含冲击波流场干涉图的图像处理方法研究

本文研究了含冲击波流场干涉图的图像处理方法,它包括干涉条纹的快束细化算法,冲击波波阵面的提取算法和时间序列冲击波波阵面的配准算法.根据这些算法,在PC-VISION 100图像处理机上开发了应用软件.实验结果表明,该方法可用于定量计算冲击波的传播速度和压力分布,增加了流场干涉图的用途.     

激光作用铝靶爆轰波流场观测与理论分析

进行了强激光作用铝靶实验,采用纹影照相法,观察爆轰波流场演化过程,分析了爆轰波衰减规律。根据冲击波运动的自相似性,采用点爆炸模型描述了激光作用下爆轰波流场的演化,计算了波阵面速度、压力和温度。结果显示:爆轰波阵面沿迎着激光光源方向较快传播,波阵面形状由最初的半椭球形逐渐向半球形转变,在演化过程中扰动区结构复杂,存在多个密度间断层。在爆轰波开始传播阶段,波阵面的压力和温度较高但下降很快。     

流场三维层析与高速干涉摄影

瞬态三维流场的层析,需要拍摄多方向干涉图,然后利用计算机进行层析。高速摄影能够记录瞬态过程的外部形象变化,给予定性的观察而不能定量描述,即使利用纹影、阴影技术也只能给出流场冲击波的二维定性结构。人们曾利用漫射全息干涉法来获得多方向干涉图,从而进行流场的三维定量计算。在这种方法中,由于干涉条纹是空间地定域于一曲面上,从各方向难于翻拍到清晰的干涉图,观察角度由于受到测域区域的     

时间序列差分干涉术研究激波流场

本文用一种新的时间序列差分干涉术研究模型在超音速风洞中产生的激波流场.实.验中用调Q序列脉冲红宝石激光器作光源,用转镜扫描高速摄影机作为记录设备,结合单Wollaston棱镜差分干涉的方法获得了激波流场的序列差分干涉图.利用图象处理技术对干涉图进行图象处理,人机交互的方式采集数据,用条纹位移转换法将差分干涉图的条纹位移量转换为绝对干涉图的条纹位移量,然后用拉普拉斯变换法计算出了密度的相对分布值.     

时间序列散斑干涉场中相位函数的计算

散斑干涉或电子散斑干涉计量应用于连续运动或变形物体时,就会产生一个时变的散斑干涉场,通过摄像系统连续地采集这一时变散斑场,可获得一系列时间序列散斑干涉图,通过对序列散斑图上各点在时间轴上光强值的变化进行分析,提出了一种基于时间序列的分析方法,用以提取干涉场的相位值,进而获得物体全场变形信息。     

Numerical Simulation of Explosive Dust Detonation with CE/SE Method

采用CE/SE方法数值模拟悬浮在空气中的RDX炸药粉尘的两相爆轰过程.炸药颗粒在爆轰波阵面后的高温高速气流中加速并升温,释放能量支持爆轰波传播.数值模拟爆轰波管中的粉尘爆轰,得到爆轰波流场中的物理量分布,确定爆轰参数,数值结果与文献符合较好.数值模拟复杂通道中的炸药粉尘爆轰,预测了爆轰波的发展和传播过程以及爆轰波后的流场演化.数值结果表明CE/SE方法能成功模拟气体-固体两相爆轰,为粉尘爆轰的研究提供了新的数值预测手段.     

CE/SE方法数值模拟炸药粉尘爆轰

采用CE/SE方法数值模拟悬浮在空气中的RDX炸药粉尘的两相爆轰过程.炸药颗粒在爆轰波阵面后的高温高速气流中加速并升温,释放能量支持爆轰波传播.数值模拟爆轰波管中的粉尘爆轰,得到爆轰波流场中的物理量分布,确定爆轰参数,数值结果与文献符合较好.数值模拟复杂通道中的炸药粉尘爆轰,预测了爆轰波的发展和传播过程以及爆轰波后的流场演化.数值结果表明CE/SE方法能成功模拟气体-固体两相爆轰,为粉尘爆轰的研究提供了新的数值预测手段.     

激光与介质薄膜作用过程的等离子体诊断

本文对YAG激光诱导介质薄膜产生的等离子体采用Mach-Zehnder干涉仪及光延迟装置进行时间序列分辨多幅干涉记录,并对序列干涉图进行电子密度及等离子体冲击波速度的计算.首次得到脉宽为15ns的1.06μm激光与介质薄膜作用在150ns以内的有关结果.     

斜爆轰发动机流动机理分析

为了研究高Mach数超燃冲压发动机和斜爆轰发动机的内流场燃烧流动机理，首先用CJ爆轰理论对超燃冲压发动机的内流场特性进行了理论分析，给出了燃烧室流场的气动规律，理论分析结果与现有实验结果吻合得非常好.其次，根据理论分析结果，提出了高Mach数超燃冲压发动机和斜爆轰发动机的气动设计原则.最后，根据提出的气动设计原则，设计了高Mach数斜爆轰发动机，飞行Mach数为9，对斜激波诱导燃烧机理开展了二维数值模拟研究.数值模拟结果表明，在高Mach数下，斜爆轰发动机燃烧室内可以得到稳定的燃烧流场.      

