High Shear Stress Values Research Articles

Experimental surface flow visualization and numerical investigation of flow structure around an oblong stockpile

Emission factors are largely used to quantify particle emissions from industrial open storage piles. These factors are based on the knowledge of velocity distribution and flow patterns over the stockpile surface which still requires further research. The aim of the present work is to investigate the airflow characteristics over a single typical oblong pile and in its near-ground surroundings for various wind flow directions. Wind tunnel experiments using an oil-film surface coating technique were carried out for near-wall flow visualization. Numerical simulation results, favorably compared to PIV measurements, were used to allow comparison analysis of flow features. For the stockpile oriented 90° to the wind main direction, typical topology of flow around wall-mounted obstacles were observed, notably a wake zone downstream the pile including two main counter-rotating vortices. Further analysis of numerical wall shear stress distribution and streamlines indicates that two complex three-dimensional vortical flow structures develop downstream the pile. For other incoming wind flow directions (30 and 60°), the flow characteristics over the storage pile greatly differ as a single helical main vortex develops from the pile’s crest. Corresponding high values of wall shear stress are noticed downstream the storage pile. For each configuration studied, downwash and upwash zones are induced by the vortical structures developed. This near-wall flow topology combined with areas of high friction levels may be linked to potential dust emission from the ground surface surrounding industrial stockpiles.

On the Effects of a Flexible Structure on Boundary Layer Stability and Transition

Delaying the onset of boundary layer transition has become a major research area in the last few years. This delay can be achieved by either active or passive control techniques. In the present paper, the effects of flexible or compliant structures on boundary layer stability and transition is studied. The Orr-Sommerfeld equation coupled to a beam equation representing the flexible structure is solved for a Blasius type boundary layer. A parametric study consisting of the beam thickness and material properties is carried out. In addition, the effect of fluid wall shear stress on boundary layer stability is analyzed. It is found that high density and high Young modulus materials behave like rigid structures and therefore do not alter the stability characteristic of the boundary layer. Whereas low density and low Young modulus materials are found to stabilize the boundary layer. High values of fluid wall shear stress are found to destabilize the boundary layer. Our results are in good agreement with those published in the literature.

Large Eddy Simulation of a Stenosed Artery Using a Femoral Artery Pulsatile Flow Profile

Computational fluid dynamics simulation of stenosed arteries allows the analysis of quantities including wall shear stress, velocity, and pressure; detailed in vivo measurement is difficult yet the analysis of the fluid dynamics related to stenosis is important in understanding the likely causes and ongoing effects on the integrity of the vessel. In this study, a three-dimensional Large Eddy Simulation is conducted of a 50% occluded vessel, with a typical femoral artery profile used as the transient inlet conditions. The fluid is assumed to be homogenous, Newtonian and incompressible and the walls are assumed rigid. The stenosis is axisymmetric, however the three-dimensional study allows for a flow field that is not axisymmetric and results show significant three-dimensionality. High values of wall shear stress and oscillatory values of wall shear stress (varying in both space time) are observed. The results of the study give insight into the time-varying flow structures for a mildly stenosed artery and indicate that three-dimensional simulations may be important to gain a complete understanding of the flow field.

