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Abstract

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) Ga ≈ 6000, the ratio of the particle to fluid
density Γ ≈ 0.53 and the dimensionless moment of inertia I∗ = Ip/If (with Ip being the
moment of inertia of the particle and If that of the fluid in an equivalent volume), are
kept constant. The geometrical aspect ratio of the spheroids, χ , is varied systematically
from χ = 0.2 (oblate) to 5 (prolate). Based on tracking all degrees of particle motion, we
identify six regimes characterised by distinct rise dynamics. Firstly, for 0.83 ≤ χ ≤ 1.20,
increased rotational dynamics are observed and the particle flips over semi-regularly in
a ‘tumbling’-like motion. Secondly, for oblate particles with 0.29 ≤ χ ≤ 0.75, planar
regular ‘zig–zag’ motion is observed, where the drag coefficient is independent of χ.
Thirdly, for the most extreme oblate geometries (χ ≤ 0.25), 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 (1.33 ≤ χ ≤ 2.5), where both modes are active and a
different one, the ‘broadside’-regime (3 ≤ χ ≤ 4), where only the second mode is present.