Abstract
We performed numerical experiments to investigate the mixing of stratified suspensions composed of different particle types by gravitational sedimentation. The mixing process is controlled by a dimensionless group
Y
m
∼
U
f
/
U
St
1
, where
U
f
is a typical velocity of a macroscopic sedimenting finger and
U
St
1
is the Stokes settling velocity of a single spherical particle in the upper suspension. The effects of components of
Y
m
, in particular, terminal velocities of particles, were investigated. For
Y
m
=
100
, no large difference was observed for the difference of components of
Y
m
, and it was confirmed that the mixing rate is determined by
Y
m
, because macroscopic (vessel-scale) mixing is dominant for large
Y
m
. For
Y
m
=
5
, macroscopic mixing and microscopic (individual particle-level) mixing due to the particle terminal velocity difference are of the same order, while completely different mixing patterns were observed for positive, zero, and negative terminal velocity differences: macroscopic mixing is promoted by the increase in apparent density due to microscopic mixing, small macroscopic mixing is suppressed by the individual particle settling, and jetting mixing occurs owing to pure liquid layer formation.
