Double-component convection due to different boundary conditions in an infinite slot diversely oriented to the gravity

Onset of small-amplitude oscillatory and both small- and finite-amplitude steady double-component convection arising due to component different boundary conditions in an infinite slot is studied for various slot orientations to the gravity. The main focus is on two compensating background gradients of the components. The physical mechanisms underlying steady and oscillatory convection are analyzed from the perspective of a universally consistent understanding of the effects of different boundary conditions. In a horizontal slot with inviscid fluid addressed by Welander [P. Welander, Tellus Ser. A 41 (1989) 66], oscillatory convection sets in with the most unstable wave number and oscillation frequency being zero. Exact expressions for the critical fixed-value background gradient and the respective group velocity at zero wave number are derived from the long-wavelength expansion both for the horizontal slot with independently varying background gradients and for the inclined slot with the compensating gradients. In the horizontal slot with viscous fluid, the dissipation of along-slot perturbation-cell motion reduces efficiency of the oscillatory instability feedback and thus prevents the most unstable wavelength from being infinite. Based on this interpretation, the oscillatory instability of a three-dimensional (3D) nature is predicted for an interval of long two-dimensional (2D) wavelengths in an inclined slot, and such 3D instability is indeed shown to arise. Related general conditions for three-dimensionality of most unstable disturbances are also formulated. As the slot orientation changes from the horizontal by angle θ (?π/2), the oscillatory 2D marginal-stability boundaries in inviscid and viscous fluid are expected to eventually transform into respective steady ones. Oscillatory instability in the vertical slot with viscous fluid, first reported by Tsitverblit [N. Tsitverblit, Phys. Rev. E 62 (2000) R7591], is of a quasi-steady nature. Its (new) mechanism is identified. It is underlain by differential gradient diffusion. As the horizontal slot at θ = π, addressed by Tsitverblit [N. Tsitverblit, Phys. Fluids 9 (1997) 2458], changes its orientation to vertical, the wave number interval of linear steady instability shrinks to the vicinity of the most unstable zero wave number and vanishes. Consistently with the basic nature of finite-amplitude steady convection being the same in the horizontal and vertical slots, the respective convective flows are continuously transformed into each other. The dissimilarity between the nature of finite-amplitude steady convective flows in the horizontal slot with θ = 0, revealed by Tsitverblit [N. Tsitverblit, Phys. Lett. A 329 (2004) 445], and that in the vertical slot is shown to eventually give rise to a region of hysteresis in θ ∈ (0, π/2).     

基于异型网的数据通信和共享

本文主要介绍基于 Unix- Windows异型网环境下点对点通信的一种方法 ,通过它可以实现 Unix服务器、终端与 Windows工作站及 Windows NT服务器间点对点通信。此外还介绍了一种在 Unix- Net Ware- Windows- Windows NT- L ANMAN工作站异型网环境下文件资源共享方法。通过它可实现 Windows工作站及 Windows NT服务器及 L ANMN工作站对 Unix文件服务器及Novell文件服务器的资源共享。     

Simplified theory and analytical solution for functionally graded thin plates with different moduli in tension and compression

In this study, a simplified theory for functionally graded thin plates with different moduli in tension and compression is proposed. Based on the classical Kirchhoff hypothesis, a mechanical model concerning tension-compression subzone is established, first. Using the geometrical and physical relations and equation of equilibrium, all stress components are expressed in terms of the deflection, in which modulus of elasticity in tensile and compressive zone are regarded as two different functions while Poisson's ratios are taken as two different constants. Via the equilibrium conditions and continuity conditions, the governing equation expressed in terms of the deflection as well as the unknown neutral layer are derived, respectively. Moreover, the application in polar coordinates, the strain energy and the perturbation solution for the unknown neutral layer, are discussed in detail. The results indicate that the bending stiffness derived in this study play an important role while contacting the classical problem and this problem. The analytical solutions from equilibrium conditions and continuity conditions are consistent. Analyses of more general cases for modulus of elasticity and Poisson's ratio also show the applicability of the simplified theory. This study provides a theoretical basis for the subsequent work.     

不同海拔区域内牦牛肠道菌群结构组成多样性研究

探索不同海拔牦牛肠道菌群结构组成,为不同海拔区域间牦牛的饲养交流提供帮助. 本研究随机采集海拔2 897~4 717 m的12头牦牛的新鲜粪便样本,用16s rDNA技术对样本宏基因组测定,比较分析高低海拔牦牛肠道菌群的OTU丰度差异,研究高低海拔区域的牦牛肠道微生物菌群结构组成. 结果显示,高海拔与低海拔地区牦牛肠道菌群在门、纲、目、科、属、种层面菌群结构组成不同. 高低海拔区域的牦牛肠道菌群有8个菌在属水平差异显著（P＜005）. 结果表明,高海拔与低海拔牦牛肠道菌群结构组成存在一定差异,低海拔地区牦牛肠道菌群OTU丰度比高海拔的更为丰富.     

北京冬季大气污染物的DOAS监测与分析

基于差分光学吸收光谱(different optical absorption spectroscopy,DOAS)技术于2007年1月19日～2月8日期间对位于北京市北四环附近的北京大学校园进行了大气污染物SO2,NO2,HONO和HCH的连续监测.并对SO2典型日变化趋势、主要污染源以及气象条件对污染物浓度分布的影响进行了分析.结果表明,在天气状况较为稳定的情况下,SO2具有"V"字型日变化特征,即除下午浓度较低外,其余时间浓度均较高.研究发现北京冬季采暖期间燃煤对大气各污染物的浓度均具有显著贡献.结合风速等气象要素对污染物扩散、传输和清除进行的分析可知,监测期间风速对污染物浓度分布起主导作用,气压等其它气象.状况对污染物浓度分布也有一定的影响.     

