Axial Gradient Research Articles

Syndepositional tectonics and mass-transport deposits control channelized, bathymetrically complex deep-water systems (Aínsa depocenter, Spain)

ABSTRACTThe inception and evolution of channels in deep-water systems is controlled by the axial gradient and lateral confinement experienced by their formative flows. These parameters are often shaped by the action of tectonic structures and/or the emplacement of mass-transport deposits (MTDs). The Arro turbidite system (Aínsa depocenter, Spanish Pyrenees) is an ancient example of a deep-water channelized system from a bathymetrically complex basin, deposited in an active tectonic setting. Sedimentologic fieldwork and geologic mapping of the Arro system has been undertaken to provide context for a detailed study of three of the best-exposed outcrops: Sierra de Soto Gully, Barranco de la Caxigosa, and Muro de Bellos. These locations exemplify the role of confinement in controlling the facies and architecture in the system. Sedimentologic characterization of the deposits has allowed the identification of fifteen facies and eight facies associations; these form a continuum and are non-unique to any depositional environment. However, architectural characterization allowed the grouping of facies associations into four depositional elements: i) weakly confined, increasing-to-decreasing energy deposits; ii) progradational, weakly confined to overbank deposits; iii) alternations of MTDs and turbidites; iv) channel fills. Different styles of channel architecture are observed. In Barranco de la Caxigosa, a master surface which was cut and subsequently filled hosts three channel stories with erosional bases; channelization was enhanced by quasi-instantaneous imposition of lateral confinement by the emplacement of MTDs. In Muro de Bellos, the inception of partially levee-confined channel stories was enhanced by progressive narrowing of the depositional fairway by tectonic structures, which also controlled their migration. Results of this study suggest that deep-water channelization in active tectonic settings may be enhanced or hindered due to: 1) flow interaction with MTD-margin topography or; 2) MTD-top topography; 3) differential compaction of MTDs and/or sediment being loaded into MTDs; 4) formation of megascours by erosive MTDs; 5) basin-floor topography being reset by MTDs. Therefore, the Arro system can be used as an analog for ancient subsurface or outcrop of channelized deposits in bathymetrically complex basins, or as an ancient record of deposits left by flow types observed in modern confined systems.

MHD Peristaltic Flow of a Couple - Stress with varying Temperature for Jeffrey Fluid through Porous Medium

This paper is intended for investigating the effects of magnetohydrodynamic on the couple stress unsteady flow of incompressible Jeffrey fluid with varying temperature through a cylindrical porous channel. The analytical expression of the axial velocity, stream function and gradient pressure, was created taking into account the effect of thermal diffusion on the flow of the fluid. The analytical formulas of the velocity, temperature have been illustrated graphically for significant various parameters such as magnetic parameter, couple stress parameter, permeability parameter.

Fast acquisition abdominal MRI study for the investigation of suspected acute appendicitis in paediatric patients

ObjectivesTo assess the diagnostic accuracy of fast acquisition MRI in suspected cases of paediatric appendicitis presenting to a tertiary referral hospital.Materials and methodsA prospective study was undertaken between May and October 2017 of 52 children who presented with suspected appendicitis and were referred for an abdominal ultrasound. All patients included in this study received both an abdominal ultrasound and five-sequence MRI consisting of axial and coronal gradient echo T2 scans, fat-saturated SSFSE and a diffusion-weighted scan. Participants were randomised into groups of MRI with breath-holds or MRI with free breathing. A patient satisfaction survey was also carried out. Histopathology findings, where available, were used as a gold standard for the purposes of data analysis. Statistical analysis was performed, and p values < 0.05 were considered statistically significant.ResultsUltrasound had a sensitivity and specificity of 25% and 92.9%, respectively. MRI with breath-hold had a sensitivity and specificity of 81.8% and 66.7%, respectively, whilst MRI with free breathing was superior with sensitivity and specificity of 92.3% and 84.2%, respectively. MRI with free breathing was also more time efficient (p < 0.0001). Group statistics were comparable (p < 0.05).ConclusionsThe use of fast acquisition MRI protocols, particularly free breathing sequences, for patients admitted with suspected appendicitis can result in faster diagnosis, treatment and discharge. It also has a statistically significant diagnostic advantage over ultrasound. Additionally, the higher specificity of MR can reduce the number of negative appendectomies performed in tertiary centres.

