Flow structures and dynamics in the wakes of sliding bubbles

An experimental investigation is reported for the flow structures in the wake of an air bubble sliding under an inclined surface in quiescent water. Time-resolved particle image velocimetry (PIV) is used to study the wakes of sliding bubbles for a range of measurement planes, bubble diameters and surface inclination angles. Additionally, key aspects of the bubble’s motion are measured simultaneously using a novel method that accounts for the motion of the bubble’s interface. Thus, vortex shedding may be linked to changes in the bubble shape and path.Analysis of the measured velocity and vorticity fields reveals a wake structure consisting of a near wake that moves in close proximity to the bubble, shedding vorticity at the inversion points of the bubble path. Downstream of the bubble in the far wake, these structures evolve into asymmetrical, oppositely-oriented hairpin vortices that are generated in the near wake. These hairpin vortices bear similarities to those observed behind freely rising bubbles and near-wall bluff bodies and are found to cause significant motion of the bulk fluid. This bulk fluid motion has the potential to offer significant convective cooling of adjacent heated surfaces, such as submerged electronics components.     

Vortex dynamics and scalar transport in the wake of a flat-plate controlled by a vibrating trailing-edge flap

We investigate the onset and development of vortical flow disturbances introduced into the wake of a horizontally fixed flat-plate by means of the controlled motion of a trailing edge flap. The vibrating mechanics of the flap allows for the introduction of both impulsive and harmonic weak amplitude velocity disturbances which are propagated downstream into the wake flow of the flat-plate. Quantitative experimental and numerical predictions of both steady and unsteady wake flow velocity resulting from different flapping frequencies are made at low Reynolds numbers (Re < 10⁴). Frequency response tests of the wake confirmed the existence of two dominant frequencies where the wake flow organises with a particular arrangement of downstream moving vortex structures. Numerical predictions of steady (unforced) and forced wake velocity profiles and kinetic energy profiles are in good agreement with the experimental results. In order to understand practical implications of the dominant vortex structures in scalar transport, we have extended the numerical part of the study solving for the concentration equation of a passive scalar being injected in particular regions of the physical domain. A spatial correlation between the trajectory of vortex structures and the scalar concentration downstream the wake is observed. Moreover, the onset of tip vortex structures produced during the forcing cycle seems to be responsible of a local increase of scalar concentration near the span wise flap ends.     

Flow above the free end of a surface-mounted finite-height circular cylinder: A review

The wake of a surface-mounted finite-height circular cylinder and the associated vortex patterns are strongly dependent on the cylinder aspect ratio and the thickness of the boundary layer on the ground plane relative to the dimensions of the cylinder. Above a critical aspect ratio, the mean wake is characterized by streamwise tip vortex structures and Kármán vortex shedding from the sides of the cylinder. Below a critical aspect ratio, a unique mean wake structure is observed. Recent experimental studies in the literature that used phase-averaged techniques, as well as recent numerical simulations, have led to an improved physical understanding of the near-wake vortex flow patterns. However, the flow above the free end of the finite circular cylinder, and its relationship to the near wake, has not been systematically studied. The effects of aspect ratio and boundary layer thickness on the free-end flow field are also not completely understood, nor has the influence of Reynolds number on the free-end flow field been fully explored. Common features associated with the free end include separation from the leading edge, a mean recirculation zone containing a prominent cross-stream arch (or mushroom) vortex, and reattachment onto the free-surface. Other flow features that remain to be clarified include a separation bubble near the leading edge, one or two cross-stream vortices within this separation bubble, the origins of the streamwise tip or trailing vortices, and various critical points in the near-surface flow topology. This paper reviews the current understanding of the flow above the free end of a surface-mounted finite-height circular cylinder, with a focus on models of the flow field, surface oil flow visualization studies, pressure and heat flux distributions on the free-end surface, measurements of the local velocity field, and numerical simulations, found in the literature.     

