Do finite size neutrally buoyant particles cluster?

TL;DR

This study examines whether neutrally buoyant particles significantly larger than the dissipation scale tend to cluster in turbulence, finding they do not, challenging assumptions about Stokes number as a clustering predictor.

Contribution

The paper provides experimental evidence that large neutrally buoyant particles do not cluster in turbulence, highlighting limitations of Stokes number as a clustering indicator.

Findings

Particles do not cluster regardless of flow conditions.

Stokes number alone is insufficient to predict clustering.

Voronoi analysis confirms uniform particle distribution.

Abstract

We investigate the preferential concentration of particles which are neutrally buoyant but with a diameter significantly larger than the dissipation scale of the carrier flow. Such particles are known not to behave as flow tracers (Qureshi et al., Phys. Re. Lett. 2007) but whether they do cluster or not remains an open question. For this purpose, we take advantage of a new turbulence generating apparatus, the Lagrangian Exploration Module which produces homogeneous and isotropic turbulence in a closed water flow. The flow is seeded with neutrally buoyant particles with diameter 700\mum, corresponding to 4.4 to 17 times the turbulent dissipation scale when the rotation frequency of the impellers driving the flow goes from 2 Hz to 12 Hz, and spanning a range of Stokes numbers from 1.6 to 24.2. The spatial structuration of these inclusions is then investigated by a Voronoi tesselation analysis, as recently proposed by Monchaux et al. (Phys. Fluids 2010), from images of particle concentration field taken in a laser sheet at the center of the flow. No matter the rotating frequency and subsequently the Reynolds and Stokes numbers, the particles are found not to cluster. The Stokes number by itself is therefore shown to be an insufficient indicator of the clustering trend in particles laden flows.