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Showing papers on "Shear stress published in 1988"


Journal ArticleDOI
TL;DR: In this paper, a modification to Brown and Miller's critical plane approach is proposed to predict multiaxial fatigue life under both in-phase and out-of-phase loading conditions.
Abstract: — A modification to Brown and Miller's critical plane approach is proposed to predict multiaxial fatigue life under both in-phase and out-of-phase loading conditions. The components of this modified parameter consist of the maximum shear strain amplitude and the maximum normal stress on the maximum shear strain amplitude plane. Additional cyclic hardening developed during out-of-phase loading is included in the normal stress term. Also, the mathematical formulation of this new parameter is such that variable amplitude loading can be accommodated. Experimental results from tubular specimens made of 1045 HR steel under in-phase and 90° out-of-phase axial-torsional straining using both sinusoidal and trapezoidal wave forms were correlated within a factor of about two employing this approach. Available Inconel 718 axial-torsional data including mean strain histories were also satisfactorily correlated using the aforementioned parameter.

1,493 citations


Journal Article
TL;DR: Development of clinical techniques for relating hemodynamic and tensile properties to plaque location, stenosis, and composition should permit pathologists to provide new insights into the bases for the topographic and individual differences in plaque progression and outcome.
Abstract: Atherosclerosis affects the major elastic and muscular arteries, but some vessels are largely spared while others may be markedly diseased. The carotid bifurcation, the coronary arteries, the infrarenal abdominal aorta, and the vessels supplying the lower extremities are at highest risk. The propensity for plaque formation at bifurcations, branchings, and curvatures has led to conjectures that local mechanical factors such as wall shear stress and mural tensile stress potentiate atherogenesis. Recent studies of the human vessels at high risk, and of corresponding models, have provided quantitative evidence that plaques tend to occur where flow velocity and shear stress are reduced and flow departs from a laminar, unidirectional pattern. Such flow characteristics tend to increase the residence time of circulating particles in susceptible regions while particles are cleared rapidly from regions of relatively high wall shear stress and laminar unidirectional flow. The flow patterns associated with plaque localization are most prominent during systole. Long-term consequences are therefore likely to be greatly enhanced by elevated heart rate and may exert a selective effect on the coronary arteries. The point-by-point redistribution of wall tension at regions of geometric transition has not been quantitatively related to plaque localization. Enlargement of arteries as plaques increase in size and the associated modeling of plaque and wall configuration tend to preserve an adequate and regular lumen cross section. Hemodynamic forces appear to determine changes in vessel diameter so as to restore normal levels of wall shear stress, while wall thickness architecture, and composition are closely related to tensile stress. Hemodynamic forces may also be implicated in the symptom-producing destabilization of plaques, especially in relation to wall instabilities near stenoses. The relative roles of wall shear stress, tensile stress, and the metabolism of the artery wall in the progression and complication of atherosclerosis remain to be clarified. Development of clinical techniques for relating hemodynamic and tensile properties to plaque location, stenosis, and composition should permit pathologists to provide new insights into the bases for the topographic and individual differences in plaque progression and outcome.

1,085 citations


Journal ArticleDOI
14 Jan 1988-Nature
TL;DR: Recordings of whole-cell patch-clamp recordings of single arterial endothelial cells exposed to controlled levels of laminar shear stress in capillary flow tubes suggest that localized flow-activated hyperpolarization of endothelium involving I K.s may participate in the regulation of vascular tone.
Abstract: The endothelial lining of blood vessels is subjected to a wide range of haemodynamically-generated shear-stress forces throughout the vascular system1. In vivo and in vitro, endothelial cells change their morphology2,3 and biochemistry4 in response to shear stress in a force- and time-dependent way, or when a critical threshold is exceeded5'6. The initial stimulus–response coupling mechanisms have not been identified, however. Recently, Lansman et al.7 described stretch-activated ion channels in endothelial cells and suggested that they could be involved in the response to mechanical forces generated by blood flow. The channels were relatively non-selective and were opened by membrane stretching induced by suction. Here we report whole-cell patch-clamp recordings of single arterial endothelial cells exposed to controlled levels of laminar shear stress in capillary flow tubes. A K+ selective, shear-stress-activated ionic current (designated IK.s) was identified which is unlike previously described stretch-activated currents. IK.s varies in magnitude and duration as a function of shear stress (half-maximal effect at 0.70 dyn cm−2), desensitizes slowly and recovers rapidly and fully on cessation of flow. IK.s activity represents the earliest and fastest stimulus–response coupling of haemodynamic forces to endothelial cells yet found. We suggest that localized flow-activated hyperpolarization of endothelium involving IK.s may participate in the regulation of vascular tone.

