Direction Of Shear Stress Research Articles

Piezo-driven clamp release for synchronisation and timing of combined direct-shear stress waves

We propose a novel double clamp Tension-Torsion Hopkinson bar (TTHB) technique for measuring material responses under combined direct-shear loading. The proposed method overcomes the limitations of the existing high rate loading techniques by allowing arbitrary loading paths of tension and torsion to be prescribed to the specimen.High speed piezo actuators were employed to enable the synchronisation of direct stress and shear stress waves. The system also allows flexibility of control on the arrival times of torsional and tensile waves, thus enabling the generation of different dynamic loading paths. Direct and shear stress pulses can be generated such a way to achieve synchronised loading, torsion loading followed by tensile loading, and vice versa.The stress pulse histories of three different loading paths are presented to illustrate and validate the technique.

Ultrasonic vibration induced the improvement of mechanical properties and fracture behavior of Inconel 690 alloy joint during laser welding

The ultrasonic vibration (UV) assisted laser welding was employed to weld Inconel 690 alloy. The microstructure evolution, the tensile strength, the elongation and the fracture behavior of the weld joint were studied through a scanning electron microscopy, an electron back-scatter diffraction and a universal testing machine. The results indicated that there produced an equiaxed crystal region in the center of the weld joint where a great deal of the low angle grain boundary distributed due to the cavitation effect caused by adding the UV. In addition, the proportion of the equiaxed crystal tended to increase, the grain size seemed to be finer. Moreover, the tensile strength and the elongation of the weld joint with UV were significantly improved with an increase in the UV because of the grain refinement. The maximum tensile strength increased to be the same as that of the matrix, and the maximum elongation was about 70% of that of the matrix. The joint without UV fractured with necking, nevertheless, the fracture of joint with UV occurred in the dendrite region of the weld joint and happened in the direction of 45° to the tensile direction due to the appearance of the similar direction between slip surface and the direction of the maximum shear stress, and the change in Taylor factor and Schmid factor distribution of the dendrite in the weld joint. Besides, the ductile fracture was still the main fracture feature because of the formation of the dimples and micro-voids.

Exploration of 3D stagnation-point flow induced by nanofluid through a horizontal plane surface saturated in a porous medium with generalized slip effects

Heat transfer enhancement is a contemporary challenge in a variety of fields such as electronics, heat exchangers, bio and chemical reactors, etc. Nanofluids, as innovative heat transfer fluids, have the potential to be an efficient tool for increasing energy transport. This benefit is obtained as a result of an enhancement in effective thermal conductivity and an alteration in the dynamics of fluid flow. Thus, this paper is concerned with heat transfer enhancement via nanofluids. The intention is to find numerical double solutions to the 3D stagnation-point flow (SPF) and heat transfer incorporated nanoparticles in a porous medium with generalized slip impacts. Appropriate similarity variables are used to non-dimensionalize the leading equations, which are then numerically solved using the three-stage Lobatto IIIa integration formula. The impacts of different parameters on the dynamics of flow and characteristics of heat transfer induced by nanofluids in the presence of an unsteady parameter, nanoparticle volume fraction, velocity slip parameter, and porosity parameter are investigated. Based on the latest findings, it is closed that the velocity slip improves the heat transfer as well as shear stress in the axial direction in both solutions, while the shear stress behaves oppositely in the respective lateral direction. In addition, the velocity profile remarkably enriches for both branch solutions, while the temperature distribution elevates for the upper branch and declines for the lower branch owing to the larger porosity parameter.

