Shear Stress Concentration Research Articles

Changes in aerodynamics during vocal cord dysfunction

BackgroundChanges in laryngeal airflow dynamics during episodes of vocal cord dysfunction (VCD) have not been well described. Very little is known about how inspiratory airflow is impacted when the vocal cords transition from normal inhalation state to a paradoxical adducted state; and how much change in laryngeal airflow and resistance occur before symptoms of stridor and air hunger emerge. This study provides new insight on the effects of VCD on respiratory airflow using computational fluid dynamics (CFD) techniques. MethodsComputed tomography images of a subject with normal vocal cords opening at the time of scanning were digitally modified to mimic an episode of VCD. To quantify and compare changes in inspiratory flow during VCD attack and normal inhalation, steady-state, laminar simulations were performed for three different breathing rates. ResultsPressure-flow analysis during VCD revealed that increasing inspiratory effort is not as efficient as in normal inhalation. Airflow resistance at the epiglottis was higher in the normal state (0.04Pa.s/mL versus 0.02Pa.s/mL) than in VCD; while resistance at the glottis and trachea remained roughly the same (0.04Pa.s/mL) during normal inhalation, it escalated during VCD (0.11Pa.s/mL and 0.13Pa.s/mL at the glottis and trachea, respectively). Peak airflow velocity and vorticity occurred around the glottis during VCD, and at the epiglottis during normal inhalation. ConclusionsThis pilot study demonstrates that attempting to force more inspired air will yield greater glottal resistance during VCD. Furthermore, there were evidence of abrupt laryngeal pressure gradient, chaotic airflow and high concentration of shear stresses in the glottal region.

Nitric Oxide Transport in Normal Human Thoracic Aorta: Effects of Hemodynamics and Nitric Oxide Scavengers

Despite the crucial role of nitric oxide (NO) in the homeostasis of the vasculature, little quantitative information exists concerning NO transport and distribution in medium and large-sized arteries where atherosclerosis and aneurysm occur and hemodynamics is complex. We hypothesized that local hemodynamics in arteries may govern NO transport and affect the distribution of NO in the arteries, hence playing an important role in the localization of vascular diseases. To substantiate this hypothesis, we presented a lumen/wall model of the human aorta based on its MRI images to simulate the production, transport and consumption of NO in the arterial lumen and within the aortic wall. The results demonstrated that the distribution of NO in the aorta was quite uneven with remarkably reduced NO bioavailability in regions of disturbed flow, and local hemodynamics could affect NO distribution mainly via flow dependent NO production rate of endothelium. In addition, erythrocytes in the blood could moderately modulate NO concentration in the aorta, especially at the endothelial surface. However, the reaction of NO within the wall could only slightly affect NO concentration on the luminal surface, but strongly reduce NO concentration within the aortic wall. A strong positive correlation was revealed between wall shear stress and NO concentration, which was affected by local hemodynamics and NO reaction rate. In conclusion, the distribution of NO in the aorta may be determined by local hemodynamics and modulated differently by NO scavengers in the lumen and within the wall.

Stress distribution in root filled teeth restored with various post and core techniques: effect of post length and crown height.

To investigate interfacial stress distribution in restored root filled teeth with various post lengths and crown heights. Three-dimensional mathematical models of a root filled mandibular premolar tooth were constructed. Parts of the tooth structures were replaced with ceramic crowns having three crown heights incorporating, either a cast post and core or a resin post and cores with fibre post or metallic post with four post lengths. Finite element linear analysis was performed to calculate equivalent and shear stress distribution at the interfaces between the teeth and post and cores under mesiodistal symmetrical boundary conditions and an oblique static load of 400N. For the resin post and core with fibre and metallic posts, shear stress at the interface was greater in the cervical area than the post area, depending on the crown height. The resin post and core with metallic post had lower shear stress at the interface of cervical area than that of the fibre post model; however, the metallic post models produced a high concentration of shear stress at the interface between the post and resin composite. On the other hand, for the cast post and core, the shear stress at the interface was mainly produced in the post end area, which increased with decrease of post length. For the resin post and core, bonding integrity to the cervical area would play a critical role in the survival of the restored tooth, whereas for the cast post and core, the bond of the post would be essential.

