Numerical and experimental investigation of wave dynamic processes in high-speed train/tunnels

Numerical and experimental investigation on wave dynamic processes induced by high-speed trains entering railway tunnels are presented. Experiments were conducted by using a 1:250 scaled train-tunnel simulator. Numerical simulations were carried out by solving the axisymmetric Euler equations with the dispersion-controlled scheme implemented with moving boundary conditions. Pressure histories at various positions inside the train-tunnel simulator at different distance measured from the entrance of the simulator are recorded both numerically and experimentally, and then compared with each other for two train speeds. After the validation of nonlinear wave phenomena, detailed numerical simulations were then conducted to account for the generation of compression waves near the entrance, the propagation of these waves along the train tunnel, and their gradual development into a weak shock wave. Four wave dynamic processes observed are interpreted by combining numerical results with experiments. They are: high-speed trains moving over a free terrain before entering railway tunnels; the abrupt-entering of high-speed trains into railway tunnels; the abrupt-entering of the tail of high-speed trains into railway tunnels; and the interaction of compression and expansion waves ahead of high-speed trains. The effects of train-tunnel configuration, such as the train length and the train-tunnel blockage ratio, on these wave processes have been investigated as well.     

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     

高速铁路长隧道压缩波波前变形规律分析

高速列车进入隧道时产生的压缩波在长隧道中传播时会产生波前变形，即波前压力梯度发生变化。线路测量和学者的研究表明，隧道出口处微气压波的强度与压缩波波前压力梯度最大值成正比，而微气压波的大小又与隧道出口的爆破音直接相关，因此有必要研究压缩波在长隧道中传播时的波前变形规律。采用计算流体力学三维动态仿真计算方法，对长隧道内压缩波的生成和传播过程进行研究。证明了压缩波的波前变形不仅与初始压缩波生成时的惯性运动和气体摩擦相关，而且与列车在隧道中的运动相关，即列车运动产生的能量输入会影响压缩波波前的变形。通过多工况计算，获得了隧道内压力梯度的最大值及其出现位置与列车速度和隧道阻塞比之间的变化规律。     

The genetically optimized tunnel-entrance hood

A numerical procedure is investigated for optimizing the design of tunnel-entrance hoods used for controlling the compression wave generated when a high-speed train enters a tunnel. Long hoods are required for long tunnels and train speeds exceeding about 350 km/h. The hood must minimize the maximum pressure gradient across the compression wave-front by taking advantage of the pressure-release provided by open windows distributed along one or both of its walls. The compression wave produced by the train can be evaluated by means of a rapid computational scheme devised and validated against experiment. Optimization is achieved by representing a possible distribution of windows by a binary string. The individuals in an initial, random population of such strings are allowed to ‘mate’ and evolve by ‘natural selection’ through several generations towards an optimal configuration by application of a genetic algorithm. The genetically fittest hood is associated with the minimum possible maximum pressure gradient for prescribed values of the train speed and hood dimensions. The algorithm yields an optimal design from among a theoretically unlimited number of possibilities; it can also supply near-optimal, smoothly varying window distributions (or optimize the variation in width of a long slit-like window in the hood wall) satisfying additional constraints imposed by the designer.     

高速铁路隧道缓冲结构气动载荷与结构应力特性分析

高速列车通过隧道时,会引起车隧气动效应.在隧道洞口设置缓冲结构是简便有效的应对措施之一.而缓冲结构一般设置在隧道洞口,列车通过隧道产生气动载荷对该结构的影响也不容忽视.本文采用数值方法,利用Ansys软件的workbench模拟平台,对列车通过隧道产生的气动载荷作用在顶部单开口缓冲结构上的压应力变化进行模拟.研究结果表明:气动载荷所引起的结构附加应力作用明显.当行车速度为350 km/h时,附加应力可以达到80 kPa,而缓冲结构开口周围成为气动载荷附加应力集中区.对于双线隧道,近车壁面与远车壁面的附加压应力规律一致,但近车侧应力值要大于远车侧.与压力波在隧道内的传播特性类似,气动载荷所引起的附加压应力具有往复传播特征.另外,对顶部缓冲结构开口附近出现附加应力集中的原因进行了分析,确定缓冲结构形式是引起应力集中的决定因素.以上结论对隧道洞口缓冲结构的设计及安全巡查具有一定的指导意义.     

