# Orientation Dynamics of Sedimenting Anisotropic Particles in Turbulence

## TL;DR

This study investigates how small anisotropic particles orient and settle in turbulence, revealing non-Gaussian orientation distributions and the influence of particle shape and flow parameters on settling behavior.

## Contribution

It combines direct numerical simulations with theory to analyze the orientation dynamics and settling velocities of spheroids in turbulence, highlighting non-monotonic effects and deviations at finite Stokes numbers.

## Key findings

- Orientation distributions are non-Gaussian and localized near broadside-on orientation.
- Settling velocities depend non-monotonically on spheroid aspect ratio.
- Preferential sweeping causes settling velocities to exceed orientational averages at higher Stokes numbers.

## Abstract

We examine the dynamics of small anisotropic particles (spheroids) sedimenting through homogeneous isotropic turbulence using direct numerical simulations and theory. The gravity-induced inertial torque acting on sub-Kolmogorov spheroids leads to pronouncedly non-Gaussian orientation distributions localized about the broadside-on(to gravity) orientation. Orientation distributions and average settling velocities are obtained over a wide range of spheroid aspect ratios, Stokes and Froude numbers. Orientational moments from the simulations compare well with analytical predictions in the inertialess rapid-settling limit, with both exhibiting a non-monotonic dependence on spheroid aspect ratio. Deviations arise at Stokes numbers of order unity due to a spatially inhomogeneous particle concentration field resulting from a preferential sweeping effect; as a consequence, the time-averaged particle settling velocities exceed the orientationally averaged estimates.