Heat and mass transfer in a separated flow region for high Prandtl and Schmidt numbers under pulsatile conditions
01 Jan 1995-Vol. 1, Iss: 21, pp 26
TL;DR: In this article, heat and mass transfer in the recirculation region of a pipe under steady and pulsatile conditions were studied under uniform and parabolic entrance velocity profiles and the results demonstrate the complexity of separation flows and identify characteristic regions of high and low heat/mass transfer.
Abstract: 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.
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Dissertation•
05 Feb 2016
TL;DR: This PhD thesis presents the development of parallel methodologies, and its implementation as an object-oriented software platform, for the simulation of multiphysics systems, and poses a new paradigm in the production of physics simulation programs.
Abstract: 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.
8 citations
TL;DR: The hypothesis that the presence of ILT in AAA correlates to significantly impaired oxygen transport to the aneurysmal wall is supported and it is 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.
Abstract: 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.
3 citations
01 Jan 2016
TL;DR: Arzani et al. as mentioned in this paper investigated the role of hemodynamics in AAA progression, complex vectorial wall shear stress (WSS) patterns, and near-wall transport in abdominal aorta.
Abstract: 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.
2 citations
Cites result from "Heat and mass transfer in a separat..."
...It has been shown that for Sc & O(10(3)) mass transfer resulting from unsteady blood flow was similar to that obtained from the time-averaged components [162]....
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References
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TL;DR: These studies confirm earlier findings under steady flow conditions that plaques tend to form in areas of low, rather than high, shear stress, but indicate in addition that marked oscillations in the direction of wall shear may enhance atherogenesis.
Abstract: Fluid velocities were measured by laser Doppler velocimetry under conditions of pulsatile flow in a scale model of the human carotid bifurcation. Flow velocity and wall shear stress at five axial and four circumferential positions were compared with intimal plaque thickness at corresponding locations in carotid bifurcations obtained from cadavers. Velocities and wall shear stresses during diastole were similar to those found previously under steady flow conditions, but these quantities oscillated in both magnitude and direction during the systolic phase. At the inner wall of the internal carotid sinus, in the region of the flow divider, wall shear stress was highest (systole = 41 dynes/cm2, diastole = 10 dynes/cm2, mean = 17 dynes/cm2) and remained unidirectional during systole. Intimal thickening in this location was minimal. At the outer wall of the carotid sinus where intimal plaques were thickest, mean shear stress was low (-0.5 dynes/cm2) but the instantaneous shear stress oscillated between -7 and +4 dynes/cm2. Along the side walls of the sinus, intimal plaque thickness was greater than in the region of the flow divider and circumferential oscillations of shear stress were prominent. With all 20 axial and circumferential measurement locations considered, strong correlations were found between intimal thickness and the reciprocal of maximum shear stress (r = 0.90, p less than 0.0005) or the reciprocal of mean shear stress (r = 0.82, p less than 0.001). An index which takes into account oscillations of wall shear also correlated strongly with intimal thickness (r = 0.82, p less than 0.001). When only the inner wall and outer wall positions were taken into account, correlations of lesion thickness with the inverse of maximum wall shear and mean wall shear were 0.94 (p less than 0.001) and 0.95 (p less than 0.001), respectively, and with the oscillatory shear index, 0.93 (p less than 0.001). These studies confirm earlier findings under steady flow conditions that plaques tend to form in areas of low, rather than high, shear stress, but indicate in addition that marked oscillations in the direction of wall shear may enhance atherogenesis.
2,623 citations
TL;DR: The experiments of McDonald and his co-workers have shown that in the larger arteries of the rabbit and the dog there is a reversal of the flow, and the simple mathematical treatment has strong similarities with the theory of the distribution of alternating current in a conductor of finite size.
