Columnar structure formation of a dilute suspension of settling spherical particles in a quiescent fluid

The settling of heavy spherical particles in a column of quiescent fluid is investigated. The performed experiments cover a range of Galileo numbers ($110 \leq \text{Ga} \leq 310$) for a fixed density ratio of $\Gamma = \rho_p/\rho_f = 2.5$. In this regime the wake undergoes several transitions for increasing $\text{Ga}$ resulting in particle motions that are successively: vertical, oblique, oblique oscillating, and finally chaotic. In this work volume fractions up to $\Phi_V = \mathcal{O}\left$ ($10^{-3}\right$) are investigated. Multi-camera recordings of settling particles are recorded and tracked over time in 3 dimensions. A variety of analysis including Vorono\"i analysis and pair angle analysis are performed and show a strong clustering behavior along with an enhancement settling velocity. The experimental findings are compared to simulations.