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On Aortic Blood Flow Simulations: Scale-Resolved Image-Based CFD
Linköping University, Department of Management and Engineering, Applied Thermodynamics and Fluid Mechanics. Linköping University, The Institute of Technology.ORCID iD: 0000-0003-1942-7699
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on modeling and simulation of the blood flow in the aorta, the largest artery in the human body. It is an accepted fact that abnormal biological and mechanical interactions between the blood flow and the vessel wall are involved in the genesis and progression of cardiovascular diseases. The transport of low-density lipoprotein into the wall has been linked to the initiation of atherosclerosis. The mechanical forces acting on the wall can impede the endothelial cell layer function, which normally acts as a barrier to harmful substances. The wall shear stress (WSS) affects endothelial cell function, and is a direct consequence of the flow field; steady laminar flows are generally considered atheroprotective, while the unsteady turbulent flow could contribute to atherogenesis. Quantification of regions with abnormal wall shear stress is therefore vital in order to understand the initiation and progression of atherosclerosis.However, flow forces such as WSS cannot today be measured with significant accuracy using present clinical measurement techniques. Instead, researches rely on image-based computational modeling and simulation. With the aid of advanced mathematical models it is possible to simulate the blood flow, vessel dynamics, and even biochemical reactions, enabling information and insights that are currently unavailable through other techniques. During the cardiac cycle, the normally laminar aortic blood flow can become unstable and undergo transition to turbulence, at least in pathological cases such as coarctation of the aorta where the vessel is locally narrowed. The coarctation results in the formation of a jet with a high velocity, which will create the transition to turbulent flow. The high velocity will also increase the forces on the vessel wall. Turbulence is generally very difficult to model, requiring advanced mathematical models in order to resolve the flow features. As the flow is highly dependent on geometry, patient-specific representations of the in vivo arterial walls are needed, in order to perform an accurate and reliable simulation. Scale-resolving flow simulations were used to compute the WSS on the aortic wall and resolve the turbulent scales in the complex flow field. In addition to WSS, the turbulent flow before and after surgical intervention in an aortic coarctation was assessed. Numerical results were compared to state-of-the-art magnetic resonance imaging measurements. The results agreed very well, suggesting that that the measurement technique is reliable and could be used as a complement to standard clinical procedures when evaluating the outcome of an intervention.The work described in the thesis deals with patient-specific flows, and is, when possible, validated with experimental measurements. The results provide new insights to turbulent aortic flows, and show that image-based computational modeling and simulation are now ready for clinical practice.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013. , 66 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1493
National Category
Applied Mechanics
Identifiers
URN: urn:nbn:se:liu:diva-85682ISBN: 978-91-7519-720-3 (print)OAI: oai:DiVA.org:liu-85682DiVA: diva2:572525
Public defence
2013-01-07, Nobel (BL32), B-huset, Campus Valla, Linköpings Universitet, Linköping, 09:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, VR 2007-4085Swedish Research Council, VR 2010-4282
Available from: 2012-11-28 Created: 2012-11-28 Last updated: 2016-03-14Bibliographically approved
List of papers
1. Quantifying Turbulent Wall Shear Stress in a Stenosed Pipe Using Large Eddy Simulation
Open this publication in new window or tab >>Quantifying Turbulent Wall Shear Stress in a Stenosed Pipe Using Large Eddy Simulation
2010 (English)In: Journal of Biomechanical Engineering, ISSN 0148-0731, E-ISSN 1528-8951, Vol. 132, no 6Article in journal (Refereed) Published
Abstract [en]

Large eddy simulation was applied for flow of Re = 2000 in a stenosed pipe in order to undertake a thorough investigation of the wall shear stress (WSS) in turbulent flow. A decomposition of the WSS into time averaged and fluctuating components is proposed. It was concluded that a scale resolving technique is required to completely describe the WSS pattern in a subject specific vessel model, since the poststenotic region was dominated by large axial and circumferential fluctuations. Three poststenotic regions of different WSS characteristics were identified. The recirculation zone was subject to a time averaged WSS in the retrograde direction and large fluctuations. After reattachment there was an ante grade shear and smaller fluctuations than in the recirculation zone. At the reattachment the fluctuations were the largest, but no direction dominated over time. Due to symmetry the circumferential time average was always zero. Thus, in a blood vessel, the axial fluctuations would affect endothelial cells in a stretched state, whereas the circumferential fluctuations would act in a relaxed direction.

Place, publisher, year, edition, pages
American Society Mechanical Engineers, 2010
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-58347 (URN)10.1115/1.4001075 (DOI)000278965500002 ()
Available from: 2010-08-13 Created: 2010-08-11 Last updated: 2017-12-12
2. Quantifying turbulent wall shear stress in a subject specific human aorta using large eddy simulation
Open this publication in new window or tab >>Quantifying turbulent wall shear stress in a subject specific human aorta using large eddy simulation
2012 (English)In: Medical Engineering and Physics, ISSN 1350-4533, E-ISSN 1873-4030, Vol. 34, no 8, 1139-1148 p.Article in journal (Refereed) Published
Abstract [en]

In this study, large-eddy simulation (LES) is employed to calculate the disturbed flow field and the wall shear stress (WSS) in a subject specific human aorta. Velocity and geometry measurements using magnetic resonance imaging (MRI) are taken as input to the model to provide accurate boundary conditions and to assure the physiological relevance. In total, 50 consecutive cardiac cycles were simulated from which a phase average was computed to get a statistically reliable result. A decomposition similar to Reynolds decomposition is introduced, where the WSS signal is divided into a pulsating part (due to the mass flow rate) and a fluctuating part (originating from the disturbed flow). Oscillatory shear index (OSI) is plotted against time-averaged WSS in a novel way, and locations on the aortic wall where elevated values existed could easily be found. In general, high and oscillating WSS values were found in the vicinity of the branches in the aortic arch, while low and oscillating WSS were present in the inner curvature of the descending aorta. The decomposition of WSS into a pulsating and a fluctuating part increases the understanding of how WSS affects the aortic wall, which enables both qualitative and quantitative comparisons.

