CFD-RANS model validation of turbulent flow in a semi-idealized urban canopy

Scientific Journals
Denise Hertwig, George C. Efthimiou, John G. Bartzis, Bernd Leitl
Journal of Wind Engineering and Industrial Aerodynamics, 111, 61–72 (Impact factor: 2.049)
Publication year: 2012

Abstract

Urban flow fields computed by two steady Computational Fluid Dynamics models based on the Reynolds-averaged Navier Stokes equations (CFD-RANS) are compared to validation data measured in a boundary-layer wind-tunnel experiment. The numerical simulations were performed with the research code ADREA and the commercial code STAR-CD. Turbulent flow within and above a 1:225-scale wind-tunnel model representing a novel semi-idealized urban complexity represents the test case. In a systematic study the quality of the numerical predictions of mean wind fields is evaluated with a focus on the identification of model strengths and limitations. State-of-the-art validation metrics for numerical models were used to quantify the agreement between the data sets. Based on detailed spatial identification of locations of good or bad comparison the study showed how unsteady flow effects within street canyons are a major cause for discrepancies between numerical and experimental results.

Modelling concentration fluctuations and individual exposure in complex urban environments

Scientific Journals
G.C. Efthimiou, J.G. Bartzis, N. Koutsourakis
Journal of Wind Engineering and Industrial Aerodynamics, 99, 349-356 (Impact factor: 2.049)
Publication year: 2011

Abstract

One of the key problems in coping with deliberate or accidental atmospheric releases, which in many cases are short or/and result in high concentrations, is the ability to reliably predict the individual exposure during the event. Furthermore, for consequence assessment and countermeasures application, it is more realistic to rely on the maximum expected dosage rather than on the actual dosage. Recently, Bartzis et al. (2008) have introduced an approach relating maximum dosage to parameters such as concentration variance and turbulence integral time scale. The need for an estimation of these parameters poses new challenges to CFD models. In the CFD RANS model ADREA, new approaches have been implemented recently, where the parameterization of the dispersion of a pollutant emitted from a point source depends not only on the parameters of turbulence, but also on the pollutant travel times. In this study the new methodology is tested against MUST and FLADIS field experimental data, which consist of high resolution concentration time series enabling the production of short term dosage data. The present comparisons further strengthen the evidence that the applied methodology is capable of dealing properly with complex transient dispersion phenomena.

COST 732 in practice: The MUST model evaluation exercise

Scientific Journals
Di Sabatino, S, R. Buccolieri, H. Olesen, M. Ketzel, R. Berkowicz, J. Franke, M. Schatzmann, H. Schlünzen, B. Leitl, R. Britter, C. Borrego, A. M. Costa, S. Trini- Castelli, T. Reisin, A. Hellsten, J. Saloranta, N. Moussiopoulos, F. Barmpas, K. Brzozowski, I. Goricsan, M. Balczò, J. Bartzis, G. Efthimiou, J. L. Santiago, A. Martilli, M. Piringer, M. Hirtl, A. Baklanov, R. Nuterman, A. Starchenko
International Journal of Environment and Pollution Volume 44, Issue 1-4, Pages 403-418 (Impact factor: 0.515)
Publication year: 2011

Abstract

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.

Atmospheric dispersion and individual exposure of hazardous materials

Scientific Journals
G.C. Efthimiou, J. G. Bartzis
Journal of Hazardous Materials, 188, 375-383 (Impact factor: 6.025)
Publication year: 2011

Abstract

In this work a new approach for CFD RANS modelling of dispersion of airborne point source releases is presented. The key feature of this approach is the model capability to predict concentration time scales that are functions not only of the flow turbulence scales but also of the pollutant travel time. This approach has been implemented for the calculation of the concentration fluctuation dissipation time scale and the maximum individual exposure at short time intervals. For the estimation of travel time in the Eulerian grid the new ‘radioactive tracer method’ is introduced. The new approaches were incorporated in the CFD code ADREA. The capabilities of the new approaches are validated against the Mock Urban Setting Trial field experiment data under neutral conditions. The comparisons of model and observations gave quite satisfactory results.

Air dispersion modelling for individual exposure studies

Scientific Journals
G. Efthimiou, J.G. Bartzis, S. Andronopoulos, A. Sfetsos
International Journal of Environment and Pollution, Vol. 47, pp. 302 – 316 (Impact factor: 0.515)
Publication year: 2011

The concentration fluctuations of a dispersing hazardous gaseous pollutant in the atmospheric boundary layer, and the hazard associated with short-term concentration levels, demonstrate the necessity of estimating the magnitude of these fluctuations using predicting models. To predict and estimate the maximum expected dosage and the exposure time within which the dosage exceeds certain health limits, the knowledge of the behaviour of concentration fluctuations at the point under consideration is needed. The whole effort is based on the field experiment MUST (Biltoft, 2001) and the computational simulations have been performed with the CFD code ADREA.