Velocity field vortices and Mermin-Ho vortices in two-component spinor BEC

In light of the φ-mapping topological current theory, two important vortex structures in two-component spinor BEC—the velocity field vortices and the Mermin-Ho vortices are discussed. It is revealed that these two different kinds of vortices are created respectively from the zero points of two different order parameter configurations in the condensates, and both their topological charges, locations and motions can be determined by the φ-mapping theory.     

Some remarks on coherent structures out of chaos in planetary atmospheres and oceans

In the context of planetary atmospheres and oceans, it is natural to define "coherent structures" as "long-lived," or "solitary," Rossby vortices. These can be described by the generalized Charney-Obukhov equation (in fluid dynamics) or the analogous generalized Hasegawa-Mima equation (in plasma physics). These two equations contain KdV-type nonlinearities which (together with the compensating dispersive spreading) determine the formation of the coherent structures and explain the clear-cut cyclonic/anticyclonic asymmetry observed experimentally in long-lived planetary Rossby vortices. Examples are given of natural vortices which are (and which are not) coherent structures.     

Large-eddy simulations and vortex structures of turbulent jets in crossflow

Using the method of large-eddy simulation, the 3-dimensional turbulent jets in crossflow with stream-wise and transverse arrangements of nozzle are simulated, emphasizing on the dynamical process of generation and evolution of vortex structures in these flows. The results show that the basic vortex structures in literatures, such as the counter-rotating vortex pair, leading-edge vortices, lee-side vortices, hanging vortices, kidney vortices and anti-kidney vortices, are not independent physical substances, but local structures of the basic vortex structure of turbulent jets in crossflow-the 3-D stretching vortex rings originating from the orifice of the nozzle, which is discovered in this study. Therefore, the most important large-scale structures of turbulent jets in crossflow are unified to the 3-D vortex rings which stretch and twist in stream-wise and swing in transverse directions. We also found that the shedding frequencies of vortex rings are much lower than the one corresponding to the appearance of leading-edge and lee-side vortices in the turbulent jets.     

Three-dimensional nonlinear lattices: from oblique vortices and octupoles to discrete diamonds and vortex cubes

We construct a variety of novel localized topological structures in the 3D discrete nonlinear Schr?dinger equation. The states can be created in Bose-Einstein condensates trapped in strong optical lattices and crystals built of microresonators. These new structures, most of which have no counterparts in lower dimensions, range from multipole patterns and diagonal vortices to vortex "cubes" (stack of two quasiplanar vortices) and "diamonds" (formed by two orthogonal vortices).     

Spatial line nodes and fractional vortex pairs in the Fulde-Ferrell-Larkin-Ovchinnikov vortex state of spin-singlet superconductors

A Zeeman magnetic field can induce a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase in spin-singlet superconductors. Here we argue that there is a nontrivial solution for the FFLO vortex phase that exists near the upper critical field in which the wave function has only spatial line nodes that form intricate and unusual three-dimensional structures. These structures include a crisscrossing lattice of two sets of nonparallel line nodes. We show that these solutions arise from the decay of conventional Abrikosov vortices into pairs of fractional vortices. We propose that neutron scattering studies can observe these fractional vortex pairs through the observation of a lattice of 1/2 flux quanta vortices. We also consider related phases in noncentrosymmetric superconductors.     

Merging of barotropic symmetric vortices. A case study for gulf stream rings

Summary We made numerical experiments to study the merging dynamics of axisymmetric barotropic ringlike vortices using a quasi-geostrophic model in a β-plane. We want to explore the initial conditions and the physical parameters that cause the merging or that affect the merging rates of ringlike structures. The initial vortices have a ?realistic? shape,i.e., they are taken to simulate closely the horizontal structure of Gulf Stream rings. In the first set of experiments we change the initial conditions. The results agree with the classical solution that there is a critical initial separation distanced between the two vortices: for bigger merging distances the process cannot occur. The initial distance affects the merging rates and it determines the development of the lateral arms and the behaviour of the ?near-field? vortices. The latter form near the elongated arms of the merging vortices; sometimes the arms of the merged vortex become unstable and get detached from the central merged vorticity region to form dipolar structures with ?near-field? vortices. A second set of numerical simulations is done by changing the numerical model parameters. The results show that the merging is a nonlinear process very sensitive to the value of the nonlinear parameter, α, and that the β-effect does not alter appreciably the speed of merging but it affects the development of the arms.     

Post-solitons and electron vortices generated by femtosecond intense laser interacting with uniform near-critical-density plasmas

Generation of nonlinear structures, such as stimulated Raman side scattering waves, post-solitons and electron vortices, during ultra-short intense laser pulse transportation in near-critical-density (NCD) plasmas is studied by using multi-dimensional particle-in-cell (PIC) simulations. In two-dimensional geometries, both P- and S-polarized laser pulses are used to drive these nonlinear structures and to check the polarization effects on them. In the S-polarized case, the scattered waves can be captured by surrounding plasmas leading to the generation of post-solitons, while the main pulse excites convective electric currents leading to the formation of electron vortices through Kelvin-Helmholtz instability (KHI). In the P-polarized case, the scattered waves dissipate their energy by heating surrounding plasmas. Electron vortices are excited due to the hosing instability of the drive laser. These polarization dependent physical processes are reproduced in two different planes perpendicular to the laser propagation direction in three-dimensional simulation with linearly polarized laser driver. The current work provides inspiration for future experiments of laser-NCD plasma interactions.     

