Kármán vortex street in a spin–orbit-coupled Bose–Einstein condensate with PT symmetry

The dynamics of spin–orbit-coupled Bose–Einstein condensate with parity-time symmetry through a moving obstacle potential is simulated numerically. In the miscible two-component condensate, the formation of the Kármán vortex street is observed in one component, while ‘the half-quantum vortex street' is observed in the other component. Other patterns of vortex shedding, such as oblique vortex dipoles, V-shaped vortex pairs, irregular turbulence, and combined modes of various wakes, can also be found. The ratio of inter-vortex spacing in one row to the distance between vortex rows is approximately0.18, which is less than the stability condition 0.28 of classical fluid. The drag force acting on the obstacle potential is simulated. The parametric regions of Kármán vortex street and other vortex patterns are calculated. The range of Kármán vortex street is surrounded by the region of combined modes. In addition, spin–orbit coupling disrupts the symmetry of the system and the gain-loss affects the local particle distribution of the system, which leads to the local symmetry breaking of the system, and finally influences the stability of the Kármán vortex street. Finally, we propose an experimental protocol to realize the Kármán vortex street in a system.

INVESTIGATION OF VORTEX SHEDDING INDUCED HYDRODYNAMIC VIBRATION IN VORTEX STREET FLOWMETER

Vortex street flowmeter has been used in steady flow measurement for about three decades The benefits of this type of flowmeter include high accuracy,good linearty,wide measuring range,and excellent reliability However,in unsteady flow measurement,the pressure disturbance as well as the noise from the system or surrounding can reduce the signal to noise ratio of the flowmeter seriously Aimed to use vortex street flowmeters in unsteady flow measurement,the characteristics of the vortex shedding induced hydrodynamic vibration around the prism bluff body in a vortex street flowmeter are investigated numerically and by expriments The results show that the hydrodynamic vibrations with 180° phase shift occur at the axisymmetric points of the channel around the bluff body The most intense vibration occurs at the points on the lateral faces close to the base of the prism The results provide therefore a useful reference for developing an anti interference vortex flowmeter using the different ial sensing technique.

The "von Karman vortex street" to the west of Big Island

Satellite-tracked drifting buoy data and altimetry data are used to study the active vortex field to the west of Big Island. A pair of vortexes were observed at the trajectory of buoy in 1995. The westward propagation of the vortex pair is studied in detail by reproducing the loops of each vortex. The orbital period and radius of the pair of vortex are determined to be 10-11 d and 58-68 km. Two arrays of contra-rotating vortices are displayed in the average sea surface height anomaly (SSHA) field to the west of Big Island. Based on the calculation of the fluid dynamical parameter, the 'von Karman vortex street' is proved to be generated to the west of Big Island as the North Equatorial Current impinges upon Big Island from the east. Finally, the analysis of the buoy trajectories in a decade contributes to the conclusion of the pattern of VKVS in a statistical view.

Control of Karman Vortex Street behind a Thin Airfoil at Low Reynolds Number

To control the Karman vortex street formed behind a thin airfoil at a certain incidence, a control cylinder was placed at the suction side apart from the airfoil in the downstream region. Both smoke visualization and hot-wire measurements confirmed that the Karman vortex street was completely suppressed. The mechanism for suppression of the vortex street was examined with a hot-wire survey.

Exploring multiple phases and first-order phase transitions in Kármán Vortex Street

Kármán Vortex Street, a fascinating phenomenon of fluid dynamics, has intrigued the scientific community for a long time. Many researchers have dedicated their efforts to unraveling the essence of this intriguing flow pattern. Here, we apply the lattice Boltzmann method with curved boundary conditions to simulate flows around a circular cylinder and study the emergence of Kármán Vortex Street using the eigen microstate approach, which can identify phase transition and its order-parameter. At low Reynolds number, there is only one dominant eigen microstate W_(1) of laminar flow. At Re_(c)^(1)= 53.6, there is a phase transition with the emergence of an eigen microstate pair W^(2,3) of pressure and velocity fields. Further at Re_(c)^(2)= 56, there is another phase transition with the emergence of two eigen microstate pairs W^(4,5) and W^(6,7). Using the renormalization group theory of eigen microstate,both phase transitions are determined to be first-order. The two-dimensional energy spectrum of eigen microstate for W^(1), W^(2,3) after Re_(c)^(1), W^(4-7) after Re_(c)^(2) exhibit-5/3 power-law behavior of Kolnogorov's K41 theory. These results reveal the complexity and provide an analysis of the Kármán Vortex Street from the perspective of phase transitions.

