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This study uses a three-dimensional, compressible, turbulence model to investigate the alleviation effect on tunnel aerodynamics of equal-transect ring oblique tunnel portals with different slope values. The turbulent flow around the train body is computed using the RNG κ-ε turbulence model, and a sliding mesh method is utilized to treat the relative motion of train and tunnel. The numerical results are verified through the results of moving model experiments. The mitigation effects of ring oblique tunnel portal on the initial compression wave are analyzed. The relationship of micro-pressure wave and slope values is proposed, by which the micro-pressure wave induced by high-speed train entering tunnel with equal-transect ring oblique tunnel portal can be estimated rapidly. The accuracy of the proposed relationship is validated by previous studies and moving model experimental data. The estimation results using the proposed formula are in good agreement with data from references and moving model experimental tests. Results also show that the maximum pressure gradient and micro-pressure wave can be reduced by about 10.8% when slope value is 1:1.75 relative to slope value of 1:0.5.

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... The ground, tunnel and track were set as no-slip walls. More information can be found in Ref. [18]. ...

... This algorithm has been widely used in computational fluid dynamics for calculating incompressible flow after it was proposed in 1972 [22]. With improvement of this algorithm, now it can be successfully used for compressible flow, as well as the train/tunnel coupling aerodynamics [18,23]. The Green-Gauss cell-based method was adopted to control the gradients [24]. ...

... For each time-step, the maximum number of iterations was set as 50, and the residuals of continuity, momentum and turbulent equations were set at 10 − 4 . To realize the relative movement between the train and the tunnel, the sliding mesh technique was adopted [18]. The sliding mesh method was used between two different zones through a pair of grid interfaces to realize the relative movement and data exchange. ...

When a high-speed maglev train enters a tunnel, the pressure around it rises and changes quickly. This may lead to serious damage of the train and tunnel structures. With increases of train speed, up to 600 km/h, this issue will become worse. In this study, the three-dimensional, compressible, unsteady, k-ϵ two-equation turbulence model and sliding grid technologies were used to study the effect of train speed on the pressure waves induced by a maglev train passing through a tunnel. The numerical simulation method used was validated against results from moving model tests and semi-empirical formulations. The maglev train modelled was specified to pass through a 2 km tunnel with speeds in the range from 400 km/h to 600 km/h. The surface pressure distribution of the train and tunnel were found and are discussed. The transient pressures on the maglev train and tunnel surface are shown to have a significant relationship with the train speed. Generally, the maxima of the train surface pressures follow the power law relationship with an exponent of 2.35 to the train speed, while for the tunnel surface pressure, an exponent of 2.46 is obtained. The gradient of the initial compression wave at the tunnel entrance follows a power law relationship with an exponent of 3.51 to the train speed, while at the exit this rises to an exponent of 4.99. The amplitude of the micro-pressure wave follows a power law relationship with an exponent of 5.00 to the train speed. Having such data will provide essential support for the design of both the maglev train and tunnel.

... Chen et al. (2017) studied the aerodynamic effects of various nose lengths for two trains passing each other in a tunnel and found that the positive peak of the initial compression wave at the tunnel wall decreases logarithmically as the nose length increases. Zhang et al. (2017) found that optimizing the slope of the tunnel hood was an effective way of alleviating the aerodynamic effects. This study also proposed a formula for estimating the micro-pressure wave (MPW) when a train enters a tunnel through an equal-transect oblique tunnel portal. ...

... As a train passes through a tunnel, it creates a three-dimensional, compressible, unsteady, turbulent airflow (Liu et al., 2017a;Zhang et al., 2017). The Reynolds-averaged Navier-Stokes equation and the renormalization group k-ε double-equation turbulence model, which have been widely used to simulate the aerodynamic performance of trains passing through tunnels (Zhang et al., 2017;Niu et al., 2017), were used in this study to simulate a ST, a DT, and an LT entering a tunnel. ...

... As a train passes through a tunnel, it creates a three-dimensional, compressible, unsteady, turbulent airflow (Liu et al., 2017a;Zhang et al., 2017). The Reynolds-averaged Navier-Stokes equation and the renormalization group k-ε double-equation turbulence model, which have been widely used to simulate the aerodynamic performance of trains passing through tunnels (Zhang et al., 2017;Niu et al., 2017), were used in this study to simulate a ST, a DT, and an LT entering a tunnel. The sliding grid method was adopted to avoid the need to generate new grids repeatedly. ...

Unsteady Reynolds-averaged Navier-Stokes (URANS) simulations were performed to simulate the flows around three different train types as they enter a tunnel. The three models-a short train, a double train, and a long train-were used to analyze the influence of the train configuration on the surrounding airflows and aerodynamic performance. The numerical predictions showed good agreement with existing experimental data. The maximum positive velocity peak induced by a short train passing through a tunnel was 140% larger than that produced in open air. The amplitudes of the slipstream velocity components decreased as the height from the ground increased. Large differences were found between the velocity fields produced by long and double trains. The velocity field had a large negative peak when a long train traversed the tunnel. This peak grew along the tunnel towards the exit. When a train is running in a double-track tunnel, the airflow above the two tracks moves in opposite directions. The double train induced a 53% higher velocity than the single train, but only a 6% higher velocity than the long train.

... The test platform has obtained China Metrology Accreditation (CMA) qualification (certificate number 2014002479K). More detailed information about the platform can be found in [26,27]. A certain type of three car marshalling train (79.77 m in length) and a 70 m 2 standard single-car tunnel (350 m in length) are adopted in the test, as is shown in Figure 2. All the models are scaled down to 1:20 in the test. ...

... The aerodynamic effects caused by a real train passing through a tunnel can be effectively simulated using scaled models and moving model experimental devices if a similarity criterion is satisfied. The Mach number, blockage ratio, scaling of the train and tunnel geometry, and the Reynolds number are the main similarity parameters needed to be considered for the similarity criterion [27][28][29][30][31]. The first three parameters of the moving model tests in this manuscript are agree to the similarity criterion. ...

... The The aerodynamic effects caused by a real train passing through a tunnel can be effectively simulated using scaled models and moving model experimental devices if a similarity criterion is satisfied. The Mach number, blockage ratio, scaling of the train and tunnel geometry, and the Reynolds number are the main similarity parameters needed to be considered for the similarity criterion [27][28][29][30][31]. The first three parameters of the moving model tests in this manuscript are agree to the similarity criterion. ...

Ear complaints induced by interior pressure transients are common experiences for passengers and crew members when high-speed trains are passing through tunnels. However, approaches to assessing the risks of the pressure-related aural discomfort have not been reported until recently. The objective of this study was to evaluate the hazards of interior pressure transients of high-speed train on human ears combining the effects of operation speed and seal index. Moving model tests were conducted to obtain the pressure transients when the model train runs in the tunnel. The recorded data were transformed into the interior pressures by empirical formula. Furthermore, the aural sensations were divided into four levels hierarchically and the range for each level was derived by logistic regression analysis method and represented by three biomechanical metrics. Furthermore, a human middle ear finite element (FE) model was used to simulate its dynamics under the interior pressures. The results indicate that lifting operation speed from 250 km/h to 350 km/h in tunnel will prolong the duration of ear complaints by more than two times whereas improving the seal index from 4 s to 12 s will reduce the incidences of the onset of tinnitus and hearing loss by more than ten times. In addition, the duration of aural comfort shortens from the head car to the tail car against the running direction. It is desirable that enhancing the seal index improve the aural sensations of the passengers and crew members considering the lifting operation speed of high-speed train.

... (i) decreasing the compression wave gradient during its formation by constructing a hood at the tunnel entrance, which may be a flared [5,6], inclined [7][8][9], section-enlarged [10][11][12], vented [12][13][14] hood, or complex hood composed of a combination of those simple structures [15], as illustrated in the schematic shown in Fig. 1; ...

... They have shown that the maximum compression wave gradient can generally be reduced by 0.5-50% with installation of a hood. In recent years, some numerical studies have conducted parametric analyses [8][9][10][11][12], from which the relationships between ) / max( t p , M , (the inclination angle of the tunnel portal), (the blockage ratio of train to tunnel) , and r (the section ratio of hood to tunnel) could be introduced to engineering formulas to provide a deeper understanding of the influence of different types of hoods. ...

... indicate the orders of the reflections from the tunnel entrance. (8) where C , U , and S L are the sound speed, train speed, and distance between the opening and the pressure sensor, respectively. Therefore, some basic characteristics of the appearance of wave superposition can be proposed: ...

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.

... 5 To mitigate MPWs, the installation of a buffer hood at the tunnel entrance has been widely adopted. Traditional buffer hoods include the enlarged cross section hood, 6,7 linear horn hood, 8 gradient hood, 9,10 and hat oblique hood, 11 as shown in Fig. 1. ...

