来流脉动对自激振荡脉冲流的影响

基于瞬变流和流体网络理论建立了由往复柱塞泵,管路和自激振荡呀嘴组成的装置系统的数学模型,分析了泵源脉动对自激振荡脉冲射流振荡特性的影响,表明当来流脉动频率小于或等于嘴嘴装置的固有频率时,能提高射流的振荡,冲蚀效果,理论分析与实验结果吻合,其结论对合理设计自激振荡射流喷嘴有一定的指导意义。     

Unsteady Liquid Jets

The behaviour of unsteady liquid jets in a gas atmosphere is mainly governed by the conservation of momentum and the interaction with the environment. In this article it will be shown that many of the particular effects in the propagation and desintegration of unsteady jets are simply explained by the conservation of initial momentum. Many of the distortions and peculiar shapes of the liquid elements of the jet can be explained by a time and space development of weak initial distortions of momentum in travelling waves during propagation. The first part of the paper is devoted to the droplet formation in liquid jets of moderate Weber numbers in the order of ten. In this regime and higher the surface tension is not the governing influence of instability of jet decomposition and hence of the droplet formation. If there is a high frequency distortion of the velocity which is artificially implemented in our experiments by a special device, but under usual conditions also existing by turbulence or source distortions of the nozzle flow, the concentration of mass in packets in a kind of shock formation is the main reason for droplet formation. The frequency of droplets in space and time is given by the frequency of the distortion. The amplitude and phase is also given by the initial values of the driver for these quantities. For large amplitudes and low frequencies the jets are decomposed in very peculiar shapes, which can be very simply explained by the collision of packets of concentrated mass on the axis of the jet motion. The whole range of phenomena can be understood in a theoretically elegant manner by a Lagrangian transform of the nonlinear equations of motion together with some plausible considerations about the lateral motion. Solutions for the droplet formation and the formation of liquid films in different shapes can be achieved this way. As an illustration of the whole range of beautiful and regularly reproducible details of jet decomposition some photographs will be shown which summarize the phenomena and give an idea of the importance of systematic investigation of the sometimes very confusing appearance of fluid jets.     

Unsteady Liquid Jets

The behaviour of unsteady liquid jets in a gas atmosphere is mainly governed by the conservation of momentum and the interaction with the environment. In this article it will be shown that many of the particular effects in the propagation and desintegration of unsteady jets are simply explained by the conservation of initial momentum. Many of the distortions and peculiar shapes of the liquid elements of the jet can be explained by a time and space development of weak initial distortions of momentum in travelling waves during propagation.The first part of the paper is devoted to the droplet formation in liquid jets of moderate Weber numbers in the order of ten. In this regime and higher the surface tension is not the governing influence of instability of jet decomposition and hence of the droplet formation.If there is a high frequency distortion of the velocity which is artificially implemented in our experiments by a special device, but under usual conditions also existing by turbulence or source distortions of the nozzle flow, the concentration of mass in packets in a kind of shock formation is the main reason for droplet formation. The frequency of droplets in space and time is given by the frequency of the distortion. The amplitude and phase is also given by the initial values of the driver for these quantities.For large amplitudes and low frequencies the jets are decomposed in very peculiar shapes, which can be very simply explained by the collision of packets of concentrated mass on the axis of the jet motion.The whole range of phenomena can be understood in a theoretically elegant manner by a Lagrangian transform of the nonlinear equations of motion together with some plausible considerations about the lateral motion. Solutions for the droplet formation and the formation of liquid films in different shapes can be achieved this way.As an illustration of the whole range of beautiful and regularly reproducible details of jet decomposition some photographs will be shown which summarize the phenomena and give an idea of the importance of systematic investigation of the sometimes very confusing appearance of fluid jets.     

The trajectories of large fire fighting jets

This article describes a computer simulation of the trajectories of large water jets which allow the effects of changes in initial velocity, elevation, nozzle diameter, and head and tail winds to be examined. The rather limited information on aerodynamic drag of large jets obtained by other workers is used. The predicted trajectories compare well with the limited data available in the literature.The results also show that for a given flow rate an optimum pressure, and hence an optimum nozzle diameter, exists for maximum throw distance which has important implications for the design of the whole system including the pumps. The optimum elevation in still air lies in the range 30–40°. Wind effects are shown to be very important.     

