## 5 Summary and Conclusions  
In this work we provide thorough analysis of the turbulent statistics of the flow through an urban array. A summary of the most relevant features is given below. First of all, the effects of the urban canopy on the freestream high-speed incoming flow have been highlighted. Above the urban canopy and within the streets parallel to the flow, it is possible to identify regions of high-speed freestream flow. However, at the roof level and on each side of the urban obstacle, there are strong velocity gradients which mark the separating shear layer and results in regions of lower velocity. Regions of reversed flow have been highlighted and the well-resolved LES show the presence of coherent vortical structures such as the legs of the arch vortex. Within the streets, the flow between two obstacles can be separated into two distinct regions: A region of reversed flow close to the leeward side of the upstream block and a region of higher velocity towards the windward side of the downstream block. The region  
Figure 6: (Top) Non-dimensional mean streamwise velocity \(U/U_{H}\) and (bottom) mean wall-normal velocity \(W/U_{H}\) at \(y/W_{b}=0\). Data extracted from the experiments by [PERSON] et al. [9]. Note that \(S\) denotes street separation. Figure 7: (Top) Square of the root-mean-squared velocity fluctuations (top) \(u_{\text{rms}}^{2}/U_{H}^{2}\) and (bottom) \(v_{\text{rms}}^{2}/U_{H}^{2}\) at \(y/W_{b}=0\). Data extracted from the experiments by [PERSON] et al. [9]. Note that \(S\) denotes street separation. of reversed flow exhibits low TKE whereas the other region shows higher levels of turbulence. The streamwise and the spanwise components of the velocity fluctuations are the main contributors to the TKE distribution and they clearly show how the flow bypasses each obstacle. High levels of turbulence have also been observed on each side of each cubical obstacle and at the upper-roof level, which are the result of the interaction between the freestream high-speed flow from above the urban canopy and the shear layers. This increased TKE, mainly due to the streamwise velocity fluctuations, is responsible for the formation of the arch vortex. Globally, the flow field exhibits a coherent structure of the flow within the urban canopy, reflected in the recirculation bubbles and regions of reversed flow. As one moves downstream, the flow reaches an equilibrium and the bulk of the flow from above does not interact with the flow from the streets. This is the case for the first street in the experiments where the streets are narrower: After a region in which the flow adapts to the canopy, a skimming-flow regime is reached and the exchange of scalar quantities and fluid between the urban canopy and the region above are not favored. The mixing characteristics of the flow through an urban environment are significantly affected by its geometry. In an environment with narrow streets and relatively high buildings, as encountered in many American cities, the bulk of the flow from above the urban canopy does not interact with the flow within the streets. The skimming-flow regime is observed and consequently all the scalar quantities are trapped within the urban canopy and this can be an issue for pollutant dispersion. When the street width becomes wider, a larger fraction of the flow from above the canopy interacts with the flow from within it. Therefore, this is a more favorable situation when it comes to reducing pollutant concentration. To conclude, a coherent picture of the flow field can be identified and the correlation between the geometry and the regime of the flow has been investigated. The influences of the geometry of the urban array on the turbulence statistics and therefore on the mixing characteristics of the flow have been studied. This directly impacts the transport of scalar quantities such as pollutants and the temperature. Additional extensions of this work could follow the work by [PERSON] et al.