阶跃式声光偏转技术及其在高速干涉摄影中的应用

本文介绍了一种阶跃式声光偏转的技术及其在高速干涉摄影中的应用,它把带有干涉图信息的光束,通过一个声光偏转器件,在阶跃超声脉冲波的作用下,形成按时间顺序偏转的10个光斑(直径为Ф6mm—Ф8mm);曝光时间为10μs—3μs可调,幅间隔为10μs.用这种方法与激光干涉仪结合,记录了激光引燃起爆药瞬时燃烧场的多幅干涉图,并进行了定量计算。     

Unstable combustion induced by oblique shock waves at the non-attaching condition of the oblique detonation wave

The instability of oblique shock wave (OSW) induced combustion is examined for a wedge with a flow turning angle greater than the maximum attach angle of the oblique detonation wave (ODW), where archival results rarely exist for this case in previous literatures. Numerical simulations were carried out for wedges of different length scales to account for the ratio of the chemical and fluid dynamic time scales. The results reveal three different regimes of combustion. (1) No ignition or decoupled combustion was observed if a fluid dynamic time is shorter than a chemical time behind an OSW. (2) Oscillatory combustion was observed behind an OSW if a fluid dynamic time is longer than a chemical time behind an OSW and the fluid dynamic time is shorter than the chemical time behind a normal shock wave (NSW) at the same Mach number. (3) Detached bow shock-induced combustion (or detached overdriven detonation wave) was observed if a fluid dynamic time is longer than a chemical time behind a NSW. Since no ignition or decoupled combustion occurs as a very slow reaction and the detached wave occurs as an infinitely fast reaction, the finite rate chemistry is considered to be the key for the oscillating combustion induced by an OSW over a wedge of a finite length with a flow turning angle greater than the maximum attach angle for an ODW. Since this case has not been previously reported, grid independency was tested intensively to account for the interaction between the shock and reaction waves and to determine the critical time scale where the oscillating combustion can be observed.     

Ultra-fast diagnosis of shock waves and plasma at front and rear surfaces in the bulk of fused silica induced by an Nd:YAG pulse laser

We investigate the dynamic processes of the Nd:YAG pulse laser ablation of fused silica by ultrafast timeresolved optical diagnosis with a nanosecond time resolution. The evolution process of plasma expansion in air and shock waves propagation in the bulk are both obtained with spatial and temporal resolutions.Laser-induced damage in the bulk of fused silica with filaments and shock waves are observed. Thermoelastic wave,mechanical wave,and shock wave dependence on the laser fluence and intensity of the plasma are analyzed. The shock pressure P and temperature T calculated through the measured shock velocity D and the Hugoniot data of fused silica are measured.     

The effects of mixture preburning on detonation wave propagation

Pressure gain combustion in the form of continuous detonations can provide a significant increase in the efficiency of a variety of propulsion and energy conversion devices. In this regard, rotating detonation engines (RDEs) that utilize an azimuthally-moving detonation wave in annular systems are increasingly seen as a viable approach to realizing pressure gain combustion. However, practical RDEs that employ non-premixed fuel and oxidizer injection need to minimize losses through a number of mechanisms, including turbulence-induced shock-front variations, incomplete fuel-air mixing, and premature deflagration. In this study, a canonical stratified detonation configuration is used to understand the impact of preburning on detonation efficiency. It was found that heat release ahead of the detonation wave leads to weaker shock fronts, delayed combustion of partially-oxidized fuel-air mixture, and non-compact heat release. Furthermore, large variations in wave speeds were observed, which is consistent with wave behavior in full-scale RDEs. Peak pressures in the compression region or near triple points were considerably lower than the theoretically-predicted values for ideal detonations. Analysis of the detonation structure indicates that this deflagration process is parasitic in nature, reducing the detonation efficiency but also leading to heat release far behind the wave that cannot directly strengthen the shock wave. This parasitic combustion leads to commensal combustion (heat release far downstream of the wave), indicating that it is the root cause of combustion efficiency losses.     

