高速列车通过隧道时的压力波动问题

高速列车通过隧道时将会在隧道内引起相当复杂的气体压力波动,这是由于列车进入隧道时在隧道入口产生的压力波在隧道内来回传递并与列车经过时的气体压力扰动相互叠加的结果。从车体强度设计和列车运行安全性角度考虑,希望了解隧道内可能的最大气体正、负压力大小及其发生位置;气体压力波动与列车运行速度的关系。通过流体力学方程三维动态数值计算,仿真分析列车高速通过隧道的过程。计算结果证明了入口压力波效应与列车经过的扰动效应的叠加关系,得到列车通过时隧道内最大正压和最大负压发生的可能位置,以及最大正压值与最大负压值与车速间的关系式。可为高速铁路隧道和高速列车设计提供参考。     

高速列车单车通过隧道压力波的研究

采用计算流体力学的数值计算方法对基于三维、瞬态、可压缩Navier-Stokes方程和κ-ε两方程紊流模型进行求解,模拟高速列车单车通过隧道时列车外流场的特性,分析高速列车单车通过隧道的压力波特性及阻力变化规律。结果表明列车单车通过隧道的压力波最小负压值与速度为二次函数的关系,列车阻力主要由压差阻力构成。研究结果可为解决隧道空气动力学问题提供参考依据。     

A numerical simulation of train-induced unsteady airflow in a tunnel of Seoul subway

This study has been conducted to investigate numerically the characteristics of train-induced unsteady airflow in a subway tunnel. A three-dimensional numerical model using the dynamic layering method for the moving boundary of a train is applied. The validation of the present study has been carried out against the experimental data obtained by Kim and Kim [1] in a model tunnel. After this, for the geometries of the tunnel and subway train which are very similar to those of the Seoul subway, a three-dimensional unsteady tunnel flow is simulated. The predicted distributions of pressure and air velocity in the tunnel as well as the time series of mass flow rate at natural ventilation ducts reveal that the maximum exhaust mass flow rate of air through the duct occurs just before the frontal face of a train reaches the ventilation duct, while the suction mass flow rate through the duct reaches the maximum value just after the rear face of a train passes the ventilation duct. The results of this study can be utilized as basic data for optimizing the design of tunnel ventilation systems.     

同轴数字全息术在大气可吸入颗粒物PM2.5测量中的应用

讨论了同轴数字全息术测量PM2.5大气可吸入颗粒物的可行性,并设计了一套针对PM2.5颗粒测量的同轴数字全息实验装置。在讨论同轴全息图的数字图像处理方法基础上,利用所设计的装置测量了与PM2.5颗粒粒径相当的标准粒子,对实验结果进行了分析和讨论。实验证明,同轴数字全息术可有效应用于PM2.5颗粒的测量与分析。     

A computational analysis of the airflow in a twin-track subway tunnel with a sliding-curtain to improve ventilation performance

A computational model of an actual Seoul subway tunnel was analyzed in this study. The computational model was comprised of one natural ventilation shaft, two mechanical ventilation shafts, one mechanical air supply, a twin-track tunnel and a train. The natural ventilation shaft discharges and supplies air due to the train’s movement. The mechanical ventilation shaft and the mechanical air supply discharges and supplies, respectively, the airflow from the axial flow fans in the middle of the ducts of the shafts. A sliding-curtain was installed in the tunnel. The objective of this study was to numerically investigate train-induced airflow in the twin-track subway tunnel with natural and mechanical ventilation shafts and an installed curtain. The numerical analysis characterized the aerodynamic behavior and performance of the ventilation system by solving three-dimensional turbulent Reynolds-averaged Navier-Stokes equations. ANSYS CFX software was used for the computations. The airflow velocity from the computational results was validated by experimental results. Understanding the flow pattern of the train-induced airflow in the tunnel is necessary to improve ventilation performance. The ventilation and aerodynamic characteristics in the tunnel, including train-induced airflow, were investigated by analyzing the volume flowrate at the exits of the ventilation shafts and the velocity in the tunnel. The computational results were compared to cases with and without a curtain installed in the twin-track tunnel. As the train passed the mechanical ventilation shafts, the quantity of discharged-air in the ventilation shafts decreased rapidly. The flowrate at the exits of the ventilation shafts was gradually recovered with time, after the train passed the ventilation shafts. The airflow at the natural shaft and mechanical ventilation shaft 2, which was closest to the curtain, was increased. The computational results showed that the installed curtain can improve ventilation performance in the tunnel.     

