The air core is an interesting and not well understood phenomenon that occurs in hydrocyclones and drastically affects the efficiency of separation. This study uses numerical code based on a three dimensional finite element solver which was designed specifically for hydrocyclones. A numerical examination of different flows at varying viscosities shows interesting results, which provides qualitative proof that the numerical code is predicting the general fluid flow in the hydrocyclone. The numerical results reveal that low viscosity feeds tend to develop a larger low pressure area in the center region of the cyclone than feeds at high viscosity. The comparison at different cross-section across the hydrocyclone at different viscosities act accordingly. The simulation results demonstrate pressure contours and are used to explain the initial stages of air core formation. (C) 2007 Elsevier B.V. All rights reserved.