Average Shear Research Articles

Shaft resistance of driven cast-in-situ piles in sand

Driven cast-in-situ (DCIS) piles are classified as a large displacement pile, despite sharing certain aspects of their construction with replacement pile types. However, there are relatively few case histories of load tests on DCIS piles in the literature to verify the assumption that they behave as large displacement piles. In particular, the shaft resistance of DCIS piles in sand is uncertain due to the complex interaction between the freshly cast concrete and surrounding displaced soil after extraction of the steel installation tube. This paper describes the installation, curing, and maintained compression load testing of three temporary-cased DCIS test piles at a uniform sand site near Coventry, United Kingdom. The piles were instrumented with vibrating wire strain gauges to enable accurate measurement of the local shear stress generated on the pile shaft during maintained compression loading. The tests showed that the peak average and local shear stresses tended to mobilize at greater shaft displacements than traditional preformed displacement piles during loading. A clear reduction in normalized local shear stresses (and hence radial effective stress) at failure with distance from the pile base, i.e., friction fatigue, was evident for all piles, implying that radial stresses generated during driven installation of the steel tube are not erased upon concreting and tube withdrawal. Furthermore, the inferred normalized radial effective stresses at failure were remarkably similar to those reported for traditional preformed displacement piles in the literature.

Seismic Proofing Capability of the Accumulated Semiactive Hydraulic Damper as an Active Interaction Control Device with Predictive Control

The intensity of natural disasters has increased recently, causing buildings’ damages which need to be reinforced to prevent their destruction. To improve the seismic proofing capability of Accumulated Semiactive Hydraulic Damper, it is converted to an Active Interaction Control device and synchronous control and predictive control methods are proposed. The full-scale shaking table test is used to test and verify the seismic proofing capability of the proposed AIC with these control methods. This study examines the shock absorption of test structure under excitation by external forces, influences of prediction time, stiffness of the auxiliary structure, synchronous switching, and asynchronous switching on the control effects, and the influence of control locations of test structure on the control effects of the proposed AIC. Test results show that, for the proposed AIC with synchronous control and predictive control of 0.10~0.13 seconds, the displacement reduction ratios are greater than 71%, the average acceleration reduction ratios are, respectively, 36.2% and 36.9%, at the 1st and 2nd floors, and the average base shear reduction ratio is 29.6%. The proposed AIC with suitable stiffeners for the auxiliary structure at each floor with synchronous control and predictive control provide high reliability and practicability for seismic proofing of buildings.

Wind direction in Moscow

Discussed are the data on wind direction in the air layer from 40 to 500 m over Moscow for the period of 2004–2014. The data was obtained with the MODOS sodar installed in Lomonosov Moscow State University. It is demonstrated that this wind direction has a stable southwestern mode in ten years on average. The western component of this mode strengthens in spring and summer (the south-southwestern direction is registered more rarely, and the west-southwestern one, more frequently). The winds of northern and eastern directions are observed much more rarely than those of southern and western directions. The rarest wind direction over Moscow is the north-northeastern one. However, under conditions of the anticyclonic weather (in the zone of blocking anticyclones in summer or the Siberian high in winter), the prevailing wind direction can have the eastern component even during several weeks on average. The northern wind is most often registered in summer, and the eastern wind, in spring. The average right wind shear in the air layer of 40–500 m is 20°. The surface air layer with the quasiconstant wind direction is not more than 40 m high in two of three cases per month on average; its height reaches 60 or 80 m in several months.

Numerical analysis of the hemodynamics of an abdominal aortic aneurysm repaired using the endovascular chimney technique.

This paper presents a numerical analysis of the hemodynamics in an abdominal aorta (AA) with an aneurysm repaired by a stent graft (SG) system using the chimney technique. Computational fluid dynamics (CFD) simulations were conducted in a model of an AA repaired with a chimney stent graft (CSG) inserted into a renal artery parallel to an aortic SG and a model of a healthy AA. Comparing the simulation results of these two cases suggests that the presence of the CSG in the AA causes changes in average wall shear stress (WSS), potentially damaging recirculation zones, and additional changes in flow patterns.