Scalar dispersion by coherent structures in uniformly sheared flow generated in a water tunnel

In order to investigate the role of coherent structures as mechanisms of scalar dispersion, we studied measurements of a passive scalar plume released in a uniformly sheared turbulent flow generated in a water tunnel. The flow had homogeneous turbulence properties in the measurement domain and contained hairpin vortices similar to those in boundary layers, and so was an ideal test bed to study the effects of coherent structures on turbulent dispersion, free from the effects of inhomogeneities or boundaries. Measurements of the velocity and concentration fields were acquired simultaneously using stereo particle image velocimetry and planar laser-induced fluorescence. We found that dye was preferentially located far away from vortices and was less likely to appear in close proximity to vortices, which is attributed to the high dissipation at the periphery of the vortices. However, we also found that dye was not directly correlated with the uniform momentum zones in the flow, suggesting a more complex relationship exists between these zones, the locations of vortices, and dye transport. Considering scalar flux events rather than simply the presence of dye as our condition of interest, a conditional eddy analysis demonstrated that hairpin vortices are responsible for the large scalar flux events as well as the large Reynolds stress events in the flow. The fact that the Reynolds stress was correlated with the scalar flux further confirmed that coherent structures are dominant mechanisms for scalar transport. Furthermore, we found that the scalar flux vector was preferentially inclined by 155° and ?25° with respect to the streamwise direction, and was thus approximately orthogonal to the planes of the legs of the most common upright and inverted hairpin structures in the flow. These findings demonstrate that coherent structures play an important and intricate role in turbulent diffusion.     

基于三角波瓣混合器的超声速流场精细结构和掺混特性

在超声速吸气式混合层风洞中,采用基于纳米粒子的平面激光散射(NPLS)技术对平板混合层和三角波瓣混合器诱导的混合层流场精细结构进行了对比实验研究.上下两层来流的实测马赫数分别为1.98和2.84,对流马赫数为0.2.NPLS图像清晰地展示了Kelvin-Helmholtz涡、流向涡、波系结构以及大尺度涡结构的配对合并过程.通过对比分析时间相关的NPLS流场图像,发现了大尺度拟序结构随时间发展演化的非定常特性.基于流动显示结果,采用分形维数和间歇因子指标对流场结构和混合特性进行了定量分析.实验研究表明,三角波瓣混合器诱导的流向涡结构显著提高了上下两层来流的掺混效率,其流动远场的分形维数突破了平板混合层中完全湍流区的分形维数值,达到了1.88,流场结构表现出明显的破碎性,有利于流动在标量层面的扩散和掺混.流动间歇性分析表明,流向涡与展向涡的相互剪切作用主导着混合层的掺混特性,同时由于流向涡的卷吸作用,三角波瓣混合器诱导的混合层混合区域更大,更多的流质被卷入混合区完成混合.     

Vortex emission accompanies the advection of optical localized structures

We show that the advection of optical localized structures is accompanied by the emission of vortices, with phase singularities appearing in the wake of the drifting structure. Localized structures are obtained in a light-valve experiment and made to drift by a mirror tilt in the feedback loop. Pairs of oppositely charged vortices are detected for small drifts, whereas for large drifts a vortex array develops. Observations are supported by numerical simulations and linear stability analysis of the system equations and are expected to be generic for a large class of translated optical patterns.     

高聚物减阻溶液对壁湍流输运过程的影响

基于相同雷诺数下清水和高分子聚合物溶液壁湍流的高时间分辨率粒子图像测速技术(time-resolved particle image velocimetry, TRPIV)的对比实验, 从高聚物溶液对湍流边界层动量能量输运影响的角度分析其减阻的机理. 对比两者的雷诺应力发现高聚物的存在抑制了湍流输运过程. 这一影响与高聚物对壁湍流中占主导地位的涡旋运动和低速条带等相干结构的作用密切相关. 运用条件相位平均、相关函数和线性随机估计(linear stochastic estimation, LSE)等方法, 分析提取了高聚物溶液流场中的发卡涡和发卡涡包等典型相干结构的空间拓扑形态. 相比于清水, 高聚物溶液中相干结构的流向尺度增大, 涡旋运动的发展及低速流体喷射的强度受到削弱, 表明了添加的高聚物阻碍了湍流原有的能量传递和自维持的机理. 正是通过影响相干结构, 高聚物抑制了湍流边界层中近壁区与外区之间的动量和能量输运, 使得湍流的无序性降低, 从而减小了湍流流动的阻力.     

