Asymmetric two-dimensional diffraction grating via a vortex field in an inverted-Ytype atomic system

We propose a theoretical scheme to realize a two-dimensional(2D)diffraction grating in a four-level inverted-Y-type atomic system coupled by a standing-wave(SW)field and a Laguerre-Gaussian(LG)vortex field.Owing to asymmetric spatial modulation of the LG vortex field,the incident probe field can be lopsidedly diffracted into four domains and an asymmetric 2D electromagnetically induced grating is created.By adjusting the detunings of the probe field and the LG vortex field,the intensities of the LG vortex field and the coherent SW field,as well as the interaction length,the diffraction properties and efficiency,can be effectively manipulated.In addition,the effect of the azimuthal parameter on the Fraunhofer diffraction of the probe field is also discussed.This asymmetric 2D diffraction grating scheme may provide a versatile platform for designing quantum devices that require asymmetric light transmission.

Electromagnetic field of a relativistic electron vortex beam

Electron vortex beams(EVBs)have potential applications in nanoscale magnetic probes of condensed matter and nanoparticle manipulation as well as radiation physics.Recently,a relativistic electron vortex beam(REVB)has been proposed[Phys.Rev.Lett.107174802(2011)].Compared with EVBs,except for orbital angular momentum,an REVB has intrinsic relativistic effect,i.e.,spin angular momentum and spin-orbit coupling.We study the electromagnetic field of an REVB analytically.We show that the electromagnetic field can be separated into two parts,one is only related to orbital quantum number,and the other is related to spin-orbit coupling effect.Exploiting this separation property,the difference between the electromagnetic fields of the REVB in spin-up and spin-down states can be used as a demonstration of the relativistic quantum effect.The linear momentum and angular momentum of the generated electromagnetic field have been further studied and it is shown that the linear momentum is weakly dependent on the spin state;while the angular momentum is evidently dependent on the spin state and linearly increases with the topological charge of electron vortex beam.The electromagnetic and mechanical properties of the REVB could be useful for studying the interaction between REVBs and materials.

A Study on the Dynamic Mechanism of the Formation of Mesoscale Vortex in Col Field

The mesoscale vortex associated with a mesoscale low-level jet （mLLJ） usually causes heavy rainfall in the col field. The col field is defined as a region between two highs and two lows, with the isobaric surface similar to a col. Using a two-dimensional shallow water model, the meso-β scale vortex couplets （MβVCs） induced by eight types of mesoscale wind perturbations in an ideal col field were numerically simulated. With the sizes of -100 km, the MβVCs induced by northerly perturbation （NP） and southerly perturbation （SP） moved toward the col point. The sizes of MβVCs induced by southwesterly perturbation （SWP）, southeasterly perturbation （SEP）, northwesterly perturbation （NWP）, and northeasterly perturbation （NEP） were relatively small for the perturbations moving toward dilatation axis. The MβVC induced by easterly perturbation （EP） and westerly perturbation （WP） could not develop because they quickly moved away from the col point, before the circulation could form. The size of the circulation was determined by the distance between the vortex and the col point. The closer to the col point the vortex was, the larger the size of vortex. The comparisons of maximum vorticity and vorticity root mean square error （RMSE） of the NP, the SWP, and the WP show that the maximum vorticity and the vorticity RMSE of the NP decreased slower than other perturbations. Therefore, the weak environment of the col field favors the maintenance of vorticity and the formation of vortex. When a mesoscale vortex forms near the col point or moves toward the col point, it may maintain a quasitationary state in the stable col field.

Study of vortex in flow fields induced by surface dielectric barrier discharge actuator at low pressure based on Q criterion

Flow fields induced by a surface dielectric barrier discharge actuator at low pressure of 7 kPa are measured by particle image velocimetry. The distribution of local vortices in the flow field is revealed by the Q criterion. The reason for the generation of vortices is analyzed and the influence of pulse frequency and duty cycle on vortices is studied. The results show that the Q criterion can reveal the small-scale vortices, which cannot be indicated by the streamline. The direction transition zone where the induced jet moves from the vertical to the tangential and the shear layer between the jet and stationary air are prone to the generation of strong vortices. The influence of pulse frequency on vortices is not obvious, but the variation of duty cycle can significantly affect the strength and distribution of vortices.

