The flow field of a 2 in. hydrocyclone is shown to be significantly asymmetric without precession, through both computational fluid dynamics (CFD) and experimental observation. Hence the application of full three-dimensional modelling is demonstrated to be essential. Further, CFD predicts that the axial pressure is not below atmospheric prior to development of the air core and that such development is not pressure driven. In fact, initial insight into a cause of instability of the air-core is identified from the CFD and supported through experimental observation. The predictions use the second-order differential-stress turbulence model which has previously been identified to represent a minimum model. Lastly, the inclusion of full three-dimensional modelling and high-order turbulence modelling leads to a new understanding of particle-separation classification within the hydrocyclone, including a significant stochastic component. (C) 2004 Published by Elsevier Ltd.