Experimental and numerical analysis of aerodynamic drag reduction of high-speed train: impact of nose, bogie fairing and roof apparatus variations

Assessing aerodynamic modifications in high-speed trains requires quantifying drag reduction relative to previous designs and evaluating the improvement potential of an integrated configuration. This study investigated three-car configurations of the KTX-Cheongryong and HSEMU-370 platforms using 4.3%-scale wind tunnel tests and unsteady Reynolds-averaged Navier–Stokes simulations to characterize the aerodynamic effects of variations in the nose, bogie fairing, and roof apparatus. The baseline HSEMU-370 configuration was compared with bogie fairing and roof apparatus variants to assess its drag reduction potential. The numerical predictions agreed with the measured drag coefficients within 2.29% and showed that the nose was the dominant drag source, and that the streamlined HSEMU-370 nose alleviated both nose pressure loading and the downstream flow disturbances extending into the first bogie region. In the underbody region, the half-covered fairing of the baseline HSEMU-370 did not consistently outperform the open KTX-Cheongryong arrangement, whereas the full underbody and side coverage produced the largest reduction by suppressing cavity-induced losses. In the roof region, the streamlined low-profile roof apparatus attenuated roof-side pressure disturbances and downstream boundary layer thickening compared to the protruding configuration, while full integration yielded an additional reduction in roof-induced drag. Compared with KTX-Cheongryong, the HSEMU-370 achieved a 12.49% reduction in the maximum cumulative drag coefficient, which increased to 14.88% and 16.52% with additional underbody and roof-side modifications, respectively. These findings demonstrate that drag reduction is governed by the flow mechanisms of nose pressure loading, cavity flow behavior, and roof-side disturbance, and that further reductions can be achieved through coordinated modifications. This study highlights the importance of an integrated aerodynamic design considering the coupled effects of external components in high-speed train development.