Shear Stress Regions Research Articles

Numerical study on the effect of separate cannulation method on hemodynamics in an arteriovenous graft

Generally, the hemodialysis patient with the prosthetic arteriovenous graft (AVG) undergoes two punctures with needles on the graft in every dialysis session. As an alternative method for patients with AVGs, clinicians and nurses often return the blood filtered by a dialysis machine to an autologous vein not the graft, and that is defined as a separate cannulation method. In this study, we numerically evaluate the influence of the separate cannulation method on the hemodynamics by using clinical information of six patients. In the results, when the separate cannulation method is used, the low time-averaged wall shear stress region increases whereas the high oscillatory shear index region decreases at the venous anastomosis and proximal vein. The quantitative analysis using hemodynamic index indicates the separated cannulation method has pros and cons on the occurrence of complications in AVGs.

Inhibition of corrosion in amine air cooled heat exchanger: Experimental and numerical study

The major challenges of CO2 capture by alkanolamines are associated with corrosion process. In this paper, the corrosion characteristics of carbon steel were analyzed in the air-cooled heat exchanger (ACHE) of a petrochemical plant in Kermanshah, Iran. The chemical analysis, weight loss method, electrochemical techniques, and eddy current measurements were applied to determine corrosion products and corrosion rate. Computational fluid dynamics was also applied to search for the links between the observed patterns of attack seen in the amine plant's heat exchanger tubes and the hydrodynamics inside the tubes. Validation of the computational fluid dynamics results was compared with the visualized model of the tubes in the first pass of ACHE header. It was found that the erosion–corrosion damage was a consequence of increased levels of turbulence at tubes' inlet, caused by a rapidly turning flow. An alternative flow correction was numerically investigated applying another nozzle in the first pass header of inlet flow and were recommended for implementation. As a result of the proposed configuration, the maximum wall shear stress was reduced from 90 Pa to 40 Pa in the inlet tube. Furthermore, the length of high wall shear stress region was decreased from 15 cm to 5 cm.

Dispersion of a passive tracer in the pressure-driven flow of a non-colloidal suspension.

This paper numerically quantifies the dispersion of a solute, and in particular, the Taylor dispersion, in the pressure-driven flow of a non-colloidal suspension at moderately high volume fractions (0.2 to 0.5) through conduits of different cross-sectional shapes. An obvious intuition is that the Taylor dispersivity should increase owing to a decrease in the molecular diffusivity of the solute in the presence of particles impermeable to the solute; however, this is true only at low volume fractions. At higher volume fractions, three other physical effects become important, all of which lead to a reduction in Taylor dispersivity relative to a Newtonian fluid. The first is the blunting of the velocity profile resulting from particle migration into the low shear-stress regions, an effect that has been alluded to in the past by Roht et al. [J. Contam. Hydrol., 2013, 14, 10] and is important only at low Péclet numbers (Pe). At higher Pe, the two stronger effects are shear-induced solute self-diffusion, which arises due to shear-induced particle-particle interactions, and secondary convection, which is observed in non-axisymmetric cross-sections as a result of the second normal stress differences exhibited by concentrated suspensions. For a given volume fraction and cross-sectional geometry, a regime map, developed using a scaling analysis, delineates five regimes of dispersion involving one or a combination of the mass transfer mechanisms mentioned above. Our analysis also suggests that the cross-sectional shape can be exploited to enhance or suppress solute dispersion by modifying the secondary current strength and profile.

Hemodynamic characterization of transient blood flow in right coronary arteries with varying curvature and side-branch bifurcation angles

Hemodynamics plays a critical role in the development and progression of plaques that are prone to sites of arterial curvatures and branch bifurcations. The occurrence of atherosclerosis at curvatures of coronary arteries and at the bifurcation sites of their branches are of medical importance. In this study, we have employed computer simulation coupled with a parametric analysis to rigorously quantify the transient wall shear stress (WSS) and wall pressure gradient (WPG) in idealized and image-based right coronary artery (RCA) models with side-branches, in order to explore the effects of curvature and bifurcation on blood flow characteristics and its hemodynamic variables. Upon establishing and implementing this parametric analysis framework, it is found that the relatively lower wall shear stress and WPG regions at the curvature and branch bifurcation sites of arteries coincide with the plaques prone points of the three investigated image-based arteries, to thereby confirm the correlation of the fluid mechanical properties with the arterial geometry variation in consistence with the data from existing literatures. Moreover, the average WSS and Pressure drop (from inlet to outlet) are significantly increased with the more complex geometry of realistic arteries in comparison with idealized arterial models. It can hence be concluded that the geometry of curvature and angulation of the side branches has a significant effect on the WSS and WPG in hemodynamics analysis of RCAs and its correlation with atherosclerotic lesions.

