Segregation and armoring in bidisperse granular beds sheared by viscous liquids

Abstract

A granular bed sheared by a viscous flow undergoes a complex motion, with fluid-like (considerable rate of deformation) and solid-like (slowly deforming) regions, known as bedload and creep, respectively. This is commonly found in river beds, for instance, where, in addition, the bed consists of polydisperse grains. In such cases, the bed structure changes considerably along time, with bed hardening and grain segregation taking place simultaneously. As a consequence, the transport rate of grains decreases as time goes on. In this talk, I will show our findings on the short- and long-time responses of a bidisperse bed sheared by a viscous liquid. For that, we carried out experiments in an annular flume filled with grains and oil, and we made use of RIM (refractive index matching) visualizations (Gonzalez et al., Phys. Fluids, 2023). On the top of the flume, a rotating lid imposed a Couette-type oil flow that entrained grains of the granular bed. We show that most of segregation occurs during the very first stages of the flow, that bed hardening becomes stronger while bedload and creep weaken along time, and that creep and hardening continue over long time scales (140 h, in our experiments). We found the segregation rates, their variation with the applied shearing and time, the time evolution of creeping and bedload, and we propose characteristic times for both the segregation (upward motion of large particles) and bed hardening. In addition, we carried out CFD-DEM (computational fluid mechanics – discrete element method) simulations, and show that the direct action of fluid forces is significant for the upward motion of large grains in the middle and upper parts of the bedload layer, while only contact forces are significant in the creep layer and lower part of the bedload layer (Gonzalez and Franklin, Phys. Fluids, under review). Our results shed light on the time evolution of polydisperse granular beds under the action of fluid flows, such as happens in river beds.