Impact of blood viscosity on wall shear stress of large intracranial aneurysms

Objective To simulate the computational hemodynamics of large intracranial aneurysms and analyze the impact of blood viscosity on wall shear stress. Methods Eight large intracranial aneurysms at internal carotid artery between June 2013 and July 2013 were studied retrospectively. Among them,3 aneurysms were ruptured and 5 were unruptured.Pulsatile CFD simulations were carried out using three levels of blood viscosity(0.002,0.004,0.012 Pa·s).For each aneurysm model, the average wall shear stress(WSS) predicted by the three blood viscosity levels were compared. Results Blood viscosity has significant impaction on average wall shear stress of intracranial aneurysms and parent artery.Average wall shear stress increased significantly with the increase of blood viscosity.When blood viscosity was 0. 002,0. 004,0. 012 Pa·s,the average wall shear stress of aneurysms was(2. 79±2. 23),(5. 27±3. 72)and(11. 05±7. 56)Pa,respectively.The average wall shear stress of parent artery was(5. 61±2. 59),(9. 56±3. 74)and(22. 56±12. 41) Pa respectively.A reduction in wall shear stress between the parent artery and the aneurysm sac was found in all the cases. Conclusion Abnormal increase or decrease in blood viscosity may aggravate vascular wall damage and increase the risk of aneurysmal rupture.It is necessary to adopt patient-specific value of blood viscosity in future hemodynamic studies of intracranial aneurysms. Key words: Blood viscosity; Large intracranial aneurysm; Wall shear stress; Hemodynamics

A simplified approach to computing flow and bedload along gravel dune-like bedforms

River dunes are important bedforms. Problems associated with the development and evolution of dune bedforms include increased flood risks, channel erosion, and damage to fish habitats. The purpose of this paper is to investigate the near-bed flow structure and bedload transport along gavel dune-like bedforms. The velocity field is computed using a relatively simple multi-layer hydrodynamic model, with a parameterization of flow separation in the leeside of dunes. The computation is of high efficiency and avoids uncertainties caused by flow separation. Fractional transport rates for a sediment mixture of sands and gravel are calculated using surface-based techniques. The computed flow velocities and bed shear stresses are in good comparison with acoustic Doppler velocimeter measurements. Bedload transport is shown to increase non-linearly with distance toward the dune crest and reach the maximum at the crest. This implies that dune-length averaged bed shear stress is not suitable for bedload calculations. At low discharges, the bed shear stress is the limiting factor, resulting in insignificant bedload. At high discharges when the bed shear stress exceeds a threshold, the effect of sediment-grain hiding and sediment-size availability are important for bedload calculations. The discharge–transport relationship is highly non-linear. This paper has demonstrated selective transport and potential dune surface coarsening. The simplified modelling approach has a good potential for application to field conditions.

Viscoelastic properties of healthy achilles tendon are independent of isometric plantar flexion strength and cross-sectional area.

Changes in tendon viscoelastic properties are observed after injuries and during healing as a product of altered composition and structure. Continuous Shear Wave Elastography is a new technique measuring viscoelastic properties of soft tissues using external shear waves. Tendon has not been studied with this technique, therefore, the aims of this study were to establish the range of shear and viscosity moduli in healthy Achilles tendons, determine bilateral differences of these parameters and explore correlations of viscoelasticity to plantar flexion strength and tendon area. Continuous Shear Wave Elastography was performed over the free portion of both Achilles tendons from 29 subjects. Isometric plantar flexion strength and cross sectional area were measured. The average shear and viscous moduli was 83.2 kPa and 141.0 Pa-s, respectively. No correlations existed between the shear or viscous modulus and area or strength. This indicates that viscoelastic properties can be considered novel, independent biomarkers. The shear and viscosity moduli were bilaterally equivalent (p = 0.013, 0.017) which allows determining pathologies through side-to-side deviations. The average bilateral coefficient of variation was 7.2% and 9.4% for shear and viscosity modulus, respectively. The viscoelastic properties of the Achilles tendon may provide an unbiased, non-subjective rating system of tendon recovery and optimizing treatment strategies.

SPH Modeling of Hydraulics and Erosion of HPTRM Levee

Post-Katrina investigations revealed that most earthen levee damage occurred on the levee crest and landward-side slope as a result of either wave overtopping, storm surge overflow, or a combination of both. In this paper, combined wave overtopping and storm surge overflow of a levee embankment strengthened with high performance turf reinforcement mat (HPTRM) system was studied in a purely Lagrangian and meshless approach, two-dimensional smoothed particle hydrodynamics (SPH) model. After the SPH model is calibrated with full-scale overtopping test results, the overtopping discharge, flow thickness, flow velocity, average overtopping velocity, shear stress, and soil erosion rate are calculated. New equations are developed for average overtopping discharge. The shear stresses on landward-side slope are calculated and the characteristics of soil loss are given. Equations are also provided to estimate soil loss rate. The range of the application of these equations is discussed.

