The role of turbulence in the ventilation of streets

Predicting pollutant transport in the urban environment remains a scientific challenge. The main goal is to provide a correct prediction of turbulent flow, which is more complex in the urban environment featuring streets and intersections than it is above a flat terrain. The oncoming wind controlled by frontal systems is already highly turbulent in nature, and its interaction with buildings further increases its turbulence. This also increases the complexity of the prediction. The general solution to the turbulent flow of an incompressible fluid, such as atmospheric air near the earth’s surface, is not yet known, and thus remains an unresolved mathematical problem of the millennium. The prediction of the transport of dangerous substances for a single simulated case (e.g. for one wind direction) places high time demands even on the world’s best supercomputers as it may take several days to calculate the forecast numerically. However, in the event of a sudden leak of a dangerous substance due to an accident or terrorist attack, it is necessary to predict the transport in the order of tens of minutes.

In the Laboratory of Environmental Aerodynamics in Nový Knín we study the pollutant transport in cities. Controlled wind tunnel experiments allow us to examine in detail the ventilation of the so-called street canyons at a reduced scale. As the name suggests, the street canyon is a street that is flanked on both sides by sufficiently tall buildings. If the wind blows perpendicular to the longitudinal axis of the canyon, the buildings create a significant obstacle to the ventilation of pollutants (such as exhaust gases from traffic). Inside the canyon, there is usually a vertical vortex that circulates the pollutants between its windward and leeward sides, and only at certain moments it is ventilated upwards over the building roofs (see video). At first glance, this ventilation principle may seem very random and difficult to predict. However, our team has shown that in this seemingly chaotic flow, organized recurring structures are created, which fundamentally contribute to the ventilation of pollutants from city streets. The number of these structures, or their frequency of occurrence, is mainly influenced by the shape and height of the roofs. For example, saddle roofs cause a more frequent occurrence of these structures, and thus faster exhaust ventilation than flat roofs.

Utilizing this knowledge will make it easier, and therefore faster, to predict the transport of hazardous substances in city streets. This may contribute to a faster and more effective response by rescue services. Our findings can also be used to improve air quality in cities, both existing and planned development.

Contact: Dr. Štěpán Nosek