The dynamics of coherent modes of gradient drift instabilities in a small magnetron discharge plasma

We report on the dynamic behavior of gradient-driven drift waves in a strongly obstructed magnetron discharge. The magnetron has a magnetic topology that results in a toroidal plasma within the gap and supports the development of very coherent modes of rotating plasma structures. The modes and their rotation are present over a wide range of conditions, and the rotation is retrograde to the usual externally imposed E×B direction. This feature seems to be unique to this device and is attributed to a field reversal due to the strong anode-directed electron diffusion that arises from large axial plasma density gradients. A multi-fluid model is proposed, and a Fourier analysis of the linearized equations results in the identification of conditions that support the growth of these instabilities and their transitions across mode symmetries, controlled experimentally by varying the discharge voltage. The model also provides insight on the possible mechanism driving cross-field particle transport. Experiments are carried out with a segmented anode to confirm the localized current flow concomitant with the presence of a coherent structure. These segment currents together with high speed videography unambiguously confirm the direction of plasma rotation and reveal the existence of a stochastic regime between voltage-controlled mode transitions. An analysis of the segment currents in this regime indicates that the lower frequency state decays into a spectrum of coherent higher frequency states that exhibit features consistent with a three-wave nonlinear parametric mixing process.

Design of a Magnet and Gradient Coils for a Tabletop Liquid-Helium-Free, Persistent-Mode 1.5-T MgB$_2$ Osteoporosis MRI

We have finalized the design of a full-scale tabletop 1.5-T/90-mm MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> finger MRI magnet system for osteoporosis screening based on our preliminary test results of small coils and superconducting joints. The magnet will operate in persistent mode at 10 K with an additional 5 K temperature margin. The magnet design which includes six main coils and an iron shield satisfies the required specification of a field intensity of 1.5 T, homogeneity of ≤5 ppm over a 20-mm diameter of spherical volume, and a fringe field of ≤5 gauss at 0.5 m in radius from the magnet center. An active protection method using external heaters will be applied to prevent a local hot spot in the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> windings from being overheated when quench occurs. Active shield transverse and axial gradient coils for this tabletop osteoporosis MRI, having primary and shield coil pairs, are designed to minimize stray fields that can induce eddy currents on nearby metal surface and thus imaging artifacts. This paper covers design and analysis of: 1) the main coils and iron shield; 2) coil former; 3) quench protection; and 4) active shield gradient coils. We also discuss design changes of the cryostat and equipment plan for the overall system. The magnet system will be completed and then, equipped with other MRI hardware components including an in-house-made gradient coil assembly and RF coils for demonstration of 1.5-T finger MRI in 2020.

From arc-melted ingot to MTR fuel plate: A SEM/EBSD microstructural study of U3Si2

