Nonuniform Magnetic Field Research Articles

Electron vortex beams in nonuniform magnetic fields

We consider the quantum theory of paraxial nonrelativistic electron beams in nonuniform magnetic fields, such as the Glaser field. We find the wave function of an electron from such a beam and show that it is a joint eigenstate of two ($z$-dependent) commuting gauge-independent operators. This generalized Laguerre-Gaussian vortex beam has a phase that is shown to consist of two parts, each being proportional to the eigenvalue of one of the two conserved operators and each having different symmetries. We also describe the dynamics of the angular momentum and cross-sectional area of any mode and how a varying magnetic field can split a mode into a superposition of modes. By a suitable change in the frame of reference, all of our analysis also applies to an electron in a quantum Hall system with a time-dependent magnetic field.

Two-phase modeling of the effect of non-uniform magnetic field on mixed convection of magnetic nanofluid inside an open cavity

In the present paper, the effect of a non-uniform magnetic field on the mixed convection heat transfer of magnetic nanofluid (water-Fe3O4) flow in an open cavity is studied. Current-carrying wires are used as a magnetic field. Momentum equations and Buongiorno's two-phase model are modified by adding the Kelvin force and magnetophoresis terms, respectively. The effect of changing various parameters such as Magnetic number (0≤Mn≤5 × 107), Reynolds number (Re=200, 400, 600) and horizontal distance of wires from cavity walls on the flow pattern, nanoparticle volume fraction distribution, and the average Nusselt number have been investigated. The results showed that applying a non-uniform magnetic field on the magnetic nanofluid flow, due to the effect of Kelvin force, changed the flow pattern and created the new vortices in the fluid flow. By applying a weak magnetic field (up to Mn=5 × 106), the average Nusselt number decreased, and with increasing the magnetic number the average Nusselt number increased significantly. The highest enhancement of the average Nusselt number was obtained with 57.07%.

Influence of Hall current effect on hybrid nanofluid flow over a slender stretching sheet with zero nanoparticle flux

AbstractThe present article explores steady, incompressible, and electrically conducting viscous hybrid‐nanofluid flow through an impermeable slender stretching sheet. We have opted for water (H2O) as base fluid and two nanoparticles namely Al2O3 and graphene for the hybrid‐nanofluid. The consequence of nonuniform magnetic field and Hall current is accounted for in the flow distribution. Zero mass‐flux boundary conditions have been included here. The leading partial differential equations of the acknowledged model revise to similarity variables. Next, the subsequent equations are numerically solved by a shooting scheme based on Runge–Kutta fourth‐order procedure. The consequences of boosting flow factors on transport systems are achieved accurately through the requisite figures and charts. Concentration outlines are dual in nature when the wall‐thickness factor intensifies. The rate of heat and mass transmit augments with wall‐thickness factor.

Effect of simultaneous application of chaotic laminar flow of nanofluid and non-uniform magnetic field on the entropy generation and energetic/exergetic efficiency

Effect of simultaneous application of chaotic laminar flow of nanofluid and non-uniform magnetic field on the entropy generation and energetic/exergetic efficiency

Acoustic absorption performance of cavity-backed magnetoactive elastomer film

This work aims to investigate the acoustic absorption performance of magnetoactive elastomer (MAE) film, which consisting of carbonyl iron particles (CIPs) and polydimethylsiloxane matrix. MAE film is a kind of smart material. The magnetized CIPs can be forced to move when suffering from a non-uniform magnetic field, and finally, cause the morphology change of MAE film. The magnetic-dependent deformation of MAE film was investigated using the digital image correlation method. The result shows that the deformation is center-symmetrically distributed and increases with the growth of current. Increasing the CIPs content and the film thickness can enlarge the magnetic-dependent deformation of the MAE film. The maximum deformation of sample 4 (75 wt. % CIPs) under the current of 4 A is nearly 0.89 mm. Then, the acoustic absorption performance of cavity-backed MAE film was fully studied. For CIPs embedded cavity-backed MAE film, increasing the current can raise its maximum acoustic absorption frequency (MAAF). The MAAF of sample 4 changes from 2.33 to 2.88 kHz when the current rise from 0 to 4 A. Meanwhile, increasing the CIPs content and film thickness can effectively reduce the MAAF of cavity-backed MAE film. Moreover, reducing the film thickness can increase the maximum acoustic absorption coefficient of cavity-backed MAE film. With exceptional acoustic absorption performance, the MAE film can find extensive applications in intelligent control and acoustic absorption devices.

