The impact of semi-open settings on ventilation in idealized building arrays

Semi-open settings (or hanged walls) above the pedestrian level in street canyons can provide protection to pedestrians from heavy-rain and strong sunshine, but may also influence street ventilation and thus pollutant dispersion. By performing computational fluid dynamics (CFD) simulations and employing ventilation concepts of net escape velocity (i.e. diluting rate for exhale effect) and the age of air (i.e. freshness of air for inhale effect), this paper investigates the effects of low-level semi-open settings on ventilation in idealized block arrays with rectangular or square form (building height/street width H/W = 1.25) under neutral atmospheric conditions. Four upstream wind directions of 0 degrees (parallel), 15 degrees, 30 degrees, 45 degrees between the approaching wind and the main street axis and four widths of semi-open settings (Ws= 0 m, 2 m, 4 m, 6 m) are investigated. Results indicate that the increasing width of semi-open settings produce greater age of air (i.e. older air) and smaller net escape velocity (i.e. less diluting rate) than those without semi-open settings. The wider semi-open settings experience the worst ventilation. In addition, the approaching wind directions of 0 degrees and 15 degrees lead to worse ventilation than 30 degrees and 45 degrees, for both rectangular and square forms.