Non-spherical granular flow down a rough incline: understanding the role of particle flatness and elongation

Dense granular flow on rough slopes provides a simplified yet powerful analogue for gravity-driven natural hazards such as landslides and debris flows, in which particle shape is known to strongly influence the flow mobility and runout but remains difficult to parameterize. Using discrete element method simulations, we systematically investigate the effect of particle flatness (and elongation) on dense granular flows over rough inclined planes. Particles with identical volume but increasing flatness, from spherical to strongly flattened, are first considered. We follow the framework of Pouliquen’s flow rule [1] to identify the critical stopping conditions and then perform an analysis of steady uniform flows, which allows us to extract the velocity scaling with flow thickness and slope angle. We find that the velocity scaling for each particle shape collapses, but the corresponding mobility parameter exhibits a nontrivial dependence on the particle flatness. This shape dependence is characterized by an initial weak sensitivity near the spherical limit, a rapid mobility reduction at intermediate flatness, and a saturation regime for highly flattened particles. Microstructural analyses reveal that this behavior originates in shape-induced constraints on the particle kinematics, including suppressed particle rotation and the emergence of strong orientational ordering, with flat particles preferentially aligning parallel to the shear plane. Furthermore, comparing with recent results of elongated particles [2], we show that flat grains exhibit a characteristic bimodal distribution of preferred orientations, reflecting a distinct alignment mechanism under shear, which nonetheless leads to a comparable macroscopic reduction in mobility. Comparison with elongated particles also indicates that the effects of flatness and elongation may be unified by considering how the particle shape becomes different from perfect sphere. Indeed, when characterized by sphericity, the flow mobility data for both particle types collapse onto a unified trend. Future work will confirm whether this finding applies to other particle shapes. Our work provides a physically grounded route for incorporating particle shape effects into predictive models of landslides and debris flows.

 

References

[1] Pouliquen O. Scaling laws in granular flows down rough inclined planes. Physics of Fluids, 1999, 11(3): 542-548.

[2] Liu J, Jing L, Pähtz T, et al. Effects of particle elongation on dense granular flows down a rough inclined plane. Physical Review E, 2024, 110(4): 044902.