Stream-wise wind penetration of urban areas under the influence of thermal buoyancy

Abstract

Buoyancy-driven flows affect urban ventilation and stream-wise penetration of background winds, but there is a lack of relevant understanding. By employing validated computational fluid dynamics (CFD) simulations, this study identifies a universal transition from wind-dominated to buoyancy-dominated flow type along the stream-wise direction. In the upstream wind-dominated region, stream-wise wind penetration is significant, with double-vortex structures being established in the street canyons. In the downstream buoyancy-dominated region, intense plumes rise, with span-wise inflows from secondary streets. Based on the local Richardson number (RiL), this study quantifies the stream-wise wind penetration depth as the stream-wise distance from the canopy entrance to the location where RiL first reaches one. The predicted penetration depth depends on the Richardson number (Ri) and spatial locations. Within the urban canopy layer, the penetration depth of the certain case remains nearly constant at different heights. Above the canopy layer, it generally increases with the height on the middle building plane, while exhibits a local “undershoot” near the roof-level on the street planes. The vertically-averaged penetration depth (Lp,v) is negatively correlated with the Ri and span-wise distance, given by the fitting formula Lp,v/H = 29.6Ri−0.7035e−(Y/H) for vertical building planes, and Lp,v/H = 37.37Ri−0.3893(Y/H)−0.19 for vertical street planes. Similarly, the horizontally-averaged penetration depth (Lp,h) within the building array is also negatively correlated with the Ri and vertical elevations, given by the fitting formula Lp,h/H = 31.96Ri−0.3302e−0.0292(Z/H). The results help to shed insights on non-isothermal urban ventilation mechanisms and facilitate effective ventilation strategies for urban environments.