Large eddy simulation of a stenosed artery

A large eddy simulation is conducted of a 50% stenosed artery. The inlet boundary condition is a physiologically accurate pulsatile flow, representative of that found in a femoral artery. The transient, three dimensional results show the non-axisymmetric nature of the flow field; high values of wall shear stress, oscillatory in both space and time are also observed. The identification of such fluid dynamic quantities within a stenosed vessel is important in understanding the likely causes and ongoing effects on the integrity of the vessel. References S. A. Berger and L-D. Jou. Flows in Stenotic Vessels. Annu. Rev. Fluid Mech. 32:347--382. 2000. doi:10.1146/annurev.fluid.32.1.347 M. Li, J. Beech-Brandt, L. John, P. Hoskins, W. Easson. Numerical Analysis of Pulsatile Blood Flow and Vessel Wall Mechanics in Different Degrees of Stenoses. J. Biomechanics 40:3715--3724. 2007. doi:10.1016/j.jbiomech.2007.06.023 S. S. Varghese and S. H. Frankel. Numerical Modeling of Pulsatile Turbulent Flow in Stenotic Vessels. J. Biomech. Eng. 125 (445--460). 2003. doi:10.1115/1.1589774 S. S. Varghese, S. H. Frankel and P. F. Fischer. Direct numerical simulation of stenotic flows. Part 2. Pulsatile flow. Journal of Fluid Mechanics., 582:281--318. 2007. doi:10.1017/S0022112007005836 B. Xiao and Y. Zhang. Numerical simulation of pulsatile turbulent flow in tapering stenosed arteries. Int. J. Numerical Meth. Heat Fluid Flow, 19(5):561--573. 2009. doi:10.1108/09615530910963526 B. Lieber and D. Giddens. Post-stenotic core flow behavior in pulsatile flow and its effects on wall shear stress. J. Biomech., 23(6):597--605. 1990. doi:10.1016/0021-9290(90)90052-5 M. Paul, Md. M. Molla and G. Roditi. Large eddy simulation of pulsatile blood flow. Medical Engineering and Physics, 31:153--159. 2009. doi:10.1016/j.medengphy.2008.04.014 W. W. Nichol and M. F. O'Rourke. McDonald's Blood Flow in Arteries: Theoretical, Experimental and Clinical Principles, Hodder Arnold, London, 5th edition. 2005. D. N. Ku. Blood flow in arteries. Annu. Rev. Fluid Mech. 29:399--434. 1997. doi:10.1146/annurev.fluid.29.1.399 D. Lilly. A Proposed Modification of the Germano Subgrid-Scale Closure Model. Physics of Fluids, 4:633--635. 1992. doi:10.1063/1.858280 B. Geurts. Elements of direct and large eddy simulation. R. T. Edwards, 2004.

Strength Effects in Micropillars of a Dispersion Strengthened Superalloy

AbstractThe present paper investigates the uniaxial compression behavior of highly alloyed, focused ion beam (FIB) manufactured micropillars, ranging from 200 up to 4000 nm in diameter. The material used was the Ni‐based oxide‐dispersion strengthened (ODS) alloy Inconel MA6000. Stress–strain curves show a change in slip behavior comparable to those observed in pure fcc metals. Contrary to pure Ni pillar experiments, high critical resolved shear stress (CRSS) values were found independent of pillar diameter. This suggests that the deformation behavior is primarily controlled by the internal obstacle spacing, overwhelming any pillar‐size‐dependent mechanisms such as dislocation source action or starvation.

Analysis of Flow Patterns in a Patient-Specific Aortic Dissection Model

Aortic dissection is the most common acute catastrophic event affecting the thoracic aorta. The majority of patients presenting with an uncomplicated type B dissection are treated medically, but 25% of these patients develop subsequent aneurysmal dilatation of the thoracic aorta. This study aimed at gaining more detailed knowledge of the flow phenomena associated with this condition. Morphological features and flow patterns in a dissected aortic segment of a presurgery type B dissection patient were analyzed based on computed tomography images acquired from the patient. Computational simulations of blood flow in the patient-specific model were performed by employing a correlation-based transitional version of Menter's hybrid k-epsilon/k-omega shear stress transport turbulence model implemented in ANSYS CFX 11. Our results show that the dissected aorta is dominated by locally highly disturbed, and possibly turbulent, flow with strong recirculation. A significant proportion (about 80%) of the aortic flow enters the false lumen, which may further increase the dilatation of the aorta. High values of wall shear stress have been found around the tear on the true lumen wall, perhaps increasing the likelihood of expanding the tear. Turbulence intensity in the tear region reaches a maximum of 70% at midsystolic deceleration phase. Incorporating the non-Newtonian behavior of blood into the same transitional flow model has yielded a slightly lower peak wall shear stress and higher maximum turbulence intensity without causing discernible changes to the distribution patterns. Comparisons between the laminar and turbulent flow simulations show a qualitatively similar distribution of wall shear stress but a significantly higher magnitude with the transitional turbulence model.

