矩形喷口欠膨胀超声速射流对撞的实验研究

在不同喷口间距和射流压力下开展了矩形喷口欠膨胀超声速射流对撞实验并与自由射流进行了对比. 实验表明:超声速射流对撞的辐射噪声中存在四种不同的啸音模式, 且随喷口距离和射流压力的变化在不同模式间切换. 在射流压力大于0.5 MPa且喷口间距小于50 mm时, 射流对撞面在两个喷口外形成两道正激波之间, 啸音基频维持在3 kHz左右. 随喷口间距的增大或射流压力的降低, 射流对撞面在一侧喷口外的弓形激波与另一侧喷口外的正激波之间. 对撞面也有可能出现在两个弓形激波之间, 对应的啸音基频约为9 kHz, 但容易受扰动而回到喷口一侧或是在喷口之间大幅度振荡. 当射流压力小于0.36 MPa且喷口间距大于70 mm后, 对撞面在两个喷口之间大幅度振荡, 产生基频在1 kHz左右并随射流压力的降低和喷口间距的增大而降低的啸音. 关键词： 超声速射流 啸音 射流对撞 激波     

On the sound sources of screech tones radiated from choked circular jets

The generation mechanism of the screech tone in the helical oscillation mode is mainly investigated using a series of instantaneous schlieren photographs. From the photographs, five evanescent sound sources are observed as prominent points along the jet axis. The sound source for the dominant helical oscillation mode is found to be the second prominent point which moves along a circular orbit in a plane perpendicular to the jet axis and just downstream of the rear edge of the third shock cell. It is shown that the speed of a moving sound source is supersonic and that the Mach cone generated by the moving sound source forms the helical-shaped wave front of the screech tone for the helical oscillation mode of the jet. This idea of the moving sound source is well supported by a measured directionality of the screech tone. Sound sources of the other oscillation modes appearing in the other pressure ratio ranges are also described.     

The connection between sound production and jet structure of the supersonic impinging jet

An experimental investigation into the sound-producing characteristics of moderately and highly underexpanded supersonic impinging jets exhausting from a round convergent nozzle is presented. The production of large plate tones by impingement on a square plate with a side dimension equal to 12 nozzle exit diameters is studied using random and phase-locked shadowgraph photography. Discrete frequency sound is produced in the near-wall region of the jet when a Mach disk occurs upstream of the standoff shock wave. Tones cease when the plate distance is approximately 2.2 free-jet cell lengths and the first and second shock waves are located in the free-jet positions. The production of impulsive sound appears to be associated with the collapse of the standoff shock wave during a portion of the oscillation cycle. Results from unsteady plate-pressure measurements indicate that plane-wave motion occurs in the impingement region and a secondary pressure maximum is observed on the plate adjacent to the flow region where sound appears to originate.     

Near-field acoustic characteristics of screech jet exhausted from a nozzle with a hard reflecting plate

The standing wave in the near field of the screech jet exhausted from a nozzle with a hard plate works on the jet flow as the forcing wave by the location of a reflecting plate, and then jet flow is considered to be changed. Moreover, the reflector location from the nozzle changes the sound pressure contours of the near field. Intensity maps of the screech tone which indicate the propagation to the jet axial direction or the radial direction of the jet by the presence of the reflector plate have not been explored. In the present paper, acoustic characteristics in the near field of the screech tone with the reflecting plate are studied using an optical wave microphone, which can measure the sound propagating for both vertical and horizontal directions to the jet axis. As a result, the standing wave in the near field of the screech jet with the reflector has two types: One is the standing wave between the hydrodynamic pressure fluctuation propagating jet downstream and the sound pressure propagating upstream, and the other is the standing wave by the difference between the wavelength of the sound wave and the wavelength at the place close to the jet.     

