Pulsatile Blood Flow Simulations in Computed Tomography(CT) Scan-Based and Idealized Geometries of Human Aorta

Blood flow in human aorta is an unsteady and complex phenomenon. The complex patterns are related to the geometrical features like curvature, bends, and branching and pulsatile nature of flow from left ventricle of heart. The aim of this work was to understand the effect of aorta geometry on the flow dynamics. To achieve this, 3D realistic and idealized models of descending aorta were reconstructed from Computed Tomography (CT) images of a female patient. The geometries were reconstructed using medical image processing code. The blood flow in aorta was assumed to be laminar and incompressible and the blood was assumed to be Newtonian fluid. A time dependent pulsatile and parabolic boundary condition was deployed at inlet. Steady and unsteady blood flow simulations were performed in real and idealized geometries of descending aorta using a Finite Volume Method (FVM) code. Analysis of Wall Shear Stress (WSS) distribution, pressure distribution, and axial velocity profiles were carried out in both geometries at steady and unsteady state conditions. The results obtained in thesis work reveal that the idealization of geometry underestimates the values of WSS especially near the region with sudden change of diameter. However, the resultant pressure and velocity in idealized geometry are close to those in real geometry

Perspectives interculturelles et interlinguistiques sur le discours académique – Crosscultural and Crosslinguistic Perspectives on Academic Discourse. Volume 1

The Department of French Studies of the University of Turku (Finland) organized an International Bilingual Conference on Crosscultural and Crosslinguistic Perspectives on Academic Discourse from 2022 May 2005. The event hosted specialists on Academic Discourse from Belgium, Finland, France, Germany, Italy, Norway, Spain, and the USA. This book is the first volume in our series of publications on Academic Discourse (AD hereafter). The following pages are composed of selected papers from the conference and focus on different aspects and analytical frameworks of Academic Discourse. One of the motivations behind organizing the conference was to examine and expand research on AD in different languages. Another one was to question to what extent academic genres are culturebound and language specific or primarily field or domain specific. The research carried out on AD has been mainly concerned with the use of English in different academic settings for a long time now – mainly written contexts – and at the expense of other languages. Alternatively the academic genre conventions of English and English speaking world have served as a basis for comparison with other languages and cultures. We consider this first volume to be a strong contribution to the spreading out of researches based on other languages than English in AD, namely Finnish, French, Italian, Norwegian and Romanian in this book. All the following articles have a strong link with the French language: either French is constitutive of the AD corpora under examination or the article was written in French. The structure of the book suggests and provides evidence that the concept of AD is understood and tackled to varying degrees by different scholars. Our first volume opens up the discussion on what AD is and backs dissemination, overlapping and expansion of current research questions and methodologies. The book is divided into three parts and contains four articles in English and six articles in French. The papers in part one and part two cover what we call the prototypical genre of written AD, i.e. the research article. Part one follows up on issues linked to the 13 Research Article (RA hereafter). Kjersti Fløttum asks wether a typical RA exists and concentrates on authors’ voices in RA (self and other dimensions), whereas Didriksen and Gjesdal’s article focuses on individual variation of the author’s voice in RA. The last article in this section is by Nadine Rentel and deals with evaluation in the writing of RA. Part two concentrates on the teaching and learning of AD within foreign language learning, another more or less canonical genre of AD. Two aspects of writing are covered in the first two articles: foreign students’ representations on rhetorical traditions (Hidden) and a contrastive assessment of written exercices in French and Finnish in Higher Education (Suzanne). The last contribution in this section on AD moves away from traditional written forms and looks at how argumentation is constructed in students’ oral presentations (Dervin and Fauveau). The last part of the book continues the extension by featuring four articles written in French exploring institutional and scientific discourses. Institutional discourses under scrutiny include the European Bologna Process (Galatanu) and Romanian reform texts (Moilanen). As for scientific discourses, the next paper in this section deconstructs an ideological discourse on the didactics of French as a foreign language (Pescheux). Finally, the last paper in part three reflects on varied forms of AD at university (Defays). We hope that this book will add some fuel to continue discussing diverse forms of and approches to AD – in different languages and voices! No need to say that with the current upsurge in academic mobility, reflecting on crosscultural and crosslinguistic AD has just but started.

