Michael Schatzmann

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.

WIND TUNNEL MEASUREMENTS OF CONCENTRATION FLUCTUATIONS IN AN URBAN STREET CANYON

Abstract A wind tunnel study was performed to examine some turbulent characteristics and statistical properties of the concentration field developing from the steady release of a tracer gas at street level in a canyon amidst urban roughness. The experiment was conducted with the approaching wind direction perpendicular to the street axis and, with a street width to building height aspect ratio equal to one. Concentration time series were recorded at 70 points within the test street cross-section and above. Mean concentrations, variances and related turbulent quantities, as well as other statistical quantities including quantiles were computed. Concentration spectra and autocorrelation functions were also examined. The emphasis is put here on the results concerning mean concentrations and the variance of concentration fluctuations. The main objective of this paper is to put forward potential benefits of the experimental approach taken in this study. Through a simple and already widely studied configuration it is aimed to show how, for modelling purposes, this approach can help improving our understanding of the mechanisms of dipersion of pollution from car exhausts in built-up areas and, with further measurements, how it could assist in drawing specifications for siting monitoring networks.

Wind-tunnel and numerical modeling of flow and dispersion about several building shapes

Abstract The flow and dispersion of gases emitted by sources located near different building shapes separately studied in various wind tunnels were determined by the commercial prognostic model FLUENT and FLUENT/UNS using the standard k–e, the RNG k–e, and the Reynolds-stress RSM turbulence closure approximations. Inlet conditions and boundary conditions were specified numerically to the best information available for each fluid modelling simulation. Calculations are compared against the wind-tunnel measurements, but no special effort was made to force-fit agreement between the numerical and experimental data by post adjusting, coefficients, surface roughness, initial conditions, etc., beyond the specifications supplied by the laboratory researchers. The intent of these calculations were to determine if a relatively robust commercial CFD package using “reasonable” boundary and initial conditions could be used to simulate wind engineering situations without massaging the results interactively.

Validation and application of obstacle-resolving urban dispersion models

Abstract The development of micro-scale meteorological models has progressed in recent years. Some of them are already commercially available. With little hesitation, consulting engineers apply them to complex real-world problems. How accurate are the results? Using the example of urban dispersion models, the paper tries to give a critical assessment of the present ‘state of application’.

Quality assurance and improvement of micro-scale meteorological models

Models have begun to play an important role in environmental assessment and urban climate studies. Their increasing use, however, is paralleled by a growing awareness that the majority of these models have never been the subject of rigorous evaluation. Consequently there is a lack of confidence in the modelled results. The paper reports on a European initiative (COST 732) which was launched 2005 with the purpose of establishing a generally accepted procedure for the improvement and quality assurance of micro-scale meteorological models that are applied for predicting flow and transport processes in urban or industrial environments.

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.

COST 732 in practice: the MUST model evaluation exercise

The aim of this paper is to describe the use of a general methodology tailored to the evaluation of micro-scale meteorological models applied to flow and dispersion simulations in urban areas. This methodology, developed within COST 732, has been tested through a large modelling exercise involving many groups across Europe. The major test case used is the Mock Urban Setting Test (MUST) experiment representing an idealised urban area. It is emphasised that a full model evaluation is problem-dependent and requires several activities including a statistical validation that requires a careful choice of the metrics for the comparison with measurements.

An advanced integral model for cooling tower plume dispersion

Abstract An advanced integral model is developed for predicting cooling tower plume rise from single natural draft cooling towers. The theoretical formulation of the model is aimed at avoiding many of the pitfalls and unnecessary assumptions of existing models. The model is based on a careful integration of the three-dimensional partial differential equations of conservation across the plume cross-section; radial profiles of temperature, velocity, and total water are assumed to be Gaussian in shape. The model includes a treatment of plume thermodynamics and tower downwash effects. The model has been calibrated with a wide range of laboratory data. Verification of the model with single-tower field data from Chalk Point, Paradise, Lunen, Gardanne and Philippsburg reveals good results.

Dispersion in urban environments : Comparison of field measurements with wind tunnel results

A wind tunnel study was performed to determine the dispersion characteristics of vehicle exhaust gases within the urban canopy layer. The results were compared with those from a field monitoring station located in a street canyon with heavy traffic load. The agreement found was fair. In the second part of the paper it is shown how wind tunnel data can be utilized to supplement and thereby enhance the value of field data for model validation purposes. Uncertainty ranges were quantified which are inherent to mean concentration values measured in urban streets.

The atmospheric impact on fluxes of nitrogen, POPs and energy in the German Bight

The external forcing of the German Bight system is largely due to the atmosphere. Energy fluxes that drive mixing processes and biological productivity, as well as atmospheric nutrient inputs outside the Elbe estuary, are important factors for biomass production. This study is based on the KUSTOS experiments focusing on air-sea exchange with intensive observations of a) radiative fluxes at the surface of the drifting water body; b) atmospheric surface layer parameters determining the mixing conditions in the planetary boundary layer; c) the speciation of atmospheric nitrogen compounds; d) and changes in aerosol and gas composition during transport over sea. These episodic data were complemented by a) synoptic data analysis of water and air temperature, wind, pressure and water vapour pressure over the sea; b) corresponding oceanic data on heat advection and mixed layer depth from an oceanic model driven by observations in the atmosphere; c) computations of the highly variable heat and radiative fluxes with the mesoscale atmospheric model METRAS; d) long-term atmospheric deposition measurements of nutrients in the German Bight; e) investigations of the atmospheric processes responsible for the formation of coarse particulate nitrate by means of a new aerosol submodel in the METRAS transport model. We present detailed seasonal or annual budgets for fluxes of heat, momentum, nitrate, ammonium, persistent organic pollutants. The atmospheric fluxes of heat and chemical matter are compared with load and fluxes in the water column in order to identify when and where the atmospheric impact is relevant and detectable. Spatial and temporal variability is discussed for the fluxes of heat, momentum and nitrogen. From the budgets we identify categories of potential atmospheric impact. Apart from the category “no atmospheric impact≓ valid e.g. for Cr, As, Ni and phosphate, we identify 4 others: 1) “atmosphere driven≓: short term, local dominant impact for Heat and momentum; 2) “episodic atmospheric impact≓: long term, local and dominant impact with large fluxes involved for radiation, PCB, Pb; 3) “persistent atmospheric pollutant≓: long term dominant but regionally indifferent impact for α- and γHCH; 4) “steadily perturbing the marine ecosystem≓: long term, widespread impact superimposed on the dynamic system driven by marine biology for nitrate, ammonium.