Computation of Lagrangian coherent structures from experimental fluid trajectory measurements in a mechanically agitated vessel

In mechanically agitated vessels, bulk flow circulation which plays a leading role in macroscale mixing is controlled by hidden Lagrangian coherent structures (LCSs). We use a numerical finite-time Lyapunov exponent (FTLE) approach, for the first time, to resolve such LCSs. Experimental 3D Lagrangian trajectories obtained from a unique positron emission particle tracking (PEPT) technique are used to drive the FTLE model. By computing forward and backward FTLE fields and extracting repelling and attracting FTLE ridges in various azimuthal planes of the flow, a highly complex flow topology is unravelled which varies significantly with azimuthal position. We demonstrate how LCSs organise and quantify the chaotic behaviour of fluid particle paths that underpin mixing through the exchange of fluid between zones of different kinematics. This new Lagrangian approach driven by unique PEPT data is able to unfold some of the complexities of turbulent flow that are beyond the capability of traditional methods.(c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

In this study, the hidden Lagrangian coherent structures in mechanically agitated vessels were resolved using a numerical finite-time Lyapunov exponent approach for the first time. The highly complex flow topology, which varies significantly with azimuthal position, was revealed by computing forward and backward FTLE fields and extracting repelling and attracting FTLE ridges from experimental 3D Lagrangian trajectories.