Rheology of mobile sediment beds sheared by viscous, pressure-driven flows

We present a detailed comparison of the rheological behaviour of sheared sediment beds in a pressure-driven, straight channel configuration based on data that was generated by grain-resolved direct numerical simulations and the experimental measurements of Aussillous et al. (J. Fluid Mech., vol. 736, 2013, pp. 594-615). The highly-resolved simulation data allows to compute the stress balance of the suspensions and the stress exchange between the fluid and particle phase. Using this knowledge, we obtain the depth-resolved profiles of the relevant rheological quantities. The scaling behavior of these quantities are examined and compared to data coming from rheometry experiments. We show that rheological properties that have previously been inferred for annular Couette-type shear flows with neutrally buoyant particles still hold in the dense regime for our setup of sediment transport in a Poiseuille flow. Subdividing the total stress into parts from particle contact and hydrodynamics suggests a critical particle volume fraction of 0.3 to separate the dense from the dilute regime. In the dilute regime, i.e., the sediment transport layer, long-range hydrodynamic interactions are screened by the porous medium and the effective viscosity obeys the Einstein relation.