Transverse Gradient Research Articles

In situ experiments in microgravity and phase-field simulations of the lamellar-to-rod transition during eutectic growth

In recent experiments on the solidification of the binary eutectic alloy succinonitrile-(D)camphor carried out on board of the International Space Station (ISS), a transition from rod to lamellar patterns was observed for low growth velocities. The transition was interpreted in terms of a competition between a propagative instability of lamellae and a drift induced by a transverse temperature gradient. Phase-field simulations of a symmetric model alloy support this scenario: for a fixed transverse temperature gradient, the transition from rods to lamellae occurs for a critical composition at fixed velocity, and for a critical velocity at fixed composition. Since the alloy and control parameters used in experiments and simulations are different, our results strongly suggest that this morphological transition is generic for eutectic alloys.

Nonlinear Forced Vibration Analysis of Two-Directional Functionally Graded Truncated Conical Shells

It is crucial to comprehend the nonlinear vibration behavior of sophisticated engineering constructions to aid in the examination, creation and production procedures. The primary objective of this study, therefore, is to focus on exploring the nonlinear forced vibrational characteristics of two-directional functionally graded (TDFG) cone-shaped shell for the first time. To accomplish this, the motion equations of the TDFG cone-shaped shell are derived based upon the framework of first-order shear deformation theory (FSDT) in conjunction with von Kármán assumption. TDFG cone-shaped shell models have mechanical properties that can change smoothly and continuously across the thickness and length of the shell, following a power law distribution pattern. To discretize the partial differential equations and transform them into ordinary differential equations, the Galerkin approach was used and ultimately, these equations were solved using the multiple time scales method. The accuracy and effectiveness of this analytical model are indicated through comparison with existing solutions. Finally, some comprehensive parametric investigations are carried out to gain insight into the impacts of several factors on the nonlinear behavior of the shell, highlighting the significant influence of transverse and longitudinal gradient indices on the nonlinear frequency response, which is not considered before. The new insights gained from this research could serve as valuable benchmark outcomes as well as contribute to a more comprehensive understanding of the nonlinear vibrations in future analysis and design processes.

Effect of transverse gradient extrusion process on texture and mechanical properties of Mg–3Al–1Zn alloy sheets

Transverse gradient extrusion (TGE) processes with different die inclined angles of 15°, 30°, 45°and 60° were used to produce Mg–3Al–1Zn sheets, thereby tailoring texture and mechanical properties of Mg alloy sheets. Microstructure, deformation texture and final mechanical properties of the obtained sheets were detected and discussed. Furthermore, the rheological behavior of Mg alloys during extrusion was analyzed by finite element method. As the TGE die inclined angle changed, the flow velocity along the transverse direction (TD) and effective strain difference along extrusion direction (ED) were changed during extrusion, and therefore the microstructure and texture of extruded sheets are altered. With increase of TGE die inclined angles, fine grain size and uniform microstructure were obtained. The base poles of most grains for the extruded sheets gradually deviate from normal direction (ND) to ED at center region and inclined from ED to TD at 1/4 edge region. A large elongation of 33.9 % and a low yield stress of 125 MPa can be obtained in the TGE-45 sheet.

Shaping Micro-Bunched Electron Beams for Compact X-ray Free-Electron Lasers with Transverse Gradient Undulators

Laser-modulator-based micro-bunching of electron beams has been applied to many novel operating modes of X-ray free-electron lasers from harmonic generation to attosecond pulse production. Recently, it was also identified as a key enabling technology for the production of a compact XFEL driven by a relatively low-energy beam. In traditional laser modulator schemes with low-energy and high-current bunches, collective effects limit the possible working points that can be employed, and thus it is difficult to achieve optimal XFEL performance. We propose to utilize transverse longitudinal coupling in a transverse gradient undulator (TGU) to shape micro-bunched electron beams so as to optimize their performance in a compact X-ray free-electron laser. We show that a TGU added to a conventional laser modulator stage enables much more flexibility in the design, allowing one to generate longer micro-bunches less subject to slippage effects and even lower the slice emittance of the micro-bunches. We present a theoretical analysis of laser-based micro-bunching with an added TGU, simulation of compression with collective effects in such systems, and finally XFEL simulations demonstrating the gains in peak power enabled by the TGU. Although we focus on the application to compact XFELs, what we propose is a general phase space manipulation that may find utility in other applications as well.

