Capillary instability of the cylindrical interface between ferrofluids in a magnetic field with circular field lines

Capillary breakup of a viscous magnetic fluid layer subjected to a gradient magnetic field under hydroweightlessness is studied within the linear theory. The cylinder surface of a current-carrying conductor serves as the inner boundary of the layer. The outer boundary of the layer is the coaxial interface with an immiscible nonviscous fluid of lower permeability. The particular subject of investigation is the effect of the relative thickness of the layer and that of the magnetic Bond number on the characteristic time of growth of the fastest-increasing harmonic and on the size of droplets forming under the ultimate conditions of capillary breakup (i.e., at large and small Ohnesorge numbers).     

合成冷/热射流控制超声速边界层流动稳定性

为了探究超声速边界层流动稳定性及其转捩控制机理,提出基于合成冷/热射流的边界层速度-温度耦合控制方法,并通过数值模拟研究了Ma=4.5超声速平板边界层不稳定波的传播,采用线性稳定性理论中的时间模式分析了壁面吹吸、射流温度、扰动频率、扰动振幅等对不稳定波控制效果的影响.结果表明:无射流控制时,边界层内同时存在不稳定的第一模态扰动波和第二模态扰动波,且二维波形式的第二模态占主导地位;壁面吹吸作用下,仅出现更加不稳定的第二模态,第一模态被抑制;速度-温度耦合控制下,射流温度对扰动模态的不稳定区域大小及扰动增长率影响显著,射流温度与来流温度不同时,温度的脉动使得流动转捩为湍流的速度加快,边界层速度型更加饱满,抗干扰能力增强,流动稳定性提高;高频的吹吸扰动对流场的控制效果优于低频扰动,扰动频率超过400 Hz时,第二模态扰动波时间增长率降低,扰动分量对边界层速度剖面和温度剖面的修正加快,第二模态更加稳定;扰动振幅减小为主流速度的1%时,仅出现时间增长率较小的第二模态,控制效果较好,进一步减小时,第一模态重新出现,并且波数范围与第二模态先重合后分离,对应的时间增长率先增加后减小.研究结果为边界层转捩控制技术提供了新的思路.     

Mo覆盖层对MgO/CoFeB/Mo结构磁各向异性的影响

研究了Mo覆盖层厚度对MgO/CoFeB结构磁各向异性的影响. 研究发现, 加平行磁场生长出来的MgO/CoFeB/Mo样品表现为面内各向异性, 并且随着CoFeB的厚度减小, 面内各向异性逐渐减弱; 在CoFeB厚度减小到1.1 nm时, 仍可以保持面内各向异性, 垂直方向的外加饱和场逐渐减少; 厚度在0.9 nm及以下的情况下, 面内各向异性消失. 改变Mo覆盖层厚度, 当t_Mo= 1.6 nm时, 垂直方向的饱和场最小. 当生长过程的磁场变为垂直磁场时, 不同厚度的Mo覆盖层对MgO/CoFeB 的磁各向异性影响不同. Mo厚度在1 nm及以下时MgO/CoFeB/Mo样品表现为面内各向异性, Mo覆盖层厚度在1.2和5 nm之间时样品出现了垂直磁各向异性; 并且垂直方向的矫顽力也发生了变化, Mo覆盖层厚度为1.4 nm时样品的磁滞损耗会大一些.     

Dewetting in associating lattice gas model confined by hydrophobic walls

The phase behavior of a two dimensional fluid confined within hydrophobic walls is obtained by Monte Carlo simulations. The fluid is described by the associating lattice gas model which reproduces the density and diffusion anomalous behavior of water.The confined fluid exhibits a liquid-liquid critical temperature which decreases with the decrease of the distance between the confining walls. In contact with the wall a dewetting is observed. The thickness of this interfacial layer is independent of the distance between the two walls. Even for very small distances between the two walls no total depletion is observed and consequently no drying transition is present.     

Nonequilibrium and morphological characterizations of Kelvin–Helmholtz instability in compressible flows

We investigate the effects of viscosity and heat conduction on the onset and growth of Kelvin–Helmholtz instability (KHI) via an efficient discrete Boltzmann model. Technically, two effective approaches are presented to quantitatively analyze and understand the configurations and kinetic processes. One is to determine the thickness of mixing layers through tracking the distributions and evolutions of the thermodynamic nonequilibrium (TNE) measures; the other is to evaluate the growth rate of KHI from the slopes of morphological functionals. Physically, it is found that the time histories of width of mixing layer, TNE intensity, and boundary length show high correlation and attain their maxima simultaneously. The viscosity effects are twofold, stabilize the KHI, and enhance both the local and global TNE intensities. Contrary to the monotonically inhibiting effects of viscosity, the heat conduction effects firstly refrain then enhance the evolution afterwards. The physical reasons are analyzed and presented.     

