Numerical investigation of flow structures resulting from the interaction between an oblique detonation wave and an upper expansion corner

Wedge-induced oblique detonation waves (ODWs) have been studied widely, but their interactions with complicated geometries have not been fully addressed. In this study, we investigate ODW interaction with a deflected upper corner due to confinement change upstream of the ODW. Numerical simulations are conducted using the reactive Euler equations with a two-step induction-reaction kinetic model. Two ODWs without the upper wall deflection are first simulated to resolve the basic structures with inflow Mach numbers M-0 = 6 and 7. Thereafter, we introduce a deflected upper confinement, resulting in a new wave configuration. This wave is characterized by a post-turning, triangular recirculation zone coupled with a gaseous wedge connecting the deflection point and ODW surface. A parametric study is performed to analyse the effects of the deflection location, deflection angle and activation energy of the heat release reaction. The results reveal that the wave configuration is due to the evolution of ODW decoupling in an expanded supersonic flow. We further study the surface stability and structural unsteadiness arising for M-0 = 6. Upstream-travelling transverse waves are observed for the first time, and effects of different parameters on the surface instability are analysed via fast Fourier transforms. Two destabilizing mechanisms of ODW structures are proposed, one from the post-surface thermal choking and the other from the enhanced surface instability

Research of the Three-dimensional Combustion Diagnostics Based on Chemiluminescence

&nbsp; 基于化学自发光的燃烧诊断技术对于发动机燃烧诊断非常重要。由于发动机燃烧室流场较为复杂，具有不稳定、湍流、三维等特性，绝大多数激光诊断技术无法用于精确地描述整个燃烧流场。而火焰的化学自发光与燃烧反应本身密切相关，基于化学发光的三维计算层析 (3D Computed Tomography of Chemiluminescence: 3D-CTC) 技术利用了化学反应产生的激发态自由基成像，结合CT (Computed Tomography) 技术，能够反演获得火焰三维结构和放热率的三维浓度分布。该技术不需要激光源，结构较为简单，成本低且操作性强，适合三维火焰结构以及燃烧的非稳态流场特性研究。对燃气轮机、冲压发动机等恶劣实验环境的复杂燃烧场诊断优势明显，有很强应用前景。 &nbsp; 本文对基于化学自发光的三维燃烧诊断技术开展了系统的研究：通过对甲烷预混火焰化学发光的定量测量实验与模型计算的对比进行了燃烧化学发光反应机理的初步研究；同时建立了单CCD耦合光纤束的3D-CTC多视角成像系统，并利用该系统进行了稳态火焰重构，极大提高该技术的适用性；针对发动机火焰，提出了大视场角下的模糊成像改进模型，并在算法上提出了基于BP神经网络的三维重构改进方法；将发展的3D-CTC技术进一步用于燃烧诊断，对燃气轮机的模型燃烧室进行了放热率的时空特性研究分析。 &nbsp; 首先，对甲烷预混火焰中的化学发光组份（OH*，CH*，C2*，CO2*）反应机理进行了初步探究。对不同当量比（&phi;=0.7 - 1.33）下，甲烷-空气预混火焰进行了辐射量化测量，并与一维火焰数值模拟对比。实验中采用波长分辨的光学收集系统，实现对多组份自由基的同时定量；数值模拟中系统分析了现有OH*，CH*的各反应通道和常数，并对比了放热分布与激发态物质（OH*，CH*，C2*，CO2*）的相互关系。研究结果表明，在甲烷-空气层流火焰中，OH*，CH*最合适标识放热率，C2*次之，而CO2*与放热率分布几乎无相关性。在自发光反应机理方面，利用模拟和实验结果，定量对比了不同反应通道的OH*和CH*生成量，评估了两自发光组份的主要生成反应路径。 &nbsp; 同时，探究3D-CTC的原理，建立了基于传统平行光束成像假设的重建方法，通过模拟分析了视角数目及噪声对重建精度的影响，并在甲烷/空气预混火焰平面炉上开展验证试验。针对平行光束成像模型的局限性建立了基于透镜清晰成像理论的的重建方法，提出了采用单个CCD耦合多光纤束的多视角成像系统以及三维空间标定方法，并于甲烷扩散环形火焰上进行了方法验证。通过对重建结果的尺寸以及形状分析发现其具有较好的重建准确性。 &nbsp; 然后，针对于发动机燃烧室等严苛的诊断环境，提出了大视场角下的焦外模糊成像模型和反卷积处理方案，并与传统的3D-CTC成像模型进行对比，验证实验表明，对于近距离大尺寸火焰的重建，考虑了成像的模糊效应后能够更好的反映实际火焰的结构信息。而且，针对内流火焰诊断中可能存在的遮挡等视角受限情况，进行了成像模型优化，分析了局部不完全成像对重建效果的影响程度。此后，从3D-CTC的层析重建算法着手，引入了基于BP神经网络的三维重建算法，与之前所用ART（Algebraic Reconstruction Technique）算法相比尽管效率较低却准确度高、适应性强，更有前景。 &nbsp; 最后，利用发展的3D-CTC技术开展火焰诊断，对燃气轮机模型燃烧室进行了放热率的时空特性研究。主要包括旋流火焰的放热率在时间上的热声振荡现象和空间的三维形态转变两方面。在燃烧形态转变方面，利用3D-CTC技术测量了全局当量比约0.65，总流速2.9 m/s至18.3 m/s的四个工况下旋流燃烧的CH*发光三维分布强度。以此表征放热率的三维分布，实现对旋流火焰放热空间形态转变的研究。实验是通过8个视角下的火焰二维成像来得到其三维CH*分布信息。为验证重建保真度，将重建的三维结果进行二维可视化，并与高速摄影下的二维时均结果进行对比，结果表明重建误差在5%以内。研究中，分析了不同雷诺数下放热率的空间变化规律，所有的实验工况下都显示了V形的附着火焰形态和沿着喷嘴方向的内部回流区。随着雷诺数的增加主要放热区域扩展明显且最大放热率随着流向移动。在旋流燃烧的热声震荡方面，利用CH*的二维高速摄影，对旋流燃烧的放热率不稳定性进行研究，发现放热率的振荡频率随着雷诺数的增大逐渐增加</p

