Abstract :

Granular segregation, a sorting phenomenon of particles by size within a polydisperse granular flow, creates many industrial problems and has strong implications for geophysical flows. In particular in sediment transport, the river bed spontaneously organizes itself into different size layers of grains, which modifies the mobility of the sediment bed.

In this presentation, a numerical and theoretical study of vertical size segregation in sediment transport is proposed. A discrete element method (for the granular phase) coupled with a fluid model is used to study the infiltration dynamics of fine grains in a bed formed of coarser grains. The mechanisms controlling the segregation velocity are analyzed. This infiltration dynamics is then analyzed with an advection-diffusion model which perfectly reproduces the discrete simulations and which makes it possible to clearly define the form of the advection and diffusion terms.

Secondly, the presentation will be devoted to the consequences of segregation on sediment transport, by studying the mobility of an already segregated bed (coarse particles above fine particles). Discrete simulations show that bidispersity increases transport. This effect is quantitatively described by a rheological analysis where it is shown that the buried fine particles play the role of a “conveyor belt” for the above large particles.