Rheological behavior of noncompatibilized and compatibilized PP/PET blends with SEBS‐g‐MA

AbstractThe rheological behaviors of noncompatibilized and compatibilized polypropylene/polyethylene terephthalate blends (80/20) in relation with their morphology were studied at two constant levels using maleic anhydride‐modified styrene‐ethylene‐butylene‐styrene polymer. By scanning electron microscopy of cryofractured surfaces, the morphology of the blends was examined after etching. The frequency sweep and step strain experiments were carried out for the blends. The frequency sweep results indicated that increasing the compatibilizer causes behavioral changes of the rheological properties, which could be related to the aggregation of the dispersed particles with rubbery shell. Also, the frequency sweep and step strain experiments in linear region, after cessation of simple steady shear flow with various preshear rates (higher shear stress values than G′p), were done on compatibilized blend. The results showed that the morphology characteristics, defined by the aggregation of the dispersed particles based on rheological experimental data, were destroyed and replaced by an alignment in the flow direction for present imposed shear rates. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Modeling of flow‐induced shear stress applied on 3D cellular scaffolds: Implications for vascular tissue engineering

Novel tissue-culture bioreactors employ flow-induced shear stress as a means of mechanical stimulation of cells. We developed a computational fluid dynamics model of the complex three-dimensional (3D) microstructure of a porous scaffold incubated in a direct perfusion bioreactor. Our model was designed to predict high shear-stress values within the physiological range of those naturally sensed by vascular cells (1-10 dyne/cm(2)), and will thereby provide suitable conditions for vascular tissue-engineering experiments. The model also accounts for cellular growth, which was designed as an added cell layer grown on all scaffold walls. Five model variants were designed, with geometric differences corresponding to cell-layer thicknesses of 0, 50, 75, 100, and 125 microm. Four inlet velocities (0.5, 1, 1.5, and 2 cm/s) were applied to each model. Wall shear-stress distribution and overall pressure drop calculations were then used to characterize the relation between flow rate, shear stress, cell-layer thickness, and pressure drop. The simulations showed that cellular growth within 3D scaffolds exposes cells to elevated shear stress, with considerably increasing average values in correlation to cell growth and inflow velocity. Our results provide in-depth analysis of the microdynamic environment of cells cultured within 3D environments, and thus provide advanced control over tissue development in vitro.

Failure of fibrous anisotropic materials under combined loading

Abstract A failure criterion for anisotropic materials under combined stress is developed and demonstrated. The generality and accuracy of the present theory are illustrated by examination through the use of material systems under various loading conditions. Calculated results are compared with the experimental data throughout four quadrants. It agrees with observation quite well at high values of shear stress, where the Tsai–Hill theory becomes too conservative. The present criterion is also compared with other criteria. The comparison shows that this criterion has a good agreement with the experimental data even when the shear stress component is greater than the shear strength.

Finite Element Analysis of Acrylic Denture Produced from Metal Stock Tray

Aims: To determine stress analysis exerted by denture on the edentulous alveolar bone in the premolar region represented by stone cast produced from metal stock tray using linear finite element method. Materials and methods: Metal perforated stock tray was used to produce stone cast that represent the denture base and the underlying bone using silicone impression material. The dimension of the cross section of the stone cast was measured using Dimax program, then the data collected was entering Excel program for making the figures. The dimension of the cross section of the stone cast entering finite element program and other new auxiliary program prepared specially for this study for finite element analysis. Results: After the data was analyzed, the results were drawn in figures using surfer 7 program. High compressive, tensile, deflection, shear and maximum principal stress values were observed below the point of force application and in the underlying structure with different values and locations according to the analysis in the mucous membrane and in the underlying bone in the X and Y axes. Conclusions: By using finite element analysis, the more stress concentration and deflection were found below the point of force application, in the mucous membrane, at the beginning of the cortical bone and

Transition Pathway Analysis of Homogeneous Dislocation Nucleation in a Perfect Silicon Crystal

AbstractTransition pathway sampling was carried out for homogeneous dislocation nucleation in perfect crystal Si. The sampling algorithm employed was Nudged Elastic Band method. Results obtained were compared with corresponding results for Cu. The stress and activation barrier ranges were found to be much higher for Si than those reported for Cu. The results also showed that while for lower values of resolved shear stress the dislocation embryo approaches that of a perfect dislocation, for higher resolved shear stress values the embryo is far from perfect. That is, the shear displacement of most particles is considerably less than the Burger’s vector. This investigation also demonstrated for the first time that Athermal shear stress for homogeneous dislocation nucleation in Si does not exist, as the crystal undergoes twinning at such high stresses.

