  Size segregation in bedload transport is studied numerically with a coupled
fluid-discrete element model. Starting from an initial deposit of small
spherical particles on top of a large particle bed, the segregation dynamics of
the bed is studied as it is driven by the fluid flow. Focusing on the
quasi-static part of the bed, the small particles are observed to segregate as
a layer of constant thickness at a velocity constant in time and independent of
the number of small particles. The segregation velocity is observed to be
directly linked to the inertial number at the bottom of the layer, and to
increase linearly with the size ratio. While the macroscopic behavior is
independent of the concentration in small particles, an analysis in the
framework of the continuous model of (Thornton et al. 2006) shows that the
dynamics results from an equilibrium between the influence of the local
concentration and the inertial number forcing. Deriving an analytical solution
of the continuous model, it is shown that the diffusion coefficient should have
the same dependency on the inertial number as the segregation flux.
Implementing the segregation and the diffusive fluxes in the continuous model,
it is shown that they quantitatively reproduce the discrete simulations. These
results improve the understanding of the size segregation dynamics and
represent a step forward in the upscaling process of polydisperse granular flow
in the context of turbulent bedload transport.
