# Lagrangian coherent structures and entrainment near the turbulent/non-turbulent interface of a gravity current

## TL;DR

This study uses Lagrangian coherent structures theory and advanced 3D particle tracking to analyze how large-scale vortices influence the turbulent/non-turbulent interface and entrainment in stratified gravity currents.

## Contribution

It introduces an automated 3D extraction algorithm for vortical LCSs and demonstrates their weak dependence on stratification and significant role in modulating entrainment.

## Key findings

- VLCSs characteristics are weakly affected by stratification.
- VLCSs modulate the TNTI height and entrainment.
- VLCSs organize flow structures on both sides of the interface.

## Abstract

In this paper, we employ Lagrangian coherent structures (LCSs) theory for the three dimensional vortex eduction and investigate the effect of large-scale vortical structures on the turbulent/non-turbulent interface (TNTI) and entrainment of a gravity current. The gravity current is realized experimentally and different levels of stratification are examined. For flow measurements, we use a multivolume three-dimensional particle tracking velocimetry technique. To identify vortical LCSs (VLCSs), a fully automated 3D extraction algorithm for multiple flow structures based on the so-called Lagrangian-Averaged Vorticity Deviation method is implemented. The size, the orientation and the shape of the VLCSs are analyzed and the results show that these characteristics depend only weakly on the strength of the stratification. Through conditional analysis, we provide evidence that VLCSs modulate the average TNTI height, affecting consequently the entrainment process. Furthermore, VLCSs influence the local entrainment velocity and organize the flow field on both the turbulent and non-turbulent sides of the gravity current boundary.