Showing papers on "Shear stress published in 1974"

Flow and bed topography in channel bends

Abstract: Uniform flow in river bends of rectangular or arbitrary cross section has been calculated by the slip velocity method and the results are compared with experiments. It is concluded that the helical motion introduced by the channel curvature may be fairly well described. The results are then applied to the more complex problem of flow in a river bend with movable bed. Next, the problem of flow in a meandering channel with fixed side walls has been investigated, using a special system of curvilinear coordinates. The solution of the linearized flow equations yields a method for calculation of the distribution of the bed shear stress. In the first approximation the large-scale bottom geometry is calculated by considering the lateral sediment balance. The helical flow will move the sediment particles towards the curvature center, while the transverse slope of the bed will give a movement in the opposite direction.

Discontinuous and dilatant viscosity behavior in concentrated suspensions. II. Theory and experimental tests

Abstract: An analysis of forces which are expected to arise in colloidal systems under shear has been used to model the flow instability which leads to discontinuous and dilatant viscosity behavior in concentrated suspensions of polymeric resins. Observation of this phenomenon was reported in the first paper of this series. The analysis is carried out on the presumption that the dominant forces leading to the flow instability are van der Waals-London attraction, electric double layer repulsion and the shear stress acting on groups of particles. Conditions for the onset of the instability are cast in terms of a torque balance and an energy balance, and a set of dimensionless numbers are obtained to characterize the phenomenon. Experimental data obtained from tests on a number of concentrated suspensions are given in support of the theory.

The Structure of the Two-Dimensional Internal Boundary Layer over a Sudden Change of Surface Roughness

Abstract: The effects of an abrupt change of surface roughness on the mean flow and turbulence structure in the neutral surface layer are numerically investigated by a higher-order turbulence closure theory, which includes dynamical equations for Reynolds stresses and the viscous dissipation rate. The closed system of governing equations, together with the specified initial and boundary conditions, is solved by an explicit finite-difference method on a digital computer. The numerical model predicts the distributions of mean wind, shear stress, turbulent energy and other quantities, with no a priori assumptions regarding the distributions of any of these variables in the transition region. The distributions of the nondimensional wind shear, the dissipation and mixing length scales, and the ratio of stress to turbulent kinetic energy are shown to differ significantly from their equilibrium flow variations.

Some properties of truncated turbulence signals in bounded shear flows

Abstract: Measurement results of turbulent shear flows are re-examined. Various statistical properties of the truncated streamwise and normal velocity components u and v and of their product uv have been determined in an attempt to characterize quantitatively the motions of the flow. Average values and probability density distributions both of the truncated and untruncated signals have been taken.

A dislocation model for yield in polyethylene

Abstract: The critical resolved shear stress (CRSS) for (010) [001] slip in oriented high- density polyethylene (HDPE) has been measured as a function of temperature between 0° and 100°C The variation of the CRSS with temperature has been interpreted in terms of yield by means of the thermal activation of dislocations at the edges of the crystal platelets under the action of an applied shear stress

Adhesion of red cells to foreign surfaces in the presence of flow.

Abstract: A controlled surface shear stress produced by laminar flow in a rotating disk or parallel plate flow system has been used to study red cell adhesion to glass, siliconized glass, polythylene and Teflon. The results indicate that the shear stress at the surface must exceed a minimum critical value before cells will detach, and that this minimum critical shear stress increases as the time of cellsurface contact is increased. The results also indicate that the minimum critical shear stress for detachment from clean surfaces correlates with the critical surface tension of the material for the four materials studied. Studies with proteincoated surfaces indicate that albumin causes a moderate reduction in the strength of adhesion. The magnitude of the effect was found to be influenced significantly by the substratum material.

Bifurcation analysis of the onset of necking in an elastic/plastic cylinder under uniaxial tension

Abstract: The bifurcation problem governing the onset of axisymmetric necking in a circular cylindrical specimen in uniaxial tension is analysed. The specimen is made of an incompressible elastic/plastic material. One end is subject to a prescribed uniform axial displacement relative to the other and both ends are shear free. The true stress at bifurcation is greater than the stress at which the maximum load is attained by an amount which depends on (a) the radius to length ratio of the specimen, (b) the ratio of the elastic shear modulus to the tangent modulus, and (c) the derivative of the tangent modulus with respect to the stress. Bifurcation takes place immediately following attainment of the maximum load when the specimen is sufficiently slender.

Rheological properties of calcium carbonate-filled polypropylene melts

Abstract: An experimental study was carried out to investigate the viscoelastic behavior of calcium carbonate-filled polypropylene melts, using the Han slit rheometer. In the analysis of the experimental data, the pressure gradient was used to determine the wall shear stress, and the exit pressure to determine the elasticity of the filled polymers. The study shows that the materials studied follow a power law in viscous behavior over the range of shear rates investigated and that the viscosity increases and elasticity decreases as filler concentration is increased. Also investigated was the effect of temperature on the viscoelastic properties of filled polypropylene melts.

