The gas-solid flows in a two-dimensional downer of 10 m in height and 0.10 m in width were simulated using a CFD-DEM method, where the motion of particles was modeled by discrete element method (DEM) and the gas flow was described by Navier-Stokes equations. The simulations revealed a rich variety of developing flow structures in the downer under different operating conditions. The two-phase flow development can be clearly characterized by the micro-scale particle distributions in the downer. Near the inlet, the particle distribution is dominated by the distributor design. Then, the particles disperse in the column, forming a homogeneous transit region. After that clusters start to form and modulate the gas-solid flow field till the fully-developed state. The particle-scale simulation disclosed that the clusters are composed of loosely collected particles, and these particles have the same flow direction as the bulk flow so that no particle backmixing can be observed. As the particles in the downer have the tendency to maintain the inertia, the capability of lateral transfer of particles is relatively weak, which was illustrated by tracking the movement of the single particles and clusters. The simulations of the inlet effect on the hydrodynamics in the downer showed that the gas-solid flow structure and the mixing behavior are sensitive to the inlet design. An inappropriate design or operation would probably cause the undesired flow phenomena such as the wide distribution of residence times. The time-averaged hydrodynamics based on the transient simulations showed good agreement with the experimental findings in the literature. The simulation based on the CFD-DEM coupled approach provides a theoretical way to comprehensively understand the physics at micro- to macro-scales in the co-currently downward gas-solid flows. (c) 2009 Elsevier B.V. All rights reserved.