Three-dimensional vortical structure and mixing mechanism of a circular jet

The three-dimensional vortical structure and the mixing mechanism of a circular water jet were investigated by a flow visualization technique. The jet was excited by axial and azimuthal perturbations to stabilize and enhance axisymmetric and streamwise vortices. A laser fluorescent dye and a laser light sheet were used to visualize the jet. The three-dimensional views of vortical structure were constructed by applying the Taylor hypothesis to the jet cross-sectional images. The views reveal the details of the complicated vortical structure. From the three-dimensional views, the areas of jet-boundary surface were calculated to discuss the jet mixing mechanism. The areas of the unmixed region were also discussed to evaluate the mixing rate on the inside of the jet. The result suggests that the enhancement of axisymmetric and streamwise vortices is very effective to increase mixing.     

Evolution of three-dimensional coherent structures in compressible axisymmetric jet

A high order difference scheme is used to simulate the spatially developing compressible axisymmetric jet. The results show that the Kelvin-Helmholtz instability appears first when the jet loses its stability, and then with development of jet the increase in nonlinear effects leads to the secondary instability and the formation of the streamwise vortices. The evolution of the three-dimensional coherent structure is presented. The computed results verify that in axisymmetric jet the secondary instability and formation of the streamwise vortices are the important physical mechanism of enhancing the flow mixing and transition occurring.     

Active control of coaxial jet mixing with manipulation of primary vortical structures by arrayed micro flap actuators

Active mixing control of a methane/air isothermal coaxial jet was achieved using micro magnetic flap actuators arranged on the inner surface of the outer annular nozzle. The spatio-temporal evolution of vortical structures and the scalar mixing were studied through the particle image velocimetry and planar laser-induced fluorescence methods. In contrast to studies on jet control using acoustic forcing, the mechanical movement of the flap directly generated large-scale intense vortices. The mixing was enhanced significantly by the vortices formed in the inner shear layer, although the control input was given to the outer shear layer. It was found that the primary vortex rings dominated the near-field mixing, while streamwise vortices were responsible for the downstream mixing. It was also demonstrated that the radial range of the inner fuel transportation could be manipulated flexibly by adjusting the shedding interval of the vortices. Especially, the mixing was enhanced most significantly when the primary vortices were most densely populated near the nozzle exit at the control Strouhal number of unity.     

Spectral characteristics of unstable flow in the mixing layer of supersonic underexpanded jet over its initial region

An experimental study of spectral characteristics of unstable flow in the mixing layer of supersonic axisymmetric underexpanded jets with Mach number M_a = 1 is reported. The destabilization of the flow is related to the formation, in the mixing layer of the jets, of disturbances in the form of streamwise vortical structures of the Taylor — Goertler type. As a result, in the mixing layer there forms an azimuthally non-uniform stationary distribution of total pressure. The Fourier transform of azimuthal sweeps of non-uniformity in pressure distributions was used to calculate the amplitude-wave spectra. An analysis of the spectral characteristics has allowed us to evaluate the longitudinal increment of amplitude growth of the disturbances and their dependence on the wavenumber and on the nozzle pressure ratio. The range of wavenumbers in which the streamwise vortical structures grow in amplitude is identified. This work was financially supported by the Russian Foundation for Basic Research (Grant No. 05-08-01215).     

Numerical visualization of vortical structures in a spatially evolving turbulent compressible mixing layer

Large-eddy simulations are performed to numerically visualize the generation of streamwise vortical structures and its interaction with spanwisely rolled-up coherent vortical structure during the spatial development of a turbulent supersonic/subsonic mixing layer at convective Mach numberM _c =0.51. Time-dependent three-dimensional compressible conservation equations were solved with a subgrid-scale turbulence model. The numerical code used the finite-volume technique, which adopted alternately in temporal discretization the second-order, explicit MacCormack’s and modified Godunov’s schemes. Both transverse and spanwise perturbations were imposed initially for promoting the formation of spanwise rollers and counter-rotating streamwise vortices, respectively. Numerical visualizations are presented in terms of time-sequence isopressure surfaces and vorticity contours of spanwise and streamwise components. The results show that the spatial growth of three-dimensional vortical structures, in particular, the formation of chain-link-fence type structures, is adequately captured by the present computations. Vorticity dynamics is further analyzed, for the first time, to identify the dominant roles played by the convection effect followed by the vortex stretching effect on affecting the evolution of streamwise and spanwise vortical structures, respectively, forM _c <0.6.     

