Preferential rotation of chiral dipoles in isotropic turbulence

TL;DR

This study demonstrates that chiral dipole particles exhibit a preferential rotation in isotropic turbulence, with their spinning behavior influenced by particle shape and flow strain, supported by experiments and simulations.

Contribution

It introduces a novel experimental and modeling approach to understand the rotation of chiral dipoles in turbulence, linking particle shape to spinning behavior.

Findings

Chiral dipoles show a non-zero mean spinning rate in turbulence.

The particle shape influences alignment with strain eigenvectors.

Model predictions agree well with experimental measurements.

Abstract

Particles in the shape of chiral dipoles show a preferential rotation in three dimensional homogeneous isotropic turbulence. A chiral dipole consists of a rod with two helices of opposite handedness, one at each end. We can use 3d printing to fabricate these particles with length in the inertial range and track their rotations in a turbulent flow between oscillating grids. High aspect ratio chiral dipoles will align with the extensional eigenvectors of the strain rate tensor and the helical ends will respond to the strain field by spinning around its long axis. The mean of the measured spinning rate is non-zero and reflects the average stretching the particles experience. We use Stokesian dynamics simulations of chiral dipoles in pure strain flow to quantify the dependence of spinning on particle shape. Based on the known response to pure strain, we build a model that gives the spinning rate of small chiral dipoles using Lagrangian velocity gradients from high resolution direct numerical simulations. The statistics of chiral dipole spinning determined with this model show surprisingly good agreement with the measured spinning of much larger chiral dipoles in the experiments.