Local Shears Research Articles

Atomistic simulations of nano-fiber-confined metal plasticity

Atomistic simulations are used to explore dislocation mechanisms in nano-confined aluminium (Al) containing silicon (Si) nanofibers. Our simulations revealed that dislocations on four {111} slip planes nucleate at Al-Si interfaces, propagate in Al nano-channels, loop and bow around Si nanofibers. Due to multiple active slip systems and dislocation cross-slips in the confined volume, a high density of dislocations intersects to form dislocation junctions and promote the formation of dislocation forests, resulting in high flow strength and strain hardening. The plasticity of Si nanofibers is accommodated by local shears and fracture triggered by the accumulated dislocation loops. Simulations show that the cracking of Si nanofibers can be efficiently buffered by dislocation mobility in the surrounding Al. Our results are consistent with experiments on nano-fibrous Al-Si eutectic microstructure.

Blob tracking and formation in edge and SOL plasmas using Q-factor

We propose a direct means of identifying and tracking plasma blobs in the tokamak edge and scrape-off layer (SOL) regions through evaluation of a quantity Q that is a measure of the relative contribution from the square of the sheared strain and the square of the vorticity at any spatial location. The sheared strain and vorticity are related to the various velocity gradients in the plasma fluid. The blob formation from the breakup of a streamer structure can be determined from the criterion Q > 0. A blob is associated with a high negative value of Q that is indicative of a stable vortex flow of the local fluid trajectories. The validity of this criterion, obtained from a linear eigenvalue analysis of the stability of fluid trajectories, is supported by nonlinear fluid simulations that clearly identify blobs with regions where Q < 0 and exceeds a threshold value. The parameter Q can be experimentally determined from measurements of local electric field shears using gas puff imaging or Langmuir probe techniques, and the proposed criterion can provide a direct means of tracking plasma blobs. The rotation or spin of a blob in the edge and SOL regions can also be characterized by Q.

Flow and heat over a rotating disk subject to a uniform horizontal magnetic field

Abstract Magnetic field is often applied to stabilize the flow field in real life applications of fluid mechanics problems. In the present work, it is employed a horizontal uniform magnetic field to regularize the flow field triggered due to a rotating disk. The energy equation is also studied subjected to such a horizontal magnetic field. The applied horizontal magnetic field is different from the well-known applied external vertical magnetic field. It is shown that the horizontal magnetic field leads to a similarity system of equations with the help of the traditional von Kármán similarity transformations. The effects of such a magnetic field on the development of velocity and temperature fields are then numerically investigated. The existence of exact series solutions in terms of decaying exponential functions is further discussed. The critical roles of horizontal magnetic field on the physical quantities involving the local wall shears, torque and the heat transfer rate are finally highlighted.

Formation of polytypes structures in Mg single crystals

We report studies of polytypes structures (PSs) formed in the microstructure of deformed Mg single crystals. PSs emerge as co-products of lattice distortion developed in dislocations’ tangles due to reactions and rearrangements of dislocations to the lower energy configurations. TEM observations indicate that entanglements containing an excess content of 〈c〉-component dislocations that maintain dislocation interactions and the evolving pattern in a confined space nucleate PSs. The distortion and rotation of the hcp lattice induce the transition to the triclinic lattice originating PSs. The overlapping strain fields from mutually interlocked dislocations and stacking faults promote local shears and shuffles that develop polytypes domains within the PSs. Ab initio and molecular statics simulations permit understanding the processes governing triclinic and polytypic transformations in Mg and the role of dislocations in PSs’ self-assembly.

Lineament mapping for a part of the Central Sulaiman Fold\u2013Thrust Belt (SFTB), Pakistan

This study aims to analyze the lineaments using the field data and a remote sensing approach, to describe their relationship with the folds, faults, and regional tectonic stress of the central Sulaiman Fold-Thrust Belt. Joint data, from nine anticlines, has been collected using the scanline method and classified into three sets (JS1, JS2, and JS3) based on their geometrical relationships. Lineaments extracted from the 30 m digital elevation model have been classified subsequently into three lineament sets (LS1, LS2, and LS3) based on the azimuths from the corresponding joint sets. A very high correlation coefficient (rs = 0.97), between the azimuths of the field joints and the remotely sensed lineaments, has been observed which validates that the lineaments are the regional representation of the local field joints. The geometrical relationship of the lineament sets with the fold hinges indicates that the older LS1 and LS2 are strongly related to the regional folding episode, while the younger LS3 is a result of local shears. The chronological interpretation of the deformational events responsible for the lineament sets is constrained by the presence of the Kingri Fault, which induces a strike-slip component within the study area. Furthermore, the controls on the joint and lineament sets, established using multivariate statistics to decipher the effects of lithological and structural contrasts on the lineament density, reveal that an increase in the lineament density can be attributed to the competence and thickness of the rock units as well as the variable local stresses across the different folds. Based on the orientations of these lineament sets, the cumulative direction of the compressive event in the NW-SE direction (310–320) coincides with the regional stress direction of the SFTB.

