Spiral dynamics of a falling sphere with rear-side filaments

Abstract

The passive structures located behind the bluff body change the wake characteristics and fluid-dynamic forces. For example, a splitter plate or hairy filaments attached to the rear of a circular cylinder reduces the drag force by suppressing the wake instability. Meanwhile, the wake instability of the fixed body is closely related to the path instability of a freely falling body

it has been reported that the Reynolds number where the bifurcation occurs for a wake behind the fixed sphere is similar to Re where the falling mode changes for the sphere. Therefore, it can be conjectured that flexible filaments in the rear of a freely falling body would change the falling dynamics compared to that of a body without filaments. In this study, we investigate the effects of filaments when attached to the rear of falling sphere by changing the length and the number of filaments. From our experiments, the filaments reduced the average vertical velocity of the sphere compared to the sphere without filaments, indicating the increase in drag, which is contrast to the passive structures behind the fixed body that reduce drag. The falling times are prolonged because the filaments increase the length of falling trajectory and the rotation of the sphere increases effective area of the sphere with filaments.