3-硝基邻苯二甲酸系列稀土配合物的光谱学与热稳定性研究

在水-乙醇溶液中合成了稀土离子(RE=La, Nd, Eu, Tb, Er, Y)与3-硝基邻苯二甲酸(H₂L)的晶体化合物,并通过元素分析、红外光谱、紫外光谱、差热-热重分析对系列配合物RE₂L₂(HL)₂(H₂O)₆·2H₂O进行了系统的研究与表征。谱学研究结果与通过X衍射表征的晶体结构数据是一致的。配体3-硝基邻苯二甲酸具有两种配位模式。两个全脱质子的配体L2-以桥联-螯合形式与两个稀土离子配位,而另两个单脱质子的配体HL-利用其酸根(—COO-)和羧基(—COOH)分别与金属离子以端基螯合形式配位。红外数据证实了羧酸根(—COO-)和羧基(—COOH)的存在以及羧基(—COOH)与结晶水之间的氢键作用。差热-热重数据与化合物的组成结构非常吻合,配合物中的结晶水和配位水在150 ℃左右开始失去并吸热,配体则在400 ℃左右开始氧化分解。     

Current progresses in two-dimensional MXene-based framework: prospects from superficial synthesis to energy conversion and storage applications

MXenes are regarded as a type of two-dimensional (2D) inorganic material, mainly comprising a number of transition metal carbides, nitrides, or carbonitrides atomic planes. Nevertheless, the scientific community is continuously interested in exploring and structuring the engineered-based multifunctional material for numerous applications. The MXenes-based materials in this context, have emerged as highly active compounds owing to their superior surface area, substantial interlayer spacing, highly reactive surface-active sites and surface functional group, even though, recent studies have shown significant scientific and theoretical progress related to enormous prospects in MXenes, chemical nature, robust electrochemistry and high hydrophilicity of MXenes. The role of MXenes in all kinds of strategies is still in an upgrading phase for their further improvement, and is not sufficiently summarized in the literature now. To begin with this, herein, present review article is intended to critically discuss the diversity of MXenes with respect to different composition, formulation, plasmonic, complexation, and numerous geometric and morphological aspects, along with novel construction strategies to improve their surface characteristics in all aforesaid multidimensional applications. Following that, in terms of broadening the application, this review article is envisaged to endorse the use of MXenes and their hybrid configuration in a series of emerging environmental decontamination via adsorption, photodegradation, photocatalytic fuel production via hydrogen evolution, CO₂ reduction, electrocatalytic sensing, along with membrane distillation and energy storage. In addition, comprehensive information about existing obstacles and future perspectives have been addressed. Finally, an overview is succinctly summarized and discussed regarding the emerging prospects of MXenes for their potential uses in numerous research fields. At the end, it is anticipated that this review article will pave the way for the effective use of MXenes in different fields of environmental remediation, energy conversion, storage and biomedical applications as an innovative, reliable, and multifunctional material.     

近红外光谱混合模型定量分析不同物理状态样品的研究

近红外光谱(NIRS)以漫反射模式对非均质样本进行测量时,由于其光谱散射和吸收系数差异较大,建立的校正模型准确性和稳健性较低,因此,本研究提出了一种基于均质样本和模型转移方法建立混合模型的策略,解决非均质样本近红外光谱检测的问题.以烟叶样本为研究对象,分别建立了基于Shenk专利算法(Shenk′s)、分段直接标准化(PDS)和基于典型相关分析的模型转移算法(CTCCA)的烟粉+烟丝、烟粉+烟片混合模型,用于烟丝和烟片样本中烟碱含量的预测.结果表明,混合模型对烟丝和烟片样本的预测均方误差(RMSEP)较直接建模分别降低了1.39%和2.73%,预测结果有一定的改善,稳健性提高,3种方法中CTCCA表现最优.因此,采用近红外光谱均质模型和模型转移方法建立的混合模型对非均质样本的测定具有可行性,有利于在线近红外光谱分析技术的发展,可为近红外光谱模型的共享提供参考.     

不同类别非完整大数据中缺失数据填充算法研究

针对目前缺失数据填充算法精度低、运行效率低、内存占用率大的特点,提出一种新的不同类别非完整大数据中缺失数据填充算法。通过2个定理阐述了缺失数据填充算法的原理,给出信息熵的计算过程。输入根据数据集构建的决策表和不同类别非完整大数据中缺失数据的最大值、最小值、填充步长。求出其他类指标和某类指标的相关性,得到数据集,求出权重系数;计算初始数据库的信息熵,通过相关理论或经验对缺失数据区间下限进行设定;用一个很小的区间数据取代缺失数据,根据给出的步长不断扩大区间范围,绘制出每一步信息熵状况,将其与初始数据库信息熵相比,实现缺失数据填充。实验结果表明,所提算法精度高、运行效率高、内存占用率低。     

不同还原剂制备氧化亚铜及其电化学性能研究

基于绿色化学的思想,运用软模板法成功的实现了环境友好型的不同还原剂如葡萄糖、麦芽糖等在水中还原Cu2+,制得了形貌不同的纳米氧化亚铜(Cu2O)晶体,用X射线衍射仪(XRD)和原子力显微镜(AFM)对所得产物进行表征,并研究了纳米Cu2O修饰电极的电化学行为。该制备方法简单易行,为纳米Cu2O的绿色合成提供了新的途径。