External Uniform Magnetic Field Research Articles

Linear Stability Analysis of Liquid Metal Flow in an Insulating Rectangular Duct under External Uniform Magnetic Field

Linear stability analysis of liquid metal flow driven by a constant pressure gradient in an insulating rectangular duct under an external uniform magnetic field was carried out. In the present analysis, since the Joule heating and induced magnetic field were neglected, the governing equations consisted of the continuity of mass, momentum equation, Ohm’s law, and conservation of electric charge. A set of linearized disturbance equations for the complex amplitude was decomposed into real and imaginary parts and solved numerically with a finite difference method using the highly simplified marker and cell (HSMAC) algorithm on a two-dimensional staggered mesh system. The difficulty of the complex eigenvalue problem was circumvented with a Newton—Raphson method during which its corresponding eigenfunction was simultaneously obtained by using an iterative procedure. The relation among the Reynolds number, the wavenumber, the growth rate, and the angular frequency was successfully obtained for a given value of the Hartmann number as well as for a direction of external uniform magnetic field.

Modulation instability of obliquely propagating ion acoustic waves in a collisionless magnetized plasma consisting of nonthermal and isothermal electrons

We have studied the modulation instability of obliquely propagating ion acoustic waves in a collisionless magnetized warm plasma consisting of warm adiabatic ions and two different species of electrons at different temperatures. We have derived a nonlinear Schr{\"o}dinger equation using the standard reductive perturbation method to describe the nonlinear amplitude modulation of ion acoustic wave satisfying the dispersion relation of ion acoustic wave propagating at an arbitrary angle to the direction of the external uniform static magnetic field. We have investigated the correspondence between two nonlinear Schr{\"o}dinger equations $-$ one describes the amplitude modulation of ion acoustic waves propagating along any arbitrary direction to the direction of the magnetic field and other describes the amplitude modulation of ion acoustic waves propagating along the direction of the magnetic field. We have derived the instability condition and the maximum growth rate of instability of the modulated ion acoustic wave. We have seen that the region of existence of maximum growth rate of instability decreases with increasing values of the magnetic field intensity whereas the region of existence of the maximum growth rate of instability increases with increasing $\cos \theta$, where $\theta$ is the angle of propagation of the ion acoustic wave with the external uniform static magnetic field. Again, the maximum growth rate of instability increases with increasing $\cos \theta$ and also this maximum growth rate of instability increases with increasing $\beta_{e}$ upto a critical value of the wave number, where $\beta_{e}$ is the parameter associated with the nonthermal distribution of hotter electron species.

Modulational Instability of Ion Acoustic Waves in a Magnetized Plasma Consisting of Isothermally Distributed Hot and Cold Electrons

Using the standard Reductive Perturbation Method a nonlinear Schr¨odinger equation is derived to study the modulational instability of small amplitude ion acoustic waves in a collisionless magnetized plasma composed of adiabatic warm ions, Maxwell-Boltzmann distribution of hot electrons as well as Maxwell-Boltzmann distribution of cold electrons, and the plasma system immersed in an external uniform static magnetic field (B0 = B0ˆz) propagating along the z-axis.The instability condition and the maximum growth rate of instability have been investigated analytically as well as numerically. We have studied the effect of each parameter of the present plasma system on the maximum growth rate of instability. In particular, it is found that the maximum growth rate of instability decreases with the increasing value of the ion cyclotron frequency with some set of values of the parameters associated with the present plasma system. Again, we have seen that the instability region decreases with the increasing value of the ion cyclotron frequency.

The influence of magnetic field on the fluid flow in the entrance region of channels: analytical/numerical solution

The present paper investigates the steady laminar development flow of a Newtonian incompressible electrically conducting fluid in a plane channel subjected to a uniform transverse external magnetic field. In order to solve the problem, the analytical integral momentum method and numerical finite volume method (FVM) are conducted. In integral momentum solution, the Pohlhausen’s fourth-degree velocity profile is assumed. Applying the boundary conditions leads to the appearance of a new dimensionless parameter named auxiliary parameter (AP). It is shown that the value of AP depends on the magnitude of magnetic intensity and the pressure gradient. For each specified value of AP, distinct velocity distribution and as a result, a correlation for computing the magnetic development length is obtained. Besides, the FVM is applied to solve the same problem and the correlation for estimating the magnetic development length is obtained. It can be concluded that for a particular engineering application of magnetohydrodynamics channel flows, the associated value of AP can be determined by the numerical investigation. The results of numerical solution are compared with the analytical results and it can be concluded that the results for AP = − 6 are in agreement with the numerical results. Further, the effect of Hartmann and Reynolds number on the magnetic development length and Lorentz force are illustrated. It was shown that as the Hartmann number increases, the value of development length and Lorentz force are reduced.