988 citations



Journal ArticleDOI
TL;DR: This study demonstrates that shear stress in certain ranges may not be detrimental to mammalian cell metabolism, and in fact, throughout the range of shear stresses studied, metabolite production is maximized by maximizing shear Stress.
Abstract: A flow apparatus was developed for the study of the metabolic response of anchorage dependent cells to a wide range of steady and pulsatile shear stresses under well controlled conditions. Human umbilical vein endothelial cell monolayers were subjected to steady shear stresses of up to 24 dynes/sq cm, and the production of prostacyclin was determined. The onset of flow led to a burst in prostacyclin production which decayed to a long term steady state rate (SSR). The SSR of cells exposed to flow was greater than the basal release level, and increased linearly with increasing shear stress. It is demonstrated that shear stresses in certain ranges may not be detrimental to mammalian cell metabolism. In fact, throughout the range of shear stresses studied, metabolite production is maximized by maximizing shear stress.

384 citations


Journal ArticleDOI
TL;DR: Two mathematical models suggested by the micropipette technique and by the geometry peculiar to these cells in their detached post-exposure state are used to determine an effective Young's modulus for bovine endothelial cells and to discuss the dependence of this modulus upon exposure to shear stress.
Abstract: Experimental studies have shown that endothelial cells which have been exposed to shear stress maintain a flattened and elongated shape after detachment. Their mechanical properties, which are studied using the micropipette experiments, are influenced by the level as well as the duration of the shear stress. In the present paper, we analyze these mechanical properties with the aid of two mathematical models suggested by the micropipette technique and by the geometry peculiar to these cells in their detached post-exposure state. The two models differ in their treatment of the contact zone between the cell and the micropipette. The main results are expressions for an effective Young's modulus for the cells, which are used in conjunction with the micropipette data to determine an effective Young's modulus for bovine endothelial cells, and to discuss the dependence of this modulus upon exposure to shear stress.

374 citations


Journal ArticleDOI
TL;DR: In this paper, the finite element equations for a variationally consistent higher-order beam theory are presented for the static and dynamic behavior of rectangular beams, which correctly accounts for the stress-free conditions on the upper and lower surfaces of the beam while retaining the parabolic shear strain distribution.

364 citations


Journal ArticleDOI
TL;DR: In this paper, a general analysis is developed for turbulent shear flows over two and three-dimensional hills with low-slopes which allows for a wide range of upwind velocity profiles, such as those caused by wakes of up-wind hills, roughness changes, or changes in stratification.
Abstract: A general analysis is developed for turbulent shear flows over two- and three-dimensional hills with low-slopes which allows for a wide range of upwind velocity profiles, such as those caused by wakes of upwind hills, roughness changes, or changes in stratification. In this paper the atmosphere is assumed to be neutrally stable and the half-lengths of the hills, L, are large compared with their heights, H, which are very large compared with the roughness length zo. The general structure of the solution is defined by dividing the flow into two regions, each of which is divided into two sublayers: an inviscid outer region composed of an upper layer in which there is potential flow when the atmosphere is neutrally stable, and a middle layer in which the wind shear dominates; and an inner region of thickness l ≤ L in which the effects of perturbation shear stresses are confined. The latter region is divided into two: a shear stress layer where the shear stresses, although weak, determine that the maximum of the perturbation velocity is located in this layer; and an inner surface layer of thickness ls where the shear stress gradient varies rapidly and the perturbation velocity tends to zero. The details of the middle layer are given here for different kinds of upwind profiles, including logarithmic, ‘power law’ and linear profiles. It is shown that the analysis can be extended to allow for nonlinear inertial effects in the middle layer. Analytical solutions are derived for the inner region as asymptotic expansions in δ = [ln(l/zo)]−1, which is assumed to be small, and this shows that ls ∼ zo(l/zo)1/2. The results of the analytical model are compared with our own and with previously published numerical computations of the full equations (applying the same assumptions used for calculating the turbulent shear stresses as used in the analytical work), which have largely been validated against full-scale measurements. These results confirm that the relative increase of surface stress is significantly greater than the increase of wind speed near the surface except when there is no upwind shear (as for example in a logarithmic boundary layer when the roughness length tends to zero). Finally, the paper shows that the outer regions of laminar (or constant eddy viscosity) and of turbulent flows over hills are broadly similar, but that the effects of the flow in the inner region on the outer regions are much smaller in the latter case.

