The effect of streamwise braid vortices on the particle dispersion in a plane mixing layer .2. Nonlinear particle dynamics

A computational investigation of the nonlinear dynamics of heavy particles in a row of counterrotating strained vortices is presented. By tracking the particles numerically in the quasi-two-dimensional fluid velocity field, information is obtained on the nature of their trajectories, as well as on probability distribution functions and potential accumulation regions. The particle behavior is discussed as a function of the dimensionless strain rate, the particle Stokes number St, and the dimensionless gravity parameter Fr. Only for very low values of the St can the particles accumulate at the vortex centers. For moderate values of St, they remain trapped on closed trajectories around the vortex centers. Increasing St leads to periodic open trajectories that allow for spanwise transport of the particles. Further bifurcations lead to the generation of multiple trajectories, as well as to subharmonic solutions. Eventually, intermittent and chaotic particle dynamics are observed. In the chaotic regime, a simplified flow model is employed in order to derive various scaling laws for the particle concentration field. For strong levels of gravity, the accumulation of large numbers of particles is observed in the upwelling regions predicted in part I of the present investigation [B. Marcu and E. Meiburg, Phys. Fluids 8, 715 (1996)]. (C) 1996 American Institute of Physics.