Assessment of the pressure load in cavities in a single-track tunnel by model-scale experiments and CFD simulation

In the Alps, several very long single-track, twin-tube tunnels either have been opened recently (Gotthard Base Tunnel, CH) or are under construction (Koralm Tunnel, AT; Brenner Base Tunnel, AT-IT; Semmering Base Tunnel, AT). Pressurized cross-passage tubes spaced every 325 to 500 meters interconnect the main tunnel tubes and are sealed with air-tight doors which must sustain high pressure loads. The impact of the cavity between the main tube and the cross passage door on the pressure load was studied in model-scale (1:25) experiments at the Tunnel Simulation Facility Göttingen (TSG) and with Computational Fluid Dynamics (CFD). In the narrow, single-track model tunnel, a shallow cavity was installed at the location where both the train head with strong gradients in its attached pressure field and the train entry wave reflected from the opposite tunnel exit arrive at the same time. Thus, the maximum possible pressure drop is generating at this position. Three different shapes were tested, a narrow and a wide with rectangular base and a wide with trapezoidal base, all enlarging the cross-section area up to 25%. A generic model of the Austrian Railjet manufactured with 3D printing was run at speeds up to 230 km/h. CFD simulations using unsteady RANS with sliding mesh approach were set up according to the experiments. The pressure development during the Railjet passage through the plain tunnel predicted by CFD compare remarkably well with measurements. For the cross tunnel cavities, the TSG test conditions were simulated. The potential of the various configurations to mitigate the pressure load in the cavity and the impact of the cavities on the tunnel pressure during and after train passage were evaluated. The change of the extremal pressure in comparison to the plain tunnel is rather small (10-15%). However, the wide cavities show a tendency of increasing the pressure load while the narrow cavity gives rather neutral results, even showing some potential to reduce pressure waves after the train has left the tunnel