Cooperative behavior displayed by five steel disks falling in a low-density particle bed involves the formation of upward and downward convex configurations, which resembles the flying pattern of a flock of birds. In this study, we focused on overtaking behavior in two falling disks, which causes the cooperative behavior, and we investigated the effects of differences in the disk release time and the initial disk separation distance on the falling behavior of the disks experimentally. Expanded polystyrene (EPS) particles (diameter 5.08 mm, mass 1.45 mg) were used as the bed particles and steel disks (diameter 25.4 mm, thickness 5.22 mm, mass 20.2 g) were used as the falling disks. We released one to five disks with various disk release time differences (0–0.154 s) and initial separation distances (0–100 mm). We recorded the disk falling behavior in the particle bed with a high-speed video camera (500 fps) and analyzed the behavior with image analysis software. Five-disk cooperative behavior similar to that reported in the literature occurred in our experimental setup. In the two-disk experiments, we observed overtaking behavior for an initial separation distance of 10 mm and release time difference of ≤ 0.076 s, and for an initial separation distance of ≤ 60 mm and release time difference of 0.02–0.03 s. The overtaking behavior arose from the decrease in the falling velocity of the first disk released. The EPS particle packing fraction in the area above the disk one disk diameter wide and a quarter of the disk diameter high determined the disk falling velocity. This mechanism was explained by the displacement behavior of EPS particles around the disks as the disks fell.

落在低密度颗粒床中的五个钢盘所显示的合作行为涉及形成向上和向下凸出的配置，类似于一群鸟类的飞行模式。在这项研究中，我们重点研究了导致协同行为的两个下落盘的超车行为，并通过实验研究了盘释放时间和初始盘分离距离的差异对盘下落行为的影响。发泡聚苯乙烯 (EPS) 颗粒（直径 5.08 毫米，质量 1.45 毫克）用作床颗粒，钢盘（直径 25.4 毫米，厚度 5.22 毫米，质量 20.2 克）用作落盘。我们发布了 1 到 5 个具有不同磁盘释放时间差异（0-0.154 秒）和初始分离距离（0-100 毫米）的磁盘。我们用高速摄像机 (500 fps) 记录了粒子床中的圆盘下落行为，并用图像分析软件分析了该行为。在我们的实验设置中发生了类似于文献中报道的五盘协作行为。在两盘实验中，我们观察到初始分离距离为 10 mm 和释放时间差≤ 0.076 s 以及初始分离距离≤ 60 mm 和释放时间差为 0.02-0.03 s 的超车行为。超车行为是由释放的第一个磁盘的下降速度降低引起的。圆盘上方一个圆盘直径宽和圆盘直径四分之一高的区域中的 EPS 颗粒填充率决定了圆盘下落速度。