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The high-speed railway bridge, accounting for over 50% of railway lines, plays an important role in high-speed railways. When the train runs at high speed on these bridges, the strong winds will directly pose a risk to the safety of train operation, even causing train overturning accidents. Therefore, it is significantly urgent to accurately simulate the relative motion of the train-bridge system in crosswind environment. An improved delayed detached-eddy simulation study based on the shear stress transfer k-omega (I-ω) turbulence model was used to analyze the aerodynamic characteristics of trains running on bridges under two relative motion modes, i.e., the static synthesis method (SSM) and the dynamic decomposition method (DDM), and the variation rule of train aerodynamic loads. The differences of aerodynamic forces, surface pressure distributions, and flow characteristics around the train body under two motions were discussed. The results show that the applied velocity-inlet boundaries in two relative train-bridge motion modes under crosswinds lead to a large difference in the flow characteristics around bridges. Compared with the SSM, the DDM generates more small-scale vortices in the leeward side of the train, causes more disturbances, and enhances the flow instability. Under the DDM, the side force coefficient and lift force coefficient of the train are decreased by 1.99%, 66.67% (Train on the windward side) and 24.47%, 48.24% (Train on the leeward side), respectively, as compared to the SSM. The achievements obtained in the current study can provide data support for the operation standard of high-speed trains traveling on bridges. [ABSTRACT FROM AUTHOR]

Ground configurations are an essential part of rail vehicles during wind tunnel tests. Reasonable reproductions of the infrastructures are of significance for experimental accuracy. In this work, the effects of ballast length were numerically studied via improved delayed detached eddy simulation (IDDES). Three extension lengths of the ballast accompanied by a 1/8th three-unit train model were proposed, with a pseudo infinite scenario as the benchmark. The numerical algorithm was firstly validated through a wind tunnel test in terms of aerodynamic loads and pressure distribution. The results revealed that the extended length of ballast had a significant impact on the leading and tail cars while less affected the middle car, due to the relatively farther distance from ballast ends. Ignored the drag coefficient, the influence of the ballast length on the separate cars was that the aerodynamic coefficients predicted by much longer ballast got closer to the benchmark. The ballast-edge-induced unrealistic flow and the train-nose-tip-induced realistic flow could easily be merged and interacted with each other and reinforced, thus causing a noticeable low-velocity zone at the leeward side. With the increasing ballast length, those two separations gradually moved away from each other, decoupled, and receded. A minimum length of E/H = 2 is recommended considering the aerodynamic performance and practical constraints of wind tunnel width. This study provides potential guidance for the arrangement of ground configurations for rail vehicles during wind tunnel tests. [ABSTRACT FROM AUTHOR]

Field tests were carried out to measure the wheel-rail forces of a high-speed train passing by a windbreak breach with a crosswind speed and a train speed of 32 m/s and 120 km/h, respectively. Unsteady Reynolds-averaged Navier-Stokes simulations and multibody dynamic simulations were carried out to reproduce the field test experiments. The results obtained indicated that the side force and yawing moment of the train plays a key role in the safety of the head and the tail of the train when it passes by a windbreak breach. By analysing the flow structures, the reasons behind the different aerodynamic loads on the train head and tail were discussed. Based on the vehicle movement, the maximum derailment coefficient (0.8) of the train head appeared to be at a higher risk of danger as compared to the maximum derailment coefficient (0.2) of the train tail. This is due to the wind loads on the train head which compel the bogies to relatively move with the carbody along the lateral direction one more time than the train tail. Whereas, for the train tail, there is no relative motion between the carbody and bogies on the tail. [ABSTRACT FROM AUTHOR]

