like zone, in which the velocity profile is similar to that over a rigid wall, and a boundary layer ensuring the transition with the dead zone below. It is shown that the effective boundary condition felt by the Poiseuille-like layer is essentially controlled by incoming flow characteristics, such that the thickness of this layer does not obey simple relations with cavity length. Interestingly, however, the thickness of the boundary layer appears to follow a generalized Oldroyd’s scaling with cavity length.