Sonication-driven dispersion of multiwalled carbon nanotubes in water with the aid of Na+-montmorillonite

The poor dispersibility of multiwalled carbon nanotubes (MWNTs) in aqueous solvents is a serious obstacle to give full play to the excellent properties of MWNTs. How to effectively disperse MWNTs has become an urgent problem to be solved. Therefore, the addition of tertiary phase became necessary to promote such dispersion. In the present work, the sonication-driven dispersion of multi-wall carbon nanotubes (MWNTs) in aqueous Na+-montmorillonite (MMT) solution has been investigated with the help of UV-vis spectroscopy, particle agglomeration size and Transmission Electron Microscopy (TEM). The impact of ultrasonic power and time as well as MMT/MWNTs ratio on the dispersion of MWNTs was discussed. It was observed that sonication at 240 W for 40 min at MMT/MWNTs weight ratio of 2 resulted in efficient dispersion and formation of stabilized suspensions of individual nanotubes. The ultrasound provided high local shears to overcome the van der Waals interactions between tubes and lead to MWNTs debundling, which facilitate adsorption and immobilization of MMT on the surface of the MWNTs, preventing re-aggregation of MWNTs.

Effects of roof geometry on tornado-induced structural actions of a low-rise building

Continued loss of property and lives in tornadoes including the 23-person death toll resulting from the Lee County AL tornado on March 3rd 2019 reflects the ever-present vulnerability of the community to tornadoes, and necessitates improvement in building standards for more accurately predicting such extreme loads. This article investigates effects of three roof geometries: flat, gable, and hip, on maximum overall and local wind-induced uplift, shear and moment on low-rise buildings by a translating tornado. To explore local structural actions, maximum of five frames along the longer horizontal dimension of each building were considered. Maximum structural actions, independent of building orientation and its distance to the center of the tornado-track were calculated, with results showing that a flat roof building experiences the largest amount of overall uplift, overall shear, local uplift, and moment. Only local shears on a hip roof building were equal to or larger than those of a flat roof building for same frame locations. This study also showed that gable and hip roof buildings experience moments of similar magnitudes, whereas a gable roof performs better against shear and a hip roof performs better against uplift. Experimentally-calculated structural actions were compared with predictions of ASCE7-16, with results showing that ASCE7-16 under-predicts overall and local uplifts, and moments, while over-predicts shear for all roof geometries. The maximum under-prediction of overall and local uplifts using ASCE7-16′s procedure were 41% and 55%, respectively, both for a gable roof building. Internal frames experienced maximum under-prediction of uplifts, which makes them a vulnerable location of low-rise buildings that are built in compliance with ASCE7-16 building standard. Finally, correction factors were introduced to modify and improve accuracy of ASCE7-16 in prediction of structural actions on low-rise buildings with different roof geometries.

Adaptive particle method based on moments for simulating the mass transport in natural flows

In this work, two modifications of an adaptive particle method based on the moments of the internal concentration of numerical particles are presented to introduce the effect of local shears on the mass transport in natural flows. This moment method uses a Taylor expansion of the flow velocity field to derive the transport equation of moments. The first modification is based on the introduction of a method of the local frame transport in order to perform an extension of the Taylor expansion of the flow velocity to a higher order. The second modification consists in applying a splitting or a merging to non-spherical numerical particles. When a non-spherical numerical particle becomes large compared to the scale of spatial variations of the velocity gradient, it is divided into two non-spherical numerical particles. On the contrary, when particles reach some other parts of the flow where they become small compared to spatial variations of the velocity gradient, then these particles should merge. The accuracy of both modifications is evaluated by simulating the mass transport in natural flows.

