The process of mechanical aeration of water reservoirs is one of the techniques most frequently used to combat eutrophication. It consists in pumping a source of compressed air into the reservoir bottom via injectors in order to create a dynamic, and aerate the water by bringing it into contact with the surface air (see Fig. 1). We focus in this work, on one hand, on the direct problem. This concerns the numerical simulation of the resulting two-phase water–air-bubble flow. Different models can be used to describe this problem [2], [4], [5] and [9]. Using the fact that the water phase is dominant, we used a simplified model in which the water phase is governed by the Navier–Stokes equations and the aeration effects are taken into account through a local boundary condition for the velocity, on the injector holes. Our discretization method is based on a three-dimensional mixed finite element method, View the MathML source [3]. The Uzawa algorithm is used to solve the matrix system obtained.