364 citations


Journal ArticleDOI
29 Jan 1988
TL;DR: In this paper, the turbulent bottom boundary layer flow is obtained for a wave motion specified by its directional spectrum based on the linearized form of the boundary layer equations and a simple eddy viscosity formulation of shear stress.
Abstract: Based on the linearized form of the boundary layer equations and a simple eddy viscosity formulation of shear stress, the turbulent bottom boundary layer flow is obtained for a wave motion specified by its directional spectrum. Closure is obtained by requiring the solution to reduce, in the limit, to that of a simple harmonic wave. The resulting dissipation is obtained in spectral form with a single friction factor determined from knowledge of the bottom roughness and an equivalent monochromatic wave having the same root-mean-square near-bottom orbital velocity and excursion amplitude as the specified wave spectrum. The total spectral dissipation rate is obtained by integration and compared with the average dissipation obtained from a model considering the statistics of individual waves defined by their maximum orbital velocity and zero-crossing period. The agreement between the two different evaluations of total spectral dissipation supports the validity of the spectral dissipation model.

361 citations


Journal ArticleDOI
TL;DR: The temporal distribution of induced seismicity following the filling of large reservoirs shows two types of response: rapid increases in elastic stress due to the load of the reservoir and the more gradual diffusion of water from the reservoir to hypocentral depths as discussed by the authors.
Abstract: The temporal distribution of induced seismicity following the filling of large reservoirs shows two types of response. At some reservoirs, seismicity begins almost immediately following the first filling of the reservoir. At others, pronounced increases in seismicity are not observed until a number of seasonal filling cycles have passed. These differences in response may correspond to two fundamental mechanisms by which a reservoir can modify the strength of the crust—one related to rapid increases in elastic stress due to the load of the reservoir and the other to the more gradual diffusion of water from the reservoir to hypocentral depths. Decreased strength can arise from changes in either elastic stress (decreased normal stress or increased shear stress) or from decreased effective normal stress due to increased pore pressure. Pore pressure at hypocentral depths can rise rapidly, from a coupled elastic response due to compaction of pore space, or more slowly, with the diffusion of water from the surface.

334 citations


Journal ArticleDOI
TL;DR: In this paper, the fluctuating wall-shear stress was measured with various types of hotwire and hot-film sensors in turbulent boundary layer and channel flows, and the rms level of the streamwise wall shear stress fluctuations was found to be 40% of the mean value, which was substantiated by measurements of streamwise velocity fluctuations in the viscous sublayer.
Abstract: The fluctuating wall‐shear stress was measured with various types of hot‐wire and hot‐film sensors in turbulent boundary‐layer and channel flows. The rms level of the streamwise wall‐shear stress fluctuations was found to be 40% of the mean value, which was substantiated by measurements of the streamwise velocity fluctuations in the viscous sublayer. Heat transfer to the fluid via the probe substrate was found to give significant differences between the static and dynamic response for standard flush‐mounted hot‐film probes with air or oil as the flow medium, whereas measurements in water were shown to be essentially unaffected by this problem.

