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71.
The nonlinear convection forced by the boundaries of a Hele Shaw cell to align perpendicular to an imposed shear flow was analytically investigated by the boundary-layer method. The imposed shear flow may be a Couette flow that extends throughout the convecting layer or flow confined to a boundary, depending on the geometry of the Hele Shaw cell. This study examined the case in which the imposed shear flow has a boundary-layer structure and its interaction with the convecting interior. Analytical solutions for both the boundary layer and interior were obtained. The study revealed the following.For large aspect ratio A , the interaction of the imposed shear flow and convection is confined to the boundary layer. The boundary layer is a viscous rather than a thermal layer. The results showed that the range of validity of the Hele Shaw equations used in the literature is of order 1/ A 2 . For an asymptotically large aspect ratio A up to order 1/ A 2 , the velocity in the y -direction must be zero. The velocity in the x -direction and the z -direction has a parabolic dependence on y , but the temperature perturbation does not depend on y . These results may have implication for convection in porous media. 相似文献
72.
Philip J. Marriott Russell M. Kinghorn Ruby Ong Paul Morrison Peter Haglund Mikael Harju 《Journal of separation science》2000,23(3):253-258
The two current technologies for achieving comprehensive gas chromatography (GC×GC) – the thermal sweeper and the cryogenic modulator – are compared in an interlaboratory study using a multicomponent semi‐volatile aromatic compound sample. The same column set (phases, film thickness, dimensions of columns) and conditions of oven temperature program were used. Carrier gas flow settings however were different for the data reported here. The thermal sweeper has a longer overall length due to the extra ca. 30 cm length of narrow bore tubing used for the modulator/accumulator section. Data reveal that the two methods behave in an analogous manner in respect of delivering GC×GC results, with key peak parameters of peak widths and symmetry measures showing good correlation. Retention time dissimilarity on the first dimension columns in the two systems arises from different flow rates used, however the second column retention is similar, and this is due to the resulting different elution temperatures that peaks elute on the first dimension in each system. Overall, the two approaches to GC×GC appear to produce equivalent results within the scope of the application studied. Each system does have its experimental limitations; the thermal sweeper has what may be called a ‘thick film effect’, where at high temperature it can be difficult to sufficiently trap the migrating bands in the accumulator column, and the pulsing of solutes in the cryogenic system may suffer from a ‘thick wall effect’ if a column with too thick a wall dimension is used at low oven temperature. 相似文献
73.
PF Hendrix SL Lachnicht MA Callaham X Zou 《Rapid communications in mass spectrometry : RCM》1999,13(13):1295-1299
Feeding strategies of earthworms and their influence on soil processes are often inferred from morphological, behavioral and physiological traits. We used (13)C and (15)N natural abundance in earthworms, soils and plants to explore patterns of resource utilization by different species of earthworms in three tropical ecosystems in Puerto Rico. In a high altitude dwarf forest, native earthworms Trigaster longissimus and Estherella sp. showed less (15)N enrichment ((15)N = 3-6 per thousand) than exotic Pontoscolex corethrurus ((15)N =7-9 per thousand) indicating different food sources or stronger isotopic discrimination by the latter. Conversely, in a lower altitude tabonuco forest, Estherella sp. and P. corethrurus overlapped completely in (15)N enrichment ((15)N = 6-9 per thousand), suggesting the potential for interspecific competition for N resources. A tabonuco forest converted to pasture contained only P. corethrurus which were less enriched in (15)N than those in the forest sites, but more highly enriched in (13)C suggesting assimilation of C from the predominant C(4) grass. These results support the utility of stable isotopes to delineate resource partitioning and potential competitive interactions among earthworm species. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
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Ruby Chauhan Ashavani Kumar Ram Pal Chaudhary 《Journal of Sol-Gel Science and Technology》2013,67(2):376-383
Fe doped ZnS nanoparticles (Zn1?xFexS; where x = 0.00, 0.03, 0.05 and 0.10) were synthesized by a chemical precipitation method. The synthesized products were characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope, ultraviolet–visible and photoluminescence spectrometer. The X-ray diffraction and transmission electron microscope studies show that the size of crystallites is in the range of 2–10 nm. Photocatalytic activities of ZnS and 3, 5 and 10 mol% Fe doped ZnS were evaluated by decolorization of methylene blue in aqueous solution under ultraviolet and visible light irradiation. It was found that the Fe doped ZnS bleaches methylene blue much faster than the undoped ZnS upon its exposure to the visible light as compared to ultraviolet light. The optimal Fe/Zn ratio was observed to be 3 mol% for photocatalytic applications. 相似文献
77.
The cognitive domain in mathematics, defined as thinking and understanding in the process of learning mathematics, is a main focus of curricula in many countries. This study explores breadth and depth of understanding as addressed in mathematics textbooks certified as aligned to Israeli national mathematics curricula. We compare opportunities for students to engage with mathematics requiring different types and levels of understanding provided by the tasks in mathematics textbooks. Comparison of two fourth grade and two eighth grade mathematics textbooks showed significant differences in the opportunities to learn in the cognitive domain that each provides. These differences can be quantified; the quantification defines the cognitive demand of the textbook. The cognitive demand of the four textbooks varies. This reveals a potential source of inequity in students’ opportunities to learn mathematics. Results should prompt discussion around standardization and alignment of textbooks to the cognitive goals of the curriculum. 相似文献
78.
Aissa R. Ruby C. Gehin A. Abdelmoula M. Génin J.-M. R. 《Hyperfine Interactions》2004,156(1-4):445-451
Al-substituted hydroxysulphate green rust (Al-GR{SO4}) were synthesised by the coprecipitation of FeII, FeIII and AlIII cations. The Al-GR{SO4} crystals (~50 nm) are significantly smaller than the hydroxysulphate green rust GR{SO4} crystals (~500 nm). The Mössbauer spectrum of Al-GR{SO4} was adjusted with two ferrous doublets D1 and D3 and one ferric doublet D2. Doublet D3 is attributed to FeII ions that have AlIII ions as a first neighbour.
相似文献79.
Adaptive group formation in dynamic environments performed by heterogeneous swarms of simple agents is an interesting research topic. In this paper we consider an unsupervised scenario where the individuals of the swarm have limited information about their environment as well as limited communication capabilities. The particular case of a multi-agent model with self-organized reconfigurable agents where the agents are confronted with a resource collection task, different movement, and group formation tactics are analyzed experimentally. It is shown that cooperation in groups is profitable for the group members and the optimal group size depends on environmental parameters. Moreover, a simple strategy based on the agents ability to measure their own workload results in an adaptive behavior that influences the size of the groups and increases the performance of the overall system. 相似文献
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