EXPERIMENTAL 3-D SIMULATION OF THE COMPRESSION WAVE,DUE TO TRAIN–TUNNEL ENTRY

The work presented in this paper concerns the first compression wave generated in a tunnel when a high-speed train enters it. This wave is the first of successive compression and expansion waves which propagate back and forth in the tunnel. Once generated at the tunnel entrance, its amplitude and gradient vary according to the train and tunnel characteristics. These waves provoke: (a) an aural discomfort for train passengers, (b) mechanical stresses on train and tunnel structures, and (c) emission of impulsive noises outside the tunnel. A reduced-scale test method, using low-sound-speed gas mixtures, has been developed and validated by using newly available European full-scale test-results. It can reproduce quite well the three-dimensional effects due to the train geometry and its position in the tunnel. The study also clearly points out that three-dimensional effects on the front of the first compression wave are attenuated with distance from the tunnel entrance and that the wave front can be considered well established and planar for distances larger than four times the tunnel diameter. Characteristics of the planar wave are in good agreement with Japanese results. The reduced-scale train Mach number has been extended up to 0.34 to determine its test domain. Our study clearly shows that, as far as the characteristics of the wave front of well-established planar first compression wave are concerned, axially symmetrical models can advantageously replace three-dimensional models, provided that the longitudinal cross-sectional area profile is the same for both configurations. This feature yields the following train nose design procedure: first determine the cross-sectional profile of a train nose against train–tunnel interactions by means of axially symmetrical configuration, then give a three-dimensional shape for drag and stability optimisation.     

颗粒材料中致密波结构研究

采用一维两相流模型与相应颗粒构形应力函数,研究了致密波的形成及其结构．用简化两相流模型系统地讨论致密波对有关因素的依赖关系．分析指出：小于基体材料音速的致密波仅能在非理想颗粒材料中存在,从波前到波后,所有状态物理量光滑过渡．大于基体材料音速的致密波,波头可能存在间断．应力函数与致密粘性确定后,致密波速度决定致密波结构、宽度、终态压实度．采用一维两相流模型模拟了活塞驱动颗粒床形成致密波这一动态过程．用线方法（MOL）对该方程组求数值解．计算表明,经过短暂的非稳态过程,颗粒床中形成一稳态致密波．分析了活塞速度与初始孔隙率对致密波结构的影响,并对简化两相流模型与两相流模型的计算结果进行了对比．     

Controlling the level of the sonic boom generated by a flying vehicle by means of cryogenic forcing. 1. Cooling of the vehicle surface

The possibility of controlling the sonic boom level by means of cooling the surface of a flying vehicle is discussed. The effect of surface cooling on the formation of the perturbed flow structure at large distances from the vehicle is demonstrated by an example of a modified power-law body of revolution. The intensity of the intermediate shock wave and the perturbed pressure pulse near the body are seen to decrease, which expands the altitude range of the region where the sonic boom is reduced (down to 50%). At larger distances from the body, cryogenic forcing ensures a 12% decrease in the bow shock wave intensity. The possibility of controlling the process of formation of wave structures near the surface, such as barrel shock waves, is demonstrated. An explanation of the cryogenic forcing mechanism is offered. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 6, pp. 88–98, November–December, 2008.     

The reflected pressure field in the interaction of weak shock waves with a compressible foam

This paper deals with the waves that are reflected from slabs of porous compressible foam attached to a rigid wall when impacted by a weak shock wave. The interest is in establishing possible attenuation of the pressure field after a shock or blast wave has struck the surface. Foam densities from 14 to 38 kg/m³ were tested over a range of shock wave Mach numbers less than 1.4. It is shown that the initial reflected shock wave strength is accurately predicted by the pseudo-gas model of Gelfand et al. (1983), with a pressure ratio of approximately 80% of the value for reflection off a rigid wall. Evidence is presented of gas entering the foam during the early stages of the process. A second wave emerges from the foam at a later stage and is separated from the first by a region of constant velocity and pressure. This second wave is not a shock wave but a compression front of significant thickness, which emerges from the foam earlier than predicted by the pseudo-gas model. Analysis of the origin of this wave points to much more complex flows within the foam than previously assumed, particularly in an apparent decrease in average wave front speed as the foam is compressed. It is shown that the pressure ratio across both these waves taken together is slightly higher than that for reflection off a rigid wall. In some cases this compression wave train is followed by a weak expansion wave.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1990.     

