圆柱形充液室中4股贴壁燃气射流扩展特性的实验研究

为了探索高温高压周向均布4股贴壁燃气射流在受限空间中的扩展特性,设计了贴壁燃气射流在圆柱形充液室内扩展的实验装置,借助数字高速录像系统,观察了4股贴壁燃气射流在充液室中的扩展过程,发现由Kelvin-Helmholtz不稳定性引起的表面不规则一直存在于整个射流扩展过程;通过处理拍摄记录的射流扩展序列图,获得不同时刻射流扩展的轴向和径向位移; 对比了不同破膜喷射压力和喷孔结构参数对4股贴壁燃气射流扩展过程的影响。实验结果表明:喷孔面积越大,贴壁射流初期轴向扩展速度越大,但由于径向扩展达到交汇的时间较早,湍流掺混和干涉强烈,衰减也越快;破膜喷射压力越高,射流径向扩展到达交汇的时间越短; 破膜喷射压力从12 MPa升高到20 MPa,射流轴向扩展速度大幅增加,气液湍流掺混效应增强。     

应用保角变换法的亚音速矩形湍流射流理论研究

为求解亚音速矩形湍流射流的速度场结构,采用复变函数中的保角变换法将圆形喷口平面转化为矩形喷口平面;进而将已有的关于轴对称喷口射流流场的相关理论拓展到矩形喷口射流研究中,避免了在数值模拟中引入辅助方程、离散化、设置边界条件来求解三维N-S方程。通过该方法得到了矩形喷口宽高比AR分别为2、4、8时对射流结构的定量影响;求出了喷口尺寸为1×5mm2、喷口马赫数Ma=0.3时,喷口对称中心的宽、窄对称面内的速度云图。结果表明:宽高比AR越大,射流核心段长度越长,轴向衰减越慢;喷口面积为10cm2、20cm2时,加大宽高比可以缩短核心段长度且可加快轴向速度衰减,在射流宽、窄对称面都存在射流核心区与衰减区。首次提出速度分布的"马鞍面"主要集中在射流核心段内。通过与现有实验研究结果对比发现:核心段长度误差最小达到30%;轴向速度分布与实验结果吻合度也很高,误差在5%以内。因此,本文理论研究的正确性得到验证。本文所得结论可为进一步研究亚音速矩形射流结构与矩形喷口参数优化设计提供参考。     

带制退器的膛口射流噪声数值模拟与实验研究

为了研究膛口装置对膛口噪声气动特性的影响，对带膛口制退器的某小口径武器的膛口射流噪声进行了数值模拟和实验研究。采用计算流体力学CFD (computational fluid dynamics)-计算气动声学CAA (computational aeroacoustics)耦合算法对膛口噪声进行数值模拟，即对膛口流场进行瞬态CFD模拟，获取流场数据，然后利用所得到的结果采用声学方程模拟声源信息求解声场。基于数值模拟结果，分析了膛口流场变化及噪声的指向性分布，并与实验结果进行了对比。研究表明:膛口制退器的安装改变了膛口流场结构，影响了膛口射流噪声的指向性分布。计算结果与实验结果的误差小于9%，验证了该计算方法的可行性。研究结果可为膛口射流噪声的预测及膛口制退器的设计提供一定的参考。     

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。     

基于五阶WENO格式的燃气在药床中流动过程二维两相流研究

为研究内弹道初始阶段中心点火管燃气在膛内药床中的流动特性和传播规律,设计了可视化点传火实验平台,并进行了膛内假药床的点传火实验。基于加权本质无震荡(weighted essentially non-oscillatory, WENO)格式,构造了膛内轴对称二维内弹道两相流模型,对膛内燃气在假药床中的流动过程进行数值模拟。计算结果与可视化实验结果符合较好,全局压力平均误差为5.35%。表明数值计算准确地描述了燃气流动特性,完整地呈现了点火管燃气在假药床中的发展过程。在点火初始阶段,膛内压力径向效应明显,气相沿径向传播较快,药床药粒基本不会发生运动;随着燃气逐渐在膛内传播,膛内压力呈现径向一致、轴向梯度分布的特征,在压力梯度作用下,气相轴向速度开始占据主导,径向速度在膛底和中部区域减小为零,而固相速度随气相速度变化而变化;气相在到达弹底前,由于固相颗粒的壅塞,会提前出现速度反向波动现象。     

出口条件对圆形湍流射流远场区自保持性的影响

报道出口条件对圆形湍流射流自保持性影响的实验研究结果. 对来自渐缩和长管两种不同结构喷嘴的射流,在相同雷诺数条件下,沿轴线进行了速度测量; 研究的统计量包括平均速度、湍流强度、高阶矩、能谱和积分尺度. 实验结果表明,渐缩喷嘴射流比长管射流发展得更快、更容易达到自保持状态. 通过对比发现,在两射流的速度(温度)场中,平均速度(温度)、湍流强度、偏斜因子和平坦度因子都存在明显的异同. 同时发现两射流的积分尺度随轴向距离的增加都成线性增长,且在渐缩喷嘴射流中增长得更快. 通过对比两射流的边界层厚度、径向与轴向湍流强度的比值、湍动能能谱图并结合前人的研究结果,对两射流湍流场所表现出的不同的统计学行为给出了合理的解释.      

