Bernd Leitl

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’.

Car exhaust dispersion in a street canyon. Numerical critique of a wind tunnel experiment

Abstract Due to increasing car traffic in cities, problems related to car induced air pollution in street canyons have become important. Physical modeling in wind tunnels or numerical codes may be used for dispersion simulation when investigating air quality. Rafailidis [in: Annalen der Meteorologie] carried out an extensive set of test runs recently in the BLASIUS wind tunnel at the Meteorological Institute of the University of Hamburg, Germany. In the present study the wind tunnel experiments were simulated numerically using the CFD-code Fluent®. In a first approach, the idealized two-dimensional case was calculated. Several test runs have been carried out to study the effect of emission rate and source design on flow structures and dispersion in the street canyon. It could be shown that alternative emission conditions and the source design might affect the concentration field within a modeled street canyon. A second set of calculations for a simplified three-dimensional simulation of the street canyon setup was performed to investigate the presence of secondary flow patterns found during wind tunnel tests. The lateral flow structure within the street canyon observed during wind tunnel measurements was simulated, and the effect of changing boundary conditions on the secondary flow structure was studied. In the paper the advantages of CFD simulations for planning wind tunnel dispersion tests are discussed.

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.

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.

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.

Effect of roughness inhomogeneities on the development of the urban boundary layer

Wind tunnel measurements over a regular array of cubes were carried out to investigate the influence of surface inhomogeneities on the development of the urban boundary layer. Two idealised cases were investigated. The first one was in terms of a strong step change in roughness from very smooth to very rough with a low turbulent approach flow and a thin approach boundary layer. For this case it was found that a very long fetch was needed until the flow reaches equilibrium with its underlying surface. In the second case the approach boundary flow was turbulent and equilibrium was reached much faster, but the necessary fetch was still significant. The results were compared and conclusions for roughness parameterisations in numerical models were drawn.

Field measurements within a quarter of a city including a street canyon to produce a validation data set

Air pollutants and meteorological parameters were measured continuously by in situ instruments, path-averaging techniques (up to three DOAS systems), and SODAR inside a street canyon and in the surrounding area of 1 km x 1 km (Gottinger Strasse in Hanover, Germany) from 2001 until 2003 which are available in the data bank ValiData for validation of microscale models. During three IOPs tracer experiments with a SF6 line source, sampling techniques of up to 15 sites and path-averaging FTIR spectrometry were performed. Concentration measurement results at roof level were anti-correlated with SODAR mixing layer heights, while those inside the street canyon are not. Re-circulation flow patterns inside the street canyon were studied together with corresponding wind tunnel experiments.

CFD-RANS prediction of individual exposure from continuous release of hazardous airborne materials in complex urban environments

ABSTRACTOne of the key issues of recent research on the dispersion inside complex urban environments is the ability to predict individual exposure (maximum dosages) of an airborne material which is released continuously from a point source. The present work addresses the question whether the computational fluid dynamics (CFD)–Reynolds-averaged Navier–Stokes (RANS) methodology can be used to predict individual exposure for various exposure times. This is feasible by providing the two RANS concentration moments (mean and variance) and a turbulent time scale to a deterministic model. The whole effort is focused on the prediction of individual exposure inside a complex real urban area. The capabilities of the proposed methodology are validated against wind-tunnel data (CUTE experiment). The present simulations were performed ‘blindly’, i.e. the modeller had limited information for the inlet boundary conditions and the results were kept unknown until the end of the COST Action ES1006. Thus, a high uncertainty ...

Development of local-scale high-resolution atmospheric dispersion model using large-eddy simulation. Part 4: turbulent flows and plume dispersion in an actual urban area

We have developed a local-scale high-resolution atmospheric dispersion model using large-eddy simulation (LOHDIM-LES) to assess the safety at nuclear facilities and to respond to emergency situations resulting from accidental or deliberate releases of radioactive materials (e.g., a terrorist attack in an urban area). In Part 1, the unsteady behavior of a plume dispersing over a flat terrain was successfully simulated. In Parts 2 and 3, LESs of turbulent flows and plume dispersion around an isolated building and in building arrays with different obstacle densities were performed, which showed the basic performance comparable to wind tunnel experimental technique. In this study, we apply the LES model to turbulent flows and plume dispersion in an actual urban area. Although some of the turbulence and dispersion characteristics are quantitatively different from the wind tunnel experimental data, the distribution patterns are generally similar to those of the experiments. It is concluded that our LES model simulates reasonably the unsteady behavior of turbulent flows and plume dispersion even for complex heterogeneous urban areas.