A computational study on the influence of Urban morphology on wind-induced outdoor ventilation

Accurate modeling of urban wind flow is important for the assessment of the natural ventilation of the outdoor environment and therefore for outdoor air quality. Different geometrical levels of analysis can be used, ranging from regular arrays of obstacles to real and complex case studies. In order to avoid the complexity of real case studies and to obtain more generic results, regular arrays of obstacles represent a suitable level to study the relation between urban morphology and outdoor ventilation. This paper presents the first results of a numerical study with Computational Fluid Dynamics (CFD) of isothermal wind flow in generic urban configurations. Nine urban configurations are defined with regular arrays of obstacles by increasing the plan area density (from 0.1 to 0.6) and the frontal area density (from 0.02 to 0.45). By this variation, the flow structure varies from isolated obstacle flow over wake interference flow to skimming flow. Among the nine configurations defined, four test cases are selected for a preliminary study. 3D steady Reynolds-Averaged Navier-Stokes (RANS) CFD simulations are performed for the four selected configurations and the simulations are compared with experimental wind-tunnel data. An overall fairly good agreement is found between the experimental and numerical results obtained with RANS simulations. The results allow establishing some relationships between morphological and fluid dynamics parameters of urban wind flow and outdoor ventilation. It is found that the upstream building density can cause a decrease of up to the 70% of the ventilation rate in the central street. Further work will focus on extending this parametric analysis and extrapolating it to real cities.