Impact of vacuum degree on the aerodynamics of a high-speed train capsule running in a tube

Motivated by the growing scientific and engineering interest in evacuated tube railway transportation systems, in this paper we numerically study the influence of the vacuum degree on the flow field around a train capsule running in an evacuated tube with circular section. The vacuum degree is increased by lowering the nominal pressure inside the tube. The numerical simulations are fully verified by wind-tunnel experimental data of supersonic flows around a blunt body and in a scramjet combustion chamber, as well as by several numerical results in other related studies. The flow around the train capsule is characterized by a compression region in front of the train, a chocked flow near the train, and a complex highly unsteady region behind the train, where expansions waves and reflecting oblique shock waves exist. The total aerodynamic drag and the vacuum degree are found to be linearly related, revealing that lowering the nominal pressure can have a significantly beneficial effect on the aerodynamic performance of the train capsule. The aerodynamic heating due to compressibility effects and the increased pressure are more prominent along the centreline of the tube than on the tube wall. As the vacuum degree increases, the temperature and pressure differences between the front and the tail of the train and the intensity of the reflected shock waves become less significant, so that the extension of the expansion region in the train wake shortens.     

Aerodynamic pressure on a wall due to movement of a high-speed train

The questions of train aerodynamics have taken on great practical importance in connection with the rapid increase of the speeds of the modern trains. In particular, of very great importance is the aerodynamic effect of trains on people and structures located near the tracks.The present article makes an attempt to evaluate the pressure of a train on a long wall parallel to a moving train.Similar problems have been considered for circular cylinders and for spheres. Literature on the subject and the initial discussion of the question may be found in the classical hydrodynamics courses [1,2]. A more exact solution of the problem of the motion of a circular cylinder in the presence of a wall has been given by Mazur [3]. The problems of the motion of an ellipsoid and certain other bodies have been examined recently to determine the suction force acting on a ship [4–6] In these studies bodies were considered whose slenderness ratio is much less than that of trains. Particular note should be taken of the article of Blokh and Ginevskii [5] which gives an approximate method which may be used also for very elongated bodies, if account is taken of the transverse velocities which they generate.The methods of thin wing theory are used in the present study.     

Effect of surface roughness on a turbulent wall-attaching offset jet

The flow characteristics of a two-dimensional offset jet discharged parallel to a rough wall is experimentally investigated by using a split film probe. The distributions of the mean velocity and turbulent stresses in the flow field are obtained and compared with those of the wall attaching offset jet on a smooth wall. It is found that the wall-attaching region on the rough wall is longer than on the smooth wall for the same offset height and the jet speed. The normal distance of the maximum velocity point is farther away from the wall than that for the smooth wall case because of the thick wall boundary layer established by the surface roughness. It is also found that the roughness of the wall accelerates the relaxation process towards redeveloped plane wall jet and that it exhibits a quite different turbulent diffusion behavior especially near the wall from that in the wall jet over a smooth surface.     

Effect of roughness on the interaction between low-density gas and the surface of a solid

The effect of roughness on the reflection of molecules of a low-density gas from the surface of a solid is studied. An expression is derived for the transform for a single reflection from a homogeneous roughness which permits simple programming for a computer. A simple approximation to this expression is considered which is applicable over a broad range of roughness parameters and of gas-molecule angles of incidence. Based on this approximation, the angular distributions of reflected molecules are calculated and a comparison is made with similar distributions taken from [1].Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 4, pp. 68–75, July–August, 1972.     

