磁场辅助电弧焊接旋转等离子体行为的数值模拟

目的 研究外加纵向磁场对倾斜电极TIG焊接的电弧温度分布、流动模式和工件所受热力作用的影响.方法 建立磁场-电弧复合焊接热、电、磁、流动的三维数学模型.通过数值模拟和高速摄像实验,揭示倾斜电极电弧在外加磁场作用下的流动、形貌及温度演化机制.结果 外加纵向磁场后,电弧流动速度明显增加,流动模式由沿电极方向喷射变为近似沿竖直方向旋转向下的流动模式;电弧对工件的热作用均匀性提高,热作用中心向电极正下方靠近,但在焊接横向方向上存在偏离;工件受到表面的电弧旋转拖拽力和内部的旋转洛伦兹力作用,最大洛伦兹力可达50000 N/m3.结论 基于所建立数学模型的模拟结果与实验电弧形貌吻合良好,结果表明,外加纵向磁场能够显著改变电弧的形态及流动模式,提高电弧热流密度的均匀性,并能够对熔池产生有效的搅拌作用.     

棱纹面与洛仑兹力对湍流边界层特性影响的比较

用氢气泡技术对棱纹面平板以及施加行渡式洛仑兹力的平板水边界层开展流动显示实验研究，探讨棱纹面与行渡式洛仑兹力影响湍流边界层相干结构的作用机理．实验结果表明，棱纹面与洛仑兹力促进了边界层由层流向湍流的转捩；而对于湍流边界层，在一定的无量纲参数范围内棱纹面与行渡式洛仑兹力都能实现边界层的局部减阻，相应地湍流边界层粘性底层低速条带平均间距有所增加．     

Heat transfer enhancement in magnetic cooling by means of magnetohydrodynamic convection

In this work, we analyze the potential of magnetohydrodynamic (MHD) convection to increase heat transfer during magnetic cooling. To do this, we consider a section of an active magnetic regenerator, namely a flat gadolinium plate, immersed in an initially stagnant heat transfer fluid (NaOH) which is placed in a cuboid glass cell. To create the MHD flow, a small electric current is injected by means of two electrodes and interacts with the already present magnetic field. As a result, a Lorentz force is generated, which drives a swirling flow in the present model configuration. By means of particle image velocimetry and Mach–Zehnder interferometry, the flow field and its impact on the heat transfer at the gadolinium plate are analyzed. For the magnetization stage, we show that a heat transfer enhancement by about 40% can be achieved even with low currents of 3 mA.     

Thermal instability of a rotating curved plate subjected to an applied magnetic field

The effect of a magnetic field on thermal instability in mixed convection flow on a heated rotating convex surface is studied in this paper. The onset position characterized by the Goertler number G _δ depends on the Grashof number, the rotational number, the Prandtl number, the magnetic field parameter, and the wave number. The buoyancy force, the centrifugal force, the Lorentz force, and the Coriolis force are found to significantly affect the flow structure and heat transfer of the flow. Negative rotation (clockwise) destabilizes the boundary layer flow on a convex surface. However, the Lorentz force stabilizes the flow. Numerical data in this study show the same order of magnitude like experimental data.     

A new approach of using Lorentz force to study single-asperity friction inside TEM

Taking advantage of the magnetic field inside transmission electron microscope(TEM),a unique Lorentz-force-actuated method for quantitative friction tests was developed via a commercial electromechanical holder.With this approach,a submicron-sized silver asperity sliding on a tungsten flat punch was actu-ated by Lorentz force due to electrical current through the punch,with the normal force imposed by the built-in transducer of the holder.The friction force was determined by tracking the elastic deflec-tion of the fabricated cantilever from in situ video.Through correlating the friction behavior with the microstructural evolution near the silver-tungsten interface,we revealed that even when the relative motion commenced with the plastic deformation of the silver asperity,the interface can still sustain the further increasing static friction force.Exactly following the arrival of the maximum static friction force,the sliding occurred at the interface,indicating the transition from static to dynamic friction.This work enriches our understanding of the underlying physics of the dynamic friction process for metallic friction behavior.     

Some results of an experimental investigation of the structure of a turbulent stream of conducting liquid

Results are presented of an investigation, made with the aid of an induction anemometer, of some characteristics of a turbulent stream of electrically conducting liquid in an open channel. External magnetic and electric fields are applied to the flow. It has been observed that the flow structure is appreciably affected by the Lorentz force normal to the channel floor. The characteristics obtained are compared with experimental results for an ordinary liquid.     

