Decaying turbulence in a passive mean scalar gradient

We consider the generation of passive scalar fluctuations by decaying isotropic turbulence in the presence of a uniform mean scalar gradient. At high Reynolds numbers, two distinct similarity states may be established depending on the form of the energy spectrum at low wavenumber magnitude (k). In the first similarity state characterized by a low wavenumber magnitude energy spectrum proportional tok ², the mean-square scalar fluctuation grows liket ^4/5, while in the second similarity state characterized by a spectrum proportional tok ⁴, the mean-square scalar fluctuation grows approximately liket ^4/7. These two high Reynolds number asymptotic similarity states have been subsequently confirmed by large-eddy numerical simulations. As a consequence of the decreasing flow Reynolds number as the turbulence decays, these similarity states do not continue indefinitely. At very long times, a final period of decay of the turbulence occurs, and in this final period, the mean-square scalar fluctuation in the first state continues to grow liket ^1/2, while that in the second state ultimately decays liket ^–1/2.     

大相对孔径长波红外光学系统无热化设计

 针对目前许多军工仪器红外成像系统的结构简单、体积小、质量轻的无热化设计要求,采用光学被动式方法对8 μm~12 μm波段、相对孔径为1的红外光学系统进行了无热化设计。具体光学系统参数：F=1,f=60 mm,2ω=11.4°。设计结果：在-40℃~60℃工作范围内,该系统的调制传递函数（MTF）接近衍射极限,空间分辨率在20 lp/mm处,中心视场传函接近0.7,边缘视场传函大于0.6。其设计结果满足系统的无热化设计要求。     

被动谐波锁模Er3+/Yb3+共掺双包层光纤环形腔激光器

 为了实现高重复频率和高功率的光脉冲,实验采用Er³⁺/Yb³⁺共掺双包层光纤作为增益介质,稳定的中心波长为976 nm的高功率半导体激光器作为泵浦源,利用非线性偏振旋转锁模技术,得到稳定的自起振锁模光脉冲。当泵浦功率为2.4 W时,激光器输出重复频率为8.829 MHz的连续锁模光脉冲,平均输出功率为52.5 mW,自起振锁模泵浦阈值功率为0.6 W,并观测到了稳定的高阶谐波锁模、调Q和调Q-锁模光脉冲输出。     

惯性颗粒所见被动标量统计特性的DNS与模型研究

本文采用DNS方法,对惯性颗粒所见各向同性湍流中具有平均标量梯度的被动标量场统计特性进行了研究。结果表明：惯性对颗粒温度脉动强度,两相温度关联,自相关特性以及颗粒热流与两相交叉热流的统计特性具有明显的影响。在PDF方法的框架下,系统地推导了非等温气固两相流的PDF方程,且基于朗之万随机体系对方程进行了封闭,并利用前面的...     

基于弱相干态光源的非正交编码被动诱骗态量子密钥分配

基于改造的弱相干态光源,提出了一种非正交编码被动诱骗态量子密钥分配方案.该方案不主动制备诱骗态,而是根据发送端探测器是否响应,将接收端的探测结果分为响应集合和未响应集合,以此分别作为信号态和诱骗态,并利用这两个集合来估计参量和生成密钥.数值仿真表明,非正交编码被动诱骗态方案的密钥生成效率和安全传输距离都优于现有的被动诱骗态方案,且性能非常接近主动无穷诱骗态方案的理论极限值;未响应集合对密钥生成的参与使方案性能免受发送端探测效率的影响,弥补了实际探测器探测效率低下的缺陷;由于不需要主动制备诱骗态,该方案实现非常简单,适用于高速量子密钥分配的场合. 关键词： 量子光学 量子密钥分配 被动诱骗态 密钥生成效率     

Assessment of fast and thermal neutron ambient dose equivalents around the KFUPM neutron source storage area using nuclear track detectors

A set of five ²⁴¹Am–Be neutron sources are utilized in research and teaching at King Fahd University of Petroleum and Minerals (KFUPM). Three of these sources have an activity of 16 Ci each and the other two are of 5 Ci each. A well-shielded storage area was designed for these sources. The aim of the study is to check the effectiveness of shielding of the KFUPM neutron source storage area. Poly allyl diglycol carbonate (PADC) Nuclear track detectors (NTDs) based fast and thermal neutron area passive dosimeters have been utilized side by side for 33 days to assess accumulated low ambient dose equivalents of fast and thermal neutrons at 30 different locations around the source storage area and adjacent rooms. Fast neutron measurements have been carried out using bare NTDs, which register fast neutrons through recoils of protons, in the detector material. NTDs were mounted with lithium tetra borate (Li₂B₄O₇) converters on their surfaces for thermal neutron detection via and nuclear reactions. The calibration factors of NTD both for fast and thermal neutron area passive dosimeters were determined using thermoluminescent dosimeters (TLD) with and without a polyethylene moderator. The calibration factors for fast and thermal neutron area passive dosimeters were found to be 1.33 proton tracks and 31.5 alpha tracks , respectively. The results show variations of accumulated dose with the locations around the storage area. The fast neutron dose equivalents rates varied from as low as up to whereas those for thermal neutron ranged from as low as up to . The study indicates that the area passive neutron dosimeter was able to detect dose rates as low as 7 and from accumulated dose for thermal and fast neutrons, respectively, which were not possible to detect with the available active neutron dosimeters.     

漩涡阻止器水力性能的PIV试验分析及其机理研究

漩涡阻止器是用来防止或减少漩涡发生的特殊装置,它具有特殊的几何形状,能够优化水力机械内部的流动。在水泵吸水池内部,尤其在吸入口的周围存在着较大的漩涡,在吸入口的下方,安装一个“T”型的漩涡阻止器,应用PIV技术测出安装漩涡阻止器前后吸水池内部的流场分布,并对其流动特性进行深入分析,证明了该阻止器能够有效的减小漩涡发生。对这种“T”型漩涡阻止器的作用过程进行深入的研究发现,能够用摩阻理论来解释它的作用机理。     

Interruptible Exact Sampling in the Passive Case

We establish, for various scenarios, whether or not interruptible exact stationary sampling is possible when a finite-state Markov chain can only be viewed passively. In particular, we prove that such sampling is not possible using a single copy of the chain. Such sampling is possible when enough copies of the chain are available, and we provide an algorithm that terminates with probability one.     

跨音速翼型上的激波／边界层干扰自适应控制计算

在激波区使用自适应壁对跨音速翼型的激波／边界层的相互作用（干扰）进行控制，可改变机翼的气动性能，这种被动控制可通过在翼型的激波区开一凹腔，其上覆盖一弹性橡胶膜柔壁来，本文给出用Ｎａｖｉｅｒ－Ｓｔｏｋｅｒ方程数值模拟这一自适应控制翼型的跨音速粘性绕流，提出了一个适应于本特殊情况（物面边界局部地区在求解过程中有变化）的处理办法。并探讨了自适应柔壁对当代跨音速翼绕流的影响。     

充分发展湍流场中Pr数对被动标量输运的影响

采用直接数值模拟方法,研究壁湍流中分子Pr数对湍流被动标量输运的影响.发现在槽道湍流的外层,湍流雷诺平均普朗特数PrT与分子普朗特数的倒数呈线性关系;湍流亚格子普朗特数PrT与分子普朗特数的关系较为复杂,在分子普朗特数为1附近时,湍流亚格子Prt数出现极小值.