Abstract: The experiments of McDonald and his co-workers (McDonald, 1952, 1955; Helps & McDonald, 1953) have shown that in the larger arteries of the rabbit and the dog there is a reversal of the flow. Measurements of the pressure gradient (Helps & McDonald, 1953) showed a phase-lag between pressure gradient and flow somewhat analogous with the phase-lag between voltage and current in a conductor carrying alternating current, and the simple mathematical treatment given below has strong similarities with the theory of the distribution of alternating current in a conductor of finite size.
1,675 citations
TL;DR: It is concluded that in the human carotid bifurcation, regions of moderate to high shear stress, where flow remains unidirectional and axially aligned, are relatively spared of intimal thickening.
Abstract: The distribution of nonstenosing, asymptomatic intimal plaques in 12 adult human carotid bifurcations obtained at autopsy was compared with the distribution of flow streamline patterns, flow velocity profiles, and shear stresses in corresponding scale models. The postmortem specimens were fixed while distended to restore normal in vivo length, diameter, and configuration. Angiograms were used to measure branch angles and diameters, and transverse histological sections were studied at five standard sampling levels. Intimal thickness was determined at 15 degrees intervals around the circumference of the vessel sections from contour tracings of images projected onto a digitizing plate. In the models, laser-Doppler anemometry was used to determine flow velocity profiles and shear stresses at levels corresponding to the standard specimen sampling sites under conditions of steady flow at Reynolds numbers of 400, 800, and 1200, and flow patterns were visualized by hydrogen bubble and dye-washout techniques. Intimal thickening was greatest and consistently eccentric in the carotid sinus. With the center of the flow divider as the 0 degree index point, mid-sinus sections showed minimum intimal thickness (0.05 +/- 0.02 mm) within 15 degrees of the index point, while maximum thickness (0.9 +/- 0.1 mm) occurred at 161 +/- 16 degrees, i.e., on the outer wall opposite the flow divider. Where the intima was thinnest, along the inner wall, flow streamlines in the model remain axially aligned and unidirectional, with velocity maxima shifted toward the flow divider apex. Wall shear stress along the inner wall ranged from 31 to 600 dynes/cm2 depending on the Reynolds number. Where the intima was thickest, along the outer wall opposite the flow divider apex, the pattern of flow was complex and included a region of separation and reversal of axial flow as well as the development of counter-rotating helical trajectories. Wall shear stress along the outer wall ranged from 0 to -6 dynes/cm2. Intimal thickening at the common carotid and distal internal carotid levels of section was minimal and was distributed uniformly about the circumference. We conclude that in the human carotid bifurcation, regions of moderate to high shear stress, where flow remains unidirectional and axially aligned, are relatively spared of intimal thickening. Intimal thickening and atherosclerosis develop largely in regions of relatively low wall shear stress, flow separation, and departure from axially aligned, unidirectional flow. Similar quantitative evaluations of other atherosclerosis-prone locations and corresponding flow profile studies in geometrically accurate models may reveal which of these hemodynamic conditions are most consistently associated with the development of intimal disease.
1,451 citations
TL;DR: It appears that wall shear rate may be a major controlling factor in the development of atheromatous lesions in man and in animals and a net flux of cholesterol from blood to wall cannot account for the observed normally occurring (quasi-steady state) and experimentally induced atheroma.
