Effects of asymmetric terrains near tunnel entrances on aerodynamic characteristics of trains

Asymmetric terrains near tunnel entrances may affect stability of railway vehicles while entering tunnels. Numerical simulations with three-dimensional, unsteady, and compressible flow are conducted on aerodynamic characteristics of trains traveling through a tunnel with asymmetric mountains with different slope angles near its entrance, including the non-mountain (flat ground), 45°-mountain, and 90°-mountain. The aerodynamic loads (the side force, rolling moment, and yawing moment) and surface pressure of the vehicles, as well as the flow around them are analyzed. A comparison between the surface pressures of the vehicle 2 in the numerical simulation and full-scale tests showed a 2.2% difference in peak-to-peak values during tunnel entry, validating the simulation results. The aerodynamic loads of the trailing vehicle are more sensitive to the slope angle of the mountain near the tunnel entrance, compared with the leading vehicle. The aerodynamic loads increase with the increase in the slope angle. The effect of the slope angle on the rolling moment is the most remarkable, and the rolling moment of the trailing vehicle with the 90°-mountain is 256.6% larger than that without a mountain. A trend in the flow toward the side with the flat ground and the limited space around the side with the mountain cause and increase the instability of the trailing vehicle when entering the tunnel. These findings provide valuable insights for designing and optimizing tunnel entrances and enhancing the stability of trains navigating asymmetric terrains.