确定分布的展向Lorentz力调制下的槽道湍流涡结构

采用直接数值模拟方法,对槽道湍流中确定分布的Lorentz力的流动控制与减阻问题进行研究.讨论了Lorentz力作用于槽道湍流后,流场的特性和涡结构的特性,并对此类Lorentz力对槽道湍流的控制与减阻机理进行了讨论.研究发现:1)Lorentz力诱导的层流流场壁面附近存在梯度极大的展向速度剪切层,该剪切层容易形成流向涡结构;2)在给定合适参数的确定分布的Lorentz力作用下,湍流流场仅剩周期分布的准流向涡;3)与未控制流场相比,控制后的流场中,准流向涡的抬升高度大大降低,从而减小猝发强度,使壁面阻力下降.     

Visualization and characterization of small-scale spanwise vortices in turbulent channel flow

High-resolution particle-image velocimetry (PIV) measurements are made in the streamwise-wall-normal plane of turbulent channel flow at Re_τ=566, 1184 and 1759, facilitating documentation of the population trends and core diameters of small-scale spanwise vortices. Swirling strength, an unambiguous vortex-identification criterion and hence a local marker of rotation, is used to extract small-scale spanwise vortex cores from the instantaneous velocity fields. Once the small-scale vortices are properly extracted from the PIV realizations, their characteristics are studied in detail. The present results indicate that the very-near-wall region (y < 0.1h) is densely populated by spanwise vortices with clockwise (negative) rotation. This behavior supports the notion that hairpin-like vortices are generated very close to the wall and grow into the outer layer as they advect downstream. In contrast, counterclockwise (positive) spanwise vortices are scarce in the very-near-wall region, but their presence steadily increases within the logarithmic layer presumably due to a localized generation mechanism. The average core diameter of negative spanwise vortices is found to be larger than the average diameter of positive vortices, with few positive vortices having core diameters exceeding 80y.     

Self-oscillations in a jet flow and gaseous flame with strong swirl

Investigation results on unsteady flow dynamics in a gaseous jet flame with strong swirl, vortex breakdown, and precession of a vortex core obtained by panoramic optical methods are presented, as well as the results of theoretical analysis of the fastest growing modes of hydrodynamic instability. Characteristics of the most unstable self-oscillating mode in the initial region of the turbulent strongly swirling propane-air jet burning in the atmospheric air in the form of a lifted flame are determined. Analysis of data by principal component analysis and linear stability analysis revealed that evolution of the dominant self-oscillating mode corresponds to quasi-solid rotation with constant angular velocity of the spatial coherent structure consisting of a jet spiral vortex core and two spiral secondary vortices.     

Analysis of flow structures in supersonic plane mixing layers using the POD method

The proper orthogonal decomposition (POD) method was applied to analyzing the database obtained from the direct numerical simulation (DNS) of supersonic plane mixing layers. The effect of different forms of the inner products in the POD method was investigated. It was observed that the mean flow contributes to a predominant part of the total flow energy, and the energy spectrum of the turbulence fluctuations covers a wide range of POD modes. The patterns of leading (high energy) POD modes reveal that the flow structures exhibit spanwise counter rotating rolls, as well as oblique vortices. These flow patterns are insensitive to the velocity of the observer. As the convective Mach number increases, the energy spectrum becomes wider, the leading POD modes contain more complicated structures, and the flow becomes more chaotic.     

Flow visualization of vortex structure in a pulsed rectangular jet

Pulsating jet is visualized using hydrogen bubble method to clarify the vortex nature in the near field of the jet. This study focused on the development in space and time of vortex structures evolution in low aspect-ratio rectangular jet with pulsation. Pulsation means large-amplitude, low-frequency excitation which is expected to increase the mixing and spreading of the jet and to accelerate its transition from a rectangular form to an axisymmetric form. It was deemed appropriate to investigate whether jet characteristics of a pulsating, submerged jet flow can be altered by including pulsations. The difference of the vortex deformation process is discussed in relation to pulsating conditions. Consequently, the pulsation leads to the formation of vortices at regular intervals, which are larger than those occurring in a steady jet. The results show that the streamwise interaction, between leading vortex and trailing vortex rolled up at nozzle lips, strengthens with increasing pulsating frequency. The spanwise drift of the vortex becomes strongly apparent at large amplitude and high frequency conditions. The drifting start position does not change regardless of pulsating condition. The convection velocity of vortex increases at lower frequency and larger amplitude.     

