146 research outputs found

    Unstructured mesh methods for stratified turbulent flows

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    Developments are reported of unstructured-mesh methods for simulating stratified, turbulent and shear flows. The numerical model employs nonoscillatory forward in-time integrators for anelastic and incompressible flow PDEs, built on Multidimensional Positive Definite Advection Transport Algorithm (MPDATA) and a preconditioned conjugate residual elliptic solver. Finite-volume spatial discretisation adopts an edge-based data structure. Tetrahedral-based and hybrid-based median-dual options for unstructured meshes are developed, enabling flexible spatial resolution. Viscous laminar and detached eddy simulation (DES) flow solvers are developed based on the edge-based NFT MPDATA scheme. The built-in implicit large eddy simulation (ILES) capability of the NFT scheme is also employed and extended to fully unstructured tetrahedral and hybrid meshes. Challenging atmospheric and engineering problems are solved numerically to validate the model and to demonstrate its applications. The numerical problems include simulations of stratified, turbulent and shear flows past obstacles involving complex gravity-wave phenomena in the lee, critical-level laminar-turbulence transitioning and various vortex structures in the wake. Qualitative flow patterns and quantitative data analysis are both presented in the current study

    Non-oscillatory forward-in-time integrators for viscous incompressible flows past a sphere

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    A non-oscillatory forward-in-time (NFT) integrator is developed to provide solutions of the Navier-Stokes equations for incompressible flows. Simulations of flows past a sphere are chosen as a benchmark representative of a class of engineering flows past obstacles. The methodology is further extended to moderate Reynolds number, stably stratified flows under gravity, for Froude numbers that typify the characteristic regimes of natural flows past distinct isolated features of topography in weather and climate models. The key elements of the proposed method consist of the Multidimensional Positive Definite Advection Transport Algorithm (MPDATA) and a robust non-symmetric Krylov-subspace elliptic solver. The solutions employ a finite volume spatial discretisation on unstructured and hybrid meshes and benefit from a collocated arrangement of all flow variables while being inherently stable. The development includes the implementation of viscous terms with the detachededdy simulation (DES) approach employed for turbulent flows. Results demonstrate that the proposed methodology enables direct comparisons of the numerical solutions with corresponding laboratory studies of viscous and stratified flows while illustrating accuracy, robustness and flexibility of the NFT schemes. The presented simulations also offer a better insight into stably stratified flows past a sphere

    Enhancement Effect of Borate Doping on the Oxygen Evolution Activity of α‑Nickel Hydroxide

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    The availability of low-cost oxygen evolution catalysts is critical to the successful commercialization of renewable energy conversion and storage systems based on solar energy and water splitting reactions. We demonstrate here that a moderate amount (∼12 wt %) of borate doping of crystalline α-Ni­(OH)<sub>2</sub>, a common low-cost catalyst, can lower the Tafel slope of the oxygen evolution reaction (OER) in 0.1 M KOH aqueous solution from the typical value of 60 to 43 mV decade<sup>–1</sup> and the overpotential for 10 mA cm<sup>–2</sup> of current density from 390 to 340 mV. Borate doping also improves the stability of α-Ni­(OH)<sub>2</sub> in OER. The borate-doped α-Ni­(OH)<sub>2</sub> can even surpass the benchmark Ir/C precious metal catalyst in terms of activity and stability. These results demonstrate the potential of borate doping as a strategy to increase the catalytic activity of base metal water oxidation catalysts

    Recyclable Hypervalent-Iodine-Mediated Dehydrogenative Cyclopropanation under Metal-Free Conditions

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    A method is developed for the synthesis of cyclopropanes from the C­(sp<sup>2</sup>)–C­(sp<sup>3</sup>) single bonds of β-keto esters with activated methylene compounds under metal-free conditions in the presence of 5-trimethyl­ammonio-1,3-dioxo-1,3-dihydro-1λ<sup>5</sup>-benzo­[<i>d</i>]­[1,2]­iodoxol-1-ol anion (AIBX), a recyclable water-soluble hypervalent iodine­(V) reagent developed by our group. This mild, efficient method has a wide substrate scope and good functional group tolerance and is complementary to existing cyclopropanation strategies. The method can be used to construct polysubstituted ring-fused cyclopropanes and is amenable to further synthetic transformations for construction of complex biologically active molecules as well as asymmetric cyclopropanes (90% de) when a chiral ester auxiliary is used

    Fit statistics of the null model results.

