Excess Shear Stress Research Articles

Estimates of sand transport in the Oosterschelde tidal basin using current-velocity measurements

Estimates of sand transport have been made from mid-depth velocity measurements in the Oosterschelde tidal basin, southwestern Netherlands. This involves: (1) The use of a two-region composite boundary-layer model in deriving the local spatially averaged bed shear stress from velocity distributions over large bedforms (megaripples and sandwaves). The two regions are divided at 100 cm above the bed, with an outer-region roughness length of 1.5 cm and an inner-region roughness length of 0.15 cm. (2) The application of a modified Bagnold's transport equation with the transport coefficient K depending on dimensionless excess shear stress in the form of a power law. The calculations indicate very active sand movement in the present-day Oosterschelde tidal basin. Local sediment circulation may develop around shoals. Channel-floor erosion may occur in parting areas of sediment transport. The net sediment transport is mainly in the ebb direction. Estimates of sediment budget for the last 20 years show that large amounts of sand have been eroded from the Oosterschelde tidal basin and transported to the ebb-delta area. These results are comparable with other estimates from echo-sounding data, reflecting the rapid adjustment of the basin to the increased tidal prism. Such active sediment redistribution and major morphological evolution processes would have important influence on the structural organization and sequential development of estuary-ebb delta deposits. Information about the sedimentary processes in present-day estuary tidal basins is valuable for a better understanding of ancient examples.

Suspended sediment transport in an estuarine tidal channel within San Francisco Bay, California

A recently developed instrumentation system has been used to monitor simultaneously flow conditions and suspended sediment distribution in the bottom boundary layer of a tidal channel within San Francisco Bay, California. Measurements were made every 15 min over six successive flood and ebb tidal cycles. They included mean velocity profiles from four electromagnetic current meters within 1 m of the seabed; mean suspended sediment concentration profiles from seven miniature nephelometers placed within 1 m of the seabed; near-bottom pressure fluctuations; vertical temperature gradient; and bottom photographs. Additionally, suspended sediment was sampled from four levels within 1 m of the seabed three times during the tidal cycle. The instrument system was retrieved during each slack water period to exchange suspended sediment sample bags. While the instrument was deployed STD-nephelometer measurements were made thoughout the water column and water samples were collected each 1–h and bottom sediment was sampled at the deployment site. Size distributions of the suspended sediment samples, estimates of particle settling velocity ( μ s ), friction velocity ( U ∗ ), and reference concentration ( C a ) at z = 20 cm were used in the suspended sediment distribution equations to evaluate their ability to predict the observed suspended sediment profiles. Three suspended sediment particle conditions were evaluated: (1) individual particle sizes in the 4–11 Φ (62.5-0.5 μm) size range with the reference concentration C a at z = 20 cm ( C Φ ); (2) individual particle sizes in the 4–6 Φ size range, flocs representing the 7–11 Φ size range with the reference concentration C a at z = 20 cm ( C f ); and (3) individual particle sizes in the 4–6 Φ size range, flocs representing the 7–11 Φ size range with the reference concentration predicted as a function of the bed sediment size distribution and the square of the excess shear stress. An analysis was also carried out on the sensitivity of the suspended sediment distribution equation to deviations in the primary variables μ s , U ∗ , and C a . In addition, computations of mass flux were made in order to show vertical variations in mass flux for varying flow conditions.

Predicting suspended sediment concentration on continental shelves

Field observations of the near-bottom suspended sediment concentration, current and wave at a depth of 90 m on the Washington Shelf and at a depth of 1.5 m on the coastal Beaufort Sea show that wave-induced oscillatory flow plays a dominant role in resuspending bottom sediment. The suspended sediment concentration can be reasonably predicted from the measured flow data by applying a diffusion model. Good agreement between the predicted and observed concentrations is obtained by using a reference concentration proportional to the square root of the excess boundary shear stress. For a sediment bed consisting of multiple grain sizes, the differential rate at which material is eroded leads to a significant change in the surfacial grain-size composition. The fraction of the fine-grain sediment in the surface layer decreases sharply with increasing boundary shear stress. A procedure is presented to model this change of the grain-size distribution in the surface layer. The analysis indicates that neglecting the change of the surfacial grain-size distribution with flow may overestimate the erosion depth by a factor of 10 at bottom orbital velocities exceeding 70 cm s −1.

