Kinematics and dynamics of freely rising spheroids at high Reynolds numbers

We experimentally investigate the effect of geometrical anisotropy for buoyant spheroidal particles rising in a still fluid. All other parameters, such as the Galileo number (the ratio of gravitational to viscous forces), the ratio of the particle to fluid density and the dimensionless moment of inertia (with being the moment of inertia of the particle and that of the fluid in an equivalent volume), are kept constant. The geometrical aspect ratio of the spheroids, is varied systematically from (oblate) to 5 (prolate). Based on tracking all degrees of particle motion, we identify six regimes characterised by distinct rise dynamics. Firstly, for, increased rotational dynamics are observed and the particle flips over semi-regularly in a 'tumbling'-like motion. Secondly, for oblate particles with, planar regular 'zig-zag' motion is observed, where the drag coefficient is independent of. Thirdly, for the most extreme oblate geometries (), a 'flutter'-like behaviour is found, characterised by precession of the oscillation plane and an increase in the drag coefficient. For prolate geometries, we observed two coexisting oscillation modes that contribute to complex trajectories: The first is related to oscillations of the pointing vector and the second corresponds to a motion perpendicular to the particle's symmetry axis. We identify a 'longitudinal' regime (), where both modes are active and a different one, the 'broadside'-regime (), where only the second mode is present. Remarkably, for the most prolate particles (), we observe an entirely different 'helical' rise with completely unique features.