转捩边界层中的上喷下扫运动

采用高精度直接数值模拟方法,研究平板可压缩流边界层转捩后期过程中的上喷和下扫运动.模拟结果清楚地显示了边界层转捩过程中的上喷下扫运动的复杂现象,展示与典型涡系及尖峰结构的紧密联系.详细剖析多层次的上喷下扫运动的形成机制,数值结果证实了实验的新发现,并进行相应的机理分析.     

改进的三阶WENO-Z+格式及其应用

为改善三阶WENO格式的耗散特性,提高其对流场结构的分辨率,在三阶WENO-Z+格式（WENO-Z+3）基础上,构造不同形式的全局光滑因子,提出一种改进的WENO-Z+3格式（NWENO-Z+3）.选取Sod激波管、双爆轰波碰撞、激波与熵波相互作用、双马赫反射等经典算例,考察该格式的计算性能,结果表明：NWENO-Z+3格式具有更低耗散性和更高的分辨率.数值研究柱形高压气体爆炸波在单舱室和连通舱室内部的传播过程及波系演化.结果表明：改进格式NWENO-Z+3能够较好地模拟包含高压比、高密度比的爆炸波系结构.     

动力学淬火过程中的不动点及衍生拓扑现象

本文对近两年来有关淬火动力学过程中拓扑现象的研究做简要综述.这些动力学拓扑现象被动力学过程中的衍生拓扑不变量保护,与淬火前后体系的拓扑性质有密切关系.基于人工量子模拟平台的高度可控性,已在诸如超冷原子、超导量子比特、核磁共振、线性光学等众多物理体系中,通过对人工拓扑体系动力学过程的调控,观测到如动力学涡旋、动量-时间域的Hopf映射及环绕数、拓扑保护的自旋环结构、动力学量子相变、动量-时间斯格明子等诸多动力学拓扑现象.其中某些拓扑结构还可以在非幺正动力学淬火过程中稳定存在.这些研究将人们对拓扑物相的认识和研究从平衡态推广到非平衡动力学领域,具有重要的科学价值.     

基于CFD/CSD的舵面控制气动弹性数值模拟方法

在动态网格上通过耦合求解流动控制方程和结构动力学方程, 发展了一种舵面控制下飞行器运动响应过程中气动弹性数值模拟研究方法.流动控制方程采用N-S方程, 结构动力学采用线性模态叠加方法, 其中流动控制方程空间离散采用基于非结构网格的有限体积方法, 对流通量采用计算HLLC格式, 非定常时间离散采用基于LU-SGS的双时间步长方法.模拟中, 气动运动和结构变形在双时间步长方法推进过程中采用改进松耦合方法, 气动网格与结构网格之间信息交换采用无限平板样条法实现, 飞行器的运动和变形采用基于重叠网格和Delaunay图映射变形网格相结合的方法进行处理.采用多个考核算例对发展的数值方法进行考核验证, 结果表明该方法可以高效精确模拟舵面开环控制下飞行器运动响应过程中的气动弹性特性.      

基于有限体积虚拟区域方法的两相流动直接模拟

为提高计算效率，提出有限体积法离散下的虚拟区域颗粒两相流动直接模拟方法.在控制方程中加入相应的虚拟区域源项，保证了颗粒内部的刚体运动特性.该源项中含有颗粒信息部分及流体信息部分.在每次迭代后，对源项中的流体信息部分进行更新，从而更好地保证颗粒内速度的刚体分布.计算静止颗粒圆柱绕流及单个颗粒的沉降过程，验证了算法的准确性.     

爆炸波问题中偶然不确定度的量化

复杂工程建模与模拟中始终存在不确定度,分析与辨识不确定度的来源及量化不确定度对建模与模拟可信度评估具有重要意义。为此,给出了建模与模拟中误差与不确定度的概念以及不确定度量化的过程,并以爆炸波问题为例说明量化偶然不确定度的过程,得到了爆炸波问题的期望和标准差以及激波位置的概率密度函数,验证了非嵌入多项式混沌方法在复杂非线性系统不确定度量化中的有效性。     

晶体生长理论研究中的计算机模拟方法

在研究晶体生长过程时,计算机模拟方法(又称Monte-Carlo方法)有许多优点.它不必列出繁难的数理方程,可以直接对大量分子的运动进行模拟,并得出可以和实验进行比较的结果;它还可以给出晶体生长微观运动过程的比较直观的图象. 例如,用计算机可以直接给出晶体生长固液界面的平衡形     

