静止水中单个上升气泡的直接数值模拟

本文发展了基于Front Tracking的直接数值模拟方法研究气液两相界面的迁移特性,该方法对气液两相采用半隐式的分步法直接求解N-S方程,耦合Front Tracking Method获得两相界面的三维变形。针对无边界以及垂直壁面附近静止水中的单个气泡上升过程进行模拟,研究气泡运动的机理以及气泡与壁面的相互作用。数值模拟准确再现了气泡的上升过程和变形,不同Re数下气泡的上升速度计算结果同经验关联式非常吻合,验证了该方法的有效性。随后分析了气泡周围流场的结构,发现壁面对气泡周围流场的抑制是壁面对气泡作用力的主要原因,将导致气泡逐渐偏离垂直壁面。     

倾斜壁面附近上升气泡的直接数值模拟

本文利用基于Front Tracking的3D直接数值模拟方法对高粘度流体中倾斜壁面附近上升的气泡进行了直接数值模拟,以研究气泡与壁面相互作用以及壁面附近气泡运动的机理.在不同的壁面倾斜角(10°,30°和50°)下针对Eo数分别等于0.7和10的两种典型工况计算了气泡的变形、运动轨迹和上升速度并与实验进行了对比.发现倾斜壁面使得气泡被压扁,在较大的倾斜角下气泡不对称变形程度更大、由此导致的流场不对称性也更大,表明壁面垂直方向的速度受到抑制是壁面引发升力的原因.对于不同的倾角,气泡最终都会沿着倾斜表面滑动,而且倾角越大,气泡中心越接近壁面.气泡的上升速度则随着倾角的增大而单调减小,同实验结果的对比吻合良好.     

气泡碰壁受力模型和反弹规律

对Mo=10-8~10-12及Re=5~750范围内的上升气泡与壁面垂直碰撞问题进行了理论求解，研究了不同控制参数下气泡碰壁反弹的规律.气泡上升和碰撞过程的运动方程考虑了浮力、液体阻力、附加质量力和与壁面碰撞时引起的薄膜诱导力.气泡碰壁过程气泡界面与壁面形成的液膜厚度变化规律由Stokes-Reynolds方程计算得到.膜内气泡变形引起的流体压强采用Young-Laplace方程求解.结果表明，基于SRYL方程的薄膜诱导力模型可以很好地预测不同Reynolds数下气泡0到多次的反弹轨迹，计算结果与实验结果吻合良好.气泡在碰壁反弹过程中会形成丰富的薄膜形状，如酒窝状变形，丘疹状变形和涟漪状变形.气泡界面变形会引起膜内压强的变化，压强的分布规律与气泡界面形状有着重要的关系.气泡在与壁面碰撞的过程中，薄膜诱导力会起主导作用，且随着Reynolds数的增加薄膜诱导力最大量级增大.气泡碰撞壁面时，反弹次数与Reynolds数有着直接的联系，不同Morton数下的气泡均在相同Reynolds数附近发生气泡反弹次数的变化.      

基于边界元法的近平板圆孔气泡动力学行为研究

研究了带有圆孔的平板附近气泡动力学特性. 基于不可压缩势流理论, 建立了平板圆形破口附近气泡运动数值模型, 并针对气泡初始位置距离破口很近而导致计算结果发散的数值缺陷, 采用气泡壁和壁面融合的方法, 将流场分离为两个半无限域问题进行求解, 实现了在不同无量纲参数范围内的数值模拟, 数值结果与实验结果符合良好. 通过对圆孔附近气泡运动特性的研究发现, 圆孔对气泡的影响基本与壁面相反, 在膨胀阶段对气泡产生腔吸作用, 收缩阶段产生排斥, 在特定的工况下会产生对射流现象. 最后分析了气泡壁与壁面融合, 流场分离后的气泡动态特性以及各工况参数对其影响规律. 关键词： 气泡 边界元 射流 圆孔     

过渡流下近壁插入圆柱对壁面强化传热的影响

本文利用基于复合网格系统的计算方法,对Re=50~1200的近壁插入圆柱流场进行数值模拟,研究过渡流状态下在壁面附近插入圆柱对下游壁面传热强化的影响。并基于低速循环水槽流动实验台,采用粒子成像测试法(PIV)对Re=100~500的近壁插入圆柱流场进行可视化实验研究,验证了数值模拟方法的可靠性。研究结果表明:近壁插入圆柱流场在Re=100时进入过渡流状态;Re直接影响圆柱尾流中周期性涡脱和壁面涡岛的发生位置及其洗刷效应的大小,随着Re的增大,洗刷效应明显增强,因而,过渡流范围内Re越大,圆柱下游壁面传热强化越大。     

黏性液体中单个气泡上升的形状特性

采用基于Level Set方法的直接数值模拟技术对黏性液体中单个气泡的上升运动进行三维模拟.数值模拟采用拟单相流模型处理气泡内外的气液两相流动,应用Level Set方法捕捉运动气泡的变形.针对Eo数从O(0)～O(2),Mo数从O(-11)～O(2)的流动范围,重点研究了上升气泡的形状特性,并与经典的气泡形状图谱进行了比较.模拟结果表明,上升气泡的形状与无量纲参数(Eo、Mo和Re)密切相关.在高Re的扁椭球区域,数值发现了气泡形状的周期性振荡行为.     

