Numerical simulations of aggregate breakup in bounded and unbounded turbulent flows

TL;DR

This study uses direct numerical simulations to analyze how small aggregates break up in various turbulent flows, revealing universal and flow-specific behaviors depending on the breakup threshold.

Contribution

It introduces a simple hydrodynamic stress criterion for aggregate breakup and compares breakup rates across different flow configurations, highlighting universal and flow-dependent effects.

Findings

Breakup rate decreases as the threshold increases.

Universal scaling observed for small thresholds across flows.

Flow inhomogeneity significantly affects breakup rates at high thresholds.

Abstract

Breakup of small aggregates in fully developed turbulence is studied by means of direct numerical simulations in a series of typical bounded and unbounded flow configurations, such as a turbulent channel flow, a developing boundary layer and homogeneous isotropic turbulence. The simplest criterion for breakup is adopted, whereas aggregate breakup occurs when the local hydrodynamic stress $\sigma\sim \varepsilon^{1/2}$, with $\varepsilon$ being the energy dissipation at the position of the aggregate, overcomes a given threshold $\sigma_\mathrm{cr}$, which is characteristic for a given type of aggregates. Results show that the breakup rate decreases with increasing threshold. For small thresholds, it develops a universal scaling among the different flows. For high thresholds, the breakup rates show strong differences between the different flow configurations, highlighting the importance of non-universal mean-flow properties. To further assess the effects of flow inhomogeneity and turbulent fluctuations, theresults are compared with those obtained in a smooth stochastic flow. Furthermore, we discuss the limitations and applicability of a set of independent proxies.