Influence of the Karman Vortex Street on the Broadband Noise Generated from a Multiblade Fan

In the prediction theory for the broadband noise generated from a multiblade fan, the vortices in the Karman vortex street was divided into n pieces. The frequency distribution of the noise was estimated so that the Strouhal number could become constant even if the wake is spread by the diffusion. From the results of the measurement of the internal flow of the fan, it was found that the noise was related to the wake characteristics of the specific location in the scroll casing where the relative flow velocity was high. The noise operating in the vicinity of the maximum efficiency point of the fan was distributed over the domain from 500 Hz to 1250 Hz. It was experimentally proved that the influence of the Karman vortex street on the noise in the domains of high and low frequencies did not exist when the distribution of the estimated sound pressure level corresponded to the measured broadband noise.

Computational analysis of fluid-structure interaction in case of fish swimming in the vortex street

The fluid-structure interaction (FSI) in case of fish swimming in the vortex street is investigated by numerical simulations. The vortex street is generated by a D-section cylinder. A 2-D fish model is placed on the downstream centerline of the bluff cylinder at a distance of 4 diameters away from the center of the cylinder. To simulate the fish body undulation and movement, the moving mesh is generated by a coupling approach based on the radial basis function and the overset grid technology. The Navier-Stokes equation in the arbitrary Lagrangian-Eulerian form is solved by coupling with the kinematics equation. Three cases are investigated: in a stationary position without deformation, a passive locomotion without deformation, and an active deformation based on the Kármán gait model. The results indicate that the fish body is acted by an alternating force and moment when it is located in the centerline of the vortex street. Furthermore, the fish could extract sufficient kinetic energy to overcome the drag under suitable conditions even when it keeps rigid and out of the suction zone. When the fish body undulates based on the Kármán gait model, the interaction is evidently shown between the fish body and the vortices. The theoretical analysis demonstrates that the lateral force and the moment acting on the fish body vary in a cosine formula, with the lateral translation and the body rotation as a result. This study focuses on the behavior of the fish body in the bluff cylinder wake and reproduces some phenomena observed in the experiments. Besides, the Kármán gait model is also theoretically analyzed, for the further exploration of the FSI mechanism in case of fish swimming.

Kinematics and forces of a flexible body in Karman vortex street

Experiments of a flexible filament in the wake of a cylinder and in free stream were conducted in a vertical soap film tunnel. The experiments distinctly visualized the movement of the filament. Based on the experimental kinematic results, a 2-d panel method was used to calculate the forces acting on the filament. The experiment and numerical results revealed that different from that in free stream, the filament in Karman vortex street flapped at the same frequency as the vortex street, and with smaller amplitude and larger curvature. The filament suffered an evident thrust in Karman vortex street, while a drag appeared in the case of free stream. The dependence of the drag coefficient on the phase relation between the movement of the filament and the Karman vortex street was also studied.

Vortex Street in Homogeneous Dense Dusty Magnetoplasma

For studying the vortex structure in uniform dense dusty astrophysical conditions, a two-dimensional nonlinear equation is derived employing the quantum magnetoplasma hydrodynamic model and considering the strong collisional effect. The coherent vortex solution is obtained by perturbation analysis method. It is shown that the distribution of the electrostatic potential forms spatially a periodic vortex street, and is controlled temporally by the arbitrary function of time that may lead to abundant spacial distributions. It is found that the dust charge number,collision frequency, electron Fermi wavelength and quantum correction all play significant roles to the spatial distribution of vortex street.