Numerical analysis of impulse waves discharged at the exit of a model tunnel

Impulse waves are micro-pressure waves, which always occur at the tunnel exit when a high-speed train is moving inside a train tunnel. The air around the train nose is compressed and compression waves are induced. The impulse wave is discharged at the exit of a train tunnel when a compression wave propagates outside of the tunnel exit. Impulse waves are weak-strength pressure waves, which lead to noise and other environmental problems. In order to efficiently control the impulse wave at the exit of a train tunnel, numerical studies on investigating the generation and propagation of the impulse wave were carried out. A 2-D axisymmetric model tunnel was simulated at different operating conditions. Different Mach numbers of compression waves were varied to induce different magnitudes of impulse waves at the tunnel exit. In addition, compression waves with different pressure gradients were assumed at the tunnel entry to check their effects on the generation of impulse waves. In order to observe impulse waves at far field, five monitor points were installed behind the tunnel exit to record pressure histories as impulse waves moved through these locations. The detailed magnitudes and characteristics of impulse waves were obtained in the present studies.     

高速列车会车压力波对侧窗的影响

高速列车会车时产生的空气压力波动会给交会车辆的侧窗造成很大的冲击,有可能出现破窗事故,给乘客和列车运行带来安全隐患。以三维、非稳态、粘性雷诺时均方程和k-ε两方程紊流模型为基础,采用移动网格的有限体积数值计算方法,仿真分析5种车速(200km/h、250km/h、300km/h、350km/h、400km/h)条件下明线和长隧道内等速会车的动态过程。得到侧窗上完整的会车压力波变化曲线。计算结果表明,明线会车与长隧道内会车产生的压力波对列车侧窗的影响有很大的不同,长隧道内会车时在交会车辆侧窗上产生的气动负压波峰值比明线会车时产生的负压波峰值要大将近一倍,因此不能将明线上会车压力波变化结论外推到隧道内会车情况。以计算结果为基础,分析会车引起破窗的原因和评价侧窗强度的方法。在进行高速列车侧窗设计时,不但要考虑窗玻璃本身的抗冲击强度,还必须考虑列车侧窗的安装强度。相同面积的侧窗,周长大的车窗更有利。     

Field measurement and estimation of ventilation flow rates by using train-induced flow rate through subway vent shafts

This study measured and estimated the subway vent shaft air flow rate induced by moving trains in the tunnel. This work estimated the flow rate via the tunnel structure and train movement to determine the quantitative effect of vent shafts as air purification systems of natural ventilation to improve the air quality management of a subway. The amount of air suctioned into the tunnel is significantly larger than that vented from the tunnel. Thus, placing vent shafts near subway stations is desirable for natural ventilation systems. Experimental approaches to measure train-induced flow rates have not yet been published. Results of this study provide useful fundamental data to study the natural ventilation in a subway. Therefore, this study suggested the significant design factors required to control indoor air quality in a subway.     

Direct simulation of inhalable particle motion and collision in a standing wave field

Motion of inhalable particles (PM₁₀) under the effect of acoustic standing wave was numerically studied, and the inter-particle collision rate was investigated based on the direct simulation Monte Carlo (DSMC) method. The results show that the particles convergence rapidly, generating two particle number concentration peaks in a wavelength range along the wave propagation direction. Considering the inter-particle collision, it is found that when the particle size and number concentration keep constant, the collision rate first increases and then decreases with the increase of the frequency. Hence there is an optimal frequency that corresponds to the highest collision rate. As the sound intensity level increases, the collision rate increases monotonically. In cases with the same acoustic wave, the collision rate increases with the particle number concentration. The particle size is also an important factor that strongly affects the collision rate. The large inhalable particles collide more intensively than the small ones.     