超音喷流啸音发声机理的实验研究

啸音是超音速喷流噪声的三大成分之一,其特点是向上游传播的离散纯音,被认为是飞机部件声疲劳的重要因素之一。但到目前为止,啸音仍是喷流噪声研究中理解最少、预测能力最低的成分,研究表明这主要是因为啸音受环境的影响很大,如啸音的强度就受到喷流马赫数、喷流温度、喷嘴唇口厚度以及形状等的影响。本文通过实验研究了在超音速喷流情况下喷嘴唇口厚度对啸音幅值、模态等的影响。实验结果表明,随着喷嘴厚度的增加,啸音的幅值增加达到了10 db以上。啸音的频率则随着唇口厚度的增加有所减小。     

Numerical analysis of gas-dynamic instabilities during the laser drilling process

The use of high-pressure gas jets in the laser-drilling process has significant influence on the melt ejection mechanism. These jets are highly unstable and this directly relates to the gas pressure and the geometry of the hole being drilled. The evolution of gas-dynamic instabilities during the laser-drilling process was investigated numerically. A minimum length nozzle (MLN) with a 300 μm throat diameter was modelled at various gas pressures, with the gas jet impinging on a range of simulated holes with different aspect ratios. The simulations predict the formation of surface pressure fluctuations that have a broad spectrum due to both the turbulent nature of the jet and the blunt shock oscillation on the surface. The surface pressure variations and the blunt shock oscillation govern the gas dynamic conditions inside the hole, which strongly influence the melt ejection phenomena during the laser-drilling process.     

Digital spectral analysis of the noise from short duration impulsively started jets

Techniques in which a shock tube is used to produce short duration jets are discussed briefly. The method adopted involves using the shock tube as a static reservoir with the jet exhausting through a nozzle originally closed by a diaphragm. Short duration noise samples of a Mach 0·9 air jet are recorded digitally and narrow band and one-third octave spectra are evaluated. Average spectra from a number of samples are presented. Comparison with both digital and analogue spectra from the equivalent continuous jet demonstrates that it is possible to obtain meaningful spectra by averaging short duration samples of impulsively started jets. The technique is therefore suitable for the relatively cheap exploration of the noise field of jets of a wide variety of gases.     

冲击射流激波振荡与抑制

欠膨胀冲击射流具有复杂的激波结构,并伴随产生高幅值的离散频率单音.通过高速摄像获取的纹影图像并结合噪声测量,对欠膨胀冲击射流激波振荡过程、剪切层不稳定波的模态和离散频率单音的产生进行了系列研究.给出了冲击距离为5倍喷嘴出口直径的复杂流动实验结果分析,射流剪切层不稳定波有对称和非对称两种模态,发现不同模态下的离散频率单音...     

Shock associated noise of supersonic jets from convergent-divergent nozzles

Results of experimental and theoretical studies of the characteristics of shock associated noise from imperfectly expanded supersonic jets over an extensive range of underexpanded and overexpanded operating conditions are described. This kind of broadband noise is believed to be generated by the weak but coherent interaction between the downstream propagating large scale turbulent flow structures in the mixing layer of the jet and the nearly periodic shock cell system. Theoretical reasoning based on this mechanism leads to the scaling formula that the intensity of shock associated noise varies as (M_j² ? M_d²)² where M_j and M_d are the fully expanded jet operating Mach number and nozzle design Mach number, respectively. This formula holds for underexpanded as well as overexpanded jet Mach numbers. In addition, a peak frequency formula is also derived from the same model. The noise intensity, directivity and spectra of supersonic jets from a convergent-divergent nozzle of design Mach number 1·67 were measured in an anechoic facility over the Mach number range of 1·1 to 2·0. The effect of jet temperature was investigated by operating the jet at three temperature conditions. These sets of data provide sufficient information for fully assessing the relative importance and characteristics of shock associated noise of supersonic jets from convergent-divergent nozzles. Comparisons between theoretical results and measurements show very favorable agreement.     