In this study, the time-averaged and instantaneous slipstream velocity, time-averaged pressure, wake flows, and aerodynamic force of a high-speed train (HST) with different nose lengths are compared and analyzed using an improved delayed detached-eddy simulation (IDDES) method. Four train models were selected, with nose lengths of 4, 7, 9, and 12 m. To verify the accuracy of the numerical simulation results, they were compared with wind tunnel test results. The comparison results show that the selection of the numerical simulation method is reasonable. The research results show that with increasing nose length, the peak values of the time-averaged slipstream velocity of the trackside position (3 m from the center of track and 0.2 m from the top of rail) and the platform position (3 m from the center of track and 0.2 m from the top of rail) decrease continuously, and show a trend of rapid reduction at first, and then a slow decrease. As the nose length increased from 4 to 12 m, the time-averaged slipstream velocity at the trackside position and platform position are decreased by 57% and 19.5%, respectively. At a height of 1.6 m from the top of the rail, ΔCPmax (maximum pressure coefficient), |ΔCPmin| (the absolute value of minimum pressure coefficient), and ΔCP (pressure change coefficient) decrease with increasing nose length, which is similar to the peak value of time-averaged slipstream velocity, decreasing rapidly at first and then slowly. As the nose length increased from 4 to 12 m, decreases of ΔCPmax, |ΔCPmin|, and ΔCP by 26.5%, 58.5%, and 44.8% were shown, respectively. Different nose lengths also have a significant impact on wake flow. [ABSTRACT FROM AUTHOR]

In areas with strong wind, windbreaks are built along railways to reduce the impact of wind on trains. However, because of the restrictions imposed by actual terrain, windbreak structures are often not uniform, such as from a cutting to an embankment, resulting in a discontinuous transition region. When a train runs through this region, a distinct yawing phenomenon occurs. This study numerically explored the aerodynamic features of a train running through a rectangular windbreak transition region. The variations in the pressure, side force, and moment of a train were analyzed, and the flow field features were clarified. Furthermore, the yawing motion of the car body with time was described. Finally, based on EN14067-6, the critical wind speed was obtained using the safety assessment of a train running through a windbreak transition region. [ABSTRACT FROM AUTHOR]

In this study, the aerodynamic performance of two trains that intersect inside a tunnel under ambient wind conditions is investigated. A full-scale test is conducted to verify the computational method and mesh, and then a series of numerical simulations are performed to investigate the pressure variation and the aerodynamic force coefficient by comparing with the condition of no ambient air. The difference between the results with/without ambient wind in a tunnel is analysed. The results indicate that if Train A travels downwind, the positive pressure of train surface increases and the negative pressure decreases as the wind velocity increases, and the arrival time of the maximum positive pressure lags on average 0.024 s and 0.058 s for wind velocities of 20 m/s and 40 m/s, respectively. For the tunnel wall measuring points, the maximum positive pressure increases as the wind velocity increases. The maximum drag coefficient of downwind Train A decreases by 4.7% and 10.1% for wind velocities of 20 m/s and 40 m/s, respectively, whereas that of upwind Train B increases by 5.7% and 15.8%, respectively. The maximum positive side force coefficient of downwind Train A increases by 20.8% for an ambient wind velocity of 40 m/s, whereas the same coefficient decreases by 16.7% for upwind Train B. [ABSTRACT FROM AUTHOR]

Based on unsteady simulations and decomposed sliding mesh technology, train aerodynamic and dynamic behaviors near an elongated hillock beside a windbreak under a crosswind were analyzed. First, the sudden deterioration of the aerodynamic performance was explained. Then, a mitigation measure was proposed to reduce the impact of the elongated hillock on the train. Finally, the dynamic responses for the original elongated hillock and mitigation measure were compared. The results showed that two symmetric vortices along the railway were generated by the elongated hillock, and in the position between these two vortices, a strong wind speed occurred, which resulted in a sudden impact on the train, and the aerodynamic impacts were the largest on the head car. A certain distance from the elongated hillock to the windbreak could make a mitigation effect. For the original elongated hillock, there was a severe yawing phenomenon of more than 83 mm/3° from the centerline of the car body for the head car. For the mitigation measure, the safety index and vibration displacement of the head car and tail car decreased effectively. • The train aerodynamic performance and flow deterioration mechanism were studied under the sudden elongated hillock area. • Based on the causes of the deterioration of the flow field, effective mitigation measures were proposed and investigated. • The system dynamic behaviors of the train under the original and the mitigation scheme were analyzed and compared. [ABSTRACT FROM AUTHOR]

