MgB2薄膜磁通钉扎的各向异性

测量了不同温度下,外场分别平行于样品ab面和平行于c轴两种情况下的M-H磁滞回线和I-V特性曲线,从磁测量和传输测量计算出临界电流密度和钉扎力密度.同时测量了两个方向上的磁弛豫,并由此估算了它的有效钉扎势.通过比较这两个方向上的静态和动态特性,我们发现在高度c-轴取向MgB2薄膜中,钉扎力的各向异性主要来自于超导体固有的各向异性,其主要的钉扎中心是各向同性的正常态点缺陷.     

Numerical study of Taylor-Couette flow with an axial flow

The flow between two concentric cylinders with the inner one rotating and with an imposed pressure-driven axial flow is studied using numerical simulation. This study considers the identical flow geometry and flow parameters as in the experiments of Wereley and Lueptow [Phys. Fluids 11 (12) (1999) 3637], where particle image velocimetry measurements were carried out to obtain detailed velocity fields in a meridional plane of the annulus. The objectives of this investigation are to numerically verify the experimental results of Wereley and Lueptow and to further study detailed flow fields and bifurcations related to Taylor-Couette flow with an imposed axial flow. The vortices in various flow regimes such as non-wavy laminar vortex, wavy vortex, non-wavy helical vortex, helical wavy vortex and random wavy vortex are all consistently reproduced with their experiments. It is demonstrated that ‘shift-and-reflect’ symmetry holds in Taylor-Couette flow without an imposed axial flow. In case of Taylor-Couette flow with an imposed axial flow, one can find that the shift-and-reflect symmetry is roughly valid for the remaining velocity field after subtracting the annular Poiseuille flow. The axial flow stabilizes the flow field and decreases the torque required by rotating the inner cylinder at a given speed. Growth rate of the flow instability is defined and used in predicting the type of the vortices. The velocity vector fields obtained also reveal the same vortex characteristics as found in the experiments of Wereley and Lueptow.     

Two-Dimensional Analysis of Electrokinetically Driven Out-of-Plane Vortices in a Microchannel Liquid Flow Using Patterned Surface Charge

The technology developed for photolithographically patterning the electric surface charge to be negative, positive, or neutral enables the realization of complex liquid flows even in straight and uniform microchannels with extremely small Reynolds number. A theoretical model to analyze a steady incompressible electrokinetically driven two-dimensional liquid flow in a microchannel with an inhomogeneous surface charge under externally applied electric field is derived. The flow field is obtained analytically by solving the biharmonic equation with the Helmholtz-Smoluchowski slip boundary condition using the Fourier series expansion method. The model has been applied to study three basic out-of-plane vortical flow fields: single vortex and a train of corotating and a series of counterrotating vortex pairs. For model verification, the solution for the single vortex has been tested against numerical computations based on the full Navier-Stokes equations revealing the dominant control parameters. Two interesting phenomena have been observed in out-of-plane multivortex dynamics: merging of corotating vortices and splitting of counterrotating vortices. The criteria for the onset of both phenomena are discussed     

Parallel DNS for vortex structure of late stages of flow transition

A new DNS using compact high order scheme and MPI parallel computation has been conducted with 1920 × 241 × 128 grid points for non-linear stages of flow transition. The coherent vortex structure of the late flow transition stages and the mechanism of formation of single vortex ring, multiple vortex rings, and small length scales are discussed. The ring-like vortex formation from the Λ-vortex is the result of the interaction of two pairs of counter-rotating primary and secondary streamwise vortices. The formation of the multiple ring structure follows the first Helmholtz vortex conservation law. A bridge must be formed to link two Λ-vortex legs. The bridge finally develops as a new ring. This process keeps going onto form a multiple ring structure. The U-shaped vortices are part of existing coherent large vortex structure. Actually, the U-shaped vortex, which is a third level vortex, serves as a second neck to supply vorticity to the multiple rings. The multiple ring-like vortex structure is found quite stable and can travel for a long distance. The “hairpin vortex breakdown” does not happen. The small vortices can be found on the bottom of the boundary layer near the wall surface. It is believed that the small vortices are generated by the interaction of higher level vortices with the solid wall, but not by the “vortex breakdown”.     

Vortex structure of steady flow in a rectangular cavity

The vortex structure of the two-dimensional steady flow in a lid-driven rectangular cavity at different depth-to-width ratios and Reynolds numbers is investigated using a lattice Boltzmann method. The aspect ratio varies from 0.1 to 7 and the Reynolds number ranges from 0.01 to 5000. The effects of the aspect ratio and Reynolds number on the size, center position and number of vortices are determined together with the flow pattern in the cavity. The present results not only confirm the vortex structure of Stokes flow reported by previous researchers, but also reveal some new evolution features of the vortices and their structure with the Reynolds number. When the Reynolds number approaches 0, the flow shows a characteristic feature of symmetric vortex structure. On the other hand, as the Reynolds number increases, the sizes and center positions of the vortices in the near-lid region appear to be strongly affected by the inertia force, resulting in an asymmetric vortex structure in this region. The influence of the inertia force decreases along the depth for the deep cavity flow. It is found that there is a critical value of the aspect ratio, which depends on the Reynolds number. When the critical value is exceeded, flow pattern in a certain region of cavity becomes symmetric again. These large symmetric vortices are similar in shape, and their sizes approach a constant.     

