Showing papers by "Dov Levine published in 2002"

Geometry of frictionless and frictional sphere packings

Abstract: We study static packings of frictionless and frictional spheres in three dimensions, obtained via molecular dynamics simulations, in which we vary particle hardness, friction coefficient, and coefficient of restitution. Although frictionless packings of hard spheres are always isostatic (with six contacts) regardless of construction history and restitution coefficient, frictional packings achieve a multitude of hyperstatic packings that depend on system parameters and construction history. Instead of immediately dropping to four, the coordination number reduces smoothly from $z=6$ as the friction coefficient $\ensuremath{\mu}$ between two particles is increased.

Geometry of frictionless and frictional sphere packings.

Abstract: We study static packings of frictionless and frictional spheres in three dimensions, obtained via molecular dynamics simulations, in which we vary particle hardness, friction coefficient, and coefficient of restitution. Although frictionless packings of hard spheres are always isostatic (with six contacts) regardless of construction history and restitution coefficient, frictional packings achieve a multitude of hyperstatic packings that depend on system parameters and construction history. Instead of immediately dropping to four, the coordination number reduces smoothly from $z=6$ as the friction coefficient $\ensuremath{\mu}$ between two particles is increased.

Analogies between granular jamming and the liquid-glass transition.

Abstract: Based on large-scale, three-dimensional chute flow simulations of granular systems, we uncover strong analogies between the jamming of the grains and the liquid-glass transition. The angle of inclination $\ensuremath{\theta}$ in the former transition appears as an analog of temperature T in the latter. The transition is manifested in the development of a plateau in the contact normal force distribution $P(f)$ at small forces, the splitting of the second peak in the pair-correlation function $g(r),$ and increased fluctuations of the system energy. The static state also exhibits history dependence, akin to the quench-rate dependence of structural properties of glasses, due to the hyperstaticity of the contact network.

Boundary effects and self-organization in dense granular flows

Abstract: Boundary effects in gravity-driven, dense granular flows down inclined planes are studied using large-scale molecular dynamics simulations. We find that the flow behavior and structure of the flowing pile changes dramatically as we vary the roughness of the supporting base. For a rough, bumpy base, there are three principal flow regimes that depend on the inclination angle θ: at small angles θ θmax, where θmax is the maximum angle for which stable, steady state flow exists, the flow is unstable; and for θr<θ<θmax, the energy input from gravity is balanced by that dissipated through friction and the system reaches a stable, steady state flow. In the stable regime, we find no slip boundary conditions with a bulk density that is independent of the height above the base. For a chute base that is ordered, the steady state regime splits into a further three distinct flow regimes: at lower angles, the flowing system self-organizes ...

Granular Glass Transition

Abstract: The transition from a flowing to a static state in a granular material is studied using large-scale, 3D particle simulations. Similar to glasses, this transition is manifested in the development of a plateau in the contact normal force distribution P(f) at small forces, along with the splitting of the second peak in the pair correlation function g(r), suggesting compaction and local ordering. The mechanical state changes from one dominated by plastic intergrain contacts in the flowing state to one dominated by elastic contacts in the static state. We define a staticity index that determines how close the system is to an isostatic state, and show that for our systems, the static state is not isostatic.

Rheology of dense granular flow

Abstract: We have performed numerical studies of dense granular flows on an incline with a rough bottom in two and three dimensions. This flow geometry produces a constant density prole that satises scaling relations of the Bagnold, rather than the viscous, kind. No surface-only flows were observed. The bulk and the surface layer dier in their rheology, as evidenced by the change in principal stress directions near the surface; a Mohr{ Coulomb type failure criterion is seen only near the surface. In the bulk, normal stress anomalies are observed both in two and in three dimensions. We do not observe isostaticity in static frictional piles obtained by arresting the flow.