  The sedimentation of two spherical solid objects in a viscous fluid has been
extensively investigated and well understood. However, a pair of flat disks (in
three dimensions) settling in the fluid shows more complex hydrodynamic
behaviors. The present work aims to improve understanding of this phenomenon by
performing Direct Numerical Simulations (DNS) and physical experiments. The
present results show that the sedimentation processes are significantly
influenced by disk shape, characterized by a dimensionless moment of inertia
I*, and Reynolds number of the leading disk Re. For the flatter disks with
smaller I*, steady falling with enduring contact transits to periodic swinging
with intermittent contacts as Re increases. The disks with larger I* tend to
fall in a Drafting-Kissing-Tumbling (DKT) mode at low Re and to remain
separated at high Re. Based on I* and Re, a phase diagram is created to
classify the two-disk falling into ten distinctive patterns. The planar motion
or three-dimensional motion of the disks is determined primarily by Re.
Turbulent disturbance flows at a high Re contribute to the chaotic
three-dimensional rotation of the disks. The chance for the two disks to
contact is increased when I* and Re are reduced.
