AbstractWe present an immersed boundary method based on volume penalization, with which pulsatile flow in a model cerebral aneurysm is simulated. The model aneurysm consists of a curved vessel merged with a spherical cavity. The dominant vortical structures arising in the time-dependent flow are discussed and the evolution of the maximal shear stress in the aneurysm is analyzed. We approximate flow properties of blood by those of an incompressible Newtonian fluid. The flow inside the aneurysm is simulated with the use of a skew-symmetric finite-volume discretization and explicit time-stepping. We focus on effects due to variations in the amplitude of the pulsatile flow as well as due to changes in the Reynolds number (Re) by studying flow at Re = 100, 250 and 500. At Re = 500 a complex timedependence in the shear stress levels is observed, reflecting the lively development of the flow in the model aneurysm in which vortices are created continuously inside the curved vessel and in the spherical cavity of the aneurysm. An increase in the amplitude of the pulsatile flow increases the shear stress levels somewhat, but at Re = 500 the flow is mainly dominated by its intrinsic unsteadiness. Reducing the Reynolds number yields a stronger contribution of the periodic pulsatile flow forcing: at Re = 100 we find a strong dominance of shear stress levels due to the forcing, while at Re = 250 the intrinsic and pulsatile unsteadiness are of comparable importance.