轴对称阻塞喷注中的声源分布

本文讨论了轴对称阻塞喷注中的湍流混合噪声源的分布。分析表明,在阻塞喷注中,声源的分布十分复杂。 同时,本文讨论了阻塞喷注中湍流噪声与压力的关系;定量地分析了阻塞喷注中压力梯度对源强度和湍流噪声的影响。     

非阻塞喷注噪声的A计权声功率的计算

喷注噪声的频谱是相似的。同一种介质的非阻塞喷注噪声的声功率谱随喷注直径的减小或喷注速度的增加而向高频移动,因而喷注噪声A计权声功率与喷注直径和喷注速度都有关系。本文使用-A声功率的连续计权函数和非阻塞喷注噪声的经验声功率谱函数计算了喷注噪声的A计权声功率与总声功率比值随喷注马赫数的增加而减小的关系。这个关系也可以用于估算因频谱的移动多喷口所获得的噪声降低。     

有翼阻塞喷注

冲击伴随噪声是阻塞喷注噪声的重要成分。通常认为它是由阻塞喷注中的大尺度湍流结构和冲击气室的相互作用所产生的,理论分析和有关的实验指出,在圆形喷口处加置翼片,将同时影响阻塞喷注中的冲击气室和大尺度湍流结构;有效地降低冲击伴随噪声。     

喷注湍流噪声及扩散消声器

本篇报告的内容是我们从1974年以来所做的工作.主要介绍有关喷注噪声及其控制方面的紧密与实际相结合的基础理论研究工作.在整个工作中我们深深体会到,从广泛实践中抽象出来的基础理论研究的重要性及其对实践的指导作用. 一、喷注的湍流噪声[1] 随着工业的发展,气流喷注的出现更为频繁.一般,气流喷注产生很强的噪声对人的干扰极大.研究气流噪声的规律及寻求降低气流噪声的方法一直是一个重要的课题.四十年代由于喷气飞机的出现,推动了喷注噪声的研究,M.J.Lishthill在五十年代初从理论上推导出速度八次方定律,为愤庄噪声研究工作的先驱[2,3…     

阻塞阀门喷注中的冲击伴随噪声

冲击伴随噪声是阻塞阀门噪声的重要成分。通常认为它是由阻塞喷注中的大尺度湍流结构和冲击气室的相互作用所产生的。分析表明,对于确定的管道系统和阀门上游驻压,存在一个确定的阀门开启位置。这时阻塞阀门喷注的冲击伴随噪声最强。     

阻塞喷注的冲击噪声

冲击噪声是阻塞喷注噪声的重要部份。我们对它的声场和频谱作了详细的实验测量,由于采用外差分析技术,冲击噪声的离散谱和宽带谱可同时记录。实验表明,声场和频谱与驻压比的关系很复杂,但在与喷注垂直的方向上,距喷口1米处的总声压级,如驻压比在3到8.5之间,约为97+20logd分贝,基本不受驻压变化的影响,其中d是喷口直径,以毫米计。进一步又给出了全部驻压比范围内的总声压级的表达式。宽带谱在驻压比为2.5到7之间,离散谱在5到7之间最突出,驻压比小于2或大于8.5,阻塞喷注噪声则主要是湍流噪声。本文还给出了计算离散谱频率和宽带谱峰频的新公式;对实验中观察到的离散谱的非谐频成份,也作了解释。在喷注上游方向,观察到阻塞喷注噪声频谱具有一个或几个凹陷部份,表明在这些频率范围存在干涉现象,可能是湍流噪声与宽带冲击噪声之间的干涉现象,文中说明了这种干涉的可能性。     

阻塞喷注冲击噪声的离散谱特性

本文研究了中φ20mm收缩喷口、压力比在2.05至3.95之间的阻塞喷注冲击噪声的频谱特性,离散频谱的基频频率与压力比的关系和离散频谱的远场指向性;提出了通过测定离散频谱的远场指向性来确定冲击波第一气室到喷口的距离S和涡旋运行速度V_c与喷注速度V_J的比值N的方法。并求得,N=0.49—0.74(平均为0.61)。     

