Showing papers on "Shear stress published in 1982"

Movable bed roughness in unsteady oscillatory flow

Abstract: A model to predict the roughness in unsteady oscillatory flows over movable, noncohesive beds is presented. The roughness over movable beds is shown to be a function of the boundary shear stress, rather than a fixed geometrical scale as is the case for fully rough turbulent boundary shear flows over immobile beds. The model partitions the roughness into two distinct contributions. These two contributions are due to the form drag around individual bed forms and to the near-bed sediment transport. The form drag over the bed forms is treated explicitly as a function of the boundary geometry and shear stress. The ripples are predicted as a function of the local skin friction, and a semiempirical expression is derived using standard law-of-the-wall arguments, which gives the ripple or form roughness as a function of the boundary geometry. The ripple roughness is found to be proportional to the product of the ripple steepness and height. Favorable comparison of the form drag model with the results of Bagnold's (1946) fixed ripple study is found. The value of z0 associated with intense sediment transport in oscillatory flow over a flat bed is determined from Carstens et al.'s (1969) experiments. This value is found to be 7 or 8 grain diameters. An expression is derived for the roughness associated with the maximum thickness of a near-bottom sediment-transporting layer consistent with Owen's (1964) roughness hypothesis for saltation of uniform grains in air. At large values of the boundary shear stress relative to the critical value for initial sediment motion, the derived expression is similar to the results of Smith and McLean's (1977) unidirectional flow approach modified for oscillatory flow. The total roughness model is found to compare favorably with Carstens et al.'s (1969) data. In contrast to Smith and McLean's (1977) steady flow findings, the results here show that when ripples are present, they account for a significant portion of the boundary roughness.

Motion of a tank-treading ellipsoidal particle in a shear flow

Abstract: A theoretical model is developed for the motion of a human red blood cell in a shear field. The model consists of a tank-treading ellipsoidal membrane encapsulating an incompressible Newtonian liquid immersed in a plane shear flow of another incom- pressible Newtonian liquid. Equilibrium and energy considerations lead to a solution for the motion of the particle that depends on the ellipsoidal-axis ratios and the ratio of the inner- to outer-liquid viscosities. The effect of variation in these parameters is explored and it is shown that, depending on their values, one of two types of overall motion is exhibited: a steady stationary-orientation motion or an unsteady flipping motion. A qualitative agreement of the predicted behaviour of the model with experi- mental observations on red blood cells is found.

High-strain-rate plastic flow studied via nonequilibrium molecular dynamics

Abstract: Recent experiments at strain rates reaching 0.1 GHz suggest a power-law dependence of solid-phase shear stress on strain rate. Novel nonequilibrium molecular dynamics simulations of plastic flow have been carried out. These steady-state isothermal calculations appear to be consistent with the present-day experimental data and suggest that the flows of metals can be described by a single physical mechanism over a range of strain rates from 10 kHz to 1 THz.

Discharge Assessment in Compound Channel Flow

Abstract: A series of laboratory tests were performed to measure the boundary shear stresses in a compound channel consisting of a main channel and associated shallow floodplains. Four different floodplain roughnesses were used. From these results apparent shear stresses across three different assumed main channel/floodplain interface planes (vertical, horizontal and diagonal) were calculated. The data was used with data from previous studies to derive a statistical relationship between the apparent shear stress on the vertical interface and several easily calculated parameters of the channel geometry and hydraulic characteristics. The accuracies of different methods of discharge calculation were shown to be dependent upon the assumptions they make regarding the magnitude of the apparent shear stress on the particular interface chosen. An apparent shear stress ratio was proposed which was found to be a useful yardstick in selecting the best method of discharge calculation for particular channel and flow conditions. Generally it was found that at low floodplain depths using the horizontal or diagonal interfaces gave better discharge results than the vertical.

Active control of laminar-turbulent transition

Abstract: Instability waves, commonly called T-S waves, can be introduced in a laminar boundary layer by periodic heating of flush-mounted heating elements. Experiments have demonstrated that nearly complete cancellation of a T-S wave excited in this way can be achieved by using a second downstream heating element with a suitable phase shift. As one application of the technique, a single element together with a feedback loop activated by measured wall shear stress has been used to reduce the amplitude of naturally occurring laminar instability waves. A significant increase in the transition Reynolds number has been achieved.

