The three-dimensional flow fields with a central air core in a hydrocyclone are numerically simulated using a k-epsilon turbulence model. The model constants C-1, C-2 and C-mu are modified because of the anisotropic character of the turbulent viscosity in the hydrocyclone. Experiments show that the predicted velocity profiles agree well with data measured by laser Doppler anemometry (LDA). Based on this prediction, the flow field and the pressure field as well as the distribution of the rate of dissipation of the turbulent energy are discussed. The study shows that an important approach towards reducing energy dissipation in a hydrocyclone is to improve the flow pattern in the cylindrical part.