Shear-hosted gold mineralization in the Oumé-Fettèkro greenstone belt, Côte d'Ivoire: the Bonikro deposit

Abstract The Bonikro deposit is the first deposit in Côte d'Ivoire where gold has been found within a sheared granodiorite in the zones of boudins and faults. Gold is associated with scheelite and molybdenite in veins. The purpose of this paper is to highlight the characteristics of the shearing and the mineralized veins, and to reveal the timing of their gold deposition in the Bonikro deposit. The investigation in this deposit and the observations of the drill cores have demonstrated the main role played by the shear zone and its three principal mineralized veins: sheeted veins, planar veins and transversal veins. The deposit is dominated by a major structure, the Bonikro Shear Zone (BSZ), which strikes north–south. The BSZ has affected the granitoid and, as a consequence, it has produced two local shears that strike NW and NE. The sheeted veins are the oldest at Bonikro, with a thickness of 1 cm, and sub-parallel sets of quartzo-feldspathic veins composed of milky quartz (70–80%), albite (5–10%) scheelite (up to 15%) and pyrite (up to 5%). They are the main host of the scheelite and also display the more visible gold in the deposit. The planar veins support the powellite and pyrite minerals. The transversal veins occurred later, and are composed of milky quartz (40%), calcite (30%), albite (10%), biotite (15%) and sulfide (molybdenite up to 5%). The molybdenite is the main sulfide. Each vein shows typical minerals and resumes a single generation of setting. These veins are bordered in the granodiorite by a corridor of strong hydrothermal activity, as indicated by sericitization, silicification, chloritization and albitization. Gold is observed in both the granodiorite and the basalt in areas where these lithologies are sheared. It appears that the introduction of the auriferous mineralization at Bonikro post-dates the magmatism, and seems to be syn- to post-tectonic. This direct link between the structural features and the gold deposition has shown that the Bonikro deposit represents a case of shear-controlled gold mineralization.

The nature and age of Mesoproterozoic strike-slip faulting based on Re–Os geochronology of syntectonic copper mineralization, Assynt Terrane, NW Scotland

In ancient basement regions such as the Lewisian Complex, NW Scotland, the ages of brittle deformation events are commonly poorly constrained owing to a lack of datable fills. An array of NW–SE sinistral and antithetic east–west dextral faults related to a regionally recognized episode of brittle shearing cut Neoarchean gneisses and c. 2.25 Ga quartz–pyrite veins close to the trace of an unexposed, regional-scale NW–SE fault. Copper–iron mineralization occurs at an intersection between an antithetic dextral fault and an older c. 2.25 Ga quartz vein. Optical microscopy, SEM and XRD analyses reveal an array of intergrown, co-genetic copper–iron sulfides, hematite and barite. Complex millimetre-thick zoned alteration rims rich in epidote occur at contacts between the sulfides and gneisses. Rhenium–osmium copper–iron sulfide geochronology yields an age of c . 1.55 Ga for the hydrothermal mineralization event associated with faulting. Fault movements demonstrably overlap with mineralization based on the asymmetric fibrous growth forms of these minerals within local dextral shears, which acted as local channelways for mineralizing fluids during and after faulting. We tentatively propose that this regionally recognized strike-slip faulting, previously termed the ‘Late Laxfordian’, should be referred to as the ‘Assyntian’ to distinguish it from kinematically distinct Laxfordian events.

Suspension of dust particles over a stretchable rotating disk and two-phase heat transfer

Studied in the present paper is the phenomenon of interaction of suspended particles with the fluid over a stretchable rotating disk. The physical insights into the resulting flow behavior and heat transfer rate of two phase equilibrium are the main targets. How the stretching contributes to the dusty flow character will be clarified. Governing equations of motion and energy are first transformed into similarity systems representing the fluid and dust stages. In the absence of dust particles, the whole system collapses onto the well-known von Karman rotating disk subject to a wall deformation in the radial direction. Numerical simulations are next performed to resolve the fluid and dust phases and their thermal responses to the interaction. The local wall shears, the wall slippage of the colloids, the far-field behaviors, and the heat transfer rates of both fluid and dust particles are eventually investigated graphically and tabularly in detail with physical insights.