Cochlear implantation in 21 patients with common cavity malformation

Conclusion: The facial recess approach is preferred in common cavity (CC) malformation with an incomplete basal turn, and the transmastoid single-slit labyrinthotomy approach in classic CC malformation. Patients with CC benefit from cochlear implantation (CI) over time, but the audiological and speech development is poorer than in cases with normal cochleas. Objectives: To discuss the surgical aspects and performance of CI in 21 patients with CC malformation. Methods: Twenty-one CC malformations were classified into 2 types: classic CC malformation and CC malformation with an incomplete basal turn. Twenty-one patients without inner ear malformation were set as the control group. Thus, data for 42 patients were analyzed. Results: The facial recess approach was used in 3 patients with CC malformation with an incomplete basal turn, and the transmastoid single-slit labyrinthotomy approach in 18 patients with classic CC malformation. After follow-up for 36 months, the average free-field hearing threshold was higher, and the scores for the CAP, SIR, IT-MAIS, and closed-set/open-set auditory speech perception tests were lower than in the control group (p < 0.05).

Effects of residual riblets of impeller's hub surface on aerodynamic performance of centrifugal compressors

In this article, the effects of residual riblets of impeller's hub surface on the aerodynamic performance of a compressor are studied in detail. Several accurate geometrical models of impellers after their hub surface are machined, with various tool passes are built. The number of tool passes for the hub surface finish of a single passage of the impeller is denoted by N. Meanwhile, a dimensionless parameter Bmax is introduced to characterize the size of the ribs of the arbitrary impeller, which is the percentage ratio rate of the maximum cusp height of the hub surface to the exit blade width. The geometrical models are numerically studied by solving the Reynolds averaged Navier–Stokes equations using the SST turbulent model. The predictions show that a group of secondary cross-stream vortices are generated by the interaction of streamwise vortices above the hub surface with riblets’ tips. For the impeller researched in this article, with the decline of Bmax, the average viscous shear stress on the hub surface decreases gradually at first and then starts increasing and leveling off; accordingly, the polytropic efficiency of the compressor, on the contrary, increases gradually and then starts leveling off later. When Bmax comes to 0.58, the average viscous shear stress of the fluids on the hub surface reaches its minimum, and it is found that the polytropic efficiency of the compressor reaches a peak and stable value. The simulation results are validated against experimental data at the design Mu, and the maximum discrepancies are less than 3.8%.

A Computational Study on the Development of Secondary Flow Through S-shaped Curved Channel

A three-dimensional computational analysis has been carried out to investigation the pulsatile flow through a S-shaped curved channel. The numerical study is performed using Reynolds-averaged Navier-Stokes equation, where a pulsated velocity field is imposed as an inlet condition. Validation for the present study is done using unsteady pulsatile conditions through a simple 90o bend channel. Then the analysis has been extended using S-shaped curved pipe for Reynolds number of 600, velocity component ratio of 1 and different time periods varied from 5s to 25s. The temporal and spatial distribution of variables like axial velocity distribution, secondary flow vectors, wall shear stress and strength of secondary flow has been analysed. The secondary flow streamline patterns at different planes for different time periods have distinguished the effect of time period on the secondary flow. Two counter rotating vortices, in addition to the former vortex pattern have been found in the second bend for different time periods of pulsatile cycle. Fluctuation of WSS has been observed for different moments at different time periods and the position of maximum fluctuation has also been identified. Time average wall shear stress has been found to decrease with time periods from 5 to 25.

Atomistic simulations of interfacial mechanical characteristics of carbon nanotube/silicon nanocomposites

The interfacial mechanical properties of carbon nanotube (CNT)-reinforced silicon nanocomposites are investigated by using molecular dynamics simulation method. The hybrid potential that includes Tersoff_2 potential for Si–Si in the silicon matrix, AIREBO potential for C–C in the CNTs and the Lennard-Jones (LJ) potential for Si–C in the interface is used in the nanocomposite system. The effects of such parameters as the CNT chirality, the CNT diameter and the CNT embedded length, the defects (Vacancy defects, Stone–Wales defects), the size of model, the temperature, the bonding strength and the cut-off distance of the interfacial LJ potential of nanocomposites on the pull force and the average interfacial shear strength (ISS) are investigated and discussed. The results show that the toughness and the maximum tensile strength have been increased significantly by adding the CNTs into the Si matrix. Also by increasing the LJ bonding strength and the cut-off distance of the LJ potential, the pull force and the ISS are increased significantly. The CNT chirality, the CNT diameter and the CNT embedded length have a great influence on the pull force and the ISS, while the effects of temperature, the defects and the size of model are very slight.