Influence of Variable Properties on Quadratic Convective Flow of Casson Nanofluid Past an Inclined Plane

In this study, the combined effects of thermophysical fluid properties alongside nonlinear thermal and solutal convective processes in an inclined flow region containing Casson nanofluid subjected to slip and convective boundary conditions are considered. Application of induced non-uniform magnetic field strength applied perpendicular to the flow plane and the buoyancy influences are believed responsible for the quadratic convection rate. The prime PDEs are renewed to systems of ODEs via applicable transformations and similarity variables. Assuming a series trial solution, the flow distribution results were obtained numerically by collocation approach with Legendary polynomial basis function. Validation of the numerical results plays favorably when compared with the weighted residual method (Galerkin) and the existing literature. The results reveal that; Casson fluid exhibit a solid characteristic when yield stress is more than the shear stress, pronouncement of nonlinear solutal and thermal buoyancy effects predicts the acceleration of the flow fields greatly compared to the linear model, adherence between the fluid particles and flow surface displayed retardation in shear force thus enhanced the motion of Casson fluid and diminished the energy fields, surface suspension suppressesbthe flow but energizes both temperature and nanoparticle volume fraction profiles.

Consider the nonuniformity of the electromagnetic field when calibrating thin dipole and loop antennas in electromagnetic field source based on a four-wire transmission line

The influence of nonuniformity of electric and magnetic fields on the calibration accuracy of dipole and loop antennas in field sources based on four-wire transmission lines is theoretically investigated. The paper proposes to take into account the influence of the nonuniformity of the electromagnetic field of a four-wire source when calibrating thin dipole and loop antennas using the equivalence coefficients. The use of these coefficients, taking into account the distributions of the field in the source and the current on the antenna conductors, can significantly weaken the requirement for field uniformity when calibrating antennas. For some common types of antennas, formulas are derived for the equivalence coefficients in the approximation of a four-wire source by an infinite line. Approximation formulas, simplified for engineering calculations, are obtained.

Thermal efficiency investigation of a ferrofluid-based cylindrical solar collector with a helical pipe receiver under the effect of magnetic field

This experimental study investigates the advantage of using Mn–Zn Fe2O4/water ferrofluid subjected to the effect of a non-uniform magnetic field on the thermal efficiency of a fabricated cylindrical solar collector with a receiver in the shape of the helical pipe. Using ASHRAE Standard, this study has investigated the influence of the nanoparticles volume fraction (0.0–1.0%), the mass flow rate of fluid (0.00415–0.033 kg/s), and the produced non-uniform magnetic field by a set of permanent magnets (Br = 0.0–1.2 T) on the collector' thermal efficiency. The results show that for the ferrofluid, the overall thermal efficiency of the collector is increased with the volume fraction augmentation. For the case with the flow rate of 0.033 kg/s and the volume fraction of 1.0%, the maximum collector efficiency enhancement compared with water has been 48.54%. The positive effect of applying the magnetic field predominates at lower flow rates and higher volume fractions. In the case with the flow rate of 0.00415 kg/s and the volume fraction of 1.0%, by applying the magnets with Br = 1.2 T, the collector's maximum efficiency is increased by 26.8% compared with that without a magnetic field.

