Lagrangian analysis of the formation and mass transport of compressible vortex rings generated by a shock tube

In order to understand the mass transport and the dynamic genesis associated with a compressible vortex formation,a dynamic analysis of compressible vortex rings (CVRs) generated by shock tubes by using the framework of Lagrangiancoherent structures (LCSs) and finite-time Lyapunov exponents field (FTLE) is performed. Numerical calculation is performed to simulate the evolution of CVRs generated by shock tubes with 70 mm, 100 mm, and 165 mm of the driver sectionat the circumstances of pressure ratio = 3. The formation of CVRs is studied according to FTLE fields. The mass transportduring the formation is obviously seen by the material manifold reveled by FTLE fields. A non-universal formation numberfor the three CVRs is obtained. Then the elliptic LCSs is implemented on three CVRs. Fluid particles separated by ellipticLCSs and ridges of FTLE are traced back to t = 0 to identify the fluid that eventually forms the CVRs. The elliptic LCSsencompass around 60% fluid material of the advected bulk but contain the majority of the circulation of the ring. The otherparts of the ring carrying almost zero circulation advect along with the ring. Combining the ridges of FTLE and the ellipticLCS, the whole CVR can be divided into three distinct dynamic parts: vortex part, entrainment part, and advected part. Inaddition, a criterion based on the vortex part formation is suggested to identify the formation number of CVRs.

Large eddy simulation of a vortex ring impinging on a three-dimensional circular cylinder

Abstract A vortex ring impacting a three-dimensional circular cylinder is studied using large eddy simulation （LES） for a Reynolds number Re = 4 × 10^4 based on the initial translation speed and diameter of the vortex ring. We have investigated the evolution of vortical structures and identified three typical evolution phases. When the primary vortex closely approaches to the cylinder, a secondary vortex is generated and its segment parts move inward to the primary vortex ring. Then two large-scale loop-like vortices are formed to evolve in opposite directions. Thirdly, the two loop-like vortices collide with each other to form complicated small-scale vortical structures. Moreover, a series of hair-pin vortices are generated due to the stretching and deformation of the tertiary vortex. The trajectories of vortical structures and the relevant evolution speeds are analyzed. The total kinetic energy and enstrophy are investigated to reveal their properties relevant to the three evolution phases.

Three-dimensional numerical simulation of a vortex ring impacting a bump

A vortex ring impinging on a three-dimensional bump is studied using large eddy simulation for a Reynolds number Re = 4 × 104 based on the initial translation speed and diameter of the vortex ring. The effects of bump height on the vortical flow phenomena and the underlying physical mechanisms are inves- tigated. Based on the analysis of the evolution of vortical structures, two typical kinds of vortical structures, i.e., the wrapping vortices and the hair-pin vortices, are identified and play an important role in the flow state evolution. The circu- lation of the primary vortex ring reasonably elucidates some typical phases of flow evolution. Furthermore, the mechanism of flow transition from laminar to turbulent state has been revealed based on analysis of turbulent kinetic energy.

Interaction Between Large-scale Vortex Structure and Dispersed Particles in a Three Dimensional Mixing Layer

In order to understand the interaction between large-scale vortex structure and particles, a two-way coupling temporal mixing layer laden with particles at a Stokes number of 5 with different mass loading planted initially in the upper half region is numerically studied. The pseudospectral method is used for the flow fluid and the Lagrangian approach is employed to trace particles. The momentum coupling effect introduced by a particle is approximated to a point force. The simulation results show that the coherent structures are still dominant in the mixing layer, but the large-scale vortex structure and particle dispersion are modulated. The length of large-scale vortex structure is shortened and the pairing is delayed. At the same time, the particles are distributed more evenly in the whole flow field as the mass loading is increased, but the particle dispersion along the transverse direction differs from that along the spanwise direction, which indicates that the effect by the addition of particle on the spanwise large-scale vortex structure is different from the streamwise counterpart.

