Stabilized motion of a freely falling bristled disk

Abstract

Several tiny insects have peculiar porous wings composed of many bristles and perform an interesting passive flight known as parachuting. Despite numerous studies on the freefall of objects such as disks, the aerodynamic principles of the effects of a bristled configuration on the parachuting motion under external disturbances remain unexplored. Here, we experimentally investigate freely falling bristled disks over a wide range of Reynolds numbers by changing the number of bristles and the initial orientation angle and compare their kinematics with those of a full circular disk with no bristles. Given the same diameter and moment of inertia, bristled disks with a smaller area have a steady-state flow field similar to that of a circular disk by virtue of the presence of a fully formed virtual fluid barrier at low-Reynolds numbers. However, in the initial transient phase after release, the bristled disks show different damped oscillatory motions from a circular disk. Regardless of their initial orientation angle, the lateral and angular deviations of the bristled disks are smaller than those of the circular disk, producing a more stable freefall. This trend is also observed even for higher Reynolds numbers, where the bristled wings are known to be ineffective from the perspective of aerodynamic performance. By considering the vorticity fields around the disk, we suggest two vortex-related mechanisms that account for the stable falling of the bristled disk, namely, the formation of more symmetric vortex structures and the location of vortex cores closer to the disk center.