Development and feasibility study of a two-dimensional ultrasonic-measurement-integrated blood flow analysis system for hemodynamics in carotid arteries.

Prevention and early detection of atherosclerosis are critical for protection against subsequent circulatory disease. In this study, an automated two-dimensional ultrasonic-measurement-integrated (2D-UMI) blood flow analysis system for clinical diagnosis was developed, and the feasibility of the system for hemodynamic analysis in a carotid artery was revealed. The system automatically generated a 2D computational domain based on ultrasound color Doppler imaging and performed a UMI simulation of blood flow field to visualize hemodynamics in the domain. In the UMI simulation, compensation of errors was applied by adding feedback signals proportional to the differences between Doppler velocities by measurement and computation while automatically estimating the cross-sectional average inflow velocity. The necessity of adjustment of the feedback gain was examined by analyzing blood flow in five carotid arteries: three healthy, one sclerosed, and one stenosed. The same feedback gain was generally applicable for the 2D-UMI simulation in all carotid arteries, depending on target variables. Thus, the present system was shown to be versatile in the sense that the parameter is patient independent. Moreover, the possibility of a new diagnostic method based on the hemodynamic information obtained by the 2D-UMI simulation, such as a waveform of the cross-sectional average inflow velocity and wall shear stress distributions, was suggested.

Study of the Effect of Spacer Orientation and Shape in Membrane Feed Channel using CFD Modelling

Computational fluids dynamics (CFD) modelling has been carried out for a spacer-filled membrane channel using ANSYS FLUENT 14.0. The effect of spacer angles relative to the feed flow direction and different spacer shape combinations on velocity magnitude, wall shear stress, pressure drop and power number were investigated. From the results, spacer angle of 63.565° is the best orientation as it can generate the highest shear stress and reasonably lower power number compared to other spacer angles. Although the combinations of spacer shape did not significantly improve the average wall shear stress, it helped in reducing the pressure drop of the channel. The combination of triangular and circular spacers provided lower Power number, and hence lower energy consumption was required compared to pure triangular spacer. The current results indicated that a combination of triangular and circular spacers can be employed to offer better saving in energy consumption.

Reach-Scale Comparison of Habitat and Mollusk Assemblages for Select Sites in the Clinch River with Regional Context

Several hypotheses, including habitat degradation and variation in fluvial geomorphology, have been posed to explain extreme spatial and temporal variation in Clinch River mollusk assemblages. We examined associations between mollusk assemblage metrics (richness, abundance, recruitment) and physical habitat (geomorphology, streambed composition, fish habitat, and riparian condition) at 10 sites selected to represent the range of current assemblage condition in the Clinch River. We compared similar geomorphological units among reaches, employing semi-quantitative and quantitative protocols to characterize mollusk assemblages and a mix of visual assessments and empirical measurements to characterize physical habitat. We found little to no evidence that current assemblage condition was associated with 54 analyzed habitat metrics. When compared to other sites in the Upper Tennessee River Basin (UTRB) that once supported or currently support mollusk assemblages, Clinch River sites were more similar to each other, representing a narrower range of conditions than observed across the larger geographic extent of the UTRB. A post-hoc analysis suggested stream size and average boundary shear stress at bankfull stage may have historically limited species richness in the UTRB (p < 0.001). Associations between mollusk assemblages and physical habitat in the UTRB and Clinch River currently appear obscured by other factors limiting richness, abundance, and recruitment.

An Alternative Method to Evaluate Endurance Limit in Plain Steel Specimens (JIS, S15C, S45C and A286)

Crack size measurement is necessary in order to calculate stress intensity factor (SIF), K. Definition of the initial crack size in plain specimens is more complex than notched specimens because artificial defects are not introduced. In this study, the effect of difference between initial crack size calculated with both average grain size and maximum shear mode crack size on K, and fatigue limit value, σw were examined. A new alternative method to predict endurance limit of plain specimen, σw with a combination of threshold stress in shear mode and in Mode I was proposed.