Flow accelerated corrosion study in feeder pipes

AbstractThe Indian Pressurized Heavy Water Reactor (PHWR) core consists of a number of horizontal channels containing nuclear fuel bundles. Parallel coolant channels are connected to Inlet and Outlet header through feeder pipes. Coolant from Reactor Inlet Header is distributed to the coolant channels and after removing heat combines at Reactor Outlet Header. Due to space constraints the feeder pipes are joined to the channel with one or two elbows close to the end fittings of the coolant channels. The carbon steel feeder pipes carry high temperature fluid at higher velocity and are liable to undergo Flow Accelerated Corrosion (FAC). In the recent inspection it has been found that feeders having double elbow are more susceptible to FAC on the intrados of second elbow. But it was found that in some of the elbows maximum thinning due to FAC was observed on the intrados of the first elbow. Hence to resolve this, effect of first bend orientation with respect of upstream direction has been studied. Two different approaches are used for predicting the FAC rate from calculated value of wall shear stress by CFD. One method is based on evaluating of wear rate using Colburn analogy and the other using an empirical equation between wear rate and shear stress. In Colburn analogy, mass transfer coefficient is evaluated by knowing shear stress and equilibrium concentration. For a case study, wall shear stress obtained fromk-∊ turbulence model was compared withk-ω SST turbulence model and no appreciable change in the wall shear stress has been found. Hence for subsequent analysisk-∊ turbulence model was chosen because large mesh size near to the surface (first layer thickness) is permitted due to higher y+value.

Parameters sensitivity and optimization for composite scarf repair

Scarf repair is the preferred method to restore the load-carrying capacity of the damaged structure of fiber-reinforced composite material when stiffness, strength, and flushness are needed. However, shear stress concentration occurs along the scarf bond line coinciding with the location of 0° plies. Previous studies did not consider the sensitivity of parameters systematically and involve the interaction effect. In this paper, sensitivities of scarf angle, adhesive thickness, and stacking sequence are evaluated synthetically through main effect and interaction effect by using the method of design of experiment. The general strategy of decreasing scarf angle needs to remove more materials from parent laminate. Thus, a new strategy of changing the stacking sequence of repair is performed to reduce the adhesive stress concentration by means of the optimization based on genetic algorithm. In spite of this, optimization results may not have good reliability especially when manufacture, load, and other fluctuations happen. Robustness analysis based on Monte Carlo simulation is used to estimate the reliability of optimization results.

A novel anchor method for multi-tendon FRP cable: Concept and FE study

This paper proposes a novel anchor toward large-sized fiber-reinforced polymer (FRP) cable with multi tendons and optimizes the key factors that affect anchor efficiency using finite element method (FEM). The limitations of conventional anchors for FRP tendons/cables were first analyzed. A new type of conical anchor with novel continuous-fiber-reinforced load transfer component (LTC) was proposed to overcome shortcomings of conventional anchors. The four key factors affecting anchor efficiency including modulus variation, conical degree, anchor length and the thickness of LTC were analyzed respectively. The results show that the proposed anchor not only owns an essential advantage in bonding by integrating LTC and FRP cable but also realizes the variable modulus of LTC by changing the winding angle of fiber roving. The optimization of key factors reveals that the avoidance of shear and compressive stress concentration can be realized by LTC modulus variation, while the conical degree of LTC significantly affects compressive stress distribution and cable axial displacements. The anchor length controls both compressive stress distribution and peak shear stresses, whereas the LTC thickness at the loading end only affects the peak compressive stresses and axial displacement. The optimization of above four factors determines the design of the proposal anchor.

Multiphase modelling of the effect of fluid shear stress on cell yield and distribution in a hollow fibre membrane bioreactor.

