UC Berkeley

Experiments are conducted to model the flow within a cerebral saccular aneurysm located at the basilar artery bifurcation. The flow phantom modeling the aneurysm consists of a nearly spherical dome located at the flow divider of a 90° bifurcation. To ensure that the experiments are physiologically relevant, a pulsatile pumping system replicates a measured basilar artery waveform at the inlet. In addition, the Reynolds and Sexl-Womersley numbers are matched with physiological conditions. The flow rate through the outlets of the bifurcation are controlled by a pair of needle valves. This control allows study of the effect of branching ratio. Three techniques are applied to characterize the flow. Flow visualization provides pathline images that are used to qualitatively define flow structures and patterns. Particle image velocimetry is used to measure the flow velocity across multiple cross sections of the flow. Post-processing of the data allows in-plane wall shear stresses to be calculated. Lastly, stereoscopic particle image velocimetry allows the 3D velocity field and in-plane wall shear stresses to be determined on an orthogonal cross section of the flow. Combining the information obtained through the experiments, the flow behavior over a single physiological waveform is characterized.

The flow characterization centers around two major features: an inlet wall jet that originates from the neck of the aneurysm and wraps around the dome and a circulation region that dominates the center volume of the dome. The motion of these structures during the waveform are captured and detailed. Wall shear stresses are found for points along the dome over the course of a waveform. In-plane maximum wall shear stress magnitudes over the entire dome vary from 6.8-18.6 dynes/cm² in the PIV planes and 13.9-31.0 dynes/cm² in the SPIV planes. Total wall shear stress magnitudes can be calculated for three points intersection points. Mean wall shear stress magnitudes at these points range from 3.28-7.94 dynes/cm² . In particular, the flow conditions that had nearly symmetric outlet flow rates had the lower WSS magnitudes of the test cases.

The primary effect of the branching ratio on the flow behavior is to alter the location and displacement of the circulation region and inlet jet. For flows with a large outlet differentials, the inlet flow shows strong direction preference entering as a wall jet. For flows with small outlet differentials, the inlet flow forms a expanding inlet jet that does not show strong direction preferences. The WSS measurements matches well with computational fluid dynamics simulations of cerebral saccular aneurysms, reinforcing the theory that lower than normal WSS values plays a role in aneurysm growth. Future experimental research will need to focus on implementing techniques that provide the complete 3D flow field and that uses a non-Newtonian working fluid to better simulate the behavior of blood.