United States

Through numerical simulations, we investigate impact generated seismic wave transmission in gran-ular media under extremely low pressure. This mimics the conditions in the interior of asteroids and other smallplanetary bodies. We find a dependency not only on the overburden pressure on the medium, but also on thevelocity of the impact that generates the wave. This is, at extremely low values of overburden pressure, the wavespeed depends no only on the imposed pressure, but also on the increment in pressure created by the passingof the wave. We study crystalline and random packings and find very similar behaviour though with differentwave speeds as expected. We then relate our results to different mission-related events on asteroids.

Powders &amp; Grains 2021

Seismic waves in the asteroid environment

[EPJ Web of Conferences, Volume 249 (2021)](https://www.epj-conferences.org/articles/epjconf/abs/2021/03/contents/contents.html)

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Powders & Grains 2021

Powders & Grains 2021 will be held in Buenos Aires (Argentina, VIRTUAL) July 5, 13, 21, 29 and Aug 6, 2021. The aim of the conference is to give an up-to-date picture of the broad research field of granular media, powders and grains. Contributions from experts around the world will focus on research topics that cover a wide range of open problems in the field.

technical paper

A very staggering result that has been constantly highlighted in granular media is that the shear strength of granular assemblies is independent of the particle size dispersity. In other words, a packing com- posed of monodisperse particles has similar strength properties to those of polydisperse systems. This has been shown numerically for the simplified case of disc and polygon assemblies in 2D and spheres in 3D. In this pa- per, we use three-dimensional contact dynamics simulations to revisit these results for the more complex case of assemblies composed of highly polydisperse rigid polyhedra. Although non-spherical shapes induce more intricated spatial correlations than spherical shapes because of the multiple contact types (i.e., vertex-face, edge- edge, edge-face, face-face), our numerical data provide evidence that the shear strength independence as the particle size dispersity increases still holds up for assemblies of polyhedra. We explain this finding from com- pensation mechanisms at the micro-scale between geometrical and mechanical anisotropies developed within the assemblies.

Independence of shear strength with particle size dispersity still valid in polyhedral particle assemblies

This work aims to investigate the eect of fines in dierent type of bi-disperse grain size distributions in terms of minimum/maximum density, compressibility and crushing. The material adopted is the Light Expanded Clay Aggregate (LECA), an artificial granular material characterized by light, porous and crushable grains. The bi-disperse grading are firstly analysed in terms of packing density, measuring experimentally the minimum and the maximum porosity for dierent combination of sizes and volume proportions. Then, some selected mixtures are subjected to one-dimensional compression tests up to high pressures. Evolution of grain size due to grain crushing phenomena and compressibility are therefore measured and interpreted. Finally a theoretical model is adopted in order to predict the mechanical material response accounting grain crushing and granular microstructure.

Packing density of bi-disperse mixtures under one-dimensional compression

Quasi-periodic collective displacements of grains at the free surface of a tilted grain packing constitute precursors of granular avalanches. Laboratory experiments are commonly performed by slowly tilting the packing from 0° to the maximal stability angle \theta_A. In these conditions, the number of precursors is too small to assess reproducible and robust statistical analyses of the precursor activity. To go beyond this limitation, we have developed a specific experimental protocol consisting of tilting the packing with successive oscillation cycles. We use a high-resolution optical camera and process the images of the packing free surface to identify precursory events during many consecutive cycles of a single packing. We observe the same behavior for all half-cycles, forth and back: appearance of the first precursors after the same variation of inclination, exponential evolution of the weak surface activity for the first precursors and linear growth of stronger surface activity for the following ones. The experimental protocol provides both reproducible precursor measurements based on large sample statistical inferences and a quasi-stationary state after one full-cycle. This approach is very promising for highlighting the effects of external parameters, including humidity and packing geometry.


Robust experimental study of avalanche precursory events based on reproducible cycles of grain packing destabilizations

Spin rates of minor planets or asteroids are known to have been affected by several agents including but not limited to tidal fly-bys, impacts, and solar radiation. Surface processes like landslides occur as a result of such rotational changes. We study the evolution of landslides on top-shaped rubble pile asteroids like 101955 Bennu and 162173 Ryugu, with the underlying core modeled as two solid cones fused back to back. Using a depth-averaged avalanche theory applicable to granular flows we solve for axisymmetric landslides occurring at various spin rates and regolith friction. Static regions on the surface corresponding to different spin rates are identified from an equilibrium analysis. We then solve for landslides initiated at different latitudes. It is found that landslides equilibrate at lower latitudes as the spin rate is increased. Beyond a critical spin rate regolith is shed from the equator. This critical spin is higher for a lower value of the semi-apex angle of the cone.

