三维湍流流动的涡量-向量势法数值模拟

本文采用涡量-向量势法,结合常规的 k-ε双方程湍流模型,并运用重新推导的固体壁面涡量边值计算法,对扁平射流煤粉燃烧器内的冷态三维流场进行了数值模拟。计算结果与实验数据相吻合。本文工作证明,用涡量-向量势法求解三维湍流问题是可行的,该法具有不应忽视的优越性,值得进一步开发。     

风口区空气流动的有限元模拟与实验研究

通风和空调出风口的射流特性主要取决于出风口截面之前的一段管道内的流动状况,研究送风口的射流偏转特性有重要实用价值.应用湍流大涡模拟技术结合Taylor-Galerkin有限单元法对工程中常见工况下的风口区管道内的三维流场进行了数值模拟和实验研究,数值计算结果与实验结果符合较好.表明湍流大涡模拟方法适合于边界形状复杂,存在各向异性的大尺度涡的内流情况,能可靠地预报风口区空气流动的射流偏转特性.     

大速差预燃室的流场特征

一种新型的大速差同向射流预燃室已经研制成功。本文基于SIMPLE算法,发展了计算这种流场的数值解法。文中介绍了算法、边界条件的详细说明以及计算结果。计算结果证明存在大面积迴流区;呈现了重要的三维特征。本文初步讨论了设计参数对迴流特性的影响。     

固体火箭燃气射流驱动液柱过程的CFD分析

固体火箭燃气射流驱动液柱过程会产生一个复杂的非稳态多相流场,为了研究液柱对固体火箭发动机工作过程中射流流场的降温效果,并揭示燃气冲击液柱的流动演化和气水之间的相互作用,利用FLUENT软件中耦合了液态水汽化方程的VOF多相流计算模型对燃气与液柱之间的耦合流动及相变过程进行了数值模拟,并与无液柱情况下射流流场的计算结果进行了对比分析。计算结果表明,当有液柱平衡体时射流流场中的压力、温度、速度波动幅度均减小,减弱了射流流场中的湍流脉动强度;液柱与燃气之间的汽化以及液柱的阻碍作用减小了射流流场的轴向发展位移,尾管后的完全发展射流流场核心区域内的压力峰值降低了0.9 MPa,温度峰值降低了503 K,速度峰值降低了291 m/s,验证了实验中液柱对燃气射流流场的降温效果。     

轻型火箭发射噪声场的分布特性

为了解小火箭发射噪声特性及其在喷口外围的声压场分布规律,针对燃气射流产生噪声问题进行了实验研究和数值计算。讨论了超声速射流噪声的3个主要成分(湍流混合噪声、啸音和宽带激波相关噪声)及相关特点,指出它们产生的根本原因是湍流射流的速度扰动。通过分析不同实验测点的射流噪声声压级峰值,得到了燃气射流噪声在轴向和径向上的分布规律,即随着离喷口距离的增大,轴向噪声的衰减程度大于径向。在实验基础上,利用大涡模拟与FW-H(Ffowcs Williams-Hawkings)声学比拟相结合的方法对燃气射流噪声的声学特性进行计算。结果表明,此方法获得的计算结果与实验结果吻合较好,可为进一步研究射流噪声控制提供参考。     

基于Stereo-PIV技术的三维发卡涡结构定量测量研究

发卡涡是湍流相干结构研究中最为关注的内容,实现发卡涡三维结构的定量测量并进行流体动力学分析,对深入研究湍流相干结构、实现湍流精准控制等具有重要意义.本研究通过对合成射流装置进行合理控制,使得层流边界层中产生了规则的人造发卡涡结构,进而用体视图像粒子测速仪(Stereo-PIV)锁相实验技术对发卡涡结构所在的三维空间流场进行了定量测量,并得到了一个完整周期内形成的发卡涡三维结构的空间流场. 结果发现,重构所得的三维发卡涡结构质量较高, 实验技术和方案具有可行性.发卡涡结构所在空间流场情况,符合目前人们对于发卡涡、高低速条带、喷射和扫掠事件的常规认识. 此外,对近壁二次流向涡、展向涡量集中区域的展向涡头和强剪切区域、与低速喷射流体相关的汇聚流动和发散流动等有了更细致的认识.同时, 也探讨了"基于二维脉动流场的相关特征去重构发卡涡三维流场"的可行性.为进一步定量探究发卡涡结构的形成演化、不同涡结构的融合及二次诱导等壁湍流相干结构问题提供思路.      

