离子推力器放电室阳极壁面电流密度分布特性研究

为了准确掌握离子推力器放电室阳极壁面电流密度分布特性，并深入理解阳极壁面处等离子体运动特性，设计了近阳极壁面等离子体诊断的具体实施方案，并基于LIPS-200离子推力器开展了近阳极壁面处等离子体诊断试验研究，得到了主要磁极附近壁面等离子体参数，并得到阳极壁面吸收电流密度分布特性。试验结果表明：LIPS-200离子推力器阳极壁面处主要磁极附近的等离子体密度范围为，测试点的电子温度范围为，壁面电流密度范围为；柱段壁面电子温度相对锥段较低，但电流密度较大，尤其在中间极靴位置电流密度最大，约为阴极极靴处电流密度的3倍，约为屏栅极靴处电流密度的2倍，阳极电流主要在放电室中间极靴处发生损失。     

径向压力和温度分布对螺旋波等离子体波场和能量吸收影响研究

为了揭示螺旋波等离子体推力器中的等离子体源功率耦合机理,针对气体工质电离后被射频加热的稳态过程,考虑等离子体密度非均匀分布条件,采用三参数压力函数(fa,sp,tp)和温度函数(f_a,s_t,t_t)表示柱状等离子体内压力和温度的径向分布,分析了径向压力梯度、温度梯度对螺旋波等离子体内功率沉积、波电场、波磁场和电流密度的影响。考虑梯度为正,梯度为负和梯度为零三种梯度类型。结果发现:压力梯度为正时,螺旋波在等离子体临近壁面处的功率沉积减弱,但射频波透入深度增加,原因是靠近管壁处等离子体密度较低,RF波径向单位长度衰减较少,透入深度增加。温度梯度为负时,柱状等离子体中心处能量沉积变强,原因是管中心位置等离子体密度较大,电子温度较高,与RF波能量耦合增强;横向截面的电磁场、电流密度分布在不同压力和温度梯度下基本不变,证明了m=1模式的稳定性。     

离子推力器放电腔内等离子体流动规律的全粒子模型

为了详细描述等离子体在放电腔内的产生、演化过程,建立了放电腔内部等离子体流动的二维轴对称全粒子数值模型。在考虑壁面二次电子发射影响的前提下,模拟得到了稳态后放电腔内电磁场、电子与离子数密度,电流密度等一系列参数分布,且与相关实验数据进行了比较,吻合较好。模型得到了实验中难以观测到的放电腔内等离子体的产生、演化过程以及稳态下的分布规律,合理解释了放电腔工作的基本原理。     

磁路温度对霍尔推力器放电热稳定性的影响

为研究磁路高温性质变化对霍尔推力器放电热失稳的贡献及影响机理,对不同磁路温度下推力器的工作磁场强度开展了实验测量,对磁路温度变化与通道内等离子体放电行为变化的交互影响开展了Particle-in-Cell数值模拟研究。实验结果表明,当磁路温度由室温升高到600℃时,推力器的工作磁场强度发生了衰减,尽管衰减量不大(约5%)。模拟结果表明,磁路高温引起的场强衰减改变了推力器放电时的电导率及电势分布,进而对电子能量各向分布、粒子密度分布等造成了影响,促进了电子在壁面的通量及能量损失,主导了壁面等离子体沉积功率的增加,从而进一步加剧了磁路温度的增长。这是一个具有正反馈性质的过程;因此,若不能通过外部手段有效控制磁路温度,将诱发霍尔推力器的放电热失稳。     

对称交叉激波和湍流边界层相互作用的数值研究

针对两种双尖鳍外形的对称交叉激波与湍流边界层相互作用,采用N-S方程和两种湍流模型进行了计算。研究了网格收敛性、鳍的角度和湍流模型对壁面压强、Stanton数和壁面摩擦力线的影响。弱相互作用的计算结果较好,强相互作用的壁面压强和摩擦力线的计算结果与试验吻合较好,而Stanton数的计算结果较差,峰值高达试验的2.5倍左右。随着鳍的角度的增加,壁面压强和Stanton数的分布从单调分布发展为M型分布,两者的峰值不在相同的位置。湍流模型对壁面压强和壁面摩擦力线影响很小,对Stanton数计算的影响很大,SST模型比BSL模型表现好一些。     