Two- and three-dimensional numerical simulations of the clap–fling–sweep of hovering insects

The importance of three-dimensional effects for flapping wings is addressed by means of numerical simulation. In particular, the clap–fling–sweep mechanism is examined. The flow at the beginning of the downstroke is shown to be in reasonable agreement with the two-dimensional approximation. After the wings move farther than one chord length apart, three-dimensional effects become essential. Two values of the Reynolds number are considered. At Re=128, the spanwise flow from the wing roots to the wing tips is driven by the centrifugal forces acting on the mass of the fluid trapped in the recirculation bubble behind the wings. It removes the excess of vorticity and delays the periodic vortex shedding. At Re=1400, vortex breakdown occurs past the outer portion of the wings, and multiple vortex filaments are shed into the wake.     

An Experimental Study of the Modulation of the Bubble Motion by Gas–Liquid-Phase Interaction in Oscillating-Grid Decaying Turbulence

The purpose of the present research is to understand dynamic bubble–liquid interaction in a bubbly flow based on the experimental results of the modulation of the bubble motion in oscillating-grid decaying turbulence. By comparing the experimental results obtained from stagnant water and those from oscillating-grid decaying turbulence, we discussed and described detailed process of the modulation of the bubble motion in a water vessel. We discussed the enhancement of the transition of the bubble motion from 2D to 3D by combining the liquid-phase motion obtained through particle imaging velocimetry/laser-induced fluorescence (PIV/LIF) measurement and the bubble wake motion captured through the LIF/HPTS (8-hydroxypyrene-1, 3, 6-trisulfonic acid) method, under both conditions (in the stagnant water and in the oscillating-grid decaying turbulence) in which the initial bubble formation and the bubble motion (gravity-center motion and surface oscillation) were considered to be the same. In addition, by using PIV/LIF measurement along with an infrared shadow technique, we simultaneously obtained the bubble motion (2D zigzagging motion in stagnant water, and 3D motion in the decaying turbulence) and the standard deviation of the liquid-phase motion (the bubble Reynolds number: 775; the turbulent Reynolds number: 62.2). Taking all of the results together, the modulation of the bubble motion in the decaying turbulence, and the dynamic interaction between the bubble and the liquid-phase motion were experimentally and carefully investigated. Consequently, the enhancement and the modulation of the bubble wake motion were considered to be triggered by the collapse of the symmetric property of the bubble–liquid (i.e. ambient liquid-phase turbulence) interaction.     

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.      

Turbulent flow dynamics caused by a truncated cylinder

The turbulent flow field around a quite simple geometry has been analysed in detail based on a snapshot database taken from numerical simulation. Here, emphasis is placed on the dominant coherent motion and the flow dynamics in the separated wake. The method-based analysis is performed using POD, filtering and phase-averaging. The results obtained show a highly intermittent flow topology, which reveals different (at least three) recurring vortex arrangements, but with considerably stochastic character. Corresponding frequencies, the periodicity as well as correlation and interaction of predominant vortex motions are discussed. The methods employed are not limited to the configuration exemplarily chosen.     

绕水翼空化流动多尺度数值研究

空化的多尺度效应是一种涉及连续介质尺度、微尺度空化泡以及不同尺度间相互转化的复杂水动力学现象, 跨尺度模型的构建是解析该多尺度现象的关键. 本文基于欧拉-拉格朗日联合算法, 通过界面捕捉法求解欧拉体系下大尺度空穴演化, 通过拉格朗日体系下离散空泡模型求解亚网格尺度离散空泡的运动及生长溃灭. 同时, 通过判断空泡与网格尺度间的关系判定不同尺度空化泡的求解模型. 基于建立的多尺度算法对绕NACA66水翼空化流动进行模拟, 将数值结果与实验进行对比, 验证了数值计算方法的准确性. 研究结果表明, 离散空泡数量与空化发展阶段密切相关, 在附着型片状空穴生长阶段, 离散空泡数量波动较小, 离散空泡主要分布在气液交界面位置; 在回射流发展阶段, 离散空泡逐渐增加并分布在回射流扰动区; 在云状空穴溃灭阶段, 离散空泡数量增多且主要分布在气液掺混剧烈的空化云团溃灭区. 在各空化发展阶段, 离散空泡直径概率密度函数均符合伽玛分布. 空化湍流流场特性对拉格朗日空泡空间分布具有重要影响, 离散空泡主要分布在强湍脉动区、旋涡及回射流发展区域.      

