Rectangular jets constitute one of the energy dissipation methods in the overtopping of dams. The high turbulence and aeration phenomena that appear in falling jets and dissipation basins make it difficult to carry out studies based only on classical methodologies. There are studies modelling spillways with computational fluid dynamics which produces accurate results. However, the study of overflow nappe impingement jets has not been sufficiently examined. Simulations of free air–water overflow weirs are scarce, and require small mesh sizes and a high computational effort. This work seeks to address such simulation. Results obtained with ANSYS CFX are compared with laboratory measurements and empirical formulae. To identify the level of reliability of computed parameters, validation of air entrainment and velocity along free falling jets, thickness and break-up of jets, and pressures on the bottom of the plunge pool, are carried out by using a two-fluid model, turbulence models and mesh-size analysis. Good agreement is obtained with experimental and theoretical data. The results show that air entrainment in the jet is highly sensitive to the mesh size, while the choice of the turbulence model seems to have only a relative effect on the stagnation point.