Clusters and coherent voids in particle-laden wake flow

Inertial point particles suspended in a two-dimensional unsteady circular cylinder flow at R e = 100 are studied by one-way coupled three-dimensional numerical simulations. The striking clustering pattern in the near-wake is strongly correlated with the periodically shed Karman vortex cells. The particles are expelled from the vortex cores due to the centrifugal mechanism and coherent voids encompassing the local Karman cells are therefore observed. The particles clustering at the upstream side of each void hole form a smooth edge, where the particle velocity magnitude is consistently lower than at the downstream edge of the voids. The trajectories of these particles originate from the side of the cylinder where the sign of vorticity is opposite to that of the vortex encompassed by the corresponding void hole. The particles are seen to decelerate along a substantial part of their trajectories. Particle inertia is parameterized by means of a Stokes number S k and smooth edges around the void holes still exist when S k is increased, although their formation is delayed due to larger inertia. Increasing inertia contributes to a decoupling of the particle acceleration from the slip velocity, which almost coincided at S k = 1 .