Coupled Multiphase Flow Solving and its Application to Turbulent Gas-Particle Flows (Berend van Wachem, Imperial College, UK)

In this talk, a fully coupled method is derived and demonstrated for solving multiphase flow problems on collocated, unstructured grids for general domains. The fully coupled method solves the linearised continuity equation and momentum equations simultaneously, which result from averaging the local physical behaviour of the multiphase flow problem. To close the continuity equation describing the multiphase flow problem, analytical expressions for the cell face fluxes are derived by weighting the momentum equations at the cell faces, taking into account volume fractions, source terms, density, and gradients of these. Special care is taken with source terms and the presence of strong gradients, such as around interfaces. This enables the fully coupled framework to exactly conserve mass as well as exactly balance pressure gradients and body forces.

The derived framework is then used to investigate the behaviour of solid spheres, elongated axis-symmetric ellipsoidal particles, discs and fibres, and their interaction with turbulence, collisions between the particles, and the effects of the particles on turbulence in two turbulent channel flows, one with low Re number and one with higher Re number.
An analysis of the turbulent kinetic energy is presented, showing the effect of the particles on the turbulent properties of the flow. It is shown, for instance, that ellipsoids with high Stokes number cause drag reduction, because of momentum transport towards the wall caused by the particles. Finally, the results show there is a big effect of particle shape and particle-orientation, especially for particles with larger Stokes numbers.