Abstract:  When particle beds are submitted to shearing flows, the particles at the surface of the bed can move as soon as hydrodynamic forces acting on them exceed a fraction of their apparent weight. This situation occurs in a wide variety of natural phenomena, such as sediment transport in rivers or by air, and in industrial processes, such as hydrate or sand issues in oil production and granular transport in food or pharmaceutical industries. A very common feature that arises is the formation of ripples, i.e. small waves on the bed surface, or of dunes, i.e. larger mounds or ridges. This talk will examine a bed constituted of sediment particles submitted to a pipe flow. It will discuss (i) the critical condition for incipient motion of the grains, (ii) bed-load transport, and (iii) the different dune patterns that arise as the flow is increased from the laminar to the turbulent regimes. The theoretical investigation uses a two-phase model having a granular rheology for the particulate phase. Calculations are performed numerically but also analytically in asymptotic cases. These predictions are compared to experimental results obtained in laminar flow above a bed composed of spherical particles in a pipe. Bio:  Élisabeth Guazzelli is Senior Researcher (Directeur de Recherche) at the CNRS (Centre National de la Recherche Scientifique) and affiliated with the IUSTI Laboratory of Polytech’Marseille (Aix-Marseille Université), of which she is Vice Director. A physicist by training, her research interests are in the field of particulate multiphase flows, such as granular media, fluidized beds, suspensions, and sedimentation, and in these fields, she is one of the world leaders. She is responsible for a very active and diversified research group in Marseille composed of ten people. Since 2005, she has been an Associate Editor of the Journal of Fluid Mechanics. Host: Eckart Meiburg