Journal ArticleDOI
TL;DR: In this paper, a biaxial loading device (plane strain) allowed direct observation of fracture propagation from an angled slot in plates of polymethylmethacrylate (PMMA, Altuglas) and sandstone (low and high porosity).
Abstract: Various brittle geological structures form from the tip of preexisting joints. Experiments have been carried out to investigate their mechanical origin and to find out whether a planar defect can propagate in its ownplane under mode II geometrical conditions (displacement parallel to the defect plane and perpendicular to the edge) as supposed by the classical rupture mechanics model, and in particular, whether mode II can be an elementary fracture mechanism, which is not clear in the model and has not been experimentally demonstrated. A biaxial loading device (plane strain) allowed direct observation of fracture propagation from an angled slot in plates of Polymethylmethacrylate (PMMA, Altuglas) and sandstone (low and high porosity). Under uniaxial load, mode I propagation of the classical branch fracture occurred in both materials, but its development was inhibited under biaxial loading (low σ3/σ1 ratio). At higher stress/strain levels, shear zones (clearly distinguishable from the branch fracture) developed from the slot tip. In PMMA they consisted of the prograding development of en echelon shallow microcracks formed in mode I and more or less associated with plastic deformation. In sandstone a narrow shear zone propagated in the prolongation of the defect (slowly in the high-porosity samples and abruptly in the low-porosity ones) accompanied by conjugate shear fractures. The deformation mechanism was cataclasis, implying tensile (mode I) microfractures. These experiments suggest that mode II cannot exist as an elementary (primary) fracture mechanism but can only be a macroscopic fracture phenomenon which must necessarily involve tensile (mode I) microcrack formation. The latter is linked to stress concentrations in the prolongation of the existing joint. As such mode II initiated shear zones have not been generated in glass, a high density of small defects which can be mobilized under shear stress would seem essential. These ideas can be used to interpret some types of natural faults.

Journal ArticleDOI
TL;DR: In this article, simple shear and pure shear extension of the lithosphere are investigated using a numerical technique which solves for two-dimensional conductive and advective heat transport hrough time.
Abstract: Simple shear and pure shear extension of the lithosphere produce very different patterns of heat flow and topography. These differences are investigated using a numerical technique which solves for two-dimensional conductive and advective heat transport hrough time. Simple shear extension of the lithosphere is modeled as occurring along a straight shear zone. Two parameters define the simple shear model: the dip of the shear zone and its width. Likewise, the pure shear model is defined by two variables: the initial width of a vertical zone of pure shear extension and the rate of change of its width. These pairs of parameters are varied between calculations, as is the overall rate of extension. Each model results in distinct patterns of crustal thinning, lithospheric thermal structure, heat flow, thermal uplift, crustal subsidence, and topography. For the simple shear model, extension results in asymmetric uplift across the rift, while the total volume of uplift is limited by the total amount of extension. The peak heat flow and thermal uplift are centered over the intersection of the shear zone with the surface. Isostatic response to simple shear extension results in successive, formerly active shear zones being rotated into listtic faults which sole into a subhorizontal detachment. The pure shear esults how that the surface heat flow is greater for smaller widths of the zone of extension. For the same overall 1Also at the Department of Geological Sciences, Columbia University. Copyright 1988 by t,he American Geophysical Union. Paper number 7T0883. 0278-7407/88/007T-0883 $10.00 extension rate, a pure shear model with a narrow zone of extension can result in pressure release melting of the mantle long before low angle simple shear models. These results are compared with topographic and heat flow data from the northem Red Sea rift, a Neogene continental rift which is close to initiating seafloor spreading. The long wavelength topographic asymmetry across the Red Sea, which has been cited as evidence for simple shear extension of the lithosphere, is not matched by any of the models. The observed high heat flow anomalies in the Red Sea require a large component of pure shear lithospheric extension centered under the region of maximum crustal extension. In contrast, at the plate separation rate of the northern Red Sea, simple shear extension of the lithosphere along a shallow (<30 ø) dip detachment is ineffective in reproducing the observed heat flow anomalies. Only a narrowing region of pure shear extension can satisfy the width of the rift, and the peak heat flow values and generate pressure release melting.

Journal ArticleDOI
TL;DR: In this paper, a simple isoparametric finite element formulation based on a higher-order displacement model for flexure analysis of multilayer symmetric sandwich plates is presented, which accounts for non-linear variation of inplane displacements and constant variation of transverse displacement through the plate thickness.

