Dynamics of freely rising spheres: the effect of moment of inertia

The goal of this study is to elucidate the effect the particle moment of inertia (MOI) has on the dynamics of spherical particles rising in a quiescent and turbulent fluid. To this end, we performed experiments with varying density ratios Gamma, the ratio of the particle density and fluid density, ranging from 0.37 up to 0.97. At each Gamma the MOI was varied by shifting mass between the shell and the centre of the particle to vary I* (the particle MOI normalised by the MOI of a particle with the same weight and a uniform mass distribution). Helical paths are observed for low, and 'three-dimensional (3-D) chaotic' trajectories at higher values of Gamma. The present data suggest no influence of I* on the critical value for this transition 0.42 < Gamma(crit) < 0.52. For the '3-D chaotic' rise mode, we identify trends of decreasing particle drag coefficient (C-d) and amplitude of oscillation with increasing I*. Due to limited data it remains unclear if a similar dependence exists in the helical regime as well. Path oscillations remain finite for all cases studied and no 'rectilinear' mode is encountered, which may be the consequence of allowing for a longer transient distance in the present compared with earlier work. Rotational dynamics did not vary significantly between quiescent and turbulent surroundings, indicating that for the present configuration these are predominantly wake driven.