Polarization singularities in near-field of Gaussian vortex beam diffracted by a circular aperture

Polarization singularities in the near-field of Gaussian vortex beams diffracted by a circular aperture are studied by a rigorous electromagnetic theory. It is shown that there exist C-points and L-lines, which depend on off-axis displacement parameters along the x and y directions, waist width, wavelength, and topological charge of the diffracted Gaussian vortex beam, as well as on propagation distance. The results are illustrated by numerical calculations.

Study correlation vortices in the near-field of partially coherent vortex beams diffracted by an aperture

We have derived the analytical expression of the electric cross-spectral density in the near- field of partially coherent vortex beams diffracted by an aperture. Taking the Caussian Schell-model vortex beam as a typical example of partially coherent vortex beams, the spatial correlation properties and correlation vortices in the near-field of partially coherent vortex beams diffracted by a rectangle aperture are studied. It is shown that the off-axis displacement, spatial degree of coherence parameter, propagation distance, and the opening factor of the aperture affect the spectral degree of coherence and positions of correlation vortices. With the optimization algorithm, we obtain the symmetric distributing coherent vortex.

Analytical Solution of the Flow Field Induced by the Uniformly Moving Double Helical Vortex Filaments

The analytical expressions was deduced for the inviscid flow field induced by the double vortex filaments that move uniformly and rigidly without change of its form in a cylindrical tube, where the vortex filaments rotate around its axial with a constant angular velocity and translates along its axial with a constant transferal velocity. It is a key of solving problem to set up a moving cylindrical coordinate system together with the vortex filaments motion, in which the relative velocity field is presumed to be time-independent and with helical symmetry. The result shows that the absolute velocity field and pressure field are all time-periodic functions, and may degenerate into a time-independent field when the helical vortex filaments slip along the filaments themselves or is immobile. The calculation results at the location of pressure peaks and valleys on pipe wall are accordant with experimental results. When the cylindrical pipe radius tends to infinitely large quantity, it is also concluded that the double helical vortex filaments induce flow field in an unbound space.

Calculation of the Centrally Symmetric and Vortex Forces Acting on Momentum in Distortion Tensor Field: Explanation of High-Temperature Plasma and Vortex Motions

The paper deals with calculation of the centrally symmetric and vortex forces for the momentum of a particle in the distortion tensor field from the action minimum, by analogy with the calculation of forces for a charge in an electromagnetic field. It is demonstrated that: 1) The compensating interaction tensor corresponds to the distortion tensor in a solid. 2) The centrally symmetric force of the distortion tensor acts on the momentum as a charge, and is analogous to the Coulomb force. In a gas, it results in change in the momentum value of the molecules exponentially to some extent. The action of this force explains the high-temperature plasma in the gas. 3) The vortex force of the distortion tensor is equivalent to the Peach-Koehler force in a solid. It acts on the momentum flow, similar to the Lorentz magnetic force, and explains the vortex motions in space, in the form of “black holes”, and in the atmosphere, in the form of cyclones and anticyclones.

基于激光雷达探测的飞机尾流融合预测方法

为提高基于激光雷达的飞机尾流探测反演精度,根据扫描出的径向风速数据,建立了基于Kolmogorov结构函数的大气背景湍流耗散率(EDR)估计方法;然后基于两阶段消散预测模型,计入湍流对尾流消散过程的影响,实现基于历史探测数据的尾涡强度环量和涡核运动趋势的预测;通过环境探测数据与预测模型的结合,提高尾涡特性参数的反演精度。研究表明,相较于反演算法,采用本文模型预测的尾涡轨迹在径向距离和角度的精度上分别提高了59.5%、64.8%。

