| 1. |
- Prahl Wittberg, Lisa, et al.
(författare)
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Effects of aortic irregularities on blood flow
- 2016
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Ingår i: Biomechanics and Modeling in Mechanobiology. - : Springer. - 1617-7959 .- 1617-7940. ; 15:2
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Tidskriftsartikel (refereegranskat)abstract
- Anatomic aortic anomalies are seen in many medical conditions and are known to cause disturbances in blood flow. Turner syndrome (TS) is a genetic disorder occurring only in females where cardiovascular anomalies, particularly of the aorta, are frequently encountered. In this study, numerical simulations are applied to investigate the flow characteristics in four TS patient- related aortic arches (a normal geometry, dilatation, coarctation and elongation of the transverse aorta). The Quemada viscosity model was applied to account for the non-Newtonian behavior of blood. The blood is treated as a mixture consisting of water and red blood cells (RBC) where the RBCs are modeled as a convected scalar. The results show clear geometry effects where the flow structures and RBC distribution are significantly different between the aortas. Transitional flow is observed as a jet is formed due to a constriction in the descending aorta for the coarctation case. RBC dilution is found to vary between the aortas, influencing the WSS. Moreover, the local variations in RBC volume fraction may induce large viscosity variations, stressing the importance of accounting for the non-Newtonian effects.
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| 2. |
- Prahl-Wittberg, Lisa, et al.
(författare)
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The Impact of Aortic Arch Geometry on Flow Characteristics
- 2013
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Konferensbidrag (refereegranskat)abstract
- Cardiovascular defects characterized by geometrical anomalies of the aorta and its eecton the blood ow is the focus of this study. Not only are the local ow characteristicsgeometry dependent, but they are also directly connected to the rheological properties ofblood. Flow characteristics such as wall shear stress are often postulated to play a centralrole in the development of vascular disease.In this study, blood is considered to be a non-Newtonian uid and modeled via theQuemada model, an empirical model that is valid for dierent red blood cell loading.Three patient-specic geometries of the aortic arch are investigated numerically. Thethree geometries investigated in this study all display malformations that are prevalent inpatients having the genetic disorder Turner syndrome. The results show a highly complexow with regions of secondary ow that are enhanced in two of the three aortas. Moreover,blood ow is clearly diverted due to the malformations, moving to a larger extent throughthe branches of the arch instead of through the descending aorta. The geometry havingan elongated transverse aorta is found to be subjected to larger areas of highly oscillatorylow wall shear stress.
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| 3. |
- Van Wyk, Stevin, et al.
(författare)
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Atherosclerotic indicators for blood-like fluids in 90-degree arterial-like bifurcations
- 2014
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Ingår i: Computers in Biology and Medicine. - : Elsevier BV. - 0010-4825 .- 1879-0534. ; 50, s. 56-69
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Tidskriftsartikel (refereegranskat)abstract
- The identification of regions prone to atherogenesis in the arterial network is compounded by the complex, slow interaction of mechanical and biomechanical processes. In recent times simplifications to the analysis of the near wall hemodynamics have been sought-after to identify plaque prone regions. Mean parameters have been defined to analyze the common fluid mechanical hypotheses considering the role of wall shear stress (WSS) variations in the pathological changes to the endothelium. In this study well known WSS indicators are applied to varying flow conditions of blood-like fluids in a 90-degree arterial bifurcation. The conventional indicators identify two distinct, focal regions that correlate with a known plaque prone location near arterial bifurcations. The results however demonstrate that the interpretation of the indicators can be difficult under varying flow conditions unless complementary parameters are considered simultaneously. A new indicator is also suggested that extracts the peaks of the temporal WSS gradients (PTWSSGs) and is shown to co-incide well with plaque prone regions. The PTWSSG could be used as a complimentary atherogenic indicator in bifurcating arteries, thereby expanding cardiovascular disease studies to the consideration of alternative fluid mechanical hypotheses. The inclusion of a non-Newtonian model is important in predicting the WSS and temporal WSS gradient distributions near the bifurcation due to the separation bubble induced fluctuations in the shear. Atherogenic indicators could be misleading if non-Newtonian effects are excluded.
