High Shear Stress Regions Research Articles

Turbulent structures in planar gravity currents and their influence on the flow dynamics

Direct numerical simulations (DNS) of planar gravity current in the Boussinesq limit have been conducted with the objective of identifying, visualizing, and describing turbulent structures and their influence on the flow dynamics. The simulations are performed for Reynolds numbers of Re = 8950 and Re = 15,000 with 31‐ and 131‐million grid point resolutions, respectively. This range of Reynolds numbers ensures fully developed turbulent gravity currents, which have never been simulated before using DNS. The flow develops zones with different turbulence characteristics, which eventually interact with each other. The near‐wall bottom flow resembles boundary layer flow with several hairpin‐like vortices oriented in the direction of the flow and preferential patterns of low‐ and high‐speed streaks. The separation between low‐speed streaks at the front scales with the lobe size, which is about 200 wall units for Re = 15,000. Upstream of the front, the separation between low‐speed streaks scales with the well‐accepted value of 100 wall units for turbulent boundary layers. These patterns have associated regions of low and high bottom shear stresses with implications for sediment erosion and bed load transport. Most of the erosive power of the flow is found in the gravity current front. The interface between heavy and light fluids rolls up by baroclinic generation of Kelvin‐Helmholtz vortices, which undergo sudden breakup and decay to small‐scale turbulence. The effect of turbulence and three‐dimensionality on the flow dynamics is addressed by comparing two‐ and three‐dimensional simulations. Three‐dimensional simulations present active mechanisms that undermine the strong flow coherence, comparing well with experimental observations.

Analysis of micromixers to reduce biofouling on reverse‐osmosis membranes

AbstractFeatures (micromixers) that promote chaotic mixing were fabricated on reverse‐osmosis membrane surfaces and evaluated using computational models and laboratory experiments to determine their effectiveness in reducing biofouling. Computational fluid dynamics models of membrane feed channels were developed using different patterns of micromixers on the membrane surface. The shear‐stress distribution along the membrane surface was simulated for steady flows along the different micromixer configurations. In addition, the hypothetical mass transfer of a tracer from the membrane surface was used as a metric to compare the amount of scouring and mixing in configurations with and without micromixers. Epoxy micromixers were printed directly onto membrane surfaces, and different patterns were evaluated experimentally. Fluorescence hyperspectral imaging results showed that regions of simulated high shear stress on the membrane corresponded to regions of lower bacterial growth in the experiments, while regions of simulated low shear stress corresponded to regions of higher bacterial growth. In addition, the presence of the micromixers appeared to reduce the overall biofouling concentration in one series of experiments, but the results were inconclusive in another series of experiments. These results indicate that while the enhancement of mixing and shear stress via micromixers may delay or mitigate the onset of localized membrane fouling from biofilms or other contaminants, the impact of micromixers on the overall performance of reverse‐osmosis membranes needs further investigation. © 2008 American Institute of Chemical Engineers Environ Prog, 2008

Non‐Newtonian flow in branched pipes and artery models

AbstractThis work concerns finite element analysis for a class of generalized Newtonian flows through branching pipe or tube junctions in industrial applications or blood flow through arterial branches, prosthetic tube implants or in biomedical engineering devices. A two‐step morphing transformation is developed and implemented to map a generic coplanar 3D branching flow configuration to general out‐of‐plane deformed geometry including local constriction/dilation ‘defects’ suitable for modeling specific arterial flow and industrial flow applications. Unstructured 3D meshes may be generated in the reference configuration and morphed to the application configuration. The resulting meshes may be tangled and exhibit mesh quality degradation. Algorithms integrating mesh generation, morphing, untangling and cell shape control strategies are presented.Phenomenological studies are conducted with the Powell–Eyring class as a representative model to determine local details of shear‐thinning flow behavior as well as the nature and location of high shear stress regions on artery branch walls. The supporting simulations are carried out using a parallel finite element scheme with domain decomposition by a sectional partitioning that takes advantage of the medial curve shape and specialized ILU preconditioning. Copyright © 2008 John Wiley & Sons, Ltd.

