  Dense granular systems that consist of particles of disparate sizes segregate
based on size during flow, resulting in complex, coupled segregation and flow
patterns. The ability to predict how granular mixtures segregate is important
in the design of industrial processes and the understanding of geophysical
phenomena. The two primary drivers of size-segregation are pressure gradients
and shear-strain-rate gradients. In this work, we isolate size-segregation
driven by shear-strain-rate gradients by studying two dense granular flow
geometries with constant pressure fields: gravity-driven flow down a long
vertical chute with rough parallel walls and annular shear flow with rough
inner and outer walls. We perform discrete element method (DEM) simulations of
dense flow of bidisperse granular systems in both flow geometries, while
varying system parameters, such as the flow rate, flow configuration size,
fraction of large/small grains, and grain-size ratio, and use DEM data to
inform continuum constitutive equations for the relative flux of large and
small particles. When the resulting continuum model for the dynamics of
size-segregation is coupled with the nonlocal granular fluidity model (a
nonlocal continuum model for dense granular flow rheology), we show that both
flow fields and segregation dynamics may be simultaneously captured using this
coupled, continuum system of equations.