Numerical optimization study to install air curtain in a subway tunnel by using design of experiment

Subways are a major mode of public transportation in metropolitan cities. A proper ventilation system is required to maintain indoor air quality in subway tunnels. Platform screen doors improve the platform environment but degrade air quality in subway tunnels. Trains transport fine particles from the tunnel into the platform. An air curtain installation in the subway tunnel permits traffic and reduces the transfer of bacteria and fine particles. The existing tunnel of Seoul subway was investigated by using computational fluid dynamics and design of experiment method for optimum air curtain installations. The flow field of the subway tunnel was computed by using ANSYS CFX software. Minitab software was used to generate the design process and to analyze the computational results. The computational domain of the existing tunnel included two natural ventilation shafts, one mechanical shaft, and the twin tracks. The height, width, and length of each track were 6, 4, and 400 m, respectively. The air curtain installation area was located between the natural and the mechanical ventilation shafts of Rrack 1. The design variables for the optimization study were the width, velocity, and installation location of the air curtain. The object function for optimization was mass flow rate at the natural ventilation shaft. The length of the air curtain was fixed at 4 m. The predicted mass flow rates were analyzed with the design variables by using the response surface method (RSM). The optimum values of the design variables, i.e., velocity, width, and installation location were 25 m/s, 0.2 m, and 5.8195 m, respectively. The maximum mass flow rate with the optimum design values was 114.4447 kg/s. The optimum values of the design variables were validated by computing the tunnel with the optimum values from RSM. The mass flow rate in the natural ventilation shaft 1 was 114.2 kg/s, as predicted. The optimization study can be helpful to set the optimum design conditions for the subway ventilation system.     

高速动车换气口流动阻力实验台设计

探讨了高速动车换气口流动阻力实验台相关问题,根据流体力学相关原理,提出用低速风洞模拟列车外界风速场,换气口试件置于风洞试验段中,将列车外界风速、流经换气口的风量模拟量都设计成可调控的实验参数,搭建了用于换气口流动阻力及其影响因素的实验平台。实验实例表明,本文方法建立的高速动车换气口流动阻力实验台方案是可行的,测量结果是可信的。     

Numerical study of air curtain systems for blocking smoke in tunnel fires

A tunnel fire is very dangerous to drivers because of generated poisonous gases. For dealing with this hazardous situation, ventilation systems for smoke control are installed so that drivers can be safely evacuated from the site of the fire. An air curtain system is one of these ventilation systems, and such a system in a tunnel generates an air wall to block the passage of poisonous gases. In this study, airflow discharge patterns of air curtain systems were analyzed using Computational fluid dynamics (CFD), with two design parameters—to predict the ability of the air curtain to block the contaminated adverse air-flow in the tunnel. The considered two design parameters were the installed angle of the slit nozzle (N_A) and the discharged air velocity at the slit nozzle outlet (N_V). The tunnel geometry for the CFD analysis was a two-lane type with a tunnel length of 100 m and an elliptical cross section. The height of the tunnel was approximately 7.3 m and the height of the installed air curtain was about 4.9 m from the road surface. In this study, the heat release rates of the fire, the distance from the fire site, and the temperature of the working fluid were respectively assumed to about 20 MW, 50 m and 473 K, on the basis of the NFPA 502. The CFD analysis demonstrated that an N_A of 0 deg could not block the adverse air flow due to a realistic tunnel inlet-outlet static pressure difference (ΔP_S). An N_A of 20 deg was required to effectively block the adverse flow. The blocking failure first formed at the sidewall of the tunnel, and it proceeded toward the center of the tunnel cross section when the adverse wind was strong. That aspect of the blocking failure was judged to be due to the fact that the tunnel cross section is elliptical. Anyway, when the tunnel ΔP_S was increased, that showed the need for a high N_V.     

