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Helical vortices in the wake of trains [1], [8], [17]

Helical vortices in the wake of trains [1], [8], [17]

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This paper considers aspects of the aerodynamic behaviour of high speed trains. It does not specifically address the many aerodynamic problems associated with such vehicles, but rather attempts to describe, in fundamental terms, the nature of the flow field. The rationale for such an approach is that the flow fields that exist are the primary cause...

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... Up till now, the maximum commercial speed of wheel rail trains in China, such as the Fuxing, is 350 km/h. This upper limit of operational speed is further restricted due to wheel rail wear, aerodynamic resistance, and other factors [Baker, 2010]. The maglev train, a type of new high-speed transport vehicle, has the advantages of lower total resistance, lesser noise, and larger speed-up space compared to traditional wheel rail trains. ...
Article
A maglev train with a speed of 600 km/h or higher can fill the speed gap between civil aircrafts and wheel rail trains to alleviate the contradiction between the existing transportation demand and actual transport capacity. However, the aerodynamic problems arising due to trains running at a higher speed threaten their safety and fuel efficiency. Therefore, we developed a newly moving model rig with a maximum speed of 680 km/h to evaluate aerodynamic performance of trains, thus determining the range of the aerodynamic design parameters. In the present work, a launch system with a mechanical efficiency of 68.1% was developed, and a structure of brake shoes with front and rear overlapping was designed to increase the friction. Additionally, a device to suppress the pressure disturbances generated by the compressed air, as well as a double track with the function of continuously adjusting the line spacing, were adopted. In repetitive experiments, the time histories of pressure curves for the same measuring point are in good agreement. Meanwhile, the moving model test and full-scale experimental result of maglev trains passing each other in open air are compared, with an error less than 4.6%, proving the repeatability and rationality of the proposed moving-model.
... As mentioned above, the aerodynamic drag contributes to 70%−80% of the total resistance experienced for a high-speed train; however, 38% − 47% of the drag comes from bogies and the associated interference drag is caused by the complex geometric structures [7,8]. ZHANG et al [10] and NIU et al [11] reported that the flow state beneath the train body plays a crucial role in the drag force of a high-speed train and the train underbody flow highly depends on the bogie geometry, since the bogies are exposed to air. ...
... As the high-speed train runs in open air, lots of vortices are generated and spread around the train, shedding to the far field and forming velocity fluctuations around the train. Underneath the train body, the flow is highly turbulent, which is expected, for coming from the rough and constrained underbody environment between the train and the ground [7]. Consequently, these has resulted in various aerodynamic issue including the ballast flight, slipstream etc, which have caused problems to the train, track, and railway workers. ...
... The velocity profiles along line x 5 in four cases show a good trend and consistency, which implies that the effect of bogie regions on the local flow characteristics is restrained below a certain height, e. g., 0.28H from the ground. This phenomenon is described perfectly by the boundary layer around the train, which is similar to the result in Ref. [7]. For C p distributions, Case 1 and Case 2 show similar trends along lines z 1 −z 3 . ...
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An investigation of the effect of simplifying bogie regions on the aerodynamic performance of a high-speed train was carried out by studying four train models, to explore possible ways to optimise the train underbody structure, improve the underbody aerodynamic performance, and reduce the aerodynamic drag. The shear stress transport (SST)k-ω turbulence model was used to study the airflow features of the high-speed train with different bogie regions at Re=2.25×106. The calculated aerodynamic drag and surface pressure were compared with the experimental benchmark of wind tunnel tests. The results show that the SST k-ω model presents high accuracy in predicting the flow fields around the train, and the numerical results closely agree with the experimental data. Compared with the train with simplified bogies, the aerodynamic drag of the train with a smooth surface and the train with enclosed bogie cavities/inter-carriage gaps decreases by 38.2% and 30.3%, respectively, while it increases by 10.2% for the train with cavities but no bogies. Thus, enclosing bogie cavities shows a good capability of aerodynamic drag reduction for a new generation of high-speed trains
... Existing literature primarily focuses on the case of a train passing through a ground station at high speed in order to study the aerodynamic effect of a train passing through a station (Muraki et al., 2010). Baker (2010Baker ( , 2014aBaker ( , 2014b discussed and summarized the pressure fluctuation on the line side and analyzed the influence of trains on the noise barrier, bridge, and station platform aerodynamic load distribution. Zhou et al. (2014) studied the transient pressure distribution characteristics on the surface of the car body and platform screen doors (PSDs) when a train passed through a station at different speeds, and when two trains met at a station using a 1:20 scale moving model test device. ...
