Motion stabilization of freely falling bristled disk

Abstract

Some of the smallest flying insects, Thysanoptera and Mymaridae, have peculiar configuration of wings referred to as "bristled wing". Previous studies focused on aerodynamic characteristics of bristled wing, originating from its unusual configuration acting in low-Reynolds-number regime of the order of 10 or less. Interestingly, tiny insects possessing the bristled wings have been reported to perform a parachuting, one of passive flight modes. However, although numerous researches investigated the free fall motions of objects, such as disks, the dynamics of bristled wings during parachuting and their effects on how falling motion is stabilized have not been studied yet. In this study, we experimentally examine the motion stabilization of a freely falling bristled disk in a wide range of Reynolds numbers by changing the number of bristles and initial falling angles, and compare with that of a full circular disk without bristles. Our experiments show that both terminal velocities and flow fields of bristled disks are similar to those of a circular disk at the steady state of falling, as expected. However, at the initial transient phase, bristled disks show notably different falling motions from the full disk. While a full disk undergoes large disturbances in its planar and angular movements, a bristled disk shows more stable motions explicitly, regardless of Reynolds numbers or initial falling angles. By observing flow fields around the falling disks, we explain mechanisms underlying the differences in motion stabilization: symmetry of vortical structure and distances of vortices from the center of disk.