用于层析粒子图像测速的模拟粒子场成像

对层析粒子图像测速(PIV)技术中示踪粒子成像部分进行理论分析,并结合真实风洞的相应参数,通过搭建模拟粒子成像平台的方法来进行研究。设计了一套体积为80 mm×100 mm×100 mm的激光照明系统,以提供粒子场的入射光强。建立了示踪粒子的三维成像模型,从而得到层析PIV系统的模拟图像。分析了影响PIV系统成像质量的相关因素。在单像素粒子数为0.007 7的情况下,通过真实粒子图像和模拟粒子图像比较,验证了该方法的正确性。     

含铝推进剂燃烧场全息粒子图象处理系统的研究

本文主要围绕微粒场全息术的记录,再现和图象分析这三个过程,对含铝复合推进剂燃烧场的粒子分布进行了深入研究.根据全息图象再现场的三维特点,我们采用4F光学全息记录系统并结合层析分幅和特殊的图象处理技术,建立了一套独特的粒子场采集和自动图象分析体系.在实践中取得了较满意的实验结果.     

微射流粒子场的同轴Fraunhofer全息测试

由于激光全息技术进行物理测试时具有图像直观、信息量大、非接触测量、抗电磁干扰等优点，而受到人们的广泛关注。采用同轴Fraunhofer全息技术记录微射流粒子场的实验装置由激光器、爆轰实验装置、4F传像系统、记录系统、时问同步系统等组成。爆轰实验装置为一可抽真空的爆炸容器，其目的是为了产生一个微射流粒子场，实验研究中在铜飞片上打一些小坑，当冲击波到达时小坑中将能产生微射流；脉冲宽度为10ns的记录用激光器可以将直径为几十微米、运动速度为每秒几千米的微射流粒子场瞬间“冻结”，从而获得清晰的全息图像；通过调节DG535精密数字延迟脉冲发生器输出信号的延迟时间，控制脉冲激光器出光和爆轰实验装置爆炸的时间，从而达到产生运动粒子场和脉冲激光束到达时间的同步。通过同步调节延迟时间，可以获取同一空间不同时刻的粒子场信息。对拍摄的全息图经过线性处理后在连续YAG陪频激光器下再现，将再现获取的信息利用CCD相机储存在计算机上，对图片进行处理可以获取运动粒子场的分布、粒子的大小、粒子的质量等信息。     

用于偏振云粒子探测系统的典型非球形粒子散射特性模拟

为进一步了解云物理过程,探测冰云及混合相云云粒子特性,提出了一种带有偏振通道的云粒子探测器模型,可以同时探测前向散射和后向散射一定立体角内的散射能量和退偏比。模拟了椭球,圆柱和球形粒子的相函数,不同角度的退偏比,总散射截面,发现球形粒子几乎不产生退偏,其退偏与椭球、圆柱粒子退偏有数个量级的差距,探测时可以直接以退偏分辨球形粒子。计算了在探测器前后两个接收立体角内非球形粒子的散射截面和退偏比均值,粒子退偏比和散射截面随粒子的横纵比变化比较连续,一定条件下可以分辨部分粒子的形状。粒子退偏比随粒子的等效半径变化波动,不利于探测时分辨粒子大小。在模拟的粒径范围内,粒子等效半径和散射截面大致成正相关,探测时可以以此分辨粒子大小。粒子散射模拟结果表明,该探测系统除可以进行球形粒子探测能力外,具有一定的非球形粒子探测能力。     

粒子场数字全息诊断中的再现算法研究

 粒子场的数字全息诊断中,良好的再现算法能够在较短的时间内给出高质量的再现像。利用标准粒子板模拟单层面的粒子场,使用大面阵CCD实现同轴数字全息记录,得到了大尺寸的数字全息图。针对4种数字全息再现算法,本文从再现图像的质量、再现全息图的大小和计算速度3个主要方面进行了比较研究,结果表明角谱算法(FFT-AS)具有再现图像背景均匀,再现结果中无物理图像压缩,可以再现大尺寸的全息图且具有较快计算速度的优点,适合于粒子场同轴数字全息图的再现计算。     

