激光烧蚀Al靶产生的等离子体中辐射粒子的速度及激波

在低真空条件下(5Pa),通过测量脉冲激光烧蚀平面Al靶产生的等离子体辐射谱的时间分辨特征,得到辐射粒子速度的空间分布.在激光脉冲宽度为10ns,烧蚀斑直径为200μm,靶面上功率密度分别为1.91×10¹⁰,5.10×10¹⁰和7.64×10¹⁰W/cm²时,测得辐射粒子Al的速度均在10⁶cm/s量级,且随着靶面径向距离的增大而近似呈指数衰减.在距靶面的相同距离处,激光功率密度的增大反而使速度减小.利用激波模型(shockwave model)较好地解释了实验结果,并得出激波的波面基本为柱对称 关键词： 激光等离子体 平面Al靶 粒子速度分布 激波     

用格子Boltzmann模型模拟非等温流场

根据微观和宏观之间的质量、动量、能量守恒准则和在原格子Boltzmann模型基础上,建立了几个新的格子Boltzmann模型,使得在外力场中的格子Boltzmann模型得到进一步完善.通过还原宏观流体力学方程,捕捉到了浮力强迫系数与Grashof数之间的关系.所得动量方程和Navier Stokes方程相比,在黏性输运项上有明显的改进,说明黏性应力不但与流体的速度梯度和流体的压缩性有关,而且还与非定常的内能梯度和动量通量有关.该模型对非等温流场的数值结果证明了其具有很好的数值稳定性和适用性. 关键词： Boltzmann模型 平衡分布函数 流体力学方程     

旋转肥皂泡热对流能量耗散与边界层特性的数值模拟

将底部加热的半个肥皂泡作为一个新的热对流模型,结合了肥皂泡固有的球面与准二维特征,由此有助于理解行星大气流动中的复杂物理机制与热对流特性.本文使用直接数值模拟方法计算了旋转肥皂泡上的湍流热对流,研究了肥皂泡上的温度与黏性边界层以及拟热能和动能耗散规律.结合肥皂泡上温度场与速度场特征,分别根据温度脉动均方根最大值以及速度脉动边界处斜率延长线与最大值交点提出了肥皂泡上温度与黏性边界层的识别方法.研究发现,当肥皂泡从边界吸收能量时,拟热能耗散与动能耗散均集中在边界层中,肥皂泡上的温度边界层与黏性边界层厚度与瑞利数Ra存在明确的标度关系.相比经典Rayleigh-Bénrad对流(RB对流)模型,温度标度指数具有较为接近的结果,但速度标度指数存在一定的差异.此外,在混合区,均方根温度(T^*)随纬度(θ)具有近似T^*～θ^0.5的标度关系,这与RB对流模型及其相应的理论预测一致.最后通过能量平衡方程发现,肥皂泡上拟热能内耗散率ε0/T和动能内耗散率ε0/u比拟热能外耗散率ε1/T和动能外耗散率ε1/u大1个量级,拟热能与动能的内部耗...     

Ag离子注入非晶SiO2的光学吸收、拉曼谱和透射电镜研究

能量为200keV的Ag离子，以1×10¹⁶，5×10¹⁶，1×10¹⁷ cm^-2的剂量分别注入到非晶SiO₂玻璃，光学吸收谱显示：注入剂量为1×10¹⁶ cm^-2的样品的光吸收谱为洛伦兹曲线，与Mie理论模拟的曲线形状一致；注入剂量较大的5×10¹⁶，1×10¹⁷ cm^-2的谱线共振吸收增强，峰位红移并出现伴峰. 透射电镜观察分析表明，注入剂量不同的样品中形成的纳米颗粒的大小、形状、分布都不同，注入剂量较大的还会产生明显的表面溅射效应，这些因素都会影响共振吸收的峰形、峰位和峰强. 当注入剂量达到1×10¹⁷ cm^-2时，Ag纳米颗粒内部可能还形成了杂质团簇. 关键词： 离子注入 纳米颗粒 共振吸收 红移     

