Constant Axial Magnetic Field Research Articles

Numerical simulations of turbulence in prominence threads induced by torsional oscillations

Context. Threads are the main constituents of prominences. They are dynamic structures that display oscillations, usually interpreted as magnetohydrodynamic (MHD) waves. Moreover, instabilities such as the Kelvin–Helmholtz instability (KHI) have also been reported in prominences. Both waves and instabilities may affect the thermodynamic state of the threads. Aims. We investigate the triggering of turbulence in prominence threads caused by the nonlinear evolution of standing torsional Alfvén waves. We study the heating in the partially ionized prominence plasma as well as possible observational signatures of this dynamics. Methods. We modeled a prominence thread as a radially and longitudinally nonuniform cylindrical flux tube with a constant axial magnetic field embedded in a much lighter and hotter coronal environment. We perturbed the flux tube with the longitudinally fundamental mode of standing torsional Alfvén waves. We numerically solved the three-dimensional (3D) MHD equations to study the temporal evolution in both ideal and dissipative scenarios. In addition, we performed forward modeling to calculate the synthetic Hα imaging. Results. The standing torsional Alfvén waves undergo phase-mixing owing to the radially nonuniform density. The phase-mixing generates azimuthal shear flows, which eventually trigger the KHI and, subsequently, turbulence. When nonideal effects are included, the obtained plasma heating is very localized in an annulus region at the thread boundary and does not increase the temperature in the cool core. Instead, the average temperature in the thread decreases owing to the mixing of internal and external plasmas. In the synthetic observations, first we observe periodic pulsations in the Hα intensity caused by the integration of the phase-mixing flows along the line of sight. Later, fine strands that may be associated with the KHI vortices are seen in the synthetic Hα images. Conclusions. Turbulence can be generated by standing torsional Alfvén waves in prominence threads after the triggering of the KHI, although this mechanism is not enough to heat such structures. Both the phase-mixing stage and the turbulent stage of the simulated dynamics could be discernible in high-resolution Hα observations.

Goufo-Caputo fractional viscoelastic photothermal model of an unbounded semiconductor material with a cylindrical cavity

In order to describe the photothermal dynamic behavior of viscoelastic semiconductor materials, the authors of this work propose a fractional modification of the Kelvin-Voigt type. The generalized Mittag-Leffler function method with two parameters was utilized as the non-local, nonsingular kernel of the Goufo-Caputo fractional operator, which is similar to the AB fractional derivative. Also, a new concept of photovoltaic heat transfer has been developed to describe the process of photothermal heating in semiconductors, in which light energy is converted into thermal energy by absorption. This unique model is grounded in the theoretical framework of the Moore-Gibson-Thompson (MGT) equation, and it offers additional insights into the underlying process of light sensitive heat transfer, as well as the intricate interaction between heating signals, elasticity, and plasma. As a direct application of the proposed model, the photothermal interactions that occur when an infinitely rotating semiconductor material with a cylindrical hole is subjected to a laser thermal flux and immersed in a constant axial magnetic field have been investigated. These findings may have implications for developing optically absorbent nanostructures and their use in photothermal energy generation. For this reason, the comparison of modified photoelasticity models with spherical derivatives and classical models applied to semiconducting materials seems interesting.

Transition to turbulence in nonuniform coronal loops driven by torsional Alfvén waves

Both observations and numerical simulations suggest that Alfvénic waves may carry sufficient energy to sustain the hot temperatures of the solar atmospheric plasma. However, the thermalization of wave energy is inefficient unless very short spatial scales are considered. Phase mixing is a mechanism that can take energy down to dissipation lengths, but it operates over too long a timescale. Here, we study how turbulence, driven by the nonlinear evolution of phase-mixed torsional Alfvén waves in coronal loops, is able to take wave energy down to the dissipative scales much faster than the theory of linear phase mixing predicts. We consider a simple model of a transversely nonuniform cylindrical flux tube with a constant axial magnetic field. The flux tube is perturbed by the fundamental mode of standing torsional Alfvén waves. We solved the three-dimensional ideal magnetohydrodynamics equations numerically to study the temporal evolution. Initially, torsional Alfvén waves undergo the process of phase mixing because of the transverse variation of density. After only few periods of torsional waves, azimuthal shear flows generated by phase mixing eventually trigger the Kelvin-Helmholtz instability (KHi), and the flux tube is subsequently driven to a turbulent state. Turbulence is very anisotropic and develops transversely only to the background magnetic field. After the onset of turbulence, the effective Reynolds number decreases in the flux tube much faster than in the initial linear stage governed by phase mixing alone. We conclude that the nonlinear evolution of torsional Alfvén waves, and the associated KHi, is a viable mechanism for the onset of turbulence in coronal loops. Turbulence can significantly speed up the generation of small scales. Enhanced deposition rates of wave energy into the coronal plasma are therefore expected.