EXPERIMENTAL STUDY ON QUANTITATIVE MEASUREMENT OF THREE-DIMENSIONAL STRUCTURE OF HAIRPIN VORTEX BY STEREO-PIV 1)

发卡涡是湍流相干结构研究中最为关注的内容,实现发卡涡三维结构的定量测量并进行流体动力学分析,对深入研究湍流相干结构、实现湍流精准控制等具有重要意义.本研究通过对合成射流装置进行合理控制,使得层流边界层中产生了规则的人造发卡涡结构,进而用体视图像粒子测速仪(Stereo-PIV)锁相实验技术对发卡涡结构所在的三维空间流场进行了定量测量,并得到了一个完整周期内形成的发卡涡三维结构的空间流场. 结果发现,重构所得的三维发卡涡结构质量较高, 实验技术和方案具有可行性.发卡涡结构所在空间流场情况,符合目前人们对于发卡涡、高低速条带、喷射和扫掠事件的常规认识. 此外,对近壁二次流向涡、展向涡量集中区域的展向涡头和强剪切区域、与低速喷射流体相关的汇聚流动和发散流动等有了更细致的认识.同时, 也探讨了"基于二维脉动流场的相关特征去重构发卡涡三维流场"的可行性.为进一步定量探究发卡涡结构的形成演化、不同涡结构的融合及二次诱导等壁湍流相干结构问题提供思路.     

大速差射流预燃室内三维回流两相湍流的数值模拟

本文由多流体两相流模型、气相湍流κ-ε模型和颗粒湍流代数模型出发,成功地模拟了真实形状大速差射流预燃室中三维湍流回流两相流动,得到了这类复杂的气固两相流中不同纵横截面上气相速度场、颗粒速度场及浓度场和两相湍流度场的分布,并且获得了与实验定性一致的合理结果,揭示了预燃室中气固两相流动与混合的主要物理特征,探讨了大速差射流技术稳焰和强化燃烧的两相流动机理。     

大速差射流预燃室内三维回流两相湍流的数值模拟

本文由多流体两相流模型、气相湍流κ-ε模型和颗粒湍流代数模型出发,成功地模拟了真实形状大速差射流预燃室中三维湍流回流两相流动,得到了这类复杂的气固两相流中不同纵横截面上气相速度场、颗粒速度场及浓度场和两相湍流度场的分布,并且获得了与实验定性一致的合理结果,揭示了预燃室中气固两相流动与混合的主要物理特征,探讨了大速差射流技术稳焰和强化燃烧的两相流动机理。     

床面上直立圆柱的三维湍流数值模拟

从数值预报桥墩等结构物床面局部冲刷的角度发展绕直立圆柱的三维湍流的数值模拟技术. 基于Wilcox的k-ω两方程湍流模式，采用基于有限体积法的压力修正SIMPLE算法, 计算了绕床面直立圆柱的三维湍流流场，分析了光滑和粗糙床面两种情况下的流动情况. 通过系列的验证计算，表明该计算模型能够比较准确地反映不同外来流条件下绕直立圆柱的流场. 计算结果揭示了床面粗糙度对绕圆柱的湍流流动的影响.     

Flow and mixing characteristics of pulsed confined opposed jets in turbulent flow regime

A numerical study is performed on a two-dimensional confined opposed-jet configuration to gain basic understanding of the flow and mixing characteristics of pulsed turbulent opposed-jet streams. The sinusoidal pulsating flows with different temperature are imposed at opposed-jet inlets, which are mixed with each other in a confined flow channel. The current mathematical model taking the effect of temperature-dependent thermo-physical properties of fluid into account can present a good prediction for opposed-jet streams compared with experimental data. The numerical results indicate that introduction of temperature difference between opposed jet flows can lead to an asymmetric flow field immediately after jet impact, and the sinusoidal flow pulsations can effectively enhance mixing rate of opposed jets. Parameter studies are conducted for optimization of pulsed opposed jets. The effect of Reynolds number and flow pulsation as well as the configuration geometry on the mixing performance are discussed in detail. Examination of the flow and thermal field shows that the mixing rate is highly dependent on the vortex-induced mixing and residence time of jet fluid in the exit channel.     

