Differences of turbulence modulation by heavy particles on solid wall and erodible bed surface

Abstract

In this paper, wall-resolved large-eddy simulation of turbulence, Lagrangian point-force model of particle tracking, and two-way coupling approach are used to simulate the particle-laden flow over a rigid wall. The flow is a turbulent open channel flow with the particle-free friction Reynolds number of Reτ=4200. Together with the simulated results over an erodible bed from Zheng et al. [J. Fluid Mech. 918, 1–27 (2021)], the influence of the lower boundary condition of particle motion with the wall-normal gravity on turbulence modulation is thoroughly compared. It is found that high-inertia (St+=244.5) particles studied in this work moving over a rigid wall increase the mean fluid velocity and the scales of turbulence structures away from the wall, suppress turbulence fluctuations and Reynolds stress, and reduce the scales of turbulence structures near the wall as compared with the particle-free flow. Gravitational settling of particles accounts for most of the changes, and the crossing trajectory caused by particles bouncing near the rigid wall is responsible for the reduction of the scales of the near-wall turbulence structures. On the contrary, the splashing process of particles over the erodible bed leads to the decrease in the mean fluid velocity, the anisotropic variation of turbulent kinetic energy, the shrink of the outer turbulence structure, and the enlargement of the near-wall streaks. The results reveal the significance of the near-wall particle motion (rebound or splashing) on turbulence modulation.