泥沙输运模拟综述——现状及其发展趋势

随着流体力学中数值方法的飞速发展,计算模型已经成为研究流体运动,泥沙输运和不同环境(诸如河流,湖泊及沿海地区)中相应污染物归趋过程等方面非常有吸引力的工具,在过去的30多年里,发展了许多计算水动力学/泥沙输运模型.文章追溯当前具有代表性的(一维、二维、三维)模型的发展历程,描述他们各自的特点,优势及局限,力图作为对模型方面感兴趣读者的第一指南,同时也为大家讨论模型的局限性,未来的发展趋势和研究需求方面搭建一个平台.给出了模型的表达,时空特性,水动力学和沉积物的耦合方式,处理非恒定流,推移质和悬移质,泥沙交换过程,泥沙类型(粘性或非粘性)及非均匀泥沙输运的能力.总结了不同模型的应用实例,读者可以运用这些例子作为研究模型设置,模型率定及模型验证的参考.给出了选择泥沙输运模型应遵循的原则,模型输入及率定方面存在的问题及改进的途径.探讨了现有水动力学/泥沙输运模型在处理复杂湍流,泥沙携带,流动与输沙耦合,非均匀泥沙,离散和扩散系数,河岸来沙处理等方面的局限性及改进的方向.最后,对基于多相流思想的泥沙输运模型及其它一些交叉性问题作了评述与展望.     

Zinc oxide nanowires in chemical bath on seeded substrates: Role of hexamine

Ceria-doped alumina sol-gel materials were obtained by two synthesis methods at low temperature; using method A, 2-propanol-diluted cerium precursor was slowly added at the time of the aluminum sol formation in acidic environment; using method B, the cerium salt was mixed with the aluminum alkoxide before sol formation in a basic environment. The supports were characterized by N₂ physisorption, thermogravimetric and thermal differential analyses (TGA and DTA), X-ray diffraction (XRD), ²⁷Al Magic Angle Spinning-Nuclear Magnetic Resonance (MAS-NMR), 2-propanol reactions, and ammonia temperature-programmed desorption (NH₃-TPD). The samples obtained by Method B present similar values in properties such as specific areas, pore volumes, pore size distribution, and acidity compared to those of pure alumina; the alumina structure was not modified, but segregated crystallites of CeO₂ were found in samples calcined at 1000 ^∘C, as observed by XRD. The ceria-containing materials synthesized by method A show a thermal behavior similar to that of alumina, with no appreciable segregation of CeO₂ detected by XRD and modifications in the amounts of tetra, penta, and octa-hedral aluminum coordination as determined by NMR. 2-propanol reactions showed a good correlation with acid density determined by NH₃-TPD. As the percentage of ceria in the material increases, surface area, pore volume, and acidity decrease. These changes can be correlated with an increase of pentacoordinated aluminum content. The results indicate that CeO₂ is well dispersed in the alumina framework when method A is used, but synthesis method B does not have the same effect on the CeO₂incorporation.     

Reaction of diaryl-1,2-diones with triphenylphosphine and diethyl azodicarboxylate leading to N,N-dicarboethoxy monohydrazones via a novel rearrangement

[reaction: see text]. A mechanistically novel reaction of diaryl-1,2-diones with diethyl azodicarboxylate and triphenylphosphine to afford N,N-dicarboethoxy monohydrazones is described. The reaction proceeds via a nitrogen to nitrogen migration of a carboethoxy group.     

Pyridine-catalyzed stereoselective addition of acyclic 1,2-diones to acetylenic esters: synthetic and theoretical studies of an unprecedented rearrangement

A systematic study of the addition of various 1,2-acyclic diones to activated acetylenic esters catalyzed by pyridine under mild conditions is described. This reaction provides a new protocol for the stereoselective synthesis of 1,2-diaroyl maleates. The exclusive formation of the cis isomer is especially noteworthy. This reaction occurs through the initial generation of a pyridine-dimethyl acetylene dicarboxylate zwitterion and its addition to the dione followed by an unprecedented benzoyl migration. Pyridine and substituted pyridines, such as 4-dimethylaminopyridine (DMAP) and 3-methoxypyridine, are the best catalysts and anhydrous 1,2-dimethoxyethane is found to be the solvent of choice. Structural, electronic, energetic and mechanistic details of the reaction are also revealed by density functional theory calculations, which strongly support the exclusive formation of the cis isomer of the 1,2-diaroyl maleates.     

A Heuristic algorithm for reducing wavelength number of optical WDM networks

This paper proposes a Heuristic algorithm in order to reduce the total number of wavelengths required to accommodate light-paths in a WDM networks with static traffic loading. Proposed algorithm is compared with Dijkstra's algorithm for average light-path length and wavelength number of network. To see the efficiency of this 01algorithm new parameter, reduced wavelength cost (RC_X) has been defined.     

Electrochemical synthesis of copper nanoparticles on nafion–graphene nanoribbons and its application for the synthesis of diaryl ethers

Electrochemical synthesis of copper nanoparticles on nafion–graphene nanoribbons support for the synthesis of diaryl ethers via Ullmann type coupling is reported. The catalyst showed excellent performance for C–O cross coupling reactions under ligand free condition. The catalyst was characterized by various techniques such as SEM, TEM, XRD, EDX, BET, TGA, and UV spectrophotometry. It was recycled several times without significant loss in its catalytic activity.     

Gold(III) chloride promoted addition of electron-rich heteroaromatic compounds to the CC and CO bonds of enals

Electron-rich heteroaromatic compounds react with α,β-enals in the presence of gold(III) chloride to afford bis-addition products in high yields.     

EFFECT OF COMPOSITION AND PROCESSING CONDITION ON MICROSTRUCTURAL PROPERTIES AND DURABILITY OF FLUOROPOLYMER/ACRYLIC BLENDS

Fluoropolymer blends have been widely used as binders for exterior coatings because of their excellent resistance to ultra-violet (UV) radiation as well as to many corrosive chemical agents.It is known that the fluorinated component usually has a lower glass transition temperature and easily crystallizes in the final structure depending upon the blend composition and sample annealing condition.We investigated the effect of blend composition and annealing process (slow and fast cooling) on the surface mor...