  The sedimentation of a pair of rigid circular particles in a two-dimensional
vertical channel containing a Newtonian fluid is investigated numerically, for
terminal particle Reynolds numbers ranging from 1 to 10, and for a confinement
ratio equal to 4. While it is widely admitted that sufficiently inertial pairs
should sediment by performing a regular DKT oscillation
(Drafting-Kissing-Tumbling), the present analysis shows in contrast that a
chaotic regime can also exist for such particles, leading to a much slower
sedimentation velocity. It consists of a nearly horizontal pair, corresponding
to a maximum effective blockage ratio, and performing a quasiperiodic
transition to chaos under increasing the particle weight. For less inertial
regimes, the classical oblique doublet structure and its complex behavior
(multiple stable states and hysteresis, period-doubling cascade and chaotic
attractor) are recovered, in agreement with previous work [Aidun & Ding,
Physics of Fluids 15(6), 2003]. As a consequence of these various behaviors,
the link between the terminal Reynolds number and the non-dimensional driving
force is complex: it contains several branches displaying hysteresis as well as
various bifurcations. For the range of Reynolds number considered here, a
global bifurcation diagram is given.