Quantitative proton radiography and shadowgraphy for arbitrary intensities

Charged-particle radiography and shadowgraphy data can be directly inverted to obtain a line-integrated transverse Lorentz force or a line-integrated transverse refractive index gradient if intensity modulations due to scattering and absorption are negligible, and angular deflections are small. We develop a new direct-inversion algorithm based on plasma physics and compare it to a new Monge–Ampère code and an existing power diagram code (Kasim et al., 2017). The measured or source intensity is represented by electrons subject to drag, and the other intensity by fixed ions. The decrease in kinetic plus electrostatic energy determines convergence. The displacement of the electrons from their initial to their equilibrium positions determines the line-integrated force or refractive index gradient. We have implemented two approaches: PIC (particle in cell) and Lagrangian fluid, in 1-D and 2-D. The PIC code works for arbitrary intensities, can work efficiently in parallel, and can make use of existing codes. The Lagrangian code requires less memory and is faster than the PIC code without massively parallel processing, but fails in 2-D for large intensity modulations. The Monge–Ampère code is by far the fastest in 2-D, without massively parallel processing, but fails for intensities with large voids, high contrast ratios and large deflections across the boundaries, and could not obtain the degree of convergence possible with the PIC code. The power diagram code was by far the slowest and most memory intensive, and failed for large peaks in the measured intensity.

Properties of the jet in M87 revealed by its helical structure imaged with the VLBA at 8 and 15 GHz

ABSTRACT We present full-track high-resolution radio observations of the jet of the galaxy M87 at 8 and 15 GHz. These observations were taken over three consecutive days in 2009 May using the Very Long Baseline Array (VLBA), one antenna of the Very Large Array (VLA), and the Effelsberg 100 m telescope. Our produced images have dynamic ranges exceeding 20 000:1 and resolve linear scales down to approximately 100 Schwarzschild radii, revealing a limb-brightened jet and a faint, steep spectrum counter-jet. We performed jet-to-counter-jet analysis, which helped estimate the physical parameters of the flow. The rich internal structure of the jet is dominated by three helical threads, likely produced by the Kelvin–Helmholtz (KH) instability developing in a supersonic flow with a Mach number of approximately 20 and an enthalpy ratio of around 0.3. We produce a clean imaging bias-corrected 8–15 GHz spectral index image, which shows spectrum flattening in regions of helical thread intersections. This further supports the KH origin of the observed internal structure of the jet. We detect polarized emission in the jet at distances of approximately 20 milliarcseconds from the core and find Faraday rotation which follows a transverse gradient across the jet. We apply Faraday rotation correction to the polarization position angle and find that the position angle changes as a function of distance from the jet axis, which suggests the presence of a helical magnetic field.

A theory of rheological film condensation

The article provides a theoretical investigation of the laminar film-condensation of shear-thinning fluids onto an isothermal, vertical plate in the presence of gravity, interfacial shear, wall friction, and inertia. The liquid motion occurs toward gravity, overcoming the resistances of interfacial shear, wall friction, and inertia. The liquid motion induces momentum to the adjacent stagnant saturated vapor by interfacial shear. The vapor motion, in turn, influences the liquid film’s heat transfer and fluid dynamics. For film condensation, it is well-known that the film, along the length of the plate, can be grouped in two separate heat transfer regimes, viz. conduction-dominated regime and convection-dominated regime. For the first time, we have identified three distinct fluid dynamic regions across the film (viz. R1, R2, and R3) unique to power-law fluids. R1 is bounded between the condensing surface and the locus of Newtonian shear stress. R2 is bounded between the locus of Newtonian shear stress and the locus of zero shear stress. Beyond R2, R3 is stretched up to the liquid-vapor interface. The transverse temperature gradient is subtly different in these three fluid dynamic regions. We examine flow physics on a non-dimensional platform. The critical role of the power-law index in determining wall-friction and condensate’s mass flow rate and the impact of liquid Prandtl number in achieving the degree of subcooling has been nicely illustrated. We expose various limiting conditions for simplifying the robust mathematical theory. An analytical expression for the liquid’s longitudinal velocity has been derived, neglecting inertia and interfacial shear.

