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Wall Shear Stress (wall + shear_stress)
Selected AbstractsRole of shear stress on nitrite and NOS protein content in different size conduit arteries of swineACTA PHYSIOLOGICA, Issue 2 2009X. Guo Abstract Aim:, Inherent fundamental difference exists among arteries of different sizes. The purpose of this study was to evaluate the relation between regional difference of wall shear stress (WSS) in various sizes arteries and contents of nitrite and NO synthase (NOS) isoforms. Methods:, Five different conduit arteries in a wide range of diameter (1,8 mm) were examined in the hind limbs of 13 pigs. Blood flow rate and outer diameter were measured in vivo to determine WSS. Arterial tissues were harvested for the measurement of nitrite and NOS protein contents. The concentration of nitrite, a product of NO synthesis, was determined by high-performance liquid chromatography method. Western blot analysis was used to assess the protein contents of endothelial NOS (eNOS), inducible NOS (iNOS) and neuronal NOS (nNOS). Results:, Our data show that WSS increases with a decrease in artery diameter. Nitrite level increases with increasing WSS and hence decreases with artery diameter. The eNOS protein contents decrease with an increase in diameter. No significant difference for iNOS and nNOS protein contents was found with different artery diameter. A significant positive correlation between tissue nitrite and eNOS protein contents was also observed. Finally, the WSS-normalized eNOS is not significantly different in various size vessels. Conclusion:, Regional difference in blood flow has no effect on iNOS and nNOS protein contents in these conduit arteries. Regional difference in eNOS expression and nitrite contents may be related to the WSS-induced NO by the endothelium under normal physiological conditions. [source] Hemodynamic analysis of intracranial aneurysms with moving parent arteries: Basilar tip aneurysmsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 10 2010Daniel M. Sforza Abstract The effects of parent artery motion on the hemodynamics of basilar tip saccular aneurysms and its potential effect on aneurysm rupture were studied. The aneurysm and parent artery motions in two patients were determined from cine loops of dynamic angiographies. The oscillatory motion amplitude was quantified by registering the frames. Patient-specific computational fluid dynamics (CFD) models of both aneurysms were constructed from 3D rotational angiography images. Two CFD calculations were performed for each patient, corresponding to static and moving models. The motion estimated from the dynamic images was used to move the surface grid points in the moving model. Visualizations from the simulations were compared for wall shear stress (WSS), velocity profiles, and streamlines. In both patients, a rigid oscillation of the aneurysm and basilar artery in the anterio-posterior direction was observed and measured. The distribution of WSS was nearly identical between the models of each patient, as well as major intra-aneurysmal flow structures, inflow jets, and regions of impingement. The motion observed in pulsating intracranial vasculature does not have a major impact on intra-aneurysmal hemodynamic variables. Parent artery motion is unlikely to be a risk factor for increased risk of aneurysmal rupture. Copyright © 2010 John Wiley & Sons, Ltd. [source] Laminar and turbulent flow calculations through a model human upper airway using unstructured meshesINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 12 2007P. Nithiarasu Abstract In this paper, numerical investigation of airflow through a human upper airway is presented using an unstructured-based characteristic-based split (CBS) scheme. The CBS scheme used in the present study employs a fully explicit matrix-free solution procedure along with artificial compressibility. A one equation Spalrat,Allmaras (SA) turbulence model is employed to study low and moderate Reynolds number flows. A detailed discussion of the qualitative and quantitative results is presented. The results show a strong influence of the Reynolds number on the flow pattern and quantities of interest, pressure drop and wall shear stress. It is also apparent that SA model can be employed on unstructured meshes to predict the steady flow with good accuracy. Thus, the novelties of the present paper are: use of the unstructured mesh-based solution algorithm and the successful application of the SA model to a typical human upper airway. Copyright © 2006 John Wiley & Sons, Ltd. [source] Blood flow dynamics and fluid,structure interaction in patient-specific bifurcating cerebral aneurysmsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 10 2008Alvaro Valencia Abstract Hemodynamics plays an important role in the progression and rupture of cerebral