Showing papers on "Shear stress published in 2003"

Effective slip in pressure-driven Stokes flow

Abstract: Nano-bubbles have recently been observed experimentally on smooth hydrophobic surfaces; cracks on a surface can likewise be the site of bubbles when partially wetting fluids are used. Because these bubbles may provide a zero shear stress boundary condition and modify considerably the friction generated by the solid boundary, it is of interest to quantify their influence on pressure-driven flow, with particular attention given to small geometries. We investigate two simple configurations of steady pressure-driven Stokes flow in a circular pipe whose surface contains periodically distributed regions of zero surface shear stress. In the spirit of experimental studies probing slip at solid surfaces, the effective slip length of the resulting flow is evaluated as a function of the degrees of freedom describing the surface heterogeneities, namely the relative width of the no-slip and no-shear stress regions and their distribution along the pipe. Comparison of the model with experimental studies of pressure-driven flow in capillaries and microchannels reporting slip is made and a possible interpretation of the experimental results is offered which is consistent with a large number of distributed slip domains such as nano-size and micron-size nearly flat bubbles coating the solid surface. Further, the possibility is suggested of a shear-dependent effective slip length, and an explanation is proposed for the seemingly paradoxical behaviour of the measured slip length increasing with system size, which is consistent with experimental results to date.

Fluid shear stress and the vascular endothelium: for better and for worse.

Abstract: As blood flows, the vascular wall is constantly subjected to physical forces, which regulate important physiological blood vessel responses, as well as being implicated in the development of arterial wall pathologies. Changes in blood flow, thus generating altered hemodynamic forces are responsible for acute vessel tone regulation, the development of blood vessel structure during embryogenesis and early growth, as well as chronic remodeling and generation of adult blood vessels. The complex interaction of biomechanical forces, and more specifically shear stress, derived by the flow of blood and the vascular endothelium raise many yet to be answered questions:How are mechanical forces transduced by endothelial cells into a biological response, and is there a "shear stress receptor"?Are "mechanical receptors" and the final signaling pathways they evoke similar to other stimulus-response transduction systems?How do vascular endothelial cells differ in their response to physiological or pathological shear stresses?Can shear stress receptors or shear stress responsive genes serve as novel targets for the design of diagnostic and therapeutic modalities for cardiovascular pathologies?The current review attempts to bring together recent findings on the in vivo and in vitro responses of the vascular endothelium to shear stress and to address some of the questions raised above.

Heparan sulfate proteoglycan is a mechanosensor on endothelial cells.

Abstract: The objective of this study was to test whether a glycosaminoglycan component of the surface glycocalyx layer is a fluid shear stress sensor on endothelial cells (ECs). Because enhanced nitric oxide (NO) production in response to fluid shear stress is a characteristic and physiologically important response of ECs, we evaluated NOx (NO2− and NO3−) production in response to fluid shear stress after enzymatic removal of heparan sulfate, the dominant glycosaminoglycan of the EC glycocalyx, from cultured ECs. The significant NOx production induced by steady shear stress (20 dyne/cm2) was inhibited completely by pretreatment with 15 mU/mL heparinase III (E.C.4.2.2.8) for 2 hours. Oscillatory shear stress (10±15 dyne/cm2) induced an even greater NOx production than steady shear stress that was completely inhibited by pretreatment with heparinase III. Addition of bradykinin (BK) induced significant NOx production that was not inhibited by heparinase pretreatment, demonstrating that the cells were still able to pr...

Computational Model for Cyclic Mobility and Associated Shear Deformation

Abstract: In saturated clean medium-to-dense cohesionless soils, liquefaction-induced shear deformation is observed to accumulate in a cycle-by-cycle pattern (cyclic mobility). Much of the shear strain accumulation occurs rapidly during the transition from contraction to dilation (near the phase transformation surface) at a nearly constant low shear stress and effective confining pressure. Such a stress state is difficult to employ as a basis for predicting the associated magnitude of accumulated permanent shear strain. In this study, a more convenient approach is adopted in which the domain of large shear strain is directly defined by strain space parameters. The observed cyclic shear deformation is accounted for by enlargement and/or translation of this domain in deviatoric strain space. In this paper, the model formulation details involved are presented and discussed. A calibration phase is also described based on data from laboratory sample tests and dynamic centrifuge experiments (for Nevada sand at a relative density of about 40%).