Performance Analysis of Coastal Wind Farms in Jiangsu, China

The operational performances of coastal wind farms, located at the east coast of Jiangsu Province of China, are reported in this paper. The result shows that although a rich wind resources predicted in this area, a strong variation of wind resources characteristics are observed between wind farms. Annual averaged wind speeds of five wind farms at hub height range from 4.8 to 6.6 m/s. In addition, the wind shear coefficient shows largely different although they appear to agree with coastal region characteristics. Ru Dong not only has the highest wind shear coefficient but also a largest range of wind shear coefficient variation amplitude. The Capacity Factors of the selected wind farms range from 16% to 27%. Ru Dong shows a lowest Capacity Factor of 16% which may due to the low average wind speed and high wind shear coefficient. On the contrary, Da Feng has a lower Capacity Factor with sensible wind resources indicating either an improved wind turbine technology or an optimized wind farm operational maintenance is needed to increase wind farm efficiency. The result obtained in this paper provides valuable information for future offshore wind farm development in China.

Preliminary Computational Hemodynamics Study of Double Aortic Aneurysms under Multistage Surgical Procedures: An Idealised Model Study

Double aortic aneurysm (DAA) falls under the category of multiple aortic aneurysms. Repair is generally done through staged surgery due to low invasiveness. In this approach, one aneurysm is cured per operation. Therefore, two operations are required for DAA. However, post-first-surgery rupture cases have been reported. Although the problems involved with managing staged surgery have been discussed for more than 30 years, investigation from a hemodynamic perspective has not been attempted. Hence, this is the first computational fluid dynamics approach to the DAA problem. Three idealized geometries were prepared: presurgery, thoracic aortic aneurysm (TAA) cured, and abdominal aortic aneurysm (AAA) cured. By applying identical boundary conditions for flow rate and pressure, the Navier-Stokes equation and continuity equations were solved under the Newtonian fluid assumption. Average pressure in TAA was increased by AAA repair. On the other hand, average pressure in AAA was decreased after TAA repair. Average wall shear stress was decreased at the peak in post-first-surgery models. However, the wave profile of TAA average wall shear stress was changed in the late systole phase after AAA repair. Since the average wall shear stress in the post-first-surgery models decreased and pressure at TAA after AAA repair increased, the TAA might be treated first to prevent rupture.

Flood Hydraulics and Impacts on Invasive Vegetation in a Braided River Floodplain, New Zealand

This study evaluates the effects of floods and their hydraulic characteristics on invasive vegetation in the Ahuriri River floodplain, a braided gravel-bed river in the eastern high country of the South Island, New Zealand. Invasive vegetation alters the natural braiding tendency of rivers such as the Ahuriri and has severe negative impacts on threatened and endangered native wading birds and floodplain ecology. Aerial photographs for a 10-year period (1991 to 2000) that includes the largest flood on record in 1994 (recurrence interval of almost 50 years) based on a 46-year record were analysed to determine changes in aerial cover of water, bare substrate, and several different vegetation classes. This was complemented by hydraulic modelling using HEC-RAS software for a representative 1.1-km reach to evaluate flood hydraulic characteristics that can impact vegetation, including water inundation area, depth, velocity, and shear stress. The 1994 flood (570 m 3 /s) removed up to 25% of the invasive vegetation in the study reach, with preferential removal of lupin and less removal of willow and grassland. However, flood effects varied considerably with overall minor effects on total vegetation cover over the entire lower 21-km reach. The spatial distribution of vegetation cover changed considerably in response to this flood event, in a pattern resembling a shifting-mosaic steady state model. Modelling showed that a 600 m 3 /s (50-year) flood is likely to result in almost total inundation of the study reach floodplain and significant velocities and shear stresses on floodplain vegetation. Over the study reach linear models approximate the relationships between flood peak magnitude with average water area, depth, velocity, and shear stress. Depth and velocity do not increase as fast as flow area or shear stress, and differences between hydraulic characteristics for the 600 and 155 m 3 /s floods ranged from 37% for velocities to 179% for flow inundation areas. Much of the vegetation appeared to recover relatively quickly after flood events so that vegetation removal effects are short-lived. Although large floods can remove a considerable amount of invasive vegetation in some reaches of the river, natural flood events are unlikely to significantly reduce invasive vegetation cover throughout the river.

Non-linear partial least squares and its application to minimising scale formation at a steel hot mill

Scale is highly detrimental to surface quality for tinplate products. As there are a large number of process variables at a typical hot mill, non-linear partial least squares was used to study the influence of these process parameters on tertiary scale formation. It was found that a logit model containing just two partial least squares components provided an adequate best fit to the process data with the most significant variables being the average temperatures at which the steel strip was entering the finishing and rolling mills, the average crop shear temperature and the percentage of phosphorus present within the strip.