Experimental study on the tuned operation of a free piston driver

An experimental study of the tuned operation of a free-piston driver is described. Two series of experiments were carried out. The first was performed to validate a theory which has been developed recently to predict the operation with a small free piston-driver named NAL-CTA. The driver has a transparent window at the end of the compression tube to allow observation of piston motion. In the second, a theoretically determined length of piston buffer was used to tune the operating condition. Piston collision speeds of less than 3 m/sec were observed. A quasi-one-dimensional numerical code including leakage of driver gas through the piston clearance gap was derived. The numerical result agreed well with the experimental result. It is concluded that tuned operation, by using an appropriate length of the piston buffer, can be extrapolated to large-size tunnels. Received 6 March 1998 / Accepted 29 October 1998     

自由活塞压缩管ALE方法数值模拟

当前国际上实现高焓气体流动的实验手段之一是自由活塞驱动类脉冲设备,包括自由活塞激波风洞和自由活塞膨胀管.采用自由活塞压缩管作为激波风洞和膨胀管的驱动段时,其驱动能力在很大程度上决定了该类设备的性能.本文采用计算流体力学中任意拉格朗日——欧拉方法(arbitrary Lagrangian Eulerian)数值模拟了压缩管内部的自由活塞运动和气体流动特征.采用移动网格技术来适应活塞运动边界,耦合求解网格运动和气体流动过程,并通过双时间步长方法进行流体运动的时间积分.为了满足几何守恒律(geometric conservation law),对移动网格的法向矢量和表面面积计算进行了修正.不同时刻的活塞位置试验测量结果及欧拉方法预测结果,以及基于简单波理论获得的运动活塞底部气体压力、活塞速度与活塞位置都与当前的ALE方法十分一致.该工作为下一步数值模拟自由活塞激波风洞和自由活塞膨胀管中包括压缩管、激波管和喷管等不同部位的耦合流动提供了基础.      

Free-piston driver optimisation for simulation of high Mach number scramjet flow conditions

The University of Queensland (UQ) is currently developing high Mach number, high total pressure scramjet flow conditions in its X2 and X3 expansion tube facilities. These conditions involve shock-processing a high-density air test gas followed by its unsteady expansion into a low-pressure acceleration tube. This relatively slow shock-processing requires the driver to supply high pressure gas for a significantly greater duration than normally required for superorbital flow conditions. One technique to extend the duration is to operate a tuned free-piston driver. For X2, this involves the use of a very light piston at high speeds so that, following diaphragm rupture, the piston displacement substitutes for vented driver gas, thus maintaining driver pressure much longer. However, this presents challenges in terms of higher piston loading and also safely stopping the piston. This article discusses the tuned driver concept, the design of a very lightweight but highly stressed piston, and details the successful development of a new set of tuned free-piston driver conditions for X2.     

Simulation of a complete reflected shock tunnel showing a vortex mechanism for flow contamination

Simulations of a complete reflected shock tunnel facility have been performed with the aim of providing a better understanding of the flow through these facilities. In particular, the analysis is focused on the premature contamination of the test flow with the driver gas. The axisymmetric simulations model the full geometry of the shock tunnel and incorporate an iris-based model of the primary diaphragm rupture mechanics, an ideal secondary diaphragm and account for turbulence in the shock tube boundary layer with the Baldwin-Lomax eddy viscosity model. Two operating conditions were examined: one resulting in an over-tailored mode of operation and the other resulting in approximately tailored operation. The accuracy of the simulations is assessed through comparison with experimental measurements of static pressure, pitot pressure and stagnation temperature. It is shown that the widely-accepted driver gas contamination mechanism in which driver gas ‘jets’ along the walls through action of the bifurcated foot of the reflected shock, does not directly transport the driver gas to the nozzle at these conditions. Instead, driver gas laden vortices are generated by the bifurcated reflected shock. These vortices prevent jetting of the driver gas along the walls and convect driver gas away from the shock tube wall and downstream into the nozzle. Additional vorticity generated by the interaction of the reflected shock and the contact surface enhances the process in the over-tailored case. However, the basic mechanism appears to operate in a similar way for both the over-tailored and the approximately tailored conditions.Communicated by R. R. Boyce     

