Transport and Clustering of Inertial Particles in a Turbulent Channel Flow

Small inertial particles in wall-bounded turbulent flows are ubiquitous in natural and industrial settings, and exhibit a rich spectrum of behaviors depending on the flow regime and on the distance from the wall. We present experimental observations of the velocity response and topological distribution of dispersed, inertial particles in the turbulent downward flow through a vertical channel. The working fluid is air laden with size-selected glass spheres, which are imaged over wall-normal and wall-parallel windows and analyzed by particle tracking velocimetry. The particle spatial distribution is analyzed by radial distribution functions and Voronoi tessellation. The wall-normal profiles are strongly modified at higher (although still dilute) mass loading, indicating the particles modify the underlying turbulent flow. The particles accumulate near the wall by turbophoresis and form low-speed streaks, while the channel core is characterized by fractal-like clusters, also preferentially aligned in streamwise direction but travelling faster than the fluid. Spatial velocity correlations of the particle velocity fields allows to quantify the contribution from random uncorrelated motions. The findings are discussed in the context of modeling strategies for dispersed multiphase flows.