Modelling the recirculation zone in street canyons with different aspect ratios, using cfd simulation

Poor air quality in cities was indicated as the cause of seven million deaths, one out of eight globally, only in 2012. Urban streets are the epicentre of the problem, as high concentrations are frequently measured, caused mainly by the traffic and secondly by the general urban background pollution. The flow inside street canyons has some specific characteristics that tend to obstruct the removal of pollutants from the city level. The small scale models that are used to study pollution in streets have to consider about these flow patterns, which many times are referred with the collective term recirculation zone. Our main hypothesis is, that the flow regime in a street canyon governs the recirculation zone and consequently the dispersion. Thus, a quasi-universal expression for the recirculation zone could expand the applicability of these simple models in cases of higher complexity. To understand better the recirculation zone, CFD simulations are validated and used. We use vortex detection methods, namely vorticity, streamlines and the λ₂ and Q criteria, to visualize the vortex structures. The final results are rather inconclusive, although the combined use of the above methods certainly give some insights to the flow structure. In any case more exploratory analysis is necessary in order to validate the further develop the hypothesis of this work.