Multi relaxation time lattice Boltzmann simulations of deep lid driven cavity flows at different aspect ratios

In this paper, the multi relaxation time (MRT) lattice Boltzmann equation (LBE) was used to compute lid driven cavity flows at different Reynolds numbers (100–7500) and cavity aspect ratios (1–4 cavity width depth). Steady solutions were obtained for square cavity flows, however for deep cavity flows at 1.5 and 4 cavity width depth, unsteady solutions prevail at Re = 7500, where periodic flow exists manifested by the rapid changes of the shapes and locations of the corner vortices in strong contrast of the stationary primary vortex. The merger of the bottom corner vortices into a primary vortex and the reemergence of the corner vortices as the Reynolds number increases are more evident for the deep cavity flows. For the four cavity width depth cavity, four primary vortices were predicted by MRT model for Reynolds number beyond 1000, which were not predicted by previous single relaxation time (SRT) BGK LBE model, and this was verified by complementary Navier–Stokes simulations. Also, MRT model is more suitable for parallel computations than its BGK counterpart, due to the more intense local computations of the multi relaxation time procedure.     

Upstream wake-secondary flow interactions in the endwall region of high-loaded turbines

A detailed experimental and numerical investigation of the unsteady interaction of secondary flow vortices in turbine endwall region was performed with the effect of upstream periodic wakes. The flow field was investigated respectively in a linear turbine cascade and a turbine rotor. The study revealed the physical mechanisms of unsteady interaction between upstream wake and secondary vortices. The influence of the upstream wake on the performance of turbine endwall region was also discussed.The flow field at the exit of the turbine blade row showed a decrease in passage vortex strength and loss due to the upstream wake transport. Two interaction mechanisms are proposed whereby passage vortex loss decreases. They are the upstream wake-pressure side leg of the horseshoe vortex interaction and the upstream wake-passage vortex interaction. The transport of upstream wake can suppress the development of pressure side leg of the horseshoe vortex and passage vortex because of the “negative jet” influence of the wake.     

Computations of steady,ellipsoidal vortex rings with finite cores

A numerical study of vortex rings of ellipsoidal shape is presented that have finite cores and are solution to the Navier-Stokes equations. The cross section of the core of the vortex ring is non-circular and is a result of the numerical solution described in the paper. The cross-sectional shape is defined by the strength of the vorticity distribution in the core and by the axis ratio of the ellipsoid, defining the outer dimensions of the vortex ring. Hill's spherical vortex and O'Brien's ellipsoidal vortices are limiting cases of the vortex rings described in this paper.     

雷诺数对高负荷低速涡轮性能影响的试验研究

为了研究雷诺数对高负荷低压低速涡轮性能的影响规律,以某双级高负荷低压低速涡轮为研究对象,在涡轮部件试验器上开展了变雷诺数试验研究。控制其他相似参数保持一致,通过来流节流、出口引射的方式改变涡轮内雷诺数水平。共采集了6个雷诺数状态下的气动参数,分析了雷诺数对涡轮总特性及测量截面流场参数的变化规律。结果表明,该涡轮效率随着雷诺数的降低而显著降低,研究雷诺数范围内,效率降低了2.8%;在低雷诺数下,第二级导叶通道涡、尾缘涡等涡系尺度及强度均发展壮大,气动损失增加;第二级动叶轮周功减小,做功能力损失6.3%。     

Vortex visualization for practical engineering applications

In order to understand complex vortical flows in large data sets, we must be able to detect and visualize vortices in an automated fashion. In this paper, we present a feature-based vortex detection and visualization technique that is appropriate for large computational fluid dynamics data sets computed on unstructured meshes. In particular, we focus on the application of this technique to visualization of the flow over a serrated wing and the flow field around a spinning missile with dithering canards. We have developed a core line extraction technique based on the observation that vortex cores coincide with local extrema in certain scalar fields. We also have developed a novel technique to handle complex vortex topology that is based on k-means clustering. These techniques facilitate visualization of vortices in simulation data that may not be optimally resolved or sampled. Results are included that highlight the strengths and weaknesses of our approach. We conclude by describing how our approach can be improved to enhance robustness and expand its range of applicability.     

Extracting 3D vortices in turbulent fluid flow

This paper presents a computational framework to extract salient patterns, called vortex structures, from 3D turbulent fluid flows. These structures can be characterized as regions of dominating rotational motion in the velocity fields and intensity concentrations in the corresponding vorticity fields. A pointwise linear representation is employed to approximate the kinematics of the flow field, and the fluid motion is classified according to motion analysis or topological patterns. The regions of vortex structures are identified as those dominated by rotational motion or those of focus-type singularity. The 2D vortices, as a special case of 3D vortices, are detected by searching for regions of vorticity concentrations     

Numerical study on mechanisms of second sweep and positive spikes in transitional flow on a flat plate

Ring-like vortex is a flow structure at late stages of a transitional boundary layer. Independent to the initial disturbance conditions corresponding to K- and N-scenarios of transition, the vortical structure shows some universal features. The nonlinear evolution of the ring-like vortices, detail flow structures around ring-like vortex and their effects on the surrounding flow were studied by direct numerical simulation with high order accuracy. A detailed enforced spatial transition on a flat-plate boundary layer in the compressible flow was studied. This study reveals the mechanism of the second sweep generation, mechanism of the positive spike formation and mechanism of high shear layer distribution.     

