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Adel M. Malek - One of the best experts on this subject based on the ideXlab platform.
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induction of aneurysmogenic high positive wall Shear Stress Gradient by wide angle at cerebral bifurcations independent of flow rate
Journal of Neurosurgery, 2019Co-Authors: Adel M. Malek, Alexandra Lauric, James HippelheuserAbstract:OBJECTIVEEndothelium adapts to wall Shear Stress (WSS) and is functionally sensitive to positive (aneurysmogenic) and negative (protective) spatial WSS Gradients (WSSG) in regions of accelerating and decelerating flow, respectively. Positive WSSG causes endothelial migration, apoptosis, and aneurysmal extracellular remodeling. Given the association of wide branching angles with aneurysm presence, the authors evaluated the effect of bifurcation geometry on local apical hemodynamics.METHODSComputational fluid dynamics simulations were performed on parametric bifurcation models with increasing angles having: 1) symmetrical geometry (bifurcation angle 60°–180°), 2) asymmetrical geometry (daughter angles 30°/60° and 30°/90°), and 3) curved parent vessel (bifurcation angles 60°–120°), all at baseline and double flow rate. Time-dependent and time-averaged apical WSS and WSSG were analyzed. Results were validated on patient-derived models.RESULTSNarrow symmetrical bifurcations are characterized by protective nega...
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Wall Shear Stress Gradient analysis within an idealized stenosis using non-Newtonian flow.
Neurosurgery, 2007Co-Authors: Clemens M. Schirmer, Adel M. MalekAbstract:OBJECTIVE: The endothelium is functionally regulated by the magnitude and spatiotemporal Gradients of wall Shear Stress (WSS). Although flow separation and reversal occur beyond high-grade stenoses, little is known of the WSS pattern within clinically relevant mild to moderate stenoses. METHODS: An axisymmetric geometry with 25, 50, and 75% stenosis criteria (quantified in accordance with the North American Symptomatic Carotid Endarterectomy Trial) was used to generate a high-resolution, hybrid, tetrahedral-hexahedral computational mesh with boundary-layer enrichment to improve near-wall Shear Stress Gradient (WSSG) computation. Time-dependent computational fluid dynamic analysis was performed using a non-Newtonian Carreau-Yasuda model of blood to yield the Shear-dependent viscosity. RESULTS: Transition to secondary flow patterns was demonstrated in stenoses of 25, 50, and 75%. A focal region with near-wall flow reversal and retrograde WSS was identified within the stenosis itself and was found to migrate cyclically during the cardiac pulse. A zone of zero WSS and divergent WSSG that shifts in toward the throat with increasing stenotic severity was identified. Focal zones of high WSSG with converging and/or diverging direction were uncovered within the stenosis itself, as were expected changes in the distal poststenotic region. These zones of divergent WSSG shift over a substantial length of the stenosis during the course of the cardiac cycle. CONCLUSION: Luminal WSS demonstrates dynamic direction reversal and high spatial Gradients within the distal stenosis throat of even clinically moderate lesions. These findings shed light on the complex vessel wall hemodynamics within clinical stenoses and reveal a mechanical microenvironment that is conducive to perpetual endothelial functional dysregulation and stenosis progression.
P.b. Kosasih - One of the best experts on this subject based on the ideXlab platform.
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A Transition-Turbulent Lubrication Theory Using Mixing Length Concept
Journal of Tribology, 1993Co-Authors: P.b. Kosasih, A. K. TieuAbstract:This paper applies the recently introduced Reynolds Stress expression (Tieu and Kosasih, 1992) in the transition-turbulent lubrication analysis. The Reynolds Stress is modeled using the mixing length expression which is able to account for the effect of local Shear Stress Gradient, and it can be extended to apply in the transilion regime. This theory is then used to determine three-dimensional velocity distributions between parallel plates. From the results, a set of coefficients covering transition-turbulent regime used in conjunction with the modified Reynolds equation is presented
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An Expression of Reynolds Stresses in Turbulent Lubrication Theory
Journal of Tribology, 1992Co-Authors: A. K. Tieu, P.b. KosasihAbstract:This paper proposes an alternative model of Reynolds Stresses for turbulent lubrication theory. The approach relies on Prandtl’s mixing length theory which is based on a modified Van Driest mixing formula [1]. However, unlike the previous theories [2, 3] the proposed equation is capable of accounting for the effect of Shear Stress Gradient on the mixing length. Thus it is well suited to turbulent flow analysis in bearings where the presence of Shear Stress Gradient due to the effect of pressure Gradient should be considered. A series of velocity measurements in thin channels in the low Reynolds number turbulent flow range are analysed using the theory. The data analysis shows a strong effect of Shear Stress Gradient on the viscous sublayer in the low Reynolds number regime. As a result, a new model of mixing length applicable to the turbulent lubrication analysis in thin film at low or high Reynolds numbers or under low or high Shear Stress Gradient is presented.
