PIV studies of large scale structures in the near field of small aspect ratio elliptic Jets

The near flow field of small aspect ratio elliptic turbulent free jets (issuing from nozzle and orifice) was experimentally studied using a 2D PIV Two point velocity correla tions in these jets revealed the extent and orientation of the large scale structures in th e major and minor planes. The spatial filtering of the instantaneous velocity field using Gaussian convolution kernel shows that while a single large vortex ring circumscribing the jet seems to be present at the exit of nozzle, the orifice jet exhibited a number of smaller vortex ring pairs close to jet exit. The smaller length scale observed in the case of the orifice jet is rep resentative of the smaller azimuthal vor tex rings that generate axial vortex field as they are convected. This results in the axis-switching in th e case of orifice jet and may have a mechanism differ ent from the self induction process as observed in the case of contoured nozzle jet flow.     

Analysis of jet entrainment mechanism in the transitional regime by time-resolved PIV

Abstract  

The entrainment mechanism in the near field of daisy-shaped and circular orifice jets have been investigated in the transitional regime using time-resolved 2D PIV measurements. The objective is to improve the knowledge from one previous investigation at initial Reynolds number of 800, based on the construction of a pseudo-time resolved PIV fields using the combination of non time-resolved PIV measurements and time-resolved visualizations (Nastase and Meslem J Vis 11(4):309–318, 2008). As expected in the previous work, the entrainment in the circular jet is correlated to the periodic Kelvin–Helmholtz (K–H) ring passing and the entrainment is produced in the braid region where the streamwise structures develop. In the daisy jet, we found that the entrainment rate is not correlated to the periodic K–H vortex passing. The observed small variation of the entrainment rate amplitude in the daisy jet could be related to the K–H dynamics. However, at the studied low Reynolds number the contribution of the K–H vortex on the daisy jet entrainment seems negligible comparing with the streamwise structures role. Furthermore, the real-time resolved measurements allow an indepth analysis of the role played by the K–H ring in the entrainment of circular jet. It is shown that the entrainment is not only produced in the braid region but is also present in the upstream part of the K–H ring. In the downstream part of the ring, the entrainment is dramatically reduced. This new observation opens a question which still has to be answered with time-resolved 3D PIV measurements. The question is “Whether the depression formed due to the ring passing or the streamwise structures rolled-up on the ring is responsible for entrainment at the upstream part of the ring?”     

Vortex dynamics and entrainment mechanisms in low Reynolds orifice jets

Classical planar 2D-PIV measurements and time-resolved visualizations enriched by low-level processing are used for the reconstruction of the Kelvin-Helmholtz vortex passing in the near field of a circular and a 6-lobed orifice jet flow. In the circular jet, the entrainment is produced in the braid region, being interrupted in the presence of the Kelvin-Helmholtz ring. The latter compresses the streamwise vortices and alters their self-induction role. Conversely, the 6-lobed orifice geometry allows the cutting of the Kelvin-Helmholtz structures into discontinuous ring segments. Consequently, into these discontinuity regions streamwise large scale structures are developing. These streamwise structures are permanent thus controlling and enhancing the jet entrainment which is not altered by the Kelvin-Helmholtz structures passing.     

Asymmetric glottal jet deflection: differences of two- and three-dimensional models

Flow is studied through a channel with an oscillating orifice mimicking the motion of the glottal-gap during phonation. Simulations with prescribed flow and wall-motion are carried out for different orifice geometries, a 2D slit-like and a 3D lens-like one. Although the jet emerges from a symmetric orifice a significant deflection occurs in case of the slit-like geometry, contrary to the 3D lens-like one. The results demonstrate the dependency of jet entrainment and vortex dynamics on the orifice geometry and the interpretation of asymmetric jet deflection with regard to the relevance of the Coanda effect in the process of human phonation.     

A computational study of asymmetric glottal jet deflection during phonation

Two-dimensional numerical simulations are used to explore the mechanism for asymmetric deflection of the glottal jet during phonation. The model employs the full Navier-Stokes equations for the flow but a simple laryngeal geometry and vocal-fold motion. The study focuses on the effect of Reynolds number and glottal opening angle with a particular emphasis on examining the importance of the so-called "Coanda effect" in jet deflection. The study indicates that the glottal opening angle has no substantial effect on glottal jet deflection. Deflection in the glottal jet is always preceded by large-scale asymmetry in the downstream portion of the glottal jet. A detailed analysis of the velocity and vorticity fields shows that these downstream asymmetric vortex structures induce a flow at the glottal exit which is the primary driver for glottal jet deflection.     