U3Si2/Al fuel plates are widely used in Material Testing Reactors (MTRs) as Low Enriched Uranium driver fuel. In this paper, a reinvestigation of the microstructure of U3Si2 particles is proposed to take full advantage of the new capabilities offered by Electron BackScattered Diffraction (EBSD) techniques.Using EBSD we demonstrate that most particles are single crystalline in as-fabricated plates. To understand this characteristic, linked to the microstructure of the starting material, an in-depth study of U3Si2 ingots produced by arc-melting was performed at a laboratory scale; they were extensively characterized by EBSD, scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. It is shown that U3Si2 grain may be large (up to several thousands of micrometers) and that they exhibit a strong preferential orientation, linked to the axial thermal gradient created in the arc-melting chamber. A significant impact of the cooling rate after arc-melting on the ingot microstructure is noticed: grains are smaller and more columnar when the cooling rate is high. A deviation from 3U/2Si stoichiometry caused for example by impurities induces the formation of a secondary phase, which exhibits a square spiral morphology.We then demonstrate that the cooling rate of U3Si2 ingots has a direct influence on the characteristics of the powders obtained by crushing these ingots. Indeed the powder obtained from “slow” cooled ingots is found very close to the powder used for industrial MTR plates. On the contrary particles obtained from “fast” cooled ingots, are polycrystalline and more resistant to crushing.Thus, this work provides significant advances both in the characterisation of technological products like U3Si2/Al MTR plates and in more basic knowledge about the U3Si2 phase formation.

Surface-induced flow: A natural microscopic engine using infrared energy as fuel.

Fluid commonly flows in response to an external pressure gradient. However, when a tunnel-containing hydrogel is immersed in water, spontaneous flow occurs through the tunnel without any pressure gradient. We confirmed this flow in a wide range of plant- and animal-derived hydrogels. The flow appears to be driven by axial concentration gradients originating from surface activities of the tunnel wall. Those activities include (i) hydrogel-water interaction and (ii) material exchange across the tunnel boundary. Unlike pressure-driven flow, this surface-induced flow has two distinct features: incident infrared energy substantially increases flow velocity, and narrower tunnels generate faster flow. Thus, surface activities in hydrogel-lined tunnels may confer kinetic energy on the enclosed fluid, with infrared as an energy source.

Relaxation time constant based optical coherence elastography.

This study aimed at visualizing relative relaxation time constant (RTC) in soft tissue by using optical coherence elastography (OCE). We proposed a forced vibration model as a theoretical base to express RTC using axial gradient of periodic vibration phase captured by phase sensitive optical coherence tomography (PhS-OCT). Validation of the model had been accomplished by experiments with isotropic and double-layered phantoms. A fresh chicken breast sample treated with focused ultrasound was prepared to test performance of the RTC-OCE in real tissue. All results were cross-validated with indentation test and traditional strain-based elastography. This study first utilized RTC mapping in 2D and 3D that covers the information of both elasticity and viscosity. The generated RTC mapping revealed the same mechanical difference internal sample which is correlated with conventional strain mapping. RTC mapping is potentially to be served as new biomarker for disease diagnosis in the future.

Current density and criticality branch-chain for a reactive Poiseuille second-grade hydromagnetic flow with variable electrical conductivity

Abstract This study investigates the current density and branch-chain thermal criticality for a non-Newtonian hydromagnetic second-grade fluid flow in a Poiseuille device with varying electrical conductivity. The combustible reactive fluid is fully exothermic without material consumption, and the fluid is induced by electric and magnetic fields in the presence of axial gradient pressure. With suitable variables, the equations are translated to dimensionless form and solved by semi finite discretization difference scheme to provide solutions for the heat and velocity dependent parameters. The thermal ignition and slice bifurcation for the criticality, and the current density results are plotted for various dependent terms. It is obtained from the investigation that terms that increases current density in the flow system need to be augmented to improve the current carrying hydromagnetic fluid in order to improve its industrial usage.

