The flow patterns within a hydrocyclone separator system have been studied experimentally using particle image velocimetry (PIV) and computationally via large eddy simulations (LES). Various aspects of the flow field such as the process of air core structure formation, pressure and velocity distributions within the system have been characterized. Detailed flow field information in the form of velocity vector maps of entire planes have been obtained experimentally and reproduced computationally. Good qualitative agreements between velocity vector fields, air core structure and averaged velocity distributions have been achieved. LES is deemed to be an adequate methodology for modelling the air core structure and flow patterns of the surrounding liquid in a hydrocyclone separator system. (C) 2010 Elsevier Ltd. All rights reserved.