Journal ArticleDOI
TL;DR: A shear stress transducer might exist in endothelial cells, which perceives the shearing force on the membrane as a stimulus and mediates the signal to increase cytosolic free Ca++.
Abstract: Vascular endothelial cells modulate their structure and functions in response to changes in hemodynamic forces such as fluid shear stress. We have studied how endothelial cells perceive the shearing force generated by blood flow and the substance(s) that may mediate such a response. We identify cytoplasmic-free calcium ion (Ca++), a major component of an internal signaling system, as a mediator of the cellular response to fluid shear stress. Cultured monolayers of bovine aortic endothelial cells loaded with the highly fluorescent Ca++-sensitive dye Fura 2 were exposed to different levels of fluid shear stress in a specially designed flow chamber, and simultaneous changes in fluorescence intensity, reflecting the intracellular-free calcium concentration [( Ca++]i), were monitored by photometric fluorescence microscopy. Application of shear stress to cells by fluid perfusion led to an immediate severalfold increase in fluorescence within 1 min, followed by a rapid decline for about 5 min, and finally a plateau somewhat higher than control levels during the entire period of the stress application. Repeated application of the stress induced similar peak and plateau levels of [Ca++]i but at reduced magnitudes of response. These responses were observed even in Ca++-free medium. Thus, a shear stress transducer might exist in endothelial cells, which perceives the shearing force on the membrane as a stimulus and mediates the signal to increase cytosolic free Ca++.

Journal ArticleDOI
TL;DR: In this article, the structure of skin layer in injection-molded polypropylen was studied by means of wide-angle X-ray diffraction, small-angle x-ray scattering, melting behavior, density, dynamic viscoelasticity, and tensile test.
Abstract: The structure of skin layer in injection-molded polypropylen which displayed a clear two-phase structure of skin and core has been studied by means of wide-angle x-ray diffraction, small-angle x-ray scattering, melting behavior, density, dynamic viscoelasticity, and tensile test. In skin layer, the c-axis and a*-axis were highly oriented to the machine direction (MD), and the plane of the lamellar structure of about 160 A in thickness was in normal to MD. The density was about 0.907 g/cm3, which was nearly the same as that of core layer. Although the majority of crystallites melted in the same temperature range as in that of the core layer, there was about 5.3% higher temperature melting structure (Tm = 182°C). The dynamic tensile modulus E′ in MD decreased more slowly with increasing temperature than that of the core layer and held high modulus in the range of ca. 30°C, just above the temperature at which E′ of the core layer suddenly dropped. E′ in MD was higher than that in TD in the temperature range below 33°C, which was slightly higher than the primary absorption temperature, and the order reversed above 33°C. The tensile yield stress in MD was 1.5 times higher than that of the core layer. The skin layer in MD ruptured just after yielding and did not show necking. The tensile yield stress in TD was about half of that in MD about 0.7 times that of the core layer. The necking stress in TD was about 0.6 times that of the core layer. In general, a polypropylene melt crystallizes under a high shear stress in injection molding. From these facts, it was concluded that the skin layer is composed of so-called “shishkebab”-like main skeleton structures, whose axis is parallel to MD, piled epitaxially with a*-axis-oriented imperfect lamellar substructure.

Journal ArticleDOI
TL;DR: In this paper, an experimental investigation is undertaken to evaluate the stress conditions required to initiate liquefaction and the influence of various parameters on those stress conditions, and an expression for a factor of safety against the initiation of liquidation is proposed.
Abstract: Liquefaction of loose, saturated sands may be caused by cyclic or static (monotonically increasing) undrained loading. Most previous studies of static liquefaction behavior have emphasized liquefaction susceptibility and the behavior of liquefied soils. An experimental investigation is undertaken to evaluate the stress conditions required to initiate liquefaction and the influence of various parameters on those stress conditions. The static liquefaction resistance, defined as the shear stress increase under undrained conditions required to initiate liquefaction, is observed to increase with increasing relative density and confining pressure, and to decrease dramatically with increasing initial shear stress level. At high initial shear stress levels, initiation of liquefaction is observed to result from increases in shear stress under undrained conditions of only a few percent of the initial shear stress. The distinction between the initiation and the effects of liquefaction is discussed, and an expression for a factor of safety against the initiation of liquefaction is proposed.