Large-eddy simulation of impinging jets with embedded azimuthal vortices

We have carried out large-eddy simulations of an impinging jet with embedded azimuthal vortices, a model of the wake of a helicopter hovering in ground effect. The azimuthal vortices are generated by sinusoidal forcing of the velocity at the jet exit. They strengthen while they are advected towards the ground; when they are close to the solid surface, a layer of opposite-sign vorticity is formed at the wall, and lifted up to form a secondary vortex that interacts with the primary one. Regions of reversed flow are caused by the strong, localised, adverse pressure gradient. After this interaction, the primary vortices begin to decay, mostly due to the Reynolds shear stresses, which contribute to the turbulent diffusion of vorticity term in the budget of the phase-averaged azimuthal vorticity. This mechanism is extremely robust, and plays the most important role in the vortex decay even if no turbulence is initially present in the jet, or if the no-slip condition is removed. A three-dimensional instability also plays a role: removing it leads to slower decay. Our results also point out some challenges for turbulence models for the unsteady Reynolds-averaged Navier–Stokes equations.     

Numerical Simulations of Two Coaxial Vortex Rings Head-on Collision

Vortex rings have been a subject of interest in vortex dynamics due to a plethora of physical phenomena revealed by their motions and interactions within a boundary. The present paper is devoted to physics of a head-on collision of two vortex rings in three dimensional space, simulated with a second order finite volume scheme and compressible. The scheme combines non-iterative approximate Riemann-solver and piecewise-parabolic reconstruction used in inviscid flux evaluation procedure. The computational results of vortex ring collisions capture several distinctive phenomena. In the early stages of the simulation, the rings propagate under their own self-induced motion. As the rings approach each other, their radii increase, followed by stretching and merging during the collision. Later, the two rings have merged into a single doughnut-shaped structure. This structure continues to extend in the radial direction, leaving a web of particles around the centers. At a later time, the formation of ringlets propagate radially away from the center of collision, and then the effects of instability involved leads to a reconnection in which small-scale ringlets are generated. In addition, it is shown that the scheme captures several experimentally observed features of the ring collisions, including a turbulent breakdown into small-scale structures and the generation of small-scale radially propagating vortex rings, due to the modification of the vorticity distribution, as a result of the entrainment of background vorticity and helicity by the vortex core, and their subsequent interaction.     

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=$4$x$10^4$ 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.     

Flow visualization and scanning PIV measurement of three-dimensional structure in near field of strongly buoyant jet

Abstract  

The near-field structure of strongly buoyant jet issuing from a square nozzle at low Froude and Reynolds numbers is studied by using LIF flow visualization and time-resolved scanning PIV. These experimental techniques allow the visualization of unsteady three-dimensional flow phenomenon occurring in the near-field of strongly buoyant jet. It is found that the buoyant jet is unstable to the positive buoyancy forces, which promote the inflow motion near the nozzle exit. The surrounding low temperature fluid moves into the nozzle inside along the nozzle corner and mixes with the high temperature fluid deep into the nozzle. Then, the flow pattern inside the nozzle becomes highly complex to promote the laminar to turbulent transition of the jet. The statistical flow characteristics of the strongly buoyant jet are evaluated from the scanning PIV measurement, and the result indicates the presence of axisymmetric distributions of mean flow and velocity fluctuations in the circle of diameter equal to the square side of the nozzle.     

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.     

Cavitating vortex generation by a submerged jet

The surface geometry of a cavitating vortex is determined in the limit of inviscid incompressible flow. The limit surface is an ovaloid of revolution with an axis ratio of 5: 3. It is shown that a cavitating vortex ring cannot develop if the cavitation number is lower than a certain critical value. Experiments conducted at various liquid pressures and several jet exit velocities confirm the existence of a critical cavitation number close to 3. At cavitation numbers higher than the critical one, the cavitating vortex ring does not develop. At substantially lower cavitation numbers (k ? 0.1), an elongated asymmetric cavitation bubble is generated, with an axial reentrant jet whose length can exceed the initial jet length by several times. This flow structure is called an asymmetric cavitating vortex, even though steady motion of this structure has not been observed.     

