Towards the integration of stent-wall contact modelling and arterial wall deformability in fluid-structure interaction simulations of percutaneous pulmonary valves

The pulmonary valve (PV) is located between the right ventricle and the main pulmonary artery (PA), regulating blood flow from the heart into pulmonary circulation. Congenital heart diseases often cause an impairment of PV functionality, thereby requiring the replacement of the native valve with a prosthesis. Over the last years, percutaneous pulmonary valves (PPVs) have emerged as promising PV substitutes. The computational investigation of PV and PA hemodynamics enables the examination of blood flow patterns, allowing for biomechanical evaluations. Specifically, fluid-structure interaction (FSI) simulation allows for investigating the interaction between blood flow and biological structures such as PV leaflets and PA wall. While several studies evaluated PA hemodynamics in valve-less patient-specific scenarios, only few works addressed the computational simulation of native or prosthetic PVs by means of FSI simulations. Additionally, none of them focused specifically on PPVs or accounted for PA deformability. In this context, the aim of this thesis is to develop an FSI model of PPVs, with a specific focus on simulating the contact between the PPV stent and the PA wall, while also accounting for PA distensibility. Contact modelling was addressed by considering a geometrical model for the Melody PPV assembled within a straight PA model. PPV leaflets, stent and PA wall were discretized with shell, tetrahedral and hexahedral elements, respectively. Blood flow within the PA was driven by physiological boundary conditions, prescribed in terms of pressure drop and flow rate. First, purely structural preliminary tests were conducted with Ansys LS-DYNA implicit structural solver to assess the efficacy of LS-DYNA Mortar tied contact in simulating the interaction between valve stent and the wall. Then, FSI simulations were conducted by coupling LS-DYNA structural implicit and incompressible fluid dynamics (ICFD) solvers. The integration of PA deformability in PPV FSI simulations was investigated in a patient-specific scenario. A patient-specific PA geometrical model was assembled with Melody valve leaflet model. Valve leaflets and PA wall were meshed with shell elements. Valve leaflets were modelled as linear elastic. The PA wall was modelled as rigid in the valve region and as hyperelastic elsewhere. A physiological flow-rate waveform and resistance boundary conditions were imposed at the inflow and outflow sections, respectively. FSI simulations were conducted with the previously mentioned coupling strategy. A rigid wall simulation was also conducted for comparative purposes. In preliminary tests on contact modelling, the evaluation of interface force vectors and contact interface displacement proved the feasibility of modelling the contact between PPV stent and PA wall in FSI simulations. In patient-specific simulations addressing wall deformability, results were analyzed in terms of wall shear stress descriptors and vortical structures. Minor quantitative differences and no substantial qualitative differences emerged between deformable-wall and rigid-wall simulations. In conclusion, the proposed FSI approach proved to be flexible in exploring highly complex simulation scenarios involving the pulmonary circulation, successfully capturing not only the interaction between blood flow and PPV leaflets but also the interaction between the PA wall, PPV stent, and blood flow.