Journal ArticleDOI
TL;DR: In this article, a microfabricated floating-element shear-stress sensor for measurements in turbulent boundary-layers is reported using surface micromachining of polyimide.
Abstract: A microfabricated floating-element shear-stress sensor for measurements in turbulent boundary-layers is reported. Using surface micromachining of polyimide, a 500- mu m*500- mu m probe has been fabricated incorporating a differential-capacitor readout circuit. A model for the sensor response is described and is used for the design of an element to measure shear stresses of 1 Pa in air. The sensor is packaged for calibration in laminar flow, and electrical results obtained match the expected response. >

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the viscosity, modulus, and yield stress for 0-6 wt% aqueous solutions of Carbopol 941 using constant shear rate, constant hear stress, and dynamic oscillatory experiments.
Abstract: Viscosity, modulus, and yield stress for 0–6 wt% aqueous solutions of Carbopol 941 were investigated using constant shear rate, constant shear stress, and dynamic oscillatory experiments. The microgel character of the polymer was evident from the solid-like behavior of the solutions above 1 wt%. Yield stress increased with concentration, but yield occurred at a critical shear strain of 40%, independent of concentration. The static stress-strain relationship became non-linear at ~ 25% strain, in fair agreement with the onset of non-linear response in the storage modulus at ~ 10% strain. Small strain moduli from static and low frequency measurements agreed rather well; modulus values obtained from the recoverable strain after yielding were 30–40% smaller. Solutions flowed at near-constant stress in the low shear rate regime; at higher rates the stress increases with shear rate more rapidly. The viscosity did not obey the Cox-Merz rule. Steady-state viscosity scaled with polymer concentration to the 3/4 power. Results were interpreted using a cellular, deformable sphere model for the polymer, in analogy to emulsions and foams.

Journal ArticleDOI
TL;DR: The laminar steady flow of non-Newtonian fluid (biviscosity fluid) through an axisymmetric stenosis is calculated and it is suggested by the authors that this force can become one of the causes of post-stenotic dilatation.
Abstract: The laminar steady flow of non-Newtonian fluid (biviscosity fluid) through an axisymmetric stenosis is calculated using the finite element methods. The flow pattern, the separation and reattachment points, and the distributions of pressure and shear stress at the wall are obtained. Then, the axial force acting on the stenosis is evaluated. It is suggested by the authors that this force can become one of the causes of post-stenotic dilatation. Calculated results show that the non-Newtonian property of blood weakens the distortion of flow pattern, pressure and shear stress at the wall associated with the stenosis and that the non-Newtonian property of blood decreases the axial force acting on the stenosis.

Journal ArticleDOI
TL;DR: The existence of a critical shear stress for 3T3 cell detachment suggests that cell adhesion to surfaces cannot be fully understood with singleShear stress methods because cells may attach with a wide range of strengths which are either all above or all below the applied shear Stress.
Abstract: Adhesion of cells to substrates strongly influences many of their functions and therefore plays an important role in a variety of processes, including growth, phagocytosis, hemostasis, and the response of tissue to implanted materials. In previous studies, the influence of substrate hydrophilicity on cell adhesion has not been separated from effects due to major differences in other properties of the substrate, such as charge, rigidity, and the specific chemical composition of the materials. In addition, very few careful studies of the force required for cell detachment from various substrates have been performed. In this study, 3T3 cell detachment from a chemically homologous series of copolymers based on hydroxyethylmethacrylate (HEMA) and ethylmethacrylate (EMA) was measured with a spinning-disc apparatus. The spinning-disc technique allowed measurements of cell detachment over a wide range of applied shear stress on each sample. Cell detachment did not occur until a critical value of shear stress was exceeded. The critical shear stress of detachment decreased linearly with increasing HEMA content, from 18 dynes/cm2 on poly-EMA to 0 on the polymers containing 83% or more HEMA. "Plating efficiency," calculated as the fraction of cells initially applied which remained after dip rinsing the surfaces, did not vary significantly among most of the copolymers. Dip rinsing, however, exposes the cells to only one, relatively low shear stress (estimated to be somewhat less than 3 dynes/cm2). The existence of a critical shear stress for 3T3 cell detachment suggests that cell adhesion to surfaces cannot be fully understood with single shear stress methods because cells may attach with a wide range of strengths which are either all above or all below the applied shear stress. The influence of surface hydrophilicity on cell adhesion and the variety of forces which may contribute to this phenomenon are discussed.

