The paramount importance of dense medium cyclones (DMCs) has led to their continued development since their invention and commercialisation by the Dutch State Mines in the immediate post-war period. The size of DMC units has grown significantly in recent times, with 1500mm units now commercially available. Given the high level of technical difficulty and costs involved in undertaking an empirical evaluation of such large units, a fundamentally-based model was developed. Using suitably validated CFD and CFD/DEM fundamentally-based models of air-water-magnetite-coal flows in a DMC, a much greater understanding of DMC separations was developed. The approach used, the validation results, as well as the effects of geometric changes on cyclone partition coefficient curves are discussed. In addition, the results are presented in terms of internal contours of important parameters, e.g., medium and coal velocities and direction, vortex position, pressure. The effects of adjusting cyclone geometry (vortex finder and spigot, etc) are quantified. Examples of conditions that lead to unwanted surging phenomena are given and, during the presentation, further details would be communicated by animated videos. The use of the results and of a new approach to making all results readily available to industry is included.