Evolution of a forced counter rotating vortex pair for two selected forcing frequencies

In this paper, results from an experimental study of the natural and forced evolution of a pair of counter rotating wing-tip vortices are reported. The vortices were generated using a pair of opposed wing-tips in a wind tunnel and measurements made up to 77 tip-spacings downstream of the models at a chord Reynolds number of 1.3 × 10⁵. The wake was interrogated using 2D particle image velocimetry and the long-wave Crow instability observed. Velocity data were recorded throughout the lifetime of the instability from initial growth through linking, formation of vortex rings and their subsequent decay. Forcing was achieved using pulsed air jets blowing in the span-wise direction from the wing tip and imparting spatially periodic perturbations to the vortices. Forcing at a frequency within the range amplified by the Crow instability was found to enhance the instability growth rate whereas forcing at a frequency outside the amplified range was found to inhibit instability growth. In the latter case the imparted wavelength was observed to die out with a preferred wavelength growing in its place.     

Spatial perturbation of a wing-tip vortex using pulsed span-wise jets

The separation distance required between transport aircraft to avoid wake vortices remains a limiting factor on airport capacity. The dissipation of the wake can be accelerated by perturbing co-operative instabilities between multiple pairs of vortices. This paper presents the results of a preliminary experimental investigation into the use of pulsed span-wise air jets in the wing tip to perturb a single tip vortex in the very near field. Velocity measurements were made using PIV and hot-wire anemometry. The results demonstrate that the vortex position can be modulated at frequencies up to 50 Hz and, as such, the method shows promise for forcing instability in multiple vortex wakes.     

Wake transition of two-dimensional cylinders and axisymmetric bluff bodies

This article presents a short review of the three-dimensional transition of wakes from two-dimensional bodies, such as cylinders of various cross-sectional shape, and axisymmetric tori or rings. The nature and sequence of instabilities are compared and contrasted, especially with reference to the base case of the circular cylinder wake. The latter has been the subject of intense interest and scrutiny for well over a century, and has implicitly assumed the role of providing the generic transition scenario for turbulent wake flow. For elongated cylinders with streamlined leading edges, the analogues of the instability modes for a circular cylinder become unstable in the reverse order, which may have implications for the route to wake turbulence for such bodies. As well, the analogue of mode B has a significantly increased relative spanwise wavelength and appears to have a different near-wake structure. At the other extreme, for a normal flat plate, the wake first becomes unstable to a nonperiodic mode that appears distinct from either of the dominant circular cylinder wake modes. For tori, which have a local geometry approaching a two-dimensional circular cylinder for high aspect ratios (ARs), the sequence of transitions with increasing Reynolds number is a strong function of AR. For intermediate ARs, the first occurring wake instability mode is a subharmonic mode. Possible underlying physical mechanisms leading to some of these instabilities are also examined. In particular, support is provided for the role of idealized physical instability mechanisms in controlling wavelength selection and amplification for the dominant wake instability modes. The results presented in this article focus on relevant research undertaken by the Monash group but draws in results from many other international groups.     

Characterising shortwave instabilities on a vortex dipole

The paper presents a method for characterising shortwave instability on a vortex dipole. Such instabilities cause the initially straight vortex lines to deform in a sinuous manner. In order to quantify the phenomenon, it is necessary to (a) characterise the vortex dipole and (b) characterise instabilities developed on it. In this study, the vortex dipole characteristics were quantified by using a nonlinear least squares fit to a Lamb–Oseen vortex profile with the velocity field measured by means of particle image velocimetry. The instability was recorded by capturing images of hydrogen bubbles, which were used to mark the vortex centre line, with a CCD camera. It was characterised by applying a fast Fourier transform and seeking dominant wavenumber components and a representative amplitude based on a range of wavenumbers within a bandwidth of interest. The method was tested on simulated and real data. Using the simulated data, the shortwave instability growth rate was calculated with an uncertainty of better than 1% and the mean wavelength deduced with an uncertainty of 5.3%. Using real data, a constant initial growth rate was deduced in agreement with the established theory. Further work might improve the algorithms so that spatial variations in wavelength and growth rate can be determined.     

