Concentrations of inertial particles in the turbulent wake of an immobile sphere

TL;DR

This study uses direct numerical simulations to analyze how inertial particles concentrate around an immobile sphere in laminar and turbulent wakes, revealing mechanisms of clustering and their impact on particle collisions.

Contribution

It identifies specific concentration patterns and mechanisms of particle clustering in the wake of a sphere, including the shadow effect and vortex-induced expulsion, in both laminar and turbulent flows.

Findings

Particles form preferential concentrations in wake regions.

Concentration profiles differ between laminar and turbulent wakes.

Particle collisions are enhanced by increased concentration and velocity differences.

Abstract

Direct numerical simulations are used to study the interaction of a stream of small heavy inertial particles with the laminar and turbulent wakes of an immobile sphere facing an incompressible uniform inflow. Particles that do not collide with the obstacle but move past it, are found to form preferential concentrations both in the sphere boundary layer and in its wake. In the laminar case, the upstream diverging flow pattern is responsible for particle clustering on a cylinder that extends far downstream the sphere. The interior of this surface contains no particles and can be seen as a shadow of the large obstacle. Such concentration profiles are also present in the case of turbulent wakes but show a finite extension. The sphere shadow is followed by a region around the axis of symmetry where the concentration is higher than the average. It originates from a resonant centrifugal expulsion of particles from shed vortices. The consequence of this concentration mechanism on monodisperse inter-particle collisions is also briefly discussed. They are enhanced by both the increased concentration and the presence of large velocity differences between particles in the wake.