On the Mach number and temperature dependence of jet noise: Results from a simplified numerical model

Numerical simulations of sound radiation from perturbed round jets are used, firstly to explore the structure of the sound sources and then to carry out a parametric study of the effect of jet Mach number and jet temperature. The simplified model problem includes a steady base jet flow, maintained in the absence of disturbances, superimposed with instability waves that are free to interact nonlinearly. Simulations over a range of subsonic jet Mach numbers show that a nonlinear mechanism dominates over a linear mechanism for low-frequency sound radiation, while for supersonic Mach numbers the linear mechanism is dominant. Additional insight is gained from a frequency-wavenumber analysis, including a transformation in the radial direction. With this decomposition, the acoustic field is located by the arc of a circle in plots of radial against streamwise wavenumber for discrete frequencies. The transformation is applied to both the pressure field, showing the sound directivity, and to selected source terms, showing characteristic directivity patterns for the streamwise and radial quadrupole terms. Decreasing the Mach number leads to a reduction in amplitude of the sources and of the sound radiation. Simulations with broadband forcing show that the qualitative effects of Mach number and jet heating are captured by this approach, which requires less resolution than a direct numerical simulation. A significant increase in the strength of the acoustic radiation for cold jets is observed, which is worthy of further investigation.     

流经矩形喷嘴的超音速射流啸叫模式切换的实验研究

通过实验分析比较了对于相同高度不同宽度的四种矩形喷嘴, 当压力在0.2 MPa到0.8 MPa 之间变动时, 欠膨胀超音速自由射流的啸叫特性和对应的流场纹影结构.结果表明: 不同宽高比喷嘴的超音速自由射流辐射噪声中的单频离散啸叫存在两种不同的啸叫模式, 且随着射流压力的变化会出现模式间的切换.所谓模式切换是指不同模式的轮流占优和消失的现象.啸叫模式间的切换及占优区间的宽度随着喷嘴宽高比的减小而缩短.其中, 宽高比为2的射流啸叫模式中的一种模式所占的射流压降区间异常小, 此现象未在相关文献中提及; 喷嘴宽高比为4的射流啸叫占优区间内, 啸叫基频-射流压力曲线在0.49 MPa时出现了间断、跳跃现象.随着压力的降低激波纹影的轴线出现了抖动, 不同宽高比下流场结构的稳定性随压力变化的规律各异.射流压力在0.70 MPa到0.45 MPa区间内, 随着宽高比减小, 第一波节格栅的激波致密度减弱, 且出现轴向脉动, 第二波节后方的流场变得紊乱; 当射流压力低于0.45 MPa 时, 激波串结构随着宽高比的增大而趋于稳定, 在此压力区间内周期性激波格栅结构较射流压力在0.45 MPa以上时有所减弱.结合啸叫频谱及纹影图分析, 可初步认为, 第二、三波节也会对啸叫频率的声压幅值起到反馈增强作用.     

在RHIC和LHC能区双喷注诱发的马赫锥:大爆炸物质的声速

A study of Mach shocks generated by fast partonic jets propagating through the quark-gluon plasma(QGP)is reviewed briefly.We predict a significant deformation of Mach shocks in central Au Au collisions at RHIC and LHC energies compared to those created by a jet propagation through a static medium. Moreover,a new hydrodynamical study of jet energy loss is presented.     

Effect of chevron count and penetration on the acoustic characteristics of chevron nozzles

Experimental investigations have been carried out on chevron nozzles to assess the importance of chevron parameters such as the number of chevrons (chevron count) and chevron penetration. Acoustic measurements such as overall sound pressure level, spectra, directivity, acoustic power, and broadband shock noise have been made over a range of nozzle pressure ratio from sub-critical to underexpansion levels. Shadowgraph images of the shock-cell structure of jets from various chevron nozzles have also been captured for different nozzle pressure ratios. The results indicate that a higher chevron count with a lower level of penetration yields the maximum noise suppression for low and medium nozzle pressure ratios. Of all the geometries studied, chevron nozzle with eight lobes and 0° penetration angle gives the maximum noise reduction. Chevron nozzles are found to be free from screech unlike regular nozzles. Acoustic power index has been calculated to quantitatively evaluate the performance of the various chevron nozzles. Chevron count is the pertinent parameter for noise reduction at low nozzle pressure ratios, whereas at high nozzle pressure ratios, chevron penetration is crucial. The results illustrate that by careful selection of chevron parameters substantial noise reduction can be achieved.     

