LEE Makina Mühendisliği Lisansüstü Programı
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Sustainable Development Goal "Goal 3: Good Health and Wellbeing" ile LEE Makina Mühendisliği Lisansüstü Programı'a göz atma
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ÖgeModeling of twophase blood flow and fluidstructure interactions in cerebral aneurysms(Graduate School, 20221216) Pahlavani, Hamed ; Özdemir, İlyas Bedii ; 503152005 ; Mechanical EngineeringThis thesis is composed of 7 chapters, each of them dealing with different aspects of numerical tools (e.g., CFD and FSI) for prediction and assessment of cerebral aneurysm rupture. Computation fluid dynamics has been widely used to investigate the effect of singlephase blood model in the risk assessments, but no further application of twophase blood model and FSI were available. For this reason, the thesis was proposed to evaluate further applications, with the aim of better understanding of the diseases. Rupture risk assessment can be classified as (a) Flow properties (e.g., inflow penetration depth, flow complexity and flow impingement zones) and (b) wall shear stress based hemodynamic indexes (e.g., OSI and TAWSS). Chapter 1 is introductory and reviews the cerebral aneurysms, the mechanisms leading to the disease, and current computational tools in order to predict and assess of aneurysm rupture. Chapter 2 gives a very deep understanding about the mathematical theory behind the singlephase, twophase flow and FSI. Considering the nonNewtonian nature of blood, two nonNewtonian viscosity models (Casson for singlephase and Carreau– Yasuda for twophase blood assumption) are discussed here. Then it proceeds with FSI concept where an appraisal of the FSI approach and its implementation, the governing equations regarding the singlephase blood assumption and mechanics of deformable vessel structure are discussed in detail. One of the most important aspects of this thesis is to use opensource solvers for numerical implementations. Regarding the implementation of singlephase and twophase blood CFD analysis, OpenFOAM is used which is free and opensource software for CFD from the OpenFOAM Foundation. For the implementation of an FSI problem, the preCICE multiphysic coupling toolkit is used in order to couple OpenFOAM (FVM CFD solver) and CALCULIX (FEM structure solver). Furthermore, two wall shear stress based hemodynamic indexes (TAWSS and OSI) are introduced which can be used in order to make a bridge from numerical results to rupture risk assessments. Two patientspecific cerebral aneurysms are given in chapter 3 where the first patient was a female of 41 years old, who had anterior communicating artery aneurysm with concomitant subarachnoid hemorrhage and left frontobasal hematoma, and the second patient was a female of 62 years old, who had dolichoectatic carotid and vertebral arteries. The 3D images in digital imaging and communications in medicine format were anatomically remodeled into patientspecific 3D geometries in the STL format. The FVM mesh, boundary conditions and numerical implementations used for CFD and FSI analysis of two aneurysms are discussed in detail in this chapter. Chapter 4 investigates the blood transport in the cerebral aneurysm using singlephase and twophase models. In twophase EulerEuler approach, the blood is represented by two interpenetrating continua where the dispersed red blood cells of nonNewtonian characteristics are suspended in the continuous Newtonian plasma. The results of two phase model, where the RBCs phase is assumed to be Carreau–Yasuda fluid, are validated against the experimental data. Furthermore, comparative analyses were performed in two patientspecific aneurysms, which indicated that for a given pulsatile flow rate, the twophase blood approach has vitally advantageous over the singlephase assumption, and revealed a deeper inflow penetration, more complex flow structures and denser flow diversion zones in the aneurysm sac. It was obvious that the high OSI values calculated by the twophase model covered much wider regions than the single phase predicted. It was equally crucial that these regions coincided with the TAWSS values lower than the threshold that the singlephase approach can predict. Apparently, the singlephase model failed to spot sites of high rupture risk. The results were further exploited to identify the RBCs aggregation regions as, for example, the concave structures and narrow paths in the saccular aneurysms, for their possible use as the precursors of the thrombus formation. Chapter 5 investigates the effect of variations in the haematocrit level on the blood flow in two cerebral aneurysms using the twophase EulerEuler approach and the Carreau–Yasuda viscosity model. The results showed that the maximum inflow jet penetration was achieved at the lowest haematocrit level, and this accompanied with strong flow impingements at the narrow corners deep inside the aneurysm sac and undesired complex flow patterns spreading from entrance to the aneurysm dome. The decrease in H level also changed the characteristics of the velocity profile inside the dome from a single to a doublepeak profile, which increased the likelihood of a daughter aneurysm formation. Furthermore, the TAWSS and OSI indicators showed that lowering the H values could change an initially lowrisk case into a very high rupture risk situation. The twophase EulerEuler approach was used to enlighten the effect of variations in the haematocrit level to cure the blood flow issue in two cerebral aneurysms. A comprehensive description of the twophase EulerEuler approach and the relevant viscosity specifications were described in the previous chapter. The same patient specific aneurysms and the numerical implementations of the twophase model discussed before were used here. However, this chapter presents an appraisal of the approach and interpretations of the flow complexity, features of the inflow diversion zone, penetration depths and the shear stress parameters based on varying Hematocrit values. Chapter 6 investigates dynamics of the wall movements of a patientspecific aneurysm dome using the interactions of the nonNewtonian blood flow and the deformable vessels. The patient under consideration had an anterior communicating artery aneurysm with a concomitant subarachnoid hemorrhage and left frontonasal hematoma. A finite volume CFD solver was used with a 3D mesh of roughly 300000 cells and three boundary patches; the inlet, deformable walls and outlets. A linear elastic material model was considered for the deformation of the aneurysm wall and, in the structural computations, a finite element solver was employed with a solid domain of approximately 12000 elements. An opensource code was exploited for the coupling between the CFD and finite element solvers. Results showed that at the peak systole, the vortical structure of the flow in the aneurysm dome was complicated. Furthermore, the instabilities in the flow field produced intense shear forces, due to which a possible weakening of the wall material will certainly lead to an increase in the risk of the rupture and bleeding. The nonuniformity of the flow field acquired large values of the von Mises stresses, resulting in prominent wall displacements, which also matched to the high OSI and low TAWSS values. The maximum displacements exhibited a nonstationery movement everywhere in the dome though mostly remained in the region of the impingement. And finally, chapter 7 covers conclusions and remarks respectively.