Interaction of Three Settling Balls in an Incompressible Viscous Newtonian Fluid

Three balls sediment in a narrow vertical channel with rectangular cross section containing an incompressible viscous Newtonian fluid. We have obtained drafting, kissing, and tumbling several times and then a stable configuration.

the dimension of computational domain = (0,1.5)x(0,0.25)x(0,6) initially, then it moves vertically with the center of the lower one
The time interval: [0, 15]
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 mean speed is 2.4653 for 13 < t < 15.
the averaged particle Reynolds number is about 49.304 for 13 < t < 15.