Numerical study on arc-droplet coupled behavior in magnetic field controlled GMAW process

The metal transfer behavior is one of the most pivotal links of gas metal arc welding (GMAW) for improving the welding quality. Arc impacts the current path and heat input on the droplet surface, in reverse, the metal droplet detachment makes the arc to flicker. It is hard to make the metal transfer process clear with only experimental methods since the limited measuring space and high temperature in the welding region. In this work, with the help of computational fluid dynamics software ANSYS FLUENT, a magnetohydrodynamic model including argon arc plasma and steel wire and the droplet is constructed by using two independent governing equations of gas and metal phase for the mass, momentum, and energy balance. Comparing the magnetic field controlled GMAW with conventional GMAW, the temperature field, current density distribution, electric potential field, static pressure field and also metal vapor mass fraction distribution are highlighted. It is shown that the metal transfer frequency decreases and droplet diameter increases with 180 A welding current when applying an external constant axial magnetic field 0.002 T. So, the arc length decreases before the detaching moment, the current density expands in the arc region and the voltage decreases. The centrifugal movement impedes the downward flow and transfers heat from the droplet center to the outside region.

Isothermal Motion of Hydromagnetic Cylindrical Shock Waves in Self Gravitating Rotating Gas

In this paper the Chisnell2 Chester1 Whitham3 (CCW1-3) method has been used to study the isothermal propagation of cylindrical hydromagnetic diverging shock waves in self gravitating rotating gas in presence of constant axial magnetic field. Assuming density decreasing atmosphere and exploring magnetic field as weak and strong, the analytical expressions have been derived for both weak and strong shocks and finally the effects of overtaking disturbances behind the flow on the motion of shock have also been included

Propagation of singular surfaces in a non-ideal gas with axial magnetic field

In this paper, a system of equations describing the motion of a one-dimensional, cylindrically symmetrical, inviscid, non-ideal gas with a constant axial magnetic field is considered and the singular surface theory is used to study different modes of wave propagation and its culmination into a shock wave. The transport equation for the jump in the velocity gradient is derived and solved numerically to study the affects of the ratio of specific heat, non ideal parameter and initial magnetic fields on jump in the velocity gradient.

Higher radial modes of azimuthal surface waves in magnetoactive cylindrical plasma waveguides

Azimuthal surface waves are eigenmodes of cylindrical plasma–dielectric–metal structures both in the presence of and without an axial static magnetic field. They are actively studied due to possible applications in plasma electronics, nanotechnologies and biomedical diagnostics. Higher radial modes are known to propagate at higher frequencies and shorter wavelengths compared to those of the zeroth mode, a feature which is of interest for practical applications. To gain the advantage of the excitation of higher radial modes of azimuthal surface waves one has first to know their dispersion properties. This paper generalizes the results of earlier papers by including a static axial magnetic field and considering the higher radial modes. The presence of the constant axial magnetic field removes the degeneracy in the wave spectrum with respect to the sign of the azimuthal wavenumber.

Simulated evolution of three-dimensional magnetic nulls leading to generation of cylindrically-shaped current sheets

The performed magnetohydrodynamic simulation examines the importance of magnetofluid evolution, which naturally leads to current sheets in the presence of three-dimensional (3D) magnetic nulls. The initial magnetic field is constructed by superposing a 3D force-free field on a constant axial magnetic field. The initial field supports 3D magnetic nulls having the classical spine axis and the dome-shaped fan surface and exerts non-zero Lorentz force on the magnetofluid. Importantly, the simulation identifies the development of current sheets near the 3D magnetic nulls. The morphology of the current sheets is similar to a cylindrical surface where the surface encloses the spine axis. The development is because of favorable deformation of magnetic field lines constituting the dome-shaped fan surface. The deformation of field lines is found to be caused by the flow generated by magnetic reconnections at current sheets which are located away from the cylindrically shaped current sheets.