发汗冷却中流动与换热的数值模拟

本文采用改进的低Re数k-ε模型-低Re数kθ-εθ模型以及局部非热平衡模型,将主流区和多孔壁面区进行耦合,通过数值模拟研究了发汗冷却过程中的流动与换热问题。计算结果表明:多孔壁面中靠近主流区的部分温度梯度很大;随着冷却剂流量的增大,壁面上的最高温度明显下降;就本文所选取的参数而言,在发汗冷却所用的冷却剂的量占总流量的1％左右时,冷却效果非常明显。     

格子Boltzmann方法中的边界条件

比较并讨论了几种格子Boltzmann方法(LBM)中的边界条件处理方法，对标准的壁面全反弹方法进行了改进，提高了壁面反弹方法的精度．通过采用d2q9模型对Poiseuille流和非定常Coueue流进行数值模拟，获得了与分析解比较吻合的结果．针对不同的松弛时间τ和Re数，还给出了采取不同边界条件处理时的收敛精度，验证了小Re数时，LBM可以完全模拟N-S方程，由计算发现松弛时间τ接近1时的精度最高，τ较大时，误差较大。     

有倾角的竖直壁面附近气泡与自由面相互作用研究

本文采用边界元方法, 研究了具有倾角的竖直壁面附近气泡与自由面的相互耦合作用. 首先基于不可压缩势流理论, 建立了边界元气泡动力学模型, 并针对无限壁面附近的气泡和自由面作用问题, 采用镜像法模拟了倾斜壁面的作用. 然后, 基于本文所建立的数值模型, 分别计算了不同倾角的壁面对气泡和自由面水冢形态的影响, 发现倾斜角度会导致自由面的边界条件不连续, 从而对其运动产生明显的非线性影响. 最后, 分别分析了无浮力和有浮力情况下倾斜角度对其影响规律. 关键词： 水下爆炸 气泡动力学 倾斜壁面 自由面     

锯齿型通道湍流流动和换热的自维持振荡

采用RNG k-ε湍流模型对锯齿形通道内流动和换热进行了数值模拟。对以时均方程法模拟得到的流场、温度场以及U-V相图、Nu-θ相图进行了分析。研究表明,在本文计算的Re范围内,数值计算结果仍能反映出周期性通道内流动和换热的自维持振荡特性,无量纲速度U,通道平均Nu数随时间作周期性振荡,并且振荡幅度、频率随Re数的增大而增大。     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

Sorting of bending magnets for the SSRF booster

The Shanghai Synchrotron Radiation Facility(SSRF)booster ring,a full energy injector for the storage ring,is deigned to accelerate the electron beam energy from 150MeV to 3.5GeV that demands high extraction efficiency at the extraction energy with low beam loss rate when electrons are ramping.Closed orbit distortion(COD)caused by bending magnet field uniformity errors which affects the machine performance harmfully could be effectively reduced by bending magnet location sorting.Considering the affections of random errors in measurement,both ideal sorting and realistic sorting are studied based on measured bending magnet field uniformity errors and one reasonable combination of bending magnets which can reduce the horizontal COD by a factor of 5is given as the final installation sequence of the booster bending magnets in this paper.     

Measurement of nonlinear coefficient of optical fiber based on small chirped soliton transmission

We measure the waveform and phase curves of short optical pulses before and after transmission over different lengths of fibers by use of the pulse analyzer with the frequency-resolved optical gating (FROG),and numerically simulate pulse evolution under the experimental conditions.The nonlinear coefficient of the fiber is given by comparing the experimental results with the numerical ones.Difference between the experiment and numerical simulation is analyzed.     

Resonance scattering of canonical elastic shells in absorbing fluid medium

Resonance scattering of elastic spherical shell and cylindrical shell while the surrounding fluid medium has absorption is studied. The normal mode solution derived using exact elastic theory and the separation of variables is still applicable. However, the scattering form function has to be modified for the absorbing medium, otherwise the unreasonable result would be obtained. The backscattering form function in the absorbing medium is redefined, and the form function of elastic spherical and cylindrical shell with vacuum or solid matter filled is calculated in various absorption conditions. The results show that the absorption of surrounding fluid leads to notable attenuation of the coincidence resonances in the mid-frequency, but it has a little influence on the low-frequency resonance scattering induced by the filler inside the shell.     

Single-beam self-referenced phase-sensitive surface plasmon resonance sensor with high detection resolution

A versatile and low-cost single-beam self-referenced phase-sensitive surface plasmon resonance(SPR)sensing system with ultra-high resolution performance is presented.The system exhibits a root-mean-square phase fluctuation of ±0.0028.over a period of 45 min.i.e.a resolution of±5.2×10-9 refractive index units.The enhanced performance has been achieved through the incorporation of three design elements:a true single-beam configuration enabling complete self-referencing so that only the phase change associated with SPR gets detected,a differential measurement scheme to eliminate spurious signals not related to the sensor response,and the elimination of retardation drifts by incorporating temperature stabilization in the liquid crystal phase modulato .Our design should bring the detection sensitivity of non-labeling SPR biosensing closer to that achievable by conventional fluorescence-based techniques.     

Influences of SiO2 protective layers and annealing on the laser-induced damage threshold of Ta2O5 films

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition.The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated.The results show that SiO2 protective layers exert little influence on the electric field intensity(EFI)distribution,microstructure and microdefect density but increase the absorption slightly.Annealing iS effective on decreasing the microdefect density and the absorption of the films.Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT.Moreover,the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.     

A stereo matching algorithm using multi-peak candidate matches and geometric constraints

Gray cross correlation matching technique is adopted to extract candidate matches with gray cross correla- tion coefficients less than some certain range of maximal correlation coefficient called multi-peak candidate matches. Multi-peak candidates are extracted corresponding to three closest feature points at first. The corresponding multi-peak candidate matches are used to construct the model polygon. Correspondence is determined based on the local geometric relations between the three feature points and the multi-peak candidates. The disparity test and the global consistency checkout are applied to eliminate the remaining ambiguous matches that are not removed by the local geometric relational test. Experimental results show that the proposed algorithm is feasible and accurate.