A biomechanical model of pendelluft induced lung injury

Lung ventilation using high frequency oscillatory techniques have been documented to attain adequate gas exchange through various gas transport mechanisms. Among them, the pendelluft flow is considered one of the most crucial mechanisms. In this work, we computationally investigate the induction of abnormal mechanical stresses and a regionally trapped volume of gas due to pendelluft flow. Large eddy simulation was used to model the turbulence in an upper tracheobronchial lung geometry that was derived from CT scans. The pendelluft flow was captured by modeling physiological boundary conditions at the truncated level of the lung model that is sensitive to the coupled resistance and compliance of individual patients. The flow–volume and volume–pressure loops are characterized by irregular shapes and suggest abnormal regional lung ventilation. Incomplete loops were observed indicating gas trapping in these regions signifying a potential for local injury due to incomplete ventilation from a residual volume build-up at the end of the expiration phase. In addition, the gas exchange between units was observed to create a velocity gradient causing a region of high wall shear stress surrounding the carina ridges. The recurrence of the pendelluft flow could cause a rupture to the lung epithelium layer. The trapped gas and wall shear stress were observed to amplify with increasing compliance asymmetry and ventilator operating frequency. In general, despite the significant contribution of the pendelluft flow to the gas exchange augmentation there exists significant risks of localized lung injury, phenomena we describe as pendelluft induced lung injury or PILI.

A note on stress fields and crack growth in porous materials subjected to a contact load

In materials where inherent heterogeneities like cells and pores are relatively large compared to other relevant mechanical dimensions, such as the contact surface in indentation tests or the length of an existing crack, stress and strain fields predicted by classical continuum elasticity theories may become too harsh because of absence of internal lengths in the equations that characterize the underlying microstructure. To overcome this deficiency a gradient enhanced elasticity continuum theory may be applied, which include length parameters in the constitutive equations that limit the magnitude of deformation gradients and is able to capture internal length effects. In this study, microscopic stress fields in porous materials subjected to a cylindrical contact load are estimated with such a gradient enhanced continuum model. To judge the model’s ability to capture the mechanical behavior in this class of materials, calculated stress fields in the contact region, given by the gradient theory, are contrasted with microscopic stress fields computed in discrete high-resolution finite element models of cellular wood-like structures having varying average pore sizes but identical macroscopic geometry and boundary conditions as the gradient model. X-ray computed tomography experiments on wood illustrate the phenomenon. It is observed, in both experiment and finite element models, that the region of high shear stresses, where a crack may grow despite a confining pressure, is located deeper down in the material than what is predicted in classical continuum theories. On the other hand, the gradient enhanced model produces remarkably similar stress/strain fields as the finite element models and experiment and is thus seemingly able to capture microstructural size effects.

Effects of shear-dependent transport properties on lumen surface concentration of LDL particles in stenosed carotid artery

Growth of stenosis is mainly due to the high concentration of plasma lipoprotein such as low density lipoprotein (LDL) near the art ery walls. Accurate prediction of LDL concentration especially near the stenosis and where the shear stress is low, can help to predict the plaque growth. In this paper, a novel model is introduced to predict the LDL concentration near a plaque. This model is based on variable diffusion coefficient of LDL due to the non-Newtonian behavior of the blood flow in low shear stress regions such as flow around plaques. The new model for diffusion coefficient consists of two parts: the stationary and the shear- induced particle diffusivity due to rotation of red blood cells. The results show that the new model predicts the LDL concentration well and unlike the constant diffusion coefficient which is used by others, produces more physical and meaningful results.

