The Experts below are selected from a list of 264 Experts worldwide ranked by ideXlab platform
George A. Truskey - One of the best experts on this subject based on the ideXlab platform.
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Normal and shear stresses influence the spatial distribution of intracellular adhesion molecule-1 expression in human umbilical vein endothelial cells exposed to sudden Expansion Flow.
Journal of biomechanics, 2006Co-Authors: V.z. Mckinney, Kristina D. Rinker, George A. TruskeyAbstract:Patterns in cell adhesion molecule expression by endothelial cells may play a role in atherogenesis. Previous studies have shown dependence of intracellular adhesion molecule-1 (ICAM-1) expression in human umbilical vein endothelial cells (HUVEC) on shear stress and have indirectly linked ICAM-1 expression to spatial gradients in shear stress. The spatial distribution of ICAM-1 in HUVEC pre-exposed to Flow for 8h was determined using fluorescence microscopy and a sudden Expansion Flow chamber with a 2.66 Expansion ratio to simulate gradients in wall shear stress found near arterial branches in vivo. When ICAM-1 expression in the disturbed Flow region was compared to theoretical stress distributions obtained from a computational model of sudden Expansion Flow, a modest trend (R2 = 0.327, p < 0.01)was observed between ICAM-1 and shear stress but the correlation between ICAM-1 and shear stress gradient was insignificant. In contrast, a moderately strong trend (R2 = 0.873, p < 0.01) was evident between ICAM-1 expression and the component of normal stress induced by the Expansion. Thus, in this in vitro model, normal stress arising from sudden Expansion Flow modulates the effect of shear stress on ICAM-1 expression.
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Characterization of a Sudden Expansion Flow Chamber to Study the Response of Endothelium to Flow Recirculation
Journal of biomechanical engineering, 1995Co-Authors: George A. Truskey, Kevin M. Barber, Thomas Robey, Lauri A. Olivier, Marty P. CombsAbstract:In order to simulate regions of Flow separation observed in vivo, a conventional parallel plate Flow chamber was modified to produce an asymmetric sudden Expansion. The Flow field was visualized using light reflecting particles and the size of the recirculation zone was measured by image analysis of the particles. Finite element numerical solutions of the two and three-dimensional forms of the Navier-Stokes equation were used to determine the wall shear stress distribution and predict the location of reattachment. For two different size Expansions, numerical estimates of the reattachment point along the centerline of the Flow chamber agreed well with experimental values for Reynolds numbers below 473. Even at a Reynolds number of 473, the Flow could be approximated as two-dimensional for 80 percent of the chamber width. Peak shear stresses in the recirculation zone as high as 80 dyne/cm 2 and shear stress gradients of 2500 (dyne/cm 2 )/cm were produced. As an application of this Flow chamber, subconfluent bovine aortic endothelial cell shape and orientation were examined in the zone of recirculation during a 24 h exposure to Flow at a Reynolds number of 267. After 24 h, gradients in cell orientation and shape were observed within the recirculation zone. At the location of reattachment, where the wall shear stress was zero but the shear stress gradients were large, cells plated at low density were still aligned with the direction of Flow. No preferred orientation was observed at the gasket edge where the wall shear stress and shear stress gradients were zero. At higher cell densities, no alignment was observed at the separation point. The results suggest that endothelial cells can respond to spatial gradients of wall shear stress.
Maria A. Founti - One of the best experts on this subject based on the ideXlab platform.
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Effects of Increasing Particle Loading in an Axisymmetric, Vertical, Liquid-Solid Sudden Expansion Flow
Journal of Fluids Engineering, 1999Co-Authors: Maria A. Founti, Thomas Achimastos, Athinodoros KlipfelAbstract:Measurements of particle and fluid velocities are reported for a turbulent, liquid-solid, sudden Expansion Flow Flowing in the direction of gravity and laden with solid particles, at loadings equal to 1, 2, 3, 4, and 5 percent per volume. The measured two-phase Flow velocities are compared to the characteristics of the corresponding single phase Flow. Forces and Flow mechanisms affecting particle dispersion in the various Flow regimes are identified and it is indicated that there exist regions where the transverse Saffman lift force attains high values and controls particle dispersion. A consistent correlation between the mean reattachment point and the volumetric particle loading is indicated. All the two phase Flows examined reattached upstream the corresponding mean reattachment location measured for the single phase Flow. Increasing particle concentration affected locally the Flow behaviour, with most obvious consequences within the recirculation zone and the near wall region.
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Experimental and computational investigations of nearly dense two-phase sudden Expansion Flows
Experimental Thermal and Fluid Science, 1998Co-Authors: Maria A. Founti, A. KlipfelAbstract:Abstract The work investigates experimentally and computationally the effects of particle-to-particle collisions on the characteristics of the particle motion in a vertical downward Flowing sudden Expansion Flow. The investigated Flow is nearly dense, laden with spherical glass particles at 5% per volume. In order to simulate the particle-to-particle collision phenomena a model has been developed based upon the simultaneous Lagrangian tracking of all the particles which move inside the Flow domain. Computational results with and without the particle-to-particle collision model are presented and compared to experimental data. The results establish Flow regimes where particle-to-particle collisions become significant in that they modify the turbulent kinetic energy of the particles. It is shown that particle-to-particle collisions induce small variation of the fluid-particle slip velocities and reduce the turbulent kinetic energy of the dispersed phase mainly in the shear layer zone where the turbulent kinetic energy of the carrier phase attains maximum values.