Edward G. Cape - One of the best experts on this subject based on the ideXlab platform.
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pulsatile wall Shear Stress Gradient and aortoseptal angle implications for subaortic stenosis 147
Pediatric Research, 1997Co-Authors: Michael D. Vanauker, Theresa A. Tacy, Gunnlaugur Sigfússon, Pedro J. Del Nido, Edward G. CapeAbstract:A better understanding of the etiology of discrete subaortic stenosis (SAS) would be useful in identifying patients at risk and in surgical decision making since the onset, progression, and recurrence of SAS are difficult to predict. Previous studies have shown that congenital defects and morphologic abnormalities associated with SAS (such as steepened aortoseptal angle [AoSA]) cause significant, localized elevations in peak wall Shear Stress and the wall Shear Stress Gradient (WSSG). High WSSG has been associated with cellular changes in the endothelium. Since the magnitude of wall Shear Stress varies thoughout the cardiac cycle, we addressed the hypothesis that these morphologic abnormalities have similar effects on the time-averaged Shear Stress and WSSG. Methods: A finite element model of the left ventricular outflow tract was implemented on the Cray C90 at the Pittsburgh Supercomputing Center for typical pulsatile flow conditions. AoSA was varied between 120 and 150 degrees, maximum aortic velocities from 0.5 to 1 m/s, and aortic diameter from 1 to 1.5 cm. Shear Stress and WSSG were directly calculated from the velocity field at discrete points in time. The time-averaged wall Shear Stress and WSSG at the location of the peak was determined. Results: At an aortic velocity of 1 m/s, the peak and time-averaged Shear Stress increased with steeper AoSA by 52% and 49.5% respectively. WSSG was more sensitive to changes in AoSA with the peak WSSG increasing by 217%, compared to the time-averaged WSSG which increased by 225%. Conclusions: Geometric variables had similar effects on both the peak and mean Shear Stress and WSSG. Although the progression and recurrence of SAS may be dependent on time-varying exposure to Shear Stress, this exposure may be quantified using either a peak or a mean value.
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Pulsatile Wall Shear Stress Gradient and Aortoseptal Angle: Implications for Subaortic Stenosis † 147
Pediatric Research, 1997Co-Authors: Michael D. Vanauker, Theresa A. Tacy, Gunnlaugur Sigfússon, Pedro J. Del Nido, Edward G. CapeAbstract:A better understanding of the etiology of discrete subaortic stenosis (SAS) would be useful in identifying patients at risk and in surgical decision making since the onset, progression, and recurrence of SAS are difficult to predict. Previous studies have shown that congenital defects and morphologic abnormalities associated with SAS (such as steepened aortoseptal angle [AoSA]) cause significant, localized elevations in peak wall Shear Stress and the wall Shear Stress Gradient (WSSG). High WSSG has been associated with cellular changes in the endothelium. Since the magnitude of wall Shear Stress varies thoughout the cardiac cycle, we addressed the hypothesis that these morphologic abnormalities have similar effects on the time-averaged Shear Stress and WSSG. Methods: A finite element model of the left ventricular outflow tract was implemented on the Cray C90 at the Pittsburgh Supercomputing Center for typical pulsatile flow conditions. AoSA was varied between 120 and 150 degrees, maximum aortic velocities from 0.5 to 1 m/s, and aortic diameter from 1 to 1.5 cm. Shear Stress and WSSG were directly calculated from the velocity field at discrete points in time. The time-averaged wall Shear Stress and WSSG at the location of the peak was determined. Results: At an aortic velocity of 1 m/s, the peak and time-averaged Shear Stress increased with steeper AoSA by 52% and 49.5% respectively. WSSG was more sensitive to changes in AoSA with the peak WSSG increasing by 217%, compared to the time-averaged WSSG which increased by 225%. Conclusions: Geometric variables had similar effects on both the peak and mean Shear Stress and WSSG. Although the progression and recurrence of SAS may be dependent on time-varying exposure to Shear Stress, this exposure may be quantified using either a peak or a mean value.