小口径膛口射流噪声的数值模拟

为了研究小口径武器的膛口气动噪声特性,采用CFD-CAA耦合算法对7.62 mm枪的射流噪声场进行了数值模拟。由于膛口流场结构复杂,在目前的计算发展水平下还不足以采用CAA直接法,因而本文中采用混合方法,即首先采用CFD方法计算7.62 mm枪的膛口流场,然后利用所得结果,采用声学方程计算射流噪声,具体为膛口近场采用LES进行计算,远场声场采用FW-H声拟法计算。通过对比验证实验,验证了该计算方法的可行性。然后,对7.62 mm枪射流噪声进行了数值模拟,分析了噪声指向性,绘制了声压级云图。研究表明:在本文的计算条件下,射流噪声强度主要集中在近膛口区域;且射流最大噪声主要分布在与轴线方向成30°~60°范围内。     

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。     

侧向多喷口干扰复杂流动数值模拟研究

采用具有高分辨率的NND格式,通过数值求解N-S方程对典型外形多喷口侧向喷流复杂干扰流动进行了数值模拟. 为了提高计算效率,采用了LU-SGS隐式算法. 采用分块对接网格技术,生成高质量的贴体计算网格,精确模拟喷口截面. 对比分析了不同计算格式、限制器形式、网格拓扑及流动形态(层流与湍流)对喷流干扰流场结构和压力分布特性的影响,研究和分析了喷口附近流场的涡系结构、波系结构和喷流干扰引起的气动力特性. 在上述研究的基础上,针对典型飞行器外形的侧向喷流干扰特性进行了详细的数值模拟,得到了喷口参数(喷口位置、数目等)及来流条件对喷流干扰流场结构、气动力特性的影响规律,并对其流动机理进行了相应的分析. 研究表明,发展的针对多喷口侧喷干扰的数值计算方法是成功的,可以应用于飞行器侧向喷流干扰的流场结构分析及气动力特性数值预测.      

轴对称直喷管的射流噪声特性实验研究

为了研究喷管射流噪声特性问题,应用丹麦B.K.公司3560C型多用途分析系统,对抽对称直喷管在不同压比下产生的亚音速和跨音速射流噪声进行了测量.并对外流场轴向和截面流动特性进行了测量和分析,确定了测量噪声探头放置位置,比较了离模型截面不同位置处的噪声.给出频率在5000HZ内噪声变化的规律,发现在低频时随着静压比的增加...     

Interaction between periodic disturbances and a turbulent jet

The results of a numerical and experimental investigation of the interaction between harmonic disturbances and a turbulent jet are presented. On the basis of large eddy simulation it is established that the narrow-band noise of a supersonic jet considerably increases, when the forcing amplitude amounts to thousandths and more of the total pressure of the flow within the nozzle. An analysis of the results of a laboratory experiment on the measurement of the longitudinal velocity spectra in the core of a low-velocity jet shows that the acoustic disturbances generated by a fan inside the nozzle lead to the generation of intense tonal hydrodynamic disturbances in the low-velocity jet.     

小口径轴对称收缩喷嘴射流冲击大平板噪声特性的实验研究

为研究小口径喷嘴冲击射流的噪声特性，测定了3mm口径的轴对称收缩喷嘴在各种压比情况下产生的亚音速和超音速射流冲击坚固大平板产生的噪声。发现噪声在空间呈近似四瓣分布，当喷嘴与平板距离减小时，噪声指向壁射流下游的瓣到增强，反之，噪声指向喷嘴上游的瓣得到增强。噪声随喷嘴距平板距离的增加呈增强的趋势，在距平板一定距离内有锯齿现象。噪声随喷嘴压比的增加而增强，相应于各种工况，存在一不同的压比值，此压比之前，噪声随压比的增大而迅速提高，但有起伏现象，在此压比之后，噪声平缓地随压比的增大而增强。     

Computational analysis of exit conditions on the sound field of turbulent hot jets