Atheroprotective Signaling Mechanisms Activated by Steady Laminar Flow in Endothelial Cells

Many studies of the human vascular tree have shown that atherosclerosis develops at an early age, with areas of the posterior aorta and branch sites showing the earliest fatty streaks. Closer analysis of the branch points showed that the flow dividers, regions of high shear stress with laminar flow, were relatively protected from fatty streak formation. In contrast, there was a predilection for atherosclerosis at the curvatures and lateral walls of branch points.1 These are regions where disturbed flow occurs; importantly, when the time-averaged shear stress is low, the spatial shear stress gradient is very high.1,2 Both monocyte adhesion and endothelial cell (EC) apoptosis also are highest in these areas.3–6 Recently, Cheng et al7 developed a vascular occluder that induced 3 different regions of altered flow: low, high, and low with oscillatory flow. They observed that lesions always developed in regions of low compared with high shear stress. However, the low shear stress was more likely to develop larger lesions with more lipids, expression of inflammatory mediators, and matrix metalloprotease activity than oscillatory flow. When viewed in concert with a study that showed that regions of low shear stress already had a greater number of inflammatory cells resident in the artery wall as a result of increased trafficking,8 it is clear that low flow is proinflammatory and atherogenic. More sophisticated 3-dimensional analysis of flow patterns in human carotid bifurcations has identified prototypic arterial waveforms, “atheroprone” and “atheroprotective,” representing the wall shear stresses in the carotid sinus, which is susceptible to atherosclerotic lesion development, and the distal internal carotid artery, which is resistant.9 Characteristics of the atheroprone waveform include a high oscillatory shear index (≈0.45) and low time-averaged shear stress amplitude (≈0 dyne/cm2). Thus, both the amplitude and flow pattern are key determinants of …

Formation of Saccular Cerebral Aneurysms May Require Proliferation of the Arterial Wall: Computational Investigation

We have performed numerical simulations to examine saccular cerebral aneurysm formation at the outer curve of a bent artery. A U-shaped arterial geometry with torsion, which was modeled on part of the human internal carotid artery, has been employed. A new numerical model was proposed to take into account proliferation as well as degradation of the arterial wall. Proliferation of the arterial wall was modeled by surface area expansion in high wall shear stress region. Based on wall shear stress distribution on the artery, we have investigated aneurysm formation for the following three conditions: (a) strength degradation of the wall, (b) proliferation of the wall, and (c) both strength degradation and proliferation of the wall. A saccular aneurysm shape was not observed when considering only arterial wall degradation up to 90%. However, the saccular shape formed when proliferation of the arterial wall was also taken into consideration. The resultant shape was consistent with clinical observations. Our findings have suggested that a saccular aneurysm may not be formed by degradation of the arterial wall alone, but also require its proliferation.

Computational modeling of coupled blood-wall mass transport of LDL: effects of local wall shear stress

The work herein represents a novel approach for the modeling of low-density lipoprotein (LDL) transport from the artery lumen into the arterial wall, taking into account the effects of local wall shear stress (WSS) on the endothelial cell layer and its pathways of volume and solute flux. We have simulated LDL transport in an axisymmetric representation of a stenosed coronary artery, where the endothelium is represented by a three-pore model that takes into account the contributions of the vesicular pathway, normal junctions, and leaky junctions also employing the local WSS to yield the overall volume and solute flux. The fraction of leaky junctions is calculated as a function of the local WSS based on published experimental data and is used in conjunction with the pore theory to determine the transport properties of this pathway. We have found elevated levels of solute flux at low shear stress regions because of the presence of a larger number of leaky junctions compared with high shear stress regions. Accordingly, we were able to observe high LDL concentrations in the arterial wall in these low shear stress regions despite increased filtration velocity, indicating that the increase in filtration velocity is not sufficient for the convective removal of LDL.

