Particle Capture and Pattern Evolution on Big Drops in Three-phase Turbulence

The process of particle capture and trapping by large deformable drops in turbulent channel flow are investigated in this thesis using an Eulerian-Lagrangian approach specifically developed for this three-phase flow. The flow field in the carrier fluid and inside the droplets is obtained from Direct Numerical Simulation of the Navier-Stokes equations; the drop interface dynamics are provided by a Phase Field Model; and par- ticle trajectories are calculated via Lagrangian tracking. Drops have the same density and viscosity of the carrier fluid in order to mimic a liquid-liquid dispersion of water and low-viscosity oil. Particles are modelled as neutrally- buoyant, sub-Kolmogorov spheres that interact with each other through collisions (excluded-volume interaction). Simulation results allow a detailed characterization of the particle dynamics during the interface capture and trapping stages. Particle capture is driven by the capillary forces of the interface in combination with near-interface turbulent motions: Particles are transported towards the interface by jet-like turbulent motions and, once close enough, are captured by interfacial forces in regions of positive surface velocity diver- gence. These regions appear to be well correlated with high-enstrophy flow topologies that contribute to enstrophy production via vortex compression or stretching. Upon capture, particles sample preferentially regions of positive surface velocity divergence, which correlate with jet-like fluid motions directed towards the interface. At later times, however, particles are observed to move from these regions under the action of the tangential stresses to the areas where the surface divergence vanishes and form the two-dimensional cluster. long- term trapping regions correlate well with the surface area characterized by higher-than-mean curvature. This finding is important since the presence of tiny particles at the interface is known to affect locally the surface tension, particularly in the presence of concentration gradi- ents: present results suggest that particle-induced modifications of the surface tension should be stronger where the curvature of the interface is higher.