Influence of lubricant on the motion of a body in an electromagnetic railgun accelerator. III. Temperature distribution in the armature, rail, and liquid film

The effect of a conducting liquid lubricant on the heating of a rail and projectile (armature) is studied theoretically and experimentally. It is shown that both the Joule and friction heating of the accelerated body can be reduced significantly by using resistive liquid films. When the contact resistance of the film is high, its temperature is determined by two competing processes: Joule heating and heat removal by the moving film. As a result, the dependence of the film temperature on the magnetic Reynolds number, its thickness, and its resistance is nonmonotonic. In the limiting case where the velocity skin effect is completely suppressed and the magnetic Reynolds number is sufficiently high, the film temperature is extremely low. In intermediate cases, however, the film temperature can turn out to be rather high and exceed the melting point of the armature. Viscous dissipation in the liquid film has no significant effect on the temperature of the rail-armature interface until the melting of the armature is determined by Joule heating within it. In the case where the velocity skin effect is strongly suppressed, viscous dissipation along with Joule heating in the resistive film can become one of the major factors controlling the attainable velocity of bodies in railguns. Zh. Tekh. Fiz. 69, 117–125 (October 1999)     

Spin polarized current controlled dynamics of domain walls in magnetic films with in-plane anisotropy

We study the dynamic properties of asymmetric vortex Bloch walls and classical 1D Néel walls controlled by a spin-polarized current in magnetic films with in-plane anisotropy. It is shown that fairly high velocities of domain walls (up to 100 m/s) can be obtained for the current density in the range j = 10⁶–10⁸ A/cm². The nonlinear dependence of the wall velocity on the film thickness and the linear dependence of the velocity on the current density and inverse damping parameter are found.     

Influence of lubricant on the motion of a body in an electromagnetic railgun accelerator. II. Hydrodynamics of a conducting lubricant

The flow of a liquid conducting film in the gap between the rail and the metallic projectile (armature) in a railgun is considered. In contrast to the usual problems in the theory of the hydrodynamics of lubricants, the statement of the problem here takes into account such additional forces acting on the liquid as the ponderomotive force and the inertial force arising as a consequence of the accelerated motion of the armature. As a result, even in the case of a gap of constant width not only is the existence of flows with negative velocities possible, but also of flows with negative flow rates. An analytical solution of the problem is obtained with the convective terms neglected. The influence of the convective terms is estimated by integrating the lubrication equations numerically. Primary attention is devoted to a calculation of the lubricant flow rate, viscous dissipation in the liquid layer, and the pressure distribution along the gap. Calculations are performed for specific cases of linearly expanding and linearly narrowing gaps. Zh. Tekh. Fiz. 69, 109–116 (October 1999)     

Pressure pulse induced by a pulsed electric current in a cylindrical liquid conductor

An analogue of z pinch is considered for a finite-conductivity liquid in a cylindrical nonconducting tube through which an exponentially decaying current pulse is passed. The distributions of magnetic induction, current density, and volume electromagnetic force pressure in the conducting liquid cylinder are found by solving an equation for a quasi-stationary electromagnetic field and the equation of magnetohydrostatics with the operational method.     

双液滴撞击平面液膜的流动与传热特性

采用耦合的水平集-体积分数法(CLSVOF)对双液滴连续撞击恒定壁温壁面上的热液膜的流动和换热特性进行了数值模拟及分析,得到了双液滴撞击热液膜后形态演变的过程.分析了液滴垂直间距、撞击速度、液膜厚度以及液滴直径对双液滴撞击液膜后的流动与传热特性的影响,结果显示,壁面平均热流密度随液滴撞击速度的增大而增大,液滴垂直间距、液膜厚度和液滴直径对平均热流密度的影响较小,但会对热流密度在撞击区域和交界区的分布产生重要影响.     

Diffusion of a pulsed field and electromagnetic forces in ferromagnets

Self-similar solutions of the magnetic field distribution in a conducting ferromagnet with a non-linear permeability are considered. The velocity of magnetic field penetration is calculated in various models of ferromagnetic properties of the conductor. Using the Maxwell tensor, equations are derived for bulk forces taking into account the possibility of saturation of the ferromagnet. A numerical solution for the distributions of the magnetic induction and of the density of the bulk force in a ferromagnetic conductor is obtained using the Preisach model taking hysteresis into account.     

静止条件下轨道炮膛内磁场分布特性分析

为了分析轨道炮静止条件下膛内磁场分布特性, 建立了轨道炮二维计算模型, 基于磁扩散方程与安培定律, 得到导轨和电枢各区域电流密度值, 并通过毕奥-萨伐尔定律对轨道炮电枢前端各考察点磁通密度进行理论计算, 基于电磁感应法进行了膛内磁场测量实验, 实验测量值与理论计算值基本一致, 结果表明, 膛内磁场大小主要由流经电枢和导轨的的电流决定, 电枢前端中心轴线上各考察点, 随着与电枢前端面距离的增大, 磁通密度峰值呈衰减趋势, 但衰减速度逐渐变小。研究结果有助于轨道炮膛内强磁场屏蔽与智能弹药设计。     

NMR velocimetry of falling liquid films

First results on NMR velocimetry of falling liquid films are presented. A film of average thickness 1 mm and width 40 mm is sustained by a continuous flow of silicon oil over a vertical plate made from PMMA. The spatial distribution of velocities is measured using a double spin--echo imaging pulse sequence supplemented by a bipolar velocity encoding gradient. Spin density and velocity images as well as two-dimensional velocity maps of different situations, i.e., undisturbed and disturbed falling film flow, are discussed. Experimental and theoretical velocity data for undisturbed film flow are compared.     

