Numerical realization and structure characterization on random close packings of cuboid particles with different aspect ratios

Abstract

Packing processes of cuboid particles with different aspect ratios (h/b) under 3D vibrations were numerically reproduced by discrete element method using multi-sphere model. The influences of vibration conditions (e.g. vibration time, frequency, and amplitude) and particle shape on the packing densification were comprehensively analyzed. The macro property such as packing density (ρ) and micro properties such as coordinate number (CN), radial distribution function (RDF), contact types and force networks were systematically characterized and compared, and the densification mechanism was discussed. Meanwhile, the randomness of cuboid packing systems was evaluated by nematic order parameter (S2) and cubic order parameter (S4). The results show that dense packings of cuboids with different aspect ratios can be realized by properly controlling vibration conditions. In ρ ~ h/b curve, three peaks can be identified at h/b = 0.5, 1.0 and 1.5, corresponding to the packing densities of 0.700, 0.736 and 0.707, respectively. After vibration, the nematic order parameter S2 decreases in each case and cuboids with too high or too low aspect ratios are easier to form orientational order, while the cubic order parameter S4 increases but it is not positively correlated to the value of packing density. The formation of more face-to-face contacts leads to higher averaged CN and packing density. RDF indicates that special local structures can be formed during packing densification. Force networks indicate that the vibration energy transferred from the container enables cuboid particles near the bottom to generate orientation ordering, where more strong forces are observed, which can promote the rearrangement of cuboid particles for packing densification.