Train Nose Research Articles

Aerodynamic drag optimization of a high-speed train

This paper considers the optimization of the nose shape of a high-speed train to minimize the drag coefficient in zero-yaw-angle conditions. The optimization is performed using genetic algorithms (GA) and is based on the Aerodynamic Train Model (ATM) as the reference geometry. Since the GA requires the parameterization of each optimal candidate, 25 design variables are used to define the shape of the train nose and, in particular, to reproduce that of the ATM. The computational cost associated to the GA is reduced by introducing a surrogate model in the optimization workflow so that it evaluates each optimal candidate in a more efficient way. This surrogate model is built from a large set of simulations defined in a Latin Hypercube Sampling design of experiments, and its accuracy is improved each optimization iteration (online optimization). In this paper we detail the whole optimization process, ending with an extense analysis of results, both statistical (analysis of variance (ANOVA) to identify the most significant variables and clustering using Self-Organized Maps (SOM)), and aerodynamic. The latter is performed running two accurate simulations using Scale-Adaptive Simulation (SAS) turbulence model. The optimal design reduces the drag coefficient a 32.5% of the reference geometry.

Modeling pressure variations generated by short train passing in open air

The pressure variation generated when a high-speed train passes in open air affects structures on the ground and causes environmental problems. It was assumed in past studies that the effect of the train nose passage is independent of the tail passage for long trains (with 16 cars). In these studies, only the effect of the nose passage was analyzed with a point source model. However, as the superposition of the effect of the nose and tail is not negligible for short trains (consisting of less than 10 cars), the entire pressure variation should be analyzed. For this purpose, the positions of sources representing the nose and tail need to be considered. In this study, a model experiment and theoretical analysis on the pressure variation when a short train passes in an open air environment were performed. The experimental results show that the negative peak amplitude is greater than the positive peak amplitude for observation points located over 1/10 of the train length away from the train, because of superposition. The distance between the two zero-pressure points is different from the train length. The effects of nose and tail configurations are considered in the proposed line source model and point source model based on the typical potential theory; the prediction results of the models showed good agreement in the waveforms and peak values of the pressure variation compared with those in model experiments. Therefore, we herein demonstrate that the line source model is valid, and the point source model is applicable to short trains when considering the positions of the point sources.

The effect of numerical divergence schemes on the flow around trains

The influence of divergence schemes in typical turbulence modelling on the aerodynamic behaviours was investigated in this study. Four common divergence schemes have been used to predict the aerodynamic behaviours of a typical train in crosswinds, including the first order upwind differencing (UD), second order limited-linear differencing (LL), linear-upwind differencing (LU) and Linear-upwind stabilized transport (LUST) schemes. Shear Stress Transport (SST) k–ω turbulence model and Improved Delayed Detached Eddy Simulation (IDDES) were applied to simulate the aerodynamic characteristics of the train. Both SST and IDDES with the above three kinds of the second order divergence schemes are capable of predict accurately the pressure coefficient. Results obtained using SST with the second order discretisation schemes gave similar streamline trends around the train nose and as anticipated, results from the first order UD scheme showed poor comparison. Results obtained using IDDES showed sensitivity to the divergence schemes, and different schemes performing differently in predicting the surface pressure on different parts of the car body. Considerable differences in the force coefficient using IDDES were observed between the LUST and the other schemes. The second order upwind LU scheme is not recommended for IDDES of the flow around trains.

Improved Delayed Detached Eddy Simulation of the Slipstream and Trackside Pressure of Trains with Different Horizontal Profiles

The slipstream caused by high-speed trains may harm pedestrians and workers trackside. In general, the characteristics of the slipstream are influenced mainly by the nose shape of the train. The present study explores the slipstream caused by high-speed trains with three different horizontal nose profiles based on the results of three-dimensional, improved delayed detached eddy simulation (IDDES) with an unsteady turbulence model and a set of 1/8th scaled train models. The results obtained using this numerical methodology are in good agreement with those obtained from corresponding wind tunnel tests. The trackside pressure changes around the train models are also captured and analyzed. The analysis reveals that the width of the nose can significantly influence the magnitude and arrival time of slipstream velocity and pressure peaks. The results and proposed numerical methodology can be used as guidelines for the design of high-speed train nose shapes.