Kinetic Simulation of Nonequilibrium Kelvin-Helmholtz Instability

The recently developed discrete Boltzmann method(DBM), which is based on a set of uniform linear evolution equations and has high parallel efficiency, is employed to investigate the dynamic nonequilibrium process of Kelvin-Helmholtz instability(KHI). It is found that, the relaxation time always strengthens the global nonequilibrium(GNE), entropy of mixing, and free enthalpy of mixing. Specifically, as a combined effect of physical gradients and nonequilibrium area, the GNE intensity first increases but decreases during the whole life-cycle of KHI. The growth rate of entropy of mixing shows firstly reducing, then increasing, and finally decreasing trends during the KHI process. The trend of the free enthalpy of mixing is opposite to that of the entropy of mixing. Detailed explanations are:(i) Initially,binary diffusion smooths quickly the sharp gradient in the mole fraction, which results in a steeply decreasing mixing rate.(ii) Afterwards, the mixing process is significantly promoted by the increasing length of material interface in the evolution of the KHI.(iii) As physical gradients are smoothed due to the binary diffusion and dissipation, the mixing rate reduces and approaches zero in the final stage. Moreover, with the increasing Atwood number, the global strength of viscous stresses on the heavy(light) medium reduces(increases), because the heavy(light) medium has a relatively small(large) velocity change. Furthermore, for a smaller Atwood number, the peaks of nonequilibrium manifestations emerge earlier, the entropy of mixing and free enthalpy of mixing change faster, because the KHI initiates a higher growth rate.     

黏性各向异性磁流体Kelvin-Helmholtz不稳定性:二维数值研究

利用corner transport upwind和constrained transport算法求解非理想磁流体动力学方程组,对匀强平行磁场作用下,黏性各向异性等离子体自由剪切层中的Kelvin-Helmholtz不稳定性进行了数值模拟.从流动结构、涡结构演化、磁场分布、横向磁压力、抗弯磁张力等角度对各向同性和各向异性黏性算例结果进行了讨论,分析了黏性各向异性对Kelvin-Helmholtz不稳定性的影响.结果表明,黏性各向异性比黏性各向同性更利于流动的稳定.其稳定性作用是由于磁感线方向上剪切速率降低导致界面卷起程度和圈数的降低,并使卷起结构中小涡产生增殖、合并,破坏了涡的常规增长,从而导致流动的稳定.黏性各向异性对横向磁压力的影响比对抗弯磁张力更大.     

太赫兹波在飞行器等离子体鞘套中的传输特性

针对临近空间飞行器的黑障问题,根据模拟的RAM C-Ⅲ飞行器周围的流场分布结果,计算了等离子体电子密度和碰撞频率,并根据其分布建立了非均匀的等离子体模型。在此基础上,利用散射矩阵方法分析了太赫兹波在等离子体中的传输特性随着等离子体密度、等离子体厚度、等离子体碰撞频率的变化以及外加磁场对传输特性的影响。结果表明,太赫兹波的传输损耗随着等离子体电子密度和等离子体厚度的增加而增加,而碰撞频率的增加会使得透射率先减小后增加。在外加磁场的作用下,左旋太赫兹波的传输特性会得到改善;而对于右旋太赫兹波,磁场的施加会引入吸收峰,并且随着磁感应强度的增加向高频方向移动。     

直流大气压电晕放电电子密度的光谱研究

利用自制针—板式放电装置,在大气中进行电晕放电实验。用发光区域照片光斑的大小,讨论了电晕层厚度与电源电压的关系。在相同针板间距下,电晕层厚度随着电压的升高而增大;在相同电压下,电晕层厚度随着针板间距的增大而减小。由于高能电子密度能够通过氮分子第二正带系337.1 nm的光谱强度大小反映,因此对氮分子第二正带系337.1 nm谱线的强度用发射光谱法进行了测量。实验结果发现在针尖附近高能电子密度最大,并且高能电子密度随电压的升高而增大;电压一定时,高能电子密度随针板间距的增大而减小。在针板间距和电源电压不变的情况下,高能电子密度随针尖曲率半径的减小而增大。     

Multilayer transition in a spin 3/2 Blume-Capel model with RKKY interaction

Using Monte Carlo simulation and mean-field theory, we have studied the effect of RKKY interaction on the multi-layer transition and magnetic properties of a spin-3/2 Blume-Capel model of a system formed by two magnetic multi-layer materials, of different thicknesses, separated by a non-magnetic spacer of thickness M. It is found that the multi-layer magnetic order-disorder transition temperature depends strongly on the thicknesses of the magnetic layer and the non-magnetic layer. The transition temperature increases with increasing thickness of the magnetic multi-layers and decreases with increasing thickness of the non-magnetic one. Furthermore, there exists a critical thickness M_L of the non-magnetic spacer beyond which the effect of the RKKY interaction becomes negligible and separate transitions occur in the two magnetic layers. The critical thickness M_L decreases on increasing the magnetic crystal field and/or the Fermi level k_f. Moreover, the multi-layer transition temperature undergoes oscillations as a function of the Fermi level. The susceptibility critical exponents are computed within Monte Carlo simulations.     

