Anisotropic clustering of inertial particles in homogeneous shear flow

TL;DR

This paper investigates how mean shear flow influences the anisotropic clustering of inertial particles in turbulence, revealing that shear can induce significant anisotropy even at small scales, depending on particle relaxation times.

Contribution

It introduces a novel angular distribution function tool to quantify anisotropic clustering in shear flows through DNS data, extending understanding beyond isotropic turbulence.

Findings

Shear flow preferentially orients particle clusters.

Anisotropic clustering persists at small scales, influenced by particle relaxation time.

Anisotropy can reach over 30% in probability variation at viscous scales.

Abstract

Recently, clustering of inertial particles in turbulence has been thoroughly analyzed for statistically homogeneous isotropic flows. Phenomenologically, spatial homogeneity of particles configurations is broken by the advection of a range of eddies determined by the Stokes relaxation time of the particles which results in a multi-scale distribution of local concentrations and voids. Much less is known concerning anisotropic flows. Here, by addressing direct numerical simulations (DNS) of a statistically steady particle-laden homogeneous shear flow, we provide evidence that the mean shear preferentially orients particle patterns. By imprinting anisotropy on large scales velocity fluctuations, the shear indirectly affects the geometry of the clusters. Quantitative evaluation is provided by a purposely designed tool, the angular distribution function of particle pairs (ADF), which allows to address the anisotropy content of particles aggregates on a scale by scale basis. The data provide evidence that, depending on the Stokes relaxation time of the particles, anisotropic clustering may occur even in the range of scales where the carrier phase velocity field is already recovering isotropy. The strength of the singularity in the anisotropic component of the ADF quantifies the level of fine scale anisotropy, which may even reach values of more than 30% direction-dependent variation in the probability to find two close-by particles at viscous scale separation.