Effect of Surface Hardening Technique and Case Depth on Rolling Contact Fatigue Behavior of Alloy Steels

Surface hardening techniques are widely used to improve the rolling contact fatigue (RCF) resistance of materials. This study investigated the RCF resistance of hardened, ground steel rods made from three different aircraft-quality alloy steels (AISI 8620, 9310, and 4140) and hardened using different techniques (atmosphere carburizing, vacuum carburizing, and induction hardening) at different case depths. The RCF life of the rods was determined using a three ball-on-rod rolling contact fatigue test machine. After testing, the microstructures of the rods were examined using metallographic techniques. The stress distributions and plastic deformation zones for the specimens under RCF were calculated using an elastoplastic model for plastically graded materials. Relationships between surface hardness, case depth, and RCF life were investigated. The longest lives were observed for the vacuum-carburized AISI 9310 specimens, and the shortest lives were observed for the induction-hardened AISI 4140 specimens. It is concluded that the most important factors in determining the RCF lives of high-cleanliness surface-hardened alloy steels are (1) the hardness in the region of highest octahedral shear stress (in this case, ∼0.13 mm beneath the surface) and (2) the depth of high hardness (>613 HV), which determines the plastic deformation zone size.

Experimental Characterisation of Fluid Mechanics in a Spinner Flask Bioreactor

The spinner flask bioreactor has been widely used in in vitro cell culturing processes due to its superiority in providing a homogeneous culture environment compared to traditional culturing methods. However, there is limited understanding of the flow fields in these bioreactors, and optimum culture conditions are yet to be determined. This article presents the experimental characterization of the flow field within a spinner flask at varying speeds (10 RPM to 80 RPM) and impeller positions. An optical, non-invasive measurement technique, Particle Image Velocimetry (PIV), was employed to illustrate the fluid flow and calculate the stresses and vorticity associated with the flow within the flask. The largest recirculation structure was observed in the meridional plane at the highest impeller position while the highest shear stress region was observed at the base of the spinner flask. The study provides an overview of the fluid structure within the spinner flask in the meridional and azimuthal planes. Furthermore, the results presented in this study give an accurate quantification of the range of stresses for the given impeller speeds. These results provide estimates of the biomechanical properties within the type of spinner flask used in many published cell studies.

Endothelial biocompatibility and accumulation of SPION under flow conditions

Magnetic targeting is considered a promising method to accumulate the nanoparticles at the sites of atherosclerotic lesions, but little is known about the biological effects of magnetic nanoparticles on the vascular wall. Here, we investigated endothelial cell growth and vitality upon treatment with SPION (0–60 mg/mL) using two complementing methods: real-time cell analysis and live-cell microscopy. Moreover, the uptake of circulating superparamagnetic iron oxide nanoparticles (SPIONs) was assessed in an in vitro model of arterial bifurcations. At the tested concentrations, SPIONs were well tolerated and had no major influence on endothelial cell growth. Our results further showed a uniform distribution of endothelial SPION uptake independent of channel geometry or hemodynamic conditions: In the absence of magnetic force, no increase in accumulation of SPIONs at non-uniform shear stress region at the outer walls of bifurcation was observed. Application of external magnet allowed enhanced accumulation of SPIONs at the regions of non-uniform shear stress. Increased uptake of SPIONs at non-uniform shear stress region was well tolerated by endothelial cells (ECs) and did not affect endothelial cell viability or attachment. These findings indicate that magnetic targeting can constitute a promising and safe technique for the delivery of imaging and therapeutic nanoparticles to atherosclerotic lesions.

Flume Experiments to Constrain Bedload Adaptation Length

Spatially variable channel geometry in natural rivers produces nonuniform flow and spatial gradients in the shear stress field. The travel distance required for the flow to acquire the capacity bedload concentration and attain a new equilibrium bedload transport rate upon encountering a region of higher or lower shear stress is defined as the bedload adaptation length (Lb). Estimates of Lb are used by some numerical morphodynamic models to account for nonequilibrium bedload transport in the computation of local transport rates. However, current methods for estimating this parameter are uncertain and often crude. The authors therefore conducted experiments designed to measure Lb for a uniform sediment mixture in a laboratory flume. Instantaneous bedload transport rates were determined by counting passing sediment particles on digital imagery collected at variable distances downstream from a zero-transport boundary in a small flume. The flume was operated at three bed slopes in order to assess Lb over a range of hydraulic conditions. Bedload adaptation length was found to be about 30±8 particle diameters at a relatively low excess dimensionless shear stress (θ−θc=0.018, where θ is the dimensionless shear stress and θc=0.0436 is the critical dimensionless shear stress) and about 100±30 particle diameters at a moderate level of excess dimensionless shear stress (θ−θc=0.032). The experiments failed to resolve Lb at higher shear stresses. These results support physically based models that cast Lb as an increasing function of excess shear stress. They also suggest that Lb may be small relative to the resolution of the numerical mesh used in many modeling applications. In such cases, model performance may be insensitive to the choice of any arbitrary small value of Lb.

