Showing papers on "Shear stress published in 1996"

Rheology of magnetorheological fluids: models and measurements

Abstract: Numerical and analytical models of magnetorheological fluid phenomena that account explicitly for the effects of magnetic nonlinearity and saturation are described. Finite-element analysis was used to calculate the field distribution in chains of magnetizable particles. The field-dependent stress required to shear the chains was then obtained using the Maxwell stress tensor. Three regimes are identified: at low applied fields, the stress increase quadratically, as expected from linear magnetostatics. In intermediate fields, the contact or polar regions of each particle saturate, reducing the rate of increase of the stress with increasing field. At high fields, the particles saturate completely, and the stress reaches its limiting value. Approximate analytical expressions for the yield stress and shear modulus in these regimes are also derived. The predictions of these models are compared to magnetorheological experiments in the literature and from our laboratory. These models predict successfully the magn...

Association Between Intima-Media Thickness and Wall Shear Stress in Common Carotid Arteries in Healthy Male Subjects

Abstract: Background Atherosclerotic lesions lie in regions of low wall shear stress. No relationship between wall shear stress and intima-media thickness in vivo has been reported. Aims of the present study were to verify the reproducibility of wall shear stress measurement in vivo and to evaluate its association with intima-media thickness in the common carotid artery in healthy subjects. Methods and Results Wall shear stress was calculated according to the following formula: Shear Stress=Blood Viscosity×Blood Velocity/Internal Diameter. Blood viscosity was measured by use of a cone/plate viscometer. Blood velocity, internal diameter, and intima-media thickness were measured by high-resolution echo Doppler. Twenty-one healthy male subjects were investigated. Peak and mean shear stress values were 29.5±8.2 and 12.1±3.1 dynes/cm−2 (mean±SD), respectively. Peak shear stress was inversely related to intima-media thickness (r=.62), age (r=.77), systolic blood pressure (r=.61), and body mass index (r=.59) (P<.0001 for ...

Shear Stress Modulates Expression of Cu/Zn Superoxide Dismutase in Human Aortic Endothelial Cells

Abstract: A major determinant of the level of cellular superoxide anion (O2-.) is the dismutation of O2-. to hydrogen peroxide by the enzyme superoxide dismutase (SOD). Three forms of SOD exist, but in endothelial cells, the major form outside of the mitochondria is the cytosolic copper/zinc-containing superoxide dismutase (Cu/Zn SOD). Since fluid shear stress is an important determinant of the function and structure of endothelial cells in vivo, we examined the effect of laminar shear stress on the expression of Cu/Zn SOD in cultured human aortic endothelial cells. Laminar shear stress of 0.6 to 15 dyne/cm2 increased Cu/Zn SOD mRNA in a time- and dose-dependent manner in human aortic endothelial cells. Shear stress also increased both Cu/Zn SOD protein content and the enzyme activity. Nuclear runon assays showed that nuclei from human aortic endothelial cells exposed to laminar shear stress had a 1.6-fold greater transcriptional activity of the Cu/Zn SOD gene compared with cells not exposed to shear, indicating that an increase in Cu/Zn SOD mRNA induced by laminar shear stress is at least in part mediated by increased transcription. In contrast, shear stress had no effect on Cu/Zn SOD mRNA levels in human aortic smooth muscle cells. These findings show that physiological levels of shear stress increase expression of Cu/Zn SOD in the endothelium. This adaptation to shear stress might augment the effect of locally produced NO. and thereby promote the antiatherogenic and anti-inflammatory properties of the endothelial cell.

Dynamically propagating shear bands in impact-loaded prenotched plates—I. Experimental investigations of temperature signatures and propagation speed