• The pressure integral method 15–4 is for the head car and tail car. • The pressure integral method 14–4 is for the middle car. • The error of M x of head car is less than 10% at different yaw angles. • The error of M x of tail car is less than 15% at different yaw angles. The present work studies the aerodynamic loads of a train for different yaw angles from 5.87° to 30° by using the detached-eddy simulation (DES) method. Furthermore, based on the pressure integral method, a reasonable layout of the pressure taps on the train under a crosswind is studied to obtain the aerodynamic forces and moments that are used for a comparison with the results of the wind tunnel test. Considering the lower errors of the pressure integral results and fewer number of pressure taps comprehensively, the pressure integral method in the case of 15–4 (15 represents the tap quantity along the length direction, 4 represents the tap quantity along the height/width direction) on the head car and tail car, the case of 14–4 on the middle car are relatively good among the nine cases discussed in this paper. Compared to the results of the wind tunnel test, for the head car, the maximum error of the side force coefficient (C y), rolling moment coefficient (M x), and yawing moment coefficient (M z) is all less than 10% for yaw angles from 5.87° to 90°, and the maximum errors of lift force coefficient (C z) is less than 20% for yaw angles from 10.2° to 90°; for the middle car, the maximum error of C y is less than 20% for yaw angles from 5.87° to 90°, and the maximum error of C z , M x and M z is all less than 15% for yaw angles from 5.87° to 90°; for the tail car, the maximum error of C y , C z , M x and M z is all less than 15% for yaw angles from 10.2° to 90°. [ABSTRACT FROM AUTHOR]

With increasing train speeds, the wind environment along routes has become complex, variable, and extreme. Constructing windbreak facilities is an effective strategy to enhance train operational safety in windy zones. Various windbreak designs exhibit notable differences in improving train dynamic performance, and economic considerations are also crucial. Utilizing the non-constant compressible Reynolds time-averaged equation URANS (Unsteady Reynold-averaged Navier–Stokes) alongside the shear-stress transport (SST) k-ω turbulence model, this study simulates the non-constant aerodynamic characteristics of a 350 km/h high-speed train traversing through different forms of windbreak corridors in a 30 m/s wind zone. The train's dynamic response was captured using a combined CFD–MBD (Computational Fluid Dynamics and Multi-Body Dynamics) offline time-domain simulation, the accuracy of which was verified experimentally. Results indicate that vortices of various positions and shapes form in the flow field along the windbreak corridor depending on the size of the openings. Using the no-windbreak corridor (Case 1) and the fully enclosed corridor (Case 4) as control groups, it was observed that smaller openings lead to more stable airflow, enhancing peak damping and fluctuation effects, albeit with varying stages of aerodynamic load fluctuation. The 1/3-opening windbreak corridor (Case 3) effectively mitigated the sudden aerodynamic load changes at the wind zone transition of the 2/3-opening windbreak corridor (Case 2), with the primary fluctuation area in Case 3 being the wind section. Dynamic analyses revealed that Case 2 exhibited insufficient lateral aerodynamic performance, raising derailment concerns. In contrast, Case 3 ensured travel safety and comfort effectively, while Case 4 offered excessive protective capacity. This study's findings serve as a valuable reference for designing windbreak corridors and ensuring the safe operation of trains in windy regions. • The internal flow field varies with the opening size of windbreak corridors. • Peak and fluctuating aerodynamic loads depend on opening size of windbreak corridors. • The abrupt change of wind load is the largest in the transition area of windbreak corridor. • The aerodynamic load impact of the head car is the largest. • Lateral aerodynamic performance of maximum opening windbreak corridor is insufficient. [ABSTRACT FROM AUTHOR]

