核反应堆不同温度流体热混合的机理及实验研究

探讨了在核反应堆热工流体力学中具有普遍意义的不同温度流体达到热混合均匀状态的过程和机理。１０ＭＷ高温气冷实验堆（ＨＴＲ－１０）热气导管内的流体热混合主要是径向对流扩散过程。用流体温度空间分布的方差表示混合效果，通过雷诺比拟方法求得在点热源下游流动流体的径向扩散系数解析解，与在模拟热气导管内的点热源下游流动流体的扩散混合实验的结果相符合。在雷诺数Ｒｅ为（１．００～３．５０）×１０５范围内，径向湍动系数εｒ的相应范围是（１．００～４．００）×１０－３ｍ２／ｓ。反映混合效果的湍动系数与流速ｕ之比εｒ／ｕ随雷诺数Ｒｅ变化不大，解释了在高温堆堆芯底部结构中Ｒｅ对流体混合效果影响不大的现象。     

Strategies of Collaboration in Multi-Swarm Peer-to-Peer Content Distribution

In modern Peer-to-Peer (P2P) content distribution applications, multiple swarms typically exist, each corresponding to the dissemination of one content among interested peers. A common design in the existing P2P applications is to allow peers in one swarm to help each other, while different swarms are only coupled when sharing the upload bandwidth at the dedicated content servers/publishers. In recent years, a number of proposals have emerged which advocate inter-swarm collaboration and resource sharing, where peers in one swarm may contribute their storage and bandwidth resources to help peers in the swarm of another content. Such inter-swarm collaboration can improve content availability and optimize resource uti-lization in the entire system, at the cost of additional overhead for content preloading and inter-swarm coor-dination. This paper presents a survey of studies on effective inter-swarm collaboration mechanisms in the existing literature. This paper first discusses strategies of collaboration in P2P file sharing applications, and then presents multi-channel collaborative design for P2P live and Video-on-Demand (VoD) streaming. In particular, this paper elaborates our recent design of collaboration strategies among multiple streaming channels in a P2P VoD system, and shows that the server cost can be reduced by up to 25% while high streaming qualities are guaranteed in the entire system, even during extreme scenarios such as unexpected flash crowds. This paper also discusses representative approaches to implement inter-swarm collaborations in various P2P content distribution systems.     

Missing Data Imputation for Traffic Flow Based on Improved Local Least Squares

Complete and reliable field traffic data is vital for the planning, design, and operation of urban traffic management systems. However, traffic data is often very incomplete in many traffic information systems, which hinders effective use of the data. Methods are needed for imputing missing traffic data to minimize the effect of incomplete data on the utilization. This paper presents an improved Local Least Squares (LLS) approach to impute the incomplete data. The LLS is an improved version of the K Nearest Neighbor (KNN) method. First, the missing traffic data is replaced by a row average of the known values. Then, the vector angle and Euclidean distance are used to select the nearest neighbors. Finally, a regression step is used to get weights of the nearest neighbors and the imputation results. Traffic flow volume collected in Beijing was analyzed to compare this approach with the Bayesian Principle Component Analysis (BPCA) imputation approach. Tests show that this approach provides slightly better performance than BPCA imputation to impute missing traffic data.     

基于Relief的组合式特征选择

ReliefF是公认的效果较好的filter式特征评估方法,但该方法一大缺点是不能辨别冗余特征．提出两种基于Relief的组合式特征选择算法：ReCorre和ReSBSW,这两种算法均首先利用ReliefF算法过滤掉无关特征,然后分别采用相关分析(Correlation)以及顺序后向搜索(SBS)的Wrapper算法去除冗余特征．在实际数据集以及人造数据集上进行了实验,分析比较了Relief,ReCorre以及ReSBSW算法的性能．实验结果得出如下结论：ReliefF方法对无关特征较多的数据集能够很好的降维,但对于实际数据中特征间关系较复杂的情况,只能去掉很少的无关特征,并会去除一部分相关特征,ReliefF不能处理冗余特征,ReCorre可以在ReliefF基础上去除大部分冗余特征．ReSBSW算法可得到较好的泛化性能,但算法计算量很高,不适合大规模数据集．     

Improved Adaptive Compression Arbitration System for Wireless Sensor Networks

Previous studies have indicated that data compression in wireless sensor networks is not always beneficial to energy conservation due to the additional computational energy costs. This work gives an energy-efficient arbitration mechanism that enhances the performance of compression algorithms by avoiding unnecessary energy losses. The adaptive compression arbitration system uses a new prediction modeling and adaptation. Tests show that the modeling method gives better predictions with the adaptive mechanism...     

Quantitative Accuracy Assessment and Optimization of SPECT Geometrical Calibration

Accurate geometrical calibration is critical to obtaining high resolution and artifact free reconstructed images for modern animal single photon emission computed tomography(SPECT) systems.Although there have been many published works on the calibration of various SPECT systems,few studies have been done to evaluate the efficacy of the proposed calibration methods in a quantitative manner.This paper presents a numerical method to assess both the uniqueness and the quantitative accuracy of SPECT calibration,...     

Optimized Implementation of the FDK Algorithm on One Digital Signal Processor

This paper presents an optimized implementation of the FDK algorithm on a single fixed-point TMS320C6455 digital signal processor(DSP).Software pipelining and proper configuration of the data transfer enables a 2563 volume to be reconstructed in about 42 seconds from 360 projections with very good accuracy.This implementation reveals the potential of modern high-performance DSPs in accelerating image reconstruction,especially when cost and power consumption are emphasized.     

核电厂严重事故序列选取准则探讨

严重事故的预防和缓解是新一代核电厂的重要安全问题。本文主要讨论核电厂严重事故研究和管理中确定严重事故主导事故序列时应考虑的准则问题,包括定性和定量准则以及准则相应的意义。以此为基础,本文整理形成一套推荐中国核电厂进行严重事故主导序列筛选时采用的准则,并以1000MW非能动先进压水堆(AP1000)标准设计为研究对象进行应用尝试。分析表明,严重事故选取准则应从定性和定量两方面阐述。从定性的角度,严重事故的选取应根据核电厂状态划分,在超设计基准事故范围内,选取可能导致堆芯损伤和安全壳旁路,并且在后果上具有包络性的事故序列。从定量的角度,应确定导致核电厂严重事故风险的绝对筛选值和相对筛选值,同时还要包络陡边效应,补充确定论分析和工程经验的成果结论。根据建议准则选取的AP1000严重事故主导序列与其概率安全分析报告中进行重点分析的严重事故序列基本吻合。     

液体爆炸分散过程中界面破碎的实验研究

设计了一种有上下平面约束的液体爆炸分散装置, 通过阴影照相获得气液界面破碎形态变化的时间序列, 并应用平面激光诱导荧光(PLIF)得到了径向膨胀液体环内诱导的荧光照片. 采用二维图像处理、分形几何和数值模拟相结合的方法, 较系统地分析了实验结果. 最后, 讨论了液体环及其气液界面运动数学模型建立中的问题.     

低浓度颗粒流Boltzmann方程的同伦分析方法解

同伦分析方法(homotopy analysis method, HAM)是求解强非线性问题的有力手段. 针对颗粒流的动理学理论中的非线性微分积分方程——?Boltzmann方程, 采用 HAM方法选取局域Maxwell速度分布函数作为初始猜测解, 得到了低浓度颗粒流的Boltzmann方程的一阶近似解, 与传统的Chapman-Enskog方法得到的一阶近似解表达式的结构一致, 初步显示了HAM方法求解Boltzmann方程的有效性, 为一般Boltzmann方程的HAM方法求解奠定了基础.