External Uniform Magnetic Field Research Articles

Technical review, types of imaging, part 4--magnetic resonance imaging.

Technical review, types of imaging, part 4--magnetic resonance imaging.

Analytical and numerical study of MHD natural convection in a horizontal shallow cavity with heat generation

A study is presented of two-dimensional MHD natural convection flow in an internally heated horizontal shallow cavity in the presence of an external uniform vertical magnetic field. The main feature of the present flow is a symmetric double-cell Hadley circulation with the fluid ascending in the center of the cavity and descending near the vertical cold walls. All walls are electrically insulated, with the horizontal being adiabatic and the vertical isothermal. The method of the matched asymptotic expansions is used to obtain solutions of the flow and heat transfer problem. This analysis which is valid for large cavity aspect ratios and for any magnetic field strength is particularly helpful for the inexpensive determination of the flow and heat transfer characteristics. In addition, numerical simulations are performed for a range of Hartmann, Prandtl and Rayleigh numbers in order to verify the accuracy and validity of the analytical results, to calculate the constants arising by the analytical approach and to obtain details of the flow in the vicinity of the vertical side walls. The comparison of the analytical and numerical results is very good indicating the correctness of the present analysis and its applicability.

Mechanical Properties of Magneto‐Sensitive Elastomers in a Homogeneous Magnetic Field: Theory and Experiment

SummaryThe aim of this study is to present a microscopic theory which describes the mechanical behaviour of magneto‐sensitive elastomers (MSEs) under a uniform external magnetic field. For this we use a model where magnetic particles are located on the sites of the regular rectangular lattice. We introduce a structure parameter to describe isotropic, chain‐like and plane‐like particle distributions. Our theory is based on the equation for free energy as a function of strain. The magneto‐induced deformation and the shear modulus are calculated as functions of the magnetization. It is shown that interaction between the magnetic particles results in the contraction of an elastomer along the homogeneous magnetic field. With increasing magnetic field the shear modulus for the shear deformation perpendicular to the magnetic field increases for all spatial distributions of magnetic particles. Furthermore, we have compared our result for the shear modulus to the experimental data and found good agreement.

Thermal Entanglement in the Spin-S Heisenberg XYZ Model

The thermal entanglement in the spin-S Heisenberg XYZ model is studied in detail by using the entanglement measure of negativity. The effects of spin on the thermal entanglement, the threshold temperature, the critical uniform external magnetic field, the nonuniform external magnetic field and the entanglement extremum are discussed, respectively. It is shown that with increasing spin, the entanglement will increase, and then decrease slowly. In addition, we find that entanglement will approach a constant Nc with the increase of DM interaction, the constant increases with the increase of spin, and both the threshold temperature Tc and critical uniform external magnetic field Bc will increase with the increasing spin. Thus high-spin system can inhibit the influence of the external environment better.

Behavior of nanoparticle clouds around a magnetized microsphere under magnetic and flow fields.

When a micron-sized magnetizable particle is introduced into a suspension of nanosized magnetic particles, the nanoparticles accumulate around the microparticle and form thick anisotropic clouds extended in the direction of the applied magnetic field. This phenomenon promotes colloidal stabilization of bimodal magnetic suspensions and allows efficient magnetic separation of nanoparticles used in bioanalysis and water purification. In the present work, the size and shape of nanoparticle clouds under the simultaneous action of an external uniform magnetic field and the flow have been studied in detail. In experiments, a dilute suspension of iron oxide nanoclusters (of a mean diameter of 60 nm) was pushed through a thin slit channel with the nickel microspheres (of a mean diameter of 50 μm) attached to the channel wall. The behavior of nanocluster clouds was observed in the steady state using an optical microscope. In the presence of strong enough flow, the size of the clouds monotonically decreases with increasing flow speed in both longitudinal and transverse magnetic fields. This is qualitatively explained by enhancement of hydrodynamic forces washing the nanoclusters away from the clouds. In the longitudinal field, the flow induces asymmetry of the front and the back clouds. To explain the flow and the field effects on the clouds, we have developed a simple model based on the balance of the stresses and particle fluxes on the cloud surface. This model, applied to the case of the magnetic field parallel to the flow, captures reasonably well the flow effect on the size and shape of the cloud and reveals that the only dimensionless parameter governing the cloud size is the ratio of hydrodynamic-to-magnetic forces-the Mason number. At strong magnetic interactions considered in the present work (dipolar coupling parameter α≥2), the Brownian motion seems not to affect the cloud behavior.

