Gas jet as a waveguide for air-coupled ultrasound

The directional characteristics of an ultrasonic signal have been studied during propagation within an axial gas jet. The effects of nozzle shape, nozzle diameter, and variations in jet velocity, temperature and gas composition have been investigated. At high flow velocities of an air jet, divergence of the ultrasonic beam was observed. This was attributed to the effects of refraction, caused by increased acoustic velocities in the direction of the flow. An effective waveguide was also demonstrated by cooling the air jet to below ambient temperatures, so that the acoustic velocity in the air jet was lower than that in the surrounding atmosphere. This could also be achieved by using carbon dioxide mixed with air, whereas the use of helium led to increased divergence. The result is likely to be of use in air-coupled ultrasonic materials inspection.     

Noise generation by instabilities in low Reynolds number supersonic jets

An experimental investigation of noise generation by instabilities in low Reynolds number supersonic air jets has been performed. Sound pressure levels, spectra and acoustic phase fronts were measured with a traversing condenser microphone in the acoustic field of axisymmetric, perfectly expanded, cold jets of Mach numbers 1·4, 2·1 and 2·5. Low Reynolds numbers in the range from Re = 3700 to Re = 8700 were obtained by exhausting the jets into an anechoic vacuum chamber test facility. This contrasts with Reynolds numbers of over 10⁶ for similar jets exhausting into atmospheric pressure. The flow fluctuations of the instability in all three jets have been measured with a hot-wire and the results are documented in a previous paper by Morrison and McLaughlin. Acoustic measurements show that the major portion of the sound radiated by all three jets is produced by the instability's rapid growth and decay that occurs near the end of the potential core. This takes place over a relatively short distance (less than two wavelengths of the instability) in the jet. In the lower two Mach number jets the instability has a phase velocity less than the ambient acoustic velocity. In the Mach number 2·5 jet the instability phase speed is 1·11 times the ambient acoustic velocity. In this case the acoustic phase fronts indicate the possibility of a Mach wave component. It was also determined that low level excitation at the dominant frequency of the instability actually decreased the radiated noise by suppressing the broad band component.     

Uniformly valid solution for acoustic propagation in weakly tapered circular waveguides: liquid jet example

The propagation of ultrasound down laminar liquid jets has potential applications to the stimulation of liquid drop production [J. B. Lonzaga, C. F. Osterhoudt, D. B. Thiessen, and P. L. Marston, J. Acoust. Soc. Am. 121, 3323-3330 (2007)] as well as to the coupling of ultrasound to objects through contact with a jet. In normal gravity, a jet issuing from a nozzle becomes tapered as the jet accelerates downward. A uniformly valid solution for the acoustic propagation in a weakly tapered, liquid jet waveguide in air with a turning point is derived using Langer's transformation and the method of multiple scales. The loss of energy from transmission into the air and from thermal viscous absorption is neglected. A solvability condition is used to obtain the leading-order correction due to the taper of the waveguide. This asymptotic solution is validated using finite-element numerical calculations. The ultrasonic wave amplitude is enhanced in the region of the jet close to the cutoff of the excited mode.     

大气压冷等离子体射流研究进展

有许多种方法可用于在大气中产生等离子体射流,冷等离子体（离子温度在室温附近）射流即是其中的一种.近年来,人们发现氦气或其它惰性气体通过毛细管介质阻挡放电形成的冷等离子体射流具有类似子弹的传输特性,在有机材料表面改性、等离子体医学等领域获得了广泛的应用.通过专门设计的一系列实验,我们逐渐揭示了其产生机理,并深入研究了传输特性.文章简要介绍近年来我们所做的有关大气压冷等离子体的实验过程以及获得的一些重要结论.在对这种等离子体深入了解的基础上,作者还开发了一种新装置,该装置的最大特点是既利用了氦气在辅助放电方面的特性,又不消耗这种昂贵的资源;并且它还特别适合于在臭氧层修复、等离子体医学等方面的应用.     