Spontaneous Core Rotation in Ferrofluid Pipe Flow.

Ferrofluid flow along a tube of radius R in a constant axial magnetic field is revisited. Our analytical solution and numerical simulations predict a transition from an initially axial flow to a steady swirling one. The swirl dynamo arises above some critical pressure drop and magnetic field strength. The new flow pattern consists of two phases of different symmetry: The flow in the core resembles Poiseuille flow in a rotating tube of the radius r_{*}<R, where each fluid element moves along a screw path, and the annular layer of the thickness R-r_{*}, where the flow remains purely axial. These phases are separated by a thin domain wall. The swirl appearance is accompanied with a sharp increase in the flow rate that might serve for the detection of the swirling instability.

Sensitivity analysis for MHD effects and inclination angles on natural convection heat transfer and entropy generation of Al2O3-water nanofluid in square cavity by Response Surface Methodology

The natural convection heat transfer and entropy generation of Al2O3-water nanofluid, in a square cavity with inclination angle θ and the presence of a constant axial magnetic field B0 are examined in this paper. The governing equations are solved numerically by finite volume method. Also an effective parameters analysis was performed by using of the Response Surface Methodology (RSM). The effects of the Rayleigh number (103, 104, 105 and 106), Hartmann number (0, 10, 30 and 50) and also inclination angles (0°, 30°, 60° and 90°) are investigated. It is observed that the mean Nusselt number and the total entropy generation increase when the Rayleigh number increases. It is also found that, regardless of the Ha parameter, by increasing of the inclination angles, the mean Nusselt number and entropy generation rate increase until inclination angle 30° and then they decrease. Also, for low Ra numbers, by increasing the Ha parameter, the mean Nusselt number increases until Ha=10 and then decreases. The analysis showed that the sensitivity of the Nusselt number and the entropy generation to Ha parameter was too small, and as a result it was negligible. Also, the sensitivity of the mean Nusselt number and the entropy generation to inclination angle, θ, increases by increasing of this angle. It is also observed that the mean Nusselt number and the entropy generation were more sensitive to the inclination angle θ than the Ha parameter.

Magneto-absorption effects in magnetic-field assisted laser ablation of silicon by UV nanosecond pulses

A constant magnetic field can significantly improve the quality and speed of ablation by nanosecond laser pulses. These improvements are usually attributed to the confinement of laser-produced plasma by the magnetic field and specific propagation effects in the magnetized plasma. Here we report a strong influence of constant axial magnetic field on the ablation of silicon by 20-ns laser pulses at wavelength 355 nm, which results in an increase of ablation depth by a factor of 1.3 to 69 depending on laser parameters and magnitude of the magnetic field. The traditional plasma effects do not explain this result, and magneto-absorption of silicon is proposed as one of the major mechanisms of the significant enhancement of ablation.

Combined Effects of Magnetic Field and Thermal Radiation on Fluid Flow and Heat Transfer of Mixed Convection in a Vertical Cylindrical Annulus

Magnetohydrodynamic (MHD, also for magnetohydrodynamics) mixed convection of electrically conducting and radiative participating fluid is studied in a differentially heated vertical annulus. The outer cylinder is stationary, and the inner cylinder is rotating at a constant angular speed around its axis. The temperature difference between the two cylindrical walls creates buoyancy force, due to the density variation. A constant axial magnetic field is also imposed to resist the fluid motion. The nonlinear integro-differential equation, which characterizes the radiation transfer, is solved by the discrete ordinates method (DOM). The MHD equations, which describe the magnetic and transport phenomena, are solved by the collocation spectral method (CSM). Detailed numerical results of heat transfer rate, velocity, and temperature fields are presented for 0≤Ha≤100, 0.1≤τL≤10, 0≤ω≤1, and 0.2≤εW≤1. The computational results reveal that the fluid flow and heat transfer are effectively suppressed by the magnetic field as expected. Substantial changes occur in flow patterns as well as in isotherms, when the optical thickness and emissivity of the walls vary in the specified ranges. However, the flow structure and the temperature distribution change slightly when the scattering albedo increases from 0 to 0.5, but a substantial change is observed when it increases to 1.