With the continuous improvement of train speeds, aerodynamic braking is a good choice to effectively reduce the braking distance. In this study, unsteady aerodynamic characteristics of vehicles with braking plates inside and outside the tunnel were evaluated by CFD using an improved delayed detached eddy simulation (IDDES). Mesh convergence studies were also performed, and results were compared with two wind tunnel tests to verify the numerical method. By comparing and analyzing the effect of plates on the force, braking distance, and flow field around vehicles inside and outside the tunnel, it is found that the aerodynamic braking force of vehicles inside and outside the tunnel increases nearly 1.5 times and 2.2 times due to the opened plates, respectively. The positive lift force of vehicles is also increased, especially inside the tunnel, which reduces the friction between the wheels and rail tracks. The opened plates also cause the formation of many vortices with excessive energy, which may aggravate the vehicle vibration and noise, especially inside the tunnel. By comparing the impact of the operating environment (inside or outside the tunnel) on the aerodynamic characteristics of each vehicle, it is also found that the closer the vehicle to the downstream, the more sensitive it is to the tunnel. The findings of this study can help to design the braking plate and guide the operation of rail vehicles. • Aerodynamic characteristics of one-fifth scaled uniform vehicle models were studied. • Effect of aerodynamic braking plates on rail vehicles was investigated. • Numerical results were verified by wind tunnel test and grid resolution analysis. • Aerodynamic braking force of vehicles in the open air and tunnel increased. • Downstream vehicles are relatively more sensitive to the tunnel. [ABSTRACT FROM AUTHOR]

Crosswind stability of trains is significantly affected by running safety and pantograph–catenary interaction, which are dependent on the depths of railway cuttings. Under crosswind with wind speed of 30 m/s and yaw angle of 30°, the aerodynamic loads and surrounding flow of the train and pantograph in the cuttings with different depths (0, 3, 6, 9, and 12 m) were investigated using numerical simulations. A dynamic model of the pantograph–catenary interaction was established and employed to analyze the contact force. The head car was the most critical even after accounting for the pantograph under crosswind. The head car was better protected against crosswind as the cutting became deeper. The pantograph–catenary interaction was optimal in the cutting with a depth of 6 m. The time-averaged contact force was excessively large in the shallow cutting, whereas the contact loss occurred in the deep cutting. The depth of the cutting had a different effect on running safety and pantograph–catenary interaction. Hence, a balance between the running safety and pantograph–catenary interaction should be considered in the cutting. According to the fitting results, the appropriate range of cutting depth is 4.6–6.5 m. Overall, these conclusions provide insight into the effect of cutting depths on the crosswind stability of trains and indicate the importance of the pantograph–catenary interaction in the design of railway cuttings. • The effect of railway cutting depths on crosswind stability of trains was studied. • The crosswind stability of the train was assessed by numerical simulations. • The running safety improved with the increase in the depth of the cutting. • A medium depth of the cutting was beneficial for the pantograph-catenary contact. [ABSTRACT FROM AUTHOR]

The present work mainly focuses on the evolution of the airflow passing embankments of different heights, with and without trains. Using the delayed detached-eddy simulation (DDES) method, the effects on the velocity and acceleration of the airflow caused by the embankments' upstream slope were studied using a set of embankments with different heights. The aerodynamic loads experienced by each car of the train, on both the windward and leeward rails, were calculated and the effects of different embankments examined. To distinguish how the geometry of embankments affected the flow patterns, distributions of streamline, velocity and the vortices formed around the head car and embankment are shown. The differences in pressure distribution on central loops of each car caused by the different embankment heights were analyzed at a yaw angle of 19.8°. Using the wind speed at 4 ​m above the actual rails and the reference at 10 ​m above the ground, the effect of wind speed on the aerodynamic forces and moments of the train, independent of the embankments' height, can be obtained. This method has the potential to simplify the calculations needed to evaluate the safety of trains running on different embankment heights in strong wind environments. • Evolutions of the airflow passing embankments of different heights were compared and analyzed. • Variations in the form of the growth laws for different cars above these embankments were found. • A relationship was established between the wind speeds in two different measuring points. [ABSTRACT FROM AUTHOR]

