Passive directors in turbulence

TL;DR

This paper investigates the behavior of small spheroids in turbulence, revealing that their orientation angles are often unexpectedly large due to fractal attractors and scar line structures, challenging previous assumptions.

Contribution

It uncovers the mechanisms behind large relative angles of passive directors in turbulence, highlighting the role of fractal attractors and scar line formations.

Findings

Angles between nearby spheroids are often large, contrary to expectations.

Two mechanisms identified: fractal attractors and scar line-like steps.

Turbulent strain dynamics lead to complex, fractal director patterns.

Abstract

In experiments and numerical simulations we measured angles between the symmetry axes of small spheroids advected in turbulence ("passive directors"). Since turbulent strains tend to align nearby spheroids, one might think that their relative angles are quite small. We show that this intuition fails in general because angles between the symmetry axes of nearby particles are anomalously large. We identify two mechanisms that cause this phenomenon. First, the dynamics evolves to a fractal attractor despite the fact that the fluid velocity is spatially smooth at small scales. Second, this fractal forms steps akin to scar lines observed in the director patterns for random or chaotic two-dimensional maps.