Cluster and inertial particle dynamics in turbulent channel flows

Turbulent flows laden with small inertial particles exhibiting preferential concentration are time varying inhomogeneous systems from a macroscopic perspective, with a fluctuating particle concentration field that can modulate other phenomena relying on local concentration or path integrated quantities. Particle local concentration can reach values many times the average concentration, with clusters that vary in size, from the dissipative to the integral turbulence length-scales. Important to physical processes mediated by clustered particles are also the timescales in which particle clusters remain spatially coherent. In this talk we will provide an overview of recent work by our group on vertical turbulent channel flows laden with particles, including data analysis methodologies, experimental methods, and discussion of results for varying flow and particle parameters. We focus on particles denser-than-fluid and smaller than the dissipative scales, and mostly in dilute regimes, to analyze the effect of particle to flow time scales ratio (St) and the contribution of gravitational effects. In particular, we will present time-averaged preferential concentration and cluster geometrical statistics as well novel results from time-resolved data on cluster and particle dynamics. We have observed that the time clusters remain coherent exhibits a power-law probability distribution seemingly independent of the three or two-dimensional nature of the data, Stokes number or average concentration within clusters, and that the lifetime of clusters from which particles migrate until disintegration scales with the turnover time of eddies with equivalent volume. Ongoing work focusing on instantaneous concentration rate of change and local particle velocities, conditioned on local concentration, aim to understand the role of enhanced settling on the coherence time-scales of clusters, and to discern between local two-way coupling and gravitational effects on enhanced settling particle velocities.