Wedge Anchorage for CFRP Strips

For several decades wedge anchorages have been used with great success for the prestressing of steel strands in civil engineering structures. Due to the anisotropy of carbon fiber-reinforced plastics (CFRP) these wedges are not efficient with CFRP tendons. High lateral and shear stress values develop in the CFRP–wedge interface that lead to a premature failure of the CFRP. A comparison of different anchorage methods corroborates the outstanding advantages of wedge anchorages over other anchoring techniques. To benefit from these advantages, a new wedge design is presented that eliminates the high lateral and shear stress values in the CFRP–wedge interface of conventional wedges. Numerical simulations were performed to show the advantageous lateral and shear stress distributions. Tensile tests on 1.2 mm thick CFRP strips and a bonded CFRP–wedge interface reached ultimate loads higher than the guaranteed tensile load. The tests on 2.5 mm thick CFRP strips were performed with an adhesively bonded wedge–CFRP ...

An investigation of the flow and scour mechanisms around isolated spur dikes in a shallow open channel: 1. Conditions corresponding to the initiation of the erosion and deposition process

The present study investigates the flow physics and the role played by the main coherent structures in the scouring processes around a vertical spur dike in a straight channel at conditions corresponding to the start (flat bed) of the scouring process. Large eddy simulation (LES) is performed at a relatively low channel Reynolds number (Re = 18,000), in the range where most flume studies with clear water scour conditions are conducted. Similar to these studies, the incoming flow is fully turbulent and contains realistic turbulence fluctuations. Visualization experiments are conducted to better understand the nature of the interactions between the dominant coherent structures playing a role in the erosion process. It is found that the structure of the horseshoe vortex (HV) system at the base of the spur dike changes considerably in time and in vertical sections perpendicular to the trajectory defined by the axis of the main necklace vortex. However, its intensity is the largest at vertical sections situated around the tip of the spur dike. It is in this region that the core of the main necklace vortex oscillates aperiodically between two preferred modes. In one of them (zero‐flow mode), the necklace vortex is closer to the spur dike and more compact, and the near‐bed jet flow beneath it is weak. In the other one (back‐flow mode), a strong near‐bed jet flow convects the primary necklace vortex away from the spur dike, and its core is more elongated and less compact. This explains the large amplification (by about 1 order of magnitude compared to the surrounding turbulent flow) of the turbulent kinetic energy and pressure fluctuations inside the HV system in the region situated around the tip of the spur dike and the double‐peak distribution of the turbulent kinetic energy. Past the spur dike, in the legs of the necklace vortex, the intensity of the bimodal oscillations decreases such that they are not observed in spanwise sections situated at more than one channel depth behind the spur dike. It is found that the legs of the horseshoe vortices can interact, at times, with the vortex tubes shed in the detached shear layer (DSL) and with the tip of the spur dike. These events typically result in a significant change in the coherence of the HV system. The largest bed shear stress values in the mean flow are present in the strong acceleration region near the tip of the spur dike, but high bed shear stress values are also observed beneath the upstream part of the DSL. The bed shear stress fluctuations around the local mean values can be very high, especially in the region situated beneath the upstream part of the DSL. At random times, some of the vortices shed in the DSL merge or interact with eddies from the recirculation region. This leads to an increase in their strength and to a large increase of the bed shear stress along their path.