Interaction of flux tubes in a type I superconductor

The repulsive interaction between multiquantum flux tubes in type I superconducting lead films has been studied magnetooptically. Two or three flux tubes were trapped in a long and narrow region of slightly reduced film thickness, and the flux tube positions were recorded as a function of an applied Lorentz force pressing the tubes against each other. At distances larger than about the flux tube diameter our results agree well with the magnetic monopole model, which only takes into account the magnetic stray field outside the superconducting film. At smaller distances deviations from the monopole model occur due to the influence of the cross-sectional shape of the flux tubes and of the wall of the tubes upon the energy of the flux tube configuration. These influences have been taken into account by means of a subdivision model in which the flux-containing region of each tube is divided into a large number of flux portions each of which is approximated again by a magnetic monopole. For sufficiently high compression due to the Lorentz force, we have observed fusion of two individual flux tubes. Upon reduction of the Lorentz force, the fused flux tubes separate again. The process of fusion and separation shows distinct hysteresis. This behavior is also well explained by our subdivision model.     

EMHD free-convection boundary-layer flow from a Riga-plate

The electro-magnetohydrodynamic (EMHD) free-convection flow of a weakly conducting fluid (e.g. seawater) from an electromagnetic actuator is considered. The actuator is a so called Riga-plate which consists of a spanwise aligned array of alternating electrodes and permanent magnets mounted on a plane surface. This array generates a surface-parallel Lorentz force which decreases exponentially in the direction normal to the (horizontal) plate. The free-convection boundary-layer flow induced by this body force is investigated numerically and analytically. It is shown that a certain length and velocity scale exists on which the flow characteristics are independent of the material properties of the fluid, as well as of the structural and functional parameters of the actuator. These universal velocity profiles are calculated numerically at different distances x from the leading edge and are discussed in some detail, both for the impermeable and the permeable Riga-plate when; in the latter case, a uniform lateral suction or injection of the fluid is applied. For the flow characteristics analytical approximations are reported. The asymptotic suction profiles approached for large values of x are given in exact analytical form. From a mathematical point of view the basic equations of the present boundary-value problem resemble those of the classical Blasius problem with an inhomogeneous forcing instead of an external flow and, accordingly, a homogeneous asymptotic condition.     

Analysis of a Lorentz force based vibration exciter using permanent magnets mounted on a piezoelectric stack

This work presents performance analysis of a Lorentz force based non-contact vibration exciter by mounting a couple of permanent magnets on a piezoelectric stack. A conductor is attached to the structure to be excited and is placed midway between unlike poles of a couple of permanent magnets. The permanent magnets are placed on a piezoelectric stack. This stack, because of its nano-positioning capabilities, can impart an accurate and adjustable harmonic vibratory motion to the couple of permanent magnets. The piezoelectric stack, because of its high stiffness remains uncoupled with the dynamics of the structure. Due to the relative motion between the magnets and the conductor, Lorentz force is generated within the conductor. This Lorentz force is responsible for vibration of the structure in a plane parallel to the pole faces of the magnets. This keeps the magnetic field almost independent of the vibration of the structure and the chance of the structure hitting the magnet during large vibration is totally eliminated. If the amplitude of displacement of the stack is kept constant, the non-contact excitation force in this exciter remains proportional to the excitation frequency. Though use of this exciter eliminates mass (apart from that of the conductor attached to the structure) and stiffness coupling, a known damping term gets added to that of the excited structure.     

In situ analysis of three-dimensional electrolyte convection evolving during the electrodeposition of copper in magnetic gradient fields

A novel three-dimensional particle tracking velocimetry technique was used to examine the flow during electrodeposition of Cu. For the first time electrode-normal, circumferential, and radial velocities were spatially resolved during deposition in superimposed low and high magnetic gradient fields. In this way the complex interaction of magnetic field gradient force and Lorentz force induced convective effects could be measured and analyzed. Magnetic field gradient force induced electrolyte flow was detected only in high gradient magnetic fields, and it was found to be directed toward regions of gradient maxima. Since this electrode-normal flow causes enhanced transport of Cu(2+) ions from the bulk electrolyte to those regions of the working electrode where maxima of magnetic gradients are present, a structured deposit is formed during diffusion-limited electrodeposition. Lorentz force driven convection was observed during deposition in the low and the high magnetic gradient experiments. The overall fluid motion and the convection near the working electrode were determined experimentally and discussed with regard to the acting magnetic forces and numerical simulations.     