A numerical study on the effect of hematocrit on hemodynamic characteristics in arteriovenous graft

Stenosis at an arteriovenous graft is related with the critical ranges of hemodynamic characteristics. Hematocrit has a significant effect on the blood viscosity. During hemodialysis, hematocrit is changed by the dialysis machine. The effect of hematocrit on hemodynamic characteristics is investigated by numerical study. A multiphase non-Newtonian blood model was used to analyze the changes of hematocrit. The hematocrit of blood flows at injection needle changed 40%, 50%, and 60%. As a result, the blood viscosity increased by about 6% point. Also, the high wall shear stress region (over 3 Pa) increased about 6% point when the hematocrit at the vein anastomosis increased by about 2% point. When the hematocrit increased by 4% at the vein anastomosis, an extremely high wall shear stress region (over 7.5 Pa) increased by 3 times. Thus, the variation of hematocrit should be predicted using a multiphase blood model to avoid the critical range of wall shear stress when hematocrit changes regionally.

Effect of hinge gap width of a St. Jude medical bileaflet mechanical heart valve on blood damage potential--an in vitro micro particle image velocimetry study.

The hinge regions of the bileaflet mechanical heart valve (BMHV) can cause blood element damage due to nonphysiological shear stress levels and regions of flow stasis. Recently, a micro particle image velocimetry (μPIV) system was developed to study whole flow fields within BMHV hinge regions with enhanced spatial resolution under steady leakage flow conditions. However, global velocity maps under pulsatile conditions are still necessary to fully understand the blood damage potential of these valves. The current study hypothesized that the hinge gap width will affect flow fields in the hinge region. Accordingly, the blood damage potential of three St. Jude Medical (SJM) BMHVs with different hinge gap widths was investigated under pulsatile flow conditions, using a μPIV system. The results demonstrated that the hinge gap width had a significant influence during the leakage flow phase in terms of washout and shear stress characteristics. During the leakage flow, the largest hinge gap generated the highest Reynolds shear stress (RSS) magnitudes (~1000 N/m²) among the three valves at the ventricular side of the hinge. At this location, all three valves indicated viscous shear stresses (VSS) greater than 30 N/m². The smallest hinge gap exhibited the lowest level of shear stress values, but had the poorest washout flow characteristics among the three valves, demonstrating propensity for flow stasis and associated activated platelet accumulation potential. The results from this study indicate that the hinge is a critical component of the BMHV design, which needs to be optimized to find the appropriate balance between reduction in fluid shear stresses and enhanced washout during leakage flow, to ensure minimal thrombotic complications.

Haemodynamical stress in mouse aortic arch with atherosclerotic plaques: Preliminary study of plaque progression

Atherosclerotic plaques develop at particular sites in the arterial tree, and this regional localisation depends largely on haemodynamic parameters (such as wall shear stress; WSS) as described in the literature. Plaque rupture can result in heart attack or stroke and hence understanding the development and vulnerability of atherosclerotic plaques is critically important. The purpose of this study is to characterise the haemodynamics of blood flow in the mouse aortic arch using numerical modelling. The geometries are digitalised from synchrotron imaging and realistic pulsatile blood flow is considered under rigid wall assumptions. Two cases are considered; arteries with and without plaque. Mice that are fed under fat diet present plaques in the aortic arch whose size is dependent on the number of weeks under the diet. The plaque distribution in the region is however relatively constant through the different samples. This result underlines the influence of the geometry and consequently of the wall shear stresses for plaque formation with plaques growing in region of relative low shear stresses. A discussion of the flow field in real geometry in the presence and absence of plaques is conducted. The presence of plaques was shown to alter the blood flow and hence WSS distribution, with regions of localised high WSS, mainly on the wall of the brachiocephalic artery where luminal narrowing is most pronounced. In addition, arch plaques are shown to induce recirculation in the blood flow, a phenomenon with potential influence on the progression of the plaques. The oscillatory shear index and the relative residence time have been calculated on the geometry with plaques to show the presence of this recirculation in the arch, an approach that may be useful for future studies on plaque progression.

