The dynamics of finite-size settling particles

Direct numerical simulations of the gravity-induced settling of finite-size particles in triply periodic domains has been performed under dilute conditions. We consider rigid, heavy, finite-size spherical particles. Depending on the particle settling regime the particles may exhibit strong clustering. The particles inside clusters experience larger settling velocities than the average. The mean particle wake is significantly attenuated by the motion of the particles

The dynamics of finite-size settling particles

This book contributes to the fundamental understanding of the physical mechanisms that take place in pseudo turbulent particulate flows. In the present work we have considered the sedimentation of large numbers of spherical rigid particles in an initially quiescent flow field. We have performed direct numerical simulations employing an immersed boundary method for the representation of the fluid-solid interface. The results evidence that depending on the particle settling regime (i.e. Galileo number and particle-to-fluid density ratio) the particles may exhibit strong inhomogeneous spatial distribution. It is found that the particles are preferentially located in regions with downward fluid motion. The particles inside clusters experience larger settling velocities than the average. The flow in all flow cases is observed to exhibit characteristic features of pseudo-turbulence. The particle-induced flow field is further found to be highly anisotropic with dominant vertical components. The results indicate that, in the present flow configurations, the collective and mobility effects play significant role for the particle and fluid motion

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 particles are known (M. Jenny, J. Du\v{s}ek and G. Bouchet, Journal of Fluid Mechanics 508, 201 (2004).) to show a variety of motions. It is known that the wake undergoes several transitions for increasing $\text{Ga}$ resulting in particle motions that are successively: vertical, oblique, oblique oscillating, and finally chaotic. Not only does this change the trajectory of single, isolated, settling particles, but it also changes the dynamics of a swarm of particles as collective effects become important even for dilute suspensions, with volume fraction up to $\Phi_V = \mathcal{O}\left(10^{-3}\right)$, which are investigated in this work. Multi-camera recordings of settling particles are recorded and tracked over time in 3 dimensions. A variety of analysis are performed and show a strong clustering behavior. The distribution of the cell areas of the Vorono\"i tessellation in the horizontal plane are compared to that of a random distribution of particles and shows clear clustering. Moreover, a negative correlation was found between the Vorono\"i area and the particle velocity; clustered particles fall faster. In addition, the angle between two adjacent particles and the vertical is calculated and compared to a homogeneous distribution of particles, clear evidence of vertical alignment of particles is found. The experimental findings are compared to simulations.Comment: 8 pages, 6 figure