Axisymmetric landslides on top-shaped asteroids

Coordination number controls elastic moduli, seismic velocity, and force transmission in sands and is thus a critical factor controlling the resistance of sands to deformation. Previous studies quantified relationships between coordination number, porosity, grain size, sphericity, and effective stress in pluviated or modeled sands. Here, we determine if these relationships hold in naturally-deposited beach sands. We collect samples while preserving their microstructures and use x-ray computed microtomography images to characterize grain properties. Similar to pluviated and modeled sand studies, we find that average coordination numbers and porosities for freshly deposited natural sands are 8.1 ± 2.8 and 0.37 ± 0.01, respectively. The range and standard deviation in coordination numbers of the natural beach sands are, however, significantly higher than observed in pluviated and modeled sand studies. At the same effective stress and porosities, coordination number is linearly proportional to grain surface area except for the smallest and largest grains. Coordination number depends non-linearly on sphericity. We attribute the higher ranges and standard deviations of coordination numbers in the natural sands to its broader grain size distribution, and we propose that the largest grains limit grain rearrangement, which influences spatial distributions of coordination numbers in natural sands.

Coordination numbers in natural beach sand

This presentation includes a numerical technique to prepare dense 2D disc samples with loose contact network using DEM under periodic boundary conditions. We mainly focus on a numerical procedure to control void ratio (e) and coordination number (Z*) for two kinds of samples. It has been achieved by using various interparticle friction coefficient values during isotropic compression and shearing. One dense and one loose sample has been prepared by varying the preparation procedure. For the loose sample we control interparticle friction coefficient values to control density while for the dense sample we add an extra step of dilation thus controlling void ratio and coordination number quasi-independently. As a result of this difference in preparation the dense sample macroscopic angle of friction under shearing has been dictated by its initial void ratio instead of coordination number.

Sample preparation of dense granular materials. Influence of void ratio e and coordination number Z* on the mechanical behaviour at failure

Fluid-driven grain flow through a confined channel filled with non-buoyant grains is herein observed to exist in three regimes according to total imposed flow rate. (1) At low imposed flow rates, no grain flow occurs as the fluid stress is insufficient to mobilise the grains and Darcy flow is observed. (2) At a sufficient imposed flow rate, grains begin to flow at the top of the channel with self-similar Gaussian velocity profiles that become faster and encroach deeper into the channel with increased flow rate. (3) At high flow rates, significant grain flow occurs at the base of the channel, distorting the Gaussian profile, resulting in a gradual transition towards a more symmetric, full-channel flow. Each regime, and the transitions between them, is discussed in relation to experimental grain velocity measurements.

From Darcy to Gaussian to fully mobilised grain flow in a confined channel

Granular packings exhibit significant changes in rheological and structural properties when the rotational symmetry of spherical or circular particles is broken. Here, we report on experiments exploring the differences in dynamics of a grain-scale intruder driven through a packing of either disks or pentagons, where the presence of edges and vertices on grains introduces the possibility of rotational constraints at edge-edge contacts, as well as differences in grain-scale dynamics and stress transmission through the granular material. We observe that the intruder’s stick-slip dynamics are comparable between the disk packing near the frictional jamming fraction and the pentagonal packing at significantly lower packing fractions. We connect this stark contrast in packing fraction with the average speed, rotation, and vorticity fields of grains during slip events, finding that rotation of pentagons is limited and the flow of pentagonal grains is largely confined in front of the intruder, whereas disks rotate more on average and circulate around the intruder to fill the open channel behind it. We lastly observe that the packing of pentagons does not transmit stresses as far around the system as does the packing of disks, even at comparable applied forces from the intruder, and that disks transmit back-bending stress chains more frequently and with greater spatial extent than pentagons. Measures of spatial extent of stress transmission collapse when packing fraction is rescaled with respect to the packing fraction of open channel formation, though intruder dynamics do not collapse. Our results indicate that grain-scale rotation constraints significantly modify collective motion of grains on mesoscopic scales and correspondingly enhance resistance to penetration of a local intruder.

Particle dynamics in two-dimensional point-loaded granular media composed of circular or pentagonal grains

Granular asteroids are naturally occurring gravitational aggregates (rubble piles) bound together by gravitational forces. For this reason, it is reasonable to use the theoretical concepts and numerical tools developed for granular media to study them. In this paper, we extend the field of applicability of the Contact Dynamic (CD) method, a class of non-smooth discrete element approach, for the simulation of three-dimensional granular asteroids. The CD method is particularly relevant to address the study of dense granular assemblies of a large number of particles of complex shape and broad particles size distribution since it does not introduces numerical artefacts due to contact stiffness. We describe how the open-source software LMGC90, interfaced with an external library for the calculation of self-gravity, is used to model the accretion process of spherical and irregular polyhedral particles.

A Contact Dynamics open source code for the simulation of granular asteroid with realistic regolith shapes. Application to the accretion process

Experiments on the discharge of a silo with an inclined outlet are performed using flattened seeds in order to evaluate the validity of a previous theoretical formulation developed in our work group [1]. In that description, funnel flow regime is assumed to be based on a free fall parabolic arc. The shape of this arc is described with a parameter which is the only one involved in the flow rate formulation. An experimental analysis of the behavior of this parameter is carried out based on the geometry and shape of the grains within the silo. Also, video analysis of the silo discharge is performed in order to investigate the velocity profiles at the outlet
of the hopper for these non-spherical particles. Experiments are contrasted with analytical predictions derived from the proposed formulation in order to assess and discuss its validity for the case of flattened particles.

Seismic waves in the asteroid environment

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