Showing papers on "Shear stress published in 1994"

Direct simulation of initial value problems for the motion of solid bodies in a Newtonian fluid. Part 2. Couette and Poiseuille flows

Abstract: This paper reports the results of a two-dimensional finite element simulation of the motion of a circular particle in a Couette and a Poiseuille flow. The size of the particle and the Reynolds number are large enough to include fully nonlinear inertial effects and wall effects. Both neutrally buoyant and non-neutrally buoyant particles are studied, and the results are compared with pertinent experimental data and perturbation theories. A neutrally buoyant particle is shown to migrate to the centreline in a Couette flow, and exhibits the Segre-Silberberg effect in a Poiseuille flow. Non-neutrally buoyant particles have more complicated patterns of migration, depending upon the density difference between the fluid and the particle. The driving forces of the migration have been identified as a wall repulsion due to lubrication, an inertial lift related to shear slip, a lift due to particle rotation and, in the case of Poiseuille flow, a lift caused by the velocity profile curvature. These forces are analysed by examining the distributions of pressure and shear stress on the particle. The stagnation pressure on the particle surface are particularly important in determining the direction of migration.

Nucleation and growth of faults in brittle rocks

Abstract: We present a model for the nucleation and growth of faults in intact brittle rocks. The model is based on recent experiments that utilize acoustic emission events to monitor faulting processes in Westerly granite. In these experiments a fault initiated at one site without significant preceding damage. The fault propagated in its own plane with a leading zone of intense microcracking. We propose here that faults in granites nucleate and propagate by the interaction of tensile microcracks in the following style. During early loading, tensile microcracking occurs randomly, with no significant crack interaction and with no relation to the location or inclination of the future fault. As the load reaches the ultimate strength, nucleation initiates when a few tensile microcracks interact and enhance the dilation of one another. They create a process zone that is a region with closely spaced microcracks. In highly loaded rock, the stress field associated with microcrack dilation forces crack interaction to spread in an unstable manner and recursive geometry. Thus the process zone propagates unstably into the intact rock. As the process zone lengthens, its central part yields by shear and a fault nucleus forms. The fault nucleus grows in the wake of the propagating process zone. The stress fields associated with shear along the fault further enhance the microcrack dilation in the process zone. The analysis shows that faults should propagate in their own plane, making an angle of 20°–30° with the maximum compression axis. This model provides a physical basis for “internal friction,” the empirical parameter of the Coulomb criterion.

Fluid shear stress induces a biphasic response of human monocyte chemotactic protein 1 gene expression in vascular endothelium

Abstract: The focal distribution of atherosclerotic lesions in the arterial tree is related to the local shear stress generated by blood flow, but the molecular basis of the atherogenic response of endothelial cells in these lesion-prone areas is still unclear. We report that shear stress mediates a biphasic response of monocyte chemotactic protein 1 (MCP-1) gene expression in vascular endothelial cells (EC). Northern blot analysis indicated that the level of MCP-1 mRNA in human umbilical vein EC (HUVEC) subjected to a shear stress of 16 dynes/cm2 (1 dyne = 10 microN) for 1.5 hr increased by 2- to 3-fold when compared with static cells. The MCP-1 gene expression decreased to the basal level at 4 hr and then declined further to become completely quiescent at 5 hr after the onset of shear. Once the gene expression was fully suppressed, it remained quiescent even after static incubation for 1.5 hr and would not respond to reshearing after this static incubation. However, if the postshearing incubation extended from 1.5 to 24 hr, the MCP-1 mRNA returned to the basal level and was then able to increase after the reapplication of shear stress. Nuclear run-on experiments showed that the shear-induced increased MCP-1 mRNA in HUVEC was regulated at the transcriptional level. By using cycloheximide, it was shown that de novo protein synthesis was not necessary for the induction of MCP-1 by shear stress. The biphasic response of MCP-1 gene expression was found in experiments in which the applied shear stress was 6, 16, or 32 dynes/cm2, and it was observed not only in HUVEC but also in HeLa cells, glioma cell lines, and skin fibroblasts. This in vitro study demonstrates that the response of MCP-1 gene to shear stress represents an immediate early gene activation and suggests that this gene is probably suppressed in EC that have been exposed to a constant shear stress.

