# Numerical Analysis of Flow in Building Arrangement: Computational Domain Discretization
## 1 Introduction  
### Building Arrangements Analysis  
The atmospheric boundary layer (ABL) is characterized by particular sensitivity to the type of terrain topography, as well as building geometry and arrangement. The ABL significantly contributes to climate and wind comfort, which is not only pedestrian comfort but also an issue of health risks resulting from dispersion of pollution. That is why the analysis of building arrangement has recently become an important issue in urban planning. The most widespread wind comfort criterion is based on mean wind velocity. However, the definition of comfort criterion should include turbulence due to mutual interactions of buildings in built-up areas. The generation of eddies in the wake behind buildings lead to an increase of flow instability. These eddies contain large and small scale turbulent structures and the resulting separation and recirculation zone is accompanied by areas of velocity fluctuations. Other attempts in the development of more advanced wind comfort criteria were done by [1; 2]. This research was directed towards the evaluation of periodic phenomena impact in external flow patterns on the flow structures in the vicinity of buildings. The results clearly indicate that the estimation of wind comfort based only on mean velocity is ineffective in the case of urban areas due to the increase of flow turbulence intensity caused by surface impact and aerodynamics of object interactions [3; 4]. In numerical and experimental studies of urban area aerodynamics, the real build-up area is usually represented by simplified models in order to explain the analysed phenomena. A single building is usually simulated by a cuboid but it does not give a comprehensive assessment of the wind environment in built-up areas, where collective wind effects generated by adjacent buildings is very important [5; 6; 7]. In this case, the flow structure is determined by the geometry of building placement that is, the distance between objects or the relative proportions of their height. The mutual interaction of buildings in build-up areas leads to an increase of flow variability where the strong effect of periodicity results from the generation of eddies in the wakes behind the objects (large-scale and small-scale turbulent structures). Moreover, separation and recirculation zones are accompanied by areas of strong velocity fluctuations. In most cases, the studies of aerodynamic interactions in built-up areas are performed for few (usually two) cuboids [6; 7; 8; 9]. The knowledge concerning the influence of building arrangement in the flow field on wind comfort relied mostly on experimental studies but in recent years researchers have focused on utilizing the capabilities of numerical methods in this field. Along with the rapid development of high-performance computers, Computational Fluid Dynamics (CFD) turned out to be one of the most promising tools in such research and as such, plays an important role in the environmental analysis [5; 10; 11; 12; 13; 14]. CFD generates detailed information about flow parameters in entire numerical domains under precisely controlled conditions. Precision and credibility of CFD depend however on many factors, such as proper selection of boundary conditions and mesh generation, selection of numerical models and interpretations of the results. It is necessary to meet a number of requirements in order to carry out numerical modelling of wind impact on buildings. Flow calculations around buildings require knowledge of ABL characteristics where not only data concerning mean wind velocity is important but also turbulence levels and large-scale wind gusts data is required. However, meteorological data for an urban area, in most cases, does not give sufficient detail required as input data to CFD. This lack of data directly causes difficulties in the formulation of proper boundary conditions and this requires a careful fit of the turbulence model parameters and roughness conditions in a given boundary layer.