Response mechanism of matrix microstructure evolution and mechanical behavior to Mg/Al composite plate by hard-plate accumulative roll bonding

In the traditional accumulative roll bonding (ARB) process of heterogeneous composite laminates, due to the excessive strain caused by accumulative roll bonding, many problems such as reduction of plasticity and cracks on the edge of composite plates cannot be ignored while increasing the strength. In this paper, a method of developing a Mg/Al composite sheet by hard-plate accumulative roll bonding (ARB (HPR)) is proposed. The addition of hard plate can change the stress state of the composite laminate during the rolling process, The shear stress in the rolling direction is partially converted into the compressive stress in the normal direction, which then increases the welding pressure between the heterogeneous composite laminates, the interface is closely bonded, and the warpage and edge crack caused by severe plastic deformation is significantly reduced. The results show that this method can significantly improve the tensile strength and plasticity of the composite laminate. The yield strength is 226 MPa, and the elongation is 21.5%, which is substantially higher than that of the traditional accumulative roll bonding. Many dynamically recrystallised grains were formed, the grains were uniform and delicate, and the grain boundaries were clear. To sum up, the Al/Mg composite sheet rolled by hard-plate accumulative roll bonding has improved in many aspects, such as mechanical properties and sheet formability, which provides a new idea for the research on the forming and manufacturing of heterogeneous composite laminates.

Effects of wall disturbances on the statistics of supersonic turbulent boundary layers

In the present study, we perform direct numerical simulations to investigate the spatial development and basic flow statistics in the supersonic turbulent boundary layers at the free-stream Mach number of 2.0 over smooth and disturbed walls, the latter of which enforces extra Reynolds shear stress in the streamwise direction to emulate the drag increment and mean streamline curvature effects of rough walls. Such disturbances escalate the growth rate of turbulent boundary layer thickness and the shape factor. It is found that under the rescaled global coordinate, the mean velocity, Reynolds stress, and pressure fluctuation variance manifest outer-layer similarity, whereas the average and fluctuation variances of temperature and density do not share such a property. Compressibility effects are enhanced by the wall disturbances, yet not sufficiently strong to directly impact the turbulent kinetic energy transport under the presently considered flow parameters. The generalized Reynolds analogy that relates the mean velocity and temperature can be satisfied by incorporating the refinement in modifying the generalized recovery coefficient, and that associates the fluctuating velocity and temperature work reasonably well, indicating the passive transport of temperature fluctuations. The dispersive motions are dominant and decay exponentially below the equivalent sand grain roughness height ks, above which the wall disturbances are distorted to form unsteady motions responsible for the intensified density and pressure fluctuations in the free-stream traveling isentropically as the acoustic radiations.

Adiabatic shear localization induced by dynamic recrystallization in an FCC high entropy alloy

Due to their strong strain hardening, face-centered cubic (FCC) high/medium entropy alloys (H/MEAs) have remarkable resistance to shear localization even under forced dynamic shear. In this work we investigated the dynamic mechanical behavior of an FCC Al0.1CoCrFeNi HEA via a series of split Hopkinson pressure bar (SHPB) experiments at 77 K and room temperature. This HEA exhibited very strong strain hardening capacity, remarkable strain rate sensitivity and moderate temperature dependence. Such a combination of mechanical properties should be strongly unfavorable for adiabatic shear bands (ASBs). However, its dynamic compressive failure at 77 K was found to be caused by crack propagation within the primary ASBs along the directions of maximum shear stress. Detailed examination of the specimens using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) revealed convincing microstructural evidences for massive dynamic recrystallization (DRX) prior to ASB. We suggested that the expansion and coalescence of these DRX softened regions served as the root cause for the occurrence of ASBs and the final catastrophic fracture of specimens. The severe local shear flow, substantial adiabatic temperature rise and extremely fast quenching within the ASB resulted in ultrafine grains therein. Our results and analyses provided a plausible understanding of the formation mechanisms of ASBs in the FCC HEA under impact loading.