Effects of sweep rates of external magnetic fields on the labyrinthine instabilities of miscible magnetic fluids

The interfacial instability of miscible magnetic fluids in a Hele-Shaw Cell is studied experimentally, with different magnitudes and sweep rates of the external magnetic field. The initial circular oil-based magnetic fluid drop is surrounded by the miscible fluid, diesel. The external uniform magnetic fields induce small fingerings around the initial circular interface, so call labyrinthine fingering instability, and secondary waves. When the magnetic field is applied at a given sweep rate, the interfacial length grows significantly at the early stage. It then decreases when the magnetic field reaches the preset values, and finally approaches a certain asymptotic value. In addition, a dimensionless parameter, Pe, which includes the factors of diffusion and sweep rate of the external magnetic field, is found to correlate the experimental data. It is shown that the initial growth rate of the interfacial length is linearly proportional to Pe for the current experimental parameter range and is proportional to the square root of the sweep rate at the onset of labyrinthine instability.     

旋转磁场对凝固组织形成的影响

研究了旋转磁场作用下Pb-45%Sn亚共晶合金的凝固组织.实验发现,旋转磁场的频率恒定时,凝固组织的晶粒尺寸随着磁场强度的增强而线性减小,同时,初生相的生长形态从枝晶转变为椭球状.X射线测试结果表明,初生相Pb发生了点阵膨胀,并且晶格常数随着磁场强度的增强先变大后减小,磁场强度在此存在一个临界值.能谱分析显示,随着磁场强度的增强,初生相Pb内Sn的含量逐渐降低.根据电磁场理论和扩散定律,对上述现象进行了理论分析,揭示出旋转磁场引起了液相强烈流动,加快了溶质原子的扩散以及对熔体的加热效应,导致了形核率的提高和长大速度的降低. 关键词： 旋转磁场 液相流动 晶格常数 溶质分配     

强磁场对Mn-Sb包晶合金相变及凝固组织的影响

以Mn-56.5 wt%Sb包晶合金为研究对象, 进行了不同磁场、不同冷速条件下的凝固实验. 通过对液相线温度、包晶温度的考察, 发现强磁场可以提高Mn-56.5 wt%Sb合金的液相线温度, 且该上升值随磁感应强度的增加而增加, 当所施加的磁感应强度为11.5 T时, 液相线温度升高大约3 ℃, 但施加磁场后包晶反应温度没有明显改变. 对该合金的凝固组织进行定量金相分析发现, 施加磁场后MnSb相明显减少, 该结果与磁场对相变温度的影响相一致. 另外通过X射线衍射分析发现, 强磁场诱发包晶反应生成相MnSb的c轴垂直于磁场方向取向, 而Mn₂Sb相的(311)面平行于磁场方向取向. 对不同冷速凝固的Mn-56.5 wt%Sb合金组织进行定量金相分析结果显示, 强磁场对合金凝固过程的作用效果受到冷却速度的影响. 随着冷却速度的增加, 强磁场对该合金凝固组织中MnSb相的相对含量变化影响效果减弱. 关键词： 强磁场 包晶合金 凝固 相变温度     

NiO-thickness dependent magnetic anisotropies in Fe/NiO/Au(001) and Fe/NiO/MgO(001) systems

The in-plane magnetic anisotropy of Fe/NiO bilayers was studied quantitatively as a function of NiO thickness using the magneto-optical Kerr effect with a rotating field. For NiO thicker than the ordering transition thickness, the total in-plane fourfold anisotropy of the Fe layer decreases with NiO thickness in Fe/NiO/Au(001), but increases in Fe/NiO/MgO(001). Our result indicates that the exchange coupling in an Fe/NiO bilayer might induce an additional in-plane fourfold anisotropy, and the opposite thickness dependent behaviors may be attributed to the different Ni²⁺ antiferromagnetic spin orientations for NiO films grown on Au(001) and MgO(001) surfaces.     