EFFECTS OF AXIAL MAGNETIC FIELD AND THERMAL CONVECTION ON A COUNTERROTATING VON KARMAN FLOW

The effects of thermal convection and of a constant axial magnetic field on a von Karman flow driven by the exact counter-rotation of two lids are investigated in a vertical cylinder of aspect ratio Gamma(= height/radius) = 2 at a fixed Reynolds number Re(= Omega R-2/v) = 300. Direct numerical simulations are performed when varying separately the Rayleigh and Hartmann numbers in the range [0, 1800] and [0, 20], respectively, in the limit of the Boussinesq approximation and of a small magnetic Reynolds numbers, Re-m << 1. Without a magnetic field, the base flow symmetries of the von Karman flow are broken by thermal convection that becomes dominant in the range of Ra [500, 1000]. Three-dimensional solutions are characterized by the occurrence of a steady, m = 1, azimuthal mode exhibiting a cat's eye vortex in the circumferential plane. When increasing the Rayleigh number in the range [500, 1000], the vortex pulsates in an oscillatory manner, due to variations of the flow intensity. Otherwise, increasing the axial magnetic field intensity stabilizes the flow, and the oscillatory motion can be inhibited. Numerical solutions show that the critical Rayleigh number for transition increases linearly with the Hartmann number. Finally, results show that when varying the Rayleigh number, the structure of the electric potential can be strongly modified by thermal convection. Such an observation suggests new induction mechanisms in the case of small nonzero values of the magnetic Reynolds number.

Axial Magnetic Effect and Chiral Vortical Effect with free lattice chiral fermions

Following the recent work by Braguta et al., we perform an indirect calculation of the chiral vortical conductivity σV with free lattice overlap fermions by measuring the response of the energy flow to the constant axial magnetic field. We find that the measurements are contaminated by very large finite-volume artifacts. However, the continuum result for finite- temperature contribution to the chiral vortical conductivity σV = T2/12 is still reproduced in the limit of infinite volume at fixed temperature.

Converging cylindrical shocks in ideal magnetohydrodynamics

We consider a cylindrically symmetrical shock converging onto an axis within the framework of ideal, compressible-gas non-dissipative magnetohydrodynamics (MHD). In cylindrical polar co-ordinates we restrict attention to either constant axial magnetic field or to the azimuthal but singular magnetic field produced by a line current on the axis. Under the constraint of zero normal magnetic field and zero tangential fluid speed at the shock, a set of restricted shock-jump conditions are obtained as functions of the shock Mach number, defined as the ratio of the local shock speed to the unique magnetohydrodynamic wave speed ahead of the shock, and also of a parameter measuring the local strength of the magnetic field. For the line current case, two approaches are explored and the results compared in detail. The first is geometrical shock-dynamics where the restricted shock-jump conditions are applied directly to the equation on the characteristic entering the shock from behind. This gives an ordinary-differential equation for the shock Mach number as a function of radius which is integrated numerically to provide profiles of the shock implosion. Also, analytic, asymptotic results are obtained for the shock trajectory at small radius. The second approach is direct numerical solution of the radially symmetric MHD equations using a shock-capturing method. For the axial magnetic field case the shock implosion is of the Guderley power-law type with exponent that is not affected by the presence of a finite magnetic field. For the axial current case, however, the presence of a tangential magnetic field ahead of the shock with strength inversely proportional to radius introduces a length scale \documentclass[12pt]{minimal}\begin{document}$R=\sqrt{\mu _0/p_0}\,I/(2\,\pi )$\end{document}R=μ0/p0I/(2π) where I is the current, μ0 is the permeability, and p0 is the pressure ahead of the shock. For shocks initiated at r ≫ R, shock convergence is first accompanied by shock strengthening as for the strictly gas-dynamic implosion. The diverging magnetic field then slows the shock Mach number growth producing a maximum followed by monotonic reduction towards magnetosonic conditions, even as the shock accelerates toward the axis. A parameter space of initial shock Mach number at a given radius is explored and the implications of the present results for inertial confinement fusion are discussed.

