Numerical flow visualization of a Single Expansion Ramp Nozzle with hypersonic external flow

Numerical simulation of scramjet asymmetric nozzle flow is carried out to visualize and investigate the effects of interaction between engine exhaust and hypersonic external flow. The Single Expansion Ramp Nozzle (SERN) configuration studied here consists of flat ramp and a cowl with different combinations of ramp angle and cowl geometry. UsingPARAS 3D, simulations are performed for a free stream Mach number of 6.5 that constitutes the external flow around the vehicle. Appropriate specific heats ratio has been simulated for the jet and free stream flow. External shock wave due to jet plume interaction with free stream flow, the internal barrel shock wave and the shear layer emanating from the cowl trailing edge and sidewalls are well captured. Wall static pressure distribution on the nozzle ramp for different nozzle expansion angles has been computed for both with and without side fence. Axial thrust and normal force have been evaluated by integrating the wall static pressure. Effect of cowl length variation and side fence on the SERN performance has also been studied and found to be quite significant. Based on this study, an optimum ramp angle at which the SERN generates maximum axial thrust is obtained. SERN angle of 20° was found to be optimum when the flight axis coincides with nozzle axis.     

Flow structures around an inclined substrate subjected to a supersonic impinging jet in laser cutting

In laser cutting, the flow structure around a substrate significantly affects the material removal rate, the cutting depth and the surface finish of the cutting front. In this paper, the phenomena of shock wave that is induced by a supersonic impinging jet emanating from a straight nozzle onto a substrate with varying inclined angles has been simulated numerically and visualized experimentally. The numerical model offers fairly good prediction in comparison with the experiments. It transpires that the angle of inclination has a significant and dramatic effect on the flow structure and that a large wall pressure with a steep gradient can be built up when the angle is large.     

不同喷嘴组合超高压射流破岩钻进特性分析

本文对不同喷嘴组合方式时超高压射流井底流场特性进行数值模拟.根据得出的井底流场的速度矢量图和井底压力分布图,分析超高压钻头喷嘴组合形式对流场结构及钻进效果的影响.结果表明,三个垂直的边喷嘴组合破岩效率较高,但清洗井底效果不佳,冲蚀井壁严重;中心加-个喷嘴可解决清洗井底及冲蚀井壁的问题,但要求高压流体排量增加;两个垂直边喷嘴与-个中心倾斜喷嘴的组合及-个垂直边喷嘴和-个中心倾斜喷嘴的组合流场结构对钻进较为有利.     

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.     

Research on the jet characteristics of the deflector–jet mechanism of the servo valve

In view of the complicated structure of the deflector-jet mechanism,a mathematical model based on the turbulent jet flow theory in the deflector-jet amplifier is proposed.Considering the energy transformation and momentum variation,an equation of the flow velocity distribution at the key fluid region is established to describe the morphological changes of the fluid when it passes through the deflector and jets into the receiver.Moreover,the process is segmented into four stages.According to the research results,the oil enters the deflector and impinges with the side wall.Then one part of the oil's flow velocity decreases and a high pressure zone is formed by the oil accumulation,the other part of the oil reverses out of the deflector along the side wall.Prior to entering the receiver,the flow is a kind of plane impinging jet.Virtually,the working pressure of the receiver is generated by the impact force,while the high speed fluid flows out of the receiver and forms a violent vortex,which generates negative pressure and causes the oil to be gasified.Compared with the numerical simulation results,the turbulent jet model that can effectively describe the characteristics of the deflector-jet mechanism is accurate.In addition,the calculation results of the prestage pressure characteristic have been verified by experiments.     

不同喷流对激波/边界层干扰控制特性对比

采用数值模拟的方法对比研究了定常、方波脉冲、正弦脉冲这3种喷流对Ma=2流动激波/边界层干扰的控制效果, 分析了不同喷流方式对受控流动流场结构、压力分布的影响. 结果表明, 不同喷流方式都能够实现对受控流动的有效控制, 可以增大激波距离, 减弱激波强度, 减小激波发生器壁面压力. 在控制效果方面, 定常喷流效果最佳, 方波脉冲喷流效果次之, 但定常喷流所需要的质量流量也最大, 方波脉冲质量流量次之. 3种喷流的控制效果与所需质量流量的关系表明, 控制收益的增加随着质量流率的增大而减小.      

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.     

