We present particle-resolving Navier-Stokes simulation results for a variety of multiphase flow processes. These simulations are based on an Immersed Boundary Method approach, which accurately captures the flow around each particle and in each pore space. We will discuss several different applications, among them particle sedimentation, particle-turbulence interaction, submerged granular collapse processes, and flocculation in turbulent channel flows. The main focus will be on the influence of cohesive forces in such flows, especially the formation and break-up of aggregates.

For the settling of polydisperse cohesive sediment, the simulations reproduce several earlier experimental observations by other authors, such as the accelerated settling of sand and silt particles due to particle bonding, the looser packing of the cohesive sediment deposits, and the consolidation process of the deposit. For cohesive sediment in homogeneous isotropic turbulence, we observe a transient flocculation phase, followed by a statistically steady equilibrium phase. Flocculation proceeds most rapidly when the fluid and particle time scales are balanced and a suitably defined Stokes number is O(1). The equilibrium floc size distribution exhibits a preferred size that depends on the cohesive number. We observe that flocs are generally elongated by turbulent stresses before breakage.

Finally, we review a computational study of the flocculation dynamics and turbulence modulation in a channel flow laden with small, finite size, cohesive particles. We conduct a series of four-way coupled, grain-resolved direct numerical simulations, across which the strength of the cohesive force as well as particle inertia (density) are varied. The strength of cohesion is shown to govern the size and number of aggregates. Interestingly, despite the prevalence of shear-induced migration at higher particle Stokes numbers, the maximum floc size is shown to be insensitive to the particle density. We additionally analyze floc properties such as size, shape, and lifespan as a function of the wall normal distance. The higher mean velocity gradient and turbulent activity near the wall is found to facilitate initial aggregation by bringing particles together, while also prompting the breakup of large aggregates. Hence, for long times the largest particles are found near the channel center, where the turbulence is not as intense. Enabled by the fully coupled nature of the simulations, we also quantify the effect of aggregation on the turbulence.

When liquids non-wet solids, their mobility can become surprisingly high. Surprisingly? Of course, we expect that minimizing the effects of contact lines (the line that bounds “regular” drops and makes them adhesive and frictional) should boost the ability of drops to move – which is already worth being discussed and illustrated. But there is more than that, in particular when liquids are viscous – a case where they are even more difficult to move at small scales. Then, it turns out that such liquids can reach, if non-wetting, unprecedented speeds precisely because they are viscous! So, yes, it seems that we may keep the word surprisingly in the first sentence of this abstract.