Simultaneous reconstruction of flow and temperature cross-sections in gases using acoustic tomography

This paper describes the use of air-coupled ultrasonic tomography for the simultaneous measurement of flow and temperature variations in gases. Air-coupled ultrasonic transducers were used to collect through-transmission data from a heated gas jet. A transducer pair was scanned in two-dimensional sections at an angle to the jet, and travel time and amplitude data recorded along various paths in counter-propagating directions. Parallel-beam tomographic reconstruction techniques allowed images to be formed of variations in either temperature or flow velocity. Results have been obtained using heated jets, where it has been shown that it is possible to separate the two variables successfully.     

屏蔽气体对氩等离子体冲击射流的影响

在空气环境中利用氩等离子体射流进行材料加工时,环境空气卷吸进入射流可能会引起金属材料氧化。采用同轴屏蔽气体保护是减小该不利影响的一种可行方案。为此本文对空气环境中层流氩等离子体冲击射流特性受屏蔽气体影响问题进行了数值模拟研究,重点考察了屏蔽气体速度等对材料加工区氧含量的影响。     

Jet noise control using the dielectric barrier discharge plasma actuators

We study experimentally how plasma actuators operating on the basis of surface barrier high-frequency discharge affect jet noise characteristics. The results of investigations of air jets (100?C200 m/s) have demonstrated that the studied plasma actuators have control authority over the noise characteristics of these jets. An actuator??s effect on the jet in the applied configuration is related to acoustic discharge excitation and to a large extent is similar to the well-known Vlasov-Ginevsky effect. It has been shown that jet excitation in the case of St ?? 0.5 using the barrier-discharge plasma actuator leads to broadband amplification of jet sound radiation. The jet excitation in the case of St > 2 leads to broadband noise reduction if the action is sufficiently intensive.     

纳秒脉冲放电等离子体射流特性

采用单针式电极,使用单极性重复频率脉冲电源,在常压氦气、氩气、氮气和空气中得到等离子体射流,并改变电压、流量和气体种类,分别观察不同的实验条件对等离子体射流的影响。实验结果表明:射流长度随施加电压的增加而增长;随着流量的连续变化,射流长度先逐渐变长,达到峰值后由于湍流影响,长度又逐渐缩短,达到一定流量后趋于饱和。此外,不同工作气体中的等离子体射流呈现截然不同的外观,氦气和氩气中射流呈针状模式,长度可达7 cm以上;而在氮气和空气中,射流呈现为长度不超过2 cm的刷状模式。     

Acoustic characteristics of interacting supersonic jets

This study was intended primarily to reveal more information about the noise producing mechanisms of supersonic jets. Two identical, small, cold air, supersonic, overexpanded jets were tested at selected angles, varying from parallel to 90 degrees intersecting, and at various distances apart. Schlieren photographs of the jet structure and far field sound data were obtained. Close spacing of the parallel jets caused acoustic attenuation, which reached a maximum at one diameter centerline spacing, where the sound of two jets nearly equals that of a single jet. In every case the intersecting jets merged into a single supersonic jet.The overall sound power level of intersecting jets is generally higher than that of two independent jets, because of the turbulent mixing of the two jet flows. A maximum level is reached when the jets intersect at a point near the middle of the flow region containing repetitive shocks. For the parallel jets and intersecting jets at large separation, the sound levels are lower in the plane containing the jet centerlines. For intersecting jets at small separation, however, this shielding effect is reversed.     

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.     

用速度浓度同步测量系统研究瞬态气体撞壁射流混合过程

用速度浓度同步测量系统研究瞬态气体撞壁射流混合过程谢辉，苏万华，史绍熙，林荣文（天津大学内燃机燃烧学国家重点实验室天津３０００７２）关键词撞壁射流，射流混合，速度浓度同步测量１引言对于中小型直喷式柴油机，燃油喷雾与壁面撞击不可避免，研究喷雾和壁面的相...     