Numerical simulation of rainwater accumulation and flow characteristics over windshield of high-speed trains

In this paper, a Euler-Lagrangian particle/fluid film/VOF coupled multiphase flow model is presented. Numerical simulations are conducted, and the rainwater accumulation and flow characteristics over two types of windshields are studied based on the presented model. The results show that an uneven water film is formed over the windshield, with rain water accumulation occurring for the concave windshield but not for the convex windshield. At low speeds, the average fluid-film thickness for a concave windshield is larger than that of a convex windshield; however, a minor difference occurs between these two values at high speeds, and a critical velocity is observed for the two types of windshields. When the train velocity is less than the critical velocity, the fluid film at the lower part of the windshield and the train nose flows downward, and beyond the critical velocity, the fluid film over the entire windshield and train nose flows upward.

Numerical studies on aerodynamics of high-speed railway train subjected to strong crosswind

Under the action of strong crosswind, the aerodynamic behavior of a rail vehicle at high speed will be changed significantly, which could directly affect the safe operation of the vehicle. With the help of the shape of train used in China, the aerodynamic characteristics of trains with scale of 1:1 is investigated using computational fluid dynamics numerical simulation method, which consists of the variation of aerodynamics force and moment with wind yaw angle, wind speed, train speed, and nose shape. After an initial validation against Baker’s results from wind tunnel test, the numerical model is then used to investigate the aerodynamic characteristics of the trains. The numerical results indicate that lift coefficient of the M train is slightly higher than TMC1 and TMC2 trains. Regardless of aerodynamics force coefficients, TMC1 reaches the maximum at a yaw angle of 75°. Aerodynamics force coefficient increases with both wind speed and train speed, but the change of which is not linear. Comparing aerodynamic force with different geometric dimensions of train nose, it is shown that height–width ratio is insensitive to side force and rolling moment, but sensitive to lift force from the yaw angle 0°–90°. The side force coefficient, as we most concern, is less than other results, when the length–width ratio is 1 and height–width is 0.87.

Aerodynamic forces on railway acoustic barriers

Aerodynamic measurements were conducted to assess the magnitude of pressures measured on a range of acoustic barriers during normal working traffic. A week of measurements were made at a number of tests sites, capturing data for a range of train types. The different test sites allowed for a novel assessment of the influence of barrier distance from centre of track and any influence from other trackside infrastructure. Results exhibited a characteristic pressure development measured on the acoustic barrier, as observed in previous studies, with similar magnitudes recorded for all high speed passenger trains. Pressure magnitudes were largest around the train nose and tail regions. A twelve car train had an additional large peak due to the coupling of two six car sets. As barrier distance from centre of track was increased the magnitude of pressures measured was shown to decrease. Fitting a curve of best fit to the results indicated that the same curve form could be used for both the positive and negative nose pressures. Differences in peak pressure magnitudes were observed for results measured underneath the footbridge suggesting a potential influence of the footbridge on the aerodynamic flow development. CEN standards were shown to offer a good estimation to the pressure coefficient measured on the barrier at most positions analysed. Analysis of peak pressure magnitudes in relation to the number of train passes across the barrier life span, calculated through a rainflow analysis, suggested that small pressure fluctuations were the dominant feature across all measurements. Larger fluctuations depended on train type and as such load calculations should account for the different types of traffic observed on the railway.