射频离子推力器热特性仿真分析

为了分析射频离子推力器热特性,建立了射频离子推力器整体热模型,基于二维流体模型,对11cm射频离子推力器开展了放电室等离子体仿真,获得了电子温度、电势分布等关键参数;以等离子体仿真结果和实测束电流为输入,获得了各热源的热通量;通过有限元计算获得了关键部组件的温度分布,与实验结果进行了对比分析。研究结果显示:放电室内电子温度约为3.6eV～3.9eV,等离子体电势最高20V,发热损耗主要来自带电粒子轰击放电室壁面和栅极造成的能量沉积、激发原子的热辐射以及射频线圈自身的发热损耗,温度仿真与实测结果一致性良好,最大误差7%,仿真得到的温度分布可以作为输入参数进一步研究栅极受热形变及对束流的影响。     

等离子体气动激励诱导空气流动的PIV研究

为了揭示等离子体气动激励与边界层相互作用的物理机制,作者进行了等离子体气动激励诱导空气流动的PIV研究。实验结果表明：毫秒、微秒等离子体气动激励诱导空气流动以“启动涡”和“壁面射流”的形式出现;当激励电压为12kV时,最大诱导速度约为3m／s;激励电压越大,“启动涡”和“壁面射流”的强度越大;脉冲激励的作用强度和作用范围要强于定常激励。该结论为提高等离子体流动控制的作用能力提供了指导。     

射频离子推力器热特性仿真分析

为了分析射频离子推力器热特性，建立了射频离子推力器整体热模型，基于二维流体模型，对11cm射频离子推力器开展了放电室等离子体仿真，获得了电子温度、电势分布等关键参数；以等离子体仿真结果和实测束电流为输入，获得了各热源的热通量；通过有限元计算获得了关键部组件的温度分布，与实验结果进行了对比分析。研究结果显示：放电室内电子温度约为3.6eV~3.9eV，等离子体电势最高20V，发热损耗主要来自带电粒子轰击放电室壁面和栅极造成的能量沉积、激发原子的热辐射以及射频线圈自身的发热损耗，温度仿真与实测结果一致性良好，最大误差7%，仿真得到的温度分布可以作为输入参数进一步研究栅极受热形变及对束流的影响。     

带交错叉排肋的矩形通道压力特性数值模拟

针对上下壁面交错又排布置肋通道的阻力特性，利用数值计算的方法研究通道的压力系数分布。入口雷诺数变化范围为60000-150000；采用非结构化网格进行网格划分，利用realizable湍流模型和增强壁面函数封闭方程；计算结果表明，带肋通道的流动阻力较大，沿着流动方向，压力系数迅速地减小，随着雷诺数的增加，压力系数增加，带肋的下壁面压力系数分布非常的不均匀，不带肋的左侧壁面压力系数分布相对均匀。     

机身边界层控制研究

对带壁面抽吸的三维可压缩机身边界层控制进行了计算研究。采用了非正交曲线坐标系边界层方程,通过修改矢量流函数关系式,将壁面法向速度转变成为控制方程的一个可调参数,从而消除了边界条件与方程未知参量之间的非线性耦合;所发展的横向分段推进数值技术,可用于计算横流速度在横向多次反号的复杂边界层流动;设计不同的壁面抽吸系数分布,以研究边界层控制对多种边界层特性的影响。结果表明:壁面抽吸对边界层的控制效果是显著的,合理的抽吸分布能有效地减小机身阻力。     

THE INFLUENCE OF THE LOCAL INTERSTELLAR MEDIUM ON THE SOLAR WIND DYNAMICS IN THE INNER HELIOSPHERE

The consequences of the interaction between the solar wind and the local interstellar medium for the wind region enclosed by the heliospheric shock are reviewed. After identifying the principal mechanisms to influence the dynamics of the solar wind, an approach allowing the simultaneous incorporation of neutral atoms, pick-up ions, cosmic rays and energetic electrons into a multifluid model of the expanding wind plasma is outlined. The effects of these particle species are discussed in detail, with special emphasis on the electron component which behaves more like a quasi-static hot gas rather than an expanding fluid. This electron gas is effectively trapped within a three-dimensional trough of a circumsolar electric potential whose outer fringes are possibly determined by the density distribution of anomalous cosmic rays. The electrons are proven to be a globally structered component of great importance for the solar wind momentum flow contributing to a triggering of the solar wind dynamics by asymmetric interstellar boundary conditions. Finally, the consequences for the relative motion of the Sun and the local interstellar medium as well as for the solar system as a whole are described.     

