Granular Flows

Those who have played with sand on the beach or with sugar in their kitchen are aware that a collection of solid grains can behave macroscopically like a liquid and flow. However, the description of this peculiar fluid still represents a challenge due to the lack of constitutive laws able to describe the rich phenomenology observed. In this paper we first review the properties of dry granular flows and we present recent advances in our understanding of their rheological behavior. The success and limits of a simple visco-plastic model recently developed is presented. In a second part of the paper, we go beyond the simple and ideal situation where the material is made of grains having the same size and interacting by contact interactions only. We present studies on more complex and realistic granular medium like polydispersed media, like cohesive granular media and like granular pastes made of grains immersed in a liquid. We analyse to which extend the progress made in our description of monodispersed dry granular flows can help understanding these more complex granular media.