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[1] The fluidal behavior of pyroclastic flows is commonly attributed to high gas pore pressures and associated fluidization effects. We carried out experiments on flows of fluidized volcanic ash at 170°C, which is hot enough to reduce cohesive effects of moisture. The flows were generated in a 3-m-long, horizontal lock-exchange flume. The ash was fluidized and expanded uniformly in the flume reservoir by up to 43% above loose packing and was then released. Each flow defluidized progressively down the flume until motion ceased. Initial expansion E and initial height h 0 were varied independently of one another. The flows traveled in a laminar manner. Flow fronts exhibited three main phases of transport: (1) a brief initial phase of gravitational slumping, (2) a dominant, approximately constant velocity phase, and (3) a brief stopping phase. Phase 2 frontal velocities scaled with ffiffiffiffiffiffiffi gh 0 p , like other types of dam-break flow. Deposition from initially expanded flows took place by progressive sediment aggradation at a rate that was independent of distance and varied only with E. Despite rates of shear up to 80 s À1 , aggradation rates were identical to those determined independently, at the same value of E, in quasi-static collapse tests. Sedimentation caused the flows to thin progressively during transit until they ran out of volume. The dynamics were governed to a first order by two dimensionless parameters: (1) the initial aspect ratio h 0 /x 0 in the lock reservoir and (2) the ratio t sett /t grav of two timescales: a particle settling time t sett and a gravitational acceleration time t grav .Citation: Girolami, L., T. H. Druitt, O. Roche, and Z. Khrabrykh (2008), Propagation and hindered settling of laboratory ash flows,

In this paper, we used a 3-D discrete-element model, Grains3D, which allows the simulation of unsteady granular flows of monodisperse soft spherical particles in a common situation (i.e., down a rectangular channel). A series of numerical dam-break experiments was performed to predict the behavior of granular columns that propagate down a rough horizontal surface from different initial conditions (varying the initial aspect ratio). Numerical results were compared to those obtained experimentally by Lajeunesse et al. (Phys Fluids 17:103302, 2005) from a similar configuration. Runout distance, temporal flow evolution, deposit morphology and internal flow structures of similar laboratory experiments were quantitatively reproduced as well as prediction of empirical and theoretical scaling laws. This paper highlights that such fully 3-D simulations of soft-spheres can remarkably capture dam-break collapses performed in a rectangular channel. Moreover, Grains3D can provide a complete physical description of such complex unsteady systems which will be the topic of future on-going studies.

International audienc

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This work deals with the fluidization and sedimentation of fine solid particles, of random shape and size, similarly to those commonly involved in geophysical mass flows, such as pyroclastic flows.While heated to avoid the effect of moisture and the formation of clusters, particles were first uniformly fluidized by a hot gas flow, up to a high expansion rate, then let sedimented after stopping the gas supply. Three different materials are explored, involving contrasted geometries, each characterized by a specific particle volume fraction at packing Φ pack . Within the range of values of the solid volume fraction Φ s /Φ pack studied here, from 0.65 to 0.95, the dense suspension forms a fully fluidized homogeneous mixture, with no segregation, for which the fluidization and sedimentation velocities are equal. Despite a significant discrepancy between the intrinsic properties of the different materials used, all measured velocities are observed to collapse into a single master curve f (Φ s /Φ pack ) provided that they are normalized by the relevant scaling. Regarding to the sedimentation velocity, Φ pack turns out to be sufficient to characterize the material made with a random distribution in particle shape and size. Furthermore, these new findings suggest that such fluidized gas-solid mixtures behave as a homogeneous equivalent fluid with a bulk apparent viscosity that only depends on Φ s /Φ pack .

The modeling of the fluidization or sedimentation velocity of a suspension of solid particles is revisited by examining experiments conducted in either a liquid or a gas. A general expression is found in the case of negligible fluid inertia, i.e. at low Reynolds or Archimedes number. It is built as the product of the velocity of an isolated particle by three non-dimensional corrections that each takes into account a specific physical mechanism. The first correction reflects the variation of the buoyancy with the particle concentration. The second correction describes how the drag force increases with the concentration in case of negligible particle inertia. The third one accounts for the further increase of the drag when the particle inertia is increased. Remarkably, each correction only relies on a single of the three independent non-dimensional groups that control the problem: (1) the particle volume fraction Φ s ; (2) the ratio Φ s /Φ pack where Φ pack is the bed packing concentration; (3) the Stokes number St 0 , which characterizes the inertia of the particles and controls their agitation. Moreover, the onset of the instability that separates the homogeneous regime from the heterogeneous one is found to be controlled similarly by the Stokes number.Empirical expressions of the corrections are given, which provide a reliable tool to predict fluidization and sedimentation velocities for all values of the three non-dimensional numbers. The present results emphasize the crucial role of particle inertia, which is often disregarded in previous modeling approaches, such as that of Richardson and Zaki.

Abstract:The aim of this paper is to evaluate the vulnerability after point source contamination and characterize water circulations in volcanic flows located in the Argnat basin volcanic system (Chaîne des Puys, French Massif Central) using a tracer test performed by injecting a iodide solution. The analysis of breakthrough curves allowed the hydrodispersive characteristics of the massive lava flows to be determined. Large Peclet numbers indicated a dominant advective transport. The multimodal feature of breakthrough curves combined with high values of mean velocity and low longitudinal dispersion coefficients indicated that water flows in an environment analogous to a fissure system, and only slightly interacts with a low porosity matrix (ne < 1%). Combining this information with lava flow stratigraphy provided by several drillings allowed a conceptual scheme of potential contaminant behaviour to be designed. Although lava flows are vulnerable to point source pollution due to the rapid transfer of water within fractures, the saturated scoriaceous layers located between massive rocks should suffice to strongly buffer the transit of pollution through dilution and longer transit times. This was consistent with the low recovery rate of the presented tracer test.