基于半解析涡环模型的农用单旋翼直升机流场快速计算

农业航空领域中,定量获取风场(流场)速度分布对单旋翼直升机辅助授粉作业、农药施用效率提升具有重要意义。基于此提出一种半解析理论方法能够快速完整计算单旋翼流场速度。以涡环模型为基础,将农用直升机旋转桨盘下的尾涡系等效为涡环连续叠加所形成的圆柱涡面,再由涡环速度诱导公式半解析计算直升机旋翼空间各点速度。理论模型与仿真和文献试验数据比较,结果表明:涡环模型简单快速,普通计算机(CPU 2 GHz,内存2 GB)上2.8 s完成计算;悬停时与计算流体力学仿真结果下洗速度径向平均误差小于1.9 m/s,平均相对误差小于39.1%,轴向平均误差小于2.26 m/s,平均相对误差小于54.6%;悬停时,诱速从中央往桨尖递增;前飞时,平飞速度越大诱速越小;流场主要影响因素依次为前飞速度、旋翼半径、飞机总重、空气密度。为定量计算农用直升机旋翼风场、辅助授粉作业和田间施药喷头布置提供一种快速方法和参考。

双涡控径向分级燃烧室流场特性

针对新设计的一种双涡控径向分级燃烧室流场特性,采用粒子图像测速仪试验的方式,获得不同压降(1.5%~5.5%)下该型燃烧室不同展向截面和流向截面上流场结构、流线、流速分布和回流区特性。试验结果表明,燃烧室在凹腔内形成了耦合展向涡与流向涡的三维涡系;主燃级旋流器后外旋螺旋流场与中心回流区共同构成三维涡球结构;凹腔与主燃级流场在主燃孔进气作用下相互耦合。随着压降变化,燃烧室冷态流场存在2种流态,压降小于3.5%时,旋流器旋转气流明显强于凹腔气流,凹腔气流被压至外壁流出,与主流掺混较为困难,在火焰筒下游与主流相互作用;压降大于3.5%时,旋流器旋转气流减弱,凹腔气流一同与旋转气流流出并相互作用,凹腔气流往主流掺混相对较为容易。

波流环境中多孔射流三维数值模拟

建立了波流环境中多孔射流的三维大涡数学模型,探讨了波流共同作用下多孔射流的扩散和稀释机制,分析了孔间距对双孔射流、射流与环境水体相互作用的影响,并给出了最优相对孔间距。模拟结果表明:在波谷相位时刻,由于横流与波浪相互作用强烈,流场内的三维涡旋结构丰富、尺度较大、数目众多,有利于射流与环境水体、射流与射流之间的掺混,这是波流环境比单一横流环境中多孔射流运动扩散效果更佳的主要原因;随着孔间距的增大,双孔射流之间的相互作用慢慢削弱,而射流与环境水体之间的相互作用却在不断增强;双孔射流在孔间距为7.0倍管口直径时扩散效果最好。

基于雷达探测数据的尾涡危险区评估

针对尾涡危险区域的研究可以有效避免后机尾涡危险遭遇事件的发生,提出了一种基于激光雷达探测数据的尾涡危险区计算方法。根据雷达实测数据反演飞机尾涡的演化轨迹和速度环量;基于后机遭遇前机尾涡的空气动力学响应构建滚转力矩系数模型;考虑尾涡在实际场景下的演化情况,结合反演得到的尾涡特征参数,计算飞机遭遇尾涡时的滚转力矩并计算尾涡危险区。结果表明:基于激光雷达探测数据反演尾涡特征参数,左涡和右涡的位置、环量并不一致,据此结合滚转力矩系数模型计算得到的整体危险区域也具有不对称性,当单涡更稳定时,对应的危险区域也更大。以B738为后机,选取某一尾涡探测片段验证,对于涡心坐标位置为左涡(390 m,8.61°),右涡(435 m,8.85°)的尾涡,左涡、右涡的环量分别为256.83 m^(2)/s和223.35 m^(2)/s,左涡相对右涡实际划设的危险区覆盖范围更大,因此后机与前机左右尾涡的安全阈值区域不同,可据此对横向间隔做出不同调整,因此所提方法有利于为分析后机遭遇安全性、缩短尾流间隔提出重要参考。

