Past statistical studies demonstrating the likelihood of slow blood flow in most ruptured aneurysms have suggested that thrombogenesis plays an important role in ruptures of cerebral artery aneurysms. In the authors’ previous study, it was reported that the degree of platelet aggregation in an aneurysm had a signifi cant correlation with the flow pattern in the aneurysmal dome. It is, therefore, crucial to investigate flow structures in various different aneurysms in order to understand better the relationship between thrombogenesis and ruptures. In this study, patterns of blood fl ow in three models of cerebral artery bifurcation aneurysms were numerically analysed and compared to discern the likelihood of platelet aggregation. The three model aneurysms had comparable aspect ratios (depth/neck width) but one model was larger in volume than the other two. Experimentally captured images of visualised flow in one of the three models were available and the calculated flow patterns in this model were seen to agree well with the images. Strong impingements of incoming main flows against aneurysmal necks were observed in all models regardless of the bifurcation angle and direction of the aneurysmal protrusion. These impingements presumably caused haemolysis, with ADP originating from haemolysed red blood cells inducing platelet aggregation. Dispersion of flow paths was observed only in the largest model and, consequently, fluid motion was slower than in the other two models. Thus, platelet aggregation was considered to be more active in the largest model. Validity of this discussion was confi rmed by application of a platelet aggregation model, which had been proposed in the authors’ previous study. It was concluded that the volume of the aneurysmal dome had a signifi cant infl uence on formation of a low-speed region, which is held to be responsible for active platelet aggregation. Geometric features such as the bifurcation angle and direction of aneurysmal protrusion are considered to be secondary factors contributing to active platelet aggregation.
cerebral aneurysm, rupture, haemodynamics, computational fl uid dynamics platelet aggretation thrombus formation
 Ujiie, H., Tamano, Y., Sasaki, K. & Hori, T., Is the aspect ratio a reliable index for predicting the rupture of a saccular aneurysm? Neurosurgery, 48(3), pp. 495–503, 2001. doi:10.1097/00006123-200103000-00007
 Weir, B., Amidei, C., Kongable, G., Findlay, J.M., Kassell, N.F., Kelly, J., Dai, L. & Karrison, T.G., The aspect ratio (dome/neck) of ruptured and unruptured aneurysms. Journal of Neurosurgery, 99(3), pp. 447–451, 2003. doi:10.3171/jns.2003.99.3.0447
 Nader-Sepahi, A., Casimiro, M., Sen, J. & Kitchen, N.D., Is Aspect Ratio A reliable Predictor of Intracranial Aneurysm Rupture? Neurosurgery, 54(6), pp. 1343–1348, 2004. doi:10.1227/01. NEU.0000124482.03676.8B
 Ujiie, H., Tachibana, H., Hiramatsu, O., Hazel, A.L., Matsumoto, T., Ogasawara, Y., Nakajima, H., Hori, T., Takakura, K. & Kajiya, F., Effects of size and shape (aspect ratio) on the hemodynamics of saccular aneurysms: a possible index for surgical treatment of intracranial aneurysms. Neurosurgery, 45(1), pp. 110–130, 1999. doi:10.1097/00006123-199907000-00028
 Takahashi, N., Ujiie, H., Yotoriyama, T., Suzuki, Y., Hori, T. & Kaibara, M., Flow visualization study of the endothelialized glass aneurysm model implanting canine carotid artery (in Japanese with English abs.). Journal of Japanese Society of Biorheology, 18(4), pp. 143–148, 2004.
 Shimano, K., Hayashi, T., Ujiie, H., Ono, T. & Enomoto, Y., Modelling of platelet aggregation in aneurysm, Proc. of 7th Int. Conf. On Modelling In Medicine and Biology, ed. C.A. Brebbia, WIT Press: Southampton, pp. 43–52, 2007.
 Torii, R., Oshima, M., Kobayashi, T., Takagi, K. & Tezduyar, T.E., Influence of wall elasticity on image-based blood flow simulations (in Japanese with English abs.). Transactions of Japan Society of Mechanical Engineers, Series A, 70(697), pp. 1224–1231, 2004.