... Therefore, a two-car formation with a length of 51.42 m, including the head train and the tail train, was selected. 10,31 Detailed structures such as bogies, pantographs, doors, and windows were not considered. The streamline head length (L h ) was set to 6.18 m, based on the design of the CRH380B model. ...

As high-speed trains exceed 400 km/h, tunnel aerodynamics pose significant challenges. The hat oblique tunnel buffer hood with enlarged cross section and ventilation windows (HEW) is a promising solution to mitigate micro-pressure waves (MPWs). However, there is limited research on HEW ventilation window configurations. Thus, field measurements and numerical simulations were conducted using the slip grid technique and an improved delayed eddy simulation turbulence model, with validation against field data. The study investigated the effects of aperture rate and ventilation window arrangement, analyzing the initial compression wave, pressure gradient, MPW, and flow field in the tunnel buffer hood under various ventilation window setups. Findings emphasize that increasing the aperture rate or placing ventilation windows near the tunnel entrance reduces MPWs when a high-speed train enters the buffer hood. However, it intensifies MPWs when the train transitions from the buffer hood to the tunnel. Optimal MPW mitigation is achieved with approximately 15% aperture rate and a ventilation window distance from the slope end of 0.3–0.4 times the enlarged cross section length. Double ventilation windows outperform single or three windows in MPW reduction, with longitudinally arranged windows at the top facilitating more efficient high-pressure air escape compared to circumferential windows.

... Winslow et al. (2005) found that small decreases in growth rate are possible (up to approximately 15%) for scarf walls extending beyond the tunnel entrance by a distance on the order of the tunnel height. Zhang et al. (2017) found that optimising the slope value of the tunnel hood is an important way to alleviate aerodynamic effects. A formula for estimating the MPW when a train enters a tunnel with an equal-transect oblique tunnel portal was proposed in that study. ...

... As a train passes through a tunnel, the air flow is a three-dimensional, compressible and unsteady turbulent flow Zhang et al., 2017). The RANS equation and the renormalisation group (RNG) k-ε double-equation turbulence model, which have been widely used to simulate the aerodynamic performance of trains passing through tunnels (Zhang et al., 2012;Niu et al. 2017b), were used in this study to simulate an ST, a DT and an LT entering a tunnel. ...

Considering the special connecting ways of double unit trains, the detached eddy simulation (DES) method was utilized in the present paper to study the different aerodynamic performance between single and double unit trains with 6 cars. The numerical results are verified by wind tunnel experiments. Results show that, the overall aerodynamic drag of the SUT and DUT is almost the same, where the drag of the fourth car of the DUT is much larger than that of the SUT. The drags of the bogies in the second, third and fourth car vary greatly between the SUT and DUT. Due to the different distribution of the pressures above and below the train, the positive lift of the third car of the DUT is much larger than that of the SUT, which may lead to instability of the DUT train.

... Winslow et al. (2005) found that small decreases in growth rate are possible (up to approximately 15%) for scarf walls extending beyond the tunnel entrance by a distance on the order of the tunnel height. Zhang et al. (2017) found that optimising the slope value of the tunnel hood is an important way to alleviate aerodynamic effects. A formula for estimating the MPW when a train enters a tunnel with an equal-transect oblique tunnel portal was proposed in that study. ...

... As a train passes through a tunnel, the air flow is a three-dimensional, compressible and unsteady turbulent flow Zhang et al., 2017). The RANS equation and the renormalisation group (RNG) k-ε double-equation turbulence model, which have been widely used to simulate the aerodynamic performance of trains passing through tunnels (Zhang et al., 2012;Niu et al. 2017b), were used in this study to simulate an ST, a DT and an LT entering a tunnel. ...

Unsteady Reynolds-averaged Navier-Stokes (URANS) simulations were performed to simulate trains with different marshalling forms and lengths entering a tunnel. Three models, including a short train, a double train and a long train, were used to analyse the influence of the train configuration on the pressure variations during a train’s passage. The results of the numerical predictions were validated against existing experimental data, with which they showed good agreement. The differences in the maximum pressure peak distribution and the pressure fluctuations were analysed by means of Mach diagrams. The results show that the grouping length exerts a considerable influence on the amplitude of the pressure on the train body and that the influence of the grouping length on the pressure variation on the tunnel wall varies with the location in the tunnel. The tunnel space can be divided into three and four zones with regard to the influences on the maximum positive and negative pressure values, respectively. The different marshalling forms also influence the maximum peak values and local profiles of the pressure history curves, although this influence is much slighter than that of the train's grouping length.

... These curved smooth 3D surface streamlines are designed using CAD technology. Compromise must be made between the curvature details on each surface and the data transfer effectiveness during the CAD design stage (Zhang et al., 2017b). In this study, we use the 3D design software CATIA TM to build and optimize geometric models. ...

... The RNG k-ɛ turbulence model, widely used for engineering applications, is adopted herein. (Li et al., 2017;Zhang et al., 2017b;Rogowski et al., 2018). ...

... Winslow et al. (2005) found that small decreases in growth rate are possible (up to approximately 15%) for scarf walls extending beyond the tunnel entrance by a distance on the order of the tunnel height. Zhang et al. (2017) found that optimising the slope value of the tunnel hood is an important way to alleviate aerodynamic effects. A formula for estimating the MPW when a train enters a tunnel with an equal-transect oblique tunnel portal was proposed in that study. ...

... As a train passes through a tunnel, the air flow is a three-dimensional, compressible and unsteady turbulent flow Zhang et al., 2017). The RANS equation and the renormalisation group (RNG) k-ε double-equation turbulence model, which have been widely used to simulate the aerodynamic performance of trains passing through tunnels (Zhang et al., 2012;Niu et al. 2017b), were used in this study to simulate an ST, a DT and an LT entering a tunnel. ...

... To realise the movement of the maglev train in the tunnel and explore the train/tunnel coupling aerodynamic characteristics, the extensively used method sliding mesh technique (Niu et al., 2018;Zhang et al., 2017;Zhang et al., 2022aZhang et al., , 2023aZhang et al., , 2023bHan et al., 2022;Xiong et al., 2022) was used in this paper. The sliding grid can be divided into two or more cell areas, which are in contact with each other by means of boundary surfaces, and the cell areas move relative to each other by means of setting speeds (Uystepruyst et al., 2011). ...

The significant increase in train speed contributes to stronger vehicle/tunnel coupling aerodynamic effect, especially on the intensity of the micro-pressure wave (MPW) emitted at high-speed maglev tunnel exit. Hence when the train speed reaches 600 km/h and more, how to effectively mitigate the MPW becomes a challenge for aerodynamic researchers. In this study, a novel vented tunnel hood with the decreasing open ratio along the enlarged cross-section wall was proposed, while one consistent and two inconsistent layouts of the hoods applied at the tunnel portals were attempted to obtain a better hood combination for the mitigation of MPW. In addition, the sliding mesh technique was used to simulate the train passing through the single-track high-speed maglev tunnel. The validation of the methodology has been carried out to compare with the previous moving model test results. The peak variations of pressure wave and MPW were analysed in combination with the grid-independence study and numerical validation. The new hoods installed consistently at the tunnel portals (entrance and exit), can reduce the maximums of MPWs at required locations, i.e., 20m and 50m from the tunnel exit, by 66.9% and 40.9% respectively, when compared to the existing unvented tunnel hood; however, when the new hood at the tunnel exit is replaced by the existing tunnel hood without vents, the maximums of MPWs at the corresponding 20m and 50m are significantly increased by 79.1% and 71.0%. After changing the setup of the inconsistent hoods, i.e., the tunnel entrance hood is unvented and this novel hood is installed at the tunnel exit, the corresponding MPW peaks can be reduced by 84.0% and 71.1%. Therefore, the hoods at both of tunnel entrance and exit can affect the variations of MPWs, and a reasonable arrangement of the hood openings at tunnel portals can effectively mitigate the MPW emitted at the high-speed maglev tunnel exit.

... Finally, as displayed in Fig. 22, the amplitude of the MPW is alleviated. Herein, the amplitudes of MPWs at M2 and M5 are selected for analysis since they are key indices used to measure the intensity of noise pollution (Han et al., 2022;Zhang et al., 2017). As shown in Fig. 22, the amplitudes of the MPW at M2 and M5 decrease as Table 3 lists in detail the reduction in the MPW. ...

... To verify the accuracy of the presented method in the simulation, the numerical results are compared with that of the moving model test conducted by Central South University [39]. Te model ratio in the test is 1 : 20, and the original lengths of the train and the double tracks tunnel are 51.7 m and 1000 m, respectively. ...

