Establishment of steady separated flow over a compression–corner in a free–piston shock tunnel

The time required to establish steady separated compression–corner flow is examined under hypervelocity conditions in a free-piston shock tunnel. This time is reasonably well described using previous perfect gas analyses. The results suggest that, provided the nozzle reservoir enthalpy is 20 MJ kg or less, there is sufficient time to establish steady separated flow before driver gas contamination becomes a significant problem in the present facility. Received 13 November 1996 / Accepted 20 May 1997     

Vibration isolation in a free-piston driven expansion tube facility

The stress waves produced by rapid piston deceleration are a fundamental feature of free-piston driven expansion tubes, and wave propagation has to be considered in the design process. For lower enthalpy test conditions, these waves can traverse the tube ahead of critical flow processes, severely interfering with static pressure measurements of the passing flow. This paper details a new device which decouples the driven tube from the free-piston driver, and thus prevents transmission of stress waves. Following successful incorporation of the concept in the smaller X2 facility, it has now been applied to the larger X3 facility, and results for both facilities are presented.     

Numerical study of wave processes in a pressure-wave refrigerator

The paper provides some results concerning the numerical study of the strongly transient gasdynamic processes in a pressure-wave refrigerator (PWR). A hierarchical set of numerical models from the simplest one-dimensional to a fully three-dimensional formulation is introduced. The computations show that one-dimensional solutions give a reasonable foundation for the understanding of PWR operational principles but cannot satisfactorily predict the refrigeration efficiency. Good agreement with experiment is achieved by considering two- and three-dimensional effects of gas mixing by overlap of rotating nozzles and expansion tubes. Received Received 14 February 1996 / Accepted 1 June 1996     

Contact surface tailoring condition for shock tubes with different driver and driven section diameters

The contact surface tailoring conditions normally used for shock tubes do not apply to shock tubes with different driver and driven section diameters. A theoretical model is presented that predicts the contact surface tailoring condition for a convergent shock tube, designed to have a larger driver cross-section area than the driven section. The tailoring condition previously developed for shock tubes with uniform driver and driven diameters can be recovered from this model. Representative on- and off-model performance is verified experimentally in a high-pressure convergent shock. Tailoring conditions calculated with the model are also given for commonly used driven gases (Ar, N₂ and air) and He–N₂ driver mixtures as a function of driver/driven area ratio.      

Dynamic fragmentation of blast mitigants

Experimental evidence from a wide range of sources shows that the expanding cloud of explosively disseminated material comprises “particles” or fragments which have different dimensions from those associated with the original material. Powders and liquids have often been used to surround explosives to act as blast mitigants, and this is the main driver for our research. There are also many other areas of interest where an initially intact material surrounding an explosive charge is dynamically fragmented into a distribution of fragment sizes. Examples of such areas include fuel air explosives and enhanced blast explosives as well as quasi-static pressure mitigation systems, and our studies are thus also relevant to these applications. In this paper, we consider the processes occurring as an explosive interacts with a surrounding layer of liquid or powder and identify why it is important to model these processes as a multiphase material problem as opposed to a single phase, single material velocity problem. We shall present results from this class of numerical modelling. In this paper we shall explore what determines the particle or fragment size distribution resulting from explosive dissemination of a layer of material and discuss reasons why clouds from disseminated liquids and powders look similar. We shall support our analysis with results from recent explosives trials and introduce early results from some ongoing small scale explosive mitigation experiments.     

A two-stage free-piston driver

The overall cost of free-piston driven facilities can be substantially reduced if the contraction between the compression and shock tubes is replaced with a constant area section. However, with such an implementation, a new driver concept is required in order to achieve a realistic facility length. This paper describes a new free-piston driver type for expansion tubes which satisfies the above criteria. The technique is known as the two-stage free-piston driver where the driver gas is compressed in two distinct stages with a unique compound piston design. A new facility has been constructed (X-2) which is described in some detail. A quasi-one-dimensional numerical model of the compression process is also developed which agrees well with driver tube experimental results. This new driver is coupled to an expansion tube arrangement where super-orbital test flows are generated. The results show that a two-stage free-piston driver is capable of driving hypervelocity expansion tubes and therefore new facilities of increased size but reduced cost are now possible. Received 18 May 1998 / Accepted 18 October 1998     

Test-time extension behind reflected shock waves using CO2–He and C3H8–He driver mixtures