Effect of Imaging Blurring on 3D Computed Tomography of Chemiluminescence

Chemiluminescence measurements are commonly employed in the study of flame geometry and excited species concentrations. Computed tomography of chemiluminescence (CTC) is potential for the three-dimensional diagnostics of combustion both on high spatial and temporal resolution. It uses 2D multidirectional projections as inputs incorporated with iterative algorithms to get the 3D distribution. In general, a good simulation of projection processing by charge-coupled device (CCD) is important for the reconstruction solution. Bokeh effect out of focus may have an effect on the projections, which is produced by lenses of finite aperture. In order to verify the influence of imaging blurring on 3D-CTC, three different projection models were studied, including clear-imaging model, out-of-focus imaging model, and deconvolution model. The results suggest that the model taken the consideration of Bokeh effects has the best reconstruction accuracy, but it is time consuming. The projection deconvolution processing can improve reconstruction accuracy without increasing computation time

automatic query classification via constructing semantic lexicon

为解决传统的文档分类方法和手工分类方法都不适宜于处理查询分类的问题,提出了一种基于Web的自动构建特定主题的语义词典的方法来分类搜索查询,通过基于主题的Web信息采集和bootstrapping,由某个主题的少量关键词逐步扩充,最终得到该主题的语义词典及词典中每个单词的相对词频.Web中信息的冗余和各主题语义上的差别使各主题的语义词典中单词的种类和数量存在很大差异,这种差异可以用来对用户的搜索查询进行分类.实验结果表明,利用语义词典可以较准确地将用户的查询分类,同时该分类方法基本上不需要人工介入,且可适应搜索查询覆盖面广和实时性强的特点,较好地解决了搜索查询分类的问题

旋流火焰燃烧温度场二维吸收光谱诊断

针对旋流火焰的复杂流场结构,结合可调谐二极管吸收光谱技术(tunable diode laser absorption spectroscopy, TDLAS)和多光谱层析成像术(hyperspectral tomography, HT),发展了具有空间分辨能力的二维吸收光谱测量技术(tunable diode laser absorption tomography, TDLAT),实现了甲烷/空气旋流火焰不同高度的二维温度场测量.该TDLAT系统吸收波长为7 185.6, 7 444.3, 6 807.8和7 466.3 cm~(-1)四线,采用分时-直接吸收探测策略,测量频率2.5 kHz,采用13×13路正交光路(空间分辨率7 mm),采用模拟退火算法进行数据重建.经与理论计算结果对比分析,重建结果真实地反映了旋流火焰温度场的二维分布

