A systematic comparison of fully resolved and unresolved particulate flow simulations using the lattice Boltzmann and discrete element methods

Numerical simulations are widely used to study the behavior of suspension flows. Fully resolved simulations, in which the detailed flow around individual particles is computed, are accurate but computationally expensive. Unresolved methods reduce the computational cost significantly by only resolving the bulk flow and modeling the small-scale flow around particles. However, the degree to which modeling rather than computing the small-scale flow field information affects the predicted behavior of suspension flows is largely unknown. Here, we examine the steady homogeneous regime by simulating the pressure drop over a porous medium and the apparent viscosity of a sheared suspension as well as the transient heterogeneous regime by simulating a particle-induced Rayleigh–Taylor instability. From these simulations, we observe that unresolved methods are able to predict macroscopic quantities in steady state problems involving homogeneous suspensions but fail to capture particle entrainment, deformation, and breakup effects in transient problems involving heterogeneous suspensions. Our results suggest that the effect of the small-scale flow fields plays an important role in the onset and growth of instabilities in suspension flows which cannot be modeled in a trivial way. This has consequences for practical applications where such instabilities are essential, such as particle mixing. Further research into the mechanisms by which such instabilities are triggered as well as ways to include these effects in computationally inexpensive unresolved models is needed.