Transport and mixing by finite-size particles in turbulent flows

Suspensions of solid particles in a viscous fluid are ubiquitous in natural and engineering settings, including sediment transport in river beds, blood flow in the human body, oil products transport in pipelines and pulp fibers in papermaking. Multiphase flows consisting of finite-size particles is a challenging topic due to multi-way coupling and interactions between the phases. Predicting these flows requires a vast knowledge of how the particle distribution and dynamics are affected by the flow field and how the global behavior of the suspension is, in turn, affected by the presence of a solid phase. In the present work, the focus is on some basic physical understanding of these flows, for different physical and mechanical properties of the particles and of the domain bounding their motion and that of the carrier fluid phase. To this purpose, particle-resolved direct numerical simulations (PR-DNS) are performed in different flow regimes and configurations. The algorithm is based on the Immersed Boundary Method (IBM) for the fluid-solid interactions with lubrication, friction and collision models for the close range particle-particle (particle-wall) interactions, including the possibility to resolve for heat transfer equation in both the dispersed and the carrier phases. Several conclusions are drawn from this study, revealing the importance of particle volume fraction and inertia on the global behavior of a suspension. In particular, the presence of particles of size of few Kolmogorov scales alters the kinetic energy transfer across the different scales of turbulence in homogeneous flows, thus modulating the turbulence; it is also shown that increasing particle inertia attenuates turbulence, while boosting particle-particle interactions by increasing the volume fraction will lead to turbulence augmentation. We have extended the range of parameter space covered in the study of pressure-driven channel flows of particle suspensions and showed that in highly inertial regime, the increased turbulent mixing makes the particle distribution more homogeneous across the domain so that the turbulent stress takes over the particle-induced stress as the main mechanism of momentum transfer. Finally, the effect of particle-fluid interactions on the heat transfer in suspensions is investigated. We have shown that addition of finite-size particles at a moderate concentration enhances the heat transfer efficiency, while at denser conditions it limits the convective heat flux and has a reducing effect instead. The study of sediment transport shows that \textit{sweep} events are mainly responsible for the dislodgement of heavy sediment particles in river beds and role of impulse from the fluid forces is highly correlated with the size of particles.