稳定分层流密度界面处湍流混合与分形结构

采用室内混合箱研究稳定分层流(上层淡水、下层盐水) 无剪切密度界面处的湍流混合与分形结构. 湍流通过浸没在盐水层中的振动格栅产生, 密度界面结构通过在盐水层中添加荧光剂或染料可视化, 共进行12 组实验. 实验观测并记录了:(1) 淡、盐水密度界面距混合箱底部的平均高程(h);(2) 淡、盐水层的密度(ρ₀,ρ), (3) 淡、盐水密度界面. 其中, 淡、盐水密度界面通过照片、录像进行记录. 观测结果用于计算:(1) 盐水层密度;(2) 卷挟速度, (3) 整体理查孙数(Ri_o), (4) 二维、三维密度界面, (5) 二维、三维密度界面的分形维度. 结果分析发现:(1) 湍流卷挟率随Rio 增大而减小, 并且满足Ri_o的-3=2 或-7=4 幂律;表明随着湍流强度的减弱, 混合的速度也越来越缓慢;(2) 二维密度界面分形维度大于1, 三维密度界面分形维度大于2;表明二维、三维密度界面存在分形结构;(3) 分形维度随Ri_o的增大而减小;表明随着湍流强度的减弱, 密度界面也越来越趋于光滑.      

基于有效端元集的双线性解混模型

光谱解混是用于定量分析高光谱图像中成分含量的一项重要技术方法,主要包括线性解混模型和非线性解混模型。线性解混模型构造简单,但并未考虑不同成分光子间的相互影响,导致解混结果在很多实际图像上不够精确。常用的非线性混合模型中的双线性解混算法,随着图像中端元数量增加,虚拟端元的数量也随之快速增加,计算精度受到很大的影响。论文报道改进了双线性解混的模型,并提出一种有效端元子集的选择算法。首先结合欧氏距离和光谱夹角,按照与混合像元的距离,将所有端元排序;然后利用排序结果和误差变化情况选择实际参与混合的端元子集。从而降低了未参与特定混合像元混合的端元对解混结果的影响,提高了解混精度。对模拟图像的测试效果证明了该算法可以减小光谱的重构误差,对实际航拍高光谱溢油图像的分析结果也进一步说明了算法的有效性。     

Solutions to Buoyancy-Drag Equation for Dynamical Evolution of Rayleigh-Taylor and Richtmyer-Meshkov Mixing Zone

With a self-similar parameter b( A_t ) = H_i /λa_i, where A_t is the Atwood number, H_i andλ_i are the amplitude and wavelength of bubble (i = 1) and spike (i =
2) respectively, we derive analytically the solutionsto the Buoyancy-Drag equation recently proposed for dynamical evolution of Rayleigh-Taylor and Richtmyer-Meshkov mixing zone.Numerical solutions are obtained with a simple form of b(A_t ) = 1/(1 + A_t ) and comparisons with recent LEM (linear electric motor) experiments are made, and an agreement is found with properly chosen initial conditions.     

Polarisation-sensitive four-wave mixing and soliton self-frequency shift effect in the highly birefringent photonic crystal fibre

By adjusting the polarisation state of the pump at 805 nm parallel to slow (x) and fast (y) axes of the highly birefringent photonic crystal fibre with zero dispersion wavelengths 790 nm and 750 nm, this paper demonstrates the efficient polarisation-sensitive four wave mixing involved in pump, anti-Stokes and Stokes signals and soliton self-frequency shift effects induced by the phase-matching between red-shifted solitons and blue-shifted dispersive waves. If the reduction of coupling efficiency to the circular pump laser mode or other circular fibres due to asymmetry of the core is neglected, more than 98% of the total input power is kept in a single linear polarisation. Controlled dispersion characteristic of the doublet of fundamental guided-modes results in achieving light field strongly confined in principal axes of photonic crystal fibre, and enhancing the corresponding nonlinear-optical process through the remarkable nonlinear birefringence.     

Phase evolution of （K,Na）NbO3 powder prepared by high temperature mixing under hydrothermal conditions

（Na, K）NbO3 （KNN） powders were successfully prepared by high temperature mixing method （HTMM） under hydrothermal conditions to study the effect of reaction time on the formation of KNN for three K＋/（K＋ ＋Na＋） ratios of 0.6, 0.7 and 0.8. The products were characterized by X-ray diffraction （XRD）, scanning electron microscope （SEM）, transmission electron microscopy （TEM） and selected area electron diffraction （SAED）, to show the change of phase and morphology of the as-prepared particles with the K＋/（K＋ ＋ Na＋） molar ratio in the solution. Pure Na-rich KNN monoclinic phase and pure K-rich KNN orthorhombic phase could be obtained quickly after mixing the solutions at high temperature when the K＋/（K＋ ＋Na＋） molar ratio was either 0.6 or 0.8. When the K＋/（K＋ ＋Na＋） molar ratio was 0.7, however, the K-rich KNN orthorhombic phase grain formed first, followed by the Na-rich KNN monoclinic phase grain, with the two phases coexisting in the final product.     

