On the zigzag dynamics of freely moving axisymmetric bodies

The zigzag path of light flat cylinders rising freely in water at rest is investigated experimentally. The body dynamics are described in terms of the orientation of the cylinder axis relative to its path as a function of its aspect ratio and Reynolds number. We show that for thick bodies the inclinations of both the axis and the velocity oscillate almost in phase, whereas for thin bodies they are rather in quadrature. Most notably, for bodies of intermediate aspect ratio, the phase difference evolves continuously between these two limits when the body flatness changes. In contrast, this phase difference is nearly independent of the Reynolds number. Comparison with potential flow theory shows that these features can only be explained by taking into account vortical effects.