激波和剪切层相互作用下的超音速射流

欠膨胀超音速射流处于螺旋模式下的中度欠膨胀时,其入射剪切层的激波具有很高强度,激波和剪切层发生了强烈的相互作用,远场辐射的拢动波出现了大间隔、交错的上下行类似螺旋锥面波形图像,该扰动波具有很强的向上游传播的指向性,导致上游噪声高于垂直喷嘴方向的声压级.而在相对压比较低的低度欠膨胀情况下,或高压比下的高度欠膨胀的情形,入射剪切层激波强度相对较弱,远场辐射没有大间隔、交错的上下行远场辐射 关键词： 超音速射流 啸叫 扰动波 激波 剪切层     

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.     

An experimental investigation of the noise produced by air jet impingement on flat plates

The sound power level produced by an air jet impinging upon a flat solid boundary is investigated. Measurements are recorded for nozzle diameters and exit velocities in the ranges $\text{0·75 in < D < 1·5}$ in and $\text{270 ft/s <}\text{U}\text{?}\text{< 700 ft/s}$. The effect of varying the nozzle to plate spacing is investigated for plates normal to and inclined to the jet axis.At low velocities and large nozzle diameters the sound power level was found to increase continuously as the nozzle to plate spacing decreased, the maximum increase being 7 dB above that of the free jet. Under certain conditions, usually small nozzle diameter and high velocities, a maximum in sound power level was observed in the nozzle to plate spacing range, increases of 27 dB above that of the free jet levels being recorded and a distinct tonal character being detected.The reasons for this phenomenon cannot be fully explained, but it is likely that this can be avoided by inclining the plate to the jet axis.     

Visualization and force measurement of high-temperature,supersonic impulse jet impinging on baffle plate

The flow visualization and force measurement of a supersonic impinging jet on a center-holed vertical baffle plate were investigated. Center-holed baffle plates of 2d to 5d in diameter, with a 1d center hole were tested, where d is the bore of the launch tube. The standoff distance of the baffle plates from the open end of the launch tube were varied to be from 2d to 5d. The supersonic impulse jet, with an incident shock wave of Mach 2.89 was produced by a high-enthalpy blast-wave simulator. The direction-indicating color schlieren method produced a two-dimensional density gradient of the flow field around the baffle plate. In addition, the flow fields were numerically analyzed, using two-dimensional asymmetric Euler equations. The results of the numerically-analyzed and the experimentally-visualized flow field agreed well. The visualized flow field indicates that the baffle plate should be at least 3.5d in diameter to deflect the supersonic impinging jet at an angle greater than a right angle. We have concluded that the representative method of designing muzzle brakes for military purpose accurately predicts the force yielded by the supersonic impinging impulse jet on the vertical baffle plate only when there is a large ratio of the baffle-plate diameter to the bore of the launch tube.     

层流与湍流等离子体冲击射流特性比较

本文采用数值模拟方法,对层流与湍流氩等离子体射流在空气环境中冲击平板时的流动与传热特性进行了对比研究.结果表明,在平板和射流进口间的距离较大时,平板的存在只对其附近的射流参数分布有较大影响,层流等离子体冲击射流的温度与轴向速度的轴向梯度明显小于湍流等离子体冲击射流情形;由于在平板表面形成的径向壁面射流对引射的附加贡献,层流和湍流等离子体冲击射流对环境空气的引射量明显增加.     

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.     

爆炸载荷作用下钢-混凝土-钢组合板的动态响应数值模拟

钢-混凝土组合结构的抗爆性能已成为防护工程和反恐防爆等领域的研究热点。以钢-混凝土-钢组合板为例,利用有限元软件ABAQUS对爆炸载荷作用(爆距为2.5~7.5 m,TNT炸药量为50~100 kg)下该结构的破坏形态和动力学性能进行了数值模拟。研究结果表明,组合板的破坏形态与炸药量和爆距有关。炸药量越大,爆距越小,组合板的破坏程度越明显。当TNT炸药量为100 kg、爆距为2.5 m时,组合板发生明显的翘曲,出现了塑性铰。钢板的存在有效地抑制了核心混凝土的剥落。在爆距相同的条件下,炸药量越大,组合板的变形越明显,跨中挠度和峰值速度也越大。当炸药量相同(100 kg)时,与爆距为7.5 m相比,爆距为5.0 m时组合板的跨中挠度为其1.53倍,爆距为2.5 m时组合板的跨中挠度为其5.01倍。     

Numerical analysis of supersonic gas-dynamic characteristic in laser cutting

The influence of the processing parameters on the dynamic characteristic of supersonic impinging jet in laser cutting is studied numerically. The numerical modeling of a supersonic jet impinging on a plate with a hole is presented to analyze the gas jet–workpiece interaction. The model is able to make quantitative predictions of the effect of the standoff distance and exit Mach number on the mass flow rate and the axial thrust. The numerical results show that the suitable cutting range is slightly different for different exit Mach number, but the optimal cutting parameter for certain exit total pressure is nearly changeless. So the better cut quality and capacity can be obtained mainly by setting the suitable standoff distance for a certain nozzle pressure.     

