Boulder-induced form roughness and skin shear stresses in a gravel-bed stream

Boulder spacing in mountain rivers and near-wake flow zones within the boulder array is very useful for fish habitat and growth of aquatic organisms.The present study aims to investigate how the boulder array and spacing influence the near-bed flow structures in a gravel-bed stream.Boulders are staggered over a gravel-bed stream with three different inter-boulder spacing namely(a)large(b)medium and(c)small spacing.An acoustic Doppler velocimeter was used for flow measurements in a rectangular channel and the results were compared with those acquired from numerical simulation.The time-averaged velocity profiles at the near-wake flow zones of boulders experience maximum flow retardation which is an outcome of the boulder-induced form roughness.The ratio of velocity differences associated to form and skin roughness and its positive magnitude reveals the dominance of form roughness closest to the boulders.Form roughness computed is 1.75 to 2 times higher than the skin roughness at the near-wake flow region.In particular,the collective immobile boulders placed at different inter-boulder spacings developed high and low bed shear stresses closest to the boulders.The low bed shear stresses characterised by a secondary peak developed at the trough location of the boulders is attributed to the skin shear stress.Further,the spatial averaging of time-averaged flow quantities gives additional impetus to present an improved illustration of fluid shear stresses.The formation of form-induced shear stress is estimated to be 17%to 23%of doubleaveraged Reynolds shear stress and partially compensates for the damping of time-averaged Reynolds shear stress in the interfacial sub-layer.The quadrant analysis of spatial velocity fluctuations depicts that the form-induced shear stresses are dominant in the interfacial sub-layer and have no significance above the gravel-bed surface.

Experimental and simulation study on shear stress-induced erythrocyte damage based on vortex oscillator

Background:Shear stress-induced erythrocyte damage,namely hemolysis,is an important problem in the development of blood-contacting medical devices such as mechanical circulatory support devices.Computational fluid dynamics simulation combined with hemolysis prediction models have been widely used to predict hemolysis.With the development of hemolysis prediction models,the new hemolysis prediction model requires more experimental data to verify.In addition,the difference of in vitro blood-shearing device also affect the accuracy of hemolysis prediction.Methods:To address these problems,a new in vitro blood-shearing device(vortex oscillator)was used to further verify the accuracy of the hemolysis prediction models,and to guide the optimal design of blood-contacting medical devices such as mechanical circulatory support devices.Firstly,the flow field information such as wall stress and velocity of the vortex oscillator under different speeds was analyzed.Secondly,different hemolysis prediction models were used to calculate hemolysis,and the predicted data was compared with the experimental data.Results and Conclusion:In this study,the flow field information inside the vortex oscillator at high rotational speeds was systematically investigated,and the prediction of hemolysis was carried out.The results showed that the predicted data of hemolysis was significantly different from the experimental data,which indicated that it was urgent to establish a standardized in vitro blood-shearing platform to provide a reference for accurate hemolysis prediction.

Effects of temperature on critical resolved shear stresses of slip and twining in Mg single crystal via experimental and crystal plasticity modeling

Magnesium(Mg)single crystal specimens with three different orientations were prepared and tested from room temperature to 733 K in order to systematically evaluate effects of temperature on the critical resolved shear stress(CRSS)of slips and twinning in Mg single crystals.The duplex non-basal slip took place in the temperature range from 613 to 733 K when the single crystal samples were stretched along the<0110>direction.In contrast,the single basal slip and prismatic slip were mainly activated in the temperature range from RT to 733 K when the tensile directions were inclined at an angle of 45°with the basal and the prismatic plane,respectively.Viscoplastic self-consistent(VPSC)crystal modeling simulations with genetic algorithm code(GA-code)were carried out to obtain the best fitted CRSSs of major deformation modes,such as basal slip,prismatic slip,pyramidalⅡ,{1012}tensile twinning and{1011}compressive twinning when duplex slips accommodated deformation.Additionally,CRSSs of the basal and the prismatic slip were derived using the Schmid factor(SF)criterion when the single slip mainly accommodated deformation.From the CRSSs of major deformation modes obtained by the VPSC simulations and the SF calculations,the CRSSs for basal slip and{1012}tensile twinning were found to show a weak temperature dependence,whereas those for prismatic,slip and{1011}compressive twinning exhibited a strong temperature dependence.From the comparison of previous results,VPSC-GA modeling was proved to be an effective method to obtain the CRSSs of various deformation modes of Mg and its alloys.

