Collisional Aggregation due to Turbulence

TL;DR

This paper reviews recent advances in understanding how turbulence influences particle collisions in fluids, impacting processes like rain formation, planet development, and sand storms, highlighting the roles of fractal clustering and velocity singularities.

Contribution

It synthesizes recent findings on turbulence-induced collisional aggregation, emphasizing the roles of fractal clustering, caustic singularities, and limitations of existing models.

Findings

Turbulence enhances collision rates through fractal clustering.

Caustic singularities significantly increase relative velocities.

Current models like the Smoluchowski equation have notable limitations.

Abstract

Collisions between particles suspended in a fluid play an important role in many physical processes. As an example, collisions of microscopic water droplets in clouds are a necessary step in the production of macroscopic raindrops. Collisions of dust grains are also conjectured to be important for planet formation in the gas surrounding young stars, and also to play a role in the dynamics of sand storms. In these processes, collisions are favoured by fast turbulent motions. Here we review recent advances in the understanding of collisional aggregation due to turbulence. We discuss the role of fractal clustering of particles, and caustic singularities of their velocities. We also discuss limitations of the Smoluchowski equation for modelling these processes. These advances lead to a semi-quantitative understanding on the influence of turbulence on collision rates, and point to deficiencies in the current understanding of rainfall and planet formation.