On the effect of nonuniform seeding on particle dispersion in two-dimensional mixing layers

The effect of nonuniform seeding on the dispersion of fluid elements and heavy particles has been investigated in two-dimensional, incompressible mixing layers. The cross-stream dispersion of fluid elements can be enhanced using nonuniform seeding with most particles near the saddle point because of the greater lateral extent of streamlines in this region, though the increase in dispersion compared to a uniform seeding occurs only after a few vortex turnover times. Compared to fluid elements, additional mechanisms—ejection from vortex cores, separatrix crossing, and the effect of the initial particle velocity—must be considered in the analysis of nonuniform seeding on heavy particle dispersion. The influences of these additional mechanisms are first investigated in a Stuart vortex. With increasing response time, vortex ejection and separatrix crossing shift the streamwise position maximizing lateral transport towards the vortex core. While changes in the initial particle velocity increase/decrease displacement, lateral dispersion may still be enhanced by appropriate nonuniform seeding of particles near the saddle point. Numerical simulations of the incompressible Navier–Stokes equations are then used to study cross-stream dispersion in a temporally evolving two-dimensional mixing layer. Stokes numbers St in the calculations were 0.05, 1, 10, and 100 where St is defined as the ratio of the particle response time to the time scale formed using the vorticity thickness of the initial mean flow. Particles were initially distributed nonuniformly at the interface between the two streams or along a line parallel to the interface. Simulation results show that the seeding location maximizing lateral dispersion is both time and Stokes number dependent, with larger increases in dispersion for the interface seeding. For Stokes numbers of order unity cross-stream dispersion exhibits a weak dependence on initial position since particles are efficiently ejected from the vortex core with subsequent motion confined to the nearby region outside the separatrix in one of the freestreams. Simulation results also show that substantial increases in particle dispersion can be obtained using nonuniform seedings relative to that obtained from an initially uniform distribution.