Liquid jet response to internal modulated ultrasonic radiation pressure and stimulated drop production

Experimental evidence shows that a liquid jet in air is an acoustic waveguide having a cutoff frequency inversely proportional to the jet diameter. Ultrasound applied to the jet supply liquid can propagate within the jet when the acoustic frequency is near to or above the cutoff frequency. Modulated radiation pressure is used to stimulate large amplitude deformations and the breakup of the jet into drops. The jet response to the modulated internal ultrasonic radiation pressure was monitored along the jet using (a) an optical extinction method and (b) images captured by a video camera. The jet profile oscillates at the frequency of the radiation pressure modulation and where the response is small, the amplitude was found to increase in proportion to the square of the acoustic pressure amplitude as previously demonstrated for oscillating drops [P.L. Marston and R.E. Apfel, J. Acoust. Soc. Am. 67, 27-37 (1980)]. Small amplitude deformations initially grow approximately exponentially with axial distance along the jet. Though aspects of the perturbation growth can be approximated from Rayleigh's analysis of the capillary instability, some detailed features of the observed jet response to modulated ultrasound are unexplained neglecting the effects of gravity.     

Quantitative visualization of high-speed 3D turbulent flow structures using holographic interferometric tomography

Using holographic interferometry the three-dimensional structure of unsteady and large-scale motions within subsonic and transonic turbulent jet flows has been studied. The instantaneous 3D flow structure is obtained by tomographic reconstruction techniques from quantitative phase maps recorded using a rapid-switching, double reference beam, double pulse laser system. The reconstruction of the jets studied here reveal a three-dimensional nature of the flow. In particular an increasing complexity can be seen in the turbulence as the flow progresses from the jet nozzle. Furthermore, a coherent three-dimensional, possibly rotating, structure can be seen to exist within these jets. The type of flow features illustrated here are not just of fundamental importance for understanding the behavior of free jet flows, but are also common to a number of industrial applications, ranging from the combustion flow within an IC engine to the transonic flow through the stages of a gas turbine.     

Liquid jet directionality and droplet behavior during emulsification of two liquids due to acoustic cavitation

The present study numerically investigates liquid-jet characteristics of acoustic cavitation during emulsification in water/gallium/air and water/silicone oil/air systems. It is found that a high-speed liquid jet is generated when acoustic cavitation occurs near a minute droplet of one liquid in another. The velocity of liquid jet significantly depends on the ultrasonic pressure monotonically increasing as the pressure amplitude increases. Also, the initial distance between cavitation bubble and liquid droplet affects the jet velocity significantly. The results revealed that the velocity takes maximum values when the initial distance between the droplet and cavitation bubble is moderate. Surprisingly, the liquid jet direction was found to depend on the droplet properties. Specifically, the direction of liquid jet is toward the droplet in the case of water/gallium/air system, and vice versa the jet is directed from the droplet in the case of water/silicone oil/air system. The jet directionality can be explained by location of the high-pressure spot generated during the bubble contraction.     

Measurement of jet shapes in high-Q^2 deep inelastic scattering at HERA

The shapes of jets with transverse energies, , up to 45 GeV produced in neutral- and charged-current deep inelastic scattering (DIS) at GeV have been measured with the ZEUS detector at HERA. Jets are identified using a cone algorithm in the - plane with a cone radius of one unit. The jets become narrower as increases. The jet shapes in neutral- and charged-current DIS are found to be very similar. The jets in neutral-current DIS are narrower than those in resolved processes in photoproduction and closer to those in direct-photon processes for the same ranges in and jet pseudorapidity. The jet shapes in DIS are observed to be similar to those in interactions and narrower than those in collisions for comparable . Since the jets in interactions and DIS are predominantly quark initiated in both cases, the similarity in the jet shapes indicates that the pattern of QCD radiation within a quark jet is to a large extent independent of the hard scattering process in these reactions. Received: 2 April 1998 / Published online: 7 April 1999     

