Effectiveness of shelterbelts in mitigating aerodynamic deterioration of high-speed trains entering tunnels in crosswinds

Shelterbelts can effectively mitigate the aerodynamic deterioration of high-speed trains traversing tunnel–flat transition sections under crosswinds. An improved delayed detached-eddy simulation method combined with overset grid technology is employed to establish a numerical shelterbelt model incorporating realistic geometric features of tree trunks and canopies. The influence of tree spacing on temporal aerodynamic loads (AELs), surface pressure distributions, and flow field evolution is investigated, and a nonlinear mapping relationship between tree spacing and aerodynamic load amplitudes is established. The results demonstrate that shelterbelts significantly suppress abrupt fluctuations in aerodynamic loads. Compared with the case without a shelterbelt, the aerodynamic load coefficient amplitudes for the leading car are reduced by 13.61%–51.94%. The relationship between the amplitudes and tree spacing follows a logistic regression model, with the protective effect becoming negligible when the spacing increases to 8 m. The shelterbelt reduces the mean pressure coefficient difference across the leading car by 25.05% and inhibits the shedding of large-scale vortex structures on the leeward side, resulting in more uniform turbulent kinetic energy. It also promotes the dissipation of large-scale vortex structures into smaller scales, suppressing the broad low-pressure zone and enabling faster flow stabilization after the train leaves the transition section. The findings provide a theoretical basis and practical guidelines for the layout design of shelterbelts at high-speed railway tunnel portals.