Preferential concentration of particles in decaying isotropic turbulence in dilute regime

Direct numerical simulations (DNS) of incompressible decaying isotropic turbulence with small particles were performed. We were able to reach a Taylor microscale Reynolds number, Reλ = 71 for a mesh of 128^3. The viscosity was chosen to be ν = 78×10-4, in line with the numerical works of Kaneda et al. (Physics of Fluids, 2003). The gravitational settling of an initially random distribution of small solid particles in decaying turbulence is investigated using an Eulerian-Lagrangian framework. A pseudospectral method is used to solve the fluid governing equations combined with a Lagrangian point-particle model for tracking the particles. There is an increase in the settling velocity of the particles in a turbulent flow compared to the settling velocity of the particles in a static fluid. This increase in the settling velocity diminishes with time in the case of decaying turbulence. Particle dynamics is modified by the decaying background flow, causing the distribution of the particles to attain a more uniform state. The decay rate of particles with St = O(1) is found to be the highest, hence the de-clustering of the particles is also found to peak at St = 1. In the case of low Froude numbers, the trademark columnar accumulation of particles disappears when the turbulence decays with time. The concentration of the heavy particles in the increased influence of gravity show no correlation with the low vorticity regions for both forced and decaying turbulence cases.