Detecting coherent structures using braids

TL;DR

This paper introduces a topological braid-based method for detecting invariant regions in fluid flows using limited trajectory data, which is especially useful in geophysical applications like oceanography.

Contribution

The paper presents a novel topological approach that identifies coherent structures from few trajectories by analyzing braid representations and loop growth, differing from traditional stretching-based methods.

Findings

Effective detection of invariant regions with limited trajectory data

Applicable to oceanic float data where velocity fields are poorly known

Identifies regions where trajectories travel together without entangling

Abstract

The detection of coherent structures is an important problem in fluid dynamics, particularly in geophysical applications. For instance, knowledge of how regions of fluid are isolated from each other allows prediction of the ultimate fate of oil spills. Existing methods detect Lagrangian coherent structures, which are barriers to transport, by examining the stretching field as given by finite-time Lyapunov exponents. These methods are very effective when the velocity field is well-determined, but in many applications only a small number of flow trajectories are known, for example when dealing with oceanic float data. We introduce a topological method for detecting invariant regions based on a small set of trajectories. In the method we regard the two-dimensional trajectory data as a braid in three dimensions, with time being the third coordinate. Invariant regions then correspond to trajectories that travel together and do not entangle other trajectories. We detect these regions by examining the growth of hypothetical loops surrounding sets of trajectories, and searching for loops that show negligible growth.