Cyclic Threshold Shear Strains in Soils

Abstract: Based on a synthesis of published laboratory data, two types of cyclic threshold shear strain are examined and their approximate magnitudes identified for different types of soils. They are the linear cyclic threshold shear strain, γtl and the volumetric cyclic threshold shear strain, γtv, with γtv>γtl. These strains represent boundaries between fundamentally different categories of cyclic soil behavior. For cyclic strains below γtl, soil behaves essentially as a linearly elastic material. Between γtl and γtv, soil becomes markedly nonlinear but remains largely elastic because permanent changes of its microstructure still do not occur or are negligible. Above γtv, soil becomes increasingly nonlinear and inelastic, with significant permanent microstructural changes taking place under cyclic loading. That is, γtv, = the threshold separating cyclic strains that cause or do not cause significant permanent changes of soil microstructure. In practical terms, these microstructural changes are manifested in resid...

Shear stress-induced reorganization of the surface topography of living endothelial cells imaged by atomic force microscopy.

Abstract: We report the first topographical data of the surface of living endothelial cells at sub-light-microscopic resolution, measurements essential for a detailed understanding of force distribution in the endothelium subjected to flow. Atomic force microscopy was used to observe the surface topography of living endothelial cells in confluent monolayers maintained in static culture or subjected to unidirectional shear stress in laminar flow (12 dyne/cm2 for 24 hours). The surface of polygonal unsheared cells was smooth, with mean excursion of surface undulation between peak height (over the nucleus) and minima (at intercellular junctions) of 3.4 +/- 0.7 microns (mean +/- SD); the mean height to length ratio was 0.11 +/- 0.02. In cells that were aligned in the direction of flow after a 24-hour exposure to laminar shear stress, height differentials were significantly reduced (mean, 1.8 +/- 0.5 micron), and the mean height to length ratio was 0.045 +/- 0.009. Calculation of maximum shear stress and maximum gradient of shear stress (delta tau/delta x, where tau is shear stress at the cell surface) at constant flow velocity revealed substantial streamling of aligned cells that reduced delta tau/delta x by more than 50% at a nominal shear stress of 10 dyne/cm2. Aligned cells exhibited ridges extending in the direction of flow that represented imprints of submembranous F-actin stress-fiber bundles mechanically coupled to the cell membrane. The surface ridges, approximately 50 nm in height and 200 to 1000 nm in width, were particularly prominent in the periphery of the aligned cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Shear stresses in magnetorheological fluids: Role of magnetic saturation

Abstract: The interparticle forces and resulting shear stresses in a magnetorheological fluid are calculated. The field due to a linear chain of particles in a fixed average magnetic induction Bave is determined from a finite element analysis in which the nonlinearity and saturation of the particle magnetization are incorporated. The shear stresses are then computed from the field using Maxwell’s stress tensor. The stresses obtained for all but the lowest magnetic inductions are controlled by the saturation of the magnetization in the contact regions of each particle. Identifying the maximum shear stress as a function of shear strain with the yield stress gives values in agreement with results reported for typical fluids. For high magnetic inductions the yield stress plateaus due to the complete saturation of the particle magnetization; the stress scales as the square of the saturation magnetization in this regime.

Turbulence structure over two‐dimensional bed forms: Implications for sediment transport

Abstract: Turbulence measurements from the flow over two-dimensional fixed dune shapes are presented along with analysis and discussion of the ramifications of the observations for transport of sediment as bed load over bed forms The spatial structure of the local transport rate determines the shape and stability of bed forms such as ripples and dunes, and the transport of sediment is a highly nonlinear process that is profoundly affected by the statistics of the temporal fluctuations in the near-bed flow field The measurements presented herein show strong spatial evolution of the joint probability distribution of the streamwise and bednormal fluctuating velocity components Unlike measurements in uniform boundary layers, these distributions and the higher moments of the velocity do not scale with the local shear velocity, indicating that it is probably inappropriate to use the shear stress to characterize the sediment flux This conclusion is supported by observations of sediment flux over a dune

A new vectorial bedload formulation and its application to the time evolution of straight river channels

