Effect of noncircular orifice plates on the near flow field of turbulent free jets

In this paper, we experimentally investigate the near-field flow characteristics of turbulent free jets respectively issued from circular, triangular, diamond, rectangular, and notched-rectangular orifice plates into air surroundings. All the orifice plates have identical opening areas or equivalent diameters（De） and their aspect ratios（AR） range from 1 to 6.5. Planar particle image velocimetry（PIV） is used to measure the velocity field at the same Reynolds number of Re = 5 × 10^4,where Re = Ue De/ν with Ue being the exit bulk velocity and ν the kinematic viscosity of fluid. The mean and turbulent velocity fields of all the five jets are compared in detail. Results show that the noncircular jets can enhance the entrainment rate, reflected by the higher acceleration rates of mean velocity decay and spread, shorten the length of the unmixed core,expedite the increase of turbulent intensity compared with the circular counterpart shortened unmixed core, and increase turbulent intensity comparing to the circular counterpart. Among the five jets, the rectangular jet（AR = 6.5） produces the greatest decay rate of the near-field mean velocity, postpones the position at which the 鈥榓xis-switching鈥檖henomenon occurs. This supports that axis switching phenomenon strongly depends on jet initial conditions. In addition, the hump in the centerline variation of the turbulence intensity is observed in the rectangular and triangular jets, but not in the circular jet, nor in diamond jet nor in notched-rectangular jet.     

Enhanced Correlation of Electron-Positron Pair in Two and Three Dimensions

Early time electron-positron correlation in vacuum pair-production in an external field is investigated. The entangled electron and positron wave functions are obtained analytically in the configuration and momentum spaces. It is shown that, relative to that of the one-dimensional theory, two- and three-dimensional calculations yield enhanced spatial correlation and broadened momentum spectra. In fact, at early times the electron and positron almost coincide spatially. The correlation also depends on the direction of the applied field. For the spatial correlation, the transverse correlation is stronger than the longitudinal correlation.     

气体火花开关电极烧蚀研究

采用Mo,WCu和W分别作为三种气体火花开关的主电极材料,进行放电条件下电极烧蚀实验,研究开关电极烧蚀率和烧蚀形貌,分析电极烧蚀特征。结果表明,Mo,WCu和W开关的主电极烧蚀率分别为3.32×10-2 C-1·m-2,2.63×10-2 C-1·m-2和1.74×10-2 C-1·m-2,W开关主电极烧蚀率最小。实验后开关的主电极中心烧蚀严重,呈现明显裂纹和烧蚀坑。Mo主电极表面呈现明显熔融态,阴极表面形成大量裂纹(宽度达10μm)和孔隙(孔径达10μm);WCu和W主电极表面形成少量圆球状W突起(粒径达20μm及以上)。开关外壳内壁沉积了喷溅颗粒。WCu开关外壳沉积颗粒较大(粒径达10μm),Mo开关外壳沉积颗粒居中(粒径为2μm),W开关外壳沉积颗粒最小(近1μm)。因此可优先选用具有优异抗烧蚀性能的W作为气体火花开关电极材料。     

一次风风速对高温预热空气下的煤粉MILD燃烧的影响

本文研究了一次风风速对高温预热空气下的煤粉MILD燃烧的影响。首先通过与国际火焰研究基金会(IFRF)的煤粉MILD燃烧实验数据对比,验证了数值模拟方法的可行性。然后在不同一次风入射角(0°、+5°和-5°)下,保持一次风风量不变,通过改变管径将风速从26 m/s增至48 m/s。研究发现,当一次风与二次风平行(0°)或背离(+5°)入射时,提高一次风风速会使炉内的峰值温度下降;当一次风朝向(-5°)二次风入射时,提高速度会使炉内温度峰值先下降后升高。总体而言,一次风与二次风背离入射时,温度峰值最低;一次风朝向二次风入射时,温度峰值最高。     

