Coupled 2D–3D CFD approach for innovative simulation of train slipstream in tunnels

The aerodynamic slipstream generated by high-speed trains in tunnels results from the combined effects of the piston wind generated before the train arrival and the local near-train flow. Traditional moving-mesh CFD simulations can reproduce these phenomena with fidelity but at a considerably high computational cost, limiting their use in large parametric or design studies. This work introduces and validates a novel CFD framework that decouples the train–tunnel aerodynamics into two components: a two-dimensional moving-mesh simulation for the piston-wind evolution and a three-dimensional steady-domain simulation with fixed mesh for the near-field slipstream. The methodology is validated against full-scale and reduced-scale reference cases, showing good agreement with experimental data and fully unsteady simulations. The framework also proves its applicability through representative case studies with different tunnel blockages. The proposed approach achieves substantial computational savings, up to two orders of magnitude for the piston-wind estimation and nearly threefold for the near-field simulation, while maintaining physical consistency with the confined flow. It therefore offers an efficient and reliable tool for aerodynamic analysis in railway tunnels, enabling sensitivity and comparative studies that are currently impractical with conventional CFD and supporting engineering and industrial applications.