Design of cylindrical saddle-shaped transverse gradient coils with small aspect ratios and high linearity for micro comagnetometers

Design of cylindrical saddle-shaped transverse gradient coils with small aspect ratios and high linearity for micro comagnetometers

Interaction of a Sparce Particle Stratum with a Constantly Heated Plane in Presence of a Transverse Temperature Gradient

While modelling dispersions in containers or tubes it may be necessary to find distortions brought by the suspended particles in the temperature distribution in a vessel. Essential step of such a calculation is to determine of temperature field emerging when the particles are placed near the plane wall of the vessel. For simplicity one may suppose additionally that the carrying medium is stationary and that the particles are spherical. Solving this problem, the authors replace the plane by a fictitious particle that is mirror-positioned with respect to a given one. This allows to use multipole expansion for representation of the temperature that is a harmonic function in the case discussed. The obtained solution is used to find effective heat conduction coefficient of particles’ stratum placed in a half-space bounded by a plane with constant temperature. To do this, the authors average the temperature in the medium by the particles’ positions and compare the result with the solution of reference problem about temperature distribution in a half-space with a uniform stratum of other thermal conductance. The calculation is provided under the assumption that suspended spheres are placed rarely and therefore interact only with the plane but not with each other. A correction term is obtained that must be included in the expression for heat conduction coefficient if the total medium longitude in the direction orthogonal to the plane is finite.

Effects of transverse concentration gradients on the vented explosion of methane-air mixtures

Experiments were performed in a 90-L vessel with a side vent to investigate the effects of transverse concentration gradients on the vented explosion of non-uniform methane-air mixture. Controlled by a mass flow meter, methane was injected uniformly into the vessel through 18 nozzles that are evenly distributed at the top to configure a methane-air mixture with an average methane volume fraction of 10.9%. Various transverse concentration gradients were obtained by altering the free diffusion time of the methane in the vessel. The deflagration flame was recorded through a high-speed camera at a frequency of 1000 Hz, and the internal overpressure at different locations was captured through three piezoelectric sensors. Experimental results reveal that larger concentration gradient resulted in lower flame propagation rate, longer time for the burst of the vent cover, and stronger pressure oscillations. Concentration gradient affects the typical flame structure, especially the formation and development of the tulip flame. Under the present experimental conditions, Helmholtz oscillations occurred in all tests. Concentration gradient has a negative correlation with the frequency of Helmholtz oscillations but a positive correlation with the duration of oscillations. In conclusion, the variation of concentration gradient obviously affects flame propagation and pressure evolution during vented explosions, which deserves more attention in the design of explosion protection.

Design of gradient coils based on the target field method in a high-permeability shield considering sidewall apertures

The combination of high-permeability ferrite and coils is often used to nullify ambient magnetic fields and is widely used in applications such as magnetic resonance imaging and high-sensitivity magnetic sensors. We propose the design method of transverse gradient coils based on the target field method in a high-permeability shield considering sidewall apertures (TFM-HPSCSA), which are installed in the ferrite shield to zero the residual magnetic field gradient. TFM-HPSCSA coils enable an innovative combination of magnetic field gradient linearity and structural luminescence. The coil current density distribution is set to avoid the central sidewall apertures. As a result, the contour distribution of the stream function under the optimized weight coefficients is calculated to obtain the wire alignments of the dBy/dx, dBy/dy and dBy/dz gradient coils. The deviations of the experimentally measured values from the finite element simulations of the three types coils are 0.63%, 1.17% and 0.48%, respectively, which verified the validity and the accuracy of the proposed method. In comparison to the traditional saddle gradient coil designed under the free space condition, the proposed dBy/dx coil produces a 30.5% improvement in the percentage range of regions with 5% magnetic field gradient nonlinearity. The increased linear range of the gradient magnetic field contributes to the improvement of the residual field nulling range in magnetic sensors and magnetic shielding systems in practical applications.

