Large eddy simulation of fiber flocculation in a diffuser: Effects of fiber inertia and reinjection kinematics

This study investigates flocculation in dilute suspensions of rigid fibers flowing through an asymmetric diffuser using an Eulerian–Lagrangian approach. The analysis focuses on flow-induced ballistic flocculation under varying fiber inertia and inlet (reinjection) kinematics. The fiber length exceeds the Kolmogorov length scale of the carrier flow, and finite inertia leads to a non-negligible slip velocity relative to the fluid. Large-eddy simulation (LES) is applied with a dynamic subgrid-scale model to resolve the flow field and turbulence. One-way coupling between the fibers and the flow is assumed, while fiber–fiber interactions are modeled using short-range attractive forces that promote floc formation. The results show that ballistic deflection significantly accelerates flocculation in the diffuser region, establishing ballistic deflection as the dominant mechanism. In addition, inlet fiber kinematics and inertia strongly influence flocculation within the straight inflow channel. • Ballistic deflection is the main mechanism driving fiber flocculation in a diffuser. • Fiber inertia and reinjection kinematics notably influence floc formation. • Higher fiber inertia promotes flocculation due to increased slip velocity. • Early-stage flocculation in the inflow channel is enhanced when the fibers are reinjected with random orientation.