Lagrangian dispersion in experimental stratified turbulence

TL;DR

This study investigates how stratification influences particle dispersion in turbulence, revealing constrained vertical movement, isotropic velocity spectra at high frequencies, and a transition from Gaussian to non-Gaussian statistics across scales.

Contribution

It provides experimental insights into Lagrangian dispersion in stratified turbulence at ocean-like conditions, highlighting spectral and statistical behaviors across different regimes.

Findings

Vertical dispersion constrained by stratification.

Velocity spectrum follows a 1/f^3 decay at high frequencies.

Flow exhibits Gaussian statistics at wave timescales and non-Gaussian at smaller scales.

Abstract

Lagrangian measurements of tracer particle dispersion in stratified turbulence are presented from a large-scale experiment achieving both high buoyancy Reynolds numbers and low Froude numbers -- a regime characteristic of oceanic conditions. Stratification has a pronounced effect on the vertical particle dispersion, which is observed to be constrained to distances on the order of $w_{\mathrm{std}}/N$, where $w_{\mathrm{std}}$ is the standard deviation of the vertical velocity and $N$ is the Brunt-V\"ais\"al\"a frequency. As expected in strongly nonlinear, stratified turbulence, the frequency spectrum of the Lagrangian velocity becomes isotropic at frequencies higher than $N$. The spectral decay follows a $1/f^3$ scaling, which contrasts with the $1/f^2$ behavior typical of homogeneous isotropic turbulence. At time scales corresponding to internal waves, the statistics of velocity increments remain Gaussian, consistent with the weakly nonlinear regime of wave turbulence. At smaller scales, however, the flow exhibits strongly non-Gaussian statistics, indicative of fully nonlinear turbulent dynamics driven by wave breaking.