Identification of lagrangian coherent structures in the turbulent boundary layer

Using Finite-Time Lyapunov Exponents (FTLE) method, Lagrangian coherent structures (LCSs) in a fully developed flat-plate turbulent boundary layer are successfully identified from a two-dimensional (2D) velocity field obtained by time-resolved 2D PIV measurement. The typical LCSs in the turbulent boundary layer are hairpin-like structures, which are characterized as legs of quasi-streamwise vortices extending deep into the near wall region with an inclination angle θ to the wall, and heads of the transverse vortex tube located in the outer region. Statistical analysis on the characteristic shape of typical LCS reveals that the probability density distribution of θ accords well with t-distribution in the near wall region, but presents a bimodal distribution with two peaks in the outer region, corresponding to the hairpin head and the hairpin neck, respectively. Spatial correlation analysis of FTLE field is implemented to get the ensemble-averaged inclination angle θ _R of typical LCS. θ _R first increases and then decreases along the wall-normal direction, similar to that of the mean value of θ. Moreover, the most probable value of θ saturates at y ⁺=100 with the maximum value of about 24°, suggesting that the most likely position where hairpins transit from the neck to the head is located around y ⁺=100. The ensemble- averaged convection velocity U _c of typical LCS is finally calculated from temporal-spatial correlation analysis of FTLE field. It is found that the wall-normal profile of the convection velocity U _c(y) accords well with the local mean velocity profile U(y) beyond the buffer layer, evidencing that the downstream convection of hairpins determines the transportation properties of the turbulent boundary layer in the log-region and beyond. Supported by the National Natural Science Foundation of China (Grant Nos. 10425207 and 10832001)     

静电场对强激光场非序列双电子电离的影响

利用半经典重碰模型计算并研究了He原子在强激光场与静电场的混合场下的非序列双电子电离过程. 由于静电场破坏了激光场电场的反演对称性，在平行于激光场偏振方向的He²⁺离子动量分布不再具有对称双峰结构. 由于超短脉冲具有相似的非对称性，研究静电场对原子在强场中非序列电离的影响，有助于分析超短脉冲非序列电离的过程，进一步了解非序列双电子电离的机理.     

三层不对称人工电磁材料界面处表面等离子体激元的理论研究

从P和S偏振出发.研究了由常规材料/左手材料/负介电常数材料、及常规材料/左手材料/负磁导率材料这两种三层不对称结构界面上表面等离子体激元(Surface plasmon polaritons,SPPs)的存在区域、色散曲线及其激发.观察到表面等离子体激元的性质强烈地依赖于人工电磁材料的组成参量,例如介质板的厚度和等离子体的频率.最后,使用衰减全反射(Attenuated total reflection,ATR)技术,探究了激发和观察表面等离子体激元的可能性.并针对P和s偏振两种情况计算了衰减全反射光谱.     

计算机仿真环境下电子双缝衍射的实验方案

在一般的近代物理实验教材中，关于电子衍射实验采用的是晶体表面反射衍射或金属箔透射衍射。本文提出一套基于双缝衍射的、用于研究微观粒子波粒二象性的电子衍射实验方案，并且鉴于在一般的实验中不具备进行电子双缝衍射实验的条件，本实验方案是在电子双缝衍射计算机模拟的仿真环境下进行的。     

Simulating high Reynolds number flow in two-dimensional lid-driven cavity by multi-relaxation-time lattice Boltzmann method

By coupling the non-equilibrium extrapolation scheme for boundary condition with the multi-relaxation-time lattice Boltzmann method, this paper finds that the stability of the multi-relaxation-time model can be improved greatly, especially on simulating high Reynolds number (Re) flow. As a discovery, the super-stability analysed by Lallemand and Luo is verified and the complex structure of the cavity flow is also exhibited in our numerical simulation when Re is high enough. To the best knowledge of the authors, the maximum of Re which has been investigated by direct numerical simulation is only around 50,000 in the literature; however, this paper can readily extend the maximum to 1000,000 with the above combination.     

二维抛物型问题的Strang型交替分段区域分裂格式

给出求解二维抛物型方程的Strang型的交替分段区域分裂格式。交替分段思想可以将区域分为一些不重叠的子区域,Strang型算子分裂技巧通过将高维问题的求解分解为几个低维问题的求解来降低其求解的复杂度。方法是无条件稳定的,理论分析了截断误差。数值算例说明格式的有效性及时空的二阶精度.     

Impact of Internal Heat Source on Mixed Convective Transverse Transport of Viscoplastic Material under Viscosity Variation

This communication addresses the impact of heat source/sink along with mixed convection on oblique flow of Casson fluid having variable viscosity. Similarity analysis has been utilized to model governing equations, which are simplified to set of nonlinear differential equations. Computational procedure of shooting algorithm along with 4th order Range-Kutta-Fehlberg scheme is opted to attain the velocity and temperature distributions. Impact of imperative parameters on Casson fluid flow, temperature, significant physical quantities such as skin friction, local heat flux and streamlines are displayed via graphs.     

求解Boltzmann模型方程的气体运动论大规模并行算法

研究各流域三维流动问题的Boltzmann模型方程计算方法,建立直接求解分子速度分布函数的气体运动论耦合迭代数值格式;基于变量依赖关系、数据通信与并行可扩展性分析,使用区域分解并行化方法,建立气体运动论数值算法并行方案,发展求解各流域三维绕流问题的气体运动论并行算法.拟定高低不同马赫数下来自不同流域的三维球体及返回舱绕流算例,进行高性能Fortran(HPF)大规模并行计算,将计算结果与有关实验数据、相关理论预测等进行比较分析,研究揭示不同流区复杂绕流现象及流动机理.研究表明,所发展的气体运动论并行算法具有很好的并行独立性,基本达到线性加速的并行效果,显示出良好的并行可扩展性.     

大型菲涅耳透镜的设计和制造

介绍一种大口径六环组合光学玻璃大型菲涅耳透镜的光学设计原理、整体工艺方案、各个球面环带的工艺计算和加工方法 ;讨论了大型菲涅耳透镜的光学胶合以及整体扇型切割成型方法 ;给出了研制成功大型菲涅耳透镜的光学检测结果。     

抗爆容器内爆炸流场数值模拟

 采用计算流体动力学中的二阶精度TVD差分格式和特殊算子分裂法,按轴对称问题,对半球顶圆柱筒密闭式抗爆容器内部爆炸流场进行了数值模拟。计算得到的容器壁面载荷分布与实验结果基本一致。不同时刻爆炸流场压力分布图像清晰地描述了容器壁面的冲击波加载过程。