Rotational dynamics of bottom-heavy rods in turbulence from experiments and numerical simulations

TL;DR

This study combines experiments and simulations to investigate how bottom-heavy, asymmetrical particles behave in turbulent flows, revealing how slight mass inhomogeneity influences their orientation and tumbling dynamics.

Contribution

It introduces a comprehensive experimental and numerical analysis of bottom-heavy particles in turbulence, including a new theoretical prediction for their orientation and tumbling behavior.

Findings

Bottom-heavy particles tend to align vertically in turbulence.

A perturbative theory accurately predicts particle orientation and tumbling rates.

The tumbling rate distribution's heavy tail is minimally affected by bottom-heaviness.

Abstract

We successfully perform the three-dimensional tracking in a turbulent fluid flow of small asymmetrical particles that are neutrally-buoyant and bottom-heavy, i.e., they have a non-homogeneous mass distribution along their symmetry axis. We experimentally show how a tiny mass inhomogeneity can affect the particle orientation along the preferred vertical direction and modify its tumbling rate. The experiment is complemented by a series of simulations based on realistic Navier-Stokes turbulence and on a point-like particle model that is capable to explore the full range of parameter space characterized by the gravitational torque stability number and by the particle aspect ratio. We propose a theoretical perturbative prediction valid in the high bottom-heaviness regime that agrees well with the observed preferential orientation and tumbling rate of the particles. We also show that the heavy-tail shape of the probability distribution function of the tumbling rate is weakly affected by the bottom-heaviness of the particles.