Wall jet similarity of impinging planar underexpanded jets

Velocity profiles and wall shear stress values in the wall jet region of planar underexpanded impinging jets are parameterized based on nozzle parameters (stand-off height, jet hydraulic diameter, and nozzle pressure ratio). Computational fluid dynamics is used to calculate the velocity fields of impinging jets with height-to-diameter ratios in the range of 15–30 and nozzle pressure ratio in the range of 1.2–3.0. The wall jet has an incomplete self-similar profile with a typical triple-layer structure as in traditional wall jets. The effects of compressibility are found to be insignificant for wall jets with Ma < 0.8. Wall jet analysis yielded power-law relationships with source dependent coefficients describing maximum velocity, friction velocity, and wall distances for maximum and half-maximum velocities. Source dependency is determined using the conjugate gradient method. These power-law relationships can be used for mapping wall shear stress as a function of nozzle parameters.     

逆向喷流流场模态分析及减阻特性研究

逆向喷流减阻的基本原理是利用逆向高速喷流与飞行器绕流的相互作用，使飞行器周围的流场结构发生变化，致使飞行器的气动特性发生改变，从而改善飞行器的气动性能。利用数值模拟方法对轴对称球头、截锥的逆向喷流流场开展了研究，考虑了高温非平衡化学反应对流场的影响。模拟了球头和截锥在不同总压比时流场不同的模态：长穿透流模态（LPM）和短穿透流模态（SPM），得到了不同模态下钝体表面压力、气动力系数和不同模态之间转换的瞬态效应．简单分析了喷流在减阻方面的应用，给出了几个喷口参数与减阻效率之间的关系，提出了喷流减阻工程应用时应考虑的主要因素。     

Variational boundary integral approach for asymmetric impinging jets of arbitrary two‐dimensional nozzle

We consider asymmetric impinging jets issuing from an arbitrary nozzle. The flow is assumed to be two‐dimensional, inviscid, incompressible, and irrotational. The impinging jet from an arbitrary nozzle has a couple of separated infinite free boundaries, which makes the problem hard to solve. We formulate this problem using the stream function represented with a specific single layer potential. This potential can be extended to the surrounding region of the jet flow, and this extension can be proved to be a bounded function. Using this fact, the formulation yields the boundary integral equations on the entire nozzle and free boundary. In addition, a boundary perturbation produces an extraordinary boundary integral equation for the boundary variation. Based on these variational boundary integral equations, we can provide an efficient algorithm that can treat with the asymmetric impinging jets having arbitrarily shaped nozzles. Particularly, the proposed algorithm uses the infinite computational domain instead of a truncated one. To show the convergence and accuracy of the numerical solution, we compare our solutions with the exact solutions of free jets. Numerical results on diverse impinging jets with nozzles of various shapes are also presented to demonstrate the applicability and reliability of the algorithm.     

Particle statistics in a gas–solid coaxial strongly swirling flow: A direct numerical simulation

A direct numerical simulation of a strongly coaxial swirling particle-laden flow is conducted with reference to a previous experiment. The carrier phase is simulated as a coaxial swirling flow through a short nozzle injecting into a large container. The particle phase is carried by the primary jet, and simulated in the Lagrangian approach. The drag force, slip-shear force and slip-rotation force experienced by particles are calculated. A partial validation of the results is followed. The results are analyzed in Eulerian approach focusing on the statistical behavior of particle motion. The relative importance of the drag, slip-shear and slip-rotation forces under different Stokes numbers is indicated quantitatively. The particle velocity profiles, fluctuations, Reynolds stress, and turbulence intensity are demonstrated and analyzed respectively. An important “choke” behavior for large particles within the mainstream is found and interpreted. Additionally, the patterns of particle distribution and the helical structures of particle motion under different Stokes numbers are demonstrated qualitatively and analyzed quantitatively.     

