What is it about?
The investigation of aerodynamic characteristics in high-speed trains presents substantial challenges, especially as increasing operational speeds exacerbate both aerodynamic drag and noise. Aerodynamic noise scales approximately with the sixth power of velocity, while aerodynamic drag increases quadratically, together imposing critical constraints on the design and operation of high-speed trains. These issues primarily arise from the complexity of three-dimensional flow structures and the presence of multi-scale interactions. A significant knowledge gap persists in understanding the characteristics of turbulent boundary layers and the distribution of skin friction drag along train surfaces, particularly under high-speed conditions and in multi-unit train configurations. This study aims to characterize the three-dimensional turbulent boundary layer structures and quantify their spatial distribution around multi-unit high-speed trains, offering fundamental insights into aerodynamic behavior at operational speeds of 400 km/h.
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Why is it important?
The unique operating environment of high-speed trains—characterized by close-to-ground motion and multi-unit configurations—creates aerodynamic conditions that are fundamentally distinct from those of aircraft or road vehicles. These conditions give rise to complex flow interactions influenced by factors such as the ground effect, train geometry, and inter-carriage gaps. Consequently, studying boundary layers along train surfaces presents significant challenges, primarily due to the highly three-dimensional, unsteady, and high-Reynolds-number nature of the flow. While current aerodynamic optimization efforts mainly focus on controlling large-scale flow separation to reduce pressure drag, methodologies for optimizing skin friction drag remain underdeveloped. Therefore, quantifying the friction drag contribution of various structural components, analyzing surface friction drag distribution, and investigating the spatial variation of boundary layer parameters are essential for advancing friction drag reduction technologies in future train designs.
Perspectives
This study reveals the distribution characteristics of the skin friction drag coefficient on multi-unit high-speed trains at 400 km/h, provides detailed statistics on the frictional drag of different components, and analyzes the spatial distribution features and instability mechanisms of the boundary layer on various train surfaces. It also investigates the momentum transport and the generation of turbulent kinetic energy within the near-wall region at different vertical heights by conducting quadrant analysis of Reynolds shear stress, which enables a deeper understanding of the flow characteristics. Despite the valuable insights provided, friction drag reduction techniques for high-speed trains remain in the early stages of development, with many challenging difficulties still to be overcome. This will undoubtedly be a massive system engineering project, and I hope that an increasing number of researchers will focus on this field in the future, contributing to its advancement.
Shishang Zhang
Central South University
Read the Original
This page is a summary of: Numerical analysis of turbulent boundary layer characteristics of multi-unit high-speed trains at 400 km/h, Physics of Fluids, April 2025, American Institute of Physics,
DOI: 10.1063/5.0269334.
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