Coupled microplane and micromechanics model for describing the damage and plasticity evolution of quasi-brittle material

Quasi-brittle materials exhibit significant heterogeneity due to their complex mesoscopic components and microdefects, and their macroscopic mechanical behavior depends strongly on crack initiation, propagation and friction-slip. In this study, by coupling microplane and micromechanics, a new constitutive model that can consider the mesoscopic damage and plasticity mechanisms of quasi-brittle materials is proposed. The microplane is used throughout the whole construction process for the model: first, a representative volume element (RVE) is established to characterize the mesoscopic structure of quasi-brittle materials, and the advantages of microplanes are fully exploited to characterize the equivalent cracks‒matrix system. Then, the relationship between macroscopic and microplane strains (stresses) is elucidated. Second, considering the unilateral effect for cracks, the expressions for the free energy potential functions for a microplane and for the RVE are established. Third, for each microplane, the damage criterion characterized by the damage driving force is constructed for the open and closed cracks, and in particular, a new plasticity criterion characterizing the static friction property coupled damage and dilatancy behavior during strain softening deformation is constructed. By considering the characteristics of the microplane to relate macroscopic and mesoscopic stress and strain, an efficient return mapping algorithm based on the Newton‒Raphson method is derived. Finally, the stress‒strain curves and the evolution of damage and plasticity are simulated under conventional triaxial compression, uniaxial tensile, direct shear and true triaxial compression stress paths, and the effectiveness of the model is verified by comparison to the experimental data. To better reflect the inherent correlation between mesoscopic damage and macroscopic mechanical behavior, a microplane configuration that can visually characterize the mesoscopic evolution characteristics of damage is established, and it is shown that the simulated results correspond well to the macroscopic deformation and failure pattern of granite samples under different loading paths.

Lithospheric Stress Due to Mantle Convection and Mantle Plume over East Africa from GOCE and Seismic Data

The Afar and Ethiopian plateaus are in a dynamic uplift due to the mantle plume, therefore, considering the plume effect is necessary for any geophysical investigation including the estimation of lithospheric stress in this area. The Earth gravity models of the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) and lithospheric structure models can be applied to estimate the stress tensor inside the Ethiopian lithosphere. To do so, the boundary-value problem of elasticity is solved to derive a general solution for the displacement field in a thin elastic spherical shell representing the lithosphere. After that, general solutions for the elements of the strain tensor are derived from the displacement field, and finally the stress tensor from the strain tensor. The horizontal shear stresses due to mantle convection and the vertical stress due to the mantle plume are taken as the lower boundary value at the base of the lithosphere, and no stress at the upper boundary value of the lithospheric shell. The stress tensor and maximum stress directions are computed at the Moho boundary in three scenarios: considering horizontal shear stresses due to mantle convection, vertical stresses due to mantle plume, and their combination. The estimated maximum horizontal shear stresses’ locations are consistent with tectonics and seismic activities in the study area. In addition, the maximum shear stress directions are highly correlated with the World Stress Map 2016, especially when the effect of the mantle plume is solely considered, indicating the stress in the study area mainly comes from the plume.

Discontinuous yielding in bare and embedded reinforcing bars: Implications on the determination of steel and bond shear stresses from strain measurements

Distinct jumps from the yield strain to the strain at the onset of hardening occur in bare reinforcing bar exhibiting a yield plateau once the yield limit is reached. These strain discountinuities known as Lüders bands nucleate at single locations and travel along the bar axis at a constant load. In contrast, no such strain jumps were observed in recent experiments with such reinforcing bars embedded in concrete and instrumented using distributed fibre optical strain sensing (DFOS). Instead, the strain increased gradually at locations where the yield limit was exceeded. This observation, made possible by the high sensing resolution of DFOS, cannot be explained entirely by the known effect of tension stiffening. While bond stresses cause a variation of steel stresses along the bar axis and, hence, indeed prevent Lüders bands from propagating along embedded bars, distinct strain jumps would still be expected at the locations where the yield stress is reached. The different steel stress–strain behaviour is rather attributed to the three-dimensional stress state induced by the locally concentrated bond forces at the bar ribs. These forces presumably (i) cause the direction of maximum shear stress to vary over the bar cross-section, leading to a loss of the macroscopic yield phenomena, and (ii) reduce the nominal yield stress in axial tension. As a consequence of the altered yield behaviour, a simple conversion from strains to stresses using a steel stress-strain relationship derived from uniaxial tensile tests leads to incorrect results for the strain range slightly below the yield limit up to the onset of hardening. These findings are most relevant for experimental studies investigating bond with DFOS, since researchers rely on the validity of the steel material law to derive and validate bond stress-slip relationships.This paper presents the results of the mentioned experiments, discusses the observed differences between bare and embedded reinforcing bars along with potential explanations, and outlines the implications on the determination of steel and bond shear stresses based on strain measurements in reinforcing bars embedded in concrete.

