Mechanisms for Selective Radial Dispersion of Microparticles in the Transitional Region of a Confined Turbulent Round Jet

The dispersion of particles of different diameters in a confined turbulent round jet is dominated by the local interactions between dispersed phase and large-scale, time dependent flow structures which populate the near-field of the jet. In this work, we address first the problem of identifying the flow structures which form in a three-dimensional, turbulent confined round jet, considering also the influence of the solid walls. Second, we examine particle dispersion specifically focusing on their preferential distribution. The three-dimensional, time dependent flow field is calculated using a finite-difference LES solver of the Navier-Stokes equations in a cylindrical reference frame, whereas the dispersion of particles is computed using a one-way coupling Lagrangian approach. The flow field is characterized by mutually interacting, transitional structures of different temporal and spatial scales. We investigate the behavior of particles, in turn characterized by different size and time-scales, with the object of examining their selective response to the various scales of the flow. Our analyses confirm that multiple organized structures grow up following different types of instability, and successively interact, creating the conditions for the developing of a three-dimensional vorticity field downstream the near-field of the jet. We show that the vortical structures interact selectively with the different size particles, producing different distribution patterns and dispersion rates qualitatively depending on the particle-to-fluid Stokes number. Following the model proposed previously, we complete the description of the particle dispersion mechanism based on the action of the large spanwise vortices, evidencing the effect of the secondary structures (ribs) on the smaller particles. We characterize the different distribution pattern of particle swarms and we analyze from a qualitative viewpoint particle dispersion behavior with the dynamics of the different transitional structures providing guidelines for dispersion control.