“…1 illustrates a schematic view of the present problem. Figure.1 a schematic figure of current problem [15] The forces acting on the spheres are the fluid-drag, F D , fluid lift force, F L , buoyancy force, F B , gravitational force, F g , solid-solid resistance force due to rolling, F R , virtual mass force, F VM due to relative acceleration of the fluid around the particle. It is reasonable to postulate that since smooth walled tubes have been studied, F R is expected to be negligible (or alternately F D + F R can be regarded as the total resistance to sphere motion, which is included in the drag coefficient).…”