Dynamics of single and twin circular jets in cross flow

2D-PIV is used to investigate the near field jet dynamics of a single and a tandem twin jet in cross flow at a blowing ratio of 3. The flow characteristics that were studied included jet trajectory and penetration, windward and leeward jet spread, and the size, location and magnitude of the reverse flow region. The tandem jet setup resulted in configuration where the rear jet was shielded by the front jet that allowed the rear jet to penetrate deeper into the cross stream. The penetration of front jet was found to be comparable to that of the single jet in the near field. Asymmetric jet spread was observed both in the windward and leeward side with higher jet spread in the leeward side for both the single and tandem jet configuration. Furthermore, for the tandem jet condition, the presence of rear jet was found to affect the windward spread of the front jet minimally. The windward and leeward spreads of the rear jet were observed to be decreased by the presence of the front jet. Reverse flow zones confined between the front and rear jets in tandem setup was also observed to be stronger than that of a single jet. A turbulent kinetic energy (TKE) analysis indicates that the behavior of a jet in tandem setup follows the same trend as that of an axisymmetric turbulent jet with low turbulence in the jet core and higher turbulence in the jet shear layer that subsequently transitions to high turbulence production in the jet core as the jet shear layers on windward and leeward side of the jet coalesce.     

Jet characteristics in confined swirling flow

Jets in confined swirling flow are investigated in a facility where the swirling flow in the tube is produced by a vane-type swirler. The jet is located centrally in the swirler, and the diameter ratio of the tube to the jet is 14. Both the jet and the swirling flow are fully turbulent. Results show that the confined jet is highly dissipative in nature. Consequently, the flow in the tube does not resemble a free jet with axial pressure gradient. The presence of swirl increases the rate of dissipation and the jet decays even faster. A fairly isotropic turbulence field is observed in the confined swirling flow. However, the introduction of the jet does not significantly affect this behavior and near isotropy of the turbulence field is again observed at 30 jet diameters downstream.     

Opposed round jets issuing into a small aspect ratio channel cross flow

Round jets (diameter D) discharging into a confined cross flow (dimension 3.16D × 21.05D) are investigated experimentally. Two configurations are considered: (1) a single jet (momentum flux ratio, J = 155) and (2) two opposed jets with two different momentum flux ratios (J = 60, and 155). A two-component laser-Doppler anemometer is used to make a detailed map of the normal stresses and mean velocities in the symmetry plane of the jets. In addition, smoke-wire and laser-sheet visualization are used to study the flow.

The rate of bending of the single confined jet is found to be higher than the rate of bending of an unconfined jet with the same momentum flux ratio. In the far field, the jet centerline velocity is observed to decay more slowly than the unconfined jet, indicating poor turbulent diffusion of linear momentum. Annular shear layer vortices are visualized on the upstream edge of the jet in the near field. In the far field, the flow visualization suggests that the jet loses its integrity and fragments into independent regions that are convected by the cross flow.

In the opposed jet configuration at the high momentum flux ratio (J = 155), the jets impinge in the center of the duct, and a pair of vortices is observed upstream of the impingement region. The flow visualization implies that the impingement vortices form quasi periodically and have a finite life span. In the impingement region, the jets are observed to penetrate alternately beyond the symmetry plane of the duct. In the two-jet configuration with J = 60, the jets do not impinge on each other owing to the higher rate of bending. Instead, the flow visualization indicates that the shear layers of the jets penetrate to the central region and periodically pinch off regions of the potential-like cross-flow fluid where they meet. The pinch-off regions of cross-flow fluid are convected by the turbulent flow for large distances, yet remain essentially unmixed.     

Density effects on jet characteristics in confined swirling flow

Density effects on isothermal jet mixing in confined swirling flow are investigated. The experiment is carried out with helium/air as the jet fluid in the same facility as that used by So et al. (1984a, b) and the test conditions are chosen to be the same as before. Contrary to the homogeneous mixing results, the helium jet is preserved up to 40 jet diameters downstream. The behavior of the mean and turbulence field depends highly on the initial jet velocity. Since the jets are fully turbulent and the jet momentum fluxes for inhomogeneous mixing are less than those for homogeneous mixing, the cause of this difference in behavior is directly attributed to the combined action of density difference and swirl. In spite of this, near isotropy of the turbulence field is again observed at 40 jet diameters downstream.     