Grass pea (Lathyrus sativus L.) starch as an alternative for cereal starches: Rheological properties and retrogradation susceptibility

The study compares selected rheological properties of starch isolated from two cultivars of grass pea with those of wheat and corn starch. The phase transition temperatures on pasting (TO, TP, TE) of both grass pea starches were lower in comparison to normal corn starch, but higher when compared to wheat starch. Pasting temperature of the starch slurries were significantly lower for grass pea as compared to cereals, and paste viscosity of grass pea starch was significantly higher than of wheat starch. Grass pea starches, exhibit much higher values of storage and loss moduli during heating of starch suspensions. Higher values of shear stress as a function of shear rate obtained during the determination of flow curves for grass pea starch pastes are confirmed by high consistency coefficients in the Herschel-Bulkley model. The mechanical spectra of starch gels allow us to state, that grass pea starch gels are characterised by higher values of storage and loss moduli in comparison to cereal starches.

Enfuvirtide effects on human erythrocytes and lymphocytes functional properties

Enfuvirtide (T-20) is the first inhibitor of human Immunodeficiency Virus type-1 (HIV-1) entrance on a target cell approved for clinical use. Recent studies indicated that its action mechanism involves the interaction with the membrane surface, increasing the concentration in the site of action. In the present study, the in vitro interaction between enfuvirtide and blood cells of healthy human donors, namely erythrocytes and lymphocytes, and the peptide effect on plasma and lymphocyte suspensions supernatant ions were evaluated, in order to better characterize the action of this peptide. Enfuvirtide causes a decrease in the concentration of hemoglobin and in the percentages of methemoglobin and carboxyhemoglobin, together with increased values of P50, pCO2, and [HCO3-], and significant decreases of pO2 and pH, in blood plasma. The supernatants of lymphocyte suspensions derived from blood incubated with enfuvirtide presented a decrease in pH and [HCO3-]. Fluorescence anisotropy measurements of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-(4-(trimethylamino)-phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH), used to assess erythrocyte and lymphocyte membrane fluidity, did not yield enfuvirtide-induced changes (an effect could be expected due to peptide partition to lipid bilayers). Erythrocytes incubated with high enfuvirtide concentrations showed a significant decrease in osmotic fragility. As for erythrocyte deformability, enfuvirtide leads to increased elongation indexes for low shear stress values, whereas for high shear stress values it has the opposite effect. Despite the observed statistically significant variations in several parameters, these enfuvirtide-induced changes are not expected to lead to any detectable biomedical outcome for enfuvirtide-treated patients.

Granules morphology and rheological behavior of green banana (Musa cavendishii) and corn (Zea mays) starch gels

In this work, it was used starch obtained from green banana (Musa cavendishii) and commercial corn (Zea mays) starch in order to compare the granule morphology and the rheological behavior of these gel-starches. Images of starch granules morphology were obtained from scanning electron microscope (SEM). The banana starch granules presented an oval and ellipsoidal shape with irregular diameters. Neverthless, the granules of corn starch showed a poliedric shape, with different sizes. The rheological behavior of gel starch solutions showed a non-newtonian character with a pseudoplastic behavior. Herschel-Bulkley model gave a good description on the rheological behavior of the gel starch. Banana gel-starch solutions showed higher values of shear stress and apparent viscosity when compared with corn gel-starch solutions. A progressive decrease in shear stress and viscosity occurred with the addition of sodium chloride and sucrose.

Role of Arginase Pathway in Response to Shear Stress: New Potential Therapeutic Targets for Atherosclerosis?

