arXiv subjects

Nathalie Thomas

Publications and source records attributed to Nathalie Thomas.

8 recordsLinked to original sources

arXiv2024

Origin of granular axial segregation bands in a rotating tumbler: An interface-mixing driven Rayleigh-Taylor instability

The origin of large and small particle axial bands in long rotating tumblers is a long-standing question. Using DEM simulations, we show that this axial segregation is due to a Rayleigh-Taylor instability which is characterized by the fact that the density of a granular medium increases with mixing and decreases with segregation. For initially mixed particles, segregation and collisional diffusion in the flowing layer balance and lead to a three-layer system, with a layer of large particles over a layer of small particles, and, interposed between these layers, a layer of more densely packed mixed particles. The higher density mixed particle layer over the lower density small particle layer induces a Rayleigh-Taylor instability, evident as waviness in the interface between the layers. The waviness destabilizes into ascending plumes of small particles and descending plumes of mixed particles with large particles enriched near the surface, which become evident as small and large particle bands visible at the free surface. Rolls driven by segregation at the tilted interface between plumes maintain the pattern of frozen plumes. Published by the American Physical Society 2024

cond-mat.soft↗

arXiv2021

Mechanisms for recirculation cells in granular flows in rotating cylindrical rough tumblers

Friction at the endwalls of partially-filled horizontal rotating tumblers induces curvature and axial drift of particle trajectories in the surface flowing layer. Here we describe the results of a detailed discrete element method study of the dry granular flow of monodisperse particles in threedimensional cylindrical tumblers with endwalls and cylindrical wall that can be either smooth or rough. Endwall roughness induces more curved particle trajectories, while a smooth cylindrical wall enhances drift near the endwall. This drift induces recirculation cells near the endwall. The use of mixed roughness (cylindrical wall and endwalls having different roughness) shows the influence of each wall on the drift and curvature of particle trajectories as well as the modification of the free surface topography. The effects act in opposite directions and have variable magnitude along the length of the tumbler such that their sum determines both direction of net drift and the recirculation cells. Near the endwalls, the dominant effect is always the endwall effect, and the axial drift for surface particles is toward the endwalls. For long enough tumblers, a counter-rotating cell occurs adjacent to each of the endwall cells having a surface drift toward the center because the cylindrical wall effect is dominant there. These cells are not dynamically coupled with the two endwall cells. The competition between the drifts induced by the endwalls and the cylindrical wall determines the width and drift amplitude for both types of cells.

cond-mat.soft↗

arXiv2020

Self-Induced Rayleigh-Taylor Instability in Segregating Dry Granular Flows

Dry granular material flowing on rough inclines can experience a self-induced Rayleigh-Taylor (RT) instability followed by the spontaneous emergence of convection cells. For this to happen, particles are different in size and density, the larger particles are the denser but still segregate toward the surface. When the flow is, as usual, initially made of two layers, dense particles above, a Rayleigh-Taylor instability develops during the flow. When the flow is initially made of one homogeneous layer mixture, the granular segregation leads to the formation of an unstable layer of large-dense particles at the surface which subsequently destabilizes in a RT plume pattern. The unstable density gradient has been only induced by the motion of the granular matter. This self-induced Rayleigh-Taylor instability and the two-layer RT instability are studied using two different methods, experiments and simulations. At last, contrarily to the usual fluid behavior where the RT instability relaxes into two superimposed stable layers of fluid, the granular flow evolves to a pattern of alternated bands corresponing to recirculation cells analogous to Rayleigh-B{é}nard convection cells where segregation sustains the convective motion.

cond-mat.soft↗

arXiv2018

Evidence of reverse and intermediate size segregation in dry granular flows down a rough incline

In a dry granular flow, size segregation behave differently for a mixture containing a few large beads with a size ratio (S) above 5 (Thomas, Phys.Rev.E 62,96(2000)). For moderate large S, large beads migrate to an intermediate depth in the bed: this is called intermediate segregation. For the largest S, large beads migrate to the bottom: this is called reverse segregation (in contrast with surface segregation). As the reversal and intermediate depth values depend on the bead fraction, this numerical study mainly uses a single large tracer. Small fractions are also computed showing the link between a tracer behavior and segregation process. For half-filled rotating drum and for rough incline, two and three (3D) dimensional cases are studied. In the tumbler, trajectories of a large tracer show that it reaches a constant depth during the flow. For large S, this depth is intermediate with a progressive sinking when S increases. Largest S correspond to tracers at the bottom of the flow. All 3D simulation are in quantitative agreement with the experiments. In the flow down an incline, a large tracer reaches an equilibrium depth during flow. For large S, its depth is intermediate, inside the bed. For the largest S, its depth is reverse, near the bottom. Results are slightly different for thin or thick flow. For 3D thick flows, the reversal between surface and bottom positions occurs within a short range of S: no tracer stabilizes near mid-height and two reachable intermediate depth layers exist, below the surface and above the bottom. For 3D thin flows, all intermediate depths are reachable, depending on S. The numerical study of larger tracer fractions (5-10%) shows the 3 segregation patterns (surface, intermediate, reverse) corresponding to the 3 types of equilibrium depth. The reversal is smoother than for a single tracer. It happens around S=4.5, in agreement with experiments.