Oscillation modes of supersonic multijets exhausting from very adjacent multiple nozzles.

The oscillation modes of a supersonic circular twin jet and multijet with square configuration where the center-to-center spacing of nozzles was fixed to 1.4 times the nozzle diameter were investigated experimentally. It is found that the twin jet oscillates simultaneously in three different oscillation modes for the pressure ratios of about 4.1. From the acoustical observation, these oscillation modes are identified as one lateral oscillation mode perpendicular to a plane composed of the twin jet axes and two lateral oscillation modes parallel to this plane. For a multijet with a square configuration, only one lateral oscillation mode was observed. In this case, it is observed that when the spacing between two facing sides of the square formed by four jets is stretched, the spacing between the other two sides shrinks and vice versa.     

Visualization of impinging supersonic free jet on a tilt plate by LIF and PSP

The flow field structures of low density supersonic free jets impinging on a tilt plate are studied by hybrid use of LIF (Laser Induced Fluorescence) and PSP (Pressure Sensitive Paint). The jet through an orifice flows into a low pressure chamber and impinges on the tilt plate with angle from jet axis 45, 60 or 90 degrees. A plane including the jet axis and the normal of the plate is visualized by LIF of seeded iodine molecules, scanning a laser beam along the jet axis. On the other hand, the pressure distribution on the tilt plate is visualized by PSP. In comparing the results of the two methods, the complicated shock wave system is analyzed. Deformations of the Mach disk and the barrel shock are also confirmed.     

Ignition of hexane-air mixtures by highly under-expanded hot jets

We report an experimental study of ignition of flammable mixtures by highly unexpanded, supersonic hot jets. The high-pressure, hot-gas reservoir supplying the jet is created by impacting a projectile on a plunger to rapidly compress and ignite a rich n-hexane/air mixture, resulting in a peak reservoir pressure of more than 20 MPa. A locking mechanism was used to prevent the plunger from rebounding and the jet was created by rupturing a diaphragm covering a nozzle with an exit diameter between 0.25 and 1 mm. The jet development and ignition processes in the main chamber filled with hexane-air mixture were visualized using high-speed schlieren and OH* chemiluminescence imaging. The ignition threshold was determined as a function of composition in the jet and main chamber, the nozzle diameter, and the initial pressure in the main chamber. Unlike the case of subsonic jets in which ignition occurs at the shear layer near the nozzle exit, ignition of combustion in the main chamber was found to take place downstream of the Mach disk terminating the supersonic expansion and within the turbulent mixing region created by the startup of the supersonic jet. The results are interpreted using a constant-pressure, well-stirred reactor model simulating the mixing between the hot jet and cold ambient gas. The critical conditions for ignition are determined by the competition between energy release due to chemical reactions initiated by the hot jet gas and cooling due to mixing with the cold chamber atmosphere. The critical value (maximum for which ignition occurs) of the mixing rate was computed using a detailed chemical reaction model and found to be a useful qualitative guide to our observations.     

平面撞击流偏斜振荡的实验研究与大涡模拟

采用实验和大涡模拟对喷嘴出口雷诺数(Re= U₀ hρ/μ, 其中 U₀为出口平均速度, h为平面喷嘴出口狭缝高度, ρ和 μ分别为流体的密度与动力黏度)为25–10000, 喷嘴间距 L为4h–40h范围内的平面撞击流偏斜振荡特性进行了研究. 通过对平面撞击流模拟和实验的结果进行比较, 验证了数值模拟的可靠性, 并对平面撞击流发生偏斜振荡的无因次参数(喷嘴间距 L/h与出口雷诺数 Re)范围进行划分, 重点考察了湍流平面撞击流的偏斜振荡周期及速度-压力变化特征. 研究结果表明大涡模拟能对平面撞击流的偏斜振荡进行有效预报; 当平面撞击流发生周期性偏斜振荡时, 特定点的压力与速度也发生周期性变化, 且变化周期与偏斜振荡周期一致, 偏斜振荡本质上是由速度-压力的周期性变化和转换引起的. 关键词： 平面撞击流 偏斜振荡 大涡模拟     