Near field shock structure of dual co-axial jets

Experimental results on the shock structure of dual co-axial jets are presented. The effects of the geometric parameters of the inner nozzle, jet static pressure ratio (ratio of the exit plane static pressures of the inner and outer nozzles) and the ratio of outer to inner nozzle throat area on the shock structure were studied. A superimposed outer and inner jet structure was observed in the schlieren photographs. The inner flow is compressed by the outer flow resulting in the formation of a Mach disc and an exit shock. A parameter incorporating the effect of Mach number of the inner nozzle and jet static pressure ratio was found to correlate the observations regarding the Mach disc location.     

Experimental Investigation of Nozzle Exit Reflector Effect on Supersonic Jet

The present study describes an experimental work to investigate the effect of a nozzle exit reflector on a supersonic jet that is discharged from a convergent–divergent nozzle with a design Mach number of 2.0. An annular reflector is installed at the nozzle exit and its diameter is varied. A high-quality spark schlieren optical system is used to visualize detailed jet structures with and without the reflector. Impact pressure measurement using a pitot probe is also carried out to quantify the reflector’s effect on the supersonic jet which is in the range from an over-expanded to a moderately under-expanded state. The results obtained show that for over-expanded jets, the reflector substantially increases the jet spreading rate and reduces the supersonic length of the jet, compared with moderately under-expanded jets. The reflector’s effect appears more significant in imperfectly expanded jets that have strong shock cell structures, but is negligible in correctly expanded jet.     

自然超空泡与尾喷流相互作用的数值模拟

认识带尾喷流和自然超空泡的水下高速航行体流体动力特性并发展其预报与控制方法仍是水动力学领域极具挑战性的课题．本研究采用CFD方法对尾喷流和自然超空泡之间的相互作用进行了数值研究．针对发动机欠膨胀超音速喷流,采用现有实验结果验证了基于两方程湍流模型的二维轴对称流动数值模型的可靠性．尾喷流在空气和蒸汽环境中流动的数值计算结果表明,由于蒸汽环境中背压较低,欠膨胀尾喷流无法及时形成压缩波,使得蒸汽环境中尾喷流的过膨胀区和气相扩散区的体积比空气中大;尾喷流很难形成规则的激波格栅,波系结构相对简单．针对携尾喷流的平头航行体超空泡流状态的数值模拟结果表明,尾喷流注入超空泡后迅速充满航行体周围的空腔区域;尾喷流与超空泡尾迹区域形成的回射流相互作用最终导致超空泡断裂,断裂过程中伴随着燃气泡的下泄现象;受空泡壁面约束,尾喷流难以在狭窄的超空泡空腔内完全膨胀,尾喷流的激波波系结构有显著的变化：在喷嘴附近受到压缩,在远离喷嘴区域受到超空泡水汽掺混的破坏;空泡内压强基本维持在饱和蒸汽压附近,没有显著增加．     

Drying performance and surface forces of a pair of impinging radial jets

The drying performance and the forces generated by a pair of radial jets impinging on a plane surface are studied experimentally and compared with a pair of in-line (axial) jets. The reattachment rings of the radial jets are visualized by liquid-crystal-thermography. The results of the drying experiments are presented in terms of Sherwoodnumbers. The results of the surface force measurements are presented in terms of force coefficients. The effects of flow exit angle, exit velocity, horizontal nozzle spacing and vertical nozzle to surface distance on mass transfer and net surface force are discussed. Results show that drying performance of a pair of radial jets with an exit angle towards the surface of 10° is 18% higher than that of a pair of in-line jets. The surface force of impinging radial jets depends on the exit angle. A global pressure force on the surface is typical for jets directed towards the surface, while a global lifting force is found when the jet is initially directed away from the surface. Radial jets have a high potential in drying applications for force sensitive products and when high drying rates and/or small suction forces are of interest.     

