How does turbulence mix a stratified fluid?

Abstract

A principal topic of interest and importance in stably stratified flows is how turbulence irreversibly mixes the ambient density field. Because the density field is coupled dynamically to the velocity field through the stable stratification, this mixing affects the overall flow dynamics, and its accurate parameterisation has become a ‘grand challenge’ in environmental fluid mechanics (Dauxois et al., Phys. Rev. Fluids, vol. 6, issue 2, 2021, 020501). In order to better understand the detailed kinematics of mixing in a stably stratified fluid, Jiang et al. (2022) perform experiments using a unique laboratory facility, capable of generating controllable stratified shear flows, and providing almost instantaneous density and three-component velocity measurements on a high-resolution, three-dimensional grid. Using three-dimensional data sets from the experiments, they employ the rortex–shear decomposition to identify the morphology of instantaneous rortices in the flow fields, leading to the interpretation of the motion of the rortices and ultimately to how the rortices cause irreversible mixing of the density field. This marks one of the first studies where, in a laboratory setting, full use has been made of somewhat high-resolution, three-dimensional near-instantaneous measurements; it demonstrates what can be accomplished in the laboratory, setting a new standard for future experiments.

New insights into experimental stratified flows obtained through physics-informed neural networks

“…2022). The particular morphology of these vortices conspires to entrain and stir fluid into the mixed interfacial region, elucidating a key mechanism for shear-driven mixing (Riley 2022). However, Jiang et al.…”

Geometry of stratified turbulent mixing: local alignment of the density gradient with rotation, shear and viscous dissipation

“…The objective of this paper is to tackle these issues by using the same datasets as Jiang et al. (2022) and quantifying the relation between kinematic rotational and shearing flow structures and the dynamics and energetics of the flow, an open question (Riley 2022) essential to improve parametrisations. Specifically, we will address the two following questions.…”