An atomic-level model for studying the dynamics of edge dislocations in metals

Abstract: A model for simulating the dynamic behaviour of edge dislocations in metals at the atomic level is presented. The model extends an earlier approach based on an array of edge dislocations periodic in the Burgers vector direction and allows the external action (either shear strain or resolved shear stress), crystal energy, plastic displacement and dislocation position and velocity to be determined unambiguously. Two versions of the model for either static or dynamic conditions, i.e. zero or non-zero temperature, are described. The model is tested for elastic response of a perfect crystal and the atomic properties of a ½111 edge dislocation in a model of bcc Fe. Several examples of dislocation glide behaviour and dislocation–obstacle interactions at zero and non-zero temperature are presented and discussed.

Shear stress–induced endothelial cell polarization is mediated by Rho and Rac but not Cdc42 or PI 3-kinases

Abstract: Shear stress induces endothelial polarization and migration in the direction of flow accompanied by extensive remodeling of the actin cytoskeleton. The GTPases RhoA, Rac1, and Cdc42 are known to regulate cell shape changes through effects on the cytoskeleton and cell adhesion. We show here that all three GTPases become rapidly activated by shear stress, and that each is important for different aspects of the endothelial response. RhoA was activated within 5 min after stimulation with shear stress and led to cell rounding via Rho-kinase. Subsequently, the cells respread and elongated within the direction of shear stress as RhoA activity returned to baseline and Rac1 and Cdc42 reached peak activation. Cell elongation required Rac1 and Cdc42 but not phosphatidylinositide 3-kinases. Cdc42 and PI3Ks were not required to establish shear stress–induced polarity although they contributed to optimal migration speed. Instead, Rho and Rac1 regulated directionality of cell movement. Inhibition of Rho or Rho-kinase did not affect the cell speed but significantly increased cell displacement. Our results show that endothelial cells reorient in response to shear stress by a two-step process involving Rho-induced depolarization, followed by Rho/Rac-mediated polarization and migration in the direction of flow.

Proliferation, differentiation, and tube formation by endothelial progenitor cells in response to shear stress

Abstract: Endothelial progenitor cells (EPCs), circulating in peripheral blood, migrate toward target tissue, differentiate, and contribute to the formation of new vessels. In this study, we report that shear stress generated by blood flow or tissue fluid flow can accelerate the proliferation, differentiation, and capillary-like tube formation of EPCs. When EPCs cultured from human peripheral blood were subjected to laminar shear stress, the cells elongated and oriented their long axes in the direction of flow. The cell density of the EPCs exposed to shear stress was higher, and a larger percentage of these cells were in the G2-M phase of the cell cycle, compared with EPCs cultured under static conditions. Shear stress markedly increased the EPC expression of two vascular endothelial growth factor receptors, kinase insert domain-containing receptor and fms-like tyrosine kinase-1, and an intercellular adhesion molecule, vascular endothelial-cadherin, at both the protein and mRNA levels. Assays for tube formation in the collagen gels showed that the shear-stressed EPCs formed tubelike structures and developed an extensive tubular network significantly faster than the static controls. These findings suggest that EPCs are sensitive to shear stress and that their vasculogenic activities may be modulated by shear stress.