When the tunnel has damages such as deformation and cracking, the lining structure should be set to reinforce the tunnel. In this study, a three-dimensional numerical method is performed to study the aerodynamic effects caused by the train passing through a tunnel with segmented linings at 350 km/h. The influence of the numbers, thickness, location, and types of the segmented lining structure on MPW is analyzed. The results show that the segmented linings located at the middle of the tunnel have a better mitigation effect on MPW than that of the normal linings, which increases with the segment number and the thickness of the linings. When the damage occurs near the entrance of the tunnel, a segmented lining structure with a constricted section slightly away from the tunnel entrance can be used to reduce its adverse effect on MPW.

... As a HST passes through a tunnel, the air flow is highly unsteady (Niu et al., 2020;Liu et al., 2017;Zhang et al., 2017). ...

Significant variation exists in the aerodynamic performances of high-speed trains (HSTs) traveling in different infrastructure scenarios. When high-speed trains travel from one infrastructure scenario to another, the aerodynamic loads acting on the trains change significantly. To investigate the safety as a HST enters a tunnel under crosswind conditions, the unsteady aerodynamic performance of the HST and the flow structures around the train were numerically studied. The results demonstrated that the flow field and the pressure field were symmetrically distributed under conditions with no crosswind, while the distribution of the flow field and the pressure field were clearly asymmetric when crosswinds were present at the tunnel entrance. Furthermore, the flow structures near the train and the pressure distributed on the train surfaces outside the tunnel were most severely affected by the presence of crosswinds. Vortex structures appeared on the windward side surface of the train inside the tunnel and on the leeward side surface of the train outside the tunnel during the entrance of the train into the tunnel. Due to the sudden changes in the flow and the pressure as the train entered the tunnel, the aerodynamic loads changed drastically, and the variations of each vehicle were different, resulting in complex dynamic responses, including lateral vibrations and pitching movement. In particular, the aerodynamic performance of the rear vehicle was the worst as the train entered the tunnel when no crosswind conditions existed, while the head vehicle was the most negatively affected for safe operation of the HST when strong crosswinds were present at the tunnel entrance.

... The reason why the three-dimensional compressible k-ε turbulence model is used to solve the flow in the tunnel is that the compressibility of air cannot be ignored considering the confined space in the tunnel. It has been widely used in the study of tunnel train aerodynamics [26][27][28]. The governing equations are the continuity equation, Navier-Stokes equation and energy equation. ...

The cave is of great importance for the storage of equipment and to avoid having workers in the tunnel, but it changes the tunnel section, leads to a change of slipstream and affects the safety of trains and workers. The Re-normalization group (RNG) k-ε turbulence method is used to investigate the slipstream induced by a single train passing through a double-track tunnel at 350 km/h. The slipstream in a tunnel with and without a cave is compared. The slipstream components in three directions are reported comprehensively. The results show that the existence of a cave changes the slipstream at the tail of the train. At measurement points before and after the train passes the cave, the intensity of the slipstream at the tail is mitigated; as the train passes the cave, the tail slipstream is enhanced to a certain extent. With increasing lateral distance, the peak value of the slipstream with a cave decreases faster than that without a cave. These findings suggest that the presence of a cave mitigates the slipstream intensity, but special attention should be paid to the design of ancillary facilities, especially their relative location.

... They obtained the best matching relationship between the length of the tunnel hood and the cross-sectional area of the entrance. Zhang et al. (2017) studied the aerodynamic effect produced by the high-speed train passing through the equal-transect ring oblique tunnel portal with different slopes, and obtained the best oblique slope to alleviate the maximum pressure gradient and micro-pressure wave. Guo et al. (2022) studied the influence of various parameters of the hat oblique tunnel portal on the peak-to-peak pressure of the tunnel wall and micro-pressure wave, and used multi-objective optimization to obtain the optimal parameters. ...

With technology development of 600 km/h high-speed maglev trains, how to effectively alleviate the transient pressure wave problem caused by such a high-speed maglev train passing through the tunnel is significantly of importance. In this paper, based on the three-dimensional, unsteady, compressible Navier-Stokes equations and k-epsilon turbulence model, a high-speed maglev train with a speed of 600 km/h was simulated to pass through a single-track tunnel with a cross-sectional area of 92 m², and the numerical method used was validated by a moving model test. The propagation characteristics of pressure waves in the tunnel and the evolution properties of the initial compression wave were explored. A new enlarged cross-section hood with an arch lattice-shell was proposed. With helps of the semi-closed decompression domain formed by the outer wall of the inner arched plate and the inner wall of the enlarged cross-section tunnel hood, the novel hood can further dissipate the energy of the initial compression wave; thus, the pressure gradient at 140 m from the tunnel entrance and the micro-pressure wave amplitude at 20 m from the tunnel exit can reduce by 32.1 % and 35.6 % respectively, as compared with the existing enlarged cross-section tunnel hood. When compared to the tunnel without hoods, this novel hood can achieve 64.8 % and 74.3 % reduction at the same monitoring points.

... The numerical semi-implicit method for pressure-linked equations consistent (SIMPLEC) scheme is adopted for solving the in-compressible Navier-Stokes equations Wang et al. (2019). The unsteady Reynolds Averaged Navier-Stokes (URANS) turbulence models are usually applied for the computation of turbulent flow induced by train motion Wang et al. (2018), Zhang et al. (2017a), as the consumption of computational re-source required by URANS is acceptable for simulating train motion. ...

Tornados are one of the most common natural disasters, but their occurrence can be sudden and unpredictable. For trains operating in the areas where tornadoes frequently happen, the operation safety is challenged. Tornado generator was recently proposed as a method of numerical investigation of tornado-like vortex flows. This paper
presents a numerical approach for the simulation of train passing through a tornado-like vortex on realistic scale. It is found that the tornado-like vortex causes appearance of localized regions of a negative pressure on the train and transient variations of the aerodynamic loads acting on the train. As a result, the tornado-like
vortex causes swings on the lateral force, and subsequently on the rolling moment, which affect the passenger comfort and operation safety of the train. The method presented herein can be further applied to the study of train behavior and real time response while encountering tornadoes of different types and strength, which is
significant for evaluating the operation safety of high-speed trains.

... First, both ends of the tunnel are blocked, and then the tunnel is inflated until the pressure in the tunnel reaches 6000 Pa. If the pressure reduction in the tunnel is less than 2% after 5 min of stopping inflation, it is considered that the tunnel model has been fully sealed [Zhang et al., 2017]. ...

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.

... The RNG model, in particular, has an additional term in its function to reflect the main flow time-average strain rate, which is recognized to improve the flow field solution (Deng et al., 2019b). Similar to the previous researches Huang et al., 2020;Rabani and Faghih, 2015;Yang et al., 2019;Zhang et al., 2017a), the k-ε model with the RNG turbulence model was adopted in the present study. The governing equations are listed as follows: ...

Transient pressure induced by the trains passing through tunnels has a significant influence on tunnel structural safety. When tunnels are constructed at somewhere with high altitude and low atmospheric pressure the impact of ambient pressure cannot be ignored. In the current study, a three-dimensional, unsteady, compressible, RNG ᴋ–ε turbulence model, and the dynamic mesh was employed to explore the effect of ambient pressure on the propagation characteristics of pressure waves inside the tunnel when the train passes through the tunnel at the same speed (100 km/h). Results show that (1) The ambient pressure has limited influence on the waveform of the pressure but strongly affects the peak value of pressure wave, where positive peak value P-max and peak-to-peak pressure difference ΔP linearly increase and negative peak value P-min proportionally decreases with the increase of ambient pressures; (2) The initial compression wave ΔPN and pressure increase caused by the friction effect ΔPfr linearly increase as ambient pressures increases. However, the ratio of ΔPN to P-max is not sensitive to the ambient pressure and values of ΔPNP-max remains to be constant.

... The RNG k-ε model has an additional term in its function, which reflects the main flow-time average strain rate, in that way improving the believability and accuracy of the flow field analysis . Such method has been validated by many researchers (Chen et al., 2017;Huang et al., 2020;Rabani and Faghih, 2015;Yang et al., 2019;Zhang et al., 2017b). The detailed solver settings of the numerical simulation are presented in Table 1. ...

The continuous construction of extra-long metro tunnels and the shortened departure intervals enable double trains to simultaneously drive in the same tunnel direction to be possible. Transient pressure changes induced by trains passing through tunnels have a remarkable influence on the structural safety of tunnels. In this study, pressure waves generated by single and double trains passing through the tunnel are both explored by numerical simulation. The accuracy and feasibility of the simulation method of double train tracking operation is verified by comparison with the small-scale experimental research. Based on the verified simulation method, the pressure wave and its propagation characteristics in the aforementioned two situations are investigated. Variation of transient pressure on the surface of the train and tunnel are compared and analyzed. Results show no obvious difference on the generation and propagation mechanism of pressure waves between the two typical situations. However, the propagation of the pressure wave is found to be more complicated in the double train tracking mode. More concretely, in the double train tracking mode, the maximum transient pressure P-max, minimum transient pressure P-min, and the peak-to-peak value of transient pressure ΔP on the first train surface are observed to be same as those in the single train condition, whereas those of the second train denote different values. The transient pressure at different positions in the tunnel and the waveform after the second train entering the tunnel are different compared to that in the single train condition, while the peak value exhibits insignificant difference. Moreover, the maximum pressure, minimum pressure, and maximum peak-to-peak pressure occur inside tunnel denote no obvious difference in the two typical scenarios.

