Free-Fall Motion of Cluster of Equal Spheres in Quiescent Fluid at Low Reynolds Numbers.

Experiments for a cluster of equal spheres falling freely in quiescent glycerol at low Reynolds numbers are carried out using a large test tank. The number of spheres tested is from 3 to 8. When the number of spheres is below 6, several equal spheres, set adjacent at the initial time, arrange themselves to take regular polygonal positions on the same horizontal plane. On the other hand, in the case of numbers above 6, equal spheres, set adjacent at the initial time, do not take regular polygonal positions on the same horizontal plane. The numerical results obtained using Stokes' flow field do not predict these experimental results. However the numerical results obtained using Oseen's flow field agree well with the experimental results.