基于动态模态分解的Π形梁涡振流场特征分析

为探索Π形桥梁断面的涡振机理及气动措施的抑振原理,采用动态模态分解方法,结合计算流体动力学数值模拟,对某Π形桥的不同工况进行流场解耦和对比分析.结果表明,主梁下表面方腔内的不稳定涡是导致Π形梁涡激振动的重要因素,增设2道四分点下稳定板将空腔分割成3个较小的区域,可减少下缘分离涡的形成空间并降低空腔内涡的强度和尺度,从而有效抑制涡振.第2阶分解模态与结构振动同频,是引起涡激振动的主导模态;此模态能够直观揭示流场中主涡的形成和发展,简化流场分析的复杂性.第3、4分解阶模态与结构的涡振频率存在明显的倍频关系,能够精确捕获涡激力中的高频成分,有利于深入理解涡激振动的瞬时变化特性,制定更为精细的抑振措施.

基于场协同原理的涡发生器强化撞击流反应器混合性能分析

为了提高撞击流反应器的混合性能,采用数值模拟的方法分析了置入矩形翼涡发生器后撞击流反应器内的流场特性与混合特性,并且优化了矩形翼涡发生器参数。研究了矩形翼涡发生器尺寸参数与位置参数对撞击流反应器混合强度的影响,以及涡发生器位置参数对反应器内速度分布和湍动能分布的影响,运用传质场协同原理分析了设置矩形翼涡发生器和未设置矩形翼涡发生器的撞击流反应器内速度场和浓度场的协同性。结果表明,随着径向间距r的增加混合强度呈现出先增后减的趋势,当翼宽h、转轴间距d和迎流攻角β逐渐增加时混合强度波动变化,在矩形翼涡发生器的翼宽h=10 mm,转轴间距d=8 mm,径向间距r=30 mm,迎流攻角β=30°时,反应器内混合强度最大,得到的混合效果最佳;在两股流体充分撞击的情况下,撞击流反应器内径向射流区域的速度分布呈现为“双峰”分布规律,涡发生器位置参数的改变可以提高径向速度,在转轴间距d=8 mm、迎流攻角β=30°时径向速度最高,反应器内径向射流区域的湍动能分布呈现出“单峰”分布规律,但涡发生器位置参数的变化对于湍动能分布的影响不大;从场协同的角度分析可知,矩形翼涡发生器的加入改变了撞击流反应器内径向射流的速度大小和方向,进而改善了撞击流反应器内速度场与浓度场的协同性,提高了撞击流反应器的混合性能。

一次冷涡背景下次天气尺度系统对强对流环境场的影响

在同一东北冷涡背景下,2016年6月12—13日山西境内连续2 d出现强对流天气,12日为分散性对流而13日为大范围强对流。基于多源资料通过对比探讨次天气尺度系统及其演变对风暴环境进而对风暴结构的影响,得出以下结论:(1)两日风暴强度和风暴结构差异显著。6月12日对流孤立分散且回波强度小于55 dBZ,而13日准线性风暴发展成尺度较大的弓形回波引发大范围强对流天气,回波强度达60 dBZ。(2)此次过程的关键影响系统为冷涡背景下的次天气尺度低涡。12日次天气尺度低压扰动开始出现,距离远而未影响山西;13日低压扰动东移发展为次天气尺度切断低涡,低涡相关的地面冷锋及850 hPa切变线触发山西上游对流。(3)12日低层水汽含量低,0~3 km垂直风切变弱,13日低涡前偏南水汽输送使低层显著增湿,叠加中层干冷空气形成不稳定层结,受低涡影响0~3 km风垂直切变增强至5.0×10-3s-1,冷锋触发的对流风暴在上述环境下强烈发展并产生阵风锋,阵风锋组织风暴形成飑线,冷池与0~3 km风切变相互作用使飑线维持。(4)13日有利于飑线发展的环境要素与东北冷涡西侧的次天气尺度切断低涡系统密切相关,次天气尺度低涡是飑线形成发展的关键系统。

