Abstract
The effects of porosity on the kinematics of porous disks are investigated experimentally. A new falling motion is identified, namely, spiral irregular motion, which is characterized by the irregular centerline and spiral motion around the centerline. Multifractal analysis is introduced to quantify the self-similarity and space-filling of irregular centerlines. Generally, the capacity dimension D-0 decreases as the diameter ratio between the inner holes and the disk diameter chi increases. However, there is a deviation at
chi = 0.2. To explain this unordinary deviation, wake is visualized by particle image velocimetry. An oblique vortex ring with high vorticity is responsible for this anomaly. With chi increasing, the angle of attack increases nonlinearly and the distance R-p between paths and centerlines decreases. However, the nutation angle does not vary monotonically with chi, and a minimum appears at
chi = 0.2. The Strouhal number St and the drag coefficient C-d share the same trend with chi; hence, C-d increases monotonically with St. Both St and C-d reach a maximum at
chi = 0.15. These findings can be applied to improve the aerodynamic stability of disk-shaped passive fliers and give theoretical insight into parameter selection.