We consider suspensions of finite-size neutrally buoyant rigid spherical particles in channel flow and investigate the relevance of different momentum transfer mechanisms and the relation between the local particle dynamics and the bulk flow properties in the highly inertial regime. Interface-resolved simulations are performed in the range of Reynolds numbers 3000 <= Re <= 15 000 and solid volume fractions 0 <= phi <= 0.3. The Lagrangian particle statistics show that pair interactions are highly inhomogeneous and dependent on the distance from the wall: in their vicinity, the underlying mean shear drives the pair interactions, while a high degree of isotropy, dictated by more frequent collisions, characterizes the core region. Analysis of the momentum balance reveals that while the particle-induced stresses govern the dynamics in dense conditions, phi = 0.3, and moderate Reynolds numbers, Re < 10 000, the turbulent stresses take over at higher Reynolds numbers. This behaviour is associated with a reduced particle migration toward the channel core, which decreases the importance of the particle-induced stress and increases the turbulent activity. Our results indicate that Reynolds stresses and the associated velocity fluctuations, characteristics of near-wall turbulence, prevail at high inertia over the resistance to deformation presented by the particles for volume fractions lower than 30 %.

   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. 

We present results of interface-resolved simulations of heat transfer in suspensions of finite-size neutrally-buoyant spherical particles for solid volume fractions up to 35% and bulk Reynolds numbers from 500 to 5600. An Immersed Boundary-Volume of Fluid method is used to solve the energy equation in the fluid and solid phase.

We relate the heat transfer to the regimes of particle motion previously identified, i.e. a viscous regime at low volume fractions and low Reynolds number, particle-laden turbulence at high Reynolds and moderate volume fraction and particulate regime at high volume fractions. We show that in the viscous dominated regime, the heat transfer is mainly due to thermal diffusion with enhancement due to the particle-induced fluctuations. In the turbulent-like regime, we observe the largest enhancement of the global heat transfer, dominated by the turbulent heat flux. In the particulate shear-thickening regime, however, the heat transfer enhancement decreases as mixing is quenched by the particle migration towards the channel core. As a result, a compact loosely-packed core region forms and the contribution of thermal diffusion to the total heat transfer becomes significant once again. The global heat transfer becomes, in these flows at volume fractions larger than 25%, lower than in single phase turbulence.

We perform interface-resolved simulations to study the modulation of statistically steady-state homogeneous shear turbulence by neutrally buoyant finite-size particles. We consider two shapes, spheres and oblates, and various solid volume fractions, up to 20%. The results show that a statistically steady state is not exclusive to single-phase homogeneous shear turbulence as the production and dissipation rates of the turbulent kinetic energy are also statistically in balance in particle-laden cases. The turbulent kinetic energy shows a non-monotonic behaviour with increasing solid volume fraction: increasing turbulence attenuation up to a certain concentration of solid particles and then enhancement of the turbulent kinetic energy at higher concentrations. This behaviour is observed at lower volume fractions for oblate particles than for spheres. The attenuation of the turbulence activity at lower volume fractions is explained through the enhancement of the dissipation rate close to the surface of particles. At higher volume fractions, however, particle pair interactions induce regions of high Reynolds shear stress, resulting in the enhancement of the turbulence activity. We show that the oblate particles of the considered size have larger rotational rates than spheres with no preferential orientation. This is in contrast to previous studies in wall-bounded flows where preferential orientation close to the wall and reduced rotation rates result in turbulence attenuation and thus drag reduction. Our results shed some light on the effect of rigid particles, smaller than the near-wall turbulent structures but still comparable to the viscous length scale, on the dynamics of the equilibrium logarithmic layer in wall-bounded flows.

We use interface-resolved direct numerical simulations to study the dynamics of a single sediment particle in a turbulent open channel flow over a fixed porous bed. The relative strength of the gravitational acceleration, quantified by the Galileo number, is varied so as to reproduce the different modes of sediment transport - resuspension, saltation and rolling. The results show that the sediment dynamics at lower Galileo numbers (i.e. resuspension and saltation) are mainly governed by the mean flow. Here, the regime of motion can be predicted by the ratio between the gravity and the shear-induced boundary force. In these cases, the sediment particle rapidly takes off when exposed to the flow, and proceeds with an oscillatory motion. Increasing the Galileo number, the frequency of these oscillations increases and their amplitude decreases, until the transport mode switches from resuspension to saltation. In this case, the sediment travels by short successive collisions with the bed. Further increasing the Galileo number, the particle rolls without detaching from the bed. Differently from the previous modes, the motion is triggered by extreme turbulent events, and the particle response depends on the specific initial conditions, at fixed Reynolds number. The results reveal that close to the onset of sediment motion, only turbulent sweeps can effectively trigger the particle motion by increasing the stagnation pressure upstream. We show that for the parameters in this study, a criterion based on the streamwise flow-induced force can successfully predict the incipient movement.