The treatment of cerebral aneurisms with a porous stent has recently been proposed as a minimally invasive way to prevent rupture and favor coagulation mechanism inside the aneurism. The efficiency of a stent is related to several parameters which are not yet fully understood. The goal of this paper is to identify, through numerical simulations, how the stent structure and its positioning affect the hemodynamics properties of the flow inside the aneurism. We use the concept of flow reduction to characterize the stent efficiency. Our simulations are based on a lattice Boltzmann modeling of blood flow. Our results, which agree well with in vitro experiments, show how the various parameters play a role and help us to understand the phenomena.