## 1 Introduction
However, designing a simulation using both systems requires making certain design decisions, taking into account all relevant approximations and simplifications of the models and justifying them thoroughly [4]. FDS [5] is a CFD solver widely used for various applications, not only for solving practical problems in fire protection technology, but also for studying basic fire dynamics. It is an open-source code developed at NIST (National Institute of Standards and Technology, USA), which simulates basic physical and chemical processes related to fire, such as combustion, pyrolysis, thermal radiation, air circulation dynamics, airflows induced by fire, turbulence, fire suppression, etc. Some of the major features of the FDS code are the low-Mach large-eddy simulation, explicit second-order kinetic-energy-conserving numerics, Deardorff eddy viscosity, gray gas radiation, etc. These features have been widely used and validated for over two decades. FDS enables various types of 2D and 3D visualizations of fire courses and parameters. The complex dynamics of fire processes requires detailed resolution of the numerical mesh and a short time step to achieve sufficient calculation accuracy, which is demanding on computer performance and memory. FDS enables the simulation to be conducted in parallel on various platforms of high-performance computers to date. There is extensive literature describing the various aspects of modeling fire and toxic smoke spread using FDS, modeling the course and effects of a fire and also modeling evacuation during a fire. Most researchers use laboratory-obtained data from small-scale models to validate simulations. Although there are a high number of practical applications in which FDS is used, there are not many papers in the literature dealing with the validation of tunnel fire simulation using data from large-scale experiments [6; 7; 8; 9; 10], or such studies remain unpublished. The first validation study related to the FDS simulation of ventilation in a tunnel used a full-scale fire experiment conducted as part of the Massachusetts Highway Department Memorial Tunnel Fire Ventilation Test Program [6]. The test consisted of a single-point supply of fresh air through a 28 m\({}^{2}\) opening in a 135 m tunnel. In [7], a coupled hybrid (1D/3D-CFD) modeling methodology using the FDS 6 (version 6.7.5) was validated by full-scale fire tests in the 1600 m long Runehamar tunnel. For 6, 66 and 119 MW fires, temperature profiles, centerline velocity, backlayering lengths and maximum temperaturesupstream and downstream from the fire source were investigated. The study [8] examined three evacuation scenarios after a 20 MW fire accident in a 1500 m long railway tunnel with longitudinal ventilation and different ventilation activation times. The fire characteristics simulated by FDS were compared with empirical formulae based on small- and full-scale fire experiments. To investigate the effectiveness of water spray in blocking smoke and heat in a short underground belt transport tunnel in a mine with mechanical ventilation and a water curtain system, a full-scale fire test was conducted measuring selected fire characteristics [9]. FDS (version 6.0.1) was then used to simulate smoke-spread characteristics, temperature distribution, visibility profiles and CO distribution. In [10], a series of full-scale experiments with 1.35, 3 and 3.8 MW fires conducted in a short metro tunnel with mechanical ventilation system was described and then simulated by FDS (version 5.5). The smoke temperature and decay rate of the temperature distribution under the tunnel ceiling were investigated. There are not many papers related to the validation of an airflow generated by emergency ventilation in a real tunnel (even without fire). However, the accurate reproduction of tunnel airflow is a key issue in modeling the operation of the tunnel ventilation system at any mode or fire development and smoke propagation in the event of fire [4,11].