Abstract: On the basis of various observations, we argue that there is spatial variation of the time-averaged wall shear rate in arteries, both overall and locally. From our own observations, and those of others, we show that the distribution of early atheroma in man is coincident with those regions in which arterial wall shear rate is expected to be relatively low, while the development of lesions is inhibited or retarded in those regions in which wall shear rate is expected to be relatively high. Such a correlation is inconsistent with a proposal, made by several workers, that there is a causative relation between arterial blood mechanics and the development of atheroma, i.e. that atheroma is associated with wall damage due to the motion of blood. Instead it immediately suggests that the process is associated with shear dependent mass transport phenomena. It has been demonstrated by others that mass transport, in the inner part of the arterial wall, is dominantly to and from blood flowing within the lumen. We review theory relevant to diffusional mass transport across such a sheared interface, and examine available experimental evidence, relating to normally occurring (quasi-steady state) and experimentally induced (transient-type) atheroma, as well as the distribution of cholesterol in arteries. These results are considered in the light of simple theoretical schemes which we develop for the movement of cholesterol, in particular, although the arguments may also be relevant to other diffusing species. Shear enhances mass transport by means of a steepening effect on the concentration gradient, thus diffusion of material from a wall is promoted when material which has already diffused is swept rapidly away, so that the concentration gradient leading to further diffusion remains steep. However, the influence of shear on the diffusion of a species, say, from just within the wall of an artery to fluid in the main stream, depends upon the relative resistances to its diffusion from within the wall to surface fluid (wall phase) and from surface fluid to fluid in the main stream (blood phase); diffusion is not appreciably shear dependent if the latter resistance is small compared with the former. Assuming simplified flow conditions and that as suggested by others cholesterol is transported in blood in association with plasma protein, we can estimate resistance for diffusion of this species in the blood phase, for different stations in the arterial system. However, we possess no definite comparable information for the wall phase; we conjecture that this resistance is relatively small, and assume shear dependence of diffusional transport of cholesterol between arterial walls and intraluminal blood. We find that a net flux of cholesterol from blood to wall, as has been suggested by others, cannot account, in terms of the proposed schemes, for the observed normally occurring (quasi-steady state) distribution of atheromatous lesions in man and in animals; mass transport is inhibited in low shear regions by the thick diffusional boundary layer. Instead it appears that cholesterol, which has been shown by others to be synthesized in arterial walls, accumulates in low shear regions because its local diffusional efflux from wall to blood is inhibited by the reduced concentration gradient. Given suitable values for relevant parameters, the theoretical schemes are also able to account for adequacy of supply of precursor to the wall for cholesterol synthesis, for the preferential occurrence that we now recognize of lesions in high shear regions in response to sudden natural or experimental elevation of blood cholesterol, and for the responses to administration of labelled cholesterol (transient type phenomena); it appears therefore possible, in terms of these schemes, to unify naturally occurring and experimentally induced atheroma. It is reported by others that platelets are associated only with advanced lesions; the correlation of naturally occurring atheroma with low shear regions, and transient type lesions with high shear regions, with the fluid mechanics being unaltered in the two situations, provides no support for the implication of platelets in the development of early atheroma. It appears that wall shear rate may be a major controlling factor in the development of atheroma, i.e. that high shear, such as is associated for example with increased cardiac output in exercise, will retard progression of the process. Its progression will also be retarded by any means which reduces the accumulation of atheromatous material, by influencing its rate of net production or diffusion.
1,356 citations
TL;DR: New convection models have been developed to predict clinical from platelet thrombosis in diseased arteries, and future hemodynamic studies should address the complex mechanics of flow-induced, large-scale wall motion and convection of semisolid particles and cells in flowing blood.
Abstract: The cardiovascular system is an internal flow loop with multiple branches circulating a complex liquid. The hallmarks of blood flow in arteries are pulsatility and branches, which cause wall stresses to be cyclical and nonuniform. Normal arterial flow is laminar, with secondary flows generated at curves and branches. Arteries can adapt to and modify hemodynamic conditions, and unusual hemodynamic conditions may cause an abnormal biological response. Velocity profile skewing can create pockets in which the wall shear stress is low and oscillates in direction. Atherosclerosis tends to localize to these sites and creates a narrowing of the artery lumen--a stenosis. Plaque rupture or endothelial injury can stimulate thrombosis, which can block blood flow to heart or brain tissues, causing a heart attack or stroke. This small lumen and elevated shear rate in a stenosis create conditions that accelerate platelet accumulation and occlusion. The relationship between thrombosis and fluid mechanics is complex, especially in the post-stenotic flow field. New convection models have been developed to predict clinical from platelet thrombosis in diseased arteries. Future hemodynamic studies should address the complex mechanics of flow-induced, large-scale wall motion and convection of semisolid particles and cells in flowing blood.
546 citations