Flow diverters stents (FDS) are a family of stents commonly used by clinicians to treat cerebral aneurysms. The purpose of a FDS is to modify the blood flow within the aneurysmal cavity and induce thrombosis and tissue remodeling. To predict the success of a given FDS in a patient specific situation, or to design more effective FDS, numerical simulations are a promising approach. Unfortunately, a flow simulation in which the details of the FDS are fully resolved requires a high spatial and temporal, and thus large computational time. Coarse grained approaches have been proposed in the literature, assuming that a FDS can be described as a porous media, obeying Darcy's law. However, FDS are not 3D porous media and a solution based on the so-called screen models is more adapted. In this paper we develop a new screen-based model to represent FDS at a coarse scale, with local porosity. It is shown to accurately predicts the flow in patient specific stented aneurysms geometries, while reducing the simulation by one order of magnitude or more. Our approach could be integrated to medical imaging devices to provide clinicians with a rapid and reliable estimate of the efficiency of the FDS which they plan to implement in a patient.