The flow fields inside conventional and rotary hydrocyclones were simulated respectively. In these simulations,water only and oil-water mixture,with distinctly different viscosities,were used as continuous phases. Simulation results agreed well with the experimental measurements. Simulation results showed that the conventional hydrocyclone could effectively separate sand from water,but could not separate sand from high viscosity water/oil emulsion. This showed that the viscosity of continuous phases influenced greatly both the separation efficiency and the flow field distribution in the conventional hydrocyclone. For high viscosity oil/water sand dispersion (mixture),the rotary hydrocyclone has better separation performance than the conventional one,with a more favorable flow field distribution.