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Influence of Expansion Ratio on the Size of Recirculation Zone in Two-Phase Sudden Expansion Flows,
1992Co-Authors: Maria A. Founti, G. PapaioannidesAbstract:Abstract : The paper presents measurements of the reattachment length and of the axial velocity component in a descending turbulent axisymmetric sudden Expansion Flow, for three Expansion ratios of 1:2, 1:3 and 1:4. The measurements have been performed with a standard laser Doppler anemometer, first in the single-phase Flow and then repeated for the fluid and particles in the two-phase Flow cases by eliminating from the raw Doppler signals each time one of the two burst populations. For the two-phase Flows the measured reattachments lengths were 6-10% shorter than for the corresponding single-phase cases. Fort particle volumetric concentrations up to 0.027, the axial mean velocities and normal stresses were not affected as the concentration was increased. (Author)
James H. Whitelaw - One of the best experts on this subject based on the ideXlab platform.
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Particle dispersion in a vertical round sudden-Expansion Flow
Philosophical Transactions of the Royal Society A, 1992Co-Authors: Yannis Hardalupas, Alexander Taylor, James H. WhitelawAbstract:The dispersion of glass beads in an air Flow through sudden step Expansions in the direction of gravity has been investigated with phase-Doppler anemometry, which provided measurements of the velocity, flux and concentration characteristics. The purpose was to quantify the effect of increasing the air velocity over a factor of 5 for bead diameters of 40 µm and 80 µm and two Expansion diameter ratios, 3.33 and 5, and at mass loadings of beads up to 90% of the air mass Flow-rate. The results showed that the beads dispersed into the recirculation zone in the lee of the step by interaction with eddies characterized by length and velocity scales of the order of the Expansion step height and the downstream area-averaged velocity. Particle dispersion into the recirculation zone was reduced when the bead mean transit time across the recirculation zone was shorter than the bead relaxation time, defined as the time required for a motionless bead suddenly exposed to a constant velocity fluid stream to reach 63% of its surrounding fluid velocity. Also the centrifuging effect, caused by the mean stream line curvature of the recirculation zone, could reduce particle dispersion into the recirculation zone, when its characteristic dimensionless group was less than unity. Beads, because of their mass, left the recirculation zone by sliding down the wall and past the air reattachment point or near the step, giving rise to bimodal probability distributions of velocity.
Edson Luiz Zaparoli - One of the best experts on this subject based on the ideXlab platform.
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Numerical study of three recirculation zones in the unilateral sudden Expansion Flow
International Communications in Heat and Mass Transfer, 2008Co-Authors: R.c. Lima, Claudia Regina Furquim De Andrade, Edson Luiz ZaparoliAbstract:Abstract Numerical simulations of the two-dimensional laminar Flow over a backward-facing step channel using two commercially-available Computational Fluid Dynamics (CFD) code are reported. The subject is to analyze the three recirculation regions of the Flow in a unilateral sudden Expansion. Results are presented for laminar air Flow for Reynolds number lower than 2500. The mathematical model equations (mass conservation and momentum) were solved with finite element (FEM) and finite volume (FVM) methods and a segregated approach. To get the grid independent, intensive refinement studies were carried out. Results obtained are compared to experimental data presenting a good agreement. The numerical results of this work show a non-linear increase of the reattachment length as also shown by the experimental results extracted from literature.
Marty P. Combs - One of the best experts on this subject based on the ideXlab platform.
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Characterization of a Sudden Expansion Flow Chamber to Study the Response of Endothelium to Flow Recirculation
Journal of biomechanical engineering, 1995Co-Authors: George A. Truskey, Kevin M. Barber, Thomas Robey, Lauri A. Olivier, Marty P. CombsAbstract:In order to simulate regions of Flow separation observed in vivo, a conventional parallel plate Flow chamber was modified to produce an asymmetric sudden Expansion. The Flow field was visualized using light reflecting particles and the size of the recirculation zone was measured by image analysis of the particles. Finite element numerical solutions of the two and three-dimensional forms of the Navier-Stokes equation were used to determine the wall shear stress distribution and predict the location of reattachment. For two different size Expansions, numerical estimates of the reattachment point along the centerline of the Flow chamber agreed well with experimental values for Reynolds numbers below 473. Even at a Reynolds number of 473, the Flow could be approximated as two-dimensional for 80 percent of the chamber width. Peak shear stresses in the recirculation zone as high as 80 dyne/cm 2 and shear stress gradients of 2500 (dyne/cm 2 )/cm were produced. As an application of this Flow chamber, subconfluent bovine aortic endothelial cell shape and orientation were examined in the zone of recirculation during a 24 h exposure to Flow at a Reynolds number of 267. After 24 h, gradients in cell orientation and shape were observed within the recirculation zone. At the location of reattachment, where the wall shear stress was zero but the shear stress gradients were large, cells plated at low density were still aligned with the direction of Flow. No preferred orientation was observed at the gasket edge where the wall shear stress and shear stress gradients were zero. At higher cell densities, no alignment was observed at the separation point. The results suggest that endothelial cells can respond to spatial gradients of wall shear stress.