复杂结构光场的多自由度协同调控技术研究进展

对光场各自由度的单一或协同调控获得的结构光场由于具有新颖的物理性质,展现了重要的研究意义和应用价值。例如,轨道角动量作为一种全新的调控自由度直接影响光场相位与空间分布,常通过单独作用或共同作用构造高维空间,调控生成的涡旋光场及矢量光场已广泛应用于超大容量光通信、遥感探测、量子通信等领域。在此基础上,针对日益发展的前沿应用需求,引入新的光场调控自由度与传统自由度结合,进一步拓展高维和多维的结构光场研究成为了亟待解决的问题。本文首先从双自由度调控技术出发,以矢量涡旋光场为重点介绍了两种典型内癝自由度的耦合以及作用方式;在此基础上,结合本课题组的相关工作,系统综述了超越传统自由度并打破双自由度数目限制的复杂结构光场调控技术。

基于客观λ_(2)方法的海洋三维涡旋核心线提取

涡旋核心线是海洋中尺度涡旋结构的重要组成要素,涡旋核心线提取和可视化对于切入中尺度涡三维结构研究、开展海洋垂直物质能量运输分析具有重要意义。本文基于客观参考框架和λ_(2)准则,提出了使用客观化的流场参数得到λ_(2)区域,并提取其山谷线作为涡旋核心线的方法,实现了对海洋三维结构中尺度涡旋核心线的提取和可视化。首先,引入了最优局部参考系,使得速度、速度梯度等测度成为在变化的参考系下保持不变的客观量,提升了在海洋科学实践中的可靠性和实用性;其次,针对含有垂向速度的海洋三维流场数据,计算其空间雅可比矩阵,展示了涡旋核心区域的三维结构,实现了海洋涡旋研究从二维到三维的提升;最后,分别在多个半径大小播撒流线种子点,分析不同旋转方向的涡旋,对已提取的涡旋核心线实验结果进行验证,证明了客观λ_(2)海洋三维涡旋核心线提取方法的有效性及可行性。

三角槽道层流脉动流的流场分布特性及热量输运规律研究

为了解析小尺度周期性扩缩结构流道与脉动流结合的复合强化传热机理,以水为工质,对三角槽道层流脉动流动特性及热量输运规律进行了研究。以“二次流”为切入点、“涡及涡运动”为视角,分析了Strouhal数(St)、脉动振幅(A)等脉动参数对三角槽道层流脉动流动及传热过程的影响,揭示了二次流强度与三角槽道层流脉动流传热的关联性。结果表明:通过调节St与A优化涡旋运动状态,可将场协同角(β)缩减5°以上,增强温度梯度与速度矢量两者的协同性,最大传热效率可提升55%以上;St的设置旨在优化涡心运动路径,存在最优St值,使传热效率提升40%以上;A的设置旨在提升涡旋强度及尺寸,当A<100%时,最大传热效率提升约15%;St和A对于层流脉动传热的影响情况,还可采用二次流强度定量反馈,以寻求较优的St和A匹配数值,有效控制传热效率及能耗量。研究结果可为解决高黏度、低流速等工程环境(如光热电站熔盐系统管路、熔盐电加热器换热管)或者小尺度异型管道的流动及传热问题提供参考。

C_3-Symmetric Molecules with Axial Chirality and Handed Arrangement of Dipole Fields

Introduction Chirality is defined as the absence of inversion symmetry,however,it is actually a pseudo-scalar of objects or figures,and does not depend for its definition on any connection to the physical world[1-5]. Logically,chiral molecules may possess other inherent physical quantity that guarantees the connection to the physical world[6,7].