Experimental study on the flow regimes past a confined prism undergoing self-sustained oscillations

An experimental study based on Particle Image Velocimetry (PIV) is presented with the objective of studying the flow regimes that appear in the flow past a confined prism undergoing self-sustained oscillations at low Reynolds numbers (Re). The square-section prism, placed inside a 3D square cross-section vertical channel with a confinement ratio of 1/2.5, was tethered to the channel walls and, therefore, it was allowed to move freely transverse to the incoming flow. Re (based on the prism cross-section height) was varied in the range from 100 to 700. Three different prism to fluid density ratios (m¹) were considered: 0.56, 0.70, and 0.91. These two parameters, Re and m¹, were used to map the results obtained. In particular, it was found that five different regimes appear: (1) steady prism with steady recirculation bubble, (2) steady prism with unsteady vortex shedding wake, (3) large amplitude low frequency oscillating prism with unsteady vortex shedding wake, (4) small amplitude high frequency oscillating prism with unsteady vortex shedding wake, and (5) irregular/chaotic motion of both the prism and the wake. The PIV results and associated numerical simulations were used to analyze the different prism and wake states.     

NUMERICAL ANALYSIS ON THE VELOCITY AND PRESSURE FIELDS INDUCED BYMULTI-OSCILLATIONS OF AN UNDERWATER EXPLOSION BUBBLE

假设水下爆炸气泡的内部气体在膨胀收缩过程中满足绝热条件,周围流体无黏无旋不可压缩. 基于势流理论,采用边界元法研究气泡动力学行为,重点关注气泡引起的流场脉动载荷以及滞后流特性,给出了相关的理论推导和数值计算方法. 通过将数值结果与解析解、实验值进行对比,数值模型的收敛性和有效性能够得到保证. 利用编写的程序进行计算和分析,发现在气泡加速膨胀阶段,流场压力在气泡径向不一定是逐渐衰减,还有可能以先增后减的规律变化;气泡射流后,为了能够继续描述环状气泡的运动以及流场特性,将此时的流场分为无旋场和一个布置在气泡内部涡环的叠加,计算过程中采用了一些数值技巧处理气泡的拓扑结构,得以连续模拟多个周期的气泡运动. 环状气泡具有相对较高的上浮迁移速度,而且在其顶部和底部附近分别形成两个高压区,顶部的高压区峰值相对较大,底部的高压区范围相对较大. 环状气泡中心轴上的流场速度会在气泡中心有一个加速过程,在气泡顶部附近又迅速减小.     

气泡多周期运动时引起的流场压力与速度

假设水下爆炸气泡的内部气体在膨胀收缩过程中满足绝热条件,周围流体无黏无旋不可压缩. 基于势流理论,采用边界元法研究气泡动力学行为,重点关注气泡引起的流场脉动载荷以及滞后流特性,给出了相关的理论推导和数值计算方法. 通过将数值结果与解析解、实验值进行对比,数值模型的收敛性和有效性能够得到保证. 利用编写的程序进行计算和分析,发现在气泡加速膨胀阶段,流场压力在气泡径向不一定是逐渐衰减,还有可能以先增后减的规律变化;气泡射流后,为了能够继续描述环状气泡的运动以及流场特性,将此时的流场分为无旋场和一个布置在气泡内部涡环的叠加,计算过程中采用了一些数值技巧处理气泡的拓扑结构,得以连续模拟多个周期的气泡运动. 环状气泡具有相对较高的上浮迁移速度,而且在其顶部和底部附近分别形成两个高压区,顶部的高压区峰值相对较大,底部的高压区范围相对较大. 环状气泡中心轴上的流场速度会在气泡中心有一个加速过程,在气泡顶部附近又迅速减小.      

