Analysis of aspect ratio and initial orientation on the sedimentation dynamics of an ellipsoidal particle at intermediate Galileo numbers

The settling dynamics of a non-spherical particle in fluids are ubiquitous in both natural and industrial processes, and the underlying mechanisms are considerably more complex than those of a traditional spherical particle. This study employs high-fidelity numerical simulations to systematically investigate the coupled effects of aspect ratio (γ = 1.00–2.00) and initial inclination angle (0°–90°) on the settling behavior of an ellipsoidal particle within an intermediate Galileo (Ga: 120–220) regime. The results reveal the existence of a critical Ga (Gacrit) that marks the transition of the flow regime from a steady state to a periodic unsteady state. Furthermore, the particle aspect ratio is identified as the key parameter governing the modes of stability and instability. Spherical and near-spherical particles destabilize first, leading to regular two-dimensional vortex shedding. In contrast, elongated particles, benefiting from shape-induced drag, demonstrate superior stability and a significantly higher Gacrit. Within the investigated Ga range, pressure drag accounts for more than 85% of the total drag during particle settling, and its dominance becomes more pronounced with increasing Ga. More importantly, in the chaotic regime, the initial inclination angle acts as a critical bifurcation parameter that can trigger distinctly different final states, including steady settling, two-dimensional oscillation, or three-dimensional tumbling.