Dynamics of non-spherical particles in turbulence

Abstract

Recent experimental and numerical investigations have enabled researchers to study directly the 3D time dependent motion of spherical and quasi-spherical particles in quiescent and turbulent flows. At the same time, fibre-like particles have been also extensively investigated during the last decades due to the high relevance of these geometries in the process industries. Although these studies have permitted the scientific community to understand several scenarios, the influence of turbulence on the settling dynamics of large, inertial, planar particles remains to be understood; and this represents not only an interesting research problem in itself but also a useful tool to optimize industrial processes, as for the case of Aquavitrum Ltd. To tackle this problem we first extended the work carried for disks falling under the fluttering mode, but to other planar geometries. We found several modes of secondary motion for n-sided polygons falling in quiescent fluid that are associated with the particle dimensionless inertia and Reynolds number. We also measured the wake behind these particles and identified strong differences associated with their falling style. Other variations of disk-like particles with wavy-edge were investigated and drag correlations based on the particle geometry obtained. We also proposed a one-equation simple pendulum model to capture precisely the complete trajectory of the particle as long as the three dimensional state of the trajectory remains moderate. Then, a random jet array facility to generate turbulence was built and two turbulence states were investigated. First, we focused on the homogeneity of the stationary state to later investigated the temporal decay of turbulent kinetic energy. We observed an overall enhancement of the decay rate as compare with previous studies and found that this was caused due to turbulent confinement. Finally, the fall of inertial disks was investigated under the effect of background turbulence. We observed severe differences on the falling style of disks as compared with the quiescent case. We also found that the mean descent velocity of the particles was enhanced for the configurations tested and this was inversely correlated with the frequency of the oscillatory motion. Last, we measured particle dispersion and found the the radial position of the particles can be well captured with lognormal distributions.

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