## 2 Modeling and Parameter Adjustment
### Domain and Mesh  
The optimal configuration of the simulation domain and mesh in FDS is critical for ensuring both the accuracy of the results and the computational efficiency of the numerical model. The domain size must be sufficiently large to capture fluid dynamic interactions with the physical boundaries, thereby providing realistic boundary conditions. The selected mesh resolution directly impacts the accuracy of thermal flow modeling and smoke propagation [37], and is typically determined based on the characteristic fire diameter. Achieving a balance between mesh granularity and computational cost is essential for obtaining reliable results, which justifies the necessity for sensitivity analyses and mesh optimization strategies [34; 38]. Figure 2: Vertical section through the building along the \(Y\)-axis. The computational model presented in Figure 3 was developed in compliance with current simulation guidelines for Large Eddy Simulation (LES)-based fire dynamics simulations [34]. The domain dimensions were extended beyond the physical envelope of the building to accurately represent external airflow and climatic conditions. The mesh was structured using a combination of uniform and localized refined subdomains, ensuring both numerical accuracy and computational efficiency. Mesh cell alignment was maintained to prevent interpolation errors at mesh interfaces. Leakage areas were configured according to prior validated research [10], and design decisions were informed by the provisions of SR EN 12101-6 and SR EN 12101-13 regarding differential pressure smoke control systems. For the simulations performed on this building, the analysis domain was extended beyond the physical limits of the structure. This approach enables the representation of various environmental conditions and allows for the monitoring of airflow both inside and outside the building envelope. The final dimensions of the simulation domain were defined as 35 m \(\times\) 20 m \(\times\) 15 m.  
Multiple subdomains were defined to improve resolution in the areas of interest (Figure 3), including 1 surrounding the fire source, 36 subdomains for infiltration areas associated with doors and windows, and 2 subdomains for mechanical air supply used to generate positive pressure. The ignition source subdomain, located on the ground floor within the fire compartment, measured 6 m \(\times\) 4 m \(\times\) 1 m. The subdomains surrounding the infiltration openings--aligned with external windows and entrance doors--measured 0.75 m \(\times\) 0.75 m \(\times\) 0.75 m each. The subdomains for mechanical air supply were configured with dimensions of 3 m \(\times\) 3 m \(\times\) 2 m for the upper supply zone and 3 m \(\times\) 1.5 m \(\times\) 2 m for the lower zone. Figure 3 illustrates the dimensions of the computational domain used in the simulation. Figure 3a provides a 2D schematic of the overall geometry, while Figure 3b through Figure 3d present detailed 3D views of specific subdomains. To enhance visualization, the infiltration Figure 3d is shown at a scale of 3:1. This modification is purely illustrative and has no impact on the accuracy of the physical model. The mesh size [39] significantly influences both the accuracy and computational time of the simulation; therefore, selecting an appropriate compromise is essential to  
Figure 3: Domain size and geometry: (**a**) two-dimensional schematic of the overall simulation domain; (**b**) fire source subdomain; (**c1,c2**) mechanical air supply subdomains; and (**d**) infiltration subdomains, shown in three-dimensional view. obtain reliable results within a reasonable timeframe. To evaluate the influence of mesh resolution, several grid sizes were tested using the D*/dx ratio, which quantifies the relation between the characteristic fire diameter and cell size [40]. In this study, the D*/dx values ranged from 6.68 (coarse mesh with 0.25 m cells) to 33.4 (fine mesh with 0.05 m cells).