Aggregate structure and hydrodynamics of aggregated systems

The hydrodynamic behavior of fractal aggregates is analyzed both for an aggregate settling individually and aggregates sedimenting collectively in the range of dense arrangement, where the aggregate size depends on the overall solid concentration due to fractal structure and volume constraint. It is shown theoretically that the sedimentation velocity divided by the free settling speed of aggregates present does not depend on the aggregate size but on the fractal dimension only. This dependence is documented experimentally by comparing the hydrodynamic behavior of a solid-liquid system in both concentration ranges. The study of the sensitivity of the model to a small difference in fractal dimensions of aggregates falling in both concentration regimes is performed to show that the hydrodynamic behavior is only slightly different. Replacing the fractal aggregates by the hydrodynamic equivalent impermeable particles leads to a permeability formula for hard sphere system, which is compatible with the standard formulae.