aneurysms. The current work describes the blood flow dynamics and fluid,structure interaction in seven patient-specific models of bifurcating cerebral aneurysms located in the anterior and posterior circulation regions of the circle of Willis. The models were obtained from 3D rotational angiography image data, and blood flow dynamics and fluid,structure interaction were studied under physiologically representative waveform of inflow. The arterial wall was assumed to be elastic, isotropic and homogeneous. The flow was assumed to be laminar, non-Newtonian and incompressible. In one case, the effects of different model suppositions and boundary conditions were reported in detail. The fully coupled fluid and structure models were solved with the finite elements package ADINA. The vortex structure, pressure, wall shear stress (WSS), effective stress and displacement of the aneurysm wall showed large variations, depending on the morphology of the artery, aneurysm size and position. The time-averaged WSS, effective stress and displacement at the aneurysm fundus vary between 0.17 and 4.86,Pa, 4.35 and 170.2,kPa and 0.16 and 0.74,mm, respectively, for the seven patient-specific models of bifurcating cerebral aneurysms. Copyright © 2008 John Wiley & Sons, Ltd. [source] Effects of blood models on flows through a stenosisINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 6-7 2003Panagiotis Neofytou Abstract The paper presents a numerical investigation of non-Newtonian modelling effects on unsteady periodic flows in a two-dimensional (2D) channel with a stenosis. The geometry and boundary conditions were chosen so as to reproduce the flow features that are observed in real haemodynamic conditions. Three different non-Newtonian constitutive equations for modelling the shear characteristics of the blood namely the Casson, power-law and Quemada models, are utilized. Similarly with previous studies based on Newtonian modelling, the present simulations show the formation of several vortices downstream of the stenosis, as well as substantial variations of the wall shear stress throughout the unsteady cycle. Additionally, it is shown that: (i) there are substantial differences between the results obtained by Newtonian and non-Newtonian models, and (ii) the prediction of vortex formation, wall shear stress distribution and separation behind the stenosis is strongly dependent on the details of the non-Newtonian model employed in the simulations. Copyright © 2003 John Wiley & Sons, Ltd. [source] Viscosity corrections for concentrated suspension in capillary flow with wall slipAICHE JOURNAL, Issue 6 2010Z. Y. Wang Abstract Corrections for viscosity measurements of concentrated suspension with capillary rheometer experiments were investigated. These corrections include end effects, Rabinowitsch effect, and wall slip. The effects of temperature, particle concentration, and contraction ratio on the end effects were studied and their effects were accounted for using an entrance and exit losses model. The non-Newtonian effect and the nonlinearity of slip velocity against wall shear stress were described using a slip model. The true viscosity of a concentrated suspension with glass powder suspended in a non-Newtonian binder system was calculated as a function of shear rate and effective particle concentration, taking into consideration particle migration, which is calculated by a diffusive numerical model. Particle size was found to affect significantly the viscosity of the suspension with viscosity decreasing with increasing particle size, which can be reflected by a decrease in the value of the power-law index in the Krieger model. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Recent Perspective on Coronary Bifurcation Intervention: Statement of the "Bifurcation Club in KOKURA"JOURNAL OF INTERVENTIONAL CARDIOLOGY, Issue 4 2010YOSHINOBU MURASATO M.D. The treatment of coronary bifurcation lesion remains a challenging issue even in the drug-eluting stent era. Frequent restenosis and stent thrombosis have been recently shown to be related not only to geometrical gap or stent structural deformation but also to rheological disturbance. Low wall shear stress at the lateral side of the bifurcation is likely to cause atherosclerotic changes due to easy access of the macrophages that induce chemical mediators. The turbulent flow over stent metal may facilitate accumulation of platelets, which results in thrombosis. The jailed strut and excess metal overlap may increase these risks. Since dramatic changes of the coronary flow pattern at the bifurcation are closely related to the genesis of atherosclerosis, future bifurcation intervention technique should be considered to restore the original physiological state as well as the anatomical structure. This article summarizes the global consensus of the members of the Asian