Dispersion from road tunnel portals: comparison of two different modelling approaches

Two different models for the dispersion of tunnel exhausts are compared with each other. The first model was developed in Japan by several research institutions, and the second model was developed in Austria at the Institute of Internal Combustion Engines and Thermodynamics. To the best of the authors’ knowledge, these are the only dispersion models developed for that specific purpose. Significant differences in the modelling approach concern the basic physical assumptions as well as the formulation regarding the dispersion process. While the Japanese model incorporates the jet flow from the tunnel portal in a mass-consistent wind field model, the Austrian model incorporates the jet flow in the dispersion process itself. Further, the Austrian model assumes a “bending jet flow” in dependence on the ambient wind field, while the Japanese model does not. Since the ambient wind field fluctuates around a mean value, the position of the jet flow does also. This is an additional dispersion process treated in the Austrian model too. For the evaluation study, three tracer experiments performed at the Ninomiya, the Hitachi, and the Enrei tunnel in Japan were used. Comparison with observed concentrations reveals the ability of both models to simulate the dispersion of pollutants in the vicinity of road tunnel portals, although the Austrian model performs slightly better as indicated by e.g. the correlation coefficient, or the linear regression analysis.