Bulldozing an immersed granular material in a confined channel

The motion of an immersed granular material in a channel is characterised by complex interactions among the grains, between the grains and the permeating liquid, and between the grains and the channel walls. Here, we develop a reduced-order continuum model for the bulldozing of an immersed, sedimented granular material by a piston in a channel. In our continuum approach, the granular layer and the overlying fluid layer evolve as a system of coupled thin films. We model the granular phase as a dense, porous, viscoplastic material that experiences Coulomb-like friction with the walls. Conservation of mass and momentum under a thin-film approximation lead to an elliptic equation for the velocity of the grains that is coupled with an evolution equation for the height of the granular layer. We solve our model numerically for a variety of different scenarios to explore the interactions between wall friction, internal viscous-like stresses, and fluid flow above and through the granular layer. We complement our numerical results with a series of experiments that provide insight into the validity and limitations of the model.