Visualizing Görtler vortices

The development of Görtler vortices with pre-set wavelength of 15 mm has been visualized in the boundary-layer on a concave surface of 2.0 m radius of curvature at a free-stream velocity of 3.0 m/s. The wavelength of vortices was pre-set by vertical wires of 0.2 mm diameter located 10 mm upstream of the concave surface leading edge. The velocity contours in the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices. Three different regions can be identified based on different growth rate of the vortices. The occurrence of a secondary instability mode is indicated by the formation of a small horseshoe eddies generated between the two neighboring vortices traveling streamwise, to form mushroom-like structures as a consequence of the non-linear growth of the Görtler vortices.     

Covariance measurements in the initial region of an annular jet

This paper describes the results of correlation measurements in the initial region of a basic annular jet, which is under the effect of the jet vortices and of the wake induced vortices. Evidence has been found that the wake induced vortices are excited by the disturbances associated with the wake vortices. For these wake induced vortices the azimuthal constituent (m = 1) is dominant. The similarity observed between the behaviour of the jet vortices and that of the wake induced vortices suggests that the jet vortices are also excited by these disturbances of the wake vortices, having the same Strouhal number. Thus the jet vortices are also dominated by the azimuthal constituent (m = 1). However, evidence has been found that within the first two outer diameters an axisymmetrical constituent (m = 0) is also present within the jet vortices. Comparison of the wake induced vortices with the jet vortices shows that these two helical waves are out of phase and are axially separated.     

扑翼尾流脱落涡特性的PIV实验研究

通过色流实验和粒子成像测速技术(particle image velocimetry, PIV)对扑翼近场尾流脱落涡的结构轨迹和能量进行了定性及定量研究.结果表明:因展向流动充分性的不同, 存在两种牛角型涡系结构; 上下扑时翅翼交替产生顺时针和逆时针脱落涡, 两涡运动轨迹呈近似弧形对称, 对称轴的仰角略大于攻角; 脱落涡的涡心涡量在上下扑极点达到最大值, 环量最大值出现在到达极点前的1/5~2/5周期之间; 产生脱落涡的半周期内, 涡的平均环量都随减缩频率的增大而增大, 减缩频率较低时, 下扑平均环量大于上扑平均环量, 减缩频率较高时则相反; 振幅对涡能量影响明显, 减缩频率为2~2.5时, 振幅±40°时的涡平均环量约是振幅±30°时的两倍, 减缩频率越大振幅影响越明显.      

Vortex breaking and cutting in type II superconductors

In technological superconductors, the Lorentz force on the flux vortices is opposed by inhomogeneous pinning and so the critical current may be controlled by a combination of vortex entanglement, cutting, and cross-joining. To understand the roles of these processes we report measurements of structures in which a weak pinning layer is sandwiched between two strongly pinning leads. Quantitative modeling of the results demonstrates that in such systems the critical current is limited by the deformation of individual vortices and not by subsequent cross-joining processes.     

Free-space propagation of second harmonic beams carrying optical vortices

A vorticity of free propagating SH beam produced in nonlinear crystal by combined beam composed of two coaxial Laguerre–Gaussian vortex beams is analyzed. It is revealed that the vortical structures of SH beam in the near and far fields are different when two vortex beams are carrying vortices with topological charges of different signs. The diffraction of SH beam under free propagation is investigated. A qualitative agreement of the theoretical predictions with experimental results is obtained.     

湍流边界层内流向涡的数值模拟研究

采用准二维共振三波作为湍流边界层近壁区相干结构初值,用直接数值模拟方法计算了流动从二维结构发展到三维结构并且伴随流向涡生成的整个过程,分析结果显示流向涡对湍流动能和质量传输有着重要作用,是湍流边界层相干结构的重要特征和运动形式.     

The vortex state of an antiferromagnet with uniaxial anisotropy

For the model of an antiferromagnet with easy-axis anisotropy, we have determined the multisoliton state that corresponds to the domain boundary on the surface of which antiferromagnetic vortices of various topological charges are localized. We analyze the pattern of interaction between the vortices. We show that repulsive and attractive forces equivalent to the forces of electrostatic interaction between point charges in a plane act between the vortices with like and unlike topological charges, respectively. However, there is a steady state in this case, since these vortices in the model of a uniaxial antiferromagnet have an infinite effective mass and do not change their relative positions. We give a general solution that describes the vortex structures on the surface of the domain boundary.     

自散焦非局域非线性材料中的光学涡旋孤子

通过数值模拟的方法对非局域非线性自散焦材料中的光学 涡旋孤子的传输特性以及相互作用特性进行了研究. 研究表明, 拓扑荷|m|=1的非局域涡旋孤子是稳定的, 而拓扑荷|m|>1的非局域涡旋孤子均具有拓扑不稳定性. 在微扰存在的情况下以及在近距离相互作用的过程中, |m|>1的涡旋孤子会分裂成一系列的|m|=1的涡旋孤子. 非局域涡旋孤子与局域涡旋孤子具有相同的长距离相互作用模式, 即点涡旋相互作用模式. 但两者的短距离相互作用存在一些差别, 在相同的距离下, 两涡旋间的相互绕转的周期随着材料的非局域响应长度增大而增大. 关键词： 非局域非线性薛定谔方程 自散焦 涡旋孤子