湍流冲击射流流动与传热的数值研究进展

湍流冲击射流由于其冲击表面时具有很高的局部传热率和冲击力,被广泛应用于如表面的加热、电子元件的冷却、纸张的干燥和材料的切割等工程应用和工业过程中.由于其流动的复杂性,也常被作为一种理想的测试实例来评价湍流模型的性能.此外,湍升力射流与地面之间的空气动力作用对V/STOL (垂直或短距离起落)飞机的性能具有很大的影响.长期以来,人们从理论分析、实验测量和数值模拟方面对冲击射流进行了广泛而系统的研究,积累了丰富的资料.本文在分析了湍流冲击射流的数值研究现状的基础上,对近年来有关湍流冲击射流流动与传热的数值研究方面的文献有选择地进行了综述,重点评述了不同湍流模型对冲击射流流动与传热的预测能力,讨论了存在的问题并对该领域今后的研究方向进行了展望.      

电流动聚焦中非牛顿流体射流影响因素的实验研究

在一套流动聚焦装置上加载高压直流电场形成电流动聚焦,并开展了非牛顿流体带电射流的不稳定性特性实验研究。实验在自行设计的装置系统上完成,获得了电流动聚焦中非牛顿流体射流的流动状态,考察了不同控制参数下射流形态的变化。结果表明,由于非牛顿流体具有粘弹性,与牛顿流体相比,非牛顿流体带电射流体现了更复杂的流动特点。这些实验结果为我们理解复杂条件下非牛顿流体射流的流动机理提供了参考,也有助于深入的实验分析和理论研究奠定了基础。     

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。     

低速轴对称层流射流流动形态和失稳机制的实验研究

本文应用染色液和悬浮粒子显示方法,进一步实验研究轴对称层流射流的流动形态及其失稳机制。首次成功地在从一定口径的喷嘴流出的低速轴对称层流射流中观察到环形回流流动。给出了射流随速度演化及实验容器边界对其流动形态影响的显示照片,发现实际射流的轴对称波动及失稳过程正好对应射流在容器底部产生的环形旋涡的生长和破碎过程。本文认为由于实验空间有限尺度对流动的限制改变了原来射流的流动形态和流场空间的拓扑性质,射流与实际边界的相互作用对实际射流的失稳和转捩有重要的影响。     

横流冲击射流尾迹涡结构的实验研究

本文采用LIF（激光诱导荧光）流动显示和PIV（粒子图像速度场仪）测量对横流冲击射流的尾迹涡结构进行了实验研究。水槽实验是在三种流速比和两种冲击高度实验工况下进行的。由实验结果可得到两种明显的尾迹涡结构、，即射流尾迹涡和横流尾迹涡。横流冲击射流中形成的主要尾迹涡结构主要依赖于流速比。本文还对横流冲击射流近区范围内射流尾迹涡和横流尾迹涡的形成机理和演化特征进行了分析。     

高速液体射流流动特性的实验测试方法

基于RLPG高压喷射环境建立了射流流动特性的实验测定系统和分析方法．利用这种方法测出高压下喷口的流量系数,得到射流头部贯穿速度及射流核形态．     

具有壁面射流激励的圆管内气相流动大涡模拟

采用大涡模拟方法和单方程亚格子模式对小尺度量进行模拟。研究了不同强度壁面射流激励对圆管内气相流动的影响,模拟结果给出了射流对瞬态拟序结构发展、时平均流向速度分布的影响。随着射流强度的增加,射流入口附近流体的回流现象增强。射流强度足够大时可以减小管壁处的切应力值,同时会减小壁面附近流动速度,这种速度分布会导致气体夹带颗粒的能力下降,从而在实际两相流动中容易造成壁面附近的气粒返混现象。     

气流作用下同轴带电射流的不稳定性研究

通过对气体驱动同轴电流动聚焦的实验模型进行简化,开展了电场力和惯性力共同作用下同轴带电射流的不稳定性理论研究.在流动为无黏、不可压缩、无旋的假设下,建立了三层流体带电射流物理模型并得到了扰动在时间域内发展演化的解析形式色散关系,利用正则模方法求解色散方程发现了流动的不稳定模态,进而分析了主要控制参数对不稳定模态的影响.结果表明,在参考状态下轴对称模态的最不稳定增长率最大,因此轴对称扰动控制整个流场.外层气流速度越高,气体惯性力越大,射流的界面越容易失稳.内外层液-液同轴射流之间的速度差越大,射流越不稳定.表面张力对射流不稳定性起到促进作用.轴向电场对射流不稳定性具有双重影响:当加载电场强度较小时,射流不稳定性被抑制;当施加电压大于某一临界值时,轴向电场会促进射流失稳.临界电压的大小与界面上自由电荷密度和射流表面扰动发展关系密切.这些结果与已有的实验现象吻合,能够对实验的过程控制提供理论指导.     