We present a simplified two-dimensional model of fluid flow, nutrient transport and cell distribution in a hollow fibre membrane bioreactor, with the aim of exploring how fluid flow can be used to control the distribution and yield of a cell population which is sensitive to both fluid shear stress and nutrient concentration. The cells are seeded in a scaffold in a layer on top of the hollow fibre, only partially occupying the extracapillary space. Above this layer is a region of free-flowing fluid which we refer to as the upper fluid layer. The flow in the lumen and upper fluid layer is described by the Stokes equations, whilst the flow in the porous fibre membrane is assumed to follow Darcy’s law. Porous mixture theory is used to model the dynamics of and interactions between the cells, scaffold and fluid in the cell–scaffold construct. The concentration of a limiting nutrient (e.g. oxygen) is governed by an advection–reaction–diffusion equation in each region. Through exploitation of the small aspect ratio of each region and asymptotic analysis, we derive a coupled system of partial differential equations for the cell volume fraction and nutrient concentration. We use this model to investigate the effect of mechanotransduction on the distribution and yield of the cell population, by considering cases in which cell proliferation is either enhanced or limited by fluid shear stress and by varying experimentally controllable parameters such as flow rate and cell–scaffold construct thickness.

NUMERICAL STUDY ON THE EFFECTS OF THE NUMBER AND GEOMETRIES OF DRUG-ELUTING STENT LINKS ON THE DRUG CONCENTRATION

The paper investigated the effects of the link numbers and geometries of drug-eluting stents (DESs) on the hemodynamics and the distribution of drug concentration, and the final results could be used as a guide for the optimization of DES design. Four 3D virtual stents with different numbers and geometries of links were modeled and numerically studied with respect to the distribution of wall shear stress (WSS) and drug concentration. Results have showed (1) The geometries of links have big impact on the distribution of WSS regions and the uniform drug concentration distribution on the vascular wall, but little effect on the mean drug concentration on the vascular wall; the S-shaped link stents had less low-WSS regions and the reason is that the flow direction was considered in the design of this type of stents. (2) The drug concentration distribution on the vascular wall had remarkable difference from that in blood. (3) The 6S link stent had the least low-WSS regions and with high and uniformly distributed drug concentration, so this type of stent was concluded to be the best design in four stents. Overall, it is not reliable to use the drug concentration in blood to deduce the drug concentration on the vascular wall. In DES design, the configuration of links should be, as much as possible, in line with the blood flow direction, so it can decrease the low-WSS regions. Meanwhile, in order to get more uniformly distributed drug on the vascular wall, the distribution of drug on the stent surface should be taken into account. Additionally, because of the increment of contact area between the stents and vascular wall, the drug concentration on the vascular wall increased with the number of links.

Dynamic lithospheric response to megathrust and precursory seismicity features of megathrust

Temporal variations of seismic properties in inner trench regions before and after three megathrusts with magnitudes greater than or equal to 8.8 since 2004 are investigated to understand the nature of megathrust earthquakes. The seismicity was increased significantly, and the fault-type compositions changed after megathrusts. The seismicity displays characteristic fault-type-dependent distribution on rupture planes. The postseismic thrustal events were populated around the down-dip rupture margins due to the concentration of shear stress after coseismic ruptures. Normal-faulting earthquakes increased after the megathrusts particularly in shallow-depth regions with large slips, which may be associated with lithospheric rebound and development of splay faults. The earthquake occurrence rate (b value) displays a characteristic slip-dependent feature. The earthquake occurrence rates decrease with the slip amount of forthcoming megathrust, which may be caused by continuous accumulation of plate-driven stress and tectonic loading around the future rupture planes on the slab surface. The slip dependency of earthquake occurrence rates is enhanced with time until the occurrence of megathrust. The level of seismicity after megathrust is inversely proportional to that before megathrust, yielding comparable average seismicity over the rupture zone. It was also observed that the dynamic lithospheric response is highly correlated with slip distribution on the rupture plane. The tension axes of the normal-faulting earthquakes for 100days after the Tohoku-Oki earthquake focus to an apparent pole, suggesting a radial viscoelastic deformation of lithosphere. The temporal changes of slip-dependent b values fit well with an exponential function, suggesting an exponential increase of normal stress on the locked region with time until the occurrence of the megathrust. This observation suggests that the temporal variation of slip-dependent b values may be useful for prediction of forthcoming megathrusts at least several tens years beforehand.

