(378c) Clustering in Gas-Solid Flows: How Are Clusters Modified By Shear?

We present simulation and model results for sheared, cluster-induced turbulence (CIT). In CIT, heavy particles settling under gravity induce velocity fluctuations in their carrier fluid as a result of momentum coupling, and in turn form large-scale clusters. To understand the effect of shear on the system dynamics, we perform Euler-Lagrange simulations of homogeneous shear flow, using a discrete element method for the particle phase and direct numerical simulation for the fluid phase. We notice that for low values of the shear Stokes number (i.e., the ratio between the particle response time and the characteristic shear time scale), particle clustering persists and the system dynamics are similar to those of fully developed CIT in the absence of shear. However, at a sufficiently large shear Stokes number, particle clustering disappears entirely. We develop a Reynolds-stress model for sheared CIT to understand these effects, and show that this model also predicts vanishing clustering above a critical shear Stokes number. We compare model results with simulation data, and use both to better understand the effect of shear on the dynamics of fully developed CIT. These results (in an idealized, homogeneous system) are then used to provide insight into the fundamental physical processes affecting particle motion in more realistic, wall-bounded configurations such as those in fluidized beds and risers.