The Flow Instabilities within an Urban Intersection

The experiment uses wind-tunnel modelling method to investigate an intermittent ventilation of pollutants within an urban intersection. The model is designed after a typical inner-city area occurring frequently in European countries. It consists of apartment houses with roofs arranged to the several blocks. Blocks form regular patterns divided by intersections with height to width ratio equal to one. Project has focused on velocity characteristics inside the intersection and adjacent canyon. The skewness and kurtosis of velocity components were studied in order to identify an area with strong intermittent tendency. Velocity measurement was carried out using 2-component LDA. Pollutants have released from line source and mean concentration was obtained using slow flame ionization detector. Comparison of time-mean flow and concentration spatial distribution verified that vertical vortices, the typical flow patterns formed inside street canyons, as well as corner vortices created just behind the turning of the street are responsible for accumulation of emissions emitted by vehicle traffic. Flow instabilities corresponding to enhancement of skewness value break down these patterns and significantly ventilate street canyons removing exhalations. Longitudinal velocity indicates an unsteady acceleration near the rooftop level whereas deceleration occurred close to surface, so it produces a higher rotational speed in recirculation zone. In contrary, the strongest downward penetration of fresh air (into the canyon) is located behind the roof eaves on leeward building. With incoming downward gust, the recirculation zone is completely destroyed. Speed extremities (derived from kurtosis) also show the highest value at eaves level. Similar process appears in horizontal plane. Intensification of longitudinal wind runs up the circulation inside corner vortex, whereas lateral gust propagating just behind the turning of street diminishes the whole vortex and cleans up the area. Vertical and horizontal momentum flux was analyzed by quadrant analysis. This method has detected the localities with highest momentum transfer from free stream down to the canyon. This transfer corresponds with large air mixing. It also found out notable momentum transfer from rotational zone downward to the surface, where accumulation of pollutants occurred.