Distribution of the magnetic field induced by a current passing through slabs in the superconducting and normal states

The magnetic field induced by a current passing through a long slab is calculated. For the uniformly distributed current, simple formulas for the field components inside and outside the slab are derived. For the nonuniformly distributed current, the magnetic field is numerically calculated for a thin wide superconducting film (the thickness of the film is on the order of the magnetic field penetration depth). It is shown that the current-induced field distribution depends on the form of the exponential dependence of the current density. Experimental distributions are compared with results of a theory where a finite-thickness film is considered as two-dimensional (indefinitely thin). Comparison is also made between experimental and calculated values of the magnetic fields. For a number of points on a plane parallel to the slab and on a plane passing through its center, experimental data are compared with the results obtained by mere summation of partial contributions to the field.     

Numerical Simulation for Falling Film Flow Characteristics of Refrigerant on the Smooth and Structured Surfaces

Flow characteristics of a liquid film flowing over a smooth surface and structured surface with the Reynolds number range from 10 to 1121 are studied. The mixture of R21 and R114 refrigerants is used as the test liquid. The 3D transient simulations are taken to capture the liquid film’s dynamic characteristics and spatial distribution. Effects of the inlet dimension, inlet flow rates, surface tension, and surface structuring on the wettability, average velocity, and film thickness are studied systematically. The obtained results show that surface tension is essential for an accurate simulation, while inlet width has no effect on the liquid film parameters in the steady-state flow regime. For low flow rates, wetting area and film thickness both are small, and a suggested range of Reynolds number is chosen to simulate further heat transfer in order to balance the film thickness and dry spots generation. It is shown that a ripple surface structure hinders the liquid film movement, reflected in a lower velocity and a larger film thickness compared to the smooth surface. Lateral movement of a liquid film can also be observed at the structured surface.     

轨道炮速度趋肤效应的分析与仿真

 根据Maxwell方程推导出电磁场参数与速度之间的关系,建立了直角坐标下的电磁场和温度场扩散2维偏微分方程, 分析了固体电枢电磁轨道炮的速度趋肤效应。以矩形固体电枢为例,给出了边界条件和激励源函数。采用有限差分法对方程进行求解,并对数据进行了分析,得到轨道和电枢中磁感应强度、温度和电流密度的分布曲线。计算结果表明:电枢的运动使得电流密度集中在电枢和轨道交界面的尾部,使得该局部地区温度增加,进而引起电枢尾部的熔融与烧蚀。     

Induced electric current-based formulation in computations of low magnetic Reynolds number magnetohydrodynamic flows

We use the induced electric current as the main electromagnetic variable to compute low magnetic Reynolds number magnetohydrodynamic (MHD) flows. The equation for the induced electric current is derived by taking the curl of the induction equation and using Ampère’s law. Boundary conditions on the induced electric current are derived at the interface between the liquid and the thin conducting wall by considering the current loop closing in the wall and the adjacent liquid. These boundary conditions at the liquid–solid interface include the Robin boundary condition for the wall-normal component of the current and an additional equation for the wall potential to compute the tangential current component. The suggested formulation (denominated j-formulation) is applied to three common types of MHD wall-bounded flows by implementing the finite-difference technique: (i) high Hartmann number fully developed flows in a rectangular duct with conducting walls; (ii) quasi-two-dimensional duct flow in the entry into a magnet; and (iii) flow past a magnetic obstacle. Comparisons have been performed against the traditional formulation based on the induced magnetic field (B-formulation), demonstrating very good agreement.     

枢轨界面电磁特性的演变规律分析

枢轨电接触性能随电枢膛内滑动而演变,界面电磁特性变化是其演变的根本作用因素。以系统时变电磁场计算为基础,从脉冲电流、电枢速度和局部接触等三个方面讨论了对界面电磁特性演变的作用规律,并设计实验综合验证了电枢膛内运动时枢轨电接触界面的电磁特性演变。结果表明:脉冲电流驱使电流向接触前端聚集,电流峰值聚集随电流变化而下降;电枢速度克服脉冲电流作用向接触尾部聚集电流,尾端聚集临界速度(50m/s)下,接触表面电流分布最为均匀,高速下接触表面呈现尾端U型电流聚集模式;局部接触对电流的聚集作用随接触面增加、速度增加而减弱;在法向磁力作用下,预置接触会向前扩展并趋于稳定。实验后枢轨表面检测验证了各因素对电磁特性演变的作用规律。研究结果为深入探究枢轨滑动电接触机理奠定基础。     