Analysis of biomagnetic blood flow in a stenosed bifurcation artery amidst elastic walls

In recent years, fluid-structure interaction (FSI) analysis has generated remarkable interest of researchers in interdisciplinary sciences problems, for instance, mechanical engineering, biomedical engineering, i.e., incorporating elastic wall behaviour in human arteries. Here, in this paper, we considered incompressible Newtonian blood flow and the elastic bifurcated artery wall in a non-uniform magnetic field. The considered model of Biomagnetic Fluid Dynamics (BFD) describes both magnetization and electrical conductivity of blood. Moreover, an Arbitrary Lagrangian-Eulerian (ALE) formulation is used by two-way fluid-structure interaction coupling of the problem. Finally, for discretization, a stable finite element pair is employed to approximate the displacement, velocity and pressure spaces independently and the resulting nonlinear algebraic system is linearized by implementing the Newtons procedure. A quantitative analysis is made against Reynolds number Re and Hartmann number Ha on the bifurcated artery amidst elastic walls showing noticeable effects on recirculation. Also, the displacement field is plotted against Ha for different values of Re and their converse behaviour is observed. The decreasing behaviour for the wall shear stresses of the bifurcated artery are also drawn in context of Ha and Re. Finally, the conclusion is drawn at the end showing the significance of the elastic behaviour of artery walls.

Reconstruction of 3D MHD liquid metal velocity from measurements of electric potential on the external surface of a thick-walled pipe

Liquid metal magnetohydrodynamic pipe flow experiments have been performed in a non-uniform strong magnetic field in order to get further insight into the physics of this type of three-dimensional MHD flow and to enlarge the database for validation of numerical tools. In these experiments, in addition to measurements of the axial pressure distribution, the electric potential has been recorded at a large number of axial and circumferential positions by probes distributed around the periphery of the pipe. Since the wall of the pipe is relatively thick, the electric potential varies across the wall thickness, and surface potentials differ from those at the fluid-wall interface. However, the dense population of potential sensors on the pipe surface allows for a reconstruction of data on the fluid-wall interface, from which we can determine components of potential gradient in the fluid in a plane perpendicular to the magnetic field. The latter ones are closely related to the velocity field, because the electric potential may serve as approximate streamfunction of the flow.

Digital Microfluidics: Magnetic Transportation and Coalescence of Sessile Droplets on Hydrophobic Surfaces.

Magnetic digital microfluidics is advantageous over other existing droplet manipulation methods, which exploits magnetic forces for actuation and offers the flexibility of implementation in resource-limited point-of-care applications. This article discusses the dynamic behavior of a pair of sessile droplets on a hydrophobic surface under the presence of a permanent magnetic field. A phase field method-based solver is employed in a two-dimensional computational domain to numerically capture the dynamic evolution of the droplet interfaces, which again simultaneously solves the magnetic and flow fields. On a superhydrophobic surface (i.e., θc = 150°), the nonuniform magnetic field forces the pair of sessile droplets to move toward each other, which eventually leads to a jumping off phenomenon of the merged droplet from the solid surface after coalescence. Also, there exists a critical magnetic Bond number Bomcr, beyond which no coalescence event between droplets is observed. Moreover, on a less hydrophobic surface (θc ≤ 120°), the droplets still coalesce under a magnetic field, although the merged droplet does not experience any upward flight after coalescence. Also, the merging phenomenon at lower contact angle values (i.e., θc = 90°) appears significantly different than at higher contact angle values (i.e., θc = 120°). Additionally, if the pair of sessile droplets is dispersed to a different surrounding medium, the viscosity ratio plays a significant role in the upward flight of the merged droplet, where the coalesced droplet exhibits increased vertical migration at higher viscosity ratios.

Evaluation of separation point in a divergent channel in the presence of non-uniform magnetic field

Due to the pressure gradient increasing downstream, the boundary layer separation phenomena may occur on the divergent channel flow wall. Still, several methods have been applied to control or postponed the separation. Exerting a magnetic field on the fluid flow is one solution. In the present research, we aim to investigate the impact of magnetic fields on the separation location in a divergent channel numerically. Hence, the finite volume method using the OpenFOAM CFD toolbox is employed, and a code for 2-D divergent channel flow under a non-uniform magnetic field is developed. The obtained results have been validated in comparison with previous studies. The results are presented for a test case of the channel with divergence half-angle equals 2.5°, 30 cm length, and 2 cm inlet height. It is demonstrated when Ha increases from 0 to 2, the separation points for liquid lithium fluid flow with Re = 208.33 are postponed from 12.93 cm to 23.18 cm. Moreover, it is shown that the separation phenomena disappear entirely for the Ha value of more than 3.

