Large eddy simulation of the effect of hat fux on pollutant dispersion in an urban street canyon

Abstract

The effect of heat flux on pollutant dispersion in an urban street canyon with fixed aspect ratio of 1 is investigated under four different heating configurations: windward heating, leeward heating, ground heating, and walls and ground heating, using large-eddy simulations (LES). For each heating configuration, the Reynolds number is varied from 400 to 3,000 and the Grashof number from 80,000 to 800,000. The retention value (the ratio of pollutant remaining inside canyon to the total pollutant emitted) is used to compare the effect of heating on pollutant dispersion under each heating configuration. Numerical results show that the wind flow patterns, pollutant dispersion patterns and retention values depend not only on the canyon aspect ratio and inflow wind speed, but also on the strength of heating and the heating configuration. The most significant effect arises from the case of windward heating when the main vortex rotating in the clockwise direction is countered by the buoyancy effect near the windward wall; the pollutant accumulated near the leeward wall is therefore decreased. On the other hand, no significant change is observed in the wind flow and pollutant dispersion pattern for the cases of leeward, ground and walls and ground heating. As the strength of heating is increased gradually, the strength of the vortex rotating in the counter-clockwise direction at the lower corner of the windward wall also increases; this causes the pollutant to accumulate near the windward wall. A further increase in the strength of heating causes the formation of a third vortex above the second one, the pollutant accumulated near the windward wall is therefore increased.