This paper uses an improved delayed detached eddy simulation method to investigate the unsteady flow features of the high-speed trains with various cavity lengths at Re = 1.85×106. The improved delayed detached eddy simulation results are validated against the experimental data obtained during previous wind tunnel tests. The effects of cavity length on the resistance force, flow structures beneath the high-speed train and in the wake are analyzed. The results show that a longer cavity significantly increases the streamwise velocity level near the rear plates and forms a stronger impinging flow on the rear plates, and thus contributes to a higher value of resistance. Furthermore, a longer cavity decreases the pressure coefficients around the near wake region from the top of the ballast to the tail nose in the vertical direction and thereby increases the pressure drag of the high-speed train. Additionally, a longer bogie cavity is found to increase the longitudinal vortex scales in the near wake region. All these changes on the flow field bring to 5.8% and 11.5% drag increase when the bogie cavities are elongated by 20% and 40%, respectively, of the wheelbase.
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