Experimental study on the influence of vegetation on urban canyon ventilation

The greening of the cities is one of the most encouraged mitigation strategy to face the urban heat island phenomenon, as vegetation increases the evapotranspiration and the shading. Moreover, trees improve the air quality of cities, as they enhance the pollutant deposition rate thanks to their large surface area. However, few studies on the impact of tree planting on the natural ventilation and dispersion mechanisms inside urban street canyons are available. In the present thesis, we report the results of an experimental campaign performed in the wind tunnel facility of \'Ecole Centrale de Lyon, aimed at understanding how the presence and the density of trees affect the air exchange between the urban street canyon and the external atmosphere, and if it hinders the pollutant removal mechanisms. For this purpose, concentration, velocity, and combined concentration and velocity measurements have been performed inside a street canyon oriented perpendicular to the wind direction, inserted in an urban network reproduced inside the test section. The vehicular pollution has been simulated by a linear source of ethane, which behaves as a passive scalar. The aerodynamic behavior of trees has been reproduced by inserting plastic miniatures along the two long sides of the canyon, using four different trees density configurations: absence of trees inside the canyon, two rows of one tree positioned on the central axis of the canyon, two low density trees rows, and two high density trees rows. Results show that the concentration field is essentially two-dimensional in the empty cavity configuration, reveling that the dispersion phenomenon is governed by recirculating cells that transport the pollutant from the downwind wall to the upwind wall, while it becomes three-dimensional in all the configurations with trees, meaning that there are further recirculating cells along the longitudinal direction of the canyon. The presence of trees modifies the spatial distribution of the pollutant concentration, but has a low impact on the bulk vertical exchange velocity, which has almost the same values in the four configurations. From the velocity measurements, we find that the presence of trees decreases the turbulent kinetic energy and it homogenizes the mean vertical velocity. Combined measurements of concentration and velocity allow turbulent mass fluxes to be evaluated, both inside the canyon and at the rooftop. Comparing the mass fluxes with the concentration field, it is found that inside the canyon the turbulent component of the total mass flux is responsible for the mixing of the pollutant in the case without trees, while it is almost negligible in the case with trees. At the rooftop, the vertical mean mass flux has the highest contribution in both cases. The statistical analysis of concentration time series measured in different spatial points inside the canyon reveals that the dispersion phenomenon can be entirely modeled with a Gamma distribution, even if the Lognormal and the Weibull 2p distributions perform a good fitting as well, and that the extreme concentration events are governed by an exponential law. The experimental campaign evidences that vegetation modifies the concentration and the velocity fields inside the street canyon, but the constant value of vertical exchange velocity suggests that the overall ventilation efficiency is not decreased. Moreover, the fine three-dimensional grid on which concentration measurements are organized provides a huge database of concentration data to