A hybrid computational fluid dynamics (CFD) and computational aeroacoustics (CAA) method is used to compute the acoustic field of turbulent hot jets at a Reynolds number $Re=316,000$ and a Mach number $M=0.12$. The flow field computations are performed by highly resolved large-eddy simulations (LES), from which sound source terms are extracted to compute the acoustic field by solving the acoustic perturbation equations (APE). Two jets are considered to analyze the impact of exit conditions on the resulting jet sound field. First, a jet emanating from a fully resolved non-generic nozzle is simulated by solving the discrete conservation equations. This computation of the jet flow is denoted free-exit-flow (FEF) formulation. For the second computation, the nozzle geometry is not included in the computational domain. Time averaged exit conditions, i.e. velocity and density profiles of the first formulation, plus a jet forcing in form of vortex rings are imposed at the inlet of the second jet configuration. This formulation is denoted imposed-exit-flow (IEF) formulation. The free-exit-flow case shows up to 50% higher turbulent kinetic energy than the imposed-exit-flow case in the jet near field, which drastically impacts noise generation. The FEF and IEF configurations reveal quite a different qualitative behavior of the sound spectra, especially in the sideline direction where the entropy source term dominates sound generation. This difference occurs since the noise sources generated by density and pressure fluctuations are not perfectly modeled by the vortex ring forcing method in the IEF solution. However, the total overall sound pressure level shows the same qualitative behavior for the FEF and IEF formulations. Towards the downstream direction, the sound spectra of the FEF and IEF solutions converge.     

Experiments on the Flow Field and Acoustic Properties of a Mach number 0·75 Turbulent Air Jet at a Low Reynolds Number

In this paper we present the experimental results of a detailed investigation of the flow and acoustic properties of a turbulent jet with Mach number 0·75 and Reynolds number 3·5 10³. We describe the methods and experimental procedures followed during the measurements, and subsequently present the flow field and acoustic field. The experiment presented here is designed to provide accurate and reliable data for validation of Direct Numerical Simulations of the same flow. Mean Mach number surveys provide detailed information on the centreline mean Mach number distribution, radial development of the mean Mach number and the evolution of the jet mixing layer thickness both downstream and in the early stages of jet development. Exit conditions are documented by measuring the mean Mach number profile immediately above the nozzle exit. The fluctuating flow field is characterised by means of a hot-wire, which produced radial profiles of axial turbulence at several stations along the jet axis and the development of flow fluctuations through the jet mixing layer. The axial growth rate of the jet instabilities are determined as function of Strouhal number, and the axial development of several spectral components is documented. The directivity of the overall sound pressure level and several spectral components were investigated. The spectral content of the acoustic far field is shown to be compatible with findings of hot-wire experiments in the mixing layer of the jet. In addition, the measured acoustic spectra agree with Tam’s large-scale similarity and fine-scale similarity spectra (Tam et al., AIAA Pap 96, 1996).     

Pressure pulsations on an obstacle in a jet

At the present time, much attention is devoted to auto-oscillations that arise from the interaction between a supersonic underexpanded jet and an obstacle that it encounters at right angles [1, 2]. There are far fewer data on the pressure pulsations on an obstacle in the absence of auto-oscillations [3–6]. However, in many cases the highest total levels of the pressure pulsations are observed when the barrier is situated at fairly large distances from the nozzle opening and the pressure pulsations have a random nature. We have investigated the pressure pulsations on a plate normal to a supersonic strongly underexpanded jet. The pulsation characteristics were measured for an arrangement of the obstacle when auto-oscillations are absent. We have established dependences that generalize the results of measurement of the pulsation characteristics at both subsonic and supersonic velocities on the jet axis directly in front of the obstacle. We have also investigated the correlation between the pressure pulsations on the plate and external acoustic noise. We have obtained the dependence of the level of the acoustic noise on the value of the maximal pressure pulsations on the plate.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 163–167, January–February, 1980.     

Numerical Analysis of the Impact of the Interior Nozzle Geometry on Low Mach Number Jet Acoustics

The flow and acoustic fields of subsonic turbulent hot jets exhausting from three divergent nozzles at a Mach number M=0.12 based on the nozzle exit velocity are conducted using a hybrid CFD-CAA method. The flow field is computed by highly resolved large-eddy simulations (LES) and the acoustic field is computed by solving the acoustic perturbation equations (APE) whose acoustic source terms are determined by the LES. The LES of the computational domain includes the interior of the nozzle geometry. Synthetic turbulence is prescribed at the inlet of the nozzle to mimic the exit conditions downstream of the last turbine stage. The LES is based on hierarchically refined Cartesian meshes, where the nozzle wall boundaries are resolved by a conservative cut-cell method. The APE solution is determined on a block structured mesh. Three nozzle geometries of increasing complexity are considered, i.e., the flow and acoustic fields of a clean geometry without any built-in components, a nozzle with a centerbody, and a nozzle with a centerbody plus struts are computed. Spectral distributions of the LES based turbulent fluctuated quantities inside the nozzle and further downstream are analyzed in detail. The noise sources in the near field are noticeably influenced by the nozzle built-in components. The centerbody nozzle increases the overall sound pressure level (OASPL) in the near field with respect to the clean nozzle and the centerbody-plus-strut nozzle reduces it compared to the centerbody nozzle due to the increased turbulent mixing. The centerbody perturbed nozzle configurations generate a remarkable spectral peak at S t=0.56 which also occurs in the APE findings in the near field region. This tone is generated by large scale vortical structures shed from the centerbody. The analysis of the individual noise sources shows that the entropy term possesses the highest acoustic contribution in the sideline direction whereas the vortex sound source dominates the downstream acoustics.     