Investigation on 3Dt wake flow structures of swimming bionic fish

A bionic experimental platform was designed for the purpose of investigating time accurate three-dimensional flow field, using digital particle image velocimetry （DSPIV）. The wake behind the flapping trail of a robotic fish model was studied at high spatial resolution. The study was performed in a water channel. A robot fish model was designed and built. The model was fixed onto a rigid support frame- work using a cable-supporting method, with twelve stretched wires. The entire tail of the model can perform prescribed motions in two degrees of freedom, mainly in carangiform mode, by driving its afterbody and lunate caudal fin respectively. The DSPIV system was set up to operate in a trans- lational manner, measuring velocity field in a series of parallel slices. Phase locked measurements were repeated for a number of runs, allowing reconstruction of phase average flow field. Vortex structures with phase history of the wake were obtained. The study reveals some new and complex three-dimensional flow structures in the wake of the fish, including ＂reverse hairpin vortex＂ and ＂reverse Karman S-H vortex rings＂, allowing insight into physics of this complex flow.

Lagrangian analysis of the fluid transport induced by the interaction of two co-axial co-rotating vortex rings

In this paper,the fluid transport in the interaction of two co-axial co-rotating vortex rings are investigated.Vortex rings are generated using the piston-cylinder apparatus,and the resulting velocity fields are measured using digital particle image velocimetry.The interaction process is analysed by means of vorticity contour,as well by investigation of the Lagrangian coherent structures(LCSs)defined by the ridges of the finite-time Lyapunov exponent(FTLE).Experimental results demonstrate that two types of vortex interaction are identified,namely strong and weak interactions,respectively.For the strong interaction,the Lagrangian boundaries of the two vortex rings are merged together and form a flux window for fluid transport.For weak interaction,only the Lagrangian drift induced by the motion of the front vortex ring is observed and affects the Lagrangian boundary of the rear vortex ring.Moreover,the fluids transported in the strong interaction carry considerable momentum but no circulation.By contrast,there are nearly no fluxes occurring in the weak interaction.By tracking the variations of circulation and impulse occupied by the separated regions distinguished by the LCSs,it is found that the circulation nearly has no change,but the impulse occupied by vortex core region has significant change.In the strong interaction,the impulse of rear vortex ring decreases but the impulse of the front vortex ring increases.Based on the impulse law,it is speculated that the fluid force generated by the formation of the rear vortex rings can be enhanced.Therefore,the strong interaction between wake vortices can actually improve the propulsive efficiency of the biological systems by operating the formation of large-scale vortices.

Large Eddy Simulation of a Vortex Ring Impacting a Bump

A vortex ring impacting a three-dimensional bump is studied using large eddy simulation for a Reynolds number Re=4×104 based on the initial diameter and translational speed of the vortex ring.The effects of bump height and vortex core thickness for thin and thick vortex rings on the vortical flow phenomena and the underlying physical mechanisms are investigated.Based on the analysis of the evolution of vortical structures,two typical kinds of vortical structures,i.e.,the wrapping vortices and the hair-pin vortices,are identified and play an important role in the flow state evolution.The boundary vorticity flux is analyzed to reveal the mechanism of the vorticity generation on the bump surface.The circulation of the primary vortex ring reasonably elucidates some typical phases of flow evolution.Further,the analysis of turbulent kinetic energy reveals the transition from laminar to turbulent state.The results obtained in this study provide physical insight into the understanding of the mechanisms relevant to the flow evolution and the flow transition to turbulent state.

影响半密闭腔室弧后气体绝缘恢复的涡环现象观测与分析

多腔室灭弧间隙在配网防雷中受瞩目,其工作时电弧喷出与空气对流换热成气体射流,射流与周围流体速度差形成剪切层,进而可能演变为涡环结构,促进弧后气体冷却与绝缘恢复。涡环形成受腔室结构影响,优化结构可提高涡环形成概率,增强灭弧效果。因此,通过搭建纹影观测平台观测弧后气体的逸散过程,研究了不同腔室结构参数对弧后气体涡环形成以及气体密度恢复的影响,分析了涡环对于促进弧后气体冷却的效果。基于纹影观测结果,采用有限元仿真分析软件,仿真分析了弧后气体涡环的形成过程。仿真结果证明了合理设置腔室出口直径可增大弧后气体涡环的形成概率,涡环夹带环境冷空气进入弧后气体热核,促进弧后气体的冷却与绝缘强度恢复。

