## 1 Introduction
### Natural Ventilation  
Natural ventilation can be described as the air renovation inside a building or a room caused by natural thermal, wind, or diffusion effects through windows, doors or purpose-built ventilation openings without need for external mechanically powered systems such as fans and air conditioning systems [11]. In the context of adapting and reducing global warming, this is very desirable, as it takes advantage of natural resources without energy demand. The main factor that affects the efficiency of this technique is the design of the buildings and urban planning, because the air patterns within the target zone will change depending on them [11; 12]. This topic has been deeply studied and is well documented. Its factors and consequences have been stated in some papers [13], as well as reviews of the different forms of studying this method, which can be through experimentation, wind tunnel experiments and CFD [14]. The first approach is the most precise one for knowing the real effect of the ventilation, but it is difficult, as it requires a building where the sensors are set to take good measurements. The second one provides good results as well, but it can be expensive to experiment in a wind tunnel and not everybody can have access to one. Finally, the last approach is the most accessible and fast one, but on the downside, it can have larger errors if it is not well calibrated. Furthermore, different models or types of natural ventilation have been described [15]: single-sided ventilation, buoyancy-driven stack ventilation and wind-driven cross-ventilation. Among them, the first one involves only one aperture to the exterior, for instance, one door or window open. Here, the air renewal is produced by changes in pressure due to the wind in the surroundings of the aperture. Because of this, the effectiveness of this type of ventilation is usually low and has huge variations. This is the most common one in single rooms; thus, a lot of study has been conducted in this field, more specifically when the room is connected to a courtyard [16]. However, it has been proved that this type of ventilation has the lowest ventilation potential of the three options. On the other hand, buoyancy-driven stack ventilation has its basis in the density difference of the air. It draws air with a lower temperature from apertures near the ground level and expels hotter air from the inside of the building through higher ventilation openings, such as chimneys or atriums. Although the ventilation efficiency is good, it becomes poorer when it comes to single floors or low dwellings. Finally, in wind-driven cross-ventilation, the driven forces are caused by a pressure difference between two or more opposing apertures; as a result, the flow runs through the enclosed space from the aperture with higher pressure to the lower one. As some authors highlight [15; 17], this can be the most useful ventilation strategy as it is only needs two apertures in different rooms for it to work, and usually, the pressure difference is enough to provide an appropriate air mass exchange rate. Different studies have been conducted about this ventilation mechanism; for instance, different apertures sizes have been tested [18], different configurations of windows layout in the room [19] or even including internal obstacles [20]. Many different techniques have been proposed to improve the ventilation potential [21] but most of them rely on CFD simulations or combinations between them and wind-tunnel experiments [22]. However, these studies mostly focus on the streamline distribution inside the room and the air ventilation rate, but data about the potential temperature reduction when this approach is used are quite scarce; additionally, it is usually based on CFD experimentation, which lacks experimental validation.