Experimental investigation of freely falling thin disks. Part 1. The flow structures and Reynolds number effects on the zigzag motion

This paper describes an experimental investigation of the dynamics of a freely falling thin circular disk in still water. The flow patterns of the disk zigzag motion are studied using dye visualization and particle image velocimetry. Time-resolved disk motions with six degrees of freedom are obtained with a stereoscopic vision method. The flow separation and vortex shedding are found to change with the Reynolds number, Re . At high Reynolds numbers a new dipole vortex is shed that is significantly different from Kármán-type vortices. The vortical structures are mainly composed of leading-edge vortices, a counter-rotating vortex pair and secondary trailing-edge vortices. The amplitude of the horizontal oscillation is also dependent on the Reynolds number with a critical Reynolds number Recr ≈ 2000 , where the oscillatory amplitude is proportional to Re for Re< Recr , but becomes invariant for Re> Recr . Three-dimensional dipolar vortices were also observed experimentally.