Role of gravity on preferential clustering of microparticles in unsteady wake flows
Direct numerical simulations are carried out for particle-laden flow over the cylinder to investigate preferential clustering of particles in an unbounded vertical channel flow. The flow is examined at Reynolds numbers Re=100 and 200 for varying particle Stokes number, particle loadings and Froude numbers to quantify the combined influence of particle inertia and gravitational settling on particle motion. The unladen flow exhibits the classical vortex shedding pattern observed in flow over bluff bodies at both Reynolds numbers. Reynolds number dependent wake width, wake recovery, and velocity-deficit evolution are observed, characterizing the coherent flow structures that govern particle dynamics. In particle-laden unsteady wake flows, the non-uniform particle distribution leads to formation of coherent voids and clusters, whose shape are directly correlated with background flow dynamics. Gravity modifies particle-fluid interaction, which leads to an increase in slip velocity, weakens vortex-induced particle clustering and promotes them to travel through vortices, resulting in a transition of the void shape from individual leaf-like structure to snake-like void zone and eventually into a nearly vertical void structure. In upstream region infront of the cylinder, inertial particles form a bow-shock-like structure whose extent increases with increase in Stokes number and finite Froude conditions. Voronoi based analysis combined with local Q values is used to investigate effect of gravity and inertia on particle distribution. The dimensionless settling velocity, St/Fr^2, is identified as the governing parameter controlling the evolution of void shape, normalized void cell area and the probability distribution of Voronoi cell areas. The effect of wake dynamics, particle inertia, and gravity is reported to jointly govern preferential clustering in bluff-body wakes.