Highly-resolved simualtions of hindered settling of porous particles

Suspended sediment transport in natural and engineered aquatic systems is often governed by the settling behavior of porous and permeable particles such as flocs, aggregates, and organic-rich sediments. Understanding how particle permeability influences hindered settling is therefore essential for predicting sediment residence times, vertical fluxes, and deposition rates in rivers, lakes, and reservoirs. We investigate the settling dynamics of suspensions of highly porous and permeable particles using particle-resolved direct numerical simulations in the viscously dominated regime. The simulations employ a coupled Euler–Lagrange framework that explicitly accounts for particle permeability, allowing us to systematically quantify bulk settling velocities as a function of particle permeability and particle volume fraction. Our results show that the bulk settling velocity follows the classical Richardson–Zaki power-law scaling with volume fraction, but that particle permeability significantly modifies hindered settling. Suspensions composed of more permeable particles settle substantially faster at increasing concentrations: at a particle volume fraction of 30%, the bulk settling velocity differs by up to 116% between the least and most permeable particles considered. This enhanced settling is explained by permeability-dependent modifications of the fluid counterflows induced by particle displacement. Quantitative analysis of the mean vertical fluid velocity demonstrates that more permeable particles generate weaker upward counterflows, thereby reducing hydrodynamic resistance to settling. We further examine how velocity fluctuations, particle self-diffusivity, and suspension microstructure depend on particle permeability and concentration. Velocity fluctuations increase systematically with particle volume fraction and are strongest for the least permeable particles. Analysis of Voronoï tessellation and pairwise particle distributions reveals pronounced permeability-dependent microstructural differences, with stronger clustering and broader Voronoï cell volume distributions at low permeability, attributed to enhanced lubrication forces. In contrast, higher permeability leads to a greater likelihood of close particle proximity due to weakened interparticle pressure effects. These findings highlight particle permeability as a key control on hindered settling and suspension structure, with direct implications for process-based numerical models of sediment transport and deposition in open water environments.