Dynamic transition of flow field for accelerating and decelerating process of evacuated tube train

The high-speed maglev train in the tube transportation system will induce supersonic flow at subsonic traveling speed. Within this process, the dynamic transition of the flow field has a significant impact on the aerodynamic load of the vehicle. This research focuses on the flow field transition characteristic with respect to the tube train's accelerating and decelerating process. The quasi-steady wind tunnel simulation method with stepwise acceleration, as well as the transient overset mesh simulation method with constant acceleration and deceleration, has both been used and compared in this study. Based on the results derived from the stepwise acceleration approach, during the accelerating stage, the dissipation of choked flow follows the critical condition given by the Kantrowitz limit, while for the decelerating stage, the occurrence of choked flow follows the critical condition given by the isentropic limit. On the other hand, for a vehicle accelerating and decelerating at a constant rate, a hysteresis between the flow field and vehicle speed for the vehicle's acceleration and deceleration process can be observed. It is found that, for vehicles with a constant decelerating rate, the absolute airflow velocity ahead of the vehicle will decrease and even reverse direction for a certain period of time, which leads to a situation when the aerodynamic drag of the vehicle's streamlined head is aligned with the vehicle's moving direction. It is also discovered that, as the vehicle decelerates from a supersonic state without choked flow to a transonic state with choked flow, under the interaction between the oblique shock wave and the expansion wave, the aerodynamic load in the streamlined tail will first decrease and then increase.