Avalanches of grains with inhomogeneous distribution of inner mass (original) (raw)

Two types of avalanche behaviour in model granular media

Physica A: Statistical Mechanics and its Applications, 2000

We have described a lattice model of a sandpile that includes a coupling between evolving granular structures and dynamic responses. The coupling manifests as a granular memory. We have illustrated the role of memory by observing avalanches in a (three)-dimensional model sandpile. There are two distinct classes of dynamic events that depend on the process history and mimic, closely, two categories of avalanches recently observed in piles of glass beads. The origins of the di erent dynamic regimes are explained in terms of the statistical distribution of local stability criteria.

INTERNAL AVALANCHES IN MODELS OF GRANULAR MEDIA

Fractals, 1999

We study the phenomenon of internal avalanching within the context of recently introduced lattice models of granular media. The avalanche is produced by pulling out a grain at the base of the packing and studying how many grains have to rearrange before the packing is once more stable. We find that the avalanches are long-ranged, decaying as a power-law. We study the distriution of avalanches as a function of the density of the packing and find that the avalanche distribution is a very sensitive structural probe of the system.

Dynamics of rearrangements during inclination of granular packings: the avalanche precursor regime

Journal of Statistical Mechanics: Theory and Experiment, 2012

The dynamics of inclined 3D granular packings driven towards their stability limits is studied experimentally through the dynamics of rearrangements of grains at their surfaces. Two regimes are identified in that way: a regime of limited and independent rearrangements of grains at small angles and a regime of extended events, called 'precursors of avalanches', at large angles. Both regimes are characterized. The transition between the two regimes is attributed to the emergence of slidings smaller than one grain diameter at the surface of the packing with a stick-slip dynamics which reflects the emergence of long-range correlations at the approach of the avalanche. The observed superficial events are shown to depend strongly on the microscopic details of the interaction between grains. In addition to the study of precursors, we discuss the consequences of this macroscopic correlation length on the packing stability. We show in particular that the avalanche angle θ a depends not only on the width of the packing but also on its length.

Scaling properties of avalanches from a collapsing granular pile

Physica D: Nonlinear Phenomena, 2000

Results of an experimental investigation of the scaling properties of a collapsing rice pile induced by reducing the length of the base support of the pile are reported. It is found that two angles of repose are needed to describe the shape of the collapsing granular pile. Corresponding to these two angles of repose, the collapse of the granular pile can be characterized by local and global avalanches. Furthermore, it is found that the probability distributions of the avalanches depend on the sizes of the avalanches under consideration. E ects of the shape of the granular particles on the avalanches are also discussed.

Avalanche Dynamics in Wet Granular Materials

Physical Review Letters, 2002

We have studied the dynamics of avalanching wet granular media in a rotating drum apparatus. Quantitative measurements of the flow velocity and the granular flux during avalanches allow us to characterize novel avalanche types unique to wet media. We also explore the details of viscoplastic flow (observed at the highest liquid contents) in which there are lasting contacts during flow, leading to coherence across the entire sample. This coherence leads to a velocity-independent flow depth at high rotation rates and novel robust pattern formation in the granular surface.

A conveyor belt experimental setup to study the internal dynamics of granular avalanches

Experiments in Fluids, 2021

This paper shows how a conveyor belt setup can be used to study the dynamics of stationary granular flows. To visualise the flow within the granular bulk and, in particular, determine its composition and the velocity field, we used the refractive index matching (RIM) technique combined with particle tracking velocimetry and coarse-graining algorithms. Implementing RIM posed varied technical, design and construction difficulties. To test the experimental setup and go beyond a mere proof of concept, we carried out granular flow experiments involving monodisperse and bidisperse borosilicate glass beads. These flows resulted in stationary avalanches with distinct regions whose structures were classified as: (i) a convectivebulged front, (ii) a compact-layered tail and, between them, (iii) a breaking size-segregation wave structure. We found that the bulk strain rate, represented by its tensor invariants, varied significantly between the identified flow structures, and their values supported the observed avalanche characteristics. The flow velocity fields' interpolated profiles adjusted well to a Bagnold-like profile, although a considerable basal velocity slip was measured. We calculated a segregation flux using recent developments in particle-size segregation theory. Along with vertical velocity changes and high expansion rates, segregation fluxes were markedly higher at the avalanche's leading edge, suggesting a connection between flow rheology and grain segregation. The experimental conveyor belt's results showed the potential for further theoretical developments in rheology and segregation-coupled models.

