A Computational Study of Thrust Vectoring Control Using Dual Throat Nozzle

Dual throat nozzle （DTN） is fast becoming a popular technique for thrust vectoring. The DTN is designed with two throats, an upstream minimum and a downstream minimum at the nozzle exit, with a cavity in between the upstream throat and exit. In the present study, a computational work has been carried out to analyze the performance of a dual throat nozzle at various mass flow rates of secondary flow and nozzle pressure ratios （NPR）. Two-dimensional, steady, compressible Navier-Stokes equations were solved using a fully implicit finite volume scheme. The present computational results were validated with available experimental data. Based on the present results, the control effectiveness of thrust-vectoring is discussed in terms of the thrust coefficient and the coefficient of discharge.     

Numerical analysis of Chevron nozzle effects on performance of the supersonic ejector-diffuser system

The supersonic nozzle is the most important device of an ejector-diffuser system.The best operation condition and optimal structure of supersonic nozzle are hardly known due to the complicated turbulent mixing,compressibility effects and even flow unsteadiness which are generated around the nozzle extent.In the present study,the primary stream nozzle was redesigned using convergent nozzle to activate the shear actions between the primary and secondary streams,by means of longitudinal vortices generated between the Chevron lobes.Exactly same geometrical model of ejector-diffuser system was created to validate the results of experimental data.The operation characteristics of the ejector system were compared between Chevron nozzle and conventional convergent nozzle for the primary stream.A CFD method has been applied to simulate the supersonic flows and shock waves inside the ejector.It is observed that the flow structure and shock system were changed and primary numerical analysis results show that the Chevron nozzle achieve a positive effect on the supersonic ejector-diffuser system performance.The ejector with Chevron nozzle can entrain more secondary stream with less primary stream mass flow rate.     

Supersonic cavity flows over concave and convex walls

Supersonic cavity flows are characterized by compression and expansion waves, shear layer, and oscillations inside the cavity. For decades, investigations into cavity flows have been conducted, mostly with flows at zero pressure gradient entering the cavity in straight walls. Since cavity flows on curved walls exert centrifugal force, the features of these flows are likely to differ from those of straight wall flows. The aim of the present work is to study the flow physics of a cavity that is cut out on a curved wall. Steady and unsteady numerical simulations were carried out for supersonic flow through curved channels over the cavity with L/H = 1. A straight channel flow was also analyzed which serves as the base model. The velocity gradient along the width of the channel was observed to increase with increasing the channel curvature for both concave and convex channels. The pressure on the cavity floor increases with the increase in channel curvature for concave channels and decreases for convex channels. Moreover, unsteady flow characteristics are more dependent on channel curvature under supersonic free stream conditions.     

Effect of plate position and size for self-induced flow oscillation of underexpanded supersonic jet impinging on perpendicular plate

When the underexpanded supersonic jet impinges on the obstacle, it is well known that the self-induced flow oscillation occurs at the specific condition of the pressure ratio in the flowfield, the position of an obstacle and so on. This oscillation is related with the noise problems of aeronautical and other industrial engineering so that the characteristic and the mechanism of self-induced flow oscillation have to be cleared to control the various noise problems. But, it seems that the characteristics of t...     

Experimental Study on Shock Wave Structures in Constant-area Passage of Cold Spray Nozzle

Cold spray is a technique to make a coating on a wide variety of mechanical or electric parts by spraying solidparticles accelerated through a high-speed gas flow in a converging-diverging nozzle.In this study,pseudo-shockwaves in a modeled cold spray nozzle as well as high-speed gas jets are visualized by schlieren technique.Theschlieren photographs reveals the supersonic flow with shock train in the nozzle,Static pressure along the barrelwall is also measured.The location of the head of pseudo-shock wave and its pressure distribution along the noz-zle wall are analytically explained by using a formula of pseudo-shock wave.The analytical results show that thesupersonic flow accompanying shock wave in the nozzle should be treated as pseudo-shock wave instead of nor-mal shock wave.     

