Street canyon ventilation: Combined effect of cross‐section geometry and wall heating

Abstract

Understanding the dynamics of mass exchange between a street canyon and the overlying atmosphere is crucial to predict air quality in urban areas. Despite the large number of studies on this topic, there are many aspects that still need to be clarified. Among these, one is certainly the role of thermal stratification in street canyon ventilation. In order to fill this gap, this study evaluates how the combined effect of street canyon geometry, wall roughness and differential heating of the building facades influences pollutant dispersion within the canyon and out of it. The study was carried out in a wind tunnel, adopting an idealized urban geometry made up of square bars placed normal to the wind direction. The boundary conditions inside the canyon were modified by heating its windward and leeward walls, by changing its aspect ratio and by introducing roughness elements at the walls. A passive scalar was injected from a line source at ground level. The flow and concentration fields were measured in a cross‐section of the canyon. Characteristic exchange velocities within the canyon and towards the external flow were estimated comparing the experimental data with an analytical model for the cavity wash‐out. Results show that the transition from one recirculating cell to two counter‐rotating cells inhibits canyon ventilation, with a consequent increase in pollutant concentration at the pedestrian level. This transition occurs as the cavity aspect ratio increases and is facilitated by adding roughness elements at the windward wall. Heating the leeward wall has negligible effects on canyon ventilation. Heating the windward wall accelerates pollutant removals in square cavities, while it contributes to a worsening of air quality in narrow cavities. Finally, the wash‐out times of the cavity are discussed in terms of a relative contribution of the mean advective motion and its turbulent counterpart.

Influencing Factors on Airflow and Pollutant Dispersion around Buildings under the Combined Effect of Wind and Buoyancy—A Review

“…Qu et al [171] described various thermal exchange models on the flow field in different thermal conditions. Fellini et al [153] evaluated how the wall roughness influences pollutant dispersion within an idealized street canyon in a wind tunnel. Asphalt and concrete were used as representatives of ground surface materials to simulate the surface albedo influence on the energy balance and urban external wall temperature [143].…”

Urban microclimate in temperate climates: a summary for practitioners

“…These influence both their temperature as well as their air quality. Differential heating of canyon walls (especially the windward walls) and the addition of roughness elements (such as awnings, protrusions and setbacks) in combination with an appropriate canyon aspect ratio (especially a square canyon, where the average height of buildings is equal to the width of street) could enhance street ventilation (Fellini et al 2020).…”

Modelling chemistry and transport in urban street canyons: Comparing offline multi-box models with large-eddy simulation

“…Because of much of the human exposure to outdoor air pollutants occurs at the pedestrian level in street canyons, understanding airflow characteristics and distributions of pollutants is of vital importance in evaluating the pollutant health risk and in making policy for targeted air pollution alleviation. The concentration of a passive scalar (PS, an idealised chemically inert substance that negligibly interferes with local fluid dynamics through effects such as buoyancy) can exhibit sharp gradients at the pedestrian level of the street canyons (Fellini et al, 2020;Lietzke and Vogt, 2013;Murena et al, 2009). Furthermore, the dispersion of atmospheric pollutants within the canyon is accompanied by complex non-linear chemical reactions, evolving on the timescale comparable to the canyon circulation and residence timescale.…”