Stiffness and Shear Stress Distribution of Glulam Beams in Elastic-Plastic Stage:Theory,Experiments and Numerical Modelling

Traditional methods focus on the ultimate bending moment of glulam beams and the fracture failure of materials with defects,which usually depends on empirical parameters.There is no systematic theoretical method to predict the stiffness and shear distribution of glulam beams in elastic-plastic stage,and consequently,the failure of such glulam beams cannot be predicted effectively.To address these issues,an analytical method considering material nonlinearity was proposed for glulam beams,and the calculating equations of deflection and shear stress distribution for different failure modes were established.The proposed method was verified by experiments and numerical models under the corresponding conditions.Results showed that the theoretical calculations were in good agreement with experimental and numerical results,indicating that the equations proposed in this paper were reliable and accurate for such glulam beams with wood material in the elastic-plastic stage ignoring the influence of mechanic properties in radial and tangential directions of wood.Furthermore,the experimental results reported by the previous studies indicated that the method was applicable and could be used as a theoretical reference for predicting the failure of glulam beams.

Prediction of boundary shear stress distribution in straight open channels using velocity distribution

Conventional methods for measuring local shear stress on the wetted perimeter of open channels are related to the measurement of the very low velocity close to the boundary.Measuring near-zero velocity values with high fluctuations has always been a difficult task for fluid flow near solid boundaries.To solve the observation problems,a new model was developed to estimate the distribution of boundary shear stress from the velocity distribution in open channels with different cross-sectional shapes.To estimate the shear stress at a point on the wetted perimeter by the model,the velocity must be measured at a point with a known normal distance to the boundary.The experimental work of some other researchers on channels with various cross-sectional shapes,including rectangular,trapezoidal,partially full circular,and compound shapes,was used to evaluate the performance of the proposed model.Optimized exponent coefficients for the model were found using the multivariate Newton method with the minimum of the mean absolute percentage error(MAPE)between the model and experimental data as the objective function.Subsequently,the calculated shear stress distributions along the wetted perimeter were compared with the experimental data.The most important advantage of the proposed model is its inherent simplicity.The mean MAPE value for the seven selected cross-sections was 6.9%.The best results were found in the cross-sections with less discontinuity of the wetted perimeter,including the compound,trapezoidal,and partially full circular pipes.In contrast,for the rectangular cross-section with an angle between the bed and walls of 90°,MAPE increased due to the large discontinuities.

Stationary Flow of Blood in a Rigid Vessel in the Presence of an External Magnetic Field: Considerations about the Forces and Wall Shear Stresses

The magnetohydrodynamics laws govern the motion of a conducting fluid, such as blood, in an externally applied static magnetic field B0. When an artery is exposed to a magnetic field, the blood charged particles are deviated by the Lorentz force thus inducing electrical currents and voltages along the vessel walls and in the neighboring tissues. Such a situation may occur in several biomedical applications: magnetic resonance imaging (MRI), magnetic drug transport and targeting, tissue engineering… In this paper, we consider the steady unidirectional blood flow in a straight circular rigid vessel with non-conducting walls, in the presence of an exterior static magnetic field. The exact solution of Gold (1962) (with the induced fields not neglected) is revisited. It is shown that the integration over a cross section of the vessel of the longitudinal projection of the Lorentz force is zero, and that this result is related to the existence of current return paths, whose contributions compensate each other over the section. It is also demonstrated that the classical definition of the shear stresses cannot apply in this situation of magnetohydrodynamic flow, because, due to the existence of the Lorentz force, the axisymmetry is broken.

Inner-outer decomposition of wall shear stress fluctuations in turbulent channels

Fluctuating wall shear stress in turbulent channel flows is decomposed into small-scale and large-scale components.The large-scale fluctuating wall shear stress is computed as the footprints of the outer turbulent motions,and the small-scale one is obtained by subtracting the large-scale one from the total,which fully remove the outer influences.We show that the statistics of the small-scale fluctuating wall shear stress is Reynolds number independent at the friction Reynolds number larger than 1000,while which is Reynolds number dependent or the low-Reynolds-number effect exists at the friction Reynolds number smaller than 1000.Therefore,a critical Reynolds number that defines the emergence of universal small-scale fluctuating wall shear stress is proposed to be 1000.The total and large-scale fluctuating wall shear stress intensities approximately follow logarithmic-linear relationships with Reynolds number,and empirical fitting expressions are given in this work.

