Dynamics of a slowly-varying sand bed in a circular pipe

The long wave-length dynamics and stability of a bed of sand occupying the lower segment of a circular pipe are studied analytically up to first-order in the small parameter characterizing the slope of the bed. The bed is assumed to be at rest, with at most a thin sand layer (the bedload) moving at the sheared interface. When the sand bed is plane, with depth independent of position z along the axis of the pipe, the velocity of the liquid is known from previous studies of stratified laminar flow of two Newtonian liquids (the lower one with infinite viscosity representing the sand bed). When the depth of the sand bed varies with z, secondary flows develop in the cross-sectional (x, y) plane, and these are computed numerically, assuming that the sand bed remains a straight horizontal line in the cross-sectional plane. The mean shear stress acting on the perturbed sand bed is then determined both from the computed secondary flows and by means of the averaged equations of Luchini and Charru. The latter approach requires knowledge only of the flow over the unperturbed, flat sand bed, combined with an accurate approximation of the distribution of the perturbed stresses between the pipe wall and the sand bed. The perturbed stresses determined by the two methods agree well with each other. Using these stresses, it is then possible to apply standard theories of bed stability to determine the balance between the destabilizing effect of inertial (out-of-phase) stresses and the stabilizing effects of gravity and relaxation of the particle flux, and various examples are considered

Physical modeling of the dam-break flow of sedimenting suspensions

We develop a physical model of the dam-break flow of fine non-cohesive particles initially fluidized by a gas. By revisiting previous experiments, we show that the dynamics of such flows involves two uncoupled phenomena. On the one hand, the settling of the particles is the same as that of a non-flowing suspension, so that the mass flux of particles that deposit can be related solely to the properties of the suspension. On the other hand, the flow of the gas-particle mixture is similar to that of an equivalent fluid of constant density and negligible viscosity. The momentum lost by the flowing mixture is equal to the product of the deposited mass flux and the longitudinal velocity. These properties allow us to model the time duration of the flow as the time taken by the particles to settle and the slope of the final deposit as the ratio between the growth rate of the deposit height and the velocity of the front of the dam-break flow. Finally, these findings lead to the formulation of consistent shallow-water equations involving specific terms of mass and momentum transfer at the bottom wall, which can be used to compute the dense lower layer of ash flows generated by a volcanic eruption. They also provide tools for the interpretation of field measurements by geologists

Dynamique et sédimentation des écoulements pyroclastiques reproduits en laboratoire

Les écoulements pyroclastiques constituent des mélanges de cendres et de gaz volcaniques chauds fortements mobiles. Leur comportement fluide est attribué aux effets de fluidisation développés durant la propagation. Au cours de ce travail, nous avons mis en place un dispositif expérimental permattant de reproduire la dynamique de ces écoulements en laboratoire. Ces expériences ont été effectuées sur un chenal horizontal et à des températures suffisament élevées pour permettre l'utilisation de matériaux naturels (cendres volcaniques). Dans un premier temps, les propriétés d'expansion et de sédimentation (1-D) des cendres naturelles ont été étudiées dans le réservoir du dispositif. Les vitesses de sédimentation ont été déterminées après expansion initiale du mélange puis sédimentation (par coupure rapide de l'alimentation en gaz). Les écoulements ont ensuite été générés en ouvrant la porte coulissante du réservoir (après expansion des cendres jusqu'à une valeur donnée : 6-43 vol% dans le réservoir). Chaque écoulement est alors soumis à une défluidisation progressive durant la propagation jusqu'à son arrêt définitif. Cette étude a révélé que le transport non turbulant des cendres augmente fortement avec l'expansion initiale. Pour une expansion donnée la vitesse de sédimentation d'un écoulement est identique à celle mesurée à partir des expériences (1-D) effectuées dans le réservoir. La propagation et la sédimentation de l'écoulement, gouvernées par deux paramètres adimensionnés : le rapport d'aspect initial du mélange expansé dans le réservoir et le rapport d'un temps d'accélération gravitattionnelle et d'un temps de sédimentation, reflète un fort contrôle de la sédimentationCLERMONT FD-BCIU Sci.et Tech. (630142101) / SudocSudocFranceF

Rheological behaviour and runout of short-lived, fast-moving flows of hot dense suspensions

International audienceThis study aims at extending the previous work of Girolami et al. [11-13] which is still suffering from a lack of reliable predictions for the description of hot, dense suspensions obtained by fluidizing with air a bed of fine solid particles, originally cohesive at room temperature, then released down a rectangular flume. Here, we first present the effective viscosities of the suspensions which increase as a power law of the particle volume fraction Φs. Thus, we show that their rheological behaviour is solely controlled by Φs and its value at packing Φpacking , whatever the material involved in the mixture. Finally, we present a new modeling of the flows runout and highlight that the flows duration is solely controlled by the settling time (i.e. the time necessary for a suspension of a given Φs to settle at a velocity U sed over a distance h exp equals to the expansion height), the initial suspension geometry a, and the key parameter Φs /Φpacking. This prediction allows to distinguish two different flow regimes, with a transition around Φs /Φpacking ≃ 0.85, which seems to be correlated to the variations of the mixture rheology

