Open-flow mixing: Experimental evidence for strange eigenmodes

TL;DR

This paper experimentally studies chaotic mixing in channel flows, demonstrating the existence of strange eigenmodes in certain configurations and contrasting them with non-self-similar mixing dynamics caused by boundary effects.

Contribution

It provides experimental evidence for strange eigenmodes in open-flow mixing and models their emergence using idealized maps, highlighting the impact of boundary conditions.

Findings

Strange eigenmodes observed in mixing with separated walls.

Self-similar concentration patterns decay over time.

Boundary effects prevent eigenmode formation in other configurations.

Abstract

We investigate experimentally the mixing dynamics in a channel flow with a finite stirring region undergoing chaotic advection. We study the homogenization of dye in two variants of an eggbeater stirring protocol that differ in the extent of their mixing region. In the first case, the mixing region is separated from the side walls of the channel, while in the second it extends to the walls. For the first case, we observe the onset of a permanent concentration pattern that repeats over time with decaying intensity. A quantitative analysis of the concentration field of dye confirms the convergence to a self-similar pattern, akin to the strange eigenmodes previously observed in closed flows. We model this phenomenon using an idealized map, where an analysis of the mixing dynamics explains the convergence to an eigenmode. In contrast, for the second case the presence of no-slip walls and separation points on the frontier of the mixing region leads to non-self-similar mixing dynamics.