离散速度方向模型在微尺度气体流动中的应用

离散速度方向模型是一种简化Boltzmann方程的新方法。该方法通过减少Boltzmann方程的维数来降低数值求解的计算量。在DVD模型中,分子速度的方向是离散的,而分子的速率仍然是连续的,这样就可以用一组三维的速率分布函数来代替Boltzmann方程中六维的速度分布函数。由于减少了三个动量维,同Boltzmann方程相比,DVD模型的数值计算量可以降低几个数量级。本文用数值的方法对DVD模型进行了研究。数值结果显示,在广泛的Knudsen数下,DVD方法可给出精确的计算结果。同线性化Boltzmann方程的计算结果相比,最大的误差不超过6%,在连续介质领域中,误差甚至不超过1%。     

约化场强对氮-氧混合气放电等离子体演化特性的影响

采用零维等离子体动力学模型,计算了不同约化场强条件下N₂/O₂放电等离子体的演化特性.结果表明,平均电子能量与约化场强有着近似的线性关系,在约化场强为100 Td时,平均电子能量约为2.6 eV、最大电子能量达35 eV;约化场强是影响电子能量函数分布的主要因素.气体放电过程结束后,振动激发态氮分子的粒子数浓度不再变化,电子激发态的氮分子、原子和氧原子的粒子数浓度达到一峰值后开始降低;放电结束后的氧原子通过复合反应生成臭氧.约化场强升高,由于低能电子减少的影响,振动激发态氮分子的粒子数浓度降低,当约化场强由50 Td增加75 Td,100 Td时,粒子数浓度由3.83×10¹¹ cm^-3降至1.98×10¹¹ cm^-3和1.77×10¹¹ cm^-3,其他粒子浓度则相应增大. 关键词： 等离子体 约化场强 粒子演化 数值模拟     

流体-亚稳态原子传输混合模型模拟空心阴极放电特性

利用流体-亚稳态原子传输混合模型研究了氩气矩形空心阴极放电稳态时的参数. 数值计算得到了压强为10 Torr时的电势、电子、离子和亚稳态氩原子密度以及电子平均能量的分布. 结果表明电子和离子密度峰值为4.7×10¹² cm^-3, 亚稳态原子密度峰值为2.1×10¹³ cm^-3. 本文同时对流体-亚稳态原子传输混合模型和单一流体模型模拟得到的放电参数进行了比较. 结果表明, 分步电离是新电子产生的重要来源, 亚稳态原子对空心阴极放电特性有重要影响. 与单一流体模型相比, 混合模型计算得到的电子密度升高, 阴极鞘层宽度和电子平均能量降低. 关键词： 空心阴极放电 流体-亚稳态原子传输模型 电子密度 分步电离     

氧气空心阴极放电模拟

本文利用流体模型对气压为266 Pa的氧气环境下空心阴极放电的放电特性及不同粒子的生成损耗机制进行了模拟研究.模型中包含11种粒子和48个反应.在该模拟条件下,周围阴极所对应的负辉区产生重叠,表明放电中存在较强的空心阴极效应.计算得到了不同带电粒子与活性粒子的密度分布.带电粒子密度主要位于放电单元中心区域,电子和负氧离子O~-是放电体系中主要的负电荷,其密度峰值分别达到5.0×10¹¹ cm^-3和1.6×10¹¹ cm^-3;O₂~+是放电体系中主要的正电荷,其密度峰值为6.5×10¹¹ cm^-3.放电体系中同时存在丰富的活性氧粒子,并且其密度远高于带电粒子,按其密度高低依次为基态氧原子O、单重激发态氧分子O₂(a~1Δ_g)、激发态氧原子O(~1D)、臭氧分子O₃.对电子、O~-和O₂~+的生成和损耗的反应动力学过程进行了深入分析,同时给出了不同活性...     

