Pollutant fluxes in two-dimensional street canyons

The turbulent dispersion of a passive tracer emitted by a line source simulating vehicular traffic in an idealized urban canyon in neutral conditions is studied in the water channel by means of simultaneous measurements of velocity and concentration fields. The experiments are conducted for two geometrical arrangements of two-dimensional obstacles reproducing the skimming flow (AR=1) and the wake-interference regime (AR=2), where AR is the canyon aspect ratio. The results show a strict connection between the dynamics of the shear layer developing at the top of the canyon and the vorticity field. For AR=1, it is found that the shear layer flaps upwards and downwards according to two different frequencies. The greatest of them matches the vortex shedding frequency as measured at the canyon top, while the lower is comparable to H/u_* (H is the building height and u_* a reference friction velocity). The shear layer flapping modulates in time the pollutant exchange rate between the canyon and the outer layer. The different characteristics of the shear layer found for the two flow regimes also explain the larger pollutant re-entrainment observed for AR=1, which turns out to be greater than the emission rate at the source. Sweep and ejection modes are identified via quadrant analysis and used to quantify the weights of the factors involved in the turbulent exchanges of tracer and momentum between the canyon and the outer layer. It is found that the venting of polluted fluid at the canyon top increases substantially passing from AR=1 to 2, while an opposite trend is observed for the entrainment of polluted fluid. The vertical flux of pollutant at the canyon top for AR=2 is largely of turbulent nature, with the contribution of the mean flux being practically negligible. On the other hand, the latter becomes comparable or even exceeds the magnitude of the turbulent flux when AR=1. The maps of the first three statistical moments of the pollutant concentration for the two geometrical arrangements are also reported and discussed.