Role of Local Flow Conditions in River Biofilm Colonization and Early Growth

AbstractDirect numerical simulations of a turbulent boundary layer flow over a bed of hemispheres of height h are performed using an immersed boundary method for comparison with river biofilm growth experiments performed in a hydraulic flume. Flow statistics above the substrates are shown to be in agreement with measurements performed by laser Doppler velocimetry and particle image velocimetry in the experiments. Numerical simulations give access to flow components inside the roughness sublayer, and biofilm colonization patterns found in the experiments are shown to be associated with low shear stress regions on the hemisphere surface. Two bed configurations, namely staggered and aligned configurations, lead to different colonization patterns because of differences in the local flow topology. Dependence with the Reynolds number of the biofilm distribution and accrual 7 days after inoculum is shown to be associated to local flow topology change and shear stress intensity. In particular, the shear stress τ on the surface of the hemispheres is found to scale as , where Ret = u*h/ν, with u* as the log law friction velocity and ν as the fluid kinematic viscosity. This scaling is due to the development of boundary layers along the hemisphere surface. Associated with a critical shear stress for colonization and early growth, it explains the increasing delay in biomass accrual for increasing flow velocities in the experiments. Copyright © 2014 John Wiley & Sons, Ltd.

Microvascular Endothelial Cells Migrate Upstream and Align Against the Shear Stress Field Created by Impinging Flow

At present, little is known about how endothelial cells respond to spatial variations in fluid shear stress such as those that occur locally during embryonic development, at heart valve leaflets, and at sites of aneurysm formation. We built an impinging flow device that exposes endothelial cells to gradients of shear stress. Using this device, we investigated the response of microvascular endothelial cells to shear-stress gradients that ranged from 0 to a peak shear stress of 9–210 dyn/cm2. We observe that at high confluency, these cells migrate against the direction of fluid flow and concentrate in the region of maximum wall shear stress, whereas low-density microvascular endothelial cells that lack cell-cell contacts migrate in the flow direction. In addition, the cells align parallel to the flow at low wall shear stresses but orient perpendicularly to the flow direction above a critical threshold in local wall shear stress. Our observations suggest that endothelial cells are exquisitely sensitive to both magnitude and spatial gradients in wall shear stress. The impinging flow device provides a, to our knowledge, novel means to study endothelial cell migration and polarization in response to gradients in physical forces such as wall shear stress.

Analysis of early thrombus dynamics in a humanized mouse laser injury model

Platelet aggregation and thrombus formation at the site of injury is a dynamic process that involves the continuous addition of new platelets as well as thrombus rupture. In the early stages of hemostasis (within minutes after vessel injury) this process can be visualized by transfusing fluorescently labeled human platelets and observing their deposition and detachment. These two counterbalancing events help the developing thrombus reach a steady-state morphology, where it is large enough to cover the injured vessel surface but not too large to form a severe thrombotic occlusion. In this study, the spatial and temporal aspects of early stage thrombus dynamics which result from laser-induced injury on arterioles of cremaster muscle in the humanized mouse were visualized using fluorescent microscopy. It was found that rolling platelets show preference for the upstream region while tethering/detaching platelets were primarily found downstream. It was also determined that the platelet deposition rate is relatively steady, whereas the effective thrombus coverage area does not increase at a constant rate. By introducing a new method to graphically represent the real time in vivo physiological shear stress environment, we conclude that the thrombus continuously changes shape by regional growth and decay, and neither dominates in the high shear stress region.