... In a tunnel, the pressure transient induced by the piston effect is strong , and factors such as the tunnel length and buffer structures at the entrance of the tunnel will affect the transient loads in the tunnel Zhang et al., 2017aZhang et al., , 2017bNiu et al., 2020). Using compressible, unsteady, and sliding mesh technologies, Liu et al. (2017b) studied the aerodynamic loads, including the transient pressure, lateral force, and overturning moment, that were caused by a single train moving through a double-track tunnel or two trains passing each other in a tunnel. ...

The pressure integral method is frequently used to obtain the train aerodynamic forces in experiments, but the effect of the pressure pipe length on the pressure amplitude is not understood. In this paper, based on field tests without pressure pipes, the dominant frequency (DF) ranges of the pressure pulsations on the train surface under various conditions, including open-air, crosswind, and tunnel conditions, were analyzed. Then the effect of the pressure pipe length on the pressure amplitude with various pulsation frequencies was investigated. Finally, in a full-scale test under crosswinds, the selected pressure pipe length was applied to verify its reliability and to study the train aerodynamic performance. The results showed that the maximum DF occurred when two trains passed
each other (near 60 Hz), and the DF under crosswinds was the smallest (less than 1 Hz). When the pressure pulsation frequency was less than 1 Hz, the error range of the pressure amplitude was less than 5% with a pressure pipe length of ≤8 m. The pressure pipe is a polyvinyl chloride (PVC) pipe with an outer diameter of 2 mm and an inner diameter of 1.8 mm. The full-scale test results for the windproof ability of different windbreak walls and the aerodynamic forces of the train showed that the current pressure pipe length was reasonable and could reflect the actual operating conditions of the train under crosswinds.

... The results from each test should subsequently be examined for accuracy, with an accuracy criterion of 2% applied to the target speed of the train. Test results that did not meet this criterion were subsequently eliminated from consideration (Zhou et al., 2014;Zhang et al., 2017). Three repeat experiments were performed when a heated tunnel was used. ...

A moving model (1/20 scale) was used to study the effect of high temperature at the entrance of a tunnel on the aerodynamic performance of a high-speed train passing through it. The air in the tunnel was heated using highly-precise, self-controlling, ultra-thin, silicone rubber heating devices that were firmly attached to the inner surface of the tunnel (covering the first 2 m of the tunnel from the entrance). When the specified temperature was reached, tests were performed using a model train moving at different speeds. The experimental results were used to compare and analyze the pressure fluctuations and propagation of pressure waves under normal and localized high-temperature conditions. The results show that the peak-to-peak value of the transient pressure on the tunnel surface decreases by 5–7% in the presence of a normal ambient and high temperature tunnel mode. In addition, the corresponding difference on the surface of the head of the train is 3–4%. Hoping this experimental study will encourage theoreticians to investigate the mechanism responsible for the generation of transient pressures in localized high temperature railway tunnels. The results can provide guidance and support for researchers seeking to improve passenger safety and comfort in high temperature railways.

... However, when it is necessary to predict the flow field around a train passing through a tunnel, a moving model need to be considered. On this account, sliding mesh or dynamic mesh techniques were used in many published works [13][14][15][16] successfully simulating moving trains passing through tunnels. Thanks to this, the aerodynamic effect of different railway tunnel conditions observed provided insight for designers of both tunnels and trains. ...

The aerodynamic performance of a high-speed train passing through tunnel junctions under severe crosswind condition was numerically investigated using improved delayed detached-eddy simulations (IDDES). Three ground scenarios connected with entrances and exits of tunnels were considered. In particular a flat ground, an embankment, and a bridge configuration were used. The numerical method was first validated against experimental data, showing good agreement. The results show that the ground scenario has a large effect on the train’s aerodynamic performance. The bridge case resulted in generally smaller drag and lift, as well as a lower pressure coefficient on both the train body and the inner tunnel wall, as compared to the tunnel junctions with flat ground and embankment. Furthermore, the bridge configuration contributed to the smallest pressure variation in time in the tunnel. Overall, the study gives important insights on complicated tunnel junction scenarios coupled with severe flow conditions, that, to the knowledge of the authors, were not studied before. Beside this, the results can be used for further improvements in the design of tunnels where such crosswind conditions may occur.

... These studies found that the micro-pressure wave is of low frequency and short duration, its magnitude being approximately proportional to the cube of the train speed, and the initial compression wave in a tunnel can reflect some characteristics of the micro-pressure wave [51]. In recent years, with the rapid development of China's high-speed railway, a new round of research has been carried out and more methods for controlling the micro-pressure wave have been proposed [20,[106][107][108][109]. ...

The coupling and complexity of railway train / tunnel system are further aggravated by increasing train speed, which produces a series of aerodynamics problems, such as aerodynamic drag, slipstream, pressure wave and micro pressure wave. Aerodynamic effects of tunnels will result in a significant increase in train energy consumption, shorten life of railway train / tunnel system, and increase maintenance cost. This paper provides a review of aerodynamics of railway train / tunnel system. Challenges in railway train / tunnel system aerodynamics and their related factors are discussed firstly. Aerodynamic performance and flow field characteristics of trains in tunnels are presented. Relationship of aerodynamic effects and parameters of railway train / tunnel system, and the control methods for reducing aerodynamic effects in tunnels are explained. A traffic safety evaluation of the train in tunnels, such as vehicle body structure, passengers’ ear comfort, etc., is introduced and analysed. Finally, future outlooks and research topics are proposed.

... The results show that the maximum magnitude of the impulse wave increases with the flange length at the far field. To relieve the strong transient pressure and the micro-pressure wave caused by a train traveling in a tunnel, some articles set up hoods at the tunnel opening and studied the relief effects of differently shaped of hoods on the pressure [13][14][15]. The results show that hoods at the opening can effectively reduce the pressure gradient in a tunnel and reduce the micro-pressure wave amplitude at the tunnel exit. ...

In this study, the spatial distribution of the transient pressure and the slipstream caused by a 1/10 scaled metro train passing through a tunnel was studied with moving model test. We hereby investigate the mechanism underlying the mitigation of the transient pressure on both the train surface and tunnel wall, as well as that of the slipstream in the tunnel. Experimental results showed that the airshaft at different locations in a tunnel had different pressure relief effects. The most significant pressure amplitude decreased by 36.0% with the airshaft locating in the middle of the tunnel. Meanwhile, the slipstream speed was also relieved from 0.45 to 0.36 after an airshaft. We also assessed and analyzed the impact of train speed on the transient pressures and slipstream. It was found that the increase of the train speed would increase the transient pressure and slipstream speed, but it did not effect their spatial distribution.

... (vi) Influence of the parameters of tunnel portals on the pressure gradient and amplitude of micropressure waves [160,161]: ...

High-speed railway aerodynamics is the key basic science for solving the bottleneck problem of high-speed railway development. This paper systematically summarizes the aerodynamic research relating to China’s high-speed railway network. Seven key research advances are comprehensively discussed, including train aerodynamic drag-reduction technology, train aerodynamic noise-reduction technology, train ventilation technology, train crossing aerodynamics, train/tunnel aerodynamics, train/climate environment aerodynamics, and train/human body aerodynamics. Seven types of railway aerodynamic test platform built by Central South University are introduced. Five major systems for a high-speed railway network—the aerodynamics theoretical system, the aerodynamic shape (train, tunnel, and so on) design system, the aerodynamics evaluation system, the 3D protection system for operational safety of the high-speed railway network, and the high-speed railway aerodynamic test/computation/analysis platform system—are also introduced. Finally, eight future development directions for the field of railway aerodynamics are proposed. For over 30 years, railway aerodynamics has been an important supporting element in the development of China’s high-speed railway network, which has also promoted the development of high-speed railway aerodynamics throughout the world.

... The test platform has obtained CMA (China Metrology Accreditation) qualification (Certificate number 2014002479K). More detailed information about the platform could be found in the reference (Zhang et al., 2017a). Operating speeds of the model train are 250 km/h, 300 km/h and 350 km/h. ...

... The authors identified an optimal ratio between the hood section and tunnel section where the temporal gradient of the pressure wave could be reduced by half. Zhang et al. [20] analyzed the mitigation effects of a ring oblique tunnel portal on the initial compression wave, and the relationship between the MPWs and slope values. The authors established that the maximum pressure gradient and MPW could be reduced by approximately 10.8% when the slope value was 1:1.75 compared to 1:0.5. ...