涡环旋转伞流固耦合特性分析

以一种典型的涡环旋转伞为研究对象,采用任意拉格朗日-欧拉流固耦合方法模拟其在无限质量条件下的充气展开过程。计算得到了涡环旋转伞的充气展开和转速、开伞动载等时程变化曲线以及稳态阶段伞周围流场变化规律、伞衣织物的结构强度等流固耦合特性。结果表明:涡环旋转伞在来流12m/s时稳定转速约为3.1r/s,伞衣幅充满外形饱满,与伞塔试验结果吻合;稳态阶段涡环旋转伞上方产生大量涡核,涡核中心的连线类似于空间螺旋线;涡环旋转伞的阻力系数大于一般结构轴对称降落伞;伞衣幅与伞绳连接区域以及边缘区域应力明显高于伞衣幅平均应力水平。

陷窝诱导涡结构数值模拟分析

为了澄清陷窝诱导涡结构及其对尾流的扰动方式,针对布置深宽比0.2陷窝的两平板间充分发展流动进行了稳态数值模拟。应用张涵信的旋涡沿轴线的非线性分叉理论分析表明陷窝内涡结构为失稳破裂的半涡环,总结了陷窝对尾流的扰动方式。研究发现,陷窝诱导的旋涡分离为螺旋点/鞍点/螺旋点分离。物面的分离螺旋点形成,在空间演化为垂直物面发展的对称类龙卷风涡结构。对称类龙卷风涡在对称面闭合形成半涡环。半涡环经历了从稳定升起、沿流向随着涡粘性扩散和涡粘性耗散下变得不稳定、到最后在强逆压梯度下泡型涡破裂的过程。半涡环涡破裂涡量散开诱导形成陷窝尾流的弱纵向涡和陷窝尖后缘绕流产生的边涡旋转同向,加强了陷窝后流场的对流强度。

涡环旋转伞充气过程及气动特性分析

为了探索灵巧子弹药的减速导旋伞——涡环旋转伞的充气性能和气动特性,利用任意拉格拉日-欧拉流-固耦合方法,模拟一种典型涡环旋转伞在无限质量和低速气流条件下的充气过程,得到伞衣动态变化过程、转速和投影直径时程变化曲线及充满后稳态下的流场变化特性。将充满伞衣幅的有限元模型转化为气动特性仿真模型,利用计算流体力学方法得到涡环旋转伞在低速气流作用下的气动力参数及流场流线、压力分布等特性。将两种方法得到的结果进行对比分析,结果表明:涡环旋转伞在来流12 m/s时能顺利充气展开并实现旋转,伞衣幅充满外形饱满,稳定转速约为3.3 r/s;阻力系数为1.36,大于一般结构轴对称降落伞;导旋力矩系数为0.87;流场分布具有中心对称性。

风琴管喷嘴喉部结构对射流流场流动结构影响的数值研究

本文提出了一种采用间歇涡环模型描述射流流场的大尺度流动结构特性，并依据边界元方法优化设计风琴管等自激振荡型喷管的内流结构曲面和其喉部结构的数值实验方法。计算结果表明内流过渡曲面为棉球面而出口形状为锥面的风琴管式喷嘴喷射出的圆形射流流场具有良好的大尺度涡环结构。

障碍物对火焰结构影响的Realizable k-ε模型数值模拟

在管道内设置长方体障碍物,对火焰翻越障碍物的情形进行数值模拟研究,分析火焰褶皱现象。基于可实现k-ε模型(Realizable k-εmodel)和EDC(涡流耗散概念模型),建立湍流加速火焰现象的数值模型,实现管道火焰流场的可视化,研究障碍物对火焰结构、火焰流场的加速过程,分析了导致火焰发生褶皱的机理,得出火焰在翻越障碍物后会形成两个相反方向的涡流。

Experimental Research on Flash Boiling Spray of Dimethyl Ether

The high-speed digital imaging technique is applied to observe the developing process of flash boiling spray of dimethyl ether at low ambient pressure, and the effects of nozzle opening pressure and nozzle hole diameter on the spray shape, spray tip penetration and spray angle during the injection are investigated. The experimental results show that the time when the vortex ring structure of flash boiling spray forms and its developing process are determined by the combined action of the bubble growth and breakup in the spray and the air drag on the leading end of spray; with the enhancement of nozzle opening pressure, the spray tip penetration increases and the spray angle decreases. The influence of nozzle hole diameter on the spray tip penetration is relatively complicated, the spray tip penetration is longer with a smaller nozzle hole diameter at the early stage of injection, while the situation is just opposite at the later stage of injection. This paper establishes that the variation of spray angle is consistent with that of nozzle hole diameter.