A PIV study of a supersonic impinging jet

The characteristics of supersonic impinging jets are investigated using Particle Image Velocimetry (PIV). The purpose of the experiments is to understand the jet induced forces on STOVL aircraft while hovering close to the ground. For this purpose, a large diameter circular plate was attached at the nozzle exit. The oscillations of the impinging jet generated due to a feedback loop are captured in the PIV images. The instantaneous velocity field measurements are used to describe flow characteristics of the impinging jet. The important flow features such as oscillating shock waves, slipstream shear layers and large scale structures are captured clearly by the PIV. The presence of large scale structures in the impinging jet induced high entrainment velocity in the near hydrodynamic field, which resulted in lift plate suction pressures. A passive control device is used to interfere with the acoustic waves travelling in the ambient medium to suppress the feedback loop. As a consequence, the large scale vortical structures disappeared completely leading to a corresponding reduction in the entrainment.     

Enhancement of thin air jets produced by ring-shaped dielectric barrier discharges using an auxiliary electrode

A ring-shaped dielectric barrier discharge (DBD) was explored as a small form factor ionic wind device. Using a concentric ring electrode geometry, the DBD produced a converging ionic wind that leads to a vertical flow away from the DBD electrodes. The vertical flow was channeled through an outlet nozzle to produce a thin air jet, and a grounded auxiliary electrode was placed at the nozzle to enhance the exit velocity. The inner diameter of the ring-shaped DBD electrode and the auxiliary electrode ranged 3.18–9.54 mm and 1.0–4.0 mm, respectively. Results showed that the auxiliary electrode generated an ionic wind velocity up to 3.7 m/s and increased the conversion efficiency from discharge to flow power by a factor of 30 by strengthening the electric field where the ions are accelerated.     

薄膜自激拍打射流的流动特性实验

在射流喷嘴出口处安装一端固定的柔性薄膜,在流速足够大时,射流和柔性膜相互诱导产生自激拍打作用,针对这一现象,提出了一种新型的自激拍打射流混合技术。在直径D=40 mm的渐缩喷嘴上固定长度L=(0.5～2)D和厚度δ=50μm的氟化乙烯丙烯共聚物(fluorinated ethylene propylene, FEP)薄膜,使用压差计测量了光滑渐缩喷嘴以及薄膜拍打运动所引起的压力损失。通过激光片光源和高速相机进行薄膜运动状态的显示和拍打幅度的测定,探究了拍打幅度受薄膜长度和Re的影响及其变化规律,利用测得的拍打幅度(A)和频率(f)作为Strouhal数(St=fA/U_o,U_o是射流的出口速度)的特征尺寸进行研究。在Re=3×10⁴的条件下,使用热线风速仪测量了在不同薄膜长度下拍打射流轴向速度沿中心线的分布,并对湍流度、概率密度函数等特征进行分析。此外,经过数字迭代滤波后获得射流沿中心线的积分尺度、 Taylor尺度和Kolmogorov尺度等统计量。实验结果显示,拍打射流的湍流度高于自由射流,意味着前者对周围流体具有更强...     

DNS study of swirling intensity effect on flow pattern of a circular jet

Abstract  

The figures show the 3D flow pattern of a circular jet with different swirling intensity. Reynolds number is approximately 4300 computed based on the nozzle diameter (d), jet velocity (U), and air fluid property at 1 atm and 300 K. The overall computational domain is set to be 4 × 4 × 12 d in spanwise, height, and streamwise direction. The governing equations are the fully compressible Navier–Stokes equations, firstly differenced by eighth-order explicit scheme and then advanced temporarily by using the fourth-order explicit Runge–Kutta method. 3D characteristics non-reflecting boundary condition including transverse source contribution is imposed on all other boundaries except the inflow boundary handled by assigning fixed profiles of temperature and velocity. To ensure the simulation resolution, here over 16 million grids are employed in sum, combined with a handful of grids located at buffer zones of outflow boundaries. To correctly represent the vortex in the flow field, velocity gradient tensor invariant Q is used here. And ψ refers to the swirling intensity defined as the ratio of tangential momentum to axis momentum. As shown in velocity profile, the flow pattern of the jet changes from a close mode to a totally open mode as ψ increases from 0.4 to 1.5. Accordingly, the recirculation zone gradually moves upstream and backflow velocity is enlarged as well. It is inteseting to found that the obvious drops of the momentums in two shown directions always occur at the same position downstream, no matter how large the ψ value is. Therefore, a momentum compensatory mechanism is expected to exist in the vortex-abundant zone. With the increase of ψ value, the increased strain rate in tangential direction can induce vortex more quickly, intensifying the entrainment and velocity-attenuation, which can be observed in Q value profile.     