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| 4. |
- van Wyk, Stevin, 1980-
(författare)
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Blood Flow variations in Large Arteries due to non-Newtonian rheology
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The blood is a complex fluid that contains, in addition to water, cells, macro-molecules and a large number of smaller molecules. The physical properties of the blood are therefore the result of non-linear interactions of its constituents, which are influenced by the local flow field conditions. Hence, the local blood viscosity is a function of the local concentration of the blood constituents and the local flow field itself. This study considers the flow of blood-like fluids in generalised 90-degree bifurcating pipes and patient-specific arterial bifurcations relevant to the large aortic branches in humans. It is shown that the Red Blood Cell (RBC) distribution in the region of bifurcations may lead to large changes in the viscosity, with implications on the concentrations of the various cells in the blood plasma. This in turn implies that the flow in the near wall regions is more difficult to estimate and predict than that under the assumption of a homogeneous fluid. The rheological properties of blood are complex and are difficult to measure, since the results depend on the measuring equipment and the inherent flow conditions. We attempt to model the viscosity of water containing different volume fractions of non-deforming RBC-like particles in tubes. The apparent viscosities of the mixtures obtained from these model experiments have been compared to the predictions of the different rheological models found in the literature. The same rheological models have also been used in the different simulations, where the local RBC concentration and local shear rate are used in the viscosity models. The flow simulations account for the non-linearity due to coupling between the flow and fluid rheology. Furthermore, from a physiological perspective, it is shown that oscillatory wall shear stresses are affected by changes in RBC concentration in the regions of the bifurcation associated with atherogenesis. The intrinsic shear thinning rheological property of the blood, in conjunction with stagnation in separated flows, may be responsible for elevated temporal wall shear stress gradients (TWSSG) influencing endothelial cell behaviour, which has been postulated to play a role in the development of atherosclerosis. The blood-like fluid properties along with variations in the RBC concentration could also lead to variations in the developing flow structures in the larger arteries that could influence the work the heart has to bear.
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| 5. |
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| 6. |
- van Wyk, Stevin, et al.
(författare)
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Haemodynamics in a 3D 90-degree bifurcation
- 2011
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Ingår i: Proceedings of the ECCOMAS Thematic International Conference on Simulation and Modeling of Biological Flows, Brussels, Belgium, September 21-23, 2011.
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Konferensbidrag (refereegranskat)abstract
- The transport behaviour of the haematocrit in the larger arteries is important in defining the variations in viscosityof blood. In this study, a finite volume method is used in order to simulate the blood flow and haematocrit transportthrough a large 3D human-like 90-degree bifurcation. The simulations are carried out to investigate the importance ofexplicitly modelling the non-Newtonian viscosity of blood regarding defining the flow. It is expected to be especiallyimportant in the regions surrounding a bifurcation. The main focus is to compare non-Newtonian to Newtonianbehaviour of the flow through important parameters such as pressure losses, mean viscosity variations and bulktransport properties of haematocrit. The study considers a broad range of physiological and pulsatile flow conditions,and displays the importance of modelling blood flow as a non-Newtonian fluid. The results have a relevant impactregarding the possible discrepencies in important physiological parameters such as wall shear stress (WSS), whencoupling the haematocrit field data back to the viscosity models.
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| 7. |
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| 8. |
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| 9. |
- van Wyk, Stevin, et al.
(författare)
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Non-Newtonian perspectives on pulsatile blood-analog flows in a 180 degrees curved artery model
- 2015
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 27:7
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Tidskriftsartikel (refereegranskat)abstract
- Complex, unsteady fluid flow phenomena in the arteries arise due to the pulsations of the heart that intermittently pumps the blood to the extremities of the body. The many different flow waveform variations observed throughout the arterial network are a result of this process and a function of the vessel properties. Large scale secondary flow structures are generated throughout the aortic arch and larger branches of the arteries. An experimental 180. curved artery test section with physiological inflow conditions was used to validate the computational methods implemented in this study. Good agreement of the secondary flow structures is obtained between experimental and numerical studies of a Newtonian blood-analog fluid under steady-state and pulsatile, carotid artery flow rate waveforms. Multiple vortical structures, some of opposite rotational sense to Dean vortices, similar to Lyne-type vortices, were observed to form during the systolic portion of the pulse. Computational tools were used to assess the effect of blood-analog fluid rheology ( i.e., Newtonian versus non-Newtonian). It is demonstrated that non-Newtonian, blood-analog fluid rheology results in shear layer instabilities that alter the formation of vortical structures during the systolic deceleration and onwards during diastole. Additional vortices not observed in the Newtonian cases appear at the inside and outside of the bend at various times during the pulsation. The influence of blood-analog shear-thinning viscosity decreases mean pressure losses in contrast to the Newtonian blood analog fluid.
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| 10. |
- Van Wyk, Stevin, et al.
(författare)
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Rheology of red blood cell flow in large geometries
- 2013
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Konferensbidrag (refereegranskat)abstract
- When studying disease development in arteries, it is important to understand the local variations in blood rheology. Blood flow in large arteries is often assumed to behave as a homogeneous fluid, an assumption that is not entirely correct. The local viscosity changes with the local concentration of Red Blood Cells (RBCs) and the rate of shear strongly influences the Wall Shear Stress (WSS) and its gradients, physiological parameters important in the study of atherosclerosis. Moreover, the flow behavior of RBCs is influenced by the geometric structure of the flow environment. In experiment, rheological properties across a tube cross-section are difficult to measure if non-invasive techniques are to be used. Therefore, rheometric devices are constructed of simple geometries to measure the bulk rheology. In this study, the Lattice Boltzmann Method is used to model the blood as a particle suspension of RBCs. The RBC Volume Fractions (VF) investigated corresponds to 1, 2 and 5%, and both a channel and a tube flow are considered. The results display large differences in RBC distributions and velocity profiles. Estimated from existing viscosity models, the viscosity distributions are found to display variations of up to 5% when comparing the two geometries. This is of importance since errors in quantifying the viscosity can lead to miscalculations of the physiological variables.
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