Oscillatory modes of interaction of supersonic overexpanded jets with barriers

The self-oscillatory interaction of supersonic jets with barriers has mainly been studied for under-expanded jets. There are only a few experimental studies examining the case of overexpanded jets, with little computational work done in this direction. To fill this gap, we performed numerical simulations of overexpanded supersonic jets with barriers. The calculations were performed by the Godunov method on fine grids using parallel programming techniques. In the course of numerical simulations, the gasdynamic parameters of the jet and the geometric parameter of the barrier were varied. The barrier had the shape of a cylindrical cavity of depth l = (0 − 18)r _a , where r _a is the nozzle exit radius (the case l = 0 corresponds to a flat-end barrier). Based on the results of the numerical simulations, the conclusion on whether the self-oscillation process occurs was drawn and the dependence its characteristics (frequency and amplitude) on the governing gasdynamic and geometric parameters were obtained. Good agreement with experimental data on the fundamental tone frequency was demonstrated. A low-frequency oscillation mode was mostly realized. In this case, the jet experienced periodic suctions into and ejections from the cavity, counter the oncoming jet flow, with the formation of a structure consisting of three discontinuity surfaces (two shock waves and a separating surface contact).     

On classification of flow regimes in a channel with sudden expansion

Supersonic flows of gas in the vicinity of the bottom region known as flows with sudden expansion have been considered. On the basis of extensive experimental studies, authors have proposed a complete classification of flow regimes: stationary, oscillating, and transient. Hysteresis of the regimes change at total gas pressure increasing and decreasing in front of the nozzle has been found. Typical shock-wave configurations emerging at the jet flowing in a channel at different modes have been determined. The type of shock-wave structure and the nature of interaction of the mixing layer of a jet with the wall or reverse flow flowing into the channel from ambient medium determine the appropriate mode. Combination of physical and numerical experiment with bottom pressure calculation according to the developed semi-empirical model have revealed new flow regimes that were not studied earlier.     

Study on concurrent measurements of velocity and density distributions for high-speed CO2 flow

Velocity and density distributions of a high-speed and initial CO₂ jet flow have been analyzed concurrently by a developed three-dimensional digital speckle tomography and a particle image velocimetry (PIV). Two high-speed cameras have been used for the tomography and one for the PIV since a shape of a nozzle for the jet flow is bilaterally symmetric and the starting flow is fast and unsteady. The speckle movements between no flow and CO₂ jet flow have been obtained by a cross-correlation tracking method so that those distances can be transferred to deflection angles of laser rays for density gradients. The three-dimensional density fields for the high-speed CO₂ jet flow have been reconstructed from the deflection angles by the real-time tomography method, and the two-dimensional velocity fields have been calculated by the PIV method concurrently and instantaneously.     

Identification of Regions with Inhomogeneous Particle Behavior in Dilute Gas‐solid Flows

The behavior of the particulate phase in a highly turbulent gas flow has been investigated in a vertical channel. Variations of the flow configuration (1. Flow past a cylinder, 2. flow past a wall‐mounted obstacle and 3. flow around a horizontally injected jet) have been subject to both experiments and numerical simulations. The velocity vector field of the solid phase has been measured by digital particle image velocimetry (DPIV). The measurements have been focused on particle‐obstacle collisions and crossflow in the vicinity of the jet nozzle using the lately developed twinpeak detection method. By application of this method regions of highly inhomogeneous particle behavior could be detected mainly upstream of the flow perturbation. Numerical results have been obtained by an Eulerian‐Lagrangian method on boundary‐fitted grids. Particle‐particle interactions as well as interphase exchange of momentum have been taken into account. The simulation results showed to be well in accordance with the experimental results.     

Numerical investigation of the interaction of the turbulent dual-jet and acoustic propagation

In order to study the interaction between two independent jets, a three-dimensional(3D) transient mathematical model is developed to investigate the flow field and acoustic properties of the two-stream jets. The results are compared with those of the single-stream jet at Mach number 0.9 and Reynolds number 3600. The large eddy simulation(LES) with dynamic Smagorinsky sub-grid scale(SGS) approach is used to simulate the turbulent jet flow structure. The acoustic field is evaluated by the Ffowcs Williams–Hawkings(FW-H) integral equation. Considering the compressibility of high-speed gas jets, the density-based explicit formulation is adopted to solve the governing equations. Meanwhile, the viscosity is approximated by using the Sutherland kinetic theory. The predicted flow characteristics as well as the acoustic properties show that they are in good agreement with the existing experimental and numerical results under the same flow conditions available in the literature. The results indicate that the merging phenomenon of the dual-jet is triggered by the deflection mechanism of the Coanda effect, which sequentially introduces additional complexity and instability of flow structure. One of the main factors affecting the dual-jet merging is the aperture ratio, which has a direct influence on the potential core and surrounding flow fluctuation. The analysis on the noise pollution reveals that the potential core plays a fundamental role in noise emission while the additional mixing noise makes less contribution than the single jet noise. The overall sound pressure level(OASPL) profiles have a directive property, suggesting an approximate 25° deflection from the streamwise direction, however, shifting toward lateral direction of about 10° to 15° in the dual-jet. The conclusion obtained in this study can provide valuable data to guide the development of manufacturing-green technology in the multi-jet applications.     