LARGE EDDY SIMULATION ON THE EVOLUTION AND THE FAST-TIME PREDICTION OF AIRCRAFT WAKE VORTICES

随着我国人民生活水平的提高,航空运输的重要性与日俱增,航班延误问题也日益严重.尾流间隔（保障后机不受前机尾流影响的最小安全间隔）是制约机场效率的关键因素.针对这一工程应用问题,采用大涡模拟方法研究飞机尾涡在大气中的演变特性.研究工作首先发展了飞机尾涡演变的大涡模拟方法,将自适应网格技术应用于飞机尾涡演变的大涡模拟,大幅减少所需的网格量,提高计算效率.提出了升力面尾涡生成方法,在不增加计算量的情况下实现了尾涡卷起过程和远场衰减的组合模拟.在系列算例分析研究基础上,创建了基于大涡模拟计算结果的尾流间隔快速预测系统.该系统可以根据实时大气风场和进出港的前后飞机机型,快速预测并输出所需的尾流间隔.经过与场地测试数据比较表明,在北京市2014年的平均风速条件下,本系统预测的尾流间隔可在现有标准基础上缩减7%~50%,能够有效提高机场容量.     

Thermal characteristics of the wake shear layers from a slightly heated circular cylinder

In this paper, we investigate the thermal characteristics of wake shear layers generated by a slightly heated circular cylinder. Measurements of the fluctuating temperature were made in the region x/d = 0.6 to x/d = 3 (where x is the downstream distance from the cylinder axis and d is the cylinder diameter) using a single cold-wire probe. The Reynolds number Re was varied in the range 2,600–8,600. For Re = 5,500, simultaneous measurements were made with a rake of 16 cold wires, aligned in the direction of the mean shear, at x/d = 2 and 3. The results indicate that the passive temperature can be an effective marker of various instabilities of the wake shear layers, including the Kelvin–Helmholtz (KH) instability. The temperature data have confirmed the approximate Re ^m dependence of the KH instability frequency (f _KH) with different values of m over different ranges of Re, as reported previously in the literature. However, it is found that this power-law dependence is not exact, and a third-order polynomial dependence appears to fit the data well over the full range of Re. Importantly, it is found that the wake shear-layer instabilities can be grouped into three categories: (1) one with frequencies much smaller than the Bénard–Kármán-vortex shedding frequency, (2) one associated with the vortex shedding and (3) one related to the KH instability. The low-frequency shear-layer instabilities from both sides of the cylinder are in-phase, in contrast to the anti-phase high-frequency KH instabilities. Finally, the observed streamwise decrease in the mean KH frequency provides strong support for the occurrence of vortex pairing in wake shear layers from a circular cylinder, thus implying that both the wake shear layer and a mixing layer develop in similar fashion.     

Transition phenomena in the wake of a square cylinder

The transition phenomena in the wake of a square cylinder were investigated. The existence of mode A and mode B instabilities in the wake of a square cylinder was demonstrated. The critical Reynolds numbers for the inception of these instability modes were identified through the determination of discontinuities in the St–Re curves, and were found to have mean values of 160 and 204 for the onset of mode A and B instabilities, respectively. The spectra and time traces of the wake streamwise velocity component were found to display three distinct patterns in laminar, mode A and mode B flow regimes. Streamwise vortices with different wavelength at various Reynolds numbers were observed through different measures. The symmetries and evolution of the secondary vortices were observed using laser-induced-fluorescent dye. It was found that, just like the case of a circular cylinder, the secondary vortices from the top and bottom rows were out-of-phase with each other in the mode A regime, but in-phase with each other in the mode B regime. From the flow visualization, it was qualitatively proven that there is stronger interaction between braid regions in the mode B regime. At the same time, analysis of PIV measurements quantitatively demonstrated the presence of the stronger cross flow in mode B regime when compared to the mode A regime. It suggests that the in-phase symmetry of the mode B instability is the result of strong interaction between the top and bottom vortex rows. It was also observed that although the vorticity of the secondary vortices in the mode A regime was smaller, its circulation was more than twice that of mode B instability. Compared to primary vortices, the circulations of both mode A and mode B vortices were much smaller, which indicates that the secondary vortices most likely originate from the primary vortices. The wavelengths of the streamwise vortices in the mode A and B regimes were measured using the auto-correlation method, and were found to be 5.1 (±0.1)D, 1.3 (±0.1)D, and 1.1 (±0.1)D at Re=183 (mode A), 228 and 377 (both mode B), respectively. From the present investigation, mode A instability was likely to be due to the joint-effects of the deformation of primary vortex cores and the stretching of vortex sheets in the braid region. On the other hand, mode B instability was thought to originate from the “imprinting” process.     

