Motivated by the understanding of shape effects in granular materials, we numerically investigate the macroscopic and microstructural properties of anisotropic dense assemblies of frictionless polydisperse rigid pentagons in shear flow, and compare them with similar systems of disks. Once subjected to large cumulative shear strains their rheology and microstructure are investigated in uniform steady states, depending on inertial number I, which ranges from the quasistatic limit (I similar to 10(-5)) to 0.2. In the quasistatic limit both systems are devoid of Reynolds dilatancy, i.e., flow at their random close packing density. Both macroscopic friction angle., an increasing function of I, and solid fraction., a decreasing function of I, are larger with pentagons than with disks at small I, but the differences decline for larger I and, remarkably, nearly vanish for I similar to 0.2. Under growing I, the depletion of contact networks is considerably slower with pentagons, in which increasingly anisotropic, but still well-connected force-transmitting structures are maintained throughout the studied range. Whereas contact anisotropy and force anisotropy contribute nearly equally to the shear strength in disk assemblies, the latter effect dominates with pentagons at small I, while the former takes over for I of the order of 10(-2). The size of clusters of grains in side-to-side contact, typically comprising more than 10 pentagons in the quasistatic limit, very gradually decreases for growing I.

}, keywords = {CONTACT DYNAMICS METHOD; DENSE GRANULAR FLOWS; GEOMETRIC ORIGIN; BEHAVIOR; PACKING; MEDIA; SHAPE}, issn = {1292-8941}, doi = {10.1140/epje/i2018-11608-9}, url = {http://link.springer.com/10.1140/epje/i2018-11608-9}, author = {Emilien Az{\'e}ma and Farhang Radja{\"\i} and Jean-Noel~ Roux} } @article {154, title = {Internal friction and absence of dilatancy of packings of frictionless polygons}, journal = {Physical Review E}, volume = {91}, year = {2015}, month = {Jan-30-2015}, pages = {Article Number: 010202}, abstract = {By means of numerical simulations, we show that assemblies of frictionless rigid pentagons in slow shear flow possess an internal friction coefficient (equalto0.183\±0.008 with our choice of moderately polydisperse grains) but no macroscopic dilatancy. In other words, despite side-side contacts tending to hinder relative particle rotations, the solidfraction under quasistatic shear coincides with that of isotropic random close packings of pentagonal particles. Properties of polygonal grains are thus similar to those of disks in that respect. We argue that continuous reshuffling of the force-bearing network leads to frequent collapsing events at the microscale, thereby causing the macroscopic dilatancy to vanish. Despite such rearrangements, the shear flow favors an anisotropic structure that is at the origin of the ability of the system to sustain shear stress.

}, issn = {1539-3755}, doi = {10.1103/PhysRevE.91.010202}, author = {Emilien Az{\'e}ma and Farhang Radja{\"\i} and Jean-Noel~ Roux} } @article {277, title = {Internal Structure of Inertial Granular Flows}, journal = {Physical Review Letters}, volume = {112}, year = {2014}, month = {Feb-21-2014}, pages = {Article Number: 078001}, abstract = {We analyze inertial granular flows and show that, for all values of the inertial number I, the effective friction coefficient \μ arises from three different parameters pertaining to the contact network and force transmission: (1)\ contact anisotropy, (2)\ force chain anisotropy, and (3)\ friction mobilization. Our extensive 3D numerical simulations reveal that \μ increases with I mainly due to an increasing contact anisotropy and partially by friction mobilization whereas the anisotropy of force chains declines as a result of the destabilizing effect of particle inertia. The contact network undergoes topological transitions, and beyond I≃0.1 the force chains break into clusters immersed in a background \“soup\” of floating particles. We show that this transition coincides with the divergence of the size of fluidized zones characterized from the local environments of floating particles and a slower increase of \μ with I.

}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.112.078001}, author = {Emilien Az{\'e}ma and Farhang Radja{\"\i}} } @article {317, title = {Identification of rolling resistance as a shape parameter in sheared granular media}, journal = {Physical Review E}, volume = {84}, year = {2011}, month = {Jul-28-2011}, pages = {Article Number: 011306 Part: 1}, abstract = {Using contact dynamics simulations, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. In simple shear conditions, we consider four aspects of the mechanical behavior of these systems in the steady state: shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. Our main finding is that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.

}, issn = {1539-3755}, doi = {10.1103/PhysRevE.84.011306}, author = {Estrada, Nicolas and Emilien Az{\'e}ma and Farhang Radja{\"\i} and Taboada, Alfredo} }