Influence of initial conditions at the nozzle exit on the structure of round jet

Experimental data concerning the influence of initial conditions at the nozzle exit on the structure and development characteristics of round jets are reported. Features in the development of laminar and turbulent round jets emanating from variously elongated nozzles at identical Reynolds numbers are revealed. Smoke visualization pictures obtained for jets formed under different initial conditions (with different distributions of mean and pulsating flow velocities at the nozzle exit) are discussed. It is shown possible to make the zone of laminar flow in the jet stream more extended, and to delay the jet turbulization process in space, by making the flow-velocity profile more parabolic at the exit of elongated nozzle. Features in the development of vortical structures in a jet under an acoustic action are identified. It is shown that, for a turbulent round jet to be produced right at the nozzle exit, the nozzle length must be increased in excess of a certain value so that to provide for spatial growth of turbulent boundary layer thickness, finally ending in the formation of a fully turbulent flow velocity profile across the channel. This work was supported by the Russian Foundation for Basic Research (Grant No. 08-01-00027), the Ministry of Education and Science of the Russian Federation (project RNP 2.1.2.3370), and by the grants of President of the Russian Federation (NSH-454.2008.1 and. MK-420.2008.1).     

Three-dimensional consideration of jet impingement onto the kerf in relation to laser cutting process: Effect of jet velocity on heat transfer rates

In laser cutting process, an assisting gas is used to improve the mass removal rate from the cutting kerf and protect the kerf surfaces from the high temperature exothermic reactions, such as oxidation reactions, during the cutting process. Therefore, heat transfer rates from the kerf wall and the skin friction along the kerf surface are important for quality cutting. In the present study, jet emerging from a conical convergent nozzle and impinging onto the kerf surface is investigated in relation to the laser cutting process. The flow field in the kerf, the heat transfer rates from the kerf wall, and the skin friction along the kerf surface are computed for four average jet velocities at the nozzle exit and two kerf wall wedge angles. The ratio of the stand-off-distance (distance between the nozzle exit and the kerf top surface) to nozzle diameter is selected as H/D=2.2., where H is the stand-off-distance and D is the nozzle exit diameter. The kerf wall temperature is kept at 1500 K to resemble the laser cutting process. It is found that the Nusselt number increases sharply at the kerf inlet and decreases towards the kerf exit for the kerf wall angle of 0°. However, it increases gradually in this region for the kerf wedge angle of 4°. The skin friction decreases along the kerf surface.     

Roles of initial condition and vortex pairing in jet noise

This is a study of the effect of initial condition on sound generated by vortex pairing in a low Mach number, cold air jet (0·15 ⩽ M ⩽ 0·35). Data has been taken, both flow velocity fields and sound pressure far fields, in a quality anechoic facility, with careful documentation of the effect of initial condition on the sound field of jets of two different geometries (i.e., circular and elliptic). Explanations are presented for most of the observed effects by applying Möhring's theory of vortex sound to vortex filament models of coherent structures in the jets. The explanations also draw upon experience with coherent structure dynamics. The sound source of interest here is that associated with the pairing of shear layer vortices. The evolution of these vortices is greatly affected by the initial condition as is their resultant sound field. The elliptic jets with laminar boundary layers show azimuthal directivity, namely, sound pressure levels in the minor axis plane were greater than in the major axis plane. This difference decreases as the nozzle boundary layer undergoes natural transition with increasing jet speed. When the nozzle boundary layer is tripped, making it fully turbulent and removing the shear layer mode of pairing, the elliptic jet sound fields become nearly axisymmetric. What appears to be the most acoustically active phase of vortex pairing has been modeled, and the resulting sound field calculated for the circular jet. Supporting evidence is found in the experimental data for the validity of this model. The model explains the connection between the initial condition and the far field sound of jets. Interestingly, a general result of Möhring's theory is that motions of vortex rings (of any arbitrary shape) can produce only axisymmetric sound fields if the rings remain in a plane. This implies that the observed asymmetric directivity of the laminar elliptic jet sound field must be due to non-planar ring motions of the vortical structures. The primary contribution of this paper is to examine quantitatively the role of vortex pairing in the production of jet noise; the results are used to reemphasize that “pairing noise” cannot be dominant in most practical jet sound fields, contrary to claims by other researchers.     