Turbulent Flow and Dispersion of Inertial Particles in a Confined Jet Issued by a Long Cylindrical Pipe

In this work we examine first the flow field of a confined jet produced by a turbulent flow in a long cylindrical pipe issuing in an abrupt angle diffuser. Second, we examine the dispersion of inertial micro-particles entrained by the turbulent flow. Specifically, we examine how the particle dispersion field evolves in the multiscale flow generated by the interactions between the large-scale structures, which are geometry dependent, with the smaller turbulent scales issued by the pipe which are advected downstream. We use Large-Eddy-Simulation (LES) for the flow field and Lagrangian tracking for particle dispersion. The complex shape of the domain is modelled using the immersed-boundaries method. Fully developed turbulence inlet conditions are derived from an independent LES of a spatially periodic cylindrical pipe flow. The flow field is analyzed in terms of local velocity signals to determine spatial coherence and decay rate of the coherent K–H vortices and to make quantitative comparisons with experimental data on free jets. Particle dispersion is analyzed in terms of statistical quantities and also with reference to the dynamics of the coherent structures. Results show that the particle dynamics is initially dominated by the Kelvin–Helmholtz (K–H) rolls which form at the expansion and only eventually by the advected smaller turbulence scales.     

Unsteady phenomena of an oscillating turbulent jet flow inside a cavity: Effect of aspect ratio

Self-sustained oscillatory phenomena in confined flow may occur when a turbulent plane jet is discharging into a rectangular cavity. An experimental set-up was developed and the flow analysis has been made using mainly hot-wire measurements, which were complemented by visualisation data. Previous studies confirmed that periodic oscillations may occur, depending on the location of the jet exit nozzle inside the cavity, and also the distance between the side-walls. The present study deals with the symmetrical interaction between a turbulent plane jet and a rectangular cavity and the influence of the geometrical characteristics of the cavity on the oscillatory motion. The size and aspect ratio of the cavity were varied together with the jet width compared to that of the cavity. The study is carried out both numerically and experimentally. The numerical method solves the unsteady Reynolds averaged Navier–Stokes equations (URANS) together with the continuity equation for an incompressible fluid. The closure of the flow equations system is achieved using a two-scale energy-flux model at high Reynolds number in the core flow coupled with a wall function treatment in the vicinity of the wall boundaries. The fundamental frequency of the oscillatory flow was found to be practically independent of the cavity length. Moreover, the oscillations are attenuated as the cavity width increases, until they disappear for a critical value of the cavity width. Contour maps of the instantaneous flow field are drawn to show the flow pattern evolution at the main phases of oscillation. They are given for several aspect ratios of the cavity, keeping constant values for the cavity width and the jet thickness. The proposed approach may help to investigate further the oscillation mechanisms and the entrainment process occurring in pressure driven jet–cavity interactions.     

Concentration fields in a confined two-gas mixture and engine in-cylinder flow: laser tomography measurement by Mie scattering

Gaseous mixture concentration characterisation by Mie scatter laser tomography has been widely used, except when experimental conditions turn out to be difficult especially in confined flow. This paper describes a complete formulation of the image processing required for a confined flow including effects from wall reflection, quality of the seeding, thermodynamic flow characteristics and instantaneous laser profile disparity. Two applications are described; the first one on a free and confined jet (turbulent, isothermal, isobaric flow) which shows the potential of the technique and the processing needed to reconstitute the mean concentration field, thus demonstrating the reliability of the processing. Second, an in-cylinder engine concentration measurement is put forward. Normal running and a stratified concept are given as examples. The results show the excellent potential of the technique to help engine developers. Thus, this paper presents the problems connected with measurement of concentration fields by laser tomography and Mie scattering from a theoretical and experimental approach. Received: 12 November 1999 / Accepted: 29 January 2001     

Computation of axisymmetric confined jets in a diffuser

The paper reports on a numerical study of turbulent confined jets in a conical duct with a 5° divergence. The flow has a large ratio of jet to ambient velocities at the entrance so that it gives rise to strong recirculation. The calculations are carried out with a general finite volume method designed for calculating incompressible elliptic flows with complex boundaries. Turbulence is simulated by the standard κ–? model. The sensitivity of the solution to numerical discretization errors is examined using three convection schemes, i.e. hybrid central/upwind differencing, QUICK and SOUCUP, on two grids consisting of 68 × 50 and 102 × 82 points respectively. An examination is also made of the influence of inlet boundary conditions on the predicted flow field. The computed results are compared with experimental data for mean axial velocity, turbulent shear stress and turbulent kinetic energy profiles. It is shown that the calculations reproduce the essential features of the flow observed in the experiments.