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排序方式: 共有102条查询结果,搜索用时 15 毫秒
1.
Wang Jin Li Guolong Li Tan Zeng Min Sundén Bengt 《Journal of Thermal Analysis and Calorimetry》2021,143(6):4057-4070
Journal of Thermal Analysis and Calorimetry - The effect of Fe3O4 nanoparticles and carbon nanotubes (CNTs) on the viscosity of a nanofluid is experimentally investigated from 278 to 313 K... 相似文献
2.
PIV measurements are performed in a channel with periodic ribs on one wall. The emphasis of this study is to investigate the flow structures in the vicinity of a rib in terms of mean velocities, Reynolds stresses, probability density functions (PDF), and two-point correlations. The PDF distribution of u′ is bimodal in the separated shear layer downstream of the rib. The maximum Reynolds shear stresses occur at the leading edge of the rib. Based on quadrant analysis, it is found that ejection motions make a dominant contribution to the Reynolds shear stress in this region. Moreover, topology-based visualization is applied to the separation bubble upstream of the rib. Salient critical points and limit cycles are extracted, which gives clues to the physical processes occurring in the flow. 相似文献
3.
4.
Terje O. Espelid 《BIT Numerical Mathematics》1994,34(1):62-79
A new approach to the integration of vertex singularities is described. This approach is based on a non-uniform subdivision of the region of integration and the technique fits well to the subdivision strategy used in many adaptive algorithms. A nice feature with this approach is that it can be used in any dimension and on any region of integration which can be subdivided into subregions of the same form. The strategy can be applied both to vertex singularities and internal point singularities. In the latter case this can be done without an initial subdivision of the region in order to put the singular point in a vertex. It turns out that the technique has excellent numerical stability properties.Dedicated to Carl-Erik Fröberg on the occasion of his 75th birthday.This work was supported by The Norwegian Research Council for Science and the Humanities. 相似文献
5.
Córdova A Ibrahem I Casas J Sundén H Engqvist M Reyes E 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(16):4772-4784
Hexose sugars play a fundamental role in vital biochemical processes and their biosynthesis is achieved through enzyme-catalyzed pathways. Herein we disclose the ability of amino acids to catalyze the asymmetric neogenesis of carbohydrates by sequential cross-aldol reactions. The amino acids mediate the asymmetric de novo synthesis of natural L- and D-hexoses and their analogues with excellent stereoselectivity in organic solvents. In some cases, the four new stereocenters are assembled with almost absolute stereocontrol. The unique feature of these results is that, when an amino acid is employed as the catalyst, a single reaction sequence can convert a protected glycol aldehyde into a hexose in one step. For example, proline and its derivatives catalyze the asymmetric neogenesis of allose with >99 % ee in one chemical manipulation. Furthermore, all amino acids tested catalyzed the asymmetric formation of natural sugars under prebiotic conditions, with alanine being the smallest catalyst. The inherent simplicity of this catalytic process suggests that a catalytic prebiotic "gluconeogenesis" may occur, in which amino acids transfer their stereochemical information to sugars. In addition, the amino acid catalyzed stereoselective sequential cross-aldol reactions were performed as a two-step procedure with different aldehydes as acceptors and nucleophiles. The employment of two different amino acids as catalysts for the iterative direct aldol reactions enabled the asymmetric synthesis of deoxysugars with >99 % ee. In addition, the direct amino acid catalyzed C(2)+C(2)+C(2) methodology is a new entry for the short, highly enantioselective de novo synthesis of carbohydrate derivatives, isotope-labeled sugars, and polyketide natural products. The one-pot asymmetric de novo syntheses of deoxy and polyketide carbohydrates involved a novel dynamic kinetic asymmetric transformation (DYKAT) mediated by an amino acid. 相似文献
6.
Córdova A Sundén H Xu Y Ibrahem I Zou W Engqvist M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(21):5446-5451
The origins of biological homochirality have intrigued researchers since Pasteur's discovery of the optical activity of biomolecules. Herein, we propose and demonstrate a novel alternative for the evolution of homochirality that is not based on autocatalysis and forges a direct relationship between the chirality of sugars and amino acids. This process provides a mechanism in which a racemic mixture of an amino acid can catalyze the formation of an optically active organic molecule in the presence of a sugar product of low enantiomeric excess. 相似文献
7.
Rueping M Haack K Ieawsuwan W Sundén H Blanco M Schoepke FR 《Chemical communications (Cambridge, England)》2011,47(13):3828-3830
The design of biologically inspired, multi-component cascade reactions enables the targeted synthesis of assorted structurally complex products. Similar to regulation in cells the reaction path is controlled by the substrate concentration and complex enantiopure products with high structural diversity are provided. 相似文献
8.
Ibrahem I Zhao GL Rios R Vesely J Sundén H Dziedzic P Córdova A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(26):7867-7879
The development of one-pot organocatalytic domino Michael/alpha-alkylation reactions between bromomalonates or bromoacetoacetate esters and alpha,beta-unsaturated aldehydes is presented. The chiral-amine-catalyzed reactions with bromomalonates as substrates give access to the corresponding 2-formylcyclopropane derivatives in high yields with excellent diastereoselectivity and up to 99 % ee. The catalytic domino Michael/alpha-alkylation reactions between 4-bromo-acetoacetate and enals provide a route for the synthesis of functionalized cyclopentanones in good to high yields with 93-99 % ee. The products from the organocatalytic reactions were also reduced with high diastereoselectivity to the corresponding cyclopropanols and cyclopentanols, respectively. Moreover, one-pot combinations of amine and heterocyclic carbene catalysis (AHCC) enabled the highly enantioselective synthesis of beta-malonate esters (91-97 % ee) from the reaction between bromomalonates and enals. The tandem catalysis included the catalytic domino reaction followed by catalytic in situ chemoselective ring-opening of the 2-formylcyclopropane intermediates. 相似文献
9.
Terje O. Espelid 《BIT Numerical Mathematics》1976,16(2):133-142
An improvement on replacement selection when used in the first phase of external sorting is studied. A modified version of a snowplow model introduced by Moore [7] is used giving a theoretical estimate of the improvement. The results are in good agreement with experiments. It is the author's opinion that Dinsmore's modification of replacement selection should be considered seriously when implementing sorting software. Under certain conditions the timegain may reach 30%. 相似文献
10.
In this paper we discuss fully symmetric integration rules of degree 7 and 9 for the 4-cube. In particular we are interested in good rules. (i.e. rules with all the evaluation points inside the cube and all the weights positive).This work was supported by the Norwegian Research Council for Sciences and Humanities. 相似文献