  In this article we present a numerical study of the dynamics of two disks
settling in a narrow vertical channel filled with Oldroyd-B fluid. Two kinds of
particle dynamics are obtained: (i) periodic interaction between two disks and
(ii) the chain formation of two disks. For the periodic interaction of two
disks, two different motions are obtained: (a) two disks stay far apart and
interact periodically and (b) two disks interact closely and then far apart in
a periodic way, like the drafting, kissing and tumbling of two disks
sedimenting in Newtonian fluid, due to the lack of strong enough elastic force.
For the formation of two disk chain occurred at higher values of the elasticity
number, it is either a tilted chain or a vertical chain. The tilted chain can
be obtained for either that the elasticity number is less than the critical
value for having the vertical chain or that the Mach number is greater than the
critical value for a long body to fall broadside-on. Hence the values of the
elasticity number and the Mach number determine whether the the chain can be
formed and the orientation of the chain.