We aim to perform a series of field measurements for high-speed railway tunnels in China to obtain the micropressure wave (MPW) at the tunnel exit and the transient pressures near the tunnel portals. The relationship between the MPW and the nose-entry wave and the effects of train speed and the tunnel exit hole on the MPW are analyzed. The results show that the MPW decreases with increasing distance from the tunnel exit, but increases rapidly with increasing train speeds. Additionally, holes in the hoods near the tunnel exit could decrease the MPW near the tunnel exit by 10-20%.

In this study, through experiments and numerical simulations, micro-pressure waves (MPWs) resulting from the movement of high-speed trains through high-temperature geothermal tunnels were studied, and a self-satisfying MPW mitigation method was proposed. First, a model experiment of a moving high-speed train under local high-temperature conditions was carried out. Then, a relationship between the MPW and length of the heating zone (LH) and the length of the train streamline (12.5 m) was established via numerical simulation. For LH < 12.5 m, the MPW amplitude gradually decreases with increasing LH. For LH > 12.5 m, the MPW amplitude remains basically unchanged. The influence of the circumferential distribution of the heating zone along the tunnel wall on the MPWs was further studied. The research results inspired an MPW mitigation method for tunnels, considering a heating length of 12.5 m and an angle of 225°. Moreover, in a tunnel, electric energy could be generated under slipstream action through a wind turbine installed to provide heating equipment with power, thereby achieving self-satisfaction. This research proposed a new MPW mitigation method for research on MPW mitigation. Moreover, new mitigation methods could facilitate the construction of an environmentally friendly society and provide a new approach to the sustainable development of cities.

Inspired by shark’s skin in nature, a non-smooth surface could be an ideal model for changing the flow characteristics of fluids on the object surface. To analyze the effect of a non-smooth surface with concaves on the maglev train aerodynamic performances and to investigate how the concave size affects the aerodynamic forces and flow structure of a maglev train, four 1/10th scaled maglev train models are simulated using an Improved Delayed Detached Eddy Simulation (IDDES) method. The numerical strategy used in this study is verified by comparison with the wind tunnel test results, and the comparison shows that the difference was in a reasonable range. The results demonstrate that the concaves could effectively reduce the tail car pressure drag, thus reducing the total drag, and that the smaller the concave size was, the better the drag reduction effect would be. The change in the lift with the concave size was more significant than that of the drag, and the tail car lift of R1 (0.0012H), R2 (0.0024H), and R3 (0.0036H) train models was 30.1%, 43.0%, and 44.5% less than that of the prototype, respectively. In addition, different flow topologies of the wake are analyzed. The width and height of the vortex core of the counter-rotating vortices tended to decrease with the concave size. Thus, from the point of view of ensuring the operating safety of a maglev train, a non-smooth surface with small-size concaves is recommended.

The train-induced airflow in a railway tunnel is of great significance for tunnel ventilation, fire rescue, and ancillary facilities overall. Previous research mainly focused on piston wind in metro tunnels, while the slipstream in high-speed railway tunnels is more drastic and complicated owing to the faster vehicle speed and diverse blockage ratios. In this research, a computational fluid dynamics (CFD) study was carried out to explore the three-dimensional transient airflow induced by a commercial high-speed train circulating through tunnels at 350 km/h under different blockage ratios (β = 0.175, 0.160, 0.140, 0.122, and 0.112). First, the train-tunnel model was established based on the unsteady Reynolds-Averaged Navier–Stokes (URANS) method merged with the sliding mesh technique, altogether validated with experimental evidence. Then, the resultant slipstream and the individual components were investigated, and it was concluded that the longitudinal component was dominant. The variation behaviors of the slipstream and local pressure with the different blockage ratios were comparatively obtained. Finally, the relationships between the slipstream and the spatial distance in longitudinal, lateral, and vertical directions were analyzed. This study provides a guideline for the determination of the tunnel clearance, and wind load evaluation in a high-speed railway tunnel.

Numerical RANS modeling has been carried out to assess the aerodynamics of different metro train geometries through a straight tunnel. A steady-state approach was first used to choose the best geometry out of seven alternatives in terms of drag reduction when compared with a typical blunt face train design representative of European metro networks. The proposed models have different edge-rounding characteristics at the front and rear faces. Afterward, the baseline and optimized geometries are compared at different train velocities, and the flow structure surrounding the models is discussed using unsteady RANS results. The study focuses on skin and pressure drag coefficients for trains traveling at 40 km/h in a straight tunnel with a blockage ratio of 0.69. All the considered alternatives show a drag reduction between 5% and 20% relative to the baseline case.

Alleviation of the pressure transients caused by high-speed trains intersecting in a tunnel with partially reduced cross-section is investigated by numerical simulation. The RNG k-ε turbulence model and hybrid grid are adopted for the numerical simulation, which is validated by the moving model test. Two factors, the length of reduced section (L) and the train marshalling number, are considered to analyse the alleviation effect. The results show that, for reduction of the pressure transients induced by the intersection of three-car marshalling trains, the location of the change in cross-section is supposed to be near the location of the maximum value of the initial expansion wave (L → 240 m). In addition, for four-car marshalling trains intersecting in a tunnel of L = 240 m, the excavation volume of the tunnel (tunnel volume) decreases by 14.4%, and the average pressure amplitude on the train surface decreases by 7.9%. Especially the pressure amplitude at the measurement point in the streamlined area of the tail car decreased by 28.6%. The reduced section tunnel has a good effect on decreasing the pressure transients around train body and the tunnel excavation volume (construction cost), which helps the tunnel design process.

Using a 1:20 scale moving model device, pressure distributions on train and tunnel surfaces, and the distribution of micro-pressure waves within 50 m of tunnel exits were investigated. In addition, the effects of train speeds on the transient pressures and micro-pressure waves were analyzed. The results revealed that the effect of the train speed on Pmin values was more significant than that on Pmax values, on the train surface. Significant differences between symmetrical measurement points located on the same cross section of the tunnel with double tracks could be observed. Moreover, similar differences between symmetrical points located on the same cross section in the streamlined zone of the model train were observed and analyzed. The pressure changes in the measurement points located on the same cross section of the train body, other than the streamlined zone, were approximately coincident. The differences between the Pmin values of measurement points located on different cross sections of the tunnel determined the differences between the ΔP values of these measurement points. The micro-pressure waves were approximately equal for measurement points located on the same cross section. Moreover, linearity decreased when the distance between the measurement points and the tunnel exit increased.

Super high-speed evacuated tube transport is an important development direction of green, energy-saving, and low-drag rail transit in the future. A great deal of aerodynamic heat and pressure change are produced by a tube train running in a closed and long tube with a big blocking ratio at high speed, and the shock wave generated by the train running at supersonic speed further aggravates the aerodynamic phenomenon in the tube, resulting in a large increase of temperature and abrupt pressure change in the tube, endangering the structural safety of the vehicle body and affecting the normal operation of equipment in the vehicle and tube. The purpose of this paper is to find new aerodynamic phenomena, study the formation and evolution mechanism of aerodynamic heating in the tube and influence of the Mach number at subsonic, transonic, and supersonic speed. The results show that shock waves appear in the front and rear of the tube train, the shock reflection is confined to the expansion wave at the tail of the tube train, and the flow field around the tube train, pressure, temperature, and so on in the tube are significantly changed by the shock wave.

Pressure gradient and micro-pressure wave induced by a high-speed train entering a tunnel were investigated for the parametric design of equal-transect oblique tunnel portal. A moving model experimental test was conducted to verify the computational method and mesh. The influence of three main factors (portal shape, slope value, and aperture ratio) of the equal-transect oblique tunnel portal were considered in this study. The results showed that the hat oblique shape was the most efficient portal design to alleviate the pressure gradient and micro-pressure wave compared to the other designs examined in this study, and the mitigation mechanism of the hat oblique design on the initial compression wave was explained. Then the effects of the slope value and the aperture ratio of hat oblique tunnel portal on the pressure gradient and micro-pressure wave were investigated, and the parameters of the equal-transect oblique tunnel portal were optimized to alleviate the pressure gradient and micro-pressure wave induced by high-speed trains entering tunnels.

This paper reports the numerical research of tunnel hood effects on high speed train-tunnel compression wave. The three-dimensional simulation with real geometry is carried out by the implementation of a commercial computational code. The train speed is 350 km/h. The train/tunnel blockage ratio is 0.115. Nine different types of tunnel hoods were studied. The calculation results showed that the hood length, the hood cross sectional area and the ventilation holes might have significant influence on the first compression wave, and inclined entry or asymmetric distribution of the ventilation holes is not available for alleviating the impulsive wave.