A. K. Tieu - One of the best experts on this subject based on the ideXlab platform.
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A Transition-Turbulent Lubrication Theory Using Mixing Length Concept
Journal of Tribology, 1993Co-Authors: P.b. Kosasih, A. K. TieuAbstract:This paper applies the recently introduced Reynolds Stress expression (Tieu and Kosasih, 1992) in the transition-turbulent lubrication analysis. The Reynolds Stress is modeled using the mixing length expression which is able to account for the effect of local Shear Stress Gradient, and it can be extended to apply in the transilion regime. This theory is then used to determine three-dimensional velocity distributions between parallel plates. From the results, a set of coefficients covering transition-turbulent regime used in conjunction with the modified Reynolds equation is presented
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An Expression of Reynolds Stresses in Turbulent Lubrication Theory
Journal of Tribology, 1992Co-Authors: A. K. Tieu, P.b. KosasihAbstract:This paper proposes an alternative model of Reynolds Stresses for turbulent lubrication theory. The approach relies on Prandtl’s mixing length theory which is based on a modified Van Driest mixing formula [1]. However, unlike the previous theories [2, 3] the proposed equation is capable of accounting for the effect of Shear Stress Gradient on the mixing length. Thus it is well suited to turbulent flow analysis in bearings where the presence of Shear Stress Gradient due to the effect of pressure Gradient should be considered. A series of velocity measurements in thin channels in the low Reynolds number turbulent flow range are analysed using the theory. The data analysis shows a strong effect of Shear Stress Gradient on the viscous sublayer in the low Reynolds number regime. As a result, a new model of mixing length applicable to the turbulent lubrication analysis in thin film at low or high Reynolds numbers or under low or high Shear Stress Gradient is presented.
Tomoaki Terada - One of the best experts on this subject based on the ideXlab platform.
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proximal stenosis may induce initiation of cerebral aneurysms by increasing wall Shear Stress and wall Shear Stress Gradient
International Journal for Numerical Methods in Biomedical Engineering, 2014Co-Authors: Kenichi Kono, Takeshi Fujimoto, Tomoaki TeradaAbstract:Hemodynamic parameters, such as wall Shear Stress (WSS), WSS Gradient (WSSG), aneurysm formation indicator (AFI), or Gradient oscillatory number (GON), have been proposed to be linked to initiation of cerebral aneurysms. However, how such conditions occur in humans is unclear. We encountered a rare and interesting case to address this issue. A patient had a newly formed aneurysm with proximal stenosis, which was confirmed by serial imagings. We made two pre-aneurysm models: one with stenosis and the other without stenosis. We performed computational fluid dynamics simulations for these models. Owing to jet flow caused by the stenosis, the maximum WSS and WSSG on the aneurysm initiation site were approximately doubled and tripled, respectively. However, the oscillatory Shear index (OSI), AFI, and GON did not change substantially by the stenosis. Computer simulations using artificial vascular models with different degrees of proximal stenosis at different distances demonstrated that oscillatory Shear index, AFI, and GON did not change substantially by the stenosis. These results showed that proximal stenosis caused high WSS and high WSSG at the aneurysm initiation site, possibly leading to aneurysm initiation. Proximal stenosis may be a potential factor to induce initiation of one class of cerebral aneurysms by increasing WSS and WSSG.
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Proximal stenosis may induce initiation of cerebral aneurysms by increasing wall Shear Stress and wall Shear Stress Gradient.
International journal for numerical methods in biomedical engineering, 2014Co-Authors: Kenichi Kono, Takeshi Fujimoto, Tomoaki TeradaAbstract:SUMMARY Hemodynamic parameters, such as wall Shear Stress (WSS), WSS Gradient (WSSG), aneurysm formation indicator (AFI), or Gradient oscillatory number (GON), have been proposed to be linked to initiation of cerebral aneurysms. However, how such conditions occur in humans is unclear. We encountered a rare and interesting case to address this issue. A patient had a newly formed aneurysm with proximal stenosis, which was confirmed by serial imagings. We made two pre-aneurysm models: one with stenosis and the other without stenosis. We performed computational fluid dynamics simulations for these models. Owing to jet flow caused by the stenosis, the maximum WSS and WSSG on the aneurysm initiation site were approximately doubled and tripled, respectively. However, the oscillatory Shear index (OSI), AFI, and GON did not change substantially by the stenosis. Computer simulations using artificial vascular models with different degrees of proximal stenosis at different distances demonstrated that oscillatory Shear index, AFI, and GON did not change substantially by the stenosis. These results showed that proximal stenosis caused high WSS and high WSSG at the aneurysm initiation site, possibly leading to aneurysm initiation. Proximal stenosis may be a potential factor to induce initiation of one class of cerebral aneurysms by increasing WSS and WSSG. Copyright © 2014 John Wiley & Sons, Ltd.