Study on effect of transverse concentration gradients in pipeline on methane explosion characteristics

In this paper, we studied the effects of transverse concentration gradients on methane explosion characteristics using a fully-transparent pipeline and finally came to the following conclusions. The peak explosion overpressure changed in a waveform of “M” along different measuring points, but a set of non-equivalent concentration gradients maximized this peak value. Higher methane concentrations in the Q2 region could cause the secondary peak overpressure near the both ends of the pipe. In this process, the flame outside the pipe tends to form a near-spherical shape. However, at lower methane concentrations in the Q2 region, the secondary peak overpressure was only detected near the outlet. Its occurrence at different sites may relate to the turbulence effect heightened by concentration gradients. Besides, under the same gradient, the smallest propagation speed was measured on the right of the ignition source when the flame propagated fastest on the left, which was attributed to different inhibiting effects of reflected waves on flame propagation.

Characterization of concomitant gradient fields and their effects on image distortions using a low-field point-of-care Halbach-based MRI system.

Concomitant gradient fields have been extensively studied at clinical field strengths. However, their effects have not yet been modeled for low-field point-of-care (POC) systems. The purpose of this work is to characterize the effects associated with concomitant fields for POC Halbach-array-based systems. The concomitant fields associated with a cylindrical gradient coils designed for a transverse and a signal model including the tilting effect of the effective magnetic field are derived. The formalism is used to simulate and predict concomitant field related distortions. A 46-mT Halbach-array-based system with a maximum gradient strength of 15 mT/m is used to verify the model using two-dimensional spin-echo sequences. The simulations and experimental results are in good agreement with the derived equations. The fundamental characteristics of the concomitant field equations are different to conventional MRI systems: Image distortions occur primarily in the transverse directions and a cross-term only exists when applying transverse gradient pulses simultaneously. The level of image warping in the frequency encoding direction is insignificant for the POC systems discussed here. However, when trying to achieve short echo-times by using strong phase encoding and readout-dephasing gradients, the combination can result in image warping and blurring which should be accounted for in image interpretation.

Optimizing noble gas pressure for enhanced self-compensation in spin-exchange relaxation-free comagnetometers.

The coupling of electron spin and nuclear spin through spin-exchange collisions compensates for external magnetic field interference in the spin-exchange relaxation-free (SERF) comagnetometer. However, the compensation ability for magnetic field interference along the detection axis is limited due to the presence of nuclear spin relaxation. This paper aims to enhance the self-compensation capability of the system by optimizing the pressure of the noble gas during cell filling. Models are established to describe the relationships between the nuclear spin polarization, the polarizing magnetic field of nuclei, the magnetic field suppression factors, and the pressure of the noble gas in the K-Rb-21Ne atomic ensemble. Experiments are conducted using five cells with different pressure. The results indicate that in the positive pressure area, the nuclear spin polarization decreases while the equivalent magnetic field experienced by the noble gas increases with increasing pressure. The magnetic field suppression factor for transverse fields increases as the pressure increases, leading to a decrease in the ability to suppress low-frequency magnetic field interference. Moreover, at the cell temperature of 180°C and a transverse residual field gradient of 4.012 nT/cm, the system exhibits its strongest capability to suppress transverse magnetic field interference when the pressure of 21Ne is around 0.7 atm.

Temperature gradient of composite PK girder based on monitoring and long-term simulation