Influence of Hartmann Number on Convective Flow of Maxwell Fluid between Two Hot Parallel Plates through Porous Medium Subject to Arbitrary Shear Stress at the Boundary

Natural convection flow of unsteady Maxwell fluid with the effects of constant magnetic force in the course of a porous media is investigated in this research work. Fluid motion between a channel of parallel plates is tempted by time dependent shear stress applied on one plate. The governing partial differential equations of a model under consideration are transformed into ordinary differential equations by Laplace transform method and then solved for temperature and velocity fields. The obtained results for temperature fields are expressed in terms of complementary error function. The influences of involved parameters likes Hartmann number, Grashf number, Prandlt number and porosity parameter, on temperature and velocity profiles are shown graphically. There is no such result regarding Maxwell fluid in the existing literature.

Dynamic Shear Stress Measurements of PMMA

In this paper,experiments of one-dimensional plane plate impact on polymethylmethacrylate(PMMA) targets are conducted,in which dynamic transverse stresses induced in the targets are measured using a new type of manganin piezoresistive stress gauge having 50 Ω low-pressure narrow grid-like foil.It is shown that this new instrument can improve measurement accuracy remarkably by reducing the so-called strain effect.Moreover,relationship between shear stress and longitudinal stress within a certain range of the latter is obtained.

Cracking behaviors and strength of rock-like samples with steps of different geometries under shear stress

Steps are distinctive features for estimating the movement of the upper and lower block of faults.However,studies about the influence of steps as a special type of discontinuity on cracking behaviors and strength of rock masses are limited.In this research,rock-like samples with steps and preexisting flaws were fabricated.Step height h and the inclination angle of gentle slope of the stepαwere set to different values.Direct shear tests were conducted on these samples under different normal stresses.The experimental results reveal that the inclination angle of the gentle slope of the stepα,step height h,and normal stress have an influence on the strength,crack initiation,and crack propagation of the samples.The experimental results show that crack behaviors and shear strength were affected by step inclination anglesαand step height h.As the normal stress increases,the improvement of the strength of samples with a large step height is larger than that of samples with a small step height,the improvement of the strength of samples withαof 10°is larger than that of samples withαof 0°and-10°.The discrete element method was used to simulate the shear test.Numerical results show five different types of displacement vectors,which can be used to determine whether the cracks are tensile cracks or shear cracks.The above conclusions can provide help for estimating mechanical properties and failure modes of rock masses with steps of different geometries.

The crosstalk between endothelial cells and vascular smooth muscle cells during low shear stress:a proteomic-based approach

Instruction Shear stress,caused by the parallel frictional drag force of blood flow,is a biomechanical force which plays an important role in the control of blood vessels growth and functions [1]. Clinical researches had found out that atherosclerotic le-

Effects of shear stress on differentiation of stem cells into endothelial cells

Stem cell transplantation is an appealing potential therapy for vascular diseases and an indispensable key step in vascular tissue engineering.Substantial effort has been made to differentiate stem cells toward vascular cell phenotypes,including endothelial cells(ECs)and smooth muscle cells.The microenvironment of vascular cells not only contains biochemical factors that influence differentiation but also exerts hemodynamic forces,such as shear stress and cyclic strain.More recently,studies have shown that shear stress can influence the differentiation of stem cells toward ECs.A deep understanding of the responses and underlying mechanisms involved in this process is essential for clinical translation.This review highlights current data supporting the role of shear stress in stem cell differentiation into ECs.Potential mechanisms and signaling cascades for transducing shear stress into a biological signal are proposed.Further study of stem cell responses to shear stress will be necessary to apply stem cells for pharmacological applications and cardiovascular implants in the realm of regenerative medicine.

Heat Transfer of Casson Fluid over a Vertical Plate with Arbitrary Shear Stress and Exponential Heating

The basic objective of this work is to study the heat transfer of Casson fluid of non-Newtonian nature.The fluid is considered over a vertical plate such that the plate exhibits arbitrary wall shear stress at the boundary.Heat transfers due to exponential plate heating and natural convection are due to buoyancy force.Magnetohydrodynamic(MHD)analysis in the occurrence of a uniform magnetic field is also considered.The medium over the plate is porous and hence Darcy’s law is applied.The governing equations are established for the velocity and temperature fields by the usual Boussinesq approximation.The problem is first written in dimensionless form using some useful non-dimensional quantities and then solved.The exact analysis is performed and hence solutions via integral transform are established.The analysis of various pertinent parameters on temperature distribution and velocity field are reported graphically.It is found that pours medium permeability parameter retards the fluid motion whereas,velocity decreases with increasing magnetic parameter.Velocity and temperature decrease with increasing Prandtl number whereas the Grashof number enhances the fluid motion.Further,it is concluded from this study that the results obtained here are more general and in a limiting sense several other solutions can be recovered.The Newtonian fluid results can be easily established by taking the Casson parameter infinitely large i.e.,whenβ→∞.

