Dynamic self-organization in particle-laden channel flow

We study dynamic flow-structuring and mean-flow properties of turbulent particle-laden riser-flow at significant particle volume fractions of about 1.5%. We include particle-particle as well as particle-fluid interactions through inelastic collisions and drag forces, in a so-called four-way coupled description. These interactions are the origin for the emergence of coherent particle swarms in a flow. The dynamic cluster-formation and cluster-disintegration are associated with the competition between turbulent dispersion and inelastic particle collisions. We establish the basic scenario of this self-organization and investigate the dominant mean-flow aspects of the resulting turbulence modulation for particles with high Stokes response-time. Large-eddy simulations of turbulent channel flow, using dynamic subgrid models and particles at a significant volume fraction and realistic mass load are presented. These simulations indicate the development of a thinner boundary layer, a flatter velocity profile, an higher effective Von Karman constant and an accumulation of particles near the walls. Moreover, it was found that neglecting particle-particle interactions, as done in so-called two-way coupling, leads to a modulated flow which displays a strong 'center-channel-jet' that is not found in physical experiments