Effect of cavities inside tube banks on acoustic resonance

In the present paper the attention is focused on the effect of small cavities inside in-line tube banks on acoustic resonance which occurred in the two-dimensional model of boiler. We measured the sound pressure level, the amplitude and the phase delay of acoustic pressures and the gap velocity. As a result, we found many peak frequencies of sound pressure level with different Strouhal numbers, mainly about S1=0.15, 0.26 and 0.52. The variation of SPL for S,=0.26, 0.52 components in the tube banks with cavities was the same as the result of no cavities. The existence of cavities inside in-line tube banks caused the resonance of St=0.15. And the acoustic resonance of the first mode in the transverse direction was generated if the small cavities existed inside the tube banks. This resonance was not generated from the tube banks of no cavities. The resonance onset velocity in the transverse mode was fairly slower than that of no cavities. It was easy to generate acoustic resonance when there were small cavities inside in-line tube banks.     

Study on the performance deterioration of mixed flow impeller due to change in tip clearance

Performance of mixed flow compressor with un-shrouded impeller strongly depends upon unsteady, asymmetrical flow fields in the axial directions. The flow through the mixed flow impeller is complex due to three-dimensional nature of geometry. In mixed flow impeller, there are clearances between the rotating impeller blades and the casing as the high pressure ratio compressors are usually open shrouded impellers. As a result, certain amount of reduction in the performance is unavoidable due to clearance flows. In the present investigations, numerical analysis is performed using a commercial code to investigate tip clearance effects on through flow. The perform- ance of mixed flow impeller with four different clearances between impeller and stationary shroud are evaluated and compared with experimental results. The impeller performance map was obtained for different operating speeds and mass flow rates with different tip clearances. The result shows that the tip leakage flow strongly inter- acts with mainstream and contributes to total pressure loss and performance reduction. The pressure and per- formance decrement are approximately linearly proportional to the gap between impeller and stationary shroud. The analysis showed scope for improvement in design of the compressor for better performance in terms of effi- ciency and operating range.     

Influence of separated vortex on aerodynamic noise of an airfoil blade

In order to clarify the mechanism by which aerodynamic noise is generated from separated flow around an airfoil blade, the relation between the attack angle and the aerodynamic noise of the blade was analyzed using a wind tunnel experiment and a CFD code. In the case of rear surface separation, the separated vortex which has a large-scale structure in the direction of the blade chord is transformed into a structure that concentrates at the trailing edge with an increase in the attack angle. The aerodynamic noise level then becomes small according to the vortex scale in the blade chord. When the flow is separated at the leading edge, a separated vortex of low pressure is formed at the vicinity of the trailing edge. The pressure fluctuations on the blade surface at the vicinity of the trailing edge become large due to the vortex in the wake. It is considered that the aerodynamic noise level increases when the flow is separated at the leading edge because the separated vortex is causing the fluctuations due to wake vortex shedding.     

空气雾化喷嘴的设计与实验研究

为提高喷嘴雾化效率,基于维多辛斯曲线理论对喷嘴出口结构进行优化,研制出一种新型渐缩式空气雾化喷嘴,并搭建了喷雾实验平台进行实验。利用图像处理技术对实验数据进行分析,提取不同结构喷嘴的喷雾粒径和雾化锥角分布,通过对比仿真和试验结果分析喷嘴气液夹角对雾化效果的影响。研究表明:喷嘴喷雾实验和仿真结果较吻合,相对误差在6.5%以内;喷嘴气液夹角α=20°时,雾化锥角最大,液滴索特尔平均直径最小,雾化效果最好。     

Uncertainty-quantification analysis of the effects of residual impurities on hydrogen–oxygen ignition in shock tubes