Incompatibility Stresses and Lattice Rotations Due to Grain Boundary Sliding in Heterogeneous Anisotropic Elasticity

Non-uniform grain boundary sliding can induce strain and rotation incompatibilities at perfectly planar interfaces. Explicit analytic expressions of stress and lattice rotation jumps are thus derived at a planar interface in the general framework of heterogeneous anisotropic thermo-elasticity with plasticity and grain boundary sliding. Both elastic fields are directly dependent on in-plane gradients of grain boundary sliding. It is also shown that grain boundary sliding is a mechanism that may relax incompatibility stresses of elastic, plastic and thermal origin although the latter are not resolved on the grain boundary plane. This relaxation may be a driving force for grain boundary sliding in addition to the traditionally considered local shears on the grain boundary plane. Moreover, the obtained analytic expressions are checked by different kinds of bicrystal shearing finite element simulations allowing grain boundary sliding and where a pinned line in the interface plane aims at representing the effect of a triple junction. A very good agreement is found between the analytic solutions and the finite element results. The performed simulations particularly emphasize the role of grain boundary sliding as a possible strong stress generator around the grain boundary close to the triple line because of the presence of pronounced gradients of sliding.

Cross and clover shaped orifice jets analysis at low Reynolds number

The jet coming from a cross-shaped orifice with an open center has been shown in the past, to provide substantial increase in the near field convective transport-mixing, in comparison to a classical round orifice jet. Detailed information has been reported in previous works on the role played in the jet mixing enhancement by the crow of vortices composed of counter-rotating pairs of secondary streamwise structures which are developing in orifice?s troughs. A trough in the cross-shaped jet generates a local shear like the one generated by a triangular tab in a square jet. In the present study we are interested by the modification of local shears in the troughs of the cross-shaped jet, when orifice geometry is modified, such as the center of the orifice becomes closed, leading to a clover-shaped orifice. The general motivation is to understand the effect of using a set of combination of longitudinal structures, themselves produced by the superposition of local shear regions, in mixing performance of a cross jet. It is shown that lower entrainment rates in the clover jet is a results of a additional internal crown of vortices which opposes the external one due to inner shears generated by closing the center of the orifice.

Improving visual quality of view transitions in automultiscopic displays

Automultiscopic screens present different images depending on the viewing direction. This enables glasses-free 3D and provides motion parallax effect. However, due to the limited angular resolution of such displays, they suffer from hot-spotting, i. e., image quality is highly affected by the viewing position. In this paper, we analyze light fields produced by lenticular and parallax-barrier displays, and show that, unlike in real world, the light fields produced by such screens have a repetitive structure. This induces visual artifacts in the form of view discontinuities, depth reversals, and excessive disparities when viewing position is not optimal. Although the problem has been always considered as inherent to the technology, we demonstrate that light fields reproduced on automultiscopic displays have enough degrees of freedom to improve the visual quality. We propose a new technique that modifies light fields using global and local shears followed by stitching to improve their continuity when displayed on a screen. We show that this enhances visual quality significantly, which is demonstrated in a series of user experiments with an automultiscopic display as well as lenticular prints.

Quantifying the mesoscopic shear strains in plane strain compressed polycrystalline zirconium

An algorithm is used to estimate mesoscopic strains in a deformed polycrystalline material. This requires comparison of microstructures before and after imposed macroscopic plastic deformations, in order to estimate the local/mesoscopic strains from the displacements of identifiable grain boundary segments. The algorithm was applied to lightly plane strain compressed (PSC) polycrystalline zirconium. Very large (up to 1.2) near-boundary mesoscopic shear strains were estimated. These were well above the estimated measurement uncertainties and remarkably larger than the extremely small (0.01–0.04) PSC strains imposed. Opposing local shears, on both sides of a grain boundary, appeared to compensate each other. Direct correlations were noted, in the same grain, between mesoscopic shear strains and (i) in-grain misorientations and (ii) subsequent grain fragmentation.

Relation among ITG Turbulence, Zonal Flows, and Transport in Helical Plasmas

Relation among the ion temperature gradient (ITG) turbulence, zonal flows, and the transport in helical plasmas is investigated by nonlinear gyrokinetic simulations. Local gyrokinetic simulations for helical field configurations are carried out employing various parameters such as the density and temperature gradients and local shears. From the simulation results, we construct a simple model function to represent ion heat diffusivity in terms of the turbulent fluctuations and zonal flow amplitude in helical plasmas.