Surface films affecting velocity profiles of slowly moving water in open channels

Rates of movement of surface films and underlying water were measured using observable solid and liquid tracers. Water flow rate, surface width, water depth, and channel length were varied in flumes in which the water was allowed to flow over a broad weir crest at the tail end. In some runs the surface was blocked 5 cm in front of the weir. When this was done, velocity of the surface film immediately upstream from the block decreased to a small fraction of its previous value and this area of slow surface velocity built upstream with time, extending to as far as several meters under steady state conditions. In this reach, the film at the air-water interface of the water causes drag on the moving water similar to that at a solid-water interface. Water surface in channels one cm wide and 100 cm long stopped moving when the shear force caused by water flowing beneath it dropped to less than 0.0013 dynes/cm2. This indicates structure in the surface which does not deform or shear at rates proportional to the force applied. Average water flow velocities from 0.3 to 1 cm per second provided shear stresses in excess of 0.002 dynes/cm2 and moved these surface films. However, the velocity distribution across the surface of the channel was not parabolic, and indicated that most of the shear in the film was taking place near the edges of the channel. Extrapolation of these observations to water film dimensions present in unsaturated soils indicates that air-water interfaces in unsaturated soils are usually static.

Effect of surface shear stress on the attachment of Pseudomonas fluorescens to stainless steel under defined flow conditions

The application of the radial-flow growth chamber to the study of the initial stages of bacterial adhesion to surfaces under flowing conditions is reported. The adhesive properties of the bacterium Pseudomonas fluorescens (NCIB 9046) to stainless steel (type AISI 316) were found to be highly dependent on surface shear stress and the time and concentration of cells used in the incubation procedure. Maximum levels of adhesion occurred in zones of lowest surface shear stress, particularly less than 6-8 Nm(-2). Adhesion was still noticeable at shear stresses even up to 130 Nm(-2). Significant detachment of cells from a monolayer attached under static conditions was found to occur at surface shear stresses in excess of 10-12 Nm(-2).

Physical factors important in the development of atherosclerosis in diabetes.

Circulation of the blood is a process controlled by physical laws. The same physical laws also determine the sites of atherosclerotic plaques. Our understanding of their role in atherogenesis is aided by a review of (1) the difference between a solid and fluid, (2) strain and shear or strain rate and shear rate as describers of motion, (3) the distinction between force as pressure and as shear stress, (4) the effects of viscosity and inertia on fluid motion, and (5) the nonlinear responses of blood and blood vessels. Moving blood affects the vessel wall principally through shear stress, the force due to its viscosity applied parallel to the surface of the vessel. Excessive shear stress occurs near the orifices of branches of the aorta, the arterial areas where atherosclerotic plaques develop. It is produced by a change in blood flow pattern at these sites. Arterial stiffening in diabetes reduces the ability of the branches of the aorta to dilate during systole, a motion that helps to control high wall shear stress. At wall shear stress levels under 400 dynes/cm2, a value only modestly higher than that normally achieved near the orifices of the major arterial branches, the endothelial lining of the arterial wall becomes disrupted, allowing entry of plasma proteins and lipids. Blood viscosity elevation in diabetes, linked to enhanced erythrocyte aggregation and reduced erythrocyte deformability, adds to the shear stress at the vessel wall, favoring increased lipid entry. Exposed to a high lipid influx secondary to excessive wall shear stress, the arterial wall must rely on physical as well as chemical means to remove the lipids. Subendothelial motion generated by diastolic arterial contraction and systolic expansion appears to cause plasmalemmal vesicle formation and dissolution, an event probably important in the removal process. The combination of arterial stiffening and hemorheologic disturbance adds to the burden of increased plasma lipids to favor the development of atherosclerosis in diabetes.

An evaluation of Bagnold's dimensionless conefficient of proportionality using measurements of sandwave movement

Only a few observations are currently available from the tidal marine environment which can be used to evaluate sediment transport formulae. Research carried out on sandwave movement indicates that most of the sediment transport is restricted to the crestal area. Mean rates of sediment transport were calculated from volumetric changes, between successive slack waters, over a nine day period spanning Spring tides. The results were used, in conjunction with boundary-layer flow measurements, to calculate the threshold velocity for the movement of sediment and also to evaluate Bagnold's (1963) dimensionless coefficient of proportionality, K. Comparisons are made with the flume data of Guy et al. (1966) for the appropriate sediment grain size. The dependence of K, upon normalised excess shear stress was found to be weaker than that suggested by Sternberg (1972). Within the range of flow velocities considered there was no indication that K tended towards a constant at high flow velocities.