Bifurcation Club and European Bifurcation Club at the KOKURA meeting. It also provides a perspective of basic sciences relating to bifurcation anatomy, physiology, and pathology, in the search for a best strategy for bifurcation intervention. (J Interven Cardiol 2010;23:295,304) [source] MRI measurement of time-resolved wall shear stress vectors in a carotid bifurcation model, and comparison with CFD predictionsJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 2 2003Panorea Papathanasopoulou MSc Abstract Purpose To study pulsatile fluid flow in a physiologically realistic model of the human carotid bifurcation, and to derive wall shear stress (WSS) vectors. Materials and Methods WSS vectors were calculated from time-resolved 3D phase-contrast (PC) MRI measurements of the velocity field. The technique was first validated with sinusoidal flow in a straight tube, and then used in a model of a healthy human carotid bifurcation. Velocity measurements in the inflow and outflow regions were also used as boundary conditions for computational fluid dynamics (CFD) calculations of WSS, which were compared with those derived from MRI alone. Results The straight tube measurements gave WSS results that were within 15% of the theoretical value. WSS results for the phantom showed the main features expected from fluid dynamics, notably the low values in the bulb region of the internal carotid artery, with a return to ordered flow further downstream. MRI was not able to detect the high WSS values along the divider wall that were predicted by the CFD model. Otherwise, there was good general agreement between MRI and CFD. Conclusion This is the first report of time-resolved WSS vectors estimated from 3D-MRI data. The technique worked well except in regions of disturbed flow, where the combination with CFD modeling is clearly advantageous. J. Magn. Reson. Imaging 2003;17:153,162. © 2003 Wiley-Liss, Inc. [source] Decreased Arteriolar Sensitivity to Shear Stress in Adult Rats is Reversed by Chronic Exercise ActivityMICROCIRCULATION, Issue 2 2002Dong Sun M.D., Ph.D. Objective: We tested the hypothesis that the decline in endothelium-dependent arteriolar dilation in adult rats is reversed by chronic exercise activity. Methods: Rats were divided into young (8,10 weeks)-sedentary (SEDY), adult (29,32 weeks)-sedentary (SEDA), and adult-exercised (EXA, treadmill exercise for 18,20 weeks) groups. Responses of isolated arterioles (,50 µm at 80 mm Hg) of gracilis muscle were assessed to increases in perfusate flow and vasoactive agents. Results: With no differences in basal tone, maximal flow-induced dilations were not different between arterioles of SEDY and SEDA rats (71 ± 2 and 72 ± 2% of passive diameter, respectively), yet the sensitivity of arterioles to shear stress (WSS50) was significantly less in SEDA than in SEDY rats (35 ± 4 vs. 23 ± 3 dyne/cm2, respectively). In vessels of EXA rats, maximal flow-induced dilation was significantly augmented (88 ± 2% of passive diameter) and WSS50 (15 ± 1 dyn/cm2) was significantly reduced. Dilation to acetylcholine was enhanced in arterioles of EXA, whereas dilation to sodium nitroprusside was not different in vessels of the three groups. Conclusion: Chronic exercise activity reverses age related reduction in sensitivity of arterioles to increases in wall shear stress. [source] Modeling Flow in a Compromised Pediatric Airway Breathing Air and HelioxTHE LARYNGOSCOPE, Issue 12 2008Mihai Mihaescu PhD Abstract Objectives/Hypothesis: The aim of this study was to perform computer simulations of flow within an accurate model of a pediatric airway with subglottic stenosis. It is believed that the airflow characteristics in a stenotic airway are strongly related to the sensation of dyspnea. Methodology: Computed tomography images through the respiratory tract of an infant with subglottic stenosis, were used to construct the three-dimensional geometry of the airway. By using computational fluid dynamics (CFD) modeling to capture airway flow patterns during inspiration and expiration, we obtained information pertaining to flow velocity, static airway wall pressure, pressure drop across the stenosis, and wall shear stress. These simulations were performed with both air and heliox. Results: Unlike air, heliox maintained laminar flow through the stenosis. The calculated pressure drop over stenosis was lower for the heliox flow, in contrast to the airflow case. This lead to an approximately 40% decrease in airway resistance when using heliox, and presumably causes a decrease in the level of effort required for breathing. Conclusions: CFD simulations offer a quantitative method of evaluating airway flow dynamics in patients with airway abnormalities. CFD modeling illustrated the flow features and quantified flow parameters within a pediatric airway with subglottic stenosis. Simulations with