Precursors and triggering mechanisms of granular avalanches

Comptes Rendus Physique, 2015

The dynamics of inclined granular packings driven towards their stability limits are studied experimentally using imaging techniques as well as acoustic methods. The formers allow to study grain rearrangements during the tilting. The implementation of both passive and active acoustic methods for probing the granular packings, with capabilities for time resolved measurements, provides information on various elastic properties of the layers along the destabilization process, including the transient precursors. Systematic experiments of granular layer destabilization for various granular media and external conditions are compared and allow to better understand the mechanisms responsible for the appearance, periodicity and intensity of precursors. La dynamique des empilements granulaires inclinés jusqu'à leur limite de stabilité peut etreétudiéeà l'aide d'expériences utilisant des techniques d'imagerie ou des méthodes acoustiques. Les premières permettent l'étude des réarrangements de grains pendant l'inclinaison. L'implémentation de méthodes acoustiques passives et actives pour le sondage d'empilements granulaires en temps réel, permet d'estimer la variation des propriétéś elastiques au cours du processus de déstabilisation, incluant le régime des précurseurs. Des expériences systématiques de déstabilisation de couches granulaires pour divers milieux granulaires et conditions extérieures sont comparées et permettent d'avoir une meilleure compréhension des mécanismes responsables de l'apparition, de la périodicité et de l'intensité des précurseurs.

Relaxation dynamics of water-immersed granular avalanches

Journal of Fluid Mechanics, 2007

We study water-immersed granular avalanches in a long rectangular cell of small thickness. By video means, both the angle of the granular pile and the velocity profiles of the grains across the depth are recorded as a function of time. These measurements give access to the instantaneous granular flux. By inclining the pile at initial angles larger than the maximum angle of stability, avalanches are triggered and last for a long time, up to several hours for small grains, during which both the slope angle and the granular flux relax slowly. We show that the relaxation is quasi-steady so that there is no inertia: the relaxation at a given time is controlled only by the slope angle at that time. This allows us to adapt a frictional model developed recently for dry or water-immersed grains flowing in stationary conditions. This model succeeds well in reproducing our unsteady avalanche flows, namely the flowing layer thickness, the granular flux and the temporal relaxation of the slope. When a water counter-flow is applied along the pile, the granular avalanches are slowed down and behave as if granular friction were increased by an amount proportional to the water flow. All these findings are also reproduced well with the same friction model by taking into account the additional fluid force.

Bidisperse granular avalanches on inclined planes: A rich variety of behaviors

The European Physical Journal E, 2007

Experiments were performed to provide insight into the flow behavior and structure of bimodal mixtures of grains in gravity-driven, free-surface flows. Unsteady unconfined flows were produced by releasing instantaneously a dry granular mass, composed of two particle sizes, over a rough inclined plane. As a result of size segregation, the small particles are found at the bottom of the flow and final deposit, the large particles are found at the free surface, but also on the lateral borders and at the front of the flow. The lateral and vertical inhomogeneous repartitions of particles lead to two main effects that are completely absent in monodispersed flows. The outline effect results from the accumulation of large beads on the periphery of the flow depending on the value of the relative friction of each particle species on the plane. This effect in turn causes a narrowing of the flow and/or an increase of length of the final deposit. The interface effect results of the interaction between layers of different size particles and causes the modification of the thickness of the deposit. These effects occur simultaneously and their combination leads to a great variety of behaviors. In this investigation, evidence of the diversity of behaviors is presented as the size ratio, relative friction and concentration of each particle species are varied.

Dynamics of Granular Material on Small Bodies

2014

The European Space Agency's ROSETTA spacecraft, en route towards its target Comet 67P/Churyumov-Gerasimenko, passed by the Asteroid (21) Lutetia on the 10 July 2010 at a distance of 3170km.OSIRIS - the Optical, Spectroscopic, and Infrared Remote Imaging System on board Rosetta - took 462 images. These images show that (21) Lutetia is covered with a thick layer of regolith. On slopes of several craters this regolith layer collapsed in landslide-like events. A possible trigger mechanism for these low-gravity avalanches is the slow impact of a small mm to cm-sized body. An experiment was conducted to investigate if such an impact is a viably mechanism to trigger an avalanche on an asteroid. The data collected during the experimental investigation show that these minor impacts can trigger a landslide-like event if the target material is tilted close to the angle of repose. The grain size distribution also influences the likelihood of an avalanche depending on the type of material un...