Neutral and charged pion properties under strong magnetic fields in the NJL model

In the framework of the Nambu--Jona-Lasino (NJL) model, we study the effect of an intense external uniform magnetic field on neutral and charged pion masses and decay form factors. In particular, the treatment of charged pions is carried out on the basis of the Ritus eigenfunction approach to magnetized relativistic systems. Our analysis shows that in the presence of the magnetic field three and four nonvanishing pion-to-vacuum hadronic form factors can be obtained for the case of the neutral and charged pions, respectively. As expected, it is seen that for nonzero magnetic field the $\pi^0$ meson can still be treated as a pseudo Nambu-Goldstone boson, and consequently the corresponding form factors are shown to satisfy various chiral relations. For definite parametrizations of the model, numerical results for $\pi^0$ and $\pi^\pm$ masses and decay constants are obtained and compared with previous calculations given in the literature.

Simulating chiral anomalies with spin dynamics

Considering that the chiral kinetic equations of motion (CEOM) can be derived from the spin kinetic equations of motion (SEOM) for massless particles with approximations, we simulate the chiral anomalies by using the latter in a box system with the periodic boundary condition under a uniform external magnetic field. We found that the chiral magnetic effect is weaker while the damping of the chiral magnetic wave is stronger from the SEOM compared with that from the CEOM. In addition, effects induced by chiral anomalies from the SEOM are less sensitive to the decay of the magnetic field than from the CEOM due to the spin relaxation process.

Multiphysics modeling and simulation of fluid‐saturated porous ferrogels at finite strains

AbstractOver the last years there has been a growing interest in the study of the behavior of field‐responsive or so called smart materials. Porous ferrogels are a class of these materials consisting of a porous polymeric matrix with dispersed micro‐ or nano‐sized ferromagnetic particles [1–3]. Due to their ability to exhibit large deformations and alter their effective material characteristics upon external magnetic stimulation, these materials are interesting for a wide range of applications in biomedical engineering, microfluidics and other innovative fields of research. The magneto‐poro‐mechanical response of porous ferrogels is a complex phenomenon that spans over multiple length‐scales and essentially depends on (i) the constitutive behavior of the individual components, (ii) their morphology and microstructural arrangement and (iii) the macroscopic shape of the specimen. In this contribution a theoretical and computational framework for the modeling of isotropic porous ferrogels at the macroscale is presented. Within this modeling approach the porous ferrogel is treated as a homogeneous continuum, whereat its complex microstructure is not resolved explicitly. A prototypical isotropic constitutive model is formulated in a conventional enthalpy‐based setting. Numerical examples show the the crucial impact of the macroscopic specimen shape on the macroscopic deformation response in an uniform external magnetic field.

Electron Energy Gain in laser ion channel affected by the space-charge potential and helical wiggler and external uniform magnetic fields

Electron Energy Gain in laser ion channel affected by the space-charge potential and helical wiggler and external uniform magnetic fields

Frequency of Formation of Vapor Bubbles in a Two-Layer Medium of Magnetic and Nonmagnetic Liquid

A method for measuring the frequency of formation of vapor bubbles formation during the boiling of opaque liquids using a two-layer system (with opaque and transparent media) is proposed and implemented. An experimental device has been developed, and experiments have been carried out to determine the frequency of formation of vapor bubbles during the boiling of opaque liquids. The frequency of the formation of the vapor bubbles is measured in the boiling of a two-layer medium of magnetic and nonmagnetic liquids on a horizontal surface with a point heat source in a uniform external magnetic field. It has been found that with an increase in a uniform constant magnetic field, the frequency of the formation of vapor bubbles in the boiling of a two-layer medium of magnetic and nonmagnetic liquids on an unlimited horizontal plate decreases, whereas the temperature of the heat-releasing surface, at which the vaporization process begins, increases for the samples of magnetic liquids with a high magnetic phase content.

Tailoring the optical properties of quantum ring irradiated by THz laser

ABSTRACTThis study set out to introduce the optical nonlinearities of a intense laser-driven two-dimensional quantum ring subjected to external uniform magnetic field. Inclusion of the effect of non-resonant, high-frequency THz laser radiation is realised by the laser-dressed potential, whereas the total optical absorption coefficients, total relative refractive index changes and third-harmonic generation (THG) coefficient are inferred from the scheme of compact density-matrix formalism and iterative method. Our results reveal that the laser field smoothens the antidot region of the ring that consequences with remarkable modifications in the electronic structure and correspondingly the optical response of the system. We have found that the height and location of the resonant peaks of total optical absorption coefficient, total relative refractive index changes and THG coefficient can be controlled by the externally applied fields and system parameters. The tunability of the intersubband transitions could be beneficial for practical device application relying on the electronic optical transitions.