Vortex generation in electroosmotic flow passing through sharp corners

A special phenomenon was found when sharp wedges were set in a microchannel where electroosmotic flow occurred, vortices were induced near the wedges when a DC electric field was imposed. The strength of the induced vortices depends on the concentration of electrolytes and the intensity of the electric field. Latex particles are used to aid the flow visualization. Formation of vortices is due to concentration depletion in the microchannel. Furthermore, the vortices are used to enhance mixing in a micromixer. Experimental results showed that the vortex structures created within the mixing section increase the mixing index from a value of 3% in the upstream region of the microchannel to 78% at the outlet of the mixing section.     

Vortex trapping on a surface with an indentation and corrugations

This paper explores the possibility of trapping single and multiple vortices on a surface and on an airfoil. Results from the vortex trajectory show that an indented surface and an airfoil with an indentation can stabilize a trapped vortex. That is, when displaced by a finite distance from the equilibrium position, the vortex always moves around its equilibrium position. A boundary with multiple corrugations can be created and incorporated onto an airfoil to trap multiple vortices. Irregular vortex motion has been found when multiple vortices are presented above the corrugations. A method is proposed, allowing the vortices to spiral towards its equilibrium positions.     

On the use of several compact methods for the study of unsteady incompressible viscous flow round a circular cylinder

Fourth order accurate methods of mehrstellen type are compared to second order accurate methods for the solution of the unsteady incompressible Navier-Stokes equations in their vorticity stream function formulation. These methods are applied to the study of separated flow around a circular cylinder at several Reynolds numbers. The impulsively started cylinder at Re = 200 and 550, is considered without symmetry restrictions. The features illustrated include the bulge phenomenon at Re = 200, the occurrence of secondary vortices depending on the schemes used at Re = 550, and of twin secondary vortices at Re = 3000. The Karman vortex street is investigated at Re = 200 with a uniform flow in the far field and with superimposed motions of the cylinder. In this last case, a frequency analysis has allowed a critical examination of results pertaining to locked-in situations with respect to confinement effects.     

Existence,stability, and scattering of bright vortices in the cubic–quintic nonlinear Schrödinger equation

We revisit the topic of the existence and azimuthal modulational stability of solitary vortices (alias vortex solitons) in the two-dimensional (2D) cubic–quintic nonlinear Schrödinger equation. We develop a semi-analytical approach, assuming that the vortex soliton is relatively narrow, which allows one to effectively split the full 2D equation into radial and azimuthal 1D equations. A variational approach is used to predict the radial shape of the vortex soliton, using the radial equation, yielding results very close to those obtained from numerical solutions. Previously known existence bounds for the solitary vortices are recovered by means of this approach. The 1D azimuthal equation of motion is used to analyze the modulational instability of the vortex solitons. The semi-analytical predictions – in particular, the critical intrinsic frequency of the vortex soliton at the instability border – are compared to systematic 2D simulations. We also compare our findings to those reported in earlier works, which featured some discrepancies. We then perform a detailed computational study of collisions between stable vortices with different topological charges. Borders between elastic and destructive collisions are identified.     

仿生魟鱼环形胸鳍波动推进的流场仿真

针对自主设计的仿生魟鱼水下机器人环形胸鳍波动推进过程进行了流场仿真研究。基于魟鱼前进运动时的胸鳍波动特征,建立了仿生环形胸鳍三维波动运动模型。利用CFD对胸鳍波动推进过程进行数值计算,分析了波动胸鳍产生的拉力以及前向运动的速度等推进性能。研究了周期波动过程中鳍面的周向压力分布,进行鳍面附近核心涡以及切面二维涡分量的计算,分析了涡的产生、发展以及尾迹涡的相关特性。结果表明,稳定后的拉力系数以近似正弦波形式进行波动,鳍面波动前后缘分布的高压和低压区域随波动沿周向从前往后传播,环形波动长鳍最终在尾迹处形成卡门涡,流场分析为仿生魟鱼水下机器人的运动控制及性能优化提供理论基础。     

Direct modeling of the interaction between vortex pairs

This work is devoted to the numerical modeling of the conjugate problem on the dynamics of two vortex pairs and calculation of the acoustic field generation at Reynolds numbers Re = 5000–10 000 on a fixed Eulerian grid. The computation was based on the CABARET scheme. For the main characteristics of integrated solutions, such as the slip period of vortex pairs and the velocity of their center of mass, a comparison is presented with the analytical solution obtained for the case of point vortices in an ideal fluid. The sensitivity of the obtained numerical solutions to the grid refinement was studied including both the hydrodynamic near field and instantaneous and root-mean-square acoustic pulsations.