Effects of air blowing in the constant cross section region on skin friction drag and aerodynamic performance of high-speed trains

Skin friction drag is a major component of aerodynamic drag in high-speed trains, but remains less explored than pressure-drag reduction. In this study, the aerodynamic and energetic effects of air blowing applied in the constant cross section region of a high-speed train are systematically investigated using validated, improved delayed detached eddy simulations. Three blowing velocities (0.1, 0.2, and 0.3 U, where U = 60 m/s) are examined. The results show that air blowing primarily reduces skin friction drag, with limited influence on pressure drag (maximum variation < 2.10%). At 0.3 U, the time-averaged skin friction drag coefficients of the head, middle, and tail cars decrease by 15.66%, 32.66%, and 22.98%, respectively, leading to an overall drag reduction of 10.05% for the three-car configuration. Quadrant analysis at y+ = 30 indicates that the drag reduction is associated with the suppression of near-wall ejection events, resulting in weakened wall shear stress. The dimensionless power-saving efficiency (η) increases with system efficiency (ηsys), with greater sensitivity at higher blowing velocities. At a blowing velocity of 0.3 U, the η reaches 0.085 when ηsys = 1.0. Moreover, the peak of the mean slipstream velocity coefficient at both trackside and platform positions does not increase, indicating that slipstream-related safety risks are not exacerbated. Overall, this study provides a physics-based guideline for the deployment of active flow control strategies on next-generation high-speed trains.