To study combustion chemistry at low temperatures in a shock tube, it is of great importance to increase experimental test times, and this can be done by tailoring the interface between the driver and driven gases. Using unconventional driver-gas tailoring with the assistance of tailoring curves, shock-tube test times were increased from 1 to 15 ms for reflected-shock temperatures below 1,000 K. Provided in this paper is the introduction of tailoring curves, produced from a one-dimensional perfect gas model for a wide range of driver gases and the production and demonstration of successful driver mixtures containing helium combined with either propane or carbon dioxide. The He/CO₂ and He/C₃H₈ driver mixtures provide a unique way to produce a tailored interface and, hence, longer test times, when facility modification is not an option. The tailoring curves can be used to guide future applications of this technique to other configurations. Nonreacting validation experiments using driver mixtures identified from the tailoring curves were performed over a range of reflected-shock temperatures from approximately 800 to 1,400 K, and some examples of ignition-time experiments that could not have otherwise been erformed are presented.     

Density reconstruction from laser schlieren signal in shock tube experiments

Using a diffraction approach the convolution-type integral equation for the laser schlieren signal created by an arbitrary disturbance at low pressure, where refractive index of disturbance is close to unity, in a shock tube (thin optical layer) has been deduced. In the equation electric circuit relaxation processes were taken into account by a response function. The equation was solved with the aid of the regularization method worked out for ill-posed problems. The density structures of the strong shock waves in air have numerically been reconstructed from experimental data ranging shock wave Mach number of –30, and –30 Pa. Received 7 April 1996 / Accepted 20 June 1996     

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     

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.     

Interpretation of Second Law of Thermodynamics in the presence of interfaces

In 1996, Muschik and Ehrentraut (J. Non-Equilib. Thermodyn. 21:175–192, 1996) proposed an amendment to the classical Second Law of Thermodynamics, which asserts that, except in equilibria, reversible process directions in state space do not exist. As a consequence of this statement, they proved that the Second Law of Thermodynamics necessarily restricts the constitutive equations and not the thermodynamic processes. In this way, the classical Coleman–Noll approach to the exploitation of Second Law (Coleman and Noll in Arch. Rational Mech. Anal. 13:167–178, 1963) follows by a rigorous proof. In the present paper, we generalize the amendment, in order to encompass the case, not considered in Muschik and Ehrentraut (J. Non-Equilib. Thermodyn. 21:175–192, 1996), in which there are surfaces across which the unknown fields suffer jump discontinuities. Due to the generalization above, we prove that the same conclusions of Muschik and Ehrentraut (J. Non-Equilib. Thermodyn. 21:175–192, 1996) can be achieved also in the presence of non-regular processes. As an application, we study the thermodynamics of a Kortweg-type fluid interface.     

Driver gas contaminationin a detonation-driven reflected-shock tunnel

A computational study has been carried out to examine the effects of driver gas contamination in the NASA HYPULSE facility at GASL when operating with a detonation driver in reflected-shock tunnel mode. Unlike high-enthalpy shock tunnels which use helium as a driver gas, the driver gas in a detonation driver consists of a mixture of water vapour and argon, which has very different chemical and thermodynamic properties than those of helium. The purpose of the present work is to quantitatively evaluate the effects of driver gas contamination on the flow properties in the test section. Two computational analyses have been performed. The first analysis examined the nozzle flow under the influence of a prescribed level of driver gas contamination. In the second analysis, the transient development of the driver gas leakage in the reflected-shock region in the shock tube is studied. The unique flow features brought about by the detonation-driver gas and the method for detecting the contamination are discussed.Received: 6 August 2002, Revised: 22 September 2003, Published online: 10 February 2004Correspondence to: R.S.M. ChueAn abridged version of this paper was presented at the 23rd International Symposium on Shock Waves at Fort Worth, Texas, July 23 to 27, 2001     

高超声速激波风洞研究进展

回顾了高超声速激波风洞的研制与发展,并依据高超声速实验研究对地面实验模拟技术的要求,分别介绍了应用轻气体、自由活塞和爆轰驱动技术研制的主要激波风洞的性能、特点和存在问题.重点介绍了爆轰驱动高焓激波风洞的3种主要运行模式:反向、正向爆轰驱动与双爆轰驱动. 根据这些运行模式的工作原理,分析了应用这些驱动技术产生的高温、高压气源的特点,探讨了不同驱动技术可能影响激波风洞性能的关键问题与解决方法.目前发展的激波风洞已经能够用于开展马赫数3$\sim$30的高超声速流动的试验模拟研究,但是试验气流的品质还不能满足高超声速科技研究的需求.为了获得可靠的实验结果, 通过不断改进、完善、提高激波风洞的性能,尽可能复现高超声速飞行条件是今后主要的研究方向.      