On the treatment of transient area variation in 1D discontinuous Galerkin simulations of train‐induced pressure waves in tunnels

To simulate the pressure wave generated by a train travelling through a tunnel, we implement a discontinuous Galerkin (DG) method for the solution of the one‐dimensional equations of variable area flow. This formulation uses a spatial discretisation via Legendre polynomials of arbitrary degree, and the resulting semi‐discrete system is integrated using an explicit Runge–Kutta scheme. A simulation of subsonic steady flow in a nozzle shows that the scheme produces stable solutions, without the need for artificial dissipation, and that its performance is optimal for polynomial degrees between 5 and 7. However, when dealing with an unsteady area, we report the presence of numerical oscillations that are not due to the steep pressure fronts in the flow but rather to the projection of a moving area, with piecewise continuous derivatives onto a fixed grid. We propose a reformulation of the DG method to eliminate these oscillations that, put in simple terms, amount to splitting the integrals where the derivatives of the cross‐sectional area are discontinuous into subintegrals where they are continuous. The resulting method does not exhibit oscillations, and it is applied here to two practical cases involving train‐induced pressure waves in a tunnel. The first application is a validation of the DG method through comparison of its computational results with pressure data measured during transit at the Patchway tunnel near Bristol (UK). The second application is a study of the influence of the nose shape and length on the pressure wave gradients responsible for sonic boom at tunnel exit portals to show that the proposed modification is able to deal with realistic train shapes. Copyright © 2012 John Wiley & Sons, Ltd.     

Heat transfer during transient compression: Measurements and simulations

Experiments have been performed to assess the utility of unsteady one-dimensional heat conduction modelling for the calculation of heat losses during a free piston compression process. Heat transfer measurements have been obtained within a gun tunnel barrel using surface junction thermocouple instrumentation. The gun tunnel was operated with a relatively heavy piston such that the shock waves induced by the piston motion were weak. Peak heat transfer values are estimated reasonably well by the unsteady one-dimensional model. However, overall quantitative agreement between the measurements and calculations has not been achieved at this stage, principally because the development of turbulent heat transport was not properly modelled. Received 21 September 2001 / Accepted 11 March 2002 – Published online 11 June 2002     

Design of a tunnel-entrance hood with multiple windows and variable cross-section

A theory is proposed for the design of a uniform tunnel-entrance hood whose cross-sectional area exceeds the tunnel area . A train entering the tunnel produces a low-frequency compression wave that can be subject to nonlinear steepening in a long tunnel. An optimized hood of length ℓ_h increases the initial thickness of the compression wave front from R/M to ℓ_h/M, where Rℓ_h is the nominal radius of the tunnel and M is the train Mach number. In addition, the pressure rise should be linear across the wave front to obtain an overall minimum value of the subjectively important pressure gradient. This is achieved in a uniform hood by distributing windows along the hood wall to vent away high-pressure air displaced by the train. We consider the problem of determining the distribution and sizes of these windows and the magnitude of the area ratio to ensure that the hood behaves optimally at low-train Mach numbers (M<0.2), when the hood can be regarded as being acoustically compact. At the projected higher Mach numbers of advanced high-speed trains (0.4, say) recent analysis for hoods of uniform cross-section by Howe in 2003 indicates that a hood optimized for low Mach number operations continues to produce an essentially linear pressure rise across a compression wave of thickness ℓ_h/M except for a low-amplitude oscillation at the very front of the wave.     

Shock unsteadiness creation and propagation: experimental analysis

The possibility of creating unsteady distortions of the tip shock by waves emitted from an aircraft is assessed experimentally. The model chosen is a cylindrical fore body equipped with a spike. This configuration is known for generating an important level of unsteadiness around the spike in supersonic regime. The wind tunnel Mach number is equal to 2. The experiments show that waves emitted from this source propagate along the tip shock and interact with it. It is then assessed that this interaction produces a periodic distortion of the shock that propagates to the external flow. Unsteady pressure sensors, high speed schlieren films, hot wire probing and laser Doppler velocimetry are used as complementary experimental means. The final result is a coherent representation of the complex mechanism of wave propagation that has been evidenced. The principle of creating unsteady shock deformation by onboard equipments could be examined as a possibly promising method of sonic boom control.     