Secondary vortices over surfaces with spanwise varying drag

A spanwise heterogeneity of streamwise drag is known to lead to the formation of large secondary motions of Prandtl's second kind. Based on the data sets extracted from direct numerical simulations (DNS) of fully developed turbulent channel flow where streamwise stripes of free-slip surface with varying spanwise extension are introduced, we investigate the topological structure of the secondary motions. We find a complex restructuring of the secondary motion with increasing extent of free-slip/no-slip region where the width of the free-slip region in viscous units appears to be one important governing parameter for the vortex formation. The most striking feature of this restructuring is a change in the rotational direction of the major vortex pair such that the related high- and low-momentum pathways are found at different locations. The present results reveal that the spanwise inhomogeneity of the Reynolds stress distribution is strongly related to the observed change of rotational direction. In addition, it is shown that the vorticity source remains largely unchanged and mainly restricted to a rather small region close to the discontinuity in the boundary condition, despite the fact that the topology of secondary motions substantially changes with variation of the spanwise length scale. This suggests a complex interplay between the vortices that are generated at the surface discontinuities and the surrounding flow.     

近壁湍流的降阶模型及应用

由条带和流向涡的循环再生构成的近壁自维持过程(self-sustaining process, SSP)是壁湍流产生和维持的重要机制. 文章通过对最小槽道的直接数值模拟(direct numerical simulation, DNS)获得近壁自维持过程的流场数据, 采用正规正交分解法(proper orthogonal decomposition, POD)对该数据进行分析, 获得了不同流向和展向尺度的特征模态, 通过将Navier-Stokes方程在这些模态上进行投影, 得到近壁自维持过程的降阶模型, 并采用DNS数据对降阶模型的预测能力进行了评价. 该模型被初步应用于大涡模拟近壁模型的构造.      

Physics of multiple level hairpin vortex structures in turbulence

Previous experimental and numerical studies have revealed that the hairpin vortex is a basic flow element of transitional boundary layer. The hairpin vortex is believed to have legs, necks and a ring head. Based on our DNS study, the legs and the ring head are generated separately by different mechanisms. The legs function like an engine to generate low speed zones by rotation, create shear layers with surrounding high speed neighbor fluids, and further cause vortex ring formation through shear layer instability. In addition, the ring head is ?-shaped and separated from quasi-streamwise legs from the beginning. Contrary to the classical concept of "vortex breakdown", we believe transition from laminar flow to turbulence is a "buildup" process of multiple level vortical structures. The vortex rings of first level hairpins are mostly responsible for positive spikes, which cause new vorticity rollup, second level vortex leg formation and finally smaller second level vortex ring generation. The third and lower level vortices are generated following the same mechanism. In this paper, the physical process from ?-vortex to multi-level hairpin vortices is described in detail.     

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.     

Viscoelastic effects on asymmetric two‐dimensional vortex patterns in a strongly coupled dusty plasma

Strongly coupled dusty plasma medium is often described as a viscoelastic fluid that retains its memory. In a flowing dusty plasma medium, vortices of different sizes appear when the flow does not remain laminar. The vortices also merge to transfer energy between different scales. In the present work, we study the effect of viscoelasticity and compressibility over a localized vortex structure and multiple rotational vortices in a strongly coupled viscoelastic dusty plasma medium. In case of single rotating vortex flow, a transverse wave is generated from the localized vortex source and the evolution time of generated waves is found to be reduced due to finite viscoelasticity and compressibility of the medium. It is found that the viscoelasticity suppresses the dispersion of vorticity. In the presence of multiple vortices, we find, the vortex mergers get highly affected in the presence of memory effect of the fluid, and thus the dynamics of the medium gets completely altered compared to a non‐viscoelastic fluid. For a compressible fluid, viscoelasticity dampens the energy in the sonic waves generated in the medium. Thus a highly viscoelastic and compressible fluid, in some cases, behaves similarly to an incompressible viscoelastic fluid. The wave‐front like rings propagate in elliptical orbits keeping the footprint of the earlier position of the point‐vortex. The rings collide with each other even within the patch vortex region forming regions of high vorticity at the point of intersection and pass through each other.     

