Urban street-canyon flows with bottom heating

Urban street-canyon flows in the presence of street-bottom heating are inve stigated using a two-dimensional numerical model with a k-epsilon turbulenc e closure scheme. The street aspect ratio (H/D, where H and D are the build ing height and the width between two buildings, respectively) varies from 0 .6 to 3.6 (in 0.2 interval) and the initial potential temperature differenc e between the street-canyon bottom and the air (Delta Theta) ranges from 0 to 16 K (in 2 K interval). Five flow regimes are identified. Regime I is ob served when the aspect ratio is very small but the bottom heating is very s trong (HID = 0.6 and Delta Theta greater than or equal to 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 observe d when the aspect ratio is relatively small or the bottom heating is weak. In regime II, the vortex intensity increases with increasing heating intens ity. Regime III is observed when the bottom heating is relatively significa nt (Delta Theta greater than or equal to 4 K) and the aspect ratio lies in the range of 1.2-2. This regime differs from regime II in that the vortex i nduced by temperature gradient on the upwind side of the upper layer has me aningful intensity and size and the maximum horizontal velocity decreases w ith increasing healing intensity. When the bottom heating is relatively sig nificant, regime IV is most commonly observed. This consists of two corotat ing vortices in the vertical within combined streamlines. Regime V exhibits two counterrotating vortices induced on both sides of the maximum temperat ure axis in the lower layer. The upper vortex is little influenced by the b ottom heating. The numerical model result is shown to be reasonably in good agreement with the wind-tunnel data. (C) 2001 Elsevier Science Ltd. All ri ghts reserved.