Microstructural evolution of Al-Zn-Mg-Cu alloy during ultrasonic surface rolling process

This paper systematically investigates the microstructural evolution of Al-Zn-Mg-Cu alloy during ultrasonic surface rolling process (USRP). Al-Zn-Mg-Cu alloys with different gradient deformation degrees were prepared by USRP conducted with 1, 2, 6, 8, 10 and, 12 processing cycles, respectively, and then the deformation structures from the surface of samples with different deformation degrees were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. The results indicated that with the increase of processing cycle, the microstructural evolution at different depths of surface deformation layer is basically the same. But the closer to the surface, the faster microstructure evolves. The basic microstructural evolution order of Al-Zn-Mg-Cu alloy during USRP is scattered dislocation, dislocation cells (DCs), elongated dense dislocation cell walls (DDWs), thick laminated structure and nanolaminated structure. According to results of crystallography and atomic level analysis, the microstructural evolution was described in detail. Scattered dislocation structure is formed by dislocation proliferation. According to the low energy dislocation structures (LEDS) principle, the scattered dislocation structure is transformed into DCs structure through dislocation movement. Under the action of large shear stress, DCs structure is elongated along the direction of shear stress to form elongated DDWs structure. The transformation of elongated DDWs into thick laminated structure and the continuous refinement of laminated structure are mainly achieved by the formation of the low angle grain boundaries (LAGBs) and the transformation from LAGBs to high angle grain boundaries (HAGBs). In some nanolaminated structure regions, the laminated structure is further refined by twin deformation. Our work provides a theoretical basis for the adjustment of the actual processing technology.

Crack initiation induced by twin-martensite and inclusion in rotatory bending fatigue of a high nitrogen martensite bearing steel

Rotating bending fatigue tests, together with microstructural characterization, have been performed on the high nitrogen martensite bearing steel Cronidur30. The results show that the fatigue stress amplitude gradually decreases with the increasing number of fatigue cycles, with a fatigue strength limit of 867 MPa after 1.0 × 107fatigue cycles. Crack sources due to both inclusions and twin-martensite volumes are found from inspection of fracture surfaces, with nearly 40% of the fatigue failures identified as originating from twin-martensite volumes. Based on a detailed microstructural examination, it is concluded that twin-martensite volumes first fracture locally along the maximum shear stress direction, and these locations then act as the sources for crack propagation and finally resulting in fatigue failure. The stress intensity factors calculation of two types of crack sources initiated by the subsurface shows that the two types of crack sources play basically the same role in fatigue crack initiation. Granular bright facet (GBF) will be formed around the crack source to meet the threshold value of crack propagation. When the initial size of these two crack sources is large enough, the crack can directly propagate without forming GBF. The novel observation of twin-martensite induced crack nucleation highlights the fact that fatigue failures of the investigated bearing steel do not only originate from inclusions, and help to provide an in-depth understanding on the fatigue mechanisms of bearing steels.

