The “wind tunnel” approach is applied to study high-speed train aerodynamics in a railway tunnel using FDS software. The main focus of the research is on the pressure distribution along the tunnel. Proven analytical dependencies based on the experimental observations for air jet centerline velocity and flow entrainment are used to evaluate the model setup. A model verification is carried out based on the pressure drop calculations due to viscous effects where the impact of the surface roughness and the tunnel length are also considered. A sensitivity analysis is performed to evaluate changes in input FDS parameters and to explore interactions between them. It is proposed to use the standard deviation, obtained from the calculated time-averaged pressure values, to specify the appropriate numeric parameter combinations, e.g. DT and PRESSURE_TOLERANCE, considering the desired results consistency and the computational time consumed. The simulated cases with and without a train inside a tunnel provide data on the aerodynamic characteristics of the models. The obtained volumetric and cross-sectional profiles for pressure and airflow velocity distribution form the basis for an informed decision regarding the tunnel design or safety solutions, for example, defining areas under maximal and minimal pressure loads. The analysis displays the necessity to carefully manage each investigated case considering the FDS features and limitations that largely affect a model setup and calculations.

The efficiency of tunnel ventilation systems is commonly evaluated through numerical modelling. In this survey, two CFD models were developed by means of Fire Dynamic Simulator and Ansys Fluent software. The simulation results were used to assess the model performance in studying the backflow distribution in a real tunnel. A full-scale experiment to evaluate the ventilation conditions in the western railway tunnel was carried out in Zentrum am Berg. The velocity values were obtained for 90 examined points located at 10 cross-sections along a 100-meter tunnel part. The results showed good agreement in velocity variation trends from field measurements and those predicted by numerical models. At cross-sections more distant from the fan outlets, the FDS and Fluent models overestimated the flow velocities to a different extent. The simulated backflow development corresponds well to the observed three specified regions (initial, transitional, and developed) with distinctive flow structures. The FDS calculations confirmed the registered spontaneous changes in flow direction at points with a prevailed flow direction in the vicinity of the jet fans. Despite some discrepancies in results, the comparative analysis of two numerical models showed their applicability in the backflow investigation.

The CFD model of the train-tunnel system, previously developed on proven analytical dependencies, is improved by the introduction of a tunnel cross-passage and the consideration of surface roughness. These additions bring the simulation setup closer to real conditions allowing to explore the FDS features in the evaluation of the aerodynamic effects occurring in a tunnel. Pressure and velocity patterns are obtained for the resulting model of a high-speed train in a tunnel with a cross-passage. The maximal and minimal pressure levels for the tunnel and the cross-passage spans are calculated to provide the data for the design phase and safety assessment. The approach to determine the most loaded surfaces of the tunnel and its inner structures, e.g. escape doors, for an estimation of their operational reliability is discussed. The study shows that the FDS software can be a helpful tool in assessing scenarios where the train-tunnel interaction is reviewed, though its applicable capabilities and set of features are largely dependent on the tasks to solve and need to be accurately adjusted.