Membrane fluidity regulates high shear stress-induced FAK activation at different subcellular compartments

Focal adhesion kinase(FAK)plays a vital role in mediating the adaptability of tumor cells under mechanical stimuli.Previous studies revealed that FAK can locate to different cell compartments,and its regulation is highly dependent on its subcellular localization.However,the local FAK activities and its regulation mechanism in different cell compartments of tumor cells in response to fluid shear stress are still unclear.In this study,5 dyn/cm^(2) and 20 dyn/cm^(2) of shear stress was applied to HeLa cells for 30 min.The activities of FAK targeting different subcellular compartments(lipids rafts,non-rafts,focal adhesions and cytoplasm)were investigated with fluorescence resonance energy transfer(FRET)technology.Results showed that the activity of FAK in response to high shear stress at focal adhesion sites was lower than that of other three areas,while no difference among four areas was observed in response to low shear stress.Furthermore,high shear stress-induced distinct FAK activation at different compartments was inhibited by decreasing membrane fluidity,but Src inhibition prevented high shear stress-induced FAK activation only in the cytoplasm.This study revealed the spatiotemporal characteristics of FAK under the different magnitude of shear stress,which provides a deeper understanding of mechanotransduction in tumor cells.

Dislocation Vibration, Morse Potential Function and Temperature Dependence of Critical Resolved Shear Stress of Some Single Crystals

By using Morse potential model,we calculate the temperature dependence of critical resolved shear stress (CRSS) of some single crystals. Dislocation-phonon interaction is considered in this paper. Theoretical calculations fit experimental law well. Investigations show: interatomic force is the physical intrinsic that controls the plastic deformation processing of materials, and thetemperature dependence of CRSS mainly comes from the temperature dependence of dislocationvibration.

Characterizing Clustering in Boulder Bed Channels and the Impact on Shear Stress Equations

Boulders and cobbles are often used in stream restoration projects to increase flow resistance and enhance channel stability and habitat diversity. Particle size metrics determined from the particle distribution are often used as a proxy for shear stress in field equations. Clustering of large particles has been thought to contribute to shear stress, but the effect of clustering is not accounted for in equations that use a representative particle size, such as the D84. In this paper, clustering is defined using the upper tail (≥84%) in a variable called Topsum. The number of clusters, average size of clusters, and shear stress are evaluated using the proposed definition of cluster. Findings suggest that the upper tail represents the roughness height better than the commonly used proxy of D84 for boulder bed streams (streams which have a D84 particle 0.05 - 0.15 meters).

Role of Interatomic Force to Critical Resolved Shear Stress of Single Crystals

Experimental results of the temperature dependence of critical resolved. shear stresses (CRSS)of Mo, Fe, Al and Mg single crystals are shown. Associating reports in recent years, we point out that the approximate exponential relationship between CRSS and the absolute temperatureat least in the region of the steep temperature dependence range of many materials is more common, even for bcc, fcc, and hcp single crystals. polycrystals and other covalent crystals,provided that the slip plane and slip direction are kept the same. Successful explanation with atomic force law shows that the interatomic forces (electronic structure) play a decisive role in determining the temperature dependence of yield stresses for a large number of materials.

The effect of laminar shear stress on the expression of matrixmetalloproteinases-9 in rat MSCs

To investigate the effects of mechanical factors on matrix metalloproteinase9(MMP-9) expressions in rat bone marrow-derived mesenchymal stem cells(MSCs) and possible mechanism signal.Rat bone marrow MSCs were isolated and cultured,then,exposed to laminar shear stress

A new analytical solution for calculation the displacement and shear stress of fully grouted rock bolts and numerical verifications

In presence of difficult conditions in coal mining roadways, an adequate stabilization of the excavation boundary is required to ensure a safe progress of the construction. The stabilization of the roadways can be improved by fully grouted rock bolt, offering properties optimal to the purpose and versatility in use. Investigations of load transfer between the bolt and grout indicate that the bolt profile shape and spacing play an important role in improving the shear strength between the bolt and the surrounding strata. This study proposes a new analytical solution for calculation displacement and shear stress in a fully encapsulated rock bolt in jointed rocks. The main characteristics of the analytical solution consider the bolt profile and jump plane under pull test conditions. The performance of the proposed analytical solution, for three types of different bolt profile configurations, is validated by ANSYS software. The results show there is a good agreement between analytical and numerical methods. Studies indicate that the rate of displacement and shear stress from the bolt to the rock exponentially decayed. This exponential reduction in displacement and shear stress are dependent on the bolt characteristics such as: rib height, rib spacing, rib width and grout thickness, material and joint properties.