Abstract 626: Shear Stress Regulates Angiotensin Type One Receptor Expression in Endothelial Cells

Several studies have shown that angiotensin II tends to favour atherosclerotic lesion formation and progression, acting mainly through angiotensin type one receptors (AT1R). Conversely, shear stress (SS) confers atheroprotective effects in the vasculature. However, whether SS regulates endothelial AT1R expression remains unknown. Using en face staining of C57BL/6 mouse aortas, we found a pronounced expression of AT1R in the inner regions of the aortic arch and at collateral artery branch points, known to have low flow and to be atheroprone, compared with neighboring high SS regions. Furthermore, staining of vessels from hypercholesterolemic LDLR−/−mice demonstrated a clear expression of AT1R only in endothelium overlying plaques. Using HUVECs exposed to a laminar SS of 15dynes/cm2, we observed a biphasic decrease in AT1R protein expression characterized by a first reduction at 1 hr (31+4% of static control, p<0.001), partial recovery at 3 hr (65+9%), and a second more prolonged decrease at 6, 12 and 24 hrs (48+9%, 36+9%, 33+5% of static control, respectively)(p<0.001). AT2R levels remained unchanged in these conditions. Shear-induced downregulation of AT1R was abolished by treatment with L-NAME (10–7M), suggesting that NO regulates AT1R in sheared endothelial cells. In accordance, stimulating static HUVECs with a NO donor for 1 hour decreased AT1R protein expression. The acute loss of AT1R at 1 hour SS suggested a rapid endocytosis and degradation of the protein. Indeed, immunocytochemical staining demonstrated a clear colocalisation of AT1R with caveolin-1 in HUVECs submitted to 5 mins SS, similar to that observed in angII-stimulated cells. Cholesterol depletion with methyl-beta-cyclodextrin prevented AT1R-caveolin colocalisation. No colocalisation of AT1R with clathrin was observed. In comparison, AT2R did not undergo endocytosis under any conditions tested. Finally, RT-PCR revealed a significant (p<0.001) decrease of AT1R mRNA in HUVECs exposed to SS during 6 (4+1% of static control), 12 (4+1%) and 24 hours (15+4%), suggesting a transcriptional downregulation of AT1R at length. Hence, our results demonstrate that SS may convey some of its atheroprotective effects through degradation and downregulation of AT1R in endothelial cells.

Disbond effects on bonded boron/epoxy composite repair to aluminium plates

The technique of repairing or reinforcing cracked metallic structures has been widely used to extend their service life. In this study, the finite element method is used to analyze the behaviour of a crack repaired by a boron/epoxy patch by computing the stress intensity factor at the crack tip in mode I and mixed mode. The effects of the mechanical and geometrical properties of the patch as well as the harmful disbond effect on the fracture parameters are highlighted. The results show that the stress intensity factor is affected by the negative disbond effect. A disbond around the crack provokes its growth causing a shorter life of a cracked plate, but a full-width disbond has the worst effect. The stress intensity factor in the presence of disbond is appreciably reduced when the patch thickness approaches that of the plate ( e R / e P → 1). Beyond this ratio, a linear relation links them. It is noted that the disbond develops in the high shear stress regions around the crack in the plate. However, the disbonding in the patching should lead to a redistribution of the residual stress. The change in the residual stress depends on the type, applied stress, crack location and relative stiffness between the patch and the plate.

Flux enhancement and cake formation in air-sparged cross-flow microfiltration

Abstract The flux enhancement in cross-flow microfiltration of submicron particles by sparged air-bubble is studied. The effects of operating conditions, such as air-bubble velocity, suspension velocity and filtration pressure, on the cake properties and filtration flux are discussed thoroughly. The results show that the pseudo-steady filtration flux increases as the air-bubble velocity and filtration pressure increase. The sparged air-bubble can significantly improve filtration flux, but the flux enhancement is more remarkable in the lower air-bubble velocity region. A gas–liquid two-phase flow model is adopted for estimating the shear stress acting on the membrane surface under various operating conditions. The cake mass can be significantly reduced by increasing the shear stress acting on the membrane surface. However, the SEM analysis illustrates that the particle packing structure becomes more compact as the air-bubble velocity increases. This results in a slightly higher average specific cake filtration resistance under higher air-bubble velocity. Consequently, a minimum flux occurs at the critical shear stress, e.g., τ w = 1.1 N/m 2 in this study, when these effects are both taken into consideration. As the shear stress increases by increasing the suspension or gas-bubble velocity, the filtration flux decreases in the low shear stress region but, on the contrary, quickly increases in the high shear stress region. Furthermore, a force balance model is derived for understanding the particle deposition on the membrane surface. The relationship among filtration flux, shear stress and overall filtration resistance is obtained and verified by experimental data.