对纳米粒子的调控

光镊子 (一组聚焦的光束 )擅长于捕获和移动毫米大小的粒子 ,但纳米量级的粒子常常可以从它的支配下滑脱 .目前东京大学的Takuyalida和HajimeIshihara两位教授提出了一个新的理论 ,他们认为让激光束的频率与半导体纳米粒子的内部能级差发生共振 ,那么光镊子就能提高一百万倍的捕获能力 ,同时光镊子还可对纳米材料中粒子的大小与形状进行分类 .众所周知 ,对微小物体的大小与形状作出分类是一件极其重要的工作 .例如DNA分子链断裂碎片的大小可以决定基因序列生长的速度 ,又如在材料科学中能对粒子的取向进行选择的话 ,就有可能对许多物质的…     

用脉冲激光全息术测量喷雾场中粒子的分布和运动速度

本文叙述了用脉冲激光全息术测量喷雾场中粒子的分布和运动速度的原理，阐明了同轴全息和离轴全息在测量雾场的应用范围，重点讨论了离轴全息。测量光学系统是４Ｆ系统，它可以测量粒子浓度大的雾场中大于５μｍ的粒子分布和运动速度。在平行光场区段，可进行不同装置、不同景深的喷雾研究，使每个粒子具有相同的放大倍数，它给再现、数据处理、粒子大小的标定都有很大的好处。文中还分析了在底片上形成干涉的各种情况；粒子直径ｄ，远场数Ｎ和从底片到粒子的距离Ｚ的关系。给出脉冲间隔为５μｓ；１０μｓ粗度为±０．１μｓ时不同直径粒子的分布和运动速度在电视屏幕上显示出来的照片。在上述实验结果的基础上讨论了粒子的识别，噪音的消除等问题。     

粒子场全息的自由度分析

根据信息论的基本原理 ,从平面全息系统成像全过程的一般方程出发 ,利用回转椭球波函数以及平面全息系统的自由度表达式 ,对在实际工作中粒子场全息测量可能遇到的几种实验记录系统进行了分析计算 ,得到了相应的系统成像自由度的大小。同时根据信息匹配的原则 ,给出了粒子场所能成像的物场范围。比较了各种构型全息系统的信息量的大小 ,并进行了简要讨论。     

基于哈特曼波前探测层析重建折射率场

介绍了哈特曼层析测量系统的原理,设计静态测试实验,对实际圆对称折射率场分布进行了层析重建.系统由哈特曼采集折射率场的投影数据,采用代数重建算法进行层析重建,取得了满意的重建结果.     

一种基于粒子滤波的雷达抗欺骗干扰方法

针对雷达的抗欺骗干扰问题,利用欺骗干扰信号强于目标信号的特点,基于粒子滤波进行了抗欺骗干扰研究。当存在欺骗干扰时,粒子滤波中各粒子的重要性权值会明显减小,据此可以检测干扰,并对受到干扰的数据点进行置零处理,使得欺骗干扰不再与匹配滤波器匹配,从而达到抑制干扰的目的。设计的粒子滤波算法不需要估计系统状态转移函数、系统量测噪声,从而使得算法更具实用性。仿真结果表明,该算法能有效地抑制欺骗干扰,且对干信比不敏感。     

Heat transfer in MHD flow of dusty viscoelastic (Walters’ liquid model-B) stratified fluid in porous medium under variable viscosity

The paper investigates the effects of heat transfer in MHD flow of viscoelastic stratified fluid in porous medium on a parallel plate channel inclined at an angle θ. A laminar convection flow for incompressible conducting fluid is considered. It is assumed that the plates are kept at different temperatures which decay with time. The partial differential equations governing the flow are solved by perturbation technique. Expressions for the velocity of fluid and particle phases, temperature field, Nusselt number, skin friction and flow flux are obtained within the channel. The effects of various parameters like stratification factor, magnetic field parameter, Prandtl number on temperature field, heat transfer, skin friction, flow flux, velocity for both the fluid and particle phases are displayed through graphs and discussed numerically.     