Abstract: The derivation of a new vectorial bedload formulation for the transport of coarse sediment by fluid flow is presented in the first part of the paper. This relation has been developed for slopes up to the angle of repose both in the streamwise and transverse directions. The pressure distribution is assumed to be hydrostatic. The bed shear stress for the onset of particle motion and mean particle velocity are obtained from the mean force balance on a particle. A new generalized Bagnold hypothesis is introduced to calculate the sediment content of the bedload layer. The new formulation possesses two innovative features. It is fully nonlinear and vectorial in nature, in addition, it behaves smoothly up to the angle of repose.A mathematical model of the time evolution of straight river channels is presented in the second half of the paper. This study focuses on the evolution process due to bank erosion in the presence of bedload only. The bed and bank material is taken to be coarse, non-cohesive and uniform in size. The sediment continuity and the fluid momentum conservation equations describe the time evolution of the bed topography and flow field. These equations are coupled through the fluid shear stress acting on the bed. This bed shear stress distribution is predicted with the aid of a simple algebraic turbulent closure model. As regards the computation of the sediment flux, the new fully nonlinear vectorial formulation is found to perform well and renders the evolution model fully mechanistic.The formation of an erosional front in the time development of straight river channels has been so far obscured in physical experiments. Herein, with the help of the new bedload formulation, the existence and migration speed of the front of erosion are inferred from the analysis of the sediment continuity equation.The model successfully describes the time relaxation of an initially trapezoidal channel toward an equilibrium cross-sectional shape, as evidenced by comparison with experimental data. This equilibrium is characterized by a constant width, vanishing sediment transport in the transverse direction, and a small but non-vanishing streamwise transport rate of bed sediment.

A shear stress based critical‐plane criterion of multiaxial fatigue failure for design and life prediction

Abstract: — The use of a previously presented general criterion of failure for high cycle multiaxial fatigue, τa/tA,B+σn.max/2σT= 1, is extended to cases where the shear and normal stress on the critical plane are non-proportional and also to give life predictions in the range of 104 to 106 cycles. The criterion takes account of whether case A cracks, growing along the surface, or case B cracks, growing in from the surface, occur.

Isotropic-to-nematic transition in wormlike micelles under shear

Abstract: We report on the linear and nonlinear rheology of surfactant solutions of elongated wormlike micelles. The surfactant solutions placed under scrutiny are made of cetylpyridinium chloride (CP + , Cl - ) and sodium salicylate (Na + , Sal - ) diluted in 0.5 M NaCl-brine. Both semidilute and concentrated regimes of entangled micelles were investigated. Rheological experiments were performed at ambient temperature (T=25 o C) for surfactant concentrations O=1%-30%. When submitted to a steady shear high enough (for shear rate γ typically higher than 1-10 s -1 ) the solutions of wormlike micelles exhibit a first-order isotropic-to-nematic transition for all surfactant concentrations O≥6%. The transition is characterized by a true plateau in the shear rate dependence of the shear stress σ (γ). For γ above the transition rate γ 1/N , σ remains constant at σ 1/N . In the concentrated regime, the transition is clearly first-order. However, the first-order character weakens upon increasing dilution, suggesting that at some critical concentration O c it becomes second-order. Below O c , the transition ceases to occur: the σ(γ)-behavior rather indicates a progressive and homogeneous orientation of the micelles throughout the sample. Moreover, in the two-phase domain (where both isotropic and nematic phases coexist) a characteristic transient behavior of the shear stress σ (γ) measured at constant γ 1/N as a function of time has been observed and investigated in detail. In agreement with the picture of the first-order phase transition (in the domain of metastability), the transient behavior could be interpreted quantitatively in terms of nucleation and one-dimensional growth process. These results are finally compared to recent predictions by Spenley, Cates and MacLeish (Ref, [7]) who described the nonlinear rheology of wormlike micelles in terms of mechanical instability of shear-banding type

Endothelial expression of thrombomodulin is reversibly regulated by fluid shear stress.