Cathode Spots Dynamic in the Initial Expansion Stage of High-Current Triggered Vacuum Arc and the Influence of Axial Magnetic Field

Cathode spots (CSs) dynamics in the initial expansion stage of high-current triggered vacuum arc and the influence of axial magnetic field (AMF) was investigated experimentally. Experiments were conducted with butt contacts in a demountable vacuum chamber. Images of CSs were photographed with a high-speed digital camera with exposure time of 2 \(\mu \) s. The uniform and constant AMF in the intercontacts region was supplied by external magnetic field coil. In high-current triggered vacuum arc, CSs expanded across the cathode surface in basically a ring-shaped pattern. Experimental results showed that the expansion ring consisted of group CSs (GCSs) and individual CSs at the beginning of the expansion process. The GCS moved outward faster than individual CS. Research results indicated that an AMF could reduce the tendency of formation of GCS. The effect of local plasma and global plasma, which could be significantly influenced by AMF, on the motion pattern and distribution of CSs in the initial expansion stage was investigated.

Embedded ferromagnetic microwires for monitoring tensile stress in polymeric materials

Considerable efforts have been made to develop testing non-destructive methods for polymer composite materials. We would like to introduce researchers in the field of smart materials to a new method of monitoring internal stresses. The method can be classified as an embedded sensing technique, where the sensing element is a glass-coated ferromagnetic microwire with a specific magnetic anisotropy. With a diameter 10–100μm, the microwire impedance acts as the controlled parameter which is monitored for a weak alternating current (AC) in the MHz range. The microwire impedance becomes stress sensitive in the presence of a weak constant axial bias magnetic field. This external parameter allows the impedance stress sensitivity to be easily tuned. In addition, a local bias field may also allow the reconstruction of stress profile when it is scanned along the microwire. The experimental results are analysed using simple magnetostatic and impedance models.

The Combined Influence of Contact Gap and Axial Magnetic Field on the Expansion Speed of Cathode Spots in High-Current Triggered Vacuum Arc

The expansion speed of cathode spots (CSs) in the initial expansion stage of high-current triggered vacuum arc was investigated experimentally. Experiments were conducted with butt contacts in a demountable vacuum chamber. Images of CSs were photographed with a high-speed digital camera with exposure time of 2 μs. The uniform constant axial magnetic field (AMF) was supplied by an external magnetic field coil. The combined influence of contact gap of 2-10 mm and AMF of 0-70 mT on CSs expansion speed in the initial expansion stage was mainly investigated. Investigation results indicated that the variation of contact gap could influence the CSs expansion speed by changing the plasma density inside the cathode arc root in two ways: loss of arc plasma and constriction of arc column. AMF could weaken these two effects caused by the variation of contact gap. In addition, experimental results indicated that there existed a critical current for a given AMF. If the arc current peak was less than the critical current, the given AMF could inhibit the arc column constriction effectively both at contact gap of 8 and 10 mm, which could maintain the dependence that CSs expansion speed increased with contact gap; otherwise, the constriction of arc column played a major role in the influence on CSs expansion speed. In this case, the CSs expansion speed was lower at contact gap of 10 mm than that at 8 mm.

Study on hydro-magnetic capillary instability with mass transfer through porous media

The linear analysis of hydro-magnetic capillary instability of a cylindrical interface between two viscous and magnetic fluids in a fully saturated porous medium has been carried out, when the fluids are subjected to a constant axial magnetic field and, when there is heat and mass transfer across the interface. The viscous potential flow theory has been used for the investigation. Viscosity enters through normal stress balance in the viscous potential flow theory and tangential stresses are not considered. A dispersion relation that accounts for the growth of axisymmetric waves is derived and stability is discussed theoretically as well as numerically. Stability criterion is given by critical value of applied magnetic field as well as critical wave number. Various graphs have been drawn to show the effect of various physical parameters such as magnetic field strength, heat transfer capillary number, vapour fraction, permeability and porosity on the stability of the system. It has been observed that heat transfer and magnetic field both have stabilizing effect while porosity has destabilizing effect on the stability of the system.

Effect of a magnetic field on the hysteresis transitions during floating-zone crystal growth

The effect of a constant axial magnetic field on the thermosolutocapillary (TSC) convection in the floating liquid zone with an undeformed free surface under microgravity (MG) conditions has been studied. It is established that several stable flow regimes may coexist under such conditions in a certain range of parameters. The boundaries of transitions between various convection regimes under MG conditions with various Hartmann numbers have been determined. It is shown that the constant axial magnetic field suppresses TSC convection, so that the region of uncertainty decreases and shifts toward higher Marangoni numbers. The dependence of the structure and intensity of flow on the floating-zone length to radius ratio has been analyzed.