  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.
