Falling styles of perforated disks

We experimentally investigate the dynamics of thin perforated disks freely falling under gravity in still water. With a stereo vision system, three-dimensional paths are reconstructed. Three motions corresponding to different paths are identified, namely spiral motion, Hula-Hoop motion and helical precession motion. Compared with previous studies, it is found that the lateral displacements of perforated disks are much smaller than those of the other regular particles under the same number of Re and I*. Flow visualizations for freely falling perforated disks are performed using sodium fluorescein dye, and significant differences can be found in the wake. Hairpin vortices can be clearly distinguished in Hula-Hoop motion, which are hardly found in the other motions. Due to the presence of vortices shed from holes, there are complex vortex interactions in the wake, such as adjacent vortices fusion and the secondary induced vortex. Mechanisms behind different wake patterns are analyzed. And different three-dimensional paths under the same I* and Re are observed due to different wake patterns.