The effect of diaphragm opening time on the feasibility of tuned operation in free piston shock tunnels

The effect of diaphragm opening time on the feasibility of tuned operation is analyzed by parametric calculations. The effect on the duration of pressure holding is also investigated. Characteristic parameters are defined to show the effects of dimensions, operation conditions and diaphragm opening time. It is shown that diaphragm opening time needs to be less than a certain limit to achieve tuned operation. The limit depends on pressure at diaphragm rupture and the ratio of diameters between a downstream shock tube and a compression tube. The permissible diaphragm opening time is also proportional to the length of the compression tube. The existence of the limit required for tuned operation is shown to be caused by the increase of pressure in the driver gas during the diaphragm opening. Moreover, the effect of diaphragm opening time on the duration of the pressure holding is shown to be weak once tuned operation becomes feasible.

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Advances in detonation driving techniques for a shock tube/tunnel

Early works on the detonation driven shock tube are reviewed briefly. High initial pressure detonable mixture can be used in backward-detonation driver when the buffer tube is attached to the end of the driver for eliminating the excessive reflected peak pressure. Experimental data showed that an improvement on attenuation of the incident shock wave generated by the forward driver can be obtained, provided the diameter of the driver is larger than that of the driven section and an abrupt reduction of cross-section area is placed just beyond the diaphragm. Also, it is clearly verified by a numerical analysis. An additional backward-detonation driver is proposed to attach to the primary detonation driver and on condition that the ratios of initial pressure in the additional driver to that in the primary driver exceed the threshold value, the Taylor wave behind detonation wave in the primary detonation driver can be eliminated completely.     

Gaseous detonation driver for a shock tunnel

The concept of a shock tunnel with gaseous detonation driver is discussed. A detonation driver presents an alternative to a free-piston driver because comparable values of high enthalpy can be attained, however, without the fast movement of a heavy piston. Wave diagrams, pressure and temperature distributions are presented. Finally, first experimental results are given.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Experimental investigation on tunnel sonic boom

Upon the entrance of a high-speed train into a relatively long train tunnel, compression waves are generated in front of the train. These compression waves subsequently coalesce into a weak shock wave so that a unpleasant sonic boom is emitted from the tunnel exit. In order to investigate the generation of the weak shock wave in train tunnels and the emission of the resulting sonic boom from the train tunnel exit and to search for methods for the reduction of these sonic booms, a 1300 scaled train tunnel simulator was constructed and simulation experiments were carried out using this facility.In the train tunnel simulator, an 18 mm dia. and 200 mm long plastic piston moves along a 40 mm dia. and 25 m long test section with speed ranging from 60 to 100 m/s. The tunnel simulator was tilted 8° to the floor so that the attenuation of the piston speed was not more than 10 % of its entrance speed. Pressure measurements along the tunnel simulator and holographic interferometric optical flow visualization of weak shock waves in the tunnel simulator clearly showed that compression waves, with propagation, coalesced into a weak shock wave. Although, for reduction of the sonic boom in prototype train tunnels, the installation of a hood at the entrance of the tunnels was known to be useful for their suppression, this effect was confirmed in the present experiment and found to be effective particularly for low piston speeds. The installation of a partially perforated wall at the exit of the tunnel simulator was found to smear pressure gradients at the shock. This effect is significant for higher piston speeds. Throughout the series of train tunnel simulator experiments, the combination of both the entrance hood and the perforated wall significantly reduces shock overpressures for piston speeds ofu _p ranging from 60 to 100 m/s. These experimental findings were then applied to a real train tunnel and good agreement was obtained between the tunnel simulator result and the real tunnel measurements.     