Plaque Rupture in the Carotid Artery Is Localized at the High Shear Stress Region

Cerebrovascular events are related to atherosclerotic disease in the carotid arteries and are frequently caused by rupture of a vulnerable plaque. These ruptures are often observed at the upstream region of the plaque, where the wall shear stress (WSS) is considered to be highest. High WSS is known for its influence on many processes affecting tissue regression. Until now, there have been no serial studies showing the relationship between plaque rupture and WSS. Summary of Case- We investigated a serial MRI data set of a 67-year-old woman with a plaque in the carotid artery at baseline and an ulcer at 10-month follow up. The lumen, plaque components (lipid/necrotic core, intraplaque hemorrhage), and ulcer were segmented and the lumen contours at baseline were used for WSS calculation. Correlation of the change in plaque composition with the WSS at baseline showed that the ulcer was generated exclusively at the high WSS location. In this serial MRI study, we found plaque ulceration at the high WSS location of a protruding plaque in the carotid artery. Our data suggest that high WSS influences plaque vulnerability and therefore may become a potential parameter for predicting future events.

Plaque Morphology in High Versus Low Shear Stress Regions of Human Carotid Stenosis

Plaque Morphology in High Versus Low Shear Stress Regions of Human Carotid Stenosis

Thrombin formation in vitro in response to shear-induced activation of platelets

Introduction Thromboembolic events caused by implanted vascular devices present serious medical challenges. In particular bileaflet mechanical heart valves (MHVs) are prone to thrombus formation in the hinge region due to a combination of high shear stress and stagnation regions. Most studies of shear-induced platelet activation and aggregation have been performed using viscometers, parallel plate flow, and other non-physiologic in vitro configurations. The present study investigated these events in a physiogically relevant environment in which thrombin formation in response to shear stress activation of platelets plays a more predominant role. Materials and methods Anticoagulated (citrated) human blood was placed in a steady flow loop containing a 400 μm round orifice or various MHVs in the leakage position. Simultaneous blood recalcification enhanced the thrombus forming potential of the blood. Aggrastat and AN51 were used to block binding to the platelet GPIIb/IIIa and GPIb receptors, respectively, and aspirin was used to block thromboxane production. Thrombin generation was measured indirectly by the thrombin-antithrombin III assay. Results and conclusions Aggrastat, AN51, and aspirin all suppressed thrombin formation. Furthermore, histological results suggested important roles for vWF and fibrinogen in a two-step model of thrombus formation. Thus, thrombin is reproducibly formed in this in vitro system, a process that can be suppressed by blocking platelet activation. This system has the potential to investigate mechanisms and interventions for medical devices that contact with blood under varying shear stress conditions.

Optimization of a Miniature Maglev Ventricular Assist Device for Pediatric Circulatory Support

A miniature Maglev blood pump based on magnetically levitated bearingless technology is being developed and optimized for pediatric patients. We performed impeller optimization by characterizing the hemodynamic and hemocompatibility performances using a combined computational and experimental approach. Both three-dimensional flow features and hemolytic characteristics were analyzed using computational fluid dynamics (CFD) modeling. Hydraulic pump performances and hemolysis levels of three different impeller designs were quantified and compared numerically. Two pump prototypes were constructed from the two impeller designs and experimentally tested. Comparison of CFD predictions with experimental results showed good agreement. The optimized impeller remarkably increased overall pump hydraulic output by more than 50% over the initial design. The CFD simulation demonstrated a clean and streamlined flow field in the main flow path. The numerical results by hemolysis model indicated no significant high shear stress regions. Through the use of CFD analysis and bench-top testing, the small pediatric pump was optimized to achieve a low level of blood damage and improved hydraulic performance and efficiency. The Maglev pediatric blood pump is innovative due to its small size, very low priming volume, excellent hemodynamic and hematologic performance, and elimination of seal-related and bearing-related failures due to adoption of magnetically levitated bearingless motor technology, making it ideal for pediatric applications.

