On the settling of a spherical particle in slightly perturbed ambient fluid

Abstract

Direct numerical simulations of the settling of a spherical particle under the action of gravity in a slightly perturbed ambient fluid have been performed. The ambient perturbations are generated using a synthetic turbulence inflow generator method, and their length scale and intensity are varied to study their influence on the particle motion. The Galileo number is 151 and the solid-to-fluid density ratio is 1.5, so that in the absence of perturbations, the particle settles following a steady vertical trajectory. It has been found that the ambient perturbations trigger the formation of double-threaded vortical structures in the wake of the particle. These structures resemble those that appear in the oblique oscillating regime that is found in the absence of perturbations at higher Galileo numbers. Due to the flow perturbations the particle is pushed randomly in all directions, and this results in a combination of slow lateral drifts along fixed directions and relatively fast excursions in random directions. The particle response has been characterized using probability density functions of the velocity in the cross-plane and persistence probability. The slow drifts are strongly influenced by the size of the perturbations and by the rotational motion of the particle, while the intensity of the perturbations seems to play a minor role.