New patterns in high-speed granular flows

We report on new patterns in high-speed flows of granular materials obtained by means of extensive numerical simulations. These patterns emerge from the destabilization of unidirectional flows upon increase of mass holdup and inclination angle, and are characterized by complex internal structures including secondary flows, heterogeneous particle volume fraction, symmetry breaking and dynamically maintained order. In particular, we evidenced steady and fully developed "supported" flows, which consist of a dense core surrounded by a highly energetic granular gas. Interestingly, despite their overall diversity, these regimes are shown to obey a scaling law for the mass flow rate as a function of the mass holdup. This unique set of 3D flow regimes raises new challenges for extending the scope of current granular rheological models

Periodic saltation over hydrodynamically rough beds: Aeolian to aquatic

International audienceWe determine approximate, analytical solutions for average, periodic trajectories of particles that are accelerated by the turbulent shearing of a fluid between collisions with a hydrodynamically rough bed. We indicate how the viscosity of the fluid may influence the collisions with the bed. The approximate solutions compare well with periodic solutions for average periodic trajectories over rigid-bumpy and erodible beds that are generated numerically. The analytic solutions permit the determination of the relations between the particle flux and the strength of the shearing flow over a range of particle and fluid properties that vary between those for sand in air and sand in water

Modèle minimal pour les dunes transverses.

National audienceNous présentons ici un modèle minimal pour décrire la stabilité des dunes transverses. Ce modèle qui s'inspire de celui proposé par NIIYA et al. (2010) est basé sur une représentation simplifiée d'une dune transverse. Celle-ci est décrite par une succession de sections longitudinales qui interagissent entre elles par l'intermédiaire de flux sédimentaires latéraux. Les coupes longitudinales sont définies à partir de caractéristiques géométriques simples. La position et la hauteur de leur sommet suffisent ainsi à les caractériser entièrement. Un bilan des flux sédimentaires dans la direction longitudinale et latérale permet d'obtenir un système de deux équations couplées décrivant l'évolution spatio-temporelle de la position et de la hauteur de la ligne de crête. Ce modèle nous permet de conduire une analyse de stabilité linéaire d'une dune transverse rectiligne et d'identifier les processus physiques susceptibles de la déstabiliser au profit d'une dune sinueuse ou de la fragmenter en une multitude de dunes plus petites. Nous avons identifié deux paramètres importants dans le processus de stabilité : le taux de capture de sédiment par la face d'avalanche et les coefficients de diffusion transverses. Ce modèle simple constitue un outil intéressant pour étudier la dynamique complexe de dunes 3D

Impact of a Projectile on a Granular Medium Described by a Collision Model

International audienceWe propose a model for the propagation of energy due to the impact of a granular projectile on a dense granular medium. Energy is transferred from grain to grain during binary collision events. The transport of energy may then be viewed as a random walk with a split of energy during successive collisions. There is a qualitative and quantitative agreement between this simple description and experimental results