Different regimes of the uniaxial elongation of electrically charged viscoelastic jets due to dissipative air drag

We investigate the effects of dissipative air drag on the dynamics of electrified jets in the initial stage of the electrospinning process. The main idea is to use a Brownian noise to model air drag effects on the uniaxial elongation of the jets. The developed numerical model is used to probe the dynamics of electrified polymer jets at different conditions of air drag force, showing that the dynamics of the charged jet is strongly biased by the presence of air drag forces. This study provides prospective beneficial implications for improving forthcoming electrospinning experiments.     

Investigation on the generation process of impact-driven high-speed liquid jets using a CFD technique

High-speed liquid jets have been applied to many fields of engineering, science and medicine. It is therefore of benefit to all these areas to investigate their characteristics by modern and inexpensive methods using a computational fluid dynamics (CFD) technique. Previously, high-speed liquid jets have been studied experimentally using a momentum exchange method, called the “impact driven method (IDM)”, by which the impact of a high-velocity projectile on the liquid package contained in the nozzle cavity produced the jet. The shock pulse reflections in the cavity caused by the impact then drove a multiple pulsed jet from the nozzle exit. In this study, a two-fluid simulation consisting of liquid and air can be successfully calculated by using a two-phase flow mixture model and a moving mesh for the projectile motion. The CFD results show good agreement to the results of previous experimental studies, both quantitatively and qualitatively. For the first time, the wave propagation within the liquid in the nozzle has been captured and analyzed, thereby demonstrating the dynamic characteristics of multiple pulsed high-speed liquid jets initiated by the IDM. This provides a breakthrough in the simulation of the supersonic injection of a liquid into air by using a well-known and user-friendly CFD software. It is useful fundamental knowledge for future studies of high-speed injection with applications in all its related fields.     

LES and RANS modelling of under-expanded jets with application to gaseous fuel direct injection for advanced propulsion systems

A density-based solver with the classical fourth-order accurate Runge-Kutta temporal discretization scheme was developed and applied to study under-expanded jets issued through millimetre-size nozzles for applications in high-pressure direct-injection (DI) gaseous-fuelled propulsion systems. Both large eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) turbulence modelling techniques were used to evaluate the performance of the new code. The computational results were compared both quantitatively and qualitatively against available data from the literature. After initial evaluation of the code, the computational framework was used in conjunction with RANS modelling (k-ω SST) to investigate the effect of nozzle exit geometry on the characteristics of gaseous jets issued from millimetre-size nozzles. Cylindrical nozzles with various length to diameter ratios, namely 5, 10 and 20, in addition to a diverging conical nozzle, were studied. This study is believed to be the first to provide a direct comparison between RANS and LES within the context of nozzle exit profiling for advanced high-pressure injection systems with the formation of under-expanded jets. It was found that reducing the length of the straight section of the nozzle by 50% resulted in a slightly higher level of under-expansion (∼2.6% higher pressure at the nozzle exit) and ∼1% higher mass flow rate. It was also found that a nozzle with 50% shorter length resulted in ∼6% longer jet penetration length. At a constant nozzle pressure ratio (NPR), a lower nozzle length to diameter ratio resulted in a noticeably higher jet penetration. It was found that with a diverging conical nozzle, a fairly higher penetration length could be achieved if an under-expanded jet formed downstream of the nozzle exit compared to a jet issued from a straight nozzle with the same NPR. This was attributed to the radial restriction of the flow and consequently formation of a relatively smaller reflected shock angle. With the conical nozzle used in this study and a 30 bar injection pressure, an under-expanded hydrogen jet exhibited ∼60% higher penetration length compared to an under-expanded nitrogen jet at 100 μs after start of injection. Moreover, the former jet exhibited ∼22% higher penetration compared to a nitrogen jet issued through the conical profile with 150 bar injection pressure.     