A Statistical Method to Evaluate the Effect of Shear Stress on Endothelial Gene Expression in vivo

Objective To determine the effect of hemodynamic shear stress on endothelial gene expression in vivo. Methods Computational fluid dynamics (CFD) simulations were performed in three realistic models of the porcine abdominal aorta and its iliac artery branches. Using these calculations, a shear stress probability map in the iliac arteries was generated. This map showed the likelihood that a particular location would experience high, medium, or low time average shear stress. Based on this map, three anatomical regions likely to experience high, medium, and low shear stress levels were identified. The median time-average shear stress in these regions were 32.1, 15.2, and 9.7 dyn/cm2, respectively. Subsequently, endothelial cell RNA was collected from these regions in five additional porcine iliac arteries. Gene expression of select genes was measured in these samples using quantitative real time PCR. Results for the high and low shear regions are reported relative to the medium shear region. Results Expression of the genes tubulin, beta-catenin, c-jun, MCP-1, and VCAM was elevated in both the low and high shear stress regions relative to the medium shear region. ICAM expression was downregulated in both the low and high shear regions relative to the medium shear region. These effects were significant in nearly all cases. Conclusions This statistical method permits evaluation of phenotypic differences that arise due to hemodynamic forces in in vivo specimens. Arterial regions subjected to distinct levels of time average shear stress exhibit differences in endothelial gene expression. Supported by NIH grant H50442.

Computational and Experimental Evaluation of the Fluid Dynamics and Hemocompatibility of the CentriMag Blood Pump

The CentriMag centrifugal blood pump is a newly developed ventricular assist device based on magnetically levitated bearingless rotor technology. A combined computational and experimental study was conducted to characterize the hemodynamic and hemocompatibility performances of this novel blood pump. Both the three-dimensional flow features of the CentriMag blood pump and its hemolytic characteristics were analyzed using computational fluid dynamics (CFD)-based modeling. The hydraulic pump performance and hemolysis level were quantified experimentally. The CFD simulation demonstrated a clean and streamlined flow field in the main components of the CentriMag blood pump. The predicted results by hemolysis model indicated no significant high shear stress regions in the pump. A comparison of CFD predictions and experimental results showed good agreements. The relatively large gap passages (1.5 mm) between the outer rotor walls and the lower housing cavity walls provide a very good surface washing through a secondary flow path while the shear stresses in the secondary flow paths are reduced, resulting in a low rate of hemolysis ([Normalized Index of Hemolysis] NIH = 0.0029 +/- 0.006) without a decrease of the pump's hydrodynamic performance (pressure head: 352 mm Hg at a flow rate of 5.0 L/min and a rotational speed of 4,000 rpm).

Shear stress affects the intracellular distribution of eNOS: direct demonstration by a novel in vivo technique

The focal location of atherosclerosis in the vascular tree is correlated with local variations in shear stress. We developed a method to induce defined variations in shear stress in a straight vessel segment of a mouse. To this end, a cylinder with a tapered lumen was placed around the carotid artery, inducing a high shear stress field. Concomitantly, regions of low shear stress and oscillatory shear stress were created upstream and down-stream of the device, respectively. This device was used in mice transgenic for an eNOS3GFP fusion gene. We observed a strong induction of endothelial nitric oxide synthase-green fluorescent protein (eNOS-GFP) mRNA expression in the high shear stress region compared with the other regions (P < .05). Quantification of eNOS-GFP fluorescence or of immunoreactivity to the Golgi complex or to platelet endothelial cell adhesion molecule 1 (PECAM-1) showed an increase in the high shear stress region (P < .05) compared with nontreated carotid arteries. Colocalization of eNOS-GFP with either the Golgi complex or PECAM-1 also responded to alterations of shear stress. In conclusion, we showed a direct response of mRNA and protein expression in vivo to induced variations of shear stress. This model provides the opportunity to study the relationship between shear stress alterations, gene expression, and atherosclerosis.

