Drag Reduction in a Turbulent Channel Flow of Cohesive Particles

We present a particle resolved direct numerical simulation of a turbulent channel flow laden with cohesive particles. Due to attractive inter-particle forces, cohesive particles in turbulent flows undergo a complex process of aggregation, breakup, and restructuring. This process of flocculation leads to the formation of large aggregates whose sizes are governed by the balance between cohesive and hydrodynamic forces. We quantify the impact of flocculation on the flow by reporting classical first- and second-order turbulence statistics. Notably, we observe that an increase in cohesion results in drag reduction. This effect is found to arise from two mechanisms: (i) turbulence attenuation in the channel center due to large aggregates, and (ii) the removal of a wall-adjacent layer of individual particles, which, when present, increases drag through high slip velocities. We find that in cohesive suspensions, aggregates scavenge these near-wall particles without adhering to the walls themselves. Finally, leveraging the particle-resolved framework, we report Lagrangian statistics and examine the interactions between aggregates and coherent flow structures.