磁斯格明子拓扑特性及其动力学微磁学模拟研究进展

具有非平庸拓扑性的新型磁结构斯格明子,由于其拓扑稳定性、尺寸小、低电流驱动等方面的显著优势,有望应用于自旋电子学储存器件.拓扑和凝聚态物理学的结合,使得斯格明子展现出很多有趣的拓扑物理现象,吸引了众多的研究兴趣,同时这些性质也是其电流驱动下动力学特点的重要影响因素.本文从斯格明子的拓扑物理学基础及其自旋电子学器件应用相关动力学两个方面介绍了相关研究进展.在拓扑物理基础方面,介绍了斯格明子的拓扑霍尔效应、斯格明子霍尔效应以及自旋轨道转矩等拓扑性质,由此讨论了斯格明子的动力学性质及其计算方法;在动力学方面,从非均匀电流驱动生成斯格明子、电流驱动下的稳定输运、产生湮灭过程的人工控制几个赛道存储应用关心的问题简要地介绍了相关微磁学模拟研究最新进展.     

磁畴壁手性和磁斯格明子的拓扑性表征及其调控

磁性斯格明子由于拓扑的保护性,具有很高的稳定性和较小的临界驱动电流,有望应用于未来的赛道存储器件中.而在中心对称体系,由于偶极作用的各向同性,磁泡的拓扑性和螺旋度都呈现出多样性的特征.其中非平庸的磁泡即等同于磁性斯格明子.我们通过近期实验结果,结合微磁学模拟的方法,发现在中心对称体系中磁斯格明子的拓扑性会受到体系垂直各向异性的调控.另外在加磁场的演变过程中,会很大程度上依赖于基态畴的畴壁特性.磁场的倾斜或者一定的面内各向异性也会改变磁斯格明子的形态.通过对材料的基态磁结构及磁各向异性的调节,辅助以面内分量的控制,可以对基态磁畴、进而对磁斯格明子的拓扑性实现调控.这对磁斯格明子在电流驱动存储器件中的应用具有重要意义.     

飞秒激光作用下异质结的线性吸收谱研究

以二奈嵌苯分子吸附在TiO₂表面所组成的异质结构为例,介绍了在飞秒激光作用下由染料分子和半导体组成的异质结构中从分子基态到半导体导带的超快电子传输过程,在理论上分析了分子内部传输和直接电子传输过程对线性吸收谱的贡献.与分子内部传输过程项相比较分析了电子的超快直接传输在不同的分子及半导体结构下对线性吸收谱的影响. 关键词： 飞秒激光 线性吸收谱 超快电子转移     

Cellular automaton model for evacuation process with obstacles

A bidimensional cellular automaton model is used to simulate the process of evacuation of pedestrians in a room with fixed obstacles. A floor field is defined so that moving to a cell with lower floor field means approaching an exit door. The model becomes non-deterministic by introducing a “panic” parameter, given by a probability of not moving, and by a random choice to resolve conflicts in the update of pedestrian positions. Two types of exit doors are considered: single (where only one person can pass) and double (two persons can pass simultaneously). For a double door, the longest evacuation time turns out to occur for a very traditional location of the door. The optimum door position is determined. Replacing the double door by two single doors does not improve evacuation times noticeably. On the other hand, for a room without obstacles, a simple scaling law is proposed to model the dependence of evacuation time with the number of persons and exit width. This model fails when obstacles are present, as their presence introduces local bottlenecks whose effect outweighs the benefits of increasing door width beyond a certain threshold.     

Moving mesh methods for blowup in reaction–diffusion equations with traveling heat source

In this paper moving mesh methods are used to simulate the blowup in a reaction–diffusion equation with traveling heat source. The finite-time blowup occurs if the speed of the movement of the heat source remains sufficiently low, and the blowup procedure is not fixed at one point not like that for stationary heat source. As time goes to the blowup time, the blowup profile converges to a stationary state. In the simulation a new moving mesh algorithm is designed to deal with the difficulty caused by the delta function in the traveling heat source. The convergence rates are verified and new blowup figures are generated from the numerical experiments.     

Interferences of ultrashort free electron wave packets

Interferences of free electron wave packets generated by a pair of identical, time-delayed, femtosecond laser pulses which ionize excited atomic potassium have been observed. Two different schemes are investigated: threshold electrons produced by one-photon ionization with parallel laser polarization and above threshold ionization electrons produced by a two-photon transition with crossed laser polarization. Our results show that the temporal coherence of light pulses is transferred to free electron wave packets, thus opening the door to a whole variety of exciting experiments.     

Simulating Free Surface Flows with SPH

The SPH (smoothed particle hydrodynamics) method is extended to deal with free surface incompressible flows. The method is easy to use, and examples will be given of its application to a breaking dam, a bore, the simulation of a wave maker, and the propagation of waves towards a beach. Arbitrary moving boundaries can be included by modelling the boundaries by particles which repel the fluid particles. The method is explicit, and the time steps are therefore much shorter than required by other less flexible methods, but it is robust and easy to program.     