Curvature-induced secondary flow in 2D depth-averaged hydro-morphodynamic models: An assessment of different approaches and key factors

Curvature-induced secondary flows are ubiquitous in nature and have long attracted scientist attention. Modelling such kind of secondary flows is not straightforward. While full 3D models fit the purpose at the cost of great computational demand, simplified models often pose concerns about their effectiveness and the representation of key processes. In the present study, helical flow secondary currents are included in a two-dimensional depth-averaged hydro-morphodynamic model on cartesian unstructured meshes. The non-uniform vertical distribution of velocity in streamwise and spanwise directions is accounted for introducing dispersive terms in the shallow water equations, an anisotropic diffusivity tensor in the advection-diffusion equation, and a correction to the direction of bed shear stress and bedload transport. Different approaches available in the literature are recast in similar form and compared to each other in terms of flow field, tracer transport, and bed evolution, using data from laboratory experiments and real-world case studies. The model includes a novel, pure 2D implementation of the non-linear saturation mechanism that limits the growth of secondary flows in relatively sharp bends. A substantial part of the paper is devoted to discuss key factors in secondary flow modelling, including implementation tricks, guidelines to mesh design, the suitability of local and non-local approaches, and the role of bathymetry. The final goal is to provide useful guidelines for 2D hydro- and morphodynamic modelling in river bends.

Study on mesoscopic failure mechanism of grout-infilled sandstone under uniaxial compression using an improved AE localization technique

In this paper, to study the influence of grouting on meso-failure mechanism, acoustic emission (AE) localization technique was used to monitor the micro-cracking during the uniaxial compression tests on single flawed sandstone specimens with or without grouting. Due to the inhomogeneous geometrical structure of the flawed specimens, the mesoscopic damage evolution is also inhomogeneous during the loading process. As a result, the wave velocity of the specimens varied temporally and spatially, which greatly increased the difficulty in accurate AE localization. To solve this problem, the wave velocities versus the axial load for the studied sandstone were first measured, and the stress concentration effect near the flaw was also theoretically analyzed. Accordingly, the localization arithmetic was also improved to match the complex wave velocity field. Results show that compared to the specimens without grouting, more microcracks (particularly the shear microcracks) are induced in the grout-infilled specimens prior to the final failure of the specimens. Moreover, the effect of grouting on microcrack initiation also varies with the oblique angle of the flaw. For θ (flaw inclination) ≤30°, grouting significantly increases the shear microcrack at macrocrack propagation stage which states that the axial tensile cracking was limited by the supporting effect of the grout material and made the failure mode trend to shear mode. For θ = 45° and 60°, grouting improved the ratio of shear microcrack during the whole failure process, indicating that the direction of maximum shear stress is close to the flaw direction. Hence, failure develops in stable shear mode. For θ = 75° and 90°, the primary microcrack generated during the first macrocrack initiation changed to shear mode, but the grouting has little influence on the subsequent crack propagation. So that, the reinforcement effect is poor in this set of specimens.

Effect of imposed shear on the stability of a viscoelastic liquid flowing down a slippery plane

We investigate a linear stability analysis for the Oldroyd-B liquid draining down a slippery inclined plane where a constant shear stress is applied at the liquid surface in the streamwise direction. The aim is to expand the previous study (Shaqfeh et al., 1989) in the presence of imposed shear stress when the bounding plane is slippery. The stability analysis is executed analytically as well as numerically for low to high values of the Reynolds number, which detects the existence of surface and shear modes. In the low Reynolds number regime, we use the analytical long-wave asymptotic expansion to find the onset of primary instability for the surface mode. On the other hand, we implement the numerical Chebyshev spectral collocation technique to determine the short-wave instability when the Reynolds number changes from a moderate to a high value. It is found that the critical Reynolds number for the surface mode decreases with the increasing values of the slip length and imposed shear stress, and thereby, the surface instability occurs at a lower Reynolds number than that when the slip length and the imposed shear stress are excluded. More specifically, the imposed shear stress has a destabilizing effect on the surface mode when acting in the co-flow direction but a stabilizing effect when acting in the counter-flow direction. Furthermore, the viscosity ratio exhibits a stabilizing effect on the surface mode near the onset of instability but exhibits a destabilizing effect far away from the onset of instability. In the high Reynolds number regime, the primary instability induced by the shear mode can be stabilized by the viscosity ratio and slip length, but can be destabilized by applying a constant shear stress in the co-flow direction. In a special case, the result for the surface mode corresponding to Walters’ liquid B ′ ′ can be recovered from that of the Oldroyd-B liquid if the viscoelastic parameter is kept at a very low value. • Analytical and numerical studies of shear-imposed Oldroyd-B liquid over a slippery plane. • The surface mode is destabilized but the shear mode is stabilized by the slip length. • Both the surface and shear modes are destabilized if a constant shear stress is applied in the coflow direction. • Both the surface and shear modes are stabilized if a constant shear stress is applied in the counter-flow direction. • Fully elastic instability exists and can be suppressed by the imposed shear stress if it acts in the counter-flow direction. • Fully elastic instability exists and can be intensified by wall slip. • The result of the long-wave surface mode for Walters’ liquid B ′ ′ is recovered from that of the Oldroyd-B liquid if the viscoelastic parameter is very small.