Journal ArticleDOI
TL;DR: In this article, a numerical model of oscillatory rough-turbulent boundary layer flow, featuring closure via the turbulent energy equation, is used to examine the principal features of wave-current interaction above the seabed.
Abstract: A numerical model of oscillatory rough-turbulent boundary layer flow, featuring closure via the turbulent energy equation, is used to examine the principal features of wave-current interaction above the seabed. Results are presented from case studies carried out with water depth of 10 m, bed roughness of 0.5 cm and wave period of 8 s. The steady surface current in the absence of waves is nominally 100 cm/s, and waves having near-bottom velocity amplitudes of 50, 100, and 150 cm/s are superimposed on this current at angles ϕ = 0, π/4, and π/2. Velocity, turbulent energy, shear stress, and eddy viscosity distributions are compared for the steady current alone, for waves alone, and for waves superimposed on the current, and the interaction in the wave-current boundary layer is examined. Cycle-averaged vertical profiles of horizontal velocity illustrate the extent to which the mean flow is retarded by wave-current interaction. For the general case in which the waves are superimposed at an arbitrary angle ϕ, the mean current veers to form an angle with the wave direction greater than that of the initial steady current. Finally, the enhancement of the bed shear stress and the increase in oscillatory boundary layer thickness associated with wave-current interaction are both quantified.

Journal ArticleDOI
TL;DR: In this article, a detailed hot-wire measurement of a turbulent natural convection boundary layer is made paying close attention to the characteristics of the near-wall region which has not been clarified quantitatively.

Journal ArticleDOI
TL;DR: In this article, the authors used an experimental geometry, which includes a slot loaded to produce a mode 3 shear stress concentration, for the formation of brittle shear fracture in two fine-grained rock types.

Journal ArticleDOI
TL;DR: In this paper, a method is described for generating reproducible and stable foam capable of retaining its structure for prolonged times. But the experimental study of foam is complicated by its inherently unstable nature and the presence of a liquid film slip layer at the wall.
Abstract: The experimental study of foam is complicated by its inherently unstable nature and by the presence of a liquid film slip layer at the wall. A method is described for generating reproducible and stable foam capable of retaining its structure for prolonged times. An experimental technique has also been devised which eliminates problems associated with wall slippage and allows measurement of material functions without use of any empiricisms for the wall region. In a steady‐shear flow, foam behaves like a Bingham plastic with a viscosity inversely proportional to shear rate indicating the presence of a yield stress τ0. The value of the viscosity, which is significantly higher than the parent liquid viscosity, is an increasing function of gas volume fraction φ. Yield stress values obtained by extrapolating viscosity versus shear stress data agree with direct τ0 measurements obtained by using a stress relaxation technique. The yield stress is also found to increase with φ. Small amplitude oscillatory shearing ...

Journal ArticleDOI
TL;DR: In this article, it is shown that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones, the earthquake events being the result of instabilities in material undergoing plastic flow.
Abstract: Adiabatic or catastrophic plastic shear has been reported in metals, polymers, and metallic glasses. The phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner. The driving force for the plastic instability is the stored elastic strain energy of the loading system, and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding, although the precise mechanism is different. It is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones, the earthquake events being the result of instabilities in material undergoing plastic flow. It is argued that for a particular strain rate there exists a critical temperature, TC, which is depth dependent; for temperatures below TC the material is strain rate softening and, for a soft enough loading system, may undergo catastrophic plastic shear. For temperatures above TC the material is strain rate hardening and is always stable during plastic shear. The cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material, and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km. The precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the TC gradient. In particular, a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 MPa above 300 km to a maximum of ≈ 50 MPa below 300 km. The differences in foci distribution between subduction zones such as Tonga, New Hebrides, and Peru result from minor differences in the geothermal gradients within the slabs. The model predicts the development of triple seismic zones high in the slab, double seismic zones down to approximately 300 km, and single seismic zones down to ≈ 800 km. Such a distribution is to be expected of relatively young, cool slabs; as the slab heats up, the seismic activity retreats up the slab. The paper only proposes a deformation mechanism for earthquake generation, it does not address the stress field within the slab but only the distribution of strength. Thus the distribution of focal plane mechanisms is not considered, only the locations where earthquakes due to plastic instabilities are possible. The absence of earthquakes does not necessarily mean that the slab does not exist, it only means that the slab is too hot to undergo plastic instability. This means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab (i.e. > circa 3°C km−1).