Influence of channel walls on the formation of vortices behind a wedge

A study is made of the influence of channel walls on the occurrence of periodic vortex formation in the wake of a wedge in a turbulent flow of incompressible fluid. The results of numerical simulation of the flow are confirmed by experimental data. The influence of the blocking of the flow by the body on the existence of the regime of vortex formation is explained by means of a linear analysis of the stability of the non-parallel flow in the near wake.     

Investigations of wake flows of a flat plate in steady,oscillatory and combined flows

Numerical study on near wake flows of a flat plate in three kinds of oncoming flows is made by using the discrete vortex model and improved vorticity creation method. For steady oncoming flow, both gross and detailed features of the wake flow are calculated and discussed. Then, in harmonic oscillatory oncoming flow two different wake flow patterns withK _c=2,4 and 10 are obtained respectively. Our results present a new wake flow pattern for lowKc numbers (Kc<5) describing vortex shedding, pairing and moving in a period of the oscillatory flow starting from rest. The calculated drag and inertia force coefficients are closer to experimental data from the U-tube than the previous results of vortex simulation. For in-line combined oncoming flow the vortex lock-in and dynamic characteristics are simulated. The results are shown to be in good agreement with experiments. The project supported by National Natural Science Fundation of China and LNM of Institute of Mechanics. CAS     

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.     

Vortex formation mechanisms in the wake behind a sphere for 200 < Re < 380

Direct numerical simulation and visualization of three-dimensional separated flows of a homogeneous incompressible viscous fluid are used to comprehensively describe different mechanisms of vortex formation behind a sphere at moderate Reynolds numbers (200 ≤ Re ≤ 380). For 200 < Re ≤ 270 a steady-state rectilinear double-filament wake is formed, while for Re > 270 it is a chain of vortex loops. The three unsteady periodic flow patterns corresponding to the 270 < Re ≤ 290, 290 < Re ≤ 320, and 320 < Re ≤ 380 ranges are characterized by different vortex formation mechanisms. Direct numerical simulation is based on the Meranzh (SMIF) method of splitting in physical factors with an explicit hybrid finite-difference scheme which possesses the following properties: secondorder approximation in the spatial variables, minimal scheme viscosity and dispersion, and monotonicity. Two different vortex identification techniques are used for visualizing the vortex structures within the wake.     

壁面约束对裙带气泡动力学的影响

基于流体体积法(volume of fluid,VOF),数值模拟了装满黏性液体的圆柱形汽缸中的裙带气泡的浮升运动,研究了侧壁面约束对裙带气泡浮升动力学的影响.用雷诺数(Re)、韦伯数(We)、长宽比(χ)、裙带厚度(T/d)和裙带长度(L/d)等参数来表征不同约束比条件下(1.1≤Cr≤10)裙带气泡的运动和变形特性,分别在全局参考系和局部参考系下分析了壁面对气泡内外流场的影响.模拟结果显示,当Cr≥8时,裙带气泡的行为特性与在无界流域条件下的情况相当,可视作壁面无关的.当Cr8时,壁面对裙带气泡的浮升速度和形状演化有显著影响.随着壁面的靠近,裙带气泡受到的阻力增大,造成浮升速度下降.约束比降低使裙带厚度增厚而长度变短直至裙带消失,裙带气泡受挤压而被拉长并逐渐变为椭圆球帽形最后到子弹形.相反,约束比增大时,裙带气泡尾流效应增强,气泡边缘处流场产生明显的循环流动(涡环),促使裙带的形成.研究表明壁面会加剧裙带气泡产生破碎,印证了前人的推断.模拟结果与已有的经验公式吻合良好,分析了前人公式的适用性.     