The present work focused on the aerodynamic performance of double unit trains (DUT) and traditional single unit trains (SUT) under crosswinds. The aerodynamic coefficient as well as the characteristics of the time-averaged and instantaneous flow of the SUT and DUT were calculated using the improved delayed detached eddy simulation (IDDES) method for 1/8th scale models. The numerical results were verified by wind tunnel tests. The time-averaged flow pattern on different cross-sections and horizontal planes were compared for the SUT and DUT. The pressure coefficients on the loops of different cars were also analyzed quantitatively. The vortices around the SUT and DUT were visualized, and the frequency characteristics of each car of the SUT and DUT were computed. The coupling region changed the forces on the train and increased the overall drag of the train. Faster vortex development was found on the last three cars of the DUT. Furthermore, a higher velocity belt was generated by the gap of the coupling region. The coupling region gave a significant influence of the power spectrum densities (PSD) on the last four cars and a relatively slight impact on the first two cars. • The aerodynamic forces of double unit trains under crosswind. • Leeward time-averaged flow patterns projected on different y-z planes. • Velocity distribution on horizontal planes of at different heights. • Ringed Pressure coefficients on train surface along the train length. • Different instantaneous characteristics between the SUT and DUT. [ABSTRACT FROM AUTHOR]

• Changepoints and piecewise linearities of pressure on trains were evaluated. • A strategy of discretization based on Lagrange rectangular elements was suggested. • A strategy of discrete integration was presented to measure aerodynamic loads. • The strategies can be used in full-scale tests and model tests. • The strategies can accurately obtain means and fluctuations of aerodynamic loads. Discrete integration occupies an important place in train aerodynamic tests in crosswinds. Improved delayed detached eddy simulations based on shear stress transport k - ω turbulence models were carried out to calculate the side force, lift, rolling moment around the lee rail, and surface pressure on bluff and streamlined vehicles. The changepoints and piecewise linearities of the pressure coefficients were evaluated, and the maximum coefficient of determination in the elements was 0.9973. A realizable strategy of the discretization based on the Lagrange rectangular elements was suggested, including the largest lengths and numbers of the elements. From this, a strategy of the discrete integration was presented to measure the aerodynamic loads, considering the real orientation of the elements. The maximum errors of the mean aerodynamic load coefficients of the bluff and streamlined vehicles were 4.1% and 2.2% (except the mean lift coefficient of the bluff vehicle), respectively. The errors were less than those in the previous studies, especially for the streamlined vehicle, which reduced by up to 8.7%. The unsteady aerodynamic loads with no delay obtained by the strategies were near to natural ones in the frequency range that people would be concerned about in crosswinds (at the Strouhal number of less than 0.4). Some suggestions were made for using the strategies in the full-scale tests and model tests, which provided a foundation for further studies of the running safety in natural crosswinds. [ABSTRACT FROM AUTHOR]

The effect of windbreak wall types on their windproof effects on trains has been systematically investigated by comparing the flow field around a high-speed train behind three typical windbreak walls. These three typical windbreak walls are widely used along the windy railway lines in China, which include the earth embankment–type windbreak wall (EW), the road cutting–type windbreak wall (RW), and the straight reinforced concrete–type windbreak wall (SW). We compared the time-averaged crosswind-induced flows and aerodynamic performances of high-speed trains behind these three windbreak walls with the same height using numerical simulations. The results revealed that the windbreak wall geometric shape effects on the aerodynamic load coefficients varied according to the type of aerodynamic load and carriage marshalling positions but barely varied with yaw angles. The total drag coefficients of the train in the EW and RW were approximately 50–60% of that in the SW under the two smaller yaw angles and were only 30–40% for the largest yaw angle. For the absolute value of the side force and rolling moments coefficients, the maximum values for the head and middle cars both appeared in the RW, and the corresponding minimum values were obtained in the SW. The maximum and minimum values for the tail car were obtained at the SW and EW, respectively. The maximum of the rolling moment coefficient among three carriages in the SW was approximately 45–60% of that in the EW and only 30–40% of that in the RW. The time-averaged train surface pressure coefficient and flow patterns were similar between the EW and RW, which showed apparent differences from those in the SW. The SW provided a strong blocking effect on the incoming windward airflow and avoided the direct impact on the train. Compared with the RW, the uplifting effect of the EW's windward slope on the incoming flow further reduced the crosswind effect on trains. In addition, the dominating frequency characteristics of the aerodynamic loads were significantly affected by windbreak wall types. These findings provide a systematic understanding of the time-averaged aerodynamics of trains behind these three typical windbreak walls. • Simulations of crosswind-induced flows around high-speed trains behind three typical windbreak walls were conducted. • Time-averaged flow characteristics were analyzed for three different windbreak walls. • Train aerodynamic performances behind three typical windbreak walls were compared in detail. • The mechanism of windproof performance differences between three typical windbreak walls was systematically explored. [ABSTRACT FROM AUTHOR]