Numerical simulations of the separated flow around a freight train passing through a tunnel using the sliding mesh technique

The main aim of this investigation is to analyse the flow around a freight train as it passes through a tunnel. The separated flow around the train nose is related to energy losses, lateral vibration, noise and streamline deviation, and it also influences the velocity magnitudes around the train. Such effects are expected to become more important with the prospect of increasing freight train speeds. The numerical simulations performed in this study use a Class 66 locomotive connected to eight container wagons, scaled to 1/25th, moving at a train speed of 33.5 m/s through a tunnel with a blockage ratio of 0.202. The k–ω model combined with a high advection scheme solves the governing equations on a structured hexahedral mesh using the sliding mesh technique. The pressure histories at the tunnel walls and train surface as well as the velocity field around the train were validated with experimental data obtained using a moving model. The longest separation bubble is found at the middle-height and middle-width of the locomotive due to extended corners at these regions. When the train enters the tunnel, the separation length is reduced by 32% at the roof and 31% at the sides, compared to open air. The maximum separation length is found at the sides of the train where it reattaches at 19% of the locomotive length, influencing the velocity peak at a short distance from the train surface. The larger the separation length, the higher the length/duration of this peak. When the train head is halfway through the tunnel, the nose velocity peak reduces by 30% compared to open air. The position of the nose inside the tunnel affects not only the slipstream velocity but also the velocity field at the tunnel portal and exit. These novel findings can be used as a benchmark for designing new freight train and tunnel shapes.

Aerodynamic Drag Reduction of a High-Speed Train Nose With Bionic Round Pits

The aerodynamic drag forces of a high-speed train with or without round pits were investigated. The aerodynamic drag reduction is mainly due to the pressure difference. The round pits installed on the coupling cover contribute nearly 2% for the aerodynamic drag reduction of high-speed trains.

Study on train nose shape optimization using linear acoustic theory

Study on train nose shape optimization using linear acoustic theory

Optimizing cross-sectional area of tunnel entrance hood for high speed rail

A micro-pressure wave (MPW) is radiated from the tunnel exit when a train enters a tunnel at high speed. The MPW causes infrasound and audible noise near the tunnel portal; hence, it must be reduced for environmental reasons. A hood with openings is a typical countermeasure. The openings must be suitably distributed to effectively reduce the MPW. The optimum distribution depends on the shape of the train nose and its velocity; therefore, it needs to be readjusted, and the hood sometimes needs to be extended when the train velocity increases or a new type train is put into operation. This study reports on numerical studies and model experiments to investigate the effective composition of an unvented hood because such a hood would eliminate the need to adjust the distribution of the openings according to the train characteristics. The results show that the optimum cross-sectional area of the hood is 2.6 times the cross-sectional area of the tunnel, and its optimum length is 1.8 times the tunnel diameter when the train velocity is 350 km/h, and the train nose length is three to six times the train radius. Finally, the unvented hood is economically advantageous compared with the currently used hood.

Experimental investigation of the aerodynamics of a freight train passing through a tunnel using a moving model

The objective of this study was to investigate the aerodynamic effects of a freight train passing through a tunnel. The nose entry generates a complex pattern of reflective pressure waves (piston effect) which can lead to intense aerodynamic forces. Previous research on the topic has focused on passenger trains because of higher speeds. The experiments of this study use a 1/25th scaled moving model at the TRAIN Rig at a speed of 33.5 m/s with a blockage ratio of 0.202. The monitored pressure along the tunnel wall can increase up to almost 1000 Pa because of the initial compression wave, while it drops when an expansion wave or the tail passes by. The maximum pressure is observed at the train nose due to air stagnation (1500 Pa) where the flow is steady, while the roof and sides experience negative pressures due to unsteady flow separation. The effect of loading configuration is significant as partially loaded trains can create a second pressure peak on the tunnel walls (after the initial compression wave) and affect the flow at the tunnel entrance wall. Under the current testing conditions, the results indicated compliance with the requirements of the Technical Specification for Interoperability and a constant pressure gradient of the initial compression wave which is in contrast with the passenger train's two-part gradient. Further work on the topic could provide visual information about the exiting jet towards the portal and the separation bubble around the train.

