  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.

Fluid Dynamics