UC Santa Barbara

Cohesive granular materials suspended in turbulent flows undergo a complex process of aggregation, breakup, and restructuring, collectively known as flocculation, wherein turbulence plays a dual role: facilitating aggregate growth by enhancing collision rates, while simultaneously promoting breakup through increased hydrodynamic stresses. The result of this competition is a dynamic population of aggregates whose size distribution directly controls particle transport and dispersion, making flocculation a central process across a wide range of environmental and engineering systems. Accurate prediction and evolution of the size distribution is therefore paramount.The population balance equation (PBE) evolves the size distribution on the basis of probabilistic, rate-describing kernels that account for aggregation and breakup. These kernels are heavily parameterized and are often empirically calibrated or agnostic to local flow conditions. Improving the physical basis of these kernels by identifying and characterizing the mechanisms that govern aggregation and breakup in turbulent flows is therefore essential to developing predictive PBE models.To this end, this dissertation presents a fundamental study on the transport of cohesive granular materials in wall-bounded turbulence, where the highly anisotropic and inhomogeneous turbulent flow leads to a rich array of spatially organized mechanisms. Six particle-resolved direct numerical simulations of turbulent channel flows laden with finite-sized cohesive particles at successively increasing cohesive strength are performed and characterized. Aggregation and breakup are shown to be highly dependent on the wall-normal direction and local turbulent shear rate. The formation of increasingly large aggregates is shown to modulate the underlying turbulence and wall shear stress: smaller particles, which normally reside near the walls, are captured by larger aggregates, resulting in a drag reduction relative to the noncohesive case.Finally, a novel PBE-based framework is used to analyze aggregate dynamics and trajectories through the joint space of size and wall-normal distance. Within this space, we identify regions of net aggregate production and removal. For example, due to breakup, smaller aggregates are preferentially created near the wall, where shear rates and turbulence intensities are elevated. This results in a net production of small aggregates near the wall that is balanced by advective transport. Expanding this to all sizes reveals a newly identified circulation: larger aggregates are overproduced in the channel center, migrate toward a wall, and break. The resulting smaller aggregates are carried back to the channel center, where they once again grow, and the cycle repeats.