Proper modeling for fluid dynamics is central in understanding any industrial multiphase flow. For the study of gas-solid flow in a circulating fluidized bed (CFB) riser, the model based on the Eulerian description of the phases is widely used. Gas-solid flows in fluidized bed units are heterogeneous, and resolving them in numerical simulations requires a very fine mesh spacing and a short time step size. Such constraints on the mesh and time step size result in very time consuming calculations even for pilot scale fluidized bed studies. A small scale CFB riser 3 m in height and 0.40 m in width was examined in order to study effects of different scales such as mesh and time step size. When using coarse scales, the information about the mesoscale structures is lost illustrating the dependence of results on the discretization scales. The same set of equations is thus not valid for the simulation using coarse meshes. A study on space averaging was done to formulate the correction factor to the drag model with some defined notations. It was found that the correction factor for the drag model was dependent on the location in the flow domain. (C) 2011 Elsevier B.V. All rights reserved.