有序混合光束和迷宫式多次反射光学系统

本文给出有序混合光束结构和多程光学系统的具体考虑,并指出它们在多波长高激发态激光光谱学研究和其它领域中的意义.     

强梯度复杂流场中的粒子动力学响应试验研究

示踪粒子在(高)超声速流场中的动力学响应是粒子成像测速等粒子示踪测量技术的关键问题之一.现有文献对粒子动力学响应的试验测量往往是通过单个斜激波响应的测量方法.然而,当示踪粒子用于测量高速飞行器发动机内部复杂的激波串流场时,粒子将经历由多道激波导致的速度、压力、黏性等剧烈变化.本文结合目前(高)超声速飞行器的研究热潮,重点关注示踪粒子在应用于发动机内部具有连续激波的复杂流场测量中存在的跟随性评估方面,开展了一系列的相关试验研究.包括测量超声速风洞的喷管出口速度分布以验证测试系统的性能,在马赫4.2和3.0流场中测量了粒子对二维10°和15°单斜劈绕流中的斜激波动力响应,并测量了模拟发动机内部连续梯度的双斜劈粒子斜激波动力响应.结合粒子动力学的理论模型,得到了各状态的粒子弛豫时间、弛豫距离、Stokes数.基于图像方法、统计学规律分析了激波非定常抖动对测量结果的影响,并对测量结果进行了修正.结果显示,相同斜劈角度下,马赫数越高,粒子的弛豫时间、弛豫距离就越大.但是在相同的来流马赫数下,斜劈角度越大,粒子的弛豫时间、弛豫距离反而减小.在强梯度之后由于流场的雷诺数和黏性系数变化剧烈,粒子的跟随性降低了大约5.7%,stokes数增加了约1%.虽然在本文条件下Stokes数仍满足超声速流场对粒子跟随性的要求,但粒子响应的降低无疑是值得关注的,尤其是当其被应用于具有更多连续梯度的复杂流场测量中.     

Single voxel vascular transport functions of arteries,capillaries and veins; and the associated arterial input function in dynamic susceptibility contrast magnetic resonance brain perfusion imaging

PurposeThe composite vascular transport function of a brain voxel consists of one convolutional component for the arteries, one for the capillaries and one for the veins in the voxel of interest. Here, the goal is to find each of these three convolutional components and the associated arterial input function.Pharmacokinetic modellingThe single voxel vascular transport functions for arteries, capillaries and veins were all modelled as causal exponential functions. Each observed multipass tissue contrast function was as a first approximation modelled as the resulting parametric composite vascular transport function convolved with a nonparametric and voxel specific multipass arterial input function. Subsequently, the residue function was used in the true perfusion equation to optimize the three parameters of the exponential functions.Deconvolution methodsFor each voxel, the parameters of the three exponential functions were estimated by successive iterative blind deconvolutions using versions of the Lucy-Richardson algorithm. The final multipass arterial input function was then computed by nonblind deconvolution using the Lucy-Richardson algorithm and the estimated composite vascular transport function.ResultsSimulations showed that the algorithm worked. The estimated mean transit time of arteries, capillaries and veins of the simulated data agreed with the known input values. For real data, the estimated capillary mean transit times agreed with known values for this parameter. The nonparametric multipass arterial input functions were used to derive the associated map of the arrival time. The arrival time map of a healthy volunteer agreed with known arterial anatomy and physiology.ConclusionClinically important new voxelwise hemodynamic information for arteries, capillaries and veins separately can be estimated using multipass tissue contrast functions and the iterative blind Lucy-Richardson deconvolution algorithm.     

On mechanical motion damping of a magnetically trapped diamagnetic particle

We discuss eddy-current-induced limitations of the attenuation of mechanical motion of a diamagnetic particle trapped by an inhomogeneous magnetic field in the Earth's gravitational potential. We show that the mechanical frequency of the particle oscillation is independent on the particle properties and is proportional to the free fall acceleration constant, similarly to the classical mechanical pendulum. The frequency can be used to measure the gravity field. The eddy-current induced attenuation constant does not depend on the mass of the particle and reduces with the particle volume. The quality factor of the mechanical motion can be as high as 10⁹ and is comparable with the attenuation due to interaction of the particle with incompletely evacuated air. A possibility of usage of the particle as a quantum mechanical system is discussed.     

