Granular materials are widely encountered in both nature and industry, significantly impacting our daily lives. Although substantial progress has been made in experimental and theoretical studies over the past two decades, several influencing factors remain insufficiently understood. In this work, we aim to analyse some uncertain factors influencing the granular column collapse phenomenon. First, we investigate the role of basal friction in granular column collapse through a series of numerical simulations using Smoothed Particle Hydrodynamics (SPH). Our study systematically examines the influence of basal friction on the deposit geometry, proposing an expression to predict run-out distance. The numerical results are compared with experimental findings from previous studies. Additionally, we analyse the effects of basal friction on final height, deposit regime transitions, and energy conversion, offering new insights into plate-grain friction mechanism. Second, as space exploration advances, understanding the collapse of granular materials under non-Earth gravity conditions becomes increasingly relevant. We investigate the effects of varying gravity levels on the collapse behaviour of granular columns, using dimensional analysis to assess how different gravity levels influence material behaviour. Two models are proposed to predict collapse time, accounting for gravitational acceleration (g). Our findings suggest that gravity has minimal influence on deposit run-out distance and final height, supported by observations of natural landslides across the Solar System. Moreover, as the aspect ratio increases, both the flow mobility angle (θ) and the modified flow mobility angle (θ') decrease, independent of gravity level. Our small-scale results align with large-scale results across varying gravity levels, indicating that the collapse run-out depends on sample volume and initial potential energy rather than gravity. Third, we address the limitations of previous studies on particle shape, which often were coupled with other non-particle shape factors (such as volume and stiffness) or used unrealistic particle geometries (primarily consisting of convex shapes without concave features). We utilized spherical harmonic (SH) functions and a high-precision 3D printing machine to fabricate ideal particles, isolating particle shape effects on flow dynamics. Subsequently, we designed a laboratory platform to investigate the influence of particle shape on flow dynamic properties. We also input the STL files of particles generated by the SH functions into Discrete Element Method (DEM) software for numerical analysis. Our study explored the effects of particle shape (varying in Df and D2, where Df and D2 are obtained by fitting the results of spherical harmonic descriptors and spherical harmonic degree) on deposit morphology, deposit geometry (run-out distance, final height, and its related scaling laws constants), energy conversion, and interlocking ability during collapse. Additionally, we quantitatively analysed the influence of particle geometric parameters, such as sphericity, particle aspect ratio, convexity, and roundness on deposit run-out distance, final height, and flow mobility. Furthermore, we proposed a model to directly predict run-out distance using particle relative roughness (Rr), derived from Df and D2, which shows strong agreement with numerical results. This is the first attempt to predict run-out distance from a particle shape perspective. Our findings enhance the understanding of dry granular collapse phenomenon and its underlying mechanisms. This research serves as a valuable reference for the application of granular materials in geotechnical and other related fields.
Yucheng Li (Wed,) studied this question.