Evaluation of CFD turbulence models on the aerodynamics of a regional train

The energetic performance of railway vehicles is becoming increasingly important, including for low-speed trains operating on regional and inter-city routes. Aerodynamic drag significantly affects train power consumption, making its accurate estimation essential for the design of energy-efficient rolling stock. Numerical simulations are widely used to estimate aerodynamic resistance; however, simplified fluid-dynamics approaches may lead to inaccurate predictions, particularly for the complex geometries typical of regional trains. This paper compares two numerical methods — Reynolds-Averaged Navier–Stokes (RANS), widely adopted in industry, and the state-of-the-art Improved Delayed Detached-Eddy Simulation (IDDES) — against wind tunnel experiments. Two train geometries are analysed: a conventional regional-train shape and a modified, more streamlined configuration. The analysis focuses on the global aerodynamic drag predicted by the numerical approaches and compares it with experimental measurements. Both methods show satisfactory agreement with the experiments, although noticeable differences emerge. Significant discrepancies are observed in highly turbulent regions, such as the tail, roof, and bogie areas, with IDDES consistently predicting higher drag levels than the RANS model. This work provides new insights into the aerodynamics of regional trains, a vehicle category that has received limited attention so far, and offers results of practical relevance to the railway industry.