## 2 Materials and Methods  
### Ventilation Tests in the Polana Tunnel  
The Polana tunnel [18; 19; 20] is a single-tube, bi-directional, 898 m long tunnel with a horse-shoe-shaped cross-section that has been in operation since 2017 (Figure 1). It is located near the D3 motorway in the north of Slovakia near the border with Poland. The D3 motorway is part of the European multimodal transport corridor VI, ensuring a fast connection of Northern and Eastern Europe. Direct and indirect losses caused by a potential fire in the Polana tunnel could therefore exceed the national level. The tunnel is 6.8 m high and 10 m wide with a cross-section area of 60.3 m\({}^{2}\), and an 8 m wide carriageway. Two emergency lay-bys are located at a distance of 373 and 635.6 m from the left (western) portal of the tunnel, one for each direction. At the lay-bys level, the tunnel is 7.5 m high and 12.8 m wide. The lay-by niches are 50 m long. Both lay-bys are constructed as one-sided and, with their asymmetric geometry, significantly affect the airflow and smoke stratification inside the lay-bys and in their vicinity (see, e.g., studies [21; 22]). The tunnel has a 2% ascending slope. At the end of each lay-by there is a prism-shaped emergency bumper, which is shown in Figure 1 (top, left). The tunnel is equipped with various safety installations, such as a linear heat detector, measurement of physical quantities, emergency ventilation, smoke detectors, emergency broadcasting, lighting and signaling, camera surveillance, etc. The longitudinal ventilation consists of 4 pairs of axial jet fans located at distances of 101, 201, 716 and 801 m from the left portal of the tunnel. They are installed at a height of 5.1 m above the road. Each individual jet fan has a fan wheel diameter of 0.8 m, a shroud length of 3.7 m, a maximum volume flow rate of 19 m\({}^{3}\)/s, an airflow velocity at the fan outlet of 38.5 m/s and a thrust in the main direction of 850 N. The aim of the ventilation in case of a fire is to create and maintain  
Figure 1: Polana tunnel: tunnel cross-section with locations of grid anemometers AG1–5 and the jet fans (**top**, **left**); tunnel tube with emergency lay-by and the lay-by’s vertical wall acting as an obstruction against the airflow highlighted by the arrow (**top**, **right**); and tunnel scheme with the determined west–east orientation, locations of jet fans JF1–8, grid G consisting of anemometers AG1–5, lay-bys LB1–2 and tunnel anemometers AT1–3 in the tunnel (**bottom**). tenable conditions for tunnel users and to ensure their safe evacuation, i.e., by regulating the longitudinal airflow and stabilizing the longitudinal airflow velocity in the tunnel to a target value. In the Polana tunnel, the aim is to reach such conditions within 120 s by a stepless continuous airflow regulation using axial jet fans, which are controlled by frequency converters [23]. The measurement of airflow velocity installed in the Polana tunnel consists of 3 ultrasonic anemometers with a measurement accuracy of 0.1 m/s. They are located at distances of 340, 465 and 565 m from the left portal of the tunnel. These devices work on the basis of measuring the transit delay of ultrasonic pulses. Each anemometer contains transmitter/receiver units for transmitting and receiving ultrasonic pulses, mounted on both sides of the tunnel at a certain angle to the direction of the airflow. In 2017, a series of ventilation tests was conducted in the Polana tunnel. The objective of one of these tests was to measure the longitudinal airflow velocity in the empty tunnel using the grid of 5 anemometers (AG1-5) located 300 m from the left portal of the tunnel [23] (Figure 1). Three ventilation scenarios were tested, corresponding to three modes of ventilation operation described in Table 1 and Figure 2. The aim of the test was to create a relatively stable airflow in the tunnel at the given ventilation power, to maintain it for a certain period of time, and thus to enable the measurement of the longitudinal velocity of the steady flow generated by the jet fans.