What is it about?

High-speed trains use a lot of energy to overcome air resistance. A major part of that resistance comes from the very thin layer of air next to the train surface. As the train moves, this layer becomes turbulent—full of small, fast fluctuations—which increases “rubbing” forces on the body. In this work, we ran detailed 3D computer simulations of a realistic multi-unit high-speed train (several cars coupled together) at different incoming flow speeds. Our goal was to understand how near-surface airflow changes with speed and where the largest losses come from. We found that the share of different drag sources is fairly stable across speeds: skin-friction drag accounts for about 40% of the total. But the amount of drag rises sharply as speed increases. From 300 to 400 km/h, the friction-related part alone increases by more than 70%, helping explain why power demand grows so rapidly at very high speed. The overall friction pattern along the train stays similar, yet the point where the roof flow turns turbulent moves farther downstream at higher speeds. The roof also shows clear low-friction streaks and swirling motions caused by the streamlined nose and curved shoulder regions. At lower speeds, pressure changes along the roof tend to amplify disturbances and spread them more widely across the roof. At higher speeds, the strongest turbulent “bursts” occur closer to the wall. Finally, by rescaling key quantities into a common, comparable form, we provide simple trends that link speed and position along the train. These results can guide aerodynamic design to reduce energy use and improve performance.

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Why is it important?

This study illustrates how inflow velocity influences turbulent boundary layer behavior over HST surfaces, providing an important theoretical basis for boundary-layer modeling and the development of friction-reduction technologies for HSTs.

Perspectives

This paper is part of a research thread I have been building around high-speed-train boundary layers. Rather than treating the boundary layer as a “black box” that simply produces drag, I see it as a structured, evolving flow system that can be described, compared, and eventually engineered. In that sense, this study is an extension of my earlier work: it pushes from observing surface quantities toward establishing a more coherent framework that links train geometry, streamwise position, and operating speed to the underlying turbulence dynamics. What motivates me is the practical reality that high-speed rail is already widely deployed and continues to expand. Even small aerodynamic improvements can translate into large energy savings at fleet scale. Yet the most stubborn part of the problem is skin-friction drag: unlike pressure drag, it is difficult to reduce without creating new penalties (noise, stability, maintenance, or performance under real operating conditions). That difficulty is exactly why I think careful boundary-layer research matters. If we want credible friction-reduction strategies, we first need reliable “rules of the game”—where transition happens, how near-wall events respond to speed, and which flow structures dominate on the roof and nose–shoulder regions. Personally, I view the normalization and trend-building in this work as a step toward that goal. My hope is that these results make boundary-layer behavior on multi-unit trains more predictable and comparable across speeds, and that they can serve as a foundation for future design tools—whether for shaping, passive devices, or surface treatments—grounded in flow physics rather than trial-and-error.

Shishang Zhang
Central South University

Read the Original

This page is a summary of: Velocity-dependent turbulent boundary layer dynamics on multi-unit high-speed trains: Instability analysis and normalized characterization, Physics of Fluids, December 2025, American Institute of Physics,
DOI: 10.1063/5.0304203.
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