基于纹影法的气体温度场可视化系统的设计

纹影法是介质折射率变化可视化的实用方法.本文基于分束镜等光学器件设计了一种新的折叠式纹影光路,可改善传统光路的低清晰度问题,同时利用光线的正入射消除了重影干扰.利用热风拆焊台出风口热流进行定标实验,获得纹影图像灰度值的差值与温度的函数关系,并利用纹影图像绘制灰度三维图像,实现了气体温度场可视化处理,提供了一种非接触测量气流温度与温度场摄像系统的测量方案.     

高超音速流场中锥体边界层转捩图象的纹影显示及处理

本文介绍了一种大画幅纹影成像技本,应用该技术,显示了高超音速流场中锥体边界层转捩。将所得到的纹影图象在显微密度计图象处理系统中进行处理与测量,并结合纹影法原理进行了分析计算,得到了边界层诸截面相对密度变化曲线以及边界层转捩位置等实验结果,该结果与有关气动理论和实践结果进行了比较。     

双光程纹影法对火焰温度场的研究

通过手机载入一个黑色背景的白点图作为光源,制作出大小、亮度等参数均可连续调节的小而均匀的光源,利用该光源搭建了反射式双光程纹影系统。利用该系统定性地观察了手掌、电烙铁、吹风机、防风火机等的纹影图像,图像效果明显。用迭代法定量计算了防风火机的火焰流场的空气折射率梯度、气体密度和温度,得到的火焰温度与热电偶测量的温度符合较好。     

可视化聚焦纹影系统实现流场中的三维扰动成像

可视化成像系统可以实现流场中扰动信息的捕获。针对光学纹影系统,分别从理论和实验上对比研究了流场中的三维超声扰动成像。因三维超声场在光传播方向上引起的流体介质密度变化不同,光波穿过密度变化的流场时,其相位累积变化使图像不能真实反映声场的扰动特征。线型栅的聚焦纹影系统因其线型栅会产生多刀口滤波的作用,故不能对流场中的三维超声扰动场实现完整成像。为了获得流场中的三维扰动特征,提出了采用环型源栅和刀口栅的改进聚焦纹影系统。结果表明:由改进聚焦纹影装置得到的图像扰动特征与实际流场中超声扰动特征相一致;结合图像重构技术,实现了流场中三维复杂扰动的重构。     

采用纹影法的微型航空发动机尾喷温度场测量方法

航空发动机作为飞机上的核心动力装置,对其运行状态的监测,可以提高飞机的安全性与经济性。航空发动机尾喷温度场蕴含着燃烧室内部燃料燃烧程度等有效信息,对其温度场的监测可实现对发动机运行状态的评估。传统的温度场测量方法一般采用单点温度计测量法、激光诱导荧光法(LIF)、可调谐半导体激光法(TDLAS)等,单点温度计测量法测量数据较离散、LIF设备应用复杂、TDLAS仪器设备价格昂贵。因此针对传统测量方法所存在的问题,提出了纹影测量法。首先需要纹影系统对温度场进行纹影成像,得到正确的纹影图像;根据纹影图像求得截面的折射率分布;根据折射率分布算得温度场分布,实现对温度场的测量。纹影测量法是一种非接触、高精度的测量方法,且无需特殊的激光收发装置。实验证明该方法可有效地应用于实验室微型涡喷发动机尾喷区域温度场的测量,可进一步为航空发动机的状态监测提供支持。     

计算流体成像技术重构方法及其应用

建立系统的计算流体成像(CFI)技术,并论证其为流动数值模拟与风洞实验相互佐证的重要手段。采用纯数值模拟技术重构得到流场计算阴影、纹影以及干涉图,主要技术包括复杂湍流模拟技术、三线性插值技术和流动图像重构技术等。同时将CFI技术应用于激波管内绕障碍物三维复杂流动计算图像重构,并与双向全息实验结果进行了比较分析。研究证明该CFI技术可以用于复杂几何边界、分区域、非定常流动图像重构,不仅能够为流场结构特征分析提供重要参考依据,而且能够用于数值模拟与风洞实验结果的比较印证。     

基于背景纹影波前传感技术的气动光学波前重构与校正

背景纹影波前传感(backgroud oriented schlieren based wavefront sensing, BOS-WS) 是利用背景纹影技术测量光学波前二维分布的新型实验手段, 可定量测量光线通过干扰场后产生的光学畸变并给出光程差. 为了利用BOS-WS技术获得光线因气动光学效应产生的畸变信息, 并通过已知畸变信息还原原始图像, 进而探索一种新型的超声速成像制导方法, 本文通过理论分析, 验证了利用背景纹影技术测量光学波前的方法, 探索了利用已知波前信息预测畸变位移场以及利用已知位移场进行波前重构的计算方法. 通过数值模拟比较了一阶梯形积分算法和Southwell方法在波前重构上的误差大小和结果合理性, 并通过误差分析证明了Southwell方法更加精确合理. 通过蜡烛火焰上方流场畸变实验和透镜对波前的扰动实验, 创造性地探索了利用已知光学光程差还原畸变位移场及用其校正畸变图像的方法, 并通过验证性实验证明了校正方法的有效性.     

