Experimental study of freely falling thin disks: Transition from planar zigzag to spiral

Using a stereoscopic vision method, we have experimentally investigated the time evolution of a free thin disk motion with six degrees of freedom for the first time. It is found that, as the dimensionless moment of inertia I∗ decreases, the trajectory of the disk transits from planar to nonplanar. New types of free falling motions were identified for small I∗ values, including the spiral state and the transitional state. An extended Re−I∗ phase diagram corresponding to different flow regimes was given. The underlying physics associated with the transition is found to be connected to the interactions between the moving object and induced vortices.