湍流喷注噪声定律的发展

本文对流体动力噪声的Lighthill理论进行了讨论,并导出与其U~8定律等值的压力定律,即噪声总功率为 W=8KD~2((P_1-P_0)~4/(ρ_0c_0P_1~2)) K即Lighthill常数,D喷口直径,P_1和 P_0分别为气室和大气压力。这个式子适用于低压冷空气喷注。进一步推广,求得高压阻塞喷注的湍流噪声、温度不同、喷注媒质不同也都适用的定律,以90°方向、距离1米处的声压级表示(dB,0dB=20μPa),得 L=80 10log((R-1)~4/(R~2-R 0.5)) 20log(TM_0/T_0M) 20logd其中,R=P/P_0,d=直径(mm),T,M为工作媒质的温度和分子量而T_0,M_O为室温及空气分子量。压力定律完全符合实验结果,它更便于在实际中应用。过去作者等提出的经验公式非常接近理论公式。     

多孔材料的出流和多孔扩散消声理论

多孔扩散消声器是七十年代发展起来的一种新型消声器。它具有体积小、结构简单和较大的减噪本领。目前这种消声器已在国外推广使用,但缺乏理论。本文详细讨论了气流通过多孔材料由于摩擦所产生的压降,提出对多孔材料的阻塞出流可以用材料的有效通道面积和驻点压降来描述。还研究了多孔扩散消声器的原理,认为扩散消声器的噪声能量由两部分组成:一部分是大量小孔喷注所发的噪声;另一部分是小孔喷注汇合成为面积较大速度较低的扩散喷注产生的噪声。在一般情况下,后者是主要的。混合后的噪声可由混合后所形成的喷注中的气流驻点压力(或气流速度)来确定。文中对此做了系统的计算,并列举出多种扩散消声器。根据这些计算,便可以设计满足实用的各种扩散消声器。 关键词：     

S_2流面非正交曲线坐标可压湍流方程组及其求解

前言 文献[1]在文献[2、3]的基础上导出了非正交曲线坐标可压湍流的基本控制方程组。在国内、外的湍流计算中,运动方程中一般都含有压力项,该项对收敛性影响较大,不太容易处理。本文用焓和熵(热力学第二定律)代替运动方程中的压力项,通过求解能量方程和     

Turbulent structure in supersonic jets with a high Reynolds number

The turbulent properties of a supersonic jet were studied related to a high level of pressure pulsation found in model jets of a reentry flight vehicle approaching the landing ground. This study comprised measurements of total pressure at a small-size target using a dynamic pressure probe placed in a free jet. The most comprehensive data about jet turbulence can be obtained by direct transformation of the pressure reading at the stagnation point near the target into the normalized velocity. The oscillogram of normalized velocity produces the velocity average value, root-mean-square value as well as turbulence intensity and turbulence spectrum. It was demonstrated that a high level of turbulence for a high-head jet retains along the supersonic core length and at the beginning of subsonic interval.     

A prediction method for the effects of flight on subsonic jet noise

The noise of a single-stream circular jet and that of a coaxial jet with coplanar nozzles of 2·5 area ratio have been measured under simulated flight conditions in the RAE 24 ft wind-tunnel. The majority of tests were conducted with the single-stream jet and primary section of the coaxial jet at a nominal temperature of 880 K. The data have been used to quantify the effect of jet temperature and were combined with measurements from an earlier test series to establish a prediction method for the effect of flight on the noise of single-stream subsonic jets. This method is based on jet noise theory modified by experimentally derived constants. For coaxial jets it is concluded that the noise reductions, which are independent of the secondary stream velocity, are predicted to an acceptable degree by the method suggested for unheated single-stream jets. The prediction methods are suitable for both OASPL's and spectra.     

