Numerical investigation of movement patterns of particles falling in a viscous fluid

To study the movement patterns of particles, the long-term sedimentation of multiple circular particles in a narrow channel filled with a viscous fluid is investigated numerically. The attention is focused firstly on the equilibrium movement pattern of particles at an intermediate terminal Reynolds number (Re_t), and then on its connection with behaviors of multiple particles. For three particles released in a symmetric pattern, at the first stage, all particles tend to fall at the same speed, then after a dramatic interaction, particles split into two groups (1 + 2). The single particle moves steadily with a faster velocity along the central line, while the other two particles self-organize into a two-particle system, moving as a steady oblique doublet, periodic oscillation pair, or a steady symmetrical doublet. For the sedimentation of six particles, there are more separations after the first stage. The small groups owning 1, 2 and 3-particle move in their own typical equilibrium movement patterns. It seems that at intermediate Reynolds numbers multiple particles tend to break up into small groups that can maintain their own equilibrium movement patterns. It is shown that the study on the movement patterns of one/two particles can provide essential information for the self-organization behavior of multiple particles.