## 1 Introduction to Urban Flows  
The study of complex turbulent flow in an urban environment is of particular interest in various domains and has been an active research area over the past years. Indeed, the presence of buildings affects the flow field and results in complex patterns for the dispersion of scalar quantities, such as contaminants. Thus, the spread of pollution in urban areas is of concern for public health. However, developing predictive models is challenging due to the wide range of factors which can affect the flow field, such as moving cars, population density, trees, buildings or street canyons. Describing the structure of flow through an idealized urban area and the operation of parameterizing it are therefore necessary to have a better understanding of pollutant spreading and to determine predictive models of the evolution in air quality within the city [1]. Although using an idealized urban environment simplifies the real urban flow, it is necessary to focus on the physical mechanisms governing the flow structure and the turbulence characteristics. The structure of flow through an urban environment has been described by [PERSON] and [PERSON] [2] both at a local scale and at a larger scale. The wind turbulence in street canyons can affect pedestrian mobility and comfort as studied by [PERSON] [3], whereas the same wind flow at a larger scale represents the wind environment of the city and is a major factor for the design and implementation of new buildings. It thus affects the removal of exhaust gas and pollution from the urban canopy and is therefore of great interest in order to improve the quality of air within the urban area. [PERSON] et al. [4]performed wind-tunnel tests to investigate pollutant dispersion as a passive scalar within a street network. The mass transfer between the streets was measured as well as the mean velocity profile of the flow above the urban canopy upon various geometrical conditions in order to emphasize the dependence of these processes on the geometrical properties of the street network. Pollutant dispersion is controlled by turbulent mixing and the mean-flow transport within the street network. The turbulence and the mixing characteristics within such an environment have been investigated by [PERSON] [5] using direct numerical simulation (DNS). It has been shown that the wind blowing within the urban canopy is slower than the flow above the buildings because of the canopy drag. However, the wind blowing within the streets creates a mixing circulation which favored exchanges of scalar quantities within the urban environment. Numerous numerical studies have been conducted in order to describe the flow and scalar dispersion within the urban area. [PERSON] and [PERSON] [6] investigated the flow characteristics within urban streets canyons using a two-dimensional numerical model build with the turbulent closure scheme. They described the flow field by the number and intensity of vortices within the street canyons. On the other hand, DNS was performed by [PERSON] et al. [7] for the flow around a wall-mounted square cylinder under various inflow conditions. They highlighted the turbulence features for a laminar-inflow case and for a turbulent-inflow simulation are different; they also studied the horseshoe vortex forming around the square cylinder as it had been observed by [PERSON] and [PERSON] [8] using smoke-wire visualizations. These phenomena have also been studied experimentally by [PERSON] et al. [9] in a simplified urban area. Indeed, it is possible to assess the flow characteristics by performing wind-tunnel tests because these experiments enable a good control of the input parameters. They highlighted the multi-scale characteristics of the flow field through a simplified urban environment and identified different features of the flow within this urban array such as the wake, the shear layers and the horseshoe vortex mentioned above.