Spatial distribution of magnetic field and persistent currents in Condon domain phase

The distributions of magnetic field above the surface of the sample due to existence of the diamagnetic domain structure are found. It is shown, that the constant magnetic induction splitting inside of a sample is caused by the magnetization current density, localised in the boundaries between adjacent domains, close to the sample surface. The properties of this current are studied. The influence of the domain wall thickness on the spatial distribution of magnetic field and magnetization current density is present. A possibility of detection of the changes in the magnetic field distribution in vacuum, close to the surface of the sample, by means of Hall probes, is discussed. The measurement of the spatial distribution of magnetic field can give lacking information about characteristic sizes of magnetic domain formation at the conditions of the strong de Haas van Alphen effect.     

A revisit to the neutron skin thickness of neutron-rich nuclei in the statistical abrasion ablation model

The cross sections of the fragments produced in the projectile fragmentation reactions of the even calcium isotopes from A=36 to A=52 are calculated using the statistical abrasion ablation model.The neutron skin thickness are studied by investigating the fragments isotopic cross section distributions.The neutron-skin thicknesses of the calcium isotopes have a good linear correlation to the peak positions of their fragment isotopic cross section distributions.The correlation between the neutron skin thickness and the neutron density distributions of 48Ca is investigated by introducing a parameter to adjust the diffuseness parameter in the fermi-type density distribution.     

液滴撞击液膜的流热耦合界面追踪方法数值模拟

采用界面追踪方法研究蒸馏过程中液滴撞击高温薄液膜的流动和传热特性,将数值结果与解析解和实验进行比较验证模型的正确性,研究气液界面和热流分布的演变过程.同时,分析液滴We数和无量纲液膜厚度对传热的影响.液滴撞击后的热流密度分布显示：液膜可分为撞击区、过渡区和静态区.由于液滴的撞击作用,强制对流是撞击区内主要的传热机制.增大液滴的韦伯数或减小无量纲液膜厚度会加强热量传递.随着液滴韦伯数的增加,冲击引起的扰动增强,在动量和能量共同作用下,平均热流密度明显增大,撞击区冠状水花越明显.无量纲液膜厚度越小,平均热流密度越大,且有更长的时间保持高热流密度换热.     

A new model analysis of the third harmonic voltage in inductive measurement for critical current density of superconducting films

The critical current density J c is one of the most important parameters of high temperature superconducting films in superconducting applications,such as superconducting filter and superconducting Josephson devices.This paper presents a new model to describe inhomogeneous current distribution throughout the thickness of superconducting films applying magnetic field by solving the differential equation derived from Maxwell equation and the second London equation.Using this model,it accurately calculates the inductive third-harmonic voltage when the film applying magnetic field with the inductive measurement for J c.The theoretic curve is consistent with the experimental results about measuring superconducting film,especially when the third-harmonic voltage just exceeds zero.The J c value of superconducting films determined by the inductive method is also compared with results measured by four-probe transport method.The agreements between inductive method and transport method are very good.     

高速弹丸的磁感应测速方法

 由固定磁体感应弹速测试技术与数字化波形存储器、微机相结合组成的速度测量自动化系统，已在23 mm口径二级轻气炮上应用成功。采用压缩磁场结构设计和严格的信号计时处理方法，使1~7.5 km/s范围的时间判读精度达到0.03%，整套系统的速度测量误差约0.1%，给二级轻气炮这类动高压设备提供了一种精确可靠的速度测试手段。     

Magnetic field penetration into half-space for type-II superconductors of the second kind

Equations that simulate the magnetic induction and current density distributions in half-space in view of the power I-V characteristic are derived. The magnetization front velocity is determined for a given mean rate of external magnetic field variation at the boundary of the sample. An integral condition for the electrical resistance (nonlinearly depending on the magnetic field) under which the magnetic flux penetrates into the sample with a finite rate is found. An analytical solution that simulates the power variation of the magnetic field at the boundary is given. The Bean generalized model describing the current density distribution near the critical current is considered. It is shown that solutions like shock waves may arise beyond the applicability domain of the Bean model.     

Critical angles and minimum yields for planar channeling

Abstract

Local regions on the surface of a sample of ZnO: Zn phosphor powder were deteriorated by low energy heavy ions to a depth of a few hundred Angstrom units. These regions behave as ‘thin films’ in which a negligible small amount of light is produced when an energetic projectile passes through it. The sample was then scanned across energetic ion beams. The scintillation response diminished when the beam impinged onto a film. The decrease, ΔL, in light intensity is the amount of light produced in a phosphor region whose thickness is equivalent to the film thickness. For ¹H and ⁴He the luminescent efficiency, ΔL/ΔE (the ratio of the light produced to the observed energy loss), is approximately independent of energy indicating that the scintillation response depends primarily on the inelastic projectile-electron collisions rather than the elastic projectile target atom encounters.

At a given velocity, a plot of ΔL versus the atomic number of the projectile exhibits oscillatory structure for which a tentative explanation is suggested.