Settling of fractal aggregates in fluids of uniform density and across density gradients

The settling dynamics of fractal aggregates in constant-density environments and through miscible density interfaces are investigated via particle-resolved direct numerical simulations, which provide the settling velocity as a function of the fractal dimension, Galileo number, and particle and fluid densities. In a fluid of uniform density the settling velocity increases with the fractal dimension and the Galileo number. This behaviour is captured by an empirical relationship that holds over a broad range of parameter values. In the presence of a miscible density interface, consistent with earlier observations we observe that lighter fluid is carried into the denser layer by the aggregate’s pore spaces, which we quantify based on the concept of $\alpha$ -shapes. This causes the aggregate to slow down, until the lighter pore fluid is replaced by the denser fluid via a combination of diffusion and convection. The degree of the aggregate’s slowdown depends on the ratio of the density differences between the aggregate and the two fluids, and it can again be captured by an empirical relationship. The duration of the slowdown is determined by the pore fluid replacement time, which in turn depends on the relative importance of convection and diffusion, and hence on the aggregate’s geometry. A relationship is derived that captures the dependence of this replacement time on the shape of the aggregate, the ratio of the density differences, and the Galileo number.