Numerical analysis of aerodynamic characteristics of high-speed train with different train nose lengths

In this study, based on the SST κ-ω turbulent model, the IDDES method is used to simulate the unsteady aerodynamic performance of trains with respect to different lengths of the tapered nose of the train (8 m and 12 m). The numerical simulation used in this study is verified through wind tunnel tests. The effects of the length of the tapered nose of the train on the aerodynamic performance, such as the train forces, boundary layer, velocity distribution, pressure distribution, and flow structure around the train, are elucidated via comparing and analyzing the obtained results. The results indicate that the effect of the length of the tapered nose of the train on the drag force of the tail car and lift force of the head car is stronger than the effect on other cars, and the Cd value of the tail car decreases by 30.53% and the Cl value of the head car increases by 87.98%. Increase in the length of the tapered nose of the train decreases the fluctuation of the drag and lift forces of the train, especially the head car. It is also observed that the boundary layer thickness around the train is decreased with the increase in the length of the tapered nose of the train. Moreover, it is observed that vortex drag is the primary factor in the aerodynamic drag of the tail car and that vortex drag primarily depends on the length of the tapered nose of the train.

Application of the Bionic Concept in Reducing the Complexity Noise and Drag of the Mega High-Speed Train Based on Computer Simulation Technologies

Regarding the continuous development of high-speed trains and the increase of running speeds, the aerodynamic design of high-speed trains has become significantly important, while reduction of drag and noise comprises a significant challenge in order to optimize aerodynamic design of high-speed trains. The design form factor of a high-speed train is highly influenced by aerodynamic aspects including aerodynamic drag, lift force, and noise. With the high-speed train as the object, the paper aims to take bionic concept as the entry point, selecting the hummingbird as the bionic prototype and extracting bionic elements to establish a bionic train model. Then, the finite volume method was used for numerical simulation and analysis of the aerodynamic performance and aerodynamic noise of the bionic high-speed train. Computational results prove that drag and noise of the bionic head type were lower than those of the original train; drag of the head train of the bionic model was reduced by 2.21% in comparison with the original model, while the whole-train drag was reduced by 3.53%, indicating that drag reduction effects are available and implying that the bionic head type could reduce drag and noise. Noise sources of the bionic train are mainly located at positions with easy airflow separation and violent turbulence motion. Large turbulence energy is in bogie areas and mainly exists at the leeward side of the bogie area. Obviously, the bogie area is the major noise source of the train. Aerodynamic noise of the bionic train in far-field comprises a wide-frequency range. Noises were concentrated within 613 Hz~3150 Hz. When the bionic high-speed train ran at 350 km/h, through comparative analysis of total noise levels at observed points of the high-speed train, it is found that this position with the maximum noise level was 25 m away from the head train nose tip, with the maximum value of 88.4 dB (A). When the bionic train ran at 600 km/h, the maximum sound pressure level at the longitudinal point was 99.7 dB (A) and the average noise level was 96.6 dB (A). When the running speed increased from 350 km/h to 600 km/h, the maximum noise level increased by 11.3 dB (A) and the average noise level increased by 11.6 dB (A). Computation results of aerodynamic noise at the point which is 7.5 m away from the rail center show that the maximum aerodynamic noise level existed at the first-end bogie of the head train, while the noise level was larger at the position closer to the ground.

Assessment of the mesh refinement influence on the computed flow-fields about a model train in comparison with wind tunnel measurements

A consistent mesh refinement study, relating to the prediction of aerodynamic forces about an experimentally validated reference train geometry, is presented in this paper. The flow about a high-speed train has a multi-scale character which poses challenges for the design of computationally effective meshes. The purpose of this study is to assist in the development of guidelines for effective drag prediction of high-speed trains using numerical simulation. These guidelines should assist CFD practitioners by identifying the regions of the mesh that are critical for the correct estimation of drag as well as providing information on appropriate mesh characteristics, such as volume and surface element length scales. Numerical assessments are validated against an experimental drag measurement program and the extent to which RANS is sufficiently predictive for industrial design is discussed. The results obtained in the work suggest that the mesh about the train nose is essential for the proper assessment of the aerodynamic drag acting on the vehicle.

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.

