Radka Kellnerová

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 (

Analysis of Scalar Fluxes and Flow Within Modelled Intersection Depending on the Approach Flow Direction

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Comparison of wavelet analysis with velocity derivatives for detection of shear layer and vortices inside a turbulent boundary layer

>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.

Contribution of Advective and Turbulent Contaminant Transport to the Intersection Ventilation

The objective of this study is to determine processes of pollution ventilation in the X-shaped street intersection in an idealized symmetric urban area for the changing approach flow direction. A unique experimental setup for simultaneous wind tunnel measurement of the flow velocity and the tracer gas concentration in a high temporal resolution is assembled. Advective horizontal and vertical scalar fluxes are computed from averaged measured velocity and concentration data within the street intersection. Vertical advective and turbulent scalar fluxes are computed from synchronized velocity and concentration signals measured in the plane above the intersection. All the results are obtained for five approach flow directions. The influence of the approach flow on the advective and turbulent fluxes is determined. The contribution of the advective and turbulent flux to the ventilation is discussed. Wind direction with the best dispersive conditions in the area is found. The quadrant analysis is applied to the synchronized signals of velocity and concentration fluctuation to determine events with the dominant contribution to the momentum flux and turbulent scalar flux.

PIV measurement of turbulent flow within a street canyon: Detection of coherent motion

The influence of the approach flow direction on contaminant spreading and ventilation within an intersection in an idealised symmetrical urban area was investigated in this study. Advective horizontal and vertical scalar fluxes are computed from measured data for five flow directions. The highest advective contaminant fluxes are measured in the bottom parts of street-canyons. The important role of the vertical turbulent scalar flux in ventilation of intersection is expected. Quadrant analysis of vertical flux of longitudinal momentum is used to determine a domination of sweep or ejection events above the intersection.

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

Turbulent flow inside street canyons was studied using Laser Doppler Velocimetry (LDV) in wind-tunnel experiment. Spatial distribution of momentum flux and moments of third and fourth order of velocity components obtained from one-point measurement are investigated. The area of interest covers an inner part of street canyon and adjacent intersection. Time-averaged momentum flux is divided into particular quadrants using Quadrant Analysis method. Comparison between flux events and skewness provides some insight into the spatial distribution of strong intermittent tendency. Therefore, we can explain better the spatially local dominance of sweep, respectively ejection that corresponds to intermittent gust moving inward, respectively outward from canyon.

Quadrant analysis of boundary layer above pitched and flat roofs

Detection of highly energetic structure in the turbulent flow can be easily done by wavelet analysis, especially with Morlet function as a mother wavelet for detection of repetitious low-frequency structures and Mexican hat for detection of vortices. Since wavelet analysis can be simply applied on one-point measurement, the better understanding of its meaning is welcome. In this project, turbulent boundary layer was investigated with focus on intermittent dynamics. PIV snapshot were used for comparison with Wavelet analysis results, since PIV was performed with high sample frequency (500 Hz). Instantaneous snapshots of velocity and vorticity were studied also by POD and spectra of particular expansion coefficients were computed. Vorticity from planar velocity vectors was used as a suitable methods for recognition of strongly deformed streamlines. Nevertheless, to identify a trajectory of vortex and estimate the true advective velocity of vortex core in the flow, swirling strength or the second invariant could have been used as a proper tool.

Ventilation of idealised urban area, LES and wind tunnel experiment

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.