Secondary wake instabilities of a blunt trailing edge profiled body as a basis for flow control

Flow in the wake of a blunt trailing edge profiled body, comprised of an elliptical leading edge and a rectangular trailing edge, has been investigated experimentally, to identify and characterize the secondary instabilities accompanying the von Kármán vortices. The experiments, which involve laser-induced fluorescence for visualization and particle image velocimetry for quantitative measurement of the wake instabilities, cover Reynolds numbers ranging from 250 to 2,150 based on thickness of the body, to include the wake transition regime. The dominant secondary instability appears as spanwise undulations in von Kármán vortices, which evolve into pairs of counter-rotating vortices, with features resembling the instability mechanism predicted by Ryan et al. (J Fluid Mech 538:1–29, 2005). Feasibility of a flow control approach based on interaction with the secondary instability using a series of discrete trailing edge injectors has also been investigated. The control approach mitigates the adverse effects of vortex shedding in certain conditions, where it is able to amplify the secondary instability effectively.     

Nonlinear evolution of instabilities behind spheres and disks

We have performed precise and systematic experiments with PIV in order to measure the velocity field in the wake of a solid sphere and of a disk in a water channel, in the range of intermediate Reynolds number in which stationary and oscillatory instabilities appear, including the hairpin shedding regime. From these experimental data, we study the modal decomposition of the streamwise vorticity in an unsteady case and we describe the full nonlinear evolution of the bifurcating branches. We compare these results with recent theoretical and numerical studies on instability in the vortex shedding process at these intermediate Reynolds numbers.     

Active flow control for reduction of fluctuating aerodynamic forces of a blunt trailing edge profiled body

Vortex shedding in the wake of two-dimensional bluff bodies is usually accompanied by three dimensional instabilities. These instabilities result in streamwise and vertical vorticity components which occur at a certain spanwise wavelength. The spanwise wavelength of the instabilities (λ_Z) depends on several parameters, including profile geometry and Reynolds number. The objective of the present work is to study the three dimensional wake instabilities for a blunt trailing edge profiled body, comprised of an elliptical leading edge and a rectangular trailing edge, and to manipulate these instabilities to control the aerodynamic forces. Results of numerical simulations of flow around the body at Re(d) = 400, 600, and 1000, as well as planar Laser Induced Fluorescence (LIF) flow visualizations at Re(d) = 600 and 1000 are analyzed to determine the wake vorticity structure and λ_Z. Based on the findings of these analyses, an active flow control mechanism for attenuation of the fluctuating aerodynamic forces on the body is proposed. The flow control mechanism is comprised of a series of trailing edge injection ports distributed across the span, with a spacing equal to λ_Z. Injection of a secondary flow leads to amplification of the three dimensional instabilities and disorganization of the von Kármán vortex street. Numerical simulations indicate that the flow control mechanism can attenuate the fluctuating aerodynamic forces at lower Reynolds numbers (Re(d) = 400 and 600) where λ_Z is constant in time. However, the control mechanism loses its effectiveness at Re(d) = 1000, due to the temporal variations of λ_Z.     

Exotic vortex wakes—point vortex solutions

Von Kármán was the first to present a quantitative model of the “vortex street” wake as a double row of point vortices, to determine which configurations propagate in the direction of the rows, and to consider the linear stability theory for such states. In the early literature one works with infinite rows of vortices. The vortex street is assumed to continue to infinity both upstream and downstream. Another analytical approach is to use periodic boundary conditions in the direction of the wake. This representation was used by Domm in his analysis of the instability of the Kármán vortex street. Birkhoff and Fisher in 1959 were the first to treat vortices in a periodic strip as a dynamical system in its own right. We have used the periodic system to address problems of vortex wake patterns, in particular vortex wakes that are more complicated than the traditional two-vortices-per-strip configurations. We use the term “exotic” for such wakes. We submit that this approach can yield a number of insights, including results of direct relevance to experiments, in the same sense that von Kármán's analysis has been helpful to the understanding of the regular vortex street wake, and we present the results obtained to date following this program.     