Influence of Burner Geometry on Heat Transfer Characteristics of Methane/Air Flame Impinging on Flat Surface

An experimental study has been conducted to find the heat transfer characteristics of methane/air flames impinging normally to a flat surface using different burner geometries. The burners used were of nozzle, tube, and orifice type each with a diameter of 10 mm. Due to different exit velocity profiles, the flame structures were different in each case. Because of nearly flat velocity profile, the flame spread was more in case of orifice and nozzle burners as compared to tube burner. Effects of varying the value of Reynolds number (600–2500), equivalence ratio (0.8–1.5) and dimensionless separation distance (0.7–8) on heat transfer characteristics on the flat plate have been investigated for the tube burner. Different flame shapes were observed for different impingement conditions. It has been observed that the heat transfer characteristics were intimately related to flame shapes. Heat transfer characteristics were discussed for the cases when the flame inner reaction cone was far away, just touched, and was intercepted by the plate. Negative heat fluxes at the stagnation point were observed when the inner reaction cone was intercepted by the plate due to impingement of cool un-burnt mixture directly on the surface. Different heat transfer characteristics were observed for different burner geometries with similar operating conditions. In case of tube burner, the maximum heat flux is around the stagnation point and decay is faster in the radial direction. In case of nozzle and orifice burner, the heat transfer distribution is more uniform over the surface.     

Low frequency sound produced by a ventilated supercavitating vehicle

An analytical investigation is made of the low frequency noise produced by gas jet impingement on the gas-water interface of a ventilated supercavity. Enclosure within a supercavity enables an underwater vehicle to attain high-speed forward motion. Whereas high frequency components of the cavity self-noise can interfere with the vehicle guidance system, low frequency sound tends to radiate in the water to large distances from the cavity. A canonical mathematical problem is examined that extends a previous study by Foley et al. (Journal of Sound and Vibration329 (2010) 415-424.) of sound generation by a specially modified, model scale supercavitating vehicle involving gas jet impingement at normal incidence to the interface. Our analysis determines the influence on low frequency sound production of cavity aspect ratio and the manner in which the efficiency of sound production increases with decreasing distance of the jet impact region from the circular ‘cavitator’ at the vehicle nose, where the supercavity is formed.     

Experimental and numerical investigation of laser shock synchronous welding and forming of Copper/Aluminum

A novel laser shock synchronous welding and forming method is introduced, which utilizes laser-induced shock waves to accelerate the flyer plate towards the base plate to achieve the joining of dissimilar metals and forming in a specific shape of mold. The samples were obtained with different laser energies and standoff distances. The surface morphology and roughness of the samples were greatly affected by the laser energy and standoff distances. Fittability was investigated to examine the forming accuracy. The results showed that the samples replicate the mold features well. Straight and wavy interfaces with un-bonded regions in the center were observed through metallographic analysis. Moreover, Energy Disperse Spectroscopy analysis was conducted on the welding interface, and the results indicated that a short-distance elemental diffusion emerged in the welding interface. The nanoindentation hardness of the welding regions was measured to evaluate the welding interface. In addition, the Smoothed Particle Hydrodynamics method was employed to simulate the welding and forming process. It was shown that different standoff distances significantly affected the size of the welding regions and interface waveform characteristics. The numerical analysis results indicated that the opposite shear stress direction and effective plastic strain above a certain threshold are essential to successfully obtain welding and forming workpiece.     

A PIV study of a supersonic impinging jet

The characteristics of supersonic impinging jets are investigated using Particle Image Velocimetry (PIV). The purpose of the experiments is to understand the jet induced forces on STOVL aircraft while hovering close to the ground. For this purpose, a large diameter circular plate was attached at the nozzle exit. The oscillations of the impinging jet generated due to a feedback loop are captured in the PIV images. The instantaneous velocity field measurements are used to describe flow characteristics of the impinging jet. The important flow features such as oscillating shock waves, slipstream shear layers and large scale structures are captured clearly by the PIV. The presence of large scale structures in the impinging jet induced high entrainment velocity in the near hydrodynamic field, which resulted in lift plate suction pressures. A passive control device is used to interfere with the acoustic waves travelling in the ambient medium to suppress the feedback loop. As a consequence, the large scale vortical structures disappeared completely leading to a corresponding reduction in the entrainment.     

