Interaction of Two Settling Balls in an Incompressible Viscous Newtonian Fluid: One on top of another initially

Two balls sediment in a narrow vertical channel with rectangular cross section containing an incompressible viscous Newtonian fluid. We have observed drafting, kissing, and tumbling twice and then two balls sediment in a stable configuration.

the dimension of computational domain = (0,1)x(0,0.25)x(0,4) initially, then it moves vertically with the center of the lower one
The time interval: [0, 10]
One is on top of another initially
the diameters = 0.2
the density of balls = 1.1
the density of fluid = 1
the viscosity = 0.01
the mesh size for the velocity field = 1/96
the mesh size for the pressure = 1/48
the time step=0.001
the averaged speed of 2 balls (8 < t < 10) = 2.4362
the averaged particle Re (8< t < 10) = 48.724