Optical counting of guided particles in a vortex beam as an optical tube by laser-two-focus method

In this study, microscopic particles such as aerosols were counted by Laser-Two-Focus method, L2F, after being trapped and guided by an optical tube. This prevents particles diffusivity as they pass through the Gaussian beam in the L2F method by optical forces such as radiation pressure and photophoretic forces. In optical tube, particles can guid to the center of the beam where the intensity gets zero. A single-charged Bessel Gaussian beam, BG₀₁, is used as the particle guidance beam in this method, which is generated by passing the first-order Laguerre-Gaussian beam, LG₀₁, from an axicon lens. LG₀₁ beam are also produced by using holographic interference grid mask. The results of the theory and simulations showed that by optical guiding of particles in the L2F-method measurement, their transverse turbulence can be reduced by about 60% and then the probability of measuring all particles to be increased by about 30%. Measurements of in-laboratory aerosols less than 3.5 μm with this method showed a 20% increase in their condensation of them compared to the conventional L2F method.     

Measurement of indoor air quality for ventilation with the existence of occupants in schools

This paper evaluates the performance of ventilation for the removal of indoor pollutants as a function of ventilation rate and the number of occupants in a test room and school classroom. An experimental apparatus consists of a test room, a tracer gas supply system, a gas detector, and a fan for ventilation air supply with a controller. The ventilation performance is evaluated in a step-down method based on ASTM Standard E741-83 using CO₂ gas as a tracer gas in the test room of 35 m³. For the ventilation air flow rate of 1.0 ACH, a recommended ventilation flow rate of Korea school standard for acceptable indoor air quality in the case of one person, CO₂ gas concentration decreases up to 55% within 50 minutes without occupancy and increases up to 75% in the case of one occupant. Also indoor air quality at the school classroom is investigated experimentally.     

Research on formation and stability of keyhole in stationary laser welding on aluminum MMCs reinforced with particles

The formation and stability of keyhole in stationary laser welding on aluminum metal matrix composites reinforced with particles are studied using a numerical simulation. The interaction between molten pool and reinforcement particles is evaluated by using the particle–fluid coupling model in the numerical simulation. In order to study the effect of different volume fractions of particles on the keyhole stability and fluid flow inside the molten pool, keyhole formation process, variation of free surface, temperature distribution, and fluid flow are calculated numerically, respectively. The calculation results show that the keyhole is stable at the beginning under different conditions and then the protrusion occurs inside the keyhole with increasing calculation time. The flow behavior of molten pool affected by particles and forces acting on the surface could explain the forming of humps inside the keyhole, which directly cause the variation of the keyhole. As the volume fraction of TiB₂ particles increases, the keyhole is more likely to be instable and the oscillation occurs at an earlier time. Fluctuations of the surface tension and recoil pressure due to the uneven distribution play an important role in the instability of the keyhole.     

Experimental investigation on dilute gas-solid multiphase jet in crossflow

In the current study, an experimental setup was built to investigate the gas flow and particle distribution in a normal jet crossflow to a main flow in a confined test section. The experiments were conducted under two test conditions: with Re_c/Re_jet of 7.9×10⁴/3.1×10⁴ and 7.0×10⁴/1.8×10⁴. Four classes of particles were used in both tests. The planar gas flow field and particle distribution on the symmetric cross-section were measured by a DPIV system. Mean fluid velocity results and transient flow visualization images were used to analyze the jet influence on the gas flow field. The analysis of the time-average particle concentration reveal that the jet control method may set a gas barrier in the flow field, which the tiny particles are able travel around, large particles are able travel through, and only 10-micronscale particles could be successfully blocked. The results show that the wall jet control method can be applied in inertia particle separator.     

Numerical study on characteristics of turbulent two-phase gas-particle flow using multi-fluid model

The purpose of this research is to study numerically the turbulent gas-particle two-phase flow characteristics using the Eulerian-Eulerian method. A computer code is developed for the numerical study by using the k-ɛ-k _p two-phase turbulent model. The developed code is applied for particle-laden flows in which the particle volume fraction is between 10⁻⁵ and 10⁻² for the Stokes numbers smaller than unity. The gas and particle velocities and the particle volume fraction obtained by using this code are in good agreement with those obtained by a commercial code for the gas-particle jet flows within a rectangular enclosure. The gas-particle jet injected into a vertical rectangular 3D enclosure is numerically modeled to study the effect of the Stokes number, the particle volume fraction and the particle Reynolds numbers. The numerical results show that the Stokes number and the particle volume fraction are important parameters in turbulent gas-particle flows. A small Stokes number (St ≤ 0.07) implies that the particles are nearly at the velocity equilibrium with the gas phase, while a large Stokes number (St ≥ 0.07) implies that the slip velocity between the gas and particle phase increases and the particle velocity is less affected by the gas phase. A large particle volume fraction (α _p ≥ 0.0001) implies that the effect of the particles on the gas phase momentum increases, while a small particle volume fraction (α _p ≤ 0.0001) implies that the particles would have no or small effect on the gas flow field. For fixed Stokes number and particle volume fraction, an increase of the particle Reynolds number results in a decrease of the slip velocity between the gas and particle velocities.     