列车荷载作用下黏弹性半空间体的随机动力响应Dynamic response of viscoelastic half-space subjected to train loads

针对列车荷载作用下黏弹性半空间体响应的问题,利用虚拟激励法将系统的随机分析转化为确定性分析。根据列车荷载构造了相应的虚拟激励形式,通过傅里叶积分变换法把半空间体控制方程转入波数‐频率域,并推导获得了系统虚拟响应的积分形式解。当相速度接近或大于瑞利波速时,积分形式解中被积函数往往具有奇异性和高振荡性,使得数值计算相当困难。对此,将被积函数图形化以确定函数的积分限,并通过自适应数值积分算法解决被积函数的振荡性。数值算例中,进行了随机列车荷载作用下半空间体的响应分析,讨论了荷载移动速度及频率等参数变化对响应的影响,给出了响应的时间和空间分布规律。本文方法可进一步推广至移动矩形荷载等载荷模型,对移动荷载作用下环境振动行为预测具有很好的借鉴意义。     

Evolution of roughness on the surface of a strained elastic material due to stress-driven surface diffusion

The free energy of a stressed crystal is assumed to consist of elastic strain energy and surface energy, and the chemical potential for surface diffusion at constant temperature is obtained under this assumption. A gradient in chemical potential results in diffusive mass transport along the surface. The result is applied in considering the phenomena of instability of a flat surface in a stressed material under fluctuations in surface shape, and the transient evolution of surface roughness due to an initial perturbation in the nearly flat free surface of the material, both under plane strain conditions.     

Dynamics of trailing vortices in the wake of a generic high-speed train

The three-dimensional dynamics of a pair of counter-rotating streamwise vortices that are present in the wake of an ICE3 high-speed train typical of modern, streamlined vehicles in operation, is investigated in a 1/10th-scale wind-tunnel experiment. Velocity mapping, frequency analysis, phase-averaging and proper orthogonal decomposition of data from high-frequency multi-hole dynamic pressure probes, two-dimensional total pressure arrays and one-dimensional multi-hole arrays was performed. Sinusoidal, antisymmetric motion of the pair of counter-rotating streamwise vortices in the wake is observed. These unsteady characteristics are proposed to be representative of full-scale operational high-speed trains, in spite of the experimental limitations: static floor, reduced model length and reduced Reynolds number. This conclusion is drawn from favourable comparisons with numerical literature, and the ability of the identified characteristics to explain phenomena established in full-scale and scaled moving-model experiments.     

Response of railway track system on poroelastic half-space soil medium subjected to a moving train load

Based on the dynamic poroelastic theory of Biot, dynamic responses of a track system and poroelastic half-space soil medium subjected to moving train passages are investigated by the substructure method. The whole system is divided into two separately formulated substructures, the track and the ground, and the rail is described by introducing the Green function for an infinitely long Euler beam subjected to the action of moving axle loads of the train and the reactions of the sleeper. Sleepers are represented by a continuous mass and the effect of the ballast is considered by introducing the Cosserat model for granular medium. Using the double Fourier transform, the governing equations of motion are then solved analytically in the frequency-wave-number domain. The time domain responses are evaluated by the inverse Fourier transform computation for a certain train speed. Computed results show that the shape of the rail displacements of the elastic and poroelastic soil medium are in good agreement with each other of the low train velocity, but the result of the poroelastic soil medium is significantly different to that of the elastic soil medium for the high train velocity which is higher than Rayleigh-wave speed in the soil. The influence of the soil intrinsic permeability on soil responses is discussed with great care in both time domain and frequency domain. The dynamic responses of the soil medium are considerably affected by the fluid phase as well as the load velocity.     

Investigation of aerodynamic effects on the high-speed train exposed to longitudinal and lateral wind velocities

The wind stability of the high-speed train has gained an increasing interest in the last few years. In this paper, an investigation of the effects of stochastic winds with longitudinal and lateral components on the high-speed train is described. The longitudinal and lateral wind time histories at the position of a moving vehicle, for a variety of wind directions, are first simulated. An algorithm for computing the unsteady aerodynamic load time histories is then derived for a moving vehicle. A typical railway vehicle has been modeled using the vehicle dynamic simulation package ‘Simpack’, and the unsteady wind loads of the same vehicle are applied to the vehicle model to investigate the dynamic response behavior. The simulated vehicle behavior is assessed against the indicator of load reduction factor, which indicates wheel unloading and therefore potential roll over. The characteristic wind curves (CWC) and its spread range are then obtained to evaluate the operational safety of the high-speed train. The results demonstrate that the operational safety of the high-speed train will be overestimated if the lateral wind velocity is not considered, especially for the small angles between vehicle and wind.     