一种新型大气压毛细管介质阻挡放电冷等离子体射流技术

介绍一种结构设计简单、操作运行方便的新型毫米量级大气压冷等离子体射流发生技术.这种射流可以在大气压条件下，利用多种工作气体(如Ar,He,N₂)，通过毛细管介质阻挡放电(DBD)的方式实现.使用频率为33kHz，峰值电压为1—12kV的双向脉冲电源，利用Ar,He,N₂等工作气体，在毛细管内形成了稳定的冷等离子体射流.放电区域的光辐射空间分布利用商用CCD摄像机记录，从中研究放电形态和空间分布，观察到了在DBD区域的流动气体放电和在毛细管出口处形成的等离子体射流 关键词： 冷等离子体射流 毛细管介质阻挡放电 射流射程 射流激发温度     

Gas bubble pulsation in a semiconfined space subjected to ultrasound

In the case of ultrasound application in biological tissues, gas bubbles might form and collapse within cells, in the intercellular spaces and on tissue surfaces. In this work the effect of confined space on the behavior of the gas bubble in the presence of ultrasonic field is studied. A numerical model for bubble pulsation in a planar liquid layer, bounded by two rigid walls, is developed. Surface tension at the interface between the host liquid and the gas in the bubble is considered as well. A mathematical statement and solution technique based on the boundary integral method are presented. In some cases, the bubble divides into two symmetrical parts and high-velocity jets are generated, aimed at the walls. The final velocity of the jets strongly depends on the surface tension of the host liquid. Two new parameters that predict the occurrence of jet formation are developed.     

复杂流场的超声-激光测量原理研究

本文从复杂流场—旋涡场参量的超声—激光测试方法的需要出发,论述了超声波产生的声相位光栅对激光产生的偏转效应.并研制了适用于产生空气超声相位光栅的大功率高频超声换能器、位移灵敏接收器、数字相位差测定仪等设备,采用了超声发射的匹配技术等,从而获得了明显的空气超声-激光偏转效应,并且测定了两光束的偏转时间差.本文的结果为利用超声-激光的空气声光偏转效应测量空气旋涡流场参量提供了实验依据.     

胶子喷注和夸克喷注性质的蒙特卡洛研究

用蒙特卡洛方法研究了91.2GeV e⁺e^－碰撞产生的3喷注事件.用3个喷注之间的夹角来标识各个喷注,分别计算了3个喷注的能量及能量分布,并在相同能量下计算了3个喷注的多重数,横动量及其分布.通过与能量相同的2喷注事件中单夸克喷注的上述性质的比较,得到了从3喷注事件中挑选胶子喷注和夸克喷注的一种简便方法.这样挑选出来的胶子和夸克喷注在性质上与QCD的理论预言一致,并且胶子和夸克喷注的平均多重数比值的计算结果与实验观测值符合     

Comparative visualized investigation of impact-driven high-speed liquid jets injected in submerged water and in ambient air

This paper is a comparative study on the characteristics of high-speed liquid jets injected in surrounding water and air using shadowgraph technique. One of the main objectives is to investigate the effects of liquid’s physical properties, used to generate the high-speed liquid jets, on jet generation’s characteristics. Moreover, comparative investigations on effects of those liquid jets after injected in water and air are reported. The high-speed liquid jets were generated by the impact of a projectile launched by a horizontal single-stage power gun. The impact-driven high-speed liquid jets were visualized by shadowgraph technique and images were recorded by a high-speed digital video camera. The process of impact-driven high-speed liquid jet injection in air and water, oblique shock waves, jet-induced shock waves, shock waves propagation, the bubble behavior, bubble collapse-induced rebound shock waves and bubble cloud regeneration were clearly observed. It was found that different properties of liquid (surface tension and kinematic viscosity) affect the jet maximum velocity and shape of the jet. Bubble behaviors were only found for the jet injected in water. From the shadowgraph images, it is found that the maximum average jet velocity, expansion and contraction velocities of bubble in axial direction increase when the value of the multiplied result of surface tension by kinematic viscosity increases. Therefore, surface tension and kinematic viscosity are the significant physical properties that affect characteristics of high-speed liquid jets.