Estimation of appropriate capacity of ventilation system based on the air infiltration rate in Korean classrooms

The appropriate capacity of a ventilation system based on the air infiltration rate in Korean classrooms is investigated to obtain optimal design conditions for ventilation systems. Theoretical and the experimental analyses are performed to estimate the proper ventilation capacity with a consideration of the air infiltration, the indoor air quality, and the ventilation rate. The air infiltration rate of the classroom is measured within the range of 0.5–1.5 1/h, and the required ventilation rate should be decided not by the contaminants (Formaldehyde and TVOC) emitted from the construction materials but by the carbon dioxide (CO₂) emitted from human breath. The appropriate capacity of the ventilation system based on the air infiltration rate of the classroom for elementary schools is 500CMH and for middle and high schools is 800 CMH. The measured and the estimated values of CO₂ concentrations are very similar and the modeling equation of CO₂ concentration can be used as a reference for the proper estimation of ventilation rate in Korean schools.     

Unsteady aerodynamic loads on high speed trains passing by each other

In order to study unsteady aerodynamic loads on high speed trains passing by each other 350km/h, three-dimensional flow fields around trains during the crossing event are numerically simulated using three-dimensional Euler equations. Roe’s FDS with MUSCL interpolation is employed to simulate wave phenomena. An efficient moving grid system based on domain decomposition techniques is developed to analyze the unsteady flow field induced by the restricted motion of a train on a rail. Numerical simulations of the strain passing by on the double-track are carried out to study the effect of the train nose-shape, length and the existence of a tunnel on the crossing event. Unsteady aerodynamic loads-a side force and a drag force-acting on the train during the crossing are numerically predicted and analyzed. The side force mainly depends on the nose-shape, and the drag force depends on tunnel existence. Also. a push-pull (i.e. impluse force) force successively acts on each car and acts in different directions between the neighborhood cars. The maximum change of the impulsive force reaches about 3 tons. These aerodynamic force data are absolutely necessary to evaluate the stability of high speed multi-car trains. The results also indicate the effectiveness of the present numerical method for simulating the unsteady flow fields induced by bodies in relative motion.     

Spectroscopic and microscopic characterization of atmospheric particulate matter

The aim of this study was to investigate the chemical properties of atmospheric particulate matter collected during post biomass burning events from November to December 2015. The types of particulate samples collected from the study area were total suspended particulates (TSP) and particulates less than 2.5?µm (PM_2.5). Multiple methods were used for the qualitative and quantitative characterization. Fifteen major and minor trace metal analyses were performed using acid digestion followed by inductively coupled plasma mass spectrometry analysis. Infrared spectroscopy was used for the identification of functional groups of organic compounds present in the samples. In addition, field emission scanning electron microscopy–energy dispersive X-ray spectroscopy was used for morphological characterization and microanalysis of individual particles. The average measured concentration of trace metals in TSP and PM_2.5 follows the order of Zn?>?Fe?>?Al?>?Cu?>?Pb?>?Mn?>?V?>?Cr?>?Ni?>?Sn?>?As?>?Hg?>?Cd?>?Ag?>?Co. Enrichment factor analysis showed strong anthropogenic contributions of Hg, Ag, Zn, Cd, Pb, As, Cu, and Sn in the samples. Field emission scanning electron microscopy–energy dispersive X-ray spectroscopy shows that three types of particles were present that originated from crustal and anthropogenic sources. Furthermore, the infrared measurements indicate that most compounds were organic acids and aromatic hydrocarbons.