Minuet motion of a pair of capsules interacting in simple shear flow

We study the three-dimensional hydrodynamic interaction of a pair of identical, initially spherical capsules freely suspended in a simple shear flow under Stokes flow conditions. The capsules are filled with a Newtonian liquid (same density and viscosity as the suspending fluid). Their membranes satisfy the neo-Hookean constitutive law. We consider the rarely studied case where the capsule centres are initially located in (or near) the plane defined by the flow direction and the vorticity vector, i.e. in two different shear planes. The motion and deformation of the capsules are modelled by means of a boundary integral technique to compute the flows, coupled to a finite element method to calculate the force exerted by the membranes on the fluids. We follow the motion and deformation of the capsules as they are convected towards each other after a sudden start of the flow. Our main finding is that, depending on their initial position and deformability, the two capsules may oscillate slowly about the flow gradient axis, get nearer to each other at each oscillation to finally interact strongly and separate. This minuet motion had not been identified previously. We identify the regions of space where either simple crossing or minuet occurs. This phenomenon has a marked influence on the irreversible trajectory drift of two capsules after crossing: the minuet process leads to a significant trajectory displacement along the flow gradient when none was expected, based on the previous studies where the two capsules had a significant relative velocity.

Contact line motion in axial thermocapillary outward flow

We study the contact line dynamics of a viscous droplet deposited at the centre of a substrate subject to an axial thermal gradient. The temperature of the substrate decreases with distance from the centre, so the Marangoni stress induced at the liquid–air interface displaces the liquid radially outward. The flow experiences two stages. In the first stage, the droplet evolves towards an axially symmetric ring whose radius increases with time as . Using the lubrication approximation, we perform numerical simulations that confirm this law for the motion of the front and show that the maximum thickness of the profile decreases as . We explain the evolution law of the contact line by balancing Marangoni and viscous stresses. In the second stage, the contact line becomes unstable and develops smooth corrugations whose amplitude increases with time and that eventually become long fingers. The temporal evolution of the Fourier spectra of the contour shows a shift of the most unstable mode from smaller to larger azimuthal wavenumbers.

Plasma Anisotropy upon Emission of Charged Particles

Anisotropic perturbation of the plasma of a hollow-cathode reflex discharge is experimentally studied upon emission of electrons through the axial channel of the reflecting cathode. The energy spectrum of emitted electrons is broadened along the emission direction when the external accelerating field is increased. The spectral broadening is related to the directional motion of plasma electrons toward the emitting surface of plasma. Axial gradients of concentration and electric potential are possible reasons for the directional motion of electrons.

Ultralow-loss domain wall motion driven by a magnetocrystalline anisotropy gradient in an antiferromagnetic nanowire

Searching for new methods controlling antiferromagnetic (AFM) domain wall is one of the most important issues for AFM spintronic device operation. In this work, we study theoretically the domain wall motion of an AFM nanowire, driven by the axial anisotropy gradient generated by external electric field, allowing the electro control of AFM domain wall motion in the merit of ultra-low energy loss. The domain wall velocity depending on the anisotropy gradient magnitude and intrinsic material properties is simulated based on the Landau-Lifshitz-Gilbert equation and also deduced using the energy dissipation theorem. It is found that the domain wall moves at a nearly constant velocity for small gradient, and accelerates for large gradient due to the enlarged domain wall width. The domain wall mobility is independent of lattice dimension and types of domain wall, while it is enhanced by the Dzyaloshinskii-Moriya interaction. In addition, the physical mechanism for much faster AFM wall dynamics than ferromagnetic wall dynamics is qualitatively explained. This work unveils a promising strategy for controlling the AFM domain walls, benefiting to future AFM spintronic applications.

Increased Presence of Cerebral Microbleeds Correlates With Ventricular Enlargement and Increased White Matter Hyperintensities in Alzheimer's Disease.