New aspects of tectonic stress inversion with reference to the TENSOR program

Abstract: Analysis of tectonic stress from the inversion of fault kinematic and earthquake focal mechanism data is routinely done using a wide variety of direct inversion, iterative and grid search methods. This paper discusses important aspects and new developments of the stress inversion methodology as the critical evaluation and interpretation of the results. The problems of data selection and separation into subsets, choice of optimization function, and the use of non-fault structural elements in stress inversion (tension, shear and compression fractures) are examined. The classical Right Dihedron method is developed in order to estimate the stress ratio R, widen its applicability to compression and tension fractures, and provide a compatibility test for data selection and separation. A new Rotational Optimization procedure for interactive kinematic data separation of fault-slip and focal mechanism data and progressive stress tensor optimization is presented. The quality assessment procedure defined for the World Stress Map project is extended in order to take into account the diversity of orientations of structural data used in the inversion. The range of stress regimes is expressed by a stress regime index R', useful for regional comparisons and mapping, All these aspects have been implemented in a computer program TENSOR, which is introduced briefly. The procedures for determination of stress tensor using these new aspects are described using natural sets of fault-slip and focal mechanism data from the Baikal Rift Zone. Analysis of fault kinematic and earthquake focal mechanism data for the reconstruction of past and present tectonic stresses are now routinely done in neotectonic and seismotectonic investigations. Geological stress data for the Quaternary period are increasingly incorporated in the World Stress Map (WSM) (Muller & Sperner 2000; Muller et at. 2000; Sperner et at. 2003). Standard procedures for brittle fault-slip data analysis and stress tensor determination are now well established (Angelier 1994; Dunne & Hancock 1994). They commonly use fault-slip data to infer the orientations and relative magnitude of the principal stresses. A wide variety of methods and computer programs exist for stress tensor reconstruction. They are either direct inversion methods using least square minimization (Carey-Gailhardis & Mercier 1987; Angelier 1991; Sperner et at. 1993) or iterative algorithms that test a wide range of possible tensors (Etchecopar et at. 1981) or grid search methods (Gephart 1990b; Hardcastle & Hills 1991; Unruh et at. 1996). The direct inversion methods are faster but necessitate more complex mathematical developments and do not allow the use of complex minimization functions. The iterative methods are more robust, use simple algorithms and are also more computer time intensive, but the increasing computer power reduces this inconvenience. This paper presents a discussion on the methodology of stress inversion with, in particular, the use of different types of brittle fractures in addition to the commonly used fault-slip data, the problem of data selection and the optimization functions. Two methodologies for stress inversion are presented: new developments of the classical Right Dihedron method and the new iterative Rotational Optimization method. Both methods use of the full range of brittle data available and have been adapted for the inversion of earthquake focal mechanisms. The interpretation of the results is also discussed for two important aspects: the quality assessment in view of the World Stress Map standards and the expression of the stress regime numerically as a Stress Regime Index for regional comparisons and mapping. All aspects discussed have been implemented in the TENSOR program (Delvaux 1993a), which can From: NIEUWLAND, D. A. (ed.) New Insights into Structural Interpretation and Modelling, Geological Society, London, Special Publications, 212, 75-100. 0305-8719/03/$15 © The Geological Society of London 2003. 76 D. DELVAUX & B. SPERNER be obtained by contacting the first author. A program guideline is also provided with the program package. Stress inversion methodologies Stress analysis considers a certain volume of rocks, large enough to sample a sufficiently large data set of slips along a variety of different shear surfaces. The size of the volume sampled should be much larger than the dimensions of the individual brittle structures. For geological indicators, relatively small volumes or rock (100-1000 m") are necessary to sample enough fault-slip data, while for earthquake focal mechanisms, volumes in the order of 1000-10 000 krrr' are needed. Stress inversion procedures rely on Bott's (1959) assumption that slip on a plane occurs in the direction of the maximum resolved shear stress. Inversely, the stress state that produced the brittle microstructures can be partly reconstructed knowing the direction and sense of slip on variably oriented fault planes. The slip direction on the fault plane is inferred from frictional grooves or slickenlines. The data used for the inversion are the strike and dip of the fault plane, the orientation of the slip line and the shear sense on the fault plane. They are collectively referred to as fault-slip data. Focal mechanisms of earthquakes are also used in stress inversion. The inversion of fault-slip data gives the four parameters of the reduced stress tensor: the principal stress axes al (maximum compression), a2 (intermediate compression) and a3 (minimum compression) and the Stress Ratio R = (a2 a3)/(al a3). The two additional parameters of the full stress tensor are the ratio of extreme principal stress magnitudes (a3/al) and the lithostatic load, but these two cannot be determined from fault data only. We refer to Angelier (1989, 1991, 1994) for a detailed description of the principles and procedures of fault-slip analysis and palaeostress reconstruction. Weare aware of the inherent limitations of any stress inversion procedures that apply also to the discussion proposed in this paper (Dupin et al. 1993; Pollard et al. 1993; Nieto-Samaniego & Alaniz-Alvarez 1996; Maerten 2000; Roberts & Ganas 2000). The question was raised as to whether fault-slip inversion solutions constrain the principal stresses or the principal strain rates (Gephart 1990a). We will not discuss this question here, and leave readers to form their own opinions on how to interpret the inversion results. The brittle microstructures (faults and fractures) are used in palaeostress reconstructions as kinematic indicators. The stress inversion scepticals (e.g. Twiss & Unruh 1998) argue that kinematic indicators are strain markers and consequently they cannot give access to stress. Without entering in such debate, we consider here that the stress tensor obtained by the inversion of kinematic indicators is a function that models the distribution of slip on every fault plane. For this, there is one ideal stress tensor, but this one is only certainly active during fault initiation. After faults have been initiated, a large variety of stress tensors can induce fault-slip by reactivation. Stress and strain relations In fault-slip analysis and palaeostress inversion, we consider generally the activation of pre-existing weakness planes as faults. Weakness planes can be inherited from a sedimentary fabric such as bedding planes, or from a previous tectonic event. A weakness plane can be produced also during the same tectonic event, just before accumulating slip on it, as when a fault is neoformed in a previously intact rock mass. The activated weakness plane F can be described by a unit vector n normal to F (bold is used to indicate vectors). The stress vector a acting on the weakness plane F has two components: the normal stress v in the direction of nand the shear stress T, parallel to F. These two stress components are perpendicular to each other and related by the vectorial relation a= v + T. The stress vector a represents the state of stress in the rock and has rrl , a2 and a3 as principal stress axes, defining a stress ellipsoid. The normal stress v induces a component of shortening or opening on the weakness plane in function of this sign. The slip direction d on a plane is generally assumed to be parallel to the shear stress component T of the stress vector a acting on the plane. It is possible to demonstrate that the direction of slip d on F depends on the orientations of the three principal stress axes, the stress ratio R = (a2 a3)/(al a3) and the orientation of the weakness plane n (Angelier 1989, 1994). The ability of a plane to be (re)activated depends on the relation between the normal stress and shear stress components on the plane, expressed by the friction coefficient: If the characteristic friction angle cP of the weakness plane F with the stress vector a acting on it overcomes the line of initial friction, the weakness plane will be activated as a fault. Otherwise, no movement will occur on it. This line is defined by the cohesion factor and the initial friction angle cPo. TECTONIC STRESS INVERSION AND THE TENSOR PROGRAM 77 Data types and their meaning in stress