A numerical study of wave structures developed on the free surface of a film flowing on inclined planes and subjected to surface shear

In this work, we determine the different patterns of possible wave structures that can be observed on a thin film flowing on an inclined plane when at the free surface a shear force (surface traction) is applied. Different wave structures are obtained dependening on the selected combination of downstream and upstream boundary conditions and initial conditions. The resulting initial boundary value problems are solved numerically using the direct BEM numerical solution of the complete two‐dimensional Stokes system of equations. In our numerical results, the initial discontinuous shock profiles joining uniform fluid depths are smoothed due to the two‐dimensional character of the Stokes formulation, including the effect of the gravitational force as well as the interfacial surface tension force. In this way, physically feasible continuous surface profiles are determined, in which the initial uniform depths are joined by smooth moving wave structures. Numerical solutions have been attained to reproduce the different patterns of possible wave structures previously reported in the literature and extended to identify some other new structures and features defining the behaviour of the surface patterns. Copyright © 2006 John Wiley & Sons, Ltd.     

Numerical computation of the electromagnetic field in the electrodes of a three‐phase arc furnace

In this paper we introduce and numerically solve a mathematical model for numerical simulation of electro‐magnetic field in a three‐phase electric reduction furnace. The model allows us to compute the current distribution on a cross‐section of the three electrodes. A combined boundary element/finite element method is used. Numerical results for real industrial furnaces are shown. As a by‐product we compute the torque on the electrodes due to the Lorentz electromagnetic force. Copyright © 1999 John Wiley & Sons, Ltd.     

Effect of External Nonuniform Magnetic Field on Flux Creep Process in Superconductor

The effect of an external nonuniform magnetic field on the flux creep rate in a high-temperature superconductor with trapped magnetic flux was studied. The magnetic relaxation was suppressed when the superconductor was put into a field of permanent magnets or when it approached a ferromagnet. The effect arises when the field sources (being magnetized, the ferromagnet produces its own field) are placed near the superconductor surface, where the flux line ends are located. For these cases, we carried out the calculations of vortex and current density distributions, which demonstrate that reverse currents flow in the near-surface regions of the sample. This verifies the hypothesis suggested earlier about the influence of counter Lorentz forces retarding the creep of the vortices. In the interpretation of the results, we also take into consideration the magnetic force acting on the vortex ends in the external nonuniform magnetic field that allows us to explain the experimental results, in which the current structure in the sample is unipolar.     

Magnetic field contribution to the Lorentz model

The classical Lorentz model of dielectric dispersion is based on the microscopic Lorentz force relation and Newton's second law of motion for an ensemble of harmonically bound electrons. The magnetic field contribution in the Lorentz force relation is neglected because it is typically small in comparison with the electric field contribution. Inclusion of this term leads to a microscopic polarization density that contains both perpendicular and parallel components relative to the plane wave propagation vector. The modified parallel and perpendicular polarizabilities are both nonlinear in the local electric field strength.     

Revisiting Arc,Metal Flow Behavior in Flux Activated Tungsten Inert Gas Welding

Tungsten inert gas (TIG) welding is commonly used for quality joining of ferrous and non-ferrous materials. But lower depth of penetration is one major shortcoming of TIG and more numbers of passes are required for thick plate joining. Use of activated flux layer on workpiece remarkably enhances penetration. In the present work, arc and metal flow behavior are studied to revisit and validate the presence and role of governing forces responsible for higher penetration in flux activated TIG (ATIG) welding. Arc behavior, material flow patterns are studied during conventional and activated TIG welding for AISI 304 stainless steel using four different activated fluxes by carrying out welding (when arc is moving) as well as keeping the arc halted (stationary arc) for all cases. Results clearly demonstrate the presence of electromagnetic Lorentz force and surface tension induced reverse Marangoni flow (strongly centripetal). These forces also push the metal flow downward near the center region of the molten pool and in many occasions penetration reaches beyond plate thickness in ATIG welding.     

