Abstract:

When the vertical vibration is applied to a U-tube filled with granular materials, the particles exhibit instability of directional migration of particles from one branch to another, which has an important influence on the transportation of granular materials. In order to investigate the dynamical behaviors and mechanisms of granular instability in the U-tube, the visualized simulation of dynamical process was carried out based on discrete element simulation. The effect of initial particle filling condition on the granular instability was analyzed, the evolution of velocity field during the development and saturation phases of granular instability was revealed, and the influence of wall friction on the granular instability was examined. The results showed that the granular instability was associated with the self-amplifying effect and the development phase of granular instability was affected by the initial filling conditions of particles. The surface level difference in the two branches of the U-tube in the final saturation phase was independent of the initial filling conditions of particles, which was in accordance with the experimental reports. In the development phase of granular instability, during the downward motion of the U-tube, the particles in the horizontal section expand towards the two vertical branches, transferring the particles to the branch with more particles but lower velocity of particles in the bottom. In the saturation phase, however, when the U-tube moved downward, the inertial effect in the branch with more particles resulted in higher particle velocities in the bottom, which inhibited the migration of the particles from the horizontal section into this vertical branch, causing the growth of granular surface level difference to cease. The granular instability did not occur when the sliding friction coefficient of the tube wall was too small or too large.

Key words: granular materials, instability, numerical simulation, U-tube, vertical vibration, discrete element method

摘要：

对填充颗粒物料的U形管施加竖直振动时，颗粒将呈现由一个分支向另一个分支定向迁移的不稳定性,该现象对颗粒物料的输运产生重要影响。为探究U形管中颗粒不稳定性的动力学行为及机理，基于离散元方法对颗粒动力学过程进行可视化模拟，分析了初始颗粒填充状况对颗粒不稳定性的影响，揭示了颗粒不稳定性的发展与饱和阶段速度场演化特性，考察了壁面摩擦对颗粒不稳定性的影响。结果表明，颗粒不稳定性与自放大效应有关，其发展阶段受到颗粒初始填充状况的影响；而最终饱和阶段U形管两分支颗粒表面高度差与颗粒初始填充状况无关，这与实验报道一致。在颗粒不稳定性的发展阶段，U形管向下运动过程中，水平段颗粒向两竖直分支膨胀，颗粒传输到颗粒量较多而底部颗粒速度较小的分支；在饱和阶段，U形管向下运动时，颗粒量较多分支中的惯性效应引起颗粒底部速度较大，抑制了水平段颗粒向其中的传输，导致颗粒表面高度差的增长停止。壁面滑动摩擦系数过小或过大时，颗粒定向迁移的现象均无法发生。

关键词: 颗粒物料, 不稳定性, 数值模拟, U形管, 竖直振动, 离散元方法