Abstract: The vascular endothelium, by virtue of its position at the interface between blood and the vessel wall, is known to play a critical role in the control of thrombosis and fibrinolysis. Thrombomodulin (TM) is a surface receptor that binds thrombin and is a potent activator of the protein C anticoagulant pathway. Although TM expression is known to be regulated by various cytokines, little is known about its response to ever-present biomechanical stimuli. We have explored the role of fluid shear stress, imparted on the luminal surface of the endothelial cell as a result of blood flow, on the expression of TM mRNA and protein in both bovine aortic endothelial (BAE) and bovine smooth muscle (BSM) cells in an in vitro system. We report in the present study that TM expression is regulated by flow. Subjecting BAE cells to fluid shear stress in the physiological range of magnitude of 15 (moderate shear stress) and 36 (elevated shear stress) dynes/cm2 resulted in a mild transient increase followed by a significant decrease in TM mRNA to 37% and 16% of its resting level, respectively, by 9 hours after the onset of flow. In contrast, shear stress at the low magnitude of 4 dynes/cm2 did not affect TM mRNA levels. The sensitivity of TM mRNA expression by flow was found to be specific to endothelium, since it was not observed in BSM cells exposed to steady laminar shear stress of 15 dynes/cm2. Furthermore, unlike BAE cells, BSM cells did not exhibit altered cell shape nor align in the direction of flow after 24 hours of shear stress at 15 dynes/cm2.(ABSTRACT TRUNCATED AT 250 WORDS)

Incipient sediment motion on non-horizontal slopes

Abstract: The study investigates the threshold condition for the initiation of cohesionless sediment transport on a nonhorizontal streamwise slope. Theoretical study of the stability of a sediment particle lying on a non-horizontal bed slope shows that the critical shear stress is a function of the streamwise bed slope. Experiments conducted with a closed-conduit flow show that the equation derived from force analysis adequately evaluates the critical shear stress of sediments lying on slopes ranging from steep positive to negative (adverse). Tests conducted with a closed-conduit flow avoid the problems associated with those conducted with an open channel flow. In the latter flow condition, uniform flow is often difficult to achieve in a steep channel, and impossible to achieve when the streamwise bed slope is adverse.

Shear‐induced particle migration in Couette and parallel‐plate viscometers: NMR imaging and stress measurements

Abstract: Couette and parallel plate viscometers are two commonly used flow geometries to characterize shear viscosity of concentrated suspensions. In Couette flow, it is well documented that prolonged shearing causes a decrease in the apparent viscosity of concentrated suspensions due to shear‐induced particle migration from the annulus region to the stagnant region under the bob. In this study, the technique of nuclear magnetic resonance imaging (NMRI) was used to measure the evolution of suspension concentration profiles in Couette and parallel‐plate flow devices upon shearing. Neutrally buoyant suspensions of nearly monodisperse, non‐Brownian spherical particles at a volume fraction of 0.5 in a Newtonian fluid were used. The same flow cells and suspensions were also used in a rheometer to measure the changes in shear stress under identical experimental conditions such that a direct comparison can be made between the stress and concentration data. For Couette flow, the NMRI data correlated very well with the str...

The role of the margins in the dynamics of an active ice stream

Abstract: A transverse profile of velocity was measured across Ice Stream B, West Antarctica, in order to determine the role of the margins in the force balance of an active ice stream. The profile extended from near the ice-stream center line, through a marginal shear zone and on to the slow-moving ice sheet. The velocity profile exhibits a high degree of shear deformation within a marginal zone, where intense, chaotic crevassing occurs. Detailed analysis of the profile, using analytical and numerical models of ice flow, leads to the following conclusions regarding the roles of the bed and the margins in ice-stream dynamics: (i) The overall resistive drag on the ice stream is partitioned nearly equally between the margins and the bed and, thus, both are important in the force balance of the ice stream. (ii) The ice within the chaotic zone must be about 10 times softer than the ice in the central part of the ice stream. (iii) The average basal shear stress is 0.06 × 105 Pa. This implies that the entire bed cannot be blanketed by the weak, deformable till observed by Engelhardt and others (1990) near the center of the ice stream — there must be regions of increased basal drag. (iv) High strain rates and shear stresses in the marginal zones indicate that strain heating in the margins may be significant. While the exact quantitative values leading to these conclusions are somewhat model and location-dependent, the overall conclusions are robust. As such, they are likely to have importance for ice-stream dynamics in general.

