The Experts below are selected from a list of 11820 Experts worldwide ranked by ideXlab platform
Clement Kleinstreuer - One of the best experts on this subject based on the ideXlab platform.
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Computational design of a bypass graft that minimizes wall shear stress gradients in the region of the distal anastomosis
Journal of vascular surgery, 1997Co-Authors: Ming Lei, Joseph P Archie, Clement KleinstreuerAbstract:Purpose: Recent experimental and theoretic studies show that large wall shear stress gradients characterize disturbed flow patterns associated with the location of myointimal hyperplasia, atheroma, or both. Graft-to-artery anastomoses that minimize wall shear stress gradients may reduce the degree of myointimal development and the propensity for thrombosis. This study analyzes the distribution of distal anastomotic wall shear stress gradients for conventional geometries and for the optimized geometry assuming idealized merging of the graft with the artery. Methods: A validated computational fluid dynamics program was used to solve the transient three-dimensional partial differential equations and auxiliary equations that describe laminar incompressible blood flow. Time-averaged wall shear stresses and wall shear stress gradients were calculated for three distal graft-artery anastomoses: a standard end-to-side, a Taylor patch, and an optimized geometry. The latter was obtained iteratively by minimizing the local wall shear stress gradients and was analyzed under resting and exercise inflow Waveforms. Results: Both the standard and Taylor patch anastomoses have relatively high wall shear stress gradients in the regions of the toe and heel. For all flow Inputs studied nonuniform hemodynamics in the optimized graft design are largely eliminated, and the time-averaged wall shear stress gradients are greatly reduced throughout the anastomotic zone. At resting flow the Taylor patch produces slightly lower wall shear stress gradients in the anastomotic region than the standard end-to-side anastomosis. The optimized design reduces wall shear stress gradients to almost one half of that of the standard and Taylor patch geometries. At exercise flow wall shear stress gradients almost triple in the standard anastomosis and increase approximately 30% in the Taylor patch. In contrast, the geometrically optimized design is basically independent of the type of flow Input Waveform in terms of time-averaged wall shear stress gradients and disturbed flow patterns. Conclusion: This study demonstrates that it is possible to design a terminal graft geometry for an end-to-side anastomosis that significantly reduces wall shear stress gradients. If the wall shear stress gradient is confirmed to be a major hemodynamic determinant of intimal hyperplasia and restenosis, these results may point to the design of optimal bypass graft geometries.
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flow Input Waveform effects on the temporal and spatial wall shear stress gradients in a femoral graft artery connector
Journal of Biomechanical Engineering-transactions of The Asme, 1996Co-Authors: Clement Kleinstreuer, Ming Lei, Joseph P ArchieAbstract:Employing a validated finite volume code, a computer-aided design of the distal end of a femoral graft-artery junction has been considered to simulate transient three-dimensional blood flow for various flow Input Waveforms. The study relies on the hypothesis that large sustained wall shear stress gradients play a major role in the rapid recurrence of intimal hyperplasia plus atheroma after bypass surgery, leading to early graft failure. Two new dimensionless parameters have been introduced to correlate flow Waveform characteristics with the severity of nonuniform hemodynamics and hence the potential risk for restenosis. The transient and, more importantly, the time-averaged wall shear stress gradient distributions shown, map out the junction areas which are still susceptible to restenosis, especially the toe region. Future geometric modifications will further reduce disturbed flow patterns and hence the probability of graft failure.
Ming Lei - One of the best experts on this subject based on the ideXlab platform.
