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High-resolution three-dimensional simulations (involving 100 million degrees of freedom) were employed to study transient turbulent flow in a carotid arterial bifurcation with a stenosed internal carotid artery (ICA). The geometrical model was reconstructed from MRI images, and in vivo velocity measurements were incorporated in the simulations to provide inlet and outlet boundary conditions. Due to the high degree of the ICA occlusion and the variable flow rate, a transitional and intermittent flow between laminar and turbulent states was established. Time- and space-window proper orthogonal decomposition (POD) was applied to quantify the different flow regimes in the occluded artery. A simplified version of the POD analysis that utilizes 2D slices only--more appropriate in the clinical setting--was also investigated.

1. Full-scale simulations of the virtual physiological human (VPH) will require significant advances in modelling, multiscale mathematics, scientific computing and further advances in medical imaging. Herein, we review some of the main issues that need to be resolved in order to make three-dimensional (3D) simulations of blood flow in the human arterial tree feasible in the near future. 2. A straightforward approach is computationally prohibitive even on the emerging petaflop supercomputers, so a three-level hierarchical approach based on vessel size is required, consisting of: (i) a macrovascular network (MaN); (ii) a mesovascular network (MeN); and (iii) a microvascular network (MiN). We present recent simulations of MaN obtained by solving the 3D Navier-Stokes equations on arterial networks with tens of arteries and bifurcations and accounting for the neglected dynamics through proper boundary conditions. 3. A multiscale simulation coupling MaN-MeN-MiN and running on hundreds of thousands of processors on petaflop computers will require no more than a few CPU hours per cardiac cycle within the next 5 years. The rapidly growing capacity of supercomputing centres opens up the possibility of simulation studies of cardiovascular diseases, drug delivery, perfusion in the brain and other pathologies.

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