air and heliox conditions mirrored the known clinical benefits of heliox as compared with air. We anticipate that computer simulation models will ultimately allow a better understanding of changes in flow caused by specific medical and surgical interventions in patients with conditions associated with dyspnea. [source] Numerical Comparative Study on the Hemodynamic Performance of a New Helical Graft With Noncircular Cross Section and SwirlGraftARTIFICIAL ORGANS, Issue 1 2010Anqiang Sun Abstract The helically distributed ribbons of thrombus, formed in the commercially available SwirlGraft (Veryan Medical, London, UK), are the result of its wall shear stress distribution, which has zonary areas of low wall shear stress. In order to overcome the inherent deficiency of the SwirlGraft, a new helical graft with a noncircular cross section was proposed and compared numerically with the SwirlGraft in terms of wall shear stress distribution, helicity of the swirling flow created, and pressure drop over the grafts. The numerical results showed that due to the modification to the geometrical configuration of the SwirlGraft, wall shear stress in the new helical graft model was enhanced, and the zones of low wall shear stress existing in the SwirlGraft were completely eliminated. The present numerical study also predicted a slightly steeper pressure drop and reduction in helicity in the new helical graft model in comparison with the SwirlGraft model. Based on the study, we believe that the new helical graft with a noncircular cross section may reduce the possibility of acute thrombus formation in the graft because the enhanced wall shear stress can impede the stay and adherence of platelets and leukocytes to the surface of the graft. [source] Hemocompatibility Evaluation With Experimental and Computational Fluid Dynamic Analyses for a Monopivot Circulatory Assist PumpARTIFICIAL ORGANS, Issue 4 2009Masahiro Nishida Abstract:, The hemocompatibility of a newly developed monopivot circulatory assist pump was evaluated by the computational fluid dynamic (CFD) analyses with the particle tracking velocimetry measurement. Results were compared with those of the hemolysis test and in vitro antithrombogenic test to prevent hemolysis and thrombus formation inside the pump. The results of the CFD analysis and the particle tracking velocimetry had a good agreement with each other. The flow distributions by the CFD analysis indicated that the radial jet out of the impeller was adequately weak so that the wall shear stress was lower than 300 Pa on the volute casing wall. It corresponded with the hemolysis tests results, indicating that the hemolysis level was lower than that of the commercially available pump. However, the flow distributions also indicated that the pivot that was easy to stagnate was washed out, not only by the secondary flow through the back gap of the impeller, but also by the vortices generated by the secondary vanes. It corresponded with the in vitro antithrombogenic test results, indicating that thrombus formation could be removed only by redesigning the geometry of the secondary vanes. [source] Design of cellular porous biomaterials for wall shear stress criterionBIOTECHNOLOGY & BIOENGINEERING, Issue 4 2010Yuhang Chen Abstract The microfluidic environment provided by implanted prostheses has a decisive influence on the viability, proliferation and differentiation of cells. In bone tissue engineering, for instance, experiments have confirmed that a certain level of wall shear stress (WSS) is more advantageous to osteoblastic differentiation. This paper proposes a level-set-based topology optimization method to regulate fluidic WSS distribution for design of cellular biomaterials. The topological boundary of fluid phase is represented by a level-set model embedded in a higher-dimensional scalar function. WSS is determined by the computational fluid dynamics analysis in the scale of cellular base cells. To achieve a uniform WSS distribution at the solid,fluid interface, the difference between local and target WSS is taken as the design criterion, which determines the speed of the boundary evolution in the level-set model. The examples demonstrate the effectiveness of the presented method and exhibit a considerable potential in the design optimization and fabrication of new prosthetic cellular materials for bioengineering applications. Biotechnol. Bioeng. 