Shear bond of composites-to-brick applied with highly deformable, in relation to resin epoxy, interface materials

The paper presents comparison of a work of stiff and flexible bonds fastening composite strengthening to masonry. In the first approach (traditional), barely deformable interface material made of stiff epoxy resin is used as shear bonds of composites-to-brick. In the second one (innovative), highly deformable interface material made of flexible polymer is used as repair shear bonds of composites-to-brick. Behavior of both materials was compared using single-lap shear tests made on four kinds of fiber fabrics (glass, carbon, basalt and steel) applied to clay brick units. The results indicated that highly deformable interface materials allow increasing load capacity, because deformable adhesive layers reduce shear stress concentrations in bond, redistributing stress more evenly along the whole lap joint. Usefulness of the theory which allows calculating the bond shear stress–slip characteristic was also discussed in accordance to the highly deformable interface materials.

Analytical modeling of local stresses at Balsa/timber core joints of FRP sandwich structures

In order to optimize sandwich structures, core materials of different density and thus different stiffness and strength are often combined. The material discontinuities at joints of dissimilar core materials may lead to abrupt changes of the shear angles and local bending of the face sheets at these locations, both of which may cause significant stress concentrations. Two analytical models have been developed to estimate local stress concentrations at butt and scarf joints of dissimilar sandwich cores. The first model predicts local axial stresses in the face sheets at scarf joints while the second model predicts local shear stresses in cores at both butt and scarf joints. The models are based on an existing closed form model and have been validated by FEM for Balsa–Douglas fir butt and scarf joints, both implemented in a sandwich with glass fiber-reinforced polymer (GFRP) face sheets. The models are applicable for scarf joints with angles of termination between 20° and 60°. They also demonstrate the positive effect of scarf joints compared to butt joints. The axial stress concentrations in the face sheets and shear stress concentrations in the cores almost disappear if a butt joint is changed to a scarf joint configuration.

Impacts of wind waves on sediment transport in a large, shallow lake

AbstractLake Okeechobee is a large, shallow subtropical lake, located in south Florida. Over the last several decades, Lake Okeechobee has experienced accelerated eutrophication due to excessive phosphorus loads from agricultural run‐off. Recycling of phosphorus from bottom sediments through resuspension is critical to addressing eutrophication of the lake and for water quality management. The present study investigates the impacts of wind waves on sediment transport in Lake Okeechobee, using measured data and the Lake Okeechobee Environmental Model (LOEM). The LOEM was fully calibrated and verified with more than 10 years of measured data in previous studies. Analysis of the measured data indicates significant wave height (SWH) and suspended sediment concentration are closely correlated to the wind speed in the lake. The nonlinear interaction of high‐frequency wind waves with relatively low‐frequency currents in the boundary layer plays a key role in sediment deposition/resuspension. Without considering the effects of wind waves, the bottom shear stress can be greatly underestimated. The spatial variations of key variables for sediment modelling, including SWH, water depth, orbital velocity, current velocity, bottom shear stress and sediment concentration, are discussed. In general, the near‐bottom wave velocity (and the associated bottom shear stress) is greater than or the same order of magnitude as the near‐bed current velocity (and the associated bottom shear stress) in this shallow water system. Although the sediment zones of Lake Okeechobee were described in previous studies, few published papers discussed its formation mechanisms. The findings of the present study include that the multiyear averaged bottom shear stress with wind‐wave effect plays a key role in forming the spatial patterns of the sediment zones. The study results are currently being used in lake management and in developing strategies for reducing phosphorus in the lake.

Microencapsulation of Aqueous Compounds Using Hexamethylenediamine and Trimesoyl Chloride: Monodisperse Capsule Formation and Reaction Conditions on Membrane Properties

Efficient microencapsulation of aqueous compounds for dispersion in aqueous media remains challenging. To prepare aqueous microcapsules, we employed interfacial polymerization with hexamethylenediamine (HMDA) and trimesoyl chloride (TMC), which are commonly used for the formation of impermeable reverse-osmosis membranes. We employed an emulsification technique based on a high-viscosity external phase and uniform shear fields to produce monodisperse aqueous drops on the order of 10 μm. The effects of shear stress and HMDA concentration on the drop sizes were studied. The HMDA amount and emulsion drop size were both found to be important for subsequent capsule formation. The stoichiometric ratios of amine to acid chloride affected both the redispersibility of the microcapsules and the release rates of Coomassie Brilliant Blue (Coomassie Blue). Although high encapsulation efficiencies were achieved, the release rate varied with the sizes of the encapsulated molecules/ions. Optimum capsule formation and control of release were obtained with a stoichiometric ratio [c(NH2)/c(COCl)] of 5.