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    China’s High-speed railway (HSR) network had experienced rapid expansion during 2009 to 2013, and how the HSR expansion affects China’s economy has been considerable concerned by both policymakers and researchers. Using firm-level data, this study accessed the effects of HSR on productivity distribution and resource misallocation among manufacturing firms. Incorporating difference in difference idea into a multilevel model, the results suggest significant misallocation rectifying effect of HSR at firm level. This effect is stronger for capital-intensive firms. City-specific analysis indicates that the effects of HSR on firm-level resource misallocation varies with city size, and firms grouped in small cities gain more misallocation rectification than those grouped in big and medium cities. Market potential, which is an important way through which the HSR affects efficient allocation of production resources, is found boost the marginal effect of HSR by reducing labor market segmentation and increasing agglomeration.</div

    Summary statistics on market potential.

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    China’s High-speed railway (HSR) network had experienced rapid expansion during 2009 to 2013, and how the HSR expansion affects China’s economy has been considerable concerned by both policymakers and researchers. Using firm-level data, this study accessed the effects of HSR on productivity distribution and resource misallocation among manufacturing firms. Incorporating difference in difference idea into a multilevel model, the results suggest significant misallocation rectifying effect of HSR at firm level. This effect is stronger for capital-intensive firms. City-specific analysis indicates that the effects of HSR on firm-level resource misallocation varies with city size, and firms grouped in small cities gain more misallocation rectification than those grouped in big and medium cities. Market potential, which is an important way through which the HSR affects efficient allocation of production resources, is found boost the marginal effect of HSR by reducing labor market segmentation and increasing agglomeration.</div

    Impact of HSR on misallocation of manufacturing firms.

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    Impact of HSR on misallocation of manufacturing firms.</p

    Summary statistics.

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    China’s High-speed railway (HSR) network had experienced rapid expansion during 2009 to 2013, and how the HSR expansion affects China’s economy has been considerable concerned by both policymakers and researchers. Using firm-level data, this study accessed the effects of HSR on productivity distribution and resource misallocation among manufacturing firms. Incorporating difference in difference idea into a multilevel model, the results suggest significant misallocation rectifying effect of HSR at firm level. This effect is stronger for capital-intensive firms. City-specific analysis indicates that the effects of HSR on firm-level resource misallocation varies with city size, and firms grouped in small cities gain more misallocation rectification than those grouped in big and medium cities. Market potential, which is an important way through which the HSR affects efficient allocation of production resources, is found boost the marginal effect of HSR by reducing labor market segmentation and increasing agglomeration.</div

    Impact of market potential on agglomeration and labor market segmentation.

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    Impact of market potential on agglomeration and labor market segmentation.</p

    Seven types of passenger train service.

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    China’s High-speed railway (HSR) network had experienced rapid expansion during 2009 to 2013, and how the HSR expansion affects China’s economy has been considerable concerned by both policymakers and researchers. Using firm-level data, this study accessed the effects of HSR on productivity distribution and resource misallocation among manufacturing firms. Incorporating difference in difference idea into a multilevel model, the results suggest significant misallocation rectifying effect of HSR at firm level. This effect is stronger for capital-intensive firms. City-specific analysis indicates that the effects of HSR on firm-level resource misallocation varies with city size, and firms grouped in small cities gain more misallocation rectification than those grouped in big and medium cities. Market potential, which is an important way through which the HSR affects efficient allocation of production resources, is found boost the marginal effect of HSR by reducing labor market segmentation and increasing agglomeration.</div
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