高雷诺数下倒虹吸闸墩绕流水力特性

为研究倒虹吸出口闸墩绕流特性,以南水北调某典型倒虹吸为例,基于Flow3D软件建立倒虹吸模型,根据现场原型实测水位、流速进行模型验证;利用该模型模拟了雷诺数6.17×10^(6)~9.38×10^(6)范围下闸墩绕流特性。结果表明,高雷诺数条件下,随着雷诺数的增加,闸墩绕流造成的液面波动幅度增加,中墩液面波动显著大于边墩;闸墩后的尾涡量持续增加,雷诺数对斯特劳哈尔数影响不大;边侧闸墩受力的非对称性明显,边墩闸室间特征流速偏流比存在先增大再减小的变化规律,偏流效应造成的离心距不断增加。本研究可针对性改善输水工程水流流态。

微小口径涡街流量计设计与测量特性研究

涡街流量计是基于流体振荡原理对流量进行测量的流量计,但在低流速下输出的信号很微弱,在实际测量过程中会叠加各种现场产生的噪音。并且目前工业上使用小口径涡街流量计在小流量工况下存在测量局限性,一般最小口径仅到DN15。针对目前涡街流量计存在的问题对DN10及以下的微小口径涡街流量计结构及参数进行研究,以满足工业上的测量新需求。基于模拟仿真计算平台,建立不同尺寸参数的微小口径涡街流量计数值计算模型并进行仿真计算,提出优化的涡街流量计结构尺寸。通过对不同压力监测点的压力波动强度及频率的研究,分析传感器的最佳布置位置。最后以电磁流量计为标准表对所研制的微小口径涡街流量计在微量程段进行了测试,对该被检流量计的误差、精度等参数进行实验,同时对雷诺数与斯特劳哈尔数之间的关系及仪表系数进行分析,结果表明,仿真所获的测量特性与实验测得的特性具有较好的一致性,且该微小口径涡街流量计相对误差优于0.5%,重复性等指标能够满足微量程段内的测量标准。

双组分玻色-爱因斯坦凝聚体中PT对称势下的异步量子Kármán涡街

数值研究了相混合态双组分玻色-爱因斯坦凝聚体(Bose-Einstein condensate,BEC)中PT (parity-time)对称势下的动力学.在障碍势不同的宽度和速度下发现了异步量子Kármán涡街、斜向漂移涡旋偶极子、V字形涡旋对、无规则量子湍流以及各种尾迹的组合模式.研究了作用在移动障碍势上的拖拽力,分析了涡旋对产生的力学机理.在不同障碍势宽度和速度下,系统地模拟了异步量子Kármán涡街和其他尾迹模式的参数区域.同样分析了PT对称势中具有增益-损耗强度的虚部对异步量子Kármán涡街稳定性的影响.最后,提供了一个实现异步量子Kármán涡街的实验方案.

大流量倒虹吸水力特性研究

为保证大流量倒虹吸能够安全稳定的输水,采用流体动力学软件(CFD)对某工程三维数学模型进行数值模拟,并结合物理模型试验,分析了倒虹吸充水过程中水流变化特点和稳定后的水力特性。结果表明,倒虹吸水流流速最大位置位于管道下坡段,压强最大位置位于管道水平段;充水过程中,倒虹吸边部管道内水流流速较中部水流大8%左右;在水流稳定后倒虹吸下游闸室后存在卡门涡街现象,造成水流流态紊乱,导致下游水面震荡,需要对下游闸墩进行延长或进行墩头结构优化,以减轻涡街强度和水面震荡的影响。

桥墩体型优化数值模拟

南水北调中线工程某段桥墩,水流流过桥墩后会在墩尾产生卡门涡街现象,涡的出现会增大水流阻力,影响输水渠道的过流能力。为了减小桥墩绕流阻力,增大渠道的过流能力,利用Fluent数值模拟软件对不同尾墩的桥墩进行数值模拟,并从流速、涡量、升阻力系数等方面对不同墩尾绕流进行分析。结果显示,随着墩尾流线半径(r)的增大,墩尾的绕流流动变的简单、流速波动降低、流线弯曲度减小、无明显的涡产生、桥墩的升阻力系数下降。综合结果分析,当r=9d时,桥墩对渠道的过流能力影响最小。

仿真研究气泡流中的卡门涡街

利用COMSOL软件中的层流模型研究了气泡流中的卡门涡街现象。在中低雷诺数范围内,利用三种方法获得了气泡流的斯特劳哈尔数。相比纯水,气泡流具有较大的斯特劳哈尔数(St≈0.24)。在研究的参数范围内(障碍物尺寸2~5 cm、入射流速0.02~0.07 m/s),涡脱频率和入射流速成正比,和障碍物直径成反比,波长和障碍物直径成正比。

