Multi-Particle Dispersion in Rotating-Stratified Turbulent Flows

TL;DR

This study investigates how pairs and groups of particles disperse in rotating-stratified turbulent flows using direct numerical simulations, revealing asymmetries, extreme fluctuations, shear layer formation, and reduced irreversibility with increased stratification and rotation.

Contribution

It provides new insights into particle dispersion asymmetries, fluctuations, and structural formations in anisotropic, rotating-stratified turbulence regimes, extending understanding beyond isotropic turbulence.

Findings

Dispersion differs forward and backward in time, with asymmetry decreasing at higher stratification and rotation.

Fluctuations in particle separation become more extreme as N and f increase.

Shear layers become more pronounced with increased stratification and rotation.

Abstract

The transport of matter by turbulent flows plays an important role, in particular in a geophysical context. Here, we study the relative movement of groups of two (pairs) and four (tetrahedra) Lagrangian particles using direct numerical simulations of the stably-stratified Boussinsesq equations, with Brunt-V\"ais\"al\"a frequency $N$ and Coriolis parameter $f$. We cover regimes close to homogeneous isotropic turbulence, to flows dominated by stratification and rotation, keeping fixed the ratio $N/f = 5$. The flows studied are anisotropic, so the relative motion between two particles depends not only on the initial separation between the particles, but also on their orientation with respect to the vertical axis. In all cases considered, we demonstrate that the relative particle motion differs depending on whether dispersion is considered forward or backwards in time, although the asymmetry becomes less pronounced when stratification and rotation increase. On the other hand, the strong fluctuations in the dispersion between two particles become more extreme when $N$ and $f$ increase. We also find evidence for the formation of shear layers, which become more pronounced as $N$ and $f$ become larger. Finally, we show that the irreversibility on the dispersion of a set of particles forming initially a regular tetrahedron becomes weaker when the influence of stratification and rotation increase, a property that we relate to that of the perceived rate-of-strain tensor.