Bridge structures exposed to the environment are inevitably affected by the thermal load which is closely related to the type of section. In this study, the temperature field of a composite Pasco-Kennewick (PK) section girder was monitored in situ for the first time. Meanwhile, a refined finite-element model (FEM) of the temperature field was established and verified with measured temperatures. The temperature distribution and variation characteristics were analyzed using measured temperatures. On this basis, the simplified profiles of vertical temperature gradient (VTG) and transverse temperature gradient (TTG) were proposed. The representative values of temperature differences with a 100-year return period were predicted by extreme value analysis (EVA) with the samples of temperature differences obtained through 23-year numerical simulation. Finally, comparisons were made between the temperature gradients of composite PK section and the recommendations in current specifications. The results show that the vertical temperature distributions and variations on different webs have apparent differences. Meanwhile, remarkable transverse temperature differences exist on the top plate and bottom plate. Five VTGs and four TTGs are proposed with totally different simplified profiles. The representative values of positive temperature differences in TTGs are larger than those in VTGs, as well as the absolute values of negative temperature differences. The proposed temperature gradients are significantly distinct from the recommendations in current specifications. These temperature gradients are expected to improve the stipulation about the temperature action of bridges.

Assessment of Total Pressure and Swirl Distortions in a Busemann Inlet at Mach 6

Flow distortions in high-speed inlet systems are complex, and high-performance air-breathing propulsion systems. In this paper, large eddy simulations are performed to study the total pressure and swirl distortions in a Busemann inlet at freestream Mach number 6. The on-design flow condition with both the Attack Angle and Sideslip Angle equal to zero and two off-design conditions (Attack Angle = 6 deg, Sideslip Angle = 0 deg and Attack Angle = 6 deg, Sideslip Angle = 6 deg) are considered to explore the flow characteristics inside the inlet duct as well as the distortions at the inlet exit plane. It is found that under the on-design flow condition, the shock structures and boundary layer development are nearly axisymmetric about the inlet axis. The captured freestream is compressed smoothly through inlet duct. The total pressure loss is limited primarily to within the boundary layer region, and nearly no swirling flow is introduced during the flow compression process. Under the off-design flow conditions, the shock structures inside the inlet duct become non-axisymmetric, and localized strong shock–boundary layer interactions occur. In the case of the off-design flow condition with Attack Angle = 6 deg, Sideslip Angle = 0 deg, a large flow separation zone appears owing to the incidence of a strong curved shock on the wall surface at the leeward side in the inlet duct, and the low-kinetic-energy flow contained in this flow separation zone leads to an obvious total-pressure reduction at the exit plane of inlet. Meanwhile, a large-scale swirling flow is formed at the exit plane of inlet owing to the appearance of a nonuniform transverse pressure gradient. Under the off-design conditions, a pair of vortex is observed at the exit plane of inlet. The shock wave–boundary layer interactions under the off-design conditions are stronger than those under the on-design condition, which results in more intense total pressure and swirl distortions. The averages of the fluctuating distortions are more evident than the temporal-averaged total-pressure and swirl distortions. These results show that turbulent flow fluctuations are important in determining the overall distortion level in a Busemann inlet.

Species-specific adaptations, individual condition, and life-history-induced dispersal strategies of fish larvae along a transverse gradient in a large river

Early life-history properties reveal a considerable variation among riverine fish species and may play a major role in the downstream drift of larval fish. This role, however, remains largely unexplored. Assemblages of drifting fish larvae and their condition during the peak seasons were analyzed along a cross-section from littoral to midstream zones in a free-flowing section in the middle Yangtze River. Assemblage structure and larval body injuries revealed pronounced transverse patterns. These patterns were mainly explained by hydraulic gradients. Highest species occurrence was recorded in the littoral zone. Larval abundance of most species and larval body injury rates were higher in littoral and sublittoral zones. Relative inshore and nearshore larval abundance revealed a significant negative relationship with species-specific initial body size, and varied among species. Larval abundance, body injury rate, and feeding intensity were all higher at night. Our findings revealed considerable habitat preferences of drifting larval fish and suggest important adaptations for reducing starvation- and predation risks. These factors should be considered in species conservation and river management.