This study addresses the influences of residual radical impurities on the computation and experimental determination of ignition times in H₂/O₂ mixtures. Particular emphasis is made on the often-times encountered problem of the presence of H-atoms in the initial composition of H₂/O₂ mixtures in shock tubes. Two methods are proposed for quantifying experimentally H-residual impurities in shock tubes, namely, an a priori method that consists of detecting OH traces upon shocking unfueled mixtures, and a posteriori method in which the amount of impurities is inferred by comparing fueled experimental autoignition data with calculations. A stochastic Arrhenius model that describes the amount of H-radical impurities in shock tubes is proposed on the basis of experimental measurements as a function of the test temperature. It is suggested that this statistical model yields a probability density function for the residual concentration of hydrogen radicals in standard shock tubes. Theoretical quantifications of the uncertainties induced by the impurities on autoignition times are provided by using the 5-step short chemistry of Del Álamo et al. [1]. The analysis shows that the relative effects of H-impurities on delay times above crossover become more important as the dilution increases and as the temperature and pressure decrease. Below crossover, the effects of H-impurities on the ignition delay vanish rapidly, and are negligible compared to the departures produced by the non-ideal pressure rise that is seen in some shock-tube experiments at such low temperatures. The influences of kinetic uncertainties on the ignition time are typically negligible compared to the effects of the uncertainties induced by H-impurities when the short mechanism is used, except for air at high temperatures for which kinetic uncertainties dominate. Furthermore, calculations performed with the short mechanism show that correlations between the uncertainties in the rates of branching and termination steps have only some small influences on the ignition-time variabilities near crossover, where a global sensitivity analysis shows an increasing importance of the recombining kinetics. Computational quantifications of uncertainties are carried out by using numerical simulations of homogeneous ignition subject to Monte-Carlo sampling of the concentration of impurities. For the conditions analyzed, these computations show that the variabilities produced in ignition delays by the uncertainties in H-impurities are comparable to the experimental data scatter and to the effects of typical uncertainties of the test temperature when the Stanford chemical mechanism [2] is used. The calculations also unveil that the utilization of two other different chemical mechanisms, namely San Diego [3] and GRI v3.0 [4], yields variations in the ignition delays which are within the range of the uncertainties induced by the H-impurities. Finally, the effects of residual impurities in kinetic-isolation experiments and in supersonic-combustion ramjets are briefly discussed.     

受限通道中三维激波与侧壁面作用的数值模拟研究

为了了解超燃冲压发动机中三维斜激波与壁面的相交情况,以冷态模拟为基础,对两种不同形状的斜劈结构产生的斜激波进行数值模拟。当斜劈展向与气流方向成45.0°时,激波和膨胀波结构呈现三维特性,并且激波在两侧壁面处的反射现象不同。以激波面上游为视角,当激波与壁面相交成锐角时会产生马赫反射;而激波与壁面相交成钝角则会以叠加膨胀波的形式减弱激波,并且使激波面的法线方向与壁面的法线方向垂直。由于两侧壁面的影响,随着高度的增加激波强度减弱,同时气体经过斜激波后偏转角度也减小。对于构型Ⅱ,有斜劈的区域产生三维斜激波会延伸到无斜劈区域,此时下壁面与斜激波相交成钝角,同样会以叠加膨胀波的形式减弱斜激波。     

车用汽油机配气机构噪声的研究

本文介绍了EQ6100型发动机配气机构噪声试验结果。讨论了配气机构噪声形成的机理,指出了影响配气机构噪声的因素及降低配气机构噪声的基本途径。     

Prediction of aerodynamic noise in a ring fan based on wake characteristics

A ring fan is a propeller fan that applies an axial-flow impeller with a ring-shaped shroud on the blade tip side. In this study, the entire flow field of the ring fan is simulated using computational fluid dynamics (CFD); the accuracy of the CFD is verified through a comparison with the aerodynamic characteristics of a propeller fan of current model. Moreover, the aerodynamic noise generated by the fan is predicted on the basis of the wake characteristics. The aerodynamic characteristic of the ring fan based on CFD can represent qualitatively the variation in the measured value. The main flow domain of the ring fan is formed at the tip side of the blade because blade tip vortex is not formed at that location. Therefore, the relative velocity of the ring fan is increased by the circumferential velocity. The sound pressure levels of the ring fan within the frequency band of less than 200 Hz are larger than that of the propeller fan. In the analysis of the wake characteristics, it revealed that Karman vortex shedding occurred in the main flow domain in the frequency domain lower than 200 Hz; the aerodynamic noise of the ring fan in the vortex shedding frequency enlarges due to increase in the relative velocity and the velocity fluctuation.     