Effects of the non-uniform magnetic field on the shear stress and the microstructure of magnetorheological fluid

A particle-level 3D simulation was carried out to explore the difference between the shear stress and the microstructure of magnetorheological fluid in the present of different distributions of non-uniform magnetic field. The non-uniform magnetic field used in the simulation was generated by a permanent magnetic. Three types of non-uniform magnetic field distributions with the same average magnitude were obtained by locating the simulation box at different positions above a permanent magnet. The quasiequilibrated structures were first examined before the beginning of the shear simulation. Then, the typical evolution of shear stress under non-uniform magnetic field was investigated. The shear stress curve included a sharper growth region in the beginning compared to the stress curve under the vertical uniform magnetic field. At the same shear rate, the shear stresses under non-uniform magnetic fields were different from these under the vertical uniform magnetic field. The chain-like structures inside magnetorheological fluid were responsible for the difference of the shear stresses. Specially, the chain-like structures with inclination θ>90° and azimuth φ≈90° were beneficial for improving the shear stress. The volume average magnetic field was employed to quantitatively describe the characteristics of the non-uniform magnetic field. The shear stress curve of magnetorheological fluid under the volume average magnetic field matched well with the stress curve under the corresponding non-uniform magnetic field.

Non-linear effects and electron heating dynamics in radio-frequency capacitively coupled plasmas with a non-uniform transverse magnetic field

A non-uniform transverse magnetic field is used to increase the plasma density and create an asymmetry in radio frequency capacitively coupled plasmas for plasma sputtering and plasma vapor deposition. Based on one-dimensional particle-in-cell/Monte Carlo collision simulations, the effect of the magnetic field magnitude on the non-linear behavior and the electron heating dynamics is studied for a pure helium plasma at a pressure of 30 mTorr. The results show that increasing the magnetic field magnitude can generate a more positive DC self-bias. As a result, non-linear oscillations of the electron current density and the electric field close to the grounded electrode are enhanced. An electric field reversal is induced when the powered electrode sheath collapses to balance electron and ion fluxes toward this boundary due to the strong confinement of electrons. Anomalous energetic electron beams are observed propagating from the collapsed sheath toward the plasma bulk. It is shown that such beams are reflections of the beams originating from the opposite expanding sheath based on the analysis of single particle motions. We show that energetic electron beams can be reflected by the transverse magnetic field.

Compact broadband Compton spectroscopy used for intense laser-driven gamma rays.

A compact broadband Compton spectrometer is designed to measure the continuous spectrum of gamma-ray sources driven by an intense laser. The incident gamma rays are converted into electrons in low-Z materials by Compton scattering. Produced by a pair of stepped magnets, a weaker-front-stronger-rear nonuniform magnetic field in the electron magnetic spectrometer is used to spectrally resolve the scattered electrons, leading to a broadband gamma-ray spectral coverage of 2-20MeV in a compact volume. Flat imaging-plate detectors are placed near the focused imaging points of the magnetic spectrometer to record the dispersed electrons, thereby achieving an optimal spectral resolution of 6%-13% in the energy range of 3-20MeV. The spectrometer is used successfully to measure the gamma-ray spectrum generated by the high-energy electron beams produced by a femtosecond-laser-driven wakefield.

Local quantum uncertainty in a two-qubit Heisenberg spin chain with intrinsic decoherence

Quantum correlation plays an important role in quantum information processing, for which various quantifiers have been proposed so far. In this paper, we address the dynamics of local quantum uncertainty (LQU) as a reliable quantifier of quantum correlation in a two-qubit Heisenberg spin chain in the presence of nonuniform external magnetic field and Dzyaloshinski-Moriya interaction with intrinsic decoherence. The influences of the initial states, external magnetic field strength, Dzyaloshinski-Moriya interaction strength and intrinsic decoherence rate on the dynamics of LQU have been in detail investigated. Our analytical results show that the dynamics of LQU is strongly depended on the form of initial states. For an initial correlated state, the dynamical behaviors of LQU exhibit either monotonic decay or damped oscillations with respect to time. While for an initial separable state, quantum correlation quantified by LQU can be created due to the Dzyaloshinski-Moriya interaction and Heisenberg anisotropic interaction. Besides, the relationship between LQU and l-norm coherence or concurrence is also demonstrated in the present model.

