Spatial distribution of particles in turbulent channel flow of dilute suspensions

We experimentally investigated the kinematics and suspension of particles in the near-wall region of a horizontal turbulent channel flow of water. The particles were glass beads with a diameter of 0.014H, where H is the full channel height. The experiments involved dilute particle suspensions at bulk volumetric concentrations (phi) of 0.05, 0.12, and 0.27%, and at Reynolds numbers (Re) of 20,200, 40,400, and 60,500. Measurement of Lagrangian position and velocity of the particles was carried out using 3D particle tracking velocimetry. At the lowest Re, the results showed a large near-wall accumulation of particles due to gravitational settling at all three bulk concentrations. The analysis of turbulence statistics suggested negligible effect due to inter-particle collisions even when the local volumetric particle concentration reached 2%. With increasing Re to 40,400 and 60,500, different concentration profiles were observed for phi = 0.05% with respect to the profiles of phi = 0.12 and 0.27%. A bi-model particle concentration distribution, with an inner and outer peak, was present for phi = 0.05% at Re = 40,400 and 60,500. In contrast, the particles at phi = 0.12 and 0.27% had a wall-peaking profile. The autocorrelation function of the streamwise Lagrangian velocity and wall-normal dispersion of the particles showed greater stability of particle motions in the near-wall region for phi = 0.12 and 0.27% with respect to phi = 0.05% at the two higher Re. The observations are associated with a stronger particle-wall lubrication at the higher volumetric concentrations of phi = 0.12 and 0.27%. (C) 2020 Elsevier Ltd. All rights reserved.