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Transient pressure variations on train and platform screen door (PSD) surfaces when a high-speed train passed through an underground station and adjoining tunnel were studied using a moving model test device based on the eight-car formation train model. The propagation characteristics of the pressure wave that was induced when the train passed through the station and tunnel at a high speed were discussed, and the effects of the train speed and station ventilation shaft position on the surface pressure distribution of the train and PSDs were analyzed and compared. The results showed that the pressure fluctuation law is different for the train and PSD surfaces, and the peak pressure increases significantly with an increase in the train speed. Ventilation shafts changed the pressure waveform on the surface of the train and PSDs and greatly reduced the peak pressure. A single shaft at the rear end of the platform and a double shaft at the station had the most significant effect on relieving transient pressure on the surface of the train and PSDs, respectively. Compared with the case with no shaft, these two shafts reduced the maximum amplitude pressure variation of the train and PSD surfaces by 46.3% and 67.4%, respectively.
... Moreover, the increasing speed of trains comes at a price, leading to increased importance of their aerodynamics considering safety and drag reduction. The experimental approaches to tackle aerodynamically induced issues for trains concern the full-scale measurements (Baker et al., 2004Sterling et al., 2009), the physical modelling for scaled models involving wind tunnel experiments (Kwon et al., 2001;Baker, 2010;Niu et al., 2017) and moving-train systems (Baker et al., 2001;Zhou et al., 2014;Bell et al., 2015). The flow duplicated in a reliable experiment helps exploration of the aerodynamic characteristics for trains but is costly (particularly with a full-scale model). ...
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The turbulent flow past a simplified Intercity-Express 3 high-speed train at Re_H=6×10^4 is investigated by a combination of wind tunnel experiments and numerical simulations using the large-eddy simulation (LES), the improved delayed detached eddy simulation (IDDES) and the unsteady Reynolds-averaged Navier-Stokes (URANS) simulation. This work aims to compare the predictive capabilities of LES, IDDES and URANS for the flow over a streamlined high-speed train. Numerical simulations are compared to experimental data for validation. Results show that the well-resolved LES is more accurate among the numerical methods used. Compared to the well-resolved LES, IDDES and URANS using the coarser mesh can produce similar mean flow, although IDDES and URANS are found to be slightly inaccurate for the coherent wake structures near the wall. However, for the near-wall flow instability concerning wake dynamics, Reynolds stresses, turbulence kinetic energy and the fluctuation of pressure, IDDES is found to be inapplicable. Overall, this study suggests that the well-resolved LES is appropriate to the flow of a streamlined high-speed train. Moreover, IDDES and URANS are proved to apply to the mean field of the studied flow.
... As there has been a rapid development in high-speed railway systems in recent decades, the crosswind stability of trains has become a main concern [1][2][3][4]. In order to ensure a high-speed railway track's smoothness, the bridge mileage accounts for a large proportion.. ...
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Wind barriers can effectively reduce the risk of overturning and derailment of high-speed trains running on a bridge under crosswind. However, it can adversely affect the wind resistance of the bridge. There are few studies on the aerodynamic performance of curved wind barriers. In this paper, the effects of curved wind barriers with four curvatures (0, 0.2, 0.35, and 0.50) and different train-bridge combinations on the crosswind aerodynamic characteristics of a train-bridge system are investigated. The results show that the curved wind barrier can significantly reduce the wind speed below a certain height on the bridge deck. The curved wind barrier with small curvature can better reduce the aerodynamic force of the train; however, it greatly increases the aerodynamic force of the bridge. A wind barrier with a curvature of 0.35 is recommended because it takes into account the aerodynamic characteristics of the train and bridge at the same time. The porosity of a wind barrier greatly influences the aerodynamic performance of the train on the track of the windward side of the bridge, while the wind barrier has little effects on the train on the track of the leeward side of the bridge. The aerodynamic performance of the train on the track of the windward side of the bridge is less affected by whether or not a train on the track of the leeward side of the bridge is present.