4种鹭的繁殖生态

1999-2001年对池鹭、牛背鹭、白鹭、夜鹭混群的迁入状况、繁殖行为、雏鸟生长等进行了观察研究。结果表明：鹭鸟数量逐年增多，种类由2种增加到4种，4种鹭鸟在筑巢、取食和生活习性等方面具有一定的种群协调性。4种鹭鸟的食物组成存在一定差异。夜鹭以蛙类和鱼类为主；白鹭以鱼类和甲壳类为主，另有蛙类和昆虫类；牛背鹭的食物主要由昆虫类和蛙类组成；池鹭以鱼类为主。幼鸟体重生长率与成体体型大小成负相关，比较分析4种鹭鸟体重增长曲线，以池鹭和牛背鹭生长较快。     

一类具有弱奇异核的偏积分微分方程的Chebyshev小波数值方法（英文）

本文提出一种基于第四类Chebyshev小波配置法,求解了一类具有弱奇异核的偏积分微分方程数值解.利用第四类移位Chebyshev多项式,在Riemann-Liouville分数阶积分意义下,导出Chebyshev的分数次积分公式.通过利用分数次积分公式和二维的第四类Chebyshev小波结合配置法,将具有弱奇异核的偏积分微分方程转化为代数方程组求解.给出了第四类Chebyshev小波的收敛性分析.数值例子证明了本文方法的有效性.     

Clifford M\"{o}bius变换与Hypergenic函数

首先,在实Clifford代数空间Cl_n+1,0(R)中给出了与Clifford Mbius变换相关的一些定理.其次,证明了hypergenic函数与Clifford Mobius变换的复合可以得到一个加权的hypergenic函数.     

Robustness measurement of scale-free networks based on motif entropy

As a classical complex network model, scale-free network is widely used and studied. And motifs, as a high-order subgraph structure, frequently appear in scale-free networks, and have a great influence on the structural integrity, functional integrity and dynamics of the networks. In order to overcome the shortcomings in the existing work on the robustness of complex networks, only nodes or edges are considered, while the defects of high-order structure in the network are ignored. From the perspective of network motif, we propose an entropy of node degree distribution based on motif to measure the robustness of scale-free networks under random attacks. The effectiveness and superiority of our method are verified and analyzed in the BA scale-free networks.     

Near-zero thermal expansion in β-CuZnV2O7 in a large temperature range

We report a new type of near-zero thermal expansion material β-CuZnV₂O₇ in a large temperature range from 173 K to 673 K. It belongs to a monoclinic structure (C2/c space group) in the whole temperature range. No structural phase transition is observed at atmospheric pressure based on the x-ray diffraction and Raman experiment. The high-pressure Raman experiment demonstrates that two structural phase transitions exist at 0.94 GPa and 6.53 GPa, respectively. The mechanism of negative thermal expansion in β-CuZnV₂O₇ is interpreted by the variations of the angles between atoms intuitively and the phonon anharmonicity intrinsically resorting to the negative Grüneisen parameter.     

Improving sound diffusion in a reverberation tank using a randomly fluctuating surface

Underwater reverberation environments that satisfy the conditions of uniformity and isotropy of the diffuse field can be used to measure the acoustic characteristics of underwater targets. This study combines two practical indicators — the standard deviation of the absolute sound pressure field (to indicate uniformity) and the analysis of the wavenumber spectrum in the spherical harmonics domain (to indicate isotropy) — for an accurate evaluation of the diffusion of the sound field in a reverberation tank. A method is proposed that can improve the narrow-band diffusion of the sound field by employing a randomly fluctuating surface. An acoustic experiment was performed in a reverberation water tank (1.2 m×1 m×0.8 m), where a randomly fluctuating surface was generated by making waves. The experimental results show that as the wave motion contributes effectively to the random reflection of sound rays in all directions, the uniformity and isotropy are improved significantly when the surface is fluctuating randomly. This work helps to ensure accurate measurements of the characteristics of underwater targets in reverberation tanks.