Rotational response of superconductors: Magnetorotational isomorphism and rotation-induced vortex lattice

The analysis of nonclassical rotational response of superfluids and superconductors was performed by Onsager [Onsager, Nuovo Cimento, Suppl. 6, 279 (1949)] and London [Superfluids (Wiley, New York, 1950)] and crucially advanced by Feynman [Prog. Low Temp. Phys. 1, 17 (1955)]. It was established that, in the thermodynamic limit, neutral superfluids rotate by forming---without any threshold---a vortex lattice. In contrast, the rotation of superconductors at angular frequency $\mathbit{\ensuremath{\Omega}}$---supported by uniform magnetic field ${\mathbf{B}}_{L}\ensuremath{\propto}\mathbit{\ensuremath{\Omega}}$ due to surface currents---is of the rigid-body type (London law). Here we show that, neglecting the centrifugal effects, the behavior of a rotating superconductor is identical to that of a superconductor placed in a uniform fictitious external magnetic field $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\mathbf{H}}=\ensuremath{-}{\mathbf{B}}_{L}$. In particular, the isomorphism immediately implies the existence of two critical rotational frequencies in type-2 superconductors.

Pair correlations in a bidisperse ferrofluid in an external magnetic field: theory and computer simulations

The pair distribution function g(r) for a ferrofluid modeled by a bidisperse system of dipolar hard spheres is calculated. The influence of an external uniform magnetic field and polydispersity on g(r) and the related structure factor is studied. The calculation is performed by diagrammatic expansion methods within the thermodynamic perturbation theory in terms of the particle number density and the interparticle dipole-dipole interaction strength. Analytical expressions are provided for the pair distribution function to within the first order in number density and the second order in dipole-dipole interaction strength. The constructed theory is compared with the results of computer (Monte Carlo) simulations to determine the range of its validity. The scattering structure factor is determined using the Fourier transform of the pair correlation function g(r) − 1. The influence of the granulometric composition and magnetic field strength on the height and position of the first peak of the structure factor that is most amenable to an experimental study is analyzed. The data obtained can serve as a basis for interpreting the experimental small-angle neutron scattering results and determining the regularities in the behavior of the structure factor, its dependence on the fractional composition of a ferrofluid, interparticle correlations, and external magnetic field.

Quark gap equation in an external magnetic field

The nonperturbative quark gap equation under the rainbow truncation and with two versions of a phenomenological one-gluon exchange interaction is studied in the presence of a uniform external magnetic field, with emphasis on the small field limit. The chiral quark condensate, magnetic moment and susceptibility are calculated and compared to recent lattice data.

The Quantum and Klein-Gordon Oscillators in a Non-commutative Complex Space and the Thermodynamic Functions

In this work we study the quantum and Klein-Gordon oscillators in a non-commutative complex space. We show that a particle described by such oscillators behaves similarly as an electron with spin in a commutative space in an external uniform magnetic field. Therefore the wave-function \(\psi (z,\bar{z} )\) takes values in C 4, spin up, spin down, particle, antiparticle, a result which is obtained by the Dirac theory. We obtain the energy levels by exact solutions. We also derive the thermodynamic functions associated to the partition function, and show that the non-commutativity effects are manifested in energy at the high temperature limit.

Linear sigma model and the formation of a charged pion condensate in the presence of an external magnetic field

We discuss the charged pion condensation phenomenon in the linear sigma model, in the presence of an external uniform magnetic field. The critical temperature is obtained as a function of the external magnetic field, assuming the transition is of second order, by considering a dilute gas at low temperature. As a result we found magnetic anti-catalysis in the Bose-Einstein condensation for lower values of the external magnetic field, and catalysis for higher values of the external magnetic field. This behavior confirms previous results with a single charged scalar field.

