A NEW VALIDATION DATA SET FOR POLLUTANT DISPERSION MODELS B. Leitl 1, M. Schatzmann 1, H. Thielen2, R. Martens2 University of Hamburg, Meteorological Institute, Hamburg, Germany 2 Gesellschaft für Anlagen- und Reaktorsicherheit (GRS), Köln, Germany e-mail:
[email protected] 1
1. INTRODUCTION The emergence of powerful computers stimulated the development of increasingly sophisticated numerical dispersion models. In recent years they have begun to play an important, often dominant, role in environmental assessment studies undertaken to investigate and quantify the effects of human activity on ecosystems or local climate. Typically all these models contain substantial empirical input, not only in the turbulent closure scheme but also in other parameterisations. The widespread use of numerical models is paralleled by a growing public awareness that the majority of these models have never been the subject of rigorous evaluation and quality assurance. This results into a lack of confidence in the modelled results, which has often been justified, by systematic studies in which • applications of the same model by different modellers to a given problem (Hall et al., 1997) and • applications of different models by either the same or different modellers to the same problem (Ketzel et al., 2001) revealed significant differences. Nevertheless the models are used in the preparation of decisions with profound economic and political consequences. As was demonstrated in Schatzmann et al. (2002), this problem is particularly serious for micro-scale dispersion problems which are characterised by large local concentration gradients due to the receptor points being close to pollutant sources. Reason for the deficit in quality assurance of models is seldom the ignorance of the model developer but more often the lack of appropriate validation data, thus preventing a model from being carefully tested. 2. REQUIREMENTS FOR VALIDATION DATA SETS There are some basic requirements for a validation data set: • The data set must be complete in that all boundary and input conditions are clearly defined. In other words, the degree of freedom a numerical modeller has in setting up a test run should be minimized. • The data set should comprise a sufficiently large number of individual cases in which the most important input parameters are varied in a systematic way over the whole range of interest. If a model successfully replicates all of these, it can be assumed that it would correctly predict also cases which were not covered in the test runs. • The data set should be sufficiently large in that it allows the assessment of whether or not a model is able to replicate complete fields of the variables. Models which show good agreement with measurements at one or two locations might fail to do so at the third. Above all when a property of interest is expected to show significant local variation, the density of measurement points must be large.
•
The data itself must be of known quality. This means the data set must come with a documentation which includes not only a clear description of the experimental set-up, but also a quantification of the uncertainties inherent to the data. Field experiments are seldom able to match the requirements stated above. Basic model testing is therefore predominantly carried out with wind tunnel data. 3. EXPERIMENTS A number of completely documented reference data sets is already available for simplified urban-type dispersion problems (see for instance the CEDVAL data bank (Leitl, 2000) under www.mi.uni-hamburg.de/cedval), but hardly any laboratory data have been published for momentum-free ground-level releases in flat uniform terrain. The lack of such data is contrasted by a variety of practical dispersion problems that can be assigned to ground-level sources. Such problems include the dispersion of gases emitted from waste dumps or radon releases from mining relics. In order to remedy the lack of qualified reference data, the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety recently sponsored a number of systematic experiments in the wind tunnel laboratory of Hamburg University. In the environmental wind tunnel ‘BLASIUS’ a neutrally stratified boundary layer flow over flat and open terrain was modelled. After carefully measuring and completely documenting the modelled boundary layer, three idealized ground level sources were set into the tunnel floor. Passive tracer gas was emitted from (1) a point source, (2) a line source and (3) a rectangular area source. The resulting tracer plumes downwind from the sources were measured for several lateral and vertical profiles. At present the wind tunnel data are being processed. They will soon be made available to the scientific community via the CEDVAL data bank. The boundary layer was generated through a combination of vortex generators and roughness elements. Velocity time series were measured in several heights above ground and turbulent macro scales determined. The macro scales in combination with the roughness length (determined from the logarithmic wind profile) led to the conclusion that the scale of the boundary layer was 1:450. Only values transformed to full scale will be reported in this paper. Figs. 1 to 3 show a selection of properties of the wind tunnel boundary layer. Shown are normalised vertical profiles of the mean wind velocity at positions upstream from the sources and at the end of the test section (Fig.1). The roughness length of the boundary layer was determined to be 0.1m and the power law exponent to be 0.18. In Fig. 2 the turbulence intensities (defined by Ii = σi(z)/U(z), with σi the standard deviation of velocity fluctuations of wind component i ) are given. Over the lowest 40 m above ground the wind tunnel data fits well to the reference curves derived from measurements in the real atmosphere. Above this height some deficits in the Iw –profile were observed which could have rectified by further adjustments of the vortex generators and roughness elements. Since the dispersing clouds keep close to the ground, additional effort to improve the agreement was not taken. The longitudinal power spectrum measured at 20 m above ground is given in Fig 3. It compares reasonably well with the curves determined by Kaimal, v.Karman or Simiu and Scanlan (1986) in corresponding field experiments. Fig. 4 shows a sketch of the 3 sources which were investigated. As can be seen in Fig.5 the sources were set flash into the tunnel floor. The source design deserves mentioning. In order
to assure an emission flux absolutely homogeneous over the complete source area, the sources were made up by numerous hypodermic needles with an inner diameter between 0.2 and 0.3 mm. A steady and continuous tracer flow through the needles was ensured by maintaining a large pressure drop over the length (70 mm) of the needles (Meroney et al., 1996). The needles were distributed in regular arrays over the source cross section The largest distance between 2 needles was 5 mm (area source). Due to the large number of needles, the source flows were virtually free of vertical momentum (The exit velocity varied between 0.03m/s (area source) and 0.45 m/s(line source)). Flow visualisation experiments and test measurements proved that the plumes remained at ground for these velocities. 4. RESULTS The results are presented in form of normalised concentrations (C measured − Cbackgrund )⋅ U reference [in 1/m²] C* = C Source ⋅ Qtracer as functions of distance from the source (x), distance from the centreline (y) or distance from the ground (z). C stands for concentrations, the velocity Uref refers to a reference height of 10 m and Q is the tracer volume flux in m³/s. Figs. 6 and 7 show normalised ground level concentrations for each of the 3 sources at identical downstream positions and Fig. 8 concentration fields in vertical crosssections perpendicular to the wind. The comparison of results reveals that with increasing source distance the lateral profiles of the 3 plumes become more and more identical. The specific source configuration is in the near field of the plume of significance only . The longitudinal concentration decay in the far field of the plumes follows the -2/3 – law known from Berljands (1982) analytical solution for point sources. More detailed results will be presented in the talk. 5. REFERENCES Berljand, M. E. 1982: Moderne Probleme der atmosphärischen Diffusion un der Verschmutzung der Atmosphäre, Akademie-Verlag, Berlin 1982 Hall, R.C. (Ed.), 1997: Evaluation of modelling uncertainty - CFD modelling of near-field atmospheric dispersion. EU Project EV5V-CT94-0531, Final Report. WS Atkins Consultants Ltd., Woodcote Grove, Ashley Road, Epsom, Surrey KT18 5BW, UK. Ketzel, M., Louka, P., Sahm, P., Guilloteau,E., Sini, J.-F., and Moussiopoulos, N., 2001: Inter-comparison of numerical urban dispersion models - Part II: Street canyon in Hanover, Germany. Proc. 3rd Int. Conf. on Urban Air Quality, Loutraki/Greece. Leitl, B., 2000: Validation Data for Microscale Dispersion Modelling. EUROTRAC Newsletter, 22, 28-32. Meroney, R.N., Pavageau, M, Rafailidis, S. and Schatzmann, M. 1996: Study of Line Source Characteristics for 2-d-Physical Modelling of Pollutant Dispersion in Street Canyons. Journ. Wind Eng. and Ind. Aerodynamics, 62, 37-56. Schatzmann, M., and Leitl, B. 2002: Validation and application of obstacle resolving urban dispersion models. Atmospheric Environment,.36, 4811-4821.
Simiu, E., Scanlan, R. H. 1986: Wind Effects on Structures – Part A: The Atmosphere. Wiley & Sons Inc.
Acknowledgements: The authors are grateful for financial support from the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety through a grant from the Federal Agency for Radiation Protection.
225
200 Mean Profile 1 Mean Profile 2
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IU ,z0 =0.5m WindTunnel
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Fig.1 Mean vertical wind profil 10
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Fig. 2: Turbulence intensity profil
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Fig. 3: Longitudinal turbulence Spectrum
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Fig. 4: Top view on the three sources investigated
Fig. 5: View into the wind tunnel equipped with the area source.
Fig. 6: Ground level concentration decay with source distance at the plume centre-line for the three sources investigated.
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Fig. 7: Lateral ground level concentration profiles for several source distances
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Point Source at X = 225 m C/C0*U/Q [1/m2]: 1.0E-05
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Fig. 8: Concentration fields in vertical planes perpendicular to the flow measured at corresponding downstream positions. Top: Point source, Centre: Line source, Botton: area source.