Feedback control for the lattice hydrodynamics model with drivers’ reaction time

In this paper, a new lattice hydrodynamic model (LH model) of traffic flow under consideration of reaction time of drivers and a corresponding feedback control scheme are proposed. Based on the model, stability analysis is conducted through linear stability analysis of transfer function. The obtained phase diagram indicates that the reaction time of driver can affect the instability region of traffic flow. Under the action of a feedback control, the unstable region is shrunken to reach suppressing jams. The numerical simulations are performed to validate the effect of reaction time of driver in the new LH model. The study results confirm that the reaction time of driver significantly affects the unstability of traffic system, and the feedback control can suppress traffic jams. Furthermore, it is found that the traffic system from the chaotic traffic state to periodic steady one is successfully realizing the control of traffic system.     

Radiative interaction between driver and driven gases in an arc-driven shock tube

An electric-arc driven shock tube was operated with hydrogen as the driven gas and either hydrogen or helium as the driver gas. The electron density was measured behind the primary shock wave spectroscopically from the width of the hydrogen beta line. The intensity of the radiation produced by the driver and driven gases and directed along the axis of the shock tube was measured with a photomultiplier tube. The temperatures behind the primary shock wave were 3 to 4 times those calculated from the Rankine-Hugoniot relations. A proposed explanation for this difference is developed, involving strong heating of the driven gas at early times due to higher shock velocities and radiative energy transfer from the driver arc. The electron density ahead of the shock wave agreed roughly with the calculation based on the precursor phenomenon due to radiative transfer.     

探索发展激波风洞爆轰驱动技术

发现了燃烧驱动激波管中入射激波马赫数异常升高的起因. 实验显示爆轰驱动能力强于燃烧驱动, 从而推动爆轰驱动技术的发展. 采用卸爆管消除爆轰波反射高压以及双爆轰驱动段全部消除爆轰波后的Taylor稀疏波, 使反向和前向爆轰驱动模式具有实用价值. 反向爆轰驱动技术还成功用来延长激波风洞试验时间.      

A new lattice model of the traffic flow with the consideration of the driver anticipation effect in a two-lane system

In this paper, a new lattice model of the traffic flow is proposed with the consideration of the driver anticipation effect for a two-lane system. The linear stability condition is derived by employing linear stability analysis. The analytical result shows that the driver anticipation effect can improve the stability of the traffic flow in a two-lane system. The mKdV equation near the critical point is obtained to describe the propagating behavior of a traffic density wave with the perturbation method. The simulation results are also in good agreement with the analytical results, which show that the traffic jam can be suppressed efficiently when the driver anticipation effect is considered in a two-lane system.     

Exploring cold regions autonomous operations

The US Army must update its vehicle fleet to be better equipped for potential future military conflicts in northern climates (US Army, 2017). This process involves considering manned, optionally manned, and unmanned vehicles as viable options in the future. Optionally manned and unmanned vehicles in the armed forces have substantial benefits because they can operate without direct driver input or are able to perform missions deemed too dangerous for troops. Optionally manned vehicles allow the driver to shift some, or all, focus away from the task of driving the vehicle. In some cases, these autonomous vehicles may perform better than a human driver by rapidly sensing and reacting to terrain changes. Onboard sensing and decision making are equally applicable to both fully autonomous and teleoperated vehicles. This work will focus on the terrain sensing, waypoint navigation, and teleoperation potential of an optionally manned or unmanned vehicle. Results from a vehicle demonstration on two different terrain conditions will provide the basis for additional terrain sensing and autonomous vehicle development work in the coming year.     

The gas-dynamic effects of a hemisphere-cylinder obstacle in a shock-tube driver

One form of gene and drug delivery can be achieved by accelerating micro-particles to a sufficient momentum to penetrate the outer layer of the skin, and target the tissue below. In hand-held clinical systems, the particles are accelerated in the transonic flow developed within a miniature shock-tube. These devices require a gas reservoir to be positioned co-axially inside the driver. This paper reports the examination of a nominally constant-area shock-tube containing a co-axial obstacle in the driver section (Case 2). As a control, the flow-field in a prismatic shock-tube of the same cross-sectional area, with the same initial conditions is also studied (Case 1). Static and Pitot pressures measured in each of the above cases are compared to one-dimensional theory and numerical computations. Significant deviations between Cases 1 & 2 are observed in the driver, but not in the driven section.