The effects of sub-critical fluid flow on granular bed strength

Granular-fluid interactions are prevalent on Earth. Fluid flow over granular materials imparts a shear stress, which when it is above a critical strength entrains particles and results in sediment transport. Recent studies have shown that granular beds subjected to sub-critical fluid flows give rise to anisotropic strengthening in the same direction as the sub-critical flow. Flows in the opposite direction cause particle mobilization at a lower flow strength than that along the pre-conditioning flow direction. To investigate the physical mechanisms that control directional strengthening, we perform discrete element method simulations of granular beds subjected to model fluid flows in two (2D) and three (3D) dimensions, varying the inter-particle static friction and magnitude of fluctuations from turbulent fluid flows. We find that inter-particle friction is crucial for promoting and sustaining large local fabric and stress anisotropy in the granular bed. We also find that sub-critical fluid flow does not cause compaction, which is one of the common mechanisms for the strengthening of granular beds. Instead, we find that the directional strength of granular beds is highly correlated with the stress and fabric anisotropy of the beds. This research enhances our understanding of erosion of granular beds caused by fluid flows with varying directions, underscoring the importance of history-dependence in granular beds.