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Computational design of a bypass graft that minimizes wall shear stress gradients in the region of the distal anastomosis
Journal of vascular surgery, 1997Co-Authors: Ming Lei, Joseph P Archie, Clement KleinstreuerAbstract:Purpose: Recent experimental and theoretic studies show that large wall shear stress gradients characterize disturbed flow patterns associated with the location of myointimal hyperplasia, atheroma, or both. Graft-to-artery anastomoses that minimize wall shear stress gradients may reduce the degree of myointimal development and the propensity for thrombosis. This study analyzes the distribution of distal anastomotic wall shear stress gradients for conventional geometries and for the optimized geometry assuming idealized merging of the graft with the artery. Methods: A validated computational fluid dynamics program was used to solve the transient three-dimensional partial differential equations and auxiliary equations that describe laminar incompressible blood flow. Time-averaged wall shear stresses and wall shear stress gradients were calculated for three distal graft-artery anastomoses: a standard end-to-side, a Taylor patch, and an optimized geometry. The latter was obtained iteratively by minimizing the local wall shear stress gradients and was analyzed under resting and exercise inflow Waveforms. Results: Both the standard and Taylor patch anastomoses have relatively high wall shear stress gradients in the regions of the toe and heel. For all flow Inputs studied nonuniform hemodynamics in the optimized graft design are largely eliminated, and the time-averaged wall shear stress gradients are greatly reduced throughout the anastomotic zone. At resting flow the Taylor patch produces slightly lower wall shear stress gradients in the anastomotic region than the standard end-to-side anastomosis. The optimized design reduces wall shear stress gradients to almost one half of that of the standard and Taylor patch geometries. At exercise flow wall shear stress gradients almost triple in the standard anastomosis and increase approximately 30% in the Taylor patch. In contrast, the geometrically optimized design is basically independent of the type of flow Input Waveform in terms of time-averaged wall shear stress gradients and disturbed flow patterns. Conclusion: This study demonstrates that it is possible to design a terminal graft geometry for an end-to-side anastomosis that significantly reduces wall shear stress gradients. If the wall shear stress gradient is confirmed to be a major hemodynamic determinant of intimal hyperplasia and restenosis, these results may point to the design of optimal bypass graft geometries.
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flow Input Waveform effects on the temporal and spatial wall shear stress gradients in a femoral graft artery connector
Journal of Biomechanical Engineering-transactions of The Asme, 1996Co-Authors: Clement Kleinstreuer, Ming Lei, Joseph P ArchieAbstract:Employing a validated finite volume code, a computer-aided design of the distal end of a femoral graft-artery junction has been considered to simulate transient three-dimensional blood flow for various flow Input Waveforms. The study relies on the hypothesis that large sustained wall shear stress gradients play a major role in the rapid recurrence of intimal hyperplasia plus atheroma after bypass surgery, leading to early graft failure. Two new dimensionless parameters have been introduced to correlate flow Waveform characteristics with the severity of nonuniform hemodynamics and hence the potential risk for restenosis. The transient and, more importantly, the time-averaged wall shear stress gradient distributions shown, map out the junction areas which are still susceptible to restenosis, especially the toe region. Future geometric modifications will further reduce disturbed flow patterns and hence the probability of graft failure.
Joseph P Archie - One of the best experts on this subject based on the ideXlab platform.
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Computational design of a bypass graft that minimizes wall shear stress gradients in the region of the distal anastomosis
Journal of vascular surgery, 1997Co-Authors: Ming Lei, Joseph P Archie, Clement KleinstreuerAbstract:Purpose: Recent experimental and theoretic studies show that large wall shear stress gradients characterize disturbed flow patterns associated with the location of myointimal hyperplasia, atheroma, or both. Graft-to-artery anastomoses that minimize wall shear stress gradients may reduce the degree of myointimal development and the propensity for thrombosis. This study analyzes the distribution of distal anastomotic wall shear stress gradients for conventional geometries and for the optimized geometry assuming idealized merging of the graft with the artery. Methods: A validated computational fluid dynamics program was used to solve the transient three-dimensional partial differential equations and auxiliary equations that describe laminar incompressible blood flow. Time-averaged wall shear stresses and wall shear stress gradients were calculated for three distal graft-artery anastomoses: a standard end-to-side, a Taylor patch, and an optimized geometry. The latter was obtained iteratively by minimizing the local wall shear stress gradients and was analyzed under resting and exercise inflow Waveforms. Results: Both the standard and Taylor patch anastomoses have relatively high wall shear stress gradients in the regions of the toe and heel. For all flow Inputs studied nonuniform hemodynamics in the optimized graft design are largely eliminated, and the time-averaged wall shear stress gradients are greatly reduced throughout the anastomotic zone. At resting flow the Taylor patch produces slightly lower wall shear stress gradients in the anastomotic region than the standard end-to-side anastomosis. The optimized design reduces wall shear stress gradients to almost one half of that of the standard and Taylor patch geometries. At exercise flow wall shear stress gradients almost triple in the standard anastomosis and increase approximately 30% in the Taylor patch. In contrast, the geometrically optimized design is basically independent of the type of flow Input Waveform in terms of time-averaged wall shear stress gradients and disturbed flow patterns. Conclusion: This study demonstrates that it is possible to design a terminal graft geometry for an end-to-side anastomosis that significantly reduces wall shear stress gradients. If the wall shear stress gradient is confirmed to be a major hemodynamic determinant of intimal hyperplasia and restenosis, these results may point to the design of optimal bypass graft geometries.
