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We experimentally investigated cohesion of artificially bonded granular materials made of spherical glass beads cemented by solid paraffin bonds. By means of laboratory tests designed and carried out for investigation at different scales, we measured the tensile yield strength for solid bonds both at the inter-particle micro-scale and cemented samples at the meso-scale. A parametric study has been performed by varying some of the granular material properties (bead diameter, paraffin content as well as the dimension of the sample for the meso-scale tensile tests. We finally propose a discusion on: (i) the relationship between the microscopic and macroscopic cohesion forces relying on classical homogenisation laws ; (ii) the potential impact of size effects based on a simple phenomenological model.

International audience; Empirical predictions of discharge rates for dry non-cohesive grains are commonly based on the Beverloo law (Chem Eng Sci 15:260–269, 1961). The present work extends this practical configuration to submerged and cohesive cases to investigate the flow behaviour of granular media with applications in the geophysical process of sinkhole formation. The analysis of the hydrostatic collapse of soil in the presence of underground conduits is performed with a 2D GPU-parallelized simulation coupling the lattice Boltzmann method and the discrete element method to describe the fluid and the solid phases, respectively. The discharge rate of a large submerged granular sample is analysed by varying orifice sizes and inter-particle cohesion strengths. For the submerged cohesionless case, we first study the revisited Beverloo relationship that includes the terminal velocity of a single falling particle in the fluid, proposed in the experimental work of Wilson et al. (Pap Phys 1307: 2812, 2014). We consistently take into account the interstitial fluid with an effective orifice size smaller than in the dry case. Then, the additional contribution of grain cohesion is examined. Our main finding is that the empirical prediction remains valid provided that the orifice cut-off increases with cohesion. Finally, the evolution of fluid pressure during the discharge, at the vicinity of the orifice, is studied and favourably compared with the recent experimental study of Guo et al. (Granul Matter 19(3): 1–8, 2017). By considering the pressure drop around the orifice as a driven-term, we succeed in predicting the solid flow rate with a similar Beverloo approach.

[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.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE; International audience; The current study presents experiments for the initial stages of fluidization induced by a localized fluid injection. The process was studied by recording high-speed videos in a 2-D-region far from the boundaries using Planar Laser Induced Fluorescence and Refractive Index Matching. The experimental setup allowed for several parameters to be systematically studied including particle sizes, initial bed heights, and injection port diameters. The results show that the critical flow rate required for fluidization is primarily dependent on the initial height of the granular bed. However, this dependence is not a linear relation but progressively plateaus for larger heights. Conversely, the diameter of the chimney relates only slightly to the injection port diameter and significantly more to the diameter of the particles.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE; International audience; This paper presents an experimental characterization of the flow of an elasto-viscoplastic fluid (Carbopol) over a dead zone constituted of the same material. The studied configuration consists of a closed rectangular channel with a cavity in its base. A curved solid-fluid interface forms into the cavity, separating a yielded flowing layer above from an unyielded dead zone below. The hydrodynamics of the flow is investigated by means of high-resolution optical velocimetry (PIV). We focus in particular on the velocity profiles and shear-rate evolution in a quasi-longitudinal flow domain located around cavity mid-length. Our measurements show a non-monotonous evolution of the shear rate, which increases from zero at the solid-liquid interface, passes through a peak (sometimes leveling off at its maximum value), and returns to zero in a plug zone sufficiently far above the cavity. Two main flow zones can be distinguished: a Poiseuille zone, in which velocity profiles are the same as in the flow over a rigid wall, and a boundary layer ensuring the transition with the dead zone. Hence, consistently with our previous work (Luu et al., Phys. Rev. E, 2015), the flow self-organizes to partially smooth out the bottom perturbation. The characteristic thicknesses of the flow zones are shown to evolve with cavity length and hydrodynamic properties of the incoming flow. The study also points out the influence of elastic effects, notably on the shape (asymmetry) of the solid-fluid interface.

International audience; We present an experimental study investigating the transition zone between a liquid-like unyielded region and a solid-like yielded region in a yield-stress fluid. The configuration consists of a rectangular closed-channel flow disturbed by the presence of a step. Upstream of the step, a solid-liquid interface between a dead zone and a flow zone appears. In this study, we use a model fluid, namely polymer micro-gel Carbopol, which exhibits Herschel-Bulkley viscoplastic rheology. Exploiting the fluid transparency, the flow is monitored by particle image velocimetry (PIV) using an internal visu-alization technique. The main outcome of this study is to show that, except in a thin transition layer close to the solid-liquid interface, the flow behaves as an apparent Poiseuille flow with an apparent slip condition at the base. The slip frontier is found to be almost independent of the flow rate while the corresponding slip velocity increases with the flow rate.

International audience; We describe a new spreading regime during the drop impact of model yield-stress fluids (Carbopol microgel solutions) on rough hydrophobic surfaces, in a range of parameters where classical Newtonian drops usually splash. For large surface roughness and high impact velocity, we observe that the maximal inertial spreading diameter of the drops can be as much as twice larger than on smooth surfaces in the same conditions, corresponding to apparent basal friction reductions of more than 80%. We interpret this large drag reduction using a simple energy balance model and a dynamic slip length that depends on both the surface roughness and the drop's dynamics.