Wind tunnel tests on rail vehicles, especially the crosswind evaluation, are often used for the determination of critical wind characteristics. To increase the reliability of the wind tunnels for these requirements, correlation data between various wind tunnels is necessary. Along those lines, a wind tunnel campaign on a 1/20th high-speed train was conducted at three separated facilities, including the study of two floor configurations within the same wind tunnel. A symmetry check was made by monitoring pressure signals at both sides of the vehicle, and the load coefficients that fall within negative and positive yaw angles. The results show good consistency with aerodynamic load coefficients measured at separated wind tunnels. The standard deviation of C M x,lee increases with the yaw angle exponentially. Pressure taps for geometric transitions and suction peaks tend to have a larger level of symmetry discrepancies than other taps. Pressure distributions for longitudinal centerline and loops in all facilities have a similar trend, while the dispersion degree slightly differs in various regions. These differences are partly explainable by the experimental settings, i.e., the boundary layer development above the floor, the offset installation of the model, and the floor configurations. Therefore, special attention is suggested to be paid to the floor configurations and boundary layers above the floor when performing related experimental works. • The aerodynamics of the same train model is tested in multiple wind tunnels. • The load coefficients of the vehicle measured in different facilities show a good correlation. • The symmetry analysis of aerodynamic load and pressure distribution is presented. • Pressure discrepancies between multiple wind tunnels are revealed and potential reasons are analyzed. [ABSTRACT FROM AUTHOR]

High-speed railway lines are widely distributed worldwide. The surrounding wind environment of such railway lines is complex, which significantly affects the safety of high-speed train operations. Using a combination of the computational fluid dynamics method of improved delayed detached eddy simulation and sliding grid technology, the effects of one- and two-side windbreak walls on the unsteady aerodynamic performance of a high-speed train running under a crosswind on a double-track railway line with an embankment and the flow-field characteristics around the train were systematically studied. The grid independence and numerical methods used in this study were verified. The results showed that when the windbreak wall was absent, the aerodynamic performance of the train running on the upstream railway line was the poorest, while the train running on the downstream railway line exhibited large aerodynamic fluctuations. The windbreak wall significantly reduced the aerodynamic force and its fluctuation; the one- and two-side windbreak walls had different effects on the unsteady aerodynamic performance of trains running on the upstream and downstream railway lines under a crosswind. The one-side windbreak wall could restrain the aerodynamic fluctuation of the train more and is thus preferred from the perspective of construction effort and cost. • Effects of one- and two-side windbreak walls on the unsteady aerodynamic performance of a moving train under a crosswind were studied. • Aerodynamic performance of the train running on the upstream railway line was the poorest. • One-side windbreak wall restrain the aerodynamic fluctuation more and is preferred from the perspective of effect and cost. [ABSTRACT FROM AUTHOR]

