Some optical specificity of ultrathin crystalline films

The changes of optical properties under boundary presence in molecular crystal nanofilm were theoretically investigated in this work. The dispersion law and states of excitons as well as their space distribution along boundary direction have been determined using adjusted Green's function method and also by combined analytical and numerical calculations. On the basis of real and imaginary part of relative permittivity, both absorption and refraction indices were determined, and the influences of boundary parameters on occurrence of a very selective and strictly discrete absorption were analyzed.     

An experimental study of interacting coherent structures in the turbulent near wake behind a pair of square cylinders

An experimental investigation (LDA technique) of coherent structures in the turbulent plane near wake behind a pair of square cylinders in side-by-side arrangement is presented with particular emphasis put on the use of 2D invariant structural measures.     

Reaction of stannenes and phosphastannenes with aldehydes and ketones: New tin four-membered ring derivatives

Stannene Tip₂Sn=CR₂ 1 (Tip=2,4,6-triisopropylphenyl, CR₂=fluorenylidene) enters a [2+2] cycloaddition reaction with benzophenone to afford the four-membered ring derivative2. This stannaoxetane undergoes a [2+2] decomposition with formation of the corresponding stannanone8 and alkene9 and an easy hydrolysis by initial cleavage of the Sn–C bond. Diphenylacetaldehyde also gives with1 a stannaoxetane, which has been characterized by its hydrolysis products. Phosphastannene Tip₂Sn=PAr13 (Ar=2,4,6-tri-tert-butylphenyl) reacts with benzaldehyde according to a [2+2] cycloaddition pattern leading to stannaphosphaoxetane14, whereas ene-products19–21 were obtained with acetaldehyde, acetone, and acetophenone.Hétérochimie Fondamentale et Appliquée, UPRES A 5069, Université Paul Sabatier, 31062 Toulouse cedex 04, France. Faculté des Sciences et Techniques, Route d'Immouzzer, BP 2022, Fs-Saiss, Fs, Maroc. Laboratoire de Synthse Organique, Faculté des Sciences, 40 Avenue du Recteur Pineau, 86022 Poitiers, France. Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp 1098–1106, August, 1999.     

Large Eddy Simulation of turbulent confined coaxial swirling jets

Large eddy simulations of swirling flow in a coaxial-jet combustor are reported. Two experimental test cases have been chosen from the literature. In both cases the configuration consists of two coaxial jets which enter into an expansion duct with the annular jet being swirled, the inner jet unswirled. The main features of the flow are well predicted in the simulations. The mean velocities and the turbulent fluctuations are compared with the experimental data and show good agreement. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Scalar Mixing and Large-Scale Coherent Structures in a Turbulent Swirling Jet

The paper presents large eddy simulations of co-annular swirling jets into an open domain. In each of the annuli a passive scalar is introduced and its transport is computed. If the exit of the pilot jet is retracted strong coherent flow structures are generated which substantially impact on the transport and mixing of the scalars. Average and instantaneous fields are discussed to address this issue. A conditional averaging technique is devised and applied to velocity and scalars. This allows to quantify the impact of the coherent structures on the mixing process.     

DNS of laminar separation nubble flows with free-stream fluctuations

A series of Direct Numerical Simulations (DNS) of Laminar Separation Bubble (LSB) flow in the presence of external disturbances has been performed. In all simulations, the primary mechanism for the transition to turbulence was found to be a two-dimensional Kelvin-Helmholtz (KH) instability of the separated shear layer that was triggered by the external fluctuation(s). The KH instability caused the shear layer to roll up. Inside the rolled up shear layer, entrained disturbances triggered elliptic instabilities which led to a rapid transition to fully three-dimensional (3D) turbulence. In the simulations with inflow oscillations the streamwise variation of the location of transition was found to decrease with the amplitude of the inflow oscillation. In the simulations with free-stream turbulence the size of the separation bubble – measured by the shape factor – was found to drastically decrease with increasing free-stream turbulence level. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Large Eddy Simulation of Turbulent Flow Through Submerged Vegetation

Large Eddy Simulations (LES) are performed for an open channel flow through idealized submerged vegetation with a water depth (h) to plant height (h _p) ratio of h/h _p = 1.5 according to the experimental configuration of Liu et al. (J Geophys Res Earth Sci, 2008). They used a 1D laser Doppler velocimeter (LDV) to measure longitudinal and vertical velocities as well as turbulence intensities along several verticals in the flow and the data are used for the validation of the present simulations. The code MGLET is used to solve the filtered Navier–Stokes equations on a Cartesian non-uniform grid. In order to represent solid objects in the flow, the immersed boundary method is employed. The computational domain is idealized with a box containing 16 submerged circular cylinders and periodic boundary conditions are applied in both longitudinal and transverse directions. The predicted streamwise as well as vertical mean velocities are in good agreement with the LDV measurements. Furthermore, fairly good agreement is found between calculated and measured streamwise and vertical turbulence intensities. Large-scale flow structures of different shapes are present in the form of vortex rolls above the vegetation tops as well as locally generated trailing and von- Karman-type vortices due to flow separation at the free end and the sides of the cylinders. In this paper, the flow field is analyzed statistically and evidence is provided for the existence of these structures based on the LES.     

Analysis of DNS and LES of Flow in a Low Pressure Turbine Cascade with Incoming Wakes and Comparison with Experiments

The flow around a low-pressure turbine rotor blade with incoming periodic wakes is computed by means of DNS and LES. The latter adopts a dynamic sub-grid-scale model. The computed results are compared with time-averaged and instantaneous measured quantities. The simulation sreveal the presence of elongated flow structures, stemming from the incoming wake vorticity, which interact with the pressure side boundary layer. As the wake approaches the upstream half of the suction side, its vortical structures are stretched and align with the main flow, resulting in an impingement at virtually zero angle of attack. Periodically, in the absence of impinging wakes, the laminar suction side boundary layer separates in the adverse pressure gradient region. Flow in the laminar separation bubble is found to undergo transition via a Kelvin–Helmholtz instability. Subsequent impingement of the wake inhibits separation and thus promotes boundary layer reattachment. LES provides a fair reproduction of the DNS results both in terms of instantaneous, phase-averaged, and time-averaged flow fields with a considerable reduction in computational effort. This revised version was published online in July 2006 with corrections to the Cover Date.     

Three-dimensional mathematical model and its application in the neighborhood of the Three Gorges Reservoir dam in the Yangtze River

The calculation of flow and sediment transport is one of the most important tasks in river engineering. The task is particularly difficult because a number of complex physical phenomena should be accounted for more realistically in a model with a predictive power. Three-dimensional calculations of river flow and suspended sediment transport are performed in this paper with application in the Three Gorges Reservoir in the Yangtze River. A period of 76 years after the dam is built is simulated and the results are compared with laboratory measurements obtained by Tsinghua University whereby the model is verified and calibrated. Generally speaking, the calculated results agree well with the experiments, demonstrating that the present model can be used for flow and sediment transport prediction in rivers. The project supported by the National Natural Science Foundation of China (50009004)