Abstract: The initiation and propagation of shear bands are investigated by subjecting prenotched plates to asymmetric impact loading (dynamic mode-II). The materials studied are C-300 (a maraging steel) and Ti-6Al-4V. A shear band emanates from the notch tip and propagates rapidly in a direction nearly parallel to the direction of impact. When the impact velocity is higher than a critical value, the shear band propagates throughout the specimen. The shear band arrests inside the specimen when the impact velocity is below this critical value. In the latter case and for the C-300 steel, a crack initiates and propagates from the tip of the arrested shear band at an angle to the direction of shear band propagation. Microscopic examinations of the shear band and crack surfaces reveal a ductile mode of shear failure inside the shear band and an opening mode of failure for the crack. The coexistence of shear banding and fracture events in the same specimen signifies a transition in the modes of failure for this material under the conditions described. For Ti-6Al-4V, the only mode of failure observed is shear banding. While the transition is induced by changes in loading conditions, the different behaviors of these two materials suggest it is also related to material properties. The experimental investigation focuses on both the thermal and the mechanical aspects of the propagation of shear bands. Real time temperature histories along lines intersecting and perpendicular to and along the shear band path are recorded by means of a high speed infrared detector system. Experiments show that the peak temperatures inside the propagating shear bands increase with impact velocity. The highest temperature measured is in excess of 1400 °C or approximately 90% of the melting point of the C-300 steel. For Ti-6Al-4V, the peak temperatures are approximately 450 °C. In the mechanical part of the study, high speed photography is used to record the initiation and propagation of shear bands. Recorded images of propagating shear bands at different impact velocities provide histories of the speed of shear band propagation for the C-300 steel. A strong dependence of shear band speed on the impact velocity is found. The highest speed observed for the C-300 steel is approximately 1200 ms−1 or 40% of its shear wave speed.

Relationship between rheology and morphology of model blends in steady shear flow

Abstract: Immiscible polymer blends display a complex flow behavior caused by the coupling between morphology and rheology. The flow induced microstructure has been studied on model systems of nearly inelastic polymers. For these systems, the elastic properties of the blend are mainly governed by the interface. Measurements of the storage modulus and of the first normal stress difference, both reflecting this enhanced elasticity, have been used to probe the blend morphology. From oscillatory measurements after cessation of flow the mean diameter of the disperse phase, as generated by the previous flow, has been calculated using the model of Palierne [Rheol. Acta 29, 204 (1990)]. A procedure based on a direct fitting of the dynamic moduli with the model is compared with one that uses a weighted relaxation spectrum. The steady state normal stress data, on the other hand, have been related to the morphology of the blend by means of the model of Doi and Ohta [J. Chem. Phys. 95, 1242 (1991)]. Since this model predicts a direct proportionality between the contribution of the interface to the normal stress and the specific interfacial area, the size of the droplets can be calculated once the proportionality constant is known. The conditions for which the model is valid have been determined and the required constant is obtained by comparing the results with those from the dynamic moduli. The resulting droplet sizes have been used to develop a data reduction scheme: The specific interfacial area is found to be inversely proportional to the ratio of interfacial tension over shear stress for several blends with various concentrations and viscosity ratios.

Estimating Local Bed Shear Stress from Velocity Observations

Abstract: Replicate velocity observations using conventional equipment under typical field conditions are used to evaluate the precision of different methods for estimating local boundary shear stress from velocity measurements. The bed shear velocity u, can be estimated within 3% using the depth-averaged velocity in the vertically averaged logarithmic velocity profile. To be accurate, this method is limited to relatively simple flow geometries which may be expected to have the appropriate velocity structure. Estimates of u, made using a single near-bed velocity observation are less precise by a factor of 3 because of the larger uncertainty associated with a single observation. Accuracy of this method requires appropriate flow conditions only near the bed, so it may be applied in a wider range of flow conditions, including spatially variable flow. Estimates of u, from the slope of the near-bed velocity profile are the least precise and require the most restrictive flow conditions for accuracy but offer the advantage that they may be made without independent knowledge of the bed roughness. different methods for estimating roe should be useful in judging the precision of estimates made under similar conditions. These estimates of precision, together with considerations of convenience and model accuracy, provide the basis for select- ing the most appropriate or advantageous method for different conditions and purposes.

Growth model of the inner core coupled with the outer core dynamics and the resulting elastic anisotropy

Abstract: We present a growth tectonic model of Earth's inner core and the resulting model of the seismic anisotropy. The inner core grows anisotropically if the convection in the outer core is of Taylor column type. The anisotropic growth produces a flow field of the poloidal zonal order 2 type as a result of the isostatic adjustment of the viscous inner core. Crystals in the inner core align themselves under the stress field produced by the flow. We model the anisotropic structure of the inner core, using the theory of Kamb [1959] and elastic constants of Stixrude and Cohen [1995b]. We consider models for both hcp iron and fcc iron, which are the probable crystal structures for the inner core iron according to Stixrude and Cohen [1995a]. We have found that the c axis for hcp iron and [111] direction for fcc iron align in the polar direction. The alignment is consistent with seismic observations, which have revealed that the P wave velocity is faster in the polar direction. Our model predicts that the degree of the alignment decreases near the inner core boundary in accord with recent body wave observations. The radial dependence of the alignment would result from the following three effects: (1) crystals near the surface have not undergone stressed state long enough to acquire anisotropy after precipitation, (2) stress near the surface is different from that in the interior of the inner core due to shear stress free boundary condition, and (3) partially molten structure results in transversely isotropic stress condition near the inner core surface due to compaction. Thus the application of Kamb's theory successfully explains the seismic anisotropy in the inner core provided that the crystals have been subjected under the same stress condition for the timescale of the order of 109 years.