Dynamic properties of shear thickening colloidal suspensions

Abstract: The transient shear rheology (i.e., frequency and strain dependence) is compared to the steady rheology for a model colloidal dispersion through the shear thickening transition. Reversible shear thickening is observed and the transition stress compares well to theoretical predictions. Steady and transient shear thickening are observed to occur at the same value of the average stress. The critical strain for shear thickening is found to depend inversely on the frequency at fixed applied stress for low frequencies (high strains), but is limited to an apparent minimum critical strain at higher frequencies. This minimum critical strain is shown to be an artifact of slip. Lissajous plots illustrate the transition in material properties through the shear thickening transition, and the energy dissipated by a shear thickening suspension is analyzed as a function of strain amplitude.

Yield stress and wall slip phenomena in colloidal silica gels

Abstract: Evidence of wall slip and magnitude of yield stress are examined for colloidal gels consisting of hydrophobic silica, polyether, and lithium salts using geometries with serrated, smooth, hydrophilic and hydrophobic surfaces. Serrated plates, which provide minimal wall slip, are used to compare different methods of measuring yield stress: conventional extrapolation of shear stress in steady shear experiments and dynamic experiments at large strain amplitudes. In the latter, the yield stress is denoted by the maximum in the elastic stress, the product of the elastic modulus and strain (G′γ), when plotted as a function of strain amplitude. Although excellent agreement is observed in the yield stress values using both these techniques, the dynamic method seems preferable considering its experimental ease, accuracy, and lack of extrapolation. In the presence of smooth geometries, the silica gels show evidence of wall slip with a concomitant decrease in yield stress. Using underestimation of yield stress as a m...

Arterial Wall Shear Stress: Observations from the Bench to the Bedside

Abstract: Shear stress is the tangential force of the flowing blood on the endothelial surface of the blood vessel. Shear is described mathematically or ideal fluids, and in vitro models have enabled researchers to describe the effects of shear on endothelial cells. High shear stress, as found in laminar flow, promotes endothelial cell survival and quiescence, alignment in the direction of flow, and secretion of substances that promote vasodilation and anticoagulation. Low shear stress, or changing shear stress direction as found in turbulent flow, promotes endothelial proliferation and apoptosis, shape change, and secretion of substances that promote vasoconstriction, coagulation, and platelet aggregation. The precise pathways by which endothelial cells sense shear stress to promote their quiescent or activated pathways are currently unknown. Clinical applications include increasing shear stress via creation of an arteriovenous fistula or vein cuff to promote bypass graft flow and patency. Since an abnormal level of shear stress is implicated in the pathogenesis of atherosclerosis, neointimal hyperplasia, and aneurysmal disease, additional research to understand the effects of shear stress on the blood vessel may provide insight to prevent vascular disease.