An experimental study of jet noise part II: Shock associated noise

The characteristics of the sound field of shock-containing under-expanded jet flows are studied by measuring the noise from a convergent nozzle operated over an extensive envelope of supercritical jet operating conditions. The measurements were conducted in an anechoic facility. They are complementary to the turbulent mixing noise experiments (described in Part I) for subsonic and fully-expanded (shock-free) supersonic jets. The overall results from shock-containing jets are compared directly with the corresponding results from shock-free jets, and the effects of nozzle pressure ratio and jet exhaust temperature on broadband shock-associated noise are assessed independently. For a supersonic jet, the regimes of jet operating conditions, observer angles, and frequencies over which the sound field is dominated by shock-associated noise are identified. Finally, the spectral results are compared in a preliminary manner with the spectra predicted by an existing theoretical model, and good agreement is obtained in most cases.     

Acoustic properties of heated twin jets

A thorough experimental study of the noise characteristics of twin jets is presented in this paper. Twin round jets are investigated at typical jet engine conditions: that is, with heated high velocity flow. By varying the nozzle to nozzle spacing, it is possible to discriminate between the effects of turbulent mixing and acoustic shielding. As a result of this investigation, it was established that the turbulent mixing effects (both interaction noise generation and mixing suppression) occur for closely spaced nozzles. While acoustic shielding occurs at all nozzle spacings, it plays the dominant role at wide nozzle spacings. The levels of this acoustic shielding afforded by an adjacent jet can be sufficient to cause a nearly complete masking of the noise of the shielded jet. A significant discovery of this investigation was the importance of the layer of cooler, slower moving ambient air that exists between the twin jet plumes. This inter-jet layer causes acoustic refraction and reflection, and as the nozzle separation increases, the layer extends to shield more of the jet noise sources.     

Measurement of wind noise levels in streamlined probes

This paper investigates the wind noise pressure spectra measured by aerodynamically designed devices in turbulent flow. Such measurement probes are often used in acoustic measurements in wind tunnels to reduce the pressure fluctuations generated by the interaction of the devices with the incident flow. When placed in an outdoor turbulent environment however, their performance declines noticeably. It is hypothesized that these devices are measuring the stagnation pressures generated by the cross flow components of the turbulence. Predictions for the cross flow contribution to the stagnation pressure spectra based on measured velocity spectra are developed, and are then compared to the measured pressure spectra in four different probe type devices in windy conditions outdoors. The predictions agree well with the measurements and show that the cross flow contamination coefficient is on the order of 0.5 in outdoor turbulent flows in contrast to the published value of 0.15 for measurements in a turbulent jet indoors.     

层/湍流等离子体射流波动特性实验研究

本文应用电压传感器、光电倍增管及水冷皮托管,对产生射入空气中的纯氩层流和湍流等离子体射流的弧电压波动、发生器出口处的射流光强波动以及沿射流轴线的滞止压力波动进行了测量。结果显示层流等离子体射流各参数的波动幅度远小于湍流射流的对应值;弧电压的波动幅度随气流量的变化明显,但随电流的变化很小;弧电压的波动幅度与其平均值之比随电流增加呈下降的趋势。     

Characteristics of helium DC plasma jets at atmospheric pressure with multiple cathodes

A novel DC plasma torch with multiple cathodes is developed for generating laminar, transitional and turbulent plasma jets. The jet's characteristics, including jet appearance, voltage fluctuation, thermal efficiency, specific enthalpy, and distributions of temperature, pressure, and velocity, are experimentally investigated. The results show that as the gas flow rate increases, the plasma jet transforms first from the laminar state to the transitional state and second to the turbulent state. Compared with the transitional/turbulent jet, the laminar jet possesses not only a better stability and a longer hightemperature zone but also a higher average/core temperature and a higher specific enthalpy at the nozzle's outlet. With the change of jet states from the laminar to the turbulent flow, the core pressure and velocity at the nozzle's outlet increase,while the decaying rates of temperature/pressure/velocity along the jet's axial direction increase sharply. Furthermore, applications of laminar, transitional and turbulent jets for zirconia spray coating are described. The test results indicate that the long laminar jet is favorable for the deposition of a high-quality coating because the powder particles injected into the laminar jet may have better heating and lower kinetic energy.     

