Stochastic Simulation of Scalar Mixing Capturing Unsteadiness and Small-scale Structure Based on Mean-flow Properties

A key limitation of Reynolds-Average Navier-Stokes (RANS) simulation of mixing and reaction in turbulent flows is the lack of resolution of small-scale structure and associated unsteadiness. Various subgrid models formulated in state space have been developed to complement the RANS solution in this regard. We here introduce a physical-space formulation that captures unsteady advective and diffusive processes at all scales of the turbulent flow. The approach is a 3D construction based on the Linear Eddy Model (LEM), involving three orthogonally intersecting arrays of 1D LEM domains, and coupled so as to capture the 3D character of fluid trajectories. To illustrate the model performance of the 3D LEM-based formulation, here termed LEM3D, multi-stream mixing in a turbulent round jet is simulated using measured mean-flow properties as input. Comparison to scalar cross-correlation coefficients and other measured properties of this mixing configuration indicate that the LEM3D approach, in conjunction with flow properties that are provided by steady-state models, is a useful representation of complex turbulent mixing processes that would otherwise be difficult to capture within a steady-state CFD framework.     

Plasmonic surface enhancement of dual fluorescence and phosphorescence emission from organic semiconductors: effect of exchange gap and spin-orbit coupling

Dual singlet-triplet fluorescence-phosphorescence emitting compounds demonstrate that plasmonic surface enhancement is controlled solely by the underlying oscillator strength of a transition: metal-free compounds with weak spin-orbit coupling show no enhancement in phosphorescence efficiency even though fluorescence is amplified.     

Determining the absolute configuration of two marine compounds using vibrational chiroptical spectroscopy

Chiroptical techniques are increasingly employed for assigning the absolute configuration of chiral molecules through comparison of experimental spectra with theoretical predictions. For assignment of natural products, electronic chiroptical spectroscopies such as electronic circular dichroism (ECD) are routinely applied. However, the sensitivity of electronic spectral parameters to experimental conditions and the theoretical methods employed can lead to incorrect assignments. Vibrational chiroptical methods (vibrational circular dichroism, VCD, and Raman optical activity, ROA) provide more reliable assignments, although they, in particular ROA, have been little explored for assignments of natural products. In this study, the ECD, VCD, and ROA chiroptical spectroscopies are evaluated for the assignment of the absolute configuration of a highly flexible natural compound with two stereocenters and an asymmetrically substituted double bond, the marine antibiotic Synoxazolidinone A (SynOxA), recently isolated from the sub-Arctic ascidian Synoicum pulmonaria. Conformationally averaged nuclear magnetic resonance (NMR), ECD, Raman, ROA, infrared (IR) and VCD spectral parameters are computed for the eight possible stereoisomers of SynOxA and compared to experimental results. In contrast to previously reported results, the stereochemical assignment of SynOxA based on ECD spectral bands is found to be unreliable. On the other hand, ROA spectra allow for a reliable determination of the configuration at the double bond and the ring stereocenter. However, ROA is not able to resolve the chlorine-substituted stereogenic center on the guanidinium side chain of SynOxA. Application of the third chiroptical method, VCD, indicates unique spectral features for all eight SynOxA isomers in the theoretical spectra. Although the experimental VCD is weak and restricted by the limited amount of sample, it allows for a tentative assignment of the elusive chlorine-substituted stereocenter. VCD chiroptical analysis of a SynOxA derivative with three stereocenters, SynOxC, results in the same absolute configuration as for SynOxA. Despite the experimental challenges, the results convincingly prove that the assignment of absolute configuration based on vibrational chiroptical methods is more reliable than for ECD.     

Taurine

The structure of taurine (2‐amino­ethane­sulfonic acid), C₂H₇NO₃S, has been redetermined at 150 K, and is compared with the structures of glycine and β‐alanine which, like taurine, are enzymatically conjugated with bile acids.     

Mean curvature flow through singularities for surfaces of rotation

In this paper, we study generalized “viscosity” solutions of the mean curvature evolution which were introduced by Chen, Giga, and Goto and by Evans and Spruck. We devote much of our attention to solutions whose initial value is a compact, smooth, rotationally symmetric hypersurface given by rotating a graph around an axis. Our main result is the regularity of the solution except at isolated points in spacetime and estimates on the number of such points.     

官能团化刚性大环的超分子化学

重点介绍具有纳米尺寸的苯炔刚性大环的超分子化学: 包括聚集效应, 主客体化学、液晶行为及二维、三维超分子结构等.     

A Toroidal Zr70 Oxysulfate Cluster and Its Diverse Packing Structures

Herein, we report the discovery of a toroidal inorganic cluster of zirconium(IV) oxysulfate of unprecedented size with the formula Zr₇₀(SO₄)₅₈(O/OH)₁₄₆⋅x(H₂O) (Zr₇₀), which displays different packing of ring units and thus several polymorphic crystal structures. The ring measures over 3 nm across, has an inner cavity of 1 nm and displays a pseudo-10-fold rotational symmetry of Zr₆ octahedra bridged by an additional Zr in the outer rim of the ring. Depending on the co-crystallizing species, the rings form various crystalline phases in which the torus units are connected in extended chain and network structures. One phase, in which the ring units are arranged in layers and form one-dimensional channels, displays high permanent porosity (BET surface area: 241 m² g⁻¹), and thus demonstrates a functional property for potential use in, for example, adsorption or heterogeneous catalysis.     

MICROMECHANICS OF MACROELECTRONICS

The advent of flat-panel displays has opened the era of macroelectronics. Enthusiasm is gathering to develop macroelectronics as a platform for many technologies, ranging from paper-like displays to thin-film solar cells, technologies that aim to address the essential societal needs for easily accessible information, renewable energy, and sustainable environment. The widespread use of these large structures will depend on their ruggedness, portability and low cost, attributes that will come from new material choices and new manufacturing processes. For example, thin-film devices on thin polymer substrates lend themselves to roll-to-roll fabrication, and impart flexibility to the products. These large structures will have diverse architectures, hybrid materials, and small features; their mechanical behavior during manufacturing and use poses significant challenges to the creation of the new technologies. This paper describes ongoing work in the emerging field of research - the mechanics of macroelectronics, with emphasis on the mechanical behavior at the scale of individual features, and over a long time.