Compact Raman CH3F, NH3 optically pumped FIR laser

Conclusion The compact superradiant FIR laser is developed using Herriott cell with long optical path as a FIR cell. The employment of the multipass cell gave an opportunity to increase the IR to FIR conversion efficiency in comparison with traditionally constructed FIR lasers and to observe many intensive FIR lines. The given of multipass cell can be useful for employment in continuously tunable FIR lasers.     

A ppb level sensitive sensor for atmospheric methane detection

A high sensitivity sensor, combining a multipass cell and wavelength modulation spectroscopy in the near infrared spectral region was designed and implemented for trace gas detection. The effective length of the multipass cell was about 290 meters. The developed spectroscopic technique demonstrates an improved sensitivity of methane in ambient air and a relatively short detection time compared to previously reported sensors. Home-built electronics and software were employed for diode laser frequency modulation, signal lock-in detection and processing. A dual beam scheme and a balanced photo-detector were implemented to suppress the intensity modulation and noise for better detection sensitivity. The performance of the sensor was evaluated in a series of measurements ranging from three hours to two days. The average methane concentration measured in ambient air was 2.01 ppm with a relative error of ± 2.5%. With Allan deviation analysis, it was found that the methane detection limit of 1.2 ppb was achieved in 650 s. The developed sensor is compact and portable, and thus it is well suited for field measurements of methane and other trace gases.     

Highly-efficient 20 TW Ti:sapphire laser system using optimized diverging beams for laser wakefield acceleration experiments

We developed a highly efficient and compact Ti:sapphire multipass amplifier that uses divergent seed beams to generate 20 TW/40 fs laser pulses with a repetition rate of 10 Hz. The laser system consists of a mode-locked oscillator, a regenerative amplifier, and a multipass power amplifier. The thermal lensing effect is very important in this system, especially in the multipass amplifier, as it limits the conversion efficiency. In order to compensate the thermal lensing effect, we calculated the optimum divergence of the seed beam and used the result for the multipass amplifier, where the thermal focusing is taken into account. In this way, we achieved a very high conversion efficiency of 41%, which is close to the theoretical limit. The laser system was then used with a capillary gas cell to generate stable high-energy electron beams with electron energies of about 150 MeV and a beam divergence of 4 ± 1 mrad. In this paper, details of the laser system development and experimental results for electron generation are presented.     

磁性材料中的光散射

磁性材料,由于多数是可见光不透明的,长期以来与光散射研究似乎无关。然而这些介质中的磁性激发和声子一样是能够与入射光发生作用,造成非弹性散射的。研究这些散射能提供介质中磁性激发的各种信息。近几年散射谱仪技术的发展,尤其是多通Brillouin散射谱仪的出现,为开发磁介质光散射提供了技术基础。这些研究陆续取得了一系列引人注目的成果,Brillouin散射,因此称为磁学研究的新的特殊方法。考虑到磁学工作者的兴趣,本文首先概述光散射的基本概念,分析光与磁性激发的耦合机制,然后介绍基于多通Fabry-Perot干涉仪的Brillouin散射实验。最后讨论对几种典型磁性材料的研究结果,用以说明磁介质光散射的应用性,前景及特点。     

A high energy, long pulse Nd: Yag laser multipass cell for raman scattering diagnostics

A new multipass cell design is proposed for use with high energy, pulsed lasers and for application to spontaneous Raman scattering diagnostics. Using a Nd: YAG laser pulse of 8 ns, 500 mJ at 1.06 μm, the frequency-doubled 60 mJ pulse is trapped in the multipass cell for 550 ns until it decays to 10% of its initial energy. This gives a resultant gain of 42 over the single pass case and corresponds to a multipass cell efficiency of 97.7% per pass.