Effects of Reynolds number on train aerodynamics considering the air compressibility: A wind tunnel study

The study on Reynolds number (Re) effects is crucial for high-speed trains for optimizing the aerodynamics, enhance safety, and reduce energy consumption. In this paper, a wind tunnel test was carried out to investigate the influence of Re on the train aerodynamic performance considering the shift of the air compressibility, which has not yet been explored to date. The test was conducted in a low-speed & large-scale tunnel with a stationary floor and the vehicle model was based on a 1/8th scaled train with 3 units. The Re ranges from Re=0.75×106∼ Re=3.12×106 by accelerating the uniform wind speed from U=27.8 m/s to U=115 m/s with zero-yaw. The Mach number (Ma) of the maximum speed scenario has exceeded 0.3, indicating that the airflow can be considered as compressible range. The results show that the aerodynamic characteristics of high-speed trains exhibit a self-similarity region of the Re, which is dependent on the flow velocity. The aerodynamic loads little changes when the Re ≥ 1.51×106, which corresponds U ≥55.6 m/s. Therefore, the compressibility of the airflow within the range up to U=115 m/s has a negligible effect on aerodynamic loads, thus can be disregarded. However, the surface pressure significantly decreases when the incoming flow surpasses 0.3Ma and transitions into a compressible state. While the compressibility has a relatively minor impact on macroscopic aerodynamics, it cannot be overlooked when considering detailed flow field, such as surface pressure.