燃气射流冲击传热特性的数值模拟

针对射流传热问题,利用基于RNGk-ε湍流模型的数值方法模拟了射流垂直冲击平板的流动过程,并与实验数据比较,验证了模型的可行性。在此基础上,以火箭喷管入口参数为入口条件,建立了超音速燃气射流垂直冲击平板和冲击浸没平板的计算模型,分析了不同冲击条件下努塞尔数分布规律和温度分布规律, 论述了超音速射流传热的特性及影响传热特性的因素。得到了冲击距离为(14~18)D的努塞尔数取值范围,并表明冲击距离和射流温度是影响传热效率的关键因素;冲击距离增加,传热效率降低,冲击平板表面的射流温度越高,传热效率越高。     

射流冲击水垫塘入射点旋涡掺气特性的研究

通过实验研究了不掺气射流冲击水垫塘入射点旋涡掺气的特性。实验结果表明：空气从射流入射点被卷入后，沿射流剪切层扩散；入射点掺气浓度随水垫深度的减小而增加；剪切层内水气混合体的流速分布具有误差函数形式；在剪切层内、外区，其掺气浓度分别符合高斯分布，与理论计算吻合良好。另外，还从理论上导出了射流入射点附近水气混合层的厚度，并给出了计算掺气量的公式。     

高压水射流冲击煤体的力学特征

以质量守恒与动量守恒定律为基础, 建立了高压水射流冲击煤体的力学模型。运用此模型分析了高压水射流在冲击煤体的过程中, 未破水体、破碎水体、煤体的破碎区与扩孔区的力学特征, 利用严格的力学守恒关系得出高压水射流冲击煤体的简化常微分方程组。将理论计算结果与现场实验和数值模拟结果进行对比, 结果表明:理论计算结果与数值模拟结果和实验结果基本一致。此模型具有明确的力学意义, 且能够反映真实的冲击过程。     

Computations for a jet impinging obliquely on a flat surface

A SIMPLE-C algorithm and Jones-Launder k-ε two-equation turbulence model are used to simulate a two-dimensional jet impinging obliquely on a flat surface. Both the continuity and momentum equations for the unsteady state are cast into suitable finite difference equations. The pressure, velocity, turbulent kinetic energy and turbulent energy dissipation rate distributions are solved and show good agreement with various experimental data. The calculations show that the flow field structure of the jet impinging obliquely on a flat surface is strongly affected by the oblique impingement angle. The maximum pressure zone of the obliquely impinging jet flow field moves towards the left as the oblique impingement angle is decreased.     

Parametric study on impinging-jet liquid sheet thickness distribution using an interferometric method

 The effects of several influencing parameters on the thickness distribution of a liquid sheet formed by two impinging jets are examined using a simple interferometric method which can provide sharp and clear fringe images in spite of very simple optical setup. The parameters changed in this study are the impinging angle, orifice diameter, liquid jet velocity, and liquid viscosity. The experimental results are compared with existing theoretical predictions. A larger impinging angle, smaller orifice diameter, and a higher azimuthal angle are found to result in thinner liquid sheets, although jet velocity itself does not have any effect on the sheet thickness. Higher viscosity, on the other hand, is found to yield a thicker liquid sheet, which has not been considered in the theoretical analysis so far. The theoretical models used for comparison predict the same trend as the experimental results, but the thickness values are overestimated at low azimuthal angles. This discrepancy is gradually lessened as the azimuthal angle is increased. More refined theoretical modeling that decreases this difference and includes the effect of physical properties of liquid is necessary. Received: 26 May 2000/Accepted: 8 November 2000     

Turbulent flow structure and bubble distribution in an axisymmetric nonisothermal impinging gas-liquid jet