The Vortex-Wave Equation with a Single Vortex as the Limit of the Euler Equation

In this article we consider the physical justification of the Vortex-Wave equation introduced by Marchioro and Pulvirenti (Mechanics, analysis and geometry: 200 years after Lagrange, North-Holland Delta Ser., Amsterdam, North-Holland, pp. 79–95, 1991), in the case of a single point vortex moving in an ambient vorticity. We consider a sequence of solutions for the Euler equation in the plane corresponding to initial data consisting of an ambient vorticity in L ¹ ∩ L ^∞ and a sequence of concentrated blobs which approach the Dirac distribution. We introduce a notion of a weak solution of the Vortex-Wave equation in terms of velocity (or primitive variables) and then show, for a subsequence of the blobs, the solutions of the Euler equation converge in velocity to a weak solution of the Vortex-Wave equation.     

Vorticity dynamics and sound generation in two-dimensional fluid flow

An approximate solution to the two-dimensional incompressible fluid equations is constructed by expanding the vorticity field in a series of derivatives of a Gaussian vortex. The expansion is used to analyze the motion of a corotating Gaussian vortex pair, and the spatial rotation frequency of the vortex pair is derived directly from the fluid vorticity equation. The resulting rotation frequency includes the effects of finite vortex core size and viscosity and reduces, in the appropriate limit, to the rotation frequency of the Kirchhoff point vortex theory. The expansion is then used in the low Mach number Lighthill equation to derive the far-field acoustic pressure generated by the Gaussian vortex pair. This pressure amplitude is compared with that of a previous fully numerical simulation in which the Reynolds number is large and the vortex core size is significant compared to the vortex separation. The present analytic result for the far-field acoustic pressure is shown to be substantially more accurate than previous theoretical predictions. The given example suggests that the vorticity expansion is a useful tool for the prediction of sound generated by a general distributed vorticity field.     

层流圆管潜射流生成蘑菇形涡结构特性数值研究

基于不可压Navier-Stokes方程, 采用计算流体力学方法, 数值模拟与分析了层流圆管潜射流在密度均匀黏性流体中的演化机理及其表现特征, 定量研究了蘑菇形涡结构无量纲射流长度L^*、螺旋型涡环半径R^*及其包络外形长度d^*等几 何特征参数随无量纲时间t^*的变化规律. 数值结果表明, 蘑菇形涡结构的形成与演化过程可分为三个不同的阶段: 启动阶段、发展阶段和衰退阶段. 在启动阶段, L^*和d^*随t^* 线性变化, 而R^*则近似为一个常数; 在发展阶段, 蘑菇形涡结构的演化具有自相似性, L^*, R^*和d^*与t^*1/2均为同一正比关系, 而且雷诺数和无量纲射流时间不影响该正比关系; 在衰退阶段, L^* 和R^* 正比于t^*1/5, 而d^*则近似为一个常数. 此外, 还对蘑菇形涡结构二次回流点、 动量源作用中心及其几何中心的速度变化规律、垂向涡量分布特征和 涡量-流函数关系进行了分析. 关键词： 圆管潜射流 蘑菇形涡结构 演化机理     

Particle Mass Loading Effect on a Two-Phase Turbulent Downward Jet Flow

The particle mass loading effect on the flow structure of a two-phase turbulent jet flow was studied. A particle mass loading ratio ranging from 0 to 3.6 was used as the control parameter. The polystyrene solid particles used had nominal diameters of 210 and 780 μm. The flow Reynolds number, which was based on the pipe nozzle diameter and the fluid-phase centerline mean velocity, was 2 × 10⁴ in the current test. A two-color laser Doppler anemometer (LDA), combined with the amplitude discrimination method and the velocity filter method, was employed to measure the mean velocity distributions for the particle and fluid phases, and the turbulent intensities and Reynolds stresses of the flow. The two-phase jet flow field was measured from the initial pipe exit to 90 D downstream. Another one-component He? Ne laser LDA system was also applied to obtain the energy spectra and temporal correlations of the two-phase jet flow.