Multiscale Analysis of Surface Thrombosis In Vivo in a Left Ventricular Assist System

Thrombosis limits the success of ventricular assist devices as the demand for alternatives to heart transplants is increasing. This study mapped the occurrence of thrombosis in a left ventricular assist system (LVAS) to better understand the biologic response to these devices. Nine calves divided into two groups were implanted with LVAS for 28 to 30 days. One group was anticoagulated, whereas the second group received no long-term anticoagulation. The blood-contacting poly(urethane urea) surfaces of blood sacs in the LVAS were examined for macroscopic thrombi upon retrieval. The sac was partitioned into eight sections and imaged for thrombi by scanning electron microscopy. No difference in thrombosis was observed macroscopically between the groups. Anticoagulation appeared to result in reduction of platelet-like structures, but the presence of fibrin-like structures remained similar between groups. Regional differences correlating with high and low shear stress regions were observed. At the macroscale, fewer thrombi were recorded in the high shear stress ports. At the microscale, features resembling fibrin were observed primarily in the ports and platelet-like features were common in lower shear stress regions. These variations in thrombosis with anticoagulation and location are likely due to varied fluid dynamics within the LVAS blood sac.

PERFORMANCE CHARACTERIZATION AND CFD ANALYSIS OF AN AXIAL BLOOD PUMP

This paper describes the performance characterization of an axial blood pump that is developed in our laboratory. Using computational fluid dynamics (CFD), regions of flow separation and high shear stress were identified since they are of concern in the development of cardiac assist devices. CFD is an efficient and cost effective tool in assisting the designer to reduce the number of experimental trials needed. Preliminary CFD studies showed the existence of substantial backflow in the impeller passage. The impeller geometry was improved using CFD modeling. Regions of flow separation were eliminated while regions of scalar stress of up to 150 Pa were observed near to the impeller tip. The final prototype can deliver a flow rate of 5 L/min at a pressure head of 14 kPa when operating at a speed of 10,000 rpm. The model was fabricated using rapid prototyping techniques and performance characterization of the pump has demonstrated that the CFD prediction of the pump performance curve and the pressure developed along the impeller agrees reasonably well with experimental results.

Letter Regarding Article by Korshunov and Berk, “Strain-Dependent Vascular Remodeling: The ‘Glagov Phenomenon’ Is Genetically Determined”

To the Editor: Drs Korshunov and Berk1 described the modulation of vascular remodeling in response to a sudden decrease of shear stress induced by partial ligation of the carotid artery of different mouse strains. Although a similar study was previously conducted with similar mouse strains in which the carotid artery was totally occluded,2 Korshunov and Berk argued that their partial ligation model is more relevant to human carotid intima-media thickness and coronary atherosclerosis and concluded from their measurements that the Glagov phenomenon has a genetic background. To strengthen the relevance of their data, the authors compared their results—outward remodeling under low flow conditions—to our data in human atherosclerotic coronary arteries.3 Indeed, we found that in segments with lumen preservation because of outward vascular remodeling, atherosclerotic plaques were observed at relatively low shear stress regions.3 Because vascular remodeling is defined as an average over the cross …

PIV Measurements of Flow in a Centrifugal Blood Pump: Steady Flow

Magnetically suspended left ventricular assist devices have only one moving part, the impeller. The impeller has absolutely no contact with any of the fixed parts, thus greatly reducing the regions of stagnant or high shear stress that surround a mechanical or fluid bearing. Measurements of the mean flow patterns as well as viscous and turbulent stresses were made in a shaft-driven prototype of a magnetically suspended centrifugal blood pump at several constant flow rates (3-9 L/min) using particle image velocimetry (PIV). The chosen range of flow rates is representative of the range over which the pump may operate while implanted. Measurements on a three-dimensional measurement grid within several regions of the pump, including the inlet, blade passage, exit volute, and diffuser are reported. The measurements are used to identify regions of potential blood damage due to high shear stress and/or stagnation of the blood, both of which have been associated with blood damage within artificial heart valves and diaphragm-type pumps. Levels of turbulence intensity and Reynolds stresses that are comparable to those in artificial heart valves are reported. At the design flow rate (6 L/min), the flow is generally well behaved (no recirculation or stagnant flow) and stress levels are below levels that would be expected to contribute to hemolysis or thrombosis. The flow at both high (9 L/min) and low (3 L/min) flow rates introduces anomalies into the flow, such as recirculation, stagnation, and high stress regions. Levels of viscous and Reynolds shear stresses everywhere within the pump are below reported threshold values for damage to red cells over the entire range of flow rates investigated; however, at both high and low flow rate conditions, the flow field may promote activation of the clotting cascade due to regions of elevated shear stress adjacent to separated or stagnant flow.