Article: Valance, A; and Louge, M; (2015) “Granular Physics”, Comptes Rendus Physique, 16(1):1-2
Abstract: Granular media play a major role in geophysics and industrial processes. Their interactions are complicated by relatively small-scale separation between individual particles and system size, by the presence of other interpenetrating phases such as water or air, by the large number of grains involved in realistic applications, and by the importance of microscopic contact forces, such as solid friction, which are challenging to measure or control. Yet significant progress has been made in the last two decades toward the understanding of granular media, thanks to the curiosity of physicists and engineers. This thematic issue gathers contributions from researchers dealing with diverse aspects of granular mechanics, from static assemblies to flowing suspensions, and from theory to natural phenomena. These review articles illustrate rather different approaches to these complicated systems.
In quasi-static assemblies of grains, the three reviews of Radjai, Behringer, and Desrues & Endo discuss how force chains, jamming and localization contribute to stresses and deformation of the granular medium. In this highly non-linear problem, a challenge is to relate the geometrical fabric to force transmission at grain contacts, so that a continuum description of the material can bring hope to model the assembly at the macroscopic scale. These reviews complement a recently released thematic issue of Comptes rendus Mecanique edited by François Nicot and Félix Darve dealing with “Granular and polycrystalline solid mechanics”, in which different macroscopic descriptions of granular assemblies are introduced.
As a gradual transition to flowing systems, van Hecke reviews how, as contacts are created and lost in relatively slow deformations of the system, hysteretic cycles of jamming and yield are governed by small fluctuations. Delannay, Duranteau & Tournat show that avalanches, which mark the transition between a static and a flowing system, are foretold by precursor events consisting of intermittent particle rearrangements that can be observed in numerical simulations and detected through their acoustic emissions in experiments.
When grains are involved in rapid motion and shearing, such as flows on inclines, Kumaran considers theoretical approaches derived from a granular analogue of the Maxwell–Boltzmann kinetic theory of dense gases. Jop then summarizes successes and limitations of a pragmatic bulk rheology for dense flows. Gray & Gajjar review how heterogeneous flows of grains segregate by size. They outline a mixture framework that captures segregation and mixing observed in experiments, including the formation of shocks and fans in particle species concentration.
This thematic issue also discusses large-scale natural granular-fluid systems. Turnbull, Bowman & McElwaine review experiments and modeling of highly destructive debris flows, which hurl mixtures of rock, water, and sediments down mountain slopes. Sovilla, McElwaine & Louge provide a composite description of the evolving structure in high-speed powder snow avalanches from recent detailed measurements on a whole mountain. Valance, Rasmussen, Ould El Moctar & Dupont review models and experiments of Aeolian sand transport. Finally, Courrech du Pont reviews the intriguing dynamics of mobile sand dunes.