## 2 Materials and Methods
### Verification of Air Flow in CFD Simulation Using the PIV Method  
The main mechanism of particle motion is forced convection formed by the ventilation system. The PIV (Particle Image Velocimetry) method was used to compare the CFD simulation with the experimental measurements. This is an immediate vector or scalar measurement of velocity in a cross section. The principle was to make a monitored plane using a pair of lasers with an adjustable switching frequency. The Dantec Dual PIV Laser creates a plane in which the movement of particles between individual switches is monitored. The inaccuracy of the PIV method measurement is determined by the maximum cross-correlation deviation of the two light pulses and the maximum value is up to 5%. The velocity vector tracking particles are delivered to the laminar field flow space using the Safex F2010 smoke generator, particle motion was recorded by a high FPS camera, and the devices used for scanning the speed profiles are shown in Figure 13.  
The particles were blown into the mounting area of the ceiling lamp to ensure as much mixing of the supply air with the smoke particles as possible (Figure 14a) and were subsequently recorded in the red area above the operating table (Figure 14b). The volume flows of the ventilation system were identical to the boundary conditions of simulated variant 5.  
Figure 14: Creation of airflow velocity profiles: (**a**) smoke generation to the measuring area; (**b**) area measured by the PIV method.  
Figure 13: Devices for scanning speed profiles using the PIV method: (**a**) Safex F2010 smoke generator; (**b**) Dantec Dual PIV laser; (**c**) High FPS camera.  
The velocity profile of the airflow generated in the measured area is shown in Figure 15a. The average air velocity in the monitored area was 0.14 m\(\cdot\)s\({}^{-1}\) (b). Figure 15b shows the same area in the CFD simulation for variant 5, using k-\(\omega\) model 5 \(\times\) 10\({}^{6}\) elements in the mesh, with inflation on the surface of the operating lamp. The sizes of the mesh elements ranged from 3-30 mm in order to achieve the lowest possible Y+ values in the viscous sublayer area; in addition, skewness quality and orthogonal quality were also monitored. It must be taken into account that airflow in the operating room is a dynamic process with slightly variable velocity profiles. Therefore, the velocity profile of the airflow in the simulation is not identical to the experimental measurement. While maintaining the same speed from LARC, the speed similarity behind the operating light is high. The average air velocity in the monitored area was 0.13 m\(\cdot\)s\({}^{-1}\). Figure 15c presents the use of the same number of elements in the mesh but is calculated by the k-\(\epsilon\) model. For this turbulence model, the average airflow rate was only 0.04 m\(\cdot\)s\({}^{-1}\). Quality control for the mesh was also performed by [PERSON] et al. using half (2.5 \(\times\) 10\({}^{6}\)) and twice (10\({}^{7}\)) the number of elements [46]. The differences in the results with twice the number of elements were negligible. A higher degree of similarity between experimental measurement and simulation was observed in flow model k-\(\omega\). The difference in the average airflow rate between the experimental measurement of the PIV method and the CFD simulation with turbulence model k-\(\omega\) was approximately 7%.