Zbyněk Jaňour

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 (

Dispersion of Light and Heavy Pollutants in Urban Scale Models: CO2 Laser Photoacoustic Studies

>

Model and real pollutant dispersion: concentration studies by conventional analytics and by laser spectrometry

>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.

Urban Air Pollution and Its Photochemistry Studied by Laser Spectroscopic Methods

Laser photoacoustic spectrometry and a line permeation pollution source were used in a study of the dispersion of pollution in an urban agglomerate using simulation in a wind tunnel. Applications of this measuring technique utilize the high sensitivity and broad dynamic range (3 orders of magnitude in this case) of the photoacoustic detection method. The minimum detected absorbance in the photoacoustic detection method employed in this work was at the level of ≅4.3 × 10-6 (≅ 8 μg/m3 CH3OH). The effectiveness and flexibility of the permeation method of generation of various concentrations of gases was verified for simulation of emission pollution sources in a wind tunnel. The line permeation pollution source developed in this work, with a concentration flux of (8.3 × 10-5 ± 2 × 10-6) g/s at 20 °C, generated a concentration level in the model used from a background value of 80–90 μg/m3 up to values of ≅ 1000 μg/m3 of methanol. A simple model street canyon together with the pollution source was employed to carry out a number of measurements of spatial profiles. The dispersion of the pollutant was studied at the bottom and on the walls of the street canyon together with the concentration variation with changes in the wind speed. The laboratory model was used to demonstrate the differences in ventilation of the street canyon. Using the laser sheet visualisation method, a video technique was applied to show the real flow in the street canyon for various reference velocities. Spatial measurement of the concentration distribution inside the street canyon was carried out on the model with reference velocity of 1.5 m/s. The dependence of the concentration field on the Reynolds number was estimated from the measurements of concentrations in the vicinity of the line permeation source.

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

The distribution of pollutants in two urban scale models (point emission source and street canyon with extensive transport) was investigated by means of CO2 laser photoacoustic spectroscopy in the region of the atmospheric window (9–10 μm). The experimental results of physical modeling are in a good agreement with the numerical calculations performed in the frame of computational fluid dynamic (CFD) modeling. Methanol, ethanol, and ozone (examples of light pollutants), as well as sulfur hexafluoride and 1,2 dichlorethane (examples of heavy pollutants), were selected on the basis of their high resolution spectra acquired by Fourier transform and laser diode spectroscopy.

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.

Numerical Solution of 2D and 3D Atmospheric Boundary Layer Stratified Flows

The differential absorption LIDAR together with spot analyzers (infrared absorption and chemiluminescence method) was used for concentration measurements in an urban street canyon in Prague. Measurements in the real atmosphere were compared with measurements of concentration distribution inside a model. Experiments in a wind-tunnel-simulated atmosphere were carried out using laser photoacoustic spectrometry and the laser visualization method. A comparison of concentrations on the windward and leeward sides of the model and real street canyon was performed.

A GIS-based approach to spatio-temporal analysis of environmental pollution in urban areas: A case study of Prague's environment extended by LIDAR data

This work compares approaches both of mathematical and physical modelling of pollutant dispersion in simulated atmospheric boundary layer (ABL) with results of remote sensing of atmospheric pollutants. Measurements were performed over a highway outside a city and in an urban street canyon with extensive traffic under different meteorological conditions (autumn versus summer period). Time-resolved spatial distributions of pollutants (NO2 and O3) were measured by the combined DIAL (differential absorption light detection and ranging)/SODAR (sound detection and ranging) method and using spot analyzers appropriately located on the leeward and windward sides near the urban street canyon bottom. Qualitative agreement was found between the results obtained by remote sensing in the real atmosphere and those obtained by physical modelling in the simulated atmosphere of a wind tunnel for the autumn period. On the other hand, the analysis of the monitoring results and outputs of the physical modelling shows disagreement for the summer period. Besides neglecting the thermal effect during the sunny period, chemical reactions or photochemical processes taking place in the street canyon can affect the dispersion and distribution of pollutants very significantly. To improve the description of the system investigated, the Computational Fluid Dynamics (CFD) environment was tested for a basic implementation of photochemical reactions into the commonly used mathematical models of turbulence and dispersion processes as well.