高压旋转射流流动特性的实验研究

测量了叶轮导引高压射流喷嘴在不同叶轮螺旋倾角和喷嘴收缩角下高压水的喷射形状参数,通过分析给陋了此种射流的外边界曲线方程形式及射流扩散角与叶轮螺旋倾角、喷嘴收缩角的关系,最后对射流机理进行了探讨,并给出一种微团轨道模型研究方法。     

钝后缘风力机翼型的环量控制研究

钝后缘风力机翼型具有结构强度高、对表面污染不敏感等优点,但其较大的阻力系数使得翼型的整体气动特性不够理想. 利用环量控制方法对钝后缘风力机翼型进行了流动控制,以改善钝后缘风力机翼型的气动特性,减弱尾迹区脱体涡强度. 通过对钝后缘风力机翼型环量控制方法进行相关的数值模拟,对比研究了环量控制方法的增升减阻效果, 研究了环量控制下翼型升阻力特性随射流动量系数的变化规律,并对不同射流动量系数下环量控制方法的气动品质因子和控制效率进行了分析. 研究结果表明:环量控制方法能够大幅提升钝后缘风力机翼型的升力系数,同时有效地降低翼型的阻力系数; 翼型的升力系数随射流动量系数的增大而增大,表现出很明显的分离控制阶段和超环量控制阶段的变化规律; 射流能耗的功率系数随射流动量系数的增大而增大,且增长速率逐渐增大;实施环量控制方法后叶片的输出功率同样随射流动量系数增大而增大,但增长速率逐渐降低. 总体来说,环量控制方法可以有效地改善钝后缘风力机翼型的气动特性以及功率输出特性,在大型风力机流动控制中具有很好的应用前景.      

A procedure for the design of nozzles used for the production of turbulent liquid jets

Where turbulent liquid jets are used for cutting and mining purposes the pressure generated by impact must be maximized. Initial jet behaviour has an important influence on subsequent jet impact pressures at medium range. Nozzle wall boundary layer history has a strong influence on the initial jet, and certain boundary layer features can be linked to poor jet performance. The procedure outlined in this paper was developed to eliminate new nozzle designs or changes in operating conditions on the grounds of badly behaved nozzle boundary flow. The design procedure consists of a potential flow analysis and a boundary layer analysis coupled to empirical correlations for boundary layers in accelerated flows. The procedure is exemplified by application to the design of a nozzle to be used for the specific purpose of mining china clay.     

漂浮运动对风力机气动特性的影响分析

海上浮式风机为蕴藏丰富的深海风能开发提供了有效的解决方案,浮式基础存在的大幅度纵荡、纵摇和艏摇运动可以改变风轮与流场间的相互作用,从而影响风力机的气动特性。基于叶素-动量理论及其修正方法,以NREL 5MW风力机为研究对象,考虑浮式基础运动对叶片不同径向位置处相对入流风速的影响,提出了风载荷的计算模型,通过编程计算获得了叶轮转矩和风力机功率,并比较了不同运动形式对风力机功率波动的影响。结果表明,纵摇对其功率特性影响最大,这为海上浮式风机的优化设计提供理论依据与数据基础。     

On coherent structures and mixing characteristics in the near field of a rotating-pipe jet

Mixing characteristics and coherent structures populating the near-nozzle area of a rotating-pipe jet at the Reynolds number of 5300 were studied by Large-eddy simulation (LES). The swirl rate, defined as the ratio of the tangential velocity of the inner pipe wall to the bulk axial velocity, varied from 0 to 1, corresponding to a weak-to-moderate swirl intensity, insufficient to induce reverse flow near the nozzle. The visualization shows that for the non-swirling jet the near-wall streaky structures generated in the pipe interact with the shear layer, evolving into hairpin-like structures that become tilted at low rotation rates. For higher swirl, they cannot be recognized as they are destroyed at the nozzle exit. No large-scale coherent structures akin to Kelvin–Helmholtz vortical rings in the ‘top-hat’ jets are identifiable close to the nozzle. Using the single and joint probability density functions of velocity and passive scalar (temperature) fields we quantify the events responsible for the intensive entrainment at various swirl numbers. The isosurface of the temperature field indicates the meandering and precessing motion of the rotating jet core at the axial distance of 6D downstream, where D is the diameter of the pipe. The Fourier analysis with respect to the azimuthal angle and time reveals an interplay between the co- and counter-rotating modes. These findings explain the previously detected but not fully clarified phenomenon of the weakly counter-rotating jet core at low swirl rates.