Preferential concentration of particles in a decaying isotropic turbulence in the dilute regime

Abstract

The gravitational settling of small solid particles in an incompressible decaying turbulence is investigated using the Eulerian–Lagrangian framework. We were able to reach a Taylor microscale-based Reynolds number, Rek ¼ 71, on a mesh size of 1283. There is an increase in the mean settling velocity of the particles in a turbulent flow compared to that in a static flow. This increase in the settling velocity decreases with time in the case of decaying turbulence. The particle dynamics is found to be modified by the decaying turbulent flow, resulting in a relatively uniform distribution. The highest decay rate of particles is observed when St ¼ Oð1Þ. Consequently, the de-clustering of the particles is also found to be peaked at St ¼ 1. The trademark columnar accumulation of particles also disappears at low Froude numbers when the turbulence decays over time. For light particles, the distribution correlates well with low-vorticity regions, which disappears for heavy particles as gravity dominates the distribution for heavy particles in both forced and decaying turbulence. We have also analyzed the distribution quantitatively by plotting the Voronoi tessellation. We have found the distribution of Voronoi cell areas to be highly skewed to very low values due to the effect of clustering of particles