Scale-dependent particle clustering in transitional wake flow

Understanding of inertial particle dispersion and mixing in wake flows is essential given its practical relevance yet inadequate. We employ one-way coupled simulations to investigate the preferential concentration of point particles in an unsteady three-dimensional circular cylinder wake at Re=200. The presence of streamwise-oriented vortical braids induced by mode A instability gives rise to the diverse clustering and void topology and heavily influences the particle dynamics in the near wake. Volume-averaged Voronoï analysis is used to investigate the Sk-dependency on the sizes of clusters and voids. A prominent inertia effect presents at cluster scale while the void scale exhibits self-similarity. Newly proposed physics-based thresholds of clusters and voids are distinctive from the probability-distribution-based threshold at cluster scale. An inner layer of particle aggregation is formed by the neighboring Kármán rollers while the co-existing streamwise braids shape the discrete clumps on the otherwise thin ribbon-like clusters. Particle dynamics diminishes as Sk increases and particle velocity is further reduced by streamwise vortices. The inertial effect varies non-monotonically but reveals strongest at Sk=1 as most particles reside in high-strain/low-vorticity regions.