激光诱导荧光聚焦纹影系统及超声速燃烧流场应用

纹影是一种常用的流动显示技术，广泛应用于可压缩流动显示及超声速燃烧流场实验.然而，在变Mach数超声速燃烧实验中，燃烧室总温随来流Mach数变化.受准稳态/非定常温度变化影响，光学玻璃窗口的折射率发生显著改变，影响基于密度梯度的纹影成像质量.同时，普通纹影为光程体积沿程积分，难以同二维燃烧场成像信息进行直接比较以开展燃烧与流动耦合研究.聚焦纹影技术可抑制燃烧室内高温引起的玻璃窗口折射率变化，并实现毫米级的急剧聚焦深度，获得二维流场结构，同时配合纳秒级脉宽Nd:YAG激光光源可冻结高超声速流场.在传统聚焦纹影系统基础上发展了激光诱导荧光聚焦纹影系统并应用于变Mach数超声速燃烧实验，创新点在于使用激光诱导荧光染料，以荧光作为光源消除原本激光光源中的相干噪声，同时发展了边缘增强图像处理方法.实验结果表明激光诱导荧光聚焦纹影系统及边缘增强图像处理方法能够有效消除激光光源相干噪声，捕捉二维超声速燃烧流场结构.      

利用彩虹滤光片使空气折射率梯度可视化

纹影法是一种将相分布转换为可见图像的光学方法,它可以用于测量一个区域内的空气折射率。彩虹纹影法则是将传统的刀口换为彩虹滤光片,可以使纹影图像变为彩色。相对于灰度来说,人眼对色彩的变化更加敏感,可以更加敏锐的观测到变化。利用MATLAB也可以定量的得到测量场中空气的折射率梯度。     

高超音速流场中锥体边界层转捩图象的纹影显示及处理

边界层转换是气动力研究的一个令人关注的问题,给出模型物面流态对分析和应用试验数据都是十分有用的。用光学方法显示边界层能获得直观的图象,将其进行图象处理与分析计算后能得到一些定量的结果。本文介绍了一种大画幅纹影成象技术,该技术具有较高的显示灵敏度和摄影分辨率,能将原来的φ24(毫米)纹影图变成为φ110或更大。应用该技术,在气动中心五所高马赫数的激波风洞中,显示高超音速流场中锥体     

Visualization and analysis of muzzle flow fields using the Background-Oriented Schlieren technique

Several experimental and numerical studies on muzzle blast and flow fields have been performed. However, due to the extremely short duration and the spatiotemporal evolution of these flows, experimental quantitative techniques are limited. As a consequence, the number of validated numerical calculations is limited as well. On the other hand, despite the development of computer models that have succeeded in predicting the measured pressure and velocity, they show unrealistic temperatures and densities. Therefore, temperature and/or density measurements are required to validate these codes, thus the motivation of this research. The present paper focuses on the development of a density-sensitive and non-intrusive measurement technique and the implementation of a quantitative flow visualization method based on Background-Oriented Schlieren (BOS) combined with a high-speed camera. In BOS, the experimental setup of conventional Schlieren (mirrors, lenses, and knife-edge) is replaced by a background pattern and a single digital camera. The muzzle flow fields and the flow field around a 5.56-mm projectile in flight were successfully visualized. Indeed, the implemented experimental high-speed BOS setup has demonstrated its ability to capture clearly the salient features of the precursor and the propellant gas flow fields and their interactions. The captured structures such as vortex, barrel shock, Mach disk, and blast wave show a good agreement with that issued from a realized conventional Schlieren setup and the bibliography, confirming the BOS capability to visualize complex density flow fields.     

Schlieren成像和声场可视化

Schlieren技术是利用声场引起透明媒质光折射率的变化而实现声场可视化的光学成像技术。它具有对声场无干扰、快速、瞬态成像的特点。本文利用二维光学Fourier变换分析了Schlieren技术的成像原理,在采用连续激光和高速ICCD的Schlieren成像系统中,实验研究了平面波声场和线聚焦声场中换能器光学校准方法和声压的定量检测技术。发展声场瞬态和动态成像技术,观测了声波的聚焦过程和固-液界面的声场分布和变化。这些结果表明Schlieren技术是一种有效的声场可视化和定量检测的光学成像技术。     

Reconstruction of the density field using the Colored Background Oriented Schlieren Technique (CBOS)

In this paper the improved Background Oriented Schlieren technique called CBOS (Colored Background Oriented Schlieren) is described and used to reconstruct density fields of three-dimensional flows. The Background Oriented Schlieren technique (BOS) allows to measure the light deflection caused by density gradients in a compressible flow. For this purpose the local image displacements of the image of a background pattern observed through the flow is used. In order to increase the performance of the conventional Background Oriented Schlieren technique, the monochromatic background is replaced by a colored dot pattern. The different colors are treated separately using suitable correlation algorithms. Therefore, the accuracy and the spatial resolution can be increased. A tomographic reconstruction method is then used to reconstruct the density field in three-dimensional flows from CBOS measurements.     