The conception of structure-borne-sound-based near-field holography

A procedure for the projection of structure-borne sound fields is introduced, following the basic idea behind Near-Field Acoustic Holography. It is shown that the bending wave field of an infinite plate subjected to point force excitation can be reconstructed by means of Fourier transform-based wave field extrapolation. All required information on the wave field is obtained by vibrational response measurement in the near-field of the excitation, using a circular geometry with a limited number of positions. The procedure is verified both numerically and experimentally, to very satisfying results. The experiments indicate that the reconstruction at the source becomes erratic when standoff distances greater than half of the minimum wavelength under consideration are used. Reconstruction of the far-field plate response is found to be less restrictive.     

Oscillatory thermal structures in a reattaching jet flow

Spatially varying surface temperature oscillations in a nominally steady reattaching slot jet at a Reynolds number (Re) of 5000 are analyzed using Proper Orthogonal Decomposition (POD) for two nozzle-to-surface spacings, and three exit openings of the nozzle. The surface temperature data in these experiments were recorded using infrared thermography at a frequency of 20 Hz along two selected lines on the impingement surface corresponding to geometrically similar and dissimilar regions within the reattachment curtain. The magnitude of temperature oscillations were found to increase with an increase in exit opening for the larger nozzle spacing. The POD analysis results indicate that a majority of the temperature flucutations are well captured by 15 dominant modes. In many cases, the three dominant modes accounted for approximately 60 percent of the variance in surface temperature fluctuation.     

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.     

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...     

FTIR free-jet set-up for the high resolution spectroscopic investigation of condensable species

An existing experimental set-up combining Fourier transform infrared (FTIR) spectroscopy and free-jet cooling has been modified significantly to allow high resolution studies of the spectrum of monomer species which are liquid under standard conditions. Evaporation of the liquid samples is controlled by a condenser apparatus which is described. A supersonic planar expansion issuing from a narrow aperture is preferred for its very high cooling rate. Such an expansion, probed with a pitot tube, has a zone of limited temperature gradient close to the nozzle exit. The continuum isentropic model appears well suited to describing the thermodynamic properties of the flow up to a high number of nozzle diameters downstream. High resolution spectra of benzene and methanol have been recorded in the 3 μm wavelength range, and their analysis demonstrates a well defined rotational temperature in the 20–25 K range.     

Compressible Flow and Transonic Shock in a Diverging Nozzle

The paper is devoted to the study of compressible flows and transonic shocks in diverging nozzles in the framework of the full compressible Euler system. Consider a nozzle having a shape as a diverging truncated sector with generic opening angle: if the upstream flow at the entrance is supersonic and is near to an axial symmetric flow, and if all parameters of the upstream flow and the receiver pressure at the exit are suitably assigned, then a transonic shock appears in the nozzle. To determine the transonic shock and the flow in the nozzle leads to a free boundary value problem for a nonlinear partial differential equation. We prove that the receiver pressure can uniquely determine the location of the transonic shock, as well as the flow behind the shock. Such a conclusion was conjectured by Courant and Friedrichs, and is confirmed theoretically in this paper for the divergent nozzles. The main advantage of this paper compared with the previous studies on this subject is that the section of the nozzle is allowed to vary substantially, while the transonic shock is not assumed to pass a fixed point. The situation coincides with the requirement in Courant-Friedrichs’ conjecture. To describe the compressible flow we use the full Euler system, which is purely hyperbolic in the supersonic region and is elliptic-hyperbolic in the subsonic region. Solving the free boundary value problem of an elliptic-hyperbolic problem forms the main part of this paper. In our demonstration some new approaches, including the introduction of a pseudo-free boundary problem and the corresponding relaxation, design of a delicate double iteration scheme, are developed to overcome the difficulties caused by the divergence of the nozzle.     

超声速等离子体射流的数值模拟

基于可压缩的全Naiver-Stokes方程，利用PHOENICS程序对由会聚 辐射阳极形状等离子体炬产生的超声速等离子体射流进行了数值模拟.考虑了等离子体的黏性、可压缩性以及变物性对等离子体射流特性影响.研究了超声速等离子体射流的流场结构特性以及不同环境压力对等离子体射流产生激波结构的影响.结果表明，超声速等离子体射流在喷口附近形成的周期性激波结构是其和环境气体相互作用的结果. 关键词： 等离子体炬 超声速等离子体射流 PHOENICS