Spiral dynamics of a freely falling sphere with flexible filaments

The passive structures located behind a bluff body change the wake characteristics and fluid-dynamic forces of the bluff body. For example, a splitter plate or hairy filaments attached to the rear of a circular cylinder reduce the drag force by suppressing wake instability. In this study, we investigate the effects of flexible filaments attached to the rear of a freely falling sphere by changing the length and number of filaments. According to our experiments, the filaments reduce the vertical falling speed of the sphere, compared with the sphere without filaments. The flexible filaments of a freely falling body induce the increase in net drag along the vertical direction, which is in contrast to the passive structures behind a fixed body that reduce drag. The trajectory of the falling sphere becomes more spiral because of the wavy deformation of the flexible filaments, thereby increasing the total length of the falling trajectory.