THIS SEMINAR WAS ORGANIZED BY NORDITA SOFT MATTER
Sedimentation is a relevant process in a wide range of environmental flows and engineering applications. Typical examples include particulate flows in fluidised beds, dust storms, and the sinking of dissolved organic matter and micro-organisms in oceanic waters. However, due to the large parameter space it is generally difficult to predict how a settling suspension will behave. Indeed, particles may differ in density, shape, size, stiffness, and the collective dynamics changes with their concentration. In addition, the carrier fluid may be quiescent or turbulent. In this last case, particles interact with eddies of different size and, consequently, the ratio between particle and turbulence characteristic length and time scales become important parameters of the problem. Previous studies deal with the settling of dilute suspensions of heavy sub-Kolmogorov particles in homogeneous isotropic turbulence. In this regime (usually known as one-way coupled, as there is no back-influence of the solid phase on the fluid), the suspension is found to settle substantially faster, on average, than isolated particles in quiescent fluid. This is due to a phenomenon called fast-tracking: as they fall, the particles are preferentially swept towards regions of low vorticity and high strain rate, oversampling downward moving fluid, therefore increasing their settling speed. The next question to answer is what happens when particles larger than the dissipative scales are considered. To this aim, we have performed interface-resolved direct numerical simulations, using an immersed boundary method to account for the solid phase. In particular, we study semi-dilute suspensions of Taylor-sized rigid spheres. We investigate the effects of the Galileo number (i.e., the ratio between buoyancy and viscous forces), density ratio, and relative turbulence intensity (defined as the ratio between the turbulence root-mean-square velocity and the terminal speed in quiescent fluid of an isolated particle). We find that the dynamics strongly depends on the features of the background turbulence, and that the mean settling speed is always lower than in quiescent fluid, due to unsteady effects and to an increased drag nonlinearity. The reduction in mean settling speed (in comparison to quiescent cases) is larger at large relative turbulence intensities. A correlation is found between the mean settling speed and the relative turbulence intensity. We then investigate the effects of particle shape on the settling dynamics. Most studies on sedimentation in quiescent fluid and at finite inertia, considered only spherical particles. However, the orientational dynamics of non-spherical particles can substantially alter particle-pair interactions, clustering, and the collective behavior of the suspension. We here study suspensions of oblate particles with volume fractions from 0.5% to 10%, at Galileo number of 60. For this Galileo, the mean settling speed of rigid spheres is a decreasing function of the volume fraction. However, for oblates we find intense clustering and the formation of columnar-like structures of particles in semi-dilute conditions. This phenomenon leads to a large increase of the mean settling speed in comparison to the case of an isolated oblate. Hindrance becomes the dominant effect only for solid volume fractions larger than 5%. An interesting orientational dynamics is also discussed.