Remodeling of the Radial Artery in Response to a Chronic Increase in Shear Stress

Abstract: Chronic changes in large artery blood flow rates induce corresponding adjustments in arterial diameter, but little is known about structural adaptations of the vessel wall in humans. We used a high-resolution echo-tracking system to measure radial artery internal diameter, wall thickness, and mean blood flow on both arms of 11 patients with end-stage renal disease. Measurements were performed on the wrist side of the arteriovenous fistula. The contralateral radial artery was investigated as control. Wall cross-sectional area, circumferential wall stress, and mean wall shear stress were calculated. Results indicate a sixfold increase in blood flow on the side of the arteriovenous fistula compared with the control side, with a 1.4-fold increase in internal diameter. The diameter enlargement was sufficient to normalize wall shear stress. Changes in diameter were not associated with arterial wall hypertrophy because wall cross-sectional area was not increased and rather suggest a "remodeling" of the arterial wall. For the same level of blood pressure, circumferential wall stress was increased on the side of the arteriovenous fistula. These results suggest that the structural adaptations of the arterial wall to a chronic increase in blood flow normalize wall shear stress and overcome stretch-induced changes in the particular circumstance of arteriovenous fistula.

Short communication radiation effects on MHD free convection flow of a gas past a semi—infinite vertical plate

Abstract: Free convection heat transfer due to the simultaneous action of buoyancy, radiation and transverse magnetic field is investigated for a semi‐infinite vertical plate Solutions are derived by expanding the stream function and the temperature into a series in terms of the parameter ζ = x1/2 L–1/2, where L is the length of the plate Velocity and temperature functions are shown on graphs and the numerical values of functions affecting the shear stress and the rate of heat transfer are entered in a table The effects of the magnetic field parameter λ and the radiation parameter F on these functions are discussed

Steady flow in an aneurysm model: correlation between fluid dynamics and blood platelet deposition.

Abstract: Laminar and turbulent numerical simulations of steady flow in an aneurysm model were carried out over Reynolds numbers ranging from 300 to 3600. The numerical simulations are validated with Digital Particle Image Velocimetry (DPIV) measure­ ments, and used to study the fluid dynamic mechanisms that characterize aneurysm deterioration, by correlating them to in vitro blood platelet deposition results. It is shown that the recirculation zone formed inside the aneurysm cavity creates condi­ tions that promote thrombus formation and the viability of rupture. Wall shear stress values in the recirculation zone are around one order of magnitude less than in the entrance zone. The point of reattachment at the distal end of the aneurysm is charac­ terized by a pronounced wall shear stress peak. As the Reynolds number increases in laminar flow, the center of the recirculation region migrates toward the distal end of the aneurysm, increasing the pressure at the reattachment point. Under fully turbulent flow conditions (Re = 3600) the recirculation zone inside the aneurysm shrinks considerably. The wall shear stress values are almost one order of magnitude larger than those for the laminar cases. The fluid dynamics mechanisms inferred from the numerical simulation were correlated with measurements of blood platelet deposition, offering useful explanations for the different morphologies of the platelet deposition curves.

The effect of surfactant on the rise of a spherical bubble at high Reynolds and Peclet numbers

Abstract: Experiments and numerical simulations of rising spherical bubbles in quiescent surfactant solutions are presented. The rise velocities versus the concentration in the bulk are measured using three surfactants, Triton X100, Brij30 and SDS for different bubble sizes, between 0.4 and 1 mm equivalent radius. We also present a brief description of the finite‐difference numerical method developed to solve the full Navier‐Stokes equations around the contaminated bubble for Reynolds numbers ranging from 50 to 200. The distributions of the tangential velocity, the vorticity, the pressure and the surfactant concentration on the bubble surface are calculated. In the case of high Peclet numbers surfactant molecules, which adsorb on the surface are convected and collected at the rear part of the bubble forming a stagnant cap where the no‐slip condition holds. The concentration on the bubble interface is obtained for surfactants having a desorption rate much slower than the convective rate. The sudden increase of the shear stress and pressure at the leading edge of the cap contributes mainly to decrease the rise velocity. This rapid slowdown of the bubble occurs when nearly half of the bubble surface is covered by the surfactant layer, and this is due to the particularly high values obtained for the shear stress and the pressure at the leading edge of this cap‐angle. Measured and calculated rise velocities for bubbles of 0.4 mm equivalent radius show good agreement when the sorption kinetics controls the surfactant exchange between the bulk and the surface. Calculated critical concentrations needed to cover completely the bubble agree with the measurements even for larger bubbles.

