Sensitivity to outflow boundary conditions and level of geometry description for a cerebral aneurysm

Mathematical models, namely the flow boundary conditions, as well as the detail of the bounding geometry, can highly influence the computed flow field. In this work, an anatomically realistic portion of cerebral vasculature with a saccular aneurysm, and its geometric idealisation, are considered. The importance of the geometric description, namely including the side branches or modelling them as holes in the main vessel, is studied. Several approaches to prescribe the outflow boundary conditions at the side branches are analysed, including the commonly used traction-free condition, zero velocity, meaning that the side branch is neglected, and the coupling with simple 0D models. All these outflow conditions are applied both at the end of the considered 3D side branch, or directly at the junction of the side branch with the main vessel. The 3D side branch is also accounted for by using 1D models. All these effects are compared quantitatively and qualitatively. Results indicate a reduced influence of the fluid boundary condition model when the branches are considered, however the solution is very sensitive in the case of clipped branches. The 0D and 1D models provide good representations of the side branches.