Stress Analysis for Anisotropic Hardening in Finite-Deformation Plasticity

Abstract: : Kinematic hardening represents the anisotropic component of strain hardening by a shift of the center of the yield surface in stress space. The current approach in stress analysis at finite deformation includes rotational effects by using the Jaumann derivatives of the shift and stress tensors. This procedure generates the unexpected result that oscillatory shear stress is predicated for monotonically increasing simple shear strain. A theory is proposed which calls for a modified Jaumann derivative based on the spin of specific material directions associated with the kinematic hardening. This eliminates the spurious oscillation. General anisotropic hardening is shown to require a similar approach. (Author)

Lateral Bed Load Transport on Side Slopes

Abstract: Lateral bed load transport is studied in laboratory flume and wind tunnel. Introduction of laterally inclinable wind tunnel permits accurate measurements of shear stress on side slope and of lateral bed load. The result of wind tunnel test was examined by an open channel experiment. A universal formula for the lateral bed load q* l is presented as a function of inclination of side slope, θ, and dimensionless shear stress τ*/τ* c ; and the expression is q* l /tanθ = α(τ*/τ* c )β. The numerical constants α and β are found to be 0.0039 and 1.5, respectively, for τ*/τ* c > 1.07. The formula may add some information to bank protection engineering, prediction of river meander, and to widening of braided river.

Incipient Motion of Sand Particles on Side Slopes

Abstract: Several criteria have been theoretically proposed for the critical shear stress of noncohesive sediment on side slopes. However, no laboratory work has ever been done on this problem. A theoretical study of the incipient motion of sediment particles on side slopes is presented, including the effects of viscous sublayer, turbulence and lift force. As a special case of level beds, the theory describes the experimental values reported by several workers. A specially designed rotary wind tunnel permits the evaluation of the critical shear stress on side slopes, and the experiment supports the theory. These works reveal that the lift force can be neglected in estimating the critical shear stress using the proposed theory if the averaged experimental values are concerned, but inclusion of the lift force in the theory is recommended for safety and for describing the lower limit of the possibility of incipient motion.

The Mechanism of Raindrop Splash on Soil Surfaces

Abstract: From the results of high-speed photography of 4.6-mm-diam drops impacting various soil materials and from soil mechanics principles, a new concept in describing the mechanism of soil detachment from raindrops impacting on saturated soil surfaces is proposed. The impulsive loading caused by the impacting drop does not permit time for drainage; thus there is no change in total soil volume or bulk density. The soil surface is deformed under the impulsive load application of the drop; however, the vertical strain under the impact area is compensated by a bulge around the perimeter of the depression. The vertical force of the drop is transformed to lateral shear caused by radial flow of the impacting drop. Splash angle is determined by the depth of the cavity and the size of the bulge surrounding it. Splash angle was highly correlated with soil shear strength as measured by the fall-cone method. Low soil strength resulted in (i) a larger cavity and surrounding bulge, (ii) a greater detachment of soil particles due to the shear stress of the radial flow, and (iii) a greater splash angle with the horizon.

Submerged laminar jet impingement on a plane

Abstract: Submerged laminar jet impingement on a plane is studied using computation. Steady-state Navier-Stokes equations for the axisymmetric case are solved numerically. The extent of the infinite flow is approximated by applying the boundary conditions at a finite but sufficiently large distance. The tube-exit velocity profile is assumed to be either a fully developed parabolic profile or a flat profile. For the former case, two different nozzle heights from the target plane are considered. The presence of a toroid-shaped eddy at low values of Reynolds number, Re, leads to some interesting observations such as the manner in which the wall shear stress depends on Re. An increase in the height of the nozzle exit from the target plane decreases the wall shear stress, more so at lower values of Re. A change from the parabolic exit velocity profile to the flat profile leads to a decrease in wall shear stress due to decreased momentum flux. The study was motivated by experiments designed to measure the yield shear strength of the vascular endothelium wherein a small saline jet was used to erode the tissue by normal impingement.

Boundary layer flow of a dusty fluid over a semi-infinite flat plate

Abstract: Both the drag force due to slip and the transverse force due to slip-shear have been considered in boundary layer equations. The solution has been found in a power series of non-dimensionalx, x being the distance in the down-stream direction. Solutions for high slip region and small slip region characterised byx≪1 andx≫1 respectively, have been found separately. In the high slip region the effect of increase in concentration parameter α of the dust particles is to increase the magnitude of the longitudinal fluid phase velocityu. Also the magnitude of the longitudinal particle slip velocityup-u is becoming maximum on the plate and decreasing along the plate withx. The transverse particle velocityvp is independent of α but it is directly proportional to β, the transverse force coefficient. An interesting result is thatvp is assuming small positive value on the plate. The transverse force has taken an important role in migration of particles away from the plate. In the small slip region the flow of dust particles is mainly governed by the fluid-phase. The effect of α on the flow field in this region is to decrease the boundary layer thickness. In this region the particles are having some tendency to accumulate near the plate. Lastly, it has been found that the shearing stress, skinfriction and the dimensionless drag-coefficient on the plate increase with increase of α.