Wave propagation in a non-ideal dusty gas

In this paper we have studied the behavior of wave motion as propagating wavelets and their culmination into shock waves in a non-ideal gas with dust particles. In the absence of non-ideal effect the gas satisfies an equation of state of Mie–Gruneisen type. An expansion wave resulting from the action of receding piston is considered and the solutions to this problem showing effects of dust particles and non-idealness are obtained. The propagation of weak waves is considered and the flow variables in the region bounded by the piston and the characteristic wave front are found out. The expansive action of a receding piston undergoing an abrupt change in velocity is discussed. Cases of central expansion fan and shock fronts are studied and the solutions up to first order in the physical plane are obtained. The effects of non-idealness and dust particles are discussed in each case.     

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.      

Reflection and transmission of a compression wave at a tunnel portal

Analyses are made of the interaction of the nonlinearly steepened, compression wavefront generated by a high-speed train in a tunnel with the tunnel portal ahead of the train. The ‘micro-pressure’ pulse emitted from the portal can rattle structures in nearby buildings, and the expansion wave reflected back towards the train can cause discomfort to passengers. It is concluded that the usual simplified approximation of one-dimensional propagation within the tunnel provides an adequate representation of interactions of the wave with the portal, and also with ‘windows’ in the tunnel wall near the portal. It is shown how a discrete distribution of windows can be used to produce a reflected expansion wave that varies linearly across the wavefront, and how the thickness of that wavefront can be made many times larger than the thickness of the incident compression wave profile. A detailed analysis of the wave radiated from the portal reveals that cumulative nonlinear effects of propagation over long distances make little or no contribution to the free-space radiation of the micro-pressure wave.     

Controlling the level of the sonic boom generated by a flying vehicle by means of cryogenic forcing. 2. Distributed injection of a supercooled gas from the vehicle surface

The possibility of improving the efficiency of cryogenic forcing on the parameters of the hanging shock determining the length of the region of minimization of the sonic boom (middle zone) generated by a modified power-law body is studied. The effect of distributed injection of the coolant from the body surface on the formation of a perturbed flow near the body and at large distances from the body is considered. The scheme of distributed injection and the regime of coolant exhaustion are demonstrated to exert a significant effect on the length of the middle zone of the sonic boom. A scheme of cryogenic forcing is determined, which ensures reduction of bow shock wave intensity by more than 40% at distances corresponding to 7000 body diameters. The mechanism of cryogenic forcing on the flow structure near the body is discussed. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 2, pp. 136–144, March–April, 2009.     

Visualization of shock-wave formation processes during shock reflection at obstacles with multiple steps

According to standard textbooks on compressible fluid dynamics, a shock wave is formed by an accumulation of compression waves. However, the process by which an accumulated compression wave grows into a shock wave has never been visualized. In the present paper, the authors tried to visualize this process using a model wedge with multiple steps. This model is useful for generating a series of compression waves and can simulate a compression process that occurs in a shock tube. By estimating the triple-point trajectory angle, we demonstrated visually that an accumulated compression wave grows into a shock wave. Further reflection experiments over a rough-surface wedge confirmed the tendency for the triple point trajectory angle to reach the asymptotic value _s in the end.This work was first presented at the Symposium on Shock Waves, Japan 2002     

Reduction of the sonic boom level by heating the flow in front of the body

A numerical study of the possibility of reducing the sonic boom level in the case of local heat release to a supersonic gas flow at Mach number equal to 2 ahead of a body is described. The computations are performed for a spherical heat supply zone located on the flight trajectory ahead of the tip of an axisymmetric thin body. For the numerical study the combined method of “phantom bodies” is used. Different magnitudes of heat supply to the incoming flow are tested. These calculations are performed with allowance for interaction of shock waves emanating from the heated gas region and from the body in the far field. The computational results show that the local heat supply to a supersonic gas flow ahead of a body can reduce the sonic boom level by more than 20 %. The reduction of the sonic boom level is ensured by changing the free-stream parameters ahead of the body and by preventing the coalescence of shock waves from the heat supply zone and from the body in the far field.     

Numerical study of flow characteristics of the high speed train entering into a tunnel

When a high speed train enters into a tunnel, the aerodynamic forces severely change and, consequently, the stability and performance significantly deviate from the value of design point which is usually set at cruising speed on the plain ground. The compression wave is also generated ahead of the train due to the piston-like action of tunnel entry motion. The present work is to understand the flow field such as variation of aerodynamic forces and generation of compression wave during tunnel entry motion by applying three-dimensional unsteady Navier–Stokes equation solver. To account for the relative motion of stationary tunnel and moving train, the sliding multi-block method has been implemented.