倾转旋翼机旋翼对机翼气动干扰的建模及分析

建立了一个耦合旋翼自由尾迹模型和机翼面元模型的悬停状态旋翼/机翼气动干扰迭代计算方法,用于分析旋翼对机翼的气动干扰影响.在该方法中,为较好地模拟大桨盘载荷及大扭转桨叶的气动特征,旋翼桨叶采用Weissinger-L升力面模型;为考虑厚度效应及机翼的升力影响,建立了包含源面元和偶极子面元的厚度机翼模型;为正确模拟旋翼桨尖涡与机翼表面间的贴近干扰,采用了一个"分析数值匹配法"的"贴近涡/面干扰模型".应用上述方法,对单独倾转旋翼下洗流分布以及旋翼对机翼的气动干扰影响进行了计算.结果表明,在旋翼下洗流场的干扰下,机翼各剖面都产生向下载荷,但并非简单地随拉力系数的增大而增大;机翼受到的旋翼干扰影响与旋翼下洗流沿桨叶展向变化密切相关.     

Kinetic Models for Whistler Wave Scattering of Electrons in the Solar Corona and Wind

Kinetic models are necessary to describe the physical processes associated with non-Maxwellian velocity distribution functions (VDFs) of electrons or ions in the solar corona and wind. It is shown that pitch-angle scattering of electrons in the solar wind needs to be considered in kinetic solar wind models. Coulomb collisions are not efficient enough to provide this scattering, but resonant interaction with whistler waves is. A solar wind model for undisturbed fast wind is presented, and the influence of scattering on flare electron propagation is investigated. Furthermore, it is found that resonant interaction of electrons with whistler waves is capable of producing suprathermal tails of electron distributions even under quiet conditions without flare activity.     

风扇湍流宽频噪声特性的数值计算分析

基于Gliebe发展的风扇自噪声经验预测模型与Mugridge-Morfey发展的湍流-叶片干涉噪声分析预测模型,以GE公司R4风扇为研究对象,详细分析了风扇自噪声与转子尾迹湍流-出口导叶干涉宽频噪声(湍流干涉噪声)的特性.研究了不同的风扇设计参数和湍流参数对风扇自噪声与湍流干涉噪声的影响.计算结果表明:湍流干涉噪声对风扇宽频噪声起着主要作用,湍流干涉噪声比转子自噪声大约6~10dB,比出口导叶自噪声大约20dB.此外,出口导叶来流速度对湍流干涉噪声的影响最大,来流速度增大15%时,声功率可增大4dB左右.湍流强度对湍流干涉噪声影响较大,当湍流强度增加30%时,声功率增大约1.5dB.      

Space weather: its effects and predictability

Terrestrial technology is now, and increasingly, sensitive to space weather. Most space weather is caused by solar storms and the resulting changes to the Earth's radiation environment and the magnetosphere. The Sun as the driver of space weather is under intense observation but remains to be adequately modelled. Recent spacecraft measurements are greatly improving models of solar activity, the interaction of the solar wind with the magnetosphere, and models of the radiation belts. In-situ data updates the basic magnetospheric model to provide specific details of high-energy electron flux at satellite orbits. Shock wave effects at the magnetopause can also be coarsely predicted. However, the specific geomagnetic effects at ground level depend on the calculation of magnetic and electric fields and further improvements are needed. New work on physical models is showing promise of raising geomagnetic and ionospheric predictability above the synoptic climatological level. This revised version was published online in August 2006 with corrections to the Cover Date.     

