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

TL;DR

This study compares the rheological behavior of sheared sediment beds in pressure-driven flows using detailed simulations and experiments, revealing how rheological properties relate to particle concentration and flow regime.

Contribution

It provides the first detailed stress and rheology profiles for sediment beds in pressure-driven flows, linking simulation and experimental data to established rheological models.

Findings

Rheological properties align with those from Couette flow in dense regimes.

A critical particle volume fraction of 0.3 separates dense and dilute regimes.

Effective viscosity in dilute regimes follows Einstein's relation.

Abstract

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 means of fully coupled, grain-resolved direct numerical simulations and experimental measurements reviously published by Aussillous {\it et al.} (J. Fluid Mech., vol. 736, 2013, pp. 594-615). The highly-resolved simulation data allows to compute the stress balance of the suspension in the streamwise and vertical directions and the stress exchange between the fluid and particle phase, which is information needed to infer the rheology, but has so far been unreachable in experiments. Applying this knowledge to the experimental and numerical data, we obtain the statistically-stationary, depth-resolved profiles of the relevant rheological quantities. The scaling behavior of rheological quantities such as the shear and normal viscosities and the effective friction coefficient are examined and compared to data coming from rheometry experiments and from widely-used rheological correlations. We show that rheological properties that have previously been inferred for annular Couette-type shear flows with neutrally buoyant particles still hold for our setup of sediment transport in a Poiseuille flow and in the dense regime we found good agreement with empirical relationships derived therefrom. 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 media and the effective viscosity obeys the Einstein relation.