## 2 Materials and Methods
### Computer Modeling of the Ventilation Test  
In this section, we follow the procedure for creating the FDS model of the Polana tunnel proposed in [18] in order to create an improved FDS model that better estimates the airflow velocity generated by the ventilation. Since the simulation results obtained in this paper need to be compared with those reported in [18], we used the same simulator version (FDS, version 6.5.2; MPI, version 3.0; MPI library, version Intel(r) MPI Library 5.1.3) as was used there. The geometrical representation of the Polana tunnel and its equipment (tunnel tube, emergency lay-bys, jet fans, vertical traffic signs, emergency bumpers in the lay-bys) for the FDS environment was modeled and adjusted with regard to the selected computational \begin{table}
\begin{tabular}{c c c} \hline \hline
**Ventilation** & **Jet Fans Operation** & **Jet Fans Operating** \\
**Scenario** & **Jet Fans Operation** & **Direction** \\ \hline Mode 1 & All jet fans operated at full power & Towards the west \\ Mode 2 & Three and five jet fans operated at full & Towards the east \\ Mode 3 & All jet fans operated at full power & Towards the east \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Description of 3 ventilation operation modes tested during the ventilation test. Figure 3: Average airflow velocity v [m/s] measured by the grid anemometers AG1–5 (black curve) and the tunnel anemometers AT1–3 (gray curve). The 6 min time intervals, where the airflows were considered stabilized, are highlighted by arrows. mesh resolution. All the dimensions and locations as well as the jet fan's performance are in accordance with the Polana tunnel. In the final simulation, a 30 cm computational mesh resolution was used; therefore, the following description of the simulation parameters corresponds to this resolution. The calculation was parallelized using the MPI (Message Passing Interface) model due to its high computational efficiency. The discussion on the model's sensitivity to the resolution of the computational mesh and the simulation parallelization is included in Appendix A. According to this analysis, the decomposition of the computational domain into 12 meshes of the same size and mesh resolution, and the computational mesh resolution of 30 cm seems to be optimal; therefore, these settings were used in this study. The same settings were also used for the final simulations in [18]. The computational domain had dimensions of 900 m \(\times\) 18 m \(\times\) 8.1 m and consisted of 3000 \(\times\) 60 \(\times\) 27 = 4,860,000 mesh cells. The mesh cell number of each particular computational mesh was 405,000. The axial jet fans were modeled by the HVAC subsystem of FDS 6, which is used for modeling HVAC (Heating, Ventilation and Air Conditioning) systems [5]. Each jet fan is represented by a pair of 0.9 m \(\times\) 0.6 m rectangular vents with a prescribed normal velocity, using the HVAC to model the inlet and outlet of the jet fan. The modeled jet fan shroud was represented by 4 rectangular OBSTRUCTIONs of 1 mesh-cell thickness surrounding the jet fan. The length of the jet fan shroud was set to 3.9 m due to the selected mesh resolution. The maximum volume flow in the HVAC settings was slightly adjusted [18] to achieve the same pressure rise in the tunnel [17] and to transfer the same momentum to the computational domain. Therefore, in the HVAC settings, a volume flow rate of 19.75 m\({}^{3}\)/s was used to conserve \(\text{Av}_{\text{j}}^{2}\), where A is the area of the modeled (0.54 m\({}^{2}\)) and actual (0.5 m\({}^{2}\)) jet fan, and \(\text{v}_{\text{j}}\) is the jet velocity at the fan outlet. The time dependence of the volume flow rate of the jet fans in operation was set by the RAMP corresponding to their performance during the ventilation test. The gradual jet fan onset was modeled by a 115 s interval of linear increase in the power of jet fan included in the RAMP directing the operation of the jet fan.