Modulation of wall-bounded turbulent flows by large particles : effect of concentration, inertia, and shape

The effect of particles on turbulence is a key phenomenon in many practical industrial applications encountered in petroleum engineering, chemical reactors and food or solid processing (transport of slurries in pipes, reactive fluidized beds, and pneumatic transport of particles), environmental engineering (such as sand storm and Particulate Matter (PM) Pollution), and biological fluid mechanics (e.g. drug delivery in blood flow and inhaled particles through the respiratory system). The experiments of Matas et al. (PRL, 2003) have highlighted the non-monotonous effect of neutrally buoyant particles on the laminar-turbulent flow transition, depending on the particle-to-pipe size ratio and on the suspension volumetric concentration. A small amount of finite size particles allowed sustaining the turbulent state and decreasing the transition threshold significantly. The complex mechanisms related to particle flow interactions are often difficult to elucidate experimentally. During the last 4 decades, direct numerical simulations have proven to be a powerful tool for understanding the features of single-phase turbulent flows. Currently, it starts to play an important role in the investigation of suspension flows as well. Almost a decade after the experiments of Matas et al. (PRL, 2003), particle-resolved numerical simulations are able to evidence that at moderate concentration, particles have a significant impact on the unsteady nature of the flow, enhancing the transverse turbulent stress components and modifying the flow vortical structures (Loisel et al. Phys. Fluids, 2013; Yu et al. Phys. Fluids, 2013; Lashgari et al. PRL, 2015). In this work, we use particleresolved numerical simulations to understand the effect of finite sized particles on wall-bounded (pressure-driven or plane Couette) turbulent flows, slightly above the laminar-turbulent transition limit. We find that in turbulent Couette flow, wall-normal profiles of the flow velocity and Reynolds stress components reveal that there is no significant difference between single phase and two-phase flows at equivalent effective Reynolds number, except that the wall shear stress is higher for the two-phase flow. At concentration up to 10%, neutrally buoyant spherical particles have a negligible effect on both the intensity and intermittency of the Reynolds stress. However temporal and modal analysis of flow fluctuations, suggest that besides increasing small scale perturbation due to their rigidity, particles have an effect on the regeneration cycle of turbulence (streak formation, streak breakdown and streamwise vortex regeneration). Indeed, the shape of the streaks and the intermittent character of the flow (amplitude and period of oscillation of the modal fluctuation energy) are all altered by the particle presence, and especially by the inertial particles (Wang et al. Phys. Rev. Fluid, 2017). When the particle shape deviates from sphericity (spheroids with aspect ratios ranging between 0.5 and 2), the features of turbulent suspension flow are not significantly impacted. The transfer of particles between different coherent structures (along the regeneration cycle period) is analyzed at the end of the thesis. Nevertheless in channel flow, neutrally-buoyant spherical particles have a drastic impact on the regeneration cycle of turbulence, decreasing thereby the transition threshold. Particles enhance the intensity of the Reynolds stress although the frequency of burst events is decreased. Particles enhance the lift-up effect and act continuously within the buffer layer. Moreover, they increase the vorticity stretching, leading to smaller and more numerous wavy streaks for suspension flows compared to the single-phase configuration.