Shear stress related blood damage in laminar couette flow.

Abstract: Artificial organs within the blood stream are generally associated with flow-induced blood damage, particularly hemolysis of red blood cells. These damaging effects are known to be dependent on shear forces and exposure times. The determination of a correlation between these flow-dependent properties and actual hemolysis is the subject of this study. For this purpose, a Couette device has been developed. A fluid seal based on fluorocarbon is used to separate blood from secondary external damage effects. The shear rate within the gap is controlled by the rotational speed of the inner cylinder, and the exposure time by the amount of blood that is axially pumped through the device per given time. Blood damage is quantified by the index of hemolysis (IH), which is calculated from photometric plasma hemoglobin measurements. Experiments are conducted at exposure times from texp=25 - 1250 ms and shear rates ranging from tau=30 up to 450 Pa ensuring Taylor-vortex free flow characteristics. Blood damage is remarkably low over a broad range of shear rates and exposure times. However, a significant increase in blood damage can be observed for shear stresses of tau>or= 425 Pa and exposure times of texp>or= 620 ms. Maximum hemolysis within the investigated range is IH=3.5%. The results indicate generally lower blood damage than reported in earlier studies with comparable devices, and the measurements clearly indicate a rather abrupt (i.e., critical levels of shear stresses and exposure times) than gradual increase in hemolysis, at least for the investigated range of shear rates and exposure times.

Triaxial Compression of Sand Reinforced with Fibers

Abstract: Results from drained triaxial compression tests on specimens of fiber-reinforced sand are reported. It is evident that the addition of a small amount of synthetic fibers increases the failure stress of the composite. This effect, however, is associated with a drop in initial stiffness and an increase in strain to failure. Steel fibers did not reduce initial stiffness of the composite. The increase in failure stress can be as much as 70% at a fiber concentration of 2% (by volume) and an aspect ratio of 85. The reinforcement benefit increases with an increase in fiber concentration and aspect ratio, but it also depends on the relative size of the grains and fiber length. A larger reinforcement effect in terms of the peak shear stress was found in fine sand, compared to coarse sand, when the fiber concentration was small (0.5%). This trend was reversed for a larger fiber concentration (1.5%). A model for prediction of the failure stress in triaxial compression was developed. The failure envelope has two segments: a linear part associated with fiber slip, and a nonlinear one related to yielding of the fiber material. The analysis indicates that yielding of fibers occurs well beyond the stress range encountered in practice. The concept of a macroscopic internal friction angle was introduced to describe the failure criterion of a fiber-reinforced sand. This concept is a straightforward way to include fiber reinforcement in stability analyses of earth structures.

Transient elastography in anisotropic medium: Application to the measurement of slow and fast shear wave speeds in muscles

Abstract: From the measurement of a low frequency (50–150 Hz) shear wave speed, transient elastography evaluates the Young’s modulus in isotropic soft tissues. In this paper, it is shown that a rod source can generate a low frequency polarized shear strain waves. Consequently this technique allows to study anisotropic medium such as muscle. The evidence of the polarization of low frequency shear strain waves is supported by both numeric simulations and experiments. The numeric simulations are based on theoretical Green’s functions in isotropic and anisotropic media (hexagonal system). The experiments in vitro led on beef muscle proves the pertinent of this simple anisotropic pattern. Results in vivo on man biceps shows the existence of slow and fast shear waves as predicted by theory.

Three-dimensional computational fluid dynamics modeling of alterations in coronary wall shear stress produced by stent implantation.

