In This work is the result of research conducted on a dam in Cameroon. The aim is to study the variation of flow through the intake of a penstock in order to highlight its influence on the entire structure. For this fact, a study is conducted by a 2D numerical approach under FLUENT (6.3.26). It is clear from this work that the shape of the structure upstream and the intake have a real impact on the structure of the flow, it is the same for the flow rate of water that arrives upstream. The structure of the flow is unstable, the maximum speed is observed not far from the wall. A large recirculation zone is observed on the step at the inlet of the intake, it is very active on the acceleration of the flow velocity towards the pipe; likewise, it is responsible for the birth of air bubbles which will have consequences later on the phenomenon of cavitation of the blades of our turbine.