Three-Dimensional Turbulent Wall Jets

Abstract: Experiments on three-dimensional turbulent bluff wall jets originating from circular, elliptic, square, rectangular, and equilateral triangular nozzles have shown that in the region of fully-developed flow, the distribution of the axial velocity in the central as well as transverse planes is similar. The velocity scale varies inversely with the axial distance whereas the length scales vary linearly with the axial distance from the respective virtual origins. The boundary shear stress in the center line has been measured and correlated. The square root of the area of the nozzle serves as a convenient length scale in correlating the data for the nozzles of different shapes.

A Study of Convection Capped by a Stable Layer Using Doppler Radar and Acoustic Echo Sounders

Abstract: Simultaneous atmospheric acoustic echo sounding and Doppler radar measurements were used to study some of the characteristics of an unstable boundary layer capped by an inversion. The resultant set of observations shows that the turbulent energy dissipation rate was constant with height up through the inversion layer. Since production from buoyancy and shear were found to be negligible in the upper part of the convective layer, the turbulent energy must be supplied by energy flux convergence. The stress at the top of the momentum layer was estimated to be approximately 2.5 dyn cm−2, assuming the energy flux divergence, term was small above the inversion. In addition, the height where the downward heat flux above the inversion went to zero and the height where the shearing stress went to zero were different. For the case analyzed in this paper, the apparent height at which the heat flux went to zero was 0.6 times the height where the shearing stress became zero.

An analysis of strain accumulation on a strike slip fault

Abstract: An analysis of strain accumulation on a strike slip fault is given. The fault between two lithospheric plates is assumed to be locked to a finite depth; owing to plastic flow the fault is free to slide at greater depths. The base of each plate is also a free boundary. The periodic stress accumulation and stress release associated with the elastic rebound hypothesis are modeled. A solution for the two-dimensional stress and strain field near the fault is obtained. It is shown that significant strain accumulation extends to distances of the order of the length of the fault break. The rates of accumulation of stress and strain energy for the San Andreas fault are obtained. Predicted distributions of shear strain are compared with observations. Because of the scatter in the data and possible complexities where observations have been obtained, poor agreement with theory is found.

Finite Element Stress Analysis of a Restored Axisymmetric First Molar

Abstract: A first molar with full crown preparation and three marginal configurations-chamfer, chisel, and shoulder with a bevel-was analyzed by the finite element method. The maximum, minimum, and shear stresses were plotted as a function of the radius along various horizontal planes. The chamfer geometry exhibited the most uniform stress distribution.

Antiplane shear crack terminating at and going through a bimaterial interface

Abstract: The antiplane shear problem of two bonded elastic half planes containing a crack perpendicular to the interface is considered. The cases of a semi-infinite crack terminating at the interface, a finite crack away from and terminating at the interface, two cracks one on each side of the interface, and a finite crack crossing the interface are separately investigated. The nature of the stress singularity for the crack terminating at and going through the interface is studied, and it is shown that at the irregular point on the interface, for the former the power of singularity is not -1/2 and for the latter the stresses are bounded. For a material pair of aluminum-epoxy some numerical results giving the stress intensity factors, the density functions, and the crack opening displacements are presented.

Time dependent strain following faulting of a porous medium

Abstract: A fracture that redistributes shear stress in a porous medium produces local fluid pore pressure changes. Decay of this pore pressure due to flow causes strain. Using the theory of Biot, I calculate the time dependent stress, strain, and pore pressure fields after a plane fracture whose offset varies sinusoidally in the direction of slip. The results permit construction of arbitrary one-dimensional slip functions by Fourier synthesis. In particular, I investigate the simple crack made from two edge dislocations. The fracture surface is slowly reloaded after an initial shear stress drop, and the shear strength of the material adjacent to the fracture varies with time. This behavior is useful in explaining earthquake foreshocks, aftershocks, and delayed creep. For the simple crack the frequency of aftershocks associated with the reloading decays like t−1.

Influence of Suction and Blowing on Entrainment of Sand Particles

Abstract: The influence of suction and blowing on the rate of entrainment of sand particles was studied experimentally. For the range of grain sizes (130 - 570 μm) suction always reduced sediment transport at a constant mean flow velocity, whereas the rate of transport increased with blowing. The results were compared with the theory of Martin on the stability of a particle under the influence of pore-water flow. It was found that the coefficient, C , relating the force on the particles at the surface of the bed to the pressure gradient in the bed, varied strongly with the grain size. The influence of suction and blowing on the bed shear stress was computed with a simple kinematic model and compared with the theory of Turcotte and others.

Repeated load testing of a granular material

Abstract: Both the resilient and permanent strains were measured and shown to reach equilibrium values after some 10 4 stress cycles under drained test conditions. The effect of changes in both the mean level and the amplitude of the deviator stress were studied and samples were tested at various constant cell pressures. The measured strains were shown to be dependent on the applied stress conditions expressed as dynamic deviator stress, mean normal stress, and cell pressure. The well-known nonlinearity between stress and resilient strain was noted and quantified for this particular material. The simple relationship previously established between resilient modulus and either cell pressure or the sum of the principal stresses was investigated and found to apply only under specific circumstances. More generally, resilient modulus depended on both the normal and shear stress conditions.