Fall of a large sphere in a suspension of small fluidized particles

International audienceThe investigation of the fall of a sphere at finite Reynolds number in a concentrated suspension of small fluidized particles leads to unexpected results. By analyzing the drag force, it is shown that the average surface stress on the sphere is independent of the size of the sphere. It is proportional to an effective viscosity determined from the sedimentation velocity of the particles multiplied by the velocity of the sphere and divided by the size of the particles. These results question the role of concentration inhomogeneities that occur on a large scale in the overall flow around a moving obstacle and on a small scale near its surface

Digital Rock Physics: computation of hydrodynamic dispersion

International audienceHydrodynamic dispersion is a crucial mechanism for modelling contaminant transport in subsurface engineering and water resources management whose determination remains challenging. We use Digital Rock Physics (DRP) to evaluate the longitudinal dispersion of a sandpack. From a three-dimensional image of a porous sample obtained with X-ray microtomography, we use the method of volume averaging to assess the longitudinal dispersion. Our numerical implementation is open-source and relies on a modern scientific platform that allows for large computational domains and High-Performance Computing. We verify the robustness of our model using cases for which reference solutions exist and we show that the longitudinal dispersion of a sandpack scales as a power law of the Péclet number. The assessment methodology is generic and applies to any kind of rock samples

Grains3D, a flexible DEM approach for particles of arbitrary convex shape — Part I: Numerical model and validations

International audienceWe suggest a novel variant of Discrete Element Method (DEM) to simulate the flow dynamics of granular material made of non-spherical particles. Our approach is limited to particles of convex shape but permits to consider any combination of shape and size, which makes it very versatile. The contact detection strategy relies on the use of the Gilbert–Johnson–Keerthi algorithm to compute the distance between two convex bodies. The validation of the method is based on two different test cases in three dimensions: (i) the formation of a packed pile in a cylindrical container and (ii) flow dynamics in a horizontal rotating drum. In both cases, four shapes are investigated: a sphere, a cylinder, a cube and a tetrahedron. We evidence that our numerical code, Grains3D, supplies reliable and reasonably accurate computed solutions, both for spherical and non-spherical particles. In particular, in the latter case, we show how angularity promotes the appearance of the avalanching regime at relatively high Froude number compared to the spherical counterpart

Particle velocity fields and depositional processes in laboratory ash flows, with implications for the sedimentation of dense pyroclastic flows

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Sur l'érosion interne de la fondation perméable des digues de protection contre les inondations

International audienceThis work focuses on the mechanisms that trigger internal erosion of the pervious foundation of flood protection dikes. The origin of these permeable layers is generally attributed to the presence of a paleo-valley and paleo-channels filled with gravelly-sandy sediments beneath the river bed and dikes. These layers may extend into the protected area. Visual observations of leaks, sand boils and sinkholes in the protected area testify to internal erosion processes in the underground soil. Local geological conditions are part of the information to be sought to explain these processes: presence of permeable soils and position of interfaces. Results obtained on Agly dikes (France), using two classical geophysical methods (EMI and ERT), were analyzed using cored soils and showed that it is not enough to simply conclude to the presence of backward erosion piping. The possibility of internal erosion, such as suffusion or contact erosion, must also be considered as the cause of leaks, sand boils and sinkholes. As the results obtained are explained by the presence of a paleo-valley and paleo-channels beneath the river bed and dikes—commonly encountered in this context—the methodology presented and the results obtained are likely to be relevant for many dikes.Ce travail concerne les causes des processus d'érosion interne de la fondation perméable des digues de protection contre les inondations. L'origine de ces couches perméables est généralement attribuée à la présence d'une paléo-vallée et de paléo-chenaux remplis de sédiments sablo-graveleux sous le lit de la rivière et les digues. Ces couches peuvent s'étendre dans la zone protégée. Des observations visuelles de fuites, de sand poils et de fontis dans la zone protégée témoignent de processus d'érosion interne dans le sol de subsurface. Les conditions géologiques locales font partie des informations à rechercher pour expliquer ces processus : présence de sols perméables et position des interfaces. Les résultats obtenus sur les digues de l'Agly (France), à l'aide de deux méthodes géophysiques classiques (EMI et ERT), ont été analysés à l'aide de sols carottés et ont montré qu'il ne suffit pas de conclure à la présence d’une érosion régressive de conduit. La possibilité d'une érosion interne, telle que la suffusion ou l'érosion de contact, doit également être considérée comme la cause de fuites, de sand poils et de fontis Comme les résultats obtenus s'expliquent par la présence d'une paléo-vallée et de paléo-chenaux sous le lit de la rivière et les digues - que l'on rencontre fréquemment dans ce contexte - la méthodologie présentée et les résultats obtenus sont susceptibles d'être pertinents pour de nombreuses digues