Large-Eddy Simulation of Turbulent Flow and Plume Dispersion in a Spatially-Developing Turbulent Boundary Layer Flow

Abstract

In one of local-scale dispersion problems, we have an important issue in the accurate prediction of airborne contaminant dispersion from industrial or nuclear facilities for safety and consequence assessments of nuclear facilities. For evaluating radiological consequences of radioactive materials, it is need to predict not only the material concentration in the air at the evaluation point for internal dose but also on the three-dimensional distribution of the plume and surface deposition for external dose. In a flat terrain, time-averaged concentration of a plume can be easily predicted by a conventional Gaussian plume model. However, in Japan, most nuclear facilities are located in complex coastal terrain. Therefore, it is important to predict the spatial distribution of concentrations considering effects from terrain and buildings. Another issue related to atmospheric dispersion in a local-scale is the potential problem that hazardous and flammable materials are accidentally or intentionally released into the atmosphere, either within or close to populated urban areas. For the assessment of human health hazards from such toxic substances, the existence of high concentration peaks in a plume should be considered because it is the instantaneous, not average, concentration that is fatal to humans. In such a situation, it is necessary to accurately predict the unsteady behavior of a plume, considering the effects of individual buildings. For the safety analysis of flammable gases, certain critical threshold levels should be evaluated. Therefore, in such a situation, not only the average levels but also instantaneous magnitudes of concentration should be accurately predicted. There are various methods for predicting plume dispersion in atmospheric boundary layers, e.g. wind tunnel experiments and Computational Fluid Dynamics (CFD). It is well known that wind tunnel experiments are a rational tool for predicting plume dispersion behavior under local topography and/or building conditions. For the case of accidental or intentional release of contaminated materials within urban areas, many studies using a wind tunnel have been made to investigate the spatial extent of contaminated areas and the characteristics of mean and fluctuating concentrations around an individual building. In the safety assessment for the construction of nuclear facilities, prediction of the spatial distribution of radionuclide concentrations over complex terrain containing buildings is required by using a wind tunnel (Meteorological Guide for Safety Analysis of Nuclear Power