Study of Shear Transfer in Modified Push-Off Members Using Finite Elements Method

This paper aims to investigate numerical study onto the behavior of modified push-off specimens under the action of direct shear stress. Based on the tow-dimensional finite element model developed in this research, the contribution of the aggregate interlock to resist the shear stress along the shear plane, the effect of existing of the compressive stress acting across the shear plane, the effect of the parallel reinforcement in resisting shear stress, the effect of the shear reinforcement parameter, the strains in the concrete and steel and the actual distribution of the shear stress along the shear plane were studied. To verify the accuracy and applicability of the suggested finite element model, a comparison between the results obtained in this study and those obtained experimentally by other authors was carried out. Comparison showed that the finite element results were in good agreement with the experimental results. It has been found that, for modified push-off specimens of groups without shear reinforcement across the shear plane the diagonal tension crack within the shear plane occurred at the load level which is closely to the ultimate shear strength respectively, while for specimens with both shear and parallel reinforcement , the first crack formed at about (33.7% - 53.0%) of the ultimate strength , also the investigation showed that the presenting of the shear reinforcement normal to the shear plane are significantly increased the shear transfer stress for all levels of loading.

Development of a Flexible MEMS Sensor for Subsonic Flow.

Detection and control of flow separation is a key to improving the efficiency of fluid machinery. In this study, we developed a flexible MEMS (microelectromechanical systems) sensor for measuring the wall shear stress and flow angle in subsonic airflow. The developed sensor is made of a flexible polyimide film and a microheater surrounded by three temperature sensor pairs. The sensor measures the wall shear stress from the heater output and the flow angle from the temperature gradient around the heater. The geometry and design of the heater and temperature sensors were determined based on numerical simulations. To evaluate the validity of the sensor, we conducted an experiment to measure the wall shear stress and the flow angle in a wind tunnel in different velocities ranging from 30 m/s to 170 m/s, equivalent to Mach numbers from 0.1 to 0.5. The heater output was proportional to one-third power of the wall shear stress. Additionally, the bridge output correlating the temperature difference between two opposing temperature sensors showed sinusoidal variation depending on the flow angle. Consequently, we have clarified that the developed sensor can measure both the wall shear stress and flow direction in subsonic flow.

Integrating particle tracking with computational fluid dynamics to assess haemodynamic perturbation by coronary artery stents.

AimsCoronary artery stents have profound effects on arterial function by altering fluid flow mass transport and wall shear stress. We developed a new integrated methodology to analyse the effects of stents on mass transport and shear stress to inform the design of haemodynamically-favourable stents.Methods and resultsStents were deployed in model vessels followed by tracking of fluorescent particles under flow. Parallel analyses involved high-resolution micro-computed tomography scanning followed by computational fluid dynamics simulations to assess wall shear stress distribution. Several stent designs were analysed to assess whether the workflow was robust for diverse strut geometries. Stents had striking effects on fluid flow streamlines, flow separation or funnelling, and the accumulation of particles at areas of complex geometry that were tightly coupled to stent shape. CFD analysis revealed that stents had a major influence on wall shear stress magnitude, direction and distribution and this was highly sensitive to geometry.ConclusionsIntegration of particle tracking with CFD allows assessment of fluid flow and shear stress in stented arteries in unprecedented detail. Deleterious flow perturbations, such as accumulation of particles at struts and non-physiological shear stress, were highly sensitive to individual stent geometry. Novel designs for stents should be tested for mass transport and shear stress which are important effectors of vascular health and repair.