Abstract: Rates of coronary restenosis after stent implantation vary with stent design. Recent evidence suggests that alterations in wall shear stress associated with different stent types and changes in local vessel geometry after implantation may account for this disparity. We tested the hypothesis that wall shear stress is altered in a three-dimensional computational fluid dynamics (CFD) model after coronary implantation of a 16 mm slotted-tube stent during simulations of resting blood flow and maximal vasodilation. Canine left anterior descending coronary artery blood flow velocity and interior diameter were used to construct CFD models and evaluate wall shear stress proximal and distal to and within the stented region. Channeling of adjacent blood layers due to stent geometry had a profound affect on wall shear stress. Stagnation zones were localized around stent struts. Minimum wall shear stress decreased by 77% in stented compared to unstented vessels. Regions of low wall shear stress were extended at the stent outlet and localized to regions where adjacent axial strut spacing was minimized and the circumferential distance between struts was greatest within the stent. The present results depict alterations in wall shear stress caused by a slotted-tube stent and support the hypothesis that stent geometry may be a risk factor for restenosis by affecting local wall shear stress distributions.

Why earthquakes correlate weakly with the solid Earth tides: Effects of periodic stress on the rate and probability of earthquake occurrence

Abstract: [1] We provide an explanation why earthquake occurrence does not correlate well with the daily solid Earth tides. The explanation is derived from analysis of laboratory experiments in which faults are loaded to quasiperiodic failure by the combined action of a constant stressing rate, intended to simulate tectonic loading, and a small sinusoidal stress, analogous to the Earth tides. Event populations whose failure times correlate with the oscillating stress show two modes of response; the response mode depends on the stressing frequency. Correlation that is consistent with stress threshold failure models, e.g., Coulomb failure, results when the period of stress oscillation exceeds a characteristic time t n ; the degree of correlation between failure time and the phase of the driving stress depends on the amplitude and frequency of the stress oscillation and on the stressing rate. When the period of the oscillating stress is less than t n , the correlation is not consistent with threshold failure models, and much higher stress amplitudes are required to induce detectable correlation with the oscillating stress. The physical interpretation of t n is the duration of failure nucleation. Behavior at the higher frequencies is consistent with a second-order dependence of the fault strength on sliding rate which determines the duration of nucleation and damps the response to stress change at frequencies greater than 1/t n . Simple extrapolation of these results to the Earth suggests a very weak correlation of earthquakes with the daily Earth tides, one that would require >13,000 earthquakes to detect. On the basis of our experiments and analysis, the absence of definitive daily triggering of earthquakes by the Earth tides requires that for earthquakes, t n exceeds the daily tidal period. The experiments suggest that the minimum typical duration of earthquake nucleation on the San Andreas fault system is ∼1 year.

The size effect in metal cutting

Abstract: When metal is removed by machining there is substantial increase in the specific energy required with decrease in chip size. It is generally believed this is due to the fact that all metals contain defects (grain boundaries, missing and impurity atoms, etc.), and when the size of the material removed decreases, the probability of encountering a stress-reducing defect decreases. Since the shear stress and strain in metal cutting is unusually high, discontinuous microcracks usually form on the metal-cutting shear plane. If the material being cut is very brittle, or the compressive stress on the shear plane is relatively low, microcracks grow into gross cracks giving rise to discontinuous chip formation. When discontinuous microcracks form on the shear plane they weld and reform as strain proceeds, thus joining the transport of dislocations in accounting for the total slip of the shear plane. In the presence of a contaminant, such as CCI4 vapour at a low cutting speed, the rewelding of microcracks decreases, resulting in decrease in the cutting force required for chip formation. A number of special experiments are described in the paper that support the transport of microcracks across the shear plane, and the important role compressive stress plays on the shear plane. Relatively recently, an alternative explanation for the size effect in cutting was provided based on the premise that shear stress increases with increase in strain rate. When an attempt is made to apply this to metal cutting by Dineshet al (2001) it is assumed in the analysis that the von Mises criterion pertains to the shear plane. This is inconsistent with the experimental findings of Merchant. Until this difficulty is taken care of, together with the promised experimental verification of the strain rate approach, it should be assumed that the strain rate effect may be responsible for some notion of the size effect in metal cutting. However, based on the many experiments discussed here, it is very unlikely that it is totally responsible for the size effect in metal cutting as inferred in Dineshet al (2001).