Introduction: Alterations of wall shear stress can predispose the endothelium to the development of atherosclerotic plaques. Ample evidence indicates that arginase expression and/or activity correlates with several risk factors for cardiovascular disease including atherosclerosis. However, the role of arginase pathway in response to shear stress has never been investigated. Methods: To evaluate the regulation of arginases by different shear stress patterns without neuroendocrine factors, we perfused carotid arterial segments to the following hemodynamic conditions: unidirectional high and low shear stress, and oscillatory shear stress. After 3 days of flow exposure, vascular function was analyzed by diameter measurements, arginase expression and localization by western blot and immunohistochemistry, respectively. We compared these well-controlled measurements to an in vivo model of shear stress-induced atherogenesis. In brief, the carotid artery of ApoE-/- mice, fed with high cholesterol diet, was exposed to high, low and oscillatory shear stress values by the placement of a shear stress modifier for 9 weeks. Immunohistochemistry was used to determine the expression of arginase I and II and to characterize the arginase expression in different cell types encountered in atherosclerotic plaques. Results: Our preliminary results from ex vivo perfusion arteries showed for the first time that exposure of carotid segments to high shear stress conditions (athero-protective) significantly decrease arginase II protein expression as compared to both low and oscillatory flow conditions. Immunohistochemisty analysis confirmed a pronounced decrease in the expression of arginase II on ECs and SMCs exposed to this high shear stress. Our preliminary data suggest that arginase I and II are also regulated by shear stress in vivo. Arginases were up-regulated on EC, SMC and macrophages of carotid segments exposed either to low stress or to oscillatory shear stress conditions. Conclusions: The present study demonstrates that arginase expression is already modulated by 3 days exposure to different shear stress patterns in carotid arteries perfused ex vivo. Similar findings are also observed in a model of shear stress-induced atherogenesis in vivo. Histopathological analysis of carotid lesions in ApoE-/- mice exposed to shear stress and chronically treated with arginase inhibitors may further elucidate the role of arginases in modulating both plaque size and vulnerability

Field based analysis of sediment entrainment in two high gradient streams located in Alpine and Andine environments

This paper presents an analysis of critical thresholds for bedload transport based on field measurements conducted in two small, high gradient streams: the Rio Cordon (Italian Alps) and the Tres Arroyos (Chilean Andes). The threshold of incipient motion was identified by using marked particles displacement and both flood and flow competence approaches. The findings are expressed in terms of Shields parameter, dimensionless discharge, and specific stream power, and are used to identify the effects of relative grain size, relative depth, and bedform resistance. Overall, particle entrainment tends to be size selective, rather than exhibiting equal mobility, and the high values of dimensionless critical shear stress observed at both study sites confirm the additional roughness effects of step–pool morphologies that are very effective in reducing the bed shear stress and causing an apparent increase in critical shear stress.

Reach-scale channel geometry of mountain streams

The basic patterns and processes of steep channels remain poorly known relative to lower-gradient channels. In this analysis, characteristics of step-pool, plane-bed, and pool-riffle channels are examined using a data set of 335 channel reaches from the western United States, Nepal, New Zealand, and Panama. We analyzed differences among the three channel types with respect to hydraulics, channel geometry, boundary roughness, and bedforms. Step-pool channels have significantly steeper gradients, coarser substrate, higher values of shear stress and stream power for a given discharge, and larger ratios of bedform amplitude/wavelength ( H/ L). Pool-riffle channels have greater width/depth ratios and relative grain submergence ( R/ D 84) than the other channel types. Plane-bed channels tend to have intermediate values for most variables examined. Relative form submergence ( R/ H) is statistically similar for step-pool and pool-riffle channels. Despite the lesser relative grain submergence and greater bedform amplitude of step-pool channels, mean values of Darcy–Weisbach friction factor do not change in response to changes in relative grain submergence. These patterns suggest that adjustments along mountain streams effectively maximize resistance to flow and minimize downstream variability in resistance among the different channel types.

Time-dependent 3D simulations of the hemodynamics in a stented coronary artery

Stenting is becoming the major interventional cardiology procedure worldwide. However restenosis remains a major limitation to the effectiveness of stents. Alterations to the local hemodynamics in the stented segment of the artery could be a potential factor in the development of in-stent restenosis. The characterization of wall shear stress and of blood flow patterns in a stented artery is therefore necessary for a good understanding of the role of hemodynamics in the development of in-stent restenosis. We have used a time-dependent 3D numerical model of a stented coronary artery to study the characteristics of the blood flow and the shear stress distribution. Our results show that the presence of the stent produces significant secondary flow that is limited to an annulus in the near wall region. Low shear stress zones were localized in the vicinity of the struts while the tips of the struts exhibited high values of shear stress. These results support the hypothesis that local hemodynamics may affect the development of in-stent restenosis and could influence the choice of stent geometries for future stent designs.