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 many times.

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 = 1.25
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
for 10 < t < 15, the averaged mean speed is 5.565, the minimum speed of the ball is 2.713 and the maximum speed of the ball is 8.980.
for 10 < t < 15, the averaged particle Reynolds number is about 111.3, the minimum is 54.26 and the maximum is 179.6.