Application of Computational Fluid Dynamics in Modeling Blood Flow in Human Thoracic Aorta

The aim of this study was to simulate blood flow in thoracic human aorta and understand the role of flow dynamics in the initialization and localization of atherosclerotic plaque in human thoracic aorta. The blood flow dynamics in idealized and realistic models of human thoracic aorta were numerically simulated in three idealized and two realistic thoracic aorta models. The idealized models of thoracic aorta were reconstructed with measurements available from literature, and the realistic models of thoracic aorta were constructed by image processing Computed Tomographic (CT) images. The CT images were made available by South Karelia Central Hospital in Lappeenranta. The reconstruction of thoracic aorta consisted of operations, such as contrast adjustment, image segmentations, and 3D surface rendering. Additional design operations were performed to make the aorta model compatible for the numerical method based computer code. The image processing and design operations were performed with specialized medical image processing software. Pulsatile pressure and velocity boundary conditions were deployed as inlet boundary conditions. The blood flow was assumed homogeneous and incompressible. The blood was assumed to be a Newtonian fluid. The simulations with idealized models of thoracic aorta were carried out with Finite Element Method based computer code, while the simulations with realistic models of thoracic aorta were carried out with Finite Volume Method based computer code. Simulations were carried out for four cardiac cycles. The distribution of flow, pressure and Wall Shear Stress (WSS) observed during the fourth cardiac cycle were extensively analyzed. The aim of carrying out the simulations with idealized model was to get an estimate of flow dynamics in a realistic aorta model. The motive behind the choice of three aorta models with distinct features was to understand the dependence of flow dynamics on aorta anatomy. Highly disturbed and nonuniform distribution of velocity and WSS was observed in aortic arch, near brachiocephalic, left common artery, and left subclavian artery. On the other hand, the WSS profiles at the roots of branches show significant differences with geometry variation of aorta and branches. The comparison of instantaneous WSS profiles revealed that the model with straight branching arteries had relatively lower WSS compared to that in the aorta model with curved branches. In addition to this, significant differences were observed in the spatial and temporal profiles of WSS, flow, and pressure. The study with idealized model was extended to study blood flow in thoracic aorta under the effects of hypertension and hypotension. One of the idealized aorta models was modified along with the boundary conditions to mimic the thoracic aorta under the effects of hypertension and hypotension. The results of simulations with realistic models extracted from CT scans demonstrated more realistic flow dynamics than that in the idealized models. During systole, the velocity in ascending aorta was skewed towards the outer wall of aortic arch. The flow develops secondary flow patterns as it moves downstream towards aortic arch. Unlike idealized models, the distribution of flow was nonplanar and heavily guided by the artery anatomy. Flow cavitation was observed in the aorta model which was imaged giving longer branches. This could not be properly observed in the model with imaging containing a shorter length for aortic branches. The flow circulation was also observed in the inner wall of the aortic arch. However, during the diastole, the flow profiles were almost flat and regular due the acceleration of flow at the inlet. The flow profiles were weakly turbulent during the flow reversal. The complex flow patterns caused a non-uniform distribution of WSS. High WSS was distributed at the junction of branches and aortic arch. Low WSS was distributed at the proximal part of the junction, while intermedium WSS was distributed in the distal part of the junction. The pulsatile nature of the inflow caused oscillating WSS at the branch entry region and inner curvature of aortic arch. Based on the WSS distribution in the realistic model, one of the aorta models was altered to induce artificial atherosclerotic plaque at the branch entry region and inner curvature of aortic arch. Atherosclerotic plaque causing 50% blockage of lumen was introduced in brachiocephalic artery, common carotid artery, left subclavian artery, and aortic arch. The aim of this part of the study was first to study the effect of stenosis on flow and WSS distribution, understand the effect of shape of atherosclerotic plaque on flow and WSS distribution, and finally to investigate the effect of lumen blockage severity on flow and WSS distributions. The results revealed that the distribution of WSS is significantly affected by plaque with mere 50% stenosis. The asymmetric shape of stenosis causes higher WSS in branching arteries than in the cases with symmetric plaque. The flow dynamics within thoracic aorta models has been extensively studied and reported here. The effects of pressure and arterial anatomy on the flow dynamic were investigated. The distribution of complex flow and WSS is correlated with the localization of atherosclerosis. With the available results we can conclude that the thoracic aorta, with complex anatomy is the most vulnerable artery for the localization and development of atherosclerosis. The flow dynamics and arterial anatomy play a role in the localization of atherosclerosis. The patient specific image based models can be used to diagnose the locations in the aorta vulnerable to the development of arterial diseases such as atherosclerosis.