Lagrangian statistics and coherent structures in two-dimensional turbulence

TL;DR

This study investigates Lagrangian statistics and coherent structures in a quasi-two-dimensional turbulent system, revealing how vortices and saddles influence particle motion and energy cascades.

Contribution

It provides new insights into Lagrangian behaviors and coherent structures in 2D turbulence through experimental measurements of particle trajectories and flow features.

Findings

Enstrophy cascades to small scales while energy moves to larger scales.

Long-lived centers trap particles, affecting their trajectories.

Saddles induce slow particle motions, influencing flow dynamics.

Abstract

Measurements of Lagrangian single-point and multiple-point statistics in a quasi-two-dimensional stratifed layer system are reported. The system consists of a layer of salt water over an immiscible layer of Fluorinert and is forced electromagnetically so that mean-squared vorticity is injected at a well-defined spatial scale. Simultaneous cascades develop in which enstrophy flows predominately to small scales whereas energy cascades, on average, to larger scales. Lagrangian velocity correlations and one- and two-point displacements are measured for random initial conditions and for initial positions within topological centers and saddles. The behavior of these quantities can be understood in terms of the trapping characteristics of long-lived centers, the slow motion near strong saddles, and the rapid fluctuations outside of either centers or saddles.