Analysis and testing of Bingham plastic behavior in semi-active electrorheological fluid dampers

Abstract: Electrorheological- (ER-) fluid-based dashpot dampers have smart capabilities because ER fluids undergo large changes in yield stress as electric field is applied. Our objective is the development and experimental validation of quasi-steady dashpot damper models, based on an idealized nonlinear Bingham plastic shear flow mechanism, for purposes of preliminary design and performance predictions. The data required for the Bingham plastic model is normally supplied by ER fluid suppliers, that is, plastic viscosity and dynamic yield stress as a function of applied field, as determined from a shear stress versus shear strain rate diagram. As force is applied to the dashpot damper, the ER fluid flows through an annulus between the concentric inner and outer electrodes. The idealized Bingham plastic shear flow mechanism predicts that three annular flow regions develop as a function of the local shear stress. In the central pre-yield or plug region, the local shear stress is less than the dynamic yield stress, so that the plug behaves like a rigid solid. The remaining two annular regions, adjacent to the electrodes, are in the post-yield condition and correspond to the shear stress exceeding the dynamic yield stress, so that the material flows. Equivalent viscous damping performance of an ER fluid dashpot damper is strongly coupled with the plug behavior. For a constant force, as the applied field increases, so does the plug thickness and equivalent viscous damping. For a constant applied field, as the force increases, the plug thickness and equivalent viscous damping both decrease. The passive and active or field-dependent damping behavior of an ER-fluid-based dashpot damper can be designed for a specific application using these quasi-steady Bingham plastic models.

Stress-induced azimuthal anisotropy in borehole flexural waves

Abstract: A nonlinear elastic model for acoustic waves in a stressed medium is used to calculate tectonic stress-induced changes in borehole flexural dispersions. Our theoretical analysis shows that a horizontal uniaxial stress in the formation causes a crossover in flexural dispersions for the radial polarization aligned parallel and normal to the stress direction. This crossover in flexural dispersions is caused by stress-induced radial heterogeneities in acoustic wave velocities that are different in the two principal stress directions. Other sources of borehole flexural anisotropy caused by finely layered dipping beds, aligned fractures, or microstructures found in shales, exhibit neither such radial heterogeneities nor flexural dispersion crossovers. Consequently, a crossover in flexural dispersion can be used as an indicator of stress-induced anistropy. In this situation, the fast shear polarization direction coincides with the far-field uniaxial stress direction. The analysis also yields an expression for the largest shear stress parameter in terms of the fast and slow seismic shear-wave velocities with shear polarization parallel and perpendicular to the far-field stress direction.

Turbulence production in flow over a wavy wall

Abstract: Measurements of the spatial and time variation of two components of the velocity have been made over a sinusoidal solid wavy boundary with a height to length ratio of 2a/λ = 0.10 and with a dimensionless wave number of α+ = (2π/λ)(v/u ⋆) = 0.02. For these conditions, both intermittent and time-mean flow reversals are observed near the troughs of the waves. Statistical quantities that are determined are the mean streamwise and normal velocities, the root-meansquare of the fluctuations of the streamwise and normal velocities, and the Reynolds shear stresses. Turbulence production is calculated from these measurements. The flow is characterized by an outer flow and by an inner flow extending to a distance of about α−1 from the mean level of the surface. Turbulence production in the inner region is fundamentally different from flow over a flat surface in that it is mainly associated with a shear layer that separates from the back of the wave. Flow close to the surface is best described by an interaction between the shear layer and the wall, which produces a retarded zone and a boundary-layer with large wall shear stresses. Measurements of the outer flow compare favorably with measurements over a flat wall if velocities are made dimensionless by a friction velocity defined with a shear stress obtained by extrapolating measurements of the Reynolds stress to the mean levels of the surface (rather than from the drag on the wall).

