A direct numerical simulation study of flow modulation and turbulent sedimentation in particle-laden downward channel flows

Fully resolved direct numerical simulations of turbulent downward channel flow laden with finite-size spherical particles are performed using the lattice Boltzmann method. Unlike upward particle-laden channel flows, the potential energy of settling particles serves as the driving force in the downward channel flows. Furthermore, the particles have an overall positive slip velocity at the center which causes the lateral hydrodynamic force to drive particles away from the center region. Both changes in the flow driving mechanism and the particle distribution affect the details of turbulence modulation in the downward channel, when compared to the upward channel flow. In this study, we focus on the effect of different particle terminal velocities, i.e., different particle settling Reynolds numbers, on the turbulent modulation of particle-laden downward channel flows. Indeed, the simulation results for downward channel flow show larger local particle concentration in the near-wall region, relative to the upward channel. It is also found that the level of particle near-wall accumulation increases with the particle terminal velocity. Opposite to the upward channel flows, the fluid-phase mean velocity in the downward channel flows is increased by heavy particles in the channel center, but reduced in the buffer layer. The reduction of mean velocity in the buffer layer is caused by the particle accumulation in low-speed streak regions. For the largest particle settling Reynolds number case (Re-T = 30) investigated, strong accumulation of particles in the buffer layer interrupts the near-wall turbulence structures and thus leads to the reductions of fluid turbulence intensity and Reynolds stress.