Mechanism and capability of ventilation openings for alleviating micro-pressure waves emitted from high-speed railway tunnels

Micro-pressure waves are a major environmental problem related to modern high-speed railway systems. The strength of this harmful noise is proportional to the amplitude of the compression wave gradient generated by a high-speed train entering a tunnel. Employing an accurate numerical method, the mechanism and effects of ventilation openings on these compression waves are parametrically investigated. The numerical results indicate that after installing an opening, the compression wave is principally developed as multiple series of wave families, and thus, the pressure gradient curve is formed by numerous peaks and troughs. The gradient peaks UP0 and UV0 are generated successively by the train nose entering the tunnel and passing over the opening, respectively, and dominate the maximum pressure gradient. The vent ratio of the opening can be optimized by balancing these two peaks. However, the vent location and train Mach number can significantly affect the optimizing and the aerodynamic behaviour of the optimized opening, which is attributed to wave superposition. Three original engineering equations are proposed for understanding the effects of the vent ratio, vent location and train Mach number on the gradient peaks, respectively, and the denoising capability of the opening is evaluated.

Effect of crosswinds on aerodynamic characteristics around a generic train model

ABSTRACTIn this article, simulations of crosswinds over a rectangular prism with two types of train nose shapes (i.e. blunt and ellipse) that are running on flat ground case are discussed. The incident flow angle is varied from 0° to 90°. The flow around the train is solved using the incompressible form of the unsteady Reynolds Navier-Stokes (URANS) equations combined with the Shear-Stress-Transport turbulence model. The Reynolds number used, based on the height of the train is 3.7 × 105. Two distinctive flow regimes appear which represent a slender body behaviour at a smaller range of incident flow angles (below 45°) and bluff body behaviour at a much higher range of incident flow angles (above 45°). The safety guidelines for train’s operation suggested that both the train’s nose geometry and the wind speed condition are the two major factors that limit the maximum allowable speed for a particular train to be travelling.

Aerodynamic Study of Two Opposing Moving Trains in a Tunnel Based on Different Nose Contours

It is well known that the train nose shape has significant influence on the aerodynamic characteristics. This study explores the influence of four kinds of nose shapes (fusiform, flat-broad, bulge-broad, ellipsoidal) on the aerodynamic performance of two opposing high-speed trains passing by each other through a tunnel at 250 km/h. The method of three dimensional, compressible, unsteady Reynolds-averaged Navier-Stokes equations and RNG k-ε double equation turbulence model was carried out to simulate the whole process of two trains passing by each other inside a tunnel. Then the pressure variations on tunnel wall and train surface are compared with previous full-scale test to validate the numerical method adopted in this paper. The assessment characteristics, such as transient pressure and aerodynamic loading, are analyzed to investigate the influence of nose shape on these assessment parameters. It is revealed that aerodynamic performance of trains which have longitudinal nose profile line B (fusiform, flat-broad shape) is relatively better when passing by each other in a tunnel. The results can be used as a guideline for high-speed train nose shape design.

Analysis of the aerodynamic effects of different nose lengths on two trains intersecting in a tunnel at 350 km/h

The three-dimensional compressible Navier-Stokes equation and k-ε turbulence model are used to simulate the flow and pressure waves caused by two trains passing each other in a tunnel. The simulation results were verified by comparison with the results of a full-scale experiment. This simulation results indicate that the positive peak of the initial compression wave on the tunnel wall has a logarithmically decreasing trend with increasing nose length. This tendency is more obvious near the tunnel portal than in the tunnel, and the positive peak caused by a train with a nose length of 12m at a distance of 20m from the tunnel entrance is 10.26% less than that caused by a train with a nose length of 4m. Throughout the intersection process, the peak-to-peak amplitude of body surface pressure decreases with increasing longitudinal distance from the front nose tip. The influence of different nose lengths on the surface pressure on the train body is mainly concentrated at the front and rear of the train. The fluctuation amplitude of the surface pressure on the head car with a 4m nose is 1.63 times that of a head car with a 12m nose. The amplitudes of the lateral force and overturning moment are also influenced by the nose length, with the strongest effect on the head car and a stronger effect on the middle car than on the tail car. A shorter train nose results in a more significant influence on the train total drag. As the nose length changes from 4m to 7m, the maximum total drag decreases by 6.71%; however, as the nose length changes from 9m to 12m, the maximum total drag decreases by only 0.16%.