# Modelling Pollutant Dispersion in Urban Canyons to Enhance Air Quality and Urban Planning
## 3 Results  
### Airflow Pattern  
This first section addresses the findings related to airflow patterns for different street layouts. Firstly, Figure 4 shows the airflow pattern for each AR studied for \(Re\) = 676,796 which is the standard case for a three-floor building with 10 m of height and an external wind velocity of 1 m/s. It is noticeable that the number and position of the air recirculation zones are strongly dependent on street morphology, as narrower streets produce more vortices. For AR = 0.75 to 2.5, only one recirculation is formed at the centroid of the street. As the AR increases, the vortex displaces towards the street roof level. Then, for AR = 3, a new recirculation is formed, causing the internal zone of the street to split into two different vortices. In the beginning, the lower vortex is significantly smaller than the other one. However, when the AR increases from AR = 3 to AR = 4, the lower vortex becomes larger. For instance, for AR = 3.5, both vortices are the same size. Then, this tendency continues until AR = 4, when a new vortex appears. Therefore, for an AR of 4 and 4.5, there are three vortices in the street. Figure 4 gives a very useful insight into the air movement inside street canyons as it allows to split them according to their recirculations. Each of these volumes is considered independent because the interaction between them is minimal, so different acclimatisation techniques can be used in each space. These findings may differ from the previous results in the literature. This is because of the Reynolds number range used (6.7\(\cdot\)10\({}^{5}\)), which is far from the typical value of 12,000 used traditionally in other studies [33; 57]. However, the results in this work are similar to those of other authors who have tested the dependence of flow on the Reynolds number, using values similar to the one in this study [30; 33; 45]. They also concluded that for an \(\mathbf{AR}\) which traditionally was supposed to have multiple vortices (\(\mathbf{AR}\) = 2), only one vortex is formed for typical Reynolds number values. Another important matter is the dependence of the number and position of vortices on the Reynolds number. Figure 5 provides the airflow pattern for \(Re\) = 2,030,387 which corresponds to a three-floor building with 10 m height and an external wind velocity of 3 m/s. When comparing to the base case of \(u\) = 1 m/s, it is noticeable that the airflow pattern of several aspect ratios has changed. For instance, AR = 3 now only has one vortex, with the location approximately like that of the upper vortex formed for \(u\) = 1 m/s. In addition, for AR = 4 and AR = 4.5, the bottom vortex disappears, forming only two recirculations. However, increasing the Reynolds number not only affects the number of vortices but also their size. The upper recirculation tends to increase in size while maintaining the centre fixed, thus the lower vortices displace towards the ground level. An example of this is for AR = 3.5, because previously both vortices were almost identical, whilst in this case, the upper vortex is significantly bigger. Figure 4: Air pattern for different \(\mathbf{AR}\)s of street canyons with \(\mathbf{Re}\) = 676,796. Therefore, it can be stated that when the Reynolds number is increased (for instance, higher wind velocities or larger streets), the number of vortices is reduced for every AR. In those cases where only one vortex is formed for a low \(Re\), the airflow pattern remains identical. However, for narrower streets, less vortices appear. Streets in the multi-vortex regime for a low \(Re\) may transition into a single-vortex regime. The effect of reducing the \(Re\) is presented in Figure 6 for \(Re\) = 507,597 which corresponds to a three-floor building with 10 m of height and an external wind velocity of 0.75 m/s. Opposed to the previous results, there are two main effects when reducing the \(Re\). Firstly, the creation of new vortices, for example, AR = 3.5 now has three vortices instead of only two.