Rotations of small, inertialess triaxial ellipsoids in isotropic turbulence

TL;DR

This study investigates how small, inertialess triaxial ellipsoids rotate in isotropic turbulence, revealing shape-dependent rotation variance, vorticity dominance, and axis-specific persistence of angular velocities.

Contribution

It provides new insights into the rotational dynamics of inertialess triaxial ellipsoids in turbulence, expanding on previous work by analyzing shape effects and axis-specific behaviors.

Findings

Particle enstrophy increases with elongation.

Vorticity dominates particle rotation.

Longest axis rotations are most persistent.

Abstract

The statistics of rotational motion of small, inertialess triaxial ellipsoids are computed along Lagrangian trajectories extracted from direct numerical simulations of homogeneous isotropic turbulence. The particle angular velocity and its components along the three principal axes of the particle are considered, expanding on the results presented by \citet{ChevillardMeneveau13}. The variance of the particle angular velocity, referred to as the particle enstrophy, is found to increase for particles with elongated shapes. This trend is explained by considering the contributions of vorticity and strain-rate to particle rotation. It is found that the majority of particle enstrophy is due to fluid vorticity. Strain-rate-induced rotations, which are sensitive to shape, are mostly cancelled by strain-vorticity interactions. The remainder of the strain-rate-induced rotations are responsible for weak variations in particle enstrophy. For particles of all shapes, the majority of the enstrophy is in rotations about the longest axis, which is due to alignment between the longest axis and fluid vorticity. The integral timescale for particle angular velocities about each axis reveals that rotations are most persistent about the longest axis, but that a full revolution is rare.