cond-mat.soft↗

arXiv2015

Influence of Rough and Smooth Walls on Macroscale Flows in Tumblers

Walls in discrete element method simulations of granular flows are sometimes modeled as a closely packed monolayer of fixed particles, resulting in a rough wall rather than a geometrically smooth wall. An implicit assumption is that the resulting rough wall differs from a smooth wall only locally at the particle scale. Here we test this assumption by considering the impact of the wall roughness at the periphery of the flowing layer on the flow of monodisperse particles in a rotating spherical tumbler. We find that varying the wall roughness significantly alters average particle trajectories even far from the wall. Rough walls induce greater poleward axial drift of particles near the flowing layer surface, but decrease the curvature of the trajectories. Increasing the volume fill level in the tumbler has little effect on the axial drift for rough walls, but increases the drift while reducing curvature of the particle trajectories for smooth walls. The mechanism for these effects is related to the degree of local slip at the bounding wall, which alters the flowing layer thickness near the walls, affecting the particle trajectories even far from the walls near the equator of the tumbler. Thus, the proper choice of wall conditions is important in the accurate simulation of granular flows, even far from the bounding wall.

cond-mat.soft↗

arXiv2015

Influence of Rough and Smooth Walls on Macroscale Granular Segregation Patterns

Size bidisperse granular materials in a spherical tumbler segregate into two different patterns of three bands with either small particles at the equator and large particles at the poles or vice versa, depending upon the fill level in the tumbler. Here we use discrete element method (DEM) simulations with supporting qualitative experiments to explore the effect of the tumbler wall roughness on the segregation pattern, modeling the tumbler walls as either a closely packed monolayer of fixed particles resulting in a rough wall, or as a geometrically smooth wall. Even though the tumbler wall is in contact with the flowing layer only at its periphery, the impact of wall roughness is profound. Smooth walls tend toward a small-large-small (SLS) band pattern at the pole-equator-pole at all but the highest fill fractions; rough walls tend toward a large-small-large (LSL) band pattern at all but the lowest fill fractions. This comes about because smooth walls induce poleward axial drift of small particles and an equator-directed drift for large particles, resulting in an SLS band pattern. On the other hand, rough walls result in both sizes of particles moving poleward at the surface of the flow, but due to radial segregation, small particles percolate lower in the flowing layer where there is a return drift toward the equator while large particles remain at the surface near the pole, resulting in an LSL band pattern. The tendency toward either of the two band patterns depends on the fill level in the tumbler and the roughness of the tumbler's bounding wall.

cond-mat.soft↗

arXiv2014

An efficient time domain representation for Single-Carrier Frequency Division Multiple Access

This paper presents a physical model for Single Carrier-Frequency Division Mutliple Access (SC-FDMA). We specifically show that by using mutlirate signal processing we derive a general time domain description of Localised SC-FDMA systems relying on circular convolution. This general model has the advantage of encompassing different implementations with flexible rates as well as additional frequency precoding such as spectral shaping. Based on this time-domain model, we study the Power Spectral Density (PSD) and the Signal to Interference and Noise Ratio (SINR). Different implementations of SC-FDMA are investigated and analytical expressions of both PSD and SINR compared to simulations results.

cs.IT↗

arXiv2004

Relation between dry granular flow regimes and morphology of deposits: formation of levees in pyroclastic deposits

Experiments on dry granular matter flowing down an inclined plane are performed in order to study the dynamics of dense pyroclastic flows. The plane is rough, and always wider than the flow, focusing this study on the case of laterally unconfined (free boundary) flows.We found that several flow regimes exist depending on the input fluxand on the inclination of the plane. Each flow regime corresponds to a particular morphology of the associated deposit. In one of these regimes, the flow reaches a steady state, and the deposit exhibits a levee/channel morphology similar to those observed on small pyroclastic flow deposits. The levees result from the combination between lateral static zones on each border of the flow and the drainage of the central part of the flow after the supply stops. Particle segregation featuresare created during the flow, corresponding to those observed on the deposits of pyroclastic flows. Moreover, the measurements of the deposit morphology (thickness of the channel, height of the levees, width of the deposit) are quantitatively related to parameters of the dynamics of the flow (flux, velocity, height of the flow), leading to a way of studying the flow dynamics from only measurements of the deposit. Some attempts to make extensions to natural cases are discussed through experiments introducing the polydispersity of the particle sizes and the particle segregation process.

cond-mat.stat-mech↗