Continuum and discrete multi-phase particle-in-cell (MP-PIC) simulation results of gas-particle flow in a 2D periodic domain are presented. Grid refinement revealed heterogeneous flow structures that increase the domain-averaged gas-particle slip velocity to about 2.5-3 times to that for homogeneous flow. Similar need for grid refinement was demonstrated for both continuum and discrete approaches. Both approaches yielded nearly identical results. If coarse grids are used in simulations, both approaches should therefore employ corrections to account for the effects of sub-grid scale structures. Published by Elsevier B.V.