射流式氧发生器的三维模拟研究

实现了对射流式氧发生器的三维仿真模拟,给出了氧发生器内部流场结构、各组分的分布状态等信息。研究了射流孔结构对氧发生器性能的影响。指出即便是具有相同比表面积的不同射流孔排布方式,也会对发生器性能产生影响。此外,逆向射流式氧发生器反应器中气体从双侧进入对于减小发生器对气体的阻力具有重要作用。     

Utilization of ultrasound to enhance high-speed water jet effects

Continuous high-speed water jets are presently used in many industrial applications such as cutting of various materials, cleaning and removal of surface layers. However, there is still a need for further research to enhance the performance of pure water jets. An obvious method is to generate water jets at ultra-high pressures (currently up to 700 MPa). An alternate approach is to eliminate the need for such high pressures by pulsing of the jet. This follows from the fact that the impact pressure on a target generated by a slug of water is considerably higher than the stagnation pressure of a corresponding continuous jet. Ultrasonically forced modulation of a continuous stream of water represents the most promising method of pulsed jet generation because of its simplicity and practicality. A pulsed jet is generated by modulating a continuous stream of water by ultrasonic waves. A velocity transformer connected to a piezoelectric transducer is located axially inside a nozzle to induce longitudinal pulsations in the water. An extensive laboratory research program is in progress to understand the basic principles of the process and to optimize the nozzle design for several applications. The results reported in this paper show that the performance of such a pulsed jet is far superior to that of a continuous jet operating at the same parameters. Experimental results obtained with the ultrasonic vibration of a tip situated inside the nozzle indicate that using this technique one can achieve performance of the jet even order of magnitude higher in comparison to continuous jet at the same hydraulic parameters. Performance of ultrasonically modulated jets in cutting of various materials was tested in laboratory conditions. In this paper, results of measurement of dynamic pressure in the nozzle and force effects of modulated jets are presented together with results obtained in cutting of various materials using ultrasonically modulated water jets. The results are compared with those obtained with continuous jets at the same operating parameters. Potential of forced modulation of the jet in applications of cleaning, paint and coating removal from surfaces and concrete cutting in the process of repair of concrete structures is mentioned.     

大气压微波等离子体射流装置放电特性研究

基于同轴传输线结构设计了两种不同喷嘴结构的大气压微波等离子体射流(MW-APPJ)装置,其工作频率2.45 GHz,工作气体为氩气,分别研究了两种不同喷嘴结构对等离子体放电特性产生的影响。仿真结果表明,MW-APPJ在气体喷嘴处会产生高强度的电场,经过优化结构,实现在频率2.45 GHz下,喷嘴处的场强满足氩气电离的击穿场强阈值要求。同时,利用多物理场耦合仿真软件对装置的气流分布进行了稳态模拟,并通过实验对比分析了两种喷嘴结构下大气压氩等离子体射流的基本特性。实验结果表明,不同的喷嘴结构会影响等离子体装置的反射系数随输入功率的变化规律,但并不影响等离子体射流长度随输入功率的变化规律和反射功率随进气流量的变化规律;同时,在大气压下,稳态微波等离子体射流呈现出类金属性,等离子体中的电子只能在很薄的区域中吸收微波能量,因而造成微波的反射功率较大。     

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     

Production of long,laminar plasma jets at atmospheric pressure with multiple cathodes

Plasma jets from conventional non‐transferred arc plasma devices are usually operated in turbulent flows at atmospheric pressure. In this paper, a novel non‐transferred arc plasma device with multiple cathodes is introduced to produce long, laminar plasma jets at atmospheric pressure. A pure helium atmosphere is used to produce a laminar plasma jet with a maximum length of >60 cm. The influence of gas components, arc currents, anode nozzle diameter, and gas flow rate on the jet characteristics is experimentally studied. The results reveal that the length of the plasma jet increases with increasing helium content and arc current but decreases with increasing nozzle diameter. As the gas flow rate increases, the length of the plasma jet initially increases and then decreases. Accordingly, the plasma jet is transformed from a laminar state to a transitional state and finally to a turbulent state. Furthermore, the anode arc root behaviours corresponding to different plasma jet flows are studied. In conclusion, the multiple stationary arc roots that exist on the anode just inside the nozzle entrance are favourable for the generation of a laminar plasma jet in this device.