Multi-scale interactions in turbulent mixed convection drive efficient transport of Lagrangian particles

TL;DR

This study uses coupled numerical simulations to reveal how multi-scale interactions in turbulent mixed convection enhance vertical transport of particles, especially near boundaries, leading to increased interior concentrations.

Contribution

It demonstrates the role of multi-scale interactions in turbulent mixed convection in promoting efficient particle transport, a coupling absent in pure flow or free convection.

Findings

Vertical transport is driven by ejection events and large-scale plume structures.

Interior particle concentrations increase significantly with higher Richardson number.

Coupling of flow structures enhances suspension compared to limiting cases.

Abstract

When turbulent convection interacts with a turbulent shear flow, the cores of convective cells become aligned with the mean current, and these cells (which span the height of the domain) may interact with motions closer to the solid boundary. In this work, we use coupled Eulerian-Lagrangian direct numerical simulations of a turbulent channel flow to demonstrate that under conditions of turbulent mixed convection, interactions between motions associated with ejections and low-speed streaks near the solid boundary, and coherent superstructures in the interior of the flow interact and lead to significant vertical transport of strongly settling Lagrangian particles. We show that the primary suspension mechanism is associated with strong ejection events (canonical low-speed streaks and hairpin vortices characterized by $u'<0$ and $w'>0$), whereas secondary suspension is strongly associated with large scale plume structures aligned with the mean shear (characterized by $w'>0$ and $\theta'>0$). This coupling, which is absent in the limiting cases (pure channel flow or free convection) is shown to lead to a sudden increase in the interior concentration profiles as $\mathrm{Ri}_\tau$ increases, resulting in concentrations that are larger by roughly an order of magnitude at the channel midplane.