Numerical analysis of turbulent boundary layer characteristics of multi-unit high-speed trains at 400 km/h

The surface boundary layer characteristics of an eight-car high-speed train at 400 km/h is investigated using improved delayed detached eddy simulation. This investigation aims to characterize the three-dimensional turbulent boundary layer structures and quantify their spatial distribution around multi-unit high-speed trains, providing fundamental insight into the aerodynamic features at operational speeds of 400 km/h. The numerical results indicate that at an operational velocity of 400 km/h, friction drag accounts for 40.39% of the total aerodynamic drag, with the train body being the dominant contributor. Specifically, the lateral surfaces constitute 24.11% of the total drag, significantly exceeding the contributions from both top and bottom surfaces. The evolution of the boundary layer manifests pronounced three-dimensional features, which starkly diverge from the postulates of classical flat-plate theory, particularly evidenced by the distinctive “inverted gourd” profile exhibited by the lateral surface boundary layer. Flow field analysis reveals complex vortical structures, where unstable separation vortices from the underbody region near bogies and deflectors propagate along the lateral surface at a vertical angle of 3.31° ± 0.2°, generating alternating shear stress bands that correlate strongly with vertical height variations. The streamwise vortices within the track-train gap diffuse outward along the train surface while transporting low-momentum fluid upward, influencing boundary layer stability and transition characteristics across varying vertical positions.