Shock-structures in the acoustic field of a Mach 3 jet with crackle

The acoustic waveforms produced by an unheated supersonic and shock free jet operating at a gas dynamic Mach number of 3 and an acoustic Mach number of 1.79 are examined over a large spatial domain in the (x,r)-plane. Under these operating conditions, acoustic waveforms within the Mach cone comprise sawtooth-like structures which cause a crackling sound to occur. The crackling structures produced by our laboratory-scale nozzle are studied in a range-restricted environment, and so, they are not the consequence of cumulative nonlinear waveform distortions, but are rather generated solely by local mechanisms in, or in close vicinity to, the jet plume. Our current work focuses on characterizing the temporal and spectral properties of these shock-structures. A detection algorithm is introduced which isolates the shock-structures in the temporal waveforms based on a pressure rise time and shock strength that satisfy user defined thresholds. The average shapes of the shock-structures are shown to vary along polar angles centered on the post-potential core region of the jet. Spectral characteristics of the crackling structures are then determined using conventional wavelet-based time–frequency analyses. Differences between the global wavelet spectrum and the local wavelet spectrum computed from instances when shocks are detected in the waveform show how shock-structures are more pronounced at shallow angles to the jet axis. The findings from this energy-based metric differ from those obtained using the skewness of the pressure and the pressure derivative.     

Study of an Underexpanded Plasma Jet

Supersonic jets (Mach number M = 1.4 – 2.0) of dense plasma were produced by a plasmatron in a pulsed regime. The shock structure was investigated by fast mirror cameras, by the schlieren technique and by probe and spectroscopic methods. It was found that during jet expansion the bow shock ahead the gas-plasma interface and the shock behind it are superimposed with the quasistationary shock structure which includes the Mach disk. Whereas the plasma in the discharge chamber is in LTE (n_E ≈ 5 × 10²³ m^?3, T ≈ 14000 K), the electron density of the expanding jet plasma falls to a plateau value of n_E ≈ 5 × 10¹⁹ m^?3 below the LTE limit. At this phase, the jet plasma formed a ball-like turbulent plasmoid which moves at a subsonic velocity through the air up to some milliseconds.     

Nonlinear development of acoustical instabilities in supersonic jets

In contrast with the roll-up of fluid interfaces through Kelvin-Helmholtz instability, recent numerical simulations with small amplitude perturbations of supersonic jets reveal another very different coherent mode of nonlinear acoustical instability of jets through the appearance of regular zig-zag shock patterns which traverse the interior of the jet and amplify as time evolves. In this paper, through a combination of appropriate ideas from linear and nonlinear high frequency geometric optics, the authors develop a quantitative theory which predicts the nonlinear development of zig-zag modes with a structure like those observed in the numerical simulations. The perturbation analysis is developed via a systematic application of nonlinear small amplitude high frequency geometric optics to the complex free surface problem defined by the perturbed jet; this procedure automatically yields simplified asymptotic equations which are analyzed explicitly and lead to the development of regular amplifying “zig-zag” shock structures in the jet. For a given streamwise period, Mach number, and jet width, the asymptotic theory gives explicit criteria for the number and structure of different regular zig-zag shock patterns which amplify with time. For Mach numbers M < 1, there are no amplifying acoustic zig-zig modes while for M > 1, there are a finite number of such modes depending on Mach number, jet width, and streamwise period. Explicit criteria to select the most destabilizing of these nonlinear eigenmodes are developed as well as several new quantitative predictions regarding the nonlinear development of acoustical instabilities in supersonic jets including the phenomenon of “super-resonance” for special values of the streamwise period.