A consistent calibration process for the Matsuoka–Nakai friction angle under direct simple shear conditions for clay hypoplasticity

In geotechnical engineering, direct simple shear (DSS) tests are used to determine strength and stiffness parameters of a material. DSS predictions of a constitutive model influence failure mechanisms in plane-strain, finite-element applications. In this article, we introduce a closed-form solution to determine the Matsuoka–Nakai equivalent critical friction angle from direct simple shear stress and normal stress data. In order to apply it to clay hypoplasticity (Mašín, 2013), we carry out DSS simulations to investigate rotations of principal stresses and the principal stress state at critical state for plane-strain conditions.Finally, we interpret DSS predictions of clay hypoplasticity for different overconsolidation ratios and demonstrate that the location of the CSL in the vertical stress–void ratio plane coincides with the location of the CSL in mean effective stress–void ratio plane.

The interactions of a circular synthetic jet with a turbulent crossflow

The three-dimensional flow of a circular synthetic jet interacting with a turbulent crossflow is investigated with unsteady Reynold-Averaged Navier–Stokes simulations. The effects of jet momentum are examined using three blowing ratios (CB=0.32, 0.67, and 1.10) at constant actuation frequency (f=300 Hz), approach crossflow Reynolds number, Reθ=900, and boundary layer thickness, δ/d=7.25, where d is the jet diameter. The results showed that the expelled jet is accompanied by a reverse flow region on the downstream side which undergoes alternating expansion and contraction during the jet cycle. The size of the reverse flow region and depth of penetration increased with increasing jet momentum. For the low momentum jet, the expelled flow structure evolved into a hairpin vortex which significantly enhanced the wall shear stress in the spanwise direction and near the jet exit. The higher momentum jets, on the other hand, exhibited strong vortex loops around the expelled jet column that transitioned into trailing vortex pairs and a tilted vortex ring further downstream. Along the wall, both horseshoe vortex upstream of the jet exit and tertiary vortices downstream were found attached to the wall. The strong tertiary vortices promoted downwash of fluids which significantly enhanced the wall shear stress along the symmetry plane for the medium and high momentum jets. The impact of the tertiary vortices increased with the jet momentum, offering greater potential for flow separation control.

Behaviour of Horseshoe-Shaped Tunnel Subjected to Different In Situ Stress Fields

At great depths, tunnel openings experience a tectonic stress field rather than overburden stress. This paper aims to examine the impact of different in situ stress ratios and multiple tunnel depths below the surface on the excavation induced-stresses and displacements around tunnel openings. Thus, a series of models has been built, using a two-dimensional elasto-plastic finite-elements code, RS2D, to conduct parametric stability analysis. The performance of tunnel opening is examined by evaluating the induced stress-deformation around the opening. The results indicate that ratio of wall convergence, roof sag and floor heave increase as in situ stress ratio and tunnel depth below surface increase. Additionally, the induced-stresses increase as depth and state of in situ stress increase. In addition, the extent of yielding zones into rock mass around tunnel roof and floor deteriorates as tunnel depth and in situ stress ratio increase. Moreover, the normal stress along rock joints is sharply dropped when joints pass in the vicinity of tunnel opening (e.g., centre of opening). As well, the direction of shear stress along joints is reversed. Consequently, inward shear displacement of rock, on the underside of the weakness plane, is produced as a result of slip occurrence.