This study investigates the characteristics of flow (forming mechanism, influencing factors, and so on) and pollutant dispersion when the aspect ratio, H/W (H is the building height and W is the width between buildings), varies and the building wall or street bottom is heated. For this, we developed two-dimensional numerical model with one and a half order turbulent closure scheme and employ the wall function as heating source to represent effectively the heat transfer between air and the building wall or street bottom.
Seven cases with different aspect ratios are considered and each case has no heating, upwind building wall heating, street bottom heating, or downwind building wall heating. In the cases of upwind building wall heating, one vortex is generated regardless of the aspect ratio. When H/W is more than 1.5 (in the case of no heating, two or three vortices are generated), the upward motion induced by upwind building wall heating in the upwind lower layer eliminates the downward motion which appears in no heating case. In the cases of street bottom heating, flow fields are similar to those of upwind building wall heating cases on the whole, because the axis of maximum temperature is shifted to the upwind side by the horizontal advection. However, when H/W is 3, the horizontal wind speed is not strong enough to shift the axis. As the maximum temperature axis is centered at the middle of the street canyon, the vortex in the lower layer is divided into two, but the vortex in the upper layer is not influenced by this axis. In the cases of downwind building wall heating, two vortices are generated except for the aspect ratio less than 0.5. The vortex in the upper layer is mechanically induced by the ambient wind, while the vortex in the lower layer is thermally induced by the downwind building wall heating.
The distribution of pollutant which is released at the street-level is quite dependent upon the given flow field. The pollutant concentration in the case of upwind building wall heating is lowest of the four heating cases. This is because as the vortex becomes stronger in its intensity (horizontal and vertical wind velocity), more pollutant escapes from the street canyon to the ambient air.