Particle-Resolved Direct Numerical Simulations of Fractal Aggregates Settling through Density-Stratified Fluid

The settling of fractal aggregates with complex geometries through fluid of varying density plays a crucial role in many industrial and environmental systems. To study the influence that aggregate shape and particle density have on settling dynamics, by performing Particle-Resolved Direct Numerical Simulations (PR-DNS) we investigate the settling of fractal aggregates through a density interface. We find that the velocity of the aggregates passing through the interface is determined by the fractal dimension, the relative density of the particles to the fluid, and the Galileo number. Crucial to determining the velocity is the retention of fluid from the less dense upper layer in the aggregates' pore spaces, which imposes an upwards buoyancy force on the aggregate as it passes into the denser lower layer. The relative density between the internal fluid and the aggregate itself, as well as the timescale over which the denser external fluid diffuses and convects into the aggregate's pore space, can greatly affect the relationship between aggregate compactness and the settling velocity, and whether a more porous aggregate settles faster or slower than a less porous one. From this, we introduce general relationships governing the settling velocity of aggregates based on the fractal dimension, Galileo number, and particle and fluid densities.