Flow and dispersion phenomena in urban street canyons in the presence of trees

In order to clarify the impact of trees on flow and concentration fields inside street canyons, measurements have been performed with a small scale wind tunnel model of an idealized urban street canyon. The model setup consists of two parallel aligned rows of houses forming the canyon and a tracer gas emitting line source for simulating traffic exhaust releases. Model trees with spherical crowns have been arranged along the canyon center axis. The setup was exposed to an approach flow perpendicular to the street axis and tracer gas concentrations at both canyon walls were measured. In the presence of trees, increasing values of exhaust concentrations at the leeward wall and decreasing concentrations at the windward wall were found. Velocity measurements reveal an extensive impact of trees on the flow field. Typical vortex structures observed in obstacle free street canyons were either significantly weakened or no longer present.