Stilianos Rafailidis

Study of line source characteristics for 2-D physical modelling of pollutant dispersion in street canyons

Abstract The University of Hamburg initiated a wind tunnel study of car exhaust dispersion from street canyons in an urban environment to investigate how pollution dispersion is affected by street geometry. Particular emphasis at the beginning of this work was put on the design of a line source to represent traffic exhaust. Pollution dispersion was studied in two dimensions (i.e., infinite-length streets were assumed). The case of an isolated street canyon in open country was examined first. The same street canyon geometry was subsequently studied in an urban environment, i.e., with additional canyons of similar geometry upstream and downstream of the test street. The dynamic and dispersion characteristics of the flow in the two cases were quite different. In the canyon amidst open country we observed better canyon ventilation than in the urban roughness case.

Some remarks on the validation of small-scale dispersion models with field and laboratory data

The objective of the paper is to contribute to the discussion on appropriate procedures for the evaluation of numerical models. This is done using as an example micro-scale atmospheric dispersion models as they are commonly applied for the prediction of local mean concentrations and higher percentile values. The paper starts with a description of problems associated with the direct comparison of numerical model results and measured data. After some remarks on numerical solutions to the problem, it concentrates on experimental strategies suitable for assuring the quality of validation data, and for quantifying systematic differences between simulated and experimental results.

Three-dimensional modelling of concentration fluctuations in complicated geometry

The strong fluctuating component in the measured concentration time series of a dispersing gaseous pollutant in the atmospheric boundary layer, and the hazard level associated to short-term concentration levels, demonstrate the necessity of calculating the magnitude of turbulent fluctuations of concentration using computational simulation models. Moreover the computation of concentration fluctuations in cases of dispersion in realistic situations, such as built-up areas or street canyons, is of special practical interest for hazard assessment purposes. In this paper, the formulation and evaluation of a model for concentration fluctuations, based on a transport equation, are presented. The model is applicable in cases of complex geometry. It is included in the framework of a computational code, developed for simulating the dispersion of buoyant pollutants over complex geometries. The experimental data used for the model evaluation concerned the dispersion of a passive gas in a street canyon between 4 identical rectangular buildings performed in a wind tunnel. The experimental concentration fluctuations data have been derived from measured high frequency concentrations. The concentration fluctuations model is evaluated by comparing the models predictions with the observations in the form of scatter plots, quantile-quantile plots, contour plots and statistical indices as the fractional bias, the geometrical mean variance and the factor-of-two percentage. From the above comparisons it is concluded that the overall model performance in the present complex geometry case is satisfactory. The discrepancies between model predictions and observations are attributed to inaccuracies in prescribing the actual wind tunnel boundary conditions to the computational code.

Numerical investigation of the pollution dispersion in an urban street canyon

The pollution levels in a real urban street canyon are determined numerically, using the Reynolds Averaged Navier-Stokes (RANS) approach, in the framework of Remote Optical Measurement Techniques (ROMT) evaluation under realistic conditions, in order to identify the most appropriate placing of the ROMT instruments to be tested in the street canyon. As the operation of instruments is based on optical absorption along a beam, a balance must be sought between beam-path length and height and the inherent concentration characteristics of the pollution dispersion field in the canyon. For modelling, a Computational Fluid Dynamics (CFD) code is utilised, which solves the 3D unsteady Reynolds Averaged Navier-Stokes (RANS) within a computational domain that includes all buildings and streets in the vicinity of the canyon. Different scenarios are examined as regards wind direction, resulting in highly varying in-canyon concentration field characteristics. Different instrument placements are also investigated, as well as beam-lengths and heights, to establish the most appropriate location and instrument configuration for the forthcoming field campaign. Results show that although oblique-to-street-axis placement of the beam-path ensures a higher overall concentration-indication due to longer beam-path, the high dependence of indication on the wind direction makes this case less favourable compared to the perpendicular-to-street-axis placement.

Interparticle stress, fluid pressure, and bubble motion in gas-fluidised beds

Abstract Direct measurements of stresses transmitted between particles and surfaces in bubbling fluidised beds are reviewed, to show that they are only significant when gross bubble-induced motion transmits particle momentum to the surface. New measurements of gas pressure around a tube in a two-dimensional bed show that the motion of a bubble near a tube, in both two- and three-dimensional beds, results from the local gas pressure rather than from interparticle effects. The implications of these observations are explored, to identify the limits to the analogy between fluidised beds and Newtonian liquids. By a further extension of the Davies and Taylor analysis of the rise of spherical-cap bubbles in liquids, a result is derived which enables the bubbling behaviour of beds containing heat transfer tubes or other immersed bodies to be simulated.