高焓膨胀管中压缩管最佳长度设计分析

中国空气动力研究与发展中心超高速所即将建成的高焓喷胀管采用自由活塞驱动,为了能够使该设备有效运行试验时间达到最长,需要对设备中的压缩管长度进行优化。针对压缩管中的自由活塞运动,本文通过数值求解单一组分和混合驱动气体条件下的活塞运动准一维常微分控制方程组,分析了不同试验设计参数和不同驱动气体介质对压缩管定压驱动时间的影响。采用数值解结合脊线(局部极值曲线)理论获得了定压驱动时间局部极值曲面,基于压缩管四种摩尔比例驱动气体(100%H2,85%H2+15%N2,100%He,85%He+15%Ar)和三组压缩比(λ=60,100,140)下的极值曲面,研究发现,定压驱动时间脊线高度随着压缩管的有效长度(初始活塞头部到压缩管末端膜片的距离)和内径之比L/D增加而增加。本文研究最终获得了给定参数条件下满足最长定压驱动时间要求的最佳压缩管长度。     

Aerodynamic calibration of TCM2 facility and study of a bow shock layer by emission and laser spectroscopy

Flow properties in the TCM2 free piston shock tube/tunnel are determined by time-resolved pressure and heat flux measurements in numerous points of the shock tube and the nozzle, and in the free flow for two stagnation enthalpy conditions (3.5 and 11 MJ/kg). These measurements demonstrate the homogeneity of the flow during more than 1 ms. The cleanness of the useful test time is shown with time-resolved emission measurements at critical wavelengths. NO fluorescence profiles are established with local and planar laser-induced fluorescence in the shock layer around a cylindrical model. It allows to determine the shock stand-off distance for both enthalpy conditions. The problems of quenching and amplified spontaneous emission are considered. The importance of atomic oxygen and atomic nitrogen densities as well as temperature effects is also shown. Evaluation of the temperatures behind the shock front through spectroscopic data agrees with calculations. The proof of the presence of vibrationally excited NO ahead of the shock layer is given. Received 14 March 2000 / Accepted 18 June 2001     

高速斜入水和水平入水气炮水箱实验系统

为开展模型高速斜入水和水中高速航行的水流场实验研究,研制了立式和卧式气炮与水箱组合的实验系统。通过快速阀和活塞阀控制气炮激发和驱动状态,一级气炮采用高压空气直接驱动弹托和模型,二级气炮采用高压空气驱动重活塞压缩使集气腔中产生高压气体,再驱动弹托和模型达到预定速度。通过调节水箱和发射管角度,使高速模型斜入水或水平入水。其中,立式可变发射角二级气炮可发射质量1～1000 g的模型至2500 m/s最大速度,卧式一级气炮可发射质量1～100 kg的模型至300 m/s最大速度。和小气室、高燃气压力火药驱动方式相比,新型气炮采用大体积、中低驱动压力气室,高压气体更接近等熵膨胀做功,调节高压气体压力,能较好地满足模型质量和速度的宽范围要求。结合光反射通断法测速、高速摄影和阴影流场显示等测量技术,得到立式气炮压缩管重活塞运动速度、压缩管末端压力时间曲线和模型倾斜与水平入水的流场阴影图像。结果表明:重活塞速度在膜片破裂前和理论计算值符合较好,但破膜后差异较大。立式气炮流场阴影图像反映了模型斜入水产生的空中和水中激波以及在气水界面的反射激波、空泡形成和侧向气水界面的破碎与飞溅等现象。从卧式气炮的模型水平入水阴影图像提取气泡轮廓,清楚地看出尾部气泡气水界面的波动和失稳。和商业计算软件Fluent计算结果相比,空泡上游区域基本重合,但尾流区域强湍流导致两者存在明显差异。和水洞实验相比,气炮水箱实验系统近真实地再现高速入水过程伴随的冲击和动态空化等物理现象和模型尺度效应。     

Attenuation of weak shock waves along pseudo-perforated walls

In order to attenuate weak shock waves in ducts, effects of pseudo-perforated walls were investigated. Pseudo-perforated walls are defined as wall perforations having a closed cavity behind it. Shock wave diffraction and reflection created by these perforations were visualized in a shock tube by using holographic interferometer, and also by numerical simulation. Along the pseudo-perforated wall, an incident shock wave attenuates and eventually turns into a sound wave. Due to complex interactions of the incident shock wave with the perforations, the overpressure behind it becomes non-uniform and its peak value can locally exceed that behind the undisturbed incident shock wave. However, its pressure gradient monotonically decreases with the shock wave propagation. Effects of these pseudo-perforated walls on the attenuation of weak shock waves generated in high speed train tunnels were studied in a 1/250-scaled train tunnel simulator. It is concluded that in order to achieve a practically effective suppression of the tunnel sonic boom the length of the pseudo-perforation section should be sufficiently long. Received 23 June 1997 / Accepted 16 September 1997     

