Large-Eddy Simulations with an Immersed Boundary Method: Pollutant Dispersion over Urban Terrain

Franck Auguste,Daniel Cariolle,Valery Masson,Christine Lac

doi:10.3390/atmos11010113

Abstract

In urban canopies, the variability of pollution may be influenced by the presence of surface heterogeneities like orography and buildings. Using the Meso-NH model enhanced with an immersed boundary method (IBM) to represent accurately the impact of the 3D shape of buildings on the flow, large-eddy simulations are performed over city of Toulouse (France) with the dispersion of a plume following a plant explosion on 21 September 2001. The event is characterized by a large quantity of nitrogen dioxide released in a vertical column after the explosion, quickly dispersed by a moderate wind prevailing in the lower atmospheric layers. Assuming a passive pollutant, the model develops a realistic plume dispersion. A sensitivity analysis of the advection scheme to the spread is presented. The limited population’s exposure to pollution developed by the model appears in good agreement with previous health studies. Beyond this case, IBM is a promising way to represent flow interaction with buildings and orography in atmospheric models for urban applications.

Highlights

Atmospheric pollution is an important societal and ecological challenge
This study reported numerical simulations of a pollutant episode in an urban environment linked to an explosion at a plant producing ammonium components
The atmospheric flow has been simulated by the Meso-NH code coupled with SURFEX and including an immersed boundary method to account for buildings

Summary

Introduction

Atmospheric pollution is an important societal and ecological challenge. The expansion of urban areas and their associated anthropogenic activities raise many questions concerning the health of urban populations. Industry constitutes an important source that can be intensified by accidental releases Predicting their dispersion requires meteorological models to consider the physics of the boundary layer like urban heat islands and urban flows, with a high accuracy to include microscale urban effects due to buildings. Due to the structured nature of buildings and cities, the implementation of an immersed boundary method (IBM; [5]) is an interesting alternative to unstructured grids classically used in computational fluid dynamics. One of the IBM techniques, the ghost-cell technique, was first developed in an ocean model by Tseng and Ferziger [7] to simulate a geophysical flow over a three–dimensional Gaussian bump It was implemented in the WRF atmospheric model [8,9], showing promising results. The IBM has been adapted to the anelastic condition by [11] in the Meso-NH model [12] and validated with analytical cases and the MUST field campaign

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