Numerical Models of Human Circulatory System under Altered Gravity: Brain Circulation

A computational fluid dynamics (CFD) approach is presented to model the blood flow through the human circul atory system under altered gravity conditions. Models required for C FD simulation relevant to major hem odynamic issues are i ntroduced such as non -Newtonian flow models governed by red blood cells, a model for arterial wall motion due to fluid -wall intera ction s, a vascular bed model for outflow boundary conditions, and a model for auto -regulation mechanism. The three -dimensional unsteady incompressible Navier -Stokes equations coupled with these models are solved iteratively using the pseudocompressibility method and dual time stepping. Moving wall boundary conditions from the first -order fluid -wall interaction model are used to study the influence of arterial wall distensibility on flow patterns and wall shear stresses during the heart pulse. A vascular bed modeling utilizing the analogy with electric circuits is coupled with an auto -regulation algor ithm for multiple outflow boundaries. For the treatment of complex geometry, a chimera overset grid technique is adopted to obtain connectivity between arterial branches. For code validation, computed results are compared with experimental data for steady and unsteady non -Newtonian flows. Good agreement is obtained for both cases. In six -type Gravity Benchmark Problems, gravity source terms are added to the Navier -Stokes equations to study the effect of gravitational variation on the human circulatory syste m. This computational approach is then applied to localized blood flows through a realistic carotid bifurcation and two Circle of Willis models, one using an idealized geometry and the other model using an anatomical data set. A three dimensional anatomica l Circle of Willis configuration is reconstructed from human -specific magnetic resonance images using an image segmentation method. The blood flow through these Circle of Willis models is simulated to provide means for studying gravitational effects on the brain circulation under auto -regulation.