Steadiness of wave complex induced by oblique detonation wave reflection before an expansion corner

Oblique detonation wave (ODW) reflection before an expansion corner leads to a sophisticated wave complex, whose steadiness is critical to achieve a practical oblique detonation engine. Both steady and unsteady wave complexes have been observed before, but the features of unsteady wave dynamics with related unsteadiness rules are still unclear so far. In this study, the ODW reflections before an expansion corner have been simulated using the reactive Euler equations with a two-step induction-reaction kinetic model, and the wave complex structures and dynamics have been analyzed correspondingly. Three subsonic zones have been distinguished, and their interactions were found to determine the wave complex steadiness. The main subsonic zone derives from the ODW reflection, which locates behind the Mach stem, while two other subsonic zones form due to the shock reflection downstream. The two downstream subsonic zones might travel upstream and combine with the main subsonic zone, resulting in two different unsteadiness modes. These wave complex dynamics were analyzed with respect to the deflection location, deflection angle and inflow Mach number, leading to the boundaries of combustion modes and ascertaining the rule of mode regime. Some transient phenomena related with the flow instability have been also discussed, clarifying fine flow structures further. (C) 2021 Elsevier Masson SAS. All rights reserved

state-of-the-art of cluster analysis of gene expression data

基因表达数据的爆炸性增长迫切需求自动、有效的数据分析工具.目前聚类分析己成为分析基因表达数据获取生物学信息的有力工具.为了更好地挖掘基因表达数据,近年来提出了许多改进的传统聚类算法和新聚类算法.本文首先简单介绍了基因表达数据的获取和表示,之后系统地介绍了近年来应用在基因表达数据分析中的聚类算法.根据聚类目标的不同将算法分为基于基因的聚类、基于样本的聚类和两路聚类,并对每类算法介绍了其生物学的含义及其难点,详细讨论了各种算法的基本原理及优缺点.最后总结了当前的基因表达数据的聚类分析方法,并对发展趋势作了进一步的展望

Effects of Cowl-Induced Expansion on the Wave Complex Induced by Oblique Detonation Wave Reflection

Oblique detonation wave (ODW) reflection on the upper wall leads to a sophisticated wave complex, whose stability is critical to the application of oblique detonation engines. The unstable wave complex characterized with a continuous moving Mach stem has been observed, but the corresponding re-stability adjusting method is still unclear so far. In this study, the cowl-induced expansion wave based on the model with an upper-side expansion wall is introduced, and the ODW dynamics have been analyzed using the reactive Euler equations with a two-step induction-reaction kinetic model. With the addition of a cowl-induced expansion wave, the re-stabilized Mach stem has been distinguished. This re-stability is determined by the weakened secondary reflection wave of lower wall, while the final location of Mach stem is not sensitive to the position of the expansion corner. The re-stabilized ODW structure is also basically irrelevant to the expansion angle, while it may shift to unstable due to the merging of subsonic zones. Transient phenomena for the unstable state have been also discussed, clarifying fine wave structures further

基于CH自由基发光的旋流火焰放热率时空特性研究

对不同工况下CH_4/air旋流火焰的放热率在时间上的热声振荡现象和空间的三维形态转变两方面进行了研究。在燃烧形态转变方面,由于旋流火焰的复杂流场分布特性,采用基于化学自发光的三维计算层析技术(3D-CTC),测量了雷诺数从5 000到20 000的三个工况下旋流燃烧的CH*发光三维火焰结构。以此表征放热率的三维分布,实现对旋流火焰放热空间形态的测量。该诊断方法通过对旋流火焰发光在8个视角下的二维成像,结合层析重建算法得到其三维CH*分布信息。为验证重建保真度,将重建后结果二维可视化与高速摄影下的二维时均结果进行对比,结果表明重建误差在5%以内。研究中,分析了不同雷诺数下放热率的空间变化规律,结果显示所有实验工况下放热率的垂直于喷嘴方向的变化程度比沿喷嘴轴向的要剧烈;而随着雷诺数增加,最大的放热区表现出了明显的向后推进趋势。在旋流燃烧的热声振荡方面,利用CH*的二维高速摄影,对旋流燃烧的放热率不稳定性进行研究,发现放热率的振荡频率随着雷诺数的增大逐渐增加