Mechanism of aerodynamic force generation related to train vibration in tunnels (Translated) (LES of large-scale flow structures around a simplified train model)

To clarify the mechanisms underlying unsteady aerodynamic forces related to train vibrations of high-speed trains running through tunnels, large-eddy simulations of large-scale flow structures around a simplified six-car train model were conducted. Since actual trains run on one of the double track lines, the position of the train model was set to deviate from the tunnel center and hence the gap between one side of the train and the tunnel wall is narrower than that on the opposite side. A train running in the open-air was also calculated for comparison. The results of this study shed light on the generation mechanism of the pressure fluctuations acting on the side of high-speed trains as follows. Firstly, in the open-air, the air velocity in the space between the underbody and the ground gradually decreases from the head toward the tail of the train. Thus, the air velocity is slower than that on both sides of the train, which generates shear flows near the bottom edges of both sides of the train. The shear flows cause large Kármán vortex-like structures forming a staggered vortex street, which in turn lead to a meandering airflow beneath the underbody of the train. Secondly, in the tunnel, the air velocity not only in the gap between the underbody and the ground but also in the narrower gap between the side of the train and the tunnel wall gradually decreases from the head toward the tail of the train. In the same mechanism as the open-air, a meandering airflow is generated throughout the side and underbody of the train and causes pressure fluctuations along the side of the train. Finally, the present LES results indicate that the wavelength of pressure fluctuations along the side of the actual train can be reasonably estimated.