Objective: To investigate whether the number of cerebral microbleeds (CMB) could be a useful indicator to predict glymphatic system dysfunction in Alzheimer’s disease (AD) patients, by comparing the degree of cerebral spinal fluid (CSF) and interstitial fluid (ISF) stasis.Methods: Forty probable AD patients were included, with those exhibiting two or more CMB were included in the multiple CMB group (mCMB, n = 21, mean = 11.1), and none or one CMB included in the non-multiple CMB group (nmCMB, n = 19, mean = 0.84). CMB was defined in axial gradient recalled echo (GRE) T2*-weighted images. Evans index (EI) was calculated to measure lateral ventricle enlargement, Voxel-based Specific Regional Analysis System for Alzheimer’s Disease (VSRAD) software was used to determine the extent of gray and white matter atrophy, and Fazekas scale (FS) was used to determine white matter hyperintensities (WMH).Results: EI was significantly larger in mCMB than in nmCMB, while the gray and white matter volume was not different between groups. Thus, the difference in lateral ventricle enlargement between AD with and without multiple CMB reflects a combination of the degree of brain atrophy and the extent of CSF stasis. FS was higher in mCMB than in the nmCMB, suggesting the failure of ISF elimination was more severe in mCMB cases.Conclusion: The difference in lateral ventricle enlargement and WMH between AD with or without multiple CMB may reflect a difference in the degree of CSF/ISF stagnation.

Influence of homogenization models on size-dependent nonlinear bending and postbuckling of bi-directional functionally graded micro/nano-beams

In this work, different homogenization schemes are employed to analyze both size-dependent postbuckling and nonlinear bending behavior of micro/nano-beams, made of a bi-directional functionally graded material (BDFGM), under external axial compression and distributed load. To such different homogenization models, including Reuss, Voigt, Mori-Tanaka, and Hashin–Shtrikman bounds schemes, together with nonlocal strain gradient elasticity theory are adopted within the framework of refined exponential shear deformation beam theory, to develop a comprehensive size-dependent BDFGM beam model. Deviation of associated physical neutral plane, from mid-plane counterpart, is also considered. Nonlocal strain gradient load-deflection responses of BDFGM micro/nano-beam are obtained by numerical solution methodology for both nonlinear bending and postbuckling behaviors corresponding to different values of the lateral and longitudinal material property indices and various small scale parameters. We observed that by decreasing the values of material property gradient indices, associated with BDFGM, difference between the estimations of various homogenization schemes is raised. We also indicated that increasing maximum deflection, decreasing the significance of nonlocal size effect on the bending strength of BDFGM micro/nano-beams, whereas strain gradient size effect becomes more important. In addition, we found that at lower material property gradient indices, bending strength reduction in BDFGM micro/nano-beams, causes by the axial gradient property is higher than lateral gradient property. At higher values of these indices, however, the trend is opposite.

On the role of the epithelium in a model of sodium exchange in renal tubules

In this study we present a mathematical model describing the transport of sodium in a fluid circulating in a counter-current tubular architecture, which constitutes a simplified model of Henle’s loop in a kidney nephron. The model explicitly takes into account the epithelial layer at the interface between the tubular lumen and the surrounding interstitium. In a specific range of parameters, we show that explicitly accounting for transport across the apical and basolateral membranes of epithelial cells, instead of assuming a single barrier, affects the axial concentration gradient, an essential determinant of the urinary concentrating capacity. We present the solution related to the stationary system, and we perform numerical simulations to understand the physiological behaviour of the system. We prove that when time grows large, our dynamic model converges towards the stationary system at an exponential rate. In order to prove rigorously this global asymptotic stability result, we study eigen-problems of an auxiliary linear operator and its dual.

Flow and deformation characteristics of a flexible microfluidic channel with axial gradients in wall elasticity.

Axial gradients in wall elasticity may have significant implications in the deformation and flow characteristics of a narrow fluidic conduit, bearing far-reaching consequences in physiology and bio-engineering. Here, we present a theoretical and experimental framework for fluid-structure interactions in microfluidic channels with axial gradients in wall elasticity, in an effort to arrive at a potential conceptual foundation for in vitro study of mirovascular physiology. Towards this, we bring out the static deformation and steady flow characteristics of a circular microchannel made of polydimethylsiloxane (PDMS) bulk, considering imposed gradients in the substrate elasticity. In particular, we study two kinds of elasticity variations - a uniformly soft (or hard) channel with a central strip that is hard (or soft), and, increasing elasticity along the length of the channel. The former kind yields a centrally constricted (or expanded) deformed profile in response to the flow. The latter kind leads to increasingly bulged channel radius from inlet to outlet in response to flow. We also formulate an analytical model capturing the essential physics of the underlying elastohydrodynamic interactions. The theoretical predictions match favourably with the experimental observations and are also in line with reported results on stenosis in mice. The present framework, thus, holds the potential for acting as a fundamental design basis towards developing in vitro models for micro-circulation, capable of capturing exclusive artefacts of healthy and diseased conditions.