Spectral flow and vortex dynamics in a temperature gradient

In the mixed state of superconductors the spectral flow of fermion zero modes in the vortex core couples the motion of vortices to that of the normal fluid. This gives rise to a heat current perpendicular to the direction of vortex motion and therefore to longitudinal thermomagnetic effects like the thermopower and the Peltier effect. Analysis of vortex motion in a temperature gradient on this basis yields excellent agreement with experimental results.     

三维边界层内诱导横流失稳模态的感受性机理

边界层感受性问题是层流向湍流转捩的初始阶段,在转捩过程中起关键性作用,尤其是三维边界层流动.因此,研究三维边界层感受性问题对进一步理解层流向湍流转捩机理以及湍流成因具有重要的理论意义.采用数值方法研究自由来流湍流与三维壁面局部粗糙相互作用下三维边界层的感受性问题,确定是否能在三维边界层内寻找一种新的横流失稳模态;确定在何种条件下三维边界层内能诱导出定常、非定常的横流失稳模态;探索自由来流湍流的强度、展向波数和法向波数以及三维壁面局部粗糙的大小和结构类型等因素在自由来流湍流与三维壁面局部粗糙作用下三维边界层内被激发出的感受性过程中有何影响,并确定何种横流失稳模态在三维边界层感受性过程中占据何种地位.对自由来流湍流与三维壁面局部粗糙作用激发三维边界层内感受性问题的深入研究,将有助于完善流动稳定性与湍流理论,为层流向湍流转捩过程的预测与控制提供合理的理论依据.     

Instability of Abrikosov vortices in a type-II superconductor-ferrite structure with a longitudinal electric field

The influence of the interaction of the Abrikosov vortices with the magnetization on the longitudinal vortex instability in a layered type-II superconductor-ferrite structure is analyzed. It is shown that in the vicinity of the orientational phase transition in the magnet, where the transverse susceptibility of the magnet is high, the longitudinal critical current in the structure can be almost 1.5 times smaller than the corresponding value in the isolated superconductor. Because of the influence of the nonlocality of the interaction between the vortices, such an effect can be observed only in structures with superconductors that have weak or moderate pinning. A structure is considered in which the thickness of the superconductor is significantly greater than the London magnetic-field penetration depth and the wavelength of the critical mode. Zh. Tekh. Fiz. 67, 28–34 (July 1997)     

PIV measurements on the change of the three-dimensional wake structures by an air spoiler of a road vehicle

The three-dimensional vortical structures formed in the wake behind a road vehicle were measured using a particle image velocimetry (PIV) technique and the change of the structures by the existence of an air spoiler was investigated. The measurements were carried out in severalx-y, y-z andz-x planes to obtain full three-dimensional flow fields, including an out-of-plane velocity component, obtained by interpolating the velocities in the other plane. Then, the velocity gradient tensor is evaluated to obtain the vortex core by theλ ₂-definition. The relationship between streamwise, longitudinal and spanwise vortices is systematically analyzed by overlapping the vortex lines and vortex cores and the whole flow topology is compared in both cases with and without an air spoiler. As a result, an air spoiler was found to weaken the C-pillar vortices producing strong wing tip vortices, which reduce downwash flow and longitudinal vortices increase in the vertical direction. The recirculation zone formed when an air spoiler is installed is higher and narrower than without a spoiler.     