Modelling of Flow and Pollution Dispersion in a Two Dimensional Urban Street Canyon

The wind-driven flow patterns and the dispersion of vehicle exhaust pollutants released at street level has been simulated with the three-dimensional (3-D) dispersion model ADREA-HF (Andronopoulos et al., 1993), for idealised two-dimensional urban fetches occupied by buildings with slanted roofs. The simulation used oncoming atmospheric boundary layer characteristics corresponding to realistic above-town wind characteristics, as measured in reference wind tunnel experiments (Rafailidis, 1997). At that stage, analysis was limited to neutral stability conditions only. Firstly, the quality assurance of the numerical model was investigated in terms of the sensitivity to different grid allocations. The modelling results were corroborated by comparison with wind tunnel measurements in a similar two-dimensional domain (Pavageau et al., 1997). The numerical modelling replicated well the high degree of non-uniformity in the dispersion field in the test street, and the results agreed satisfactorily with the experimental measurements. The reasons for the differences observed have been investigated. With the model thus validated, three different exhaust release scenarios have been tested, keeping the same overall emission rate but different spatial patterns of street-release. The effect of the different street-release scenarios was found to be only marginal, with the dispersion patterns on the sidewalls affected only locally, close to the street level.

Near-field geometry effects on urban street canyon measurements for model validation

Selection of numerical urban pollution dispersion models for regulatory purposes entails reliable prior validation against either physical modelling or field measurements. It is shown that an asymmetric geometry of the urban street canyon, in which measurement takes place, induces persistent gradients in pollution dispersion within it, irrespective of the wind vector meandering above the roofs. This influences field measurements taken there and, depending on canyon aspect ratio and roof geometry, favorable or unfavorable ventilation regions develop, on an annually-averaged bases. Therefore, inappropriate placement of the sampling probe can lead to systematic under- or over-predictions of actual air quality. Proper consideration of the potential influence of the near-field on pollution dispersion guarantees that the campaigns results remain free of systematic bias and, therefore, appropriate for regulatory model testing and validation. The repercussions of these findings for urban air quality monitoring are identified and discussed.

Physical Modeling of Car Exhaust Dispersion in Urban Street Canyons — The Effect of Slanted Roofs

Leisen et al. (1982) have shown that building roof configurations adjacent to an urban street canyon influence pollution dispersion in it. The phenomenon can not be modelled numerically properly with the currently available urban pollution dispersion codes, because one needs to tackle both slanted roofs and extensive solution domains. Therefore, to elucidate the effects further, extensive physical modelling is currently underway at Hamburg University. The results form the basis of a comprehensive experimental databank for testing and validating advanced numerical dispersion models.

A comprehensive experimental databank for the models verification of urban car emission dispersion

A summary presentation is made of representative samples from a comprehensive experimental databank on car exhaust dispersion in urban street canyons. Physical modelling, under neutral stratification conditions, was used to provide visualisation, pollutant concentration and velocity measurements above and inside test canyons amidst surrounding urban roughness. The study extended to two different canyon aspects ratios, in combination with different roof configurations on the surrounding buildings. To serve as a reliable basis for validation and testing of urban pollution dispersion codes, special emphasis was placed in this work on data quality assurance.

Influence of the Near-Field Geometry on Field Measurements in Urban Street Canyons

The potential effect of nearby buildings on air quality observations in the field is inadequately understood. Rafailidis (1998) proposed a method for assessing this influence conservatively and on a long-term basis. It is based on subdividing the wind vector above the roofs into parallel- and normal- to street contributions (Figure 1). Under windy conditions, while U n exceeds the critical threshold for canyon vortex formation, air circulation and street pollution dispersion are described satisfactorily by the wind tunnel modelling of Rafailidis (1997). Additional flushing by U. mainly advects pollution along the street, without overcoming asymmetries which may develop depending on street canyon configuration (Figure 2). To validate the potential of the above analysis and assess its impact on actual urban canyon situations, this paper examines field observations, focusing on the existence of the suggested persistent vortex circulation and canyon reaeration patterns.