The shielding effects of neighboring particles on the flocculation dynamics of cohesive sediment in homogeneous isotropic turbulence is investigated using a two-phase particle-unresolved, but turbulence-resolved, Euler–Lagrange simulations. A coupled CFD-DEM (Computational Fluid Dynamics-Discrete Element Method) framework was applied, in which the discrete element method model captures collisional interactions among particles. The high-resolution grid used in the CFD resolves all the turbulent scales. The primary particles are substantially smaller than the Kolmogorov length scale, therefore, flow around particles is not resolved and the fluid–particle interactions are modeled by force models. The present work employs the semiempirical force model of Kim and Lee (KL), in which the multibody interactions between the particles that makeup a floc are modeled as functions of pairwise interactions among particles. In comparison, the widely used free-draining approximation (FDA) uses Stokes drag of individual particles and completely ignores all inter-particle interactions within the floc. Most importantly, we observe that by allowing more accurate hydrodynamic interactions among the fractal floc members, the KL method predicts much larger flocs at equilibrium. By including the intra-floc shielding effects, the KL model predicts the floc settling velocity to substantially increase with floc size, in contrast to the FDA model. The aggregation and breakup kernels follow qualitatively similar patterns for both the FDA and KL models. For future work, a computationally efficient and accurate force model for fractal floc shapes needs to be developed for better predictions of the flocculation processes of cohesive sediment.
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