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International audience; The propagation of horizontal dam-break flows induced by the release of a rectangular column of a liquid phase into a negligible ambient fluid is investigated by combining numerical simulations and laboratory experiments. Varying both the geometry of the initial column and liquid properties, the parameter space covered allows the investigation of the transition from inertial to viscous slumping barely considered so far. In light of the Reynolds number Re based on the initial parameters of the fluid column, we report a newly investigated transient regime following the inertial slumping regime and prior to the viscous-dominated one. The transient intermediate regime is shown (i) to only exist for $Re\gtrsim10$ and (ii) to be characterized by an inertial overshoot followed by a strong deceleration of the front position accompanied by an excess and a deficit of fluid at the front and at the origin, respectively, with respect to the expected theoretical prediction of the viscous-dominated regime. Beyond the intuitive idea of an adaptive transition from the inertial to fully viscous slumping, these results highlight and quantify the influence of initial inertia on the spreading of a liquid, which may be particularly useful for modeling industrial and geophysical applications.

Co-auteur étranger; International audience; Granular flows that propagate down a mountainside, then reach the sea, a lake or a river and finally, travel underwater, is a common event on the Earth’s surface. To help the description of such events, laboratory experiments on gas-fluidized granular flows entering water are performed, analyzed, and compared to those propagating in air. The originality of this study lies in the fluidization process, which improves the laboratory modelling of geophysical flows by taking their high mobility into account. Qualitatively, the presence of the water body promotes the generation of a granular jet over the water surface, a leading and largest wave, and a particle-driven gravity current underwater. Hydrodynamic forces mainly play a dissipative role by slowing and reducing the spreading of the granular mass underwater, but a low amount of grains are still transported by the turbulent fluid as a gravity current far away. The temporal evolution of the granular jet and the particle-driven gravity current are well described by ballistic motion theory and scaling laws of homogeneous gravity currents, respectively. Most currents propagate with a constant flow-front velocity along the horizontal bottom, which is controlled by the flow height depending on the water depth. In constrast, the bulk volume concentration of particles in the current is estimated to be nearly constant, interpreted as a critical concentration above which the excess of particles cannot be maintained by the turbulent fluid. This experimental study highlights the complexity of the dynamics and deposits of granular masses when they encounter a water body.

Tsunamis generated by the collapse of a granular mass into water represent a low amount of documented tsunamis in the world. However, they can locally cause severe human and material damages as they occur near coastlines. In order to identify physical mecanisms controlling the features of induced waves, the simple configuration of a granular material entering into water has been widely investigated. This study extends the configuration to the case of a fluidized granular flow, representative of tsunamis generated by pyroclastic flows during volcanic eruptions. We show that, in dam-break configuration, the fluidization process enhances the mobility of the granular flow with larger front velocities and larger propagation distances of the mass. The consequence is a significant increase of the maximum wave amplitude in comparison to an analogous situation without fluidization. The influence of the impact velocity and of the mass on the amplitude of the wave is also supported by varying the initial height of the granular column released. Finally, we show that the level of water does not affect the characteristics of the wave, in the range of parameters considered here., Les tsunamis générés par l'entrée d'une masse granulaire dans l'eau ne représentent qu'une faible proportion des tsunamis répertoriés dans le monde. Toutefois, ils peuvent localement causer d'importants dégâts humains et matériels car ils sont généralement créés proche des côtes. Afin d'identifier les méca-nismes physiques contrôlant les propriétés des vagues induites, la configuration simplifiée d'un matériau granulaire impactant l'eau a été largement étudiée en laboratoire. Cette étude étend la configuration au cas d'un écoulement granulaire fluidisé, représentatif des tsunamis générés par des écoulements py-roclastiques lors d'éruptions volcaniques. Nous montrons qu'en configuration de rupture de barrage (dam-break), le processus de fluidisation améliore la mobilité de l'écoulement granulaire, caractérisée par une augmentation de la vitesse du front et de la distance de propagation de la masse. La consé-quence est une amplitude de vague plus grande par rapport à une situation analogue sans fluidisation. L'influence de la vitesse d'impact et de la masse sur l'amplitude de la vague est également confirmée en variant la hauteur initiale de la colonne granulaire relâchée. Finalement, nous montrons que la hauteur d'eau n'affecte pas les caractéristiques de la vague, dans la gamme des paramètres considérées ici.

International audience; Laboratory experiments on the collapse of a liquid-saturated granular column on a horizontal plane are reported. The trigger and the resulting dynamics of the collapse when occurring, as well as the shape of the final deposit, are characterized and analyzed in light of some dimensionless parameters, namely, the "column" Bond number Bo, the grain diameter to capillary length ratio d/l c , the initial aspect ratio a, the Stokes number St, and the initial volume fraction φ, by varying the properties of the interstitial fluid and of the grains, the geometry, and the compaction of the initial granular column. The main contribution of this study is to: (i) provide a diagram of the different regimes of collapse shown to be mostly controlled by capillary effect, (ii) develop simple criteria that capture the transitions between each regime in terms of critical values of the Bond number and the ratio of the grain diameter to the capillary length, (iii) extend a predictive model of the runout for dry collapses to the more general case of liquid-saturated granular collapses, and (iv) quantify the influence of a, St, and φ on the collapse dynamics and the shape of the final deposit in the capillary-free regime. A perspective description of the role of the interstitial fluid on the spreading of the granular medium, and more particularly of the driving role of the fluid, is discussed and argued on the basis of the present set of experiments.

International audience; Laboratory experiments of the granular collapse of an initially dense-packing column in a fluid are reported. By extracting the temporal evolution of the granular material height profile, both the dynamics of the granular flow and the final deposit are characterized depending on the Stokes number St, based a on dissipative process at the grain scale, the grain-fluid density ratio r, and the aspect ratio a of the initial column. A full description of the granular collapse including the transient dynamics of the flow and the characterization of the final shape deposit is proposed. The main contribution of the present study is to provide (i) the St dependence on the granular collapse beyond the effect of the aspect ratio a already reported in previous studies, (ii) the characterization of granular flow regimes in the ((d/Hi) 1/2 St, (d/Hi) 1/2 r) plane, where d/Hi is the particle diameter to initial column height ratio, and (iii) simple correlations to describe the granular collapse which would be of interest for geophysical purposes.