Ventilation strategy and air change rates in idealized high-rise compact urban areas

We regarded high-rise compact urban areas as obstacles and pathways to the approaching wind. Flow rates across street openings, open street roofs and along street networks contribute to air exchange between urban airspaces and their external surroundings. We numerically studied the ventilation and air change rates in some aligned square building arrays (the building width B = 30 mm, building heights H = 2B or 2.67B) with building area densities of λp = 0.25 or 0.4 (i.e. the ratio between the plan area of buildings viewed from above and the total underlying surface area). The main and secondary streets are parallel and perpendicular to the approaching wind respectively. Urban parameters are found important to the ventilation. The taller buildings (H = 2.67B) may capture larger inflow rates across windward street openings than the lower (H = 2B). Wider streets and smaller building area density provide more wind pathways and obtain larger flow rates along street networks. Meanwhile, the flow rates along the street may quickly decrease due to strong resistances produced by high-rise buildings, so the total street length should be limited, otherwise the ventilation in downstream regions is not good. The secondary streets always experience worse ventilation than the main streets. A building height variation benefits ventilation in the secondary streets by enhancing vertical mean flow rates across street roofs in contrast to those with uniform heights. If the base of all buildings is open from z = 0 to 0.33B, the ventilation in both the main and secondary streets becomes better.