Modulation of large-scale meandering and three-dimensional flows in turbulent slot jets

We investigate a vertically discharged shallow wall-bounded turbulent water jet both experimentally (LIF, TomoPIV) and numerically (LES). We identify the well-known meandering motion of the jet core generating large-scale planar vortices similar to the Karman vortex street. The modulation of the meandering amplitude is identified in experiments and simulations, which is attributed to the competition between sinusoidal and symmetric instability modes. TomoPIV data confirms that elongated streamwise vortices represent the smaller scale vortical structure of the jet in the near and far fields.     

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.     

Roles of initial condition and vortex pairing in jet noise

This is a study of the effect of initial condition on sound generated by vortex pairing in a low Mach number, cold air jet (0·15 ⩽ M ⩽ 0·35). Data has been taken, both flow velocity fields and sound pressure far fields, in a quality anechoic facility, with careful documentation of the effect of initial condition on the sound field of jets of two different geometries (i.e., circular and elliptic). Explanations are presented for most of the observed effects by applying Möhring's theory of vortex sound to vortex filament models of coherent structures in the jets. The explanations also draw upon experience with coherent structure dynamics. The sound source of interest here is that associated with the pairing of shear layer vortices. The evolution of these vortices is greatly affected by the initial condition as is their resultant sound field. The elliptic jets with laminar boundary layers show azimuthal directivity, namely, sound pressure levels in the minor axis plane were greater than in the major axis plane. This difference decreases as the nozzle boundary layer undergoes natural transition with increasing jet speed. When the nozzle boundary layer is tripped, making it fully turbulent and removing the shear layer mode of pairing, the elliptic jet sound fields become nearly axisymmetric. What appears to be the most acoustically active phase of vortex pairing has been modeled, and the resulting sound field calculated for the circular jet. Supporting evidence is found in the experimental data for the validity of this model. The model explains the connection between the initial condition and the far field sound of jets. Interestingly, a general result of Möhring's theory is that motions of vortex rings (of any arbitrary shape) can produce only axisymmetric sound fields if the rings remain in a plane. This implies that the observed asymmetric directivity of the laminar elliptic jet sound field must be due to non-planar ring motions of the vortical structures. The primary contribution of this paper is to examine quantitatively the role of vortex pairing in the production of jet noise; the results are used to reemphasize that “pairing noise” cannot be dominant in most practical jet sound fields, contrary to claims by other researchers.     

自由热射流流场的光学不均匀性数值研究

 通过建立物理模型，求解层流N-S方程，用数值方法模拟了二维热射流流场的有序大涡结构，计算了流场不同流向位置的光程值，分析了射流流场混合层的光学特性，得出了气动光学流场由于流场中大尺度涡结构的存在所引起的光学不均匀性，大涡结构中奇点所导致的光学畸变最大，在一定条件下大涡结构有散焦作用。     

自由热射流流场的光学不均匀性数值研究

通过建立物理模型,求解层流N-S方程,用数值方法模拟了二维热射流流场的有序大涡结构,计算了流场不同流向位置的光程值,分析了射流流场混合层的光学特性,得出了气动光学流场由于流场中大尺度涡结构的存在所引起的光学不均匀性,大涡结构中奇点所导致的光学畸变最大,在一定条件下大涡结构有散焦作用。     

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.     

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.     

Sufficient conditions for linear long-wave instability of steady-state axisymmetric flows of an ideal liquid with a free boundary in an azimuthal magnetic field

The problem of linear stability of steady-state axisymmetric shear jet flows of an inviscid ideally conducting incompressible liquid with a free surface and “frozen-in” azimuthal magnetic field is analyzed. The sufficient conditions for theoretical (on semi-infinite time intervals) and practical (on finite time intervals) instability of these flows relative to small axisymmetric long-wave perturbations are obtained by the direct Lyapunov method. An a priori lower estimate indicating (at least) an exponential increase with time of small perturbations under investigation is constructed in the case when these conditions are valid for theoretical as well as practical instability. In addition, an illustrative analytic example of steady-state flows under investigation and small axisymmetric long-wave perturbations superimposed on them is constructed (according to our estimate, these perturbations increase with time).     

Covariance measurements in the initial region of an annular jet

This paper describes the results of correlation measurements in the initial region of a basic annular jet, which is under the effect of the jet vortices and of the wake induced vortices. Evidence has been found that the wake induced vortices are excited by the disturbances associated with the wake vortices. For these wake induced vortices the azimuthal constituent (m = 1) is dominant. The similarity observed between the behaviour of the jet vortices and that of the wake induced vortices suggests that the jet vortices are also excited by these disturbances of the wake vortices, having the same Strouhal number. Thus the jet vortices are also dominated by the azimuthal constituent (m = 1). However, evidence has been found that within the first two outer diameters an axisymmetrical constituent (m = 0) is also present within the jet vortices. Comparison of the wake induced vortices with the jet vortices shows that these two helical waves are out of phase and are axially separated.     

