Breakup of small aggregates driven by turbulent hydrodynamic stress

TL;DR

This paper investigates how turbulent hydrodynamic stresses cause small solid aggregates to break up, using theoretical analysis and high-Reynolds-number simulations, and introduces new methods to predict aggregate size distributions.

Contribution

It presents a novel Lagrangian and Eulerian framework for calculating aggregate breakup rates based on hydrodynamic stress statistics, improving upon previous single-point estimates.

Findings

Turbulent fluctuations significantly influence aggregate breakup.

New definitions of fragmentation rate are proposed and validated.

The methods enable better prediction of aggregate size distributions.

Abstract

Breakup of small solid aggregates in homogeneous and isotropic turbulence is studied theoretically and by using Direct Numerical Simulations at high Reynolds number, Re_{\lambda} \simeq 400. We show that turbulent fluctuations of the hydrodynamic stress along the aggregate trajectory play a key role in determining the aggregate mass distribution function. Differences between turbulent and laminar flows are discussed. A novel definition of the fragmentation rate is proposed in terms of the typical frequency at which the hydrodynamic stress becomes sufficiently high to cause breakup along each Lagrangian path. We also define an Eulerian proxy of the real fragmentation rate, based on the joint statistics of the stress and its time derivative, which should be easier to measure in any experimental set-up. Both our Eulerian and Lagrangian formulations define a clear procedure for the computation of the mass distribution function due to fragmentation. Contrary, previous estimates based only on single point statistics of the hydrodynamic stress exhibit some deficiencies. These are discussed by investigating the evolution of an ensemble of aggregates undergoing breakup and aggregation.