On the history of torsional stress concentrations in shafts: From electrical analogies to numerical methods

This article proposes a retrospective on experimental and numerical methods developed throughout the past century to solve the torsion problem in shafts, with particular emphasis on the determination of shear stress concentration factors in discontinuities of typical use in shaft design. This article, in particular, presents the theory and related solutions distinguishing between two classes of geometries: shafts with constant cross section and axisymmetric shafts with variable diameter. Emphasis is given to approaches based on physical analog methods and, in particular, those based on electrical analogies proposed since about 1925. Experimental methods based on structural physical models and numerical formulations are also reviewed, and a number of results from different approaches are collected and compared for two typical design case studies: a constant section shaft with a keyway and an axisymmetric shaft with a shouldered fillet.

Interlaminar Stress Distributions of Notched Quasi-Isotropic AS4/PEEK Laminate

This paper investigates the interlaminar stress distributions of notched angle-ply thermoplastic laminate under tension numerically. The representative AS4/PEEK laminate with configuration of was studied by finite element code ANSYS. Computation results show: The interlaminar stress concentrations are evident and localized in the vicinity of the hole and the two free sides, and the maximum values always appear near the hole. Meanwhile, at interfaces of-45/+45 and +45/0, the concentration of peel stress is evident, while at interfaces of 90/-45 and 0/90, the concentration of shear stress is evident. Further analysis demonstrates interlaminar shearing stress is the dominating stress and may be the leading cause of laminate delamination.

Estimation of cracking and damage mechanisms of rock specimens with precut holes by moment tensor analysis of acoustic emission

This work presents an experiment on the acoustic emission (AE) of coarse grain granites with two square-shaped precut holes under uniaxial loading. Studies were carried out on the temporal–spatial evolution behavior of micro-cracks by AE mechanisms with the use of the simplex location method and the moment tensor theory, with further analysis in comparing the numerical simulations using the software RFPA\(^\mathrm{2D}\) (Rock Failure Process Analysis). The results show that during the loading process, from beginning to rock failure, shear-mode micro-cracks are prominent, constituting more than 60 % of the total events; next most common are tensile-mode micro-cracks a less than 35 % of the total events. Variations of micro-cracks of the three modes during the loading process have the same increase tendency, i.e. fewer were generated in the initial loading stage, with a rapid increase when the stress values are between 40 and 60 % of the peak stress, and a rate dimunition before rock failure. It is observed that the tensile stress concentration is prone to appear at the tops of the two holes in the form of tensile type cracks, while the shear stress concentration usually appears at the bottom in the middle region of the specimen in the form of shear type cracks. The findings of the present work may serve as guidance for the prevention of roof and floor collapse in the stope exploration of mines.

Static and Dynamic Interfacial Stresses Analysis for Three-Layer Beam Structures Using Spectral Element Method

As the structural integrity at the interfaces of a three-layer structure certainly depends on the shear and normal (peeling) stress concentrations near the free ends of the cover plate, it is very important to predict the static and dynamic interfacial stresses in an accurate and efficient way for successful designs of three-layer beam-type structures subject to various loading conditions. Thus, this paper presents a frequency-domain spectral element method by which both static and dynamic interfacial stresses can be accurately predicted. To that end, the governing equations of motion for three-layer beam-type structures are derived by using Hamilton's principle and then the spectral element model is formulated in the frequency domain by using the variational method. The high accuracy of the present spectral element model is verified in due course. Numerical studies are then conducted to investigate static and dynamic interfacial stresses for an example three-layer beam subjected to various loadings.