Instability of surfactant solution flow in a Taylor cell

The hydrodynamic instability of surfactant solutions between two coaxial cylinders was investigated by using a laser-induced-fluorescence flow visualization technique to clarify the effect of drag-reducing additives on vortex formation in Taylor-Couette flow. The test fluids were Ethoquad O/12 surfactant solutions, which have a gel-like structure called “shear-induced structurer” (SIS). Photographs of the formation of Görtler vortices were taken and compared with these of tap water. In the Taylor number range of 1.2×10⁵≤Ta≤7.1×10⁵, tap water and 10 ppm surfactant solution flows consisted of Taylor vortices and much smaller Görtler vortices at the rotating inner wall. However, for 50 and 100 ppm surfactant solutions, Taylor vortices were not apparent and Görtler vortices were collapsed. Measurements of the wavelength of Görtler vortices lead to the conclusion that surfactant solutions have a stabilizing effect on Görtler instabilities. This effect depends on surfactant concentration and becomes considerable with increasing acceleration of the inner cylinder.     

Propagation dynamics of off-axis noncanonical vortices in a collimated Gaussian beam

It is widely accepted that an off-axis noncanonical vortex moves across the free-space diffracting Gaussian beam without rotation. But our analysis shows that the vortex swirls a while before it approaches infinite. By neglecting the divergence of the host beam, we focus on this rotation characteristics of the vortices in linear homogeneous media. For the symmetrical host beam, it is found that the vortex moves along an elliptical trajectory, while the topological charge and the angular momentum of the vortex core relative to the beam axis are conserved. For the asymmetrical host beam, the vortex trajectory is rather complicated, since the noncanonical parameter varies as the light propagates, resulting in topological charge inversion. But we find that the vortices are always confined in a rectangular area, and the rotation direction is determined by the topological charge.     

受限空间内旋流回流区特性

为了研究受限空间内旋流回流区的三维结构特性,采用realizable k-ε模型模拟了旋流数等于0.884时,不同受限空间内的旋流流场.受限率是影响回流区形态的重要因素,受限率大于6时,中心回流区与下游回流区是两个独立的区域,有两对涡结构;受限率在3～6之间时,中心回流区与下游回流区合并到一起,存在两对独立的涡结构;受...     

Viscous near-wall flow in a wake of circular cylinder at moderate Reynolds numbers

Here we present the results of experimental investigation of a cross flow around a circular cylinder mounted near the wall of a channel with rectangular cross section. The experiments were carried out in the range of Reynolds numbers corresponding to the transition to turbulence in a wake of the cylinder. Flow visualization and SIV-measurements of instantaneous velocity fields were carried out. Evolution of the flow pattern behind the cylinder and formation of the regular vortex structures were analyzed. It is shown that in the case of flow around the cylinder, there is no spiral motion of fluid from the side walls of the channel towards its symmetry plane, typical of the flow around a spanwise rib located on the channel wall. The laminar-turbulent transition in the wake of the cylinder is caused by the shear layer instability.     

高超声速椭圆锥边界层横流转捩特性大涡模拟

横流效应显著影响高超声速飞行器的三维边界层转捩过程, 深化对该流动机制的认识有助于提升和改善飞行器气动性能及热力学环境. 针对HIFiRE5椭圆锥绕流问题, 采用大涡模拟方法计算分析了超声速边界层横流转捩特性, 并揭示其中的流动机理. 参考HIFiRE5风洞模型试验条件, 数值模拟中椭圆锥来流入口处施加人工速度扰动以激发边界层内不稳定扰动波, 进而预测了高超声速边界层流动横流失稳、转捩过程等基本流动特征, 并基于转捩热流分布形态对比, 获得了与试验数据基本吻合的计算结果. 研究发现, 椭圆锥中心线流动汇聚形成的流向涡结构非常容易失稳, 另外在中心线及侧缘之间的中部区域存在较强的横流不稳定性, 两种机制共同作用影响边界层转捩过程. 此外, 分析了来流扰动幅值对边界层横流失稳转捩的影响, 并发现静来流条件下, 横流区域出现两组独立的定常横流涡结构, 而强噪声来流条件下, 中心线主涡和中部横流涡均发生失稳转捩, 且在椭圆锥表面形成多峰状的转捩阵面. 最后, 深入分析流场的压力脉动动力学特性, 揭示了三维边界层发生失稳转捩的非线性演化机制.