Sedimentation of swarms of particles at low and moderate Reynolds numbers

The sedimentation of a cloud of particles in a viscous fluid at low and moderate Reynolds numbers has been studied using an Eulerian-Lagrangian multiphase flow approach. We looked at the volume fraction dependence of the settling cloud and find a similar dependence in the simulations as in the theoretical predictions of (Nitsche and Batchelor 1997). The average cloud settling velocity and the velocity fluctuations around this average are found to have a functional dependence on ö^1/3 at negligible Reynolds number. The velocity fluctuations display strong anisotropy with the magnitude of the vertical component almost three times the magnitude of the horizontal component. Similarities in the interaction between a system of two particle clouds and a system of two immiscible droplets was established with an observed increase in the velocity of the trailing cloud due to drag reduction in the wake of the leading cloud. The formation of the stagnation points at the leading front of the cloud is pointed to as the cause of shape deformation in these systems. Particle leakage at low Reynolds number was established and found to be directly related to the initial number of particles At higher Reynolds numbers, the cloud of particles evolved into an open torus and subsequently losing its axi-symmetry and breaking-up into a number of secondary clouds. This process is a type of Rayleigh-Taylor instability and the number of secondary drops was found in our simulations to be dependent on the shape of the boundaries of the flow domain used rather than the nature of the boundaries. Breakup at moderate Rec is found to occur after a critical aspect ratio is reached and a scaling was proposed for dependence of the breakup length and breakup time on Rec