Shelter behind two-dimensional solid and porous fences

Using a pulsed-wire anemometer, extensive measurements have been made in the wakes of two-dimensional solid and porous fences immersed in the constant-stress region of a simulated rural atmospheric boundary-layer. The porosity (ratio of open to total area) of the perforated fences ranged from 0.0 to 0.5 and each porosity was modelled using three forms of openings, i.e. vertical slats, horizontal slats and circular holes. Comparative measurements made using the more conventional hot-wire anemometer are given. The results show the superiority of the pulsed-wire anemometer in correctly measuring the highly turbulent and sometimes recirculating wake flows. This indicates that previously reported experiments may be in considerable error. Results from the present experiment are given in the form of dimensionless mean and turbulence velocity profiles. In addition, contour plots of shelter parameters behind the fences are given for comfort and shelter analyses. Downstream of the re-attachment region, velocity deficits and excess shear and normal stress perturbations (quantities useful in shelter analyses) are plotted in a self-preserving form and simple equations that fit the experimental points are given. Power spectra measured in the presence of the solid fence are given and the structure of the turbulent wake deduced from these is discussed.

Velocity profiles over a rippled bed and the threshold of movement of sand

Measurements of velocity profiles have been obtained within 2 m of the sea bed over a rippled sand in Start Bay, Devon, U.K. Simultaneous observation by underwater television has enabled the threshold of movement of the sand to be determined. The threshold criteria are considerably higher than laboratory measurements for a flat bed. As the sand moves the ripples change their shape. This causes a change in the bottom roughness length, the drag coefficient and the apparent threshold of sand movement during the tidal cycle. The bottom roughness length and the threshold of movement also increase with decreasing tidal amplitude. The calculated suspended sediment concentration at the height of bed roughness length was proportional to the normalized excess shear stress with a constant of proportionality of 0·78×10−4. The concentration at a height of 10cm was proportional to u*5.

Plate motion and thermal instability in the asthenosphere

This paper investigates the effect of shear heating in the asthenosphere on the thermal structure of the upper mantle. Equations describing the motion of the lithosphere over the asthenosphere in the presence of a strongly temperature-dependent stress-strain rate relation are derived and solved with the help of several approximations. These approximations are shown to be valid under conditions appropriate for the earth. Two sets of solutions are found. For one set (the “subcritical” solutions) a normal shear stress—velocity relation is found for small stresses. The velocity increases as the stress increases, reaching a maximum velocity σ c for a critical stress σ c. The subcritical solutions have a negligible effect on the thermal structure of the earth, even at the critical stress. The other set of solutions (the “supercritical” solutions) has the bizarre property that a decrease of applied shear stress leads to an increase of velocity. Thus, as the shear stress goes to zero, the velocity becomes infinite. At larger shear stresses the velocity decreases until it reaches σ c at a stress σ c (the two sets of solutions share this point in common). There are no steady solutions of any kind for shear stresses in excess of σ c . We discard the supercritical solutions as candidates for the thermal structure of the earth on the basis of their instability to small perturbations of applied stress and temperature. The realm of subcritical solutions (stress less than σ c , velocity less than σ c ) thus defines a regime of plate motion in which the thermal effects of shear heating are negligible. If the shear stresses acting on plates exceed σ c , however, new physical processes must come into play to dissipate the excess heat generated. Assuming that the velocities of plates on the earth today are less than σ c , relative to the deep mantle, a strict upper limit of a few tens of bars can be derived for σ c , corresponding to effective viscosities of ca. 10 19 poise in the asthenosphere.

Si単結晶のひっかき条痕部の転位組織

The present paper deals with the formation mechanism of dislocations by the scratch deformation. (111) single crystal of Si is scratch-deformed by a diamond stylus along [\bar101] and [1\bar21] directions at room and liquid nitrogen temperatures. Thin foils for transmission electron microscope observations were made by thinning down the scratched specimen only from the back side through the jet chemical polishing.The results obtained are as follows: The specimen scratch-deformed at −196°C has less dislocation density than that deformed at room temperature and dislocations have the tendency to align in a certain crystallographic direction by lower temperature deformation. The dislocation half loops observed at the scratched part at −196°C lie on {111} slip planes and have the Burgers vector of a⁄2〈110〉 which is consistent with the direction of the maximum Schmid factor by the scratch deformation. These dislocations are supposed to be induced by slip deformation due to the local excess shear stress during the scratching.

Vibration-induced arterial shear stress: the relationship to Raynaud's phenomenon.

The enhancement of arterial wall shear stress by mechanical vibration is examined from the viewpoint of its relationship to circulatory disorders found in those using vibrating hand tools. With the use of recent results, it is shown that certain arterial wall physiological processes are influenced by arterial wall shear and that for sufficiently large shear stresses, eg, approximately 1,000 dynes/sq cm, endothelial damage does occur. Calculations are presented that predict the influence of vibration on arterial wall shear. For typical arterial flow conditions, it is estimated that shear stresses in excess of 1,200 dynes/sq cm may be produced by a vibrating tool at 20 hertz with a half-amplitude of 2.5 cm and allowance for 50% attenuation of the vibration impulse.