Distinct element simulations of granular flows in two-and three-dimensional hoppers are compared with imaging data from conventional photography and gamma-ray tomography where information of the order of the particle size can be extracted. A novel feature of these comparisons is that both particle and vessel dimensions are matched exactly between the experiments and the computer simulations, thereby leaving little scope for speculation regarding ’scale effects’ which are often used to justify scepticism over the validity of simulation predictions. Another novel feature of the work is that quantitative comparisons are provided during the entire period of filling and discharge events rather than selecting an arbitrary ’snapshot’ in time, as is often the case in such simulation studies. Microstructural inspection of two-dimensional photographs of systems with large disc particles provides quantitative information which shows good agreement with simulation in terms of packing height, static and flowing voidage, stagnant/flow boundaries in funnel flow and heap/repose angles. Three-dimensional solids fraction data from packed beds of 7 mm diameter maple peas obtained by transmission gamma-ray tomography show encouraging agreement with simulation. An important result of these investigations is the degree of correlation between the flowing voidage and flow velocity of particles which are individually both affected by variations in particle size and shape but are mutually compensating in their effects on the simulated and measured discharge rates. In general, the simulations produce a less dilated assembly moving at smaller velocities.