A new Adomian decomposition technique for a thermal analysis forced non-Newtonian magnetic Reiner-Rivlin viscoelastic fluid flow

This paper presents a new semi-analytical method, called the Adomian Decomposition Method (ADM), as well as Finite Element Methods, to study forced Reiner-Rivlin non-Newtonian Magnetohydrodynamic (MHD) fluid motion confined between two disks. The innovation presented in this paper is the utilization of both analytical and numerical methods, namely ADM and FEM, to solve coupled linear differential equations, which enables the calculation and examination of parameters such as heat transfer and fluid velocity between the two disks by simplifying these equations. This model incorporates the magnetic field, and the system of partial differential equations (PDEs) acts as the governing equation in this study, which are then transformed into a set of non-linear ordinary differential equations (ODEs) using von Karman analog variables. The Adomian decomposition method can be used to solve ODEs that are related to boundary conditions. The main findings of this article suggest that as the dimensionless force parameter increases, the displacement of the fluid velocity decreases, as the particles collide with each other, the temperature gradient around the disks decreases inversely. Moreover, when the stress tensor increases, the heat transfer rate reaches its maximum value, and the transverse velocity gradient between different disks decreases.

Optimal bi-planar gradient coil configurations for diamond nitrogen-vacancy based diffusion-weighted NMR experiments

IntroductionDiffusion weighting in optically detected magnetic resonance experiments involving diamond nitrogen-vacancy (NV) centers can provide valuable microstructural information. Bi-planar gradient coils employed for diffusion weighting afford excellent spatial access, essential for integrating the NV-NMR components. Nevertheless, owing to the polar tilt of roughly 55^{circ } of the diamond NV center, the primary magnetic field direction must be taken into account accordingly.MethodsTo determine the most effective bi-planar gradient coil configurations, we conducted an investigation into the impact of various factors, including the square side length, surface separation, and surface orientation. This was accomplished by generating over 500 bi-planar surface configurations using automated methods.ResultsWe successfully generated and evaluated coil layouts in terms of sensitivity and field accuracy. Interestingly, inclined bi-planar orientations close to the NV–NMR setup’s requirement, showed higher sensitivity for the transverse gradient channels than horizontal or vertical orientations. We fabricated a suitable solution as a three-channel bi-planar double-layered PCB system and experimentally validated the sensitivities at 28.7 mathrm mT/m/A and 26.8 mathrm mT/m/A for the transverse G_{x} and G_{y} gradients, and 26 mathrm mT/m/A for the G_{z} gradient.DiscussionWe found that the chosen relative bi-planar tilt of 35^{circ } represents a reasonable compromise in terms of overall performance and allows for easier coil implementation with a straight, horizontal alignment within the overall experimental setup.

Emergent cerebral venous stenting: A valid treatment option for fulminant idiopathic intracranial hypertension

BackgroundFulminant idiopathic intracranial hypertension (FIIH) is characterized by rapid, severe, progressive vision loss and often treated surgically. Cerebral transverse venous stenting (CTVS) is efficacious in IIH patients, but emergent CTVS in FIIH is rarely reported. We present our experience with emergent CTVS in patients with FIIH. MethodsSince 01/2019, an institutional protocol allowed emergent CTVS in FIIH patients with bilateral transverse sinus stenosis and gradient pressure > 15 on digital subtraction angiography (DSA). We retrospectively analyzed a prospective registry of all IIH patients with details of neurological and neuro-ophthalmological assessments before and after treatment, and subjective assessments of headache and tinnitus were made pre-and post-procedure. Results259 IIH patients, including 49 who underwent CTVS, were registered. Among them, five female patients met inclusion criteria for FIIH and underwent emergent CTVS. FIIH patients were younger (18.8 ± 1.64 vs 27.7 ± 4.85, p < 0.01), mean BMI was lower (30.8 ± 10.57 vs 34.6 ± 4.3, p < 0.01), and lumbar puncture opening pressure higher (454 ± vs 361 ± 99.4, p < 0.01) than that of IIH patients. They presented with acute visual loss, severe headache, papilledema, significant bilateral transverse sinus stenosis on CT-venography, and mean dominant side gradient pressure of 26.4 ± 6.2 on DSA. CTVS was performed without significant complications, resulting in remarkable improvement in headache, optical coherence tomography, and visual fields within 1 week. At 1-year follow-up (four patients) and 6-month follow-up (1 patient), there was complete resolution of papilledema and headache, and marked improvement in visual acuity. ConclusionsIn these patients, emergent-CTVS was a safe and effective treatment option for FIIH. Further evaluation is warranted.