Suspensions of finite-size rigid spheres in different flow cases

Sammanfattning: Dispersed multiphase flows occur in many biological, engineering and geophysical applications such asfluidized beds, soot particle dispersion and pyroclastic flows. Understanding the behavior of suspensionsis a very difficult task. Indeed particles may differ in size, shape, density and stiffness, theirconcentration 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 due to the interactionsbetween particles and eddies of different size, ranging from the smallest dissipative scales up to thelargest integral scales. Most of the investigations on the topic have dealt with heavy small particles (typicallysmaller than the dissipative scale) and in the dilute regime. Less is known regarding the behavior ofsuspensions of finite-size particles (particles that are larger than the smallest lengthscales of the fluid phase).In the present work, we numerically study the behavior of suspensions of finite-size rigid spheres indifferent flows. In particular, we perform Direct Numerical Simulations using an ImmersedBoundary Method to account for the solid phase. Firstly is investigated the sedimentation of particles slightly larger than theTaylor microscale in sustained homogeneous isotropic turbulence and quiescent fluid. The results show thatthe mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. By estimatingthe mean drag acting on the particles, we find that non stationary effects explain the increased reductionin mean settling velocity in turbulent environments.We also consider a turbulent channel flow seeded with finite-size spheres. We change the solid volumefraction and solid to fluid density ratio in an idealized scenario where gravity is neglected. The aim isto 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 effectof increasing the density ratio is a shear-induced migration toward the centerline. However, at very high density ratios (~100) the two phases decouple and the particles behave as a dense gas.Finally we study the rheology of confined dense suspensions of spheres in simple shear flow. We focus onthe weakly inertial regime and show that the suspension effective viscosity varies non-monotonically with increasingconfinement. The minima of the effective viscosity occur when the channel width is approximately an integernumber of particle diameters. At these confinements, the particles self-organize into two-dimensional frozen layers thatslide onto each other.