高超声速平板边界层流动显示的试验研究

本文在高超声速脉冲式风洞内对基于纳米示踪的平面激光散射技术(nano-based planar laser scattering, NPLS)的应用进行了探索, 并在此基础上对平板边界层流动结构的精细测量进行了研究. 试验来流Ma=7.3, 总压4.8 MPa, 总温680 K. 通过时序的分析和调试, 对各分系统实现了高精度的同步控制; 定量的粒子注入及混合, 实现了粒子的均匀撒播, 对主流获得了均匀的显示效果; 对于边界层流动, 获得了精细的瞬态流动结构图像, 显示了层流到湍流的转捩过程, 并分析了其时空演化特性.     

声学CT复杂温度场重建研究

为提高声学CT复杂温度场重建能力,提出一种利用Markov径向基函数逼近和Tikhonov正则化的温度场重建算法,简称MTR算法。该算法首先用Markov径向基函数的线性组合,逼近介质中的复杂声速场分布,然后利用介质中多路径声波传播时间和Tikhonov正则化法,求解声速场分布,进而利用声速与温度的关系获得温度分布。对单热点、三热点和五热点温度场模型进行了仿真重建,结果表明MTR算法热点定位精度高,重建误差小。开发了声学CT温度场重建实验系统,用电加热器在内装1200 kg大豆的实验粮仓中形成热点,MTR重建结果能正确反映热点位置,热点温度重建误差1.3%。可见,MTR算法复杂温度场重建能力强,可望用于实际储粮温度分布监测。      

超声速预混可燃气流的点火与燃烧

在激波风洞一激波管组合设备上开展了碳氢燃料超声速预混可燃气流的点火与燃烧实验研究。实验结果表明:利用激波对燃料进行预热,并以高温燃气作为引导火焰,可以有效缩短汽油空气超声速可燃混气的点火延迟时间,使之缩短到 0.2 ms以下。利用纹影照片对超声速燃烧流场结构作出了分析;研究了超声速预混可燃气流的温度以及当量比对超声速燃烧流场结构、点火与火焰传播特性的影响。     

Temperature and velocity measurement of a 3-D thermal flow field using thermo-sensitive liquid crystals

It is an important challenge to analyze a three-dimensional thermal flow field in engineering, science, and agriculture. For such an analysis, it is essential to measure physical quantities such as temperature and velocity over the entire thermal flow field. This paper presents a measurement system based on color image processing for temperature and velocity vector distributions in a three-dimensional thermal flow field. Flow visualization is accomplished by the use of thermo-sensitive liquid crystal tracers. An algorithm for the color-to-temperature transformation using a multi-layer feed-forward neural network is applied to three-dimensional natural convection in a rotating cylindrical cell. Two-dimensional temperature distributions in a slit plane are obtained by using the algorithm. A three-dimensional temperature distribution is consequently constructed by interpolating the two-dimensional distributions using the B-spline function. In addition, the Spatio-Temporal correlation method is applied to the natural convection to obtain a three-dimensional velocity vector distribution.     

Visualization and diagnostics of thermal plasma flows

Flow visualization is a key tool for the study of thermal plasma flows. Because of their high temperature and associated self emission, standard and high speed photography is commonly used for flow and temperature field visualization. Tracer techniques through the injection of a seed powder in the plasma flow have also been often used for the study of flow structure. Shadowgraphs and Schlieren techniques have been used particularly when cold flow regions are present in the close proximity of the plasma discharge. They also provide key information about the flow structure in the fringes of the discharge. Laser strobe techniques are commonly used for the characterization of particle trajectories under plasma conditions. A brief review is presented of available plasma and particulate diagnostic techniques with detailed measurements reported for a radio frequency (r.f.) induction plasma discharge using enthalpy probe techniques.     

Interacting wakes of two normal flat plates an investigation based on phase averaging of LDA signals

The flow around two bluff bodies in tandem is an interesting phenomenon whose nature strongly depends on the distance between the bodies. In this work oil smoke visualizations of the flow around two normal flat plates in tandem configuration are compared with phase averaged Laser Doppler Anemometry measurements of the flow field for a full period. Phase averaging is exploited to resolve the mean time evolution of the flow and the signal from a single constant temperature hot wire probe is employed as phasing reference. The flow-field information obtained from visualizations agrees well with quantitative indications provided by measures, and the comparison allows to precisely understand what information can be extracted from simple visualizations. As a result, new insight in flow mechanism is obtained from the analysis of the flow data showing that, for the analyzed plate distance, the gap between the two plates behaves substantially as a cavity while the vortex formation process takes place in the wake region behind the aft plate.