11 research outputs found
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 ïŹow of sedimenting suspensions
We develop a physical model of the dam-break ïŹow of ïŹne non-cohesive particles initially ïŹuidized by a gas. By revisiting previous experiments, we show that the dynamics of such ïŹows involves two uncoupled phenomena. On the one hand, the settling of the particles is the same as that of a non-ïŹowing suspension, so that the mass ïŹux of particles that deposit can be related solely to the properties of the suspension. On the other hand, the ïŹow of the gas-particle mixture is similar to that of an equivalent ïŹuid of constant density and negligible viscosity. The momentum lost by the ïŹowing mixture is equal to the product of the deposited mass ïŹux and the longitudinal velocity. These properties allow us to model the time duration of the ïŹow as the time taken by the particles to settle and the slope of the ïŹnal deposit as the ratio between the growth rate of the deposit height and the velocity of the front of the dam-break ïŹow. Finally, these ïŹndings lead to the formulation of consistent shallow-water equations involving speciïŹc terms of mass and momentum transfer at the bottom wall, which can be used to compute the dense lower layer of ash ïŹows generated by a volcanic eruption. They also provide tools for the interpretation of ïŹeld measurements by geologists
Dynamique et sédimentation des écoulements pyroclastiques reproduits en laboratoire
Les Ă©coulements pyroclastiques constituent des mĂ©langes de cendres et de gaz volcaniques chauds fortements mobiles. Leur comportement fluide est attribuĂ© aux effets de fluidisation dĂ©veloppĂ©s durant la propagation. Au cours de ce travail, nous avons mis en place un dispositif expĂ©rimental permattant de reproduire la dynamique de ces Ă©coulements en laboratoire. Ces expĂ©riences ont Ă©tĂ© effectuĂ©es sur un chenal horizontal et Ă des tempĂ©ratures suffisament Ă©levĂ©es pour permettre l'utilisation de matĂ©riaux naturels (cendres volcaniques). Dans un premier temps, les propriĂ©tĂ©s d'expansion et de sĂ©dimentation (1-D) des cendres naturelles ont Ă©tĂ© Ă©tudiĂ©es dans le rĂ©servoir du dispositif. Les vitesses de sĂ©dimentation ont Ă©tĂ© dĂ©terminĂ©es aprĂšs expansion initiale du mĂ©lange puis sĂ©dimentation (par coupure rapide de l'alimentation en gaz). Les Ă©coulements ont ensuite Ă©tĂ© gĂ©nĂ©rĂ©s en ouvrant la porte coulissante du rĂ©servoir (aprĂšs expansion des cendres jusqu'Ă une valeur donnĂ©e : 6-43 vol% dans le rĂ©servoir). Chaque Ă©coulement est alors soumis Ă une dĂ©fluidisation progressive durant la propagation jusqu'Ă son arrĂȘt dĂ©finitif. Cette Ă©tude a rĂ©vĂ©lĂ© que le transport non turbulant des cendres augmente fortement avec l'expansion initiale. Pour une expansion donnĂ©e la vitesse de sĂ©dimentation d'un Ă©coulement est identique Ă celle mesurĂ©e Ă partir des expĂ©riences (1-D) effectuĂ©es dans le rĂ©servoir. La propagation et la sĂ©dimentation de l'Ă©coulement, gouvernĂ©es par deux paramĂštres adimensionnĂ©s : le rapport d'aspect initial du mĂ©lange expansĂ© dans le rĂ©servoir et le rapport d'un temps d'accĂ©lĂ©ration gravitattionnelle et d'un temps de sĂ©dimentation, reflĂšte un fort contrĂŽle de la sĂ©dimentationCLERMONT 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 Ίs. Thus, we show that their rheological behaviour is solely controlled by Ίs and its value at packing Ίpacking , 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 Ίs 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 Ίs /Ίpacking. This prediction allows to distinguish two different flow regimes, with a transition around Ίs /Ίpacking â 0.85, which seems to be correlated to the variations of the mixture rheology
Sédimentation de particules, de formes et de tailles aléatoires, préalablement fluidisées par un gaz
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 PĂ©clet 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 â 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 GilbertâJohnsonâKeerthi 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 dikesâcommonly encountered in this contextâthe methodology presented and the results obtained are likely to be relevant for many dikes.Ce travail concerne les causes des processus d'Ă©rosion interne de la fondation permĂ©able des digues de protection contre les inondations. L'origine de ces couches permĂ©ables est gĂ©nĂ©ralement attribuĂ©e Ă la prĂ©sence d'une palĂ©o-vallĂ©e et de palĂ©o-chenaux remplis de sĂ©diments sablo-graveleux sous le lit de la riviĂšre et les digues. Ces couches peuvent s'Ă©tendre dans la zone protĂ©gĂ©e. Des observations visuelles de fuites, de sand poils et de fontis dans la zone protĂ©gĂ©e tĂ©moignent de processus d'Ă©rosion interne dans le sol de subsurface. Les conditions gĂ©ologiques locales font partie des informations Ă rechercher pour expliquer ces processus : prĂ©sence de sols permĂ©ables et position des interfaces. Les rĂ©sultats obtenus sur les digues de l'Agly (France), Ă l'aide de deux mĂ©thodes gĂ©ophysiques classiques (EMI et ERT), ont Ă©tĂ© analysĂ©s Ă l'aide de sols carottĂ©s et ont montrĂ© qu'il ne suffit pas de conclure Ă la prĂ©sence dâune Ă©rosion rĂ©gressive de conduit. La possibilitĂ© d'une Ă©rosion interne, telle que la suffusion ou l'Ă©rosion de contact, doit Ă©galement ĂȘtre considĂ©rĂ©e comme la cause de fuites, de sand poils et de fontis Comme les rĂ©sultats obtenus s'expliquent par la prĂ©sence d'une palĂ©o-vallĂ©e et de palĂ©o-chenaux sous le lit de la riviĂšre et les digues - que l'on rencontre frĂ©quemment dans ce contexte - la mĂ©thodologie prĂ©sentĂ©e et les rĂ©sultats obtenus sont susceptibles d'ĂȘtre pertinents pour de nombreuses digues