Controlling Magnon Interaction by a Nanoscale Switch.

The ability to control and tune magnetic dissipation is a key concept of emergent spintronic technologies. Magnon scattering processes constitute a major dissipation channel in nanomagnets, redefine their response to spin torque, and hold the promise for manipulating magnetic states on the quantum level. Controlling these processes in nanomagnets, while being imperative for spintronic applications, has remained difficult to achieve. Here, we propose an approach for controlling magnon scattering by a switch that generates nonuniform magnetic field at nanoscale. We provide an experimental demonstration in magnetic tunnel junction nanodevices, consisting of a free layer and a synthetic antiferromagnet. By triggering the spin-flop transition in the synthetic antiferromagnet and utilizing its stray field, magnon interaction in the free layer is toggled. The results open up avenues for tuning nonlinearities in magnetic neuromorphic applications and for engineering coherent magnon coupling in hybrid quantum information technologies.

Experiments and Numerical Implementation of a Boundary Value Problem Involving a Magnetorheological Elastomer Layer Subjected to a Nonuniform Magnetic Field

Abstract This study investigates experimentally and numerically the response of a magnetorheological elastomer (MRE) layer placed atop an electromagnetic coil. The MRE layer is deflected upon application of a current in the coil, which creates highly nonuniform magnetic fields. Isotropic and transversely isotropic layers (i.e., containing chains of magnetic particles) are tested experimentally, and the isotropic layer exhibits the largest deflection. To enhance the energetic efficiency of the model device, an iron core is introduced inside the electromagnetic coil, thereby leading to an increase in the resulting magnetic field near the center of the MRE layer. In parallel, the boundary value problem —including the MRE layer, the coil, the core (if present) and the surrounding air—is modeled numerically. For this, a magneto-mechanical, vector potential-based variational formulation is implemented in a standard three-dimensional finite element model at finite strains. For the material description, a recently proposed analytical homogenization-guided model is used to analyze the MRE in the “coil-only” configuration. It is then employed to predict the response of the layer in the “coil plus core” configuration, thus circumventing the need for a separate material characterization procedure. The proposed numerical simulation strategy provides a deeper understanding of the underlying complexity of the magnetic fields and of their interaction with the MRE layer. This study also reveals the importance of modeling the entire setup for predicting the response of MRE materials and, as a result, constitutes a step toward designing more efficient MRE-based devices.

Wetting dynamics of a sessile ferrofluid droplet on solid substrates with different wettabilities

There are several numerical approaches to define a permanent magnet in terms of mathematical equations, and each approach has progressed since its inception, but still endures some limitations on specific numerical phenomena. This study seeks to propose a novel numerical representation of a permanent magnet without incorporating its effect through boundary conditions, which overcomes the limitations of previous studies and enables us to introduce a magnetic field of desired strength at any location. A self-correcting method is modified to incorporate the magnetic field effects, while a simplified lattice Boltzmann method is utilized to solve the governing equations for flow field and interface. The validity of the proposed method is ensured by simulating some benchmark phenomena with and without the external magnetic field. This study also investigates the wetting dynamics of a sessile ferrofluid droplet deposited on solid substrates with different wettabilities. The influence of uniform and non-uniform magnetic fields on droplet spreading is discussed in detail. It is observed that for a non-uniform magnetic field in vertical direction, the ferrofluid droplet on a hydrophilic surface does not observe the spherical cap approximation unless the magnetic field strength is below saturation magnetization. Moreover, if the magnet is located above, the droplet undergoes large deformations and achieves pointy shapes with sharp tips on less wettable surfaces.

Forced convection Fe3O4/water nanofluid flow through a horizontal channel under the influence of a non-uniform magnetic field

Forced convection Fe3O4/water nanofluid flow through a horizontal channel under the influence of a non-uniform magnetic field