Wall shear stress temporal gradient and anastomotic intimal hyperplasia.

Abstract: Unusual wall shear stress patterns on the bed of end-to-side arterial anastomoses appear to be the primary factor in the development of intimal hyperplasia that often leads to arterial bypass graft failure. In this in vitro study using the photochromic tracer technique, the shear stress variation on the bed of a 30 degrees anastomosis was examined before and after the development of hyperplasia. With the disease-free model, a rapid downstream shift in the stagnation point was seen on the bed during the systolic phase of the flow cycle, which led to sharp temporal changes in the shear stress from positive to negative values, ie, rapid changes in the direction of the shearing force. The resulting negative temporal gradients were roughly eight times larger than those seen in a straight tube, and it is suggested that this may lead to endothelial deformation or injury and eventually to intimal thickening. With the diseased model, the simulated tissue overgrowth on the bed appeared to act as a flow divider that restricted the motion of the stagnation point, and this drastically reduced the rapid changes in the direction of the shearing forces. Thus, it seems that the development of bed anastomotic intimal hyperplasia may be a response designed to reduce shear-induced endothelial deformation or injury.

Aggregation and dispersion of spheres falling in viscoelastic liquids

Abstract: This paper focuses on the settling on one sphere near another or near a wall. We find maximum differences between Newtonian and viscoelastic liquids, with repulsion between nearby bodies in the Newtonian case and attraction in the viscoelastic case. Side-by-side arrangements of sedimenting spheres are unstable in exactly the same way that broadside-on settling of long bodies is unstable at subcritical speeds in a viscoelastic fluid. The line of centers between the spheres rotates from across to along the stream as the spheres are sucked together. The resulting chain of two spheres is a long body which is stable when the line between centers is parallel to the fall, but this configuration breaks up at subcritical speeds where inertia again dominates. To explain the orientation of particles in the supercritical case, we correlate the aggregative power of a viscoelastic fluid with a zero shear value of the coefficient of ratio of the first normal stress difference to the shear stress and for exceptional cases we introduce the idea of the memory of shear-thinning leading to corridors of reduced viscosity.

Traction-Carpet Stratification in Turbidites-Fact or Fiction?

Abstract: The "spaced stratification" that characterizes some thick deep-water sandstones consists of a stack of layers 5-10 cm thick that are inversely graded at their base and structureless at their top. These layers have been attributed to deposition from a succession of collapsed inertia layers (traction carpets). Prior to collapse, each layer was believed to have been maintained by shear from the overriding turbidity current. This depositional model is based on the mistaken assumption that the distribution of shear stress in such a traction carpet is indistinguishable from the distribution of shear stress in a grain flow on a steep slope. Instead, shear stress in an externally sheared traction carpet should be approximately constant from the top to the base of the layer, and deposition sho ld be a gradual process, leaving no record of sequentially active inertia layers. Inertia layers that do form under large turbidity currents are probably never as thick as 5-10 cm, based on available data from shear-cell experiments. Instead, inertia layers are probably about 1 cm thick, similar to the thickness of the inversely graded basal part of each layer in field examples of "spaced stratification". The rest of each inversely graded stratum probably forms due to rapid fallout from suspension. The basal, inversely graded 1 cm is believed to record strong shear that may accompany impingement on the bed of large turbulent eddies. Hence, "spaced stratification" may not be a structure produced beneath steady flows, but may record strong y fluctuating hydrodynamic conditions and vigorous burst/sweep cycles in large turbidity currents.

Shear-Induced Isotropic-to-Nematic Phase Transition in Equilibrium Polymers

Abstract: We report on a first-order isotropic-to-nematic phase transition induced by shear in concentrated solutions of elongated flexible wormlike micelles. As shear is applied to a disordered micellar solution, the transition shows up in steady-shear experiments as a kink in the shear stress behaviour. Using small-angle neutron scattering under shear, we show that beyond the transition rate, a nematic phase manifests itself by the growth of crescentlike scattering patterns in the direction perpendicular to the flow. A dynamical phase diagram is finally proposed for this system of equilibrium polymers.

