The coupled effect of mechanical and thermal conditions on pedestrian-level ventilation in high-rise urban scenarios

Abstract

Complex urban structure and thermal conditions jointly determine the complexity of urban air flow, but the coupling effect of mechanical and thermal processes on pedestrian-level ventilation has rarely been documented. Using large-eddy simulation (LES) of the idealized wind environment in parametric urban-like geometric scenarios, we evaluated the coupling effect of structural properties and the strength of sensible heat flux of urban blocks on urban ventilation. The study focused mainly on high-rise and high-density urban scenarios, and it was found that for an array of tall buildings, the width of streets perpendicular to the input wind should be considered in defining flow regimes. Canonical flow regimes could be “horizontally” applied to pedestrian-level ventilation because the horizontal wake interference between perpendicular and parallel streets slows down the mean wind in the parallel street canyons. Thermal conditions enhance pedestrian-level ventilation through intensifying vertical mixing by thermal turbulence under weak background wind. However, if the initial thermal conditions are fixed, higher absolute wind speed means stronger horizontal convection, which will weaken the vertical mixing caused by thermal turbulence and eventually lead to the weakening of pedestrian-level ventilation.