Laboratory studies of aeolian sediment transport processes on planetary surfaces

International audienceWe review selected experimental saltation studies performed in laboratory wind tunnels and collision experiments performed in (splash-) laboratory facilities that allow detailed observations between impinging particles on a stationary bed.We also discuss progress in understanding aeolian transport in nonterrestrial environments. Saltation studies in terrestrial wind tunnels can be divided into two groups. The first group comprises studies using a short test bed, typically 1–4m long, and focuses on the transitional behavior near the upwind roughness discontinuity where saltation starts. The other group focuses on studies using long test beds — typically 6 m or more — where the saturated saltation takes place under equilibrium conditions between wind flow and the underlying rough bed. Splash studies using upscaled model experiments allow collision simulations with large spherical particles to be recorded with a high speed video camera. The findings indicate that the number of ejected particles per impact scales linearlywith the impact velocity of the saltating particles. Studies of saturated saltation in several facilities using predominantly Particle Tracking Velocimetry or Laser Doppler Velocimetry indicate that the velocity of the (few) particles having high trajectories increases with increasing friction velocity. However, the speed of the majority of particles that do not reachmuch higher than Bagnold's focal point is virtually independent of Shields parameter—at least for lowor intermediate u⁎-values. In this case mass flux depends on friction velocity squared and not cubed as originally suggested by Bagnold. Over short beds particle velocity shows stronger dependence on friction velocity and profiles of particle velocity deviate from those obtained over long beds. Measurements using horizontally segmented traps give average saltation jump-lengths near 60–70 mm and appear to be only weakly dependent on friction velocity, which is in agreement with some, but not all, older or recent wind tunnel observations. Similarly some measurements performed with uniform sand samples having grain diameters of the order of 0.25–0.40mmindicate that ripple spacing depends on friction velocity in a similar way as particle jump length. The observations are thus in agreementwith a recent ripple model that link the typical jump length to ripple spacing. A possible explanation for contradictory observations in some experiments may be that long observation sequences are required in order to assure that equilibrium exists between ripple geometry and wind flow.Quantitative understanding of saltation characteristics onMars still lacks important elements. Based upon image analysis and numerical predictions, aeolian ripples have been thought to consist of relatively large grains (diameter N 0.6mm) and that saltation occurs at high wind speeds (N26 m/s) involving trajectories that are significantly longer than those on Earth (by a factor of 10–100). However, this is not supported by recent observations from the surface of Mars, which shows that active ripples in their geometry and composition have characteristics compatible with those of terrestrial ripples (Sullivan et al., 2008). Also the highest average wind speeds on Mars have been measured to be b20 m/s, with even turbulent gusts not exceeding 25 m/s. Electrification is seen as a dominant factor in the transport dynamics of dust onMars, affecting the structure, adhesive properties and detachment/entrainment mechanisms specifically through the formation of aggregates (Merrison et al., 2012). Conversely for terrestrial conditions electric fields typically observed are not intense enough to significantly affect sand transport rates while little is known in the case of extra-terrestrial environments

Report on the Program “Fluid-mediated particle transport in geophysical flows” at the Kavli Institute for Theoretical Physics, UC Santa Barbara, September 23 to December 12, 2013

International audienceThe KITP program held at UC Santa Barbara in the fall of 2013 addressed the dynamics of dispersed particulate flows in the environment. By focusing on the prototypes of Aeolian transport and turbidity currents, it aimed to establish the current state of our understanding of such two-phase flows, to identify key open questions, and to develop collaborative research strategies for addressing these questions. Here we provide a brief summary of the program outcome. Introduction Flows of a continuous fluid phase containing dispersed particles represent a ubiquitous phenomenon, with numerous applications in nature and technology. They can give rise to a great variety of qualitatively distinct flow regimes governed by different balances of inertial, viscous, gravitational and interparticle forces, depending on such aspects as the density ratio between particles and fluid, the nature of the particle-particle interactions, on whether the flows are dilute or concentrated, conservative or nonconservative, and Newtonian or non-Newtonian in nature, to name just a few. Even the narrower field of geophysical particle-laden flows covers a wide variety of phenomena, ranging from Aeolian transport, dust storms and powder snow avalanches to volcanic ash plumes, sediment transport in rivers, estuaries and oceans, and dense pyroclastic and debris flows. While all of the above flows have distinctly different features, they nevertheless share a number of common aspects as well. To advance our capabilities to describe flows of this nature, the community will have to draw heavily on such fundamental research areas as the physics of suspensions and granular flows. The KITP program aimed to review the current state of our understanding of such flows, to identify the key open questions that remain, and to develop collaborative research strategies for addressing these questions via a combination of laboratory experiments, computational investigations and field observations

Granular flows on a dissipative base

International audienceWe study inclined channel flows of sand over a sensor-enabled composite geotextile fabric base that dissipates granular fluctuation energy. We record strain of the fabric along the flow direction with imbedded fiber-optic Bragg gratings, flow velocity on the surface by correlating grain position in successive images, flow thickness with the streamwise shift of an oblique laser light sheet, velocity depth profile through a transparent side wall using a high-speed camera, and overall discharge rate. These independent measurements at inclinations between 33 • and 37 • above the angle of repose at 32.1 ± 0.8 • are consistent with a mass flow rate scaling as the 3/2 power of the flow depth, which is markedly different than flows on a rigid bumpy boundary. However, this power changes to 5/2 when flows are forced on the sand bed below its angle of repose. Strain measurements imply that the mean solid volume fraction in the flowing layer above the angle of repose is 0.268 ± 0.033, independent of discharge rate or inclination

Rheology of confined granular flows

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