... The distribution of the wall shear stress, slipstream, and wake dynamic structures is related to the distribution of boundary layer profiles. 7,14 Some literature studies have also compared and analyzed the influence of the train length on the aerodynamic performance of the train, but the analysis is not thorough enough, and most of them focus on the final integral physical quantity (force). Jia et al. 11 investigated the effect of the train lengths (three cars, four cars, five cars, and eight cars) on the boundary layer, wake vortices, surface pressure, and aerodynamic drag coefficients, but the analysis except the aerodynamic drag coefficients is all qualitative research. ...
... The height from the ground has a greater influence on the distribution of δ * and θ on the side surface in Fig. 19, and the lower the position, the larger the value of δ * and θ, which is consistent with previous studies. 13,14 The displacement and moment thicknesses in the 1/10th scale wind tunnel experiment of two-car ICE3 are used for the quantitative comparison, and the displacement thickness and moment thickness at x = −2.5H on the side surface (z = 0.5H) are δ * /H = 0.0312 and θ/H = 0.025, respectively. The results in the wind tunnel experiment (two cars) are nearly at the same level with the results of C2 (five cars) and larger than those of C1 (three cars) shown in Figs. ...
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The improved delayed detached eddy simulation is adopted in the present study to investigate the influence of the train length on its aerodynamic performance. The low y+ wall treatment and the cubic constitutive relation are adopted to resolve the viscous flows and model the anisotropic turbulence within the boundary layer. The analysis implied that the distribution region and intensity of velocity fluctuation are strengthened, resulting in a larger turbulence kinetic energy distribution and a higher boundary layer thickness as the train length increases. A reduction in the streamwise velocity and the negative pressure with the increasing train length on the tail train is observed, resulting in lower drag and lift coefficients. As the length of the train increases, both the mean and instantaneous slipstream velocities are increased. The boundary layer thickness and the skin friction coefficient are compared with flat plate theory, reduced-scale, and full-scale experiments, proving the ability of numerical simulation to model the boundary layer velocity profile and skin friction coefficient distribution correctly. The wake structures are identified by the Spectral Proper Orthogonal Decomposition method, the dominant mode frequency decreases, and the wavelength becomes larger as the length of the train becomes longer due to the thickening boundary layer.
... Some research mainly focused on the flow field distributions around high-speed trains and the aerodynamic loads acting on the vehicles [24][25][26]. These studies did not involve the dynamic responses of the vehicle systems under aerodynamic loads. ...
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The dynamic characteristics of a railway vehicle system under unsteady aerodynamic loads are examined in this study. A dynamic analysis model of the railway vehicle considering the influences of aerodynamic loads was established. The model not only considers the forced excitation effect of unsteady aerodynamic loads but also accounts for the effect of unsteady aerodynamic loads on the change of the wheel–rail contact normal forces as well as changes of the wheelset creep coefficients and creep forces/moments. Therefore, this model also considers the influences of unsteady aerodynamic loads on the self-excited vibration characteristics of the vehicle system. The time-history curves, phase trajectory diagrams, Poincaré sections, and Lyapunov exponents of the vehicle system running on a smooth straight track under unsteady aerodynamic loads were determined. The results show that when the critical speed is exceeded, the vehicle system usually performs quasi-periodic motion under unsteady aerodynamic loads, which is significantly different from the periodic motion under steady aerodynamic loads. In different cases, the amplitude and phase of motion are significantly different. The amplitude of the motions can be increased by more than 159%, and the difference of phase can be up to 173°. (The phase is almost reversed.) The dynamic responses of the vehicle system under unsteady aerodynamic loads contain abundant frequency components, including the frequency of the self-excited vibration, the frequency of the forced excitation, and combinations of their integer multiples. The vibration forms corresponding to the main harmonic components under unsteady and steady aerodynamic loads were compared, and the self-excited vibration component of the vehicle system under unsteady aerodynamic loads was identified. The variations in the critical speed with various parameter combinations were computed. The variation range of the critical velocity can reach 73%.
... Up to date, various turbulent models, which includes RANS, IDDES, and LES, have been used in the train aerodynamic related numerical studies, where the IDDES model is the most widely used turbulent model in train aerodynamic studies (Xia et al., 2018;Wang et al., 2020b). This is because the train-induced separation, as well as the breakup of large vortices result in a flow field with different vortex scales (Baker, 2010;Baker et al., 2001). Yao et al. analyzed the aerodynamic profile of CRH2 high-speed train through wind tunnel experiment and numerical simulation. ...