基于振动分析的船舶螺旋桨鸣音故障诊断

文章针对某船在试航过程中艉部区域出现的不明金属敲击异响,通过测量异响区域船体的振动信号,分析异响出现和消失时振动频谱结构特征,确定故障原因是螺旋桨鸣音。研究发现,附着在螺旋桨上的海生物发展到一定程度后,会改变桨叶的随边形状,流体在螺旋桨背后形成卡门涡街,作用于桨叶发出金属敲击声。

问题导入式教学在圆柱绕流卡门涡街讲授中的应用

以圆柱绕流卡门涡街讲授为例,在工程流体力学授课中采用问题导入式(project-based learning,PBL)教学法,提出了一种“问题+知识点”为主线的进阶型教学模式,实现了课程的高阶性和创新性挑战,阐述了PBL教学法的具体应用。通过理论与实际相结合,搭建起了课堂与工程实际之间的桥梁,加大学生学习投入,让学生体验“跳一跳才能够得着”的学习挑战。

Magnus效应旋转圆柱水动力性能分析

为研究Magnus效应旋转圆柱水动力性能,采用大涡模拟(LES)模型对Magnus效应旋转圆柱进行数值模拟,分析旋转圆柱的长径比、转速比、雷诺数对其升力及三维流场的影响,模拟旋转圆柱升阻力变化趋势和尾部流场涡的脱落过程。结果表明:当转速比大于1.5时,旋转圆柱的升阻比随长径比一直增加;升阻比随转速比的变化规律是以1.7为界先升后降;旋转圆柱脱落的旋涡轨迹与来流形成一定的夹角,且角度会随转速比逐渐增大;当转速比大于1.75时,卡门涡街消失,旋转圆柱尾涡长度逐渐缩短,最终只有圆柱上表面发生涡旋脱落,下表面的涡不再脱落,附着在圆柱表面;雷诺数会改变旋转圆柱尾部流场,使升阻比先增加,达到峰值后以相反趋势下降。

基于光纤布拉格光栅的涡街流量传感技术研究

流量是科学研究及工业生产中的一项重要参数。在众多流量测量仪表中,涡街流量计是一种常用类型。由于光纤布拉格光栅(Fiber Bragg Grating,FBG)具有灵敏度高、体积小和抗电磁干扰强等特点,基于FBG的涡街流量传感技术具有重要研究意义。概述了FBG涡街流量传感机理,分析了涡街发生体下游的流体状态与升力变化。从已有研究成果入手,分析提炼了基本型涡街发生体作用下的旋涡属性、涡街信号处理、消除管道振动和小流量测量四个关键性问题。最后,总结并展望了该项技术利用光学方法抑制干扰信号、小口径管道涡街流量测量和FBG传感封装与增敏三个方面,还提出了未来可能发展的研究方向,以期发展新型涡街流量计。

外排土场宏观尺度的区域微气象因子影响特征

为了研究露天煤矿外排土场对周围微气象因子的影响关系,进而为矿区生态修复提供基础,以矿区周围气流运动及水分分布为影响因子,进一步探索外排土场尺度及形态的生态效应。以红沙泉露天煤矿为例,运用Fluent流体模拟软件,采用大涡模拟和组分传输模型,分别建立不同高度与角度的外排土场模型进行模拟。通过红沙泉露天煤矿所处地区气象站点,得到该地区2020年逐月温度与露点数据,从而得出该地区逐月凝结高度。分析不同条件下得到的气流速度云图,水分质量分数云图,获得不同外排土场尺寸对周围气流运动及水分分布的影响规律,并与该地区凝结高度进行对比。数值模拟结果表明:随着外排土场高度增加,气流爬坡速度缓慢增加,爬升最大高度也随之增长,迎风坡以及外排土场顶部的水分质量分数均增大,背风坡形成越来越明显的卡门涡街现象,涡流数量增加且影响范围变大,涡流区域内气流速度、水分质量分数均增加并达到最大值,外排土场角度增大对气流最大爬升高度影响较大,对周围微气象因子影响不大。在极限高度360 m条件下,当外排土场角度达到26°临界值时,会使气流最大爬升高度达到该地区夏季凝结高度;或在外排土场极限角度22°条件下,当外排土场高度达到380 m临界值时,会使气流最大爬升高度达到该地区夏季凝结高度,从而对矿区周围降水产生促进作用。