State of Stress from Focal Mechanisms Before and After the 1992 Landers Earthquake Sequence

Abstract: The state of stress in the Eastern California Shear Zone (ECSZ) changed significantly because of the occurrence of the 1992 M_w 6.1 Joshua Tree and the M_W 7.3 Landers earthquakes. To quantify this change, focal mechanisms from the 1975 Galway Lake sequence, the 1979 Homestead Valley sequence, background seismicity from 1981 to 1991, and the 1992 Landers sequence are inverted for the state of stress. In all cases, the intermediate principal stress axis (S2) remained vertical, and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes (S_1 and S_3) and small variations in the value of the relative stress magnitudes (ϕ). In general, the stress state in the ECSZ has S_1 trending east of north and ϕ = 0.43 to 0.65, suggesting that the ECSZ is a moderate stress refractor and the style of faulting is transtensional. South of the Pinto Mountain fault, in the region of the 1992 Joshua Tree earthquake, the stress state determined from the 1981 to 1991 background seismicity changed on 23 April and again on 28 June 1992. In the central zone, S_1 rotated from N14° ± 5°E to N28° ± 5°E on 23 April and back again to N16° ± 5°E on 28 June. Thus, the Landers mainshock in effect recharged some of the shear stress in the region of the M_w 6.1 Joshua Tree earthquake. Comparison of the state of stress before and after 28 June 1992, along the Landers mainshock rupture zone, showed that the mainshock changed the stress orientation. The S1 trend rotated 7° to 20° clockwise and became progressively more fault normal from south to north. Along the Emerson-Camp Rock faults, the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor. The complex distribution of P and T axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock. The San Bernardino Mountains region of the M_w 6.2 Big Bear earthquake has a distinctively different state of stress, as compared to the Landers region, with S_1 trending N3° ± 5°W. This region did not show any significant change in the state of stress following the 1992 M_w 6.2 Big Bear sequence.

Effects of temperature and surface roughness on time‐dependent development of wall slip in steady torsional flow of concentrated suspensions

Abstract: A flow visualization technique was applied to investigate the time and temperature‐dependent development of wall slip and the rheological behavior of a concentrated suspension, containing 63% by volume solid glass spheres and a poly (butadiene‐acrylonitrile‐acrylic acid) terpolymer matrix, using steady torsional flow. Flow visualization allowed the concomitant determination of the wall slip velocity and the shear viscosity of the concentrated suspension. The deformation rate, shear stress, and the wall slip velocity values during torsional flow were time dependent and asymptotically reached steady‐state values. The characteristic time necessary to reach steady state decreased with increasing shear rate and temperature. Increasing temperature also increased the wall slip velocity. The flow visualization technique was further utilized to determine the yield stress of the suspension directly, which was found to decrease with increasing temperature. Increased surface roughness prevented the wall slip of the concentrated suspension under certain conditions; however, it frequently resulted in the fracture of the suspension samples. Sample fracture became more pronounced with the preshearing of the samples.

Molecular mechanics modeling of shear and the crystal orientation dependence of the elastic precursor shock strength in pentaerythritol tetranitrate

Abstract: The elastic precursor shock strengths of pentaerythritol tetranitrate explosive crystals were measured for [100], [101], [110], and [001] orientations using velocity interferometer system for any reflector instrumentation for samples 3–6 mm thick. Input shock strength was 1.14 GPa. Measured precursor amplitudes were 0.38, 0.58, 0.98, and 1.22 GPa, respectively, for the four orientations. Critical shear stress for the slip system with the maximum resolved shear stress for each shock orientation was computed. Details of the elastic and plastic wave profiles are discussed. Molecular mechanics modeling of the shear induced by the uniaxial strain of a plane shock wave in this molecular crystal was also performed using the amber code. This may be the first application of molecular mechanics computation to a shear problem. The modeling correctly predicts the dependence of the precursor amplitude on crystal orientation for the cases considered. The results confirm the importance of steric hindrance to shear in controlling the orientation‐dependent strength in molecular crystals and sensitivity to shock initiation of detonation in molecular explosive crystals. Details of the molecular deformations and contributions to the energy barrier to inelastic shear for different orientations are given. The computational results also explain why the {110} 〈111〉 slip system is observed in quasistatic deformation in spite of having the longest Burgers vector. The dynamics of sterically hindered, shock‐induced shear is considered.