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

Classical scaling relationships for rheological quantities of monodisperse particles have become increasingly popular for closures of two-phase flow modelling. We extend these considerations to more realistic sediment compositions. We investigate the rheological behavior of sheared sediment beds composed of polydisperse spherical particles with a diameter size ratio of up to 10 in a laminar Couette-type flow. The data were generated by fully coupled, grain resolved direct numerical simulations and yield depth-resolved profiles of the relevant rheological quantities. A comparison against experimental data shows excellent agreement for the monodisperse case. We improve upon the parameterization of the μ(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 very low viscous numbers. Considering the low shear rates of our data, we found that the friction coefficient governing the quasi-static state tends to a finite value for vanishing shear, which decreases the critical friction coefficient by a factor of three for all cases investigated.