Mesoscale Flow Structures and Fluid-Particle Interactions

During the last two decades, the insight has gradually grown-thanks to advances in both experimental techniques and computational simulation tools-that many dispersed two-phase flows are dominated by dynamic mesoscale coherent structures in which particles, bubbles, or drops organize themselves. Evidence from experiments, hydrodynamic analyses, and computational simulations on the presence and dynamics of such structures, strands, and clusters is presented. Their origin being less well understood, the general consensus is that fluid-particle interaction forces play a dominant role in bringing and keeping the dispersed-phase particles together. This chapter then revisits the topic of the fluid-particle interaction in all of its aspects and constituents such as single-particle steady-state drag, added mass, Basset force, lift, and particle rotation, while also the effects of particle and fluid acceleration and carrier phase turbulence are reviewed. The common practice of linearly adding the various correlations obtained for very specific canonical cases is to be rejected for other flow conditions than just creeping flow. Particular attention is paid to the way the pertinent fluid-particle interaction correlations are utilized in computational simulations of both the Euler-Lagrange (point-particle tracking) and Euler-Euler (two-fluid) type, where a distinction is made between Reynolds-averaged Navier-Stokes-based simulations and large-eddy simulations. Tracking a particle immersed in a 3D turbulent fluid in the presence of other particles by means of the steady-state drag force is a dubious approach. Undoubtedly, the best representation of the fluid-particle interaction can be obtained from periodic-box direct numerical simulations.