湍流边界层中下扫流与“反发卡涡”

用氢气泡法观测湍流边界层的下扫流和有关的流动结构.实验中发现一种新型涡结构，它的特征与典型的发卡涡正好相反.发卡涡的头部指向下游，而它的头部指向上游； 发卡涡的两腿之间，由于涡的诱导产生上升流，而它则在其两腿之间，由于涡的诱导产生下扫流. 关键词： 湍流边界层 流动显示 流动结构 发卡涡     

可压缩轴对称射流流场近区的数值模拟

采用高精度差分方法对二维轴对称可压缩射流流场进行了数值模拟,研究了大尺度涡结构的形成和发展过程及其在射流雪展过程中的作用,计算结果显示了射流失稳后首先出现Ｋｅｌｖｉｎ－Ｈｅｌｍｈｏｌｔｚ非稳定特征,之后向下游发展的过程中, 一笥效应的增长导致涡的配对和对并;指出射流场中压力波动的分布是大尺度涡结构分布的直接结果。     

Combined experimental and computational study of laminar, axisymmetric hydrogen–air diffusion flames

We investigate the structure of two-dimensional, axisymmetric, laminar hydrogen–air flames in which a cylindrical fuel stream is surrounded by coflowing air, using laser-diagnostic and computational methods. Spontaneous Raman scattering and coherent anti-Stokes Raman scattering (CARS) are used to measure the distributions of major species and temperature. Computationally, we solve the governing conservation equations for mass, momentum, energy, and species, using detailed chemistry and transport. The fuel is diluted with nitrogen (1:1) to reduce heat transfer to the burner, to match the zero temperature gradient at the fuel exit. Three average fuel exit velocities are studied: 18, 27, and 50 cm/s. Comparisons of the measured and computed results are performed for radial profiles at a number of axial positions, and along the axial centerline. Peak major species mole fractions and temperatures are quantitatively predicted by the computations, and the axial species profiles are predicted to within the experimental uncertainty. In the radial profiles studied, base-case computations excluding thermal diffusion of light species were in excellent agreement with the measurements. While the addition of thermal diffusion led to some discrepancy with the measured results, the magnitude of the differences was no more than 25%. The computations predicted the axial centerline profiles from the burner exit to the maximum temperature well, though the experimental temperatures in the downstream mixing region decreased somewhat faster than the computed profiles. Radiative losses are seen to be negligible in these flames, and changes in transport properties and variations in initial flow velocities generally led to only modest changes in the axial profiles. The results also show that the detailed axial profiles of major species and temperature at different fuel jet velocities scale quantitatively with the jet velocity.     

On the influence of dissipative effects on instabilities of differentially-rotating plasmas

The stability of differentially-rotating cylindrical plasmas in the axial homogeneous magnetic field is studied in the framework of one-fluid dissipative magnetohydrodynamics. The dispersion relation of small-scale axisymmetric perturbations, taking into account the effects of the plasma thermal stratification, its resistivity and its viscosity, is derived. In the limiting cases of negligible resistivity and of negligible viscosity, the criteria of plasma stability are obtained. It is shown that in the case of small viscosity, the azimuthal flow of resistive plasma in the axial magnetic field is unstable due to the buoyancy effect if both the plasma pressure and its entropy either increase or decrease in the radial direction.     

Experimental evidence for non-linear growth in compressible mixing layer

An experimental study of compressible mixing layers(CMLs)was conducted using planar laser Mie scattering(PLMS)visualizations from condensed ethanol droplets in the flow.Large ensembles of digital images were collected for two flow conditions at convective Mach numbers Mc=0.11 and 0.47.The coherent vortices,braids and eruptions in the mixing zone were observed,interpreted as evidence of multi-scale,three-dimensional structures at a high Reynolds number.The mixing layers with a large visualized range present two stages along the streamwise direction,corresponding to the initial mixing and the well-developed stage.A new method,the gray level ensemble average method(GLEAM),by virtue of the similarity of the mixing layer,was applied to measure the growth rate of the CML thickness.New evidence for a nonlinear growth of CML is reported,providing an interpretation of previous observations of the scattering of the growth rate.     

蒸汽浸没射流引起的紊流区轴向温度分布研究

本文采用紊态扩散模型分析了蒸汽浸没射流凝结过程,通过假设紊流区为单相水轴对称流动,利用自由紊动射流理论得到了紊流区轴向温度的计算公式。根据先前给出的温度特性半宽的实验关联式,得到蒸汽浸没射流引起的紊流区单相水射流的虚源,从而给出了计算紊流区轴向温度的半经验公式。本文中利用无量纲穿透长度确定了紊流区的起点位置,在此紊流区范围内,轴向温度的计算值和实验值吻合得较好,相对误差基本在±5%以内。