CONVERGENT FLOWS AND LOW-VELOCITY SHOCKS IN DR21(OH)

DR21(OH) is a pc-scale massive, 7000 Msun clump hosting three massive dense cores (MDCs) at an early stage of their evolution. We present a high angular-resolution mosaic, covering 70" by 100", with the IRAM PdBI at 3 mm to trace the dust continuum emission and the N2H+ (J=1-0) and CH3CN (J=5-4) molecular emission. The cold, dense gas traced by the compact emission in N2H+ is associated with the three MDCs and shows several velocity components towards each MDC. These velocity components reveal local shears in the velocity fields which are best interpreted as convergent flows. Moreover, we report the detection of weak extended emission from CH3CN at the position of the N2H+ velocity shears. We propose that this extended CH3CN emission is tracing warm gas associated with the low-velocity shocks expected at the location of convergence of the flows where velocity shears are observed. This is the first detection of low-velocity shocks associated with small (sub-parsec) scale convergent flows which are proposed to be at the origin of the densest structures and of the formation of (high-mass) stars. In addition, we propose that MDCs may be active sites of star-formation for more than a crossing time as they continuously receive material from larger scale flows as suggested by the global picture of dynamical, gravity driven evolution of massive clumps which is favored by the present observations.

Plastic deformation transfer through the amorphous intercrystallite phase in nanoceramics

A three-dimensional model is proposed for plastic deformation transfer through the amorphous intercrystallite phase in mechanically loaded nanoceramics. In this model, glide dislocation loops are pressed against amorphous intercrystallite boundaries by the applied local shear stress and initiate in them local longitudinal plastic shears, which causes emission of new glide dislocation loops into neighboring grains. The energy characteristics of these processes and the critical applied stress required for barrierless nucleation of grainboundary and intragrain loops are calculated. As an example, a nanoceramic based on cubic silicon carbide is considered. It is shown that plastic deformation transfer through the amorphous intercrystallite phase in such nanoceramics is energetically favorable and can occur athermically over wide ranges of values of the applied stress and the structural characteristics of the material.

Wind profiler observations of a monsoon low-level jet over a tropical Indian station

Abstract. Three-year high-resolution wind observations of the wind profiler have been utilized to characterize the diurnal and seasonal features of the monsoon Low-Level Jet (LLJ) over a tropical station, Gadanki (13.5° N, 79.2° E), with a focus on the diurnal variability of low-level winds. The Boreal summer monsoon winds show a conspicuously strong westerly LLJ with average wind speed exceeding 20 m s−1. The L-band wind profiler measurements have shown an advantage of better height and time resolutions over the conventional radiosonde method for diurnal wind measurements. An interesting diurnal oscillation of LLJ core has been observed. It is varying in the height range of 1.8±0.6 km with the maximum and minimum intensity noticed during the early morning and afternoon hours, respectively. The jet core (wind maxima) height is observed to coincide with the inversion height. Strong wind shears are normally located beneath the LLJ core. The sole wind profiler observations are capable of identifying the monsoon phases, such as onset, break and active spells, etc. The mutual influence between the LLJ and the boundary layer has been discussed. One notices that the observed LLJ diurnal structures depend on the local convective activity, wind shears and turbulence activity associated with boundary layer winds. The day-to-day change in the LLJ structure depends on the latitudinal position of the LLJ core.

Petschek‐Type Reconnection Exhausts in the Solar Wind Well beyond 1 AU:Ulysses

We have identified 91 Petschek-type exhausts associated with local, quasi-stationary magnetic reconnection in the solar wind in plasma and magnetic field data from the Ulysses spacecraft obtained over a wide range of heliocentric distances (1.4-5.4 AU) and latitudes (S79°-N65°). The characteristic signature of an exhaust was a brief (minutes) interval of accelerated or decelerated plasma flow within a bifurcated current sheet in which changes in magnetic field and flow velocity were correlated at one edge and anticorrelated at the other. Transitions from outside to inside the exhausts were always slow-mode-like, the exhausts appearing to an observer as encounters with closely spaced, forward-reverse, slow-mode wave (shock) pairs. The exhausts almost universally occurred in low-speed or interplanetary coronal mass ejection plasma having low proton β (<1 and often 1) at relatively large shears (90°-180°) in the heliospheric magnetic field. Magnetic field strength decreases within the exhausts were highly variable from event to event and were roughly correlated with the magnitude of the local magnetic shears; this indicates the presence of substantial guide fields in at least some of these events. Many of the exhausts occurred at current sheets separating plasma having distinctly different Alfven speeds and were thus asymmetric. Local exhaust widths at Ulysses varied up to a maximum of ~2 × 106 km but statistically did not vary significantly with heliocentric distance. We offer several possible explanations of this observation.