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The influence of different ground simulation systems on the air flow around a high-speed train with zero yaw angle is investigated. Force values, force development graphs, surface pressures, the underbody flow and the wake are studied in detail with Computational Fluid Dynamics, which is initially validated by wind tunnel testing. It shows that the stationary ground has severe deviations from the full moving ground on the aerodynamic performance due to the inaccurate pressure distribution on the underbody. This is mainly attributed to the high level of interaction between the underbody and the boundary layer development. In addition, a ground boundary layer separation bubble can be observed under the tail end of the train for the stationary ground on account of insufficient energy to overcome the increasing adverse pressure gradient. In order to guarantee a correct underbody flow, a partially moving ground is proposed, including the ''3-moving ground'' and the ''1-moving ground''. Such ground simulation systems are well compatible with the fixed rail tracks and the bottom support struts compared to the full moving ground. As a conceivable method to reduce the influence of the boundary layer, raising the high-speed train model with different ground clearances is also studied. Overall, the 3-moving ground is suggested to be the best choice for the ground simulation systems in high speed train wind tunnel testing.

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In this work, the impact of the isentropic and Kantrowitz limits on the aerodynamic behavior of evacuated tube transportation (ETT) was numerically explored. Two tube train systems with different blockage ratios ( β), that is, β = 0.09 and β = 0.25, were employed for the comparative study of aerodynamic drag and flow structure. The results revealed three distinct aerodynamic behaviors, corresponding to the three speed regions separated by the two critical Mach numbers. Furthermore, the influence of head and tail lengths on drag reduction was investigated in these three speed ranges. An increase in head length appeared to be more sensitive to drag reduction at a speed of 600 m/s, while a long tail was found to induce a pronounced drag reduction at 200 m/s. In addition, the combined effect of the head and tail lengths on drag reduction was close to the superposition of their individual optimization effect. Based on the results, this study concludes that the individual designs of the head and tail of ETT systems may be rather demanding to achieve the desired optimization when considering distinct cruising speeds.

The influence of ground effect on the wake of a high-speed train (HST) is investigated by an improved delayed detached-eddy simulation. Aerodynamic forces, the time-averaged and instantaneous flow structure of the wake are explored for both the stationary ground and the moving ground. It shows that the lift force of the trailing car is overestimated, and the fluctuation of the lift and side force is much greater under the stationary ground, especially for the side force. The coexistence of multiscale vortex structures can be observed in the wake along with vortex stretching and pairing. Furthermore, the out-of-phase vortex shedding and oscillation of the longitudinal vortex pair in the wake are identified for both ground configurations. However, the dominant Strouhal number of the vortex shedding for the stationary and moving ground is 0.196 and 0.111, respectively, due to the different vorticity accumulation beneath the train. A conceptual model is proposed to interpret the mechanism of the interaction between the longitudinal vortex pair and the ground. Under the stationary ground, the vortex pair embedded in a turbulent boundary layer causes more rapid diffusion of the vorticity, leading to more intensive oscillation of the longitudinal vortex pair.