A combined numerical and experimental study of riser hydrodynamics is reported for the fast fluidisation regime. The experimental data sets are used for validation of the CFD-DEM simulations. Special attention is paid to the formation of heterogeneities in the particle distribution, i.e., the formation of clusters. Application of Particle Image Velocimetry (PIV) in combination with Digital Image Analysis (DIA), provides a complete data set for the particle flow in a pseudo-2D riser. A CFD-DEM model is employed to simulate the co-current gas-particle flow. Particle clusters are detected and characterised in our CFD-DEM simulation results and compared to experimental data. Core-annulus flow is well predicted by the model and other hydrodynamic parameters such as cluster frequency, internal solids volume fraction and aspect ratio were also in good agreement with experimental data. (C) 2016 Elsevier Ltd. All rights reserved.