# The effect of polydispersity in a turbulent channel flow laden with finite-size particles

## TL;DR

This study uses direct numerical simulations to analyze how polydispersity in particle sizes affects turbulence and particle distribution in channel flows, finding minimal impact on flow statistics but notable effects on particle fluctuations and distributions.

## Contribution

It provides new insights into the effects of particle size distribution on turbulence and particle behavior in channel flows through detailed numerical simulations.

## Key findings

- Flow statistics are similar for monodisperse and polydisperse suspensions.
- Particle velocity fluctuations increase near walls with larger size dispersion.
- Smaller particles penetrate viscous and buffer layers more effectively.

## Abstract

We study turbulent channel flows of monodisperse and polydisperse suspensions of finite-size spheres by means of Direct Numerical Simulations using an immersed boundary method to account for the dispersed phase. Suspensions with 3 different Gaussian distributions of particle radii are considered (i.e. 3 different standard deviations). The distributions are centered on the reference particle radius of the monodisperse suspension. In the most extreme case, the radius of the largest particles is 4 times that of the smaller particles. We consider two different solid volume fractions, 2% and 10%. We find that for all polydisperse cases, both fluid and particles statistics are not substantially altered with respect to those of the monodisperse case. Mean streamwise fluid and particle velocity profiles are almost perfectly overlapping. Slightly larger differences are found for particle velocity fluctuations. These increase close to the wall and decrease towards the centerline as the standard deviation of the distribution is increased. Hence, the behavior of the suspension is mostly governed by excluded volume effects regardless of particle size distribution (at least for the radii here studied). Due to turbulent mixing, particles are uniformly distributed across the channel. However, smaller particles can penetrate more into the viscous and buffer layer and velocity fluctuations are therein altered. Non trivial results are presented for particle-pair statistics.