锥形阀消力池淹没射流的大涡结构及特征

结合盘石头水库输水洞模型试验 ,应用现代紊流理论 ,对输水洞下游锥形阀消力池内的复杂流动结构进行了观测试验和定性分析 ,并从紊流力学的角度对耗散作了机理性分析 .认为锥形阀消力池内流态的显著特点是锥底存在较大范围的卷吸区 ,流态具有强三维性 ,大尺度涡拟序结构对池内的流速分布、压强分布及消能起主导作用 。

新型裙板对重型车气动特性的影响

应用数值模拟的方法,采用SST k-omega湍流模型,仿真分析了一种新型变速裙板对重型车气动特性的影响。本研究中不考虑侧风影响,仅模拟了以25m/s车速正常行驶时的流场特性。计算结果表明,采用加速裙板后模型较原始型风阻降低1.3%。鉴于原始型风阻系数已经较低,该结果达到了预期效果。分析发现,加速裙板的影响集中在尾流区,改善了尾部流场的环形漩涡结构,从而降低了风阻。

喷气涡流纺织物与传统环锭纺织物性能对比

研究喷气涡流纺纱线结构特点及其织物性能特点。对比分析了喷气涡流纺与传统环锭纺纱线结构上的差异;用同原料、同号数、不同纺纱方法生产的纱线织造同规格的织物,对比了两种织物的表面形态、力学性能、耐磨性、保暖性、透气透湿性。结果表明:与传统环锭纺织物相比,喷气涡流纺织物纹路清晰,毛羽少,耐磨性、抗起球性、保温性更好,但强力较低。认为:喷气涡流纺织物较适宜用作运动服、内衣和时尚面料。

活塞和圆盘涡环的运动学特征分析

采用数值模拟方法,使用流函数和拉格朗日相关结构对由2种生长机制所得涡环的典型物理特征进行分析.结果表明:由2种生长机制所得涡环均属于薄核涡环,其涡核的涡量分布满足高斯分布,并且具有统一的物理模型;但由于2种涡环生长过程中的环境差异而导致其运动学特征存在差异,主要表现在涡环的传输速度和无量纲涡核半径有所不同.

原液着色涡流纱与不同捻系数环锭纱的颜色匹配

选用蓝色涤纶纤维纺制了线密度为36.90 tex的环锭纱和涡流纱,环锭纱捻系数设置为280,320,360,400,440。将涡流纱和环锭纱织成1+1罗纹的针织小样,采用Datacolor测色,绘制光谱反射率曲线及色差图,并对L,a,b数据进行直线拟合。结果表明:捻系数为440的环锭纱与涡流纱的颜色匹配度较好,色差最小,且织物顶破强力接近。两种纱线之间存在色差的主要原因为纱线表面结构。

Effects of the computational domain on the secondary flow in turbulent plane Couette flow

A series of direct numerical simulations of the fully developed plane Couette flow at a Reynolds number of 6000（based on the relative wall speed and half the channel height h） with different streamwise and spanwise lengths are conducted to investigate the effects of the computational box sizes on the secondary flow（SF）. Our focuses are the number of counter-rotating vortex pairs and its relationship to the statistics of the mean flow and the SF in the small and moderate computational box sizes. Our results show that the number of vortex pairs is sensitive to the computational box size, and so are the slope parameter, the rate of the turbulent kinetic energy contributed by the SF, and the ratio of the kinetic energy of the SF to the total kinetic energy. However, the averaged spanwise width of each counter-rotating vortex pair in the plane Couette flow is found, for the first time, within 4（1 ± 0.25）h despite the domain sizes.