Abstract:  When solid heavy particles, e.g. sand grains, are entrained by a fluid flow over a non-erodible ground, they form  isolated dunes showing a remarkable crescentic shape with horns  pointing downstream. These dunes are commonly observed in deserts, with height of a few meters and velocity of a few tens of meters per year. Such dunes, known as barchan dunes, are also encountered in liquid flows, at  a much smaller, centimetric scale (e.g. in petroleum engineering). Their striking dynamical and stability properties are not well understood yet. We will present experiments on the dynamics of these dunes in turbulent channel flows, including measurements of the fluid velocity and stresses above the dune. These results will be discussed in the light of theoretical analyses of the physical processes of particle erosion and deposition, and of the turbulent flow above low hills. References: Charru F. and Hinch E. J. 2006 J. Fluid Mech. 550,  111 Mouilleron H., Charru F. and Eiff O. 2009 J. Fluid Mech. 628, 229 Lajeunesse E., Malverti L. and Charru F. 2010  J. Geophys. Res. 115,   F04001 Mini Bio:  PhD (1991) at the Institute of Fluid Mechanics of Toulouse (IMFT) now Professor of Mechanics at the Paul Sabatier University, Toulouse. Research interests: interfacial instabilities, viscous flows, granular flows (about 30 papers) Teaching: mechanics, thermodynamics, hydrodynamic instabilities Author of a book on "Hydrodynamic Instabilities" to appear next Spring at Cambridge University Press (with the help of Bud Homsy) Member of the French National Comittee of Scientific Research (CNRS) French Delegate at the General Assembly of the Intl Union of Theoretical and Applied Mechanics (IUTAM). Host:  Prof. Gary Leal, Chemical Engineering