Effect of loading-path on the evolution of yield surface for anisotropic metals subjected to large pre-strain

A theory of plasticity previously formulated by the author to discuss the free-end torsion problem has been extended to the more general case of combined axial-torsion of a thin-walled tube. This paper is devoted to the discussion of evolution of the yield surface due to different (proportional and non-proportional) loading paths with pre-strains in the large strain range. Experimental yield surfaces with axial and torsion pre-strains of magnitudes up to 40 and 45%, respectively, are presented, and theoretical results are compared with the experimental data.     

Flow topology and unsteady features of the wake of a generic high-speed train

The unsteady wake of a high-speed train is investigated experimentally. From a practical point of view, the wake region is of considerable importance as it is where slipstream velocities—velocities induced by the vehicles movement through air—are largest. In turn, this can create a considerable risk for passengers and track-side workers as the train passes. The flow is quantified in a 1:10 scale wind-tunnel experiment using high-frequency 4-hole dynamic pressure cobra probes, surface-pressure measurements and flow visualisation. The dominant feature of the time-average wake topology consists of a clearly identifiable counter-rotating streamwise vortex pair. Although the wake structure and evolution should perhaps be considered as a whole, the near wake exhibits periodic unsteadiness, at a Strouhal number of 0.2, that could be attributed to periodic shedding from the sides and to a lesser extent the top surface. This periodicity feeds into the trailing vortices, consistent with lateral and vertical displacement of the cores as they advect downstream and thus affecting maximum slipstream velocities.     

Application of artificial intelligent to predict surface roughness

Experimental Techniques - This article proposes for predicting the surface roughness of AISI 1040 steel material using the artificial intelligent. Cutting speed, feed rate, depth of cut, and nose...     

Influence of surface roughness on the flow through a staggered tube bank

The influence of surface roughness on the cross-flow through a staggered tube bank is investigated in the range of Reynolds numbers 4 ×10⁴ <Re, < 10⁷. At two roughness conditions the total pressure drop, the static pressure distribution and the local skin friction were measured. In the subcritical flow régime no effect of surface roughness on the pressure drop could be observed. Exceeding a critical Reynolds number, which itself is depending on surface roughness, the curve splits in terms of surface roughness parameterk _s/D. In the transcritical range the flow becomes independent of the tube diameterD, while the roughness heightk _s is the new characteristic length. By means of the local pressure and skin friction distribution the boundary layer behaviour was studied. Particularly the boundary layer separation and the friction forces were considered. The experiments are completed by investigations on the flow past a single row of tubes.

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Zusammenfassung Im Bereich der Reynolds-Zahlen 4 × 10⁴ < Re < 10⁷ wurde der Einfluß der Oberflächenrauhigkeit auf die Umströmung eines versetzten Rohrbündels untersucht. Unter zwei verschiedenen Rauhigkeitsbedingungen wurden der Gesamtdruckverlust sowie die örtliche Druck- und Wandschubspannungsverteilung gemessen. Im unterkritischen Strömungsbereich konnte kein Einfluß der Oberflächenrauhigkeit auf die Strömung beobachtet werden. Erst beim Überschreiten einer sogenannten kritischen Reynolds-Zahl, die ihrerseits wieder von der Oberflächenrauhgikeit abhängig ist, teilt sich die Kurve für den Druckverlust nach Maßgabe des Rauhigkeitsparametersk _s/D auf. Im transkritischen Strömungsbereich wird die Durchströmung des Bündels vom RohrdurchmesserD unabhängig. Als neue charakteristische Länge ist die Rauhigkeitshöhek _s anzusehen. Anhand der lokalen Druck- und Wandschubspannungsverteilung wurde das Grenzschichtverhalten studiert. Im einzelnen wurden Aussagen über die Grenzschichtablösung sowie über den Anteil der Reibungskräfte am Gesamtdruckverlust gewonnen. Ergänzend wird über Versuche an einer einzelnen Rohrreihe berichtet.

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