Rotation of anisotropic particles in Rayleigh-B\'enard turbulence

TL;DR

This study investigates how anisotropic particles rotate and align in Rayleigh-Bénard turbulence, revealing shape-dependent tumbling rates and orientations influenced by turbulent fluctuations and shear flows.

Contribution

It uncovers the shape-dependent angular dynamics of inertialess anisotropic particles in Rayleigh-Bénard turbulence, linking particle orientation to flow properties and boundary layer effects.

Findings

Maximal tumbling rates occur for weakly oblate particles.

Prolate particles align with the fluid velocity.

Oblate particles orient with the temperature gradient.

Abstract

Inertialess anisotropic particles in a Rayleigh-B\'enard turbulent flow show maximal tumbling rates for weakly oblate shapes, in contrast with the universal behaviour observed in developed turbulence where the mean tumbling rate monotonically decreases with the particle aspect ratio. This is due to the concurrent effect of turbulent fluctuations and of a mean shear flow whose intensity, we show, is determined by the kinetic boundary layers. In Rayleigh-B\'enard turbulence prolate particles align preferentially with the fluid velocity, while oblate ones orient with the temperature gradient. This analysis elucidates the link between particle angular dynamics and small-scale properties of convective turbulence and has implications for the wider class of sheared turbulent flows.