Place, publisher, year, edition, pages
Elsevier, 2012
Keyword
Human aorta, Atherosclerosis, Wall shear stress, Computational fluid dynamics, Scale resolving turbulence model, Reynolds decomposition
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-84887 (URN)10.1016/j.medengphy.2011.12.002 (DOI)000309028800016 ()
Note

Funding Agencies|Swedish research council|VR 2007-4085VR 2010-4282|National Supercomputer Centre (NSC)|SNIC022/09-11|

Available from: 2012-11-01 Created: 2012-10-26 Last updated: 2017-12-07
3. Wall shear stress in a subject specific human aorta - Influence of fluid-structure interaction
Open this publication in new window or tab >>Wall shear stress in a subject specific human aorta - Influence of fluid-structure interaction
2011 (English)In: International Journal of Applied Mechanics, ISSN 1758-8251, Vol. 3, no 4, 759-778 p.Article in journal (Refereed) Published
Abstract [en]

Vascular wall shear stress (WSS) has been correlated to the development of atherosclerosis in arteries. As WSS depends on the blood flow dynamics, it is sensitive to pulsatile effects and local changes in geometry. The aim of this study is therefore to investigate if the effect of wall motion changes the WSS or if a rigid wall assumption is sufficient. Magnetic resonance imaging (MRI) was used to acquire subject specific geometry and flow rates in a human aorta, which were used as inputs in numerical models. Both rigid wall models and fluid-structure interaction (FSI) models were considered, and used to calculate the WSS on the aortic wall. A physiological range of different wall stiffnesses in the FSI simulations was used in order to investigate its effect on the flow dynamics. MRI measurements of velocity in the descending aorta were used as validation of the numerical models, and good agreement was achieved. It was found that the influence of wall motion was low on time-averaged WSS and oscillating shear index, but when regarding instantaneous WSS values the e.ect from the wall motion was clearly visible. Therefore, if instantaneous WSS is to be investigated, a FSI simulation should be considered.

Place, publisher, year, edition, pages
World Scientific Publishing, 2011
Keyword
computational fluid dynamics; wall deformation; windkessel model; pressure wave; magnetic resonance imaging
National Category
Applied Mechanics
Identifiers
urn:nbn:se:liu:diva-71720 (URN)10.1142/S1758825111001226 (DOI)000299096300006 ()
Note
funding agencies|Swedish research council| VR 2007-4085 VR 2010-4282 |National Supercomputer Center (NSC)| SNIC022/09-11 |CMIV||Available from: 2011-11-02 Created: 2011-11-02 Last updated: 2016-03-14
4. Large eddy simulation of LDL surface concentration in a subject specific human aorta
Open this publication in new window or tab >>Large eddy simulation of LDL surface concentration in a subject specific human aorta
2012 (English)In: Journal of Biomechanics, ISSN 0021-9290, E-ISSN 1873-2380, Vol. 45, no 3, 537-542 p.Article in journal (Refereed) Published
Abstract [en]

The development of atherosclerosis is correlated to the accumulation of lipids in the arterial wall, which, in turn, may be caused by the build-up of low-density lipoproteins (LDL) on the arterial surface. The goal of this study was to model blood flow within a subject specific human aorta, and to study how the LDL surface concentration changed during a cardiac cycle. With measured velocity profiles as boundary conditions, a scale-resolving technique (large eddy simulation, LES) was used to compute the pulsatile blood flow that was in the transitional regime. The relationship between wall shear stress (WSS) and LDL surface concentration was investigated, and it was found that the accumulation of LDL correlated well with WSS. In general, regions of low WSS corresponded to regions of increased LDL concentration and vice versa. The instantaneous LDL values changed significantly during a cardiac cycle; during systole the surface concentration was low due to increased convective fluid transport, while in diastole there was an increased accumulation of LDL on the surface. Therefore, the near-wall velocity was investigated at four representative locations, and it was concluded that in regions with disturbed flow the LDL concentration had significant temporal changes, indicating that LDL accumulation is sensitive to not only the WSS but also near-wall flow.

Place, publisher, year, edition, pages
Elsevier, 2012
Keyword
Low-density lipoprotein, Wall shear stress, Disturbed flow, Atherosclerosis
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:liu:diva-72895 (URN)10.1016/j.jbiomech.2011.11.039 (DOI)000300863600019 ()
Note
funding agencies|Swedish Research Council| VR 2007-4085 VR 2010-4282 |National Supercomputer Centre (NSC)| SNIC022/09-11 |CMIV||Available from: 2011-12-09 Created: 2011-12-09 Last updated: 2017-12-08

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