Shock associated noise of inverted-profile coannular jets,part III: Shock structure and noise characteristics

The basic objective of the work described in this paper is to obtain an understanding of the characteristics of shock associated noise from inverted-profile coannular jets in terms of the properties of the shock cell structure and the jet flow. To achieve this, a first-order shock-cell model is developed. Based on the concept that shock-associated noise is generated by the weak interaction between the large-scale turbulent structures in the mixing layers of the jet and the repetitive shock-cell system, formulae for the peak frequencies as well as noise intensity scaling are derived. The calculated results of these formulae agree very favorably with experimental results.     

Jittering wave-packet models for subsonic jet noise

Three simplified wave-packet models of the coherent structures in subsonic jets are presented. The models comprise convected wave-packets with time-dependent amplitudes and spatial extents. The dependence of the radiated sound on the temporal variations of the amplitude and spatial extent of the modulations are studied separately in the first two model problems, being considered together in the third. Analytical expressions for the radiated sound pressure are obtained for the first and third models.Results show that temporally localised changes in the wave-packet can lead to radiation patterns which are directional and which comprise high-amplitude bursts; such intermittency is observed in subsonic jets at the end of the potential core, and so the models may help explain the higher noise levels and intermittent character of the sound radiated to low emission angles for subsonic jets. By means of an efficiency metric, relating the radiated acoustic power to the fluctuation energy of the source, we show that the source becomes more powerful as its temporal localisation is increased. This result extends that of Sandham et al. (Journal of Sound and Vibration 294(1) (2006) 355–361) who found similar behaviour for an infinitely extended wavy-wall.The pertinence of the model is assessed using two sets of data for a Mach 0.9 jet. One corresponds to a direct numerical simulation (DNS) of a Reynolds number 3600 turbulent jet and the other to a large eddy simulation (LES) of a Reynolds number 4×10⁵ jet. Both time-averaged and time-dependent amplitudes and spatial extents are extracted from the velocity field of the numerical data. Computing the sound field generated by the wave-packet models we find for both simulations that while the wave-packet with a time-averaged envelope shows discrepancies of more than an order of magnitude with the sound field, when the wave-packet ‘jitters’ in a way similar to the intermittency displayed by the simulations, we obtain agreement to within 1.5 dB at low axial angles. This shows that the ‘jitter’ of the wave-packet is a salient source feature, and one which should be modelled explicitly.     

The distribution of headways in traffic noise simulation

In recent years researchers in jet turbulence and jet noise have become increasingly interested in what is termed “large scale coherent jet structures”. There is now considerable evidence that azimuthally coherent structures can be generated by acoustically forcing a jet from upstream. However, the evidence for such structures in unforced jets, except close to the nozzle at low Reynolds numbers, is, at best, circumstantial. The role of such structures in subsonic jet noise production is also completely unproven. In an attempt to establish a link between azimuthally coherent structures and the jet noise field a number of experimenters have recently made azimuthal cross-correlation measurements of either the near field pressure or far field noise, and used the observed coherence to infer the existence of an azimuthally coherent source field. The term azimuthally coherent is used here to infer that the source region is dominated by low order azimuthal components, with relatively little contribution coming from the higher azimuthal components. The purpose of this paper is to question the interpretation of that data. Specifically the sound field generated by a simple ring source with various types of azimuthal coherence is considered theoretically. It is shown that the azimuthal coherence of both the near and far field pressures is principally a function of the Helmholtz number and in many cases of practical interest is relatively insensitive to any coherent structure of the source.