The present and the future expectation in parallel computing pose a new generational change in simulation and computing. Modern High Performance Computing (HPC) facilities have high computational power in terms of operations per second -today peta-FLOPS (10e15 FLOPS) and growing toward the exascale (10e18 FLOPS) which is expected in few years-. This opens the way for using simulation tools in a wide range of new engineering and scientific applications. For example, CFD&HT codes will be effectively used in the design phase of industrial devices, obtaining valuable information with reasonable time expenses. However, the use of the emerging computer architectures is subjected to enhancements and innovation in software design patterns. So far, powerful codes for individually studying heat and mass transfer phenomena at multiple levels of modeling are available. However, there is no way to combine them for resolving complex coupled problems. In the current context, this PhD thesis presents the development of parallel methodologies, and its implementation as an object-oriented software platform, for the simulation of multiphysics systems. By means of this new software platform, called NEST, the distinct codes can now be integrated into single simulation tools for specific applications of social and industrial interest. This is done in an intuitive and simple way so that the researchers do not have to bother either on the coexistence of several codes at the same time neither on how they interact to each other. The coupling of the involved components is controlled from a low level code layer, which is transparent to the users. This contributes with appealing benefits on software projects management first and on the flexibility and features of the simulations, later. In sum, the presented approaches pose a new paradigm in the production of physics simulation programs. Although the thesis pursues general purpose applications, special emphasis is placed on the simulation of thermal systems, in particular on buildings energy assessment and on hermetic reciprocating compressors.

Parallel object-oriented algorithms for simulation of multiphysics : application to thermal systems

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Mechanistic numerical study of trhombus growth

The objective of this paper is to analyze the association of intraluminal thrombus (ILT) presence and morphology with oxygen transport in abdominal aortic aneurysms (AAA) and local hypoxia. The biomechanical role of the ILT layer in the evolution of the aneurysm is still not fully understood. ILT has been shown to create an inflammatory environment by reducing oxygen flux to the arterial wall and therefore decreasing its strength. It has been also hypothesized that the geometry of the ILT may further affect AAA rupture. However, no previous research has attempted to explore the effect of morphological features of ILT on oxygen distributions within the AAA, in a systematic manner. In this study, we perform a comprehensive analysis to investigate how physiologically meaningful variations in ILT geometric characteristics affect oxygen transport within an AAA. We simulate twenty-seven AAA models with variable ILT dimensions and investigate the extent to which ILT attenuates oxygen concentration in the arterial wall. Geometric variations studied include ILT thickness and ILT length, as well as the bulge diameter of the aneurysm which is related to ILT curvature. Computer simulations of coupled fluid flow-mass transport between arterial wall, ILT, and blood are solved and spatial variations of oxygen concentrations within the ILT and wall are obtained. The comparison of the results for all twenty-seven simulations supports the hypothesis that the presence of ILT in AAA correlates to significantly impaired oxygen transport to the aneurysmal wall. Mainly, we observed that ILT thickness and length are the parameters that influence decreased oxygen flow and concentration values the most, and thick thrombi exacerbate hypoxic conditions in the arterial wall, which may contribute to increased tissue degradation. Conversely, we observed that the arterial wall oxygen concentration is nearly independent of the AAA bulge diameter. This confirms that consideration of ILT size and anatomy is crucial in the analysis of AAA development.

The Effects of Geometric Features of Intraluminal Thrombus on the Vessel Wall Oxygen Deprivation

Author(s): Arzani, Amirhossein | Advisor(s): Shadden, shawn C. | Abstract: Abdominal aortic aneurysm (AAA) is a permanent local enlargement of the abdominal aorta. Complex anatomies, presence of side branches, and pulsatility of blood flow creates a complex chaotic flow field in AAAs. The progression of AAA can lead to rupture, which is one of the leading causes of death in the elderly. In this study, the flow topology in AAAs, role of hemodynamics in AAA progression, complex vectorial wall shear stress (WSS) patterns, and near-wall transport in AAAs were investigated.Patient-specific computational fluid dynamics (CFD) was used to obtain blood flow information. Lagrangian coherent structures (LCS) were computed to study the flow physics. The utility of these structures in studying chaotic mixing and transport, flow separation, and vortex wall interaction was demonstrated in different patients. The effect of exercise on flow topology and quantitative mixing was evaluated. The evolution of a systolic vortex formed in the proximal region, strongly influenced the flow topology in the aneurysms. Intraluminal thrombus (ILT) deposition and lumen progression were quantified in several patients using magnetic resonance imaging over a 2--3 year followup. Point-wise spatial correlation of hemodynamic parameters to ILT deposition, revealed a negative correlation between oscillatory shear stress and ILT deposition. This was attributed to persistence recirculation, which can lead to unidirectional backward WSS. Complex vectorial variations in WSS was studied. Namely, variations in WSS magnitude, direction, and vector in space and time were quantified and compared. Several new WSS measures were introduced to better quantify WSS vectorial variations. The concept of Lagrangian wall shear stress structures (WSS LCS) was introduced. WSS was scaled to obtain a first order representation of near-wall velocity. Tracers representing biochemicals in thin concentration boundary layers were tracked on the aneurysm surface based on the WSS vector field. Formation of coherent structures from WSS tracers were shown. The WSS LCS organize near-wall transport in high Schmidt number flows and could be used to predict regions of high near-wall stagnation and concentration. A wall shear stress exposure time (WSSET) measure was introduced to quantify near-wall stagnation and concentration. Excellent agreement between WSSET and surface concentration obtained from 3D continuum mass transport was obtained. Finally, the important roles that WSS fixed points play in cardiovascular flows was discussed.

Hemodynamics and Transport in Patient-specific Abdominal Aortic Aneurysms

Abstract Heat and mass transfer phenomena were studied in the sudden expansion region of a pipe under steady and pulsatile conditions. The Prandtl number was varied from 100 to 12 000 and the flow was characterized for both uniform and parabolic entrance velocity profiles. A uniform velocity profile was used for pulsatile flow. It was found that heat transfer in the recirculation region was maximal near the area where wall shear was minimal. Blunting of the inlet profile caused the point of maximum heat transfer to move upstream. There was a nonlinear effect of Prandtl number on heat transfer which plateaued for Pr > 10 3 . The wall shear rate in the separation zone varied markedly with pulsatile flows, but the wall heat transfer remained relatively constant. The time-averaged pulsatile heat transfer at the wall was approximately the same as with steady flow with the mean Reynolds number. However, the isotherms within the pulsatile flow were markedly different from steady flow. The results demonstrate the complexity of separation flows and identify characteristic regions of high and low heat/mass transfer for high Prandtl/Schmidt pulsatile flow.

Li Xianming

Papers

Heat and mass transfer in a separated flow region for high Prandtl and Schmidt numbers under pulsatile conditions