MODITIC - large-eddy simulations of dense gas dispersion in urban environments

The European Defence Agency (EDA) project B-1097-ESM4-GP “MOdelling the DIspersion of Toxic Industrial Chemicals in urban environments” (MODITIC) (2012 – 2016) has studied the release and transport of neutral and non-neutral chemicals in complex urban environments, in order to enhance the understanding of the dominating physical processes involved, and to support improvements in modelling techniques. This report describes the work conducted using large-eddy simulations (LES) to simulate release and dispersion of neutral and dense gases. The dispersion process takes place in geometries with increasing complexity, and thus an increasingly complex flow field. The main purpose of the study is to improve the methodology for high fidelity dispersion models, to study the simulated effects of dense gas release, and to validate the results against wind tunnel data. The simulations reported in this work element, WE5300, have been performed by the Swedish Defence Research Agency (FOI), the Institut National de l’Environnement Industriel et des Risques (INERIS), and the Norwegian Defence Research Establishment (FFI) using different solvers and methods to treat the dense gas release. In this report, results from the solvers OpenFOAM and CDP are presented. Results show that the methods used managed to predict the release and dispersion of both dense and neutral gas very well compared to the wind tunnel experiments. The complex flow fields were also simulated correctly. In all cases studied, there was a big difference in the dispersion pattern between dense and neutral gas. The dense gas was transported upwind from the source, against the wind, and the plume spread close to the ground and more laterally compared to the neutral gas. It was also seen that the dense gas changed the local wind field which led to reduced mixing and lower turbulence kinetic energy in areas with high concentrations. In order to assure satisfactory results, special care needs to be taken to the generation of the incoming turbulent boundary layer. This is especially true when there is no geometry (e.g. buildings) affecting the wind field.