Regulation mechanism of underbody flow field and aerodynamic drag reduction for high-speed trains via mixed-gas jets

Abstract

The bogie region is the primary source of aerodynamic resistance for high-speed trains, making it a key target for flow control strategies aimed at drag reduction. Numerical simulations were performed to explore the flow regulation mechanism and associated drag reduction effect of underbody mixed-gas jets with varying helium mass fractions (HMF) and velocities (Vjet). Results indicate that applying air jet in front of the leading bogie with increasing velocities leads to an intensified downward deflection of the underbody flow, which suppresses the initial flow separation at the front lower edge of cavity and the re-separation at the leading edge of bogie wheels. The entrainment effect induced by the descending free shear flow weakens flow reattachment on the cavity rear end and suppresses the formation of vortex structures. Since the effect of velocity on jet momentum is more pronounced than that of gas density, an increase in HMF exerts varying degrees of influence on the regulation of the underbody flow field across different velocity levels. Under a high jet velocity, the momentum reduction induced by an increased HMF imposes only a marginal degradation on the jet's ability to regulate flow separation and vortices, while concurrently inhibiting the cavity rear-face flow reattachment that is typically observed in low-velocity jets with high HMF. The pure helium jet at Vjet/U = 0.5 achieves a maximum drag reduction rate of 10.3% along with optimal net energy-saving performance. This study provides fundamental physical insights for underbody aerodynamic control of high-speed trains.