Štěpán Nosek

Ventilation Processes in a Three-Dimensional Street Canyon

The ventilation processes in three different street canyons of variable roof geometry were investigated in a wind tunnel using a ground-level line source. All three street canyons were part of an urban-type array formed by courtyard-type buildings with pitched roofs. A constant roof height was used in the first case, while a variable roof height along the leeward or windward walls was simulated in the two other cases. All street-canyon models were exposed to a neutrally stratified flow with two approaching wind directions, perpendicular and oblique. The complexity of the flow and dispersion within the canyons of variable roof height was demonstrated for both wind directions. The relative pollutant removals and spatially-averaged concentrations within the canyons revealed that the model with constant roof height has higher re-emissions than models with variable roof heights. The nomenclature for the ventilation processes according to quadrant analysis of the pollutant flux was introduced. The venting of polluted air (positive fluctuations of both concentration and velocity) from the canyon increased when the wind direction changed from perpendicular to oblique, irrespective of the studied canyon model. Strong correlations (

Impact of roof height non-uniformity on pollutant transport between a street canyon and intersections

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Quadrant analysis of turbulent pollution flux above the modelled street intersection

>0.5) between coherent structures and ventilation processes were found at roof level, irrespective of the canyon model and wind direction. This supports the idea that sweep and ejection events of momentum bring clean air in and detrain the polluted air from the street canyon, respectively.

Short-term gas dispersion in idealised urban canopy in street parallel with flow direction

The objective of this experimental study is to determine processes of pollution ventilation above the X-shaped street intersection in an idealised symmetric urban area for several approach flow directions. A unique experimental set-up for simultaneous measurement of the flow velocity and the tracer gas concentration in a high temporal resolution is assembled. Advective and turbulent vertical scalar fluxes are computed from the measured data in a horizontal plane above the street intersection. Vertical turbulent pollution transport was found to be a significant and positive contribution to the total vertical transport of pollutants form the intersection. Prevailing events in vertical scalar and momentum fluxes were determined using quadrant analysis.