• Full-scale tests were carried out on a fast streamlined train in crosswinds. • Aerodynamic loads were obtained by discrete integration of pressure differences. • Means and fluctuations of aerodynamic loads were obtained by data processing. • Fluctuations of aerodynamic coefficients are due to oscillations of wind speed. • Dominant frequencies of aerodynamic coefficients are at Strouhal numbers of 0–0.4. Unsteady aerodynamic loads have a significant influence on the running safety of a train in crosswinds. Full-scale tests were carried out to measure the pressure on a fast streamlined train in crosswinds, and the unsteady aerodynamic loads were obtained by the discrete integration of the pressure to assess the influence of different crosswinds on the loads. The maximum wind speed was 34.0 m/s in the full-scale tests, and the maximum turbulence intensity was 0.202. The means and fluctuations of the side force coefficient, lift coefficient, and rolling moment coefficient around the lee rail at different yaw angles were studied. The means first increased and then decreased with the increase in the yaw angle, and the maximum values occurred in the range of 54.5°–59.6°. The coefficients of variation of the aerodynamic load coefficients were approximately twice the turbulence intensity, which indicated that the fluctuations of the aerodynamic load coefficients were due to the oscillations of the wind speed. For the frequency characteristics of the aerodynamic load coefficients, the most dominant frequencies were at Strouhal numbers of 0–0.4. [ABSTRACT FROM AUTHOR]

• Trainset pressure on train surface is cut off by the tunnel gap. • Tunnel gap has significant influence on the slipstreams around the second tunnel entrance. • Aerodynamic force and moment coefficients nearly match for the first and second tunnel. • The aerodynamic issue of eliminating tunnel gap depends on their own critical tunnel lengths. This investigation deals with numerical solution of complex aerodynamic interactions induced by a high-speed train (HST) entering and passing through tunnels with a closely adjacent gap and without in between. We present evidences that included pressure transient on the train surface and tunnel wall and comparisons between slipstreams and aerodynamic loads. The results show that two closely adjacent tunnels with a gap does not induce pressure differences on the tunnel wall, in relation to pressures otherwise observed on the single long tunnel, but slightly affects the friction component of the initial pressure rise on the train surface. In addition, the peak values of the slipstreams on the side nearest to the train inside the second tunnel are larger than those recorded when the train passed the first tunnel, suggesting that the existence of a gap between tunnels affect the airflow within the second tunnel. Peak drag coefficients for the train travelling through the first and second tunnel match while being 9.8% lower than the corresponding value observed within a single yet longer tunnel. The study highlights the differences on train aerodynamics when in transit through closely adjacent tunnels and a long tunnel to conclude that improved aerodynamic performances should be considered as the train circulates through closely adjacent tunnels as opposed to through a single passage. The study provides an aerodynamic guideline and reference for the tunnel engineering. [ABSTRACT FROM AUTHOR]

Reasonable gap spacings between adjacent train vehicle models are of great significance for the data accuracy during a wind tunnel test. In this research, crosswind flows around 1/8th-scale high-speed train models with inter-carriage gap spacings of 0, 5, 8, 10, and 20 mm are studied using improved delayed detached eddy simulation (IDDES) with the SST (shear-stress transport) k-omega model. The numerical methodology is first validated against the wind tunnel experiment data. Then, the effects of gap spacings on aerodynamic loads and the flow field around the train are investigated. The results indicate that the side force of the tail car significantly increases with the gap spacing, in which the maximum difference occurs at 20 mm compared with the zero gap spacing, whereas the difference in the drag decreases. When the gap spacing is greater than 10 mm, the time-averaged pressure distribution, flow pattern and transient flow structures around the inter-carriage gap regions observe a significant difference from other gap spacings. Considering the overall aerodynamic effects induced by gap spacings and the practical handleability during a wind tunnel test, a gap spacing between 5 mm and 10 mm is therefore recommended for adjacent train vehicles with a 1/8th scale. • Time-averaged quantities of train vehicle models with different gap spacings were compared. • Flow patterns and structures around different gap spacings were investigated. • PSD of aerodynamic loads on train vehicles with different gap spacings was analyzed. • Reasonable gap spacings between adjacent train vehicle models were recommended. [ABSTRACT FROM AUTHOR]