On the compression process in a free-piston shock-tunnel

Preliminary results in the Marseille free-piston shock-tunnel facility are presented. The compression of the driver gas by the piston is studied experimentally for two different geometries of the end of the compression tube. Peak pressures obtained with the end of the compression tube closed, and with bursting of the diaphragm separating the high pressure from the low pressure chamber, are compared with calculated values in the cases of N₂ and He as driver gases. A phenomenon of accoustic resonance has been uncovered, generating strong pressure oscillations which, if not properly dealt with, could impair the quality of the useful flow in such a facility.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

A new, friction controlled, piston actuated diaphragmless shock tube driver

A new friction operated single piston shock tube driver design that is capable of generating shock waves of Mach number 1.1 to 2 is presented. By using different test gases and evacuating the driven section Mach 5 shock waves can easily be produced. The driver is efficient with shock wave Mach numbers within 9% of that predicted by ideal shock tube theory and the non-dimensional formation length lies between 20 and 40. The brake pad mechanism, that restrains the piston until tests commence, removes the necessity of venting an auxiliary chamber rapidly, thus speeding up the displacement of the piston. It is believed that the design is a practical, simple and cost effective way of generating reproducible shock tube tests with very short test turn around times, while removing the necessity of using a diaphragm and exposing the test gases to the atmosphere. Results for three pistons with masses of 4.4, 0.71 and 0.38 kg (brass, PVC and hollow aluminium respectively) with driver gauge pressures of between 2 and 50 bar (Mach 1.2 to 2) are given. Received 27 February 1998 / Accepted 8 July 1998     

Experimental study of toroidal shock wave focusing in a compact vertical annular diaphragmless shock tube

The paper reports results of experiments regarding toroidal shock wave focusing in a vertical shock tube as a part of a series of converging shock wave studies. This compact vertical shock tube was designed to achieve a high degree of reproducibility with minimum shock formation distance by adopting a diaphragmless operating system. The shock tube was manufactured in the Institute of Fluid Science, Tohoku University. An aspheric lens shaped cylindrical test section was connected at the open end of the shock tube to visualize the diffraction and focusing of the toroidal shock wave released from the ring shaped shock tube opening. Pressure transducers were flush mounted on the shock tube’s test section to measure pressure histories at the converging test section. Double exposure holographic interferometry was employed to quantitatively visualize the shock waves. The whole sequence of toroidal shock wave diffraction, focusing, and its reflection from the symmetrical axis were successfully studied. The transition of reflected shock waves was observed.     

Pressure waves in a separated gas-liquid layer in a horizontal duct with a step

Pressure wave propagation into a separated gas-liquid layer in a horizontal duct with a step is investigated analytically. The linear solution is derived assuming a large density ratio of liquid to gas. The solution can be found first for the gas layer and then for the liquid layer. The linear wave in a liquid layer is valid even for fairly large initial pressure ratios, and clearly exhibits the dispersive characteristics of the pressure wave in a liquid layer. As the initial pressure ratio is increased, the pressure wave in the gas layer becomes a shock wave. Thus, its effect on the wave in a liquid layer can be found analytically by modifying the boundary condition in part. The wave in a liquid layer consists of a main wave, which propagates with the shock speed in gas, and a precursor wave, whose front propagates with the speed of sound in liquid. The precursor wave has an oscillatory structure; its amplitude increases with increasing shock strength and also with liquid layer thickness.     