Experimental investigation of the role of instability waves in noise radiation by supersonic jets

Existing ideas of instability waves as the main dynamic noise sources in supersonic jets are tested for conformity with the data of acoustic measurements of this noise. Methodologically, the problem consists in the verification of the main principles of Tam’s theory of noise radiation by supersonic jets based on the ideology of instability waves in the shear layer of the jet and their key role in noise generation. Technologically, the study is based on a new technique for measuring the noise, namely, the azimuthal decomposition method developed by the authors. It is shown that on the Strouhal number range from 0.03 to 0.35 the theory satisfactorily describes the radiation pattern of the individual harmonics, while the initial amplitudes of the instability waves are in qualitative agreement with the assumption of their uniform distribution near the nozzle edge.     

Flow in a viscous jet escaping through a supersonic nozzle into a semi-infinite ambient space

The problem of an axisymmetric gas flow in a supersonic nozzle and in the jet escaping from the nozzle to a quiescent gas is solved within the framework of Navier-Stokes equations. The calculated pressure distribution is compared with that measured in the jet by a Pitot tube. The influence of the jet pressure ratio, Reynolds number, and half-angle of the supersonic part of the nozzle on nozzle flow and jet flow parameters is studied. It is shown that the distributions of gas-dynamic parameters at the nozzle exit are nonuniform, which affects the jet flow. The flow pattern for an overexpanded jet shows that jet formation begins inside the nozzle because of boundary-layer displacement from the nozzle walls. This result cannot be obtained with the inviscid formulation of the problem.     

基于模态分解的轴对称超声速射流啸声产生位置数值分析

不完全膨胀超声速射流的势核中会产生准周期的激波栅格结构, 其与剪切层内拟序结构的相互作用会产生激波噪声. 啸声是主要向上游方向传播的、具有离散频率的高强度激波噪声, 其产生是受一种非线性的声反馈环机制驱动. 精确定位啸声的声源位置是定量理解啸声反馈环机制和发展准确的啸声预测模型的一个关键所在. 为了分析近场啸声, 本文采用高精度数值方法直接求解轴对称可压缩Navier-Stokes方程, 数值模拟了完全膨胀射流马赫数为1.10和1.15的圆形声速喷管欠膨胀超声速冷射流, 得到了A₁和A₂两种轴对称模态啸声. 通过傅里叶模态分解、本征模态分解和动态模态分解, 分析了射流时序压力场和速度场, 研究了啸声关联拟序流动结构的空间演化, 精确定位了轴对称模态啸声的声源位置. 研究表明: 啸声关联拟序流动结构存在饱和态区域, 啸声声波是在其饱和态区域产生并向外传播; 在本文所涉及的射流马赫数范围内, A₁和A₂两种轴对称模态啸声的有效声源位置分别是在第4和第3个激波栅格结构的尾缘.      

Investigation of downstream and sideline subsonic jet noise using Large Eddy Simulation

The sound fields radiated by Mach number 0.6 and 0.9, circular jets with Reynolds numbers varying from 1.7×10³ to 4×10⁵ are investigated using Large Eddy Simulations. As the Reynolds number decreases, the properties of the sound radiation do not change significantly in the downstream direction, whereas they are modified in the sideline direction. At low Reynolds numbers, for large angles downstream from the jet axis, the acoustic levels are indeed remarkably lower and a large high-frequency part of the sound spectra vanishes. For all Reynolds numbers, the downstream and the sideline sound spectra both appear to scale in frequency with the Strouhal number. However their peak amplitudes vary following two different velocity exponents according to the radiation direction. The present observations suggest the presence of two sound sources: a Reynolds number-dependent source, predominant for large radiation angles, connected to the randomly-developing turbulence, and a deterministic source, radiating downstream, related to a mechanism intrinsic to the jet geometry, which is still to be comprehensively described. This view agrees well with the experimental results displaying two distinguishable components in turbulent mixing noise [1, 2].