Hall Effects on unsteady MHD flow of a Non-Newtonian fluid through a Porous medium with uniform suction and injection

In this Paper we discussed the unsteady magneto hydro dynamic flow of an electrically conducting viscous incompressible non-Newtonian Bingham fluid through a porous medium bounded by two parallel non- conducting porous plates with heat transfer considering the hall current effects into account. The fluid is driven by a uniform pressure gradient parallel to the channel plates and the entire flow field is subjected to a uniform magnetic field of strength Ho with the normal to the boundaries in the transverse xz-plane. An external uniform magnetic field is applied perpendicular to the plates and the fluid motion is subjected to a uniform suction and injection. The lower plate is stationary and the upper plate moves with a constant velocity and the two plates are kept at different but constant temperatures. Numerical solutions are obtained for the governing momentum and energy equations taking the Joule and viscous dissipations into consideration. The effects of the Hall term, the parameter describing the non-Newtonian behaviour, and the velocity of suction and injection on both the velocity and temperature distributions is studied.

Unsteady Flow of a Dusty Conducting Fluid through porous medium between Parallel Porous Plates with Temperature Dependent Viscosity and Thermal Radiation

The purpose of present problem is to studies of effects of variable viscosity and thermal radiation on unsteady laminar flow of dusty conducting fluid between parallel porous plates through porous medium with temperature dependent viscosity. The fluid is considered as unsteady laminar flow through porous medium and acted upon by a constant pressure gradient and an external uniform magnetic field is applied perpendicular to the plates. It is assumed that the parallel plates are porous and subjected to a uniform suction from above and injection from below. It is also considered the viscosity is temperature dependent. The governing nonlinear partial differential equations are solved finite difference approximation. The results for temperature field and velocity for fluids and dust particles have been obtain numerically and displayed graphically.

Calculations of magnetoactive elastomer reactions in a uniform external magnetic field

The model of a magnetoactive elastomer (MAE) based on a system of single-domain fine particles distributed in an elastically deformable matrix is considered. The dependence of a deformation of samples with different concentrations of an ultra-dispersed magnetic material with a given value of a uniformly applied external magnetic field has been calculated and compared with experimental data. It is shown that the presence of inflection points is connected to the intrinsic characteristics of the magnetic filler. Changing the strength of the external magnetic field leads to an alteration of the total energy, and as a consequence promotes the restructuring of the particle distribution, which results in a non-uniform internal magnetic field. The theoretical analysis is found to agree with the experimental data.

227 強制外力を加えた一様磁場下の一様性MHD乱流の直接数値解析(OS13-3 乱流現象の実験と数値シミュレーション(3),OS13 乱流現象の実験と数値シミュレーション)

227 強制外力を加えた一様磁場下の一様性MHD乱流の直接数値解析(OS13-3 乱流現象の実験と数値シミュレーション(3),OS13 乱流現象の実験と数値シミュレーション)

Mechanical properties of magneto-sensitive elastomers: unification of the continuum-mechanics and microscopic theoretical approaches

A new theoretical formalism is developed for the study of the mechanical behaviour of magneto-sensitive elastomers (MSEs) under a uniform external magnetic field. This formalism allows us to combine macroscopic continuum-mechanics and microscopic approaches for complex analysis of MSEs with different shapes and with different particle distributions. It is shown that starting from a model based on an explicit discrete particle distribution one can separate the magnetic field inside the MSE into two contributions: one which depends on the shape of the sample with finite size and the other, which depends on the local spatial particle distribution. The magneto-induced deformation and the change of elastic modulus are found to be either positive or negative, their dependences on the magnetic field being determined by a non-trivial interplay between these two contributions. Mechanical properties are studied for two opposite types of coupling between the particle distribution and the magneto-induced deformation: absence of elastic coupling and presence of strong affine coupling. Predictions of a new formalism are in a qualitative agreement with existing experimental data.

LAYER MULTIPLE SCATTERING CALCULATIONS FOR NONRECIPROCAL PHOTONIC STRUCTURES

We present an extension of the layer-multiple-scattering method to photonic crystals of gyrotropic spheres in a homogeneous host medium. The efficiency of the method is demonstrated on specific examples of three-dimensional chiral structures and surfaces of crystals of plasma spheres in an external static uniform magnetic field that lack, simultaneously, time-reversal and space-inversion symmetries, and exhibit a nonreciprocal spectral response.

