This review article provides an overview of dry granular flows and particle fluid mixtures, including experimental and numerical modeling at the laboratory scale, large scale hydrodynamics approaches and field observations. Over the past ten years, the theoretical and numerical approaches have made such significant progress that they are capable of providing qualitative and quantitative estimates of particle concentration and particle velocity profiles in steady and fully developed particulate flows. The next step which is currently developed is the extension of these approaches to unsteady and inhomogeneous flow configurations relevant to most of geophysical flows. We also emphasize that the up-scaling from laboratory experiments to large scale geophysical flows still poses some theoretical physical challenges. For example, the reduction of the dissipation that is responsible for the unexpected long run-out of large scale granular avalanches is not observed at the laboratory scale and its physical origin is still a matter of debate. However, we believe that the theoretical approaches have reached a mature state and that it is now reasonable to tackle complex particulate flows that incorporate more and more degrees of complexity of natural flows.

We observed experimentally a new regime for granular flows in an inclined channel with a flow-rate-controlled system. For high flow rates, the flow occurs atop a static granular heap whose angle is considerably higher than those usually exhibited by granular heaps. The properties of such superstable heaps (SSH) are drastically affected by a change in the channel width $W$. This indicates that the unusual stability of these heaps can be accounted for by the flowing layer and its friction on the sidewalls. A simple depth-averaged model, assuming Coulomb friction, shows that the SSH angle scales as $h/W$ ($W$ being the channel width), and that grain size plays no part.

Several acquisitions of x-ray microtomography have been performed on a beads packing while it compacts under vertical vibrations. An image analysis allows to study the evolution of the packing structure during its progressive densification. In particular, the volume distribution of the pores reveals a large tail, compatible to an exponential law, which slowly reduces as the system gets more compact. This is quite consistent, for large pores, with the free volume theory. These results are also in very good agreement with those obtained by a previous numerical model of granular compaction.

We study fully developed, steady granular flows confined between parallel flat frictional sidewalls using numerical simulations and experiments. Above a critical rate, sidewall friction stabilizes the underlying heap at an inclination larger than the angle of repose. The shear rate is constant and independent of inclination over much of the flowing layer. In the direction normal to the free surface, the solid volume fraction increases on a scale equal to half the flowing layer depth. Beneath a critical depth at which internal friction is invariant, grains exhibit creeping and intermittent cage motion similar to that in glasses, causing gradual weakening of friction at the walls.