Effects of particle elongation on dense granular flows down a rough inclined plane

Granular materials in nature are nearly always nonspherical, but particle shape effects in granular flow remain largely elusive. This study uses discrete element method simulations to investigate how elongated particle shapes affect the mobility of dense granular flows down a rough incline. For a range of systematically varied particle length-to-diameter aspect ratios (AR), we run simulations with various flow thicknesses h and slope angles θ to extract the well-known h_{stop}(θ) curves (below which the flow ceases) and the Fr-h/h_{stop} relations following Pouliquen's approach, where Fr=u/sqrt[gh] is the Froude number, u is the mean flow velocity, and g is the gravitational acceleration. The slope β of the Fr-h/h_{stop} relations shows an intriguing S-shaped dependence on AR, with two plateaus at small and large AR, respectively, transitioning with a sharp increase. We understand this S-shaped dependence by examining statistics of particle orientation, alignment, and hindered rotation. We find that the rotation ability of weakly elongated particles (AR≲1.3) remains similar to spheres, leading to the first plateau in the β-AR relation, whereas the effects of particle orientation saturate beyond AR≈2.0, explaining the second plateau. An empirical sigmoidal function is proposed to capture this nonlinear dependence. The findings are expected to enhance our understanding of how particle shape affects the flow of granular materials from both the flow- and particle-scale perspectives.