How particle shape controls basal slip and mobility in granular flows down smooth and rough inclines

Basal boundary conditions exert a fundamental control on the mobility of granular flows. Recent numerical simulations by Wang et al. (2025, Journal of Fluid Mechanics) demonstrated that dense granular flows over smooth or weakly rough beds develop a thin but highly active basal boundary layer. This basal layer exhibits velocity slip and enhanced particle agitation. It causes the velocity profile to deviate from Bagnold scaling and leads to increase in flow velocity, providing a physically grounded explanation for the high mobility observed in natural mass movements. Natural landslides are predominantly composed of non-spherical particles, whose shape-induced rotational constraints may significantly modify basal layer dynamics. However, the role of particle shape in controlling basal slip and near-bed kinematics remains poorly understood.
In this study, we investigate particle-shape effects on basal boundary-layer slip using discrete element simulations of dense granular flows down smooth and rough inclines. Particles are represented as superquadrics with shape exponent n=2,3,4,5, and 6, systematically transitioning from spheres to increasingly angular grains while preserving identical volume and aspect ratio (a:b:c=1:1:1). This numerical framework allows isolation of shape-controlled mechanical effects at the basal boundary. Building on recent advances in basal slip mechanics and contact-dominated friction weakening, we hypothesize that increasing particle angularity progressively suppresses basal slip by limiting particle rotation, strengthening force-chain structures, and increasing the effective thickness of the basal shear layer. In contrast, near-spherical particles are expected to promote rolling-dominated basal dynamics, leading to stronger basal slip and enhanced flow mobility. Preliminary results indicate a systematic transition in basal slip behavior and boundary-layer structure with particle shape, highlighting particle geometry as a key factor governing basal boundary conditions and mobility in granular landslides.

Reference
Wang, T., L. Jing, C. Y. Kwok, Y. D. Sobral, T. Weinhart, & A. R. Thornton. Basal layer of granular flow down smooth and rough inclines: kinematics, slip laws and rheology. Journal of Fluid Mechanics, 2025, 1025: A27