The objective of this study was to investigate the effects of oblique tunnel portals on train and tunnel aerodynamics using a 1:20 scale moving model device. Transient pressure and micro-pressure waves were measured using pressure sensors as the train model travelled through various tunnel models at a speed of 350 km/h. The mitigation physical mechanism of oblique tunnel portal on the initial compression wave was explained. Experimental results showed that oblique tunnel portals had obvious mitigation effect on- the pressure gradient and micro-pressure wave induced by the train model passing through the tunnel model. A hat oblique tunnel portal combined with a buffer structure with top holes was particularly effective.

Using compressible, unsteady and sliding meshing technologies, the aerodynamic loads including transient pressure, lateral force, and overturning moment that are caused by a single train moving through a double-track tunnel or two trains passing each other in a tunnel were calculated. The dynamic response generated by applying these loads to a train body was also analysed. The results show a large difference in the pressure on both sides of the vehicle nose when a single train moves through a double-track tunnel and when two trains pass each other in a tunnel. The pressures at the symmetric measuring points on both sides of the middle of the train body are approximately the same except at the moment of passing. When the train passes the measuring points, the pressure change at the symmetric measuring points on the tunnel wall closer to the train is obviously larger than that farther from the train; the pressure changes are approximately the same at all other times. Therefore, the propagation of the pressure wave in the tunnel has good one-dimensional characteristics. Under dynamic loads in the tunnel, especially those generated by two trains passing each other in the tunnel, the lateral and vertical displacements of the underframe and side walls all increase significantly. The lateral acceleration on the roof is significantly greater than that of the underframe, which has a rolling movement below the centre of gravity. For the measuring points on both the side wall and underframe, the vertical accelerations at both ends are larger than that in the middle of the body because of a pitching motion. The horizontal and vertical vibration acceleration for a train passing another train in a tunnel are 33.8% and 47.2% larger than those for a train operating in open air, respectively.

As a fast and efficient short distance transportation means, the subway line has been built and expanded in an increasing number of cities. The pressure in the tunnel fluctuates significantly while metro trains pass. This kind of pressure may damage the equipment and workers in the tunnel. Considering that, the metro train does not have airtightness, and that pressure can spread inside the vehicle, passengers in the vehicle would be directly affected by the alternating aerodynamic pressure, which causes discomfort to passengers. This phenomenon is exacerbated at high speeds. Therefore, it is important to estimate the aerodynamic alternating pressure generated by the metro train in the tunnel before construction. In this study, the aerodynamic performance of a metro train running between two adjacent platforms in a tunnel was simulated by using FLUENT. In this work, the effects of acceleration and speed of the metro train, and of platform spacing, on the alternating pressure on the train and in the tunnel are studied. In the analysis of the impact of train acceleration, the pressure change inside a passenger train in a 1 s timespan was used to evaluate the comfort of passengers. Maximum and average ΔP (pressure changes in amplitude) shows an exponential relationship with a (acceleration), Vc (constant speed) and Lplatform (platform spacing), especially the ΔP measured on tunnel surface. The fluctuation of the train surface pressure is more intense than that of the tunnel. The Pmin (minimum pressure) on the train surface and in the tunnel is not affected by the acceleration of the train, but it is mainly related to the highest train speed in the tunnel. When the platform spacing is higher than 1500 m, Pmax, Pmin, and △P in the tunnel and on the train surface showed little change. These findings contribute not only to the design of the metro train and tunnel system, but also to the guidance of the metro train operation.

The micro-pressure wave (MPW) radiated from a tunnel portal can, if audible, cause serious problems around tunnel portals in high-speed railways. This has created a need to develop an acoustic model that considers the topography around a radiation portal in order to predict MPWs more accurately and allow for higher speed railways in the future. An acoustic model of MPWs based on linear acoustic theory is developed in this study. First, the directivity of sound sources and the acoustical effect of topography are investigated using a train launcher facility around a portal on infinitely flat ground and with an infinite vertical baffle plate. The validity of linear acoustic theory is then discussed through a comparison of numerical results obtained using the finite difference method (FDM) and experimental results. Finally, an acoustic model is derived that considers sound sources up to the second order and Green's function to represent the directivity and effect of topography, respectively. The results predicted by this acoustic model are shown to be in good agreement with both numerical and experimental results.

A computer programme is used to predict the pressure histories on the walls of a tunnel when a train enters and passes through the tunnel at speed. The programme is capable of simulating the velocity and pressure histories in complex tunnel systems during the passage of any number of trains. Comparisons are made with pressure histories recorded by transducers mounted on the wall of the laboratory model described in the first of these two papers. By carefully choosing empirical coefficients in order to give a good ft to the data, excellent correlation is obtained for the basic case of a train in a simple tunnel. The principal features of the pressure histories in a wide range of tunnel configurations are also well simulated using the same empirical data. Comparisons are also made with full scale measurements obtained in Patchway Tunnel. The correlation is not as good as with the laboratory measurements. However, it is sufficiently close for the accompanying predictions of the influence of various modifications to the tunnel to be regarded as valid. It is shown that there are significant benefits to be gained from entrance modifications, but that these cannot alone provide a complete solution. Pressure fluctuations generated during train exit must also be taken into account. (A)

According to the 100m
2 double-line tunnel cross-section which is generally used in high-speed railway of China, this paper develops a tunnel - air - train simulation model, based on the three-dimensional incompressible Navier - Stokes equations and the standard k − ε turbulence model. This model can simulate two situations one is that a single train runs through a tunnel normally while the other is that another train runs beside a train parked in the tunnel. Time-history variation rules and space distribution characteristics of train wind are studied respectively at 120 km, 200 km, 250 km, 300 km and 350 km per hour. Furthermore, the authors discuss train wind influence on personnel safety on evacuation passageways. In addition, the authors give out analytically the results of the numerical simulation. The results show that: Train wind is complex three-dimensional flow changing with time and space, so people should avoid activities at dangerous time and places; since personnel safety may be threatened by train wind in the two situations above, therefore, effective measures should be taken to avoid accidents.

When a train enters a tunnel, a pressure pulse called a micropressure wave is radiated from the other end of the tunnel. Various methods have been applied to the tunnels on the Shinkansen lines to alleviate it. A method using the side branches in a tunnel has been adopted at the Haruna and Nakayama tunnels on the Joetsu Shinkansen. The effect of this countermeasure was confirmed at these tunnels.

Based on analyzing the generating mechanism of micro-pressure wave at high-speed railway tunnel exit, with one-dimensional unsteady-compressible- nonisentropic air flow theory and round-piston radiation theory with infinite baffle plate, the rules of micro-pressure waves generated by high-speed train near tunnel exit under different shape hoods were studied, the influences of various shapes and parameters of hoods on the waves were qualitatively and quantitatively analyzed. It is indicated that the wave intensity evidently decrease with the length and cross-section area increasing of line-type, parabola-shaped and discontinuous-type hoods, the effect of discontinuous-type hood is evidentest; when hood cross-section area is constant, but hood construction is perforated, the choice for the best structural parameters needs an integrated plan contrast although the wave intensity can be greatly reduced.

After having evaluated a 3-dimensional Eulerian code’s capability to predict the wavefront’s pressure gradient of the primary pressure waves generated by trains entering a tunnel, SNCF’s Research and Technology Department was assigned to test the tool via a study bearing upon tunnel portal extensions which attenuate the pressure gradient of the entry wavefront. As a first step, taking into account the numerical and computational constraints, the model was sized to permit reasonably quick simulations of many train-tunnel entry cases. As a second step, the parametric study was made on the particular case of the Terranuova le Ville tunnel and served to assess the influence of various geometrical parameters relating to short, flared or perforated tunnel portal extensions.

Entering a tunnel a high-speed train generates
a pressure wave, which propagates along the tunnel and is partly reflected at the opposite tunnel portal. This wave leads to some severe problems like loads on the installations inside of the tunnel, discomfort of the passengers or even micro-pressure waves at the end of the tunnel. The tunnel-simulation facility Göttingen (TSG) was built in order to analyse these pressure changes and to develop systems, which smooth the pressure increase and reduce the pressure-depending problems in train-tunnel entry. The TSG is a moving-model rig, which allows a very realistic investigation of train-tunnel interaction. The train used is an ICE3-model made of carbon fiber scaled 1:25. The train speed ranged from 30 up to 45 m/s. The results of the experiments done in the TSG show, that the pressure gradient can be reduced by about 45 % using an extended, vented tunnel portal.

In this paper, the characteristics of train-tunnel interaction at a tunnel entrance has been investigated numerically. A three-dimensional numerical model using the remeshing method for the moving boundary of a passenger train in Iran railway was applied. The turbulent flows generated by the moving train in a tunnel were simulated by the RNG κ−ε turbulence model. The simulations have been carried out to understand the effect of the train speed as well as the influences of the hoods and air vents on the pressure waves, drag, and side force coefficients. The results show that the maximum drag coefficient occurs when the train enters the tunnel and is equal to 2.2. The air vents and enlarged hood at the portal are demonstrated to attenuate the pressure gradient and drag coefficient about 28% and 36%, respectively. Furthermore when train is entering the tunnel asymmetrically, a side force is created that pushes the train toward the tunnel wall, which the maximum side force is 900 N.

