Wang Siqiang, Zhuravkov Michael, Ji Shunying
State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology, Dalian 116023, China.
Soft Matter. 2020 Sep 7;16(33):7760-7777. doi: 10.1039/c9sm02435b. Epub 2020 Aug 3.
Granular flow is widely found in nature or industrial production. Although the external driving force significantly affects the dynamic behavior of a granular system, a large number of numerical simulations have been conducted to study granular flows driven by gravity. In this study, a superquadric equation was used to construct spherical and cylindrical elements, and the flow processes of granular materials under external pressure were simulated by the discrete element method. To examine the validity of the DEM model, the Janssen effect of spherical particles, the static packing of cylindrical particles and the flow process of spherical particles under external pressure are simulated and compared with the previous experimental and theoretical results. Subsequently, the effects of blockiness, orifice diameter, and particle friction on the flow characteristics are investigated. Results show that the flow rate of spherical particles increases as the external pressure and opening diameter increase or the particle friction decreases. However, the flow rate of cylindrical particles decreases as the blockiness parameter increases, and the external pressure has little effect on the flow rate of the cylindrical particles when the blockiness parameter is greater than 4. Furthermore, the external pressure causes a change in the flow pattern of granular systems. In a gravity-driven granular flow, cylindrical particles appear in funnel flow, and spherical particles in both mass and funnel flows. In a pressure-driven granular flow, spherical particles appear in mass flow, and cylindrical particles in both mass and funnel flows. The critical height of the transition between mass and funnel flows decreases with increasing external pressure and eventually reaches a steady state. Meanwhile, the critical height increases with the blockiness parameter, which indicates that more cylindrical than spherical particles appear in funnel flow. Finally, the basic flow characteristics of granular materials under external pressure are further analyzed by the velocity uniformity index, the normal contact force between particles, and the bottom pressure. Overall, the numerical results are useful for understanding the changes in the flow characteristics of spherical and cylindrical granular materials under external pressure, and further provide guidance for the appropriate design and optimization of cylindrical hoppers.
颗粒流在自然界或工业生产中广泛存在。尽管外部驱动力会显著影响颗粒系统的动力学行为，但已经进行了大量数值模拟来研究重力驱动的颗粒流。在本研究中，使用超二次方程构建球形和圆柱形单元，并通过离散元方法模拟颗粒材料在外部压力下的流动过程。为检验离散元模型的有效性，模拟了球形颗粒的扬森效应、圆柱形颗粒的静态堆积以及球形颗粒在外部压力下的流动过程，并与先前的实验和理论结果进行比较。随后，研究了块状度、孔径和颗粒摩擦力对流动特性的影响。结果表明，球形颗粒的流速随着外部压力和开口直径的增加或颗粒摩擦力的减小而增加。然而，圆柱形颗粒的流速随着块状度参数的增加而降低，当块状度参数大于4时，外部压力对圆柱形颗粒的流速影响很小。此外，外部压力会导致颗粒系统流动模式的变化。在重力驱动的颗粒流中，圆柱形颗粒呈现漏斗流，球形颗粒呈现整体流和漏斗流。在压力驱动的颗粒流中，球形颗粒呈现整体流，圆柱形颗粒呈现整体流和漏斗流。整体流和漏斗流之间转变的临界高度随着外部压力的增加而降低，最终达到稳定状态。同时，临界高度随着块状度参数的增加而增加，这表明漏斗流中出现的圆柱形颗粒比球形颗粒更多。最后，通过速度均匀性指数、颗粒间的法向接触力和底部压力进一步分析了颗粒材料在外部压力下的基本流动特性。总体而言，数值结果有助于理解球形和圆柱形颗粒材料在外部压力下流动特性的变化，并进一步为圆柱形料斗的合理设计和优化提供指导。
