Petra Kastner-Klein

Wind-tunnel study of concentration fields in street canyons

Abstract The paper presents results from a case study of gaseous pollutant dispersion in street canyons. Tracer-gas experiments were performed in a neutrally stratified wind tunnel. Vehicle emissions were simulated as line sources. Concentration profiles along building walls were measured. A two-dimensional street canyon was considered as the reference case. The influence of systematic parameter variations on the concentration field is studied and discussed. Building dimensions, upwind building configuration, wind direction and roof geometry were found to be important parameters. Data sets from the study may be used for evaluation of numerical models and for expert estimates of air quality in the urban environment

A wind tunnel study of organised and turbulent air motions in urban street canyons

High concentrations of car-exhaust gases in urban street canyons are typically associated with low wind velocities or situations when the wind blows perpendicular to the canyon axis. The latter flow configuration has been studied in a wind tunnel model of a street canyon. The mean flow and turbulence structure have both been investigated and comparisons have been carried out with results of full-scale flow measurements in urban street canyons. A qualitative similarity has been found between the results of atmospheric measurements and flow characteristics in the modelled street canyon. Data from all employed sources give evidence of a flow acceleration (in some cases, rather sharp) above roof level. Additionally, the effects of traffic on the organised and turbulent components of airflow in the canyon have been quantified. The experimental data show significant differences in flow and turbulence patterns corresponding to the model cases of one-way and two-way traffic.

The Modelling of Turbulence from Traffic in Urban Dispersion Models — Part II: Evaluation Against Laboratory and Full-Scale Concentration Measurements in Street Canyons

The paper addresses the problem of the parameterisation of traffic induced turbulent motion in urban dispersion models. Results from a variety of full-scale and wind-tunnel studies are analysed and interpreted within a modelling framework based on scaling considerations. The combined effects of traffic and wind induced dispersive motions are quantified for different traffic situations (variable traffic densities, vehicle velocities and vehicle types) and incorporated into the developed parameterisations. A new dispersive velocity scale is formulated and recommendations regarding its application in urban dispersion models are given. The necessity of accounting for traffic induced air motions in predictions of street-canyon pollution levels is demonstrated. Further research is needed to verify the empirical constants in the proposed parameterisations and to generalize the developed approach for a broader range of urban building configurations, meteorological conditions, and traffic situations.

Modelling of Vehicle Induced Turbulence in Air Pollution Studies for Streets

Vehicle-induced turbulence can be an important factor of pollutant dispersion in urban areas, especially under conditions of low wind speeds which are typical for street canyons. An experimental concept (Plate, 1982) for modelling the effects of vehicle-induced turbulence was applied in the present study. The movement of vehicles was simulated in a boundary-layer wind tunnel by small metal plates mounted on two belts moving along a modelled street canyon. The scaling factor was based on the ratio of turbulence production by cars to that by wind flow. The traffic was represented by the velocity, density, frontal area and drag coefficients of the vehicles. The velocity and traffic density were varied, and the influence of the vehicle-induced turbulence on concentration patterns at the canyon walls was studied. It was found that concentration decreases with an increasing ratio of vehicle to wind velocity and with an increase of traffic density. A dimensionless combination of vehicle to wind-velocity ratio and density factor was proved to be a universal parameter describing the dependence of the concentration on vehicle-induced turbulence. The wind tunnel measurements were compared with predictions by the numerical Operational Street Pollution Model (OSPM; Hertel and Berkowicz, 1989a). Differences between the wind tunnel and numerical results regarding effects of vehicle-induced turbulence are discussed. The comparison revealed general agreement between wind tunnel and numerical data. Turbulence and concentration measurements in a street canyon in Copenhagen have been additionally employed for analysis of the model results.

The Modelling of Turbulence from Traffic in Urban Dispersion Models - Part I: Theoretical Considerations

The modelling of pollutant dispersion at the street scale in an urban environment requires the knowledge of turbulence generated by the traffic motion in streets. In this paper, a theoretical framework to estimate mechanical turbulence induced by traffic in street canyons at low wind speed conditions is established. The standard deviation of the velocity fluctuations is adopted as a measure of traffic-produced turbulence (TPT). Based on the balance between turbulent kinetic energy production and dissipation, three different parameterisations for TPT suitable for different traffic flow conditions are derived and discussed. These formulae rely on the calculations of constants that need to be estimated on the basis of experimental data. One such estimate has been made with the help of a wind tunnel data set corresponding to intermediate traffic densities, which is the most common regime, with interacting vehicle wakes.

Concentration and flow distributions in the vicinity of U-shaped buildings: Wind-tunnel and computational data

Abstract The flow and dispersion of gases emitted by point sources located near a U-shaped building were determined by the prognostic model FLUENT using the RNG κ−e turbulent closure approximation. Calculations are compared against wind-tunnel measurements about such a U-shaped building and several other prognostic and diagnostic numerical models. FLUENT gives a mixed image in terms of accuracy of predicted concentrations compared to the wind tunnel experiment. For identical boundary conditions higher as well as lower concentration values are calculated for different test cases. Ground level sources show higher discrepancies than situations where the tracer was emitted from the roof of the model building. A major error source was found to be the stationary solution procedure that was chosen for all simulations.

Experimental and Numerical Verification of Similarity Concept for Dispersion of Car Exhaust Gases in Urban Street Canyons

In urban conditions, car exhaust gases are often emitted inside poorly ventilated street canyons. One may suppose however that moving cars can themselves produce a certain ventilation effect in addition to natural air motions. Such ventilation mechanism is not sufficiently studied so far. A similarity criterion relating the vehicle- and wind-induced components of turbulent motion in an urban street canyon was proposed in 1982 by E. J. Plate for wind tunnel modelling purposes. The present study aims at further evaluation of the criterion and its applicability for a variety of wind and traffic conditions. This is accomplished by joint analyses of data from numerical simulations and wind tunnel measurements.

Diffusion from a line source deployed in a homogeneous roughness layer: interpretation of wind-tunnel measurements by means of simple mathematical models

In wind-tunnel studies of atmospheric dispersion, the pollutant emission by traffic is usually considered as ground-level line source. The source performance under different flow conditions is therefore important for the adequate description of pollutant dispersion. The dispersion from a line source in the wind-tunnel model of the atmospheric boundary layer has been investigated in the present study. Specially conducted mean flow and turbulence measurements have proved the similarity between the wind-tunnel flow and the flow in the lower portion of the atmospheric boundary layer. Lateral and vertical distributions of mean concentration downwind of the source have been measured with varying source parameters and wind velocities. The concentration pattern has not revealed a significant dependence on the parameter variations. The wind-tunnel results have been further evaluated by means of analytical and numerical dispersion models. The evaluation has shown a good general agreement of measured and calculated concentration fields. Two alternative sets of equations with different parameterizations of eddy diffusivity and different values of the turbulent Schmidt number have been employed in the analytical model calculations. A better agreement with the wind-tunnel results has been achieved with a similarity-theory expression for eddy diffusivity rather than with the eddy diffusivity parameterization based on the conjugate-power-law formula. The numerical model has provided the best match for the wind-tunnel concentration data at large distances from the source. However, certain discrepancies between the wind-tunnel and numerical predictions have been marked out close to the source.