DES evaluation of near-wake characteristics in a shallow flow

Three-dimensional numerical modeling using Detached Eddy Simulation (DES) based on unsteady Reynolds-Averaged Navier–Stokes (RANS) with the k–ω SST (Shear-Stress Transport) turbulence model has been carried out to evaluate the characteristics of a shallow wake flow. The shallow wake is generated by inserting a sharp-edged bluff body in the open channel flow. A horseshoe vortex is captured in front of the body, which stretches downstream and envelops the vortices that form part of the shear layers. The mean and instantaneous flow field characteristics in the wake are examined and compared at different downstream locations to evaluate the three-dimensional features in the flow. Streamwise positive directed velocity is observed in the wake centerline at horizontal planes close to the bed. Flow features hitherto not captured in experimental studies can be identified in sections parallel to the bed and body. A typical signature of three-dimensionality, upward ejection of fluid elements from the bed towards the free surface, is also observed in the wake.     

Bubble deformation and flow structure measured by double shadow images and PIV/LIF

An experiment on bubble motion in a simple shear layer was performed in order to obtain fundamental knowledge of the force on the bubble and its lateral motion induced by the surrounding flow field. We explored the flow structure in the vicinity of the bubble in one plane and its deformation in two planes by particle image velocimetry (PIV)–laser-induced fluorescence (LIF) and a projection technique for two perpendicular planes, respectively. For our experiment, we chose a single air bubble with an equivalent bubble diameter D _eq of 2~6 mm in a vertical shear flow. Velocity measurements were made using a digital high-speed CCD camera for PIV with fluorescent tracer particles. The second and third CCD cameras were used to detect the bubbles shape and motion via backlighting from an array of infrared LEDs. We quantitatively studied the three-dimensional wake structure from measurements of the two-dimensional vortex structure and approximated three-dimensional shape deformation arranged from two perpendicular bubble images.     

Numerical study of single bubble motion in liquid metal exposed to a longitudinal magnetic field

The paper presents numerical simulations modeling the ascent of an argon bubble in liquid metal with and without an external magnetic field. The governing equations for the fluid and the electric potential are discretized on a uniform Cartesian grid and the bubble is represented with a highly efficient immersed boundary method. The simulations performed were conducted matching experiments under the same conditions so that sound validation is possible. The three-dimensional trajectory of the bubble is analyzed quantitatively and related to the flow structures in the wake. Indeed, the substantial impact of the magnetic field in the bubble trajectory results from its influence on the wake. Quantitative data describing the selective damping of vortex structures are provided and discussed. As a result of applying a longitudinal field, the time-averaged bubble rise velocity increases for large bubbles, it reaches a maximum and then decreases when further increasing the magnetic interaction parameter. For small bubbles, the time-averaged bubble rise velocity decreases when increasing the magnetic field. The bubble Strouhal number as a dimensionless frequency is reduced with the application of a magnetic field for all bubbles considered and the zig–zag trajectory of the bubble becomes more rectilinear.     

Modes of vortex formation and transition to three-dimensionality in the wake of a freely vibrating cylinder

The present work is aimed to give some insight into the relation between vortex shedding modes and transition to three-dimensionality in the wake of a freely vibrating cylinder by establishing a numerical model and analyzing the relevant results of two- and three-dimensional simulations. The compressible flow past an elastically-mounted cylinder is solved by using the immersed boundary method (IB method). The cylinder is free to vibrate in the transverse direction with zero structure damping. The response of displacement amplitude is studied with the variation of reduced velocity. Whether P+S mode exists in three-dimensional flow and the occurrence of 2P mode is caused by flow transition from two-dimensional to three-dimensional are problems of concern. Both 2P and P+S wake modes are observed in two- and three-dimensional simulations. The numerical results indicate that the flow transition from two-dimensional to three-dimensional is coupled with the cylinder vibration in the synchronization/lock-in regime. The wake formation given by three-dimensional simulations suggests that the P+S mode might exist in reality when the flow is reverted to two-dimensional by vortex induced vibration (VIV) at Re=300–350. When Reynolds number increases to 425, the wake formation undergoes transition to three-dimensionality and 2P mode is observed. The effect of mass ratio on the flow transition to three-dimensionality is studied. The relationship between wake modes and aerodynamic forces is discussed.