Suspensions of finite-size rigid particles in laminar and turbulent flows

Dispersed multiphase flows occur in many biological, engineering and geophysical applications such as fluidized beds, soot particle dispersion and pyroclastic flows. Understanding the behavior of suspensions is a very difficult task. Indeed particles may differ in size, shape, density and stiffness, their concentration varies from one case to another, and the carrier fluid may be quiescent or turbulent. When turbulent flows are considered, the problem is further complicated by the interactions between particles and eddies of different size, ranging from the smallest dissipative scales up to the largest integral scales. Most of the investigations on this topic have dealt with heavy small particles (typically smaller than the dissipative scale) and in the dilute regime. Less is known regarding the behavior of suspensions of finite-size particles (particles that are larger than the smallest length scales of the fluid phase). In the present work, we numerically study the behavior of suspensions of finitesize rigid particles in different flows. In particular, we perform direct numerical simulations using an immersed boundary method to account for the solid phase. Firstly, the sedimentation of spherical particles slightly smaller than the Taylor microscale in sustained homogeneous isotropic turbulence and quiescent fluid is investigated. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. By estimating the mean drag acting on the particles, we find that non stationary effects explain the increased reduction in mean settling velocity in turbulent environments. Moreover, when the turbulence root-mean-square velocity is larger than the terminal speed of a particle, the overall drag is further enhanced due to the large particles cross-flow velocities. We also investigate the settling in quiescent fluid of oblate particles. We find that at low volume fractions the mean settling speed of the suspension is substantially larger than the terminal speed of an isolated oblate. This is due to the formation of clusters that appear as columnar-like structures. Suspensions of finite-size spheres are also studied in turbulent channel flow. We change the solid volume and mass fractions, and the solid-to-fluid density ratio in an idealized scenario where gravity is neglected. The aim is to independently understand the effects of these parameters on both fluid and solid phases statistics. It is found that the statistics are substantially altered by changes in volume fraction, while the main effect of increasing the density ratio is a shear-induced migration toward the centerline. However, at very high density ratios (∼ 1000) the solid phase decouples from the fluid, and the particles