Comparison between wind tunnel experiment and large-eddy simulation of concentration fluctuations in pollutant dispersion in a realistic urban area

The time-dependent properties of concentration fluctuations in pollutant gas dispersion in urban areas are associated with various environmental and health assessment problems. In this study, we investigated the performance of a large-eddy simulation (LES) in predicting the time-dependent nature of pollutant gas dispersion emitted from a point source near the ground in a realistic urban area representing a highly dense city in Tokyo. A wind tunnel experiment was conducted on the gas dispersion using a fast flame ionization detector for numerical validation. We found that the LES results were qualitatively in good agreement with the experimental results for the mean concentration and higher-order moments. The vertical distributions of the concentration statistics above the main road where the mean plume path from the source was located were similar to those of the open terrain, although there was an approximately vertically constant distribution in the wake region of the high-rise buildings. The LES accuracy was quantitatively evaluated using validation metrics for the concentration statistics. A slightly poorer agreement for higher-order moments was observed despite obtaining good results for the mean concentration.