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Vacuum Tube Transportation (VTT) system utilizes the low-pressure environment to achieve aerodynamic drag reduction and reduce the energy consumption of high-speed trains. Within this system, even for trains operating at subsonic speed, the flow acceleration around the streamlined train may leads to local supersonic flow. The appearance of the supersonic flow will dramatically alter the flow field structure and aerodynamic load of the high-speed train. Therefore, this study investigates two types of representative flow fields under different blockage ratio through the improved delayed detached eddy simulation (IDDES) method. The initialization, propagation, reflection and intersection process of shock waves in transonic case are captured and analyzed. It is shown that, the boundary layer thickness and width of vortex group in wake has experienced a slight increase with the increase of vacuum level. Compared with the subsonic case, the occurrence of choked flow and shock wave in transonic flow field lead to a decrease of wake region's vortex group width. As a result, the total drag coefficient of the train is increased by approximately one order of magnitude, and the dominant St number of tail car's drag coefficient has shifted from 0.16 to 0.11.
... When running, the motion around the train includes the relative motion between the train and the air, as well as the relative motion between the stationary ground (track) and the train. Full-size testing is undoubtedly the most reliable method of assessment of maglev trains, because the full-size test perfectly reflects the actual operation of the train, however, in the case of train aerodynamics, full scale tests is complicated [3] . Full-scale testing is heavily dependent on the environment, and often requires a lot of testing to obtain reliable results. ...
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The effect of the ground condition on unsteady aerodynamic performance of maglev train was numerically investigated with an IDDES (Improved Delayed Detached Eddy Simulation) method. The accuracy of the numerical method has been validated by wind tunnel experiments. The flow structure, slipstream and aerodynamic force around the train under stationary and moving ground conditions were compared. Compared with the stationary ground condition, the vortex structure under the condition of moving ground generated by the wake region is narrower and higher because of the track. Near the nose point of the head and tail vehicles, the peak value of slipstream under the condition of moving ground is slightly higher than that under stationary ground. In the wake area, the effect of the main vortex structure on both sides of the tail vehicle and the track makes the vortex structure in the wake area stronger than that under moving ground, the slipstream peak is larger and the locus thereof is further forward. Under the two ground conditions, the vortex structure is periodically shed from both sides of the train into the wake area, and the shedding frequency of the main vortex under the moving ground condition is lower than that under the stationary ground condition. Moving ground can increase the resistance of the maglev train, reduce the lift of the maglev train, and decrease the standard deviation of the maglev train’s aerodynamic force.
... Therefore, in this paper we studied the impact of the speed-up effect of different embankment heights on the slipstream around a train. Results of previous experimental and numerical studies Baker et al. (2014a, b), Baker (2010), Muld et al. (2012), Bell et al. (2014Bell et al. ( , 2016Bell et al. ( , 2017 showed the existence of two strong peaks in the slipstream velocity around the train. The present paper aims to explore the flow around the train that is responsible for the strength and the character of these. ...
... Figure 11b is similar to Fig. 11a and it shows the instantaneous U and 1 s MA at z = 0.36H. Unlike the instantaneous U of a high-speed train without a crosswind (Baker 2010;Bell et al. 2014), the peak value of the ensemble average appeared near the head car and not in the near wake region under crosswind conditions. Furthermore, the difference in the ensemble average was significant for the different embankment heights, including the raw data and 1 s MA. ...
Article
Full-text available
A numerical study using improved delayed detached eddy simulation (IDDES) was used to investigate the influence of the embankment height on the aerodynamic performance of a high-speed train travelling under the influence of a crosswind. The results of the flow predictions were used to explore both the instantaneous and the time-averaged flows and the resulting aerodynamic forces, moments and slipstreams. An increase of the aerodynamic drag and side forces as well as the lift force of the head and middle cars were observed with rising embankment height. While the lift force of the tail car decreased with the increasing embankment height. Furthermore, the height of the embankment was found to have a strong influence on the slipstream on the leeward side of the train. The correlation between the embankment height and the slipstream velocity on the windward side, was rather small. The flow structures in the near-wake of the leeward side of the train, responsible for the aerodynamic properties of the train were analyzed, showing strong dependency on the embankment height.