L\'evy Walks and Path Chaos in the Dispersal of Elongated Structures Moving across Cellular Vortical Flows

TL;DR

This study investigates how semi-rigid filaments move in cellular vortical flows, revealing diverse transport behaviors including Levy walks, ballistic motion, and chaos, influenced by filament length and flexibility, with experimental validation.

Contribution

It uncovers the complex transport dynamics of elongated structures in vortical flows, highlighting the transition from Levy to Brownian walks and chaos in rigid filaments, supported by simulations and experiments.

Findings

Filaments exhibit Levy walks with diffusion exponents decreasing as length increases.

Transport behaviors include random walks, ballistic motion, and trapping.

Rigid filaments can still display chaotic dynamics and diverse transport states.

Abstract

In cellular vortical flows, namely arrays of counter-rotating vortices, short but flexible filaments can show simple random walks through their stretch-coil interactions with flow stagnation points. Here, we study the dynamics of semi-rigid filaments long enough to broadly sample the vortical field. Using simulation, we find a surprising variety of long-time transport behavior -- random walks, ballistic transport, and trapping -- depending upon the filament's relative length and effective flexibility. Moreover, we find that filaments execute L\'evy walks whose diffusion exponents generally decrease with increasing filament length, until transitioning to Brownian walks. Lyapunov exponents likewise increase with length. Even completely rigid filaments, whose dynamics is finite-dimensional, show a surprising variety of transport states and chaos. Fast filament dispersal is related to an underlying geometry of ``conveyor belts''. Evidence for these various transport states are found in experiments using arrays of counter-rotating rollers, immersed in a fluid and transporting a flexible ribbon.