Non-Newtonian behaviour in elastohydrodynamic lubrication

Abstract: The relation between shear stress and shear rate has been determined for elastohydrodynamic oil films. At low values the rate of shear is directly proportional to the shear stress, but at higher values the shear rate increases more rapidly than the stress. It is shown that the critical factor is the magnitude of the shear stress, not the shear rate, and that this critical magnitude depends upon the pressure and the molecular size. Above the critical stress, in the non-Newtonian region, the shape of the curve relating the stress to the rate of shear depends upon the distribution of the sizes of the molecules in the oil. It is shown that in elastohydrodynamic conditions the limits of Newtonian behaviour are frequently exceeded and that this is liable to influence the pressure distribution, the magnitude of the traction, the generation of heat, and, at high speeds, the value of the film thickness.

Differentiation, organogenesis, and the tectonics of cell wall orientation. iii. theoretical considerations of cell wall mechanics

Abstract: This paper is concerned with the relationship between cell wall orientation in dividing tissues and stress. An examination of the possible orientations of a cell-plate in a cell under axial stress reveals that only one orientation passing through the center of the cell can be completely free from shear stress, thus providing a favored plane for the positioning of an hypothetical, shear-sensitive, cell-plate precursor. From this principle of shear-free partitioning, and the three basic rules of stress behavior along free boundaries, it is possible to predict certain features of cell wall orientation along free edges and epidermal surfaces and in some simple apices. The possible significance of the widespread phenomenon of axillary induction of bud formation is discussed in terms of the generation of stresses at the base of the axillant leaf, and the general efficacy of mechanical stress as a spontaneously arising morphogenetic trigger is considered. IN PREVIOUS PAPERS a correlation was established between cell wall patterns and principal stress trajectories, as studied in the maturing ovule of Gossypium hirsutum L. This was made possible by the accurate description of principal stress trajectories in a suitable plastic model by means of photoelastic stress analysis. The role of the integuments in the ontogeny of the ovule was discussed in terms of providing for the growth of the embryo-sac in a space free from the external compressive stresses generated by the growth of

Flow and heat transfer along a plane wall with periodic suction

Abstract: The three-dimensional incompressible laminar boundary layer past a flat plate is investigated, when suction with a slightly sinusoidal transverse suction velocity distribution at the wall is applied. For the asymptotic flow conditions far downstream the components of the wall shear stress and the heat transfer including its dependence on Prandtl number are determined.

Inclusion of Transverse Shear Deformation in Finite Element Displacement Formulations

Abstract: A stiffness matrix is derived for a beam element with transverse shear deformation. It is shown that straightforward energy minimization yields the correct stiffness matrix in displacement formulations when transverse shear effects are considered. Since the TIM4 beam element does not represent the geometric boundary conditions for a cantilever beam the rotation of the normal must be retained as a grid point degree of freedom.

Stokes problem for elastico-viscous fluid

Abstract: An approximate solution to the flow past an impulsively started infinite plate in an elastico-viscous fluid is derived for the velocity and shearing stress. It is observed that the velocity increases with increasing the elastic parameterk and the shearing stress decreases with increasingk.

Turbulent flow in smooth concentric annuli with small radius ratios

Abstract: Fully developed turbulent flow through three concentric annuli was investigated experimentally for a Reynolds-number range Re = 2 × 104−2 × 105. Measurements were made of the pressure drop, the positions of zero shear stress and maximum velocity, and the velocity distribution in annuli of radius ratios α = 0.02, 0.04 and 0.1, respectively. The results for the key problem in the flow through annuli, the position of zero shear stress, showed that this position is not coincident with the position of maximum velocity. Furthermore, the investigation showed the strong influence of spacers on the velocity and shear-stress distributions. The numerous theoretical and experimental results in the literature which are based on the coincidence of the positions of zero shear stress and maximum velocity are not in agreement with reality.

The Turning Point of Elastodynamic Waves

Abstract: Summary It is demonstrated how a uniform asymptotic expansion can be developed for the elastodynamic wave equations in a spherically symmetric medium. The expansion is valid in the neighbourhood of first order turning points except at discontinuities. Because the dependent variables in the expansion are the displacement and stress, discontinuities are easily included. The extra terms from the Earth-flattening transformation are included. The optimum transformations for SH and acoustic waves diner significantly. They compensate different terms in the transformed wave equations. In the SH case, the shear stress and strain are corrected for the geometrical differences in the co-ordinate systems, whereas in the acoustic case, the hydrostatic pressure is maintained unaltered and the transformation compensates for the geometrical change in dilatation. A P wave is a mixture of dilatation and shear, and an optimum transform cannot be found.