Hierarchical instability of a vortex ring array in multipulse laser-matter interactions

In XeCl excimer Laser interactions with Co-coated steel surfaces, we have seemingly realized a quasi-linear array of vortex rings. These form from instabilities on an array of vortex filaments which emerge and then lead to a series of loops. Finally the collapse-and-reconnection process yields a cascade of nearby vortex rings. Since the filament array is subjected to randomly distributed local multipolar strains, unstable waves can develop on the vortex rings. These deformed shapes are frozen permanently by ultrafast cooling, following the last laser pulse. Using a modal analysis, an attempt is made to relate the wave structures to a parametric resonance instability which is caused by dipolar and a quadrupolar fields. Multipulse laser-matter interactions are thus capable of nonselective excitation of vortex ring instabilities of various sizes and various modal structures.     

基于相交不稳定性的尾流自消散机翼实验研究

飞机尾流是复杂的流动现象,相关控制的研究常采用简化模型,抓住主要矛盾进行尾流不稳定性的学术探索. 采用结构化矩形机翼模型,通过添加扰流片来模拟襟翼的一种作动方式,引入一对与主翼涡反向的小涡,以期诱发尾涡的瑞利-路德维希相交不稳定性. 改变模型在水槽中的拖曳速度以及机翼攻角,采用粒子图像速度场仪定量研究单主翼尾涡发展特性以及双涡相互作用特性. 研究表明,未添加扰流片时,尾涡环量在45 个翼展内相对于初始环量衰减了10%;而添加了扰流片的实验中,在较好的实验参数组合情况下,主翼尾涡环量较初始环量降低35%~45%. 结果表明添加适当扰流片产生的反向小涡能诱发与主翼尾涡的相交不稳定性,在尾流涡系中引入自消散机制,加速机翼尾涡的消散过程,达到提早消弱尾涡的目的.      

Flow-induced vibrations of low aspect ratio rectangular membrane wings

An experimental study of a low aspect ratio rectangular membrane wing in a wind tunnel was conducted for a Reynolds number range of 2.4×10⁴–4.8×10⁴. Time-accurate measurements of membrane deformation were combined with the flow field measurements. Analysis of the fluctuating deformation reveals chordwise and spanwise modes, which are due to the shedding of leading-edge vortices as well as tip vortices. At higher angles of attack, the second mode in the chordwise direction becomes dominant as the vortex shedding takes place. The dominant frequencies of the membrane vibrations are similar to those of two-dimensional membrane airfoils. Measured frequency of vortex shedding from the low aspect ratio rigid wing suggests that membrane vibrations occur at the natural frequencies close to the harmonics of the wake instabilities. Vortex shedding frequency from rigid wings shows remarkably small effect of aspect ratio even when it is as low as unity.     

Absolute and convective instabilities of two-dimensional bluff body wakes in ground effect

Local and global instabilities are investigated of wakes of general two-dimensional bluff bodies placed near and parallel to a plane boundary or ground. A spatio-temporal linear stability analysis is first applied to a four-parameter family of local wake profiles to investigate the fundamental local stability characteristics of the wake in ground effect. The analysis shows significant dependencies of the stability characteristics of the wake on the distance from the wake centreline to the ground (normalised by the wake width), and also on the velocity ratio of the near- and far-ground sides of the wake. The analysis is then compared with earlier experiments on a circular cylinder to examine, according to the transition scenario of the steep global modes, the streamwise variation of the local stability characteristics of the wake in ground effect. The comparison indicates that the near wake region of the cylinder changes from being absolutely unstable to being convectively unstable when the cylinder comes down into the near-ground range in which the von Kármán-type vortex shedding from the cylinder is suppressed, being qualitatively consistent with the transition scenario for general wake-type flows. A possible explanation is also given for the counter-intuitive relation between the thickness of the boundary layer on the ground and the critical gap distance for the cessation of the von Kármán-type vortex shedding in ground effect.     

The structure and dynamics of reacting plane mixing layers

The reacting two-dimensional plane mixing layer has been studied in two configurations: a rearward facing step and a two-stream mixing layer. Observations have been made of the steady state behavior, and the unsteady behavior when the flow was forced by a specific acoustic frequency. The steady behavior of the mean properties of the reacting flows is similar to that of non-reacting free shear flows except for the global effects of thermodynamic property changes. The structure of these flows is qualitatively similar to that of non-reacting flows. Vortices form by the two-dimensional Kelvin-Helmholtz instability and grow by subharmonic combination until the mixing layer interacts with the walls. Entrainment is dominated by the two-dimensional vortex motion. Three-dimensional instabilities give rise to secondary vortices which are coherent over several Kelvin-Helmholtz structures and dominate the fine scale mixing process. The mixing transition corresponds to a loss of coherence in the layer. Unsteady behavior occurs when there are resonant interactions with the Kelvin-Helmholtz instability or the instability associated with the recirculation vortex in the rearward facing step flow. Modeling efforts are reported which show promise of simulating the essential features of plane mixing layers.A version of this paper was presented at the ASME Winter Annual Meeting of 1984 and printed in AMD-Vol. 66     