2010;107:737,746. © 2010 Wiley Periodicals, Inc. [source] Advanced models for erosion corrosion and its mitigation,MATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 2 2008G. Schmitt Erosion corrosion, i.e., flow-induced localized corrosion (FILC) is initiated when flow dynamic forces surpass the fracture energy of protective layers or scales on metals. With a new model the maximum interaction energies between flowing media and solid walls can be quantified in terms of "freak" energy densities created during singular events (freak events) of perpendicular impacts by near-wall microturbulence elements. The freak energy densities are in the megaPascal range and match well in the order of magnitude with fracture energies of protective layers and can be estimated from Wavelet diagnostics of electrochemical current noise measured at microelectrodes under mass transport controlled conditions. This solves the problem that wall shear stresses, generally used to quantify critical flow intensities for FILC initiation, range several orders of magnitude (Pa range) below the fracture energies of protective layers. The new advanced model allows for the first time to quantify the maximum fluid dynamic forces exerted on solid walls under different turbulent and disturbed flow conditions (one-phase liquid flow on jet impinged surfaces and on coupons in rotated cages, surfaces impacted by slug flow and gas-pulsed impinging jets). Drag reducing additives were shown to reduce freak energy densities to values significantly below fracture energies of protective layers and hence inhibit initiation of FILC. The onset of FILC can be monitored online with the newly developed CoulCount method, an easy-to-use, non-invasive diagnostic tool which evaluates electrochemical current noise between jet impinged electrode pairs made from the metals to be tested. [source] Computational Fluid Dynamics and Vascular AccessARTIFICIAL ORGANS, Issue 7 2002Ulf Krueger Abstract: Anastomotic intimal hyperplasia caused by unphysiological hemodynamics is generally accepted as a reason for dialysis access graft occlusion. Optimizing the venous anastomosis can improve the patency rate of arteriovenous grafts. The purpose of this study was to examine, evaluate, and characterize the local hemodynamics and, in particular, the wall shear stresses in conventional venous end-to-side anastomosis and in patch form anastomosis (Venaflo) by three-dimensional computational fluid dynamics (CFD). We investigated the conventional form of end-to-side anastomosis and a new patch form by numerical simulation of blood flow. The numerical simulation was done with a finite volume-based algorithm. The anastomotic forms were constructed with usual size and fixed walls. Subdividing the flow domain into multiple control volumes solved the fundamental equations. The boundary conditions were identical for both forms. The velocity profile of the patch form is better than that for the conventional form. The region of high static pressure caused by flow stagnation is reduced on the vein floor. The anastomotic wall shear stress is decreased. The results of this study strongly support patch form use to reduce the incidence of intimal hyperplasia and venous anastomotic stenoses. [source] Investigating the Feasibility of Stem Cell Enrichment Mediated by Immobilized SelectinsBIOTECHNOLOGY PROGRESS, Issue 6 2007Nichola Charles Hematopoietic stem cell therapy is used to treat both malignant and non-malignant diseases, and enrichment of the hematopoietic stem and progenitor cells (HSPCs) has the potential to reduce the likelihood of graft vs host disease or relapse, potentially fatal complications associated with the therapy. Current commercial HSPC isolation technologies rely solely on the CD34 surface marker, and while they have proven to be invaluable, they can be time-consuming with variable recoveries reported. We propose that selectin-mediated enrichment could prove to be a quick and effective method for recovering HSPCs from adult bone marrow (ABM) on the basis of differences in rolling velocities and independently of CD34 expression. Purified CD34+ ABM cells and the unselected CD34, ABM cells were perfused over immobilized P-, E-, and L-selectin-IgG at physiologic wall shear stresses, and rolling velocities and cell retention data were collected. CD34+ ABM cells generally exhibited lower rolling velocities and higher retention than the unselected CD34, ABM cells on all three selectins. For initial CD34+ ABM cell concentrations ranging from 1% to 5%, we predict an increase in purity ranging from 5.2% to 36.1%, depending on the selectin used. Additionally, selectin-mediated cell enrichment is not limited to subsets of cells with inherent differences in rolling velocities. CD34+ KG1a cells and CD34, HL60 cells exhibited nearly identical rolling velocities on immobilized P-selectin-IgG over the entire range of shear stresses studied. However, when anti-CD34 antibody was co-immobilized with the P-selectin-IgG, the rolling velocity of the CD34+ KG1a cells was significantly reduced, making selectin-mediated cell enrichment a feasible option. Optimal cell enrichment in immobilized selectin surfaces can be achieved within 10 min, much faster than most current commercially available systems. [source] | ||||||||||||||||