霍尔推力器等离子体磁鞘特性

为进一步研究霍尔推力器壁面二次电子发射对发动机性能的影响,建立流体模型数值模拟了霍尔推力器磁化二次电子等离子体的鞘层特性,采用Sagdeev势的方法得到了鞘层的玻姆判据,以这个判据为鞘层边界条件数值讨论了磁场、二次电子发射以及不同等离子体对推力器等离子体鞘层结构的影响。结果表明,磁场和二次电子发射系数的增加都将使鞘层中粒子密度增加,鞘层电势升高,鞘层厚度减小;对于氩和氙二种等离子体,氙等离子体产生的鞘层势垒高,粒子密度大,鞘层厚度增加。这些结果直接影响推力器的电子传导机制及性能。     

Type-III radio bursts and their interpretation

The observations of type-III solar radio bursts are briefly reviewed to set requirements on a model for their interpretation. The most important of these requirements is that the source must be an electron stream which is in a state of continuous quasilinear relaxation and which initially must have a nearly monotonically decreasing velocity distribution. The problem of constructing a model is broken into three parts: (1) The plasma wave source which depends on the interaction of the electron stream with electron plasma waves. (2) The radiation source which depends on the interaction of plasma waves and transverse electromagnetic waves or in a magnetized plasma the ordinary and extraordinary modes of magnetoionic theory. (3) The propagation of radiation between the source and the observer which depends on the transmission of radiation through a scattering refracting absorbing magnetized plasma.Progress on a model for the plasma wave source is reviewed and it is concluded that no existing models are adequate. The equations which would lead to an adequate model are written down, but not solved. These include, in addition to collisional damping, Landau damping both by the exciting stream and the background plasma, and spontaneous and induced processes for a three-dimensional distribution of plasma waves. Possible limitations to a quasilinear approach such as pile-up of plasma waves and nonlinear effects are considered. Processes which affect the gross structure of the source such as electron trajectories in coronal streamers and electron scattering by inhomogeneities are reviewed.Progress on the radiation source is considered both in the absence and presence of a magnetic field. At high frequencies (e.g., 80 MHz) observations of radiation near the fundamental and second harmonic of the plasma frequency allow a unique determination of source size and the energy density in plasma waves within the uncertainties of geometry by source ray tracing. This determination is extremely critical because the fundamental must be amplified and thus production of the fundamental is effectively a much more highly nonlinear process than production of the second harmonic. At low frequencies (e.g., 500 kHz) the second harmonic is shown to be dominant because amplification of the fundamental becomes an inefficient process.Calculations of scattering of radiation in a random medium are reviewed. It is concluded that these are adequate at high and low frequencies, but have not been carried out properly at intermediate frequencies where amplification of the fundamental may still be present. It is shown in particular that when scattering is taken into account at high frequencies all observations can be explained by isotropic emission near the second harmonic. At low frequencies the nature of the scatterers is determined by source occultations unlike the case at high frequencies where these are free parameters. This fact allows the possibility of determining true source sizes at low frequencies by subtracting out the contribution due to scattering. A mechanism for producing the possibly observed linear or highly elliptical polarization of type-III bursts, which must be imposed far from the source due to Faraday rotation, is reviewed.Finally, the questions of what remains to be done and what we can hope to obtain upon completion of this work are briefly considered.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

选区熔化过程多尺度多物理场建模研究进展

选区熔化技术是一种基于粉末床的、能够精确成形复杂零件、调控微观组织和性能的金属增材制造技术,其成形过程的计算机模拟对于生产实践具有重要指导意义。综述了国内外的研究现状,首先介绍选区熔化技术的原理及其特点;然后介绍国内外对选区熔化过程已经开展的计算机模拟技术的研究进展;并进一步重点介绍针对电子束选区熔化过程的多尺度多物理场模型,主要包括:(1)微观尺度电子束与材料相互作用的能量吸收模型;(2)细观尺度从铺粉到粉末加热、熔化、流动、沉积成形全过程的模型;(3)宏观尺度零件成形过程模型。     

飞机结构腐蚀管理全寿命模型研究

针对当前飞机的腐蚀损伤评估模式,构建了一种较为合理的腐蚀管理全寿命分析模型。该模型可将全寿命阶段飞机结构可能遭受的各种腐蚀形态、MSD和结构材料性能随时间的退化作为变量纳入到一个框架中,提高了分析精度和可靠性,为新机定寿和老龄飞机延寿提供技术支持;方法具有一般性,可推广到其他装备结构寿命评定中。     

微波与等离子体之间的相互作用

根据微波和等离子的基本理论以及它们之间相互作用的原理,建立了由微波能、谐振腔内电场、工质流场、等离子体平衡场等组成的轴对称偏微分方程组。通过对模型的计算,获得了放电管内等离子体区的形状、电场、电子浓度和温度场,以及气体温度场。