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flow Input Waveform effects on the temporal and spatial wall shear stress gradients in a femoral graft artery connector
Journal of Biomechanical Engineering-transactions of The Asme, 1996Co-Authors: Clement Kleinstreuer, Ming Lei, Joseph P ArchieAbstract:Employing a validated finite volume code, a computer-aided design of the distal end of a femoral graft-artery junction has been considered to simulate transient three-dimensional blood flow for various flow Input Waveforms. The study relies on the hypothesis that large sustained wall shear stress gradients play a major role in the rapid recurrence of intimal hyperplasia plus atheroma after bypass surgery, leading to early graft failure. Two new dimensionless parameters have been introduced to correlate flow Waveform characteristics with the severity of nonuniform hemodynamics and hence the potential risk for restenosis. The transient and, more importantly, the time-averaged wall shear stress gradient distributions shown, map out the junction areas which are still susceptible to restenosis, especially the toe region. Future geometric modifications will further reduce disturbed flow patterns and hence the probability of graft failure.
Kazuhiro Itoh - One of the best experts on this subject based on the ideXlab platform.
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sensitivity of the gradient oscillatory number to flow Input Waveform shapes
Journal of Biomechanics, 2012Co-Authors: Yuji Shimogonya, Hiroshige Kumamaru, Kazuhiro ItohAbstract:The sensitivity of the gradient oscillatory number (GON), which is a potential hemodynamic indicator for cerebral aneurysm initiation, to flow Input Waveform shapes was examined by performing computational fluid dynamics (CFD) simulations of an anatomical model of a human internal carotid artery under three different Waveform shape conditions. The local absolute variation (standard deviation) and relative variation (coefficient of variation) of the GON calculations for three Waveform shapes were computed to quantify the variation in GON due to Waveform shape changes. For all Waveform shapes, an elevated GON was evident at a known aneurysm site, albeit occurring at additional sites. No significant differences were observed among the qualitative GON distributions derived using the three different Waveform shapes. These results suggest that the GON is largely insensitive to the variability in flow Input Waveform shapes. The quantitative analysis revealed that GON displays an improved relative variation over a relatively high GON range. We therefore conclude that it is reasonable to use assumed flow Input Waveform shapes as a substitute for individual real Waveform shapes for the detection of possible GON elevations of individual clinical cases in large-scale studies, where the higher values of GON are of primary interest.
D'amico S. - One of the best experts on this subject based on the ideXlab platform.
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Fluids energy harvesting system with low cut-in velocity piezoelectric MEMS
IEEE, 2017Co-Authors: Biccario G. E., De Vittorio M., D'amico S.Abstract:Energy harvesting from environmental vibrations has established as an effective and green solution for electric energy production. In particular, fluid flows like the wind represent a steady and ubiquitous energy source. Traditional fluids harvesters are based on huge and bulky infrastructures like turbines, with a high environmental impact and a quite high cut-in speed (higher than 3÷4 m/s) for the fluids to be harvested. Transducers based on piezo-electric devices in the micrometric scale have pushed this value down by about one order of magnitude. The development of nanostructured piezo-electric transducers in the submicrometric scale offers a new generation of devices capable of converting the energy of very slow fluids (velocities lower than 1 m/s), like human breath, thanks to their high flexibility. The electronic interface circuit demanded of harvesting the energy from such a transducer is called to sense output signals of hundreds of millivolts with power equal to few microwatts or less. Active architectures must be employed even though they suffer for a start-up phase and the power demand. For building up the circuit supply voltage in few hundreds of milliseconds with the mentioned Input power, we propose the employment of two storage devices so that the powering of interface circuit is decoupled from the energy storing. For harvesting all the peaks of the Input Waveform down to 50 mV, a detector based on current sensing and offset rejection through AC coupling is proposed. © 2017 IEEE