Inhibition of MicroRNA-92a Prevents Endothelial Dysfunction and Atherosclerosis in Mice

MicroRNAs (miRNAs) are small noncoding RNAs that regulate protein expression at post-transcriptional level. We hypothesized that a specific pool of endothelial miRNAs could be selectively regulated by flow conditions and inflammatory signals, and as such be involved in the development of atherosclerosis. To identify miRNAs, called atheromiRs, which are selectively regulated by shear stress and oxidized low-density lipoproteins (oxLDL), and to determine their role in atherogenesis. Large-scale miRNA profiling in HUVECs identified miR-92a as an atheromiR candidate, whose expression is preferentially upregulated by the combination of low shear stress (SS) and atherogenic oxLDL. Ex vivo analysis of atheroprone and atheroprotected areas of mouse arteries and human atherosclerotic plaques demonstrated the preferential expression of miR-92a in atheroprone low SS regions. In Ldlr(-/-) mice, miR-92a expression was markedly enhanced by hypercholesterolemia, in particular in atheroprone areas of the aorta. Assessment of endothelial inflammation in gain- and loss-of-function experiments targeting miR-92a expression revealed that miR-92a regulated endothelial cell activation by oxLDL, more specifically under low SS conditions, which was associated with modulation of Kruppel-like factor 2 (KLF2), Kruppel-like factor 4 (KLF4), and suppressor of cytokine signaling 5. miR-92a expression was regulated by signal transducer and activator of transcription 3 in SS- and oxLDL-dependent manner. Furthermore, specific in vivo blockade of miR-92a expression in Ldlr(-/-) mice reduced endothelial inflammation and altered the development of atherosclerosis, decreasing plaque size and promoting a more stable lesion phenotype. Upregulation of miR-92a by oxLDL in atheroprone areas promotes endothelial activation and the development of atherosclerotic lesions. Therefore, miR-92a antagomir seems as a new atheroprotective therapeutic strategy.

3D reconstruction techniques of human coronary bifurcations for shear stress computations

BackgroundHeterogeneity in plaque composition in human coronary artery bifurcations is associated with blood flow induced shear stress. Shear stress is generally determined by combing 3D lumen data and computational fluid dynamics (CFD). We investigated two new procedures to generate 3D lumen reconstructions of coronary artery bifurcations for shear stress computations. MethodsWe imaged 10 patients with multislice computer tomography (MSCT) and intravascular ultrasound (IVUS). The 3D reconstruction of the main branch was based on the fusion of MSCT and IVUS. The proximal part of side branch was reconstructed using IVUS data or MSCT data, resulting in two different reconstructions of the bifurcation region. The distal part of the side branch was based on MSCT data alone. The reconstructed lumen was combined with CFD to determine the shear stress. Low and high shear stress regions were defined and shear stress patterns in the bifurcation regions were investigated. ResultsThe 3D coronary bifurcations were successfully generated with both reconstruction procedures. The geometrical features of the bifurcation region for the two reconstruction procedures did not reveal appreciable differences. The shear stress maps showed a qualitative agreement, and the low and high shear stress regions were similar in size and average shear stress values were identical. The low and high shear stress regions showed an overlap of approximately 75%. ConclusionReconstruction of the side branch with MSCT data alone is an adequate technique to study shear stress and wall thickness in the bifurcation region. The reconstruction procedure can be applied to further investigate the effect of shear stress on atherosclerosis in coronary bifurcations.

Mean and turbulent flow structure during the amalgamation process in fluvial bed forms

[1] Fluvial channels present bed forms such as dunes and ripples that alter instantaneous hydrodynamics parameters such as flow velocities, water surface profiles, bed shear stresses, and Reynolds stresses and create turbulent coherent structures that are significantly different from those presented in flat bed conditions. It is known that LES-based models are more suitable than RANS models to reproduce the complex hydrodynamics around bed forms. Herein, a LES model is applied to describe the mean and turbulent flow structure under superimposed bed forms. Three cases were simulated: RUN I (train of ripples), RUN II (superimposed bed forms), and RUN III (amalgamated bed forms). The LES modeling was performed using a free surface condition to allow the model to develop undulations and boils on the water surface caused by effect of the bed forms. Some important conclusions from this study are: the division of high and low shear stresses on the stoss side of the dune, the progression of the flow field topology from RUN I and RUN III, and the type of turbulent coherent structures found in each stage. The region of high shear stresses was related to turbulence production, in which the streamwise velocity fluctuations (where strips structures are related to streaks) were associated to the modification of the bed morphology. The turbulence Horseshoes Vortices (THV) were more frequent in RUN I than in the other two cases (where streamwise rolls were more frequent). Finally, the frequency of the bursting events increased from RUN I to RUN II and decreased from RUN II to RUN III. Implications of detailed hydrodynamics into bed forms processes are also presented and discussed.