A three-dimensional flow induced by a practical high-speed train moving into a tunnel is studied by the computation of the compressible Navier-Stokes equations with the zonal method. The transient flow field induced by tunnel entry is investigated with the focus on the compression wave which is the source of the booming noise at the tunnel exit. The results reveal a pressure increase inside the tunnel before tunnel entry, the one-dimensionality of the compression wave, the histories of the aerodynamic forces, etc. The computed pressure histories inside the tunnel agree with the field measurement data. The flow fields are also computed for cases where the train runs on differently positioned tracks into the tunnel. The results indicate that the wavefront of the compression wave is affected by the train position and this phenomenon is explained by the parameter υwatt.

Little is known of the behaviour of transient air velocities and dynamic pressure loads generated by high-speed trains in confined spaces, or whether current methodologies for assessing transient gust loads in open spaces can be used in confined spaces. Experiments have been carried out in which a moving-model high-speed train passed walls, a partially-enclosed tunnel, and single-track tunnels with a variety of cross-sectional areas and lengths. An open air control experiment has also been carried out. The train model was a simplified 1/25 scale four-carriage ICE2 train travelling at 32 m/s. Cobra Probes measured the three-dimensional air velocity components at various positions inside the structures. The results show that the peak gust magnitudes increase in all confined cases compared to the open air. In tunnels, a ‘piston effect’ appears to have been a dominant cause of the increases in the peak gust magnitudes, as well as prolonged winds occurring before and after the train passed the probes. The tunnel length impacted considerably on the flow characteristics, and the partially-enclosed tunnel showed further increases in the gusts due to high lateral and vertical velocities.

This paper relates to the parametric study of tunnel hoods in order to reduce the shape, i.e. the temporal gradient, of the pressure wave generated by the entry of a High speed train in tunnel. This is achieved by using an in-house three-dimensional numerical solver which solves the Eulerian equations on a Cartesian and unstructured mesh. The efficiency of the numerical methodology is demonstrated through comparisons with both experimental data and empirical formula. For the tunnel hood design, three parameters, that can influence the wave shape, are considered: the shape, the section and the length of the hood. The numerical results show, (i) that a constant section hood is the most efficient shape when compared to progressive (elliptic or conical) section hoods, (ii) an optimal ratio between hood's section and tunnel section where the temporal gradient of the pressure wave can be reduced by half, (iii) a significant efficiency of the hood's length in the range of 2-8 times the length of the train nose. Finally the influence of the train's speed is investigated and results point out that the optimum section is slightly modified by the train's speed.

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.

The objective of this study is to investigate numerically the characteristics of train-induced unsteady airflow in a subway tunnel with natural ventilation ducts. A three-dimensional numerical model using the dynamic layering method for the moving boundary of a train is first developed, and then it is validated against the model tunnel experimental data. With the tunnel and subway train geometries in the numerical model exactly the same as those in the model tunnel experimental test, but with the ventilation ducts being connected to the tunnel ceiling and a barrier placed at the tunnel outlet, the three-dimensional train-induced unsteady tunnel flows are numerically simulated. The computed distributions of the pressure and the air velocity in the tunnel as well as the time series of the mass flow rate at the ventilation ducts reveal the impact of the train motion on the exhaust and suction of the air through ventilation ducts and the effects of a barrier placed at the tunnel outlet on the duct ventilation performance. As the train approaches a ventilation duct, the air is pushed out of the tunnel through the duct. As the train passes the ventilation duct, the exhaust flow in the duct is changed rapidly to the suction flow. After the train passes the duct, the suction mass flow rate at the duct decreases with time since the air pressure at the opening of the duct is gradually recovered with time. A drastic change in the mass flow rate at a ventilation duct while a train passes the corresponding ventilation duct, causes a change in the exhaust mass flow rate at other ventilation ducts. Also, when a barrier is placed at the tunnel outlet, the air volume discharge rate at each ventilation duct is greatly increased, i.e., the barrier placed at the tunnel outlet can improve remarkably the ventilation performance through each duct.

An obvious pressure change and a micro-pressure wave are generated when a train enters a tunnel at high-speed, which impacts on the comfort of passengers and the environment around the tunnel. Thus, it is necessary to study the aerodynamics of tunnels in order to reduce the pressure change and micro-pressure wave. Our laboratory owns an advanced moving-model device that can simulate two trains crossing in the open air or trains entering a tunnel, with a maximum speed of 400km/h. This paper studies the pressure change, the micro-pressure wave and a series of hoods using three-dimensional numerical simulations and moving-model experiments. From a comparative study, we obtain rules governing the influence of hoods on the micro-pressure wave, and reasonable shapes and parameters of hoods are designed.

This paper reports numerical computations of the train–tunnel interaction at a tunnel entrance with real dimensions. Simulations were carried out by the FEM using the three-dimensional compressible Euler equation. The train speed was 300km/h. For a single-track tunnel, four kinds of tunnel entrance shapes were studied to investigate the formation of the compression wave front at the tunnel entrance. This study shows the possibility of a partial change in the compression wave front by means of the optimal combination of the degree of the tunnel entrance slopes and holes in the tunnel entrance ceiling. The results can be used for countermeasures against the boom noise at the tunnel exit, for the air tightness design of the train body shell and fatigue damage of the tunnel wall and structure. The compression wave fronts at the tunnel entrance directly affected the pressure drop in the tunnel and the booming noise intensity at the tunnel exit.

An optimally designed entrance portal must be capable of minimizing the maximum growth rate of the compression wave generated when a high-speed train enters a tunnel. A theoretical and experimental investigation has been made to determine the changes in compression wave characteristics produced when the portal is 'scarfed' with tapering side walls. It is concluded that portal modifications of this type are unlikely to produce a significant reduction in the maximum compression wave growth rate. Small decreases in growth rate are possible (up to about 15%) for scarf walls extending a distance beyond the tunnel entrance of the order of the tunnel height, but little or no additional improvement is achieved with longer walls.

A compression wave is generated when a high-speed train enters a tunnel. The wave propagates ahead of the train at the speed of sound. In a long tunnel nonlinear steepening of the wavefront produces the emission of a strong micro-pressure wave (mpw) from the distant tunnel exit. The mpw can produce structural damage to the tunnel and rattles in surrounding buildings. Nonlinear steepening can be countered by increasing the initial rise time of the compression wave by installing a tunnel entrance ‘hood’ consisting of a nominally uniform extension of the tunnel of larger cross-section. In this paper a theoretical examination is made of the influence on the wave of the rapid change in tunnel cross-section in the transition region between the tunnel and hood. It is shown that by optimally profiling the cross-sectional area changes across this region it is possible to minimize the amplitudes of second and third peaks of the compression wave pressure gradient. By this means the amplitudes of the secondary peaks in the micro-pressure wave are greatly reduced.

A practical analytical scheme is proposed for making rapid numerical predictions of the compression wave generated when a high-speed train enters a tunnel fitted with a vented entrance hood. The method synthesises results from several analytical procedures developed during the past few years for treating different aspects of the tunnel-entry problem, including the effects of change in cross-sectional area at the hood-tunnel junction, high-speed jet flows from windows distributed along the length of the hood, frictional losses associated with separated turbulent flow between the tunnel and hood walls and the train, and the influence of train nose shape. Details are given in this paper for the simplest case of circular cylindrical tunnels and hoods of the type used in model scale testing and design studies. Typical predictions can be made in a few seconds on a personal computer (in contrast to the tens or hundreds of hours required for simulations using the Euler or Navier–Stokes equations on a high performance supercomputer). A summary is given of selected predictions and their comparisons with experiments performed at the Railway Technical Research Institute in Tokyo at train Mach numbers as large as 0.35(∼425km/h).