Resuspension of Deposited Cohesive Sediment Beds

Abstract: Surfieial layers of estuarial fine, cohesive sediment beds are deposited from flow and often are in a state of partial consolidation. A series of laboratory investigations were carried out to elucidate the erosional behavior of deposited cohesive sediment beds in flumes using kaolinite. A significant feature of such beds is that they are stratified with respect to the density and the cohesive shear stength. Under a given bed shear stress, erosion occurs at a continuously decreasing rate up to a depth at which the bed shear stress equals the shear strength. This bed shear stress is therefore also equal to the critical shear stress for erosion at that depth. An expression for the rate of erosion relating this rate to the difference between the bed shear stress and the critical shear stress has been obtained. The critical shear stress increases both with depth and with the bed consolidation time. The rate of erosion decreases with increasing consolidation time.

A Fracture Criterion for Edge-Bonded Bimaterial Bodies

Abstract: In this paper a fracture criterion for the bond between two different linear elastic materials is proposed. This criterion is based on the singular behaviour of the normal and shear stress in the vicinity of the point of intersection be tween the bond line and the outer boundary.

Some Observations in High Pressure Rheology of Lubricants

Abstract: Experimental data are presented on viscosity, elastic shear modulus, and limiting shear stress of 12 liquid lubricants. It is shown that transition histories do affect the limiting shear stress of the materials in the form of isothermal compression resulting in a lower density and lower limiting stress than isobaric cooling. The measured limiting shear stress agrees with EHD traction data at slide-to-roll ratios of 0.1 or more. In pressure viscosity measurements of the polymer solutions, it is found that for some temperatures, the pressure viscosity coefficient of the blend is slightly less than that of the base, which results in the crossing of the viscosity-pressure isotherms at high pressures.

Practical Stress Analysis in Engineering Design

Abstract: Fundamental Relations and Concepts Forces and Force Systems Simple Stress and Strain: Simple Shear Stress and Strain Hooke's Law and Material Strength Stress in Two and Three Dimensions Strain in Two and Three Dimensions Curvilinear Coordinates Hooke's Law in Two and Three Dimensions Straight and Long Structural Components: Beams, Rods, and Bars Beams: Bending Stresses (Flexure) Beams: Displacement from Bending Beam Analysis Using Singularity Functions Beam Bending Formulas for Common Configurations Torsion and Twisting of Rods Thick Beams: Shear Stress in Beams Curved Beams Stability: Buckling of Beams, Rods, Columns, and Panels Shear Center Plates, Panels, Flanges, and Brackets Plates: Bending Theory Plates: Fundamental Bending Configurations and Applications. Panels and Annular Plate Closures Flanges Brackets Special Plate Problems and Applications Dynamic Loadings, Fatigue, and Fracture Dynamic Behavior of Structures: A Conceptual Review Elements of Seismic Design Impact Stress Propagation Fatigue Fracture Mechanics: Design Considerations Fracture Control Piping and Pressure Vessels Vessels with Internal Pressure Externally Pressured Cylindrical Vessels and Structures Buckling of Spherical Shell Axial and Bending Response Advanced and Specialized Problems Special Cylinder Problems Stress Concentration Thermal Considerations Axial Response of Straight and Tapered Bars Thin Rings and Arches Links and Eyebars Springs Irregular Shape Springs

Modelling of adiabatic shear band development from small imperfections

Abstract: A model of shear banding is presented which shows how a wide shear band develops from a narrow imperfection in an elasto-viscoplastic material subjected to dynamic shear strain. The model predicts that the width of the shear band is (i) independent of the properties of the initial imperfection and (ii) dependent upon thermal conductivity and strain rate. The dependence upon strain rate is verified qualitatively and quantitatively from experimental results. Finally, the model predicts narrowing of the region of rapid straining with ongoing deformation as is observed in experiment.

