Measurements of the Solid-body Rotation of Anisotropic Particles in 3D Turbulence

TL;DR

This study introduces a novel 3D measurement method for analyzing the rotational behavior of anisotropic particles in turbulence, revealing how particle shape influences tumbling and alignment with flow vorticity.

Contribution

A new stereoscopic imaging technique using 3D printed particles to measure the rotation and alignment of anisotropic particles in turbulent flow.

Findings

Disks tumble faster than rods in turbulence.

Crosses align with flow rotation axes, confirming flow-particle interaction theories.

Particles of different shapes exhibit distinct rotational dynamics.

Abstract

We introduce a new method to measure Lagrangian vorticity and the rotational dynamics of anisotropic particles in a turbulent fluid flow. We use 3D printing technology to fabricate crosses (two perpendicular rods) and jacks (three mutually perpendicular rods). Time-resolved measurements of their orientation and solid-body rotation rate are obtained from stereoscopic video images of their motion in a turbulent flow between oscillating grids with $R_\lambda$=$91$. The advected particles have a largest dimension of 6 times the Kolmogorov length, making them a good approximation to anisotropic tracer particles. Crosses rotate like disks and jacks rotate like spheres, so these measurements, combined with previous measurements of tracer rods, allow experimental study of ellipsoids across the full range of aspect ratios. The measured mean square tumbling rate, $\langle \dot{p}_i \dot{p}_i \rangle$, confirms previous direct numerical simulations that indicate that disks tumble much more rapidly than rods. Measurements of the alignment of crosses with the direction of the solid-body rotation rate vector provide the first direct observation of the alignment of anisotropic particles by the velocity gradients of the flow.