Biological factors in plasma from diabetes mellitus patients enhance hyperglycaemia and pulsatile shear stress-induced endothelial cell apoptosis

People suffering from Diabetes Mellitus (DM) are prone to an array of vascular complications leading to end organ damage. The hallmark of these vascular complications is endothelium dysfunction, which is caused by endothelial cell (EC) apoptosis. Although the endothelial cell (EC) dysfunction induced by hyperglycaemia and fluid shear stress has been studied, the effects of biological factors in the blood of DM patients on EC integrity have not been reported in the in vitro models that mimic the physiological pulsatile nature of the vascular system. This study reports the development of a hemodynamic lab-on-a-chip system to investigate this issue. The pulsatile flow was applied to a monolayer of endothelial cells expressing a fluorescence resonance energy transfer (FRET)-based biosensor that changes colour from green to blue in response to caspase-3 activation during apoptosis. Plasma samples from healthy volunteers and DM patients were compared to identify biological factors that are critical to endothelial disruption. Three types of microchannels were designed to simulate the blood vessels under healthy and partially blocked pathological conditions. The results showed that EC apoptosis rates increased with increasing glucose concentration and levels of shear stress. The rates of apoptosis further increased by a factor of 1.4-2.3 for hyperglycaemic plasma under all dynamic conditions. Under static conditions, little difference was detected in the rate of EC apoptosis between experiments using plasma from DM patients and glucose medium, suggesting that the effects of hyperglycaemia and biological factors on the induction of EC apoptosis are all shear flow-dependent. A proteomics study was then conducted to identify biological factors, demonstrating that the levels of eight proteins, including haptoglobin and clusterin, were significantly down-regulated, while six proteins, including apolipoprotein C-III, were significantly up-regulated in the plasma of DM patients compared to healthy volunteers. This hemodynamic lab-on-a-chip system can serve as a high throughput platform to assess the risk of vascular complications of DM patients and to determine the effects of therapeutics or other interventions on EC apoptosis.

Dependence of shear and concentration on fouling in a membrane bioreactor with rotating membrane discs

Rotating ceramic membrane discs were fouled with lab‐scale membrane bioreactors (MBR) sludge. Sludge filtrations were performed at varying rotation speeds and in different concentric rings of the membranes on different sludge concentrations. Data showed that the back transport expressed by limiting flux increased with rotation speed and distance from membrane center as an effect of shear. Further, the limiting flux decreased with increasing sludge concentration. A model was developed to link the sludge concentration and shear stress to the limiting flux. The model was able to simulate the effect of shear stress and sludge concentration on the limiting flux. The model was developed by calculating the shear rate at laminar flow regime at different rotation speeds and radii on the membrane. Furthermore, through the shear rate and shear stress, the non‐Newtonian behavior of MBR sludge was addressed. © 2013 American Institute of Chemical Engineers AIChE J 60: 706–715, 2014

Delamination damage analysis of laminated bonded tubular single lap joint made of fiber-reinforced polymer composite

Onset and growth of delamination damages in laminated fiber-reinforced polymer composite made bonded tubular single lap joints have been thoroughly studied through a finite element analysis-based simulation technique developed in the present research. The model provides sufficient scope for a comprehensive parametric study of the bonded structure in terms of three-dimensional stress analyses within the joint, studying the delamination damage onset and growth characteristics, and investigating its effect on stress distributions in the joint and both the adherends (outer and inner). Quadratic failure criterion has been used to depict delamination damage-induced cohesion failure within the adhesive layer. Whereas Tsai–Wu coupled stress criterion has been used for locating the delamination and adhesion failure prone regions at ply-interfaces of the adherends and adherend–adhesive interfaces within the joint, respectively. Three-dimensional stress analyses revealed that outer adherend of the bonded tubular single lap joints experiences comparatively higher interlaminar peel ([Formula: see text]) and shear stress ([Formula: see text]) concentrations at the free edge of the first ply-interface and is likely to delaminate at this edge. Accordingly, pre-embedded through-the-circumference delamination damages have been simulated at this edge, and contact finite element analyses have been performed in order to avoid interpenetration of delaminated surfaces. Strain energy release rate calculated using modified crack closure integral vis-à-vis virtual crack closure technique has been used as the characterizing parameter for assessing the growth of the damages. Free edge delamination in the outer adherend has been observed to propagate in a self-similar manner mainly in the in-plane shearing mode. It causes peel and shear stress concentrations in regions of the adherend–adhesive interfaces and first ply-interface of the inner adherend, localized to the clamped edge of the joint making the joint and the adherend prone to adhesion failure and delamination damage, respectively.