Effect of sub-critical fluid shear flow on granular bed strength

TL;DR

This study uses DEM simulations to explore how sub-critical fluid shear flows influence the directional strength of granular beds, revealing that surface grain dislodgement correlates with bed strength anisotropy and its temporal persistence.

Contribution

It provides new insights into grain-scale mechanisms of bed strengthening and weakening under sub-critical flows, emphasizing the role of surface grain dislodgement and fabric history.

Findings

Bed strength anisotropy correlates with dislodgement of surface grains.

Anisotropic strength persists over a finite time scale set by Shields number.

Inter-particle static friction influences the magnitude but not the existence of anisotropy.

Abstract

Interactions between fluids and granular materials are prevalent on the Earth's surface. In the case of fluid flow over a sediment bed, the fluid imparts a shear stress to the granular materials. When the applied shear stress is above a critical value, the grains become entrained in the fluid flow. Prior experimental studies have shown that granular beds subjected to a sub-critical fluid flow can strengthen in the same direction as the sub-critical flow. In contrast, granular beds can become weaker in the direction opposite to the sub-critical fluid flow. To investigate the grain-scale mechanisms that control directional strengthening and weakening, we perform discrete element method (DEM) simulations of granular beds subjected to model fluid flows in two (2D) and three (3D) dimensions with varied inter-particle static friction coefficients and conditioning flow speeds. In these studies, the sub-critical grain motion does not cause significant bed compaction. Instead, we find that the strength of a granular bed in a particular direction is highly correlated with the fraction of {\it surface} grains that can be dislodged by a fluid force applied in that direction. Further, the anisotropic bed strength only persists over a finite time scale that is set by the Shields number. We also show that inter-particle static friction is not required for bed strength anisotropy, but varying the friction affects the magnitude of the anisotropy. This research enhances the grain-scale understanding of erosion of granular beds caused by fluid flows and underscores the importance of tracking the history of the fabric of the bed surface since it couples strongly to bed strength.