Burgers-Korteweg-de Vries复合方程的格子Boltzmann方法模拟

针对Burgers-Korteweg-de Vries(cBKdV)复合方程提出一种格子Boltzmann模型.通过恰当地处理色散项u_xxx并运用Chapman-Enskog展开从格子Boltzmann方程推导出宏观方程,从而得到联系微观量与宏观量的局部平衡分布函数.对不同微分方程进行数值实验,数值解与解析解非常吻合,相比于其它数值结果,该格子Boltzmann模型的数值结果更精确,说明该数值模型的高效性.     

热动力学格子Boltzmann模型

提出了一类热动力学格子Boltzmann模型,其分布函数中含有内能项与高阶速度项。以三个HPP模型为基础,建立了一个三迭加HPP格子Boltzmann模型,并给出其局部平衡分布函数。设计了热动力学现象模拟中的外力处理方法和温度边界条件处理方法,用该模型成功地模拟了Benard热对流现象。     

Boltzmann Equation Analysis of the Electron Gas in CF4 Discharge Plasma

The electron energy distribution function in CF₄ gas discharge was numerically calculated for density reduced electric field E/N = 15 - 300 × 10^?17 Vcm² using the Boltzmann kinetic equation. The momentum transfer and vibrational cross-sections were derived from available experimental swarm parameters while the cross-sections for inelastic electron-molecule collisional processes included were taken from the literature. The two-term spherical harmonic expansion of the distribution function was used in the calculations.     

Three-dimensional finite-difference lattice Boltzmann model and its application to inviscid compressible flows with shock waves

In this paper, a three-dimensional (3D) finite-difference lattice Boltzmann model for simulating compressible flows with shock waves is developed in the framework of the double-distribution-function approach. In the model, a density distribution function is adopted to model the flow field, while a total energy distribution function is adopted to model the temperature field. The discrete equilibrium density and total energy distribution functions are derived from the Hermite expansions of the continuous equilibrium distribution functions. The discrete velocity set is obtained by choosing the abscissae of a suitable Gauss–Hermite quadrature with sufficient accuracy. In order to capture the shock waves in compressible flows and improve the numerical accuracy and stability, an implicit–explicit finite-difference numerical technique based on the total variation diminishing flux limitation is introduced to solve the discrete kinetic equations. The model is tested by numerical simulations of some typical compressible flows with shock waves ranging from 1D to 3D. The numerical results are found to be in good agreement with the analytical solutions and/or other numerical results reported in the literature.     

基于总能形式的耦合的双分布函数热晶格玻尔兹曼数值方法

双分布函数热晶格玻尔兹曼数值方法在微尺度热流动系统中得到广泛的应用. 本文基于晶格玻尔兹曼平衡分布函数低阶Hermite展开式, 创新性地提出了包含黏性热耗散和压缩功的耦合的双分布函数热晶格玻尔兹曼数值方法, 将能量场内温度的变化以动量源的形式引入晶格波尔兹曼动量演化方程, 实现了能量场与动量场之间的耦合. 研究了考虑黏性热耗散和压缩功的和不考虑的两种热自然对流模型, 重点分析了不同瑞利数和普朗特数下流场内的流动情况以及温度、速度和平均努赛尔数的变化趋势. 本文实验结果与文献结果一致, 验证了本文数值方法的可行性和准确性. 研究结果表明: 随着瑞利数和普朗特数的增大, 方腔内对流传热作用逐渐增强, 边界处形成明显的边界层; 考虑黏性热耗散和压缩功的模型对流作用相对增强, 黏性热耗散和压缩功对自然对流的影响在微尺度流动过程中不能忽略.     

Fluctuating coulomb field near ionic dielectric surfaces

Analytical and numerical studies of the energy density spatial distribution of a fluctuating electric field near the model ionic crystal surface as a function of temperature are carried out. It is established that the fluctuating field energy density decreases as h ^−3.3 with increasing distance h from the crystal surface. The fluctuating field energy increases with temperature and differs from zero at −273°C due to quantum zero-point vibrations of ions in the lattice.     

