Urban street-canyon flows with bottom heating

Urban street-canyon flows in the presence of street-bottom heating are investigated using a two-dimensional numerical model with a k– turbulence closure scheme. The street aspect ratio (H/D, where H and D are the building height and the width between two buildings, respectively) varies from 0.6 to 3.6 (in 0.2 interval) and the initial potential temperature difference between the street-canyon bottom and the air (ΔΘ) ranges from 0 to 16 K (in 2 K interval). Five flow regimes are identified. Regime I is observed when the aspect ratio is very small but the bottom heating is very strong (H/D=0.6 and ΔΘ 10 K). In regime I, as the heating intensity increases, the thermally induced vortex expands but the mechanically induced vortex contracts. Regime II is mainly observed when the aspect ratio is relatively small or the bottom heating is weak. In regime II, the vortex intensity increases with increasing heating intensity. Regime III is observed when the bottom heating is relatively significant (ΔΘ 4 K) and the aspect ratio lies in the range of 1.2–2. This regime differs from regime II in that the vortex induced by temperature gradient on the upwind side of the upper layer has meaningful intensity and size and the maximum horizontal velocity decreases with increasing heating intensity. When the bottom heating is relatively significant, regime IV is most commonly observed. This consists of two corotating vortices in the vertical within combined streamlines. Regime V exhibits two counterrotating vortices induced on both sides of the maximum temperature axis in the lower layer. The upper vortex is little influenced by the bottom heating. The numerical model result is shown to be reasonably in good agreement with the wind-tunnel data.