Evaluation of the ventilation potential of courtyards and urban street canyons using RANS and LES

We introduce the ventilation potential (VP) as a statistical, climate-dependent measure to assess the removal of scalars, such as heat and pollutants, from courtyards or urban street canyons. The VP is obtained following a three-step approach. First, the magnitude of the flux through a horizontal surface situated at the top of the courtyard or canyon is determined by means of computational fluid dynamics (CFD) simulations for various courtyard geometries and ambient wind directions. Then, this exchange flux is normalized with the free-stream wind speed and subsequently parameterized as a function of the courtyard’s length-to-width ratio and the incidence angle of the wind flow. Finally, the combination of the parameterization with site-specific wind data yields the VP. This study reveals that the normalized exchange flux is maximal when the angle between the prevailing flow direction and the main courtyard axis is about 15–30°, regardless of the courtyard length. The normalized exchange flux increases with increasing courtyard length, and approaches the optimum for courtyards with a length-to-height ratio of ten. Longer courtyards behave as urban street canyons. Unsteady (LES) simulations lead to a much higher VP and thus favor scalar removal when compared with steady (RANS) simulations. These observations can have a decisive impact on urban planning, human comfort and health.