有限磁场作用下等离子体圆柱波导中的线性理论

 考虑了有限磁场的作用,利用“匹配场法”推导出填充环形等离子体时圆柱波导的色散方程,并较详细地讨论了等离子体的厚度、密度, 以及所加磁场强度对色散特性的影响。尤其讨论了等离子体厚度对增长率的影响并得出：对应最大的增长率有一最佳的等离子体厚度。     

Growth of nanosized MnAs/Si(111) magnetoelectronic heterostructures and their magnetooptical study

Thin (6–12 nm) epitaxial MnAs films were MBE-grown on Si(111) substrates under different technological conditions. The films feature essentially different surface morphology. This manifests itself in the formation, on the silicon surface, of hexagonal-shaped crystallites, whose dimensions vary depending on the growth conditions. The volume and surface magnetic properties of the films were studied using the magnetooptical Kerr effect and optical second harmonic generation. The Kerr effect was found to scale linearly with the effective thickness of the magnetic layer. The thickness of the magnetically disordered transition layer formed near the interface with the substrate was estimated. The surface and volume hysteresis properties of the films were found to be different. A contribution to the second-harmonic intensity was observed which is an odd function of magnetization. This effect originates from the interference of the magnetic and nonmagnetic contributions to the nonlinear polarization.     

由小斜率近似计算粗糙海面对空气声源激发的浅海声场的影响

采用一级小斜率近似方法处理空气声经粗糙海面透射至浅海中的声场问题。假定空气、海水和海底为三层均匀介质,将海水层中声场分解为下行波和上行波,导出了粗糙海面条件下下行波和总声场的小斜率近似表达式。导出的声场表达式可由快速傅里叶变换计算,但会出现混叠问题。为了减轻混叠影响,提出了引入额外介质吸收的计算方法。针对高度一维变化且频谱满足PM谱的起伏海面,采用蒙特卡洛仿真计算了相应的水下声场。结果表明,随着海面起伏均方根高度的增加,水下相干场强度减小而均方声压则略有增大。      

基于开口环阵列结构的表面晶格共振产生及二次谐波增强

理论研究了二维周期排列的金开口环谐振器的磁共振模式与周期阵列的衍射模式发生强耦合所需满足的条件及其对二次谐波产生效率的影响.通过控制阵列结构在x和y方向的周期大小,使得衍射模式只在其中一个方向产生,当衍射模式的电场方向与入射光电场偏振方向一致时,衍射模式才会与开口环谐振器的磁共振模式发生强耦合作用,产生表面晶格共振进而实现近场场增强.在此基础上,进一步计算了金开口环谐振器阵列的二次谐波产生效率,随着阵列周期逐渐增大,即开口环谐振器的数密度减小,二次谐波强度呈现先增加后降低的趋势,当开口环谐振器数密度降为原来的1/4左右时,二次谐波强度可以增强2倍以上.本文的研究为金属超表面二次谐波产生效率的提高提供了一种新的可能途径.     

Sound transmission from air into shallow water through rough sea surfaces by small slope approximation

The first-order small slope approximation is applied to the problem of the sound transmission from an airborne source into shallow water through a rough sea surface.By assuming a three-layer homogeneous fluid model and decomposing the sound field in water layer into up-going and down-going waves,the expressions for the down-going wave and thus the total field are derived.The expressions can be calculated by the FFT,but the alias problem will arise.To mitigate the alias effect,additional medium absorption is introduced and afterwards compensated.Monte Carlo calculations are performed.The sea surface is assumed to be random rough with a Pierson-Moskowitz spectrum and to have height variations in only one dimension.The results show that the mean field intensity decreases as the root-mean-square surface height increases,while the mean-square field intensity increases slightly.     

圆形复合式磁控溅射阴极设计及其放电特性模拟研究

基于二维有限元算法使用COMSOL软件对圆形复合式磁控溅射阴极的磁场进行了计算,结合Matlab优化工具箱分别采用遗传算法和模拟退火算法对圆形复合式磁控溅射阴极的结构进行优化,得到靶材利用率达到最大的最优结构.对得到的最优化磁控阴极,基于自洽粒子模拟方法,使用VSim软件对不同工况下的放电特性进行了模拟.研究发现随着磁场非平衡度的增加,阴极表面电势降落最大的位置和等离子体聚集的位置,沿着阴极表面外沿不断向阴极中心移动,阴极表面磁场的强度不断减小.随着磁场非平衡度的增加,等离子体密度先增加后减小,鞘层厚度先减小后增加,等离子体的密度和鞘层厚度不仅与磁场非平衡度有关,而且与磁场强度有关.最后根据粒子模拟的结果,对复合式磁控阴极的靶材刻蚀深度进行了研究.研究发现,在优化前后靶材的刻蚀范围从60 mm扩展至整个靶面,极大地提高了靶材利用率.