The flow structure of a bubbly impinging jet in the presence of heat transfer between the two-phase flow and the surface is numerically investigated on the basis of the Eulerian approach. The model uses the system of Reynolds-averaged Navier–Stokes equations in the axisymmetric approximation written with account for the inverse effect of the bubbles on the average and fluctuating flow parameters. The influence of the gas volumetric flow rate ratio and the dimensions of the bubbles on the flow structure in a gas-liquid impinging jet is studied, In the presence of gas bubbles the liquid velocity is higher than the corresponding value in the single-phase flow. A considerable, more than twofold, anisotropy between the axial and radial turbulent fluctuations in the gas-liquid impinging jet is shown to exist. An addition of air bubbles leads to a considerable growth in the liquid velocity fluctuations in the two-phase flow (up to 50% compared with the single-fluid liquid impinging jet). An increase in the disperse phase dimensions leads to intensification of turbulence of the liquid.     

Large-Eddy Simulations of Forced Three-Dimensional Impinging Jets

Large-eddy simulations of the flow field around twin three-dimensional impinging jets were carried out to simulate the near-ground hover configuration of a vertical takeoff and landing (VTOL) aircraft. Both the impinging jet and the upwash caused by the collision of the wall jets are modeled in this study. The evolution of the vortical structures in the impinging jet flow field, due to the introduction of axisymmetric and azimuthal perturbations at the jet exit, has been investigated. The vortical structures formed in the jet shear layer due to azimuthal forcing, show significant three-dimensional vortex stretching effects when compared to the structures formed during axisymmetric forcing. Breakdown of the large-scale structures into smaller vortices also occurs much earlier during azimuthal forcing. When compared to the upwash formed during axisymmetric forcing, the azimuthally perturbed jet forms an upwash that is less coherent and results in a weaker upload or lift-off force on the aircraft undersurface. Comparison with available experimental data indicates good agreement for the centerline velocity decay, the wall pressure variation and the phase speed of the vortical structures.     

Self-sustained oscillations of a confined impinging jet

The present paper investigates the dynamics of a laminar plane jet impinging on a flat plate in a channel. An experimental parametric study is carried out to determine the flow regimes at different levels of confinement and Reynolds numbers. For very confined jets, the flow is steady whatever the Reynolds number. The overall structure of the flow is symmetric with respect to the jet axis and is characterized by the presence of recirculation zones at the channel walls. The dynamics is radically different for less confined jets. Above a critical Reynolds number, the flow bifurcates in the form of an oscillating flapping mode of the impinging jet. Analyses of the experimental results provide with a quantitative characterization of this regime in terms of amplitude, wavelength and frequency. This self-oscillating bifurcated flow induces strong sweepings of the target plate by the jet and intense vortex dipole ejections from the impacted wall. Such a regime is expected to be particularly useful in the enhancement of the local heat transfer at relatively low cost in terms of flow rate.     

Relationship between vortex and sound in under‐expanded supersonic impinging jets

The study of an under‐expanded supersonic jet impinging on a flat plate by using large‐eddy simulation is reported. A third‐order upwind compact difference and a fourth‐order symmetric compact scheme are employed to discretize the nondimensional axisymmetric compressible Favre‐filtered Navier–Stokes equations in space, whereas the third‐order Runge–Kutta method with the total variation diminishing property is adopted to deal with the temporal discretization. The numerical simulation successfully captures the shock wave and vortex structures with different scales in the flow field. Waves with high and low frequencies traveling forward and reflecting back, and sound sources in different locations can be observed. By comparison with the frequency of the impinging tone from the experiment, it can be deduced that the change of pressure and swirling strength in the shear layer, pressure change on the impinging plate, and vortex merging in the jet shear layer are interdependent with the impinging tone. The effects of nozzle lip thickness on the impinging jet flow field have been investigated. The results show that the values of pressure fluctuation and vortex swirling strength in the shear layer near the nozzle have an extremum with the variation of the nozzle lip thickness. The results provide a theoretical foundation for the design of supersonic nozzles. Copyright © 2011 John Wiley & Sons, Ltd.     

Impingement of a supersonic underexpanded jet onto obstacles with different permeabilities

Results of an experimental study of the flow structure and characteristics of flow oscillations induced by impinging of a supersonic underexpanded jet onto obstacles made of permeable and impermeable materials are presented. Spectral characteristics of the acoustic field in the regime of unsteady interaction between the jet and the obstacle are determined. The influence of permeability of porous obstacles on these characteristics is identified.