Transport and Formation of OPAs in Rivers

A numerical framework was developed to study oil-particle aggregate (OPA) formation by incorporating NEMO3D code and the A-DROP model into an open-source platform, OpenFOAM. The developed framework was then used to study oil transport and OPA formation in a two-dimensional (2D) hypothetical river at a depth of 3.0 m. The river's hydrodynamic profile was used in conjunction with the A-DROP model to simulate OPA formation, whereas the NEMO3D model was used to track the movement of the oil droplets and OPAs. Results suggest that an increase in buoyancy results in a decrease in the streamwise variance and spreading coefficient. The small (i.e., 50 μm) droplets became entrained in the deep water column at high-energy dissipation rates, which enhanced OPA formation. The large (i.e., 200 μm) droplets aggregated much more rapidly than the small ones in the same turbulence environment owing to differential sedimentation. In general, OPA formation in the upper rivers was dominated by collisions caused by differential sedimentation, while in the deep water column, collision caused by turbulence shear plays a more critical role. The aggregation rates of the formed OPAs were less than 60% within a short period (20 min) in a river with relatively mild turbulence (ϵ<5×10-4 W/kg).