Experimental investigation of free falling thin disks. Part II: Onset of three dimensional motion from zigzag to spiral

The free falling motion of thin disk with small dimensionless moment of inertia was investigated experimentally. Transition from two dimensional zigzag motions to three dimensional spiral motions takes place as a consequence of breakup of planar symmetry flow structures. The counter-rotating vortices behind the disk became asymmetry induced by the instability of three dimensional disturbances, one of the separation location moves from the edge to the disk broad surface which induced a new upright vortex. The oscillation in the direction normal to zigzag plane increases with the development of instability. Meanwhile, the oscillation of nutation angle vanishes and reaches a constant value. In addition, we have shown that small moment of inertia is responsible for growth of disturbances in the third dimension by means of generalized Kirchhoff equations.