Rheology of mobile sediment beds in laminar shear flow: effects of creep and polydispersity

TL;DR

This study investigates the rheological behavior of polydisperse sediment beds under laminar shear flow using detailed numerical simulations, extending existing models to include polydispersity and creeping regimes.

Contribution

It introduces an extended $d$-rheology model incorporating polydispersity and creep effects, validated against experimental data for sediment transport modeling.

Findings

Excellent agreement with experiments for monodisperse sediments.

Extended rheology parameters as functions of maximum particle volume fraction.

Friction coefficient approaches a finite value in the creeping regime.

Abstract

Classical scaling relationships for rheological quantities such as the $\mu(J)$-rheology have become increasingly popular for closures of two-phase flow modeling. However, these frameworks have been derived for monodisperse particles. We aim to extend these considerations to sediment transport modeling by using a more realistic sediment composition. We investigate the rheological behavior of sheared sediment beds composed of polydisperse spherical particles in a laminar Couette-type shear flow. The sediment beds consist of particles with a diameter size ratio of up to ten, which corresponds to grains ranging from fine to coarse sand. The data was generated using fully coupled, grain resolved direct numerical simulations using a combined lattice Boltzmann - discrete element method. These highly-resolved data yield detailed depth-resolved profiles of the relevant physical quantities that determine the rheology, i.e., the local shear rate of the fluid, particle volume fraction, total shear, and granular pressure. A comparison against experimental data shows excellent agreement for the monodisperse case. We improve upon the parameterization of the $\mu(J)$-rheology by expressing its empirically derived parameters as a function of the maximum particle volume fraction. Furthermore, we extend these considerations by exploring the creeping regime for viscous numbers much lower than used by previous studies to calibrate these correlations. Considering the low viscous numbers of our data, we found that the friction coefficient governing the quasi-static state in the creeping regime tends to a finite value for vanishing shear, which decreases the critical friction coefficient by a factor of three for all cases investigated.