The influence of permeability on the settling behavior of porous flocs

Currently, the Stokes-based relation for a drag coefficient is used in the majority of experimental studies to calculate the settling velocity of flocculated sediments. Despite the fact that there are more sophisticated models to calculate settling velocity, it is usually assumed that the main mechanism affecting the settling velocity is the reduced density of the sediment. In the present work, we, therefore, focus on the influence of permeability of porous sediments. We analyze available models for the settling velocity of sediments and compare them with existing experimental measurements of various sediment types. We show that even though various settling models yield similar results to capture experimental data of single flocs settling in quiescent fluid, the mechanisms of drag reduction are different. To this end, we carry out particle resolved Direct Numerical Simulations that treat porous flocs as permeable objects. Using this approach to simulate more complex scenarios such as settling in a stratified fluid and hindered settling, we illustrate that permeability of flocs is a key parameter that strongly affects the settling behavior and cannot be neglected.