Implications of an Elastic Analysis of In Situ Stress Measurements Near the San Andreas Fault

Abstract: Twenty-nine measurements of in situ stress obtained with the hydraulic fracturing technique near Palmdale, California, are the basis of an elastic analysis of the state of stress in the Mojave Desert adjacent to the San Andreas fault. The measurements were made at depths extending from 80 to 849 m and at distances from the fault between 2 and 34 km. The elastic solution indicates a state of deviatoric stress typical for continents in that the inferred depth gradient of the maximum shear stress is about 7.9 MPa/km. Extrapolation yields an average shear stress in the upper 14 km of the crust of about 56 MPa, a result that is higher than estimates of the average shear stress on the San Andreas fault based on the analysis of heat flow data. This finding is consistent, however, with estimates of fault strength based on laboratory determinations of the coefficient of friction for samples of San Andreas fault gouge if the regional state of deviatoric stress is limited by the strength of the fault zone. If so, then the coefficient of friction of the San Andreas fault zone inferred from the stress field results is about 0.45. The state of stress does not appear to vary systematically with distance from the San Andreas fault although considerable localized variation is observed. The observations suggest an upper bound of about 0.1 MPa/km for the horizontal gradient of the maximum shear stress in the direction perpendicular to the San Andreas fault, a result that implies a corresponding limit of about 1.4 MPa on the shear traction applied to the base of the seismogenic layer. Finally, we demonstrate the potential application of in situ stress data to the direct assessment of accumulated slip, which could be released in a large earthquake. We show that on the basis of a model involving a locked fault, extending to about 22 km, the total fault slip below the locked portion is less than 13 m. A more comprehensive set of stress data could permit the estimation of an even lower bound.

Analysis of plastic flow in an amorphous soap bubble raft by the use of an inter-bubble potential

Abstract: The changes in potential energy, free volume, and internal shear strain in clusters undergoing shear transformations during the shearing of amorphous soap bubble rafts have been analysed by means of a generalized inter-bubble potential initially developed by Nicolson and by Lomer. A unique inverse correlation was found between the activation free energy barriers of transformations and the initial state of free volume of the transforming sites. Two separate branches of correlation, one for concentrated shear transformations and the other for diffuse shear transformations have been identified. When proper scaling between the inter-bubble potential and the appropriate close-packed metal potential is applied to the results, the distribution of activation free energies found from the soap bubble model gives remarkably good agreement with the experimentally measured distributions in metallic glasses obtained by Argon and Kuo earlier. The inverse correlation, found between activation free energy and fre...

Long wavelength approximation to peristaltic motion of a power law fluid.

Abstract: Peristaltic motion of a power law fluid in a two-dimensional channel is studied. Assuming that the wavelength of the peristaltic wave is large in comparison to the mean half-width of the channel, a solution for the stream function is obtained as an asymptotic expansion in terms of slope parameter. Expressions for axial pressure gradient and shear stress are derived. The effect of flow behaviour indexn on the streamline pattern and shear stress is studied and the phenomenon of trapping is discussed.

Influence of layer thickness on the strength of angle-ply laminates

Abstract: Experimental results are presented showing that the strength and toughness of finite width angle-ply laminates can be increased significantly by using an alternating layer stacking sequence as opposed to a clustered configuration. The ultimate tensile stress of an alternating plus/minus theta laminate of the form (+ or - theta) sub 2 sub s can be much as 1.5 times that of a clustered configuration of the form theta sub 2/-theta sub 2 sub s. Further, the toughness of the alternating layer configuration can be as much as 2.7 times that of the clustered configuration. These differences are explained analytically through consideration of the influence of layer thickness on the magnitude of the interlaminar shear stress and by examination of failed specimens. It is shown that the two laminate configurations exhibit distinctly different failure modes for some fiber angles. Both laminate configurations exhibit catastrophic failure with the damage limited essentially to a small region defined by the length of a single crack across the width of the specimen, parallel to the fiber direction. Results are presented for T300/5208 graphite-epoxy for fiber orientations of 10, 30, and 45 deg.

Study of Shear Stress Relaxation in Well‐Characterized Polymer Liquids

Abstract: Shear stress relaxation data were measured for a range of shear strains on several entangled polymer liquids in order to test time‐strain factorability and the Doi‐Edwards predictions about strain dependence. Undiluted samples and concentrated solutions of well‐characterized linear and star polybutadienes with narrow molecular weight distributions were used. Some departures from time‐strain factorability were found, but these appear to be nonsystematic, i.e., they do not correlate in any obvious way with the polymer structure. The strain dependence, however, follows the pattern observed by Osaki and Kurata for polystyrene solutions. The Doi‐Edwards prediction gives a good account of the results at intermediate entanglement densities (35,000≲cM≲150,000 for linear and star polybutadienes), but systematic departures are found at higher values. The causes of the latter are unknown.