Stability of ion acoustic solitary waves in a magnetized plasma consisting of warm adiabatic ions and non-thermal electrons having vortex-like velocity distribution

AbstractSchamel's modified Korteweg-de Vries–Zakharov–Kuznetsov (S-ZK) equation, governing the behavior of long wavelength, weak nonlinear ion acoustic waves propagating obliquely to an external uniform static magnetic field in a plasma consisting of warm adiabatic ions and non-thermal electrons (due to the presence of fast energetic electrons) having vortex-like velocity distribution function (due to the presence of trapped electrons), immersed in a uniform (space-independent) and static (time-independent) magnetic field, admits solitary wave solutions having a sech4 profile. The higher order stability of this solitary wave solution of the S-ZK equation has been analyzed with the help of multiple-scale perturbation expansion method of Allen and Rowlands (Allen, M. A. and Rowlands, G. 1993 J. Plasma Phys. 50, 413; 1995 J. Plasma Phys. 53, 63). The growth rate of instability is obtained correct to the order k2, where k is the wave number of a long wavelength plane wave perturbation. It is found that the lowest order (at the order k) instability condition is strongly sensitive to the angle of propagation (δ) of the solitary wave with the external uniform static magnetic field, whereas at the next order (at the order k2) the solitary wave solutions of the S-ZK equation are unstable irrespective of δ. It is also found that the growth rate of instability up to the order k2 for the electrons having Boltzmann distribution is higher than that of the non-thermal electrons having vortex-like distribution for any fixed δ.

Acceleration of particles in spacetimes of black string

The particle acceleration mechanism in $S^2 \times\mathbb{R}^1$ topology, namely in the spacetime of the five-dimensional compact black string has been studied. The expression of center-of-mass energy of the colliding neutral particles near static black string has been found. The collision of charged particle moving at the innermost stable circular orbit with neutral particle coming from infinity have been considered when black string is immersed in external uniform magnetic field. It has been shown that the unlimited center-of-mass energy can be approached in case of the extremal rotation of the black string which is similar to the analogous effect in the Kerr spacetime. We have also obtained that the scattering energy of particles in the center-of-mass system can take arbitrary large values not only for extremal black string but also for nonextremal one. It has been derived that the presence of the extra dimension can, in principle, increase the upper limit of efficiency of energy extraction from the extremely rotating black string up to $203\%$ versus to $143\%$ which can be extracted from the extreme Kerr black hole.

Magnetic field expulsion from an infinite cylindrical superconductor

The solutions of the London equations for the magnetic field expulsion from superconductors are presented in this paper for the cylindrical symmetry. The result is analyzed in detail and represented numerically for the case of a uniform external magnetic field in the transverse plane. In particular, several contour plots of the magnetic energy density are depicted for the regions inside and around the superconducting area for a wide range of penetration lengths, showing how the expulsion and penetration of the magnetic field evolve with the ratio between the penetration length and the cylinder radius.

Magnetic Field Induced Chain Alignment of Ferroparticles in Magnetic Fluid

Chain alignment of suspended-ferroparticles in a carrier fluid exposed to the external magnetic field was investigated theoretically and experimentally.The results show that without the external magnetic field the ferroparticles were randomly distributed and when exposed to uniform magnetic field the turned out to be a chained-alignment pattern along the external magnetic field orientation at intensity of 11.1 kA/m.With the external uniform magnetic field intensity up to 28.6 kA/m,more ferroparticles closely aggregated to a chained-alignment pattern along the magnetic field direction.When the magnetic field intensity reached 28.7 kA/m in the gradient magnetic field,a large number of ferroparticles gathered around the coils and formed complex clusters,while a small number of ferroparticles demonstrated a chain-alignment pattern along r direction with weaker magnetic field intensity.When the magnetic field gradient was gradually increasing from 1.73 kA/m2 to 5.11 kA/m2,the cluster pattern of ferroparticles turned out to be dense along the axis.This research may enhance the applications of magnetic fluid in the fields of mechanical engineering,bioengineering and thermodynamic engineering.