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- Gårdhagen, Roland, 1978-, et al.
(författare)
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Assessment of Geometrical Influence on WSS Estimation in the Human Aorta
- 2006
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Ingår i: WSEAS Transactions on Fluid Mechanics. - 1790-5087. ; 4:1, s. 318-326
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Tidskriftsartikel (refereegranskat)abstract
- Computational fluid dynamics simulations were performed on a stenosed human aorta with poststenotic dilatation, in order to estimate wall shear stress (WSS). WSS is important due to its correlation with atherosclerosis. Both steady-state and non-stationary simulations were conducted. Three different models were created from a set of MRI images. Comparison of geometrically different models was accomplished by using geometrical landmarks and a comparison parameter. Geometrical differences had larger influence on WSS magnitude than inflow rotation in steady-state results for the models used. In non-stationary flow the largest differences in WSS are found when the flow velocity near the wall is low e.g. when the inflow is low or in recirculation regions.
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| 4. |
- Gårdhagen, Roland, 1978-, et al.
(författare)
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Subject Specific Wall Shear Stress in the Human Thoracic Aorta
- 2006
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Ingår i: WSEAS Transaction on biology and biomedicine. - 1109-9518. ; 10:3, s. 609-614
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Tidskriftsartikel (refereegranskat)abstract
- Numerous studies have shown a correlation between Wall Shear Stress (WSS) and atherosclerosis, but few have evaluated the reliability of estimation methods and measures used to assessWSS, which is the subject of this work. A subject specific vessel model of the aortic arch and thoracic aorta is created fromMRI images and used for CFD simulations with MRI velocity measurements as inlet boundary condition. WSS is computed from the simulation results. Aortic WSS shows significant spatial as well as temporal variation during a cardiac cycle, which makes circumferential values very uninformative, and approximate estimates using Hagen-Poiseuille fails predict the averageWSS. Highly asymmetric flow, especially in the arch, causes the spatial WSS variations.
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| 7. |
- Renner, Johan, 1977-, et al.
(författare)
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Conduction and convection heat transfer for aluminum ingot in preheating furnace
- 2011
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Ingår i: Proceedings of 6th Baltic Heat Transfer Conference 2011. - 9789521526398 - 9789521526381 - 9789521526404
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Konferensbidrag (refereegranskat)abstract
- Aluminium is a widely used material, which is found in a number of products e.g. thin aluminium bands that is the base material in many heat exchangers. Rolling processes are used to produce these thin aluminium bands, in order to get the right properties and to get the aluminium easier to roll, heat treatment is needed. This heat treatment of aluminium ingots prior to the rolling is in focus in this work, where computational fluid dynamics and computational heat transfer techniques is used to predict the heating process in a hot air pre-heating furnace. The used approach includes steady state computational fluid dynamics simulations combined with transient computational heat transfer simulations. The simulation results in form of spatial and temporal distributed aluminium ingot temperature was compared with temperature measurement in a thermocouple prepared ingot in the actual pre-heating furnace. Simulation results correspond well with the measurements and there are small differences. Results of the described simulation approach open the possibility to predict spatial and temporal temperature distribution in these kinds of pre-heating processes.
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| 8. |
- Renner, Johan, 1977-
(författare)
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Estimating patient specific wall shear stress in the human aorta : geometrical and post-processing considerations
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis describes a workflow to perform in-vivo wall shear stress (WSS) estimations in the human aorta using computational fluid dynamics (CFD) methods. An abnormal WSS distribution is believed to influence the development of many cardiovascular diseases, e.g. atherosclerosis. The focus in this thesis is on geometrical influence on the WSS results and interpretation methods tor non-stationary results. The work shows that results are sensitive to the choice of segmentation method (the process from medical images to a geometrical model) and a correct geometrical description of the artery is crucial in making WSS estimations. A new parameter for non-stationary WSS results has been proposed; Wall Shear Stress Angular Amplitude (WSSAA), making the analysis of non-stationary results more straight-forward. It has been shown that the workfiow can be used with confidence and that WSS can be estimated in-vivo. using the combination of MRI-based geometry definition and CFD.
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| 9. |
- Renner, Johan, 1977-, et al.
(författare)
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Post-Processing Dynamic Behavior of WSS in Aortic Blood Flow
- 2006
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Pulsating flow simulations with CFD is performed on a stenosed human aorta with post-stenotic dilatation, for development of wall shear stress (WSS) dynamic parameters. WSS is of interest due to its correlation with atherosclerosis. The dynamic behavior and dynamic capturing parameters of WSS are usable in analyzing non-stationary results from blood flow simulations. The amount of wall back-flow is shown to be an very easy parameter to interpret and it showed an "washout" effect in the post-stenotic dilatation. A new dynamic capturing parameter describing the WSS angular amplitude (WSSAA) is presented. It has both differences and similarities with the widely used oscillating shear index (OSI) parameter. WSSA have a more direct physical interpretation then OSI.
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| 10. |
- Renner, Johan, 1977-
(författare)
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Towards Subject Specific Aortic Wall Shear Stress : a combined CFD and MRI approach
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The cardiovascular system is an important part of the human body since it transports both energy and oxygen to all cells throughout the body. Diseases in this system are often dangerous and cardiovascular diseases are the number one killer in the western world. Common cardiovascular diseases are heart attack and stroke, which origins from obstructed blood flow. It is generally important to understand the causes for these cardiovascular diseases. The main causes for these diseases are atherosclerosis development in the arteries (hardening and abnormal growth). This transform of the arterial wall is believed to be influenced by the mechanical load from the flowing blood on the artery and especially the tangential force the wall shear stress. To retrieve wall shear stress information in arteries invivo is highly interesting due to the coupling to atherosclerosis and indeed a challenge. The goal of this thesis is to develop, describe and evaluate an in-vivo method for subject specific wall shear stress estimations in the human aorta, the largest artery in the human body. The method uses an image based computational fluid dynamics approach in order to estimate the wall shear stress. To retrieve in-vivo geometrical descriptions of the aorta magnetic resonance imaging capabilities is used which creates image material describing the subject specific geometry of the aorta. Magnetic resonance imaging is also used to retrieve subject specific blood velocity information in the aorta. Both aortic geometry and velocity is gained at the same time. Thereafter the image material is interpreted using level-set segmentation in order to get a three-dimensional description of the aorta. Computational fluid dynamics simulations is applied on the subject specific aorta in order to calculate time resolved wall shear stress distribution at the entire aortic wall included in the actual model.This work shows that it is possible to estimate subject specific wall shear stress in the human aorta. The results from a group of healthy volunteers revealed that the arterial geometry is very subject specific and the different wall shear stress distributions have general similarities but the level and local distribution are clearly different. Sensitivity (on wall shear stress) to image modality, the different segmentation methods and different inlet velocity profiles have been tested, which resulted in these general conclusions:The aortic diameter from magnetic resonance imaging became similar to the reference diameter measurement method.The fast semi-automatic level-set segmentation method gave similar geometry and wall shear stress results when compared to a reference segmentation method.Wall shear stress distribution became different when comparing a simplified uniform velocity profile inlet boundary condition with a measured velocity profile.The method proposed in this thesis has the possibility to produce subject specific wall shear stress distribution in the human aorta. The method can be used for further medical research regarding atherosclerosis development and has the possibility for usage in clinical work.
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