非定常粘性空化流动模型及其数值计算

基于气液两相当地均相介质模型,本文给出了一种模拟非定常粘性空化流动的计算模型。认为空化绕流流场中流动介质是一种当地均匀的气液混相物。控制方程采用了应用两相流模型的N-S方程。基于液相和气相的状态方程推导了混合介质密度的表达式。为了保证数值计算的稳定性,控制方程的数值求解采用了TVD-MacCormack格式。为了评价计算模型的可靠性,分别计算了绕台阶和管道水锤的空化流动,所得结果是合理的,说明该方法可以用于空化流动的数值计算。     

绕水翼片状空化流动结构的数值与实验研究

采用数值与实验相结合的方法研究了水翼片状空化流动结构.实验采用高速录像技术观察了片状卒泡形态,应用LDV测量了翼型周围的湍动能和速度分布;采用N-S方程和基于空泡动力学方程的空化模型计算了绕水翼片状空化流场.结果表明:在片状空化阶段,翼型吸力面上附着很薄的一层透明空泡,空泡彤态呈现于指状;随着空化数的减少,空泡尾部水汽交界面相互作用增强,并且空泡尾部出现大的旋涡,影响了空泡尾部区域压力和速度分布,片状空泡尾部的水汽混合区出现不稳定现象,同时存在小的空泡团脱落.数值模拟得到的水翼片状空化流动现象和实验观察到的结果基本一致,验证了计算模型和数值方法的可靠性.     

双级轴流泵内部三维紊动流场CFD分析

双级轴流泵可用作船舶的推进装置,具有推进效率高、抗空化能力强、工作平稳、噪声低等优点。采用标准k-ε紊流模型和SIMPLEC算法,对双级轴流泵内360°全流道的三维紊动流场的速度和压力分布进行数值模拟计算,并对装置的性能进行预估。计算结果与试验数据的比较表明:所采用的紊流模型和数值方法可用来描述实际流动现象。     

部分流低温流体循环泵空化性能预测

为了对部分流低温流体(液氮) 循环泵空化特性进行预测. 基于软件 ANSYS-FLUENT, 计算选用Stardard k-ε 湍流模型,Simplec 压力耦合方式, 进行空化前 MRF 模型定常计算, 添加两相流参数, 选取 Singhal-et-al 空化模型, 添加两相参数, 考虑液-气密度比对低温流体液氮泵内能量的传递和交换的影响, 得到气液动量、 质量和能量守恒方程, 利用 RNGk-ε 湍流模型,Simplec 压力耦合方式, 对不同进液压力条件下, 部分流低温流体(液氮) 循环泵空化特性进行全流域空化数值计算. 进行泵空化特性试验, 在额定转速下, 随着泵前流体压力的降低呈现的空化性能, 数值计算与试验测得的泵头数值最大偏差在10% 以内, 曲线吻合性较好, 泵内流场空化发生伴有显著的压头下降, 空化过程增强, 空化区变大, 液相和汽相相互拖拽能力增强, 空化核心区由叶顶背压部分扩散到整个流道, 汽液界面不清晰, 直至断流. 本文采用的计算方法和研究结果为低温流体循环泵内部流体空化的诊断和性能优化提供了一定依据.     

应用隐式SMAC格式计算射流放水阀内部非定常不可压粘性流场

为了分析射流放水阀内部的非定常不可压粘性流场,在一般曲线坐标系下求解了以逆变速度为参数的不可压Ｎ－Ｓ方程和压力椭圆方程。计算采用了基于隐式ＳＭＡＣ格式的有限差分格式。在这种格式中,非定常流场的速度是通过使用Ｃｒａｎｋ－Ｎｉｃｈｏｌｓｏｎ格式,并在每一时间步处以牛顿迭代的方法得到的;而压力椭圆方程是通过使用Ｔｓｃｈｅｂｙｓｃｈｅｆｆ ＳＬＯＲ格式并交替改变计算方向而求解。通过使用交错网格和迎风差分,诸如改进的ＱＵＩＣＫ格式;计算过程中的误差和数值不稳定能够得到有效地控制。计算结果和实验观察得到的流动图谱对比表明,二者十分吻合。     

非定常欠膨胀射流的正格式数值模拟

 采用二阶正格式方法对非定常欠膨胀射流进行了数值模拟。将二维守恒方程的正格式方法推广到轴对称Euler方程组的求解,并对不同马赫数下的燃气射流进行了数值计算。计算结果表明,该方法能够较好地捕捉到包含膨胀波、入射激波、反射激波、马赫盘、射流边界以及三波点等复杂射流流场的波系结构,与实验照片反映的流动特征以及已有的数值结果相吻合。表明该方法对间断解具有较强的捕捉能力,在激波阵面上不会出现数值振荡。     

混流泵叶轮内空化流动的数值计算

本文应用三维Navier-Stokes方程和k-ε两方程湍流模型以及全空化模型,对一台喷水推进混流泵叶轮内的空化流动进行数值模拟.介绍了空化流动的模拟方法,并计算了混流泵的空化性能指标,有效地预测了叶片上空化发生的区域和空化流动的发展情况.文中还对两种进口相对流动角作了数值计算,结果表明小的相对流动角可以提高混流泵的空化性能,但却使扬程有所下降.     

强制循环到自然循环过渡过程中流动不稳定性的研究

本文对安装有引射器的回路系统从强制循环到自然循环的过渡特性进行了实验研究和数值模拟，实验结果表明，在过渡过程中、当加热功率超过某一限度，系统内将发生密度波型汽液两相流动不稳定性，危及设备的安全运行。数值模拟采用汽液两相热力学非平衡模型以考虑过冷沸腾对流动不稳定性的影响，使用迭代解法求解差分方程，数值模拟结果与实验结果相当一致。     

水轮机活动导叶内部三维固液两相紊流计算及磨损预估

本文报导了水轮机活动导叶内部固液两相紊流的三维数值分析结果和磨损预估结果。文中基于N－S方程和两相紊流的k—ε—Ap模型,采用贴体坐标和交错网格系统,用SIMPLEC算法对水轮机活动导叶内部的固液两相紊流进行数值模拟,提出设计工况时活动导叶内部的两相紊流特性。在数值模拟的基础上,根据Finnie模型预估了活动导叶的磨损。并与Drtina的实验结果进行了比较．     

三维小空化数空泡流数值方法

采用均质平衡流空泡模型,引入基于求解液体质量份数输运方程的空泡流模型,提出新的压力密度耦合的压缩性方法和控制气液两相转换的源项,求解汽水混合介质的RANS方程和带低雷诺数修正的k-ε模型,实现了小空化数(σ=0.2~0.01)下水下航行体的空泡流数值模拟.得到清晰的空泡形态特征与内部结构,以及空泡长度和最大直径随空化数的变化规律,给出航行体运动阻力系数与空化数之间的变化关系.将计算结果与解析结果及实验数据进行比较表明,该方法保证了极小空化数下自然空泡流计算的稳定性与收敛性,提高了空泡形态特性的预报精度.     

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.     

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.     

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.     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

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.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.