Aggregation, breakup, and size-dependent transport in a turbulent channel flow with cohesive particles

TL;DR

This study investigates how cohesive particles in turbulent wall-bounded flows undergo aggregation, breakup, and transport, revealing a size-dependent circulation pattern influenced by turbulence and shear.

Contribution

First detailed analysis of aggregate dynamics in wall-bounded turbulence using simulations and population balance equations, highlighting size-dependent transport mechanisms.

Findings

Larger aggregates form in the channel center and migrate toward the wall where they break.

Smaller aggregates are transported away from the wall, grow, and cycle back.

Local imbalance in aggregation and breakup is balanced by size-dependent transport.

Abstract

Due to attractive inter-particle forces, cohesive particles suspended in turbulence undergo a complex process of aggregation, breakup, and restructuring. Despite a growing body of knowledge on the ``flocculation'' of cohesive granular materials suspended in homogeneous isotropic turbulence, little focus has so far been placed on wall-bounded flows where turbulence and shear are inhomogeneous. This study presents a first investigation of a fully developed wall-bounded flow of resolved cohesive particles. Five direct numerical simulations of turbulent channel flows laden with finite-sized particles at successively increasing cohesive strength are performed. A population balance equation (PBE) framework is used to analyze aggregate dynamics. When integrated over the full domain, the PBE is closed by aggregation and breakup alone. However, this balance is found to not hold locally in the wall-normal direction, where regions of net aggregate production and depletion are identified. This imbalance is shown to be compensated by the size-dependent wall-normal transport of aggregates, revealing a mean circulation: larger aggregates are preferentially produced in the channel center and migrate toward the wall where they break, while smaller aggregates are transported away from the wall, grow, and reenter the cycle.