Theory of the radiation pressure on dielectric surfaces

The radiation pressures exerted by light beams incident normally on dielectric surfaces are calculated by evaluation of the quantum-mechanical Lorentz force. The free-space surface of an almost transparent dielectric and the interface of a lossy dielectric with a transparent medium are treated in detail. Light beams in the forms of a short single-photon pulse and a continuous-mode narrow-band coherent excitation are considered. The use of a pulse excitation enables discrimination of the surface and bulk contributions to the force. It is shown that the surface force is directed inwards to the dielectric for entrance and exit of the pulse from and to free space, contrary to the conclusions of some earlier work. For the interface of lossy and transparent dielectrics, it is shown that the high-reflectivity mirror modelled by an appropriate limit of the lossy dielectric experiences a force enhanced by the refractive index of the transparent medium, in agreement with experiments on a mirror suspended in liquid dielectrics. The results for the Lorentz force are used to identify the effective momenta of photons in dielectrics.     

Capturing non-Brownian particle transport in periodic flows

In this paper we computationally examine the motion of a dilute suspension of slightly non-neutrally buoyant solid spheres as they migrate across the curved fluid streamlines of a viscous cellular flow. This is done by incorporating particle-fluid interactions into a continuum-based Lagrangian advection model derived from the Basset–Boussinesq–Oseen (BBO) equation, where the flow field is mimicked by using a perturbed streamfunction. Although the purely regular cellular flow is able to capture maximum velocity and particle diameter effects that are observed experimentally, it has several shortcomings. Most significantly, it is unable to capture the secondary island structures that exist in many rotating flow systems, nor the impact that these structures are observed to have on particle migration. Our results in this work demonstrate significant interplay between the underlying fluid structure and the non-trivial equilibrium locations of the non-Brownian particles, in agreement with previous experimental work. We also evaluate the effect of the Saffman lift force on the lateral migration of the solid spheres.     

Lorentz Force Eddy Current Testing: a Prototype Model

We report an investigation of the motion of a free-falling permanent magnet in an electrically conducting pipe containing an idealized defect. This problem represents a highly simplified yet enlightening version of a method called Lorentz force eddy current testing which is a modification of the traditional eddy current testing technique. Our investigation is a combination of analytical theory, numerical simulation and experimental validation. The analytical theory allows a rigorous prediction about the relation between the size of the defect and the change in falling time which represents the central result of the present work. The numerical simulation allows to overcome limitations inherent in the analytical theory. We test our predictions by performing a series of experiments. We conclude that our theory properly captures the essence of Lorentz force eddy current testing although a refinement of the experiment is necessary to reduce the discrepancy to the predictions. In spite of its apparent simplicity the present system can serve as a prototype and benchmark for future research on Lorentz force eddy current testing.     

电磁激活板的宽度对圆柱绕流控制的影响

电磁力可以改变流体边界层的结构,是控制流体运动的主动控制方法之一。基于电磁场和流体的基本方程,对置于弱电介质中的圆柱电磁激活板周围产生的Lorentz力及其对圆柱绕流的控制进行了数值模拟,着重讨论了电磁激活板的宽度对其周围的电磁场、产生的Lorentz力、流场的控制和涡量变化的影响。电磁场包覆范围为流体分离点至其后部,当N值较小时,分离点后移,但不能够完全抑制流体的分离,极板越宽对尾涡的抑制效果越好;随着N值的增大,由于极板窄的表面涡量大,所以可以首先达到完全抑制流体分离的控制效果;当N值较大时,无论极板宽窄,都可以达到完全抑制流体分离的效果。     

Anomalous radial migration of single DNA molecules in capillary electrophoresis

We report the unexpected radial migration of DNA molecules in capillary electrophoresis (CE) with applied Poiseuille flow. Such movement can contribute to anomalous migration times, peak dispersion, and size and shape selectivity in CE. When Poiseuille flow is applied from the cathode to the anode, DNA molecules move toward the center of the capillary, forming a narrow, highly concentrated zone. Conversely, when the flow is applied from the anode to the cathode, DNA molecules move toward the walls, leaving a DNA-depleted zone around the axis. We showed that the deformation and orientation of DNA molecules under Poiseuille flow was responsible for the radial migration. By analyzing the forces acting on the deformed and oriented DNA molecules, we derived an expression for the radial lift force, which explained our results very well under different conditions with Poiseuille flow only, electrophoresis only, and the combination of Poiseuille flow and electrophoresis. Factors governing the direction and velocity of radial migration were elucidated. Potential applications of this phenomenon include an alternative to sheath flow in flow cytometry, improving precision and reliability of single-molecule detection, reduction of wall adsorption, and size separation with a mechanism akin to field-flow fractionation. On the negative side, nonuniform electroosmotic flow along the capillary or microfluidic channel is common in CE, and radial migration of certain analytes cannot be neglected.