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Under strong winds, the effect of sudden windbreak transition (WT) on high-speed trains is severe, leading to a deterioration of train aerodynamics and sudden yawing motion of the car body. To address these problems, based on a high-speed train and the specific geometric conditions derived from Xinjiang railway, first, the impact of a WT on the train and reasons for sudden changes in aerodynamic forces were determined by flow structural analysis. Furthermore, based on a multibody system dynamic model, the dynamic responses to WT were analysed. The results show that the impacts of WT were the strongest on the head car. WT had a strong effect on the train due to the unreasonable structural shape and the insufficient height of the windbreak in the transition region. This led to a strong push effect on the train; subsequently, the train's dynamic characteristics deteriorated. [ABSTRACT FROM AUTHOR]

The use of double-unit trains is becoming a common means of increasing passenger capacity for a rail network. However, their expanded usage may create additional aerodynamic challenges. The present work obtains the characteristics of the slipstream caused by single and double unit trains using the detached eddy simulation (DES) method for 1/20th scaled models. The numerical results are verified by full-scale experiments. The slipstream velocities and pressures obtained by the two train models at different distances from the center of track (COT) and the top of rail (TOR) are compared. The coupling structure of the double-unit train model is found to produce a velocity peak that is much greater than that of the single-unit train model in the same position. At the area away from the COT and close to the TOR, the velocity of the far wake region is larger for the double-unit train model. The coupling structure also leads to a positive pressure change in the coupling region, and its value is comparable to or even much greater than that caused by the nose. It is considerable that the subsequent pressure criteria could take the positive pressure on the coupling region into account for the double-unit train. [ABSTRACT FROM AUTHOR]

As a high-speed train passes through a tunnel, the intense aerodynamic pressure wave outside the train penetrates into the passenger compartments, changing the pressure environment inside the train and causing discomfort to passengers. This phenomenon varies enormously among tunnels of different lengths. To evaluate the real influence of tunnel length on the interior pressure environment of the carriages and ensure passenger comfort, field measurements were conducted on a CRH2A train operating on the Hefei-Wuhan rail line in China; twelve tunnels with lengths ranging from 72 to 10766 m were investigated. Pressure sensors were deployed along the outside and inside train surfaces to investigate the variations in interior pressure with the exterior pressure environment. The results show that the interior pressure curve over time in a short tunnel has a single-trough form, very different from that in a long tunnel. The total variation of interior pressure increases monotonically with tunnel length from 72 to 10766 m, whereas the maximum pressure variation within a 3 s period occurs in the 1080 m tunnel; the variation within a 1 s period is not significantly affected by the tunnel length for tunnel lengths greater than 556 m. There is a delay in the 1 s interior pressure variation in response to an exterior pressure change; this delay is independent of the tunnel length and is approximately 1.2 s in this test. It is found that the reason for passengers’ ear discomfort in very long tunnels, which is mainly an effect of excessive changes in pressure over a long period of time, could be different from that in long or short tunnels. [ABSTRACT FROM AUTHOR]

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

In order to determine the effect of the coupling region on train aerodynamic performance, and how the coupling region affects aerodynamic performance of the couple multiple units trains when they both run and pass each other in open air, the entrance of two such trains into a tunnel and their passing each other in the tunnel was simulated in Fluent 14.0. The numerical algorithm employed in this study was verified by the data of scaled and full-scale train tests, and the difference lies within an acceptable range. The results demonstrate that the distribution of aerodynamic forces on the train cars is altered by the coupling region; however, the coupling region has marginal effect on the drag and lateral force on the whole train under crosswind, and the lateral force on the train cars is more sensitive to couple multiple units compared to the other two force coefficients. It is also determined that the component of the coupling region increases the fluctuation of aerodynamic coefficients for each train car under crosswind. Affected by the coupling region, a positive pressure pulse was introduced in the alternating pressure produced by trains passing by each other in the open air, and the amplitude of the alternating pressure was decreased by the coupling region. The amplitude of the alternating pressure on the train or on the tunnel was significantly decreased by the coupling region of the train. This phenomenon did not alter the distribution law of pressure on the train and tunnel; moreover, the effect of the coupling region on trains passing by each other in the tunnel is stronger than that on a single train passing through the tunnel. [ABSTRACT FROM PUBLISHER]

With an increase in train speed, the aerodynamic effects caused by the train could escalate, especially for a train running in a tunnel. A number of large transient pressure waves are generated owing to the confined spaces within the tunnel, resulting in possible damage to the vehicle structure and the facilities in the tunnel. Therefore, it is necessary to study the aerodynamic performance of a train running in a tunnel. A scaled moving model test system was constructed to facilitate the simulation of the aerodynamic effects caused by a train running in a tunnel. In this study, the influence of grid density, calculation time step, and turbulence model on the pressure caused by the train entering the tunnel was analyzed, which is helpful for choosing suitable values of the aforementioned parameters to simulate the aerodynamic performance of the train in the tunnel. The impacts of Reynolds number effect on the distribution of the surface pressure and peak of pressure wave along the train, and the pressure waveform were also studied through numerical simulation on three scaled model trains (full scale, 1/8, 1/20, and 1/32 scaled). The findings aid in understanding the relationship between the Reynolds and pressure amplitude, and the results of the scaled test can be applied to a full-scale train. [ABSTRACT FROM AUTHOR]