This paper presents a three-dimensional numerical simulation of the gas solid flow in the third generation dynamic separator. The industrial CFD code (Fluent) is used to model the high turbulent fluid flow and the separation efficiency inside this separator. The Reynolds Stress Model (RSM) and the Discrete Phase Model (DPM) are used respectively to predict the velocity field and pressure drop and to estimate the selectivity curve inside the high efficiency separator. While the DPM model is used to estimate the selectivity curve. Some factors which affect the performance of dynamic separator were identified. The fish-hook effect is the most marked mechanism for selecting finer cement powder. The CFD simulation results are analyzed in order to understand the fish-hook effect and the separation mechanism in the dynamic separator. It is found that the RSM model better captures the phenomenon in the dynamic separator when compared with industrial results made by the Enfidha Cement plant "CE" and that the fish-hook effect was a result of the bubble-type vortex breakdown in the separating chamber. But much difficulty has been reached to achieve convergence. (C) 2014 Elsevier B.V. All rights reserved.