Implementation of a brain injury screen MRI for infants at risk for abusive head trauma.

Head computed tomography (CT) is the current standard of care for evaluating infants at high risk of abusive head trauma. To both assess the feasibility of using a previously developed magnetic resonance imaging (MRI) brain injury screen (MRBRscreen) in the acute care setting in place of head CT to identify intracranial hemorrhage in high-risk infants and to compare the accuracy of a rapid imaging pulse sequence (single-shot T2 fast spin echo [ssT2FSE]) to a conventional pulse sequence (conventional T2 fast spin echo [conT2FSE]). This was a quality improvement initiative to evaluate infants <12months of age who were screened for intracranial hemorrhage using an MRBRscreen as part of clinical care. The MRBRscreen included axial conT2FSE, axial gradient recalled echo, coronal T1-weighted inversion recovery, axial diffusion-weighted image and an axial ssT2FSE. A comparison of ssT2FSE to conT2FSE with respect to lesion detection was also performed. Of 158 subjects, the MRBRscreen was able to be completed in 155 (98%); 9% (14/155) were abnormal. Ninety-four percent (137/145) of subjects underwent only an MRBRscreen and avoided both radiation from head CT and sedation from MRI. The axial ssT2FSE and conT2FSE results were congruent 99% of the time. An MRBRscreen in place of a head CT is feasible and potentially could decrease head CT use by more than 90% in this population. Using a rapid ssT2FSE in place of a conT2FSE can reduce total scan time without losing lesion detection. If an MRBRscreen is readily available, physicians' threshold to perform neuroimaging may be lowered and lead to earlier detection of abusive head trauma.

Анизотропия плазмы при извлечении из нее заряженных частиц

The anisotropic perturbation of a hollow-cathode reflex discharge plasma when electrons are extracted from it through an axial channel in the cathode-reflector is experimentally investigated. The expansion of the extracted electrons energy spectrum in the direction of extraction was observed as the external accelerating electric field increased. The energy spectrum expansion is associated with the plasma electrons directional movement towards the emitting surface of plasma. The axial gradients of the concentration and the electric potential are assumed to be the reasons for the directional movement of electrons.

Decomposing the intra-annual variability of flushing characteristics in a tidal bay along the North Sea

Flushing timescales in estuaries and coastal bays largely shape the distribution of dissolved and particulate matter and therefore have important environmental implications. Here we investigate the spatiotemporal variability of turnover times in a semi-enclosed tidal bay and examine the potential underlying causes. The basin considered here is the Dutch coastal bay Oosterschelde (Eastern Scheldt in English) on the east coast of the North Sea. Using a calibrated and validated 3D hydrodynamic model, conservative tracer experiments were conducted to estimate turnover times. Sensitivity scenarios were run to examine the relative roles of tides, winds, and gravitational flow in driving the intra-annual variability. Results indicate that gravitational flow and winds contribute most to the intra-annual variability of turnover times in the Oosterschelde. Freshets from the Westerschelde and Rhine Rivers extending to the mouth of the bay can reduce the axial density gradient and gravitational flow, increasing turnover times of the basin. The prevailing southwesterly winds, when of sufficient magnitude (>5 m/s), modify the spatial patterns of turnover times especially in shallow waters. Tides force flushing of the bay but do not induce significant intra-annual variability. Findings in this study underline the importance of intra-annual variability of turnover times in coastal bays and methodology proposed here facilitates intercomparison studies for other systems.