Nonlinear modeling of mean flow redistribution in curved open channels

Abstract: [1] Phenomena concerning flow, morphology, and water quality in rivers are often investigated by means of a depth-integrated flow model, coupled to a sediment transport model and a water quality model. In such depth−integrated models, the vertical structure of the flow is represented by a closure submodel, which mainly has to account for the secondary circulation, which (1) redistributes by advection the flow, the boundary shear stresses, the sediment transport, and dissolved and suspended matter, (2) causes the direction of the bed shear stress to deviate from the direction of the depth-averaged velocity and thereby influences the bed topography, and (3) gives rise to additional friction losses as compared with straight uniform flow. The commonly used linear closure submodels are shown to fail in reproducing essential features in moderately to strongly curved flow, because they neglect the feedback between the downstream velocity and the secondary circulation. A nonlinear closure submodel taking this feedback into account is proposed and shown to yield results that compare well with experimental data, even for very strongly curved flow. The feedback effects turn out to be controlled almost exclusively by a single parameter, which enables their parameterization in a relatively simple way. This control parameter also helps to objectively distinguish weak, moderate, and strongly curved flows. The proposed closure submodel has the potential of improving the performance of depth-integrated flow-sediment-water quality models without much extra computational effort.

Large amplitude oscillatory shear behavior of complex fluids investigated by a network model: a guideline for classification

Abstract: Large amplitude oscillatory shear (LAOS) behavior of complex fluids, which form microstructures depending on their deformation history, has been investigated by using a network model. According to recent experimental observations, the LAOS behavior of complex fluids could be classified by at least four types: type I, strain thinning (G � , G �� decreasing); type II, strain hardening ( G � , G �� increasing); type III, weak strain overshoot ( Gdecreasing, G �� increasing followed by decreasing); type IV, strong strain overshoot ( G � , G �� increasing followed by decreasing). To understand such complex behavior, we have applied a general network model. As there is little information available on the form of creation and loss rates of network junctions, we have modeled the creation and loss rates as exponential functions of shear stress. By adjusting the model parameters that define the creation and loss rates, the types of LAOS behavior observed in the experiments could be reproduced. Despite highly simplistic modeling, the model reproduced the types of LAOS behavior observed in the experiments, which means that the behavior can be explained in terms of the model parameters, that is, the creation and loss rates of network junctions. It is also suggested that the LAOS behavior can be effectively used as a tool for classifying complex fluids. © 2003 Elsevier B.V. All rights reserved.

Chronic Physiological Shear Stress Inhibits Tumor Necrosis Factor–Induced Proinflammatory Responses in Rabbit Aorta Perfused Ex Vivo

Abstract: Background— Regions in the vasculature exposed to steady laminar flow have a lower likelihood for atherosclerosis than regions exposed to disturbed flow with low shear stress. We previously found that laminar flow of short duration inhibited tumor necrosis factor (TNF)-α–mediated proinflammatory signaling in cultured endothelial cells (ECs). However, mechanisms responsible for the atheroprotective effects of physiological shear stress remain undefined. Therefore, we examined the effects of chronic shear stress on TNF-α–induced inflammatory responses using an ex vivo perfusion organ culture system. Methods and Results— Rabbit aortas were exposed to low or normal shear stress (0.4 or 12 dyne/cm2) at a constant pressure for 24 to 26 hours. EC and vascular smooth muscle cell (VSMC) proteins were selectively purified. After exposure to low shear stress, TNF-α (50 ng/mL, 6 hours) specifically stimulated vascular cell adhesion molecule (VCAM)-1 expression in ECs but not VSMCs. TNF-α–stimulated VCAM expression wa...

Active control of turbulent boundary layers

Abstract: An experimental investigation is made into the active control of the near-wall region of a turbulent boundary layer () using a linear active control scheme. System identification in the boundary layer provides optimal transfer functions that predict the downstream characteristics of the streamwise velocity fluctuations. Enhanced detection techniques isolate the large-scale turbulent motion and improve the downstream correlations, resulting in greater controllability. The control is applied using a spanwise array of resonant synthetic jet actuators that introduce pairs of streamwise vortices into the flow. Control results show that a maximum reduction of 30% in the streamwise velocity fluctuations is achieved. This reduction is greatest at the point of optimization but spans a few hundred viscous lengths downstream of the actuator, about 50 viscous lengths in the wall-normal direction and 150 viscous lengths in the spanwise direction. The wall pressure fluctuation and the mean wall shear stress (measured approximately using mean velocity profiles near the wall) were reduced by 15% and 7% respectively. The bursting frequency, based on VITA event detection was also reduced by up to 23%.