Gas-kinetic numerical studies of three-dimensional complex flows on spacecraft re-entry

The gas-kinetic numerical algorithm solving the Boltzmann model equation is extended and developed to study the three-dimensional hypersonic flows of spacecraft re-entry into the atmosphere in perfect gas. In this study, the simplified velocity distribution function equation for various flow regimes is presented on the basis of the kinetic Boltzmann–Shakhov model. The discrete velocity ordinate technique and numerical quadrature methods, such as the Gauss quadrature formulas with the weight function 2/π^1/2exp(?V²) and the Gauss–Legendre numerical quadrature rule, are studied to resolve the barrier in simulating complex flows from low Mach numbers to hypersonic problems. Specially, the gas-kinetic finite-difference scheme is constructed for the computation of three-dimensional flow problems, which directly captures the time evolution of the molecular velocity distribution function. The gas-kinetic boundary conditions and numerical procedures are studied and implemented by directly acting on the velocity distribution function. The HPF (high performance fortran) parallel implementation technique for the gas-kinetic numerical method is developed and applied to study the hypersonic flows around three-dimensional complex bodies. The main purpose of the current research is to provide a way to extend the gas-kinetic numerical algorithm to the flow computation of three-dimensional complex hypersonic problems with high Mach numbers. To verify the current method and simulate gas transport phenomena covering various flow regimes, the three-dimensional hypersonic flows around sphere and spacecraft shape with different Knudsen numbers and Mach numbers are studied by HPF parallel computing. Excellent results have been obtained for all examples computed.     

A minimum photon “rest mass” — Using Planck's constant and discontinuous electromagnetic waves

Reasons for taking1/2h/c ² in cgs units as an equivalent in grams for the photon “rest mass” are given. Its numerical value of3.68×10 ^?48 g corresponds to the minimum mass equivalent energy for one half-cycle of an electromagnetic dipole field distribution, which is discontinuous. For the fluid models that are discussed, this field distribution corresponds somewhat to a hydrodynamic toroidal vortex which is stationary—if we use toroidal coordinates and assume that the ring origin has the radial velocity c, that the gauge is defined by the ring origin diameter, and that free space is represented by a two-fluid model (the fluids oppositely charged). There are mappings which can transform such toroidal entities (photons) into spherical ones. The toroidal entities are possible candidates for the role of “hidden variables.”     

Laser induced collisional energy transfer in Sr—Li system

A four-state model considering the relative velocity distribution function for calculating the cross section of laserinduced collisional energy transfer in a Sr-Li system is presented and profiles of laser-induced collision cross section are obtained.The resulting spectra obtained from different intermediate states are strongly asymmetrical in an opposite asymmetry.Both of the two intermediate states have contributions to the final state,and none of the intermediate states should be neglected.The peak of the laser-induced collisional energy transfer(LICET) profile shifts toward the red and the FWHM becomes narrower obviously with laser field intensity increasing.A cross section of 1.2 × 10-12 cm 2 at a laser field intensity of 2.17 × 10 7 V/m is obtained,which indicates that this collision process can be an effective way to transfer energy selectively from a storage state to a target state.The existence of saturation for cross section with the increase of the laser intensity shows that the high-intensity redistribution of transition probabilities is an important feature of this process,which is not accounted for in a two-state treatment.     

二维平板通道中流动与传热的格子-Boltzmann模拟

格子-Boltzmann数值模拟方法(LBM),在最近十几年来得到迅速发展。本文发展了LBM的流动与传热模型,并对二维平板通道中的流动与传热进行了模拟,采用密度分布函数得到速度场,用单独的内能分布函数得到温度场,并与传统FVM方法所得到的多个特征量结果进行了比较,模拟结果与FVM解均吻合很好。鉴于LBM边界条件处理简单和易于实施等特点,该方法可望成为求解流动与传热的一种有效数值模拟手段。