并行重叠/变形混合网格生成技术及其应用

为了适用于柔性变形、相对运动等复杂动边界问题,建立了并行环境下重叠和变形相结合的动态混合网格生成技术.通过计算区域分解以及分布式并行实现了重叠和变形技术的结合,其中重叠网格采用了并行化的隐式装配方法,并发展了两种并行化查询策略.变形网格则采用了并行化的径向基函数（RBF）插值方法.并行化动态网格生成方法大幅提高了动态网格生成效率,有利于处理大规模的动边界问题.在此基础上,发展了基于变形/重叠动态混合网格的流动/运动/控制一体化数值模拟方法,进一步改进了耦合模拟软件平台——HyperFLOW.典型应用算例证明了该动态混合网格技术及一体化算法的实用性.      

高功率电磁脉冲对无人飞行器的毁伤破坏

高功率电磁脉冲干扰甚至损毁无人飞行器是应对小型无人飞行器威胁的一种非常有效的办法,高功率电磁脉冲对无人机系统进行干扰和毁伤主要通过前门耦合和后门耦合的方式。完成了高功率电磁脉冲对无人飞行器的毁伤破坏实验,完成了无人机在电磁脉冲源毁伤作用下的目标易损性分析,根据毁伤效果将高频电磁脉冲对无人飞行器毁伤等级分为三级,即干扰级、毁伤级、失效级。分析了电磁脉冲作用的主要目标元件,结果表明,无人飞行器最有可能受干扰的部分为接收机和电子调速器。在相关研究和实际应用时,应着重研究电磁脉冲对无人飞行器的接收机和电子调速器的毁伤破坏,根据接收机和电子调速器内部的电路结构来确定毁伤最佳频率。     

Asynchronous optical sampling for high-speed characterization of integrated resonant terahertz sensors

Two femtosecond Ti:sapphire lasers with slightly different repetition rates near 1 GHz are coupled to implement high-speed asynchronous optical sampling. The application of this technique is successfully demonstrated in the field of terahertz time-domain spectroscopy (TDS). A time delay of 1 ns is scanned at a frequency of 5 kHz without moving mechanical parts. Compared with that of conventional TDS schemes based on lock-in detection and moving mirrors, the readout time of integrated resonant THz sensors is reduced by a factor of 20, opening the way for high-throughput THz sensing in marker-free DNA analysis.     

Measurement of vortex motion in a type-II superconductor: A novel use of the neutron spin-echo technique

We have used the neutron spin-echo technique to measure the small energy change of neutrons which are diffracted by a moving vortex lattice in a low-pinning Nb-Ta superconducting sample. A transport current was passed in the mixed state to cause flux line movement. In the case of uniform motion, the flux velocity v(L) was given as expected by the values of electric and magnetic fields, via E = -v(L)wedgeB. We show that with a nonuniformly moving vortex lattice, one can measure the dispersion of the velocities, opening up new possibilities for investigating moving vortex lines.     

Subfemtosecond K-shell excitation with a few-cycle infrared laser field

Subfemtosecond bursts of extreme ultraviolet radiation, facilitated by a process known as high-order harmonic generation, are a key ingredient for attosecond metrology, providing a tool to precisely initiate and probe ultrafast dynamics in the microcosms of atoms, molecules, and solids. These ultrashort pulses are always, and as a by-product of the way they are generated, accompanied by laser-induced recollisions of electrons with their parent ions. By using a few-cycle infrared (λ(0)=2.1 μm) driving laser, we were able to directly excite high-energy (～870 eV) inner-shell electrons through laser-induced electron recollision, opening the door to time-resolved studies of core-level and concomitant multielectron dynamics.     

Probability analysis of position errors using uncooled IR stereo camera

This paper analyzes the random phenomenon of 3D positions when tracking moving objects using the infrared (IR) stereo camera, and proposes a probability model of 3D positions. The proposed probability model integrates two random error phenomena. One is the pixel quantization error which is caused by discrete sampling pixels in estimating disparity values of stereo camera. The other is the timing jitter which results from the irregular acquisition-timing in the uncooled IR cameras. This paper derives a probability distribution function by combining jitter model with pixel quantization error. To verify the proposed probability function of 3D positions, the experiments on tracking fast moving objects are performed using IR stereo camera system. The 3D depths of moving object are estimated by stereo matching, and be compared with the ground truth obtained by laser scanner system. According to the experiments, the 3D depths of moving object are estimated within the statistically reliable range which is well derived by the proposed probability distribution. It is expected that the proposed probability model of 3D positions can be applied to various IR stereo camera systems that deal with fast moving objects.