## 2 Materials and Methods
### CFD Analysis  
CFD is an area of engineering knowledge that belongs to the computer aided engineering (CAE) simulation programs. These analyses allow the numerical and visual simulation of flows, heat transfer, and even chemical reactions. These simulations and analyses are increasingly used to characterise living spaces, determining how the ventilation is operating and therefore the level of existing pollution. In short, CFD analysis helps to detect where the air is fouled. Thanks to this, preventive measures can be taken to avoid air quality risks [90]. These rises may be due to an increase in the number of people inside living spaces or a lack of ventilation in them [86; 90]. In this study, CFD simulations helped to detect CO\({}_{2}\) concentration. For this research, two different software packages were used: ANSYS Fluent 2021 R2 and ANSYS Discovery 2021 R2 (available at www.ansys.com). ANSYS Fluent was used for static and dynamic simulations. In addition, ANSYS Discovery was used only for dynamic simulations. The main features that had to be selected are:  
* Dimension: 3D. * Display options: Display mesh after reading. * Options: Double precision. * Processing Options: Serial. To begin with the analysis, the geometry and boundary conditions had to be previously defined to establish the convergence of the solution [86]. These boundary conditions were related to CO\({}_{2}\) concentration and ventilation flow, helping to determine the location of the measurement instruments. The case study analyses the indoor atmosphere of a medium-sized room in a single-family house was simulated. In order to perform it, a series of conditions and restrictions had to be established:  
* The building is located in Medina Sidonia, a village in the province of Cadiz (southern Spain). Its height above the sea is 337 m. In addition, its coordinates are as follows: longitude 5\({}^{\circ}\)55\({}^{\prime}\)37.81\({}^{\prime\prime}\) W and latitude 36\({}^{\circ}\)27\({}^{\prime}\)25.02\({}^{\prime\prime}\) N.  
* The usual outdoor pollution levels are low, thanks to its location near Alcornocales Natural Park on the town periphery. In some previous measurements with the standard reference equipment, the CO\({}_{2}\) concentration ranged from 380 to 410 ppm under different conditions of traffic, industrial activity, and prevailing winds. It can be noted that although some of the values are even lower than those indicated by the technical report \"CEN/TR 16798-2\", a value of 400 ppm is assumed as the average outdoor concentration, as suggested by the report. * The characteristic wind in this area comes from the east. As the main opening is in the southeast direction, the room received an adequate amount of fresh air. * The room consists of a rectangular enclosure of average dimensions (3.9 \(\times\) 2.4 \(\times\) 2.5 m). The geometry allowed the simulations to be carried out easily. It can be noted that this is a factor that directly influences the meshing performed by ANSYS. In addition, the boundary conditions defined refer to the amount of air entering the room, which was considered constant in the defined scenarios. * Only one person lives in the room. No additional CO\({}_{2}\) emissions were considered that could have an impact on the defined boundary conditions. * The test room is not forced ventilated and has no HVAC systems to renew the air inside. Therefore, ventilation is natural, with air being renewed through the openings in the envelope. The room has an exterior sliding window (height 0.93 m and width 0.63 m by leaf) and an interior door (height 2.03 m and width 0.72 m) that communicates with the rest of the rooms in the building. * We intended to locate a clonic device inside the room. For this purpose, it was essential to determine the areas where there is no air renewal, as well as the presence of vortices that may affect data collection. Four scenarios were defined for the CFD analysis.