An analysis is made of the compression wave generated when a high-speed train enters a tunnel with a flared portal. Nonlinear steepening of the wavefront in a very long tunnel is responsible for an intense, environmentally harmful, micro-pressure wave , which propagates as a pulse from the distant tunnel exit when the compression wave arrives, with amplitude proportional to the maximum gradient in the compression wavefront. The compression wave profile can be determined analytically for train Mach numbers M satisfying M2⪡1, by regarding the local flow near the tunnel mouth during train entry as incompressible . In this paper, the influence of tunnel portal flaring on the initial thickness of the compression wave is examined first in this limit. The shape of the flared portal is “optimal” when the pressure gradient across the front is constant and an overall minimum, so that the pressure in the wavefront increases linearly . This linear behaviour is shown to occur for a flared portal extending a distance ℓ into the tunnel from the entrance plane (x=0) only when the tunnel cross-sectional area S (x) satisfiesS (x)A=1[A/AE−(x/ℓ)(1−A/AE)], −ℓ

An experimental facility was developed for investigating pressure waves generated by high-speed trains. The facility launches a 1/30 scale model conforming to the actual shape of the train and enables measurements to be carried out with the same geometric configurations at full scale. The train models are launched using compressed air. A mathematical model is developed to predict the performance of the experimental facility. This model allows the optimum values of the design parameters of the facility to be determined in order to achieve a given target velocity and to control the launching velocity by adjusting the pressure of the compressed air. Measurement of the flow in the experimental facility shows that the facility performs as designed by the mathematical model and is capable of launching a train model at velocities greater than 500km/h. Pressure waves generated by a train moving into a tunnel are measured, and the experimental data agree well with field measurements. The effect of the train nose on the strength and form of the pressure waves is also discussed.

A numerical procedure for the rapid prediction of the compression wave generated by a high-speed train entering a tunnel was presented and validated by Howe et al. [Rapid calculation of the compression wave generated by a train entering a tunnel with a vented hood, Journal of Sound and Vibration 297 (2006) 267–292]. The method was devised to deal principally with compression wave generation in long hoods typically of length ∼10 times the tunnel height and ‘vented’ by means of a series of windows distributed along the hood walls. Hoods of this kind will be needed to control wave generation by newer trains operating at speeds U exceeding about 350 km/h. In this paper experimental results are presented and compared with predictions in order to extend the range of applicability of the numerical method of Howe et al. (2006) to include short hoods with lengths as small as just twice the tunnel height (the situation for most hoods currently deployed on the Japanese Shinkansen) and for U as large as 400 km/h.

The topic of this paper is to present a new methodology for the three-dimensional numerical simulation of the entrance of high-speed trains in a tunnel. The movement of the train is made thanks to a technique of sliding meshes and a conservative treatment of the faces between two do-mains. All parts of the development are thought with the aim to reduce the computational time. In particular, non reflecting boundary conditions for non-structured three-dimensional meshes are developed in order to limit the calculation domain. Validations of the methodology are presented on different test cases.

We develop the dynamic renormalization group (RNG) method for hydrodynamic turbulence. This procedure, which uses dynamic scaling and invariance together with iterated perturbation methods, allows us to evaluate transport coefficients and transport equations for the large-scale (slow) modes. The RNG theory, which does not include any experimentally adjustable parameters, gives the following numerical values for important constants of turbulent flows: Kolmogorov constant for the inertial-range spectrumC
K=1.617; turbulent Prandtl number for high-Reynolds-number heat transferP
t
=0.7179; Batchelor constantBa=1.161; and skewness factorS
3=0.4878. A differentialK-
[`(e)]\bar \varepsilon
model is derived, which, in the high-Reynolds-number regions of the flow, gives the algebraic relationv=0.0837 K2/
[`(e)]\bar \varepsilon
, decay of isotropic turbulence asK=O(t
–1.3307), and the von Karman constant[`(e)]\bar \varepsilon
, and[`(e)]\bar \varepsilon
is finite. This latter model is particularly useful near walls.

Upon the entrance of a high-speed train into a relatively long train tunnel, compression waves are generated in front of the train. These compression waves subsequently coalesce into a weak shock wave so that a unpleasant sonic boom is emitted from the tunnel exit. In order to investigate the generation of the weak shock wave in train tunnels and the emission of the resulting sonic boom from the train tunnel exit and to search for methods for the reduction of these sonic booms, a 1300 scaled train tunnel simulator was constructed and simulation experiments were carried out using this facility.In the train tunnel simulator, an 18 mm dia. and 200 mm long plastic piston moves along a 40 mm dia. and 25 m long test section with speed ranging from 60 to 100 m/s. The tunnel simulator was tilted 8 to the floor so that the attenuation of the piston speed was not more than 10 % of its entrance speed. Pressure measurements along the tunnel simulator and holographic interferometric optical flow visualization of weak shock waves in the tunnel simulator clearly showed that compression waves, with propagation, coalesced into a weak shock wave. Although, for reduction of the sonic boom in prototype train tunnels, the installation of a hood at the entrance of the tunnels was known to be useful for their suppression, this effect was confirmed in the present experiment and found to be effective particularly for low piston speeds. The installation of a partially perforated wall at the exit of the tunnel simulator was found to smear pressure gradients at the shock. This effect is significant for higher piston speeds. Throughout the series of train tunnel simulator experiments, the combination of both the entrance hood and the perforated wall significantly reduces shock overpressures for piston speeds ofu
p
ranging from 60 to 100 m/s. These experimental findings were then applied to a real train tunnel and good agreement was obtained between the tunnel simulator result and the real tunnel measurements.

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.

A theoretical and experimental study is made of the compression wave generated when a train enters a nominally uniform tunnel with a long, unvented entrance hood. The purpose of the hood is to reduce as much as practicable the maximum gradient of the compression wave front. The pressure gradient can increase in a long tunnel as a result of nonlinear wave steepening, and thereby increase the impact on residential dwellings of the acoustic ‘boom’ (or micro-pressure wave) radiated from the far end of the tunnel when the compression wave arrives. Our experiments are conducted at model scale using axisymmetric ‘trains’ projected at speeds up to along the axis of a cylindrical tunnel fitted with a cylindrical entrance hood. Theoretical predictions of the compression wave are made using the equation of aerodynamic sound containing a slender body approximation to the effective source representing the moving train, coupled with a small correction that accounts for the ‘vortex’ sources in the free shear layers in the exit flows from the hood and tunnel of the air displaced by the train. The compression wave is generated by the two successive interactions of the train nose with the hood portal and with the junction between the hood and tunnel. The interactions produce a system of compression and expansion waves, each having characteristic wavelengths that are much smaller than the hood length; the waves are temporarily reflected back and forth within the hood prior to transmission into the tunnel, and are resolved analytically by use of an approximate Green's function determined by the hood geometry. Theoretical predictions are found to be in excellent agreement with experiment, including in particular a detailed correspondence between measured and predicted interference patterns produced by the multiple reflections of waves in the hood.

The design of new high-speed railway lines requires longer and more numerous tunnel sections, where aerodynamic effects limit the maximum allowed train velocity for a given tunnel cross-section area. These effects influence the train power requirement, the traction energy costs and the pressure wave amplitude: the knowledge of the unsteady aerodynamic field around the train is therefore essential to the optimum choice of a tunnel configuration, and mainly of the cross-section diameter and of the presence and position of pressure relief ducts. In this paper, the aerodynamic phenomena generated by a train traveling at high speed through a long tunnel of small cross-section are analyzed by means of quasi one-dimensional numerical simulations of the air flow induced by a train traveling at 120 m/s in a tunnel connecting two stations 60 km apart. Several tunnel configurations at high blockage ratio are discussed, together with the positive and negative effects of pressure relief ducts and of partial air vacuum. Aerodynamic phenomena are evaluated in terms of drag, pressure wave amplitude and shock wave onset on the train tail. Results suggest that configurations consisting of twin tunnels connected by pressure relief ducts near stations and operated under partial vacuum should be preferred.

The generation and alleviation of air pressure transients caused by the high speed passages of vehicles through tunnels

- Fox

Fox, J.A., Vardy, A.E., 1973. The generation and alleviation of air pressure transients
caused by the high speed passages of vehicles through tunnels. In: Proceedings of
First International Symposium on the Aerodynamics and Ventilation of Vehicle
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Model experiment on reduction of micro-pressure wave radiated from exit

- Ozawa

Ozawa, S., Maeda, T., 1988. Model experiment on reduction of micro-pressure wave
radiated from exit. In: Proceedings of the International Symposium on Scale
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Numerical simulation about train wind influence on personnel safety in high-speed railway double-line tunnel

- L M Peng
- R Z Fei
- C H Shi
- W C Wang
- Y T Liu

Peng, L.M., Fei, R.Z., Shi, C.H., Wang, W.C., Liu, Y.T., 2013. Numerical simulation about
train wind influence on personnel safety in high-speed railway double-line tunnel. In:
25th International Conference, ParCFD 2013, Changsha, May, pp. 553-564.

Pressure rise due to the friction of a train at the entrance of a tunnel

- Yamamoto

Yamamoto, A., 1969. Pressure rise due to the friction of a train at the entrance of a tunnel.
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Micro-pressure wave around high-speed railway tunnel exit and active and passive method of reduction

- W C Zhao
- B Gao
- T Y Qi

Zhao, W.C., Gao, B., Qi, T.Y., 2004. Micro-pressure wave around high-speed railway
tunnel exit and active and passive method of reduction. J. Shi Jia Zhuang Railw. Inst.
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Micro-pressure wave around high-speed railway tunnel exit and active and passive method of reduction

- Zhao