Ultrahigh differential mobility and velocity of Néel domain walls in spin valves with planar-transverse polarizers and perpendicularly injected small currents

Transverse domain wall (TDW) dynamics in long and narrow spin valves with perpendicular current injection is theoretically investigated. We demonstrate that stable traveling-wave motion of TDWs with finite velocity survives for strong enough planar-transverse polarizers. For typical ferromagnetic materials (for example, Co) and achievable spin polarization ($P=0.6$), TDWs acquire a velocity of $10^3$ m/s under a current density below $10^7$ $\mathrm{A/cm^2}$. This efficiency is comparable with that of perpendicular polarizers. More importantly, in this case the wall has ultra-high "differential mobility" around the onset of stable wall excitation. Our results open new possibilities for developing magnetic nanodevices based on TDW propagation with low energy consumption. Also, analytics for parallel and perpendicular polarizers perfectly explains existing simulation findings. Finally, further boosting of TDWs by external uniform transverse magnetic fields is investigated and turns out to be efficient.

Method of Measuring Deformations of Magnetoactive Elastomers under the Action of Magnetic Fields

Magnetic deformation is a change in the size and shape of a sample under the action of a uniform external magnetic field. The study of this effect in various materials provides deep understanding of the nature of magnetic and mechanical interactions. Moreover, magnetic deformation is of great interest from an engineering point of view for designing new devices. In magnetoactive elastomers containing magnetic microparticles in the polymer matrix, a giant deformation is detected under the action of an external magnetic field. The generally accepted methods for measuring magnetic deformation in magnetoactive soft materials are now practically absent. The article describes the installation for the study of the magnetomechanical characteristics of magnetoactive elastomers and demonstrates its experimental capabilities. The installation allows to measure deformations in the range from 0 to 12.5 mm with a resolution of 1 micron. The deformation curves obtained using these installations are required for developing actuators and sensors based on magnetoactive elastomers, and also for improving their manufacturing technologies.

Experimental Validation of Theoretical Expressions on Additional AC Loss of Two-Strand Parallel Conductors Composed of REBa2Cu3O<italic> <sub>y</sub> </italic> Tapes via Pick-Up-Coil Method

By a pick-up-coil method, the additional ac loss due to the formation of two-strand parallel conductors consisting of REBa2Cu3O y (REBCO, RE = rare earth) superconducting tapes was experimentally investigated. We recently improved the theoretical expression of the additional ac loss assuming that the voltage–current characteristics are expressed by the n -value model. The objective of this study is to prove the validity of the improved theoretical expression by experiment. The two insulated REBCO strands were cowound into a one-layer solenoidal coil. A uniform external magnetic field was applied in the axial direction of the sample coil. The additional ac losses were measured by the pick-up-coil method in sinusoidal ac magnetic fields whose maximum amplitude and frequency are 0.1 T and 10 Hz, respectively. The observed additional ac losses corresponded with the theoretical prediction. Therefore, it was concluded that the improved theoretical expression is more accurate than the previous one. At high-frequency regions, the observed additional ac losses slightly deviated from the theoretical predictions.

Hartmann and Reynolds Numbers Effects in the Newtonian Blood Flow of a Bifurcated Artery with an Overlapping Stenosis

Abstract Blood flow through a bifurcated artery with the presence of an overlapping stenosis located at parent’s arterial lumen under the action of a uniform external magnetic field is studied in this paper. Blood is treated as an electrically conducting fluid which exhibits the Magnetohydrodynamics principle and it is characterized by a Newtonian fluid model. The governing equations are discretized using a stabilization technique of finite element known as Galerkin least-squares. The maximum velocity and pressure drop evaluated in this present study are compared with the results found in previous literature and COMSOL Multiphysics. The solutions found in a satisfactory agreement, thus verify the source code is working properly. The effects of dimensionless parameters of Hartmann and Reynolds numbers in the fluid’s velocity and pressure are examined in details with further scientific discussions.