氢氧燃烧及爆轰驱动激波管

分析并观察了沿驱动段轴向分布多火塞燃烧驱动段的性能．提出主膜处同一管截面均匀分布三火花塞引燃的点火方法．用这种点火方法驱动产生的入射激波强度重复性较高,激波后气流速度、温度和压力的定常性亦大大改善,可满足气动试验实际要求．提出在驱动段尾端串接卸爆段来消除爆轰波反射高压,从而可使反向爆轰驱动段用来产生高焓高密度试验气流．这种反向爆轰驱动产生的入射激波重复性高,激波衰减弱．在主膜处的收缩段产生的反射波可缓解爆轰波后跟随的稀疏波的不利影响,从而使前向爆轰驱动具有实用性．在产生的入射激波强度相同条件下,前向爆轰驱动所需的爆轰驱动段可爆混合气初始压力可较反向爆轰低近一个量级．     

Formation of particle jetting in a cylindrical shock tube

A dense, solid particle flow is numerically studied at a mesoscale level for a cylindrical shock tube problem. The shock tube consists of a central high pressure gas driver section and an annular solid powder bed with air in void regions as a driven section with its far end adjacent to ambient air. Simulations are conducted to explore the fundamental phenomena, causing clustering of particles and formation of coherent particle jet structures in such a dense solid flow. The influence of a range of parameters is investigated, including driver pressure, particle morphology, particle distribution and powder bed configuration. The results indicate that the physical mechanism responsible for this phenomenon is twofold: the driver gas jet flow induced by the shock wave as it passes through the initial gaps between the particles in the innermost layer of the powder bed, and the chaining of solid particles by inelastic collision. The particle jet forming time is determined as the time when the motion of the outermost particle layer of the powder bed is first detected. The maximum number of particle jets is bounded by the total number of particles in the innermost layer of the powder bed. The number of particle jets is mainly a function of the number of particles in the innermost layer and the mass ratio of the powder bed to the gas in the driver section, or the ratio of powder bed mass (in dimensionless form) to the pressure ratio between the driver and driven sections.     

Experimental study of shock tunnel flow with a stationary throat plug

A throat plug is a device to prevent fragments produced by the bursting of the primary diaphragm of a shock tunnel from entering the nozzle and damaging the model. An experimental study has been carried out to investigate the flow in the KAIST shock tunnel with two stationary throat plugs at primary shock velocities of 1.19 and 1.28 km/s. These shocks generate tailored conditions when helium and air are used as the driver and driven gases, respectively, at room temperature. Nozzle reservoir pressures and Pitot pressures at the nozzle exit are measured to examine the influence of the stationary throat plug on these properties. The throat plug that has an areal blockage of 6.2 % is found to have negligible effects on shock tunnel flow. However, the throat plug of 19.4 % blockage shows an appreciable influence by generating a pressure bump. Although it retards the establishment time of the nozzle flow, after the transient period, it does not cause deviation of the pressure from that for the clean nozzle configuration. It is found that the reduction of the cross-sectional area, which results from the presence of the throat plug, causes the pressure bump. By increasing the diameter of the driven tube in the region where the throat plug is located appropriately, the magnitude of the pressure bump is reduced significantly.     

激波风洞高低压段钢膜片破裂特性研究

激波风洞是用于高超声速飞行器气动外形设计和优化的常用地面试验装置,基于爆轰驱动技术,激波风洞能够在短时间(毫秒级)内产生高温、高压的驱动气体来模拟高超声速试验气流.主膜片位于激波风洞中的爆轰驱动段和激波管段之间,试验时膜片在爆轰脉冲压力下打开,膜片的打开状态和脱落情况对激波风洞气流品质有很大的影响. 同时,膜片也是形成激波的先决条件. 传统的风洞采用铝质膜片进行试验,在激波风洞中需要承压能力更强的膜片, 此时铝质膜片不再适用, 需要采用钢质膜片.因此, 对激波风洞中的钢膜片破裂特性进行研究很有必要.将数值计算结果与试验结果进行比较, 发现数值计算结果与试验结果吻合得比较理想,计算结果具有可靠性. 基于膜片的应力-应变模型, 建立了膜片打开的动力学模型,根据CJ爆轰理论, 采用有限元软件计算模拟了膜片破裂的过程,分析总结了膜片破裂的机制和力学特性规律.采用控制变量法对不同厚度和凹槽长度的膜片进行分析研究,得到了膜片破膜压力和有效破膜时间的变化规律. 在激波风洞试验中,根据膜片总破膜时间设计了适用于JF-12复现风洞的膜片参数.