New Physics Revealed by the Particle-Fluid-Particle Stress

The drag force is the major force considered in most engineering models in multiphase flows. In our recent work, based on the ensemble average and nearest particle statistics, the fluid-particle phase interaction force is decomposed into a particle-mean-field force and the divergence of particle-fluid-particle (PFP) stress. While the drag is one of the major forces contained in the particle-mean-field force, the PFP stress has been shown to be important in many practical flows. To explore the physics of PFP stress, particle-resolved numerical simulations have been carried out using high-order CFD-DEM-IBM. For a fluid-particle system with high density ratios of particle to fluid, the PFP stress is attractive along the direction of the flow and repulsive in the direction perpendicular to the flow. This can be considered as a quantification of the well-known drafting-kissing-tumbling phenomena observed in experiments for the first time.

To study dynamics of nearest particle interactions, the age of the nearest particle pair is introduced. The probability distribution of the ages of the nearest particle pairs is found exponential. The relaxation time of age probability distribution is also the relaxation time of PFP stress. Furthermore, anisotropy and inhomogeneity of particle distributions are seen to develop driven by the PFP stress. The mean distances to the nearest particles and evolution of Voronoi cell volume distribution are studied to confirm the variation of meso-structure of particle packings.