 Funazaki, K., Higashi, M., Yamada, K., Taniguchi, H. & Tomura, N., Flow-Structure Coupled Analysis of Cerebrovascular Artery with an Aneurysm of Realistic Geometry (in Japanese with English abs.). Transactions of Japan Society of Mechanical Engineers, Series B, 73(731), pp. 1472–1479, 2007.
 Utter, B. & Rossmann J.S., Numerical simulation of saccular aneurysm hemodynamics: Influence of morphology on rupture risk. Journal of Biomechanics, 40(12), pp. 2716–2722, 2007. doi:10.1016/j.jbiomech.2007.01.011
 Patankar, S.V., Numerical Heat Transfer and Fluid Flow, Hemisphere publishing: Washington D.C., pp. 126–131, 1980.
 Shimamura, T., A Study on Flow in Aneurysms (in Japanese), Graduation thesis, Tokyo University of Science, Tokyo, 2006.
 Satoh, A., Nakamura, H., Kobayashi, S., Miyata, A., Tokunaga, H., Wada, M. & Watanabe, Y., Surgical approaches and techniques for anterior communicating artery aneurysms: from angioanatomical point of view (in Japanese with English abs.). Surgery for Cerebral Stroke, 30, pp. 240–246, 2002. doi:10.2335/scs.30.240
 Rhie, C.M. & Chow, W.L., Numerical study of the turbulent flow past an airfoil with trailing edge separation. AIAA Journal, 21(11), pp. 1525–1532, 1983. doi:10.2514/3.8284
 Shimano, K. & Arakawa, C., Numerical simulation of incompressible flow on parallel computer with the domain decomposition technique, Parallel Computational Fluid Dynamics: New Algorithms and Applications (Proc. of the Parallel CFD'94 Conference), eds N. Satofuka, J. Periaux & A. Ecer, Elsevier: Amsterdam, pp. 189–19, 1995.
 Shimano, K., Okudera, K., Anaguchi, T., Utsumi, N., Saito, M., Sumie, C. & Enomoto, Y., Parallel Computing of Flow in Centrifugal Fan Volute Using Contravariant Physical Velocity, Parallel Computational Fluid Dynamics: Multidisciplinary Applications (Proc. of the Parallel CFD 2004 Conference), eds G. Winter, A. Ecer, J. Periaux, N. Satofuka & P. Fox, Elsevier: Amsterdam, pp. 313–320, 2005.
 Leuprecht, A. & Perktold, K., Computer simulation of non-Newtonian effects on blood flow in large arteries. Computer Methods in Biomechanics and Biomedical Engineering, 4, pp. 149–163, 2001. doi:10.1080/10255840008908002
 Shimano, K, Yonezu, S. and Enomoto, Y., Acceleration of Unsteady Incompressible Flow Calculation Using Extrapolation Method, Transactions of Japan Society of Mechanical Engineers, Series B, 74(745), pp. 1896-1902, 2008.
 Shimano, K, Yonezu, S. and Enomoto, Y., Acceleration of Unsteady Incompressible Flow Calculation Using Extrapolation Method, Proc. of the fourth International Conference on Computational Fluid Dynamics (ICCFD4), eds Deconinck, H. & Dick, E., Springer: Berlin, pp. 303–308, 2009.
 Wahap, G., Kobori, T., Takakura, Y., Arai, N., Konishi, Y. & Fukasaku, K., Numerical simulation of Flows in a Pipe with an Aneurismal Sac (Effects of Aneurismal Models and Stents).
Advanced Materials Research, 33–37, pp. 1025–1030, 2008. doi:10.4028/www.scientific.net/ AMR.33-37.1025
 Alkhamis, T.M., Beissinger, R.L. & Chediak, J.R., Red blood cell effect on platelet adhesion and aggregation in low-stress shear flow -Myth of Fact? Transactions of American Society of Artificial Internal Organs, 34, pp. 868–873, 1988.
 Evans, E.A., Waugh, R. & Melnik, L., Elastic area compressibility modulus of red blood cell membrane. Biophysical Journal, 16, pp. 585–595, 1976. doi:10.1016/S0006-3495(76)85713-X