Self-similarity of a Rayleigh–Taylor mixing layer at low Atwood number with a multimode initial perturbation

High-fidelity large eddy simulation (LES) of a low-Atwood number (A = 0.05) Rayleigh–Taylor mixing layer is performed using the 10th-order compact difference code Miranda. An initial multimode perturbation spectrum is specified in Fourier space as a function of mesh resolution such that a database of results is obtained in which each successive level of increased grid resolution corresponds approximately to one additional doubling of the mixing layer width, or generation. The database is then analysed to determine approximate requirements for self-similarity, and a new metric is proposed to quantify how far a given simulation is from the limit of self-similarity. It is determined that mixing layer growth reaches a high degree of self-similarity after approximately 4.5 generations. Statistical convergence errors and boundary effects at late time, however, make it impossible to draw similar conclusions regarding the self-similar growth of more sensitive turbulence parameters. Finally, self-similar turbulence profiles from the LES database are compared with one-dimensional simulations using the k-L-a and BHR-2 Reynolds-averaged Navier–Stokes models. The k-L-a model, which is calibrated to reproduce a quadratic turbulence kinetic energy profile for a self-similar mixing layer, is found to be in better agreement with the LES than BHR-2 results.     

Autler–Townes spectroscopy of high-lying state by phase conjugate six-wave mixing

An Autler-Townes(AT) spectroscopy based on phase conjugate six-wave mixing(SWM) is proposed to detect AT doublet of high-lying state in a Doppler-broadened cascade four-level system.It is found that the SWM spectrum is dependent strongly on the ratios between the magnitudes of the wave vectors.We discuss how the Doppler broadening affects the SWM spectrum from a time-domain viewpoint and find that,due the atomic motion,the atomic polarizations acquire different phases for atoms with different velocities as time evolves.The Doppler free SWM spectrum can be obtained only when the atomic polarization can be rephasing again at certain time after the interactions of all the incident fields.     

基于团簇结构模型的Na2O-Al2O3-SiO2三元体系熔体吉布斯混合摩尔自由能计算

以三元硅酸盐熔体团簇结构模型为基础,选取了Na₂O-Al₂O₃-SiO₂体系不同成分的团簇结构,采用半经验量子化学方法MNDO/d分别计算该三元体系熔体中不同结构的团簇基元在1473、1873、2000 K温度下的熵、焓、热容和自由能等热力学数据,计算得出不同团簇结构基元的混合自由能,并根据统计热力学波尔兹曼分布定律,推导计算得出Na₂O-Al₂O₃-SiO₂三元体系各成分下的混合摩尔自由能。三元硅酸盐熔体的热力学性质与该熔体的微观结构密切相关。     

Numerical Simulation of Mixing Characteristics and Energy Consumption in Vane Extruders with Different Structure Parameters

Numerical simulation was performed with the Computational Fluid Dynamics (CFD) analysis software POLYFLOW (ANSYS, Inc., USA) to investigate the melt transportation energy consumption and mixing characteristics of vane extruders. By modifying related parameters in the numerical simulation models, the effects of vane arrangement and eccentricity between rotor and stator were studied. The results showed that for different vane arrangements, the vane unit with four vanes distributed uniformly had the lowest energy consumption per unit melt transported in one cycle and the best mixing characteristics. With the increase of eccentricity, the average rotational power transmitted by the vanes to the melt increased, and the rate of increase increased. The same tendency occurred for the maximum shear rate and stretching rate experienced by the particles in the vane extruder. This suggested that the mixing performance of the vane unit increased significantly with the increase of eccentricity.     

Estimating the spatial distribution of artificial groundwater recharge using multiple tracers

Stable isotopes of water, organic micropollutants and hydrochemistry data are powerful tools for identifying different water types in areas where knowledge of the spatial distribution of different groundwater is critical for water resource management. An important question is how the assessments change if only one or a subset of these tracers is used. In this study, we estimate spatial artificial infiltration along an infiltration system with stage–discharge relationships and classify different water types based on the mentioned hydrochemistry data for a drinking water production area in Switzerland. Managed aquifer recharge via surface water that feeds into the aquifer creates a hydraulic barrier between contaminated groundwater and drinking water wells. We systematically compare the information from the aforementioned tracers and illustrate differences in distribution and mixing ratios. Despite uncertainties in the mixing ratios, we found that the overall spatial distribution of artificial infiltration is very similar for all the tracers. The highest infiltration occurred in the eastern part of the infiltration system, whereas infiltration in the western part was the lowest. More balanced infiltration within the infiltration system could cause the elevated groundwater mound to be distributed more evenly, preventing the natural inflow of contaminated groundwater.

Dedicated to Professor Peter Fritz on the occasion of his 80th birthday