This work presents a numerical investigation of the dynamics of freely falling annular disks released from still in infinite water. Based on the large eddy simulation, a dynamic fluid body interaction model and moving computational method are utilized to obtain the full six degrees of freedom of the disks. In particular, we performed a comprehensive analysis of the influencing factors on the falling styles of the disks, i.e., the inner to outer diameter ratio $(0\le \eta \le 0.9)$, the dimensionless moment of inertia $(4\times {10}^{-4}\le I^{*}\le 4.2\times {10}^{-4})$ and the body to fluid density ratio $(1.2\le {\rho }^{*}\le 6)$. $\eta$ is found to be a key parameter for the falling trajectory and the corresponding flow structures, with $\eta =0.6$ as a turning point from regular to random motion. From low to high $I^{*}$, although the disks still exhibit hula-hoop (HH) motion, the dispersion degree of each period and the fluid forces against the disk's inertia increase gradually. We find that regardless of ${\rho }^{*}$, the disk trajectory exhibits HH motion, while more discrete vortices are generated for heavy disks. Ultimately, the mechanism of the planar precession of HH motion is explained by combining the time evolution of horizontal displacement, fluid forces, and vortical structures. Consequently, a hysteresis effect is observed during falling.

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• Fluid-particle interactions
• Locomotion
• Vortex flows