Subradiance-protected excitation transport

We explore excitation transport within a one-dimensional chain of atoms where the atomic transition dipoles are coupled to the free radiation field. When the atoms are separated by distances smaller or comparable to the wavelength of the transition, the exchange of virtual photons leads to the transport of the excitation through the lattice. Even though this is a strongly dissipative system, we find that the transport is subradiant, that is, the excitation lifetime is orders of magnitude longer than the one of an individual atom. In particular, we show that a subspace of the spectrum is formed by subradiant states with a linear dispersion relation, which allows for the dispersionless transport of wave packets over long distances with virtually zero decay rate. Moreover, the group velocity and direction of the transport can be controlled via an external uniform magnetic field while preserving its subradiant character. The simplicity and versatility of this system, together with the robustness of subradiance against disorder, makes it relevant for a range of applications such as lossless energy transport and long-time light storage.

Uniaxial tensile behavior of aligned steel fibre reinforced cementitious composites

By applying an external uniform magnetic field to a fresh cement mixture during casting, an aligned steel fibre reinforced cementitious composites (ASFRC) was prepared. This investigation compares the performance of ASFRC with its counterpart—ordinary steel fibre reinforced cementitious composite (SFRC) containing randomly distributed steel fibres. First, the orientation of the steel fibres in ASFRC and SFRC specimens was examined using X-ray computed tomography analysis; this confirmed that the steel fibres were effectively aligned in the ASFRC. Then, uniaxial tensile tests were performed to allow a comparison of the uniaxial tensile stress–strain curves of the ASFRC and SFRC; and to determine the advantages, if any of ASFRC over SFRC in terms of uniaxial tensile strength (fUtu), ultimate strain (εUtu) and energy dissipation (Gf-A). The uniaxial tensile test results were also used to show that, if the tensile strength of ASFRC is equal to that of SFRC (actually slightly exceeding) using the aligned steel fibre technology, the dosage of steel fibres can be reduced at least 40%. It was also found that the alignment of the steel fibres affects the strain-hardening and multiple cracking behavior of the composites during uniaxial tension testing. Finally, the multiple cracking behavior of the composites was analyzed using a digital image correlation method. These results show that ASFRC exhibits a multiple cracking pattern at a much lower fibre content compared to SFRC.

Angular momentum of open quantum systems in external magnetic field

The non-Markovian dynamics of a charged harmonic oscillator linearly coupled to a neutral bosonic heat bath is investigated in external uniform magnetic field. The analytical expression is derived for the asymptotic angular momentum. The orbital diamagnetism of quantum system in a dissipative environment is studied.

Chiral mixed phase in disordered 3d Heisenberg models under magnetic fields

Using Monte Carlo simulations, we studied the effect of uniform external longitudinal magnetic field on the 3d isotropic Heisenberg disordered model. Simulations are performed for a variety of fields. Our results confirm that the mixed chiral phase must belong to the same class of universality with those of zero fields and with same group of critical exponent.

Variations of the Static Contact Angle of Ferrofluid Droplets on Solid Horizontal Surfaces in External Uniform Magnetic Fields.

The application of external uniform magnetic fields to ferrofluid droplets affects their magnetic order at the nanoscale as well as their shape at the macroscale, thus changing their contact angle with the surface. In this work, the effects of external uniform magnetic fields on the contact angles between different oil-based ferrofluid droplets and a handful of horizontal surfaces of varying wettability were studied. The contact angle is no longer constant around the ferrofluid droplet; rather, it varies in a rich yet predictable way. Droplets dispensed in the presence of the magnetic field on oleophobic surfaces adjust such that the contact angle increases at the front and back ends and decreases at the two perpendicular positions. The opposite behavior is reported for ferrofluid droplets on oleophilic surfaces. These direction-dependent changes in the contact angle can have a significant impact on the behavior of ferrofluid droplets on gradient surfaces where they can either diminish or enhance the surface tension gradient. Our work is fundamentally relevant to potential applications involving the controlled movement of ferrofluid droplets on surfaces like the lab-on-a-chip under the combined effects of a magnetic field and either a surface tension gradient or an electric field (i.e., electrowetting). It is important to understand how the two effects interact for the optimal utilization of these effects in future applications.

The magnetic properties of GaAs parabolic quantum dot in the presence of donor impurity, magnetic and electric fields

The magnetization and the magnetic susceptibility quantities of a single electron moving in a two-dimensional (2D) parabolic quantum dot are studied under the influence of external uniform electric and magnetic fields, in the presence of a donor impurity. The Hamiltonian was solved using shifted 1/N expansion method within the effective mass approximation. The results have been displayed as a function of physical parameters: confinement strength ω0, magnetic field strength ωc, temperature T and electric field strength F.