Study on an Undershot Cross-Flow Water Turbine

This study aims to develop a water turbine suitable for ultra-low heads in open channels, with the end goal being the effective utilization of unutilized hydroelectric energy in agricultural water channels. We performed tests by applying a cross-flow runner to an open channel as an undershot water turbine while attempting to simplify the structure and eliminate the casing. We experimentally investigated the flow fields and performance of water tur- bines in states where the flow rate was constant for the undershot cross-flow water turbine mentioned above. In addition, we compared existing undershot water turbines with our undershot cross-flow water turbine after at- taching a bottom plate to the runner. From the results, we were able to clarify the following. Although the effec- tive head for cross-flow runners with no bottom plate was lower than those found in existing runners equipped with a bottom plate, the power output is greater in the high rotational speed range because of the high turbine ef- ficiency. Also, the runner with no bottom plate differed from rtmners that had a bottom plate in that no water was being wound up by the blades or retained between the blades, and the former received twice the flow due to the flow-through effect. As a result, the turbine efficiency was greater for runners with no bottom plate in the full ro- tational speed range compared with that found in runners that had a bottom plate.     

基于欧拉多流体模型对喷嘴内流和射流破碎的研究

在Open FOAM的框架下,采用多流体模型和大涡模拟方法研究不同背压对喷嘴内流和射流破碎的影响.利用已修正的空化模型考虑了空化现象,利用界面压缩法对两相界面进行捕捉,进而分析燃油的一次破碎过程.结果表明:降低背压有利于喷嘴内部空化的形成与发展,但会导致喷嘴喉部有效流通面积减小,流动阻力增加,质量流量降低.射流破碎中气/液相界面处气/液两相存在很大的速度差,导致产生很强的阻力,大大促进了一次破碎过程中主喷雾区和蘑菇头的破碎.背压越高,空气密度越大,则燃油受到的阻力越大,液柱破碎越剧烈且射流破碎角越大.因此,同时提高喷射压力和背压可以促进燃油与空气的混合,也可以有效抑制空化来避免空蚀现象以及流量系数降低,从而延长喷嘴的使用寿命.     

175F-1型柴油机整机降噪研究

本文概述了柴油机声辐射机理,讨论了噪声源的识别方法和发动机外表面按其辐射声功率的大小评定主次的方法。通过对175F—1型柴油机现状的分析,提出了降低该机整机噪声的有效措施。同时强调指出:控制辐射源与控制激振源同样重要,两者都是柴油机噪声控制技术的关键。     

浅析波浪能发电的现状与发展

就国内外近年来在波浪能发电方面的研究情况进行一个初步的分析,提出未来发展建议,为助推波浪能发电向商业化发展提供动力。虽然各国波浪能发电示范研究都有了一些进展,取得了具有较高科学价值的相关数据,但从目前技术发展来看,波浪能发电装置的研发仍处在技术攻关和产业化前夕阶段,还有诸多问题需要解决。如果能研发出一种高效可靠的波浪能发电装置,将是一种可持续提供清洁能源的途径,为海洋强国提供可靠的能源保障。     

柴油机不同类型喷嘴内部空化流动特性的研究

柴油喷嘴内部空化效应是燃油液体射流雾化的重要原因之一。针对目前发动机上广泛采用的SAC型和VCO型两种柴油机多孔喷嘴,运用基于欧拉多流体法的多相流模型,通过全自动网格生成技术,对二者进行了多维数值模拟。将空化模型应用于数值模拟中,全面分析了喷射压力、喷孔背压、喷孔入口圆角半径和喷孔倾斜角等参数对喷孔内部空化流动及出口流量的影响。结果表明,增大喷射压力容易促使空化的发生,压力较大时,SAC型喷嘴出口流量变化不如VCO型的大;出口压力对空化起到抑制作用,减小出口压力有利于空化的形成,有助于喷孔出口燃油的雾化;喷孔入口圆角半径和倾斜角的增大,使得孔内流动变得更加顺畅,燃油蒸气体积变小,出口流量增加。所以孔内空化效应对缸内喷雾过程尤其是燃油液体射流的初始破碎的影响不容忽视。     

柴油机喷油器喷孔空泡雾化的研究

作者对柴油机喷孔空泡雾化的特性进行了研究，分析了喷孔内空泡产生的机理，并建立了计算模型。该模型考虑了喷射压力、喷孔几何结构、燃烧室条件、孔内流动损失与空穴因素，为喷雾的多维模拟提供了准确的初始条件。三维数值模拟计算结果表明，空泡雾化促进了喷孔出口液核的分裂，加剧了喷束的二次雾化，以考虑空雾化模型为计算初始条件，其在喷射速度、液滴尺寸（SMD）和贯穿距的计算结果更符合实际喷雾测量。