Navier-Stokes simulations of a temporally growing mixing layer are emp
loyed to investigate three-dimensional mechanisms for the dispersion a
nd accumulation of small, heavy, spherical particles. It is found that
in particular the presence of the streamwise braid vortices gives ris
e to additional dynamical effects that modify the concentration, dispe
rsion, and suspension patterns observed in two-dimensional situations.
Intense stretching and folding by the evolving three-dimensional vort
icity field, when combined with inertial effects such as ejection by t
he concentrated streamwise vortices, strongly distorts the geometry of
both clear fluid and particle laden regions. Different time scales ca
n be associated with the spanwise and streamwise vortices, so that the
se distinct vortical systems can selectively affect different classes
of particles. (C) 1996 American Institute of Physics.