Shear margins in glaciers and ice sheets

Abstract: Analytical and numerical techniques are used to examine the flow response of a sloped slab of power-law fluid (power n) subjected to basal boundary conditions that vary spatially across the flow direction, as for example near an ice-stream margin with planar basal topography. The primary assumption is that basal shear stress is proportional to the basal speed times a spatially variable slip resistance. The ratio of mean basal speed to the speed originating from shearing through the thickness. denoted as r, gives a measure of how slippery the bed is. The principal conclusion is that a localized disturbance in slip resistance affects the basal stress and speed in a zone spread over a greater width of the flow. In units of ice thickness H, the spatial scale of spreading is proportional to a single dimensionless number R n ≡ (r/n+ 1)1/n+1 derived from n and r. The consequence for a shear zone above a sharp jump in slip resistance is that the shearing is spread out over a boundary layer with a width proportional to R n. For an ice stream caused by a band of low slip resistance with a half-width of w H, the margins influence velocity and stress in the central part of the band depending on Rn in comparison to w. Three regimes can be identified, which for n = 3 are quantified as follows: low r defined as R 3 1w, for which the boundary layers from both sides bridge across the full flow width and the driving stress in the center is supported almost entirely by side drag; intermediate r, for which the driving stress in the center is supported by a combination of basal and side drag. Shear zones that are narrower than predicted on the basis of this theory (≈ R 3) would require localized softening of the ice to explain the concentration of deformation at a shorter scale.

Stress concentrations around multiple fiber breaks in an elastic matrix with local yielding or debonding using quadratic influence superposition

Abstract: A new computational technique, called the quadratic influence superposition (QIS) technique, is developed to study the stresses around arbitrary arrays of fiber breaks in a unidirectional composite loaded in simple tension, and consisting of elastic fibers in a matrix, which is either elastic-perfectly plastic or which can debond at the interface leaving residual friction. The method involves extending a recently developed break influence superposition (BIS) technique, where to model the behavior of damaged (yielded or debonded) matrix elements, we use special compensating shear stress profiles and develop the corresponding influence functions. The QIS technique appears to be at least an order of magnitude more efficient than other numerical schemes as the computation time is tied mainly to the amount of damage, and it is more accurate than a simpler version of this technique developed earlier. In illustrative examples, the method determines the Mode I fiber and matrix stress distributions around a “center crack” consisting of up to 31 contiguous fiber breaks. Incremental treatment is needed to establish the extent of the inelastic regions and the results, which achieve excellent agreement with exact shear lag analyses, clearly show that QIS calculated these correctly. Results show that the extent of the matrix damage region increases approximately linearly with applied load and nonlinearly with the number of breaks. The stress concentrations and overload profiles along nearby unbroken fibers are altered as compared to the fully elastic case with magnitudes reduced but length scales increased.

Earthquakes as a coupled shear stress‐high pore pressure dynamical system

Abstract: The migration, coalescence and localization of slip, seismicity, and zones of high pore pressure are modeled using a porosity reduction mechanism to drive pore pressure within a fault zone in excess of hydrostatic. Increased pore pressure in discrete cells creates zones of low effective stress, which induces slip that may propagate to surrounding cells depending on the local state of stress. At slip, stress is transferred using the solution for a rectangular dislocation in an elastic half-space, and pore pressures are redistributed by conserving fluid mass. Using simple assumptions about fault rheology and permeability, it is shown that the interaction between shear stress and effective stress evolves to a state of earthquake clustering with repeated events, locked zones, and large variations in fault strength. The model evolves from a uniform shear stress state on a strong fault, to a heterogeneous shear stress state on a weak fault.

Turbulent flow structure at concordant and discordant open-channel confluences

Abstract: Models of flow at river-channel confluences that consist of two concordant confluent channels with avalanche faces dipping into a scour zone are limited because this morphology may be the exception rather than the rule in nature. In this paper the mean and turbulent flow structure in the streamwise and vertical directions at both concordant and discordant laboratory confluences were examined in order to determine the effect of bed discordance on the flow field, and to assess its influence on sediment transport. Instantaneous velocities were measured with a laser Doppler anemometer using a dense spatial sampling grid. The spatial distribution of normal stress varies with bed geometry as bed discordance generates a distortion of the mixing layer between the confluent streams. Turbulent shear stress is larger in the discordant bed case and its peak is associated with the position of the mixing layer whereas for concordant beds the zone of mixing is characterised by a decrease in the Reynolds shear stress. Quadrant analysis also revealed differential dominating quadrants between the two bed geometries which will influence sediment transport routing and, consequently, the resulting bed morphology. These results highlight the need for significant modifications to current models of confluence flow dynamics in order to account for the bed configuration.