Turbulence modification by the clusters of settling particles in turbulent water flow in a horizontal duct

Simultaneous visualization is carried out for depositing fine copper particles and fluorescent tracer particles in turbulent water flow in a horizontal duct with a new technique by using the color video camera. Particle image velocimetry (PIV) and particle tracking velocimetry (PTV) techniques are applied to the images of the scattered light from the depositing particles and those of the fluorescence of the tracer particles, respectively. It is found that the increase in the streamwise and downward mean velocities and the decrease in the streamwise turbulence intensity are mainly caused by some copper-particle clusters, which modify near-wall turbulence structure, and some other clusters, which fall with higher velocity than the still-fluid settling velocity and are elongated in the vertical direction. The Stokes number based on the initial state for the group of particles introduced into the flow is found to be one of the criteria for particle clustering in the flow.