Section: New Results

Numerical methods for fluid mechanics and application to blood flows

Participants : Jean-Frédéric Gerbeau, Marc Thiriet, Irène Vignon-Clementel.

• [13] : In this work, a virtual planning of three different surgical Fontan repairs was performed to test the predictive capability of a closed-loop multi-scale model (3D-0D), constructed based on preoperative patient-specific data. Results from this multi-scale approach showed that the pre-operative caval flows should not be used as boundary conditions in post-operative simulations. The Y-TCPC repair seemed to perform better than all other TCPC models both at rest and under exercise conditions. Further work is needed to correlate results from these simulations with clinical outcomes.

• [18] : Flow reversal at an outlet, although perfectly physical, can lead to rapid numerical divergence in computational fluid dynamics. Several remedies have been proposed in the literature and are discussed in the present finite element study. The most robust one was found to be a boundary advective stabilization term. The comparison was done on simple examples, as well as realistic three-dimensional multi-branched models of blood flow.

• [22] & [23] : Treatments for coarctation of the aorta (CoA) can alleviate blood pressure gradients, but long-term morbidity still exists that can be explained by altered indices of hemodynamics and biomechanics. These articles present a combination of CFD methods (physiologically realistic boundary conditions and FSI with viscoelastic tissue support) to explore these indices in untreated and treated CoA, comparing them to normal subjects under rest and exercise conditions. These studies showed in particular that CoA disturbs normal patterns of wall shear stress and oscillatory shear index throughout the thoracic aorta (potentially linked to the development of atherosclerosis) and that restoring favorable anatomy may not restore normal hemodynamics.

• [24] : The objective of this work is to address the formulation of an adequate model of the external tissue environment when studying a portion of the arterial tree with fluid-structure interaction. The simulations are quantitatively assessed by detailed comparisons with dynamical medical image sequences, and the model results are shown to be in very good adequacy with the data.

• [26] The wide range of existing viscoelastic wall models may produce significantly different blood flow, pressure, and vessel deformation solutions in cardiovascular simulations. In this paper, we have successfully implemented and verified two viscoelastic wall models in a nonlinear 1D finite element blood flow solver and analyzed differences between these models in various idealized and physiological simulations, including exercise.

• [28] : High-intensity focused ultrasound (HIFU) is used as a thermal ablation process to eliminate tumors in different body's organs. Blood flow has a cooling effect. Conversely, ultrasounds are responsible for acoustic streaming. A three-dimensional acoustics-thermal-fluid coupling model is carried out to compute the temperature field a given hepatic cancerous region.

• [29] : Imaged-based patient-specific models of the multi-branched pulmonary arteries and superior vena cava were built for five cavopulmonary connection (i.e. Glenn) patients prior to their third surgery to alleviate their congenital heart disease. Inflow and outflow boundary conditions for computational blood flow simulations (CFD) were constructed based on an iterative procedure to match available MRI and catheterization clinical data. Common trends and differences emerged from this three-dimensional CFD study; in particular low wall shear stress was found for all subjects, which is potentially deleterious. A sensitivity analysis was performed to investigate the impact of input data (clinical and modeling) to construct boundary conditions on several clinical and mechanical indicators. Among other findings, this study suggests that although 6-10% flow split imprecision seemed reasonable in terms of patient comparison, the common practice of imposing a right pulmonary artery/left pulmonary artery flow split of 55%/45% when performing patient specific simulations should be avoided.

• [31] : A novel Y-shaped baffle has been proposed for the Fontan operation with promising initial results on idealized models or a single patient-specific model. The objective of this study is to comprehensively compare the hemodynamic performance and hepatic blood flow distribution of the Y-graft Fontan baffle with two current designs on multiple patient-specific models. Methods include virtual geometrical design, computational fluid dynamics based on preoperative patient-specific data, particle tracking and sensitivity analysis, including rest and exercise conditions. The Y-graft Fontan design achieves overall superior hemodynamic performance compared with traditional designs. However, the results emphasize that designs should be customized for individual patients before clinical application.