Micro-scale instability in a continuously stratified fluid

Experimental observations of small-scale structures caused by flow instabilities at layers of high density gradient in the wake behind a cylinder in a fluid with a continuous salt concentration stratification are reported. In the density wake it is possible to discern a number of structures such as wedge-like structures or cusps; small-scale instabilities (breakers) in the zones of interaction of attached internal waves and the high-gradient wake envelope; small-scale instability of the density boundary layer with a complicated density gradient pattern superimposed on a smooth velocity profile, and small-scale wake structures behind attached vortices in the case of a closed (central) wake envelope.Translated from Izvestiya RossiiskoiAkademii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, pp. 3–10, May–June, 1995.     

水下发射航行体尾涡不稳定性分析

在水下连续发射过程中前一发航行体尾流会对后一发航行体运动姿态稳定性产生流动干扰现象. 因此, 研究尾流中涡旋结构演变机理对解决多弹体水下连续发射流动干扰难题具有重要的意义. 本文采用改进型分离涡模型与能量方程, VOF多相流模型与重叠网格技术相结合方法, 对航行体水下发射尾流演变过程开展精细化模拟研究, 其中模拟结果和实验吻合度较好, 验证了本文数值方法的有效性. 以航行体尾流区域为重点研究对象, 分析了尾流区瞬态流场分布, 讨论了横流强度和雷诺数对尾涡结构演变以及脉动压力分布特性的影响. 结果表明: 由于尾流区高速流体核心区与低速自由流相互作用导致Kelvin-Helmholtz不稳定现象出现, 可以清晰地发现涡旋结构在剪切力的作用下发生脱落. 在横流条件下, 航行体尾端脱落的涡环与涡腿形成发卡涡, 而多个发卡涡沿轴向间隔排列组成发卡涡包存在于尾流中. 随着横流强度增大, 形成多级发卡涡包结构, 而导致脉动压力二次峰值均出现的主要原因是尾流涡旋流场演变引起的. 随着雷诺数的增大, 尾流中由圆柱形涡和U型涡组成的二次涡结构逐渐明显, 不稳定性加强.      

Wake structures behind an oscillating bubble rising close to a vertical wall

In the present study, the wake structures behind an oscillating (zigzagging in a plane) air bubble, rising in a close vicinity of a vertical wall are experimentally investigated using a high-speed two-phase particle image velocimetry. While varying the distance between the rising bubble and the wall, the spatial and temporal variations in the spanwise and streamwise vorticity components contained in the wake vortices, in addition to the bubble trajectory, are measured in a tank filled with water. In particular, the Lagrangian streamwise vorticity fields in the bubble wake have been reconstructed and investigated in detail with different conditions. Without the wall, it is confirmed that there exist counter-rotating streamwise vortex tubes in the bubble wake, agreeing with the case of a two-dimensional zigzagging bubble, as reported in the literatures. It is also found that the hairpin vortex chain structures, initially attached to the bubble rear, evolve to detached vortex ring structures as the bubble rises in an oscillating path. While the detailed vortex structures show up quite differently from the reference case depending on the distance to the wall (e.g., actual bubble-wall collision), in general the wake behind the bubble as it moves toward and away from the wall can be summarized as: (i) transition to the detached vortex ring structures is accelerated; (ii) streamwise length of vortex tubes is shortened (evolution is interfered); (iii) counter-rotating vortex tubes approaching the wall tend to slightly bounce off and slide away (being dissipated fast) from each other on the wall; and (iv) boundary-layer like secondary flow structures are induced on the wall due to additional viscous effects. These wall-induced wake modification indicates that more fluid energy is wasted due to the wall interference, rather than being used to force the lateral movement of the bubble, which agrees with the reduced amplitude and wavelength of the oscillating bubble path on the wall. Finally, this explanation has been further confirmed by estimating the vortex-induced lateral forces acting on the bubble for each case.