极端条件两相界面流与反应流机理、模型与算法研究进展

清华大学喷雾燃烧与推进实验室长期专注于极高速、强可压和高瞬变等极端条件下的两相流和反应流前沿科学问题,并致力于应用基础研究成果解决航空航天动力与推进系统的关键技术难题。综述了实验室近些年在极端条件下两相流动和含化学反应流动物理机理、数理模型与数值算法等方面的研究进展。首先,介绍了实验室发展的耦合高瞬变相变过程的强可压缩气液两相界面流的数理模型和高精度数值方法,以及针对激波受气液（曲）界面约束情况下,描述非定常激波透射/反射（如波角、波强等物理量关系）的激波动力学分析方法。其次,基于上述模型、算法与分析方法,实验室研究了激波液滴相互作用、高速液滴撞击壁面等一系列问题,解析了上述高瞬变过程中复杂波系与界面的时空演化过程。以被激波或壁面冲击的液滴内流体空化初生为例,揭示了曲界面汇聚膨胀波诱导流体空化的机理,推导了预测空化初生位置的理论公式。最后,介绍了面向发动机燃烧室内的强可压缩两相喷雾反应流动,实验室开发了基于Euler-Lagrange框架的高性能数值仿真软件TURFsim,并成功用于真实复杂几何结构的航空发动机燃烧室和超声速燃烧室的数值模拟。以典型的超声速混合层流动数值模拟为例,总结了斜激波增强混合层混合特性的规律及其物理机理,获得了极限条件火核生成及火焰的传播模式与机理,详细分析了液雾弥散与蒸发、小激波和局部爆震波的时空演化特性,提出使用"第三Damk?hler数Da_Ⅲ"定量表征燃烧模式,应用该无量纲参数成功进行了局部准等容燃烧过程的辨识与演化分析。上述研究结果对于航空航天发动机燃烧室复杂物理过程的理解及工程设计具有重要价值。      

激光驱动的冲击波自生磁场以及外加磁场的冲击波放大研究

冲击波是天体物理观测中常见的现象, 其对粒子的加速被认为是高能宇宙射线的来源. 宇宙中冲击波周围往往存在很强的磁场, 但人们对于此类强磁场的产生放大过程的理解并不充分. 本文利用二维粒子模拟程序研究了激光与磁化或者非磁化等离子体相互作用产生的冲击波现象, 给出了冲击波波前处磁场的产生放大特性. 研究发现, 作用过程中的自生磁场可以储存能量, 从而进一步加速电子; 当存在外加磁场时, 由冲击波加速的电子和离子的能量都比同条件下非磁化等离子体的能量高; 而且外加磁场藉由冲击波放大倍数则与其值有极大关系. 与天文观测中推断的磁场与背景磁场相比放大千倍这一研究结果的比较可以看出, 天体冲击波周围磁场放大主要是由局域内生磁场导致的.     

连续相位板束匀滑容差能力分析

在惯性约束聚变系统中,改进G-S算法设计的连续相位板能够对波前发生畸变的激光进行束匀滑处理,以改善远场分布。由于连续相位板对不同畸变状态的波前进行处理后的效果不一样,因此针对连续相位板的束匀滑容差能力进行了系统分析。鉴于畸变光束的传输特性,采用波前均方根梯度来量化波前畸变量,定量计算了不同畸变光经过匀滑处理后的远场光强分布情况。并比较了不同入射畸变光通过连续相位板后的远场焦斑顶部均匀性,结果表明:当入射波前畸变的均方根梯度小于0.32 wave/mm 时,连续相位板具有很好的束匀滑效果。     

化学激光器用文氏咀流场特性的数值模拟

 基于现有的文氏咀流量供给管道,提取出轴对称2维计算模型,计算了不同背压下文氏咀及下游管道的流场特性。计算结果显示,随着背压的升高,激波在管道内经历了3种状态,在直管道内为斜激波,到转接段时为拟正激波,进入文氏咀后则为正激波。在进行文氏咀流量计的设计时,应尽量将正激波控制在文氏咀喉道或向下游延伸的一小段距离内,此时是文氏咀的最佳工作状态。另外,采用壁面开孔法在文氏咀下游选取测压点时,为了准确测量截面上的静压,应将测压点选取在距离直管道入口数倍管径处,一般不低于8倍管径。     

斜激波与轴对称边界层干扰的数值模拟

使用两方程Menter-SST模型,对来流Mach数为3时的斜激波与轴对称边界层的相互干扰现象进行了数值模拟与定性、定量分析。研究了斜楔激波发生器楔角和来流单位Reynolds数变化对干扰区流动的影响,总结了参数变化引起的流动分离变化规律;此外,还计算了与三维计算中心对称面上的入射激波等效的二维情形,并将三维结果与二维情形进行对比,对比结果显示中心对称面上的壁面压力系数、分离涡尺寸、涡量分布等与相应的二维情形存在明显差异。     

Impact of an extended source in laser ablation using pulsed digital holographic interferometry and modelling

Pulsed digital holographic interferometry has been used to study the effect of the laser spot diameter on the shock wave generated in the ablation process of an Nd:YAG laser pulse on a Zn target under atmospheric pressure. For different laser spot diameters and time delays, the propagation of the expanding vapour and of the shock wave were recorded by intensity maps calculated using the recorded digital holograms. From the latter, the phase maps, the refractive index and the density field can be derived. A model was developed that approaches the density distribution, in particular the ellipsoidal expansion characteristics. The induced shock wave has an ellipsoid shape that approaches a sphere for decreasing spot diameter. The ellipsoidal shock waves have almost the same centre offset towards the laser beam and the same aspect ratio for different time steps. The model facilitates the derivation of the particle velocity field. The method provides valuable quantitative results that are discussed, in particular in comparison with the simpler point source explosion theory.