Journal ArticleDOI
TL;DR: In this article, a general definition of initial motion in mixed-size sediment is presented that allows the characteristic differences between the results to be explained in terms of sampling and scaling considerations inherent in the mixed size initial motion problem.
Abstract: Two methods are commonly used to estimate the critical shear stress of individual fractions in mixed-size sediment, one using the largest grain displaced, the other using the shear stress that produces a small value of transport rate for each fraction. The initial-motion results produced by the two methods are typically different: largest-grain critical shear stresses vary with roughly the square root of grain size, and reference transport critical shear stresses show little variation with grain size. Comparison of the two methods is seldom possible because both methods can rarely be applied to the same data. The one case known for which both methods can be used suggests that the typical differences in initial-motion results reflect more methodological influence than real differences in the initial motion of different sediments. Although the two classes of methods may not be directly compared, a general definition of initial-motion in mixed-size sediment is presented that allows the characteristic differences between the results to be explained in terms of sampling and scaling considerations inherent in the mixed-size initial-motion problem. The initial-motion criterion defined also provides a rational basis for collecting comparable and reproducible data using the two classes of method.

Journal ArticleDOI
TL;DR: In this article, the critical shear stress for incipient motion of the individual size fractions within these sediments was estimated as that shear stressed that produced a small dimensionless transport rate.
Abstract: Transport rates of five sediments were measured in a laboratory flume. Three of these sediments had the same mean size, the same size distribution shape, and different values of grain size distribution standard deviation. The critical shear stress for incipient motion of the individual size fractions within these sediments was estimated as that shear stress that produced a small dimensionless transport rate. The sorting of the sediment mixture had little effect on the critical shear stress of individual fractions, once the median size (D50) of the mixture and a fraction's relative size (Di/D50) are accounted for. Our data, combined with previously published data, show a remarkably consistent relation between the critical shear stress of individual fractions and the fraction's relative grain size, despite a broad variation in the available data of mixture sorting, grain size distribution shape, mean grain size, and grain shape. All fractions in a size mixture begin moving at close to the same value of bed shear stress during steady state transport conditions. This result is apparently true for transport systems where the transport rates of individual fractions are determined solely by the flow and bed sediment (recirculating systems), as well as for systems where the fractional transport rates are imposed on the system (feed systems). This equivalence in initial-motion results is important because natural transporting systems often show attributes of both types of behavior in an unknown combination.

Journal ArticleDOI
TL;DR: In this paper, the authors measured the reattachment length of a separated flow behind a backward-facing step for a range of Reynolds numbers (8000 < ReH < 40,000) and initial boundary-layer thickness (0 < δ/H < 2).
Abstract: Measurements of reattachment length of a separated flow behind a backward-facing step for a range of Reynolds numbers (8000 < ReH < 40,000) and initial boundary-layer thickness (0 < δ/H < 2) were performed with the purpose of explaining the scatter in existing (high quality) data sets and to understand the effect of the initial shear-layer structure on the reattachment zone. The reattachment length for the case of laminar boundary layers upstream of the step were 30% smaller than when the boundary layer upstream of the step was turbulent. Measured values of the mean wall shear stress in the reattachment zone were also measurably affected by the upstream boundary-layer state. The (rms) levels of fluctuating wall stress were not sensitive to boundary-layer state, but rather to δ/H, as was the case for the pressure profiles in part 1 (Adams and Johnston 1988).

Journal ArticleDOI
TL;DR: Cell death in the viscometer was shown to exhibit trends similar to cell death caused by excessive agitation in spinner flasks, suggesting that viscometric shear can be used to model in a more reproducible way some of the fluid mechanical aspects of damage to cells caused by agitation.